Summary Food ingredient Functionality - food proteins

Food proteins
• Introduction to proteins
- Sources of proteins
o Animal proteins
▪ Milk proteins
▪ Egg (white) proteins
▪ Animal by-products (blood/ gelatin)
o Plant proteins (legumins)
▪ Soy proteins
▪ Lupine proteins
▪ Sunflower proteins
o Novel (plant) proteins • Potato proteins • Algae proteins • ‘Leafy’ proteins
- Function of proteins in nature
o Nutrition (milk, soybeans, amino acids)
▪ Source of nitrogen (N) and essential amino acids
o Structure (collagen  gelatine)
o Metabolism (homeostasis, enzymes, antibodies)
- Functions of proteins in food
o Nutrition (meat, cheese, nuts)
o Texture (gluten in bread)
o Taste (maillard reaction)
- Proteins, peptides and amino acids




- Occurrence of proteins, peptides and amino acids
o Plant/animal raw materials
▪ Proteins: nutrition, structure, metabolism
▪ Peptides: dipeptides in meat products; glutathione in flour
▪ Amino acids & peptides: hormones
o In food products: formed during production
▪ Amino acids and peptides: Fermentation products (cheese)
o In food products: added as ingredients
▪ Proteins: whey proteins in ice-cream
▪ Peptides: hydrolysates in sport nutrition
▪ Amino acids: non-allergenic infant formula
- Questions:
o How is the function of meat (muscle) proteins in the body related to the solubility of
these proteins?  The function of muscle tissue is to give structure to the body.
Therefore these proteins are not soluble.
o How is the function of milk proteins related to the solubility of these proteins  The
function of milk proteins is to give nutrition, since the calf will drink the milk, the
proteins should be soluble.
• Amino acids
- Structure of amino acids
o Occurrence:
▪ In nature > 200 different amino acids
▪ In proteins ± 20 amino acids (α -L-amino
acids)

80

,- Stereo-isomers of amino acids
o Occurrence:
▪ In nature (α-)L amino acids
▪ After processing (α -)D amino acids (non-digestible)
- Structure of amino acids
o Occurrence:
▪ In proteins only (α -)L amino acids
▪ In certain peptides also (β-)L amino acids
- Amino acid side chains
o Examples




- Classification of amino acids:
o Side-chain




o Nutritional value




- Questions
o Indicate whether these amino
acids are:
▪ Non-polar, non-
charged
▪ Polar, non-charged
▪ Polar, charged




81

,• Peptides
- Occurrence of peptides
o In nature:
▪ blood (buffering function)
▪ plants (hormones)
o In foods:
▪ bread (glutathion)
o In foods after hydrolysis:
▪ Microbial: fermentation products (cheese, ham)
▪ Enzymatic: protein hydrolysates (infant formula)
- Function of peptides
o Peptides:
▪ Are more reactive than proteins
▪ Can have taste
▪ Are more easily taken up by the body
▪ Are studied for their bio-functional properties
- Peptides: oligomers of amino acids




- Nomenclature:
o peptides




o iso-peptides




- Peptide bond
o Occurrence:
▪ In nature  Trans
▪ After processing  Cis

82

, - Question
o What is the molecular weight (MW) of the tetrapeptide formed out of these 4 amino
acids?




• Effect of pH on charge
- Charge of proteins
o is affected by pH
o can result in repulsion, or attraction between proteins
o has strong effects on
▪ Solubility  Aggregation of protein
▪ Foaming and emulsifying properties
- Protonation equilibria




- Amino acids: basic and acidic character
o pH << pKa: the acid or base is mostly protonated (COOH/NH3+)
o pH >> pKa: the acid or base is mostly deprotonated (COO-/NH2)




- Amino acids: multiple charged groups
o Isoelectric point: the net-charge of the acidic and basic groups of a amino
acid/peptide is zero




83