The effect of microwave pasteurization on the composition of milk. I. Amino acid composition, free amino acid content, biological value

Abstract

The authors examined free amino acid and total amino acid content of milk samples pasteurized using different heat treatment procedures at a Dairy Company of Hargita county, using HPLC after derivatization with OPA/2-mercaptoethanol. They analyzed the effect of microwave treatment on the amino acids, free amino acid content and biological value compared to the conventional heat treatment technology. The amount of total essential amino acids was fully identical irrespective of whether it was about untreated raw milk or milk heat treated in different ways. Sulfur amino acids extremely sensitive to oxidation and heat, tyrosine as well as threonine changed minimally. The same was experienced for valine, isoleucine, leucine and phenylalanine content and no change was experienced for the non-essential amino acids, either, during the heat treatment. The two applied heat treatments caused practically no change in the amino acid composition of the milk protein neither in case of the essential nor in the case of the non-essential amino acids. Total free amino acid content of the raw milk was measured to be 20.67 mg/100 g milk, which reduced in the milk pasteurized in the traditional way to 8.02 mg amino acid/100 g milk, whereas in the microwave pasteurized milk to 8.96 mg amino acid/100 g milk. With the exception of arginine, the amount of each essential free amino acid decreased substantially during the heat treatment. The most striking is this decrease for phenylalanine, leucine, lysine, valine and tyrosine. The decrease is presumably a result of the technological intervention. Thus, significant difference was obtained between the raw milk and the milk samples heat treated in different ways regarding the free amino acids; no distinction could be drawn between the two heat treatment methods in the respect of free amino acids, however, consequently, the two heat treatment methods can be considered as being of same value also in this respect.