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

Подождите немного. Документ загружается.

The logarithm of the solubility (S) is proportional

to the ionic strength (m) at low concentrations:

log

S ¼ km.

0036 Protein solubility is decreased (‘salting-out‘ effect)

at higher salt concentrations due to the ion hydration

tendency of the salts. The following relationship

applies (S

, solubility at m ¼ 0; K, salting-out con-

stant): log

S ¼ log

 Km. Cations and anions in

the presence of the same counter ion can be arranged

in the following orders (Hofmeister series) based on

their salting-out effects: K

>Rb

>Na

>Cs

>Li

>NH

;SO

2

> citrate

2

> tartrate

2

> acetate



>Cl



>NO



>Br



> SCN



. Multivalent anions

are more effective than monovalent anions, whereas

divalent cations are less effective than monovalent

cations.

0037 Since proteins are polar substances, they are hy-

drated in water. The degree of hydration (grams of

water of hydration per gram protein) is variable. It is

0.22 for ovalbumin (in ammonium sulfate), 0.06 for

edestin (in ammonium sulfate), 0.8 for b-lactoglobu-

lin, and 0.3 for hemoglobin.

0038 The swelling of insoluble proteins corresponds to

the hydration of soluble proteins in that insertion of

water between the peptide chains results in an in-

crease in volume and other changes in the physical

properties of the protein. The amount of water taken

up during swelling can exceed the dry weight of the

protein by several times. For example, muscle tissue

contains 3.5–3.6 g of water per gram of protein (dry

matter).

Chemical Reactions

0039 The chemical modification of proteins is of import-

ance for a number of reasons. It provides derivatives

suitable for sequence analysis, identifies the reactive

groups in catalytically active sites of an enzyme,

enables the binding of protein to a carrier (protein

immobilization), and provides changes in protein

properties which are important in food processing.

In contrast to free amino acids, except for the rela-

tively small number of functional groups of the

terminal amino acids, only the functional groups

in protein side chains are available for chemical

reactions.

Lysine Residue

0040 Reactions involving the lysine residue can be divided

into several groups: (1) reactions leading to a posi-

tively charged derivative; (2) reactions eliminating the

positive charge; (3) derivatizations introducing a

negative charge; and (4) reversible reactions. The

last are of particular importance.

0041Reactions which retain the positive charge Alkyla-

tion of the free amino group of lysines with aldehydes

and ketones is possible, with a simultaneous reduc-

tion step. A dimethyl derivative (Prot-N(CH

)

) can

be obtained with formaldehyde.

0042Guanidination can be accomplished by using O-

methylisourea as a reactant. a-Amino groups react

at a much slower rate than e-amino groups. This

reaction is used analytically to assess the amount of

biologically available e-amino groups.

0043Derivatization with imido esters to amidines is also

possible. The reactant is readily accessible from the

corresponding nitrile. Proteins can be cross-linked

with the use of a bifunctional imido ester.

0044Treatment of the amino acid residue with amino

acid carboxyanhydrides yields a polycondensation

reaction product, made up of the protein as main

chain and peptide side chains. The length of the side

chains depends on reaction conditions. The carboxy-

anhydrides are readily accessible through interaction

of the amino acid with phosgene.

0045Reactions resulting in a loss of positive charge Ace-

tic anhydride reacts with lysine, cysteine, histidine,

serine, threonine, and tyrosine residues. Subsequent

treatment of the protein with hydroxylamine leaves

only the acetylated amino groups intact. Acylation is

possible with N-hydroxysuccinimidyl acetate, succi-

nimidyl 3-(p-hydroxphenyl)propionate (after radio-

iodination for radioactive labeling, Bolton–Hunter

reagent) or S-ethylthiotrifluoroacetate. A photocleav-

able amine-protecting group can be introduced by

reaction with 6-nitroveratryl chloroformate.

0046Sulfonylation occurs with dansyl chloride (5-

(dimethylamino)-naphthalene-1-sulfonyl chloride),

which also reacts with cysteine, histidine, and tyro-

sine. Carbamoylation with cyanate attacks a-and

e-amino groups as well as cysteine and tyrosine resi-

dues. However, their derivatization is reversible

under alkaline conditions. Thiocarbamoylation with

fluorescein isothiocyanate (FITC) or tetramethylrho-

damine isothiocyanate (TRITC) is used for fluores-

cent labeling of proteins.

0047Arylation is possible with DNFB (2,4-dinitrofluo-

robenzene) and 2,4,6-trinitrobenzenesufonic acid

(TNBS). DNFB also reacts with cysteine, histidine,

and tyrosine. 7-Chloro-4-nitrobenz-2-oxa-1,3-

diazole, which also reacts with cysteine, and 4-

fluoro-3-nitrobenzenesulfonic acid, a reactant which

has good solubility in water, are also of interest for

derivatization of proteins.

0048Deamination can be accomplished with nitrous

acid. This reaction involves a-and e-amino groups as

well as tryptophan, tyrosine, cysteine, and methio-

nine residues.

PROTEIN/Chemistry 4815

0049 Reactions resulting in a negative charge Acylation

with dicarboxylic acid anhydrides, e.g., succinic an-

hydride, introduces additional carboxyl groups into

the protein. Introduction of a fluorescent acid group

is possible by interaction of the protein with pyri-

doxal 5-phosphate, followed by reduction of an inter-

mediate Schiff base, or with fluorescamine.

0050 Reversible reactions N-Maleyl derivatives of pro-

teins are obtained at alkaline pH by reaction with

maleic anhydride. The acylated product is cleaved at

pH < 5, regenerating the protein. The half-life (t)of

e-N-maleyl lysine is 11 h at pH 3.5 and 37



C. More

rapid cleavage is observed with the 2-methylmaleyl

derivative (t < 3 min at pH 3.5 and 20



C) and the

2,2,3,3-tetrafluorosuccinyl derivative (t very low at

pH 9.5 and 0



C). Cysteine binds maleic anhydride

through an addition reaction. The S-succinyl deriva-

tive is quite stable. This side reaction is, however,

avoided when protein derivatization is carried out

with exo-cis-3,6-endoxo-1,2,3,6-tetrahydrophthalic

acid anhydride.

0051 Acetoacetyl derivatives are obtained with diketene.

This type of reaction also occurs with cysteine and

tyrosine residues. The acyl group is readily split from

tyrosine at pH 9.5. Complete release of protein from

its derivatized form is possible by treatment with

phenylhydrazine or hydroxylamine at pH 7.

Arginine Residue

0052 The arginine residue of proteins reacts with a-or

b-dicarbonyl compounds, e.g., with glyoxal, phenyl-

glyoxal hydrate, 2,3-butanedione (diacetyl) or nitro-

malondialdehyde, to form cyclic derivatives. The

nitropyrimidine resulting with nitromalondialdehyde

absorbs at 335 nm. The arginyl bond of this deriva-

tive is not cleaved by trypsin but it is cleaved in its

tetrahydro form, obtained by reduction with sodium

borohydride. In the reaction with benzil, an imino-

oxo-imidazolidine derivative is obtained after a

benzilic acid rearrangement. Reaction of the arginine

residue with 1,2-cyclohexanedione is highly selective

and proceeds under mild conditions. Regeneration of

the arginine residue is again possible with hydroxyla-

mine.

Glutamic and Aspartic Acid Residues

0053 These amino acid residues are usually esterified with

methanolic hydrochloric acid. There can be side reac-

tions, such as methanolysis of amide derivatives or

N,O-acyl migration in serine or threonine residues.

Diazoacetamide reacts with a carboxyl group and

also with the cysteine residue to carboxamidomethyl

derivatives.

0054Amino acid esters or other similar nucleophilic

compounds can be attached to a carboxyl group of a

protein with the help of a carbodiimide. Amidation is

also possible by activating the carboxyl group with an

isoxazolium salt (Woodward reagent) to an enolester

and its conversion with an amine.

Cystine Residue

0055Reductive cleavage of cystine occurs with sodium bor-

ohydride and with thiols. Thiols used to reduce cystine

to cysteine include 2-mercaptoethanol and, more ef-

fectively, dithiothreitol (DTT, Cleland’s reagent) or

dithioerythritol (DTE). More recently, bis(2-mercap-

toethyl)sulfone and tris(2-carboxyethyl)phosphine

have been introduced, which are both water-soluble

and very effective, and also tri-n-butylphosphine,

which is less water-soluble. The phosphines have the

advantage that the excess used for reduction does not

react with thiol reagents used for subsequent thiol

modification.

0056Cleavage of cystine is also possible by a nucleophi-

lic attack. The nucleophilic reactivity of the reagents

decreases in the following series: arsenite and phos-

phite > alkanethiol > aminoalkanethiol > thiophenol

and cyanide > sulfite > OH



> p-nitrophenol > thio-

sulfate > thiocyanate. Complete cleavage with sulfite

requires that oxidative agents (e.g., Cu

2þ

) are present

and that the pH is higher than 7. The resultant S-sulfo

derivative is quite stable in neutral and acidic media

and is fairly soluble in water. The S-sulfo group can be

eliminated with an excess of thiol reagent. Cleavage

of cystine residues with cyanides (nitriles) is of inter-

est since the thiocyanate formed in the reaction is

cyclized into a 2-iminothiazolidine under cleavage of

the N-acyl bond. This reaction can be utilized for the

selective cleavage of peptide chains. Initially, all the

disulfide bridges are reduced with DTT, and are then

converted to mixed disulfides through reaction with

5,5

-dithio-bis(2-nitrobenzoic acid). These mixed di-

sulfides are then cleaved by cyanide at pH 7.

0057Electrophilic cleavage occurs with Ag

and Hg

2þ

. Electrophilic cleavage with H

is possible only

in strong acids (e.g., 10 mol l

1

hydrochloric acid).

The sulfenium cation which is formed can catalyze a

disulfide exchange reaction. In neutral and alkaline

solutions a disulfide exchange reaction is catalyzed by

the thiolate anion.

Cysteine Residue

0058A number of alkylating agents, e.g., iodoacetic acid,

iodoacetamide, ethylenimine, and 4-vinylpyridine,

yield derivatives which are stable under the condi-

tions for acid hydrolysis of protein. The reaction with

ethylenimine gives an S-(2-aminoethyl) derivative

4816 PROTEIN/Chemistry

and, hence, an additional linkage position in the

protein for hydrolysis by trypsin. lodoacetic acid,

depending on the pH, can react with cysteine,

methionine, lysine, and histidine residues. Maleic

anhydride, 2-nitrobenzyl bromide, 2-nitrohydroxy-

benzyl bromide, and methyl p-nitrobenzenesulfonate

are also alkylating agents. A number of reagents

make it possible to measure thiol group content spec-

trophotometrically (azobenzene-2-sulfenyl bromide,

5,5

-dithio-bis(2-nitrobenzoic acid) (DTNB, Ellman’s

reagent), p-hydroxymercuribenzoate, N-ethylmale-

imide, N-phenylmaleimide). Especially suitable for

the specific and very sensitive labeling of cysteine-

containing peptides prior to isolation are N-[4-

-dimethylaminoazobenzene)]maleimide (DABM),

N-(7-dimethylamino-4-methyl-3-coumarinyl)malei-

mide (DACM), N-(1-pyrenyl)maleimide, and 4-

aminosulfonyl-7-fluoro-2,1,3-benzoxdiazol (ABDF).

0059 Cysteine is readily converted to the corres-

ponding disulfide, cystine, even under mild oxidative

conditions, such as treatment with iodine or potas-

sium hexacyanoferrate(III). Stronger oxidation of

cysteine, and also of cystine, e.g., with performic

acid, yields the corresponding sulfonic acid, cysteic

acid.

Methionine Residue

0060 Methionine residues are oxidized to methionine sulf-

oxide with hydrogen peroxide. The sulfoxide can be

reduced, regenerating methionine, using an excess

of thiol reagent. With performic acid, methionine

sulfone is formed.

0061 a-Halogen carboxylic acids, b-propiolactone, and

alkyl halides convert methionine into sulfonium

derivatives, from which methionine can be regener-

ated in an alkaline medium with an excess of thiol

reagent. Reaction with cyanogen bromide, which

splits the peptide bond on the carboxyl side of the

methionine molecule, is used for selective cleavage of

proteins.

Histidine Residue

0062 Diethyl pyrocarbonate reacts with histidine to

form N-(ethoxycarbonyl)histidine. With iodoaceta-

mide, N-1-(carboxamidomethyl)-, N-3-(carboxami-

domethyl)-, or N-1,N-3-

di(carboxamidomethyl)histidine are formed.

0063 Selective modification of histidine residues present

on active sites of serine proteinases is possible. Sub-

strate analogs such as halogenated methyl ketones

inactivate such enzymes (e.g., 1-chloro-3-tosyla-

mido-7-amino-2-heptanone inactivates trypsin and

1-chloro-3-tosylamido-4-phenyl-2-butanone inacti-

vates chymotrypsin) by N-alkylation of the histidine

residue.

Tryptophan Residue

0064N-Bromosuccinimide oxidizes the tryptophan side

chain and also tyrosine, histidine, and cysteine. The

reaction is used for the selective cleavage of peptide

chains at the carboxyl group of tryptophan residues

and the spectrophotometric determination of

tryptophan. Other oxidative cleaving reagents

are o-iodosobenzoic acid and 3-bromo-3-methyl-

2-(2-nitrophenylmercapto)-3H-indole. Selective

modification of histidine is possible with 2-

hydroxy-5-nitrobenzyl bromide (Koshland reagent

I) and 2-nitrophenylsulfenyl chloride.

Tyrosine Residue

0065Selective acylation of tyrosine can occur with 1-

acetylimidazole as a reagent. Diazotized p-arsanilic

acid reacts with tyrosine (ortho substitution) and with

histidine, lysine, tryptophan, and arginine. Tetrani-

tromethane introduces a nitro group into the ortho

position. Radioactive labeling of tyrosine is possible

with Na

125

I and chloramine T (ortho mono- and

di-iodo derivatives).

Bifunctional Reagents

0066Bifunctional reagents enable intra-and intermolecular

cross-linking of proteins. Examples are bifunctional

imidoester, maleimides, fluoronitrobenzene, and iso-

cyanate derivatives.

See also: Amino Acids: Properties and Occurrence;

Determination; Chromatography: Principles; High-

performance Liquid Chromatography; Enzymes:

Functions and Characteristics; Mass Spectrometry:

Principles and Instrumentation; Applications; Peptides;

Protein: Food Sources; Functional Properties;

Interactions and Reactions Involved in Food Processing;

Spectroscopy: Nuclear Magnetic Resonance

Further Reading

Belitz H-D and Grosch W (1999) Food Chemistry, 2nd edn.

Berlin: Springer-Verlag.

Davies JS (ed.) (1993–99) Amino-acids, Peptides, and

Proteins, vols 24–30. Cambridge: Royal Society of

Chemistry.

Deutscher MP (ed.) (1990) Methods in Enzymology,

vol. 182. Guide to Protein Purification. San Diego:

Academic Press.

Gray WR (1993) Dislfide structures of highly bridged

peptides: a new strategy for analysis. Protein Science 2:

1732–1748.

Ikenaka T, Odani S and Koide T (1974) Chemical structure

and inhibitory activities of soybean proteinase inhibi-

tors. In: Fritz H, Tschesche H, Greene LJ and Truscheit

E (eds) Bayer-Symposium V ‘Proteinase Inhibitors‘, pp.

325–343. Berlin: Springer-Verlag.

PROTEIN/Chemistry 4817

Jones JH (ed.) (1983–92) Amino-acids, Peptides, and Pro-

teins, vols 14–23. London: Royal Society of Chemistry.

Kellner R, Lottspeich F and Meyer HE (1999) Micro-

characterization of Proteins, 2nd edn. Weinheim:

Wiley-VCH.

Lottspeich F and Zorbas H (eds) (1998) Bioanalytik.

Heidelberg: Spektrum Akademischer Verlag.

Lundblad RL and Noyes CM (1984) Chemical Reagents for

Protein Modification, vols I and II. Boca Raton: CRC

Press.

Matsudaira PT (ed.) (1989) A Practical Guide to Protein

and Peptide Purification for Microsequencing. San

Diego: Academic Press.

ller S and Wieser H (1997) The location of disulphide

bonds in monomeric g-type gliadins. Journal of Cereal

Science 26: 169–176.

Ogata C, Hatada M, Tomlinson G, Shin WC and Kim S-H

(1987) Crystal structure of the intensely sweet protein

monellin. Nature 328: 739–742.

Shewry PR and Tatham AS (1997) Disulphide bonds in

wheat gluten proteins. Journal of Cereal Science 25:

207–227.

Shimada K and Mitamura K (1994) Derivatization of thiol-

containing compounds. Journal of Chromatography B

659: 227–241.

Singh R and Whitesides GM (1994) Reagents for rapid

reduction of native disulfide bonds in proteins. Bio-

organic Chemistry 22: 109–115.

Wingler K, Bo

cher M, Flohe

L, Kollmus H and Brigelius-

Flohe

R (1999) mRNA stability and selenocysteine

insertion sequence efficiency rank gastrointestinal glu-

tathione peroxidase high in the hierarchy of selenopro-

teins. European Journal of Biochemistry 259: 149–157.

Wittmann-Liebold B, Salnikow J and Erdmann VA (eds)

(1986) Advanced Methods in Protein Microsequence

Analysis. Berlin: Springer-Verlag.

Wright DJ (1987) The seed globulins. In: Hudson BJF (ed.)

Developments in Food Proteins, vol. 5, pp. 81–157.

London: Elsevier Applied Science.

Wright DJ (1988) The seed globulins – part II. In: Hudson

BJF (ed.) Developments in Food Proteins, vol. 6, pp.

119–178. London: Elsevier Applied Science.

Food Sources

J K P Weder and H-D Belitz

, Technische Universita

nchen, Garching, Germany

Protein

0001 Amino acids, peptides, and proteins are important

constituents of food. They supply the required build-

ing blocks for protein biosynthesis. In addition, they

directly contribute to the flavor of food and are pre-

cursors for aroma compounds and colors formed

during thermal or enzymatic reactions in production,

processing, and storage of food. Other food constitu-

ents, e.g., carbohydrates, take part in such reactions.

Proteins also contribute significantly to the physical

properties of food through their ability to stabilize

gels, foams, doughs, emulsions, and fibrillar struc-

tures.

0002Table 1 shows the most important protein sources

and their contribution to world-wide production of

protein. Cereals contribute to protein production by

more than half, followed by oil seeds and meat. Be-

sides plants and animals, algae (Chlorella, Scenedes-

mus, and Spirulina spp.), yeasts and bacteria may be

used for protein production (single-cell protein

(SCP)). Common carbon sources are glucose, molas-

ses, starch, waste water, higher n-alkanes, and metha-

nol. Yeasts of the genus Candida can be grown, for

example, on paraffins, and they deliver about 0.75 t

of protein per tonne of alkane. Bacteria of the genus

Pseudomonas deliver about 0.3 t of protein per tonne

of alcohol in aqueous methanol. Because of the high

nucleic acid content of yeasts and bacteria (6–17%

of dry mass), isolation of the protein from biomass

is necessary. The importance of SCP in the future

will depend on the total protein market, especially

on prices and functional properties of individual

proteins.

0003Table 2 provides data on the average protein con-

tent and the nutrient density – the ratio between the

amount of protein (g) and the total energy value (MJ)

of the digestible constituents – of selected foodstuffs.

The protein content varies as follows: > 20%

tbl0001Table 1 World-wide annual protein production (10

tonnes)

Source 1993 1994 1995 1996 1997 1998

Cereals 177 183 177 193 196 191

Oil seeds 53.1 60.7 60.2 62.1 65.6 68.0

Legumes 11.8 12.1 11.8 11.6 11.9 12.2

Root crops 8.5 8.1 8.4 8.9 8.4 8.4

Vegetables 5.1 5.3 5.6 5.9 6.0 6.1

Meat 37.9 39.1 40.2 40.6 42.0 42.9

Milk 18.0 18.2 18.4 18.5 18.7 18.7

Fish and aquatic

animals

11.3 12.2 12.6 13.0 13.3

Eggs 4.8 5.1 5.3 5.6 5.9 6.0

Total 328 344 340 359 368 353

Excluding fish and aquatic animals.

Data from FAO (1999) FAO Year book Production, vol. 52, 1998. FAO

Statistics Series No. 148. Rome: Food and Agriculture Organization and by

courtesy of Dr. Rehbein (Federal Research Centre for Fisheries, Hamburg,

Germany, for fish and aquatic animals), protein calculated from production

with data from Scherz H and Senser F (eds) (2000) Food Composition and

Nutrition Tables, 6th edn. Boca Raton, FL: CRC Press.

Deceased.

4818 PROTEIN/Food Sources

tbl0002 Table 2 Protein content (%) of selected foodstuffs

No.

Foodstuff Average protein

content

Nutrient

density

(gMJ

1

)

Raw

product

Edible

portion

Milk and dairy products

1 Human milk 1.13 1.13 3.92

2 Cow’s milk 3.33 3.33 12.05

3 Buffalo milk 4.01 4.01 8.94

4 Ewe’s milk 5.27 5.27 13.17

5 Goat’s milk 3.69 3.69 13.14

6 Condensed milk (7.5% fat) 6.49 6.49 11.78

7 Dried milk whole 25.20 25.20 12.50

8 Dried skimmed milk 35.00 35.00 23.05

9 Yoghurt (1.5–1.8% fat) 3.55 3.55 16.88

10Bluecheese(50%fatdm

)21.1021.1014.35

11 Camembert cheese

(40 % fat dm)

22.50 22.50 19.72

12 Cheddar cheese

(50% fat dm)

25.40 25.40 15.43

13 Edam cheese (40% fat dm) 24.80 26.10 19.93

14 Emmental cheese

(45% fat dm)

26.98 28.70 18.02

15 Feta cheese (45% fat dm) 17.00 17.00 17.24

Eggs

16 Chicken egg, whole 11.35 12.90 19.98

17 Chicken egg, yolk 16.10 16.10 11.03

18 Chicken egg, white 11.10 11.10 53.37

Meat

19 Beef (muscles only) 22.00 22.00 48.41

20 Veal (muscles only) 21.30 21.30 54.33

21 Pork (muscles only) 22.00 22.00 49.68

22 Mutton (muscles only) 20.40 20.40 43.13

23 Lamb (muscles only) 20.80 20.80 42.41

24 Corned beef, American 25.30 25.30 28.94

25 Luncheon meat 14.70 14.70 12.36

26 Meat extract 56.60 56.60 54.13

27 Sausage ‘Cervelat‘ 19.89 20.30 12.43

28 Chicken, for roasting 14.73 19.90 28.70

29 Duck 14.48 18.10 19.17

30 Goose 9.89 15.70 11.10

31 Turkey 14.75 20.20 22.48

Fish

32 Carp 9.36 18.00 37.22

33 Cod 13.28 17.70 54.55

34 Flounder 7.42 16.50 53.85

35 Halibut 16.08 20.10 49.67

36 Herring 12.74 18.20 18.80

37 Mackerel 12.16 18.70 24.66

38 Mullet 10.61 20.40 40.32

39 Redfish 8.74 18.20 41.09

40 Salmon 12.74 19.90 23.65

41 Sardine 11.45 19.40 38.98

42 Trout 10.14 19.50 45.09

43 Tuna 13.12 21.50 22.90

Cereals

44 Barley (whole grain) 9.84 9.84 7.35

45 Maize (whole grain) 8.54 8.54 6.17

46 Corn flakes 7.15 7.15 4.77

47 Oats (whole grain) 11.69 11.69 7.91

48 Rolled oats 12.53 12.53 8.09

49 Rice, polished 6.83 6.83 4.68

50 Rye (whole grain) 8.82 8.82 7.08

51 Rye flour, type 997 6.86 6.86 5.22

52 Rye bread 6.22 6.22 6.76

53 Sorghum 10.30 10.30 6.97

54 Wheat (whole grain) 11.73 11.73 8.95

55 Wheat flour, type 405 9.84 9.84 6.98

56 Wheat flour, type 1700 11.23 11.23 8.76

57 Wheat bread 7.61 7.61 7.54

Roots and tubers

58 Beetroot 1.19 1.53 8.73

59 Cassava 0.74 1.00 1.75

60 Celeriac 1.13 1.55 20.18

61 Potato 1.63 2.04 6.84

62 Potato flakes 8.60 8.60 6.26

63 Sweet potato 1.32 1.63 3.55

64 Taro 1.68 2.00 4.40

65 Topinambur 1.68 2.44 18.72

66 Yam 1.68 2.00 4.77

Leaves,stems,andflowers

67 Artichoke 1.15 2.40 25.68

68 Broccoli 2.01 3.30 29.88

69 Brussel sprouts 3.47 4.45 29.41

70 Cauliflower 1.53 2.46 25.94

71 Chinese leaves 0.94 1.19 22.85

72 Red cabbage 1.17 1.50 16.24

73 Soya bean sprouts 4.59 5.53 26.15

Vegetable fruits

74 Aubergine 1.03 1.24 17.12

75 Cucumber 0.44 0.60 11.59

76 Green peppers 0.90 1.17 13.65

77 String beans 2.25 2.39 17.24

78 Tomato 0.91 0.95 12.98

Legumes and oil seeds

79 Bean, white dry 21.09 21.30 19.32

80 Chick pea, dry 19.80 19.80 17.02

81 Lentil, dry 23.50 23.50 17.61

82 Pea, dry 22.67 22.90 19.89

83 Peanut 25.25 25.25 10.75

84 Soya bean, dry 28.00 33.73 24.98

Fruits

85 Apple 0.31 0.34 1.49

86 Apricot 0.82 0.90 4.90

87 Apricot, dried 5.00 5.00 4.91

88 Banana 0.77 1.15 3.07

89 Cherry (morello) 0.80 0.90 4.00

90 Date, dried 1.61 1.85 1.58

91 Fig 1.30 1.30 5.00

92 Fig, dried 3.50 3.54 3.34

93 Grape 0.65 0.68 2.38

94 Grape, dried (raisin) 2.46 2.46 2.09

95 Orange 0.72 1.00 5.58

96 Peach 0.70 0.76 4.33

97 Pear 0.44 0.47 2.02

98 Plum 0.56 0.60 2.92

99 Strawberry 0.80 0.82 6.03

Mushrooms

100 Champignon 2.69 2.74 41.10

101 Chanterelle 0.96 1.57 32.89

102 Edible boletus 2.88 3.60 42.32

103 Edible boletus, dried 19.70 19.70 37.66

104 Morel 1.38 1.66 41.44

105 Oyster mushroom 2.35 2.35 52.07

106 Truffles 5.53 5.53 48.99

Thesenumbersareusedin Table 3.

dm, dry matter.

Data from Scherz H and Senser F (eds) 2000 Food Composition and

Nutrition Tables, 6th edn. Boca Raton, FL: CRC Press.

PROTEIN/Food Sources 4819

tbl0003 Table 3 Amino acid compositions of selected foodstuffs

Foodstuffs

Aminoacids12345678910111213

Alanine 0.056 0.130 0.130 0.220 0.140 0.270 0.910 1.25 0.160 0.730 0.890 0.760 0.890

Arginine 0.051 0.130 0.110 0.180 0.130 0.230 0.920 1.28 0.130 1.65 0.820 0.900 1.02

Aspartic acid 0.120 0.290 0.310 0.460 0.300 0.550 1.99 2.74 0.280 0.830 1.67 1.81 2.00

Cystine 0.024 0.028 0.048 0.060 0.083 0.066 0.230 0.310 0.027 0.120 0.130 0.210 0.250

Glutamic acid 0.220 0.790 0.480 1.07 0.780 1.50 5.51 7.57 0.700 4.97 4.95 6.62 6.15

Glycine 0.036 0.076 0.110 0.074 0.180 0.560 0.770 0.086 0.510 0.460 0.470 0.550

Histidine 0.031 0.095 0.078 0.130 0.079 0.170 0.660 0.920 0.095 0.990 0.730 0.800 0.930

Isoleucine 0.077 0.220 0.200 0.280 0.230 0.450 1.61 2.24 0.220 1.19 1.52 1.81 1.48

Leucine 0.130 0.360 0.370 0.530 0.390 0.720 2.47 3.43 0.380 2.14 2.19 2.52 2.65

Lysine 0.086 0.280 0.460 0.340 0.520 1.96 2.72 0.280 2.38 1.65 2.07 2.37

Methionine 0.024 0.090 0.097 0.140 0.094 0.170 0.620 0.860 0.092 0.520 0.590 0.770 0.780

Phenylalanine 0.054 0.180 0.162 0.260 0.180 0.340 1.22 1.70 0.190 1.22 1.20 1.45 1.44

Proline 0.120 0.340 0.550 0.470 0.720 2.55 3.50 0.420 2.35 2.38 3.05 2.91

Serine 0.059 0.210 0.230 0.320 0.210 0.410 1.43 1.97 0.220 1.33 1.29 1.58 1.57

Threonine 0.063 0.160 0.180 0.240 0.230 0.330 1.16 1.61 0.160 0.920 0.840 0.980 1.13

Tryptophan 0.022 0.049 0.053 0.070 0.050 0.088 0.350 0.490 0.042 0.210 0.310 0.290 0.400

Tyrosine 0.056 0.180 0.183 0.260 0.240 0.320 1.28 1.78 0.180 1.02 1.08 1.30 1.33

Valine 0.081 0.240 0.220 0.320 0.280 0.480 1.73 2.40 0.270 1.46 1.61 1.81 1.88

Foodstuffs

Amino acids 14 16 17 18 19 20 21 22 23 24 25 28 29

Alanine 0.920 0.890 1.03 0.830 1.69 1.64 1.53 1.44 1.29 1.44

Arginine 1.00 0.890 1.28 0.680 1.54 1.54 1.53 1.44 1.39 1.63 0.790 1.39 1.10

Aspartic acid 1.58 1.46 1.76 1.23 2.34 2.40 2.43 2.15 1.89 2.27

Cystine 0.170 0.310 0.310 0.290 0.280 0.280 0.310 0.290 0.170 0.320 0.120 0.300

Glutamic acid 5.76 1.81 2.20 1.64 4.13 3.97 3.91 4.30 3.05 3.69

Glycine 0.510 0.530 0.620 0.500 1.56 1.34 1.42 1.43 1.09 1.40

Histidine 1.02 0.330 0.440 0.280 0.850 0.800 0.990 0.630 0.600 0.890 0.360 0.610 0.410

Isoleucine 1.73 0.930 1.09 0.740 1.25 1.29 1.27 1.21 1.02 1.24 0.560 1.29 0.940

Leucine 2.99 1.26 1.63 1.08 1.95 1.89 1.92 1.80 1.69 1.92 1.03 1.78 1.40

Lysine 2.39 0.890 1.30 0.740 2.31 2.05 2.20 2.00 1.89 2.13 1.01 2.04 1.56

Methionine 0.790 0.450 0.470 0.470 0.650 0.600 0.720 0.560 0.530 0.690 0.330 0.640 0.450

Phenylalanine 1.61 0.800 0.790 0.760 1.06 1.02 0.980 0.920 0.820 0.910 0.540 0.910 0.710

Proline 3.73 0.590 0.780 0.500 1.28 1.15 1.21 1.02 0.970 1.05

Serine 1.66 1.15 1.62 0.920 1.14 1.15 1.12 1.04 0.820 0.920

Threonine 1.14 0.710 1.01 0.580 1.15 1.13 1.25 1.09 0.890 1.11 0.550 1.01 0.790

Tryptophan 0.430 0.230 0.290 0.200 0.290 0.300 0.310 0.290 0.200 0.230 0.120 0.280

Tyrosine 1.61 0.590 0.780 0.460 0.890 0.880 0.910 0.770 0.820 0.850 0.460 0.760

Valine 2.12 1.12 1.24 0.980 1.32 1.31 1.42 1.18 1.12 1.27 0.710 1.18 0.870

Foodstuffs

Aminoacids 30 31 32 33 34 35 36 37 38 39 40 41 42

Alanine 0.970 1.45 1.42 1.24 1.77 1.52 1.88 1.55 1.68 1.67 1.60 1.55

Arginine 0.980 1.21 1.27 1.21 1.16 1.37 1.18 1.16 1.36 1.19 1.33 1.31 1.40

Aspartic acid 1.41 2.31 2.01 2.07 2.15 2.31 2.12 2.90 2.28 2.22 2.32 2.36

Cystine 0.270 0.180 0.250 0.150 0.240 0.240 0.230 0.310 0.220 0.290 0.220 0.170

Glutamic acid 2.34 3.19 3.13 3.18 3.01 3.23 3.17 3.47 3.30 3.23 3.04 3.33

Glycine 0.990 1.00 0.940 0.930 1.15 1.13 1.41 1.38 1.10 1.63 1.24 1.47

Histidine 0.440 0.520 0.420 0.520 0.450 0.480 0.520 0.840 0.610 0.420 0.660 0.460 0.570

Isoleucine 0.740 1.01 1.00 0.990 0.920 1.27 1.04 1.09 1.09 1.14 1.16 1.19 1.07

Leucine 1.32 1.47 1.68 1.69 1.60 1.94 1.75 1.80 1.95 1.78 1.77 1.87 1.78

Lysine 1.24 1.74 2.11 2.05 1.82 1.56 1.75 1.73 2.04 1.90 2.02 2.28 2.02

Methionine 0.380 0.530 0.590 0.600 0.580 0.800 0.660 0.640 0.660 0.640 0.700 0.640 0.660

Phenylalanine 0.660 0.770 0.890 0.840 0.700 0.680 0.750 0.840 0.860 0.840 0.910 0.910 0.920

Proline 0.760 0.640 0.820 0.600 0.810 0.840 0.790 0.790 0.750 1.00 0.850 0.850

Serine 0.620 0.990 0.990 0.860 1.27 1.05 0.900 0.950 1.04 1.01 1.10 0.970

Threonine 0.700 0.810 1.04 0.970 0.920 0.990 1.04 0.970 1.02 1.01 1.11 1.12 1.08

Tryptophan 0.200 0.160 0.210 0.240 0.210 0.260 0.210 0.270 0.320 0.200 0.260 0.240 0.240

Tyrosine 0.510 0.280 0.740 0.710 0.640 0.680 0.670 0.640 0.740 0.560 0.720 0.810 0.680

Valine 0.970 0.950 1.05 1.09 1.06 1.30 1.21 1.21 1.24 1.04 1.39 1.45 1.25

Continued

4820 PROTEIN/Food Sources

Foodstuffs

Aminoacids 43 44 45 46 47 48 49 50 51 52 53 54 55

Alanine 1.61 0.560 0.790 0.800 0.720 0.790 0.500 0.520 0.410 0.300 0.880 0.510 0.370

Arginine 1.25 0.560 0.420 0.240 0.850 0.870 0.570 0.490 0.330 0.420 0.380 0.620 0.430

Aspartic acid 2.88 0.680 0.620 0.540 1.11 1.29 0.780 0.680 0.610 0.480 0.710 0.700 0.480

Cystine 0.290 0.220 0.140 0.160 0.320 0.390 0.110 0.190 0.140 0.130 0.100 0.290 0.240

Glutamic acid 3.52 2.81 1.78 1.86 2.90 3.08 1.58 2.57 2.05 1.92 2.29 4.08 3.66

Glycine 1.17 0.540 0.430 0.340 0.780 0.850 0.410 0.500 0.430 0.320 0.430 0.720 0.420

Histidine 1.09 0.210 0.260 0.240 0.270 0.300 0.170 0.190 0.180 0.240 0.220 0.280 0.220

Isoleucine 1.21 0.460 0.430 0.330 0.560 0.610 0.340 0.390 0.320 0.260 0.580 0.540 0.460

Leucine 2.17 0.800 1.22 1.24 1.02 1.13 0.660 0.670 0.540 0.470 1.36 0.920 0.820

Lysine 2.21 0.380 0.290 0.180 0.550 0.500 0.290 0.400 0.280 0.300 0.260 0.380 0.240

Methionine 0.610 0.180 0.190 0.170 0.230 0.240 0.170 0.140 0.120 0.060 0.200 0.220 0.170

Phenylalanine 1.05 0.590 0.460 0.430 0.700 0.780 0.390 0.470 0.360 0.350 0.440 0.640 0.550

Proline 0.880 1.26 1.02 0.970 0.870 0.840 0.420 1.25 0.840 0.720 1.55 1.56 1.45

Serine 1.05 0.540 0.520 0.470 0.740 0.710 0.410 0.450 0.340 0.350 0.420 0.710 0.660

Threonine 1.18 0.430 0.390 0.320 0.490 0.530 0.280 0.360 0.310 0.250 0.440 0.430 0.320

Tryptophan 0.300 0.150 0.070 0.050 0.190 0.190 0.090 0.110 0.070 0.050 0.110 0.150 0.120

Tyrosine 0.970 0.390 0.380 0.270 0.450 0.570 0.260 0.230 0.220 0.170 0.250 0.410 0.320

Valine 1.42 0.580 0.510 0.440 0.790 0.810 0.490 0.530 0.410 0.330 0.580 0.620 0.490

Foodstuffs

Aminoacids 56 57 58 60 61 62 63 68 69 70 71 72 74

Alanine 0.490 0.240 0.086 0.110 0.350

Arginine 0.600 0.310 0.027 0.044 0.120 0.610 0.065 0.190 0.280 0.110 0.080 0.110 0.042

Aspartic acid 0.660 0.390 0.164 0.430 1.83

Cystine 0.250 0.180 0.004 0.020 0.140 0.025 0.010 0.030

Glutamic acid 3.75 3.15 0.283 0.460 1.89

Glycine 0.630 0.290 0.047 0.120 0.340

Histidine0.2500.1800.0210.0240.0400.2900.0290.0630.1100.0490.0260.0270.021

Isoleucine 0.520 0.380 0.049 0.048 0.100 0.330 0.068 0.130 0.210 0.110 0.043 0.063

Leucine 0.860 0.590 0.053 0.075 0.140 0.590 0.084 0.160 0.230 0.170 0.061 0.077

Lysine 0.350 0.200 0.082 0.074 0.130 0.690 0.066 0.150 0.250 0.140 0.058 0.071 0.034

Methionine 0.210 0.130 0.005 0.018 0.030 0.080 0.028 0.050 0.040 0.048 0.032 0.014 0.007

Phenylalanine 0.590 0.420 0.026 0.047 0.100 0.420 0.079 0.120 0.150 0.077 0.047 0.032 0.054

Proline 1.57 0.960 0.040 0.110 0.470

Serine 0.740 0.390 0.049 0.100 0.460

Threonine 0.390 0.250 0.033 0.044 0.090 0.360 0.068 0.120 0.160 0.110 0.052 0.042 0.043

Tryptophan 0.150 0.080 0.013 0.012 0.030 0.070 0.028 0.037 0.050 0.034 0.020 0.012 0.011

Tyrosine 0.370 0.210 0.025 0.080 0.500 0.071 0.035 0.039

Valine 0.600 0.390 0.047 0.073 0.130 0.490 0.110 0.170 0.240 0.150 0.070 0.046 0.073

Foodstuffs

Aminoacids 75 76 77 78 79 80 81 82 83 84 85 88 90

Alanine 0.026 0.740 1.29 0.480 0.810 1.53 0.015 0.046

Arginine 0.045 0.023 0.100 0.018 1.49 1.48 2.24 3.71 3.46 2.36 0.008 0.054 0.040

Aspartic acid 0.121 2.45 3.16 1.92 3.31 3.99 0.101 0.115

Cystine 0.024 0.001 0.230 0.280 0.250 0.450 0.430 0.590 0.001 0.002

Glutamic acid 0.337 4.33 4.49 3.46 5.63 6.49 0.025 0.105

Glycine 0.018 0.950 1.30 0.590 1.64 1.42 0.009 0.042

Histidine 0.008 0.014 0.049 0.013 0.700 0.530 0.710 0.770 0.710 0.830 0.006 0.077 0.040

Isoleucine 0.019 0.045 0.110 0.023 1.49 1.14 1.19 1.88 1.23 1.78 0.010 0.038 0.060

Leucine 0.025 0.045 0.140 0.030 2.26 1.46 2.11 2.34 2.03 2.84 0.016 0.085 0.062

Lysine 0.026 0.050 0.140 0.029 1.87 1.37 1.89 2.13 1.10 1.90 0.015 0.057 0.044

Methionine 0.007 0.260 0.260 0.220 0.350 0.310 0.580 0.003 0.009 0.022

Phenylalanine 0.014 0.054 0.073 0.024 1.40 0.960 1.40 1.39 1.54 1.97 0.009 0.034 0.051

Proline 0.016 0.980 1.22 0.490 1.43 1.82 0.010 0.040

Serine 0.028 1.38 1.51 0.980 1.83 1.69 0.012 0.049

Threonine 0.016 0.049 0.093 0.023 1.15 0.700 1.12 1.57 0.850 1.49 0.008 0.038 0.049

Tryptophan 0.004 0.009 0.027 0.006 0.230 0.160 0.250 0.350 0.320 0.450 0.002 0.018 0.049

Tyrosine 0.050 0.012 0.970 0.660 0.840 1.22 1.19 1.25 0.005 0.021

Valine 0.021 0.032 0.130 0.023 1.63 0.980 1.39 1.82 1.45 1.76 0.012 0.057 0.076

Table 3 Continued

Continued

PROTEIN/Food Sources 4821

(cheeses, meat, legumes, oil seeds); 10–20% (fish,

eggs); 5–10% (cereals); and < 5% (milk, roots, tubers,

vegetables, fruits, mushrooms).

0004 A considerable number of proteins have been isol-

ated from various foods and characterized by physical

and chemical properties. Cereals and cereal products

are amongst the most important staple foods of man-

kind. Proteins provided by bread consumption in

industrial countries meet about one-third of the

daily requirement. The major cereals are wheat,

maize, rice, barley, sorghum, oats, millet, and rye.

Wheat and rye have a special role since only they

are suitable for bread-making. With the example of

wheat, the cereal proteins have been separated by

Osborne, on the basis of their solubility, into four

fractions: the water-soluble albumins, the salt-soluble

globulins, the 70% aqueous ethanol-soluble prola-

mins, and the remaining glutelins. In the literature,

Osborne fractions derived from different cereals are

often designated by special names, e.g., gliadin and

glutenin for wheat prolamins and glutelins, respect-

ively. The levels of Osborne fractions differ amongst

cereals with albumins amounting to 4–44%, globu-

lins 3–12%, prolamins 2–48%, and glutelins 24–

77% of the whole protein fraction. Each of the

Osborne fractions consists of a larger number of

proteins. Albumins and globulins contain the

enzymes, whereas prolamins and glutelins are storage

proteins. The glutelins can be separated into two

subfractions after reduction of the disulfide bonds:

the low-molecular-weight (LMW) subunits and the

high-molecular-weight (HMW) subunits. Wheat pro-

lamins and glutelins, both fractions together also

designated wheat gluten, are responsible for the char-

acteristic rheological properties of wheat dough.

Wheat prolamins consist of o-, a-, and g-gliadins,

designated according to their electrophoretic mobil-

ity. In addition, the LMW subunit fraction of wheat

glutelins also contains some gliadins (o5-, o1,2-, and

g-gliadins). The amino acid sequences of some gluten

proteins from wheat are shown in the article ‘Protein:

Chemistry‘ (HMW subunit of glutenin, x-type and y-

type; LMW subunit of glutenin; a-gliadin; g-gliadin).

0005Meat and meat products are other important staple

foods, in particular in industrial countries. The main

meat-producing warm-blooded animals are pig,

cattle, poultry, sheep, goats, and buffalo. Meat pro-

teins, i.e., the proteins of the muscle, are divided into

three groups: proteins of the contractile apparatus

(myofibrillar proteins), soluble proteins (sarcoplasma

proteins), and insoluble proteins (connective tissue

and membrane proteins). The myofibrillar proteins

of a typical mammalian muscle amount to about

60% of total muscle protein, with myosin (29%)

and actin (13%) as their predominating components

and about 20 minor components including connectin,

tropomyosins, troponins, and actinins. The sarco-

plasma proteins form about 30% of total protein.

They contain most of the enzymes, in particular

those of glycolysis and the pentosephosphate cycle,

but also considerable amounts of creatine kinase

(2.7% of total protein), myoglobin, and some hemo-

globin. The insoluble proteins contain collagen as the

main component, besides elastin, insoluble enzymes,

and cytochrome c. In connective tissue, collagen

forms a triple-stranded helix composed of a-helices.

Foodstuffs

Aminoacids 92 94 95 96 99 100 101 102 103 105 106

Alanine 0.160 0.091 0.029 0.039 0.044 0.044

Arginine 0.090 0.305 0.073 0.017 0.037 0.200 0.076 0.260 0.650

Aspartic acid 1.14 0.087 0.122 0.090 0.191

Cystine 0.140 0.006 0.003 0.009 0.007 0.014 0.120 0.290 2.08 0.150

Glutamic acid 0.380 0.118 0.066 0.139 0.126

Glycine 0.260 0.063 0.023 0.015 0.034 0.035

Histidine 0.090 0.051 0.012 0.017 0.016 0.057 0.028 0.220 1.59 0.050 0.100

Isoleucine 0.140 0.047 0.020 0.013 0.019 0.110 0.040 0.030 0.210 0.110 0.160

Leucine 0.140 0.075 0.032 0.028 0.044 0.120 0.086 0.120 0.840 0.170 0.400

Lysine 0.140 0.071 0.039 0.029 0.034 0.170 0.039 0.190 1.35 0.150 0.490

Methionine 0.040 0.013 0.008 0.030 0.001 0.023 0.009 0.058 0.420 0.050

Phenylalanine 0.120 0.047 0.020 0.018 0.025 0.074 0.065 0.100 0.730 0.100 0.190

Proline 0.130 0.157 0.189 0.027 0.027 0.040

Serine 0.260 0.051 0.043 0.033 0.033 0.049

Threonine 0.120 0.055 0.020 0.027 0.026 0.087 0.088 0.110 0.750 0.120 0.380

Tryptophan 0.040 0.005 0.007 0.005 0.015 0.024 0.048 0.210 1.46 0.030 0.020

Tyrosine 0.220 0.010 0.013 0.020 0.029 0.066 0.058 0.120 0.860 0.180

Valine 0.290 0.071 0.033 0.039 0.025 0.090 0.052 0.078 0.560 0.140 0.250

Numberingasin Table 2;unitsaregramsofaminoacidper100gofedibleportion.

Data from Scherz H and Senser F (eds) 2000 Food Composition and Nutrition Tables, 6th edn. Boca Raton, FL: CRC Press.

Table 3 Continued

4822 PROTEIN/Food Sources

Covalent cross-links are formed between the fibers of

collagen during maturation and aging, thus improv-

ing its mechanical strength. When heated, collagen

fibers shrink or are converted into gelatine, depending

on the temperature. The structure of the gelatine

obtained after cooling depends on the gelatine con-

centration and temperature gradient. Collagen con-

tains two unusual amino acids, 4-hydroxyproline and

5-hydroxylysine. Since the occurrence of the former is

confined to connective tissue, its determination pro-

vides data on the extent of connective tissue content

of a meat product. The primary structure of bovine

skin collagen is given in the first edition of this encyc-

lopedia (Belitz H-D (1993) Protein-Chemistry. In:

Macrae R, Robinson RK and Sadler MJ (eds) En-

cyclopaedia of Food Science, Food Technology, and

Nutrition, pp. 3781–3791. London: Academic Press)

showing the characteristic three-amino-acid repeats

with glycine in the first position, often followed by

proline and hydroxyproline.

0006 Milk and dairy products form a further important

group of staple foods. Milk generally means cow’s

milk, but milk from buffalo, goats, and sheep is of

importance in some regions. Milk proteins, in par-

ticular the caseins, play an important role in process-

ing to yield dairy products such as cheese and sour

milk products. The caseins, first isolated by Hammar-

sten in 1877, make up about 80% of total milk

proteins. They have been separated later into differ-

ent fractions: a

-, a

-, k-, b-, and g-caseins, consti-

tuting 34, 8, 9, 25, and 4% of total protein,

respectively. Each of these fractions occurs in the

form of different genetic variants, designated A, B,

C, etc., depending on the breed from which they have

been isolated. In cheese-making, the specific cleavage

of k-casein by chymosin (EC 3.4.23.4) into para-k-

casein and a glycopeptide (so-called, though not

always containing a sugar moiety) reduces the solu-

bility of the casein complexes and casein micelles,

thus leading to their aggregation followed by gel

formation (curd formation). The whey proteins

(about 20% of total protein) consist of b-lactoglobu-

lins, a-lactalbumins (both in different genetic vari-

ants), serum albumin, immunoglobulins, and

proteose-peptone. Also, more than 40 enzymes

occur in the whey protein fraction, but in much

lower quantities than the other components. Whey

proteins can be incorporated into the curd using

several new processing methods of cheese-making in

order to increase the yield and also to reduce waste

water or whey treatment costs. The primary struc-

tures of some proteins from bovine milk are shown in

the first edition of this encyclopedia (Belitz H-D

(1993) see above) (a

-, a

-, b-, and k-casein; a-lac-

talbumin; b-lactoglobulin).

0007Legumes (pulses) are very important staple foods in

some parts of the world, e.g., soya beans in South-east

Asia and common beans in Latin America. Other

legumes, some of greater regional importance, in-

clude peas, peanuts, chick peas, broad beans, and

lentils. Legume proteins, when fractionated, according

to Osborne, in a similar way to cereal proteins, yield

three fractions: albumins, globulins, and glutelins.

The portion of the fractions varies, depending on

the legume species, but globulins always predomin-

ate. The globulins are subdivided, initially according

to sedimentation during ultracentrifugation, into 11S

and 7S globulins (legumins and vicilins, respectively).

Again, the subfractions derived from different

legumes are sometimes designated by special names,

e.g., glycinin and arachin for soya bean and peanut

legumin, as well as b-conglycinin and phaseolin for

soya bean and common bean vicilin, respectively.

Soya protein isolates, produced by diluted alkali ex-

traction of defatted soya bean flakes followed by acid

precipitation, are texturized and flavored for use as

meat substitutes or are added to foods to raise their

protein level and improve their processing qualities

such as the water-binding capacity or emulsion stabil-

ity. The isolates contain about 95% protein and con-

sist of 11S and 7S globulins. The similarity between

the caseins from bovine milk and the globulins from

soya beans is reflected by the production of some

typical Asian foods such as soy milk, soy curd

(tofu), and soy cheese (sufu). The primary structures

of some legume proteins are shown in the article

‘Protein: Chemistry‘ (glycinin, pea legumin; b-congly-

cinin, pea vicilin, phaseolin.)

0008Eggs are used as a food not only because of their

excellent nutritional quality but also because of

their functional properties. Eggs generally means

chicken eggs; those of other birds (geese, ducks,

plovers, seagulls, quail) are less important. Egg pro-

teins are divided into those of egg white and those of

egg yolk. Egg white proteins (about 10% of total egg

white) are ovalbumin, conalbumin (ovotransferrin),

ovomucoid, ovomucin, lysozyme, ovoglobulin G

ovoglobulin G

, and some minor components (54,

12, 11, 3.5, 3.4, 4, 4, and 2.5% of total egg white

protein, respectively). Ovalbumin, conalbumin, ovo-

mucin, and the ovoglobulins G contribute to foam

formation and foam stability. Yolk proteins (about

17% of total yolk) are phosvitin, the livetins, and

the protein moieties of high-density lipoproteins

(HDL) and low-density (LDL) lipoproteins (13, 31,

36, and 20% of total yolk protein, respectively).

Apart from phospholipids, LDL and proteins are re-

sponsible for the emulsifying effect of whole eggs or

egg yolk alone. Owing to the ability of all egg pro-

teins, except ovomucoid and phosvitin, to coagulate

PROTEIN/Food Sources 4823

when heated, egg products are important food-

binding agents.

0009 The amino acid compositions of selected foodstuffs

are shown in Table 3. The nutritional quality of a

food protein depends on the absolute content of es-

sential amino acids, the relative proportions of essen-

tial amino acids, and their ratios to nonessential

amino acids. In addition, the digestibility of the food

protein, the influence by other food components such

as dietary fibers, polyphenols, or proteinase inhibi-

tors, and also the total food energy intake are of

importance. (See ‘Protein: Requirements‘; Protein:

Quality.) The daily requirements of humans at differ-

ent ages for essential amino acids are compiled in

Table 4. During pregnancy and lactation, the first 6

months, and after 6 months, the daily requirement

increases by 13, 24, and 18%, respectively. The bio-

logical value of a protein is generally limited by the

following amino acids:

0010 lysine: deficient in proteins of cereals and other

plants;

0011 methionine: deficient in proteins of bovine milk

and meat;

0012 threonine: deficient in wheat and rye;

0013 tryptophan: deficient in casein, corn and rice.

0014 The biological values of some important food pro-

teins are given in the article ‘Protein: Quality‘. The

highest value observed so far is for a blend of 35%

egg and 65% potato proteins (one chicken egg and

500 g of potatoes).

See also: Amino Acids: Metabolism; Rheological

Properties of Food Materials; Protein: Requirements;

Functional Properties; Quality; Single-cell Protein:

Algae; Yeasts and Bacteria