Abstract
The lysine availability of animal feed sources is dependent upon the protein source and the processing treatment. Animal bioassays are the standard method for evaluating lysine availability. There is a need to improve in vitro assays for lysine availability to better reflect animal availability measures. Such assays could eventually replace the need for animal assays and substantially reduce assay time and costs. One in vitro assay for lysine availability that has been used previously measures the growth response of an Escherichia coli lysine auxotroph on a protein source. However, since E coli does not secrete proteolytic enzymes, it is necessary to predigest proteins to release free lysine or lysine peptides. The objective of this study was to determine the lysine availability of different protein sources using different enzyme pretreatments and varying the duration of digestion. Pepsin, pancreatin, protease and peptidase enzyme solutions were prepared as either single enzymes or enzyme combinations and subsequently added to various protein solutions. The lysine availability of the protein digests was assayed by measuring the growth response of the E coli lysine auxotroph in supplemented minimal medium. Lysine availability was significantly affected by the enzyme treatment (p < 0.01). The protease–peptidase enzyme combination had the most extensive digestion for all protein sources. There was no significant difference between 4 and 10 h digestion periods for the combination of protease–peptidase. The 4 h protease–peptidase treatment produced a digest with E coli-determined lysine availability highly correlated (Pearson correlation coefficient of 0.942) with animal availability values from previous work by others. Based on these results, it is possible to use a standard 4 h protease–peptidase enzyme treatment to predigest feed proteins to provide an accurate estimate of available lysine by an in vitro E coli lysine assay. © 1999 Society of Chemical Industry
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