39
Drying of Biotechnological Products
Janusz Adamiec, Władysław Kamin´ski, Adam S. Markowski,
and Czesław Strumiłło
CONTENTS
39.1 INTRODUCTION
Biotechnology is the action aiming at producing use-
ful products for various branches of the economy by
means of biological components and microorganisms,
viruses, animal and vegetable cells, as well as extra-
cellular substances found within tissues. The growing
scope of these activities includes production of a bio-
logical system, a producer strain, using the recombin-
ation technique and cell engineering. As a result of
processes taking place in the presence of microorgan-
isms, materials of various forms are produced, such as
microorganisms similar to the inlet materials, e.g.,
yeast and bacteria. The product may be a substance
with a complex chemical structure in the form of a
high-molecular polymer or organic compound (e.g.,
antibiotics, vitamins, and organic acids).
Biotechnology includes several basic processes,
the most important of which is the fermentation pro-
cess (
). In this proce ss the fund ament al
transformations of reagents take place in the presence
of microorganisms. This is a complex process of
biochemical transformations depending on many
microbiological, chemical, physical, and mechanical
factors. These factors include microorganism cultures
and strains, types and amounts of additives, pH of the
medium, temperature, mixing intensity, and so on.
The other process, which is of equal importance, is
preservation of the product of microbiological trans-
formation. The preservation process depends, among
other things, on product applicability, the way it is
utilized, storage possibility, and conditions. Some-
times the product can be used jointly with by-
products and unreacted residues of the culture med-
ium and nutrients. Then, it is a raw material for
microbiological concentrates (as a result of filtration,
concentration, or drying). When the product is used
as a purely microbiological material, it should be
separated from the fermentation medium, impurities,
and residues of reagents, and then the purified sub-
stance should be concentrated and preserved by re-
moval of solvent (water) through freezing or drying.
The aim of this chapter is to present methods of
preservation of biotechnological products by drying
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2006 by Taylor & Francis Group, LLC.
while maintaining required product quality. In the
first part, a biotechnological product is presented as
an object of drying. Particular attention is given to its
sensitivity and qualitative changes due to dewatering
and elevated temperature. In the second part, quali-
tative changes of the product are considered in the
mathematical description of the drying process, its
modeling, and optimization of process parameters.
Finally, many known drying methods are character-
ized with reference to their advantages and disadvan-
tages as applied to biotechnological products.
This chapter has been updated from the previous
version [1]. However, its structure has been preserved
and the most important trends and procedures applied
in drying of biotechnology products are emphasized.
The chapter is supplemented with the results of the
latest research, and most recent references are quoted.
Worth mentioning are the monograph Thermal
Processing of Bio-materials [2] and relevant parts of
Advanced Drying Technologies [3] that have an appli-
cative character. It is also worth referring the mono-
graph on bioproduct granulation [4] that presents
problems related to thermal impacts on a material
and drying of bioproducts. From among many gener-
ally known source materials it is worth to mention the
conference proceedings concerning preservation, con-
centration, and drying of foodstuffs [5–8] and the
studies on the role of water in food and biological
products [9–12]. Many monographs on drying of
biotechnology products were published in Russian
[13–21]. There are still up-to-date general publications
presenting the analysis of specific conditions of drying
and preservation of biotechnology products used in
many European countries [22–30], and also valuable
publications that give an account of experimental
results [31–35].
39.2 BIOTECHNOLOGICAL PRODUCTS
AS MATERIAL BEING DRIED
A typical suspension of liquid fermentation culture is
an aqueous mixture of microorganisms or biopoly-
mers, unreacted residues of nutrients, by-products,
process-controlling additives, and so on, with dry
mass content amounting to several percent. In the
process of concentration and drying of these mater-
ials, attention cannot be paid only to the removal of
water, which is the medium and solvent of a sub-
stance, but also to the removal of water, which is a
constructional element of the product particle. The
presence or absence of such water is of fundamental
importance for biotechnological product storage life,
its activity, and applicability. In general, dried mater-
ials have been classified as drying objects according to
their structure and type of water binding [18,20].
A variety of biotechnological products, from the
point of view of both their nature and structure, do
not allow precise specification of the form of material-
moisture binding.
Drying
Concentration
Fermentation
Cooling
Inoculation
Air
Outlet
gas
Isolation
extraction
Fermentation
media
Sterilization
Solvent
removal
Biotechnological
product
Intermediate
products
Mixing
aeration
FIGURE 39.1 Schematic diagram of the biotechnological process.
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2006 by Taylor & Francis Group, LLC.
In general , it is assum ed that these mate rials be-
long to colloi dal-capilla ry-poro us substa nces in which
all form s of mate rial-mo isture bonds are possibl e.
Apart from water solvent , there are also free wat er
and bound wat er. A signi ficant pa rt of water
occurri ng in the cell s of microorgan isms is free
water, while that in the macrom olecul es of biopoly-
mers and prod ucts of microbiolog ical trans form -
ations is mostly bound water (Figur e 39.2). The
biosynt hesis products are usually hydrophil ic colloid s
that can hold up a significa nt amount of bound water.
In the micr oorganis m structure, e.g., a typic al
bacter ial ce ll, there are man y impor tant elem ents
(e.g., nuc leus, cytopl asm, cell membran e) built of a
large number of various inorgani c an d organ ic com-
pounds with molec ular wei ghts that range from sev-
eral hundr eds to thousands of daltons . Among all
compon ents of a live cell, water prevai ls, amou nting
to 70 to 98 %. In micro organis ms only ab out 15 to
18% bound water was found . A major pa rt of the
water pre sent in the c ell is free wat er that is a medium
for va rious react ions and a solvent of many sub -
stance s. Fluctuat ions in wat er level can ca use distu rb-
ances in bio chemical process es that take place in the
cell because many reactions, includ ing those catalyzed
by enzymes , depen d on hyd ration of a cell as a whole
and of cell protein s. Interacti ons typica l for drying
that cause changes in moisture con tent an d tempe ra-
ture of a prod uct include many trans form ations
affecti ng its qua lity. Examp les of basic changes a re
given in
. Four types of changes are dist in-
guished: (a) biochemi cal (microbiol ogical ); (b) en -
zymatic; (c) chemi cal; and (d) phy sical.
Bio chemic al changes charact eristic of yeast or
bacter ia are strictly connected with the loss of wat er
in the cells and in their indivi dual structural elem ents.
Enzymat ic changes include mainl y the changes of
activit y cau sed by struc tural biop olymer deco mpos-
ition. Chem ical chan ges us ually resul t in a de crease of
nutritiv e values of biotec hnologica l products and in a
formati on of sub stances nox ious to the environm ent.
The effect of these chan ges can easily be obs erved.
Biochemi cal and chemi cal chang es are often revealed
as physica l chan ges: the biotec hnologica l product
loses its solubi lity or water- binding capacity; it also
loses aromatic compound s due to decompo sition or
high volat ility. Decol oratio n of the product is often
observed.
Anothe r set of quality cha nges is shown in
, which pre sents exampl es of such chan ges in
particular group s of biotech nologic al pr oducts and
their main componen ts dur ing drying. In general , it
may be stated that the basic de grading mech anism is
the denatura tion of protei n, which is the main com-
ponen t of both live microo rganisms and pro ducts of a
microb iological transform ation.
Rock land and Beuc hat [9] present ed a diagra m
illustrati ng the effe ct of moisture content (exact ly,
water activit y) on the react ions of compo unds and
the preser vatio n of the activit y of a biotech nologic al
substa nce or food stuff (cf.
) .
The level of water removal from the reaction med-
ium depends on the nature of the substance and on the
applicability of the product, i.e., whether after drying
the substance remains biologically active, is in anabi-
osis, or becomes an inactive organism with determined
structure and biochemical composition. An example
can be baker’s yeast (with a final moisture content of
about 10% by weight—active yeast, after rehydration
being able to grow and undergo biochemical transform-
ations) and fodder yeast (with a final moisture content
of about 5% by weight—dead microorganisms with no
microbiological activity after rehydration). An effect of
moisture content on the percentage of living cells is
presented in
[20].
Tem peratur e and the durati on of therm al process -
ing are the other fact ors on which micr obiologic al
H
H
O
OH
H
H
H
O
H H
Na
O
H H H H
H
H
H
H
H
H
H
H
H
O
O
O
O
O
O
+
H H
H
O
O
O
H H
O
O
H H
O
H H
H
H
H
R
R
H
H
R
O
O
O
O
O
H H
H
H
H
H
O
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
H
2
H H
N
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
N
N
N
N
NH
N
N
N
N
N
R
Water dipole
Secondary
structure bond
Side chain
Primary
structure bond
FIGURE 39.2 Schematic diagram of water particle addition
to polar protein groups.
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2006 by Taylor & Francis Group, LLC.
propert ies of preser ved bio technol ogical pro ducts de-
pend. In general , a lower process tempe rature en ables
a longer process ing tim e. How ever, as follo ws from
drying theory, unde r strictl y control led heat co ndi-
tions it is pos sible to use elevat ed tempe ratures of
the dry ing agent during drying of therm olabile ma-
terials . The effect of tempe rature and time on the
percent age of live micro organis ms is shown in
To quantitatively analyze the biotechnological
product resistance and durability, changes of one or
several quality factors considered most important are
assumed. In general, a qualitative change can be de-
termined by the index of quality degradation A; the
indices written as A
X
or A
T
refer to quality changes
resulting from the changes in moisture content and
temperature, respectively. They are the function of
time and reflect the number (concentration) of living
TABLE 39.1
Main Changes of Biotechnological Product Properties during Drying
Biochemical
(microbiological)
Enzymatic
Chemical
Physical
Atrophy of microorganisms (cells)
Loss of activity
Decrease of nutritive
values and activity
Solubility
Rehydration
Shrinkage
Loss of aroma
Yeast
Enzymes
Protein
Every biotechnological
Bacteria
Vitamins
Carbohydrates
product
Molds
Fats
Antibiotics
Amino acids
TABLE 39.2
Examples of Degradation Processes of Biotechnological Product Components during Thermal Drying
Product
Type of Reaction
Degradation Processes
Result
Live microorganisms
Microbiological changes
Destruction of cell
membranes
Denaturation of
protein
Death of cells
Lipids
Enzymatic reactions
Peroxidation of lipids
(discoloration of the
product)
Reaction with other
components (including
proteins and vitamins)
Proteins
Enzymatic and chemical
reactions
Total destruction of
amino acids
Denaturation of proteins
and enzymes
Derivation of some
individual amino acids
Partial denaturation, loss
of nutritive value
Cross-linking reaction
between amino acids
Change of protein
functionality
Enzyme reaction
Polymer carbohydrates
Chemical reactions
Gelatination of starch
Improved digestibility and
energy utilization
Hydrolysis
Fragmentation of
molecule
Vitamins
Chemical reactions
Derivation of some amino
acids
Partial inactivation
Simple sugars
Physical changes
Caramelization (Maillard-
Browning reaction)
Loss of color and flavor
Melting
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2006 by Taylor & Francis Group, LLC.
Stability isotherm
Browning reaction
Moisture sorption
Isotherm
Fatty
acids
Oxidation
Mould
Yeast
Bacteria
Enzymes
Autooxidation
Carotenoids
Chlorophyl, anthocyanins
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Water activity a
w
Moisture content/relative activity
Ionic
Covalent
Solute &
capillary
Bonding
FIGURE 39.3 Water activity–stability diagram. (From L.B. Rockland and L.R. Beuchat (Eds.), Water Activity: Theory and
Applications to Food, Marcel Dekker, New York, 1987. With permission.)
4.5
3.6
1.8
2
3
1
5
4
0
0.9
20
X, %w.b
40
60
80
A
X
=
C (t)
C (0)
100
2.6
6.0
14.9
36.6
90.0
2.7
A
X
, %
In
A
X
FIGURE 39.4 The effect of moisture content on the percentage of living cells of microorganisms: (1) Lactobacillus plantarum;
(2) Rhizobium pisum; (3) Beauveria bassiana; (4) Rhodotorula sp., Candida sp.; and (5) Saccharomyces sp. (From E.G. Tutova
and P.S. Kuts, Drying of Microbiological Products, Agropromizdat, Moscow (1987) (in Russian).)
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2006 by Taylor & Francis Group, LLC.
microo rganisms C ( t) or the activit y of a product a( t )
as related to the values characteriz ing the beginn ing
of the process C(0) or a(0):
A
X
or A
T
¼
C ( t)
C (0)
100,% (39 : 1)
or
A
X
or A
T
¼
a( t )
a (0)
100, %
When ch aracterizin g the qua lity of a prod uct of
microb iological trans formati ons (and food prod ucts
as wel l) a large number of propert ies are taken into
accoun t. In
propert y is given, namel y, the percent age of biologi c-
ally acti ve biomas s cells as a function of moisture
content A
X
, tempe ratur e A
T
, and process time t . Lit-
erature on pro duct quality refers to different pro per-
ties and transform ations that affect therm al
preser vation an d storage of biotec hnolog ical and
food produc ts [6–9].
The biotech nologic al produ cts can be class ified as
drying object s using specia l criteri a (e.g., resistance to
elevated tempe ratur e or suscept ibilit y to drying) .
Tutova and Ku ts [20] app lied the above criteri a and
proposed the class ification of biotec hnologica l pro d-
ucts into two groups:
1. Materials nonr esistant therm ally, microo rgan-
isms of high death rate due to therm al treat -
ment; they are cha racterized by a relative ly
high va lue of critical , bounda ry mois ture con -
tent. So, these are therm o- and xerola bile sub -
stances. The assum ed bounda ry values are
maximum at tempe rature, abo ut 60 8 C and
moisture co ntent, about 35 to 40% by weight.
In this group, bacterial cultur es, enzymes , vir-
uses, yeast , and fungi are included.
2. Biotechnol ogical produ cts of high therm al re-
sistance, with a therm al inact ivation rate that is
low and crit ical mois ture content reaches sev-
eral pe rcent. So, these are therm o- and xero-
stable substa nces. The assumed bounda ry
values are tempe ratur e, abo ut 150 8 C and mois -
ture con tent, 5 to 7% by weigh t. This group
contain s the products of micr obiologi cal syn-
thesis and spores (e.g., amino acids, antibioti cs,
and selected bacter ial strains) .
Anothe r class ification is prop osed, taking as a
basis the role of wat er present in the biotech nologic al
product [31]. Here, two group s are also dist inguishe d:
1. Biotechnol ogical prod ucts in which water is
one of the decisiv e elem ents for the life and
activity of micr oorganis ms (bact eria, yeast ,
and molds) or products of microbiological
transformation (enzymes). These substances
are characterized by high susceptibility to dry-
ing and their qualitative parameters are sub-
jected to significant changes during drying.
2. Biotechnological products in which water is a
solvent, a medium for microbiological changes,
and not a structural element of biopolymers
4.5
3.6
0
50
100
150
200
250
300
2.6
4.91
90
36.6
6.0
5
6
3
4
2
8
1
7
0.9
1.8
2.7
A
T
, %
t, s
In
A
T
FIGURE 39.5 The effect of temperature and time on the percentage of living microorganisms and active biopolymers: (1, 2, 3)
Bacillus thuringiensis at 90, 120, and 1708C, respectively; (4, 5) Beauveria bassiana at 50 and 608C, respectively;
(6) Lactobacillus plantarum at 558C; and (7, 8) lysine at 150 and 1908C, respectively. (From E.G. Tutova and P.S. Kuts,
Drying of Microbiological Products, Agropromizdat, Moscow (1987) (in Russian).)
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2006 by Taylor & Francis Group, LLC.
that decides their propert ies (anti biotics, amino
acids, a nd vita mins). Thes e mate rials are char-
acterized by high resistance to drying an d their
qualitativ e pa rameters undergo only sligh t
changes during drying.
One more classificat ion of biotec hnologica l pro d-
ucts can be made, taking as a crit erion the applicab ility
of the dry pro duct. The same biotec hnol ogical prod uct
can be preser ved maint aining its micr obiologi cal
activit y and biological struc ture (e.g., activ e yeast for
promot ing fermen tation process es) or be devo id of the
propert ies of a live micr oorganis m wi th its chemi cal
composi tion maintained (e.g., inactive yeast wi th high
protein content used as fodder yeast).
39.3 DEGRADATION PROCESSES IN THE
SCHEME OF DRYING MODELING
In a general scheme of the mo deling of drying, the
aspect concerning produ ct qua lity is neglect ed. In the
case of mod eling of drying biotec hnologica l prod ucts
it is necessa ry to consider the kineti cs of qua litative
changes of pro ducts an d relat e it to the dry ing kineti cs
[29,31–42] . Thus, for one or severa l selec ted qua litative
parame ters being most impor tant for a product, the
process of qualit ative degradat ion sh ould be de scribed
using mathe matica l express ions. As stated above, in
such an express ion for a given index, three most
impor tant drying parame ters shou ld be co nsidered :
(a) mois ture content ; (b) material temperatur e; and
(c) process durati on.
In major ity of qua litative chan ges it is assum ed
that their kine tics corres ponds to the first-o rder reac-
tion an d c an be de scribed by the followi ng e quation:
dA
d t
¼ r
d
¼ k
d
A (39 : 2)
where A is the numeri cal value of the charact eristic
index of degradat ion describ ed by this relationshi p
(e.g., the number of living micr oorgan isms, activity
of antibi otics, en zymes). The react ion rate constant
k
d
, as a function of tempe rature, is determ ined by the
Arrhen ius equ ation:
k
d
¼ f ( T ) ¼ k
d
,
T
¼ k
1
exp
E
a
R u
(39 : 3)
The frequency coeffici ent k
1
(s
1
) and acti vation
energy of E
a
(J/mol) are charact eristic values for a
given sub stance and depend on the tempe ratur e,
moisture content , an d reaction time.
Tut ova and Kuts [20] presen ted a diagra m of
changes in the react ion rate constant for therm al
inactivation k
d,T
depending on the temperature for sev-
eral types of biotechnology products (Figure 39.6). As
the degradat ion index, the de ath rate of microo rgan-
isms has been assum ed. The first group of lines [1]
refer to vegeta tive forms of microo rganisms drying at
a relative ly low tempe ratur e (45 to 50 8C) . The inact i-
vation temperatur e of these culture s is in the narrow
range (10 to 15K) wi th k
d, T
chan ging 17 times. Lin es
[2–4] refer to spores an d biosynt hesis products . They
are charact erized by higher thermo stability and a re
resistant to the tempe rature abo ve 1 00 8C. The kinetic
constant k
d, T
changes 6 to 8 times in a wi der range of
tempe ratures (60 to 80 K).
On the basis of experi menta l data, us ing the cal-
culation method of chemical reaction kinetics, the
kinetic parameters of thermal inactivation of many
substa nces wer e establ ished (
) [20] . How -
ever, the authors make reference to a high discrep-
ancy of experimental and calculated data, reaching 20
to 30%. Similar kinetic parameters were determined
as a result of investigations of selected biotechno-
logical products [43–46].
The qualitative changes induced by a decrease
in moisture content in a biotechnological product
Reciprocal absolute temperature 10
3
/q, 1/K
2.0
2.4
2.8
3.2
1.5
3.5
5.5
7.5
4
3
2
1
Reaction rate constant, ln(
k
d,T
)
FIGURE 39.6 Dependence of constant reaction rate of ther-
mal inactivation k
d,T
on temperature u: (1) nonsporulating
microorganisms; (2) bacterial spores; (3) antibiotics (baci-
tracin and grisin); and (4) lysine. (From E.G. Tutova and
P.S. Kuts, Drying of Microbiological Products, Agropromiz-
dat, Moscow (1987) (in Russian).)
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2006 by Taylor & Francis Group, LLC.
(dehydra tion degradat ion) can be descri bed using the
reaction rate constant k
d,X
depend ing mainly on bio-
mass mois ture co ntent X [20] :
k
d
¼ f ( X ) ¼ k
d
,
X
¼ k
0
1
X
n
(39 : 4)
where k
1
0
and n are the parame ters ch aracteris tic for
dried mate rial.
Luybe n et al. mad e sim ulation calcul ations of the
drying pro cess assum ing the de gradat ion index to be
the change of acti vity of three enzymes : alkaline phos -
phatase in skim milk , lipas e in rye, and catal ase in
wheat and catal ase from spinach in a buf fer solu-
tion [33]. They trans formed general
of qualitati ve changes (i. e., the acti vity chan ge) to
the form :
a
¼ a
0
exp
ð
t
0
k
d
dt
0
@
1
A
(39 : 5)
The react ion rate constant k
d
was calcul ated acco rd-
ing to the Arrhen ius equatio n,
. The
activati on energy E
a
an d frequency coeffici ent k
1
were made depen dent on moisture con tent X acco rd-
ing to the empirical eq uations :
E
a
¼ E
a
,
1
þ (E
a
,
0
E
a
,
1
) exp ( pX ) (39 : 6)
ln k
1
¼ ln k
1
,
1
þ ( ln k
1
,
0
ln k
1
,
1
) exp
exp (
pX ) (39 : 7)
The numeric al va lue of constant p was given for each
enzyme on the basis of exp erimental data: for cata-
lase, p
¼ 3.699; for lipase, p ¼ 4.880; and for alkaline
phosph atase, p
¼ 11.366 . The effect of mois ture con -
tent X on E
a
and k
1
. From these graphs the va lues of k
1
(s
1
)
and E
a
(J/ mol) can be read out for a bone-dry pro -
duct (subsc ript 0 ) an d for a prod uct with mois ture
content X
¼ 1 and mo re (subsc ript infinit y).
present s a change of react ion rate co nstant k
d
of enzyme inact ivation as a function of moisture co n-
tent X calcul ated from the Arrhenius equati on for
tempe rature 60 8 C.
Zimm ermann an d Bauer [34, 35] invest igated the
drying of baker’s yeast (granulated yeast in a fluidized
bed) and developed a mathematical model in which
thermal inactivation of a product was based on the
TABLE 39.3
Kinet ic Param eters of Therma l Inact ivation
Substance (temperature range) E
a
(kJ/mol) k
1
(s
1
) Heating Time t (s)
Nonspore bacteria (40–558C)
Beauveria bassiana
227
10
34
Rhizobium pisum
248
10
38
Azotobacter chroococcum
341
10
52.5
Lactobacillus plantarum
428
7
10
66
Lactobacillus acidophilus
142
10
21
Fungi
Penicillium, Trichoderma
299
10
48
60
Enzymes
Transeliminase
111
10
15.5
Polygalaturonase
251
10
37
Spore bacteria (90–1908C)
Bacillus thuringiensis
33
2.7
10
2
Antibiotics (90–2008C)
Biomycin
64.5
1.6
10
6
58
1.2
10
5
>
60
Grisin fodder preparation
45
7.9
10
4
60
27
1.75
>
60
Bacitracin
68
1.5
10
6
60
62
2
10
5
>
60
Lysine (90–2008C)
107
10
10
60
30
1.9
>
60
Source: From E.G. Tutova and P.S. Kuts, Drying of Microbiological Products,
Agropromizdat, Moscow, (1987) (in Russian).
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2006 by Taylor & Francis Group, LLC.
first-order reaction equation and the Arrhenius equa-
tion. Taking into account the form of dried material
(pellets of about 0.8 mm or 0.5 mm diameter) and
the theory of shrinking during drying, the general
inactivation of enzymes was considered to be that of
inactivation in the coating layer of pellets at the hygro-
scopic moisture content X
hygr
, and medium tempera-
ture T
m
, and inactivation of enzymes in a wet core of
material with critical moisture content X
crit
and sur-
face temperature T
S
. The above-mentioned relations
are described by the following equation:
da
dt
¼ k
d
(X
crit
,T
S
)a
r
2
R
2
þ k
d
(X
hygr
, T
m
)a 1
r
2
R
2
(39:8)
where r is the radius of the wet core (m) and R is the
pellet radius (m).
Kerkhof and Schoeber [36], modeling droplet dry-
ing in spray dryers (SDs), used earlier works by
Labuza (1972), Verhey (1972), and Ball and Olson
(1957). In the case of the first-order reaction of qual-
ity degradation of component i, the following equa-
tion for a determined volume of the reference system
is employed:
@C
i
@t
¼ k
d
C
i
1
r
2
@
2
@r
2
(r
2
w
i
)
(39:9)
where w
i
is the density of mass stream of compon-
ent i being degraded (kg/m
2
s) and r is the droplet
radius (m).
Catalase
Lipase
0
0
1
0.5
1.0
Moisture content X , kg H
2
0/kg d.m.
Activation energy
E
a
. 10
5
, J/mol
Alkaline
phosphatase
2
3
4
5
FIGURE 39.7 Activation energy E
a
of enzyme inactivation
as a function of material moisture content X. (Adapted
From K.Ch.A.M. Luyben, J.K. Liou, and S. Bruin, Enzyme
Degradation During Drying, Biotechnology and Bioengin-
eering, 24, 533–552 (1982).)
Lipase
0
0
0.5
1.0
Catalase
Alkaline
phosphatase
50
100
150
Frequency coefficient lnk
∞
, –
Moisture content X, kg H
2
0/kg d.m.
FIGURE 39.8 Frequency coefficient k
1
of enzyme inactiva-
tion as a function of material moisture content X. (Adapted
From K.Ch.A.M. Luyben, J.K. Liou, and S. Bruin, Enzyme
Degradation During Drying, Biotechnology and Bioengin-
eering, 24, 533–552 (1982).)
Catalase
Alkaline
phosphatase
Lipase
0
0.5
10
−8
10
−7
10
−6
10
−5
10
−4
10
−3
1.0
Reaction rate constant k
d,x,
s
−
1
Moisture content X, kg H
2
0/kg d.m.
FIGURE 39.9 Specific reaction rate k
d,X
of enzyme inactiva-
tion at 608C as a function of moisture content X. (From
K. Ch. A.M. Luyben, J.K. Liou, and S. Bruin, Enzyme
Degradation During Drying, Biotechnology and Bioengineer-
ing, 24, 533–552 (1982), Wiley, New York. With permission.)
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2006 by Taylor & Francis Group, LLC.
The effect of temperature on the reaction rate
constant k
d,T
can be expressed by the general formula:
k
d
,
T
¼ k
1
exp (zT)
(39:10)
which, when used in the calculation of phosphatase
inactivation in skim milk, gave better agreement with
experimental data than the Arrhenius equation [36].
In model calculations, Kerkhof and Schoeber used
the following empirical relation valid for phosphatase
inactivation in skim milk and for aqueous maltose
solution:
k
d
¼ exp (34:9 1:39=C
w
þ 0:5183T)
(39:11)
where C
w
is the water concentration inside the droplet
(kg/m
3
).
Figure 39.10 presents changes of temperature and
thermal degradation in the droplet of maltose–water
system as a function of reduced time (t/R
0
2
) calculated
by Kerkhof and Schoeber [36]. It follows from the
graph that at temperature 1008C the complete inacti-
vation takes place in a very short time; at temperature
808C the reaction begins at a slower rate. In both
cases the reaction starts when the droplet temperature
is about 708C.
Karel [37] analyzed several recent works on quali-
tative changes of dehydrated foodstuffs and biotech-
nological materials. He stressed that when optimizing
the process from the point of view of product quality
usually the interaction of several qualitative param-
eters takes place. Their change in the final product
depends not only on material temperature and
moisture content, but also on changes occurring in
other parameters. He stated that numerous processes
that cause these changes during water removal are
dependent on the ‘‘mobility’’ of components of the
material being dehydrated. He stressed that qualitative
changes and the effect of process parameters must
be analyzed as a function of the time of interaction,
which is often neglected in other studies. This state-
ment can be written in the general form of the function
of a constant reaction rate of qualitative changes:
k
d
¼ f (T(t),X (t) )
(39:12)
A synthesis of the mathematical modeling and opti-
mization of the process was presented by Strumiłło
et al. [29,32] and Kamin˜ski and Strumiłło [39]. A
natural starting point for the analysis is that the rate
constant k
d
depends on the history of the drying
process, which should be known if the description of
the degradation processes is to be complete. Thus, the
following equations are obtained experimentally:
T
¼ g
1
(t)
(39:13)
X
¼ g
2
(t)
(39:14)
By correcting k
d
, the constants in Equation 39.15 are
searched for with the use of experimental data:
k
d
¼ k
1
f (T ,X )
exp (E
a
=Ru
)
(39:15)
where k
1
and E
a
are the experimental constants. To
obtain such a relation, a dynamic approximation of
the experimental data is required.
For example, on the basis of experimental data
presented in Ref. [33], the following correlations of
constant k
d
of the degradation indices of baker’s yeast
Saccharomyces cerevisiae were obtained:
Alcohol dehydrogenase (ADH) enzyme activity:
k
d
¼ (1 0:2555X þ 9:529 10
2
X
2
þ 1:859 10
3
XT )
exp
310:4
þ 339:8X
T
þ 273:15
(39:16)
CO
2
production:
k
d
¼ [1:0023 þ exp (2:269 10
4
(T
20)
þ 9:124 10
4
(T
20)
2
)]
exp
515:27
þ 240:72X
T
þ 273:15
(39:17)
Reduced time t /
R
2
0
·10
−8
, s/m
2
Droplet
temperature T
D
Degradation A
T
Droplet temperature
T
D
,°
C
Degradation level
A
T
, %
20
25
15
10
5
100
0
20
40
60
80
0
1
2
2
FIGURE 39.10 Droplet temperature T and thermal degrad-
ation A
T
in maltose–water system vs. reduced time t/R
0
2
for
two air temperatures: (1) to 1008C, and (2) to 808C. (From
P.J.A.M. Kerkhof, W.J.A.H. Schoeber, Theoretical Model-
ing of the Drying Behavior of Droplets in Spray Dryers, in
Advances in Preconcentration and Dehydration of Foods, A.
Spicer (Ed.), Elsevier Applied Science, London, pp. 349–397
(1974). With permission.)
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2006 by Taylor & Francis Group, LLC.
O
2
deman d:
k
d
¼ [1 :0045 þ exp (7: 51 10
3
( T
20)
þ 5:276 10
4
(T
20)
2
)]
exp
525 : 35
þ 200 :25 X
T
þ 273 : 15
(39 : 18)
through Equation 39.18 material
tempe rature an d mois ture c ontent are the function s
of tim e.
In gen eral, in model a nd design calculati ons opti-
mum process parame ters of drying are determ ined.
Taking into account the qua lity changes, usually
severa l quality indice s are co nsidered and thus so me
hierarc hy of the index syst em can be assum ed or one
global index A
g
co mprising all indices of the syst em
may be creat ed [38] . The global index can be writt en as:
ln (A
g
)
¼
X
n
i
¼ 1
ln
a
i
a
i0
l
i
¼
ð
t
0
X
n
i
¼ 1
k
di
(
)d t (39 : 19)
where l
i
>
0, i
¼ 1, . . . , n is the weight syst em for the
assum ed system of qua lity indices a
i
[39] .
39.4 EXAMPLES OF DRYING METHODS
AND EQUIPMENT
A wi de choice of biotec hnolo gical produ cts indica tes
that it is possibl e to use various methods to dry them.
Howev er, taking into acco unt therm o- an d xerola bil-
ity an d a risk of many qualit ative chan ges in these
produc ts, some pro cess con ditions should be strictly
observed (e.g., tempe rature and moisture content of a
produc t should be control led, rapid changes of pres-
sure shou ld be eliminat ed, and mechani cal inter -
action s should be avoided as they may damage the
cells of micro organis ms). In addition , many process es
must be c arried out unde r sterile con ditions, using
sterile drying agents . In general, the foll owing solu-
tions, which take into accou nt the propert ies of bio-
technol ogical pro ducts, can be recomm ended:
1. Low-intens ive condition s
2. Vacuum or freez e drying
3. Applicati on of mult istage systems
4. Special techni ques based on the decreas e of
initial mate rial moisture content
Kn owing general princi ples of particular drying
methods , we shall discus s their adva ntages and disad-
vantage s in app lication to biotec hnol ogical products ,
in particular the one that may end anger high prod uct
quality.
39.4.1 S
PRAY
D
RYING
The spray- drying method is widely used, in parti cular
for liqui d suspensi ons of sub stances resistant to high
tempe ratures, such as biopolym ers and pro ducts of
microb iological trans form ations. On the other han d,
spray dry ing can also be ap plied to thermolabi le su b-
stance s provided the sprayed mate rial is highly dis-
persed and the drying agent has a high tempe rature
(the drying process takes place immediat ely at a wet
bulb tempe ratur e). Thus , the produ ct qua lity is not
deteriorat ed. The install ation should ope rate fault -
lessly and the con trol and measur ing equipmen t
should be of high qua lity. Disadvan tages of this dry-
ing method are scali ng up of dryers under mil d ther-
mal con ditions (at 150 to 200 8 C), expensi ve devices
for liqui d dispersion and dust remova l from the pro d-
uct are neede d, and inst ability of the process due to
possible dep osition of a product on chamber walls
and dust-removi ng gas pipes. The prod uct can often
be dep osited on the chamb er walls because of chan -
ging propert ies of the mate rial being sprayed ; this is
connected with differen t compo sitions of ferm enta-
tion broth, changing conversi on factors during the
fermentat ion process , and varyi ng amo unts of addi-
tives. The applic ation of specia l spray dryer constr uc-
tions can prevent the disadva ntageous effects.
An exampl e of the specia l spray dryer constru c-
tion to drying of a suspensi on of an tibiotics, fodd er
yeast, amino acids, and enzymes is shown in
[20] . Inside the dryer chamber a sweeper rotat es
along the wall. The cleani ng is made by air of prop er
parame ters, such as pressur e and tempe rature, which
is introd uced by noz zles insta lled on the swee per. The
air, or an inert ga s, preven ts the mate rial from stick -
ing to the wall and cools down to a therm olabile
substa nce. A sim ilar resul t can be achieve d in a co cur-
rent spray dryer when a specially designed air dis-
tributor producing an air cushion along the chamber
wall is installed [20]. For example, the dryer presented
in Figure 39.11 for drying of fodder lysine concen-
trate, with evaporation capacity of 1000 kg/h, is char-
acterized by the following parameters:
Diameter of the cylindrical section is 5.4 m.
Height of the drying chamber is 9.0 m.
Cone angle is 508.
Diameter of the spraying disk is 0.25 m.
Frequency of disk rotations is 8,000 to 11,000 rpm.
Drying agent temperature at the inlet is 250 to
2908C and at the outlet is 80 to 1258C.
Fan output is 16,000 m
3
/h.
For deep drying of a product in spray dryers, it is
required to use high drying temperatures. In the case
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2006 by Taylor & Francis Group, LLC.
of biotechnological products, under such conditions
degradation of product quality may take place easily.
Therefore, to preserve the quality of biotechnological
products, it is advisable to apply multistage dryers
[47]. The first stage is a spray dryer in which material
is predried under conditions ensuring a safe tempera-
ture and quick product removal from the drying
chamber. The second and the third stages are fluid-
ized or vibrofluidized bed dryers in which slow drying
or cooling of the product takes place. In such a system
a detailed control of product temperature and mois-
ture content is possible in an appropriate residence
time at the second or the third stage, which makes it
possible to obtain a product of preserved biotechno-
logical properties. Spray drying for drying of biotech-
nological products is a method still most frequently
applied in practice and research [44,48–53].
39.4.2 J
ET
-S
POUTED
B
ED
D
RYING
For drying of suspensions at a low solid-phase con-
centration, a solution called a jet-spouted bed (JSB) is
applied (Figure 39.12) [54–56]. This is a conical–
cylindrical chamber containing inert material spouted
by a high-velocity airstream. The material being dried
is sprayed by nozzles onto an inert material. Among
many advantages of the presence of inert material,
usually in the form of cubes or spheres of diameter
5
10
3
to 10
10
3
m, two are worth mentioning:
1. A heated material surface causes an intensified
drying.
2. An intensive motion of particles leads to a
mechanical cleaning of the chamber and
ensures high disintegration efficiency of dry
particles.
This method is useful in drying of thermally re-
sistant products (e.g., amino acids and antibiotics). A
JSB dryer is a competitive construction for a spray
dryer, particularly for technologies of low and med-
ium capacity, due to a higher volumetric evaporation
rate and lower capital costs (a significant reduction of
drying chamber volume). Table 39.4 presents a com-
parison of parameters and results of drying of a fod-
der antibiotic (Zn–bacitracin complex) in both dryer
types [55]. A several times higher volumetric evapor-
ation rate W
v
is observed in the JSB dryer and about
20% higher heat utilization coefficient k is reported
for a spray dryer.
Biomass (liquid)
2
1
4
Cold
air
5
3
Hot air
Air +
dry material
FIGURE 39.11 Scheme of spray dryer with chamber
sweeper: (1) drying chamber; (2) biomass distributor; (3)
rotating sweeper; (4) sweeper drive; and (5) blower. (From
E.G. Tutova and P.S. Kuts, Drying of Microbiological Prod-
ucts, Agropromizdat, Moscow, (1987) (in Russian).)
Particles
of inert material
Gas
Wet
material
Wet
material
Gas + solid
FIGURE 39.12 Scheme of jet-spouted bed dryer.
TABLE 39.4
Comparison of Optimal Drying Parameters for
Zn–Bacitracin in a Jet-Spouted Bed Dryer and
Spray Dryer
Parameter
JSB Dryer
SD (lab)
SD (full scale)
T
G0
, 8C
170
200
248
T
Gk
, 8C
90.5
85
80
X
0
, %
80.2
83.4
83
X
k
, %
2.64
6.55
5.24
W
v
, kgH
2
O/m
3
s
4.37
10
2
0.26
10
2
0.14
10
2
k
0.45
0.53
0.58
V
G
, kg air/kgH
2
O
36.5
25.4
18.9
d
p
, m
24.7
10
6
49.7
10
6
30.9
10
6
a, units/mg
4.2
4.3
4.4
W
H2O
, kgH
2
O/h
8
12
1200
Source: From A.S. Markowski, Drying Technology, 11(2), 369–387
(1993).
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2006 by Taylor & Francis Group, LLC.
39.4.3 I
MPINGEMENT
S
TREAM
D
RYING
One of the latest drying methods for susp ensions and
solution s call ed, in general , impi ngement stream dry-
ing (ISD) is a method specia lly recomm ended for dry-
ing of liquid bio technol ogical pro ducts [57]. The
method consis ts of mult iple mutual co llisions of dry-
ing agent stre ams flowi ng at high velocitie s into which
fermen tation liquid is sprayed by noz zles. Depend ing
on a specific design solut ion, the follo wing dry ers are
distinguis hed: (a) co unterr otating cou ntercurren t; (b)
corotat ing coun tercurrent; an d (c) coax ial wi th a mov-
ing impi ngement zone. An intens ive evapo ration and
the motion of suspen sion and produ ct pa rticles en sure
short reside nce time of the material in the drying zo ne,
which has an effect on high quality of pr oducts. A
schema tic diagra m of ISD in the counterr otating
countercur rent versio n is present ed in Figure 39.13.
39.4.4 D
RYING IN A
F
LUIDIZED AND
V
IBROFLUIDIZED
B
ED
The adv antage of the fluidized and vibroflui dized
system is that it is isothermal due to intens ive mixing
of the solid state, whi ch pr events local overheat ing of
the particles . It is possibl e to build multist age dryers
and ap ply independen t tempe ratur e control in par-
ticular secti ons. Thus , the prod uct can be dried uni-
formly an d changes in mate rial mo isture content can
be controlled precisely in every zone of the dryer. Thi s
is particu larly important for therm o- and xerola bile
produc ts. Advant ageous also are high intens ity of
transfer pro cesses, relat ively sim ple dryer constru c-
tion, and easy control of process parame ters.
Dis advantag es of this drying method are the risk
of mecha nical damagi ng of cell s (abras ion) or form a-
tion of mate rial agglom erates a nd de posits on the
walls, which dist urb the unifor m be d fluidiza tion
and ca use local overheat ing of mate rial an d therm al
inactivat ion of the product. A disadva ntage of drying
in a fluidized bed, pa rticular ly of a polydis perse bio-
technol ogical product, is the necessi ty of limiting the
velocity of a fluid izing agent below the velocity of
material particle entrai nment, with a related risk of
overheat ing a thermo labile mate rial near the grid
baffle of the dryer. This, howeve r, can be overcome
if a vibroflu idized bed is app lied. Du ring drying of
granula ted mate rial there is a temperatur e drop in the
granule cross secti on and thus there is a risk of over-
heatin g the surfa ce layer an d inact ivation of this part
of the pro duct. Bec ause process pa rameters must be
control led and, due to pr oduct qua lity chan ges, dry-
ing is carri ed out in batch or con tinuous fluidized bed
multistage dryers with parameters adjusted in particu-
lar sections (heating and cooling) (Figure 39.14).
As already mentioned, before drying the biotech-
nological products are usually liquids or paste-like
suspensions. Despite such a form they can be dried
in fluidized or vibrofluidized beds. Three characteris-
tic variants of process schemes are used: spraying of
liquid onto the bed of inert material (sprayed fluidized
bed) [59], which is similar to the JSB dryer pre-
sented above; use of a bed of recirculating product
(
) [58] ; and drying of granula ted biotec h-
nological product [34]. However, these methods have
limited applicability.
The application of thermosensitive microbio-
logical materials in product recirculation may influ-
ence a risk of thermal inactivation if part of the
substance being dried stays too long in the heating
zone during recirculation. In the case of spraying the
material onto the bed of inert material, its particles
are covered with a thin layer of wet material. With
Spray nozzle
Tangential nozzles
Heater Blower
Dry product
Supplementary air
Air outlet
FIGURE 39.13 Impingement stream dryer for biotechno-
logical products. (From T. Kudra and A.S. Mujumdar
Impingement Stream Dryers for Particles and Pastes. Drying
Technology, 7(2), 219–266 (1989).
Dry product
Wet material
2
3
1
4
5
6
FIGURE 39.14 Scheme of the fluidized bed horizontal mul-
tisectional dryer: (1) drying chambers; (2) threshold; (3) bed;
(4) baffles; (5) grid; and (6) air collectors.
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2006 by Taylor & Francis Group, LLC.
drying in a turbulent bed the dry product is abraded
and possibly damage the microorganism cells or
mechanically disrupt the biopolymer chains. Hence,
vegetative and spore cultures must not be dried. It
should be emphasized that for a correctly chosen inert
material and at a properly conducted drying process
the output expressed as the amount of moisture evap-
orated in unit volume of the drying chamber is 15 to
17 times higher than the output of a spray dryer at the
same temperature.
39.4.5 D
RYING IN A
F
IXED
B
ED
A characteristic feature of drying in a fixed bed is that
the material in the bed is fixed (e.g., in band, chamber,
and tunnel dryers). The advantage of this method is
the application of multisectional or multilevel dryers
that facilitate the control of products and adjustment
of drying conditions in particular drying zones. Also
advantageous are the separate feeding of hot or cold
air at a constant or varying flow rate, multiple appli-
cation of an air portion if it flows through several
sections or several band layers, possibility of cocur-
rent, countercurrent or mixed flow, and recirculation
of a drying agent.
The method has some disadvantages, namely, the
dried product is not homogeneous as far as moisture
content and microbiological quality are concerned,
process control is difficult in the case of a one-section
dryer, the product can agglomerate and get stuck to
trays in drying chambers or onto transporting bands.
Long drying time and low dryer output, large size of
the drying installation, and bad sanitary conditions
for drying of such products are additional disadvan-
tages of this method.
Drying in a fixed bed is applied when wet material
is formed and concentrated on filters or concentrated
by adding a filling material and when low drying
temperature is required. Chamber and shelf dryers
are often applied in low-output technologies, in
pilot-scale situations, and in drying of highly thermo-
and xerolabile products.
39.4.6 D
RYING IN
D
RUM
D
RYERS
In drum dryers, material can be dried convectively,
inside the drum. There are also cylindrical dryers with
contact drying of material on the external surface of
the cylinder. The first type of drum dryers are widely
used for bulk products. The material should be in the
form of granules or dense paste. The efficiency of such
drying is not high. The drum dryers of the second
type are more frequently used for drying suspensions.
A thin layer of the material is retained on the hot
cylinder surface, then dried and removed by special
scrapers. Such a drying method is suitable for prod-
ucts that do not require special control of drying or a
specific temperature regime (e.g., fodder concentrates,
waste products).
39.4.7 F
REEZE AND
V
ACUUM
D
RYING
In freeze and vacuum drying, drying takes place at
low temperatures, below 08C. In the case of drying of
biotechnological, food, and pharmaceutical products,
the temperature ranging from
5 to 408C and aver-
age vacuum are applied. The process has the follow-
ing advantages:
Moisture is removed at low temperature, which
in fact excludes thermal inactivation of the
product.
The material structure is maintained (particles and
cells are not damaged).
It is easy to obtain a sterile dried product in com-
mercial unit packages (ampules and bottles).
The main disadvantages of freeze drying include
long process duration, high energy consumption,
and complicated equipment.
In freeze drying, a very important stage is freezing,
which directly precedes sublimation. Depending on
the method of drying and physicochemical properties
of substances, an initial freezing called self-freezing is
applied. The initial freezing takes place in the dryer
chamber as a result of a slow decrease of temperature
and pressure. A very slow cooling unfavorably affects
3
9
9
7
8
9
6
6
2
1
5
4
Hot air
Compressed
air
Product
Biomass
(liquid)
Air outlet
FIGURE 39.15 Wetted bed fluidized bed dryer: (1) fluidiza-
tion chamber; (2) fluidized bed; (3) air distributor; (4) dis-
integrator; (5) nazzle; (6) screw feeder; (7) rotary feeder; (8)
cyclone; and (9) motor. (From B. Dencs and Z. Ormos,
Recovery of Solid Content from Ferment Liquor Concen-
trates in Fluidized Bed Spray Granulator, Proceedings of
Fifth Conference on Applied Chemistry, Unit Operations and
Processes, 3–7 September, Balatonfured, Hungary, 320–325
(1989). With permission.)
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2006 by Taylor & Francis Group, LLC.
cells because electro lyte concentra tion increases in the
ambie nt medium an d cells are dehyd rated e xcessively .
At a slow and average co oling rate (10 to 10 08/mi n)
large ice crystals are form ed that can mechani cally
damage cell structures such a s cy toplasmic mem-
branes. The self -freezing takes place at ultr aswift
cooling (e.g., at the rate of 10,000 8 /min) a s a resul t
of rapid pressur e dro p. The pro cess takes place in the
whole volume of the mate rial. Small ice cryst als a re
distribut ed unifor mly, and in favora ble co nditions
amorphou s ice is form ed (tran sformati on into a vit-
reous form , termed vitr ification), which does not
cause mechani cal damage of cells. The self-f reezing
not only facilit ates the technol ogical process , but also
intensifies de hydration . It is estimat ed that the ev ap-
oration rate is 3 to 5 times higher than in the pr ocess
of sublimati on. However, the self-freezi ng is recom-
mended pa rticular ly in drying of mate rials with rela-
tively low mois ture co ntent as sub stances of high
moisture content are easily foamed and requir e initial
freezing.
The opt imum cooling or freez ing rate is diff erent
for various micr oorganis ms and biop olymers. For
instanc e, in bacteria, slow freez ing (l8 /min) to
35 8 C
at the beginni ng and then rapid coo ling to the desired
low final tempe rature is recomm end ed.
In Tabl e 39.5, init ial freez ing temperatur es and
percent age of frozen mois ture as a functio n of tem-
peratur e of severa l material s are present ed. It follows
that in most sub stances the freez e-dryi ng tempe rature
should be lower than
408C because at this tempera-
ture a significant part of moisture remains unfrozen.
In freeze drying, the sublimation temperature and
the kinetics of changes of the sublimation surface (or
a displacement of a wet surface) within the material
layer are very important (
) [60–62 ]. The
optimal temperature is the one below the eutectic
temperature. In practice, however, it is difficult to
attain because at that temperature the drying time
must be prolonged and so more energy is required.
The presence of protective substances in dried mater-
ial, such as glycerin and sugars (fructose, saccharose,
lactose, or specially prepared compositions known
only under trademarks), has influence on the eutectic
temperature and the applicability of elevated temper-
atures during freeze drying. The substances protect
microorganisms against too high salt concentration
and provide more favorable conditions for ice crys-
tallization during freezing. For example, the role and
mechanism of glycerin interaction are as follows:
In the temperature range from 0 to
208C gly-
cerin decreases the crystallization rate, pro-
motes the formation of small crystals, and
sustains the state of cooling; in the process of
water crystallization, it dissolves salts and,
penetrating the cells, it contributes to the res-
toration of osmotic balance.
In the temperature range from
20 to 808C
glycerin controls the crystallization process
and makes it reversible, thus eliminating eutec-
tic solutions caused by the presence of salt so-
lutions. It also shifts the coordinates of eutectic
borders and contributes to the formation of
vitreous structures.
The effect of various protective substances on the
stability of microbiological preparations during
TABLE 39.5
Frozen Moisture as a Function of Temperature, %
Substance
Minimum Freezing Temperature (8C)
Mean Ambient Temperature (8C)
Frozen Moisture, %
5
10
20
30
40
Water
Distilled
0
100
100
100
100
100
Tap
0.3
94
97
98.5
99
99.2
Virus of aphthous fever
2.2
56
78
89
92.7
94.5
Fowl cholera strains
2.4
52
76
88
92
94
Swine erysipelas strains
2.1
59
79
89.5
93
94.5
Anthrax vaccine
1.0
80
90
95
96.7
97.5
Strains of comma bacillus
1.0
80
90
95
96.7
97.5
Lactic acid bacteria
1.4
72
86
93
95.7
96.5
Skim milk
0.5
90
95
97.5
98.3
98.8
Source: From E.G. Tutova and P.S. Kuts, Drying of Microbiological Products, Agropromizdat, Moscow, (1987) (in Russian).
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2006 by Taylor & Francis Group, LLC.
freeze- and other drying techni ques has be en dis-
cussed in many publ ications [63–66 ] and exempl ary
results are presen ted in Figure 39.17 [65] .
W hile analyzing the subli mation kinetics (Figure
39.16), one can observe that initially, while he ating
frozen mate rial, the subli mation of ice crystals star ts
at the mate rial surface and success ively pe netrates the
interior, leaving a solid matr ix an d void pores
through whi ch wat er vapor must be relea sed to the
material surfa ce. In the case of biotec hnologica l pro d-
ucts this matrix is in fact a highly concentra ted liqui d.
So, as sublimatio n pro ceeds, condu ction of heat to
the sub limation front results in furth er dehyd ration
by diff usion. Hence, the consecuti ve zon es of diff erent
moisture content appe ar in a freeze-dri ed mate rial
until all the ice cryst als are sublimed.
Since the su blimatio n rate can be increa sed by a
higher heati ng rate only to a certain e xtent, the rate of
water vapor remova l from the su blimatio n front be -
comes the key point in freeze drying. Adsor ption of
water vapor by sorben ts placed insi de a drying cham-
ber or by direct con tact of the sorbent an d drying
material (as a mixture) is one of the most promi sing
methods in this area. M olecular sieve s, zeolites, or
ceram ics can be recomm end ed here be cause their
sorption acti vity under rarefied conditio ns doe s not
drop with tempe rature rise due to the heat of sorp-
tion. The use of parti culate sorbent s pe rmits subli m-
ation to take place in a fluid ized stat e under
atmosp heric pressur e [67,68].
A typical freeze dryer consists of a drying cham-
ber, vacuu m syst em, and vapor cond enser, which can
either be separat ed or built within the drying cham-
ber. The drying ch amber constru ction and the syste m
of heat su pply for sub limation can be so lved in many
ways. A schema tic diagra m of a continuou s scraper -
type freeze dryer especi ally suit able for biotec hno-
logical mate rials is present ed in
liquid produ ct of low co ncentra tion of the solid pha se
(up to 40%) is sprayed by nozzles placed on the
rotating central pip e and then freez es on the chamb er
walls. The dryer chamber is a cylinder equipped wi th
a co oling jacket . The heat of subli mation is sup plied
by rad iators also placed on the cen tral pipe. Dry
product is scraped from the chamber walls by adjust -
able bru shes or scraper s.
present s a schema tic diagra m of a
continuous vibrogr avitatio nal freez e dryer for liquid s
and pastes [70] . In the uppe r chamb er, the vacu um-
frozen liqui d is dispersed . The granula ted mate rial is
transp orted to the low er chamb er equ ipped wi th a
system of vibrating screens with hol es of diame ters
that decreas e wi th the direction of mate rial trans fer.
The heat of subli mation is supp lied by radiators in-
stalled above each screen.
The idea of the intens ification of freeze drying by
introd ucing a mois ture-adso rbing substa nce into the
drying chamber is shown schema tica lly in
0
5
10
15
4
4
3
2
8
12
16
20
24
28
32
t, h
H, mm
1
1
2
3
4
H
FIGURE 39.16 Moisture content distribution in a slab of
freeze-dried material. (From V.G. Popovski, L.A. Bantysh,
N.T. Ivasiuk, N.H. Grinberg, and G.B. Gorshunova, Sub-
limation Drying of Food Products of Vegetable Origin,
Pishchevaya Promyshlennost , Moscow, (1975) (in Russian).)
60 60
0
1
1
1
1
2
2
2
2
3 3
3
3
4 4
4
4
0
20
20
40
40
60
60
80
80
100
100
180
180
240
Time
t, min
L. plantarum
L. murinus
L. fermentum
L. casei
Relative viability
N/N
0
,%
240
300
300
120
120
FIGURE 39.17 Effect of the protective substance’s medium
on the survival rate of Lactobacilli at various drying times:
(1) adonitol; (2) glutamate; (3) PEG 1000; and (4) nonfat
skim milk. (From C.F. De Valdez, G.S. De Giori, and A.P.
De Ruiz Holgado, Effect of Drying Medium on Residual
Moisture Content and Viability of Freeze-Dried Lactic Acid
Bacteria, Applied and Environmental Microbiology, 49, 413–
415 (1985). With permission.)
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2006 by Taylor & Francis Group, LLC.
and Figure 39.21 [20]. Figure 39.20 presents a multi-
band continuous dryer in which dried material and
sorbent are transferred countercurrently on adjacent
bands. In Figure 39.21 the sorbent is transported along
the grid baffle placed over the connecting pipes to
which containers (bottles or ampules) with dried ma-
⊗
⊗
⊗
⊗
1
7
Biomass (Liquid)
Dry product
2
6
3
4
5
8
FIGURE 39.18 Continuous scraper-type freeze dryer: (1)
vacuum chamber; (2) spray nozzles; (3) scraper; (4) radi-
ators; (5) cooling jacket; (6) freezant; (7) motor; and (8)
vacuum lock. (From K.P. Shumski, Vacuum Apparatus
and Equipment in Chemical Industry, Mashinostroyenie,
Moscow, (1974) (in Russian).)
Dry product
Biomass
8
7
3
2
1
5
6
4
FIGURE 39.19 Continuous vibrogravitational freeze dryer
for liquid and pastes: (1) vacuum granulator; (2) material
feed; (3) condenser; (4) vacuum pump; (5) feeding valve; (6)
drying chamber; (7) radiators; and (8) vibrating trays.
(From P.A. Novikov, I.F. Pikus, and E.G. Tutova, Con-
tinuous Freeze-Dryer for Liquid Materials, Russian Patent
No. 27,3734 (1970).)
Dry product
Sorbent
Sorbent
Wet material
To vacuum
pump
1
2
3
FIGURE 39.20 Scheme of the multiband continuous freeze
dryer of countercurrent dislocated material and sorbent
layers: (1) sorbent; (2) transporting band; and (3) dried
material). (From E.G. Tutova and P.S. Kuts, Drying of
Microbiological Products, Agropromizdat, Moscow, (1987)
(in Russian).)
⫻
⫻
⫻
⫻
3
2
5
4
1
To vacuum
pump
Sorbent
output
Sorbent
input
FIGURE 39.21 Scheme of the device for ampuled material
dehydration by freeze/vacuum drying: (1) sorbent reservoir;
(2) connecting pipes for ampule coupling; (3) ampules con-
taining dried product; (4) sorbent material; and (5) grid for
sorbent charging. (From E.G. Tutova and P.S. Kuts, Dry-
ing of Microbiological Products, Agropromizdat, Moscow,
(1987) (in Russian).)
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2006 by Taylor & Francis Group, LLC.
terial are attached. In both design solutions the trans-
fer of sorbent layers takes place near the dried sub-
stances, from which the sorbent sublimates moisture,
enhancing the evaporation rate significantly. The costs
of sorbents and the energy consumed during their
recovery justify the application of such solutions be-
cause the process of sublimation is intensified and
product quality is improved as a result of decrease of
the process temperature and duration.
39.4.8 D
RYING OF
M
ICROORGANISMS AND
B
IOTECHNOLOGICAL
P
RODUCTS
ON
C
ARRIERS
This is a method of drying suitable for substances
used as mixtures. One of the components of the mix-
ture is a carrier. In the literature, this drying method
is known under various labels: drying on carrier
material, drying with immobilized cells, or contact-
sorption drying. Material that participates in the
process is called a carrier, filling material, or sorbent.
The results of experimental investigations and theor-
etical assumptions concerning this drying method are
discussed elsewhere [20,24,71–77] and in a mono-
graph on microbiological concentrates [19].
Let us analyze the role of material called a carrier
and its properties:
The carrier introduced into the biotechnological
product suspension changes the form of a sub-
stance being dried (from liquid or suspension, a
granulated product is obtained).
Hygroscopicity of the material changes.
A
biotechnological
product
in
the
mixture
changes its xerolability.
As a result, the technology and method of drying
the mixture can be changed, and so a product of high
quality is obtained. The carrier is a material that
protects cells or biopolymers against mechanical or
thermal destruction during drying. The carrier as a
sorbent takes over part of moisture from biomass and
thus the amount of water to be evaporated is smaller,
which causes energy savings. Another source of en-
ergy savings is that the process can be carried out at a
medium temperature of the drying agent, which con-
tributes to the diminishing of heat losses. Two basic
features of the carrier include:
1. Its suitability for application and use in the mix-
ture together with biotechnological products.
2. Neutral interactions with product. (The mater-
ial must not destroy or deteriorate product
quality; also, it cannot be a medium on which
an uncontrolled growth and development of
product takes place.)
The carrier should have lower hygroscopicity,
which facilitates long-term storage of the product
without the use of specific methods and conditions.
It should also be easily available and cheap. The
carrier is often a by-product of another technology,
and then its application in preserving biotechnology
products contributes to the protection of the environ-
ment against solid wastes.
Tutova and Kuts [20] propose to evaluate the
sorbent suitability from the point of view of its sorp-
tive abilities. They characterize material comparing
sorptive activities (i.e., relative sorptivity K or hygro-
scopicity) that under thermodynamic equilibrium of
the system is written as follows:
K
¼
a
s
a
r
¼
X
s
,
e
X
r
,
e
(39:20)
where a
s
and X
s,e
are the absorptivity and equilibrium
moisture content of the sorbent, respectively, and a
r
and X
r,e
are the absorptivity and equilibrium moisture
content of a reference material, respectively. A refer-
ence material, or a matrix, is a filter paper with con-
stant absorptivity for the whole range of relative air
humidities. In calculations and the diagram presented
in Figure 39.22, the authors took the equilibrium
50
0
1
12
11
1
10
6
7
9
5
8
2
2
3
3
4
4
5
60
70
80
90
100
j, %
K
FIGURE 39.22 Relative absorptivity K of some sorbents:
(1) Zeolite CaA; (2, 3) activated carbon; (4), wheat bran; (5)
maize meal; (6, 7) peat; (8) kaolin; (9, 10) potato flour; and
(11, 12) silica gel. (From E.G. Tutova and P.S. Kuts, Drying
of Microbiological Products, Agropromizdat, Moscow,
(1987) (in Russian).)
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2006 by Taylor & Francis Group, LLC.
moisture content X
r,e
¼ 0.01
*
X
e,max
, wher e X
e,max
is
the maxi mum equilib rium mois ture content of the
matrix at 25 8 C. Fro m the diagra m in
,
material s of mean sorptive activit y ( K
¼ 1 to 1.5, e.g. ,
maize meal , potato flour , an d peat) wi th an ab sorp-
tivity K that does not depend on air humidi ty, w, and
material s of high sorpti ve acti vity (K > 2, e.g., ze olite,
silica gel, an d activated carbo n) wi th absorpt ivity that
depend s to a large extent on the relative air humidi ty,
w
, can be selec ted.
The method for drying bio technol ogical prod ucts
on carriers can be us ed in every techniq ue discus sed
above. The most inter esting is spray and fluidized/
vibroflui dized be d drying. Thes e method s differ not
only in the techni que of drying but also in the way of
prepari ng sub strates. In the spray dryer a mate rial
and a carrier get in contact directly in the dryer cham-
ber. Both mate rials are disper sed and at the same time
the pr oduct is adsorbed on the surfa ce of carrier
particles , mois ture transfer betw een the mate rials
takes place, and the pro cess of evap oration pro ceeds.
Durin g fluidized bed drying (and also vibroflu idized
bed, drum, and band drying) , granula ted material
should be supp lied to the dryer chamb er. In this
case the contact of the biotec hnologica l product and
carrier takes place in a separate mixin g ch amber (thi s
also can be in a fluidized bed or in a screw c onveyor) ,
from which the mixture pa sses to a batch feeder or a
granula tor an d next to the dry ing chamb er.
Exa mples of the de sign solut ions are shown sche-
matical ly in Figu re 39.23 through Fi gure 39.26 [20] .
Figure 39.23 present s disper sion of a carrie r in the
zone of drying agent feedi ng and bio mass suspen sion
sprayin g in a coc urrent spray dry er. A mixture that
forms in the contact zone is trans ported with a stre am
of dry ing agent and remove d from the dryer to a
cyclone.
and Figure 39.24b sh ow sche-
matical ly a ch amber in the spray-fl uidized bed dryer.
It presen ts also a specia l three-chan nel nozzle. Bi o-
mass is sprayed in the dr ying chamber by two nozzles,
the cen tral nozzle (Figure 39.24b) is used to spray and
contact the two material s (i.e., a biomas s suspen sion
and carri er). In the low er pa rt of the ch amber the
mixture is dried in a fluidized be d by means of a
separat e air stream.
Fig ure 39.25 present s a sch ematic diagra m of a
two-st age fluidized bed dryer with a separat e feed for
each stage with a drying agen t, and a gran ulated
biomas s and carri er mixture fed to the dr ying cham-
ber. The mixture of both mate rials is prepared in a
screw co nveyor during the transpo rt of mate rial to
the feeder -granul ator. The drying mate rial is trans -
ported along perfor ated plate s and ge ts in contact
counter- current ly with the drying agen t of variab le
parame ters.
present s an inter esting tw o-stage
dryer wi th a fluidized bed for granula tion and dry ing
of paste- like biotec hnologica l pro ducts. In the uppe r
chamber, material is prep ared for drying, that is, in
the fluidized bed granule s of biomass are mixe d wi th
carrier parti cles that stick to granule s, a dsorb part of
the mois ture, and pro tect granule s from agglom er-
ation an d sti cking to the chamber walls. The gran u-
lated material prepa red in such a way is transpo rted
to the lower c hamber wher e, in the fluidized bed, the
mixture is dried.
39.4.9 D
RYING OF
E
NCAPSULATED
B
IOTECHNOLOGICAL
P
RODUCTS
Recently, special attention has been paid to the up-
to-date technology of encapsulation of valuable sub-
stances and next drying of these systems so that they
could be stored in a stable state. So far, the technol-
ogy has been most often applied in cosmetic, pharma-
ceutical,
and
food
industries,
mainly
for
the
production of aromatic and coloring substances.
Lately, there are many references to the studies and
applications of this technology in biotechnological
products [78–86].
This technology is related to the immobilization of
biotechnological substances mentioned previously.
It consists in a durable combination and often a
6
1
3
4
2
5
Hot
air
Air outlet
Pressed air
Biomass
Carrier
Dry product
FIGURE 39.23 Scheme of the cocurrent spray dryer with a
carrier dispersion in the zone of biomass suspension spray-
ing and drying agent feeding: (1) drying chamber; (2) air
duct; (3) spray nozzle; (4) carrier feed; (5) carrier tank; and
(6) cyclone. (From E.G. Tutova and P.S. Kuts, Drying of
Microbiological Products, Agropromizdat, Moscow, (1987)
(in Russian).)
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2006 by Taylor & Francis Group, LLC.
capsulation of a sensitive component in the stable
structure that protects it against external factors.
Additionally, in the systems characterized by such
properties the protected component is released in a
way controlled by external factors and time of expos-
ure. So, complex systems of medicines (antibiotics
and vitamins) are obtained which are slowly released
in the organism. Due to this, the medicine is not
Sorbent/Carrier
Biomass
Biomass
Pressed
air
Air outlet
4
1
5
6
3
2
Dry product
Drying
agent
(a)
(b)
2
1
6
4
3
5
FIGURE 39.24 Scheme of the spray-fluidized bed dryer: (a) components mixing in a dryer chamber: (1) Mixing–drying
chamber; (2) grid; (3) spraying device (three-channel pneumatic sprayer); (4) sprayer; (5) fluidized bed; and (6) valve; (b)
scheme of the spraying device (three-channel pneumatic sprayer): (1) Pressed air feeding valve; (2) sorbent vessel; (3) biomass
feeding pipe; (4) air feeding pipe; (5) sorbent feeding pipe; and (6) blow-through valve of the external duct. (From E.G. Tutova
and P.S. Kuts, Drying of Microbiological Products, Agropromizdat, Moscow, (1987) (in Russian).)
1
9
3
4
5
8
8
2
7
7
6
6
Air
Air
Air
outlet
Biomass
Carrier
Dry product
FIGURE 39.25 Scheme of the two-stage fluidized bed dryer with carrier-porous material: (1) drying chamber I; (2) drying
chamber II; (3) screw conveyor; (4) feeder-granulator; (5) fluidized bed; (6) blower; (7) heater; (8) valve; and (9) carrier tank.
(From E.G. Tutova and P.S. Kuts, Drying of Microbiological Products, Agropromizdat, Moscow, (1987) (in Russian).)
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2006 by Taylor & Francis Group, LLC.
excreted after a short time and its action is prolonged
to many hours. The aim of encapsulation is similar in
the case of enzymes, bacteria, protein structures, and
other biotechnological products.
Drying of such systems is not a widely applied
technique and the results of investigations published
in the literature most often refer to laboratory scale.
As dry product is required in the form of very fine
spheres with the diameter around 10 mm, the process
of drying is usually carried out in spray or freeze
dryers. In the encapsulation process, one of the fun-
damental questions is a proper choice of material that
forms a structure carrying or encapsulating a biotech-
nological substance. Therefore, significant part of
studies and publications are concerned with different
behaviors of such materials as starch with differ-
ent degrees of modification and origin (e.g., potato
and maize), gelling substances (rubber of different
origin), biopolymers (e.g., chitosan and polyglycols).
Frequently, a proper choice of the material for micro-
capsule production is most important for reaching
this aim.
It is worth mentioning that in order to solve cor-
rectly the problems of encapsulation (in other words
micro- or bioencapsulation) adequate knowledge is
required, and industry and academia people represent-
ing different branches should cooperate. So, very
popular are joint conferences such as the World Con-
gress on Encapsulation and international research
teams and projects (e.g., the Bioencapsulation Re-
search Group in Nantes, France [87,88]). The contri-
bution of drying specialists to this technology is
indispensable. Only a proper selection of drying
methods and the most precise designing of drying and
control systems can guarantee that such a specific final
dry product will have the desired properties.
Summing up the above review of drying tech-
niques and dryer constructions for biotechnological
products, the following factors are worth stressing:
1. Shortening of high temperature interaction of a
product (e.g., cooling of the spray-drying
chamber walls, drying at a lower temperature
in multistage systems).
2. Possibility of a frequent control, adjustment,
and changes of drying parameters in many
points of the dryer connected with the control
of product quality changes.
Biomass
(paste)
5
1
11
9
3
4
8
10
8
Gas agent
outlet
2
6
7
Sorbent
(carrier)
Fluid bed
agent inlet
Dry product
Drying gas
agent inlet
Drying gas
outlet
FIGURE 39.26 Scheme of the apparatus for paste-like material drying in a sorbent fluidized bed: (1) chamber for granulation
and predrying of granules in a falling bed; (2) sorbent-fluidized bed; (3) drying chamber; (4) granule bed; (5) granulator; (6)
screw; (7) moist granules; (8) gas distributors; (9) granule overflow channel; (10) gate and batcher; and (11) drying device for
sorbent supply. (From E.G. Tutova and P.S. Kuts, Drying of Microbiological Products, Agropromizdat, Moscow, (1987) (in
Russian).)
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2006 by Taylor & Francis Group, LLC.
3. Possibility of a complet e ch ange of the dr y-
ing techni que by the ap plication of carriers/
sorbent s in mult icomponent bio technol ogical
concentra tes.
39.5 CONCLUSION
The subject of biotech nologic al produ cts covers a
variety of complex pro blems conce rning pro duction,
preser vation, an d storage , maint aining at the same
time high produ ct qualit y. One of the very impor tant
stages in the techno logy of biotec hnol ogical prod ucts
is the drying process , which has the decisi ve influence
on prod uct quality and preservati on. The process of
drying for the purpose of product preser vation shou ld
be strictly ad justed and c ontrolled becau se of high
sensitiv ity of sub stances to ch anges of tempe rature
and mois ture content as well as to the pro cess dur -
ation. The program of design of process parame ters ,
choice of a drying method, an d dryer co nstruction
should cover the foll owing tasks :
1. The influ ence of drying parame ters (e.g ., range
of water remova l, change of temperatur e, tim e)
on changes of pro duct qua lity and marki ng of
the critical values of these pa rameters.
2. A mathe matical descri ption of the kineti cs of
degradi ng change s a nd choice of the most im-
portant qua lity ind ices.
3. Many drying methods can be used wi th respect
to individ ual prop erties of biotech nologic al
products , e.g., ch anges of thermal an d hyd ro-
dynami c conditio ns in multistage drying instal-
lations or change of techni que and ope rating
parame ters in the case of drying with carrier/
sorbent mate rials.
It shou ld be stre ssed that the quality of a biotec hno-
logical produ ct is also decided by inst antaneous fer-
menta tion conditi ons of a unit charge of a pro duct;
thus a produ ct being dr ied can have propert ies that
change in time. Therefor e, preser vatio n of specific
produc t qualit y can be achieve d by adding specia l
protect ive substa nces to the produ ct of fermentation
that preser ve product quality during drying.
39.6 NOMENCLATURE
a activit y, units/mg
a mois ture absorp tivity, % by wei ght
A index of quality degradat ion, %
A
g
global index of quality degradation, %
C conce ntration, num ber of cell s/cm
3
, kg/m
3
d
p
Sauter mean particle diameter, m
E
a
activati on energy, J/mol
f, g functio n
i successive co mponent or qua lity index
k heat utilization coeffici ent
k
d
react ion rate constant , s
1
k
1
frequency coeffici ent, s
1
K relative sorptivit y
n number (e.g., function and indice s)
p
and
r react ion rate, units/( mg
s) or number of cells/
(cm
3
s)
r variable pelle t (dropl et) radius , m
R pelle t radius , m
R gas constant, J/(mol
K)
t time, s
T tempe rature, 8 C
w
i
densit y of mass stre am, kg/(m
2
s)
W
H2O
evaporat ive capacity, kg H
2
O/ h
W
v
volumetric evaporation rate, kg H
2
O/(m
3
s)
V
G
air flow ratio, kg of air/kg of H
2
O
z
empirical constant in Equation 39.10
X
material moisture content, % by weight
Greek Symbols
l
weight systems
u
temperature, K
Subscripts
d
degradation
G
gas
i
component
k
final
T
temperature
t
time
w
water
X
moisture
0
beginning
1 infinitive
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Document Outline
- Table of Contents
- Chapter 039: Drying of Biotechnological Products
- 39.1 Introduction
- 39.2 Biotechnological Products as Material Being Dried
- 39.3 Degradation Processes in The Scheme of Drying Modeling
- 39.4 Examples of Drying Methods and Equipment
- 39.4.1 Spray Drying
- 39.4.2 Jet-Spouted Bed Drying
- 39.4.3 Impingement Stream Drying
- 39.4.4 Drying in a Fluidized and Vibrofluidized Bed
- 39.4.5 Drying in a Fixed Bed
- 39.4.6 Drying in Drum Dryers
- 39.4.7 Freeze and Vacuum Drying
- 39.4.8 Drying of Microorganisms and Biotechnological Products on Carriers
- 39.4.9 Drying of Encapsulated Biotechnological Products
- 39.5 Conclusion
- 39.6 Nomenclature
- References
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