Chilling - Effect of Humidity on Reduction of Campylobacter on Pork

Chilling reduces the number of Campylobacter on pork meat. The humidity during chilling is expected to have an influence on the reduction of Campylobacter. In this study, bellies taken from carcasses before chilling have been inoculated either with a mixture of C. jejuni isolates or C.coli isolates and chilled with blast chilling or conventional chilling combined with two levels of relative humidity, >90 % RH and approx. 70% RH. In blast chilling, the surface is frozen in the first phase of the chilling, and in conventional chilling the surface does not freeze. The major reduction in the numbers of Campylobacter was found when the bellies were blast chilled. Low humidity and resulting drying of the surface had no effect on the reduction of Campylobacter. Introduction Blast chilling has been shown to reduce the number of pathogens on pork carcasses (Bracewell et al, 1985; Chang et al, 2003; Dalsgaard et al, 1999). Especially with regard to Campylobacter, it has been suggested that the reduction during chilling is due to sensitiveness to drying (Bracewell et al, 1985; Oosterom et al, 1983). The purpose of the present study was to quantify the effect of freezing and drying on the reduction of Campylobacter during blast chilling and conventional chilling. Materials and Methods Bellies were obtained from carcasses at a nearby slaughterhouse before chilling of the carcasses and were kept in insulated boxes until inoculation and chilling. The bellies were inoculated with either a mixture of 3 C. coli isolates or a mixture of 2 C. jejuni isolates. All isolates, except one C. jejuni isolate from a human case, have been isolated from pork carcasses during previous studies. The total number of Campylobacter after inoculation was approx. 10 cfu/cm. Chilling was conducted in 20-ft containers, rebuilt to make a simulation of commercial chilling processes possible. To simulate blast chilling, the bellies were chilled for 50-60 min. (depending on the average weight of the bellies) at –18°C with an air speed of 3 m/sec. and then left to equalize 21 hours at 5°C with an air speed of 0.5 m/sec. To simulate conventional chilling, the bellies were chilled for 22 hours at 5°C with an air speed of 3 m/sec. Both chilling methods were conducted at a Relative Humidity (RH) of 90-100 % and a RH of 65-75 %. All combinations of chilling methods and RH were conducted twice with 10 bellies at a time, 5 inoculated with C. coli and 5 inoculated with C. jejuni. From each belly, 2 samples were taken from the rind side, and 2 samples were taken from the meat side before and after chilling. Samples were taken by excising 50 cm. Numbers of Campylobacter were obtained by incubating dilutions in Boulton bouillon for 48h at 42°C, streaking 10μl from each dilution onto CCDA and enumerating after 48 h at 42°C. All counts were log transformed before statistical analysis. The two results from each belly before and after chilling were treated as a duplicate determination, and a reduction was calculated for each belly. Statistical analysis was done by analysis of variance (SAS System, release 8.02). Results Results are shown in table 1. In general, C. coli is more susceptible to chilling than C. jejuni independent of the chilling method. Blast chilling in general results in a higher reduction than conventional chilling. The effect of chilling method is significant for C. jejuni (p=0.03) and the greatest reduction is obtained by blast chilling. The effect of chilling method is not significant for C. coli. The reduction on the rind side is systematically higher than on the meat side, but this effect is not significant. Differences in reduction due to RH were small and insignificant. Weight loss during blast chilling with high and low RH was 1.7% and 2.9%. Weight loss during conventional chilling with high and low RH was 2.9% and 4.0%.