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

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lobules. In spite of this, the liver functions are well

maintained. (See Fats: Digestion, Absorption, and

Transport.)

Muscle Wasting

0013 Because of increased protein catabolism, skeletal

muscle is often wasted, and subjects limit their phys-

ical activity. In many cases, the parallel prevailing

edema conceals the wasting.

Edema

0014 Edema is a frequent sign in protein deficiency. It

generally causes swelling of the tissues but usually

appears in the feet and lower legs. The etiology of

edema has not been completely clarified. Protein

deficiency or, rather, hypoalbuminemia is only one

of the factors.

0015 Other common features of protein deficiency are

changes in the hair; it becomes easily plucked, and

there are changes in texture and color. Also common

are skin changes, with areas of desquamation and

hypo- and hyperpigmentation. Protein deficiency

also leads to mental changes, lethargy, fatigue, anor-

exia, and some degree of anemia, and is frequently

associated with infections. In children, severe protein

deficiency leads to growth retardation.

Physiological Effects

0016 Organs and systems can be changed in protein defi-

ciency, and this may alter their physiological func-

tions.

Cardiovascular System

0017 Muscle wasting in the heart leads to reduced cardiac

output and poor circulation. As a result, the pulse rate

of many patients is low and even impalpable, and the

extremities are cold and pale.

Renal Function

0018 Structural abnormalities of the kidney have not been

observed, but the glomerular filtration rate may be

diminished. The concentrating power can also be

impaired, which may be attributable to a low blood

urea concentration or an accompanying potassium

deficiency.

Pancreas

0019 Atrophic changes and fibrosis of the pancreas have

been observed both in children dying of protein mal-

nutrition and in animals fed low-protein diets. The

atrophy affects the acinar cells, responsible for exo-

crine secretion, but not the islets, responsible for

hormone (glucagon and insulin) production. A re-

duced number of secretory granules have also been

observed. These changes result in impaired pancreatic

function with a diminished secretion of pancreatic

enzymes into the intestine. Glucagon and insulin

secretion are also lowered in severe cases of protein

deficiency.

0020In another abnormality of the pancreas in protein-

deficient animals, the protein content and phospho-

lipid levels are reduced, and triglyceride and

cholesteryl ester levels are increased. This altered

pancreatic lipid composition in protein-deficient

animals may lead to changes in the integrity of pan-

creatic membranes and can predispose to injury.

Gastrointestinal Tract

0021The mucosal epithelial cells of the small intestine make

up the tissue with the highest turnover rate and are

therefore very vulnerable to protein deficiency. The

intestine wall becomes thin, the mucous membrane is

smooth and atrophic, and the villi are frequently flat-

tened. The enzyme-secreting cells and the pancreatic

acinar cells may be affected in a similar way. As a

result, the production of digestive enzymes, primarily

the disaccharidases, is reduced, and the absorptive and

digestive capacity of nutrients is impaired.

0022A feature seen in biopsis after protein deficiency is

the intense cellular infiltration in the mucosa and

submucosa, especially with lymphocytes and plasma

cells. The colon can be less affected, but the muscle

coats may also be atrophic, and the surface epithe-

lium may show infiltration of plasma cells.

Endocrine System

0023The pituitary gland responds effectively to the stimu-

lus of protein deficiency; thus, the secretion of human

growth hormone is normal or, possibly, supranormal,

and the thyroid-stimulating hormone is elevated. (See

Hormones: Thyroid Hormones; Pituitary Hormones.)

0024The thyroid may be atrophic, with reduced colloid

tissue, and the vascular cells are flattened and in-

active. The thyroid function appears to be normal,

but there are data suggesting a reduced function of

the thyroid gland. The concentrations of thyroxine-

binding proteins and of thyroxine are reduced.

0025The adrenal gland appears atrophic, but neverthe-

less, plasma cortisol levels are elevated and may lead

to metabolic disturbances. A reduced insulin secre-

tion, as seen after glucose tolerance tests, can also be

predominant in protein deficiency. (See Glucose: Glu-

cose Tolerance and the Glycemic (Glycaemic) Index;

Hormones: Adrenal Hormones.)

Immunological System

0026In protein deficiency, the lymphoid tissue – primarily

the thymus gland, but also lymph nodes and spleen – is

PROTEIN/Deficiency 4865

atrophic. First, the production of thymus-dependent

lymphocytes (T cells) is reduced and is responsible for

the diminished cell-mediated immunity to infectious

agents, as demonstrated in specific tuberculin skin

tests. Second, the phagocytic activity of neutrophils

and antibody formation are reduced. The body there-

fore becomes more susceptible to infections which can

be fatal. (See Immunology of Food.)

Treatment

0027 The dietary treatment of protein deficiency depends

on the cause of the deficiency. In patients with

nephrotic syndrome, characterized by massive losses

of albumin into urine, the intake of protein should be

increased to 90–120 g per day for adults when hepatic

synthesis of albumin can compensate in part the

urinary losses. (See Kwashiorkor; Marasmus.)

0028 In cases of acute renal failure, the protein intake

should be only 20 g per day. This reduces protein

metabolism and the production of urea. In patients

with liver cirrhosis, the protein intake should also be

reduced because a high-protein diet can precipitate

hepatic encephalopathy.

Special Feeding Methods

0029 Protein deficiency can be treated by tube feeding,

intravenous feeding or supplementary feeding. Tube

feeding is indicated in patients with severe malnutri-

tion who are unable to eat. Parenteral nutrition is

essential when the small intestine is unable to digest

and absorb nutrients, or after severe injuries such as

burns.

0030 In mild protein deficiencies, an increased oral

intake of protein is sufficient to meet dietary needs.

Several special preparations containing high levels of

protein are available on the market.

Complications

0031 Since protein deficiency is frequently accompanied by

infections, dehydration, and deprivation of vitamins

and electrolytes, these conditions need special atten-

tion. In most cases of severe protein deficiency, there-

fore, treatment has to start with fluid replacement

and, if indicated, with antibiotic therapy.

Adaptation to Low-protein Intakes

0032 Waterlow (1986) described two possibilities of nutri-

tional adaptation, which he called the first and second

line of defense. The first line of defense is the ability to

achieve nitrogen balance at various levels of protein

intake. The second line of defense is necessary after

the capacity to economize nitrogen is exhausted,

resulting in a reduced lean body mass or reduced

growth rate in children. In this section, only the first

line of defense is discussed.

Nitrogen Balance

0033Nitrogen equilibrium is a state in which, for a given

intake of nitrogen, an equivalent amount of nitrogen

is lost from the body via urine, feces, skin, sweat, etc.

When protein intake is low, dietary protein is used

more efficiently; this is exemplified by the excretion

of nitrogen, particularly in the form of urea, which is

reduced and so contributes to the restoration of nitro-

gen balance. In this situation, the pathways of amino

acid synthesis predominate.

0034The liver plays an important role in the adaptive

process, since it is the only organ which can transform

the nitrogen from amino acids into urea. The alter-

ations in the nitrogen pathway are mainly brought

about by changes in the activity of liver enzymes.

Experiments with animals have shown that in protein

deficiency, urea cycle enzymes, such as argininosucci-

nase, are low, and the amino-acid-activating enzymes

are high. Table 2 shows this finding in children with

severe protein malnutrition, soon after admission to

hospital and after recovery.

0035The metabolic activity of the gastrointestinal tract

is important in the adaptive process. Normally, one-

third of the urea produced is passed into the bowel

and can be hydrolyzed by the gut microflora. Thus,

urea nitrogen becomes available for metabolic inter-

action, e.g., for the synthesis of nonessential and

essential amino acids. However, the necessary carbon

skeletons can be the limiting factor in adaptation. In

protein deficiency, a greater proportion of urea is

retained by the body. (See Microflora of the Intestine:

Role and Effects.)

0036Recent investigations have shown that the colon is

permeable to urea and amino acids, thus supporting

the view that hydrolysis of urea plays an important

nutritional role.

tbl0002Table 2 Enzymatic activity of the livers of children with

malnutrition

Amino-acid-activating

enzymes

Argininosuccinase

Soon after admission 1.44 1.06

1–2 months later 0.91 1.46

The figures for the amino-acid-activating enzymes are the means of 18

measurements and are expressed in micromoles of phosphorus

exchanged per milligram of protein; for argininosuccinase, the figures are

the mean of 11 measurements and expressed in micromoles of urea per

milligram of protein. The changes on recovery are statistically significant.

From Passmore R and Eastwood MA (1986) Human Nutrition and Dietetics,

8th edn., p. 52. Edinburgh: Churchill Livingstone.

4866 PROTEIN/Deficiency

Plasma Albumin

0037 Plasma albumin reacts very sensitively to a reduced

protein intake. Although the albumin turnover is

reduced by about 36% in studies with adults, the

plasma albumin concentration is reduced by only

about 7%. This maintenance of intravascular circu-

lating albumin mass is attributable to a reduced

breakdown and a shift of albumin from the extra-

vascular to the intravascular compartment.

Factors Affecting Adaptation

0038 Among the factors that can affect the adaptation to

low protein intake are infections, diarrheal diseases,

and injuries. In infections, protein from muscle and

skin is needed for the immune response, and this leads

to a negative nitrogen balance. In diarrheal diseases,

the malabsorption has a negative influence on the

adaptive process; in patients with injury, the severe

losses of nitrogen have the same effect.

0039 Energy balance is also essential for nitrogen bal-

ance because of its nitrogen-sparing effect. Thus, if

protein deficiency is accompanied by energy defi-

ciency, the adaptation to a low protein intake cannot

be achieved completely.

Protein Reserves

0040 The body of a human adult (65 kg) contains 12 kg of

protein, about 50% of which is found in muscles. The

amount of body protein depends on, among other

things, the dietary protein and carbohydrate intake;

if carbohydrate is lacking, the amino acids are utilized

for gluconeogenesis. (See Glucose: Function and

Metabolism.)

0041 Protein reserves are not comparable to special fat

depots, and not all body proteins can serve as protein

reserve. Reserves are primarily organs which contain

labile body protein such as liver, plasma (with pro-

teins such as albumin and enzymes) and the gastro-

intestinal tract, which has an assessed protein content

of about 700–800 g. Although the protein turnover

rate in muscle is very slow, this tissue is a very import-

ant protein reserve owing to its large mass.

0042In protein deficiency, the labile body proteins are

metabolized first; when deficiency is long-term, all

organs are affected to various extents (Table 3). The

well-fed human adult can lose about 3 kg of protein

without any disturbances to his or her health.

See also: Anorexia Nervosa; Bulimia Nervosa; Burns

Patients – Nutritional Management; Celiac (Coeliac)

Disease; Fats: Digestion, Absorption, and Transport;

Glucose: Function and Metabolism; Glucose Tolerance

and the Glycemic (Glycaemic) Index; Hormones: Adrenal

Hormones; Thyroid Hormones; Pituitary Hormones;

Immunology of Food; Kwashiorkor; Malnutrition: The

Problem of Malnutrition; Marasmus

Further Reading

Elmadfa I and Leitzmann C (1998) Erna¨hrung des

Menschen, 3rd edn. Stuttgart: Verlag Ulmer.

Jackson AA (1990) The aetiology of kwashiorkor. In: Har-

rison GA and Waterlow JC (eds) Diet and Disease in

Traditional and Developing Societies, pp. 76–113.

Cambridge: Cambridge University Press.

Khaled MA, Kabir I and Mahalanabis D (1995) Effect of

protein energy supplementation on oxidative stress in

malnourished children. Nutrition Research 15:

1099–1104.

Kraut H (ed.) (1981) Der Nahrungsbedarf des Menschen:

1 – Stoffwechsel, Erna¨ hrung und Nahrungsbedarf,

Energiebedarf, Proteinbedarf, pp. 140–153. Darmstadt:

Steinkopff Verlag.

Latham MC (1990) Protein–energy malnutrition. In: Brown

ML (ed.) Present Knowledge in Nutrition, 6th edn,

pp. 39–46. Washington, DC: International Life Sciences

Institute – Nutrition Foundation.

Latham MC (1997) Human Nutrition in the Developing

World, FAO Food and Nutrition Series No. 29, pp.

127–146. Rome: FAO.

Passmore R and Eastwood MA (1986) Human Nutrition

and Dietetics, 8th edn pp. 40–53, 279–291, 403–407,

452, 482–501. Edinburgh: Churchill Livingstone.

Siegenthaler W, Kaufmann W, Hornbostel H and

Waller HD (eds) (1984) Lehrbuch der inneren Med-

izin, pp. 14.17–14.19. Stuttgart: Thieme Verlag.

Trowell HC, Davies JNP and Dean RFA (1982) A Nutrition

Foundation’s Reprint of Kwashiorkor, pp. 122–160.

New York: Academic Press.

Waterlow JC (1986) Metabolic adaptation to low intakes

of energy and protein. Annual Reviews of Nutrition 6:

495–526.

tbl0003 Table 3 Relative losses of protein in different organs and

tissues from rats over 7 days

Organ or tissue Loss (percentage of primary content)

Liver 40

Prostate gland 29

Seminal vesicle 29

Gastrointestinal tract 28

Kidney 20

Blood plasma 20

Heart 18

Muscle, skin, skeleton 8

Brain 5

Eyes 0

Testicle 0

Adrenal gland 0

From Kraut H (ed.) (1981) Der Nahrungsbedarf des Menschen: 1 ^

Stoffwechsel, Erna€hrung und Nahrungsbedarf, Energiebedarf, Proteinbedarf,

p. 148. Darmstadt: Steinkopff Verlag.

PROTEIN/Deficiency 4867

Heat Treatment for Food

Proteins

L Mauer, Purdue University, West Lafayette, IN, USA

Background

0001 Amino acid composition and sequence determine the

native structure, functionality, and nutritional quality

of a protein in a set environment. During food pro-

cessing, heat is often added to the protein’s environ-

ment, and this addition of energy can change any or

all of the structural, functional, and nutritional char-

acteristics of the native protein. Foods are complex

systems, and it is important to recognize that pH,

water activity, food composition, and interactions

of these with temperature also affect protein proper-

ties to varying extents. Common heat treatments

used in processing food proteins, effects of heat

on protein structure and function, effects of heat on

protein interactions with carbohydrates and lipids,

and nutritional effects of heating proteins are dis-

cussed.

Common Heat Treatments on Food

Proteins

0002 At home or on an industrial scale, the purposes of

common heat treatments of foods containing proteins

are similar: to change texture or function, improve

safety and quality, and control enzymes by altering

physical, chemical, and biological protein properties.

Foods are baked to change texture and improve

safety, vegetables are blanched to inactivate enzymes,

canning temperatures used for low acid foods are

designed to prevent toxin production by pathogenic

microorganisms, pasteurization and sterilization are

designed to kill pathogens and inactivate enzymes,

and extrusion modifies the texture of protein-

containing foods. Mild heat treatments, such as incu-

bation, generally do not cause the same extent of

change as higher heat treatments and may be used

to promote enzyme activity instead of destroying it. It

is worthwhile to note that proteins usually are most

stable to heat at their isoelectric points.

Dry-heat Food Preparation

0003 Dry-heat food preparation methods at temperatures

ranging from 160 to 230



C include baking, roasting,

grilling, and frying. In addition to microbial destruc-

tion, dry-heat methods alter the texture of protein

foods by heat gelation of proteins, denature enzymes

and pigments, and with extensive heat may form

thermally induced mutagens. When meats are heated,

myofibrillar proteins denature, then form a gel matrix,

enzymes such as myosin and actomyosin are inacti-

vated, and oxidation of denatured myoglobin pig-

ments turns cooked meats brown. On baking, the

elastic wheat gluten network in bread dough expands

to contain leavening gases until the temperature is

high enough to gelatinize starch, around 65



C. Gluten

protein denaturation and gelation occur at higher

temperatures than starch gelatinization, beginning

around 74



C, and continuing for the remaining

baking time. The Maillard reaction between proteins

and carbohydrates produces the browning of bread

crusts during baking.

Moist-heat Food Preparation

0004Moist-heat food preparation methods at tempera-

tures ranging from 65 to 100



C include blanching,

boiling, steaming, scalding, and poaching. Pressure

canning foods can reach temperatures in excess of

116



C. Blanching is a process to inactivate enzymes

by dipping foods, usually vegetables, into boiling

water for a short time period. This enzyme inactiva-

tion prevents quality loss by color, texture, or flavor

changes during frozen storage. Blanching also de-

creases initial microbial load on foods as well as

wilts products such as spinach for tighter packing.

Boiling occurs at or near 100



C, depending on eleva-

tion, and functions to inactivate enzymes, denature

proteins, change texture, and potentially destroy

toxins.

0005Higher temperatures achieved under pressure, such

as for pressure canning, inactivate enzymes, destroy

pathogenic and many spoilage microorganisms, and

prevent toxin formation during storage. Poaching

eggs leads to denaturation and coagulation of egg

white proteins. Steaming fish causes texture changes

as a result of denaturation and gelation of proteins.

Scalding milk may initiate unfolding of whey proteins

and improve select functional properties desirable in

preparing bakery foods.

Microwaving

0006Microwaving combines properties of dry- and moist-

heat food preparations with the advantage of reduc-

ing food cooking times. Heat produced by friction

of rotating food molecules, most notably water,

in response to magnetron-generated microwaves

denatures and coagulates food proteins as do other

heating methods. However, foods cooked in a micro-

wave do not brown as they would in dry-heating

methods. The crust of microwaved bread dough does

not brown, because the air in the microwave does not

4868 PROTEIN/Heat Treatment for Food Proteins

reach high enough temperatures, and the steam gen-

erated does not allow the surface to dry sufficiently

for nonenzymatic browning to occur.

Pasteurization

0007 The pasteurization process is designed to destroy any

pathogenic microorganisms that might be present in

the food product. Temperatures used for pasteuriza-

tion also reduce the total microbial load, thereby

increasing shelf-life, and may inactivate select

enzymes that lead to quality loss during storage. Pas-

teurization of milk, designed to destroy the pathogen

Coxiella burnetti, is accomplished by a time/tempera-

ture integrated process. Lower temperatures take

longer times (63



C for 30 min, 71.5



C for 15 s),

whereas higher temperatures require much shorter

process times to accomplish the same result (88.5



for 1 s). At temperatures above 78



C, esterase, a

lipase that may cause hydrolytic rancidity of milk

products, is inactivated. Pasteurization of milk also

may denature whey proteins, and higher tempera-

tures (ultrahigh temperature processing) or longer

process times (such as vat pasteurization) may cause

cooked and heated flavors primarily from the volatile

sulfur compounds released by b-elimination of

disulfide bonds in denatured whey proteins. The

high temperatures used for sterilization of aseptic

products lead to microbial safety but do not inacti-

vate all enzymes. During extended storage, these

enzymes may cause age gelation of aseptically pro-

cessed milk products. Pasteurization of liquid eggs is a

low-temperature, long-time process, either 60



C for

3.5 min or 64



C for 2.5 min, designed to destroy

pathogenic microorganisms, specifically Salmonella,

without coagulating the egg proteins.

Extrusion

0008 Extrusion is a high-temperature/high-pressure/high-

shear process used to convert soy proteins to textured

vegetable protein meat analogs. The texture changes

during extrusion cooking result from orientation and

denaturation of proteins followed by cross-linking

into a network of fiber-like proteins. The extruded

vegetable protein network mimics the fibrous texture

of meat products. Pressure and shear individually can

denature proteins, and lower temperatures may be

used in combination with these forces to achieve the

same level of denaturation as higher temperatures

alone.

Incubation

0009 Temperature is the most important factor affecting

the rate of enzyme-catalyzed reactions and also influ-

ences the stability of the enzymes. As temperature is

increased, reaction rates increase until a temperature

at which the enzyme loses activity is passed, often

around 50



C. In the range of 10–40



C, every 10



rise in temperature is accompanied by a 1.8 increase

in reaction rate for the enzyme chymosin. The enzyme

calf rennet (chymosin) is added to milk in the cheese-

making process to cleave k-casein from the casein

micelle and induce micelle aggregation. The max-

imum rate of aggregation is achieved at 40



C, and

no aggregation occurs below 18



C or above 60



At temperatures above 50



C, denaturation of the

enzyme causes it to lose activity. Therefore, incuba-

tion at 40



C will maximize chymosin activity, and

increasing temperatures above 50–60



C can be used

to stop the enzyme. Temperatures used for cheese

ripening also are controlled to maximize desirable

enzyme activity for flavor development.

Heat-induced Changes in Protein

Structure

0010Common changes in protein structure as a result of

thermal processing include denaturation, aggrega-

tion, and thermal degradation. Elevated temperatures

also increase rates of deleterious chemical reactions

in proteins that lead to oxidation, isomerization,

Maillard browning, deamidation, desulphuration,

and other b-elimination reactions; however, many of

these reactions may occur at temperatures as low as



C as well.

Denaturation

0011Heat denaturation of proteins involves configur-

ational changes in the thermodynamically stable

native structure of the protein via unfolding or alter-

ation of the quaternary, tertiary, or secondary struc-

ture as a response to heat exposure. Denaturation

may disrupt hydrogen and disulfide bonds, hydro-

phobic interactions, and salt bridges, but peptide

bonds remain intact. The primary structure, or

amino acid sequence, of the protein remains un-

changed, as does the molecular weight. A loss of

ordered structure generally occurs in the entropy-

driven transition from a native to a denatured pro-

tein. Temperatures at which denaturation occurs vary

greatly with protein source and type. Some proteins

unfold a few degrees above temperatures at which

they function, whereas others, such as wheat gluten

and milk b-casein, require much higher temperatures

for denaturation. Globular dairy whey proteins de-

nature at much lower temperatures than casein pro-

teins that have more random coil native structures.

0012Functional properties affected by heat denatur-

ation include solubility, emulsifying capacity, gelation

PROTEIN/Heat Treatment for Food Proteins 4869

capacity, foaming properties, and enzyme or bio-

logical activity. After mild (or insufficient) heat treat-

ments, protein denaturation may be reversible. For

enzymes, this reversible unfolding always occurs

prior to irreversible inactivation. If a thermal process

does not irreversibly inactivate target enzymes, the

enzymes may be able to refold and potentially cause

quality issues in food products during storage.

Residual enzyme activity in aseptically processed

foods can lead to problems such as thinning of pud-

dings due to amylase activity, age gelation in ultra-

high-temperature processed milks, and separations in

orange juice caused by pectin methylesterase activity.

Aggregation

0013 Denaturation, or at least a partial denaturation, of a

protein is usually required prior to its ability to aggre-

gate and form a gel or a precipitate. Unfolding of a

protein exposes reactive groups that may then form

intermolecular cross-links via covalent, hydrogen,

ionic, or other bonding. Gels are formed when the

cross-linked protein network is extensive enough to

form a continuous phase and trap water. Precipitates

are formed when the aggregated proteins become

insoluble and settle out of a solution.

0014 Gelation The cross-linking of proteins and entrap-

ment of water during gelation may be either revers-

ible or irreversible. The polymeric networks formed

by hydrogen bonding of gelatin molecules are ther-

moreversible. On cooling, the gelatin bonds to form a

gel, and on reheating, the hydrogen bonds break, and

gelatin returns to the dispersed phase in the solution.

Conversely, denatured globular proteins form ther-

moirreversible gels. Whey, soy, and egg white proteins

first denature and then interact via disulfide, hydro-

phobic, and ionic bonds to form gels when exposed to

heat. These gels are heat-set and may stiffen, instead

of liquefy, when exposed to additional heat. The term

‘coagulation‘ is often applied to irreversible heat-

setting of proteins.

0015 Precipitation Precipitation of food proteins often

is controlled by pH, enzymes, or salt concentration

adjustments for food ingredient isolation and appli-

cations; however, heat also may destabilize proteins,

causing them to precipitate. Solubility and hydropho-

bicity, which are affected by temperature, also influ-

ence protein precipitation. As temperatures increase,

the likelihood of protein precipitation increases as

hydrophobicity increases and solubility decreases.

Denaturation of whey proteins in milk causes them

to precipitate, adhere to the cooking vessel, and

possibly scorch with continued heating.

Thermal Degradation

0016Thermal degradation of proteins occurs at tem-

peratures higher than those for denaturation, and as

with denaturation reactions, temperatures for ther-

mal degradation vary greatly with protein type. In

addition to the quaternary, tertiary, and secondary

structure disruption in denaturation, thermal degrad-

ation also disrupts the primary structure and peptide

bonds of proteins. Effects of thermal degradation

on functionality of proteins are more severe than

the effects of denaturation. Types of protein thermal

degradation include hydrolysis, racemization, and

pyrolysis.

0017Hydrolysis Hydrolysis of proteins for use as func-

tional or flavor ingredients often is accomplished

using acid and enzyme techniques; however, high

temperatures also may be used to hydrolyze proteins

into peptides, especially at high or low pH. Hydroly-

sis of caseins may occur at 140



C, and heat-induced

hydrolysis of meat connective tissue proteins may

increase their solubility. Partially hydrolyzed proteins

may be more digestible than native proteins due to the

unfolding of the protein structure.

0018Racemization Racemization, or isomerization, of

amino acid residues from l-isomers to d-isomers

occurs when heating proteins either above 200



Cor

at alkaline pH. This may reduce protein digestibility,

because d-amino acids are less absorbed and hydro-

lyzed than l-amino acids in the digestive tract. Iso-

merase or racemase enzymes also contribute to these

conversions.

0019Pyrolysis Pyrolysis is a high temperature degrad-

ation of organic materials in nonoxidative conditions.

Amino acid pyrolysis products are formed at tem-

peratures above 250–300



C. Free radicals formed

during pyrolysis may attach to other amino acids,

thereby forming new heterocyclic compounds. Some

of these compounds are thermally induced mutagens.

Functional Changes in Heat-treated

Proteins

0020Research indicates that protein structure is optimized

for function as opposed to stability. As a result, when

addition of heat destabilizes protein structure and

stability, functionality also is affected. Effects of

heat on protein functionality can be positive or nega-

tive, and the degree of heat treatment on a specific

protein often differentiates between an increase or

decrease in the desired functionality. Addition of

some heat may slightly unfold a protein, thereby

4870 PROTEIN/Heat Treatment for Food Proteins

increasing emulsifying, gelation, and foaming proper-

ties. In designing functional and nutritional foods, the

extent of protein denaturation is controlled to attain

desired functional characteristics, such as whey pro-

teins designed for use as fat replacers. Higher heat

treatments, especially those leading to thermal deg-

radation, may in turn decrease these functional prop-

erties as the protein unfolds more or loses primary

structure. Water–protein interactions dominate pro-

tein functionality in food structure; therefore, effects

of heat on hydrophobicity, solubility, emulsifying and

gelation capacities, foaming properties, and enzyme

activity are important.

Hydrophobicity

0021 The surface hydrophobicity of proteins may either

increase or decrease as a result of heat treatments. A

thermally induced unfolding of protein molecules

exposes hydrophobic sites, thereby increasing hydro-

phobicity. Conversely, protein aggregation in re-

sponse to heat results in decreased exposure of

hydrophobic sites, thereby decreasing the surface

hydrophobicity of aggregated proteins.

Solubility

0022 The solubility of proteins depends on the nature of

the protein surfaces in contact with the environment

(usually water). To generalize, a protein with a hydro-

philic surface will be more soluble in water than a

protein with a more hydrophobic surface. As tem-

peratures rise from 0 to 40



C, most proteins exhibit

increasing solubility; however, hydrophobic proteins

such as b-casein show the opposite solubility trend

in this temperature range and may be most soluble

around 4



C. As temperatures rise above 40



C and

proteins unfold, more hydrophobic sites are exposed,

and the solubility of the proteins will decrease. These

temperature-dependent changes in solubility will

influence emulsifying, gelation, and foaming proper-

ties, since solubility influences the amount of protein

available for reactions.

Emulsifying Capacity

0023 The emulsifying capacity of proteins results from

amphipathic structure and is measured as the volume

of oil that can be emulsified per gram of protein in an

oil-in-water system. The protein must be somewhat

soluble in water in order to act as an emulsifier, and

the increase in surface hydrophobicity that occurs

with partial denaturation of most proteins will

increase their emulsifying capacities. When higher

temperatures have caused extensive denaturation

and decreased protein solubility, the emulsifying

capacity also will decrease.

Gelation Capacity

0024The gelation capacity of proteins is measured as the

amount of water that can be bound or trapped per

gram of protein. The effects of heat on gelation

capacity are similar to those on emulsifying capacity.

Partial denaturation may increase gelation capacity,

whereas extensive denaturation will decrease it.

Foaming Properties

0025The foaming capacity of a protein is measured as the

amount of interfacial area that can be created by whip-

ping the protein. Foam stability is measured as the time

required to lose either 50% of the liquid or 50% of the

volume from the foam. Generally, heating a globular

protein to achieve partial denaturation will increase

foaming properties. As the structure unfolds and ex-

poses hydrophobic sites, it may be able to adsorb more

quickly to air–water interfaces and lower interfacial

tension, thereby trapping more air and increasing the

foaming capacity. Extensive heat denaturation of pro-

teins will decrease their ability to form foams.

Enzyme Activity

0026Temperature controls the rate of enzyme-catalyzed

reactions and influences enzyme stability. As dis-

cussed in the incubation section, moderate tempera-

tures can optimize the rate of enzyme-catalyzed

reactions, and higher temperatures may denature

(unfold) the enzymes, thereby causing them to lose

activity. The temperatures at which enzymes denature

vary with the type of enzyme. Chymosin, added to

milk for cheese-making, loses activity above 50



whereas pectinmethylesterase present in orange juice

is stable to much higher heat treatments.

Heat-induced Protein–Carbohydrate

Interactions

0027Perhaps the most notable protein interaction with

other food ingredients is the browning produced in

the Maillard reaction between proteins and carbohy-

drates. At high temperatures, low water activity, and/

or extended storage times, proteins may react with

reducing sugars to form brown pigments in the Mail-

lard reaction (nonenzymatic browning). Reactive

groups in proteins for the Maillard reaction are pri-

mary amines, usually the e-amino groups of lysine

residues. Examples of reducing sugars are glucose,

fructose, lactose, and maltose. When the free e-

amino group of lysine reacts with a reducing sugar,

the lysine is no longer nutritionally available. Lysine

is often the limiting amino acid in protein quality of

grain products, and a decrease in the available lysine

due to the Maillard reaction decreases the overall

PROTEIN/Heat Treatment for Food Proteins 4871

protein quality of the food. Nonenzymatic browning

is desirable in bakery products such as breads, cooked

meats, and caramels for which browning contributes

to color and flavor development. However, too much

browning produces burnt or off-flavors in these prod-

ucts. Nonenzymatic browning also is undesirable in

dried milk powders, infant formula, dehydrated po-

tatoes, dried fruits, and white wine.

Heat Effects on Protein–Lipid Interactions

0028 Heat treatments may affect protein–lipid interactions

in terms of free-radical formation, changes in emulsi-

fying capacity, and alteration of conjugated lipopro-

tein structure. Lipid–protein free radicals may be

formed when free radicals produced by oxidation of

unsaturated lipids react with proteins. High tempera-

tures greatly increase the rate of oxidation of sulfur-

containing amino acids via reactions with oxidized

lipids. Cysteine and histidine free-radicals may then

cross-link and induce aggregation of proteins. As dis-

cussed in the emulsifying capacity section, partial

denaturation of globular proteins may expose hydro-

phobic sites and increase emulsifying capacity, thereby

increasing the ability of proteins to interact with

lipids; however, higher heat treatments will decrease

this ability. Heat also will denature proteins in conju-

gated lipoprotein structures and affect the functional-

ity of these, especially in membrane systems.

Nutritional Changes in Heat-treated

Proteins

0029 Heat-induced changes in protein structure can exhibit

both advantageous and negative effects from a nutri-

tional standpoint. Heat denaturation of globular

proteins, such as dairy whey proteins, often leads to

increased digestibility and sometimes increased nutri-

tional value as the structure is unfolded and therefore

more susceptible to proteolytic attack by digestive

enzymes. Proteinaceous antinutritional factors, com-

monly found in legume and oilseed proteins as well as

milk and egg proteins, can be inactivated with suffi-

cient heat, usually moderate heat treatments, thereby

increasing the biological availability and digestibility

of select proteins. Heat inactivation of trypsin and

chymotrypsin inhibitors present in legumes and oil-

seeds also may protect the pancreas. Once heat de-

natured, the lectins (phytohemagglutinins) present

in legumes and oilseeds no longer bind to intestinal

cells, causing malabsorption of nutrients. Heat inacti-

vation of ovomucoid and ovoinhibitor in eggs and

plasmin and plasminogen activator inhibitors in

milk prevent their protease inhibitory activities.

Once denatured by heat, egg avidin no longer inhibits

biotin absorption. Toxic proteins, including Clostri-

dium botulinum neurotoxin and Staphylococcus

aureus enterotoxin, also are inactivated by heat,

although toxin destruction usually requires higher

heat treatments than inactivation of antinutritional

factors.

0030Although heat improves safety and some nutri-

tional aspects of food, heating foods also may detract

from nutritional quality, especially during high heat

or long time processes such as charcoal grilling. Pro-

tein cross-linking can decrease proteolysis, thereby

decreasing digestibility. The d-amino acid residues

formed in racemization are less digestible than the

original l-amino acids. As a result of Maillard

browning, there is a loss of available lysine and con-

sequent decrease in protein quality. Melanoidins and

heterocyclic amines produced in this reaction and

during pyrolysis have been shown to be mutagenic

or carcinogenic and classified as thermally induced

mutagens or carcinogens. Consumption of small

amounts of these compounds over extended periods

of time may lead to serious health problems.

See also: Amino Acids: Properties and Occurrence;

Determination; Metabolism; Browning: Nonenzymatic;

Toxicology of Nonenzymatic Browning; Heat Treatment:

Chemical and Microbiological Changes; Pasteurization:

Principles; Plant Antinutritional Factors: Detoxification;

Protein: Functional Properties; Interactions and

Reactions Involved in Food Processing

Further Reading

Brown A (ed.) (2000) Understanding Food Principles and

Preparation. Belmont, CA: Wadsworth.

Creighton TE (1993) Proteins. Structure and Molecular

Properties. New York: W.H. Freeman.

in CO

(ed.) (1997) Stabilizing Protein Function. New

York: Springer.

Fennema OR (ed.) (1996) Food Chemistry, 3rd edn. New

York: Marcel Dekker.

Friedman M (ed.) (1991) Nutritional and Toxicological

Consequences of Food Processing. New York: Plenum

Press.

Hoyem T and Kvale O (eds) (1977) Physical, Chemical and

Biological Changes in Food Caused by Thermal Process-

ing. London: Applied Science.

Kinsella JE and Soucie WG (1989) Food Proteins. Cham-

paign, IL: American Oil Chemists Society.

Phillips RD and Finley JW (eds) (1989) Protein Quality and

the Effects of Processing. New York: Marcel Dekker.

Priestly RJ (ed.) (1979) Effects of Heating on Foodstuffs.

London: Applied Science.

Stadelman WJ and Cotterill OJ (eds) (1995) Egg Science and

Technology, 4th edn. New York: Food Products Press.

Zayas JF (1997) Functionality of Proteins in Foods. New

York: Springer Verlag.

4872 PROTEIN/Heat Treatment for Food Proteins

Sources of Food-grade Protein

G S Gilani and N Lee, Bureau of Nutritional Sciences,

Health Canada, Ottawa, Ontario, Canada

Background

0001 Comparison of data from different countries has in-

dicated that countries with low gross national prod-

ucts also tend to have a low food energy availability.

In addition, there are major differences in the pattern

of foods in the diet in such countries compared with

wealthier countries. In particular, the availability of

animal-protein foods is significantly lower in poor

countries. Compared to the developed world, a

greater proportion of the protein consumed by people

in developing countries is derived from low-quality

protein sources such as cereals. Moreover, the pat-

terns of indispensable amino acid (IAA) composition

in the proteins are such that there is a significant

difference between the diets of rich and poor people

with respect to lysine. Standard tables of amino acid

composition show that the values for cereals range

from 26 to 38 mg of lysine per gram of protein, while

the values for animal foods range from 70 to 100 mg

of lysine per gram of protein. A recent analysis of

the world’s balance of dietary IAA demonstrated

that average lysine intakes were low in poor, de-

veloping countries. This would indicate a consider-

able risk of inadequate levels of intake, especially in

vulnerable subgroups such as children and pregnant

or lactating women. Strategies to alleviate the pos-

sible global problems of a low lysine supply could

include increasing dietary diversity as well as either

supplementing cereal-based foods with food-grade

proteins high in lysine or fortifying cereals with

lysine.

Food-grade Proteins

0002 In this chapter, sources of food-grade protein are

defined as products made by processing certain food

ingredients in a manner that significantly increases

their protein content. They are often referred to as

protein flours, protein concentrates, and protein isol-

ates. Some examples are milk-protein isolate, whey-

protein concentrate, casein, lactalbumin, egg-white

solids, gelatin, soybean-protein flours, concentrates,

and isolates, pea-protein concentrate, canola-protein

concentrate, mycoprotein, cottonseed protein, and

wheat gluten.

0003These protein sources could be used to improve the

protein nutriture of populations by increasing the

protein content of the diet, increasing the quality of

the proteins in the diet, or a combination of both. It

should be noted that increasing the protein quantity

and/or quality of a diet is ineffective if energy require-

ments are not met.

Animal-protein Products

0004Most animal sources of food-grade protein (except

gelatin) are excellent sources of all the IAA (Table 1)

and have a high protein digestibility (See Protein:

Quality). High-protein-quality animal products in-

clude egg proteins, casein, milk protein isolate, lac-

talbumin, whey-protein concentrate, beef, tuna, and

chicken meat products. These products are especially

high in key IAA such as lysine, methionineþcystine,

threonine, and tryptophan, which are limiting in

poor-quality vegetable-protein products. Among the

animal-protein products, egg white contains the

highest amount of methionineþcystine, and lactalbu-

min and whey-protein concentrate contain the

highest amounts of lysine and tryptophan, while egg

white, tuna, beef, and chicken meat products contain

the highest amounts of threonine. Therefore, these

animal-protein products could serve as supplements

to low-quality proteins limiting in one or more of

these IAA.

0005The protein efficiency ratio (PER) values of most

high-quality animal-protein products are higher than

that of the standard for the method, which is casein

(Table 2). In contrast, proteins with protein digest-

ibility-corrected amino acid scores (PDCAAS) ex-

ceeding 100% are truncated to 100%, implying that

values exceeding 100% do not contribute any add-

itional benefit in humans. This is not true, especially

when these proteins are used as supplements to poor-

quality proteins. Therefore, there is an urgent need to

revise the PDCAAS method to permit the use of

untruncated PDCAAS values for high-quality

animal-protein products so that their supplementary

value, when combined with lower-quality protein

sources, can be identified by their PDCAAS values.

0006Gelatin is a unique animal protein product which is

devoid of tryptophan and is also limiting in all other

IAA. This complete lack of nutritional value has not

stopped some individuals from using it as the basis for

weight-loss drinks. After several illnesses and deaths

were associated with the use of these products in very-

low-calorie diets, Canada introduced a requirement

that any product containing gelatin or collagen or

hydrolyzed gelatin or collagen must carry a caution-

ary statement advising people not to use it as a sole

source of nutrition.

PROTEIN/Sources of Food-grade Protein 4873

Vegetable-protein Products

0007 The Codex Committee on Vegetable Proteins (CCVP)

has developed an international general standard for

vegetable protein products (VPP). This standard

applies to VPP intended for use in foods and prepared

by various separation and extraction processes from

proteins of vegetable sources except single-cell pro-

teins. The VPP are intended for use in foods requiring

further preparation and for use by the food-

processing industry. The VPP standard applies to

food products produced by the reduction or removal

from vegetable materials of the major constituents

(water, oil, starch, and other carbohydrates) in a

manner to achieve a protein (N 6.25) content of

40% or more. CCVP has also developed international

standards for soy-protein products (flour, concen-

trate, and isolate) and for wheat gluten. These VPP

are extensively used in international trade.

Soybean-protein Products

0008Soybeans are dehulled and solvent-extracted to

remove the oil. The residual solvent is then removed

and the flakes dried, resulting in defatted soy flakes.

Soybean-protein products are mostly made from the

defatted flakes. They are not consumed directly but

instead find wide application as versatile ingredients

in various food systems, including bakery, breakfast

cereals, beverages, infant formulas, enteral nutri-

tionals, dairy, meat, and dairy and meat analogs.

They not only boost protein content in these food

systems but also provide many functional properties,

including gelling, emulsifying, water-holding, and fat-

absorbing properties. There are four major types of

soybean protein products, including flour, concen-

trates, isolates and textured soybean protein. The

Codex Alimentarius Commission has established

international compositional standards for soybean

protein flours, concentrates, and isolates.

0009Soybean flour comes in many types including full-

fat, low-fat and defatted, as well as toasted and tex-

tured. Defatted soybean flour, the most common

type, is produced by grinding the defatted flakes. It

has a protein content of about 50% (N 6.25). The

soybean-protein flour is mainly used as an ingredient

in bakery products. However, full-fat soybean flour

has been gaining popularity in recent years as newer

technology has enabled it to be used as a starting

material for the production of soy milk and tofu and

as an ingredient in many other food systems.

0010Soybean protein concentrate is traditionally made

by aqueous alcohol extraction of defatted soy flakes.

The resulting product has a protein content of

about 70% (N  6.25), with the remaining portion

being mainly insoluble carbohydrates. The protein

tbl0001 Table 1 Protein (on a percentage dry weight basis) and IAA compositions and protein quality expressed as PDCAAS of animal-

protein products

Nutritional

parameter

Casein Milk-protein

isolate

Lactalbumin Whey-protein

concentrate

Egg

white

Tu n a Mi n c e d

beef

Chicken

breast

Gelatin

Totalprotein%(N6.25)90–9593807987.789.195.290.296.8

IAA:expressedasapercentageoftherespectiveIAAintheFAO/WHO(1991)scoringpattern

Histidine 158 153 105 110 118 289 168 179 42

Isoleucine 193 157 200 207 188 223 149 161 50

Leucine 154 141 215 191 133 123 117 115 42

Lysine 145 131 209 172 120 153 137 141 66

Methionine þ cystine 140 156 224 196 266 143 131 148 38

Phenylalanine þ tyrosine 182 157 108 124 168 108 111 113 32

Threonine 135 123 165 168 138 145 124 123 51

Tryptophan 118 127 209 191 133 110 112 113 0

Valine 194 157 163 154 194 133 130 137 57

Protein digestibility (%) 99 98 99 98 97 95 94 97 98

PDCAAS (%) 100 100 100 100 100 100 100 100 0

FAO/WHO (1991) Recommended Amino Acid Scoring Pattern (containing, in milligrams per gram of protein included: histidine, 19; isoleucine, 28;

leucine, 66; lysine, 58; methionine þ cystine, 25; phenylalanine þ tyrosine, 63; threonine, 34; tryptophan, 11; and valine, 35.).

tbl0002 Table 2 PER and PDCAAS values of some high-quality animal-

protein products

Product PER (casein ¼2.5) PDCAAS (%)

Casein þ methionine 3.1 100

Whey-protein concentrate 3.0 100

Egg-white solids 3.0 100

Lactalbumin 2.8 100

Skim milk powder 2.8 100

Milk-protein isolate 2.8 100

Chicken breast 2.8 100

Turkey breast 2.8 100

Minced beef 2.7 100

Beef salami 2.6 100

Tuna 2.6 100

4874 PROTEIN/Sources of Food-grade Protein