Testing for total bacteria in dairy powder-comparison of test incubation temperatures (a case study) : a thesis presented in partial fulfilment of the requirements for the degree of Master in Food Technology at Massey University, Manawatū, New Zealand

Abstract

The objective of this study is to identify any deficiencies in the incubation temperatures currently used in the dairy industry for the microbiological assessment of dairy samples. In New Zealand, dairy industries use the Aerobic Plate Count (APC) to enumerate mesophiles at 30°C and thermophiles at 55°C. However, there are potentially some microorganisms in dairy samples with optimal growth temperatures outside the current temperature range used by the industry for microbiological testing. Therefore, in this study, 70 milk powder samples were tested for the APC at 30°C, 37°C, 55°C and 65°C. The results showed no significant difference (p > 0.05) between the number of bacteria capable of growth at 30°C and 37°C in all samples. The average number of isolates capable of growth at 30°C and 37° was 2.27 and 2.26 log₁₀ CFU/g respectively. However, bacterial growth at 55°C (1.78 log₁₀ CFU/g) was significantly higher (p < 0.05) than growth at 65°C (1.54 log₁₀ CFU/g). B. licheniformis was found to be the dominant bacteria in the dairy powder samples when testing was done at 30°C and 37°C. G. stearothermophilus and A. flavithermus were found in dairy powder samples when tested at 55°C and 65°C. These results indicate that the current testing temperatures (30°C and 55°C) used in the dairy industry are satisfactory.