Proteins in Foods General functionalities of proteins in foods: Source of nutritionally essential amino acids Provide structure & texture to foods

Protein content of common foods Food (as eaten) Amount yielding 25 g protein* Meats, fish 4-5 oz Cheddar cheese 4 oz Black beans 10 oz (1 ½ cups) Tofu (soybean gel) 1-2 cups White bread 9 slices Egg 4 eggs Milk 3 cups Spaghetti 3 cups Lettuce 33 cups Apples 75 apples Celery 90 stalks Coffee 750 oz (94 cups) * about 50% of RDA for adults

Structure of Proteins Proteins are polymers of amino acids: (“R group”) A generic amino acid: 20 common amino acids 9 are essential (required in the diet) Amino acids are distinguished by their side chain, or “R” group : Polar vs non-polar Ionizable vs non-ionizable R group :

Amino acids with polar (hydrophilic) R groups : serine Amino acids with non-polar (hydrophobic) R groups : alanine Note: these R groups are hydrocarbons (C, H only) leucine phenylalanine

Amino acids with ionizable R groups : Those with carboxyl (COOH) or amine (NH2) groups Mostly Neg charged above pH 4 What is the pH range of foods ? Mostly Pos charged below pH 9 “ pKa ” pH at which [charged] = [uncharged] pKa Carboxyl group 3-4 Amine group 9

Amino acids with ionizable R groups : Examples: lysine glutamic acid ( Note: these are also polar )

glutamic acid, an amino acid Free amino acids are relatively rare in foods Exception: monosodium glutamate (MSG): sodium salt of glutamic acid, an amino acid Flavor enhancer (“umami” taste)

Amino acids are linked together by peptide bonds (strong covalent C--N bond) to form proteins (“polypeptides”) Peptide bonds can be hydrolyzed by protease enzymes: Upper GI tract (secreted, stomach &pancreas) Enzymes in foods

Spatial conformation of proteins Fibrous proteins (often extended helices) Globular proteins Myosin muscle protein tend to be smaller, more soluble than fibrous proteins Connective tissue protein, from which gelatin is made

Protein Conformation and Denaturation "Native" Conformation: The preferred, or most stable conformation of a protein (its "natural" shape or arrangement) Each different protein has its own native conformation, determined largely by its amino acid sequence and its folding pattern

A globular protein in aqueous solution (native conformation) water molecules hydrophobic amino acids ( ) on interior (away from H2O)

Chemical bonds involved in the 3-D conformation of proteins

Protein “denaturation” : Change in conformation Unfolding, uncoiling energy Hydrophobic amino acid (e.g. alanine, leucine) Complete denaturation is often irreversible

Physical consequences of protein denaturation: Exposes hydrophobic amino acid residues normally "hidden" on the interior. * Increases hydrophobic character of the protein * Decreases water solubility * Causes proteins to spontaneously aggregate via hydrophobic interactions

Causes of protein denaturation Unfavorable physical or chemical environment 1. Heat Heat energy disrupts bonds responsible for native 3-D structure 2. Changes in pH Can disrupt ionic or disulfide bonding responsible for native 3-D structure "Isoelectric pH" (“pI”) pH at which the number of negative charges (COO- ) equals the number of positive charges (NH3 +) on a protein molecule At this pH, the protein, as a whole molecule, is net neutral At the isoelectric pH: * Charge repulsion between proteins is minimal * Many proteins exhibit minimal solubility; clump and precipitate from solution

3. Surface or interfacial tension Causes of protein denaturation (con’t) 3. Surface or interfacial tension High energy water at surfaces (between water and air), or interfaces (between oil and water) 4. Mechanical shear High speed mixing, pumping under high pressure, etc. Protein denaturation can be desirable or undesirable, depending on the product and circumstances

Functional Properties of Proteins in Foods (as natural components or ingredients) Texturizing Responsible for texture of meats, dough-based products (pasta): Characteristics: Chewy Elastic Crispy Tough Thickeners (water binding): hydrophilic proteins only; often used with CHO thickeners Soups, sauces, gravies Beverages Gelling Some proteins can form gels under certain circumstances * Gelatin: dessert, snack gels, etc. * Casein (milk protein) : cheeses, yogurt, other dairy products * Egg albumin: custards, etc. * Tofu (soybean protein)

Surfactants (surface-active agents): Foaming agents: (air cell dispersal) Baked products (cakes, cookies, breads) Toppings Beverages Emulsifying agents (fat and water dispersal) Sauces Mayonnaise, salad dressings, etc. Sausages, meat products

Enzyme activity a. Endogenous enzymes (pre-existing in foods or produced by contaminating microorganisms) : * can adversely affect color, taste, odor, texture Denaturation of enzymes can maintain quality by preventing enzyme-induced changes ex.: pasteurization (heating) of milk denatures lipases and proteases, enzymes which degrade lipids and proteins to products that adversely affect taste and odor of milk (free fatty acids and free amino acids) b. Added desirable enzymes: * texturizing : ex.: meat tenderizers (proteases) * gelling ex.: rennin, a protease used in cheese-making

Proteins as surfactants : Use in stabilization of foams and emulsions Foams and emulsions are dispersions of two physically incompatible phases: Foam: Dispersion of air cells in liquid (water) or a semi-solid Emulsion: Dispersion of oil in water Dispersions are thermodynamically unstable due to high surface tension (interfacial tension) at the boundary between the two phases Surfactants stabilize dispersions by : * lowering surface tension * providing a physical barrier between phases Phase dispersal Incr. total surface area Incr. system energy (instability)

Phase dispersal Incr. total surface area Incr. system energy (instability) Phase dispersal “dispersed phase” (energy) “continuous phase” Phase separation Surfactants stabilize dispersions by : * lowering surface tension * providing a physical barrier between phases

A stable dispersion of water in oil : Proteins from egg and other ingredients act as the surfactants

A B C Oil phase (hydrophobic) high energy water zone (surface tension) interface between oil and water high energy water zone (surface tension) A globular protein in native conformation in aqueous solution bulk water (lower energy) B unfolded (hydrophobic) proteins tend to “stick” at the interface partial denaturation of protein caused by surface tension C high energy water zone no longer exists; unfolded protein provides barrier between water and air Unfolded protein spread out at surface

Stabilization of foams and emulsions by denatured proteins air cell or oil droplet air cell or oil droplet charge repulsion (except at pI * ) Unfolded proteins coat surface of air cells or oil droplets, stabilizing the dispersion Good protein surfactants: flexible, easily denatured by surface tension; amphipathic: possess both hydrophobic and hydrophilic regions within the same protein molecule

Stability of an emulsion or foam depends upon: Interfacial surface forces Size of the disperse phase droplets Viscous properties of the continuous phase Density difference between the two phases Food dispersions in which proteins function as surfactants egg white foam emulsions

Other Proteins as Gelling, Texturizing Agents Gelatin Product of hydrolysis of collagen, a structural protein found in connective tissues of meats Collagen gelatin Transparent, thermo-reversible gels (reverse when heated/cooled) > 140 F H20 Tofu Cooked, gelled soy proteins Not thermo-reversible Surimi Texturized fish protein (white, high water content species) Pigmented with natural or artificial colors Imitation crab, shrimp

Proteins as Pigment Precursors [ Louis Camille Maillard, 1916 ] aldehyde functional group aldehydes are notoriously reactive

Maltose Sucrose A reducing sugar NOT a reducing sugar (the O atom at carbon 1 constitutes the ether bond)

Maillard reactants (con’t) lysine

Initial reaction in Maillard browning protein protein H2O (neutral form) (aldehyde form) brown hues

Maillard browning reaction [ A non-enzymatic process ] primary amine reducing sugar Schiff base (brown) Reactive carbonyl compounds: aldehydes ketones (flavor, odor) + other proteins “Melanoidin” pigments CHO-protein complexes Grey-brown-black hues

4. Carbohydrate, type of sweetener b - amylase Flour starches glucose, maltose H20, heat Sucrose glucose, fructose (partial) Note: Artificial sweeteners (aspartame, saccharine, sucralose) will not react

Grains also contain proteins that react with maltose to produce brown pigments

Protein A “complete” protein : One which contains all essential amino acids in amounts and relative proportions needed to maintain life and support growth when consumed as the sole source of protein The biological value of individual proteins is commonly assessed using animal growth studies or nitrogen retention Measures of protein quality: Protein Efficiency Ratio Net Protein Utilization Biological Value

Animal proteins Plant proteins Generally high biological value Exception: Gelatin Limiting amounts of 4 amino acids (lacks tryptophan entirely) Plant proteins Generally lower biological value than animal proteins Reason: limited amounts of some amino acids: Wheat, rice: lysine Corn (maize): lysine, tryptophan Legumes: methionine

Complementary proteins Ex.: corn (maize) & black beans 50% beans [ Opaque-2 maize: incr. lysine, tryptophan content ] 100% beans