PROTEINS

COMPOSITION Organic Organic Amino Acids Amino Acids Central carbon atom Central carbon atom Carboxyl group (COOH) Carboxyl group (COOH) Amino group (NH 2 ) Amino group (NH 2 ) Group or side chain - distinguishes one amino acid from another Group or side chain - distinguishes one amino acid from another

AMINO ACIDS

STRUCTURE PEPTIDE BONDS-strong bonds PEPTIDE BONDS-strong bonds DIPEPTIDES - 2 amino acids DIPEPTIDES - 2 amino acids POLYPEPTIDES - many POLYPEPTIDES - many Nature of side group determines conformation Nature of side group determines conformation CONFORMATION - determined by specific amino acid and sequence – shape of protein in space CONFORMATION - determined by specific amino acid and sequence – shape of protein in space

PEPTIDE BONDS

PRIMARY STRUCTURE Specific sequence of amino acids joined by peptide bonds along the peptide chain Specific sequence of amino acids joined by peptide bonds along the peptide chain Specific sequence determines form and shape Specific sequence determines form and shape

SECONDARY STRUCTURE Alpha-helix - ordered; corkscrew Alpha-helix - ordered; corkscrew Beta pleated sheet - ordered; zig-zag Beta pleated sheet - ordered; zig-zag Random coil - disordered; side chains prevent formation of alpha or beta structure Random coil - disordered; side chains prevent formation of alpha or beta structure Three dimensional Three dimensional

TERTIARY STRUCTURE Built on secondary structure Built on secondary structure May contain alpha, beta, and random coil May contain alpha, beta, and random coil Two types: Two types: Fibrous proteins - collagen, myosin & actin Fibrous proteins - collagen, myosin & actin rods or fibers rods or fibers Globular proteins - whey, myoglobin; spherical or elliptical Globular proteins - whey, myoglobin; spherical or elliptical

QUATERNARY STRUCTURE More than one polypeptide chain More than one polypeptide chain Chains not identical Chains not identical Actomyosin in muscles, casein in milk Actomyosin in muscles, casein in milk

REACTIONS & PROPERTIES AMPHOTERIC- amino group and carboxyl group both ionized  can behave as acid or base = buffering capacity AMPHOTERIC- amino group and carboxyl group both ionized  can behave as acid or base = buffering capacity ISOELECTRIC POINT - point at which protein is electrically neutral; positive charges = negative charges ISOELECTRIC POINT - point at which protein is electrically neutral; positive charges = negative charges Food processing – knowledge used to make cottage cheese Food processing – knowledge used to make cottage cheese

REACTIONS CONT’D WATER-BINDING CAPACITY - hydrophilic; helps maintain stability of protein dispersion WATER-BINDING CAPACITY - hydrophilic; helps maintain stability of protein dispersion Some proteins readily disperse in salt solutions: Some proteins readily disperse in salt solutions: SALTING IN - when salt solution increases dispersibility of protein (brine in ham) SALTING IN - when salt solution increases dispersibility of protein (brine in ham) SALTING OUT - high salt concentration; salts compete with protein for water; during freezing process SALTING OUT - high salt concentration; salts compete with protein for water; during freezing process

DENATURATION Change in secondary, tertiary, quaternary structure of protein Change in secondary, tertiary, quaternary structure of protein Does not break bonds - unfolds them Does not break bonds - unfolds them Caused by: heat, pH change, ionic strength; freezing, surface changes Caused by: heat, pH change, ionic strength; freezing, surface changes Permanent in food Permanent in food Lose functional properties Lose functional properties

HYDROLYSIS Breaking of peptide bonds Breaking of peptide bonds Acid digestion Acid digestion Proteolytic enzymes: tenderizers Proteolytic enzymes: tenderizers Papain, bromelain for meat Papain, bromelain for meat Rennet for cheese Rennet for cheese

MAILLARD BROWNING Non-enzymatic browning Non-enzymatic browning Free carboxyl group reacts with free amino group Free carboxyl group reacts with free amino group Color and flavor of baked goods Color and flavor of baked goods Favored by: High sugar content, high protein content, high temperatures, high pH, low water content Favored by: High sugar content, high protein content, high temperatures, high pH, low water content Discoloration of food products like powdered sugar: must be “desugared” enzymatically Discoloration of food products like powdered sugar: must be “desugared” enzymatically