Food Analysis Water na Minerals

Water and Minerals

M Gonza´lez

, Instituto Canario de Investigaciones

Agrarias, La Laguna, Spain
V Gonza´lez

, University of La Laguna, La Laguna, Spain

Water

Water determination is one of the most important
and frequent measurements in the processing and
analysis of foods and responds to different necessi-
ties. The ﬁrst is commercial because it is more prof-
itable to buy products based on their dry weight. The
second one is legal because for hygienic and com-
mercial reasons the law limits the water content
in many foods. The third reason is technological:
several processes of industrial transformation need to
know this value. Finally, the fourth reason is analyt-
ical because the food composition is generally ex-
pressed with respect to dry matter to facilitate
comparison between samples.

The water content of foods varies enormously, as

can be seen in Table 1. Water is the main component
in most foods. It constitutes the medium in which
chemical reactions occur and is one of the substrates
in hydrolysis. So, eliminating water, or immobilizing
it by increasing sugar or salt concentration, inhibits
many reactions and the development of microorgan-
isms, thereby increasing food shelf-life. Water also
contributes signiﬁcantly to food texture because of
its physical interactions with proteins, polysaccha-
rides, lipids, and salts.

Knowing the water content is not enough to evalu-

ate food stability as foods with similar water content
differ in their perishability. Because water activity is a
very important factor in the stability and quality of
foods, it is frequently measured along with water
content. The water activity (a

) of a product in equi-

librium with air, in an airtight container at a given
temperature and pressure, is deﬁned as the ratio of
the partial pressure of water in the air in equilibrium
with the product to the vapor pressure of pure water
at the same temperature and pressure. Foods with a

values between 0.2 and 0.4 are most stable (Table 2).
The quality of these foods is practically unspoiled by
microorganism development, nonenzymatic brown-
ing, or lipid autooxidation. Intermediate moisture
foods (a

value between 0.6 and 0.9) are protected

against most of the alterations produced by micro-
organisms. However, most fresh foods have an a

value of 0.97, being susceptible to alteration.

Water in foods is found in three different forms:

free, absorbed, and bounded. The ease with which
water is eliminated from food depends on the form of
the water. To ensure similar results between different
water determination methods, standard methods
have been developed in which work conditions are
speciﬁed. In each case the precision of the analytical
results must be checked in order to decide if their
precision can be substantially improved by using
other methods.

Methods for the Determination of
Water in Food

When determining water some precautions must
be taken to minimize the losses and gains of
moisture that can be produced during sampling and
sample preparation. In general, any exposure of the
sample to open air and heating of the sample by
friction during homogenization should be mini-
mized. During storage the empty space in the sam-
ple container must be kept to a minimum because
water is transferred to this space to equilibrate water
content. It is also necessary to minimize temperature
ﬂuctuations because water migrates to the coldest
part of the sample. To reduce this potential error,
homogenization of the sample is required before its
analysis.

Choosing an analytical method depends on the

expected water content, volatility or sensitivity to
heat of other food components, instrument avail-
ability, speed requirements, necessary accuracy, and
aim of the analysis.

‘Water content’ is deﬁned as the amount of water

lost by a food when it reaches the true equilibrium
against zero water vapor pressure. From this deﬁni-
tion arises a quantiﬁcation method of water content
called the absolute reference method, a determina-
tion that is only possible in specialized laboratories.
The food industry only uses practical reference meth-
ods, calibrated against the absolute reference meth-
od. Moreover, water content standards are not
available because a product’s water content depends
on the humidity and temperature of the environment,
which makes participating in intercomparison exer-
cises between laboratories essential for detecting
possible experimental errors.

The Association of Ofﬁcial Analytical Chemists

(AOAC) publishes reference methods for the analysis
of moisture in foods. Some of these methods are
summarized in Table 3.

FOOD AND NUTRITIONAL ANALYSIS

/ Water and Minerals

241

Absolute Reference Methods

Among these methods, the Karl Fischer method is the
most used. It is a chemical method based on iodine
reduction by sulfur dioxide in the presence of water.

The sample is titrated with the Karl Fischer reagent,
consisting of a mixture of sulfur dioxide, iodine, and
pyridine in methanol. Because pyridine has an
unpleasant odor and is toxic, it has been replaced
by imidazole, and the methanol has been replaced by

Table 1

Water, macroelement (calcium, magnesium, phosphorus, potassium, and sodium) and oligoelement (iron, zinc, copper,

manganese, and selenium) content in some foods

Food

Amount in 100 g of edible portion

Water

(g)

(mg)

Fruits, vegetables, legumes and derived products
Broccoli, raw

89.30

0.73

316

0.41

0.049

2.5

Bananas, raw

74.91

0.26

358

0.15

0.078

1.0

Bananas, dehydrated

3.00

1.15

108

1491

0.61

0.391

3.9

Oranges, raw, California,

Valencias

86.34

0.09

179

0.06

0.037

–

Orange juice, raw

88.30

0.20

200

0.05

0.044

0.1

Potatoes, white, ﬂesh and skin,

raw

84.58

0.52

407

0.29

0.116

0.3

Tomatoes, red, ripe, raw

94.50

0.27

237

0.17

0.059

0.0

Legumes
Beans, navy, mature seeds,

sprouted, raw

79.15

1.93

101

100

307

0.89

0.356

0.6

Lentils, sprouted, raw

67.34

3.21

173

322

1.51

0.352

0.6

Nuts
Almonds

5.25

248

4.30

275

474

728

3.36

1.110

2.8

Pistachio nuts

3.97

107

4.15

121

490

1025

2.20

1.300

7.0

Cereals and derived products
Pasta, corn, dry

10.00

0.93

119

253

294

1.79

0.202

7.9

Rice, brown, long-grain, raw

10.37

1.47

143

333

223

2.02

0.277

23.4

Wheat ﬂour, white, unenriched

11.92

1.17

108

107

0.70

0.144

33.9

Wheat ﬂour, whole grain

10.27

3.88

138

346

405

2.93

0.382

70.7

Wheat, soft white

10.42

5.37

402

435

3.46

0.426

–

Bread, whole-wheat,

commercially prepared

37.70

3.30

229

252

527

1.94

0.284

36.6

Fats and oils
Butter, light, stick, with salt

42.10

1.09

450

0.06

0.000

1.0

Margarine, regular, stick,

composite, 80% fat, with salt

17.17

0.12

654

0.11

0.000

0.0

Oil, vegetable, sunﬂower

0.00

0.03

0.00

0.000

0.0

Beverages
Beer, regular

94.32

0.02

0.01

0.005

0.7

Carbonated beverage, cola

89.10

0.02

0.01

0.006

0.1

Meat, poultry, and ﬁsh
Pork, fresh, leg (ham), whole, raw

62.47

0.85

199

315

1.93

0.065

29.4

Beef, chuck, arm pot roast, raw

57.92

2.01

166

289

3.81

0.077

15.9

Chicken, broilers or fryers, breast,

meat only, raw

74.76

0.72

196

255

0.80

0.041

17.8

Egg, whole, raw, fresh

75.84

1.83

191

134

140

1.11

0.102

31.7

Fish, tuna, fresh, blueﬁn, raw

68.09

1.02

254

252

0.60

0.086

36.5

Dairy products
Brie cheese

48.82

184

0.50

188

152

629

2.38

0.019

14.5

Milk, whole, 3.3% fat

88.32

101

0.03

133

0.38

0.023

3.7

Yogurt, plain, skim milk

85.23

199

0.09

157

255

0.97

0.015

3.6

242

FOOD AND NUTRITIONAL ANALYSIS

/ Water and Minerals

methoxymethanol to stabilize the reagent. The meth-
od is precise because the water is determined specif-
ically and selectively by chemical reaction. Moreover,
this method determines both free and bounded water.

Practical Reference Methods

This section examines oven-drying methods. In these
methods the sample is heated under specific condi-
tions and the weight loss is used to calculate the wa-
ter content of the sample. For this determination
three types of ovens can be used: convection, forced
draft, and vacuum. The determined water quantity is
highly dependent upon the type of oven used, the
conditions inside the oven, and the time and tem-
perature of drying.

Excess errors can be produced in these analyses

due to the loss of volatile compounds different from

water (acids, alcohols, esters, and aldehydes) and to
water formation in oxidation and nonenzymatic
browning reactions. On the other hand, other errors
can be produced as a consequence of not allowing
enough time or temperature to evaporate all the wa-
ter from the sample.

These methods are simple and can analyze many

samples simultaneously. The analysis time can be be-
tween a few minutes and 24 h. Recently other thermic
methods have been developed that use microwave
ovens, infrared lamps, halogen lamps, or ceramic
heating elements, which reduce analysis time.

To complete the analysis, distillation procedures

must also be considered. These techniques consist in
the codistillation of sample water with a high-boiling
point solvent that is immiscible in water (toluene,
xylene, benzene). The distilled mixture is collected,
and the sample volume is measured. These methods
produce fewer losses by food decomposition than
does oven drying at high temperatures. However,
measuring by water volume can be less exact than
measuring by weight.

Table 4 compares some of the methods used to

analyze water in food by principle, type of sample,
and analysis time, including some methods based on
the physical characteristics of water in food.

Methods of Determination of Water
Activity in Foods

Methods used to measure water activity can be direct
and absolute or indirect and calibrated with regard
to the former.

Table 2

Typical water activity of some foods

Food

Water activity (a

)

Fresh meat and ﬁsh

0.95–1.00

Bread

0.95

Cheese

0.90–0.95

Margarine

0.85–0.90

Salami

0.80–0.85

Jams and jellies

0.80

Dried fruits

0.70–0.80

Nuts

0.65–0.75

Honey

0.60–0.65

0.30

Milk powder

0.20

Dried vegetables

0.20

Table 3

Ofﬁcial AOAC analysis methods applicable to moisture

AOAC method

Food

Analytical technique

931.04

Cacao products

Gravimetry

977.10; 984.20;

967.19

Cacao products; oils and fats; dried vegetables

Karl Fischer titrimetry

935.29

Malt

Gravimetry – convection oven

920.116; 984.25

Butter; frozen potatoes

Convection oven

981.05

Malting barley

Convection oven – forced-draft oven

925.45

Sugars

Convection oven – vacuum oven

948.12

Cheese

Steam bath

þ forced-draft oven

926.07

Macaroni products

Forced-draft oven – vacuum oven

950.46

Meat

Air drying – vacuum oven

968.11; 979.12;

927.05; 926.08;
934.06; 962.12;
920.186; 977.21

Roasted coffee; dried milk; cheese; dried fruits; oils and

fats; maple products; corn syrups and sugars

Vacuum oven

969.38

Honey

Vacuum oven – refractometry

977.11

Cheese

Microwave oven

985.14

Meat and poultry products

Rapid microwave oven

969.19; 986.21

Cheese; spices

Distillation

978.13

Milk

Vapor pressure osmometric (VPO) method

972.20

Prunes and raisins

Moisture meter

FOOD AND NUTRITIONAL ANALYSIS

/ Water and Minerals

243

Absolute Methods

These methods are based on measuring air temper-
ature in equilibrium with a product and some other
air characteristic like water content, wet bulb tem-
perature, or dew temperature. With these data and
the use of an enthalpy diagram of moist air or tables,
its hygrometric level can be measured and thus the
a

value of the product in equilibrium with this

air. There are chromatographic, psychometric, and
manometric methods.

Calibrated Methods

These methods use mechanical probes or hygrome-
ters that respond to the variations in the relative hu-
midity of air that are caused by variations in
dielectric resistance or capacity. These probes direct-
ly measure the relative humidity of air, and the most
commonly used is lithium chloride. In general, these
methods require frequent calibration and should only
be used within a narrow a

range.

On the other hand, the reference salts method can

indirectly determine the a

value of a product using

graphic interpolation. This method consists in in-
troducing an aliquot of the sample in an airtight
container where different saturated saline solutions
or dilutions of sulfuric acid are placed, creating a
relative humidity and therefore a known a

value.

Once equilibrium has been reached, the samples are
weighed and the weight variations are represented

according to relative humidity. An interpolation of
the curve obtained this way allows the determination
of relative humidity and, by extension, the a

value

of the food.

Importance of Minerals in the Diet and
in Processing Food

Minerals are the inorganic elements constituting
foods (excluding carbon, hydrogen, oxygen, and nit-
rogen) that remain as ashes when foods are inciner-
ated. They are signiﬁcant for their nutritional value,
toxicological potential, and interaction with the
texture and processing of foods. For these reasons
it is necessary to know and control their concentra-
tion levels in foods.

Among the 50 known minerals, between 15 and

20 minerals are natural components of foods that are
part of at least one vital biological system of a plant
or animal. Some of them are denominated macroel-
ements because of their abundance in foods; these
include calcium, phosphorus, sodium, potassium,
magnesium, and chlorine. Others are called oligoel-
ements or trace elements due to their minimal con-
centration; among these are iron, iodine, zinc,
copper, chromium, manganese, molybdenum, ﬂuo-
ride, and selenium.

Minerals are divided into two categories according

to their biological importance: those with a

Table 4

Comparison of methods used to determine water in foods

Method

Principle

Nature of sample

Analysis time

Karl Fischer

Chemical reaction of the water

All foods, especially foods very low in moisture

or high in fats and sugars

5 min

Convection oven

Removal of water and weight of the

remaining solids

Not valid for foods that contain volatiles or

have components that undergo chemical
reactions at high temperatures

0.75–24 h

Forced-draft oven

Removal of water and weight of the

remaining solids

Not valid for foods that contain volatiles or

have components that undergo chemical
reactions at high temperatures

0.75–24 h

Vacuum oven

Removal of water and weight of the

remaining solids

3–6 h

Distillation

Separation of water from the solids and

volume measurement

Valid for foods that contain volatiles

Dielectric and

conductivity

Change in capacitance or resistance to an

electric current that passes through the
sample

Foods that contain less than 30–35% of water

Hydrometry

Relationship between specific gravity and

moisture content

Liquid samples: drinks and sugar solutions

Refractometry

Water in the sample affects light refraction

Liquid samples, condensed milk, sugar

solutions, fruits, and fruit products

Infrared drying

Penetration of heat into the sample

10–25 min

Near–infrared

spectroscopy

Specific absorption of near-infrared

radiation (1400–1450, 1920–1950 nm) by
the water molecules of the food

244

FOOD AND NUTRITIONAL ANALYSIS

/ Water and Minerals

known biological role, called essential minerals, and
those with an unknown biological role, referred to as
nonessential minerals. Some minerals in the nonessen-
tial group are being investigated because there is some
evidence that they have a biological function, although
this biochemical role is not yet clear. The nonessential
minerals being investigated are vanadium, tin, nickel,
arsenic, and boron. There are also toxic elements,
which should be avoided in the diet. This group in-
cludes lead, mercury, cadmium, and aluminum. Some
essential minerals, like ﬂuoride and selenium, are
known to be harmful if consumed in excessive quan-
tities, even though they have biochemical functions
that are beneﬁcial when consumed at safe levels.

Some minerals are found naturally in foods in

variable amounts and can be modiﬁed, or others can
be added, during industrial processing of food. Table
1 indicates the amounts of some of the macro- and
microelements that can be found in various raw and
processed foods. Moreover, almost all these minerals
can be found as pollutants in foods due to environ-
mental or industrial factors, and are analyzed for
hygienic reasons, and not as natural components of
foods.

Food Treatments Prior to Mineral
Analysis

There are various methods of mineral determination
available, and they can be used according to the an-
alytical characteristic that best suits the objectives of
the analyst: accuracy, sensitivity, detection limit,
speciﬁcity, and interferences. Other factors to take
into account are the costs of the complete analysis,
instrumental availability, and time necessary for
analysis.

Before mineral analysis it is usually necessary to

treat the sample to ensure that the sample is
homogeneous and also to prepare it for the analyt-
ical procedure that follows. Various processes may be
necessary, but among the most important is sample
mineralization, often associated with the need to de-
stroy organic matter present in the sample and
always necessary to make the sample soluble. More-
over, the treatment of a sample may entail reduction
and homogenization of its size or elimination of in-
terferences. In any case, contamination of the sample
or loss of volatile compounds may occur during these
steps of the analytical process, affecting the quality of
the analytical results.

Mineralization

Mineralization is usually a crucial step before the
analysis of specific minerals because most analytical

methods require that minerals be freed from their
matrix. Nevertheless, some analytical techniques,
such as near-infrared spectroscopy, sometimes allow
mineral estimation without destroying the carbon
matrix that constitutes foods.

The destruction of the matrix, generally organic

matter, can be done by wet or dry ashing. The se-
lected mineralization procedure depends on what the
ashes will be used for and on limitations based on
cost, time, and number of samples.

Dry ashing, consisting in incinerating the sample at

high temperatures (450

1C or higher) in a mufﬂe fur-

nace, converts most minerals into oxides, sulfates,
phosphates, chlorides, and silicates. Before me-
asuring the analytes, the residuum is redissolved in
a minimal amount of acid and is diluted with dis-
tilled water to a known volume. It is a safe method
that does not require the addition of a reagent, and
once ignited it requires little of the analyst’s atten-
tion, allowing the treatment of many crucibles si-
multaneously. The main disadvantage of this method
is the long treatment time (12–18 h). Moreover, some
elements such as selenium, lead, and mercury can
partially volatilize with this procedure. For this rea-
son alternative methods must be used to determine
these minerals.

In wet ashing, organic matter is oxidized by the

addition of acids and oxidants or their combinations.
A strong oxidant such as concentrated nitric acid,
sulfuric acid, or perchloric acid is needed to destroy
organic matter. In general, reagent mixtures are cho-
sen in accordance with the type of food and the
quantiﬁcation method. Perchloric acid is an excellent
oxidant, but its use runs the risk of forming explosive
organic perchlorates if the mixture dries completely.
Some precautions must be followed when fat-rich
foods are treated.

Minerals remain in solution during wet digestion

without volatilization losses because the tempera-
tures used are lower than in dry ashing. The oxida-
tion time is short and requires a hood, hot plate, long
tongs, and safety equipment. However, this method
requires the continual attention of the operator, uses
corrosive reagents, and only a reduced number of
samples can be treated simultaneously. If perchloric
acid is used in the wet digestion, all work must be
done in a perchloric acid fume hood.

Microwave radiation systems are now available

that accelerate the mineralization process substan-
tially. This process has obvious advantages in red-
ucing decomposition time, acid needs, and the risks
of contamination or foaming.

Other alternatives include closed reactor digestion

at a high temperature and pressure; combustion in a
vessel containing oxygen; the Sho¨niger method,

FOOD AND NUTRITIONAL ANALYSIS

/ Water and Minerals

245

which is especially suitable when determining ha-
logens and sulfur; or combustion in an oxygen–argon
plasma at a low temperature.

Other Pretreatments

Most solid foods must be ground up in order to ob-
tain a homogeneity compatible with the previously
described procedures. In this process it is necessary to
adjust the grinding ﬁneness according to the miner-
alization method that is going to be used. Dry ashing
needs particles of diameter between 0.5 and 1 mm to
avoid losses by mechanical drag during the inciner-
ation. Oxygen combustion methods and especially
direct fusion methods need a grade of ﬁneness that
avoids particle volatilization losses and artiﬁcial en-
riching of samples.

According to their nature, the ground and

homogenized samples are stored cold or at room
temperature, in airtight containers made of a mate-
rial that does not allow the transfer of materials be-
tween the container and the product. Generally, a
polyethylene or Teﬂon container is a good solution.

On the other hand, mineralized and solubilized

samples sometimes need treatments that eliminate
interferences. Factors such as pH, sample matrix,
some pretreatments, or the incorporation of a
reagent can interfere with the ability of an analyti-
cal method to quantify a mineral. In this situation, it
is common to turn to the use of masking reagents or
separation processes using selective precipitation
techniques, liquid–liquid extraction, ion-exchange
resins, etc.

Sample Contamination

One of the biggest problems that can occur during
mineral analysis is contamination of the sample. The
selection of the equipment used to treat the samples
is closely related to the mineral that is going to be
quantiﬁed. Stainless steel enriches the sample in
chromium and nickel; agate mortars and mills nota-
bly increase the calcium content of the ground prod-
uct, etc. Teﬂon has some important advantages, but
due to its low resistance to abrasion not all treat-
ments can be done using this material. Sometimes
glass is inadequate; this must be taken into account
when determining sodium because the glassware
used may enrich the solution. Additionally, the re-
petitive use of glassware can be a contamination
source, and for this reason glassware used in sample
preparation and analysis must be cleaned scrupu-
lously using acids and the purest water.

Since solvents can contain large amounts of min-

erals, it is necessary to use the purest reagents and
distilled–deionized water to analyze minerals. As

reagents can be very expensive, a possible alternative
is to work with a reagent blank or a sample of the
reagents used in the sample analysis in the same pro-
portion as in the sample but without the mineral that
is going to be analyzed.

Methods of Determination of Minerals
in Foods

Chemical Methods

The determination of food minerals can be done
using chemical methods. Among them, volumetric
methods stand out because they are fast and inex-
pensive while still being adequately precise. Their
main disadvantages are that they have low sensitivity
and selectivity.

Ethylenediaminetetraacetic acid (EDTA) com-

plexometric titrations are based on the fact that
many metallic ions form stable complexes with this
tetradentate ligand, EDTA. The endpoints are de-
tected using complexing agents capable of forming
complexes with the metallic species to be determined
and having lower coordination constants than those
of the complexes that are formed with EDTA and
that also have different colors in their free and com-
plexed states.

In precipitation titrations the titration reaction

produces an insoluble precipitate. Despite the many
known precipitation reactions, very few of them have
the necessary requirements to be the basis of a volu-
metric method. Determining chloride using silver ion
precipitation, using several methodologies, is the
most appropriate application of this group of titra-
tions.

The most important applications of these methods

in food analysis are limited to the determination of
certain metals such as calcium in water and beer or
chlorides in different types of samples. Table 5 lists
some methods used to determine food minerals based
on volumetry.

Molecular Absorption Spectrophotometry

These methods, which include colorimetric methods,
are based on the measurement of radiation absorp-
tion by molecular species at a speciﬁed wavelength.
In mineral analysis, the absorbent species are usually
compounds, mostly coordination complexes, formed
in a reaction between the mineral and a chromogen
ligand. These methods are more sensitive and
selective than the previously discussed methods.
Some of the methods that use ultraviolet (UV)–vis-
ible spectrophotometry can be combined with dy-
namic techniques like ﬂow injection systems or with
a previous separation using liquid chromatography,

246

FOOD AND NUTRITIONAL ANALYSIS

/ Water and Minerals

allowing the quantiﬁcation of many elements simul-
taneously. Table 5 summarizes some of the main col-
orimetric methods used regularly in mineral analysis.

Atomic Spectroscopy

These techniques are based on the atomization of the
analyte present in a solution, using a ﬂame or a plas-
ma. The amount of an element present in the sample
is determined from the absorption or emission of the
visible or UV radiation of its atoms in the gaseous
phase. The high sensitivity and selectivity that can be
achieved with these techniques and the increasing
necessity to respond to requests for mineral quanti-
ﬁcation in foods at trace levels explain the increasing
use of some of the atomic absorption and emission
spectroscopic techniques. Table 6 summarizes some

methods applicable to the determination of minerals
using atomic spectroscopy.

Among atomic emission spectrometry (AES) meth-

ods, the classic ﬂame photometric technique is still
favored for determination of sodium and potassium
in foods.

The use of an inductively coupled plasma (ICP)

allows temperature and stability conditions that
eliminate most of the interferences found in com-
bustion sources. The sensitivities that can be ob-
tained and the speed of this technique, which can be
used to determine several elements simultaneously,
make it in an interesting, although expensive, alter-
native for the analysis of metals found in foods.
Coupling ICP with a mass spectrometer gives the best
analytical results, although it is currently a technique
restricted to specialized laboratories.

Table 5

Mineral determination using methods different from atomic spectroscopic methods

Mineral

Foods

Pretreatments

Analytical

technique

Tritimetry
Chloride

Oils

Dry ashing

Precipitation

titrimetry

Calcium

Legumes

Wet ashing (HClO

:HNO

)

EDTA titrimetry

Calcium

Tubers

Lyophilization, dry ashing

EDTA titrimetry

Colorimetry
Phosphorus

Eggs, vegetables

Phosphorus

Oils, yogurts, infant foods,

ﬁshes, grains, nuts,
vegetables, tubers, fruits

Dry ashing

Phosphorus

Meat, legumes

Wet ashing (HNO

:HClO

)

Phosphorus

Fish, grains, legumes

Wet ashing (HNO

:HClO

)

Phosphorus

Grains

Wet ashing (20% HCl)

Phosphorus

Nuts

Wet ashing (HNO

)

Iron

Wine

Wet ashing (HNO

:HClO

)

Iron

Tubers

Lyophilization, dry ashing

Iodine

Dairy products

Dry ashing

Boron

Fruits

Dry ashing

Electroanalytical techniques
Selenium and lead

Wine

Wet ashing (HNO

:HClO

)

Voltametry

Selenium

Vegetables

Wet ashing (HNO

:HClO

)

Voltametry

Fluorine

Grains

Voltametry

Chromatography
Sodium, potassium,

magnesium, and calcium

Dairy products

Dry ashing

Ionic

chromatography

Selenium

Nuts

Wet ashing (HNO

)

Gas chromatography –

mass
spectrometry

Other techniques
Sulfates

Oils

Dry ashing

Turbidimetry

Manganese, bromine, cobalt,

vanadium, arsenic, antimony,
copper, selenium, aluminum,
and lanthanum

Tubers

Neutron activation

analysis

FOOD AND NUTRITIONAL ANALYSIS

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247

Table 6

Mineral determination using atomic spectroscopic techniques

Mineral

Foods

Pretreatments

Flame photometry
Sodium, potassium and calcium

Honey

Infrared lamp drying, dry ashing

Sodium and potassium

Oils, vegetables, fruits, yogurts, ﬁsh

Dry ashing

Sodium, potassium, and calcium

Grains

Sodium and potassium

Meat

Wet ashing

(HNO

:HClO

)

Flame atomic absorption spectrometry
Sodium, potassium, zinc, iron, calcium, and copper

Honey

Wet ashing (H

:HNO

)

Zinc, cadmium, and lead

Honey

Microwave drying, dry ashing

Iron

Wine

Wet ashing (HClO

:HNO

)

Iron

Wine

Without treatment

Manganese, chromium, iron, nickel, and copper

Juices

Wet ashing (H

:HNO

)

Cadmium, copper, chromium, cobalt, iron, nickel, lead,

and zinc

Beverages, dairy products

Dry ashing

Copper, iron, and zinc

Milk

Wet ashing (HNO

)

Copper, iron, zinc, and manganese

Yogurts

Dry ashing

Calcium, magnesium, and zinc

Yogurts

Dry ashing

Copper and zinc

Vegetables, fruits, meat, ﬁsh, legumes,

cereals, spices, dairy products,
tubers, sweeteners, canned foods

Dry ashing

Copper and zinc

Fats, oils

Wet ashing (HNO

:HClO

)

Sodium, potassium, calcium, magnesium, iron, zinc,

manganese, and copper

Fish

Wet ashing (HNO

:HClO

)

Cadmium and lead

Fish

Wet ashing (30% H

:HNO

)

Iron, zinc, aluminum, titanium, and vanadium

Fish

Wet ashing (HNO

)

Selenium, lead, and cadmium

Baby foods

0.1% Triton X-100

þ 1%

HNO

þ 30% H

Cadmium, cobalt, chromium, copper, iron,

manganese, molybdenum, nickel, lead, and zinc

Seafood

Wet ashing (HNO

, H

)

Cadmium, calcium, iron, magnesium, manganese, and

zinc

Meat

Wet ashing

(HNO

:HClO

)

Lead, cadmium, iron, copper, manganese, zinc, and

cobalt

Mushrooms

Wet ashing (HNO

)

Copper, magnesium, lead, sodium, silver, bismuth,

manganese, nickel, lithium, cobalt, antimony,
calcium, zinc, iron, chromium, aluminum, and
potassium

Mushrooms

Wet ashing (HNO

:HClO

:HCl)

Potassium, sodium, calcium, magnesium, iron,

manganese, copper, and zinc

Vegetables

Wet ashing (HNO

)

Sodium, potassium, magnesium, zinc, and copper

Tubers

Lyophilization, dry ashing

Calcium, magnesium, iron, zinc, copper, and

manganese

Fruits

Microwave drying, wet ashing

(HNO

: 30% H

)

Inductively coupled plasma
Aluminum, barium, calcium, copper, iron, potassium,

iron, copper, iron, potassium, magnesium,
manganese, sodium, strontium, and zinc

Beverages

Without treatment

Calcium, cobalt, copper, chromium, iron, magnesium,

manganese, molybdenum, sodium, zinc, silver,
aluminum, arsenic, barium, beryllium, cadmium,
mercury, nickel, lead, antimony, tin, strontium,
titanium, thallium, uranium, and vanadium

Milk, ﬂour

Microwave drying, dry ashing

Calcium, phosphorous, magnesium, sodium,

potassium, aluminum, iron, zinc, and copper

Seafood, meat

Dry ashing

Chromium, iron, manganese, selenium, and zinc

Vegetables

Wet ashing (HNO

:HClO

)

Aluminum, boron, calcium, iron, magnesium,

phosphorus, potassium, sodium, and titanium

Fruits, fruit products

Microwave drying, wet ashing

(HNO

)

Hydride/cold vapor
Selenium

Flour, ﬂour products

Wet ashing

(HNO

:HClO

)

Selenium

Milk

Wet ashing (HNO

:HClO

)

Mercury

Fish

Wet ashing (HCl, HNO

, H

)

Mercury

Mushrooms

Wet ashing (HNO

)

248

FOOD AND NUTRITIONAL ANALYSIS

/ Water and Minerals

Atomic absorption spectrometry (AAS) is the most

common technique for the determination of metals in
food products.

Most commonly used instruments use a ﬂame

(ﬂame AAS (FAAS)) produced by combustion of an
air/acetylene or dinitrogen oxide/acetylene mixture.
The few interferences are easy to avoid, and the sen-
sitivities that are reached are adequate for the metals
of greatest interest to the food industry. Variants of
this technique, such as the coupling of hydride gene-
ration (HG) systems (HG–AAS), increase its scope to
higher-sensitivity determination of elements like se-
lenium, arsenic, tin, and other elements that form
hydrides. In a similar vein, the determination of mer-
cury using the cold vapor technique should be
highlighted.

An alternative to FAAS, particularly suited to the

determination of metals with higher sensitivity, is
electrothermal AAS (ET-AAS), in which a graphite
furnace substitutes the ﬂame as the atomization
source. Generally speaking, the detection limits ob-
tained with a graphite furnace are two orders of
magnitude lower than those reached with ﬂame at-
omization.

Comparing ET-AAS and ICP, techniques with sim-

ilar scopes in food analysis, conﬁrms that absorption
techniques require less expensive instruments and
usually achieve higher sensitivities. Since ICP is a
particularly fast method, a laboratory that has to
analyze many metals per sample and/or many sam-
ples can justify the investment and maintenance
associated with ICP.

Other intriguing spectrometric methods used in

metallic species analysis, such as X-ray ﬂuorescence
spectrometry and neutron activation analysis, are
unusual in food analysis.

Electrochemical Methods

In food analysis, the most common electrochemical
methods are potentiometric and voltametric.

In potentiometric methods, the potential between

a reference and an indicator electrode is measured,
which corresponds to the analyte activity. Because of
their usefulness in food analysis, ion-selective elec-
trodes (ISEs) that measure anions like bromide, chlo-
ride, and ﬂuoride or cations like potassium, sodium,
and calcium stand out among indicator electrodes.
The characteristics and advantages of ISE include the
ability to measure different anions and cations di-
rectly, the fact that they do not consume the analyte,
the fact that analyses are independent of sample
volume when taking direct measurements, and that
moreover turbidity, color, and viscosity do not affect
the measurement. Potentiometric methods are also

fast, easy to use, and inexpensive. The disadvantages
of ISEs include the following: they have a relatively
low sensitivity, proteins or other organic solutes can
interfere in the determination, and some ions can act
like ligands or can poison the electrodes.

Voltametric techniques are based on the relation

between current and voltage in an electrochemical
process. Among them, anodic stripping voltampe-
rometry permits metallic species determination with
detection limits of parts per billion or lower. The
equipment used with these techniques is much more
inexpensive than that used with spectroscopic tech-
niques that are also used in trace analysis.

Table 5 summarizes some of the electroanalytical

methods usually employed in mineral analysis of
foods.

Separation Methods: Electrophoretic and
Chromatographic Methods

For multielemental analysis of foods, separation
methods are also used, although less frequently.
Capillary electrophoresis is used to separate metallic
cations like sodium, potassium, calcium, manganese,
and magnesium. Among chromatographic methods,
determination of anionic and cationic species using
ionic chromatography, with or without the use of an
ionic suppressor, and conductimetric detection
should be highlighted. Table 5 reviews some separa-
tion methods applicable to the determination of
minerals.

The AOAC publishes reference methods for the

analysis of minerals in foods. Table 7 summarizes
some of the AOAC ofﬁcial methods of analysis
applicable to minerals.

Analytical Quality Assurance

Quality control is of the utmost importance in the
case of mineral analyses because of the low concen-
trations of the elements normally found in foods and
the ubiquitous presence of signiﬁcant levels of many
of them in the environment. In addition to the stand-
ard techniques of working in a clean laboratory to
reduce the potential for accidental contamination to
a minimum, it is essential that procedures be vali-
dated and results checked against appropriate certi-
ﬁed reference materials (CRMs). CRMs for most of
the trace and other minerals of interest in foods are
available from international reference centers such as
the Community Bureau of Reference of the European
Union, the National Institute of Standards and Tech-
nology of the United States, and the International
Atomic Energy Agency in Vienna.

FOOD AND NUTRITIONAL ANALYSIS

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249

See also: Atomic Absorption Spectrometry: Principles
and Instrumentation. Atomic Emission Spectrometry:
Principles and Instrumentation. Elemental Speciation:
Overview. Food and Nutritional Analysis: Sample Prep-
aration. Ion-Selective Electrodes: Overview. Quality
Assurance: Reference Materials. Sample Dissolution
for Elemental Analysis: Dry Ashing; Oxygen Flask Com-
bustion; Wet Digestion; Microwave Digestion. Spectro-
photometry:

Inorganic

Compounds.

Titrimetry:

Overview.