The Mechanism of Urea Action in Milk on the Fermentation Activity of Lactic Cultures

Veterinarni Medicina 39, 1994, 715-722

An increased content of urea in milk is a result of administration of excess rates of proteins to dairy cows at a deficient dietary energy portion. The presence of nitrates in the ingredients of feed ration also contributes to this situation. Nitrates act as inhibitors of a range of oxidation-reduction enzymes, that means also as inhibitors of most fermentations in the cow rumen, which decreases utilization of energy contained in feed ration and enhances disproportions in the protein-glycide supply in dairy cows. The increased content of urea in milk influences its processing to fermented products and to cheese. In the present paper the effect of urea in milk (40-60 mmol/l) on yoghurt and sour cream cultures was investigated. The higher content of urea had negative impacts on the activity of yoghurt culture. The concentration higher than 6 mmol/l decreased the counts of lactobacilli, which induced an adverse ratio of streptococci and lactobacilli. This resulted in poor fermentation, low acidity and increased content of ammonia in the product (Tab. I). The presence of urea in sour cream culture evokes intensive multiplication, and this leads to overfermentation of products (Tab. II). No inhibitory effects on the particular strains of lactic bacteria were observed, nor on Lactobacillus delbrueckii subsp. bulgaricus. These observations made us consider that the compound which negatively influences the activity of lactic cultures is not urea but ammonia produced by enzymatic breakdown of urea. This is the reason why such amounts of ammonium chloride were added to milk with yoghurt and sour cream culture that adjusted the terminal concentrations to the values recorded in preceding experiments. Incubation of samples was in agreement with the time and temperature indicated for the given cultures. The obtained results confirmed our assumptions (Tabs. IV and V) that the main reason of disturbance of the optimum activity of lactic cultures is ammonia produced by urea breakdown to the bacterial activity

FULL PAPER on request
« back