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Effect of storage temperature on the bacterial growth and pH levels of bovine colostrum
Storage of colostrum is a convenient practice that ensures supplies are readily available should they be needed, however storage temperature may affect colostrum quality. The objective of this study was to investigate the effect of storage temperature on bacterial growth and pH levels of colostrum. The study took place at Teagasc Moorepark Research Farm, Cork, Ireland. Colostrum from six Holstein-Friesian cows (≥3rd lactation) was collected from 5-12 March 2013 at the first scheduled milking post-calving (<9hr post-calving). Immediately after collection samples were separated into 100ml aliquots, which were replicated and stored in separate temperature-controlled units at one of three temperatures: 4°C, 13°C and 20°C. Aliquots were removed and frozen at 0, 6, 12, 24, 36, 48, 60 and 72 hours post storage. Consequently, they were defrosted at 4°C to determine total bacterial count (TBC) using serial dilution. Dilution rate ranged from 1:10,000 (lowest expected) to 1:10,000,000 (highest expected). Diluted samples (1ml) were incubated at 32°C for 48 hours on 3M petrifilm aerobic count plates. Subsequent recordings were obtained using a 3M Petrifilm Plate Reader. Duplicate TBC’s were prepared and an average was calculated. Simultaneously, aliquots were measured for pH using an OHM Delta 2105.2 datalogger (www.lennox.ie). Calibration was carried out before each test period and the probe was cleaned weekly using product guidelines. Data was tested for normality using PROC UNIVARIATE in SAS (v9.3) displaying positively-skewed data, thus a log transformation was performed. Transformed data was analysed using a mixed model (PROC MIXED; SAS v9.3). The model included treatment, time and their interactions. Significant differences between treatments were seen in TBC once colostrum was stored for ≥12 hours. Storage at 20°C had significantly higher TBC from 12 hours than storage at both 13°C and 4°C (P<0.001), while TBC of colostrum kept at 13°C for ≥24 hours was significantly greater than colostrum kept at 4°C (P<0.001). Additionally, colostral pH was significantly lower from 24 hours in colostrum kept at 20°C compared to 13°C and 4°C (P<0.001). Furthermore, from hour 60 of storage the pH of colostrum stored at 13°C was significantly lower than that kept at 4°C (P<0.01). Analysis revealed that a LogTBC >7.5 cfu/ml resulted in a pH drop to <6.5. Further research is warranted to determine if this affects passive transfer of immunity in calves. It is clear however that stored colostrum should be refrigerated (≤4°C) to minimise bacterial proliferation and maintain pH.
Keywords: colostrum; storage; bacteria