FOOD AND NUTRITIONAL ANALYSIS / Water and Minerals 241
Water and Minerals
M González, Instituto Canario de Investigaciones
Water in foods is found in three different forms:
Agrarias, La Laguna, Spain
free, absorbed, and bounded. The ease with which
V González, University of La Laguna, La Laguna, Spain
water is eliminated from food depends on the form of
the water. To ensure similar results between different
& 2005, Elsevier Ltd. All Rights Reserved.
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
Water
precision can be substantially improved by using
Water determination is one of the most important
other methods.
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-
Methods for the Determination of
itable to buy products based on their dry weight. The
Water in Food
second one is legal because for hygienic and com-
mercial reasons the law limits the water content When determining water some precautions must
in many foods. The third reason is technological: be taken to minimize the losses and gains of
several processes of industrial transformation need to moisture that can be produced during sampling and
know this value. Finally, the fourth reason is analyt- sample preparation. In general, any exposure of the
ical because the food composition is generally ex- sample to open air and heating of the sample by
pressed with respect to dry matter to facilitate friction during homogenization should be mini-
comparison between samples. mized. During storage the empty space in the sam-
The water content of foods varies enormously, as ple container must be kept to a minimum because
can be seen in Table 1. Water is the main component water is transferred to this space to equilibrate water
in most foods. It constitutes the medium in which content. It is also necessary to minimize temperature
chemical reactions occur and is one of the substrates fluctuations because water migrates to the coldest
in hydrolysis. So, eliminating water, or immobilizing part of the sample. To reduce this potential error,
it by increasing sugar or salt concentration, inhibits homogenization of the sample is required before its
many reactions and the development of microorgan- analysis.
isms, thereby increasing food shelf-life. Water also Choosing an analytical method depends on the
contributes significantly to food texture because of expected water content, volatility or sensitivity to
its physical interactions with proteins, polysaccha- heat of other food components, instrument avail-
rides, lipids, and salts. ability, speed requirements, necessary accuracy, and
Knowing the water content is not enough to evalu- aim of the analysis.
ate food stability as foods with similar water content  Water content is defined as the amount of water
differ in their perishability. Because water activity is a lost by a food when it reaches the true equilibrium
very important factor in the stability and quality of against zero water vapor pressure. From this defini-
foods, it is frequently measured along with water tion arises a quantification method of water content
content. The water activity (aw) of a product in equi- called the absolute reference method, a determina-
librium with air, in an airtight container at a given tion that is only possible in specialized laboratories.
temperature and pressure, is defined as the ratio of The food industry only uses practical reference meth-
the partial pressure of water in the air in equilibrium ods, calibrated against the absolute reference meth-
with the product to the vapor pressure of pure water od. Moreover, water content standards are not
at the same temperature and pressure. Foods with aw available because a product s water content depends
values between 0.2 and 0.4 are most stable (Table 2). on the humidity and temperature of the environment,
The quality of these foods is practically unspoiled by which makes participating in intercomparison exer-
microorganism development, nonenzymatic brown- cises between laboratories essential for detecting
ing, or lipid autooxidation. Intermediate moisture possible experimental errors.
foods (aw value between 0.6 and 0.9) are protected The Association of Official Analytical Chemists
against most of the alterations produced by micro- (AOAC) publishes reference methods for the analysis
organisms. However, most fresh foods have an aw of moisture in foods. Some of these methods are
value of 0.97, being susceptible to alteration. summarized in Table 3.
242 FOOD AND NUTRITIONAL ANALYSIS / Water and Minerals
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 Ca Fe Mg P K Na Zn Cu Se
(g) (mg) (mg) (mg) (mg) (mg) (mg) (mg) (mg) (mg)
Fruits, vegetables, legumes and derived products
Broccoli, raw 89.30 47 0.73 21 66 316 33 0.41 0.049 2.5
Bananas, raw 74.91 5 0.26 27 22 358 1 0.15 0.078 1.0
Bananas, dehydrated 3.00 22 1.15 108 74 1491 3 0.61 0.391 3.9
Oranges, raw, California, 86.34 40 0.09 10 17 179 0 0.06 0.037 
Valencias
Orange juice, raw 88.30 11 0.20 11 17 200 1 0.05 0.044 0.1
Potatoes, white, flesh and skin, 84.58 9 0.52 21 62 407 6 0.29 0.116 0.3
raw
Tomatoes, red, ripe, raw 94.50 10 0.27 11 24 237 5 0.17 0.059 0.0
Legumes
Beans, navy, mature seeds, 79.15 15 1.93 101 100 307 13 0.89 0.356 0.6
sprouted, raw
Lentils, sprouted, raw 67.34 25 3.21 37 173 322 11 1.51 0.352 0.6
Nuts
Almonds 5.25 248 4.30 275 474 728 1 3.36 1.110 2.8
Pistachio nuts 3.97 107 4.15 121 490 1025 1 2.20 1.300 7.0
Cereals and derived products
Pasta, corn, dry 10.00 4 0.93 119 253 294 3 1.79 0.202 7.9
Rice, brown, long-grain, raw 10.37 23 1.47 143 333 223 7 2.02 0.277 23.4
Wheat flour, white, unenriched 11.92 15 1.17 22 108 107 2 0.70 0.144 33.9
Wheat flour, whole grain 10.27 34 3.88 138 346 405 5 2.93 0.382 70.7
Wheat, soft white 10.42 34 5.37 90 402 435 2 3.46 0.426 
Bread, whole-wheat, 37.70 72 3.30 86 229 252 527 1.94 0.284 36.6
commercially prepared
Fats and oils
Butter, light, stick, with salt 42.10 48 1.09 5 34 71 450 0.06 0.000 1.0
Margarine, regular, stick, 17.17 3 0.12 1 5 18 654 0.11 0.000 0.0
composite, 80% fat, with salt
Oil, vegetable, sunflower 0.00 0 0 0.03 0 0 0 0.00 0.000 0.0
Beverages
Beer, regular 94.32 5 0.02 6 13 25 4 0.01 0.005 0.7
Carbonated beverage, cola 89.10 3 0.02 1 13 1 4 0.01 0.006 0.1
Meat, poultry, and fish
Pork, fresh, leg (ham), whole, raw 62.47 5 0.85 20 199 315 47 1.93 0.065 29.4
Beef, chuck, arm pot roast, raw 57.92 7 2.01 18 166 289 57 3.81 0.077 15.9
Chicken, broilers or fryers, breast, 74.76 11 0.72 28 196 255 65 0.80 0.041 17.8
meat only, raw
Egg, whole, raw, fresh 75.84 53 1.83 12 191 134 140 1.11 0.102 31.7
Fish, tuna, fresh, bluefin, raw 68.09 8 1.02 50 254 252 39 0.60 0.086 36.5
Dairy products
Brie cheese 48.82 184 0.50 20 188 152 629 2.38 0.019 14.5
Milk, whole, 3.3% fat 88.32 101 0.03 10 84 133 43 0.38 0.023 3.7
Yogurt, plain, skim milk 85.23 199 0.09 19 157 255 77 0.97 0.015 3.6
Absolute Reference Methods The sample is titrated with the Karl Fischer reagent,
consisting of a mixture of sulfur dioxide, iodine, and
Among these methods, the Karl Fischer method is the
pyridine in methanol. Because pyridine has an
most used. It is a chemical method based on iodine
unpleasant odor and is toxic, it has been replaced
reduction by sulfur dioxide in the presence of water. by imidazole, and the methanol has been replaced by
FOOD AND NUTRITIONAL ANALYSIS / Water and Minerals 243
methoxymethanol to stabilize the reagent. The meth- water (acids, alcohols, esters, and aldehydes) and to
od is precise because the water is determined specif- water formation in oxidation and nonenzymatic
ically and selectively by chemical reaction. Moreover, browning reactions. On the other hand, other errors
this method determines both free and bounded water. can be produced as a consequence of not allowing
enough time or temperature to evaporate all the wa-
ter from the sample.
Practical Reference Methods
These methods are simple and can analyze many
This section examines oven-drying methods. In these
samples simultaneously. The analysis time can be be-
methods the sample is heated under specific condi-
tween a few minutes and 24 h. Recently other thermic
tions and the weight loss is used to calculate the wa-
methods have been developed that use microwave
ter content of the sample. For this determination
ovens, infrared lamps, halogen lamps, or ceramic
three types of ovens can be used: convection, forced
heating elements, which reduce analysis time.
draft, and vacuum. The determined water quantity is
To complete the analysis, distillation procedures
highly dependent upon the type of oven used, the
must also be considered. These techniques consist in
conditions inside the oven, and the time and tem-
the codistillation of sample water with a high-boiling
perature of drying.
point solvent that is immiscible in water (toluene,
Excess errors can be produced in these analyses
xylene, benzene). The distilled mixture is collected,
due to the loss of volatile compounds different from
and the sample volume is measured. These methods
produce fewer losses by food decomposition than
does oven drying at high temperatures. However,
Table 2 Typical water activity of some foods
measuring by water volume can be less exact than
Food Water activity (aw)
measuring by weight.
Table 4 compares some of the methods used to
Fresh meat and fish 0.95 1.00
analyze water in food by principle, type of sample,
Bread 0.95
Cheese 0.90 0.95
and analysis time, including some methods based on
Margarine 0.85 0.90
the physical characteristics of water in food.
Salami 0.80 0.85
Jams and jellies 0.80
Dried fruits 0.70 0.80 Methods of Determination of Water
Nuts 0.65 0.75
Activity in Foods
Honey 0.60 0.65
Cookies 0.30
Methods used to measure water activity can be direct
Milk powder 0.20
and absolute or indirect and calibrated with regard
Dried vegetables 0.20
to the former.
Table 3 Official AOAC analysis methods applicable to moisture
AOAC method Food Analytical technique
931.04 Cacao products Gravimetry
977.10; 984.20; Cacao products; oils and fats; dried vegetables Karl Fischer titrimetry
967.19
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; Roasted coffee; dried milk; cheese; dried fruits; oils and Vacuum oven
927.05; 926.08; fats; maple products; corn syrups and sugars
934.06; 962.12;
920.186; 977.21
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
244 FOOD AND NUTRITIONAL ANALYSIS / Water and Minerals
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 5min
or high in fats and sugars
Convection oven Removal of water and weight of the Not valid for foods that contain volatiles or 0.75 24 h
remaining solids have components that undergo chemical
reactions at high temperatures
Forced-draft oven Removal of water and weight of the Not valid for foods that contain volatiles or 0.75 24 h
remaining solids have components that undergo chemical
reactions at high temperatures
Vacuum oven Removal of water and weight of the 3 6 h
remaining solids
Distillation Separation of water from the solids and Valid for foods that contain volatiles
volume measurement
Dielectric and Change in capacitance or resistance to an Foods that contain less than 30 35% of water
conductivity electric current that passes through the
sample
Hydrometry Relationship between specific gravity and Liquid samples: drinks and sugar solutions
moisture content
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 Specific absorption of near-infrared
spectroscopy radiation (1400 1450, 1920 1950 nm) by
the water molecules of the food
Absolute Methods
according to relative humidity. An interpolation of
the curve obtained this way allows the determination
These methods are based on measuring air temper-
of relative humidity and, by extension, the aw value
ature in equilibrium with a product and some other
of the food.
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 Importance of Minerals in the Diet and
aw value of the product in equilibrium with this
in Processing Food
air. There are chromatographic, psychometric, and
Minerals are the inorganic elements constituting
manometric methods.
foods (excluding carbon, hydrogen, oxygen, and nit-
rogen) that remain as ashes when foods are inciner-
Calibrated Methods
ated. They are significant for their nutritional value,
These methods use mechanical probes or hygrome- toxicological potential, and interaction with the
ters that respond to the variations in the relative hu- texture and processing of foods. For these reasons
midity of air that are caused by variations in it is necessary to know and control their concentra-
dielectric resistance or capacity. These probes direct- tion levels in foods.
ly measure the relative humidity of air, and the most Among the 50 known minerals, between 15 and
commonly used is lithium chloride. In general, these 20 minerals are natural components of foods that are
methods require frequent calibration and should only part of at least one vital biological system of a plant
be used within a narrow aw range. or animal. Some of them are denominated macroel-
On the other hand, the reference salts method can ements because of their abundance in foods; these
indirectly determine the aw value of a product using include calcium, phosphorus, sodium, potassium,
graphic interpolation. This method consists in in- magnesium, and chlorine. Others are called oligoel-
troducing an aliquot of the sample in an airtight ements or trace elements due to their minimal con-
container where different saturated saline solutions centration; among these are iron, iodine, zinc,
or dilutions of sulfuric acid are placed, creating a copper, chromium, manganese, molybdenum, fluo-
relative humidity and therefore a known aw value. ride, and selenium.
Once equilibrium has been reached, the samples are Minerals are divided into two categories according
weighed and the weight variations are represented to their biological importance: those with a
FOOD AND NUTRITIONAL ANALYSIS / Water and Minerals 245
known biological role, called essential minerals, and methods require that minerals be freed from their
those with an unknown biological role, referred to as matrix. Nevertheless, some analytical techniques,
nonessential minerals. Some minerals in the nonessen- such as near-infrared spectroscopy, sometimes allow
tial group are being investigated because there is some mineral estimation without destroying the carbon
evidence that they have a biological function, although matrix that constitutes foods.
this biochemical role is not yet clear. The nonessential The destruction of the matrix, generally organic
minerals being investigated are vanadium, tin, nickel, matter, can be done by wet or dry ashing. The se-
arsenic, and boron. There are also toxic elements, lected mineralization procedure depends on what the
which should be avoided in the diet. This group in- ashes will be used for and on limitations based on
cludes lead, mercury, cadmium, and aluminum. Some cost, time, and number of samples.
essential minerals, like fluoride and selenium, are Dry ashing, consisting in incinerating the sample at
known to be harmful if consumed in excessive quan- high temperatures (4501C or higher) in a muffle fur-
tities, even though they have biochemical functions nace, converts most minerals into oxides, sulfates,
that are beneficial when consumed at safe levels. phosphates, chlorides, and silicates. Before me-
Some minerals are found naturally in foods in asuring the analytes, the residuum is redissolved in
variable amounts and can be modified, or others can a minimal amount of acid and is diluted with dis-
be added, during industrial processing of food. Table tilled water to a known volume. It is a safe method
1 indicates the amounts of some of the macro- and that does not require the addition of a reagent, and
microelements that can be found in various raw and once ignited it requires little of the analyst s atten-
processed foods. Moreover, almost all these minerals tion, allowing the treatment of many crucibles si-
can be found as pollutants in foods due to environ- multaneously. The main disadvantage of this method
mental or industrial factors, and are analyzed for is the long treatment time (12 18 h). Moreover, some
hygienic reasons, and not as natural components of elements such as selenium, lead, and mercury can
foods. partially volatilize with this procedure. For this rea-
son alternative methods must be used to determine
these minerals.
Food Treatments Prior to Mineral
In wet ashing, organic matter is oxidized by the
addition of acids and oxidants or their combinations.
Analysis
A strong oxidant such as concentrated nitric acid,
There are various methods of mineral determination
sulfuric acid, or perchloric acid is needed to destroy
available, and they can be used according to the an-
organic matter. In general, reagent mixtures are cho-
alytical characteristic that best suits the objectives of
sen in accordance with the type of food and the
the analyst: accuracy, sensitivity, detection limit,
quantification method. Perchloric acid is an excellent
specificity, and interferences. Other factors to take
oxidant, but its use runs the risk of forming explosive
into account are the costs of the complete analysis,
organic perchlorates if the mixture dries completely.
instrumental availability, and time necessary for
Some precautions must be followed when fat-rich
analysis.
foods are treated.
Before mineral analysis it is usually necessary to
Minerals remain in solution during wet digestion
treat the sample to ensure that the sample is
without volatilization losses because the tempera-
homogeneous and also to prepare it for the analyt-
tures used are lower than in dry ashing. The oxida-
ical procedure that follows. Various processes may be
tion time is short and requires a hood, hot plate, long
necessary, but among the most important is sample
tongs, and safety equipment. However, this method
mineralization, often associated with the need to de-
requires the continual attention of the operator, uses
stroy organic matter present in the sample and
corrosive reagents, and only a reduced number of
always necessary to make the sample soluble. More-
samples can be treated simultaneously. If perchloric
over, the treatment of a sample may entail reduction
acid is used in the wet digestion, all work must be
and homogenization of its size or elimination of in-
done in a perchloric acid fume hood.
terferences. In any case, contamination of the sample
Microwave radiation systems are now available
or loss of volatile compounds may occur during these
that accelerate the mineralization process substan-
steps of the analytical process, affecting the quality of
tially. This process has obvious advantages in red-
the analytical results.
ucing decomposition time, acid needs, and the risks
of contamination or foaming.
Mineralization
Other alternatives include closed reactor digestion
Mineralization is usually a crucial step before the at a high temperature and pressure; combustion in a
analysis of specific minerals because most analytical vessel containing oxygen; the Shöniger method,
246 FOOD AND NUTRITIONAL ANALYSIS / Water and Minerals
which is especially suitable when determining ha- reagents can be very expensive, a possible alternative
logens and sulfur; or combustion in an oxygen argon is to work with a reagent blank or a sample of the
plasma at a low temperature. reagents used in the sample analysis in the same pro-
portion as in the sample but without the mineral that
Other Pretreatments is going to be analyzed.
Most solid foods must be ground up in order to ob-
tain a homogeneity compatible with the previously
Methods of Determination of Minerals
described procedures. In this process it is necessary to
in Foods
adjust the grinding fineness according to the miner-
Chemical Methods
alization method that is going to be used. Dry ashing
needs particles of diameter between 0.5 and 1 mm to
The determination of food minerals can be done
avoid losses by mechanical drag during the inciner-
using chemical methods. Among them, volumetric
ation. Oxygen combustion methods and especially
methods stand out because they are fast and inex-
direct fusion methods need a grade of fineness that
pensive while still being adequately precise. Their
avoids particle volatilization losses and artificial en-
main disadvantages are that they have low sensitivity
riching of samples.
and selectivity.
According to their nature, the ground and
Ethylenediaminetetraacetic acid (EDTA) com-
homogenized samples are stored cold or at room
plexometric titrations are based on the fact that
temperature, in airtight containers made of a mate-
many metallic ions form stable complexes with this
rial that does not allow the transfer of materials be-
tetradentate ligand, EDTA. The endpoints are de-
tween the container and the product. Generally, a
tected using complexing agents capable of forming
polyethylene or Teflon container is a good solution.
complexes with the metallic species to be determined
On the other hand, mineralized and solubilized
and having lower coordination constants than those
samples sometimes need treatments that eliminate
of the complexes that are formed with EDTA and
interferences. Factors such as pH, sample matrix,
that also have different colors in their free and com-
some pretreatments, or the incorporation of a
plexed states.
reagent can interfere with the ability of an analyti-
In precipitation titrations the titration reaction
cal method to quantify a mineral. In this situation, it
produces an insoluble precipitate. Despite the many
is common to turn to the use of masking reagents or
known precipitation reactions, very few of them have
separation processes using selective precipitation
the necessary requirements to be the basis of a volu-
techniques, liquid liquid extraction, ion-exchange
metric method. Determining chloride using silver ion
resins, etc.
precipitation, using several methodologies, is the
most appropriate application of this group of titra-
Sample Contamination
tions.
The most important applications of these methods
One of the biggest problems that can occur during
in food analysis are limited to the determination of
mineral analysis is contamination of the sample. The
certain metals such as calcium in water and beer or
selection of the equipment used to treat the samples
chlorides in different types of samples. Table 5 lists
is closely related to the mineral that is going to be
some methods used to determine food minerals based
quantified. Stainless steel enriches the sample in
on volumetry.
chromium and nickel; agate mortars and mills nota-
bly increase the calcium content of the ground prod-
Molecular Absorption Spectrophotometry
uct, etc. Teflon has some important advantages, but
due to its low resistance to abrasion not all treat- These methods, which include colorimetric methods,
ments can be done using this material. Sometimes are based on the measurement of radiation absorp-
glass is inadequate; this must be taken into account tion by molecular species at a specified wavelength.
when determining sodium because the glassware In mineral analysis, the absorbent species are usually
used may enrich the solution. Additionally, the re- compounds, mostly coordination complexes, formed
petitive use of glassware can be a contamination in a reaction between the mineral and a chromogen
source, and for this reason glassware used in sample ligand. These methods are more sensitive and
preparation and analysis must be cleaned scrupu- selective than the previously discussed methods.
lously using acids and the purest water. Some of the methods that use ultraviolet (UV) vis-
Since solvents can contain large amounts of min- ible spectrophotometry can be combined with dy-
erals, it is necessary to use the purest reagents and namic techniques like flow injection systems or with
distilled deionized water to analyze minerals. As a previous separation using liquid chromatography,
FOOD AND NUTRITIONAL ANALYSIS / Water and Minerals 247
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 (HClO4:HNO3) EDTA titrimetry
Calcium Tubers Lyophilization, dry ashing EDTA titrimetry
Colorimetry
Phosphorus Eggs, vegetables
Phosphorus Oils, yogurts, infant foods, Dry ashing
fishes, grains, nuts,
vegetables, tubers, fruits
Phosphorus Meat, legumes Wet ashing (HNO3:H2SO4:HClO4)
Phosphorus Fish, grains, legumes Wet ashing (HNO3:HClO4)
Phosphorus Grains Wet ashing (20% HCl)
Phosphorus Nuts Wet ashing (HNO3:H2SO4)
Iron Wine Wet ashing (HNO3:HClO4)
Iron Tubers Lyophilization, dry ashing
Iodine Dairy products Dry ashing
Boron Fruits Dry ashing
Electroanalytical techniques
Selenium and lead Wine Wet ashing (HNO3:HClO4) Voltametry
Selenium Vegetables Wet ashing (HNO3:HClO4) Voltametry
Fluorine Grains Voltametry
Chromatography
Sodium, potassium, Dairy products Dry ashing Ionic
magnesium, and calcium chromatography
Selenium Nuts Wet ashing (HNO3:H2SO4) Gas chromatography 
mass
spectrometry
Other techniques
Sulfates Oils Dry ashing Turbidimetry
Manganese, bromine, cobalt, Tubers Neutron activation
vanadium, arsenic, antimony, analysis
copper, selenium, aluminum,
and lanthanum
allowing the quantification of many elements simul- methods applicable to the determination of minerals
taneously. Table 5 summarizes some of the main col- using atomic spectroscopy.
orimetric methods used regularly in mineral analysis. Among atomic emission spectrometry (AES) meth-
ods, the classic flame photometric technique is still
favored for determination of sodium and potassium
Atomic Spectroscopy
in foods.
These techniques are based on the atomization of the The use of an inductively coupled plasma (ICP)
analyte present in a solution, using a flame or a plas- allows temperature and stability conditions that
ma. The amount of an element present in the sample eliminate most of the interferences found in com-
is determined from the absorption or emission of the bustion sources. The sensitivities that can be ob-
visible or UV radiation of its atoms in the gaseous tained and the speed of this technique, which can be
phase. The high sensitivity and selectivity that can be used to determine several elements simultaneously,
achieved with these techniques and the increasing make it in an interesting, although expensive, alter-
necessity to respond to requests for mineral quanti- native for the analysis of metals found in foods.
fication in foods at trace levels explain the increasing Coupling ICP with a mass spectrometer gives the best
use of some of the atomic absorption and emission analytical results, although it is currently a technique
spectroscopic techniques. Table 6 summarizes some restricted to specialized laboratories.
248 FOOD AND NUTRITIONAL ANALYSIS / Water and Minerals
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
(HNO3:H2SO4:HClO4)
Flame atomic absorption spectrometry
Sodium, potassium, zinc, iron, calcium, and copper Honey Wet ashing (H2SO4:HNO3)
Zinc, cadmium, and lead Honey Microwave drying, dry ashing
Iron Wine Wet ashing (HClO4:HNO3)
Iron Wine Without treatment
Manganese, chromium, iron, nickel, and copper Juices Wet ashing (H2SO4:HNO3)
Cadmium, copper, chromium, cobalt, iron, nickel, lead, Beverages, dairy products Dry ashing
and zinc
Copper, iron, and zinc Milk Wet ashing (HNO3:H2O2)
Copper, iron, zinc, and manganese Yogurts Dry ashing
Calcium, magnesium, and zinc Yogurts Dry ashing
Copper and zinc Vegetables, fruits, meat, fish, legumes, Dry ashing
cereals, spices, dairy products,
tubers, sweeteners, canned foods
Copper and zinc Fats, oils Wet ashing (HNO3:HClO4)
Sodium, potassium, calcium, magnesium, iron, zinc, Fish Wet ashing (HNO3:HClO4)
manganese, and copper
Cadmium and lead Fish Wet ashing (30% H2O2:HNO3)
Iron, zinc, aluminum, titanium, and vanadium Fish Wet ashing (HNO3:H2SO4)
Selenium, lead, and cadmium Baby foods 0.1% Triton X-100 þ 1%
HNO3 þ 30% H2O2
Cadmium, cobalt, chromium, copper, iron, Seafood Wet ashing (HNO3, H2O2)
manganese, molybdenum, nickel, lead, and zinc
Cadmium, calcium, iron, magnesium, manganese, and Meat Wet ashing
zinc (HNO3:H2SO4:HClO4)
Lead, cadmium, iron, copper, manganese, zinc, and Mushrooms Wet ashing (HNO3:H2SO4:H2O2)
cobalt
Copper, magnesium, lead, sodium, silver, bismuth, Mushrooms Wet ashing (HNO3:HClO4:HCl)
manganese, nickel, lithium, cobalt, antimony,
calcium, zinc, iron, chromium, aluminum, and
potassium
Potassium, sodium, calcium, magnesium, iron, Vegetables Wet ashing (HNO3)
manganese, copper, and zinc
Sodium, potassium, magnesium, zinc, and copper Tubers Lyophilization, dry ashing
Calcium, magnesium, iron, zinc, copper, and Fruits Microwave drying, wet ashing
manganese (HNO3: 30% H2O2)
Inductively coupled plasma
Aluminum, barium, calcium, copper, iron, potassium, Beverages Without treatment
iron, copper, iron, potassium, magnesium,
manganese, sodium, strontium, and zinc
Calcium, cobalt, copper, chromium, iron, magnesium, Milk, flour Microwave drying, dry ashing
manganese, molybdenum, sodium, zinc, silver,
aluminum, arsenic, barium, beryllium, cadmium,
mercury, nickel, lead, antimony, tin, strontium,
titanium, thallium, uranium, and vanadium
Calcium, phosphorous, magnesium, sodium, Seafood, meat Dry ashing
potassium, aluminum, iron, zinc, and copper
Chromium, iron, manganese, selenium, and zinc Vegetables Wet ashing (HNO3:HClO4)
Aluminum, boron, calcium, iron, magnesium, Fruits, fruit products Microwave drying, wet ashing
phosphorus, potassium, sodium, and titanium (HNO3)
Hydride/cold vapor
Selenium Flour, flour products Wet ashing
(HNO3:HClO4:H2SO4)
Selenium Milk Wet ashing (HNO3:HClO4)
Mercury Fish Wet ashing (HCl, HNO3, H2SO4)
Mercury Mushrooms Wet ashing (HNO3)
FOOD AND NUTRITIONAL ANALYSIS / Water and Minerals 249
Atomic absorption spectrometry (AAS) is the most fast, easy to use, and inexpensive. The disadvantages
common technique for the determination of metals in of ISEs include the following: they have a relatively
food products. low sensitivity, proteins or other organic solutes can
Most commonly used instruments use a flame interfere in the determination, and some ions can act
(flame AAS (FAAS)) produced by combustion of an like ligands or can poison the electrodes.
air/acetylene or dinitrogen oxide/acetylene mixture. Voltametric techniques are based on the relation
The few interferences are easy to avoid, and the sen- between current and voltage in an electrochemical
sitivities that are reached are adequate for the metals process. Among them, anodic stripping voltampe-
of greatest interest to the food industry. Variants of rometry permits metallic species determination with
this technique, such as the coupling of hydride gene- detection limits of parts per billion or lower. The
ration (HG) systems (HG AAS), increase its scope to equipment used with these techniques is much more
higher-sensitivity determination of elements like se- inexpensive than that used with spectroscopic tech-
lenium, arsenic, tin, and other elements that form niques that are also used in trace analysis.
hydrides. In a similar vein, the determination of mer- Table 5 summarizes some of the electroanalytical
cury using the cold vapor technique should be methods usually employed in mineral analysis of
highlighted. foods.
An alternative to FAAS, particularly suited to the
determination of metals with higher sensitivity, is
Separation Methods: Electrophoretic and
electrothermal AAS (ET-AAS), in which a graphite
Chromatographic Methods
furnace substitutes the flame as the atomization
source. Generally speaking, the detection limits ob- For multielemental analysis of foods, separation
tained with a graphite furnace are two orders of methods are also used, although less frequently.
magnitude lower than those reached with flame at- Capillary electrophoresis is used to separate metallic
omization. cations like sodium, potassium, calcium, manganese,
Comparing ET-AAS and ICP, techniques with sim- and magnesium. Among chromatographic methods,
ilar scopes in food analysis, confirms that absorption determination of anionic and cationic species using
techniques require less expensive instruments and ionic chromatography, with or without the use of an
usually achieve higher sensitivities. Since ICP is a ionic suppressor, and conductimetric detection
particularly fast method, a laboratory that has to should be highlighted. Table 5 reviews some separa-
analyze many metals per sample and/or many sam- tion methods applicable to the determination of
ples can justify the investment and maintenance minerals.
associated with ICP. The AOAC publishes reference methods for the
Other intriguing spectrometric methods used in analysis of minerals in foods. Table 7 summarizes
metallic species analysis, such as X-ray fluorescence some of the AOAC official methods of analysis
spectrometry and neutron activation analysis, are applicable to minerals.
unusual in food analysis.
Electrochemical Methods Analytical Quality Assurance
In food analysis, the most common electrochemical Quality control is of the utmost importance in the
methods are potentiometric and voltametric. case of mineral analyses because of the low concen-
In potentiometric methods, the potential between trations of the elements normally found in foods and
a reference and an indicator electrode is measured, the ubiquitous presence of significant levels of many
which corresponds to the analyte activity. Because of of them in the environment. In addition to the stand-
their usefulness in food analysis, ion-selective elec- ard techniques of working in a clean laboratory to
trodes (ISEs) that measure anions like bromide, chlo- reduce the potential for accidental contamination to
ride, and fluoride or cations like potassium, sodium, a minimum, it is essential that procedures be vali-
and calcium stand out among indicator electrodes. dated and results checked against appropriate certi-
The characteristics and advantages of ISE include the fied reference materials (CRMs). CRMs for most of
ability to measure different anions and cations di- the trace and other minerals of interest in foods are
rectly, the fact that they do not consume the analyte, available from international reference centers such as
the fact that analyses are independent of sample the Community Bureau of Reference of the European
volume when taking direct measurements, and that Union, the National Institute of Standards and Tech-
moreover turbidity, color, and viscosity do not affect nology of the United States, and the International
the measurement. Potentiometric methods are also Atomic Energy Agency in Vienna.
250 FOOD AND NUTRITIONAL ANALYSIS / Water and Minerals
Table 7 Official AOAC analysis methods applicable to the determination of minerals in foods
AOAC method Mineral Food Chemical reaction
Titrimetry
976.09; 976.10 Calcium Beer EDTA þ eriochrome black T
944.03 Calcium Flour Oxalic acid þ bromocresol green
948.09 Phosphorus Flour Molybdate þ phenolphthalein
983.19; 968.31 Calcium Poultry and beef; canned vegetables EDTA þ hydroxyl naphthol blue
960.29 Chloride Butter Mohr method
915.01 Chloride Plant material Volhard method
Colorimetry
944.02; 945.40; 950.39; Iron Flour; bread; macaroni products; o-Phenantroline
955.21 beer
960.17; 967.17 Copper Beer; distilled liquors Zinc dibenzyldithiocarbamate
970.18; 972.12 Copper Wines; beer Diethanolamine þ carbon disulfide
955.19; 962.11; 970.39; Phosphorus Distilled liquors; wines; fruits; milk- Molybdovanadate
986.24; 991.27; 991.25 based infant formulas; meat and
meat products; cheese
Flame photometry
963.08; 965.30 Potassium Distilled liquors; fruit and fruit 
products
963.09; 966.16 Sodium Distilled liquors; fruit and fruit 
products
963.13; 969.23; 990.23 Potassium and Wines; seafood; dried milk 
sodium
AAS
967.08; 970.18 Copper Distilled liquors; wines 
970.19 Iron Wines 
972.06 Aluminum Baking powders 
987.02 Potassium Beer 
987.03 Sodium Beer 
985.35 Minerals Milk-based infant formulas 
991.25 Calcium and Cheese 
magnesium
ICP emission spectroscopy
984.27 Calcium, copper, Infant formulas 
iron, magnesium,
manganese,
phosphorus,
potassium,
sodium, and zinc
See also: Atomic Absorption Spectrometry: Principles
Nielsen SS (ed.) (2003) Food Analysis, 3rd edn., pp. 81
and Instrumentation. Atomic Emission Spectrometry:
100, 103 111, 191 202. New York: Kluwer Academic/
Principles and Instrumentation. Elemental Speciation:
Plenum Publishers.
Overview. Food and Nutritional Analysis: Sample Prep- Horwitz W (ed.) (2000) Official Methods of Analysis of the
aration. Ion-Selective Electrodes: Overview. Quality
Association of Official Analytical Chemists, 17th edn.
Assurance: Reference Materials. Sample Dissolution
Arlington: AOAC International.
for Elemental Analysis: Dry Ashing; Oxygen Flask Com- Macrae R, Robinson RK, and Sadler MJ (eds.) (1993) En-
bustion; Wet Digestion; Microwave Digestion. Spectro- cyclopedia of Food Science, Food Technology and Nu-
photometry: Inorganic Compounds. Titrimetry:
trition. San Diego: Academic Press.
Overview.
Pomeranz Y and Meloan CE (1994) Food Analysis: Theory
and Practice, 3rd edn., pp. 575 624. New York: Chap-
man & Hall.
Further Reading
USDA Agricultural Research Service (2003) USDA nutrient
database for standard reference. http://www.nal.usda.-
Belitz HD and Grosch W (eds.) (1999) Food Chemistry,
gov/fnic/cgi-bin/nut search.pl.
2nd edn. Berlin: Springer.