Time and temperature of stretching as critical control points for Listeria monocytogenes during production of mozzarella cheese.

Different heating times and temperatures commonly used during curd stretching were investigated to determine their effects on the viability of Listeria monocytogenes in mozzarella cheese. Pasteurized whole milk was inoculated with two levels of L. monocytogenes (7 and 3 log CFU/g) and coagulated with citric acid and rennet. The curd was stretched at 55, 66, and 77 degrees C for 1, 3, and 5 min. Results indicated that the majority of L. monocytogenes cells remained in the cheese curds at both inoculum levels. Stretching at 66 degrees C for 3 min reduced the number of L. monocytogenes by 5 log units, whereas stretching at 55 degrees C had a minimal effect. Stretching at 77 degrees C resulted in the complete demise of L. monocytogenes cells (from 7.6 log CFU/g to < 1.0 log CFU/g) in 1 min. If the stretching temperature partially reduced microbial counts, bring (4 degrees C for 12 h) usually had a lethal effect on the remaining microorganisms, but was less effective than the stretching temperature. These results show that stretching curd at 66 degrees C for 5 min or 77 degrees C for 1 min can effectively control L. monocytogenes during the production of mozzarella cheese.