The Solubility of Milk Powder as Affected by Moisture Content

A review of the data herein shows conclusively that the development of insolubility of proteins of milk powder during storage is due to the presence of moisture. There was a complete absence of casein in suspension after centrifuging under definite conditions at the end of seven to eight weeks during which time about 100 liters of moist air (humidity 70 to 75 per cent) were circulated daily. During this period and under the conditions of the test, the water content of the powder increased from under 2 to about 11 per cent. The absorption of moisture and consequent development of insolubility of the casein was essentially the same in powders made by the Just double cylinder process and the spray process. Moisture-free air circulated through milk powder at the same rate as the humid air did not cause any change in the solubility of the proteins within a period of six months, and from the results and character of the powder at the end of that time it is believed the proteins would retain their original solubility indefinitely, or as long as moisture was entirely excluded. The original solubility of the casein of milk powder can be maintained for a year or more if the moisture content is kept below 3 per cent during the storage period. In powder with a moisture content maintained between 4 and 5 per cent there was a greater degree of insolubility throughout a period of one year than in those samples in which the moisture content was maintained at 3 or lower. Samples in which the moisture content was maintained from about 3 to about 5 per cent and in which the solubility had shown but slight change during a period of one year, became almost entirely insoluble within a few days when the moisture content was suddenly raised to 6.5 to 7 per cent. Samples in which the moisture content was maintained as near 5 per cent as possible throughout a period of one year showed greater insolubility than those samples in which the moisture content was gradually raised from 4 to 5 per cent during the same period. The material precipitated by centrifugal force from reconstituted Just process milk powder contains approximately 93 per cent protein material and 7 per cent ash. The ash is almost entirely composed of the oxides of calcium, phosphorus and magnesium. From the analysis it appears that insoluble calcium phosphate is the principle constituent of the precipitated mineral matter.