Abstract
The analytical method for the total solid content of milk by heat drying (the Japanese official method) was evaluated from several points of procedure. Soild content of milk continuously decreased by prolonged heating in an air oven and constant value which is described in the official method was difficulty attained. The continued loss of weight may be partly attributted by conversion of milk constituents into volatile components such as carbonyls, hydrogensulfide, acidic and basic components which were confirmed to be generated from milk solid during heating. Gas chromatographic analysis of moisture content remaining in milk soild after drying revealed that heating time in an air oven for the official method is reasonable to set 3 hours because residual moisture in dried solid reaches minimum after 3 hours heating. Dried solid heated for 3 hours still contains, however, about 1% of moisture which is not eliminated by prolonged heating. Slight absorption of moisture by dried soild was also observed during cooling in desiccator after heating. Vacuum drying at 98±2°C accelerated drying and decreased residual moisture of dried solid but weight loss was continued throughout drying. By vacuum drying method at 68±2°C, milk soild was insufficiently dried even by 5 hours heating. Freeze drying method was also evaluated as a method for gravimetric measurement of milk solid. Milk was well dehydrated into porous state like sponge without any sign of browning. Residual moisture decreased to about one-half of those found in heat dried solid. On drying operation, prefreezing was necessary but preevaporation on boiling water bath and cooling in desiccator after heating were not required. This drying principle may be employed as a more correct and available gravimetric method for the determination of total solid content of milk.
