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

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Federation (IDF), International Organization for

Standardization (ISO), and Association of Official

Analytical Chemists (AOAC). In most cases there

are several other methods available for determining

the properties and composition. A few of these are

included where they are based on different principles

from the standard methods.

Physical Properties

0004 Physical properties include viscosity, specific gravity,

freezing point, thermal conductivity and heat cap-

acity, vapor pressure, surface and interfacial tension,

electrical conductivity, refractive index, light absorp-

tion, and light scattering. Most of these are usually

measured only for research purposes.

Specific Gravity

0005 The specific gravity of milk varies with its compos-

ition and is usually measured by either a standard

hydrometer (lactometer) or with a pycnometer (dens-

ity bottle). A correction table for specific gravity at

different temperatures has been calculated. If the fat

content is known, the total solids content can be

calculated using Richmond’s formula:

Total solids, Tð%Þ¼0:25G þ 1:2F þ 0:14

where G ¼lactometer reading at 60



F (1505



C) and

F ¼fat%.

Freezing Point

0006 The freezing point of milk is used to detect the add-

ition of water either accidentally during milking and

processing or fraudulently. The standard method

accepted by the World Health Organization (WHO)

uses a thermostatically controlled water bath cooled

by electrical refrigeration and a thermistor probe in

place of the mercury-in-glass thermometer of the

classical Hortvet method. Two types of instrument

are available. The first determines a plateau in the

freezing curve and the other, used for routine

screening, reads at a fixed time from the start of

freezing.

0007 In the standard method, supercooled milk is in-

duced to crystallize by mechanical vibration which

causes the temperature to rise to a plateau corres-

ponding to the freezing point. Standard solutions of

sodium chloride are used for calibration. The method

gives highly reproducible results – differences be-

tween laboratories testing the same sample of milk

should not be greater than 0.005



C. In general, 1%

of additional water in milk raises the freezing point

by about T/100, where T is base freezing point of

authenticated samples. Although dietary, daily, and

seasonal variations in the composition of milk affect

the freezing point, the osmotic pressure does not

change very much, so that the freezing point normally

remains within the range 0.530 to 0.570



C (mean

0.540



C).

Viscosity

0008At all but the lowest shear rates, milk may be con-

sidered to be a Newtonian fluid, that is, shear stress is

directly proportional to shear rate and the extrapo-

lated line passes through the origin. Three main types

of viscometer are used: capillary tube (e.g., Ostwald),

coaxial cylinder (e.g., Brookfield, Couette) and fall-

ing sphere (e.g., Hoeppler). For research, a sophisti-

cated controlled stress rheometer such as a Carrimed

or Bohlin may be used. Measured values are depend-

ent on the composition of the milk, temperature, and

pH. Simple processing, for example homogenization,

has relatively little effect. Viscosity may be measured

for research into the effects of processing or chemical

additions on the behavior of milk components.

Chemical Analysis

Proximates: Total Solids, Fat, Protein, and Ash

0009Individual methods are available for the determin-

ation of the proximates of milk. However, many

laboratories use an infrared analyzer (e.g., Foss Elec-

tric) which measures the protein, fat, and lactose

content of the milk simultaneously.

0010For fat, infrared analyzers measure the radiation

absorbed by the carbonyl groups of ester bonds in the

acylglycerols (at * 5.73 mm) or by the CH groups (at

* 3.48 mm). For protein, the absorption at 6.46 mmby

secondary amide groups is measured, and for lactose

the absorption at 9.61 mm by the hydroxyl groups. To

reduce errors the milk is first homogenized.

Total Solids

0011The total solids (TS) of milk is the mass remaining

after complete removal of water and volatiles. In the

standard method, accepted by WHO, a measured

quantity of milk is dried under controlled conditions

to constant weight. The TS is expressed as a percent-

age by mass.

Fat

0012The chemical method most often used to measure the

fat content of milk is the Gerber method. A measured

quantity of milk is added to a standard amount of

concentrated sulfuric acid in a special test bottle

(Butyrometer). Isoamyl alcohol is added to extract

the fat and then the bottle is centrifuged to separate

the aqueous and fat phases. The fat phase separates

into the graduated neck of the bottle and the

3964 MILK/Analysis

percentage of the total volume may be read off. (See

Fatty Acids: Analysis.)

001 3 The Rose–Gottlieb method is another which is used

quite often. This method gives excellent results, but

requires very careful attention to detail to achieve the

best results. The milk is treated with concentrated

ammonia and then the fat is extracted using organic

solvents. Finally, the extract is dried, weighed,

and expressed as a percentage of the original weight

of milk.

Protein

0014 A number of different methods are commonly used to

determine the protein content of milk. (See Protein:

Determination and Characterization.)

001 5 In the classical Kjeldahl method, the proteins are

‘digested’ in sulfuric acid with a catalyst (selenium,

mercury, or copper salts). An acid deposit of ammo-

nium sulfate is formed, which is then dissolved in

water. The solution is made alkaline with NaOH

and heated to distil off ammonia into excess standard

acid (sulfuric). The excess acid is back-titrated with

standard NaOH to determine the amount of ammo-

nia and hence nitrogen.

Casein protein ¼ Casein N  6:38

The total protein, casein protein, and noncasein

protein are determined simultaneously by measuring

total nitrogen (TN) and noncasein nitrogen (NCN),

after precipitation of the casein fraction at its iso-

electric point by acetic acid/sodium acetate:

Casein N ¼ TN  NCN

0016 Dye-binding methods use the ability of dyes to bind

to charged amino acid residues of the proteins. The

proteins may be separated as previously described

and then treated with the dye. Dyes used include

amido black and orange G. When they bind to the

protein they form an insoluble complex which may be

removed by centrifugation. The amount of residual

color in the supernatant is then measured by spectro-

photometry.

Individual Components

Lactose

0017 Several methods may be used to measure lactose in

milk. It can be measured enzymatically by hydrolysis

of the lactose in the presence of nicotinamide adenine

dinucleotide (NAD), which is reduced to NADH

(eqns (1) and (2)). The amount formed is determined

spectrophotometrically at 340 nm.

Lactose

!

-Galactosidase

Glucoseþ-Galactose ð1Þ

-Galactose þ NADþ

!

-Galactose dehydrogenase

Galactonic acid

þNADH þ H

ð2Þ

NADH has an absorption coefficient of 6.3 mmol

1

at 340 nm, so that the exact amount of anhyd-

rous lactose can be calculated. (See Lactose.)

0018Polarimetry may also be used if the proteins are

removed first using phosphotungstic acid. The optic-

ally active lactose rotates the plane of polarized light

in proportion to its concentration.

Ash

0019The salt content and ash content of milk are not

identical. Ash is the residue after incineration at

< 550



C and comes from both the organic and inor-

ganic constituents of the milk. Chlorides are easily

volatilized if the ashing temperature is too high, and

citrate is lost completely.

Acidity

0020This is usually measured in milk at the start of cheese-

making as the baseline for the development of acidity

by the starter bacteria which produce lactic acid.

Phenolphthalein indicator is added to a measured

quantity of milk diluted to twice its volume with

carbon dioxide-free water. This is then titrated with

01111 mol l

1

sodium hydroxide (mol l

1

/9) to a first

persistent pink color. (1 ml NaOH ¼0.0010 g lactic

acid). (See Cheeses: Starter Cultures Employed in

Cheese-making.)

Lactic Acid

0021In the IDF/AOAC standard method for lactic acid in

milk, the proteins are removed by precipitation with

phosphotungstic acid and filtration. The filtrate is

acidified with sulfuric acid and lactic acid is extracted

in a special extractor using ether. After adding water,

the ether is evaporated off and the remaining solution

neutralized. After removal of the ethanol used to

wash the solution, it is treated with ferric chloride in

hydrochloric acid and a blue color develops which

may be quantified at approximately 365 nm using a

spectrophotometer.

0022Gas–liquid chromatography (GLC) and enzymatic

methods are also used. (See Chromatography: Gas

Chromatography; Enzymes: Use in Analysis.)

Individual Proteins

0023These are measured by electrophoresis of extracts of

milk on polyacrylamide gels (PAGE) using standard

protein solutions for quantitation. The caseins are

solubilized with urea and separated by electrophor-

esis on polyacrylamide slab gels containing urea to

MILK/Analysis 3965

prevent aggregation. However, this method is not

very satisfactory for the caseins. The whey proteins

(b-lactoglobulin and a-lactalbumin) are determined

in casein-free extracts by polyacrylamide slab or

disk electrophoresis. Proteins in milk may also be

analyzed by fast protein liquid chromatography

(FPLC), in which they are separated on a column of

ion exchange resin, eluted, and detected by their

absorption of ultraviolet light.

Minerals

0024 Metallic cations (including calcium and heavy-metal

contamination) may be measured by atomic absorp-

tion spectrophotometry (AAS), in which the sample is

atomized in the gas flame and the absorption is meas-

ured at the characteristic wavelength of each element.

Calcium

0025 In the standard method for determining total calcium,

the proteins are precipitated with trichloroacetic acid

and filtrate obtained containing the soluble calcium.

This is converted to the oxalate by adding saturated

ammonium oxalate and precipitated by adding acetic

acid. The calcium oxalate is washed into water, acid-

ified with sulfuric acid, and heated over a boiling water-

bath to dissolve it. The hot solution (> 60



C) is titrated

with potassium permanganate to a first permanent pink

color. (See Calcium: Properties and Determination.)

0026 To measure soluble and colloidal calcium separ-

ately, the milk must be separated into a milk serum

fraction and a whole casein fraction by, for example,

ultrafiltration. The soluble calcium is further sub-

divided into protein-bound and ionic. Ionic calcium

may be determined in protein-free extracts by AAS.

0027 Chloride, phosphate, and other salts in milk may

be measured by various titrimetric and colorimetric

methods. (See Spectroscopy: Visible Spectroscopy

and Colorimetry.)

Phosphatase Activity

0028 The enzyme alkaline phosphatase is naturally present

in milk and, because it is sensitive to heat, it is used

to measure the effectiveness of pasteurization. Milk

is diluted with a buffer (pH 10.6) containing disodium

phenylphosphate which is hydrolyzed by the

enzyme, releasing phenol. The phenol is reacted with

2,6-dibromoquinonechloroimide, producing a color

which is measured spectrophotometrically at 610 nm.

Contaminants

Microorganisms

0029 Microorganisms in milk (bacteria, yeasts, and molds)

come from various sources: from the udder of the cow

during milking, especially if the animal has mastitis,

from contaminated equipment used to handle and

transport the milk, or from personnel, whilst others

come from the air. Hygiene control measures, such as

use of antimicrobial washes and closed systems and

keeping the milk cold, help to prevent microorgan-

isms from entering the milk and growing. Heat treat-

ment of processing equipment and of the milk reduces

the levels of pathogenic organisms. The growth of

most bacteria is reduced by the use of low tempera-

tures, but certain types, called psychrotrophs will

continue to grow at around 6



C and cause problems

when milk is stored for prolonged periods before

use. (See Adulteration of Foods: Detection; Pasteur-

ization: Principles.)

0030The same principle is used to measure most micro-

bial contamination of milk. Dilutions of milk are

carefully mixed into molten agar (40



C) containing

nutrients (purified meat extracts, salts, and a carbo-

hydrate source such as glucose) – the growth medium.

This is poured into sterile Petri dishes, cooled to

set the agar into a gel, and incubated at a suitable

temperature (e.g., 30



C). Each microorganism which

grows forms a small colony of cells in the agar. Col-

onies are counted by placing the dish over an indirect

light source and marking each one on the outside of

the dish with a felt-tip pen.

0031Total microorganisms are measured by culturing

them in a complete growth medium that provides

nutrients for all types. Specific types of microorgan-

ism are determined by using media which promote

their growth but retard or inhibit others. For

example, selective media are used to measure total

coliforms (bacteria originating in the gut of animals)

and fecal coliforms separately. To measure yeasts

and molds, an antibiotic, usually chloramphenicol,

is added to the growth medium to prevent bacteria

growing. Salmonella species are measured by isol-

ation in enrichment media before being grown,

recognized, and confirmed.

0032With the need to prevent contaminated milk from

being consumed or processed, rapid tests have been

developed. The direct epifluorescence test (DEFT) is

used routinely by some laboratories. A fluorescent

marker is attained chemically to living cells. The

marked microcolonies which grow in a short time

fluoresce in ultraviolet-light and are counted auto-

matically by an image analyzer.

Psychrotrophs

0033Psychrotrophs grow very slowly in milk at low tem-

peratures. They are measured by using a growth

medium similar to that described above but incubated

at 6.5



C for 10 days. A recent modification to the

method allows the number of psychrotrophs to be

3966 MILK/Analysis

measured after 25 h at 21



C, but other nonpsychro-

trophs may interfere, reducing the accuracy of this

method.

Antibiotics and Other Microbial Inhibitors

0034 There are at least 40 antibiotics and sulphonamides

which may occur in milk. They can be detected and

identified by both microbial and chemical methods.

0035 Microbial methods An inhibitory substance in the

milk will prevent or retard the growth of test bacteria

(Bacillus stearothermophilus, Streptococcus thermo-

philus, B. subtilis, Sarcina lutea, Lactobacillus bul-

garicus, B. megaterium, etc.). In the disk-assay plate

method, a disk of absorbent paper impregnated with

the milk sample to be examined is placed on the

surface of an agar plate preinoculated with a suitable

test organism. When incubated, the growth of the

bacteria causes the agar to become cloudy. The

presence of inhibitory substances such as antibiotics

which diffuse into the agar is indicated by a clear zone

around the edge of the disk. The inhibitor may be

identified by adding an agent that blocks its action.

For example, the effect of penicillin is prevented by

the addition of penicillinase; p-aminobenzoic acid

blocks sulphonamides. With such substances in the

growth medium, the antibiotic no longer prevents the

test organism from growing.

0036 Commercially available methods include Interest,

Delvotest, and TTC.

0037 On the farm, a Charm Farm Test kit may be used.

The test organism is inoculated in tablet form into

liquid medium in a testtube with a sample of the milk.

An indicator in the buffer changes color if the bacteria

are able to grow.

0038 Chemical methods Inhibitory substances are ex-

tracted from the milk and then detected chemically

by thin-layer chromatography (TLC), high-perform-

ance liquid chromatography (HPLC), fluorescence

liquid chromatography, gas chromatography (GC),

electrophoresis, bioautoradiography, enzyme-linked

immunosorbent assay (ELISA), radioimmunoassay,

enzymatic methods, and other forms of immuno-

logical methods. They are usually more specific than

the microbiological methods. (See Chromatography:

Thin-layer Chromatography; High-performance

Liquid Chromatography; Immunoassays: Principles;

Radioimmunoassay and Enzyme Immunoassay.)

Organic Residues

0039 Organic residues occur in milk either through the diet

or by contamination of the milk directly. Pesticide

residues, polychlorinated biphenyls (PCBs), and

dioxins are all toxic and tend to accumulate in the

body. PCBs are used in a very wide range of industrial

processes and products; dioxins are a byproduct of

many chemical processes, including incineration of

chlorine-containing materials at low temperatures.

Pesticide Residues

0040There is considerable difficulty in identifying pesti-

cide residues in milk because they may be present in

very low concentrations. To measure organochlorine

(e.g., DDT) and organophosphorus (e.g., bromophos,

malathion) residues, the fat is first extracted from

the milk by a suitable solvent. The residues are then

extracted from this into acetonitrile and diluted with

water. The residues are extracted into petroleum

ether, purified through a column of fluorosil, and

eluted with petroleum ether and diethyl ether. The

residues in the eluate are measured by GC and identi-

fied by a combination of GC and TLC. This is tending

to be superseded by the more sensitive combination of

GC with mass spectroscopy (GC-MS). (See Pesticides

and Herbicides: Residue Determination.)

Polychlorinated Biphenyls

0041Two similar methods are used to measure PCBs in

milk. In the first they are isolated by solvent extrac-

tion along with organochlorine pesticides and

measured by GLC. In the second method they are

extracted with solvents and purified on alumina or

on fluorosil columns before determination by GC and

GC-MS.

Dioxins

0042After treatment with ammonia and ethanol, the fat

phase containing the dioxins is extracted into ether/

pentane, then dried and concentrated. It is cleaned up

by passing it through a series of columns of base

silica, alumina, and activated carbon. The eluate is

analyzed by high-resolution GC-MS. The method is

calibrated by using standard dioxin labeled with

carbon-13.

Radionuclides

0043Radionuclides which arise in milk are either naturally

occurring or come from nuclear accidents such as

the Chernobyl reactor disaster. Strontium-89 and

strontium-90 (

Sr,

Sr), cesium-137 (

137

Cs), and

iodine-131 (

131

I) are probably the most important

contaminants. Strontium is absorbed into bone,

cesium mainly into muscle, and iodine only into the

thyroid. These radionuclides are measured by scintil-

lation counting of their radioactivity. Samples have to

be enriched, unless the contamination is extremely

high, to get counts which are significantly above

background level. Scintillation counting consists of

MILK/Analysis 3967

placing a clarified sample into a special glass bottle

with a reagent (scintillant) which absorbs the

radiation and emits light in proportion. The light is

detected by a sensitive photomultiplier and expressed

as counts per unit time or time to standard count. (See

Radioactivity in Food.)

Quaternary Ammonium Compounds (QACs)

0044 These residues from cleaning agents and other fluids

containing surfactants (e.g., cetylpyridinium chlor-

aide (CPC) and cetyltrimethylammonium bromide

(CTAB)) may have bacteriostatic effects or cause

flavor taints. To determine QACs in milk, they are

first extracted into solvent (1,1,2,2-tetrachloro-

ethane) and then detected by the formation of a

pink color with dioctyl sodium sulfosuccinate (Aero-

sol T) which is quantified by spectrophotometry.

Species Identification

0045 Identification of the source of milk is generally used

to detect the mixing of bovine milk into nonbovine

milk and vice versa. Various methods have been tried,

including TLC, GLC, and HPLC of the triacylglycer-

ols. However, these methods are not very reliable

because the identification of the species of milk pre-

sent depends on measuring ratios of the fatty acids in

the mixture. Improvements in HPLC methodology

are helping to overcome some of these difficulties.

0046 The proteins provide a better means of identifying

the sources of the milks. The a

-casein in bovine

milk has a higher electrophoretic mobility than the

equivalent protein from other species. Therefore, in

principle, simple electrophoresis on polyacrylamide

gel should be sufficient to analyze the mixture. This

is not very reliable, and better results can be obtained

by more sophisticated electrophoretic methods such

as isoelectric focusing and immunoelectrophoresis.

The position and intensity of the electrophoretic

bands gives a measure of the identity and amount of

protein present. The availability of antisera to total

milk proteins, and more recently the availability of

monoclonal antibodies to specific proteins, means

that exact identification of the origins of proteins in

mixed milks is now relatively easy. Antibodies to

particular proteins are allowed to react with the

milk proteins. When the antibody binds to its antigen

it normally forms a line or band of precipitation, the

position and extent of which may be used to identify

and quantify the protein present. Methods that com-

bine electrophoresis with immunoreaction, such as

rocket electrophoresis, give the most precise results.

See also: Adulteration of Foods: Detection; Calcium:

Properties and Determination; Cheeses: Starter Cultures

Employed in Cheese-making; Contamination of Food;

Enzymes: Uses in Analysis; Fatty Acids: Properties;

Analysis; Immunoassays: Principles;

Radioimmunoassay and Enzyme Immunoassay; Lactose;

Pasteurization: Principles; Radioactivity in Food; Trace

Elements; Triglycerides: Structures and Properties;

Vitamins: Overview

Further Reading

Andrews AT (1986) Electrophoresis. Theory, Techniques

and Biochemical and Clinical Applications, 2nd edn.

Oxford: Oxford University Press.

Association of Official Analytical Chemists (1990) Official

Methods of Analysis (OMA), 15th edn. Arlington,

Virginia: Association of Official Analytical Chemists

(AOAC).

International Dairy Federation (1990) Inventory. Methods

of Analysis for Milk and Milk Products. Bulletin of

the International Dairy Federation no. 248, 3rd edn.

Brussels: IDF.

International Dairy Federation (1991) Detection and Con-

firmation of Inhibitors in Milk and Milk Products.

Bulletin of the International Dairy Federation no. 258.

Brussels: IDF.

Marth EH (ed.) (1978) Standard Methods for the Examin-

ation of Dairy Products, 14th edn. Washington, DC:

American Public Health Association.

Ribadeau-Dumas B and Grappin R (1989) Milk protein

analysis. Le Lait 69: 357–416.

Royal Society of Chemistry (1984) Challenges to Contem-

porary Dairy Analytical Techniques. Special publication

no. 49. London: Royal Society of Chemistry.

Dietary Importance

J Buttriss, British Nutrition Foundation, London, UK

Introduction

0001Milk has been a component of the diet in many parts

of the world for thousands of years. This article will

review the nutritional value of milk, trends in milk

consumption in the UK, and the role of milk in the

diet of different population groups.

Nutrient Composition

0002Milk can be described as one of the most nutritionally

complete foods. It provides a wide range of essential

nutrients, in particular protein and a range of vita-

mins and minerals (Table 1). It is, however, a poor

source of iron and vitamin D, and contains no starch

3968 MILK/Dietary Importance

or dietary fiber. By volume, water is the major con-

stituent of milk, comprising just over 87%. The re-

mainder consists of milk fat and solids-nonfat (SNF).

Protein

0003 The principal proteins found in milk are casein, lac-

talbumin, and lactoglobulin. Milk protein has a high

biological value since it contains all of the eight essen-

tial amino acids, which need to be provided by diet. In

addition, milk can improve the overall protein quality

of a meal when consumed with foods of lower protein

quality such as cereals and pulses. (See Amino Acids:

Metabolism.)

0004 Considerable interest has developed in a number of

the peptides present in milk. It is now clear that cows’

milk contains a number of biologically active peptide

components that may prove to be of relevance to

human health. Much of the research in this area has

sought to identify and characterize components that

influence the immune system.

Carbohydrate

0005Carbohydrate in milk is in the form of lactose, a

disaccharide comprising a molecule of glucose and a

molecule of galactose. This sugar is found naturally

only in milk and is much less sweet than sucrose.

0006In the small intestine, lactose is digested by the

enzyme lactase to its two component monosacchar-

ides. This enzyme is present in babies and young

children. In some adults, mainly noncaucasians,

enzymatic activity can decrease, making milk in

quantity less well tolerated. Such individuals are de-

scribed as being lactose-intolerant. Most can tolerate

small quantities of milk, and fermented milk products

tbl0001 Table 1 Nutrient composition (per 100 g) of pasteurized milk in the UK

Whole milk Skimmed milk Semiskimmed milk Channel Islands

milk

Energy

(kcal) 66 33 46 78

(kJ) 275 140 195 327

Protein (g) 3.2 3.3 3.3 3.6

Carbohydrate (g) 4.6 4.8 4.8 4.6

Sugars (g) 4.6 4.8 4.8 4.6

Fat (g) 3.9 0.1 1.6 5.1

Saturates (g) 2.4 0.06 1.0 3.3

Monounsaturates (g) 1.1 Trace 0.5 1.3

Polyunsaturates (g) 0.1 Trace Trace 0.1

Sodium (mg) 55 55 55 54

Dietary fiber (g) Nil Nil Nil Nil

Vitamin A (mg) 56 1 23 58

Thiamin (mg) 0.04 0.04 0.04 0.04

Riboflavin (mg) 0.17 0.18 0.18 0.19

Nicotinic acid (mg) 0.08 0.09 0.09 0.07

Potential nicotinic acid from tryptophan (mg) 0.75 0.78 0.78 0.85

Vitamin B

(mg) 0.06 0.06 0.06 0.06

Folic acid (mg) 6 6 6 6

Vitamin B

(mg) 0.4 0.4 0.4 0.4

Pantothenic acid (mg) 0.35 0.32 0.32 0.36

Biotin (mg) 1.9 2.0 2.0 1.9

Vitamin C (mg) 1 1 1 1

Vitamin D (mg) 0.03 Trace 0.01 0.03

Vitamin E (mg) 0.09 Trace 0.03 0.11

Calcium (mg) 115 120 118 130

Chloride (mg) 100 100 100 100

Copper (mg) Trace Trace Trace Trace

Iodine (mg) 15 (15) (15) N

Iron (mg) 0.05 0.05 0.05 0.05

Magnesium (mg) 11 12 11 12

Phosphorus (mg) 92 95 95 100

Potassium (mg) 140 150 150 140

Selenium (mg) 1 (1) (1) (1)

Zinc (mg) 0.4 0.4 0.4 0.4

N, no reliable information available; values in parentheses, estimated value.

From Holland B, Unwin ID and Buss DH (1989) Milk Products and Eggs. McCance and Widdowson’s The Composition of Foods, suppl, 4. London: Royal

Society of Chemistry and Ministry of Agriculture, Fisheries and Food.

MILK/Dietary Importance 3969

appear to be better tolerated. In the UK, lactose

intolerance is relatively rare in people of European

descent but is more common in those of Asian,

Far Eastern, and African descent, particularly first-

generation members of these ethnic groups. (See

Carbohydrates: Metabolism of Sugars; Food Intoler-

ance: Lactose Intolerance.)

Fat

0007 The fat in milk is in the form of minute droplets which

rise to the top when milk is left to stand. The principal

component of milk fat is triglyceride (triacylglycerol),

three fatty acids joined to a glycerol backbone. All

triglycerides contain mixtures of three types of fatty

acids: saturated, monounsaturated, and polyunsatur-

ated. The contribution of these three types of fatty

acid to milk fat in the UK is 61% saturated, 28%

monounsaturated, and 3% polyunsaturated. The per-

centages do not add up to 100% because milk fat is

not composed totally of fatty acids. In the UK, whole

cows’ milk typically contains 3.9 g of fat in every 100 g

of milk; semi-skimmed has 1.6 g per 100 g, and

skimmed milk has less than 0.1 g per 100 g. The fatty

acid profile remains the same. Milk fat contains small

amounts of the two essential fatty acids, a-linoleic acid

(1.4 g per 100 g fatty acids) and linolenic acid (1.5 g

per 100 g fatty acids). (See Fatty Acids: Properties;

Triglycerides: Structures and Properties.)

0008 There is growing interest in a fatty acid found in

milk known as conjugated linoleic acid (CLA). It is a

polyunsaturated fatty acid with one trans bond, and

is a product of rumen fermentation and so is present

only in the meat and milk of ruminant animals. Work

on rodents has provided evidence to support an

inhibitory effect of CLA against mammary tumors,

with some evidence for a similar effect against colon

tumors. This work is supported by in vitro evidence

using human tumor cell lines. CLA has also been

shown to inhibit the accumulation of fat during

growth in small and large animal models, and there

is some evidence of a protective effect in cardiovascu-

lar disease. If the health benefits suggested in animal

studies can be shown to apply to human populations,

the consequences of advice to reduce fat intake may

have important public health implications.

Vitamins

0009 All of the known vitamins are present in whole milk

(Table 1), although some are present in small quantities.

0010 The fat-soluble vitamins – A, D, E, and K – are

removed with the fat when milk is skimmed.

Consequently, they are present in only trace amounts

in skimmed milk and in reduced amounts in semi-

skimmed milk. Whole milk is a good source of vitamin

A, a pint (560 ml) providing 47% of the adult male

and 55% of the adult female UK reference nutrient

intake (RNI). Refer to individual nutrients.)

0011All of the three major types of cows’ milk (whole,

semi-skimmed, and skimmed) are good sources of

riboflavin (vitamin B

) and vitamin B

. Where the

overall diet is poor, milk can also provide useful

amounts of thiamin (vitamin B

), nicotinic acid, and

ascorbic acid (vitamin C).

0012Heat treatment can have an effect on the vitamin

content of milk. Of the heat-treated milks, fresh

pasteurized milk has the highest vitamin levels. Milk

which has undergone the ultraheat treatment (UHT)

process keeps for longer but the higher temperature

used in the processing results in lower levels of some

vitamins, particularly vitamin B

, vitamin C, and

folate. The sterilization process used for milk has a

somewhat greater effect and levels of riboflavin, vita-

min B

, and pantothenate will be lower than in milk

heat-treated by the other processes. Vitamin C and

folate are virtually absent from sterilized milk.

(See Heat Treatment: Ultra-high Temperature (UHT)

Treatments.)

0013Some loss of vitamins is inevitable when milk is

stored. Milk exposed to bright sunlight on the door-

step for several hours can lose up to 70% of its

riboflavin. Vitamin C levels also fall under these con-

ditions, and measures should be taken to limit such

exposure. There will also be gradual losses of folate

and vitamin B

from UHT and sterilized milks, even

under ideal storage conditions, because of reactions

with small amounts of oxygen remaining in the pack

or bottle. Boiling milk also reduces its vitamin con-

tent, ranging from a 5% reduction in vitamin B

to a

50% reduction in vitamin C.

Minerals

0014Milk makes a contribution to human needs for virtu-

ally all the minerals and trace elements known to be

essential for health. These are often present in a form

which is well absorbed and utilized by the body (high

bioavailability), e.g., calcium and zinc. For most

people in the western world, milk and milk products

are a major source of calcium. In the UK, the National

Food Survey, conducted by the Ministry of Agricul-

ture, Fisheries and Food (now DEFRA), indicates that

55% of the calcium in the typical British diet is contrib-

uted by milk and milk products. Milk alone contrib-

utes about 40% of the total. Although the contribution

to zinc requirements made by milk is relatively low

compared with meat, the major contributor, the zinc

is in a highly bioavailable form and there is evidence

that the combination of milk (or meat) with vegetable

foods, in which the zinc is less bioavailable, can en-

hance the bioavailability of zinc from the whole meal.

(See Bioavailability of Nutrients.)

3970 MILK/Dietary Importance

Seasonal and Breed Variations

Seasonal

0015 The nutritional value of milk is influenced by the cow’s

diet. Milk produced in summer has a slightly different

nutritional composition from that produced in winter.

For most of the grazing season (spring), fresh grass can

provide all the nutrients the cow needs. However, in

some circumstances, e.g., where a cow has a high milk

yield, additional food in the form of high-energy pellets

is provided. When grazing, a cow may eat up to 70 kg

(150 lb) of grass each day. The quality of grass is

therefore an important factor in milk production.

0016 The main diet during winter is grass conserved

during the summer in the form of either hay (sun-

dried grass) or silage (‘pickled’ grass). This is always

less nutritious than fresh grass, so it is necessary to

provide additional rations, sometimes in the form of

concentrates, usually during milking. Alternative

components of the winter diet include sugar beet

pulp, maize gluten, or treated straw. Kale, cabbage,

forage maize, or fodder beet are often specially grown

for winter feeding.

0017 As a result of the seasonal changes in diet, levels of

vitamin A, b-carotene, and vitamin E are higher in

summer milk. Iodine levels are higher in winter than

in summer. Other nutrients remain the same.

0018 The stage in the cow’s lactation also has an effect

on milk composition. Fifty days into the lactation, the

lactose content of the milk starts to fall, and the fat

and protein contents rise. However, since milk from

individual cows at different stages of lactation is

combined, these changes in composition are not

reflected in seasonal changes in milk purchased.

Breed

0019 Some breeds, such as Friesian and Holstein, are noted

for producing large quantities of milk. Others, such as

Guernsey and Jersey cows, are noted for the high fat

content of their milk (5.1 g per 100 g compared with

3.9 g per 100 g). Other nutrients, including protein,

calcium, phosphorus, carotene, and riboflavin, are

also present in slightly higher concentrations in milk

from Guernsey and Jersey cows, known as Channel

Islands milk. Semiskimmed Channel Islands milk is

also available in the UK and retains its slightly higher

calcium content compared with ordinary whole milk.

0020 As with milk from Friesian and Holstein cows,

Channel Islands milk shows seasonal variations in

retinol, carotene, and vitamin E content.

Health Implications of Raw Milk

0021 The hygienic quality of raw (untreated) milk is very

high in the UK. This is largely due to the introduction

of a centralized testing scheme by the dairy industry

in the early 1980s and a payment system to farmers

based on the hygienic and compositional quality of

raw milk produced on the farms.

0022Less than 1% of milk does not receive any form

of heat treatment. In 1985, legislation reduced the

availability of raw milk by prohibiting sales through

shops, hotels, schools, and other catering establish-

ments. Direct sales to the public are still allowed in

England, Wales, and Northern Ireland. Raw milk has

not been available in Scotland since 1983. It can be

produced only by a farm holding a licence from

OEFRA; it must be clearly labeled ‘this milk has not

been heat-treated and may therefore contain organ-

isms harmful to health.’ The address at which it was

bottled or cartoned must appear on the container.

0023The Chief Medical Officer has advised that young

children, pregnant women, elderly people, and those

who are currently unwell or have a chronic illness

should not consume raw cows’ milk.

Trends in Milk Consumption

0024According to National Dairy Council figures, in 1990

weekly per capita consumption in the UK was 2.3 l

(4.1 pints) per week, and had been falling steadily

over the previous 10 years, as indicated in Table 2,

which uses figures from a different source. The trends

in the 1990s are shown in Figure 1.

Growth in Low-Fat Milks

0025There has been continued growth in purchases of

low-fat milk in the UK. Most of this growth has arisen

via increased sales of semiskimmed milk (Figure 2).

tbl0002Table 2 Estimated liquid milk consumption (pints per person

per week)

April to March England

and Wales

Scotland Northern

Ireland

1959–1960 4.85 4.35 NA NA

1964–1965 4.94 4.38 5.05 4.89

1969–1970 4.80 4.24 5.09 4.75

1974–1975 4.73 4.58 4.80 4.73

1978–1979 4.50 4.45 4.93 4.52

1979–1980 4.42 4.40 4.91 4.45

1980–1981 4.31 4.35 4.72 4.35

1981–1982 4.29 4.36 4.66 4.31

1982–1983 4.21 4.35 4.66 4.24

1983–1984 4.21 4.31 4.69 4.28

1984–1985 4.16 4.25 4.63 4.21

1985–1986 4.13 4.25 4.59 4.19

1986–1987 4.08 4.15 4.58 4.12

1987–1988 4.05 4.11 4.47 4.05

1988–1989 4.00 4.08 4.43 4.02

NA, not available.

From Milk Marketing Board. Dairy Facts and Figures. Thames Ditton,

Surrey: Milk Marketing Board.

MILK/Dietary Importance 3971

A similar pattern is seen in many other western

countries.

Importance of Milk for Different Age

Groups

0026 As part of a balanced diet incorporating a variety of

foods, milk can make a useful contribution to require-

ments for a number of nutrients. It is particularly valu-

able in the diets of certain groups of the population.

Infants and Preschool Children

0027In discussing milks suitable for children, the 1994

report, from the government’s Committee on Medical

Aspects of Food Policy, makes the following recom-

mendations:

3.75

3.8

3.85

3.9

3.95

4.05

4.1

4.15

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

fig0001 Figure 1 Estimated total liquid milk consumption in the UK from 1989 to 1998. Values given are pints per head per week. Reproduced

from National Dairy Council (1999) Dairy Facts and Figures. London: NDC, with permission.

1986

1988

1990

1992

1994

1996

1998

fig0002 Figure 2 Percentage household milk purchases by fat content in England and Wales from 1986 to 1998. Open columns, whole milk;

filled columns, semiskimmed milk; cross-hatched columns, skimmed milk. Reproduced from National Dairy Council (1999) Dairy Facts

and Figures. London: NDC, with permission.

3972 MILK/Dietary Importance

.0028 There are advantages in continuing breast milk or

infant formula during the first year. Cows’ milk

should not be introduced as the main drink during

the first year of life. (See Infants: Breast- and Bottle-

feeding.)

0029 However, from 6 months of age, whole cows’ milk

can be introduced in small quantities as part of the

weaning diet, e.g., to mix cereals. It is important

with all milks, but particularly with cows’ milk,

which is low in iron, that care is taken to insure

that adequate iron-containing weaning foods are

also provided. Suitable foods include those based

on lean meat, fortified infant cereals, eggs, pulses,

and green leafy vegetables. (See Infants: Weaning.)

0030 Skimmed milk is not suitable for children under 5

years, and semi-skimmed milk should not be given

to children under 2 years of age because both milks

are lower in energy and vitamin A.

0031 Semiskimmed milk can be introduced as the main

drink at age 2 years, provided that the child has a

good appetite and is eating a wide range of foods.

0032 A pint of milk per day is recommended by many

health professionals for the under-fives. (See Chil-

dren: Nutritional Requirements.)

Older Children

0033 The report on Dietary Reference Values (1991), from

the government’s Committee on Medical Aspects of

Food Policy, advises that children over 2 years and

adults should make attempts to reduce their fat intake,

especially intake of saturates, such that the proportion

of food energy derived from fat is reduced from its

current level of 42% to an average of 35% and intake

of saturates is reduced to an average of 11% of food

energy. At the same time, the reduction in fat intake

should be compensated for by an increase in starchy,

fiber-rich foods. Replacing whole milk with skimmed

or semiskimmed milk is one of the ways in which fat

intake can be reduced. Other ways include reduction

in fried food, choosing leaner meat, using less spread-

ing and cooking fat/oil, and cutting down on cakes,

pastries, and fatty snack foods.

0034 For children and teenagers, milk is a nutritious

component of the diet and a major source of calcium,

which is essential for the development of a strong

skeleton.

0035 School milk schemes exist in the UK and the rest of

the European Community which enable primary

schoolchildren to receive milk at a subsidized price

whilst at school. A nursery school scheme also exists

in the UK, providing free milk.

Pregnancy and Lactation

0036 During pregnancy there is no increment in the advised

daily calcium intake in the UK (700 mg) as the

efficiency of absorption is known to increase. How-

ever, it is important that all pregnant women insure

that they obtain an average of 700 mg per day

(equivalent to a pint of milk). During lactation, cal-

cium needs are covered by a daily intake of 1250 mg

per day (1350 mg per day for pregnant teenagers).

Milk and milk products are a useful way of obtaining

the extra calcium. (See Lactation: Human Milk:

Composition and Nutritional Value.)

Slimmers

0037Calcium intakes are frequently low among slimmers.

This is avoided if skimmed milk, with its lower energy

content, is included in the diet. A pint provides over

700 mg calcium, the reference daily intake for

women.

Vegetarians

0038Milk is acceptable to most vegetarians and can make

a useful contribution to protein quality. Whole milk is

very important in the diet of young vegetarian chil-

dren. Such diets can be bulky for the young child, and

whole milk is a compact source of energy and nutri-

ents in an easily absorbed form. Milk will also make a

valuable contribution to vitamin B

intake, which

can be low in the diet of vegetarians who exclude

eggs. (See Vegetarian Diets.)

Convalescents

0039During periods of illness and convalescence, appetite

can be poor. Although energy requirement may be

lower, essential nutrients are still required. Milk is a

useful and versatile food at this time.

Elderly People

0040Milk is a valuable food for the elderly, who may also

have a reduced appetite. Milk provides many essen-

tial nutrients in an easily assimilated form. Whole

milk provides small amounts of vitamin D, which

may be particularly beneficial for the housebound

elderly who are unable to get out in the sunlight.

See also: Amino Acids: Metabolism; Bioavailability of

Nutrients; Carbohydrates: Metabolism of Sugars; Fatty

Acids: Properties; Food Intolerance: Lactose

Intolerance; Heat Treatment: Ultra-high Temperature

(UHT) Treatments; Infants: Nutritional Requirements;

Breast- and Bottle-feeding; Weaning; Lactation: Human

Milk: Composition and Nutritional Value; Triglycerides:

Structures and Properties; Vegetarian Diets