Protein

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General Info

  • It is a macro nutrient and so can be stored in the body, and it is a very important group of nutrients.
  • It can be found in the cytoplasm of all living cells, both animal and plant.
  •  1g of protein provides 17kJ (4kcal).
  •   Of the 20 amino acids commonly found in proteins, 8 are essential/indispensable in the diet.
  • An additional one must be supplied in the diets of rapidly growing infants.
  • The non-essential (indispensable) amino acids can be synthesised in the body by converting one amino acid into another within the body cells.
  • When protein foods are eaten, the proteins are hydrolysed during digestion to produce amino acids.
  • After absorption, the amino acids are transported by the blood to the cells.
  • In the cells, the amino acids recombine and proteins are formed.
  •  Functions in the body: growth and repair/maintenance of cells; formation of enzymes, anti-bodies and some hormones; formation of red blood cells.
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Excess and Deficiency

Excess

  • used for energy

Deficiency

  • not common in UK except for some cases of anaemia.
  • In Africa and Far East:
  • kwashiorkor - wasting of muscles, retarded growth, anaemia, distended abdomen caused by oedema.
  • It is common amongst young children after weaning.
  • Dried skinned milk is one of the most useful foods for the treatment of kwashiorkor.
  • PEM (Protein-Energy Malnutrition) is used to cover the whole range of conditions from protein deficiency at the one extreme to energy deficiency at the other. 
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Sources of protein

  • Protein can be obtained from both animal and plant sources.
  • In General foods from animal sources contain more protein than foods from vegetable sources, however some vegetable material do contain high sources of protein such as soya

Meat and fish:

  • The term ‘meat’ refers to the muscle tissue of animals but may be extended to include other parts of the animal, such as liver, kidney and sweetbreads.

 Bread and cereals:

  • Bread contains a significant amount of protein and is the most important and one of the cheapest sources in the British diet.

 Milk and cheese:

  • Milk is a valuable source of good quality protein and protein is present in some foods made from milk including cheese

Eggs: They are an excellent source of high quality protein.

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Physical and chemical structure of protein

  • Contains the elements hydrogen, oxygen, carbon and nitrogen. Some also contain phosphorus and sulphur.
  •  Protein molecules are large: they consist of long chains of amino acids chemically combined by peptide links/bonds.
  •  Each amino acid molecule contains at least one amino group (-NH2) and at least one acidic group (-COOH).
  • Therefore, amino acids show both basic and acidic properties and are said to be amphoteric.
  • A peptide link is formed when the amino (-NH2) group of one amino acid reacts with the acidic (-COOH) group of an adjacent amino acid.
  • A molecule of water is eliminated during the formation of the peptide link. This is known as a condensation polymerisation
  • Two amino acids formed together form a dipeptide.
  • Longer chains of amino acids are called polypeptides.
  • Protein molecules can consist of a single polypeptide chain or a number of polypeptide chains joined by cross linkages.
  • There are several orders of complexity in the structure of proteins including the primary, secondary and tertiary structures.
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Physical and chemical structure of protein

Primary structure:

  • the sequence of amino acids in the protein chain.
  • The amino acid and carboxyl groups take part in a condensation reaction to form a peptide bond.
  • Peptide bonds are very stable and can only be broken by the action of strong acids or enzymes, for example hydrolysed during digestion.

  Secondary structure:

  • amino acids are further linked by various bonds to give it a definite shape, often in the form of a spiral, namely at "R" groups.
  • The most important group involved in cross linking is the SH group, which forms a disulphide bridge.
  • Other links are formed between the amino acids which contribute to the coiling of the chain.
  • Cross links can also be made by the formation of hydrogen bonds and electrostatic bonds.
  • Cross linking determines the secondary structure (i.e. the shape) of the protein.

 

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Physical and chemical structure of protein

Tertiary structure:

  • the coiled or folded chain of amino acids is often coiled or folded further.
  • More bonds form between different parts of the polypeptide chain.
  • For proteins made from a single polypeptide chain, the tertiary structure forms their final 3D structure
  • Proteins can be classified in two groups according to the shape of the molecule: globular and fibrous.

Globular:

  • rounded in shape but not necessarily spherical.
  • The amino acid chain is folded and the molecule is kept in shape by cross linkages within the amino acid chain.
  • It is easy for water molecules to penetrate empty spaces within protein molecules.
  • Examples include ovalbumin found in egg white, haemoglobin, myoglobin and caseinogen found in milk.
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Physical and chemical structure of protein

Fibrous:

  • either straight (inelastic fibrous proteins) or coiled in a spiral (elastic fibrous proteins).
  • Usually an organised arrangement and molecules are closely packed together.
  • There are cross links between adjacent amino acid chains.
  • It is difficult for water molecules to penetrate the structure.
  • It is not soluble in water and examples include collagen in connective tissue, gluten found in wheat and elastin found in meat.
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High Biological and Low biological Value

  • Used as a measure of protein quality
  •  It is the % of absorbed protein converted into body protein.

HBV:

  • proteins that contain all the eight essential amino acids in sufficient quantity.
  • Found mainly in meat, cheese, fish, eggs, milk and soya beans.

 LBV:

proteins that lack one or more of the essential amino acids.

Found mainly in plant foods: wheat, rice, oats, pulses, lentils, beans, some nuts, vegetables (a little).

 exceptions: soya beans are HBV gelatine is LBV

 

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Complementation of Protein Foods

  • Complementation foods are proteins containing different essential amino acids (LBV and HBV) which should be eaten together in order to provide all of the essential amino acids for example, beans on toast.
  • it is the most efficient way of getting all the amino acids into a vegetarian's diet
  • The problem for vegetarians is that no single source of protein contains all the essential amino acids required for health.
  • By mixing different sources of vegetable protein together, it provides all the essential amino acids required
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Syneresis of Protein

  •  Syneresis is the shrinkage of a gel and the subsequent loss of liquid.
  •  Gels affected by syneresis can be based upon protein.
  •  An example of syneresis is scrambled egg, which if overcooked, shrinks and produces a liquid
  •  Another example is jelly left to stand for a long time and therefore produces a pool of water around the base
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The Nutritional Composition of Dairy Products

  •  Milk and Cheese: this is a valuable source of good-quality protein. Protein is present in some foods made from milk, the most important of these being cheese.
  •  Eggs: eggs are an excellent source of high-quality protein. However, they are not eaten as much as other protein foods.

 

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The Effect of Heat on Protein Foods

  • Globular proteins are affected by heat, whereas fibrous proteins are not.
  • Heat causes the secondary structure of proteins to denature, where the molecule unfolds and changes shape but the sequence of amino acids remains the same.
  •  Denaturation breaks the cross linkages which maintain the shape of the molecule. It is usually irreversible.
  • As a result, of denaturation, the properties of proteins alter: they become less soluble and more viscous. The unfolded molecules tend to form clumps, as they bond with each other. This results in the setting or hardening of protein foods, known as coagulation.
  • Egg white proteins coagulate at 60̊ C and the yolk approximately at 66̊ C.
  • The muscle fibre proteins in meat coagulate when heated resulting in the shrinkage of meat during cooking. Collagen is softened at 80-100 degrees C, in the presence of water, and is converted into gelatine. Overcooking makes the fibres stringy, elastin nor reticulin are affected by heat.
  • Maillard Reaction - browning results from the reaction of reducing sugars and amino acids. It occurs as the result of heating.  The heat turns the myoglobin in meat from red to brown.
  • Denatured proteins are more digestible as the digestive enzymes can break them down more easily.
  • Some examples of different types of cooking using heat for protein foods: Beef - braising, casserole, stewing, Chicken - barbecuing, roasting, grilling. Fish - smoking, drying, frying
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The Effect of Rennin

  •    Rennin also known as rennet is an enzyme which coagulates protein
  • Rennin is used to make junket which is clotted or coagulated milk
  •  It is also used with a bacterial started to make the curd in cheese manufacture
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The Effect of Acids and Alkalis

·         Fibrous proteins are not greatly affected by acids or alkalis but globular proteins are.

·         Acid can start to make proteins coagulate e.g. starter cultures in cheese and yoghurt which consist of lactose-fermenting bacteria. The lactic acid produced by the bacteria is responsible for "setting" or coagulating the milk, forming a curd.

·         It breaks down the tertiary structure of protein e.g. in meat - the use of acidic marinades (vinegar, lemon juice, wine or even tomato juice). The acid increases the rate at which collagen is converted to gelatine.

·         In meringue, the addition of lemon juice makes a stronger/denser/more stable foam.

·         Lemon juice can be used to denature proteins and to speed up the time it takes for egg whites to foam.

·         Acid lowers the temperature of coagulation and gives a firmer set e.g. in poached egg.

 

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the Effect of Mechanical Action on Protein Foods

 

·         Mechanical action causes denaturation (which can be reversible), followed by coagulation (irreversible), (when heated/cooked) such as in the whisking of egg white. The protein molecules unfold and form a reinforcing network around the air bubbles, therefore stabilising the foam.

·         Food products of meringues and soufflés are examples of this effect.

·         Mechanical pounding, cutting up meat, mincing helps to break up longer muscle fibres. Elastin and reticulin are not affected but are made easier to eat.

 

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The Effect of the Addition of Salt

The Effect of the Addition of Salt

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

·         During cheese making, salt is often added to the curd to increase firmness and also to suppress the growth of micro-organisms.

·         Example: salting fish

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