Relationship between Sedimentability and Composition of Casein Complex in Milk with Special Reference to Calcium and Phosphorus Contents of the Complex

Abstract

The casein complex was studied for a relationship between the sedimentability and the contents of some constituents by measuring the nitrogen, calcium, and various forms of phosphorus of the supernatant liquids of unheated or heated (85°C, 30 minutes) skim milk resultingfrom ultracentrifugation for serially increasing time. The effects of cooling and adding acid and calcium salt were also studied. The results are asfollcws.
1. About 11 per cent of the casein was not sedimented even after ultra-centrifugation at 35, 000G for 60 minutes. The precipitated nitrogen of heated milk was more than that of unheated milk. This increased precipitate in heated milk, at least a part of it, was assumed to be composed of denatured whey protein contained in the casein complex.
2. The calcium: nitrogen ratio and the phosphorus (in various forms): nitrogen ratio were essentially the same among different sizes of the casein complex. Besides, these ratios were not affected by heating.
3. The nitrogen precipitated by 60 minutes' centrifugation was decreased when the pH was lowered from 6.7 to 5.6-5.2 by addition of lactic acid. It was increased again at the isoelectric point of casein (pH4.6). Calcium and inorganic phophorus were released from the casein complex by addition of acid.
4. By addition of calcium chloride, the precipitable nitrogen was increased. At the same time, the calcium: nitrogen and the inorganic phosphorus: nitrogen ratios of the precipitate were increased, so that the calcium: inorganic phosphorus ratio remained almost constant.
5. The precipitable casein was decreased by cooling, which did not markedly affect the calcium: nitrogen and the inorganic phosphorus: nitrogen ratios of it.