Observations on the Application of the Nitroprusside Test to Heated Milk

Abstract

A critical study of the nitroprusside test as it is applied to heated milk indicated the following amts. or concn. of reagents to be optimum: Na nitroprusside - 4.5-5% aqueous soln.; NH4OH - not less than 16% strength; and (NH4)2SO4 -4-5 g./5 ml. sample of milk. Sample temp. at the time of testing was found to be an important variable. For max. color development in the test samples should be cooled to 20 C or lower. Expts. utilizing the nitroprusside test as an indicator of sulfhy-dryl behavior demonstrated that these groups disappear in skim milk heated for prolonged periods at high temps. This disappearance depends upon a reaction involving lactose and casein and is correlated with the development of caramelized flavor and browning. The distr. of sulfhydryl substances in heated skim milk was studied, using the nitroprusside test. These substances associate themselves physically and/or chemically with the casein in heated skim milk. This phenomenon may be an important factor in explaining the soft curd characteristics of highly heated milk.