Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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However, pecan kernels can be preserved against

fungal deterioration and discoloration by reducing

the moisture content rapidly, after harvest, with mod-

erately warm (< 40



C), dry air circulated constantly

around the nuts, followed by storing in ventilated

chambers containing air of relative humidity in the

range 60–68%. Chlorine or propylene oxide treat-

ments are also employed by the pecan industry to

disinfect in-shell pecans, halves, and pieces.

See also: Amino Acids: Properties and Occurrence;

Antioxidants: Natural Antioxidants; Carbohydrates:

Classification and Properties; Energy: Measurement of

Food Energy; Measurement of Energy Expenditure; Fatty

Acids: Properties; Mycotoxins: Occurrence and

Determination; Oxidation of Food Components;

Phenolic Compounds; Preservation of Food; Protein:

Requirements; Sensory Evaluation: Aroma; Storage

Stability: Parameters Affecting Storage Stability;

Tannins and Polyphenols; Triglycerides: Structures

and Properties

Further Reading

Brouk B (1975) Plants Consumed by Man. London: Aca-

demic Press.

Considine PEDM and Considine GD (eds) (1982) Pecan

and walnut. In: Foods and Food Production Encyclo-

paedia, pp. 1443–1450, 2088–2098. New York: Van

Nostrand Reinhold.

Dhar DN (1968) The Chemistry and Biochemistry of

Walnut Trees. Kanpur: Indian Institute of Technology.

Eckey EW (1954) Vegetable Fats and Oils. New York:

Reinhold.

Vaughan JG (ed.) (1970) Juglandaceae. In: The Structure

and Utilization of Oilseeds, pp. 113–116. London:

Chapman and Hall.

Young CT (1981) Nuts. In: Grayson M (ed.) Kirk-Othmer

Encyclopaedia of Chemical Technology 3rd edn, vol. 16,

pp. 248–276. New York: John Wiley.

Young CT (1996) Nuts. In: Kroschwitz J and Howe-Grant

M (eds) Kirk-Othmer Encyclopaedia of Chemical

Technology, 4th edn, vol. 17, pp. 544–579. New York:

John Wiley.

WASTAGE OF FOOD

C Hunt, University of Huddersfield, Queensgate,

Huddersfield, UK

Background

0001 Some food wastage is inevitable during storage, dis-

tribution, processing, and cooking. However, avoid-

able wastage of food at any point in the food chain,

from crop losses in harvesting and storage to food

left on the plate, could be considered undesirable in

moral, economic, and nutritional terms. There have

been relatively few studies on this topic, but interest

intensifies if commodities become scarcer and/or more

expensive. Pollution problems relating to disposal of

organic waste, including food and food by-products

also need to be addressed.

0002 Discrepancies between total food supplies and

actual food consumption of populations have long

been recognized but not fully understood. For

example, recent estimates in the UK suggest that

there is sufficient food and drink available to provide

approximately 13.4 MJ (3200 kcal) per head per day.

0003 These estimates relate to edible food available from

farms and from imports after allowance had been

made for exports, nonfood usage and any changes in

year-end stocks. Yet the actual average energy intake

in 1998 (National Food Survey) was calculated at

8.8 MJ (2110 kcal) per person per day, about 12%

of which derived from food and drink consumed

outside the home. The survey included a sample of

5973 fully participating households, and the home

food consumption was based on a 7-day food diary

of all food entering the household. The above energy

intake figure does not include food wastage in the

home. However, in further calculations relating

energy intakes to dietary reference values (DRVs),

the report assumes that 10% of edible food is wasted

via plate wastage, spoilage in the kitchen or on the

plate, or fed to domestic pets or livestock. This

assumed figure is slightly higher than that measured

in the study mentioned below.

0004Thus, on a national scale, there appears to be a

‘crude energy gap’ of approximately 30–35% com-

paring food energy available with energy consumed.

Possible contributors to this loss in the food chain

could be distribution and storage losses, and waste

in markets and shops, food factories, hotel and

catering outlets, and domestic households.

Definition

0005Food waste can be defined as ‘any potential source of

food that has knowingly been discarded or des-

troyed,’ e.g., food discarded in factories, food not

WASTAGE OF FOOD 6079

sold in shops or restaurants, kitchen scraps, and plate

waste. Food loss could be used to refer to food that

has inadvertently been destroyed or spoiled, e.g.,

stored grain eaten by rats, meat spoiled by bacterial

growth and nutrients destroyed or going into solution

in vegetable canning.

0006 Food waste and loss can be described in terms of

weight, cost,ornutritional value but does not refer to

inedible material such as egg shells, potato peelings,

and bones.

Household Waste

0007 In a major study in the UK conducted by the Ministry

of Agriculture, Fisheries and Foods (1980) food waste

was measured in 672 representative households (338

in summer and 334 in winter). The householders

collected all potentially edible food wasted in their

homes during 1 week and also kept a record of the

food other than commercial pet food, which they

gave to pets and wild birds. Each food sample

received was weighed and its energy content deter-

mined calorimetrically.

0008 Significantly more waste food was collected in

summer than in winter, equivalent to 9.3 MJ

(2220 kcal) and 7.1 MJ (1700 kcal) per household

per week, respectively. In terms of energy, cereals, fat

and meat wastage predominated, whereas in terms of

weight, milk was more important, and fat less so.

Significant quantities of otherwise edible food were

also given to pets and birds, accounting on average

for a further 2.4 MJ (570 kcal) and 3.0 MJ (710 kcal)

per household per week in summer and winter,

respectively.

0009 The energy content of all food wasted in the home

therefore averaged 11.7 MJ (2790 kcal) per house-

hold per week in summer and 10.1 MJ (2410 kcal)

in winter, equivalent to an average 0.55 MJ

(140 kcal) per person per day. This was therefore

less than one-quarter of the gap between food sup-

plies and the amount of food thought to be eaten in

the UK. When assessed against the expected usage of

food in the home, wastage accounted for 6.5% of the

energy intake in summer and 5.4% in winter. Wastage

was not related to income or geographical area but

was greater per person in smaller compared with

larger households.

0010 In another study conducted in Cambridge, UK

(1985) on 82 families, wastage of edible energy aver-

aged 3.8% (with a further 3.0% of purchases being

consumed by visitors).

0011 Domestic food wastage in the USA is probably

higher than in the UK. Few data exist, but one study

in Arizona, based on measurement of waste food in

garbage cans, estimated approximately 10% of the

weight of food that entered the home being discarded.

However this did not include food given to pets or

washed down sinks or through garbage disposal

units.

Catering and Hotel Waste

0012As mentioned previously, this is another area where

food wastage occurs. Reliance by consumers on food

provided by the caterer varies widely within and

between different groups of people and between

countries. In the UK, on average, three to four meals

per week per person are consumed outside the home.

(See Catering: Catering Systems.)

0013Catering outlets vary greatly in size ranging from

small cafe

s, snack bars, and restaurants to large in-

dustrial canteens and hospitals. Wastage can occur at

various stages of the catering operation. Firstly, poor

storage conditions for incoming goods can lead to

food being discarded, e.g., from temperatures rising

too high in refrigeration, cold storage, etc. Meat,

meat products, fruits, and vegetables are particularly

vulnerable in this respect. Also, excessive humidity

can lead to spoilage of dry goods particularly if

unpackaged. (See Catering: Nutritional Implications.)

0014Secondly, losses can occur in preparation such as in

the trimming of meat, vegetables, and fruit. Spillages

can occur, and also significant drip-thaw loss of

water-soluble protein, vitamins, and minerals can

occur in the defrosting of frozen meat.

0015Cooking can cause further loss, e.g., loss of weight

due to evaporative loss, as in roasting of meat, also a

reduction in fat and hence energy content in the

grilling of meat. Destruction of heat-labile vitamins

(B complex and C) inevitably will occur as well in

cooking, together with leaching of water-soluble vita-

mins. These losses can be minimized by good catering

practice. Unserved food may also be discarded, and

finally, plate wastage may occur, particularly if the

meal is unappetizing, e.g., not hot enough or poorly

presented. There have been a few studies of wastage

in catering. Platt et al., in a study of UK hospitals,

drew attention to alarmingly high levels of food

waste, with 25–35% of food (by weight) sent from

the kitchen being left over in serving dishes and plate

waste averaging a further 10%. In a later study of a

variety of UK catering establishments, 30% of total

edible food energy input was found to be wasted

throughout the catering operation in the hospital

sector, but was less in other sectors (cafe

s and snack

bars 4.8%; schools 6.5%; place of work 10.2%;

restaurants, hotels and pubs 15.5%; and other wel-

fare 17%), giving an overall average of 11.4%.

Higher overall wastage occurred with generally poor

control procedures, and high plate wastage was also

6080 WASTAGE OF FOOD

noticeable with high fat and high energy meals

(served particularly in hotels and restaurants).

0016 A recent study on long-stay elderly patients in

hospital maintained concerns about the nutritional

impact of waste. The data suggested that, although

adequate energy and nutrients per patient were pro-

vided from the kitchens, service loss and plate waste

resulted in actual intakes being inadequate, on

average.

0017 In this respect, arguments remain as to the relative

merits of plated meals or bulk trolley service, but

there have been recent suggestions that bulk trolleys,

when operated on an appropriately ‘personalized

service’ basis, can significantly reduce plate waste

and improve patient intakes.

Factory Waste

0018 The extent of wastage, however defined, in food-

processing factories varies greatly with country and

with the particular food products concerned.

0019 Stocks of raw food materials and of food products

after processing are held in factories for variable

periods and are potentially open to attack by

microbes, insects, rodents, etc. The extent of losses

will range from considerable to minimal, depending

on how tightly storage conditions are controlled and

monitoring procedures enacted.

0020 In developing countries, the food-processing indus-

tries are generally less well developed and regulated,

and the potential for waste is considerable. In de-

veloped countries, processing has been increasingly

concentrated in very large factory units with

enormous throughputs of food. Control and quality-

assurance procedures in the latter are relatively so-

phisticated and waste can be minimized. Inevitably,

though, there is still some loss either directly of

the food raw materials and food products or of

potentially utilizable byproducts. For example, it

has been estimated that 1–4% (by weight) of the

milk input to a modern dairy plant is wasted and

that 2–5% of total carcass protein is lost in the efflu-

ent from abattoirs and poultry plants. However, inter-

est in recovery of waste food products and

byproducts has been increasing for both economic

and ecological reasons, since very large quantities of

biodegradable materials are involved, e.g., aqueous

‘washings’ from the processing of fruit, vegetables

and cereals containing starch, sugars and protein,

blood from slaughterhouses, and whey from the

dairy industry. These wastes have been sprayed dir-

ectly on to fields, where they can act as fertilizer but

can be degraded by soil bacteria in the process, or

discharged into the sewage system, or directly into

rivers. Excessive use of the last option has been in-

creasingly controlled since the relatively high bio-

chemical oxygen demand of these biodegradable

substances can overwhelm the natural capacity of

the river to deal with them.

0021The technology exists for recovery of these mater-

ials by chemical and microbiological means either

directly for human food or by conversion to animal

feedstuffs, but the extent to which this is done is likely

to depend largely on economics and environmental

legislation.

See also: Catering: Catering Systems; Nutritional

Implications

Further Reading

Birch GG, Parker KJ and Worgan JT (1976) Food from

Waste. London: Applied Sciences.

Collison R and Banks GH (1981) Food waste in catering.

Proceedings of the Institute of Food Science and Tech-

nology 14: 181–189.

Collison R and Colwill JS (1987) Food waste in public

houses and restaurants and customer attitudes. Inter-

national Journal of Hospitality Management 6(3):

163–167.

Fenton J, Eves A, Kipps M and O’Donnell CC (1995) The

nutritional implications of food wastage in continuing

care wards for elderly patients with mental-health prob-

lems. Journal of Human Nutrition & Dietetics 8(4):

239–248.

Jelen P (1983) Reprocessing of whey and other dairy wastes

for use as food ingredients. Food Technology 37(2):

81–84.

Kelly L (1999) Audit of food wastage: differences between a

plated and bulk system of meal provision. Journal of

Human Nutrition & Dietetics 12(5): 415–424.

Knorr D (1983) Recovery of functional proteins from food

processing wastes. Food Technology 37(2): 71–76.

MAFF (1999) National Food Survey, 1998: Annual Report

on Food Expenditure, Consumption and Nutrient

Intakes. London: The Stationery Office.

Nelson M, Dyson PA and Paul AA (1985) Family food

purchases and home food consumption: comparison

of nutrient contents. British Journal of Nutrition 54:

373–387.

Wenlock RW, Buss DH and Derry BJ (1980) Household

food wastage in Britain. British Journal of Nutrition

43: 53–70.

WASTAGE OF FOOD 6081

Waste Water See Effluents from Food Processing: On-Site Processing of Waste; Microbiology of

Treatment Processes; Disposal of Waste Water; Composition and Analysis

WATER

Contents

Structures, Properties, and Determination

Physiology

Structures, Properties, and

Determination

M M Pierce, University of Nebraska Lincoln, Lincoln,

NE, USA

This article is reproduced from Encyclopaedia of Food Science,

Introduction

0001 In most food systems, water is a major component

and influences food structure, appearance, flavor and

susceptibility to spoilage.

Shape of the Molecule

0002 The water molecule is made up of two hydrogen

atoms with an oxygen central atom. The water mol-

ecule is a polar, angular structure (Figure 1). There is

a 105



angle between the covalently bonded hydro-

gen atoms. The hydrogen bonds are covalent s bonds

with a dissociation energy of 4.614  10

kJ mol

1

0003 Pure water can contain as many as 33 chemical

variants of the molecule HOH. These include the

common isotopes

O and

H, as well as

H (deuterium) and

H (tritium). Water also contains

ionic particles including hydronium ions (H

hydroxyl ions (OH



) and their isotopic counterparts.

(See pH – Principles and Measurement.)

Hydrogen Bonding

0004As mentioned, water has a bond angle of 105



, which

allows for an approximate tetrahedral structure. The

hydrogen atoms are found along two of the axes of

the tetrahedron, and the lone pairs of the oxygen

atom are located approximately where the other

two tetrahedron axes would be. This balance of two

hydrogen bond donor and receptor sites allows

for three-dimensional hydrogen bonding with a

maximum of four other water molecules.

Structure of Ice

0005It has been shown that the ice crystal structure has a

hexagonal symmetry (Figure 2). Three of the hydro-

gen bonds of the tetrahedron are in the same plane,

and the fourth is directly above or below. In the ice

crystal, these basal planes are parallel (Figure 3).

Therefore, the structure of ice is a series of parallel

planes. When ice melts, the structure collapses as

hydrogen bonds break. The arrangement becomes

more closely packed and, thus, liquid water occupies

less space than ice.

Physical Properties

0020When compared with molecules of a similar molecu-

lar weight and composition, water has unusually high

melting and boiling points. Water also has large

values for surface tension, dielectric constant, heat

capacity, and heats of phase transition and a low

value for density. Also, the thermal conductivity is

large compared with other liquids and nonmetallic

solids. These physical properties are influenced by

the highly polar nature of the water molecule, which

−

fig0001 Figure 1 The water molecule. Reproduced from Water: Struc-

ture, Properties and Determination, Encyclopaedia of Food Sci-

ence, Food Technology and Nutrition, Macrae R, Robinson RK and

Sadler MJ (eds), 1993, Academic Press.

6082 WATER/Structures, Properties, and Determination

allows such phenomena as hydrogen bonding and

ionization to occur. Table 1 lists some of the physical

properties of water.

Nature of Water in Foods

0021 When associated with food, water can be placed

into three categories, free, absorbed, and bound.

Free water is the water found within the pores or

intergranular spaces of the food. This water retains

the usual physical properties of water and often acts

as a solvent or dispersing agent for crystalline or

colloidal substances. Water absorbed on the surface

of macromolecular colloids is termed absorbed water.

The macromolecular colloids to which water may be

absorbed include starches, pectins, cellulose, and pro-

teins. This absorption is due to Van der Waals’ forces

and hydrogen bonding. Bound water is the propor-

tion of the water that is in combination with consti-

tuents of the food, e.g., water of hydration. This

classification system is arbitrary, and there are numer-

ous definitions and variations for water types.

Determination in Food

0022The amount of water in a food is often referred to

as the ‘moisture content.’ This moisture content is

important for many reasons. The most important of

these is the effect of moisture on the stability and

quality of the food substance. This refers to micro-

biological spoilage as well as undesirable chemical

reactions. Moisture is of great economic importance

because it is inversely related to the amount of dry

material. Other important moisture content issues are

processing problems, such as material balance and

optimizing processing techniques affected by mois-

ture content. Moisture content is also necessary for

calculation of nutritional information, as a means of

expressing analytical data on a uniform basis (dry

weight basis) and meeting compositional standards

and laws. (See Preservation of Food; Spoilage: Bacter-

ial Spoilage; Water Activity: Principles and Measure-

ment; Effect on Food Stability.)

Heating Methods

0023One of the oldest methods of determining moisture

content is heating, and several variations of heating

methods exist. They are all common in that the water

is driven from the food by the use of a heat source.

The loss in weight is measured and assumed to be the

moisture content of the food.

0024The accuracy of these methods is greatly affected by

drying temperature, relative humidity and movement

fig0003 Figure 3 Diagram showing the basal plane of an ice crystal.

Reproduced from Water: Structure, Properties and Determin-

ation, Encyclopaedia of Food Science, Food Technology and Nutri-

tion, Macrae R, Robinson RK and Sadler MJ (eds), (1993),

Academic Press.

fig0002 Figure 2 Diagram showing the hexagonal structure of an ice

crystal. Reproduced from Water: Structure, Properties and Deter-

mination, Encyclopaedia of Food Science, Food Technology and Nu-

trition, Macrae R, Robinson RK and Sadler MJ (eds), (1993),

Academic Press.

tbl0001Table 1 Physical properties of water

Molecular weight 18.015

Phase transitions

Melting point at 1 atm (



C) 0.00

Boiling point at 1 atm (



C) 100.00

Heat of fusion at 0



C (kJ) 6.012

Heat of vaporization at 100



C (kJ) 40.63

Heat of sublimation at 0



C (kJ) 50.91



C (liquid) 0



C (ice)

Density (kg l

1

) 0.9998 0.9168

Viscosity (Pa s) 1.787  10

3

—

Surface tension (N m

1

) 75.6  10

3

—

Heat capacity (J kg

1

) 4.2177 2.1009

Thermal conductivity (J m

1

) 5.644  10

22.40  10

Thermal diffusivity (m

1

) 1.3  10

5

1.1  10

4

WATER/Structures, Properties, and Determination 6083

of the air in the chamber, vapor pressure of the cham-

ber (vacuum), sample particle size and quantity, oven

construction, position and number of samples in the

chamber, the surface of the sample, and the rate of

diffusion of water vapor. The drying temperature

must be at least the boiling point of water at the

given vapor pressure of the chamber. When the cham-

ber is under vacuum, a lower drying temperature

must be used. The humidity of the air must be low,

and movement of the air is important. Continuous

sweeping of the chamber with dry air aids drying and

decreases drying time. Using a sample of small par-

ticle size increases the surface area and aids drying. A

thin layer of sample in a shallow aluminum pan also

increases the rate of drying. Allowing liquids to evap-

orate on a water bath before oven drying is the pre-

ferred method. To allow for uniform drying of several

samples, pans need to be evenly spaced in the center

of the oven 2–5 cm from the walls and 5–8 cm from

the door.

0006 There are two common types of ovens, air ovens

and vacuum ovens. Air ovens include both convection

and forced-air types. Vacuum ovens allow for com-

plete drying with less change to the organic compos-

ition of the product.

0007 A less traditional heating method is infrared

drying. This type of drying reduces the drying time

to 12–33% of the conventional drying time. Infrared

drying consists of a 250–500 W lamp, which heats to

2000–2500 K. The lamp is placed above the weighed

sample at a distance of about 10 cm. The sample pan

may rest on a torsion balance with an indicator scale

reading in grams and/or percent moisture. This nor-

mally has a drying time of 10–40 min. Microwave

ovens can also be used for drying and allow very

short drying times.

Distillation Methods

0008 Distillation is almost as old a method as drying. There

are two types of distillation apparatus. The first uses a

high-boiling-point water-immiscible liquid such as a

mineral oil. The sample is suspended in this oil and

heated to a temperature high enough to allow the

water to be vaporized. As the water is distilled off, it

is condensed and collected in a vessel and measured.

The second type of distillation involves the use of a

solvent that is immiscible with water. Again, the

sample is suspended, and the solvent and water are

codistilled and collected in a measuring device or

moisture trap. The two liquids separate, and the

water layer is measured. Often, a reflux system is

used to allow recycling of the solvent. Figure 4

shows a typical distillation system, and Figure 5

shows various types of moisture traps.

Chemical Methods

0009The Karl–Fischer titration is a good method for

samples that are low in moisture, high in sugar, and

high in reducing sugar and protein, or samples that

give erratic results. The Karl–Fischer method is based

upon the reduction of iodine by sulfur dioxide in the

presence of water. Methanol and pyridine are used to

dissolve the iodine and sulfur dioxide. The reaction

occurs in two steps (eqn (1)).

N  I

þ C

N  SO

þ C

N þ H

! 2C

N  HI þ C

N  SO

ð1Þ

N  OSO

þ CH

OH ! C

NðOHÞSO

ð1bÞ

Normally an excess of methanol, pyridine, and sulfur

dioxide is used, the strength being dependent on the

fig0004Figure 4 A typical distillation system for determining the water

content in food. Reproduced from Water: Structure, Properties

and Determination, Encyclopaedia of Food Science, Food Technol-

ogy and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds),

1993, Academic Press.

6084 WATER/Structures, Properties, and Determination

concentration of iodine. Karl–Fischer reagents can be

obtained commercially as two solutions: (1) iodine in

methanol and (2) sulfur dioxide in pyridine.

0010 The titration can be carried out in one of two ways.

The first requires that the reagents be mixed just

before use and the sample dispersed in a solvent

(e.g., methanol). The dispersed sample is then titrated

with the four-component reagent until a yellow–

mahogany brown end point is reached. In the second

method, the sample is dispersed in a mixture of sulfur

dioxide, pyridine, and methanol. This is then titrated

with iodine in methanol. In order to enhance the end

point, a few drops of methylene blue may be added,

giving a green end point. Alternatively, a photometric

or electrochemical method can be used instead of a

visual method.

Instrumental Methods

0011Infrared determination Infrared spectrophotometry

is based on absorption of wavelengths characteristic

of molecular vibrations. In water, the fundamental

vibrational modes are 3000 and 6100 nm with the

first overtone of the OH stretch at 1400 nm. (See

Spectroscopy: Infrared and Raman.)

0012Near-infrared determination This type of spectro-

photometry is similar in nature to infrared spec-

trophotometry. The wavelengths of interest are

1940 nm, as the primary OH bend and stretch, and

1450 nm, from the first overtone of the OH stretch.

(See Spectroscopy: Near-infrared.)

0013Gas chromatographic determination Moisture can

be determined by gas chromatography using a thermal

conductivity detector, provided that the water can be

extracted using asolvent suchas methanol/butanol and

that separation of water from other volatiles is pos-

sible. (See Chromatography: Gas Chromatography.)

0014Nuclear magnetic resonance (NMR) determina-

tion NMR determination uses a magnetic field of

constant frequency that is applied to the sample, and

the strength of the field is varied. The peak-to-peak

amplitude of the signals is measured, and moisture is

determined by using a standard curve. This method,

however, is hydrogen-specific, not water-specific, and

may be useful for materials only with a very low

solids content. (See Spectroscopy: Nuclear Magnetic

Resonance.)

0015Electrical determinations Several instruments exist

that use conductivity as a method to measure

moisture. Most of these instruments use a modified

Wheatstone bridge. In these instruments, an electrical

current is applied to a food, and the resistance of the

food is measured and related to moisture content.

0016Another electrical method is capacitance. This type

of instrument relies on the fact that the dielectric

constant of water is higher than that of most other

liquids. The sample is placed between two plates,

with alternating opposite charge at a fixed frequency,

resulting in an alternating current. Polar substances

with permanent dipoles orient themselves in this

alternating field, increasing the charge of the plates.

Problems of Determination

0017All of the methods discussed have drawbacks, and

while some are excellent methods for one product,

they may yield poor results for others. In drying, the

most common source of error is the formation of a

crust or case hardening. Lowering the oven tempera-

ture and increasing the vacuum may help. In the case

(a) (b)

(c)

fig0005 Figure 5 Moisture traps for distillation methods of water deter-

mination: (a) Stark and Dean, (b) Bidwell and Sterling, and (c)

Barrett. Reproduced from Water: Structure, Properties and De-

termination, Encyclopaedia of Food Science, Food Technology and

Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), (1993),

Academic Press.

WATER/Structures, Properties, and Determination 6085

of some liquids that are high in sugar, mixing with

sand may help. A two-stage drying procedure can also

be used, where the temperature starts low and is

increased very slowly.

0018 Distillation methods also have many difficulties

associated with them. These include adherence of

moisture to glass and improper drainage of water

into the measuring device, difficulties in reading the

meniscus, and low precision of the measuring device.

Also, water-soluble components may be distilled,

causing erroneous results.

0019 The main difficulty of the Karl–Fischer titration is

incomplete extraction of water from the sample.

Also, ascorbic acid, a common additive, is an inter-

ferent and has an additive effect. Aldehydes and

ketones release water when mixed with methanol,

causing a fading endpoint.

0025 Thus, although chemical and instrumental methods

are, in general, very fast, they have a narrow range of

products with which they can be used.

See also: Chromatography: Gas Chromatography;

Preservation of Food; Spectroscopy: Infrared and

Raman; Near-infrared; Nuclear Magnetic Resonance;

Spoilage: Bacterial Spoilage; Water Activity: Principles

and Measurement; Effect on Food Stability

Further Reading

AOAC (1990) Official Methods of Analysis: Association of

Official Analytical Chemists, 15th edn. Arlington, VA:

Association of Official Analytical Chemists.

Aurand WA, Woods AE and Wells MR (1987) Food Com-

position and Analysis. New York: AVI.

Fennema OR (1985) Food Chemistry. New York: Marcel

Dekker.

Osborne BG and Fearn T (1986) Near Infrared Spectros-

copy in Food Analysis. Harlow: Longman.

Pomeranz Y and Meloan CE (1987) Food Analysis: Theory

and Practice, 2nd edn. New York: AVI.

Sisler HH, VanderWerf CA and Davidson AW (1967) Col-

lege Chemistry, 3rd edn. New York: The Mallcom

Company.

Physiology

J R Robinson, Dunedin, New Zealand

Introduction

0001 Body water cannot be poured out to measure its

volume; it must be determined indirectly. A good

indirect method is to introduce a known quantity of

a harmless solute and measure its concentration in the

blood plasma after mixing is complete. The results

depend upon the solute chosen. Very diffusible sub-

stances such as urea, ethanol, or isotopic forms of

water, give large values that correspond to the total

amount of water in the body, 35–45 l, or 50–70% of

body weight. Proteins or other colloids that cannot

leave the blood give the volume of plasma, around 3 l,

or 4–5% of body weight. Substances of intermediate

molecular size that cannot penetrate into cells give

estimates ranging between 12 and 20 l, or 16–30% of

body weight. (See Body Composition.)

0002Provided that these substances can penetrate into

dense connective tissues and tendons, this intermedi-

ate estimate should give the sum of the extracellular

fluid outside the blood vessels and the plasma inside

them. The results are consistent with the obvious

anatomical fact that the water of the body is not one

homogeneous phase. The most important subdivision

is into two major fluids: one is intracellular, discon-

tinuous, in 10

or so separate tiny portions, each

surrounded by a cell membrane; the other is extracel-

lular, the continuous medium in which the cells live,

and which provides the medium for their communi-

cation with each other, and, through the circulating

blood, with the external environment across inter-

faces at the lungs and the alimentary tract.

0003These two major fluids are as distinct in compos-

ition as in anatomical location (Table 1). The solutes

in the intracellular fluids (ICFs) are potassium salts,

and the anions are mostly organic, chiefly phosphoric

esters and proteins, with only a little chloride. The

extracellular fluid (ECF) is more like a dilute sea

water; its chief solutes are sodium and chloride, with

small but important concentrations of potassium,

calcium and bicarbonate, together with organic

nutrients such as glucose and amino acids, and

waste products such as urea.

tbl0001Table 1 Composition (mmol per kg of water) of typical

intracellular and extracellular fluids

Muscle ICF Plasma ECF

Potassium (K

) 150 5

Sodium (Na

) 10 150

Magnesium (Mg

2þ

)101

Calcium (Ca

2þ

)102

Organic phosphate (Org P



) 130 5

Bicarbonate (HCO



)1027

Chloride (Cl



) 5 110

Phosphate (H



Protein (Prot

17

)21

pH 7.1 7.4

Osmolality (mosmol per kg of water) 285 285

Taken from Table 1.1 in Bray JJ, Cragg PA, Macknight ADC, Mills RG and

Taylor DW (1989) Lecture Notes on Human Physiology, 2nd edn. Oxford:

Blackwell Scientific with permission.

6086 WATER/Physiology

0004 The membranes surrounding the cells are of re-

markably low and highly selective permeability, but

they are not totally impermeable to sodium, potas-

sium, and other ions. Consequently, the characteristic

differences in composition have to be maintained

actively, and a major proportion of ‘resting’ metabol-

ism is required to provide energy to pump out sodium

that continually diffuses into cells, and to recover

potassium that escapes or is lost through activities

such as muscular contraction or conduction of nerve

impulses. The membranes are permeable to water,

so that the fluids on either side are in osmotic

equilibrium. (See Potassium: Physiology; Sodium:

Physiology.)

Role as Transport Vehicle for Nutrients

and Metabolites

0005 Within the body, nutrients and metabolites are in

solution, and the transport system is based on water

carriage. That part of the ECF which is the blood

plasma is contained in the arteries, capillaries, and

veins, and circulated rapidly; even at rest, the heart

pumps about 5 l min

1

from the aorta. The lymph,

which is also in vessels, circulates much more slowly

in returning escaped plasma proteins from tissue

spaces to the blood. The flowing plasma carries nu-

trients and metabolites close to cells, but cannot quite

reach them. Transport of consumables to cells, and

products away, depends on diffusion through the ca-

pillary walls and across thin layers of interstitial fluid

between the capillaries and cells in the tissues. Diffu-

sion in water is slow over distances greater than

0.1 mm, but at distances of the order of cell diam-

eters, it is remarkably rapid, 90% equilibration

taking less than 0.05 s at 10 mm, even at room tem-

perature. Between adjacent cells 1 mm apart, it would

take less than 5 10

4

s, across a synapse, 0.1 m m

wide, < 5 10

6

s. Thus, the circulating blood

provides long-distance transport of oxygen from the

lungs, metabolites from the liver, and nutrients from

the gut to the tissues; it also carries carbon dioxide

to the lungs and nonvolatile waste products to the

kidneys for disposal. Diffusion provides rapid trans-

port in the ICFs between organelles within the cells,

and rapid exchanges with the ECF in which they

live. Both these mechanisms of transport depend on

the water in which the transported materials are

dissolved. The blood also carries ‘messengers’ such

as carbon dioxide from active tissues to the respira-

tory center, and hormones from endocrine glands to

cells all over the body, which makes transport by

water extremely important for the coordinated func-

tion of a vast population of separate cells as one

organism.

Protection and Lubrication

0006There are some smaller collections of fluid that are

not inside cells and yet cannot exchange freely with

the main mass of ECF by diffusion across highly

permeable capillary membranes. Examples are the

vitreous and aqueous humors in the chambers of the

eye, the cerebrospinal fluid that surrounds the brain

and the spinal cord, and the synovial fluid in the

capsules of joints. These are by no means ICFs, but

they are secreted by layers of specialized cells (e.g., in

the ciliary body, choroid plexus, or synovial mem-

branes) which surround them and separate them

from the main body of ECF. They may be called

transcellular fluids, because the only approach to

them by diffusion from the continuous ECF is across

cellular membranes with their specialized and re-

stricted permeability. These specialized fluids serve

to maintain the size and shape of the eyeball, to

protect the brain from trauma due to sudden move-

ments by reducing its effective weight by flotation,

and to lubricate the joints.

0007The cells are tiny aquatic organisms, and the ECF is

the pond in which the live. It has been aptly called a

middleman fluid because it accommodates exchanges

of matter and energy between cells and other cells in

remote parts of the body as well as between cells inside

the body and the external environment from which

they eventually derive their nutrients and the oxygen

they need to utilize these. The famous French physi-

ologist, Claude Bernard, died in 1878 correcting the

proofs of the book in which he characterized the ECF

as an internal environment within which the cells live

a sheltered life, protected from the vicissitudes of an

external world that is variable and often hostile. Ber-

nard enunciated the principle that the constant prop-

erties of this internal environment are a necessary

condition for the free, independent life of higher or-

ganisms. Indeed, a substantial part of the subject

matter of physiology is concerned with homeostatic

mechanisms that operate to stabilize the chemical and

physical properties of the internal environment.

Role in Body Temperature Regulation

0008The rates of the chemical reactions catalyzed by

enzymes have substantial temperature coefficients,

so that proper coordination of the activities of the

body’s many enzymes depends on maintaining a fairly

constant temperature despite the variable output of

heat by active tissues. The large thermal capacity of

water minimizes local increases in temperature at

sites where heat is evolved; flowing blood carries

heat away from active sites far more rapidly than is

possible by conduction, so that the extra heat is

WATER/Physiology 6087

shared with a larger volume of blood and other

tissues. Heat produced at active sites within the

body must ultimately be conveyed to an external

surface before it can be lost; the high heat capacity

of water allows this to be achieved with only small

changes in temperature of the blood. When the sur-

roundings of the body are warmer than the skin, heat

cannot be lost at all by conduction and convection;

but the large latent heat of vaporization of water

enables large amounts of heat to be lost to hotter

surroundings without undue losses of body fluids.

The skin is cooled by the evaporation of sweat, a

special dilute solution secreted by sweat glands that

are activated when the temperature of the blood

bathing receptors in the brain rises. Maximal

sweating may exceed 1 l h

1

, and people working or

exercising in hot environments may lose 15 l per day;

but if the latent heat of vaporization were to be 10

times smaller (as were those of some liquids Hender-

son (1913) took for comparison), 10 times as much

would have to be lost, up to three or four times the

total amount in the body in the course of a single day!

Conversely, in circumstances of extreme cold, a

modest flow of blood suffices to bring heat from

within the body to combat chilling of the extremities,

and the large latent heat of fusion of water protects

these extremities from quickly freezing.

Other Inevitable Losses of Body Water

0009 The secretion of sweat is almost entirely dictated by

the need to prevent overheating of the brain; sweating

cannot be turned off to save water. There is an equally

unavoidable loss of about 0.5 l every day in the form

of vapor from moist skin between the ducts of the

sweat glands (‘insensible’ because, unlike liquid

sweat, it is not obvious). The skin must remain

moist to retain its flexibility and avoid cracking

(with the risk of bacterial infection) with bending

and stretching. The alveolar lining of the lungs must

also be moist to permit deformation and to allow

oxygen and carbon dioxide to exchange in solution

between alveolar gas and blood. Since the expired air

is saturated with water vapor at the temperature of

the respiratory tract, while the inspired air is drier, an

inevitable loss of about 0.5 l of water by evaporation

accompanies breathing at sea level. The loss is

proportional to the volume of expired gas; mountain-

eers breathing large volumes of thin air at great

heights may lose as much as 2 l in a day. This inevit-

able loss of water vapor takes with it the latent heat of

evaporation, so that they risk severe impairment of

their physical condition from chilling as well as dehy-

dration, especially if their exertions lead to additional

losses in sweat.

0010Losses in feces are unimportant unless there is diar-

rhea. However, one very important function of body

water that can only be carried out by expending it is

the removal of soluble waste products in the urine.

About 600 mmol must be excreted every day, and this

requires 0.5 l of maximally concentrated urine, or a

minimum of 2 l if, because of impaired function, the

kidneys cannot make the urine more concentrated

than the blood.

0011Hence, everyday living involves unavoidable daily

losses of 1.5–2 l of water; since water is an essential

part of the living machinery of the cells, and effective

transport around the body requires adequate volumes

of blood to circulate and of ECF to bathe the cells,

these losses must be replaced. A small amount of

water, about 300 ml, is actually generated each day

in the cells by metabolism; solid food (which contains

as much, or, if vegetable, more water than our own

tissues) provides about a liter, and the rest must be

taken in as beverages or soups.

Maintaining the Water Balance of the

Body

0012When water is lost without solute and not replaced,

the osmotic pressure of all the body fluids increases,

and their combined volume shrinks. Osmoreceptors

in the supraoptic nucleus of the hypothalamus at the

base of the brain respond by evoking the sensation of

thirst and by releasing into the blood an antidiuretic

hormone (ADH), which causes the kidneys to con-

serve water by making the urine more concentrated (if

it is not already maximally concentrated). The subject

is encouraged to drink more watery fluid, and the

kidneys retain water in the body. If the subject drinks

more water than is required, the body fluids become

diluted, thirst fades, the release of ADH ceases, and

the kidneys rapidly excrete excess water in a copious

flow of dilute urine. (See Renal Function and Dis-

orders: Kidney: Structure and Function.)

0013Not only do the kidneys act with thirst to preserve

the total amount of water in the body, but they also

regulate the distribution between cells and ECFs. As

stated above, the ICFs are primarily solutions of po-

tassium salts, and the ECFs of sodium salts. Neglect-

ing the other ions, human ECF can be likened to a

0.154 mol l

1

(0.9%) solution of sodium chloride.

Hence, for every 9 g of sodium chloride, there is a

liter of ECF. The kidneys are controlled by a number

of factors, including the adrenal hormone, aldoster-

one, so that they retain more salt in the body when the

volume of blood is diminished. Metabolic activity

keeps the extra sodium out of the cells, in the ECF,

so that its volume, and that of the blood in the vessels,

is restored. (See Hormones: Adrenal Hormones.)

6088 WATER/Physiology