Fish Nutrition & Feed Technology

Unit 3 - Proteins and Amino acids

3.8. Amino acid Composition and Protein Quality

The protein quality of a feed ingredient is dependent upon the amino acid composition of the protein and the biological availability of the amino acids present. In general, the closer the EAA pattern of the protein approximates to the dietary EAA requirement of the species, the higher its nutritional value and utilization.

For example, under certain conditions some of the amino acids may be unavailable because the proteins in the diet are incompletely digested. Thus, for carnivorous fish and shrimp species, the cellulose cell wall within plant protein sources may render the proteins present within the cell inaccessible to the digestive enzymes. In other cases, digestion may be hindered by the presence of enzyme inhibitors within the food protein like trypsin inhibitor within raw soybeans.

Although it is possible to inactivate these inhibitors by moderate heat processing, under conditions of excessive heat treatment proteins become more resistant to digestion due to peptide bond formation occurring between the side chains of lysine and dicarboxylic acid. The free epsilon amino groups of lysine are particularly susceptible to heat damage, forming addition compounds with non-protein compounds (ie. reducing sugars such as glucose) present in the food stuff (Cockerell, Francis & Halliday, 1972). This reaction is known as the Maillard reaction, and renders the lysine biologically unavailable. Substances other than reducing sugars which are known to react with the free epsilon amino group of lysine include gossypol; phenol based compound present in cottonseed meal. Thus an estimate of the biological availability of amino acids within feed proteins gives an indication of protein quality.