Applications and opportunities for ultrasound assisted extraction

in the food industry

— A review

Kamaljit Vilkhu

a,

⁎

, Raymond Mawson

a

, Lloyd Simons

a

, Darren Bates

b

a

Ultrasonics Processing Group, Food Science Australia, 671 Sneydes Road, Werribee, VIC 3030, Australia

b

Innovative Ultrasonics Pty Ltd, P.O. Box 321, Noosaville, QLD 4566, Australia

Abstract

Ultrasound assisted extraction (UAE) process enhancement for food and allied industries are reported in this review. This includes herbal, oil,

protein and bioactives from plant and animal materials (e.g. polyphenolics, anthocyanins, aromatic compounds, polysaccharides and functional
compounds) with increased yield of extracted components, increased rate of extraction, achieving reduction in extraction time and higher
processing throughput. Ultrasound can enhance existing extraction processes and enable new commercial extraction opportunities and processes.
New UAE processing approaches have been proposed, including, (a) the potential for modification of plant cell material to provide improved
bioavailability of micro-nutrients while retaining the natural-like quality, (b) simultaneous extraction and encapsulation, (c) quenching of the
radical sonochemistry especially in aqueous systems to avoid degradation of bioactives and (d) potential use of the radical sonochemistry to
achieve targeted hydroxylation of polyphenolics and carotenoids to increase bioactivity.

Keywords: Ultrasound assisted extraction; Cavitation; Particle size; Mass transfer

Industrial relevance: The application of ultrasonic assisted extraction (UAE) in food processing technology is of interest for enhancing extraction of components
from plant and animal materials. This review shows that UAE technology can potentially enhance extraction of components such as polyphenolics, anthocyanins,
aromatic compounds, polysaccharides, oils and functional compounds when used as a pre-treatment step in a unit process. The higher yield obtained in these UAE
processes are of major interest from an industrial point of view, since the technology is an

“add on” step to the existing process with minimum alteration, application

in aqueous extraction where organic solvents can be replaced with generally recognised as safe (GRAS) solvents, reduction in solvent usage, and shortening the
extraction time. The use of ultrasonic for extraction purposes in high-cost raw materials is an economical alternative to traditional extraction processes, which is an
industry demand for a sustainable development.

1. Introduction

The application of ultrasound as a laboratory based tech-

nique for assisting extraction from plant material is widely
published. Several reviews have been published in the past to
extract plant origin metabolites (

Knorr, 2003

), flavonoids from

foods using a range of solvents (

Zhang, Xu, & Shi, 2003

) and

bioactives from herbs

Vinatoru (2001)

. A limited number of

publications have included continuous ultrasonic process deve-
lopment and pilot-scale applications. The range of published

extraction applications include herbal, oil, protein and bioac-
tives from plant materials (e.g. flavones, polyphenolics), sum-
marised in

Table 1

and outlined in more detail in the following

Applications section. Much of the work is empirical in nature
and explanations of the mechanisms have been proposed. Some
workers also discuss both the mechanisms involved in UAE and
the likely issues for potential for scale up. The review by

Vinatoru

(2001)

outlines a program of work where industrial scale up was

attempted under an EU Copernicus grant (ERB-CIPA-CT94-0227-
1995). They highlight that while it is relatively easy to achieve
extraction on the laboratory bench it is very challenging to attempt
extraction on an industrial scale. Several key issues and ob-
servations relating to UAE have been identified, as follows, (1) the
nature of the tissue being extracted and the location of the

⁎ Corresponding author. Tel.: [phone]; fax: [phone].

E-mail address:

[e-mail]

(K. Vilkhu).

components to be extracted with respect to tissue structures, (2) pre-
treatment of the tissue prior to extraction, (3) the nature of the
component being extracted, (4) the effects of ultrasonics primarily
involve superficial tissue disruption, (5) increasing surface mass
transfer (

Balachandran, Kentish, Mawson, & Ashokkumar, 2006;

Jian-Bing, Xiang-hong, Mei-qiang, & Zhi-chao, 2006

), (6) intra-

particle diffusion, (7) loading of the extraction chamber with
substrate, (8) increased yield of extracted components and
(9) increased rate of extraction, particularly early in the extraction
cycle enabling major reduction in extraction time and higher pro-
cessing throughput (

Moulton & Wang, 1982; Caili, Haijun,

Quanhong, Tongyi, & Wenjuan, 2006

).

Living tissues where the desired components are localized in

surface glands can be stimulated to release the components by
relatively mild ultrasonic stressing (

Toma, Vinatoru, Paniwnyk, &

Mason, 2001

). In tissues where the desired components are located

within cells, pre-ultrasound treatment by size reduction to maximise
surface area is critical for achieving rapid and complete extraction
(

Riera, Golás, Blanco, Gallego, Blasco, & Mulet, 2004; Balachan-

dran et al., 2006;Vinatoru, 2001

). Where pre-hydration is necessary

to achieve extraction, ultrasound effectively accelerates the hydra-
tion process (

Vinatoru, 2001

). Ultrasound induced cavitation bub-

bles present hydrophobic surfaces within the extraction liquid
(Grieser, personnel communication) thereby increasing the net
hydrophobic character of the extraction medium. Thus it is possible
to extract polar components into otherwise hydrophilic aqueous
extraction media, reducing the need for generally undesirable
hydrophobic or strongly polar extraction media. The disruption of
tissue surface structures is revealed with microscopic examination
by

Vinatoru (2001)

,

Chemat, Lagha, AitAmar, Bartels, and Chemat

(2004)

,

Haizhou, Pordesimo, and Weiss (2004)

,

Balachandran et al.

(2006)

. Several of the authors in the work cited below highlight

concerns due to the potential for ultrasonic cavitation to propagate
free radicals, in particular hydroxyl radicals. Where the potential
oxidative damage is a concern radical production can be quenched
by the addition of small amounts of ethanol to lower the

temperatures within the cavitation bubbles and extinguish the
chemistry involved (Sun et al. unpublished work in progress).

This paper provides a compilation of food-related UAE

applications, highlighting the application approaches and per-
formance. Following this, a more detailed discussion is given on
UAE mechanisms, process development, equipment design and
future opportunities.

2. Applications

2.1. Herbal and oil extraction

Ultrasound has been recognised for potential industrial ap-

plication in the phyto-pharmaceutical extraction industry for a
wide range of herbal extracts.

Vinatoru (2001)

published an

overview of the UAE of bioactive principles from herbs. The
improvement in extractive value by UAE compared with
classic methods in water and ethanol for fennel, hops, marigold
and mint was 34%, 18%, 2%, and 3% respectively in water,
whereas 34%, 12%, 3%, and 7%, respectively in ethanol. In
another study, an aqueous extraction of Geniposide from
Gardenia fruit was investigated by

Jian-Bing et al. (2006)

.

When ultrasound was applied at 0.15 W cm

− 2

the extraction

yield of Geniposide was increased by 16.5%, in comparison
with a static process using 40 ml/g of the solvent volume to
fruit weight. The variability in percentage extract yield was
mainly due to the individual product structure. Large scale
ultrasonic extraction designs were proposed for stirred tank
systems with temperature control.

In recent years,

Albu, Joyce, Paniwnyk, Lorimer, and Mason

(2004)

investigated the effect of different solvents and ultra-

sound on the extraction of carnosic acid from rosemary. Using
conventional stirred extraction ethanol was significantly less
effective then ethyl acetate and butanone. The application of
ultrasound improved the relative performance of ethanol such
that it was comparable to butanone and ethyl acetate alone.

Table 1
List of ultrasound assisted extraction studies from the literature on various food components

Product

Ultrasound Process

Solvent

Performance

Author

Almond oils

Batch, 20 kHz

Supercritical carbon
dioxide

30% increased yield or extraction
time reduction

Riera et al. (2004)

Herbal extracts (fennel, hops,

marigold, mint)

Stirred batch,
20 to 2400 kHz

Water and ethanol

Up to 34% increased yield over stirred

Vinatoru (2001)

Ginseng saponins

Batch, 38.5 kHz

Water, methanol
and n-butanol

3-fold increase of extraction rate

Wu et al. (2001)

Ginger

Batch, 20 kHz

Supercritical carbon
dioxide

30% increased yield or extraction
time reduction

Balachandran et al. (2006)

Soy protein

Continuous, 20 kHz,
3 W per gram

Water and alkali
(sodium hydroxide)

53% and 23% yield increase over
equivalent ultrasonic batch conditions

Moulton and Wang (1982)

Soy isoflavones

Batch, 24 kHz

Water and solvent

Up to 15% increase in extraction efficiency

Rostagno et al. (2003)

Rutin from Chinese Scholar Trees

Batch, 20 kHz

Water and methanol

Up to 20% increase in 30 min

Paniwynk et al. (2001)

Carnosic acid from rosemary

Batch, 20 and
40 kHz

Butanone and ethyl
acetate

Reduction in extraction time

Albu et al. (2004)

Polyphenols, amino acid and

caffeine from green tea

Batch, 40 kHz

Water

Increased yield at 65 °C, compared with 85 °C

Xia et al. (2006)

Pyrethrines from flowers

Batch, 20 and
40 kHz

Hexane

Increased yield at 40 °C, compared with 66 °C

Romdhane and Gourdan
(2002)

Thereby ultra-sonication may reduce the dependence on a sol-
vent and enable use of alternative solvents which may provide
more attractive (a) economics, (b) environmental and (c) health
and safety benefits.

Ginsenosides (tri-terpene saponins) are known as the prin-

cipal ingredients of ginseng roots. Ginseng saponins are asso-
ciated with traditional herbal medicine and health foods (

Tang

& Eisenbrand, 1992

). UAE of ginseng saponins was approx-

imately 3-times faster than the traditional extraction method
involving reflux of boiling solvents in a soxhlet extractor.
Furthermore, the UAE technique was achieved at lower tempe-
ratures which are more favourable for thermally unstable com-
pounds (

Wu, Lin, & Chau, 2001

). Similar results were reported

on UAE of carvone and Limonene from caraway seeds, which
resulted in 2 fold increases in their contents (

Chemat et al.,

2004

).

Likewise, anthraquinones from roots of Morinda citrifolia

(Noni) are the active compounds which show several therapeutic
effects and used in anti-cancer medical applications. Recently,

Hemwimol, Pavasant, and Shotipruk (2006)

investigated the use

of UAE to improve the solvent extraction efficiency of an-
thraquinones from the roots of M. citrifolia. Ultrasound extraction
in an ethanol water system provided a 75% reduction in extraction
time and yield comparable with non-sonicated sample.

Supercritical fluid extraction (SFE) is an intrinsically capital

intensive process where any enhancement of extraction efficien-
cy either in terms of extraction rate or yield is economically
attractive. Over a period of many years it has been shown that
combined action of ultrasound and supercritical carbon dioxide
on extraction could be used to significantly improve extraction
rate or yield of amaranth oil from seeds (

Bruni, Guerrini, Scalia,

Romagnoli, & Sacchetti, 2002

), almond oil (

Riera et al., 2004

),

tea seed oil (

Rajaei, Barzegar, & Yamini, 2005

), gingerols from

ginger (

Balachandran et al., 2006

), operating parameters such as

temperature, pressure and CO

2

flow for Adlay seed (Coix

lachrymal-jobi L. var. Adlay) oil and coixenolide from adlay
seed (

Ai-jun, Shuna, Hanhua, Tai-qiu, & Guohua, 2006

).

UAE has been recognised for application in the edible oil

industry to improve efficiency and reduce extraction time (

Babaei,

Jabbari, & Yamini, 2006

). This potential was based on UAE

increases in oil from soybeans; carvone and limonene from cara-
way seeds. The ultrasonically induced cavitation was shown to
increase the permeability of the plant tissues. Microfractures and
disruption of cell walls in soybean flakes (

Haizhou et al., 2004

) and

caraway seeds cell wall (

Chemat et al., 2004

) provided more

evidence for the mechanical effects of ultrasound thus facilitating
the release of their contents, in contrast to conventional maceration
or extraction. These effects were identified under scanning electron
microscopy. Importance was given to the effect of solvent vapour
pressure and surface tension on cavitation intensity.

The benefit of using ultrasonic pre-treatment before extract-

ing oil from the seeds of Jatropha curcas L., and almond and
apricot seeds by aqueous enzymatic oil extraction (AEOE)
process was evaluated by

Shah, Sharma, and Gupta (2005)

,

Sharma and Gupta (2006)

. Ultrasonic pre-treatment of the

almond and apricot seeds before aqueous oil extraction and
aqueous enzymatic oil extraction provided significantly higher

yield with reduction in extraction time. Thus, implementation of
ultrasonic pre-treatment reduced oil extraction time that may
improve through put in commercial oil production process.

2.2. Protein extraction

A small pilot-scale ultrasound batch and continuous soy

protein extraction trials were reported by

Moulton and Wang

(1982)

. The continuous high-intensity application extracted

54% and 23% more protein for aqueous and alkali extraction
respectively, compared with the batch extraction using com-
parable processing times and volumes. During the trials it was
estimated that the continuous process used 70% less energy than
the batch system to extract the same amount of protein and
sonication efficiency improved with the greater load of thicker
slurry, up to 1:10 (flake to solvent) ratio.

2.3. Bioactive extraction from plant materials

2.3.1. Polyphenols

Grape marc is the solid waste of the wine-making process.

Consisting of skins, seeds, and small amount of leaves, grape
marc has long been used for alcohol, tartaric acid and more
recently, the recovery of phenolic compound. Phenolic com-
pounds are of particular interest in wine industry as it gives the
characteristics colour and flavour in wine, and in pharmaceutical
industry for its benefits on human health (

Brenna, Buratti, Cosio,

& Mannino, 1998

). Polyphenols are associated with reduced risk

of cardiovascular disease by inhibiting in-vitro oxidation of low-
density lipoproteins possess anti-ulcer, anti-mutagenic, anti-
inflammatory activity and anti-carcinogenic properties (

Flamini,

2003; Negro, Tommasi, & Miceli, 2003; Bonilla, Mayen,
Merida, & Medina, 1998; Palma & Taylor, 1999

). Phenolic

compounds include tannins and colour pigments, anthocyanins
which present at a higher level in red grape marc compared with
white grape marc and are more likely to be found on the grape
seeds (

Springett, 2001; Palma & Taylor, 1999

).

The application of ultrasound at Food Science Australia has

focused on the use of high-powered systems for extraction of
bioactives. Principle targets have been polyphenols and caro-
tenoids and in both aqueous and solvent extraction systems. The
ultrasound extraction trials have demonstrated improvements in
extraction yield ranging from 6 to 35%, as summarised in

Table 2

. Results of ultrasonically treated Shiraz and Sangiovese

grape marc showed 17 and 35% increase in phenolic com-
pounds respectively, However extraction of these compounds
yielded much higher recovery from their respective seeds
(Vilkhu, Food Science Australia unpublished data).

Supercritical carbon dioxide extraction is proposed as a

better method than ultrasound assisted extraction of polyphe-
nolic compounds from grape seeds

Palma and Taylor (1999)

. It

was believed that the lower catechin (used as a measure of
phenolic content) recovery from ultrasound method could be due
to the insufficient power of the solvent used (aqueous methanol) or
due to the degradation of samples during extraction process. Their
study was focused on the efficiency of supercritical fluid extraction
(SFE) rather than other methods used in the experiment. The

results of catechin recovery using different extraction methods
compared to a control (solvent extraction only) was not available,
therefore it was not possible to determine whether ultrasound
treatment (although having a lower recovery compared to SFE
method) contributed to the increase in catechin recovery relative to
a control. Most importantly though, the frequency of ultrasound
and other extraction conditions (e.g. temperature) was not stated,
therefore it was not known whether suitable frequencies or
application conditions were used.

In recent years it has been shown that pressurized hot water

extraction methods offered higher phenolic compound recovery
when compared to UAE, hydro-distillation and maceration with
70% ethanol (

Ollanketo, Peltoketo, Hartonen, Hiltunen, &

Riekkola, 2002

). The use of methanol during UAE produced the

lowest recovery with results not statistically different from
maceration with 70% ethanol. Potential exists for combining
ultrasound as an adjunct with the other extraction procedures to
improve efficiency and yield. More recently,

Tedjo, Eshtiaghi,

and Knorr (2002)

studied the quality attributes of grape juices

for wine-making using non-thermal processes including
ultrasound. The non-thermal processes examined offered a
suitable gentle-action alternative to other cell breakdown
methods with increased grape juice yields. Quality analyses
(e.g. sugar, anthocyanins and mineral concentration, acidity,
colour) showed that non-thermally processed juices had superior
quality to untreated samples and comparable quality to that of
enzyme treated grape juices. Likewise significantly enhanced
contents of tea polyphenols, amino acid and caffeine in tea infu-
sions were recovered with ultrasound assisted extraction when
compared with conventional extraction. The sensory quality of tea
infusion with ultrasound assisted extraction was better than that of
tea infusion with conventional extraction (

Xia, Shi, & Wan, 2006

).

2.3.2. Anthocyanins

Anthocyanins are enjoying greater prominence due to in-

creasing public concern with the use of synthetic colouring
agents. Anthocyanins represent a large group of water-soluble
plant pigments based on the 2-phenylbenzophyrylium (flavy-
lium) structure and there are more than 200 compounds in this
category (

IPCS, 2001

). Anthocyanins are the main colour pig-

ments in wild fruits and berries, and predominantly found in the
sap of mature cells in grape skin (

Springett, 2001

). The pigments

present in grape skin consist of di-glucosides, mono-glucoside,
acylated monoglucosides and acylated di-glucosides of peoni-
din, malvidin, cyanidin, petunidin and delphinidin. Anthocya-
nins content in grapes varies from 30

–750 mg/100 g (

Birdle &

Timberlake, 1997

). The wide variation in amount of these

compounds is greatly dependent upon cultivar, season, growing
conditions, degree of ripeness, storage conditions as well as
extraction procedures (

Cacace & Mazza, 2003

). UAE of crushed

Shiraz and Merlot grapes by Food Science Australia showed 15

–

18% increase in total colour in grape juice (unpublished data).

A study has been conducted on the potential to use

microwave and ultrasound treatments for the extraction of pig-
ments from strawberries. Optimal extraction was achieved using
microwaves at 624 W, with a treatment time of 60 s, together
with ultrasonic processing for 40 s and a ratio of material and
extraction solvent of 1:6. The stability of the pigment extracts
was considerably affected by pH, and achieving a maximum at
pH 5.0. Addition of sucrose or heating at temperatures up to
80 °C had little effect on pigment stability. However, pigment
stability and colour were greatly improved by addition of citric
acid (

Cai, Liu, Li, & An, 2003

).

2.3.3. Tartaric acid

Tartaric acid occurs naturally in fruits, and found in high

concentrations in grapes and tamarind (

Springett, 2001

).

Approximately 90% of the total organic acids in grapes are
tartaric and malic acids. Tartaric acid is a by-product in the wine
industry since a tremendous amount of tartaric acid from lees has
to be removed from the wine after yeast fermentation. Tartaric acid
is widely used in bakery operations, wine production, pharma-
ceutical industry, hardening of gypsum, confectionery processing
and in the chemical industry.

Palma and Barroso (2002)

optimized

the UAE conditions for the recovery of tartaric and malic acid from
red and white variety grapes for quantitative determination in
wine-making by-products. Our studies on UAE of tartaric esters
from red grape marc yielded an increase 16 to 23% from two
different varieties (Vilkhu, unpublished).

2.3.4. Aroma compounds

Over a period of many years it has been shown that ultra-

sound could be used to extract aromatic chemicals, which impart
bouquet to the wines (

Cocito, Gaetano, & Delfini, 1995

). Solvent

mixtures of n-pentane and diethyl-ether (1:2) and dichloromethane
were used to study the optimization of the sonication extraction
process. This study emphasised that UAE improved extraction
efficiency with increased reproducibility of most aroma com-
pounds compared to conventional extraction (

Vila, Mira, Lucena,

& Fernandez, 1999

).

An evaluation of UAE of isoflavones from ground soybeans

was undertaken by

Rostagno, Palma, and Barroso (2003)

, the

efficiency of the extraction was improved by 15% but this was
dependent on the organic solvent used. Notably 40

–60% water

Table 2
Examples of bioactive ultrasound assisted extraction work completed at Food Science Australia

Extract target

Product

Solvent

Process

Processing conditions

Improvement range (%)

Beta-carotene

Carrot

Aqueous

Laboratory; 24 kHz, 20

–7 W s ml

− 1

Ambient

15

–25

Ethyl-acetate

Laboratory; 24kHz, 20

–75 W s m

− 1

Ambient

8

–20

Polyphenols

Red grape marc

Aqueous

Laboratory; 24 kHz, 20

–75 W s ml

− 1

Ambient

11

–35

Polyphenols

Black tea

Aqueous

Laboratory; 24 kHz, 8

–10 W s ml

− 1

Hot processing 90 °C

6

–18

Polyphenols

Apple

Aqueous

Laboratory; 40 kHz, 20

–75 W s ml

− 1

Hot processing 80 °C

6

Gingerol

Ginger

Supercritical carbon dioxide

Laboratory; 20 kHz

Pressure 160 bar

30

was required to improve the extraction efficiency, which was
thought to be due to the relative polarity of the isoflavones and
increased ultrasound propagation in aqueous systems. Some
aromatic compounds such as rutin from the flower buds of
Chinese Scholar Tree (Sophora japonica) have improved consi-
derably with higher levels of organic solvent compared to
aqueous conditions. The difference in performance was
attributed to hydroxyl radical and hydrogen peroxide formation
in aqueous conditions resulting in degradation of the rutin. The
application of ultrasound in methanol was considered more
effective due to the higher solubility of rutin in methanol and
hydrogen peroxide is not formed by ultrasound in methanol
(

Paniwynk, Beaufoy, Lorimer, & Mason, 2001

).

In order to extract phycocyanin from Spirulina platensis

(Arthrospira platensis) cells, selection of ultrasonic frequency
was important (

Furuki et al., 2003

). The purity of phycocyanin

in its crude extract was dependant on ultrasonic frequency. For
example, phycocyanin was extracted with higher purity at 28 kHz
than at 20 kHz, due to the selective extraction of the active
component at these frequencies. It was suggested that rapid and
selective extraction of phycocyanin from S. platensis may be
possible if an optimized ultrasonic application is developed.

2.3.5. Polysaccharides and functional compounds

The extraction of carbohydrates, polysaccharides and other

functional compounds has been studied in the recent years.
Various extraction procedures with and without a short applica-
tion of ultrasound at the beginning of the extraction were used to
examine the effect of sonication on the extractability of the
hemicellulose components of buckwheat hulls (

Hromadkova &

Ebringerova, 2003

), cellulose from sugarcane bagasse (

Sun,

Sun, Zhao, & Sun, 2004

), and xyloglucan from apple pomace

(

Caili et al., 2006

). UAE of these compounds not only acce-

lerates the extraction process but also preserves structural and
molecular properties. In sugar cane bagasse hemicellulose
extraction processes, UAE improved extractability of hemi-
celluloses apparently by destruction of cell walls and cleavage
of links between lignin and the hemicelluloses (

Jing, RunCang,

Xiao, & YinQuan, 2004

). Whereas ultrasonic aqueous extrac-

tion of polysaccharides from edible fungus, Pleurotus
tuberregium, resulted in the formation of glycan

–chitin com-

plexes with higher average molecular weight than compounds
obtained by hot water extraction (

Mei, Lina, Chi-Keung-

Cheung, & Eng-Choon-Ooi, 2004

), which could be due to the

sonochemical modification of two polysaccharides. Further
improvement in immunological as well as anti-tumour activi-
ties of these complexes were reported on animal trials.

UAE can enable extraction at lower temperatures,

Xu, Zhang,

and Hu (2000)

have compared UAE with hot water extraction of

flavonoids from bamboo leaves. The laboratory scale trials
results showed that the optimal conditions for extraction were
achieved using UAE at lower temperature, rather than using hot
water bath extraction at 80 °C. More recently,

Rosângela et al.

(2007)

investigated the chemical composition of Mate tea

extracts (leaves of Ilex paraguariensis, a native tree from
Brazil). The effect of the ultrasonic treatment resulted in
improved mass yield of caffeine and palmitic acid in methanol

solvent. Ultrasound enhanced both the kinetics and yield which
was attributed to increase in the internal diffusion that controls
the transfer of solute to the solvent and also the destruction of
pores in which the solute can be trapped. However the efficiency
of the extraction will be dependent on the concentration of the
methanol solvent employed

Rostagno et al. (2003)

.

2.4. Bioactive extraction from animal materials

There is limited number of publications on UAE from animal

material. Attempts were made to extract chitin from fresh water
prawn shells (

Kjartansson, Zivanovic, Kristbergsson, & Weiss,

2006

) and lutein from egg yolk (

Xiaohua, Zhimin, Witoon, &

Joan, 2006

) by using sonication. In chitin studies from prawn

shells, it was found that the chitin yield decreased during soni-
cation, this loss was attributed to depolymerization of extracted
chitin in the wash water. Subsequently, the degree of acetylation
of chitins was unaffected by sonication, but the degree of
acetylation of chitosans produced from sonicated chitin decreased.

Egg yolk is one of the major lutein sources in our foods

(

Johnson, 2004

). Lutein in egg yolk is highly bio-available,

compared with other sources. It was reported that egg yolk
intake significantly increased plasma lutein (

Handelman,

Nightingale, Lichtenstein, Schaefer, & Blumberg, 1999

).

Recently,

Xiaohua, et al. (2006)

have reported higher extraction

yield of luetin when ultrasonic used in combination of sapo-
nificated organic solvent. Further to their report, compared with
the traditional saponification solvent extraction method, the
UAE extraction method was more effective in extracting lutein
from the sample matrix, presumably by avoiding degradation
reactions.

3. Extraction mechanisms and process development

Extraction enhancement by ultrasound has been attributed to

the propagation of ultrasound pressure waves, and resulting
cavitation phenomena. High shear forces cause increased mass
transfer of extractants (

Jian-Bing et al., 2006

). The implosion of

cavitation bubbles generates macro-turbulence, high-velocity
inter-particle collisions and perturbation in micro-porous
particles of the biomass which accelerates the eddy diffusion
and internal diffusion. Moreover, the cavitation near the liquid

–

solid interface sends a fast moving stream of liquid through the
cavity at the surface. Cavitation on the product surface causes
impingement by micro-jets that result in surface peeling,
erosion and particle breakdown. This effect provides exposure
of new surfaces further increasing mass transfer.

This phenomenon was confirmed by performed scanning

electron micrography on peppermint plant leaves and trichomes.
After these were ultrasonically treated for menthol extraction,
microscopy results indicated that there were two mechanisms
involved in extraction: (a) the diffusion of product through the
cuticle of peppermint glandular trichomes and (b) the exudation
of the product from broken and damaged trichomes (

Shotipruk,

Kaufman, & Wang, 2001

).

Acceleration in the extraction kinetics and improved

extraction yield of pyrethrine from pyrethrum was largely

attributed to ultrasonics increasing the intra-particular diffusion
of the solute, considered the rate limiting step (

Romdhane &

Gourdon, 2002

). If the substrate is dry then ultrasound may be

used to facilitate swelling and hydration and cause enlargement
of the pores of the cell wall (

Vinatoru, 2001

). Diffusion through

the plant cell walls, disruption and washing out of the cell
contents were also attributed to improved extraction perfor-
mance. The corresponding reduction in the size of the vegetal
material particles by ultrasound disintegration will increase the
number of cells directly exposed to extraction by solvent and
ultrasonic cavitation (

Vinatoru, 2001

). Intensive ultra-sonica-

tion can also serve the purpose of reducing the particle size in
tomato juice (Food Science Australia unpublished data).

As large amplitude ultrasound waves pass through a mass

media, cavitational bubble collapse can occur in close vicinity
or at the surface of the plant membranes causing microfractures
(

Vinatoru, 2001

). The occurrence of microfracture by ultra-

sound was demonstrated in soybean flakes (

Haizhou et al.,

2004

). Cavitation collapse can occur on the plant surfaces,

resulting in a micro-jet directed into the solid surface. Cavitation
at cell surfaces has the ability to punch holes through cell wall as
recently demonstrated with studies of bacterial cell sonication
(

Ugarte-Romero, Feng, Martin, Cadwallader, & Robinson,

2006

). Preferentially micro-jetting will occur onto hydrophilic

particle surfaces (

Arora, Claus-Dieter, & Knud, 2004

).

Variation in the extraction yield from different plant varieties

may result from structure, rheology (hardness of the seed
structure) or the compositional differences resulting in varying
degrees of susceptibility to ultrasound shock waves and like-
lihood that cavitation bubble will contact with the plant surface
causing micro-jetting (

Haizhou et al., 2004

). Factors such as

plant tissue turgor and the mobility of particles such as starch
granules within the cell cytoplasm can be expected to influence
ultrasound energy dispersion and extraction effectiveness
(

Zhang, Niu, Eckhoff, & Feng, 2005

).

In the study on supercritical fluid extraction enhancement by

ultrasound

Balachandran et al. (2006)

they were able to demon-

strate that the effectiveness of ultrasound was gained by the
increase in the superficial mass transfer and that effectiveness
declined sharply after the readily accessible surface solute had
been removed. However, by reducing the substrate particle size
major gains in extraction efficiency and extraction time reduc-
tion could be achieved.

Solvent selection is usually based on achieving high mole-

cular affinity between the solvent and solute. When ultrasound
is also applied the cavitation will be affected by the physical
properties of the solvent. Cavitation intensity decreases as
vapour pressure and surface tension are increased.

Haizhou

et al. (2004)

demonstrated this phenomenon in soybean oil

extraction where greater UAE was achieved by isopropanol
compared with hexane, the later having approximately 5-fold
higher vapour pressure.

4. Adjunct processes

During extraction, ultrasound may also achieve adjunct

processes, whereby the food extract, ingredient or product

functionality may be modified by physical and sonochemical
mechanisms. One such modification has been reported by

Cravotto, Binello, Merizzi, and Avogadro (2004)

in rice bran

wax conversion to policosanol (common name for a mixture of
C24

–C34 linear saturated fatty alcohols), a rich source of

nutrients and pharmacologically active compounds. Both the
first bran fraction from rice polishing and the discarded wax
from the manufacture of rice oil were convenient and profitable
starting materials for the production of policosanol.

In the date syrup industry, ultrasound was applied for im-

proving the quantity and quality of the syrup extraction.

Entezari,

Nazary, and Khodaparast (2004)

successfully optimized ultrasonic

processing conditions in laboratory trials which lead to a higher
extraction in a shorter time with improved physical quality of the
date syrup extract. Most importantly, the sonication significantly
decreased the microbial count in comparison to the conventional
method. This study also confirmed the presence of anti-microbial
substances in date fruit, and that ultrasonic waves can accelerate
their effects.

The anti-oxidative activity provided by phenolic compounds

has been shown to inhibit the oxidation of low-density proteins
(

Frankel, Waterhouse, & Teissedre, 1995

). Resveratrol (trans-3,

5, 4

′-trihydroxystilbene), a stilbene phyto-alexin, is a phenolic

compound possessing anti-oxidant activity. Resveratrol has
been shown to provide health-promoting activities such as
lowering the incidence of coronary heart disease and provide
cancer chemo-preventive activity (

Frankel, Waterhouse, &

Kinsella, 1993; Jang et al., 1997

). The combined use of ultra-

violet light and ultrasound treatments on peanut kernels was
reported (

Rudolf & Resurreccion, 2005

) for the elicitation of

trans-resveratrol, total phenolic compounds, and anti-oxidant
activity. A short exposure of ultrasound (4 min) to sliced peanuts
and further incubation for 36 h at ambient temperature resulted in
an 8-fold increase of trans-resveratrol as compared to untreated
control samples. It was also reported that the anti-oxidative
activity in stressed peanuts was negatively correlated with trans-
resveratrol concentration, indicating that as anti-oxidant activity
decreased trans-resveratrol concentration increased.

To potentially replace the conventional destructive extraction

process of menthol extraction from peppermint plants

Shotipruk

et al. (2001)

studied the feasibility of using ultrasound to extract

menthol from biologically viable peppermint plants (Mentha
xpiperita). The results showed that plants ultrasonicated for 1 h
at 22 °C in a standard 40 kHz ultrasonic bath released approx-
imately 17.8

μg of menthol per gram of leaf tissue (2% of total

product). The amount of menthol release increased with the
time of treatment and was greatly affected by the temperature of
the ultrasonic bath water. An increase from 2% to 12% of total
product was observed when the temperature was increased from
22 °C to 39 °C. When the temperature effects were isolated, the
mechanism of the product release was found to be that of
cavitation. The treated plants remained viable and were ready
for the subsequent ultrasound extraction after approximately
4 days of recuperation. However, the amount of product re-
leased was reduced in subsequent extractions. This study has
shown the possibility of using an online ultrasonic, non-des-
tructive extraction method to continuously release intracellular

plant metabolites from the plants while maintaining the plant's
viability.

The application of ultrasound treatment to yellow dent corn

at different points in the conventional wet milling process
enhanced starch separation, providing an increase in final starch
levels of 6.35 to 7.02% (

Zhang et al., 2005

). The starches

produced by ultrasonic treatments showed a significant increase
in whiteness and decrease in yellowness that were comparable
to starches produced by conventional wet milling. The ultra-
sound-treated starches exhibited higher paste viscosities. These
viscosity changes during ultrasound treatments to starch gra-
nules in the slurry were attributed to the phenomenon of cavitation.
The intense ultrasound treatment which generated localized spots
of very high temperature and pressure might lead to configura-
tional modifications of the granular structure, which could be in
the forms of diffuse erosion or pitting of the starch granules as
earlier observed by

Degrois, Gallant, Baldo, and Guilbot (1974)

.

The cork taint is one the major problem in wine corks.

Trichloroanisoles (TCA) a natural contaminant chemical during
processing of corks is responsible for wine spoilage. There is a
limited efficacy of conventional washing processes for removal
of TCA. The Ultracork process involving UAE of TCA,
followed by application of a silicone barrier coating has provi-
ded an improved approach to overcome cork taint (

Rowe, 2003

).

5. Industrial extraction application design

The use of ultrasound in food processing has been reviewed

by

Mason, Paniwynyk and Lorimer (1996)

. Recently, the design

of ultrasound processing equipment has advanced to provide
industrially robust processing capability. Enabling design and
operational features have included; (a) automated frequency
scanning to enable maximum power delivery during fluctuation
of processing conditions, (b) non-vibrational flanges on sono-
trodes for construction of high-intensity inline flow-cells and
(c) construction of radial and hybrid sonotrodes to provide
greater range in application design and product opportunities.
Presently, 16 kW is the largest available single ultrasound flow-
cell, which can be configured in-series or in parallel modules.
Industrial ultrasound manufactures within the last 2 years have
promoted industrial processing capability for food extraction
applications (

Hielscher, 2006

).

Several ultrasound reactor designs have been described by

Chisti (2003)

and

Vinatoru (2001)

, the latter specifically for

industrial extraction of plant tissue. These included (a) stirred
ultrasound horn (sonotrode) directly immersed into stirred bath
or reactor, (b) stirred reactor with ultrasound coupled to the
vessels walls and (c) recycling of product from stirred reactor
through an external ultrasonic flow-cell. These configurations
may provide both intermittent and continuous ultrasound ex-
posure, from low intensity in a large volume reactor (0.01 to
0.1 W/cm

3

) to high intensity (1 to 10 W/cm

3

) in an external

flow-cell. Mixed frequency reactors have been shown to offer
advantages with respect to process efficiency and energy dis-
tribution (

Moholkar, Rekveld, & Warmoeskerken, 2000; Swamy

& Narayana, 2001; Tatake & Pandit, 2002; Feng, Zhao, Zhu, &
Mason, 2002; Delgadino, Bonetto, & Lahey Jr., 2002

). Reactor

geometries that are asymmetrical and polygons preferably with
odd numbered sides using swept frequencies are also reported to
be more effective (

Gogate, Mujumdar, Thampi, Wilhelm, &

Pandit, 2004

; Puskas, personal communication).

Modern ultrasonic systems include automated frequency

scanning which adjusts operation of the system to the optimal
frequency to ensure that maximum power is transmitted to the
extraction vessel. The benefit of automated frequency scanning
as opposed to a fixed frequency was demonstrated by

Romdhane

and Gourdan (2002)

where the former achieved a 32% increase in

pyrethrine extraction and a 30% increase in power delivered to the
product. The presence of a dispersed phase contributes to the
ultrasound wave attenuation. The active sonication region in a
reactor is restricted to a zone located at the surface of the probe.

Where it is not a disadvantage to extract oily materials as

stable emulsions, ultrasound can be used to carry out aqueous
extraction of oily materials with yields of the order of 50%
(Food Science Australia, unpublished results).

To improve effectiveness the material to be extracted should be

reduced to as smaller particle size as practical without denaturing
the material to be extracted and commensurate with separation
from the solvent post extraction. If this is done very high yields
and extraction rates are possible with ultrasonic augmentation of
the extraction process (

Balachandran et al., 2006

).

The proposed benefits of UAE for the food industry include,

(a) overall, enhancement of extraction yield or rate, (b) en-
hancement of aqueous extraction processes where solvents
cannot be used (juice concentrate processing), (c) providing the
opportunity to use alternative (GRAS) solvents by improvement
of their extraction performance, (d) enable sourcing/substitution
of cheaper raw product sources (variety) while maintaining
bioactive levels and (e) enhancing extraction of heat sensitive
components under conditions which would otherwise have low
or unacceptable yields.

6. New opportunities for UAE in the food industry

There is an opportunity to capture new intellectual property

in the area of ultrasound processing particularly where the
technology can provide commercially attractive advantages and
outcomes unique to ultrasound processing. Ultrasound has the
unique capacity to both enhance extraction from substrates
while simultaneously encapsulating the extracted substance with
an encapsulate material in the extraction fluid by hydroxyl radical
initiated covalent bonding and microsphere formation. To suc-
cessfully accomplish this, the encapsulating material should have a
higher reductive potential than the material being extracted and be
relatively more hydrophobic. Preferably a mixed frequency ultra-
sound field is used, a relatively low frequency to facilitate extrac-
tion and a higher frequency under independent amplitude control
to facilitate hydroxyl radical production for cross linking and
microsphere formation. Proteins are suggested encapsulants as the
sonochemistry and conditions favouring sphere development have
been established. Vessel geometries, frequency combinations and
frequency modulation to achieve the desired outcomes on a large
scale suitable for scale up to industrial application would need to
be explored and optimized.

7. Conclusion

State of the art in UAE can achieve worthwhile gains in

extraction efficiency and extraction rate, which if realised on
industrial scale would represent worthwhile economic gains.
Ultrasonic equipment engineering is such that it is commer-
cially viable and scaleable to consider industrial-scale ultrasonic
aided extraction. Potential exists for applying UAE for en-
hancement of aqueous extraction and also where organic solvents
can be replaced with generally recognised as safe (GRAS) sol-
vents. UAE can also provide the opportunity for enhanced ex-
traction of heat sensitive bioactive and food components at lower
processing temperatures. There is also a potential for achieving
simultaneous extraction and encapsulation of extracted compo-
nents to provide protection through the use of ultrasonics.

Acknowledgement

This work was supported by CSIRO - Food Science

Australia, Food Futures Flagship. This work was partially pre-
sented at Food innovation: Emerging Science, Technologies &
Application (FIESTA), 3rd Innovative Foods Centre Confer-
ence held at Melbourne, Australia on 16

–17 October, 2006.

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