Behavior of Salmonella typhimurium DT104 during the manufacturing of Ergo and Ayib, Ethiopian traditional fermented milk products
Growth and survival of Salmonella typhimurium DT104 were evaluated during the manufacturing of Ergo (Ethiopian naturally fermented milk) and Ayib (Ethiopian cottage cheese). Three different initial inoculum levels of DT104 (high: ~108 cfu mL-1; medium: ~106 cfu mL-1 and low: ~104 cfu mL- 1) were used. Samples were drawn at 0, 12, 24, 36, 48, 60 and 72 h of fermentation for bacterial enumeration and pH measurements. Ayib was made using defatted fermented milk after 24 and 72 h of fermentation and at three cooking temperatures (50, 60 and 70°C) with sampling being performed at 0, 20, 40 and 60 min of cooking. DT104 survived up to 72 h of fermentation when co-inoculated with Lactic Acid Bacteria (LAB) at high and medium initial inoculum levels, though DT104 population dropped by 3.3 and 5.6 log cfu mL-1, respectively. In the control milk with no LAB, DT104 survived better with only 0.8 log cfu mL-1 reduction in cell number after 72 h of fermentation. When fermented milk (inoculated with or without LAB at 0 or 24 h of fermentation) was cooked at 50°C for Ayib-making, DT104 survived up to 60 min of cooking with survival varied based on initial inoculum level. Complete inhibition was achieved between 20 and 40 min of cooking at 60 and 70°C. Direct consumption of Ergo is not safe. The use of boiled milk with fermentation initiated by a portion of fermented milk resulting from 'back-slopping' can be recommended for Ergo-making. For Ayib-making, a minimum cooking temperature of 60°C for at least 40 min is required to ensure the wholesomeness of the product.