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Mechanisms of heat damage in proteins

6. The digestibility of individual amino acids in heated and propionylated proteins*

Published online by Cambridge University Press:  09 March 2007

Shirley A. Varnish  and
K. J. Carpenter
Shirley A. Varnish
Affiliation:
Department of Applied Biology, University of Cambridge, Cambridge CB2 3DX
K. J. Carpenter
Affiliation:
Department of Applied Biology, University of Cambridge, Cambridge CB2 3DX
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Abstract

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1. The digestibilities of protein and amino acids have been estimated by two different techniques: the analysis of faeces (conventional method) and the analysis of ileal contents (ileal technique).

2. Freeze-dried muscle protein was found by both techniques to be almost completely digested. After autoclaving, the digestibility for the same protein was estimated by the conventional and ileal techniques to be 0.65 and 0.57 repectively.

3. Unmodified lactalbumin was found by both techniques to have a digestibility of about 0.90. Propionylation of the lactalbumin reduced digestibility to 0.82 and 0.79 as indicated by faecal analysis and ileal content analysis respectively.

4. In general, the digestibilities of individual amino acids in any one protein sample were rather uniform, and reflected over-all protein digestibility. For each amino acid, digestibility, as determined by the both methods, was lower for the modified protein than for the corresponding control protein: estimates based on ileal content analyses were consistently lower than those obtained by conventional analyses. The ileal technique was considered to be both more convenient and meaningful.

5. From the results obtained by the ileal technique it appears that reduced digestibility is an adequate explanation for the reduction found in nutritional value of the autoclaved protein. In contrast, for the propionylated protein, reduced digestibility of lysine is only a partial explanation of the low availability of this amino acid as estimated by chick growth assay.

6. In our experiments we found that the type of dietary protein used did influence the amino acid composition of the ileal contents. This was most marked with the least-digestible protein. These findings do not support the views of Nasset (1962).

Type
Papers of direct relevance to Clinical and Human Nutrition
Information
British Journal of Nutrition , Volume 34 , Issue 2 , September 1975 , pp. 339 - 349
Copyright
Copyright © The Nutrition Society 1975

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