Effects of Combined Shear and Thermal Forces on Destruction of Microbacterium lacticum

Abstract

A twin-screw extruder and a rotational rheometer were used to generate shear forces in concentrated gelatin inoculated with a heat-resistant isolate of a vegetative bacterial species,Microbacterium lacticum. Shear forces in the extruder were mainly controlled by varying the water feed rate. The water 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 surviving microorganisms could be detected at the highest wall shear stress of 409 kPa, giving log reduction of 5.3 (minimum detection level, 2 × 104 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°C. TheD 75°C of the microorganism in gelatin at 65% water content was 20 min. It is concluded that the physical forces generated in the reverse screw element and the extruder die rather than heat played a major part in cell destruction. In a rotational rheometer, after shearing of 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°C, the number of surviving microorganisms in the sheared sample was 5.2 × 106 CFU/g of sample compared with 1.4 × 108 CFU/g of sample in the nonsheared control. The relative effectiveness of physical forces in the killing of bacteria and destruction of starch granules is discussed.

Phenotypic and Genotypic Investigation of Two Representative Strains of Microbacterium Species Isolated From Micro-Filtered Milk: Growth Capacity and Spoilage-Potential Assessment

“…Microbacterium is considered a heat-resistant microorganism (Bulut et al, 1999;Walsh et al, 2012). Its genome has a high GC content.…”

“…For example, the heat-resistant and psychotropic strains Microbacterium lacticum and Microbacterium flavum were isolated from pasteurized milk after a prolonged storage period (14 days) at 7.2°C (Washam et al, 1977). Given the high thermal stability of the M. lacticum strains isolated from pasteurized milk, they have also been used for thermal resistance experiments, in combination with an extrusion process, in which they were subjected to both mechanical and thermal energy (Bulut et al, 1999). Microbacterium spp.…”

Toward a Philosophy and Theory of Volumetric Nonthermal Processing

“…The idea of shear sensitivity of bacteria is well established in biotechnology (Chisti and Flickinger ). Bulut and others () have investigated the effect of combined shear and heat on Microbacterium lacticum , and suggest a concept of a “shear D value.” It is possible that some of the extra lethal effects attributed to homogeneous field exposure may be due to shear. This is a worthy topic for future investigation.…”

Bacterial Sterilization Using Cavitating Jet

“…Bulut et al (31) examined the effect of shearing stress on Microbacterium lacticum using a twin-screw extruder. Log RR increased roughly with the square of wall shear stress.…”