HEAT RESISTANCE PATTERNS OF BACTERIAL CONTAMINANTS OF READY- TO- EAT SMOKED FISH
Fish is an important animal protein food available to millions of people. Fish spoilage is brought about as a result of three basic processes- enzymatic autolysis, oxidation and microbial growth and this is as a result of not being properly handled. In terms of microbial spoilage, fish is susceptible to a wide range of pathogenic bacteria. A total of six bacteria namely: S. aureus, Bacillus sp, Pseudomonas sp, E. coli, Listeria sp and Proteus sp were isolated from twenty- seven samples of smoked fish in Nsukka town. From this six, three were chosen for further studies. The effect of heat, pH and substrate (tomato paste) were carried out on three isolates namely: S. aureus, Bacillus sp and Listeria monocytogenes by taking their culture broth and adjusting to 0.5 McFarland’s standard (108cfu/ml) and subjecting to various temperatures (40- 80oC), pH (5 – 9) and substrate (tomato paste) concentrations (20, 40 and 60%). The thermal death time (TDT)was constructed using thermal resistance data. D-value (decimal reduction time) and Z-value were taken from the thermal resistance data when log10 of survivors were plotted against time. Bacillus sp had the highest D-value of 2.70 min at the temperature of 40oC and decreased to 0.97 min at temperature of 80oC after 40 min of heating, Listeria monocytogeneshad aD-value of 2.40 min at the temperature of 40oC and decreased to 0.79 min at 80oC after 40 min of heating and S. aureushad aD-value of 1.96 min at the temperature of 40oC and decreased to 0.73 min at 80oC after 30 min of heating. Their Z-values were as follows: Bacillussp had a Z-value of 0.042, Lpisteria monocytogenes had a Z-value of 0.041 and S. aureus had a Z-value of 0.032. S. aureushad the lowest Z-value, showing that theD-values of S. aureuswere more sensitive to temperature changes than that of Bacillus sp. and Listeria monocytogenes. All the isolates showed their highest resistance properties and D-values at pH 6, 7 and 8 at 40oC but decreased as the temperature increased to 60oC for 40 min. At pH 5 and 9, all the isolates showed their least decimal reduction time. The isolates were also heated in the presence of tomato paste substrate at concentrations of 20, 40 and 60%. As the temperature and concentration increased their heat resistance properties and D-values decreased. The isolates showed their highest D-values and heat resistance properties at 20% tomato paste substrate concentration at temperature of 40oC and decreased to 60% tomato paste substrate concentration as the temperature increased to 60oC. The study demonstrated that the selected bacteria from smoked fish exhibited heat resistance properties for the wide range of temperatures used, suggesting that use of optimized environmental factors such as temperature (> 60oC) and pH (5 and 9) are required for an efficient removal of bacteria.