Prevalence of B.Cereus in Retail Raw Milk and Influence of Ph on its Survival in Pasteurized Milk During Low Temperature Storage

Abstract

A study was conducted to determine the prevalence of B.cereus in retail raw milk samples and the influence of pH on survival of the organism during low temperature storage. A total of 43 samples of raw milk were purchased from 9  Fulani milkmaids in Unguwar Rimi district of Kaduna, Nigeria. Samples were collected at the point of milking, in sterile bottles, and plated unto sterile Mannitol Egg yolk Polymyxin  (MYP) agar (Oxoid)  plates for B.cereus isolation . Counts ranged from <1.4 ± 0.0 to 0.4 ± 0.6 log₁₀ CFU/mlfor B. cereus in the raw milk samples.  Results also revealed a low prevalence rate (7%) for the raw milk samples.  B.cereus counts fell within expected microbiological levels for ready-to-eat meals (<10²).  Analysis of variance revealed non significant differences in counts amongst samples at 5% level (P>0.05).  To study the influence of pH on survival of B.cereus in milk during low temperature storage, Mcfarlands standard 1 of a B.cereus strain (about 8.5log 10 CFU/ml) isolated from  raw milk during the prevalence study,  and identified using a  microgen^TM  Bacillus  ID,  was inoculated into pasteurized  milk samples (72^oC for 15min) following  pH adjustment to 6.7, 6.4, (control samples) 6.3 and 6.8 with 0.1M lactic acid/Sodium hydroxide. Samples were stored at low temperature (4-7^oC) for a period of 72 hrs, and analyzed for B.cereus counts on sterile nutrient agar at intervals of 24 hours.  At the end of 24 hours, mean counts were observed to decrease from initial levels in samples especially at pH 6.8, except at pH 6.3, where counts remained almost unchanged. Further decreases were also observed at 48 hours, in samples at pH extremes of 6.3(about 2 log units) and 6.8, whereas slight increases were observed (less than 1 log unit) in control samples. At 72 hours, mean counts decreased in control samples while samples at 6.3 and 6.8 had increases of about 1 log unit in counts. Mean counts were not significantly different across samples during the storage period (P˃0.05), but significant differences existed when counts were analyzed across pH levels (PË‚0.05) at 5% level.  Findings of this study suggest B.cereus cells are inhibited in pasteurized whole cow milk during storage at less than 10^oC at pH levels 6.3. 6.4 and 6.7 for 24 hours, while sustained bactericidal effects are exerted for up to 48 hours at a higher pH level of 6.8. Longer storage periods however allowed for growth as a possible consequence of change in milk composition due to lysis of dead cells, and proteolytic activity of B.cereus cells. Findings also infer that B.cereus cells from mesophilic environments in raw milk are capable of  psychrotrophic  growth.