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Proteins - Principle of food chemistry
INTRODUCTION Proteins are polymers of some 21 different amino acids joined together by peptide bonds. Because of the variety of side chains that occur when these amino acids are linked together, the different proteins may have dif- ferent chemical properties and widely differ- ent secondary and tertiary structures. The various amino acids joined in a peptide chain are shown in Figure 3-1. The amino acids are grouped on the basis of the chemical nature of the side chains (Krull and Wall 1969). The side chains may be polar or non- polar. High levels of polar amino acid resi- dues in a protein increase water solubility. The most polar side chains are those of the basic and acidic amino acids. These amino acids are present at high levels in the soluble albumins and globulins. In contrast, the wheat proteins, gliadin and glutenin, have low levels of polar side chains and are quite insoluble in water. The acidic amino acids may also be present in proteins in the form of their amides, glutamine and asparagine. This increases the nitrogen content of the protein. Hydroxyl groups in the side chains may become involved in ester linkages with phos- phoric acid and phosphates. Sulfur amino acids may form disulfide cross-links between neighboring peptide chains or between dif- ferent parts of the same chain. Proline and hydroxyproline impose significant structural limitations on the geometry of the peptide chain. Proteins occur in animal as well as vegeta- ble products in important quantities. In the developed countries, people obtain much of their protein from animal products. In other parts of the world, the major portion of dietary protein is derived from plant prod- ucts. Many plant proteins are deficient in one or more of the essential amino acids. The protein content of some selected foods is listed in Table 3-1. AMINO ACID COMPOSITION Amino acids joined together by peptide bonds form the primary structure of proteins. The amino acid composition establishes the nature of secondary and tertiary structures. These, in turn, significantly influence the functional properties of food proteins and their behavior during processing. Of the 20 amino acids, only about half are essential for human nutrition. The amounts of these essen- tial amino acids present in a protein and their availability determine the nutritional quality of the protein. In general, animal proteins are of higher quality than plant proteins. Plant Proteins CHAPTER 3 Figure 3-1 Component Amino Acids of Proteins Joined by Peptide Bonds and Character of Side Chains. Source: From Northern Regional Re- search Laboratory, U.S. Department of Agricul- ture. proteins can be upgraded nutritionally by judicious blending or by genetic modification through plant breeding. The amino acid com- position of some selected animal and vegeta- ble proteins is given in Table 3—2. Egg protein is one of the best quality pro- teins and is considered to have a biological value of 100. It is widely used as a standard, and protein efficiency ratio (PER) values sometimes use egg white as a standard. Cereal proteins are generally deficient in lysine and threonine, as indicated in Table Table 3-1 Protein Content of Some Selected Foods Product Protein (g/1 OO g) Meat: beef 16.5 pork 10.2 Chicken (light meat) 23.4 Fish: haddock 18.3 cod 17.6 Milk 3.6 Egg 12.9 Wheat 13.3 Bread 8.7 Soybeans: dry, raw 34.1 cooked 11.0 Peas 6.3 Beans: dry, raw 22.3 cooked 7.8 Rice: white, raw 6.7 cooked 2.0 Cassava 1.6 Potato 2.0 Corn 10.0 3-3. Soybean is a good source of Iysine but is deficient in methionine. Cottonseed pro- tein is deficient in lysine and peanut protein in methionine and lysine. The protein of potato although present in small quantity (Table 3-1) is of excellent quality and is equivalent to that of whole egg. Table 3-3 Limiting Essential Amino Acids of Some Grain Proteins First Second Limiting Limiting Grain Amino Acid Amino Acid Wheat Lysine Threonine Corn Lysine Tryptophan Rice Lysine Threonine Sorghum Lysine Threonine Millet Lysine Threonine PROTEIN CLASSIFICATION Proteins are complex molecules, and classi- fication has been based mostly on solubility in different solvents. Increasingly, however, as more knowledge about molecular composi- tion and structure is obtained, other criteria are being used for classification. These include behavior in the ultracentrifuge and electrophoretic properties. Proteins are di- vided into the following main groups: simple, conjugated, and derived proteins. Simple Proteins Simple proteins yield only amino acids on hydrolysis and include the following classes: • Albumins. Soluble in neutral, salt-free water. Usually these are proteins of rela- tively low molecular weight. Examples Table 3-2 Amino AcJd Content of Some Selected Foods (mg/g Total Nitrogen) Amino Acid lsoleucine Leucine Lysine Methlonine Cystine Phenylalanine Tyroslne Threonine Valine Arginine Histidine Alanine Aspartic acid Glutamic acid Glycine Proline Serine Meat (Beef) 301 507 556 169 80 275 225 287 313 395 213 365 562 955 304 236 252 Milk 399 782 450 156 434 396 278 463 160 214 255 424 1151 144 514 342 Egg 393 551 436 210 152 358 260 320 428 381 152 370 601 796 207 260 478 Wheat 204 417 179 94 159 282 187 183 276 288 143 226 308 1866 245 621 281 Peas 267 425 470 57 70 287 171 254 294 595 143 255 685 1009 253 244 271 Com 230 783 167 120 97 305 239 225 303 262 170 471 392 1184 231 559 311 are egg albumin, lactalbumin, and serum albumin in the whey proteins of milk, leucosin of cereals, and legumelin in legume seeds. • Globulins. Soluble in neutral salt solu- tions and almost insoluble in water. Examples are serum globulins and (3-lac- toglobulin in milk, myosin and actin in meat, and glycinin in soybeans. • Glutelins. Soluble in very dilute acid or base and insoluble in neutral solvents. These proteins occur in cereals, such as glutenin in wheat and oryzenin in rice. • Prolamins. Soluble in 50 to 90 percent ethanol and insoluble in water. These proteins have large amounts of proline and glutamic acid and occur in cereals. Examples are zein in corn, gliadin in wheat, and hordein in barley. • Scleroproteins. Insoluble in water and neutral solvents and resistant to enzymic hydrolysis. These are fibrous proteins serving structural and binding purposes. Collagen of muscle tissue is included in this group, as is gelatin, which is derived from it. Other examples include elastin, a component of tendons, and keratin, a component of hair and hoofs. • Histories. Basic proteins, as defined by their high content of lysine and arginine. Soluble in water and precipitated by ammonia. • Protamines. Strongly basic proteins of low molecular weight (4,000 to 8,000). They are rich in arginine. Examples are clupein from herring and scombrin from mackerel. Conjugated Proteins Conjugated proteins contain an amino acid part combined with a nonprotein mate- rial such as a lipid, nucleic acid, or carbohy- drate. Some of the major conjugated proteins are as follows: • Phosphoproteins. An important group that includes many major food proteins. Phosphate groups are linked to the hydroxyl groups of serine and threonine. This group includes casein of milk and the phosphoproteins of egg yolk. • Lipoproteins. These are combinations of lipids with protein and have excellent emulsifying capacity. Lipoproteins occur in milk and egg yolk. • Nucleoproteins. These are combinations of nucleic acids with protein. These compounds are found in cell nuclei. • Glycoproteins. These are combinations of carbohydrates with protein. Usually the amount of carbohydrate is small, but some glycoproteins have carbohydrate contents of 8 to 20 percent. An example of such a mucoprotein is ovomucin of egg white. • Chromopmteins. These are proteins with a colored prosthetic group. There are many compounds of this type, including hemoglobin and myoglobin, chlorophyll, and flavoproteins. Derived Proteins These are compounds obtained by chemi- cal or enzymatic methods and are divided into primary and secondary derivatives, de- pending on the extent of change that has taken place. Primary derivatives are slightly modified and are insoluble in water; rennet- coagulated casein is an example of a primary derivative. Secondary derivatives are more extensively changed and include proteoses, peptones, and peptides. The difference between these breakdown products is in size and solubility. All are soluble in water and not coagulated by heat, but proteoses can be precipitated with saturated ammonium sul- fate solution. Peptides contain two or more amino acid residues. These breakdown prod- ucts are formed during the processing of many foods, for example, during ripening of cheese. PROTEIN STRUCTURE Proteins are macromolecules with different levels of structural organization. The primary structure of proteins relates to the peptide bonds between component amino acids and also to the amino acid sequence in the mole- cule. Researchers have elucidated the amino acid sequence in many proteins. For exam- ple, the amino acid composition and se- quence for several milk proteins is now well established (Swaisgood 1982). Some proteolytic enzymes have quite spe- cific actions; they attack only a limited num- ber of bonds, involving only particular amino acid residues in a particular sequence. This may lead to the accumulation of well-defined peptides during some enzymic proteolytic reactions in foods. The secondary structure of proteins in- volves folding the primary structure. Hydro- gen bonds between amide nitrogen and car- bonyl oxygen are the major stabilizing force. These bonds may be formed between differ- ent areas of the same polypeptide chain or between adjacent chains. In aqueous media, the hydrogen bonds may be less significant, and van der Waals forces and hydrophobic interaction between apolar side chains may contribute to the stability of the secondary structure. The secondary structure may be either the oc-helix or the sheet structure, as shown in Figure 3-2. The helical structures are stabilized by intramolecular hydrogen bonds, the sheet structures by intermolecular hydrogen bonds. The requirements for maxi- mum stability of the helix structure were established by Pauling et al. (1951). The helix model involves a translation of 0.54 nm per turn along the central axis. A complete turn is made for every 3.6 amino acid resi- dues. Proteins do not necessarily have to occur in a complete a-helix configuration; rather, only parts of the peptide chains may be helical, with other areas of the chain in a more or less unordered configuration. Pro- teins with a-helix structure may be either globular or fibrous. In the parallel sheet structure, the polypeptide chains are almost fully extended and can form hydrogen bonds between adjacent chains. Such structures are generally insoluble in aqueous solvents and are fibrous in nature. The tertiary structure of proteins involves a pattern of folding of the chains into a com- pact unit that is stabilized by hydrogen bonds, van der Waals forces, disulfide bridges, and hydrophobic interactions. The tertiary struc- ture results in the formation of a tightly packed unit with most of the polar amino acid residues located on the outside and hydrated. This leaves the internal part with most of the apolar side chains and virtually no hydration. Certain amino acids, such as proline, disrupt the a-helix, and this causes fold regions with random structure (Kinsella 1982). The nature of the tertiary structure varies among proteins as does the ratio of a-helix and random coil. Insulin is loosely folded, and its tertiary struc- ture is stabilized by disulfide bridges. Lyso- zyme and glycinin have disulfide bridges but are compactly folded. Large molecules of molecular weights above about 50,000 may form quaternary structures by association of subunits. These structures may be stabilized by hydrogen bonds, disulfide bridges, and hydrophobic interactions. The bond energies involved in Figure 3-2 Secondary Structures of Proteins, (A) Alpha Helix, (B) Antiparallel Sheet 3rd turn 2nd turn 1st turn Rise per residue forming these structures are listed in Table 3-4. The term subunit denotes a protein chain possessing an internal covalent and noncova- lent structure that is capable of joining with other similar subunits through noncovalent forces or disulfide bonds to form an oligo- meric macromolecule (Stanley and Yada 1992). Many food proteins are oligomeric and consist of a number of subunits, usually 2 or 4, but occasionally as many as 24. A list- ing of some oligomeric food proteins is given in Table 3-5. The subunits of proteins are held together by various types of bonds: electrostatic bonds involving carboxyl, amino, imidazole, and guanido groups; hy- drogen bonds involving hydroxyl, amide, and phenol groups; hydrophobic bonds in- volving long-chain aliphatic residues or aro- matic groups; and covalent disulfide bonds involving cystine residues. Hydrophobic bonds are not true bonds but have been described as interactions of nonpolar groups. These nonpolar groups or areas have a ten- dency to orient themselves to the interior of the protein molecule. This tendency depends on the relative number of nonpolar amino Table 3-4 Bond Energies of the Bonds Involved in Protein Structure Bond Energy* Bond (kcal/mole) Covalent C-C 83 Covalent S-S 50 Hydrogen bond 3-7 Ionic electrostatic bond 3-7 Hydrophobic bond 3-5 Van der Waals bond 1 -2 These refer to free energy required to break the bonds: in the case of a hydrophobic bond, the free energy required to unfold a nonpolar side chain from the interior of the molecule into the aqueous medium. acid residues and their location in the peptide chain. Many food proteins, especially plant storage proteins, are highly hydrophobic—so much so that not all of the hydrophobic areas can be oriented toward the inside and have to be located on the surface. This is a possible factor in subunits association and in some cases may result in aggregation. The hydro- phobicity values of some food proteins as reported by Stanley and Yada (1992) are listed in Table 3-6. The well-defined secondary, tertiary, and quaternary structures are thought to arise directly from the primary structure. This means that a given combination of amino acids will automatically assume the type of structure that is most stable and possible given the considerations described by Paul- ing etal. (1951). Table 3-5 Oligomeric Food Proteins Molecular Protein Weight (d) Subunits Lactoglobulin 35,000 2 Hemoglobin 64,500 4 Avidin 68,300 4 Lipoxygenase 108,000 2 Tyrosinase 128,000 4 Lactate 140,000 4 dehydrogenase 7S soy protein 200,000 9 Invertase 210,000 4 Catalase 232,000 4 Collagen 300,000 3 11S soy protein 350,000 12 Legumin 360,000 6 Myosin 475,000 6 Source: Reprinted with permission from D.W. Stanley and R.Y. Yada, Thermal Reactions in Food Protein Sys- tems, Physical Chemistry of Foods, H.G. Schwartzberg and R.H. Hartel, eds., p. 676, 1992, by courtesy of Mar- cel Dekker, Inc. DENATURATION Denaturation is a process that changes the molecular structure without breaking any of the peptide bonds of a protein. The process is peculiar to proteins and affects different pro- teins to different degrees, depending on the structure of a protein. Denaturation can be brought about by a variety of agents, of which the most important are heat, pH, salts, and surface effects. Considering the com- plexity of many food systems, it is not sur- prising that denaturation is a complex pro- cess that cannot easily be described in simple terms. Denaturation usually involves loss of biological activity and significant changes in some physical or functional properties such as solubility. The destruction of enzyme activity by heat is an important operation in food processing. In most cases, denaturation is nonreversible; however, there are some Table 3-6 Hydrophobicity Values of Some Food Proteins Hydrophobicity Protein cal/residue Gliadin 1300 Bovine serum albumin 1120-1000 oc-Lactalbumin 1050 (3-Lactoglobulin 1050 Actin 1000 Ovalbumin 980 Collagen 880 Myosin 880 Casein 725 Whey protein 387 Gluten 349 Source: Reprinted with permission from D.W. Stanley and R.Y. Yada, Thermal Reactions in Food Protein Sys- tems, Physical Chemistry of Foods, H.G. Schwartzberg and R.H. Hartel, eds., p. 677, 1992, by courtesy of Mar- cel Dekker, Inc. exceptions, such as the recovery of some types of enzyme activity after heating. Heat denaturation is sometimes desirable—for example, the denaturation of whey proteins for the production of milk powder used in baking. The relationship among temperature, heating time, and the extent of whey protein denaturation in skim milk is demonstrated in Figure 3-3 (Harland et al. 1952). The proteins of egg white are readily dena- tured by heat and by surface forces when egg white is whipped to a foam. Meat proteins are denatured in the temperature range 57 to 75 0 C, which has a profound effect on tex- ture, water holding capacity, and shrinkage. Denaturation may sometimes result in the flocculation of globular proteins but may also lead to the formation of gels. Foods may be denatured, and their proteins destabilized, during freezing and frozen storage. Fish pro- teins are particularly susceptible to destabili- zation. After freezing, fish may become tough and rubbery and lose moisture. The caseinate micelles of milk, which are quite stable to heat, may be destabilized by freez- ing. On frozen storage of milk, the stability of the caseinate progressively decreases, and this may lead to complete coagulation. Protein denaturation and coagulation are aspects of heat stability that can be related to the amino acid composition and sequence of the protein. Denaturation can be defined as a major change in the native structure that does not involve alteration of the amino acid sequence. The effect of heat usually involves a change in the tertiary structure, leading to a less ordered arrangement of the polypeptide chains. The temperature range in which denaturation and coagulation of most pro- teins take place is about 55 to 75 0 C, as indi- cated in Table 3-7. There are some notable exceptions to this general pattern. Casein and gelatin are examples of proteins that can be boiled without apparent change in stability. The exceptional stability of casein makes it possible to boil, sterilize, and concentrate milk, without coagulation. The reasons for this exceptional stability have been discussed by Kirchmeier (1962). In the first place, restricted formation of disulfide bonds due to low content of cystine and cysteine results in increased stability. The relationship between coagulation temperature as a measure of sta- Figure 3-3 Time-Temperature Relationships for the Heat Denaturation of Whey Proteins in Skim Milk. Source: From H.A. Harland, S.T. Coulter, and R. Jenness, The Effects of Various Steps in the Manufacture on the Extent of Serum Protein Denaturation in Nonfat Dry Milk Solids. /. Dairy ScL 35: 363-368, 1952. TCNPCKATURf PER CENT DeNATUMTK)N TIME OF HCATINO IN MINUTCS bility and sulfur amino acid content is shown in Tables 3-7 and 3-8. Peptides, which are low in these particular amino acids, are less likely to become involved in the type of sulf- hydryl agglomeration shown in Figure 3-4. Casein, with its extremely low content of sulfur amino acids, exemplifies this behav- ior. The heat stability of casein is also explained by the restraints against forming a folded tertiary structure. These restraints are due to the relatively high content of proline and hydroxyproline in the heat stable pro- teins (Table 3-9). In a peptide chain free of proline, the possibility of forming inter- and intramolecular hydrogen bonds is better than in a chain containing many proline residues (Figure 3-5). These considerations show how amino acid composition directly relates to secondary and tertiary structure of pro- teins; these structures are, in turn, responsi- ble for some of the physical properties of the protein and the food of which it is a part. NONENZYMIC BROWNING The nonenzymic browning or Maillard reaction is of great importance in food man- ufacturing and its results can be either desir- Table 3-7 Heat Coagulation Temperatures of Some Albumins and Globulins and Casein Coagulation Protein Temp. ( 0 C) Egg albumin 56 Serum albumin (bovine) 67 Milk albumin (bovine) 72 Legumelin (pea) 60 Serum globulin (human) 75 p-Lactoglobulin (bovine) 70-75 Fibrinogen (human) 56-64 Myosin (rabbit) 47-56 Casein (bovine) 160-200 able or undesirable. For example, the brown crust formation on bread is desirable; the brown discoloration of evaporated and steril- ized milk is undesirable. For products in which the browning reaction is favorable, the resulting color and flavor characteristics are generally experienced as pleasant. In other products, color and flavor may become quite unpleasant. The browning reaction can be defined as the sequence of events that begins with the reac- tion of the amino group of amino acids, pep- tides, or proteins with a glycosidic hydroxyl group of sugars; the sequence terminates with the formation of brown nitrogenous polymers or melanoidins (Ellis 1959). The reaction velocity and pattern are influ- enced by the nature of the reacting amino acid or protein and the carbohydrate. This means that each kind of food may show a different browning pattern. Generally, Iysine is the most reactive amino acid because of the free £-amino group. Since lysine is the limiting essential amino acid in many food proteins, its destruction can substantially reduce the nutritional value of the protein. Foods that are rich in reducing sugars are very reactive, and this explains why lysine in milk is destroyed more easily than in other Table 3-8 Cysteine and Cystine Content of Some Proteins (g Amino Acid/100 g Protein) Cysteine Cystine Protein (%) (%) Egg albumin 1.4 0.5 Serum albumin 0.3 5.7 (bovine) Milk albumin 6.4 — p-Lactoglobulin 1.1 2.3 Fibrinogen 0.4 2.3 Casein — 0.3 [...]... R-S-CH3 R-SO-CH3 R-SO2-CH3 R-S-S-R R-SO-SO-R R-SO2-SO2-R R-SH R-SOH R-SO2H R-SO3H Proteins react with polyphenols such as phenolic acids, flavonoids, and tannins, which occur widely in plant products These reactions may result in the lowering of available lysine, protein digestibility, and biological value (Hurrell 1984) Racemization is the result of heat and alkaline treatment of food proteins The... Leatherhead, England, 1972 Amadori Rearrangement a-D-Glucopyranosylamine 1-Amino-l-deoxy-aD-fructopyranose Figure 3-9 Amadori Rearrangement Source: From MJ Kort, Reactions of Free Sugars with Aqueous Ammonia, Adv Carbohydrate Chem Biochem., Vol 25, pp 31 1-3 49, 1970 Figure 3-1 0 Structure of 1-Deoxy-l-Glycino-pD-Fructose undergo hydrolysis and form the enolform (7) of 3-deoxyosulose (8) In another step the Schiff... into amino- group of lysine to yield lysmoalanine (Ziegler 1964) as shown: NH2 - C H - ( C H 2 ) 4 - N H 2 + CH2 = C-COOH » • COOH NH2 NH2 - C H - ( C H 2 ) 4 - N H - C H 2 —CH-COOH COOH NH 2 Figure 3-1 6 Formation of Amide-Type Bonds from the Reaction Between £-amine Groups of Lysine and Amide Groups of Asparagine (n = 1) Glutamine (n = 2) Source: From J Bjarnason and K J Carpenter, Mechanisms of Heat... ornithinoalanme is formed Treatment of proteins with ammonia can result in addition of ammonia to dehydroalanine to yield (3-amino-alanine as follows: CH2=C-COOH-HNH3 NH2 > NH2 - C H 2 —CH-COOH I NH2 Ammo-acrylic acid (dehydroalanine) is very reactive and can combine with the E-amino Light-induced oxidation of proteins has been shown to lead to off-flavors and destruction of essential amino acids in milk... formation of polymers, initially of small size, highly hydrated, and in colloidal form These initial products of condensation are fluorescent, and continuation of the reaction results in the formation of the brown melanoidins These polymers are of nondistinct composition and contain cr-D-Fructopyranosylamine 2-Amino-2-deoxy-o?D-glucopyranose Figure 3-1 1 Heyns Rearrangement Source: From MJ Kort, Reactions of. .. addition of the amine to the carbonyl group of the open-chain form of the sugar, elimination of a molecule of water, and closure of the ring The rate is high at low water content; this explains the ease of browning in dried and concentrated foods The Amadori rearrangement of the glycosylamines involves the presence of an acid catalyst and leads to the formation of ketoseamine or 1-amino-1-deoxyketose... the a-amino acid Table 3-1 0 Aroma and Structure Classification of Browned Flavor Compounds Aromas: Structure: Examples of compounds: Burnt (pungent, empyreumatic) Polycarbonyls(a,p-Unsat'd aldehydes-C:O-C: 0-= C-CHO) I Glyoxal Pyruvaldehyde Diacetyl Mesoxalic dialdehyde Variable (aldehydic, ketonic) Monocarbonyls (R-CHO, R-C:0-CH3) Strecker aldehydes lsobutyric Isovaleric Methiona! 2-Furaldehydes 2-Pyrrole... according Loss of lysine Milk Peanut Cotton Wheat Heating at 150° (minutes) Figure 3-6 Loss of Lysine Occurring as a Result of Heating of Several Foods Source: From J Adrian, The Maillard Reaction IV Study on the Behavior of Some Amino Acids During Roasting of Proteinaceous Foods, Ann Nutr Aliment (French), Vol 21, pp 12 9-1 47, 1967 to the scheme shown in Figure 3-9 In the reaction of D-glucose with... 3-5 Effect of Proline Residues on Possible Hydrogen Bond Formation in Peptide Chains (A) Proline-free chain; (B) proline-containing chain; (C) hydrogen bond formation in proline-free and proline-containing chains Source: From O Kirchmeier, The Physical-Chemical Causes of the Heat Stability of Milk Proteins, Milchwissenschaft (German), Vol 17, pp 40 8-4 12, 1962 of an aldoseamine with a second mole of. .. Reaction Rate of D-Glucose with DL-Leucine Source: From G Haugard, L Tumerman, and A Sylvestri, A Study on the Reaction of Aldoses and Amino Acids, / Am Chem Soc., Vol 73, pp 459 4-4 600, 1951 Millimole of OL-leucme per ml Time (minutes) Figure 3-1 5 Effect of pH on the Reaction Rate of D-Glucose with DL-Leucine Source: From G Haugard, L Tumerman, and A Sylvestri, A Study on the Reaction of Aldoses and . (Beef) 30 1 507 556 16 9 80 275 225 287 31 3 39 5 2 13 365 562 955 30 4 236 252 Milk 39 9 782 450 15 6 434 39 6 278 4 63 16 0 214 255 424 11 51 144 514 34 2 Egg 39 3 5 51 436 210 15 2 35 8 260 32 0 428 38 1 152 37 0 6 01 796 207 260 478 Wheat 204 417 17 9 94 15 9 282 18 7 1 83 276 288 1 43 226 30 8 18 66 245 6 21 2 81 Peas 267 425 470 57 70 287 17 1 254 294 595 1 43 255 685 10 09 2 53 244 2 71 Com 230 7 83 16 7 12 0 97 30 5 239 225 30 3 262 17 0 4 71 39 2 11 84 2 31 559 31 1 are. (Beef) 30 1 507 556 16 9 80 275 225 287 31 3 39 5 2 13 365 562 955 30 4 236 252 Milk 39 9 782 450 15 6 434 39 6 278 4 63 16 0 214 255 424 11 51 144 514 34 2 Egg 39 3 5 51 436 210 15 2 35 8 260 32 0 428 38 1 152 37 0 6 01 796 207 260 478 Wheat 204 417 17 9 94 15 9 282 18 7 1 83 276 288 1 43 226 30 8 18 66 245 6 21 2 81 Peas 267 425 470 57 70 287 17 1 254 294 595 1 43 255 685 10 09 2 53 244 2 71 Com 230 7 83 16 7 12 0 97 30 5 239 225 30 3 262 17 0 4 71 39 2 11 84 2 31 559 31 1 are. Albumin 1. 8 6 .3 5.9 12 .3 2.6 4.2 5.8 6.0 0.8 6.6 5 .1 4.8 10 .9 16 .5 12 .8 5.9 4.0 Casein 1. 9 3. 1 6.8 9.2 5.6 5 .3 4.4 0 .3 1. 8 5 .3 5.7 13 . 5 7.6 24.5 8.9 3. 3 3. 8 Gelatin 27.5 11 .0 2.6 3. 3 1. 7 4.2 2.2 0.0 0.9 2.2 0 .3 16 .4 14 .1 6.7 11 .4 4.5 8.8 0.8 of