Protein

Essential Amino Acids

Essential Amino Acids

Cannot be made by the body and must be taken from the food we eat. E.g. Valine

Non-essential amino acids

Can be made in the body so not essential in our food. E.g. Alanine

Sources of protein

Animal; Meat (Actin) Egg (Ablumin) cheese (Casein). Plant; Wheat (Gluten)

Primary Protein Structure

The sequence or order of amino acids in the polypeptide chains. These amino acids are linked together by peptide links. Primary structure also refers to the number of amino acids in each polypeptide chain, E.g. Insulin is a simple protein with 51 amino acids arranged in a specific order

Secondary protein structure

Involves further linking of amino acids in the polypeptide chain/s to give the protein a definite shape which is often the form of a spiral. This structure is caused by cross links that form between different chains or within the one

Disulphide links

Type of cross link, happens when 2 sulphur atoms join within one polypeptide chain or between two different polypeptide chains. The amino acid cysteine has sulphur and so two cysteine amino acids can join together by disulphide links

Hydrogen bonds

Type of cross link, Where a hydrogen atom in one chain bonds with an oxygen atom in another chain, occurs in collagen. These bonds are formed with one polypeptide chain or between two different polypeptide chains

Tertiary protein structure

Involves pattern of folding of the polypeptide chains to give a three dimensional compressed unit. The protein chains cross link forming either a fibrous or a globular protein structure.

Fibrous proteins

Polypeptide chains are straight, coiled or zigzag. These proteins are insoluble in water and not easily denatured. Example keratin, hair. Gluten, wheat. Elastin and collagen, meat

Globular proteins

Polypeptide chains are spherical. These proteins are soluble in water and easily denatured. Example, Ovalbium, egg white. Lactalbumin, milk

Denaturation

A change in the nature of the protein, unfolding of protein chains resulted in irreversible loss in both structure and shape

Heat

Most proteins coagulate when heated, heat causes protein chains to unfold and bond together, which results in hardening or setting of the protein food e.g. fried egg

Chemicals

Acids lower the Ph, e.g. lemon juice added to milk cause the milk protein caseinogen to curdle. The enzyme rennin, coagulates the milk protein caseinogen in the stomach.

Mechanical action (agitation)

Whisking an egg white causes the protein chains to unfold resulting in partial coagulation and foam forms. Moist heat(boiling) converts collagen to gelatine, tenderising meat. Dry heat(grilling) causes meat to become tougher as the meat fibres shrink and lose water. Whisking egg white(mechanical action)

Elasticity

Some proteins such a gluten in flour, are very elastic. Presence of gluten allows bread to rise during cooking by making yeast dough elastic so it can trap Co2

Solubility

Proteins are generally soluble in water except egg white ( only in cold water) and collagen( only in warm water). Moist heat (stewing) converts collagen in meat to gelatine, more tender

Maillard reaction

The non enzymic browning of food when dry heat is applied. Occurs when food is roasted, baked or grilled. Under dry heat a reaction occurs between amino acids and carbohydrates resulting in browning of food e.g. roast beef, roast potatoes, toast

Brown colour

Amino acid, carbohydrate, dry heat

Gel formation

The heat collagen presents in the bones and skin of meat is converted to gelatine on heating. Gelatine has the ability to absorb large amounts of water when heated, as protein chains uncoil and waste becomes trapped.

Sol

A Sol (a solution with evenly dispersed particles) is formed, which on cooling, forms a gel

Foam formation

Whisking an egg white causes protein chains to unfold and air bubbles form, which entrap air creating temporary foam. Heat is generated due which coagulates the egg albumin so the egg white slightly sets. The foam will collapse in time, unless heated so it sets to form a permanent foam e.g. making meringues

Biological value

A measure of the quantity of a protein in food and is determined by the number of essential amino acids present

High biological value

A complete protein, contains all essential amino acids required, generally sourced from animals apart from soya beans

Low biological value

Incomplete protein, lack some essential amino acids, generally plant sourced apart from gelatine

Supplementary value/ complimentary rate

When lbv foods are eaten together they can provide all essential amino acids, only if the deficiency of the amino acid missing in one food can be made up for by being present in the other food e.g. Beans on toast

Beans on toast

Bread is lacking lysine but is high in methionine. Beans are lacking methionine but is high in lysine. By eating beans on toast both essential amino acids are included in the meal