MACRONUTRIENTS (PROTEIN)

PROTEIN

Protein are a very important group of macronutrients. They are found in the cytoplasm of all cells, both animals and plants. Protein are organic substances and they resemble like carbohydrates contain elements carbon, hydrogen and oxygen. However, all proteins contain nitrogen and most contain sulfur and some contain phosphorous.

STRUCTURE OF PROTEINS

Protein molecules are extremely large and consists of long chain of amino-acids chemically combined together. Each amino acid molecule contains at least one amino group (NH₂) and at least one acidic group (-COOH).

A typical protein molecule contains about 500 amino acids joined together by peptide links. A peptide link is formed when the amino (NH₂) group of one amino acid interacts with the acidic (-COOH) group of an adjacent amino acid. A molecule of water (H₂O) is eliminated during the formation of the peptide link. Longer chains of amino acids are called polypeptides. Polypeptides chains are joined by cross linkages.

TYPES OF PROTEIN

ON THE BASIS OF COMPOSITION

1. SIMPLE PROTEINS

Simple proteins are made up of amino acid subunits joined together by peptide bonds. When hydrolyzed by enzymes, simple proteins yield only the amino acids from which they are comprised of.

Examples of simple proteins include albumins, globulins, glutelins and albuminoids.

2. CONJUGATED PROTEINS

Conjugated proteins are complex proteins composed of simple proteins and a non-protein substance.

ON THE BASIS OF STRUCTURE

1. GLOBULAR PROTEINS

Globular protein are rounded in shape but are not necessarily spherical. The amino-acid chain is folded and the molecule is kept in shape by cross-linkages within the amino-acid chain. Some examples of globular protein found in food are ovalbumin found in egg white and casinogen, found in milk.

2. FIBROUS PROTEIN

Molecules of fibrous protein are straighter. In fibrous protein, there is usually an organized arrangement and the molecules are closely packed together. Fibrous protein are not soluble in water. There are cross linkage between adjacent amino acids chains and it is difficult for water molecules to penetrate the structure. Example of fibrous protein in food are gluten, the insoluble protein found in wheat is an elastic protein. When pulled, gluten will stretch but it tends to return to its original shape when the force is removed.

PROPERTIES OF PROTEIN

Following are the properties of protein:

DENATURATION AND COAGULATION

Protein undergo a process known as denaturation when their secondary structure is altered but their structure is unchanged. The molecule unfold and changes shape but the sequence of amino-acids remains the same. Denaturation is brought about by various physical and chemical means and involves the breaking of the cross-linkages which maintain the shape of the molecule. The unfold molecules tend to bond with each other forming clumps. This results in the hardening or setting of the protein food and is known as coagulation.

Coagulation can be brought about by various means such as:

1. ACTION OF HEAT

Many protein coagulate when heated. For example, when an egg is cooked the protein in the white and yolk coagulate. Egg white protein coagulate at 60⁰C and yolk at approximately at 66^◦C. this coagulation is used extensively in the preparation of any dishes.

EXAMPLE: Egg custard and sponge cake

2. PRESENCE OF ACID

When milk sours, bacteria present in the milk ferment lactose, producing lactic acid. The pH of milk is lowered and this causes the milk protein caseinogen to coagulate.

EXAMPLE: Used in the manufacture of some milk products, such as yogurt and cheese.

3. ADDITION OF SALT

Certain salts such as sodium chloride, coagulate some protein. If salt is added to the cooking water used for boiling eggs, the white will not escape as readily if shell is cracked.

4. MECHANICAL ACTION

Mechanical action during the whisking of egg white causes a partial coagulation of the protein. The protein molecules unfold and forms a reinforcing network areound the air bubbles, thus stabilizing, the foam. This is used in the preparation of food.

EXAMPLE: meringues and soufflés

ESSENTIAL AND NON ESSENTIAL AMINOACIDS

Amino acids can be called the “building blocks” of protein and are an important part of every human body. There are 20 different amino acids – nine of which are called “essential” and 11 of which are labeled as “non-essential.”

ESSENTIAL AMINO ACIDS

These are the nine amino acids that your body cannot create on its own, and that you must obtain by eating various foods. Adults need to eat foods that contain the following eight amino acids: methionine, valine, tryptophan, isoleucine, leucine, lysine, threonine and phenylalanine. Histidine, the ninth amino acid, is only necessary for babies.

NON-ESSENTIAL AMINOACIDS

The other type is the non-essential amino acid, 11 of which exist and are synthesized by the body. Thus, although they are an important part of building proteins, they do not need to be included in an everyday diet. The non-essential amino-acids are alanine, arginine, aspargine, aspartic acid, cysteine, glutamic acid, glycine, ornithine, proline, serine and tyrosine.

SOURCES OF PROTEIN IN DIET

Following are the sources of protein in the diet:

• Meat and fish
• Bread and cereals
• Milk and cheese
• Eggs
• Vegetables
• Nuts

MILLARD REACTION (NON ENZYMATIC BROWNING)

This is the browning reaction which occurs during roasting, baking, grilling and frying of many foods. A chemical reaction takes place between the amino group of amino-acid or a free amino group on a protein chain and carbonyl group of a reducing sugar e.g, glucose. Brown colored compounds are formed which are responsible for the attractive color of products such as bread crust, roasted meat, fried potatoes, and baked cakes and biscuits. This reaction is generally considered desirable during cooking.