Protein in Food PDF

Amino acids There are about 20 amino acids in a protein hydrolysate. With a few exceptions, their general In the simplest case, R=H (amino acetic acid or glycine). In other amino acids, R is an aliphatic, aromatic or heterocyclic residue and...

Amino acids There are about 20 amino acids in a protein hydrolysate. With a few exceptions, their general In the simplest case, R=H (amino acetic acid or glycine). In other amino acids, R is an aliphatic, aromatic or heterocyclic residue and may incorporate other functional groups. Fig.( 2) Structure of amino acid back bone of protein  the most important “building blocks” of proteins. There are about 20 amino acids found in nature There are a number of ways of classifying amino acids. Since their side chains are the deciding factors for intra- and intermolecular interactions in proteins, and hence, for protein properties  Classification of amino acids 1-With nonpolar , uncharged side chains 2-With uncharged , polar side chains 3-With charged side chains 4-Essential amino acid 5-Non essential amino acid  Amino acids with nonpolar, uncharged side chains: glycine, alanine, valine, leucine, isoleucine, proline, phenylalanine, tryptophan and methionine. Amino acids with uncharged, polar side chains: serine, threonine, cysteine, tyrosine, asparagine and glutamine. Amino acids with charged side chains: aspartic acid, glutamic acid, histidine, lysine and arginine. Based on their nutritional/physiological roles, amino acids can be differentiated as: Essential amino acids: Valine, leucine, isoleucine, phenylalanine, tryptophan, methionine, threonine, histidine (essential for infants), lysine and arginine (“semi-essential”). Nonessential amino acids: Glycine, alanine, proline, serine, cysteine, tyrosine, asparagine, glutamine, aspartic acid and glutamic acid. Properties of amino acid a- Configuration and Optical Activity: The optical activity of proteins is due not only to asymmetry of amino acids but also to the chirality resulting from the arrangement of the peptide chain. Information on the conformation of proteins can be obtained from a recording of the optical rotatory dispersion (ORD) or the circular dichroism (CD), especially in the range of peptide bond absorption wavelengths (190–200 nm). b-Solubility: The solubility of amino acids in water are highly variable. Besides the extremely soluble proline, hydroxyproline, glycine and alanine are also quite soluble. Other amino acids () are significantly less soluble, with cysteine and tyrosine having particularly low solubilities. Addition of acids or bases improves the solubility through salt formation. C-UV-Absorption Aromatic : amino acids such as phenylalanine, tyrosine and tryptophan absorb in the UV range of the spectrum with absorption maxima at 200–230 nm and 250–290 nm. Chemical properties Chemical Reactions Esterification of Carboxyl Groups Reactions of Amino Groups Alkylation and Arylation Reactions with Carbonyl Compounds Reactions Involving Other Functional Groups Lysine Physical properties  Size  Colloidal solutions  Charge  UV absorption  Solubility Physical properties Proteins, like amino acids, are amphoteric. Depending on pH, they can exist as polyvalent cations, anions or zwitter ions. Proteins differ in their -carboxyl and -amino groups – since these groups are linked together by peptide bonds, the uptake or release of protons is limited to free terminal groups. Therefore, most of the dissociable functional groups are derived from side chains. Protein Protein building  Functional properties of protein Part 2 : Functional properties of protein: Functional properties defined as: “those physical and chemical properties of proteins that influence their behavior in food systems during preparation, processing, storage and consumption, and contribute to the quality and organoleptic attributes of food systems. Functional properties of protein Functional Property Food System Solubility Beverages, Protein concentrates/isolates Water-binding and holding ability Muscle foods, cheese, yogurt Gelation Muscle foods, eggs, yogurt, gelatin, tofu, baked goods Emulsification Salad dressing, mayonnaise, ice cream, gravy Foaming Meringues, whipped toppings, angel cake, marshmallows  In common with other food constituents, proteins contribute significantly to the physical properties of foodstuffs, especially through their ability to: 1-build or stabilize gels, foams, dough, emulsions and fibrillar structures. 2-some typical examples of functional properties of proteins in relation to important food systems. Foaming, gelling, and emulsifying properties Functional properties of protein Solubility, Hydration and Swelling Power Protein solubility is variable and is influenced 1. the number of polar and a polar groups 2. arrangement along the molecule.  A. Foam Formation and Foam Stabilization In several foods, proteins function as foam forming and foam- stabilizing components, for example in baked goods, sweets, desserts and beer. This varies from one protein to another. Serum albumin foams very well, while egg albumin does not. Protein mixtures such as egg white can be particularly well suited In that case, the globulins facilitate foam formation. Ovomucin stabilizes the foam, egg albumin and conalbumin allow its fixation through thermal coagulation. Foams: Foams: Are dispersions of gases in liquids. Proteins stabilize by forming flexible, cohesive films around the gas bubbles. During impact, the protein is adsorbed at the interface via hydrophobic Foams collapse : because large gas bubbles grow at the expense of smaller bubbles (disproportionation). The protein films : counteract this disproportionation. That is why the stability of a foam depends on the strength of the protein film and its permeability for gases. Film strength : depends on the adsorbed amount of protein and the ability of the adsorbed molecules to associate. Surface denaturation; generally releases additional amino acid side chains which can enter into intermolecular interactions. The stronger the cross-linkage, the more stable the film. formability of a protein molecule depends Diffusion rate The ease with which it is denatured. These parameters in turn depend on the molecular mass, the surface hydrophobicity, the stability of the conformation. stability of a foam depends the stability of the protein film Its permeability for gases. The amount of adsorbed protein and on the ability of the adsorbed protein to associate. Generally, Surface denaturation exposes additional amino acid side-chains that can participate in intermolecular interactions. The stronger the cross-linking, the more stable is the film. The pH of the system should be near the isoelectric points of the involved proteins because the association is promoted by a small net charge B. Gel Formation Gels are disperse systems of at least two components in which the disperse phase in the dispersant forms a cohesive network. They are characterized by the lack of fluidity and elastic deformability. Gels are placed between solutions, in which repulsive forces between molecules and the disperse phase predominate Protein molecular size Molecular weight:  Vary from 6000 to million Daltons (Da)  Photometric proteins: 50 000 to 100 000 Da  Oligometric proteins: > 100 000 Da Colloidal properties  Solution （< 1 nm）  Colloid （1 – 100 nm）  Suspension (> 100 nm)  Protein  Particle size of 2~20 nm  Protein solution has colloidal properties (high viscosity, high absorption capacity, light distraction, do not pass through a semipermeable membrane Denaturation The term denaturation denotes a reversible or irreversible change of native conformation (tertiary structure) without cleavage of covalent bonds (except for disulfide bridges). Denaturation is possible with any treatment that cleaves hydrogen. Denaturing agents: Chemical agents  Loss of native conformation Altered secondary, tertiary or quaternary structure Disruption of disulfide bonds (covalent) and non-covalent bonds (H-bond, ionic bond, hydrophobic interaction Peptide bonds are not affected.  Acids and alkalis; Altered pH  Organic solvents (ether, alcohol)  Salts of heavy metals (Pb, Hg)  Chemotropic agents Detergents Reducing/oxidizing agents. Protein denaturation have other properties :  Increased viscosity Altered functional properties Loss of enzymatic activity  Protein denaturation Proteins exist in two main states DENATURED STATE NATIVE STATE  Loss of native conformation  Usually most stable ◦ Altered secondary, tertiary or  Usually most soluble quaternary structure  Polar groups usually on the outside ◦ Disruption of disulfide bonds (covalent) and non-covalent  Hydrophobic groups on bonds (H-bond, ionic bond, inside hydrophobic interaction ◦ Peptide bonds are not affected  The proteins can regain their native state when the denaturing 25 influence is removed-Renaturation. Thermal treatment Protein denaturation:  Increased viscosity  Altered functional properties  Loss of enzymatic activity  Denatured protein is more easily digested : due to enhanced exposure of peptide bonds to enzymes. Cooking causes protein denaturation and therefore, cooked food is more easily digested. proteins must be denatured: by heat, acid, base, and/ or solvent in order to form the more extended structures that are required for film formation. 2. Maillard reaction (carbonyl - amine browning)  Changes functional properties of proteins  Changes color  Changes flavor  Decreases nutritional quality (amino acids less available)  Functional properties: Gelation  Texture, quality and sensory Solution attributes of many foods depend on protein gelation on processing.  Sausages, cheese, yogurt, custard  Gel; a continuous 3D network of proteins that entraps water  Protein - protein interaction and protein - water (non-covalent) interaction  A gel can form when proteins are denatured by Gel Heat, pH, Pressure, Shearing  Functional properties: Emulsification Emulsion: A suspension of small globules of one liquid in a second liquid with which the first will not mix.  Proteins can be excellent emulsifiers because they contain both hydrophobic and hydrophilic groups. Type of emulsion  To form a good emulsion the protein has to be able to: Rapidly adsorb to the oil-water interface Rapidly and readily open up and orient its hydrophobic groups towards the oil phase and its hydrophilic groups to the water phase Form a stable film around the oil droplet Functional properties: Gelation Texture, quality and sensory attributes of many foods depend on protein gelation on processing. Sausages, cheese, yogurt, custard Gel; a continuous 3D network of proteins that entraps water Protein - protein interaction and protein - water (non-covalent) interaction A gel can form when proteins are denatured by Heat, pH, Pressure, Shearing