Determination of microbiological effects of dry chilling of beef carcasses

Objective: The objective of this project was to determine whether dry chilling can be used as an effective antimicrobial intervention for beef carcasses at small packing plants.

Experimental Design & Analysis: Groups of 25 carcasses selected at random at a commercial beef packing plant at which no antimicrobial interventions are used were sampled when the chilling process commenced and after the carcasses were chilled for 1, 2, 4, 6, 8, 24 and 67 h, for determination of the numbers of aerobes, coliforms and Escherichia coli. Ambient air conditions including air temperature, velocity and relative humidity (RH) were monitored throughout the chilling process. E. coli collected during the chilling process were genotyped using multiple-locus variable-number tandem repeat analysis (MLVA). Genotypes of E. coli only found at 0 h (group A) and found more than once (group B), as well as five strains of E. coli O157 (group C) were inoculated on stainless steel coupons and their response to desiccation (RH 75%) was investigated.

Key Results: The numbers of aerobes, coliforms and E. coli on carcasses before chilling were 5.33 ± 0.42, 1.95 ± 0.77, 1.42 ± 0.78 log CFU/4,000 cm2, respectively. The number of aerobes on carcasses was reduced by 1 log unit each by the first hour of chilling and the subsequent 23 h of chilling. There was no significant difference (P > 0.05) between the numbers of aerobes recovered from carcasses after 24 and 67 h of chilling. The change in the numbers of the indicator organisms approximated a bi-phasic process (Fig. 1). The log total numbers of coliforms and E. coli on carcasses before chilling and after the first hour of chilling were 3.86 and 3.30, and 2.24 and 2.04, respectively. The subsequent 23 h of chilling reduced the numbers of both groups of organisms by a further log unit. No coliforms or E. coli were recovered after 67 h of chilling. All strains inoculated on stainless steel coupons and exposed to 75% RH at 35 °C were completely inactivated, irrespective of their groups (Table 1). Inactivation of E. coli of the three groups was not significantly (P > 0.05) different by exposure to 75% RH at 0 °C.

How can this information be applied in the industry? The findings of this study show that reductions of up to 2 log units of aerobes, coliforms and E. coli could be attained within a 24-h chilling period by a process with appropriate parameters (e.g. an air temperature of 0°C with an off-coil RH of 88% and air velocity at 1.65 m/s). Inactivation of E. coli genotypes and E. coli O157 by desiccation on stainless steel simulating dry chilling conditions did not differ significantly. Thus, dry chilling process can be employed as an alternative to antimicrobial solutions for controlling microbiological contamination of beef carcasses, particularly at smaller abattoirs.