Protein

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Structure of protein

  • Molecules of protein are the largest known and are responsible for growth, repair and maintenence of the body
  • Proteins are large polymers built of amino acids
  • 80 amino acids- 20 found in food protein

Amino acids are compounds consisting of

  • Carbon
  • Hydrogen
  • Oxygen
  • Nitrogen
  • Sulphur and Phosphorous

                                                    

Amino group-N H 2 - C - C O O H-Carboxyl/acid group

           Side chain(varies for each amino acid)

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Dipeptide

The simplest amino acid is when R=H and this is caleld GLYCINE

 Amino acids can combine through their amino or carboxyl groups, when they condense in this way a dipeptide is formed. This link is called a peptide bond.

Amino acids join together to form larger moelcules called PEPTIDES. When more amino acids react with the peptide a very large chain of amino acids is called a POPLYPEPTIDE. These combine in branched and twisted chains to form protein moecules. A typical protein molecule can contain 500 or more amino acids. Each protein has its own unique number and sequence of amino acids.

Formula for a dipeptide-

N - C - C O O H   +   N - C - COOH                   N - C - C - N - C -  COOH +H20

Condensation Reaction- H20 released

Dipeptide-Peptides-Polypeptides(500 + amino acids)

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Protein Structure

The way in which the amino acids are joined and held together in protein molecules will determine the protein structure- protein structrue is either fibrous or globular.

Primary structure: The sequence of amino acids in the protein chain                          

Secondary structure: The amion acids further linked by various bonds to give a definate shape- often spiral        

Tertiary structure: Helix or spiral structure ay be folded over and held by cross links to form a globule

Globular:

  • Easily affected by acids,alkalis, heat and mechanical agitation
  • Unstable
  • Soluble
  • E.g. egg white(albumen) and milk (casein)

Fibrous: Found in helical coils or elastic

  • Fairly stable
  • Resistant to acids and alkalis and moderate heat
  • Insoluble
  • E.g. gluten (strong flour), collagen, elastin
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Function of protein

Growth and maintenance: Proteins serve as building materials for growth and repair of body tissues                                                                                                                             

Enzymes: Proteins facilitate needed chemical reactions    

Hormones: Proteins regulate body processes. Some hormones are proteins or are made from amino acids

Antibodies:Proteins form the immune system molecules that fight disease

Fluid and electrolyte balance:Proteins help to help maintain the fluid and mineral composition of various body fluids                                                                                                              

Acid base balance: Proteins help maintain the acid-base balance of various body fluids by acting as buffers

Energy: Proteins provide some fuel for the body's energy needs

Transportation: Proteins help transport needed substances, such as lipids, minerals and oxygen around the body

Blood clotting: Protein provides the netting on which blood clots are built

Structural components: Proteins form integral parts of most structures such as skin, tendons, ligaments, membranes, muscles, organs and bones

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Denaturation

Structure of protein may be readily changed by a number of agents causing molecules to aggregate and precipitate- known as denaturation and is usually irreversible but can be reversible. Denaturation is caused by:

Mechanical agitation- Whisking, beating, kneeding

  • Whisking egg white(albumen) increases the volume of the egg white creating a foam (air in liquid, collidial system)
  • Kneeding bread dough helps the gluten to become more elastic before cooking, allowing the dough to stretch as the rasing agent works

pH(Acids, alkalis, chemicals)

  • Marinades- acid acts on collagen in meat, making it more tender
  • Souring- Milk and lemon juice produces lactic acid, causing curdling
  • Stability- acid used in whisked foams increase volume

Heat (hot/cold)

  • When heated, protein foods coagulate(thicken/set) at approximatley 60 degrees celsius
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Foam formation

  • Foam is formed when a gas is dispearsed through a liquid
  • A very fine 'honeycomb' mesh is formed
  • Egg white foams easily
  • The quality of an egg white is judged by it's density and stability
  • Egg white foams are used to lighten and aerate mixtures

Colloidal systems

Made of two phases- dispearse and continous

  • Foams: air in liquid e.g. whisked egg white
  • Air in solid e.g. heated whisked egg white as in meringue
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The structure of meat

  • Striated or voluntary muscle
  • Consits of long cylindrical cells, or muscle fibres which are parallel to eachother
  • Striations go across the muscle cell hence the name striated muscles
  • Individual muscle fibres formed into bundles surrounded by connective tissue
  • These bundles are formed into groups which are also surrounded by connective tissue

Muscle fibres

  • Each muscle fibre held together by a sheath or sarcolemma
  • Within this the fibre is divided into MYOFIBRILS  surrounded by fluid
  • Myofibrils are made up of two proteins- myosin(thicker filaments) and actin (thinner filaments)
  • These proteins are responsible for the contraction of muscle and for rigor mortis after death

Connective tissue

  • Two proteins: collagen and elastin
  • Collagen = main component of tendons and connective tissue surrounding muscle fibres(esp. those that do most work) and is less flexible than elastin. When heated with moisture it turns to soluble gelatine=increase in tenderness of connective tissue and therefore the meat
  • Elastin= main component of ligaments & has ability to stretch and go back to its original shape. It is insoluble & tough but there is less elastin in muscles so does not have influence on tenderness of meat
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The structure of meat 2

  • Size of muscle fibres is related to the tenderness of cooked meat:
  • Slender, small fibres are assicated with tender meat
  • Large, long fibres are aassociated with tougher meat
  • Muscle fibres increase in size as the animal gets older
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The conversion of muscle into meat

  • When animal is slaughtered the carcass stiffens, caused by muscles contracting- rigor mortis
  • The residual glycogen in the muscles produces lactic acid as a result of the process of anaerobic glycolysis which takes place in the absence of oxygen.
  • The lactic acid causes the pH to fall and muscle contraction occurs (state of rigor)
  • After a few hours rigor mortis disappears and carcass starts to soften
  • This period is called the ageing or conditioning period of meat and is very important for good quality meat
  • During this ageing process, enzymes break down large molecules, particularly proteins and lipids, releasing amino acids and free fatty acids which are essential in the development of the typical meat flavour
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