Classification of Proteins
Simple: Animal- fibrous, globular
Plant- glutelins, prolamines
Conjugated: combined amino acids and a non-protein compound
High Biological Value
contains all essential amino acids, known as complete proteins, come from animal source eg eggs, milk, meat and soya bean
Low Biological Value
contains only some of the essential amino acids, known as incomplete proteins, come from plant sources eg rice, wheat, nuts and gelatine
Properties of Protein
Effects of Heat
Most proteins are insoluble in water, acids, and alkalis. Egg white is soluble in cold water and collagen is soluble in hot water.
Effects of Heat
Dry heat: Maillard reaction occurs between proteins and carbohydrates. It is an irreversible reaction eg toast
Moist heat: Collagen dissolves during moist cooking and converts to gelatine. This helps in making tough meat tender eg stewing meat
This is a change in the nature of a protein, ie. a breakdown in its structure. It occurs during cooking and heating of food or the addition of chemicals. Causes the links in tertiary and secondary structures to unravel and lose thei structure. It is irreversible.
Coagulation of protein can come about by beating them into a foam, eg egg white. When the foam is formed the coagulation stabilises and heat makes it become rigid, eg meringues
Collagen can be converted into gelatine by moist heat. Gelatine can absorb large amounts of water to form a gel used in cheesecakes.
a supplementary energey source
needed for growth and repair
needed for the production of hormones, enzymes, and antibodies
needed to supply body with essential amino acids
Eating two LBV protein foods together can ensure that all essential amino acids are obtained. This is important for vegetarians where no animal protein is eaten. eg beans on toast. Beans are high in lysine and low in methionine. Bread is low in lysine but high in methionine.
Digestion in mouth
secreted by saliva. polypeptide chains are mechanically broken down into peptones.
Digestion in stomach
secreted by gastric juices. polypeptide chains are broken down by the enzymes rennin and pepsin into peptones.
Digestion in pancreas
secreted by pancreatic juices. peptones are broken down by the enzyme trypsin into peptides.
Digestion in small intestine
secreted by intestinal juices. peptides are broken down by the enzyme peptidase into amino acids