Amino-acid content of raw and heat-sterilized cow's milk

Abstract

It is well known that the nutritive value of milk proteins is reduced by heating, for example when milk is brought to, and kept for some time at, its boiling point. The reduction may be due either to the breakdown of certain amino acids under the influence of heat, or to combination of amino acids with other chemical constituents of milk, as in the formation of the so-called Schiff bases due to the condensation of free amino groups with the aldehyde group of sugars. This condensation, in turn, leads to the Maillard reaction, often referred to as the 'browning reaction'. It takes place in heat-sterilized milk when it is exposed for a sufficiently long time to a high temperature. Amino acids may eventually be released from certain of these condensation products by acid hydrolysis, which does not necessarily mean that the nutritive value of the heated product has not been impaired since digestive enzymes may fail to hydrolyse these protein derivatives and digestibility may thereby be reduced (Lea & Hannan, 1950). Our purpose has been to determine the effect of sterilization by heat on the amino-acid content of partly skimmed milk. EXPERIMENTAL A sample of partly skimmed raw milk was divided into two parts : one was hydrolysed immediately, the other was sterilized in an autoclave at 122-124O for 20 min. This process produced no visible browning. Two samples of the unsterilized and two of the sterilized milk, each containing 40-50 mg protein, were hydrolysed by refluxing with 200 ml of ~N -H C ~ (ammonia-free) for 22 h. The amino-acid content of the hydrolysates was determined by Moore & Stein's (195 I) original chromatographic method on ion-exchange columns (Dowex 50 x S), with the modified procedure of Schram, Dustin, Moore & Bigwood (1953). Cystine was determined separately In the course of acid hydrolysis under our conditions, methionine is partly converted into its sulphoxide, which is allowed for in the calculation Tryptophan was estimated separately in the hydrolysate by the microbiological method of Henderson & Snell(1948). Total nitrogen was estimated in duplicate in each sample by Kjeldahl's method.