2. An Evaluation of Chemical Methods for Predicting the Amino Acid Value of Proteins in Heated and Unheated Feeds to Rats

Mechanisms of heat damage in proteins

1. The preparation of a propionylated protein is described, and the effects of this treatment on amino acid composition and availability are compared with the effects of severe heat treatment (autoclaving) of a protein.

2. Using chemical analyses, changes exceeding 5% for total tyrosine, histidine, methionine and cystine contents were found after propionylation of the protein. Autoclaving of the protein resulted in changes in total serine, lysine, methionine, cystine and tryptophan contents.

3. Microbiological estimates of total amino acid contents were not in close agreement with the chemical estimates for the autoclaved protein.

4. Fluorodinitrobenzene-reactive lysine content was reduced to almost zero by propionylation, and by almost 40% by autoclaving.

5. Both propionylating and autoclaving protein reduced the amount of lysine available to the chick by about half. In contrast, the availabilities of methionine and tryptophan to the chick were unchanged by propionylation, but were reduced to 0.66 and 0.44 respectively, relative to the untreated protein, by autoclaving.

6. Because of the difficulties of obtaining reliable absolute estimates of amino acid availability using chick growth assays, our interpretation of results is mainly based on relative values. The merits of microbiological microbiological estimates of amino acid availability are assessed.