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phenomenon of aggregation/dehydration of the case-

ins caused by heating at the isoelectric point (pH 4.6).

Certain hydrocolloids (e.g., pectins, alginate, carb-

oxymethyl cellulose, or prophylene glycol) are nega-

tively charged, and, when added to yogurt before the

heat treatment stage, they interact with the positive

charges of the casein below its isoelectric point, and

the separation of the two phases in the product is

avoided. (See Casein and Caseinates: Methods of

Manufacture.)

Concentration

0005 Concentrated or strained yogurt is a popular product

in the Middle East region. The product is known as

labneh, labaneh, or lebneh (in most Arab countries),

mast or mastou (in Iraq), leben zeer (in Egypt), tan or

than (in Armenia), Greek yogurt or Greek-style yogurt

(in the UK), stragisto or tzatziki (in Greece), torba or

suzme (in Turkey), basa or zimne (in the Balkans),

and yogurt cheese (in some parts of the world). Other

closely related products, which are produced in dif-

ferent countries and using mesophilic, thermophilic,

and/or lactose-fermenting yeast starter cultures, are

ymer (in Denmark), chakka and shirkhand (in India)

and skyr (in Iceland).

0006 The traditional and rural method of production

consists of straining cold and unsweetened natural/

plain yogurt using a cloth bag, animal skin, or

earthenware vessel. The method, in comparison

with large, factory-scale operations, is slow, labor-in-

tensive, unhygienic, and cumbersome, and gives low

yields due to the residues left in the bag. Typical com-

positional standards for labneh (e.g., in Saudi Arabia

and Lebanon) range between 22 and 26 g 100 g

1

total

solids and 7 and 10 g 100 g

1

fat, respectively.

0007 The factory-scale production of concentrated or

strained yogurt includes the methods described

below.

Traditional Process

0008 The cold natural/plain yogurt is stirred and emptied

into cloth bags of about 25 kg. The bags are stacked

on top of each other in a vertical press that is located

in a refrigerated room. Pressure is applied in order to

assist in whey drainage for a duation of 12–18 h. On

the following day, the concentrated product is emp-

tied into a mixing bowl to obtain a uniform texture

prior to packaging. Alternatively, long, horizontal

filter cloths can be used; the long sides are supported

on poles and gently oscillated up and down, while

slight lateral pressure is applied. This method of pro-

cessing is known as the modified ‘Berge’ system, and

was developed in France in the mid-1960s for the

production of fresh curd cheese.

0009The application of more pressure and a longer

dewheying stage will yield a product that contains

30 g 100 g

1

total solids, and is known as ‘yogurt

cheese.’ This highly concentrated product can be

shaped into balls by hand, placed in a jar, and pre-

served in oil. In Lebanon, mainly goat’s milk is used

for the production of yogurt cheese, which is known

locally as ‘labneh anbaris.’ Herbs and spices are

added to the curd after concentration, and before it

is shaped into balls; the product is referred to as

‘shanklish’ (in the Lebanon).

Mechanical Separators

0010The production of strained yogurt by centrifugation

of heated yogurt has been used successfully in experi-

mental trials and commercially in different countries;

concentration is achieved using a nozzle or Quarg

separator. Skimmed milk should be used for the

manufacture of the yogurt, and the fermented milk

is stirred vigorously, heated to about 60



C, cooled to

about 40



C and concentrated to 18 g 100 g

1

total

solids, cooled to about 12



C, blended with cream,

and finally packaged. If whole milk is used instead,

the nozzles of the separator will clog. However, recent

developments in the design of the centrifugal separ-

ators have made it feasible to use fermented whole

milk for the production of strained yogurt.

0011After acidification (i.e., pH 4.6–4.8), the fermented

milk is heated to 60



C to inactivate the culture

and control the level of acidity, and then deaerated

for 15–20 min to assist the separation of whey in the

separator. A centrifugal pump transports the fer-

mented milk through a switchable double strainer to

break up any lumps before it enters the separator. The

concentrated product leaving the separator is blended

with cream and seasoning (e.g., salt, herbs or fruit

flavors – optional), cooled, and packaged. A typical

chemical composition for strained yoghurt is 24 g

100 g

1

total solids and 9.6 g 100 g

1

fat (about 40%

fat-in-dry-matter); the composition of the whey is 6.1 g

100 g

1

total solids, consisting mainly of lactose and

minerals, but also about 0.5 g 100 g

1

fat. Capacities

of such separators are up to 6.5 tonnes h

1

, depending

on the composition of the milk used and the acidity of

the fermented milk prior to concentration.

tbl0001 Table 1 Categories of pasteurized/UHT yogurts

Temperature Packaging Shelf-life/storage

Pasteurization (low) Hot 2–3 weeks/cold

Pasteurization (high) Cold/aseptic 1–2 months/cold

UHT Hot/aseptic Several months/ambient

6260 YOGURT/Yogurt-based Products

Ultrafiltration (UF)

0012 Two different systems of UF have been used for the

production of strained yogurt. Firstly, the standard-

ized milk is concentrated by UF to the desired solids

content before homogenization, heat treatment, and

fermentation, and secondly, the warm yogurt (about



C) is concentrated by UF. (See Filtration of

Liquids.)

0013 In the former system of production, the UF retentate

is fermented in the retail container – as with the manu-

facture of natural set yogurt – and the firmness of the

product is much greater than that of products made

using the traditional (cloth bag) method or by the UF

of warm yogurt. However, when the product is broken

and smoothed by passing it through a lactic curd hom-

ogenizer, the strained yogurt showed signs of whey

syneresis, and an excessive reduction in its firmness is

observed. These faults are not evident in the other

types of strained yogurt, and the product should be

processed differently in order to overcome such faults.

0014 The quality of strained yogurt made by UF of warm

yogurt closely resembles the traditional product in

terms of elasticity, firmness, and structure. The stages

of manufacture consist of: (1) production of yogurt

from whole milk, (2) UF of yogurt at 35–45



C using

batch or multistage systems, (3) partial cooling to



C, fruit blending (optional), and packaging, and

(4) cooling in the retail container to < 10



C. No

fat losses are observed in the permeates, and the

solids content consists almost exclusively of lactose,

organic acids, minerals and very slight losses of low-

molecular-weight nonprotein nitrogen.

0015A wide range of UF plants are available on the

market that can be used for the production of strained

yogurt on a large scale. A typical example is illustrated

in Figure 1, and according to the supplier, the manu-

facturing process is as follows. The standardized milk

(e.g., 12.4 g 100 g

1

total solids and 3.5 g 100 g

1

fat)

is preheated to 60



C, homogenized at 14.7 MPa,

heated in a plate heat exchanger (PHE) to 95



C for

5 min in a holding tank before cooling to 40–45



Cin

the regeneration section of the PHE. After the fermen-

tation period, the warm yogurt is heated at 58–60



for 3 min in the PHE, cooled to 40



C, concentrated in

a two- to four-stage UF plant, cooled in a plate cooler

to about 20



C, and finally packaged. The degree of

concentration of yogurt using a four-stage UF plant,

for example, could be adjusted to give 14, 16, 19, and

22 g 100 g

1

total solids, respectively.

Product Formulation

0016It is feasible to manufacture strained yogurt from

recombined dairy ingredients. The process involves

reconstitution of powder(s) (whole, skimmed milk,

retentate, and/or caseinate) in water and blending

it with anhydrous milk fat/butter oil, stabilizer (e.g.,

Cremodan Mousse 31 (Danisco Ingredients (UK)

Ltd.)), and salt (optional). The recombined milk is

Permeate

Standardized

yogurt milk

Culture

fig0001 Figure 1 Simplified flow chart for the manufacture of strained yogurt by UF: 1, fermentation tanks; 2, plate heat exchanger; 3, two- to

four-stage UF plant; 4, plate cooler; 5, buffer tank; 6, fruit tank (optional); 7, in-line mixer; 8, packaging machine. Reproduced courtesy

of Tetra Pak (Processing System Division) A/B, Lund, Sweden.

YOGURT/Yogurt-based Products 6261

handled and processed in a similar way to the pro-

duction of yogurt. After the fermentation stage, the

product is precooled to about 20



C and packaged,

and the final cooling to 5



C takes place in the cold

store. Typical compositions of full- and low-fat

strained yogurts are given in Table 2.(See Recom-

bined and Filled Milks.)

0017 At present, there are no compositional standards

for this product in the UK. However, Table 3 illus-

trates the variation in the chemical composition of

leading brands of strained yogurt that are marketed in

the UK.

Freezing

0018 Frozen yogurt resembles icecream, in that the fresh

stirred yogurt is stabilized, fortified with fruit base

(syrup or pieces), whipped, and frozen. In general, the

product is classified into three main categories: soft,

hard, or mousse. The recommended ratio of yogurt to

fruit base for the manufacture of soft and hard frozen

yogurts is 80:20 and 65:35, respectively. The outlet

freezer temperature of these two products is 6



and the storage temperature for soft and hard frozen

yogurts is 6



C and 25



C, respectively. Although

air is normally used at the whipping/freezing stage, a

longer-shelf-life product can be achieved by using

nitrogen rather than air.

0019The chemical composition of frozen yogurt may

differ from one manufacturer to another, or between

countries. This product is very popular in the USA,

and the variation in the chemical composition of a

number of commercial samples is shown in Table 4.

Drying

0020The objective of manufacturing powdered yogurt is

to produce a product that is stable during prolonged

storage and readily utilizable. The stages of manufac-

ture depend on the method of drying used.

0021Traditionally, natural/plain yogurt or, in some

instances, strained yogurt is mixed with ‘burghol or

bourghoul’ (i.e., parboiled cracked wheat) in a ratio

of 4:1. The mixture is kneaded daily for around a

week, salted, passed through a meat mincer, shaped

into rolls, and placed in the sun to dry. The dried

product is called ‘kishk’, and it is either sold as rolls

or ground into flour. Kishk as a dish is prepared by

rehydrating the dried product with water and then

simmering the mix gently over a fire. The product is

rather thick, similar to porridge. Chopped onions,

coriander, and/or tomatoes can be added to the

reconstituted ‘kishk,’ and such a dish is normally

consumed with bread. (See Fermented Milks: Types

of Fermented Milks.)

0022Other types of dried, fermented dairy product are

known in the Arabian peninsula as ‘oggtt’ or ‘mad-

deer’ or ‘buqle’. The desert dwellers allow goat’sor

camel’s milk to sour naturally, churn it to make

butter, and then boil the buttermilk until it thickens.

When cooled, the concentrated buttermilk is shaped

into small cakes and left in the sun to dry.

0023At present, powdered yogurt, which is sometimes

known as dried or instant yogurt, is manufactured in

relatively small quantities using normal driers. Some

of the manufacturing techniques have been patented,

and the dried product is aimed towards the do-it-

yourself consumer market, the baby food manufac-

turers, and the food/baking industries.

tbl0002 Table 2 Typical compositions (g 100 g

1

) of full- and low-fat

strained yogurts

Component Full-fat Low-fat

Fat 10.0 4.2

Solids-not-fat 13.8 17.4

Salt 0.5 0.5

Cremodan Mousse 31 0.8 0.9

Total solids 26.1 23.0

tbl0003 Table 3 Chemical composition (g 100 g

1

) of different brands of

strained yogurt that are marketed in the UK

Brands

CompositionABCDEF

Total solids 21.70 19.25 21.98 21.56 19.78 14.27

Protein 7.31 4.46 4.89 7.74 6.00 9.90

Fat 10.68 9.99 10.11 10.48 9.44 0.21

Carbohydrate

3.14 3.55 6.00 2.76 3.70 3.53

Ash 0.57 0.75 0.98 0.58 0.64 0.63

Calculated by difference.

After Tamime (unpublished data).

tbl0004 Table 4 Range of composition (g 100 g

1

) of commercial frozen yogurts sold in the US market

Flavor Fat Protein Ash Total solids pH

Vanilla 1.79–5.94 3.52–3.84 0.70–1.01 28.83–34.21 6.37–7.10

Chocolate 3.15–5.73 2.94–4.19 0.87–1.06 31.08–36.70 6.36–7.10

Strawberry 1.69–5.31 1.61–3.15 0.82–1.05 31.20–37.60 4.37–5.70

Data from Tieszen KM and Baer RJ (1989) Composition and microbiological quality of frozen yoghurts. Cultured Dairy Products Journal 24(4): 11–14,

with permission.

6262 YOGURT/Yogurt-based Products

0024 Yogurt presents no difficulties when spray dried,

but some precautionary measures that should be

considered included the following: firstly, the concen-

tration stage, before drying, should be carried out

at a low temperature (about 50–60



C) to minimize

scorching on to the surfaces of the evaporator, or

discoloration of the final powder, and secondly, the

processing conditions must be moderate in order to

ensure a high viable cell count of starter culture or-

ganisms in the dried product. The acidified product

is concentrated using an ordinary evaporator to

25–36 g 100 g

1

solids at 57–58



C. The concentrated

buttermilk, which is highly viscous, is pumped to the

spray drier at 43



C and dried using an inlet air drying

temperature between 175 and 190



C. The moisture

content in the dried product is around 4 g 100 g

1

and the tapped bulk density is 0.77–0.83 g cm

3

(See Drying: Spray Drying.)

0025According to APV Nordic A/S, which is one of the

leading drier manufacturers, yogurt can be dried in

a three-stage drying plant, and an illustration is

shown in Figure 2. The yogurt is concentrated to

35 g 100 g

1

total solids, preheated, and nozzle atom-

ized into the drying chamber. The inlet and outlet air

fig0002 Figure 2 Schematic illustration of a three-stage drying plant for the manufacture of dried yoghurt; A, product inlet; B, steam; C,

cooling water; D, air inlet; E, air outlet; F, product outlet; 1, feed tank; 2, preheater; 3, atomizer; 4, spray drying chamber; 5, integrated

fluid bed; 6, external fluid bed; 7, cyclone. Reproduced courtesy of APV Norcid A/S, Copenhagen.

tbl0005 Table 5 Range of composition (g 100 g

1

) of different types of powdered yogurt products

Product Total solids Protein Fat Total sugars Ash

Oggt

Sheep 91.9–95.6 31.7–37.3 14.5–39.3 19.3–32.5 5.3–7.6

Cow 93.7–96.3 26.2–31.3 11.0–25.4 38.7–44.4 6.0–7.2

Goat 92.5–93.1 26.3–30.4 18.9–28.9 34.8–37.3 5.3–6.5

Madeer 91.8–96.1 35.5–36.4 13.4–15.3 34.4–37.4 7.6–7.9

Yo gur t /p lain kis h k 95.6–96.0 33.0–54.4 1.0–7.9 29.5–54.0 3.8–7.0

Kishk

Commercial 87.0–94.5 8.9–23.5 1.6–16.1 31.0–68.8 2.0–9.1

Laboratory 90.5–92.5 14.5–19.7 nr

nr 4.4–8.7

Different cereals 87.5–94.8 17.6–25.8 6.4–9.7 56.3–67.9 2.2–4.9

Milk and soy 90.1–94.8 16.1–28.2 nr nr 3.5–5.3

Whey 90.3 13.3 nr nr nr

nr: data not reported.

Data compiled from Tamime AY and Robinson RK (1999) Yoghurt Science and Technology, 2nd edn. Cambridge: Woodhead, with permission.

YOGURT/Yogurt-based Products 6263

drying temperatures are 160 and 65



C, respectively.

The partially dried yogurt particles fall down on to

the integrated fluid bed drier in the bottom of the

drying chamber. Such particles form a fluidized

layer, which is further dried.

0026 Finally, the powder is transferred to an external

fluid bed drier for final drying and cooling. The spent

drying air from both the drying chamber andthe

external fluid bed is drawn through cyclone(s) to

separate the very fine powder particles (fines)

from the air. The fines are fed back to the external

fluid bed drier, where they are mixed with the bulk

of the powder. During drying, the maximum tem-

perature of the powder is 55



C and that of the

product outlet, 25



C. Such powder contains

2 g 100 g

1

moisture and has a tapped bulk density

of 0.5 g cm

3

0027 It is evident that dried-yogurt products (traditional

or industrial) are available to consumers in rural or

industrialized countries, and Table 5 illustrates some

typical compositional analyses. Note that some

products contain low quantities of fat, which indicate

that: (1) skimmed milk has been used rather than

full-fat milk, or (2) in some traditional dried prod-

ucts, the fermented milk is first churned to make

butter, and then the buttermilk is dried for preserva-

tion purposes.

See also: Casein and Caseinates: Methods of

Manufacture; Drying: Spray Drying; Fermented Milks:

Types of Fermented Milks; Filtration of Liquids; Heat

Treatment: Ultra-high Temperature (UHT) Treatments;

Recombined and Filled Milks; Starter Cultures;

Yogurt: The Product and its Manufacture

Further Reading

Bylund G (1995) Dairy Processing Handbook. Lund: Tetra

Pak Processing Systems.

Lehmann HR, Dolle E and Bucker H (1991) Processing

Lines for the Production of Soft Cheese, 3rd edn. Tech-

nical and Scientific Document No. 8. Germany: West-

falia Separator AG.

Marshall VME and Tamime AY (1997) In: Law BA (ed.),

Microbiology and Biochemistry of Cheese and Fer-

mented Milk, 2nd edn. London: Blackie Academic &

Professional.

Masters K (1991) Spray Drying Handbook, 5th edn.

Harlow: Longman Scientific and Technical.

Pisecky J (1997) Handbook of Milk Powder Manufacture.

Copenhagen: Niro A/S.

Tamime AY and Marshall VME (1997) In: Law BA (ed.)

Microbiology and Biochemistry of Cheese and Fer-

mented Milk, 2nd edn. London: Blackie Academic &

Professional.

Tamime AY and Robinson RK (1999) Yoghurt Science and

Technology, 2nd edn. Cambridge: Woodhead.

Tieszen KM and Baer RJ (1989) Composition and micro-

biological quality of frozen yoghurts. Cultured Dairy

Products Journal 24(4): 11–14.

Dietary Importance

J Buttriss, British Nutrition Foundation, London, UK

Introduction

0001Yogurt is a fermented milk product and as such is a

means of preserving the nutrients in milk. A wide

variety of yogurts are now available around the

world, ranging from very-low-fat fruit yogurts to

Greek-style yogurt with a fat content around 8 g per

100 g. Yogurt can be made from cows’, ewes’, goats’,

or buffalo’s milk. This article reviews the nutritional

composition of a range of yogurts, provides data on

yogurt consumption around the world and discusses

the importance of yogurt for different population

groups. (See Buffalo: Milk; Milk: Dietary Import-

ance; Sheep: Milk.)

0002Since yogurt is derived from milk, it provides

protein, calcium, and other minerals, and a range of

vitamins (Table 1). Levels of some vitamins, such as

vitamin B

and pantothenic acid, are reduced as they

are utilized by the bacterial culture used to produce

the yogurt. However, folic acid levels are typically

higher than in milk since folic acid is produced by

the bacteria. Refer to individual nutrients.

0003Some people have speculated that the bioavail-

ability of some minerals in yogurt is enhanced, but

research to support this claim is inconclusive.

(See Bioavailability of Nutrients.)

Sugar and Artificial Sweeteners

0004Reflected in the calorific content, yogurts with a

range of sugar contents are now available. In plain

yogurt, it is lactose, naturally present in yogurt, that

provides the sweetness. But in other forms, sugar is

added either as sucrose or as fructose and glucose

from fruit. In ‘diet’ or ‘very-low-fat’ yogurt, artificial

sweeteners are often used to reduce the calorie

content. (See Carbohydrates: Sensory Properties;

Sweeteners: Intensive.)

Trends in Yogurt Consumption

0005Yogurt and other cultured milk products have

traditionally been popular dietary items in Eastern

Europe. Popularity in the UK has increased

considerably in the past 25 years: consumption

6264 YOGURT/Dietary Importance

increased 10-fold from 0.4 kg per person per year in

1966 to 4.1 kg per person per year in 1989, according

to the International Dairy Federation (IDF) Annual

Statistics. By 1996, the per capita consumption

reached 6.18 kg. Much of this increase has been due

to the introduction in the UK of flavoured and fruit

yogurts.

0006 The UK’s National Food Survey for 1998 indicated

that yogurt consumption is 109 ml per person per

week. Comparable consumption levels for fromage

frais and other dairy desserts are 16 and 27 ml, re-

spectively. In this data set, yogurt is responsible for

72% of the market for these three types of products.

0007 Table 2 shows the consumption of yogurt, fromage

frais, and, for comparison, dairy desserts, in a

selection of countries. Again, yogurt represents

about 72% of the market.

Importance of Yogurt for Different

Population Groups

Yogurt and Health

0008Apart from its contribution to nutrient needs, the

perception of yogurt as a ‘healthy’ food has been

augmented by claims of health benefits attributed to

specific live bacteria present in some yogurts, in

particular Lactobacillus acidophilus and bifidobac-

teria. Both of these types of bacteria are to be found

in the human gastrointestinal tract, especially in

breast-fed infants, and it has been suggested that

these microorganisms may be able to colonize the gut

when consumed in yogurt, and protect against patho-

gens. It has also been speculated that they may be of

benefit in a number of intestinal disorders, including

tbl0001 Table 1 Nutritional value of varieties of yogurt

Low-fat plain (per100 g) Low-fat fruit (per100 g) Whole-milk fruit (per100 g) Low-caloriefruit (per100 g)

Energy (kcal) 56 90 105 41

(kJ) 236 382 441 177

Protein (g) 5.1 4.1 5.1 4.3

Carbohydrate 7.3 17.1 15.4 5.8

Sugars (g) 7.3 17.1 15.4 5.8

Fat (g) 0.8 0.7 2.8 0.2

Saturates (g) 0.5 0.4 1.5 0.1

Monosaturates (g) 0.2 0.2 0.8 0.1

Polyunsaturates (g) Trace Trace 0.2 Trace

Sodium (mg) 83 64 82 73

Dietary fiber (g) Nil 0.5

0.5

Vitamin A (mg) 9 11 42 Trace

Thiamin (mg) 0.05 0.05 0.06 0.04

Riboflavin (mg) 0.25 0.21 0.30 0.29

Nicotinic acid (mg) 0.15 0.14 0.13 0.13

Potential nicotinic acid

from tryptophan (mg)

1.20 0.96 1.29 1.00

Vitamin B

(mg) 0.09 0.08 0.07 0.07

Folic acid (mg) 17 16 10 8

Vitamin B

(mg) 0.2 0.2 0.1 (0.2)

Pantothenic acid (mg) 0.45 0.33 0.30 N/A

Biotin (mg) 2.9 2.3 2.0 N/A

Vitamin C (mg) 1.0 1.0 1.0 1.0

Vitamin D (mg) 0.01 (0.01) (0.04) Trace

Vitamin E (mg) 0.01 (0.01) (0.05) 0.03

Vitamin K (mg) na na na na

Calcium (mg) 190 150 160 130

Chlorine (mg) 150 130 150 120

Copper (mg) Trace Trace Trace Trace

Iodine (mg) 63 48 (48) na

Iron (mg) 0.1 0.1 Trace 0.1

Magnesium (mg) 19 15 16 13

Phosphorus (mg) 160 120 130 110

Potassium (mg) 250 210 210 180

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

Zinc (mg) 0.6 0.5 0.5 0.4

This figure will vary depending on the type of fruit used.

na, data not available.

Source: Holland B, Unwin ID and Buss DH (1989) Milk Products and Eggs. Fourth supplement to McCane and Widdowson’s The Composition of Foods,

4th edn. London: Royal Society of Chemistry and Ministry of Agriculture, Fisheries and Food, with permission.

YOGURT/Dietary Importance 6265

those precipitated by antibiotic treatment or by

diseases such as cancer and liver or kidney disease.

Claims have been made that specific bacteria used

in the production of a certain brand of yogurt

(no longer on the market) have the potential to reduce

blood levels of low-density lipoproteins (LDL). On the

basis of existing research it is not possible to substan-

tiate these various claims, although evidence is

increasing, and inconsistencies in the findings may

in part be explained by differences in strain and

species of bacterial cultures, and differences in experi-

mental design.

0009 There is, however, a substantial body of evidence to

indicate that fermented dairy products such as yogurt

are well tolerated by individuals who are lactose-

intolerant. It has been suggested that this is because

of the bacterial enzyme, b-galactosidase (produced

by the culture) in ‘live’ yogurt. This enzyme, which

is able to digest lactose to glucose and galactose, is

intracellular and hence is thought to survive gastric

digestion. However, as lactose maldigesters tolerate

yogurts with varying b-galactosidase activities

equally well, it would seem that other factors may

also be important, including the rate of gastrointest-

inal transit of yogurt.

0010 Intake of the types of lactic acid-producing organ-

isms found in yogurt has also been postulated to

prevent or inhibit intestinal growth of a variety of

foodborne, disease-causing organisms. Most of the

evidence supporting a role for cultured dairy prod-

ucts, or the bacteria used to make them, in controlling

intestinal pathogens comes from experimental animal

and in vitro studies. Findings are inconsistent, but

various mechanisms have been proposed to explain

reported protective effects. For example, the ability of

lactic acid cultures to lower intestinal pH favors

growth of lactic acid bacteria but provides a hostile

environment for pathogens. It has also been suggested

that lactic acid bacteria may produce bacteriocins,

peptides with a direct antibiotic effect.

0011In summary, while a fairly clear case has been

made for tolerance of yogurt by lactose maldigesters

unable to tolerate milk, the potential benefit of yogurt

(or specific types of yogurt) in protecting against

pathogens, in recolonizing the gut after illness, or in

lowering LDL cholesterol concentrations in the blood

needs further investigation.

See also: Bioavailability of Nutrients; Buffalo: Milk;

Carbohydrates: Sensory Properties; Folic Acid:

Properties and Determination; Lactic Acid Bacteria;

Milk: Dietary Importance; Pantothenic Acid: Properties

and Determination; Sheep: Milk; Sweeteners: Intensive;

Thiamin: Properties and Determination

Further Reading

Buttriss J (1997) Nutritional properties of fermented milk

products. International Journal of Dairy Technology

50(1): 21–27.

Chandan RC (1989) Yogurt: Nutritional and Health

Properties. VA: National Yoghurt Association.

ERC Group plc (1999) Yoghurt and Dairy Desserts: Main-

stream Europe. Newmarket, UK: ERC Group:

Hamilton-Miller JMTand Fuller R (1996) Probiotics – pana-

cea or nostrum? BNF Nutrition Bulletin 21: 199–208.

Holland B, Unwin ID and Buss DH (1989) Milk Products

and Eggs. Fourth supplement to McCance and Widdow-

son’s The Composition of Foods, 4th edn. London:

Royal Society of Chemistry and Ministry of Agriculture,

Fisheries and Food.

Ratcliffe B and McMillan S (1999) Gut microflora. BNF

Nutrition Bulletin 24: 82–91.

Renner E (1989) Micronutrients in Milk and Milk-based

Food Products. London: Elsevier Applied Science.

Robinson RK (ed.) (1991) Therapeutic Properties of

Fermented Milks. London: Elsevier Applied Science.

Tamime AY and Robinson AY (1985) Yogurt: Science and

Technology. Oxford: Pergamon Press.

The Dairy Council (2000) Milk Matters: a guide to the UK

dairy industry and its products. London: The Dairy

Industry.

tbl0002 Table 2 Yogurt, fromage frais, and dairy desserts in 1998

Yogurt and dairy dessertsmarket Segmentation (percent volume)

Thousand tonnes Percentage change

(1992^1998)

Percapita (kg) Yogurt Fromage frais Dairy desserts

Belgium 148.1 þ16.7 14.52 50.6 26.1 23.4

France 1 643.3 þ10.6 27.91 56.6 20.7 22.7

Germany 1 703.8 þ28.2 20.75 74.4 7.6 18.0

Italy 306.2 þ23.7 5.33 84.7 3.7 11.7

Netherlands 573.5 1.1 36.33 56.6 5.5 37.9

Spain 501.8 þ13.9 12.73 69.4 4.8. 25.8

UK 500.8 þ45.9 8.53 72.5 10.8 16.8

Total 5 377.5 þ18.1 16.67 66.4 11.7 22.0

Source: ERC Group Plc (1999) Yogurt and Dairy Desserts ^ Mainstream Europe. Newmarket, UK: ERC.

6266 YOGURT/Dietary Importance

ZINC

Contents

Properties and Determination

Physiology

Deficiency

Properties and Determination

M Ruz, University of Chile, Santiago, Chile

Importance of Zinc in Human Biology

0001 Zinc is the 23rd most abundant element on the

Earth’s crust. Among the transition elements, zinc is

the second in abundance in eukaryotic cells. In

mammals, zinc is found, on average, at a concentra-

tion of 0.02 mM in the extracellular space and 1 mM

in the intracellular space. The total amount of zinc in

the body of an adult ranges from 1.5 g (female) to

2.5 g (male). The distribution of zinc in the organism

of a man is detailed in Table 1.

0002 The essentiality of zinc for growth for lower forms

of life was discovered in 1869 by Raulin studying the

effects of zinc on Aspergillus niger. Todd documented

the essentiality of zinc for rats in 1934. Today, the

requirement of zinc for the growth and replication of

all cells is recognized. Zinc is required for virtually all

aspects of cell metabolism, i.e., DNA synthesis, DNA

transcription, translation of mRNA into proteins,

and structure and stabilization of translated proteins.

Thus, at a whole organism level, zinc is necessary not

only for growth but also for development and highly

relevant functions, such as immunity, tissue repair,

vitamin A metabolism, protection against oxidative

damage, neuropsychological functions, apoptosis,

cellular signaling, and hormone action, among

others. (See Zinc: Physiology.)

0003 Prior to 1963, zinc deficiency in humans was un-

recognized. Today, as a result of remarkable advances

in the area of zinc metabolism, we know that mild

zinc deficiency is a widespread condition, probably

as prevalent as iron deficiency. This has led to the

suggestion that zinc should be added to iron, iodine,

and vitamin A, to the group of the micronutrients

with the highest risk of suboptimal nutrition in the

world.

0004Severe human zinc deficiency is now a rare condi-

tion. It has been reported mainly in patients with

acrodermatitis enteropathica and in patients receiv-

ing total parenteral nutrition without added zinc.

Clinical manifestations include growth retardation,

delayed sexual maturation, alopecia, epithelial

lesions, immune deficiencies, hypogeusia, hyposmia,

delayed wound healing, altered appetite, and night

blindness. Mild zinc deficiency is much more difficult

to characterize, and much information has been

obtained from zinc supplementation studies in popu-

lations around the world. Growth retardation has

been the most extensively studied feature, and im-

munological alterations, neuropsychological mani-

festations, and increased morbidity have also been

reported. (See Zinc: Deficiency.)

0005In order to understand why zinc is crucial for life

and the importance of achieving a normal nutritional

status regarding this mineral, it is necessary to analyze

the biochemical characteristics of this ion.

General Properties

0006Zinc is a IIB element with a complete d subshell and

two additional s electrons. It is a small ion, with a

radius of 0.65 A

and an oxidation state of þ2. It has

five stable isotopes and several radioactive isotopes

with variable half-lives, some of which are shown in

Table 2.

0007Zinc is a ubiquitous element and is mainly found in

intracellular space (extracellular/intracellular ratio

1:50). The concentration of free zinc ions varies from

10

9

M in the cytoplasm of many cells to 10

3

in vesicles. Zinc has a highly concentrated charge and

a small size, and a modest binding capacity to anions

such as carboxylate and phosphate. Like others ions,

zinc has a high affinity for electrons. It is a strong

Lewis acid and can bind strongly to donors such as

thiolates and amines.

0008 Zinc is a flexible atom that is able to adopt

four, five, and six coordinated geometries. It has a

distinctive ability to form stable complexes with

several molecules. Zn readily complexes amino

acids, peptides, proteins, and nucleotides, and has

an affinity for thiol and hydroxy groups and for

ligands containing electron–rich nitrogen as a donor.

For instance, zinc is often found in internal cross-links

of intracellular proteins, where it apparently func-

tions in a similar way to the disulfide bridges of

extracellular proteins. Disulfide bridges are unstable

to reduction in the intracellular environment. Zinc

does not show any changes in its oxidation state,

and this is an important advantage in reactions in

which redox activities may represent a risk for oxida-

tive damage.

0009 Table 3 lists the important properties of zinc in a

biological context.

Link Between Properties and Major Roles

0010 The exceptional ability of the zinc atom to participate

in strong but readily exchangeable ligand binding, the

flexibility of its coordination geometry and its stable

oxidation state are extremely useful in biological

systems. In other words, zinc is important for its

ubiquity and versatility. These characteristics mostly

explain the importance of zinc involvement in bio-

logical functions.

0011The biological functions of zinc can be classified

into three main categories: catalytic, structural, and

regulatory. Although there are good examples for

each of these, this should be considered only as a

rough classification, because there is often some

degree of overlap.

Catalytic

0012The catalytic role of zinc is related to the requirement

of zinc by more than 200 enzymes. Zinc metallo-

enzymes are found in all six enzyme classes: oxidore-

ductases, transferases, hydrolases, lyases, isomerases,

and ligases. Zinc metalloenzymes are involved in

nucleic acid, protein, carbohydrate, lipid, and vita-

min metabolism.

0013A combination of properties of zinc accounts for its

utility in zinc metalloenzymes. Zn as a Lewis acid is

able to accept a pair of electrons, and it does not

participate directly in redox reactions. This is highly

valuable in reactions such as proteolysis and the hy-

dration of carbon dioxide. As result of having filled

the d-shell orbitals, no specific geometry is more

stable than any other, although the binding geometry

observed most frequently is tetrahedral, with the metal

ion coordinating three or four protein side-chains.

This flexibility is widely used by zinc metalloenzymes

during catalysis. Another important characteristic is

its rapid ligand exchange.

0014Zinc present in enzymes plays distinct roles, from

catalytic or cocatalytic to structural. It is not uncom-

mon to observe mixed functions.

0015Catalytic Zinc is located in the active site interact-

ing directly with the molecule involved in the reac-

tion. Zinc has a highly localized charge and electron

affinity, so it is a very effective attacking element. The

tbl0001 Table 1 Distribution of zinc in the human body

Total Zn (g) Percentage

Skeletal muscle 1.53 57

Bone 0.77 29

Skin 0.16 6

Liver 0.13 5

Brain 0.04 1.5

Kidneys 0.02 0.7

Heart 0.01 0.4

Hair <0.01 0.1 (approx)

Blood plasma <0.01 0.1 (approx)

Modified from Jackson MJ (1989) Physiology of zinc: general aspects. In:

Mills CF (ed.) Zinc in Human Biology. pp. 1–14. London: Springer-Verlag.

tbl0002 Table 2 Stable and radioactive isotopes of zinc

Stable isotopes Radioactive isotopes

Isotope Natural abundance (%) Isotope Half-life

Zn 48.89

Zn 9.22 h

Zn 27.91

Zn 38.5 min

Zn 4.11

Zn 243.8 days

Zn 18.57

69m

Zn 13.8 h

Zn 0.62

71m

Zn 3.97 h

Zn 46.5 h

tbl0003Table 3 Importance of zinc in biology

1. High availability

2. Strongly retained

3. Fast ligand exchange

4. No redox reaction

5. Flexible coordination geometry

6. Good electron acceptor or Lewis acid

7. Supplies hard base, hydroxide

Modified from Williams RJP (1989) An introduction to the biochemistry of

zinc. In: Mills CF (ed.) Zinc in Human Biology, pp. 15–31. London: Springer-

Verlag.

6268 ZINC/Properties and Determination

most frequently observed ligand in the active site of

zinc metalloenzymes is histidine, followed by glu-

tamic acid, aspartic acid, and cysteine. These ligands

account for three or four ligands that can be incorpor-

ated with at least one water molecule. The zinc-

bound water is a critical component for the catalytic

site. It can be ionized to zinc bound hydroxide, polar-

ized by a general base to generate a nucleophile for

catalysis, or displaced by the substrate. Thus, a given

substrate binds directly to Zn and may displace the

metal-bound H

O molecule. Zinc in this case would

act as a Lewis acid polarizing the bound substrate

facilitating the nucleophilic attack. This mechanism

is observed in aldolases and peptidases. Alternatively,

zinc may not bind to the substrate but mediate its

function through the metal-bound water molecule.

In this case, zinc lowers the pK

of the bound water

molecule. The resultant metal-bound hydroxide ion is

the direct agent attacking the substrate. Examples of

enzymes in which zinc plays catalytic roles are car-

bonic anhydrase (EC 4.2.1.1) and carboxypeptidase

A (EC 3.4.17.1).

0016 Because of the mechanisms of action and the

characteristics of the substrates of zinc metalloen-

zymes, these enzymes are used in nature preferably

for acting on small substrates that cannot be handled

by organic groups in a multipoint attachment or

when a chemically less specific but very rapid attack

is needed.

0017 Cocatalytic This term refers to the participation of

more than one metal ion in the catalysis, i.e., the

situation in which there are several metal ions

bound in proximity to each other, and although all

ions play crucial roles in the catalysis, only one is

involved directly in the attack. Examples of this

group are alkaline phosphatase, which has two zinc

atoms and one magnesium atom per mole (EC

3.1.3.1), phospholipase C, which has three zinc

atoms per mole (EC 3.1.4.3), and leucine aminopep-

tidase, which contains two Zn ions per mole (EC

3.4.11.1).

0018 Structural In its structural role, zinc is coordinated

with four amino acid chains, usually in a tetrahedral

geometry. Cysteine is the most frequently observed

ligand, followed by histidine and aspartate. In this

situation, zinc atoms contribute to the stabilization

of the tertiary or quaternary structure of the protein

but do not participate in the catalysis. One such

example is the enzyme aspartate carbamyl transferase

(EC 2.1.3.2), which has six zinc atoms per mole.

These atoms are found in the R subunits tetrahedrally

bound by four cysteine thiolates, stabilizing the

quaternary structure of the protein.

Structural

0019The structural role of zinc is related to the participa-

tion of zinc in specific cellular or subcellular compon-

ents. It is an important component in proteins involved

in gene regulation and as part of cellular membranes.

0020Zinc and gene expression Important cellular pro-

cesses such as transcription, RNA processing, and

replication are regulated by protein–DNA and pro-

tein–protein interactions. A significant number of

such regulatory proteins present domains (or motifs)

that are involved in binding to the DNA molecule. A

major breakthrough was the identification of such

motifs as zinc-containing structures generically

termed ‘zinc fingers.’

0021Zinc fingers are usually formed by zinc and cyst-

eine and histidine residues. Tetrahedral coordination

of zinc to thio-sulfur of cysteine residues and imida-

zole–nitrogen of histidine stabilizes the segment in a

structure that resembles a finger. In this structure, the

zinc atom does not interact directly with the DNA: it

acts as a type of scaffolding that confers the required

conformation to the overall structure, enabling the

DNA-binding domain to bind DNA. Site-specific

binding of Zn finger domains to DNA is one of the

mechanisms by which proteins modulate gene expres-

sion. Zinc finger proteins contain one or more zinc

finger domains and include transcription factors, hor-

mone receptors, DNA-repair enzymes, products of

oncogenesis, and tumor suppressor genes. Genes en-

coding zinc finger proteins could amount to as much

as 1% of the human genome.

0022The first transcription factor to be identified as a

zinc finger-containing protein was TFIIIA in 1985.

TFIIIA is required for the correct initiation of tran-

scription of Xenopus laevis 5S RNA genes by RNA

polymerase III. The structure of TFIIIA encompasses

zinc bound to two cysteine and two histidine residues.

This is referred as the classic zinc finger DNA-binding

motif.

0023The steroid (glucocorticoid and estrogen) receptors

are members of the multigene family, which also in-

cludes receptors for thyroid hormone, retinoic acid,

and vitamin D. The steroid hormone receptor has two

zinc-finger loops. Steroid hormone receptors are

ligand-inducible transcription enhancers that bind

specifically to short DNA sequences or hormone re-

sponse elements and regulate a number of genes. The

steroid hormones induce gene expression by binding

to receptor proteins in the cytoplasm, causing these

proteins to enter the nucleus, bind to DNA, and acti-

vate transcription of certain genes.

0024Other zinc finger-containing proteins are: tran-

scription factor Sp1, NGFI-A/EGR/Krox 24/Zif 268

ZINC/Properties and Determination 6269