Behavior and control of Listeria innocua during manufacture and storage of Turkish White Cheese

The physico-chemical changes occurring in Turkish White Cheese and the survival of Listeria innocua, total aerobic count (TAC) and lactic acid bacteria (LAB) in artificially inoculated Turkish White Cheese during manufacture and storage periods (45 days), with respect to different contamination levels of L. innocua were investigated. Turkish White Cheese was manufactured by the short-set procedure in pilot-plant-sized vats in a commercial dairy plant. Pasteurized cow's milk was inoculated with L. innocua to a final level of 3.84 (low dose) and 7.12 (high dose) log(10) CFU/ml. Bacterial loads of inoculated milk, whey, post-ripened curd and post-salted cheese was determined during processing at 23 degrees C. Cheeses were stored in 16% saline solution at 4 degrees C up to 45 days. Samples were taken from each treatment and analyzed at 5-day intervals. Total reduction in viable cell numbers of L. innocua in Turkish White Cheese with each inoculum dose was approximately 2 log(10) cycles during the storage period with a first-order-rate of inactivation at 4 degrees C. Variations in initial number of LAB in L. innocua seeded cheeses after overnight pressing were observed as 3.84, 4.24, 5.85 log CFU/ml at conditions of high-dose, low-dose inoculation of L. innocua and without inoculation as control sample, respectively. The salt concentration, pH, starter activity and storage time were determined to be the main causes of this reduction in LAB. The results had shown that L. innocua was able to survive during the manufacture and storage due to inadequate pasteurization or post-process contamination. The storage period has been defined as a critical control point (CCP) for consumption of Turkish White Cheese and calculated to be at least 90 and 178 days at refrigeration temperature, in low and high inoculum doses of L. innocua, respectively, for a safe consumption. This study, which gives the result of behavior of low and high dose of Listeria monocytogenes during storage as CCP, can even guide the future risk assessment studies on Listeria in Turkish White Cheese.