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Effect of storage temperature on survival and recovery of thermal and extrusion injured Escherichia coli K-12 in whey protein concentrate and corn meal.

Previously, we reported inactivation of Escherichia coli populations in corn product (CP) and whey protein product (WPP) extruded at different temperatures. However, information on the effect of storage temperatures on injured bacterial populations was not addressed. In this study, the effect of storage temperatures on the survival and recovery of thermal death time (TDT) disks and extrusion injured E. coli populations in CP and WPP was investigated. CP and WPP inoculated with E. coli bacteria at 7.8 log(10) CFU/g were conveyed separately into the extruder with a series 6300 digital type T-35 twin screw volumetric feeder set at a speed of 600 rpm and extruded at 35°C, 55°C, 75°C, and 95°C, or thermally treated with TDT disks submerged into water bath set at 35°C, 55°C, 75°C, and 95°C for 120 s. Populations of surviving bacteria including injured cells in all treated samples were determined immediately and every day for 5 days, and up to 10 days for untreated samples during storage at 5°C, 10°C, and 23°C. TDT disks treatment at 35°C and 55°C did not cause significant changes in the population of the surviving bacteria including injured populations. Extrusion treatment at 35°C and 55°C led to significant (p<0.05) reduction of E. coli populations in WPP as opposed to CP. The injured populations among the surviving E. coli cells in CP and WPP extruded at all temperatures tested were inactivated during storage. Population of E. coli inactivated in samples extruded at 75°C was significantly (p<0.05) different than 55°C during storage. Percent injured population could not be determined in samples extruded at 95°C due to absence of colony forming units on the agar plates. The results of this study showed that further inactivation of the injured populations occurred during storage at 5°C for 5 days suggesting the need for immediate storage of 75°C extruded CP and WPP at 5°C for at least 24 h to enhance their microbial safety.