An improved spectrophotometric determination of cysteine and cystine in milk proteins and membrane proteins of milk fat globules

A simple spectrophotometric method to determine cysteine (RSH) and cystine (RSSR) was improved to use for turbid samples without need of treatment prior to analysis. The method is based on a controlled formation of coloured complexes between RSH and osmium acid whereas RSSR is to be hydrolysed by NaCN releasing one RSH for analysis. The improved method was scrutinized for its accuracy and precision using raw milk, skimmed (i) and untreated (ii), buttermilk from heated (iii) and untreated cream (iv) and technical casein (v). Investigations into the SH/SS-status of milk, casein and buttermilk revealed that the content of free -SH groups increased as follows: v < i < ii < iii < iv. These results give evidence that physiological alterations in the mammary gland may influence the coefficient RSH/RSSR and, milk fat globules stabilise the -SH groups in raw milk via -SH/ SH- interactions between proteins and fat globules. The analyses of RSH/RSSR in the buttermilk samples are pointing to a remarkable high reductive potential attributable to the unique physico-functional properties of the membrane material of milk fat globules. There is evidence that the maximum conceivable amount of -SH groups is reached in buttermilk produced from unheated cream.