Effects of combined shear and thermal forces on destruction of Microbacterium lacticum

A twin-screw extruder and a rotational rheometer were used to generate shea r forces in concentrated gelatin inoculated with a heat-resistant isolate o f a vegetative bacterial species, Microbacterium lacticum, Shear forces in the extruder were mainly controlled by varying the water feed rate. The wat er content of the extrudates changed between 19 and 45% (wet weight basis). Higher shear forces generated at low water contents and the calculated die wall shear stress correlated strongly with bacterial destruction. No survi ving microorganisms could be detected at the highest wall shear stress of 4 09 kPa, giving log reduction of 5.3 (minimum detection level, 2 x 10(4) CFU /sample), The mean residence time of the microorganism in the extruder was 49 to 58 s, and the maximum temperature measured in the end of the die was 73 degrees C, The D-75 degrees C of the microorganism in gelatin at 65% wat er content was 20 min. It is concluded that the physical forces generated i n the reverse screw element and the extruder die rather than heat played a major part in cell destruction. In a rotational rheometer, after shearing o f a mix of microorganisms with gelatin at 65% (wt/wt) moisture content for 4 min at a shear stress of 2.8 kPa and a temperature of 75 degrees C, the n umber of surviving microorganisms in the sheared sample was 5.2 x 10(6) CFU /g of sample compared with 1.4 x 10(8) CFU/g of sample in the nonsheared co ntrol. The relative effectiveness of physical forces in the killing of bact eria and destruction of starch granules is discussed.