1 Chapter 5 Food Chemistry II: Proteins

2 CHAPTER OBJECTIVES describe structure of food proteins and functional properties relationship between isoelectric point and protein functionality composition of casein micelle and functional properties of polypeptides in the micelles

3 FUNCTION AND COMPONENTS OF PROTEIN Protein is essential to building and maintaining body tissues Amino acids are building blocks of protein Plant and animal proteins are made of up of 20 common amino acids

4 CLASSIFICATION OF AMINO ACIDS IN FOOD PROTEIN Amino acids in food protein can be classified as l Essential (indispensable) - body synthesis inadequate to meet needs l Non-essential (dispensable) - can be synthesised by the body l Conditionally essential (indispensable) - become essential under certain conditions

5 ESSENTIAL AMINO ACIDS Histidine***Methionine* (and Cysteine) IsoleucinePhenylalanine** (and Tyrosine) LeucineThreonine LysineTryptophan Valine* necessary for synthesis of cysteine ** necessary for synthesis of tyrosine *** necessary only for infants

6 COMPLETE AND INCOMPLETE PROTEINS Complete (balanced) protein – single food protein containing all 9 essential amino acids in concentrations sufficient to meet effectively the requirements of humans Incomplete (unbalanced) protein – single food protein deficient in 1 or more of 9 essential amino acids

7 COMPLEMENTARY PROTEINS 1 People can meet their minimum daily requirements for protein and essential amino acids by –  Consuming sufficient quantities of complete protein(s)  Consuming a sufficient amount of a variety of incomplete proteins  Combining complete proteins with incomplete proteins 2 Complementary proteins can be consumed over the course of a day

8 l Proteins consist of 20  -amino acids l Amino acids consist of carbon, hydrogen, oxygen, nitrogen, and partly sulphur l Amino acids differ in their side chains PROTEIN COMPOSITION

9 BASIC STRUCTURE OF AN AMINO ACID H CCOOHR NH 2 R = sidechains of different composition

10 l Characteristic side chain (R) influences physiological and physico-chemical properties of amino acids and also those of proteins l Division into five groups in relation to the different side chains: – non-polar aliphatic side chains – polar, not charged (hydrophilic) side chains – positively charged side chains – negatively charged side chains – aromatic side chains PROPERTIES OF AMINO ACIDS

11 l 1: Non polar aliphatic side chains – alanine (Ala) – glycine (Gly) – isoleucine (Ile) – leucine (Leu) – methionine (Met) – valine (Val) l 2: Polar, not charged (hydrophilic) side chains – asparagine (Asp-NH 2 ) – cysteine (Cys) – glutamine (Glu-NH 2 ) – proline (Pro) – serine (Ser) – threonine (Thr) l 3: Positively charged side chains – arginine (Arg) – histidine (His) – lysine (Lys) l 4: Negatively charged side chains – aspartic acid (Asp) – glutamic acid (Glu) l 5: Aromatic side chains – phenylalanine (Phe) – tryptophan (Trp) – tyrosine (Tyr) AMINO ACIDS IN PROTEINS

12 AMINO ACIDS IN PROTEINS

13 l Amino acids are linked by amide bonds (also peptide bonds) l Up to 10 amino acids: peptides l More than 10 amino acids: proteins C O H N CR 2 COO - H R 1 and R 2 are side chains of amino acids R1CR1C H + NH 3 LINKING AMINO ACIDS INTO PEPTIDES and PROTEINS

Food Proteins A. The Structure of Proteins

15 l Proteins can be divided into two groups: – Homoproteins, containing amino acids only – Heteroproteins, containing an extra non- protein part or prosthetic group l nucleo-, lipo-, glyco-, phospho-, hemo-, flavo-, metalo-proteins PROTEIN TYPES

16 l Proteins are characterised by their amino acid sequence and their conformation or three-dimensional structure: l Primary structure: the amino acid sequence l Secondary and tertiary structure: the three dimensional arrangement of the polypeptide chain l Quarternary structure: the arrangement of several polypeptide chains together PROTEIN STRUCTURE

Primary Structure

Secondary Structure

Tertiary Structure

Quaternary Structure

21 Primary structure: the amino acid sequence Secondary and tertiary structure: the three dimensional arrangement of the polypeptide chain Quarternary structure: the arrangement of several polypeptide chains together Proteins are characterised by their amino acid sequence and their conformation or three-dimensional structure:

22 l Fibrous proteins l Mainly structural tasks l Consist of simple and repeating secondary structures (  -helix and  -sheet structure ) l For example: keratin, collagen (  -helix), silk fibrin (  -sheet structure) l Globular proteins – Biologically active proteins – Complex tertiary structure often with several types of secondary interactions within the same polypeptide chain MAIN PROTEIN CLASSES

Food Proteins B. The Chemical Reactions and Functional Properties of Proteins 1) Buffering 2) Denaturation 3) Emulsification 4) Enzymes 5) Fat Reduction 6) Foaming 7) Gelation 8) Hydrolysis 9) Solubility 10) Water-Holding Capacity

Food Proteins 1)Buffering a process where pH change are prevented through ionization Amino acids may form ions in aqueous solution at certain pH, excess charge in protein, +ve or –ve helps to be polar soluble in water

25 l Some amino acid properties follow from the uneven charge distribution l In aqueous solution, amino acids appear as amphoteric molecules  behave as both acid & base CHCOO¯R + NH 3 AMINO ACID PROPERTIES

Food Proteins 2) Denaturation unfolding of protein structure H bonds breaking without disrupting protein covalent bond

27 heat salts acid alkalies Aggregation of protein to form a crossed linked matrix Coagulation Change from fluid (sol) to solid or semisolid (gel) state Denaturation of protein and loss of solubility Process of coagulation sol gel whipping

28 l Physical: – Heating – Cooling – Mechanical treatment – Hydrostatic pressure – Radiation l Chemical: – Acids – Bases – Metals – Organic solvents PROTEIN DENATURATION

29 Coagulation: Process of denaturation Denaturation of protein when heated up 1. Protein structure will open up 2. Refold in new arrangement 3. New permanent bonds formed Heat

30 Critical thinking Between denaturation & coagulation : -which bonds are broken :H, covalent, peptide ? -reversible -compare similarities & differences

31 l Change in solubility by exposure of hydrophilic or hydrophobic peptide units l Change in water-binding capacity l Loss of biological activity l Higher risk of chemical attack because of exposure of other peptide bonds l Change in the viscosity of solutions l Loss of crystallisation properties PROTEIN DENATURATION EFFECTS

Food Proteins 3) Emulsification - oil water interface are stabilized by hydrophilic & hydrophobic groups in proteins Oil Water Interface

33 Protein Stabilized Emulsions In order to form and stabilize an emulsion, a protein must:  Diffuse to the interface  Unfold  Expose hydrophobic groups  Interact with lipid

34 3) Emulsification

35 4)Enzymes -protein molecules which speed up chemical reactions without being used up in the process -pH,temprature moisture

Food Proteins 5) Fat Reduction  microparticulated protein Simpless  reduced calorie 1-2 kcal/gm  whey protein, milk, egg protein bakery creamers dairy products salad dressing sauces soups

Food Proteins 6) Foaming Foaming Characteristics Similar in some respects to emulsification. Denaturation of protein at an air-liquid interface. Hydrophobic groups interact the air, hydrophilic amino acids remain in the water.

Food Proteins 7) Gelation  A gel is a continuous network of macroscopic dimensions immersed in a liquid medium exhibiting no steady-state flow. Stages in heat induced gelation  Protein unfolding  Water binding  Protein-protein interactions  Water immobilization

Food Proteins 8) Hydrolysis -enzymatic & non enzymatic hydrolysis -amino acids-breakdown products 9) Solubility -affected by pH, temperature 10) Water-Holding Capacity -ability of proein molecule to bind water

Food Proteins 10) Water-Holding Capacity WHC determined by pH, salt, temperature Eg :- meat ve charge = +ve charge on a protein, protein-protein P-P interactions maximum adding salt-reduce P-P interactions more water-protein interactions Na + and Cl - ions bind with charged groups on protein fiber molecules Temp 80 C, water binding in proteins form thermally induced gels 3-dim gel network

41 l Hydration l Solubility l Viscosity l Gel formation l Texture l Dough formation l Emulsification l Foaming l Aroma binding l Interaction with other food components FUNCTIONAL PROPERTIES OF FOOD PROTEINS