The Effect of Different Somatic Cell Count Levels on the Rate of Acidification in Yoghurt Milk During Incubation Period

Mastitis and elevated somatic cell count are considered as factors that affect the quality of milk and milk products. The purpose of present study was to investigate the effect of mastitis on the rate of fermentation in inoculated yoghurt milk during incubation time. Yoghurt milk with three different levels of somatic cell count (<200000 (Low), 200000 to 800000 (Medium) and >800000 cells/ml (High)) was used to produce set style yoghurt. The non protein nitrogen in raw milk, titratable acidity and pH of inoculated yoghurt milk were monitored during incubation period. The results revealed that with increasing somatic cell count, the amount of non protein nitrogen increases and longer incubation time is required to reach the target pH or titratable acidity at the end of incubation period (p<0.05). It was suggested that by elevating somatic cell count, the concentrations of chloride, sodium, citrate and immunoglobulin increase while the concentrations of potassium, calcium, phosphorus, casein and lactose decrease which might cause alteration of buffering capacity, pH and titratable acidity of yoghurt milk. Beside higher proteolytic activity in mastitic milk that results in higher nonprotein nitrogen content in medium and high somatic cell samples can stimulate the growth of Streptococcus thermophilus. It has been concluded that at high level of somatic cell in yoghurt milk, longer fermentation is required to achieve the desired acidity and structure in final yoghurt.