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PROTEINS

PROTEINS. COMPOSITION. Organic Amino Acids Central carbon atom Carboxyl group (COOH) Amino group (NH 2 ) Group or side chain - distinguishes one amino acid from another. AMINO ACIDS. STRUCTURE. PEPTIDE BONDS-strong bonds DIPEPTIDES - 2 amino acids POLYPEPTIDES - many

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PROTEINS

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  1. PROTEINS

  2. COMPOSITION • Organic • Amino Acids • Central carbon atom • Carboxyl group (COOH) • Amino group (NH2) • Group or side chain - distinguishes one amino acid from another

  3. AMINO ACIDS

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

  5. PEPTIDE BONDS

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

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

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

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

  10. REACTIONS & PROPERTIES • 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 • Food processing – knowledge used to make cottage cheese

  11. REACTIONS CONT’D • WATER-BINDING CAPACITY - hydrophilic; helps maintain stability of protein dispersion • Some proteins readily disperse in salt solutions: • 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

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

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

  14. MAILLARD BROWNING • Non-enzymatic browning • Free carboxyl group reacts with free amino group • Color and flavor of baked goods • 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

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