THE EFFECTS OF YEAR SEASON, UREA, ACETONE AND NITRATE ADDITIONS AND NATIVE CONTENT OF MICROELEMENTS ON COWS MILK FERMENTATION

Milk fermentation is an important quality indicator in the regions in which fermented milk products are made. Besides antibiotics presence, it is influenced by many other factors. The objective of the present p aper was to evaluate the effect of some factors on fermentation of bul k milk samples. A total of 555 samples were used, 20 samples were excl uded from statistical evaluation as a result of positive finding (Bac. stearothermophilus) by IN-test method leading to a suspicion of the p resence of inhibitory substances of particularly antibiotic origin. Mi lk fermentation (yoghurt test) was determined as titratable acidity af ter addition of yoghurt culture R(x) (Laktoflora) to pasteurized milk after 3.5 hours of incubation at 43-degrees-C. The effect of urea, ace tone and nitrate additions (U, A and N) above the original values was also tested in identical paired samples as exerted on milk fermentatio n. The additions corresponded to levels which are probable to occur in metabolic production disorders which may result from wrong nutrition, and to markedly higher levels. In a random parallel formed set the co nsequences of variability of the native content of some microelements were also evaluated with respect to milk fermentation. The value of mi lk fermentation (Tab. I) was significantly higher (P < 0.001) in summe r feeding season than in winter feeding season, and it was accompanied by other significant differences (P < 0.001 and P < 0.05) in the othe r milk characteristics. In summer, the somatic cell counts and acetone content were lower while urea and nitrate contents were higher as a r esult of different diets in the feeding seasons. The increase in the o riginal urea (U) concentration in milk by 25 and 50 mg/100 ml did not have a significant effect (P > 0.05) on milk fermentation (Tab. II). U rea concentration in milk in itself, even if it increases to a somewha t higher value due to improper nutrition, does not worsen the quality of milk fermentation. The increase in the original acetone (A) concent ration in milk by 15 and 30 mg/l (Tab. III) significantly deteriorated milk fermentation (P < 0.001 and P < 0.0 1; Tab. III). Hence the resu lts indicate that the acetone concentration in itself, if markedly hig her than the physiological level (e.g. in subclinical ketosis), can in fluence milk fermentation negatively. But the marked increase in nitra te concentration (N; by 35 mg/l; Tab. IV) did not affect the quality o f milk fermentation either (P > 0.05). Only when the increase in the l evels of undesirable metabolites (U, A, N) was very large and hardly t o occur in practical farming conditions (Tab. V), the quality of milk fermentation decreased (P < 0.01). It is apparent that mere increases in undesirable metabolite concentrations in milk above the physiologic al limits (except strongly increased concentrations) do not cause a de terioration of milk fermentation in operating conditions. That is why the decrease in milk fermentation often observed in metabolic disorder s of dairy cows is a result of other overall changes in the milk pictu re from cows with metabolic disorders. Tab. VI shows a variability of native contents of some microelements. An evaluation of linear regress ion indicates that the variability of native contents of individual mi croelements documented the variability of milk fermentation as follows : Mo responsible for 11 % (r = 0.34; P < 0.01), Mn for 1 %, Al for 1 % , Cu for 15 % (r = 0.38; P < 0.01), Fe for 13 % (r = -0.37, P < 0.01), Zn for 5 % (r = -0.23; P < 0.05). Thirty-four per cent of milk fermen tation variability can be explained from joint effects of all the inve stigated microelements: Mo, Mn, Al, Cu, Fe and Zn.