Ribonucleosides: Chemical parameters for controlling the heat treatment of milk

From experimental findings concerning changes in the contents of ribonucleosides ‐ which belong to the group of minor milk constituents ‐ in heat‐treated milk it can be concluded that the concentrations of cytidine, guanosine and inosine present in raw milk show a multifold increase in milk which has been subjected to thermization and holder pasteurization, whereas the adenosine level decreases. On the other hand, these enzyme‐ (e.g. adenosine deaminase, alkaline phosphatase) catalysed changes in the content of unmodified ribonucleosides in milk are not observed in milk samples after short‐ and long‐time pasteurization, high temperature pasteurization, ultra‐high heating and sterilization. The changes in milk ribonucleoside contents found in these heating regions are mainly attributable to the catalytic activity of the milk enzymes in the course of the heating‐up phase. During short‐ and long‐time‐, as well as high temperature pasteurization i.e. in the whole pasteurization region, changes in the nucleoside levels correlating with processing temperature and holding time were not observed since in these heating regions milk enzymes, such as adenosine deaminase and alkaline phosphatase, are inactivated. Hence, in the whole region of pasteurization neither enzyme‐induced degradation‐ and conversion reactions nor thermally induced chemical degradation reactions take place with respect to unmodified ribonucleosides. However, with sterilization values exceeding F0 = 30 min thermally induced hydrolytic effects are observed in UHT milk samples which lead in particular to an increase in adenosine contents. Comparable observations were made as regards cytidine, guanosine, inosine and adenosine contents in sterilized milk samples. Summing up, it can be concluded that from the viewpoint of dairy technology cytidine, guanosine and mainly inosine can be taken into account as chemical parameters for detecting thermization and, in particular, holder pasteurization. For characterizing heat treatment in the region of pasteurization, high temperature pasteurization, UHT heating and sterilization unmodified ribonucleosides of milk appear to be unsuited. The studies on the Dimroth‐rearrangement under heating conditions of thermization and holder pasteurization, as well as short‐, long‐time‐ and high‐temperature pasteurization, ultra‐high heating and sterilization have shown that N6‐methyladenosine, which is the resulting product of the rearrangement, is suited to be used as chemical parameter for controlling milk heat‐treatment under time‐temperature conditions with sterilization values (F0) lying between ≈0.4 and 22 min. N6‐methyladenosine is, thus, suited to be used as a chemical parameter for detecting heat treatment in the upper range of high temperature pasteurization, the whole range of UHT heating and the lower range of sterilization (as normally applied in dairies). From the viewpoint of dairy technology N6‐methyladenosine is, thus, suited to be used as a further chemical parameter mainly for characterizing the upper range of UHT heating and the lower range of sterilization of heat‐treated milk.