[Measure of available and blocked lysine in industrial milks]

The classical methods to measure lysine availability give rise to some difficulties in milks due to the presence of deoxyketose-lysine (DOKL). Six methods, one of which measuring the blocked lysine, are studied. Amino acid chromatography gives a too high value of lysine when it is blocked as DOKL, the latter regenerating lysine during acid hydrolysis. The determined lysine does not correspond to the available one. Fluro-dinitro-benzene is not fully specific for free epsilon-NH2 of lysine and the reducing sugars present interfere during the reaction in converting "nitro" into "amino" groups. Guanidization is a specific reaction between the epsilon-NH2 of lysine and O-methyl-isourea with formation of homoarginine. The homarginine, calculated as lysine corresponds to the available lysine. Reduction with sodium borhydride converts the DOKL to a stable compound which, during acid hydrolysis, does not regenerate lysine and does not form furosine. The recovered lysine corresponds to the available one. Our "in vitro" digestion method with simultaneous dialysis enables the determination of available lysine, as the proteolytic enzymes used do not split up the DOKL binding. The liberated lysine corresponds to the available one. The blocked lysine is measured from furosine after acid hydrolysis. A formula, which makes up the level of furosine formation and for lysine regeneration from DOKL, is established.