Comparison of the growth kinetics and proteolytic activities of Chryseobacterium species and Pseudomonas fluorescens.

The effect of temperature on the growth kinetics and proteolytic activity of Chryseobacterium joostei and Chryseobacterium bovis was determined during this study. The results were compared with the activities of Pseudomonas fluorescens, which is regarded to be a major food spoilage psychrotolerant microorganism. For the growth studies, cultures were incubated in nutrient broth in a temperature gradient incubator (from 9 to 50 °C) and separately at 4 °C, and the optical density was measured at different time intervals. Growth temperature profiles for each organism were constructed. For determination of proteolytic activity, the cultures were incubated in fat-free ultra-high temperature processed milk in the temperature gradient incubator for 72 h (temperature range as above). Cell-free extracts were used to determine the proteolytic activity using the azocasein method. Results of the growth studies showed that C. joostei had the ability to grow over a wider temperature range than C. bovis and P. fluorescens without being affected by changes in the temperature. For the proteolytic activity, C. joostei had significantly (p < 0.001) higher activity per milligram of protein at 15.5 °C, followed by C. bovis and P. fluorescens. The results showed that C. joostei potentially has an even greater spoilage capacity in milk on the basis of growth rate and proteolytic activity than did P. fluorescens.

The growth, survival and thermal inactivation of Escherichia coli O157:H7 in a traditional South African sausage.

This study investigated the growth and survival of Escherichia coli O157:H7 inoculated into boerewors models with (B+P) and without (B-P) sulphur dioxide preservative at a low (L) and high (H) inoculum followed by storage at 0, 4 and 10 degrees C for 10 days. The pathogen's thermal inactivation at 50, 60, 65 and 70 degrees C was also evaluated in B+P. The B-P at both low and high inocula had significantly higher recoveries at all temperatures compared to B+P. The BL+P and BH+P had significant reductions in recoveries at 0 degrees C, declining to below detectable limits at days 8 and 10, respectively. At 4 degrees C, the BL+P and BH+P recoveries declined significantly at day 10. At 10 degrees C, significant increases were observed from days 0 to day 10 in both models and at low and high inocula. At 0 degrees C, the BL-P and BH-P treatments had significant declines in recoveries. The combination of sulphur dioxide preservative and low temperature demonstrated the best efficacy against E. coli O157:H7 survival. Thermal inactivation of E. coli O157:H7 was 60 min at 60 degrees C, 80 s at 65 degrees C and 60s at 70 degrees C. This study demonstrated that E. coli O157:H7 can survive in boerewors with and without preservative and is more sensitive to heat treatment at 70 degrees C.