Role of the Sulphydryl Group in the Formation of Alkaline Protein Gels

Abstract

COMMENTING on an account¹ from this laboratory of the gelation of alkaline casein dispersions, Wormell² has suggested that the sulphydryl group is the specific one concerned in gelation and that its oxidation to the disulphide bond provides the most likely cross-linkage. In, work as yet unpublished, we have also considered the possibility of non-methionine sulphur being involved in gelation. The results shown in Table 1 represent the amounts of alkalilabile sulphur liberated from the system lactic casein–sodium hydroxide–water at 25° C. when the ratio of air-dry casein to water is 2:5. At the end of the period of reaction, the glues were dispersed in their own volume of water and 1 N hydrochloric acid added, in excess of that required for precipitation. The appearance of the protein precipitated after treatment at high pH. was distinctly fibrous; milder treatments yielded flocculent precipitates. The hydrogen sulphide evolved was passed through N/10 iodine solution, the protein dispersions being boiled until a negative sulphide test was given by lead acetate. The liberated sulphur was determined by back-titration against N/100 sodium thiosulphate.