Protein

Elements of an Amino Acid

• Carbon

• Nitrogen (growth)

• Oxygen

• Hydrogen

→ variable with Sulphur, Iron & Phosphorus

Total number of Amino Acids

20

How many Essential and Non-Essential for Adults?

• 8 essentials

• 12 non-essentials

How many Essential and Non-Essential for Kids?

• 10 essential

• 10 non-essential

The 3 groups in Amino Acids

• NH₂: Amino group + alkaline

• COOH: Carboxyl group + acidic

• R: Variable: differs in different amino acids and helps distinguish each e.g. Hydrogen = R for amino acid Glycine

Peptide Bonding Process

The OH from the COOH group from one of the amino acids, bonds with the H of the NH₂, and forms H₂O ← condensation reaction.

The remaining amino acids bond to from CO-NH bond, aka a peptide link.

Formation of a Peptide Link Diagram

Essential Amino Acids

• Not made in the body

• Must be obtained from a food source

• Valine, Isoleucine, Leucine and Lysine

Non-Essential Amino Acids

• Are made in the body

• Not needed to get from a food source

• Glycine, Alanine, Proline, Serine

Primary Structure

Determines the…

• Number

• Order

• Type

… of amino acids to go on the protein chain

Secondary Structure

• The protein chain can form cross-links with itself or with another protein chain → causing it to fold + pleat

1. Hydrogen links: oxygen + hydrogen → week link

2. Di-Sulphide links: sulphur + sulphur → strong link

Tertiary Structure

The pattern of the folding of a protein chain that gives its 3D shape

1. Globular: protein chain in a globe/sphere shape e.g. albumin

2. Fibrous: protein chain in straight, spiral, zig-zag shape e.g. gluten and collagen

Actin

Meat + fish fibres

Myosin

Meat + fish fibres

Albumin

Egg whites

Casein

Cheese

Collagen

Meat + fish connective tissues

Elastin

Meat connective tissues

Gelatine

Meat bones

Gluten

Wheat

Caseinogen

Milk

Properties of Protein

1. Foam Formation

2. Elasticity

3. Denaturation

4. Maillard Reaction

5. Solubility

6. Gel Formation

Foam Formation

• Proteins in egg denature when whisked and entrap air bubbles

• Protein coagulates around the air bubbles, forming temporary foam + changes colour (white)

• Sugar assists aeration

• Foam is fully set during cooking

• Culinary Use: Meringues

Denaturation

• Protein chains unfold, then bonds again → coagulation (firm + colour change)

• Irreversible

• Caused by:

  • Salt e.g. ham

  • Heat e.g. frying egg

  • Acid e.g. marinate meat

  • Mechanical Action e.g. whisking

  • Enzymes e.g. rennin - cheese

Elasticity

• Fibrous proteins are stretchy and elastic

• Gluten supports bread dough as it rises

Maillard Reaction

• Food with amino acids + sugar turn brown under dry heat & develops flavour

• No enzymes

• E.g. roasting potatoes

Solubility

• Proteins don’t dissolve in water (insoluble)

  • Exceptions: albumin (egg whites) dissolve in cold water + collagen dissolves into gelatine in hot water

• Glutelins dissolve in acids + alkalis

• Prolamines dissolve in alcohol

Gel Formation

• Happens when a protein like gelatine is dissolved in hot water → coiled strands unwind & float freely through hot liquid ← SOL

• As mixture cools down, unfolded strands lose energy & hit each other, forming new bonds + cross-link together to make a 3D shape ← coagulation

• Liquid traps in between bonds, turning mixture from liquid to a wobbly, semi-solid gel

HBV Protein

• High Biological Value

• Contains all 8 essential amino acids

• Meat, egg (100%), milk, cheese, soya beans (74%)

LBV Protein

• Low Biological Value

• Food is low in or lacks one or more essential amino acid

• Wheat (53%), gelatine (0%), beans, nuts, peas, lentils

Supplementary Value of Protein

• Eat 2 LBV foods together to get all essential amino acids

• Example: Beans on toast

  • Beans: high in lysine, low in methionine

  • Bread: high in methionine, low in lysine

Energy Value of Protein

1 gram = 4 kcal of energy

RI of Protein

• 1 kg of body weight = 0.75 grams of protein

• Food Pyramid: 2 portions of portions day

• Kids: 30-50g

• Adults: 50-75g

• Pregnancy: 75-80g

→ too much protein can lead to liver + kidney issues

Effect of Heat on Protein

• Dry Heat: Maillard Reaction, coagulation, colour change

• Moist Heat: Tenderising (collagen turns into gelatine), coagulation, colour change

→ Overcooking makes protein foods indigestible

Structural Proteins

• Growth & repair → produce skin, body and muscle cells

Physiologically Active Proteins

• Makes hormones, antibodies, enzymes, blood proteins and nucleoproteins

Nutrient Proteins

• Excess is used for heat and energy

• Provide body with amino acids

Deamination of Protein

• Excess amino acids are broken down by the liver for energy and waste

  • COOH group: is oxidised for heat and energy

  • NH₂: Liver changes it into NH₃ = Ammonia → then changes to urea → excreted by kidneys in urine

Hydrolysis

Adding water + enzymes during digestion to break up peptide links

Breakdown of Protein

Protein → Peptones → Peptides → Amino Acids