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Determination of L- and D-methionine and L- and D-valine in soy isolate and soy flour samples.
L to D isomerization of amino acids in a commercially prepared
alkali-treated food-grade soy isolate was investigated. A soy flour
product was also investigated as a control which had not been alkali treated.
Methionine and valine collected from two soy flour and two
soy isolate samples were coupled with L-leucine-N-carboxyanhydride.
If both D- and L-forms were present this would produce diastereomeric
dipeptide derivatives. The derivatives were separated by ion-exchange
chromatography using a TSM Amino Acid Analyzer. Comparison of the
sample methionine and valine dipeptide peaks to standard dipeptide
peaks revealed the presence of L-valine, L-methionine and D-methionine
in both soy flour and soy isolate samples. Methionine was substantially
isomerized in both products. D-valine was not observed in either
soy flour or soy isolate samples. Thus, isomerization of methionine
appeared to occur at a faster rate than valine. Acid hydrolysis,
toasting of soy meal, and alkaline treatment are all conditions which
may be responsible for the isomerization of methionine.