RIBONUCLEOSIDES - CHEMICAL-PARAMETERS FOR CONTROLLING THE HEAT-TREATMENT OF MILK

From experimental findings concerning changes in the contents of ribon ucleosides - which belong to the group of minor milk constituents in h eat-treated milk it can be concluded that the concentrations of cytidi ne, guanosine and inosine present in raw milk show a multifold increas e in milk which has been subjected to thermization and holder pasteuri zation, whereas the adenosine level decreases. On the other hand, thes e enzyme- (e.g. adenosine deaminase, alkaline phosphatase) catalysed c hanges in the content of unmodified ribonucleosides in milk are not ob served in milk samples after short-and longtime pasteurization, high t emperature pasteurization, ultra-high heating and sterilization. The c hanges 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-, a s well as high temperature pasteurization i.e. in the whole pasteuriza tion region, changes in the nucleoside levels correlating with process ing temperature and holding time were not observed since in these heat ing regions milk enzymes, such as adenosine deaminase and alkaline pho sphatase, are inactivated. Hence, in the whole region of pasteurizatio n neither enzyme-induced degradation-and conversion reactions nor ther mally induced chemical degradation reactions take place with respect t o unmodified ribonucleosides. However, with sterilization values excee ding F-0 = 30 min thermally induced hydrolytic effects are observed in UHT milk samples which lead in tions were made as regards cytidine, g uanosine, inosine and adenosine contents in sterilized milk samples, S umming up, it can be concluded that from the viewpoint of dairy techno logy 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 s terilization 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 sterilizat ion have shown that N-6-methyladenosine, which is the resulting produc t of the rearrangement, is suited to be used as chemical parameter for controlling milk heat-treatment under time-temperature conditions wit h sterilization values (F-0) lying between approximate to 0.4 and 22 m in. N-6-methyladenosine is, thus, suited to be used as a chemical para meter for detecting heat treatment in the upper range of high temperat ure pasteurization, the whole range of UHT heating and the lower range of sterilization (as normally applied in dairies). From the viewpoint of dairy technology N-6-methyladenosine is, thus, suited to be used a s a further chemical parameter mainly for characterizing the upper ran ge of UHT heating and the lower range of sterilization of heat-treated milk.