Bacterial Multiplication in Packed Wiltshire Bacon

Abstract

SUMMARY Matured Wiltshire bacon normally carries 105–106 viable bacteria/g when sliced, predominantly salt tolerant micrococci and lactobacilli. Kept at 15°, the numbers rose in 4 days to 107–108/g, the flora remaining generally similar in nature and with higher counts at 20° than at 37°. The situation then stabilized; there was a slow decline in organoleptic quality, but nothing unusual occurred in two weeks. Kept at 37°, similar numbers were reached after 1 day, and the nature of the flora changed rapidly, counts at 37° much exceeding those at 20°. Organoleptic deterioration was marked after 2 days and the bacon was unacceptable after 4 days. These changes occurred whether the bacon was packed in air or in 5% oxygen, the oxygen being all consumed in 4 days or less according to temperature. Apparently, if the bacon is kept at too high a temperature for its normal psychrophilic (20° optimum) flora, this is replaced by a mesophilic (37° optimum) flora. It appears that pathogenic staphylococci might be present in this mesophilic flora. A high proportion of isolates proved to be phosphatase positive, or 44° positive; and some had both these properties. Though all, so far, have been coagulase negative, there is no evident reason why pathogenic types should not occur.

Variations in the composition of the flora on a Wiltshire cured bacon side

“…It is now well established that micrococci are the predominant bacteria on matured Wiltshire bacon (Garrard & Lochhead, 1939;Brooks et al, 1940;Ingram, 1952Ingram, , 1960. The present work has confirmed this for both the rind and meat surfaces of a bacon side.…”

“…The bacterial contamination of the meat site tended to be lower than either the singed or unsinged rind, thus confirming the observations of Tofte Jespersen & Riemann (1958) and Ingram (1960). Of all the sites this would probably have the least opportunity for outside contamination.…”

“…It is difficult to assess these results because of the different media used and also the isolates were not fully identified. Ingram (1960) found that the flora of matured bacon consisted of micrococci plus a roughly similar number of lactobacilli and a smaller proportion of Gramnegative rods. The latter accounted for 4 7 9 % of the flora of the 1. dorsi samples and 0*1-20% of the skin samples of bacon sides after 7 days' maturation.…”

The fate of a toxigenic strain ofStaphylococcus aureusin vacuum-packaged bacon

“…Thus inhibition of staphylococci by other micro-organisms is affected bytemperature; beinglessas the temperature rises. This has already been established, especially when bacon is stored at temperatures exceeding about 250 C. (Eddy & Ingram, 1962;Ingram, 1960). It was suggested by Eddy & Ingram (1962) that the principle obstacle to the multiplication of staphylococci is the preponderance of the normal microflora and they 571 posed the question, 'why does this flora inhibit staphylococci?'.…”

“…By improving hygiene in the manufacture of packaged bacon and thereby reducing the number of spoilage organisms, accidental contamination of the product with CPS might therefore result in their multiplication. Furthermore, Ingram (1960) considered that a warm vacuum pack of bacon might act as an unintentional selective medium for staphylococci since they can reduce nitrate, are proteolytic, facultatively anaerobic and have a temperature optimum for growth near 370 C. Despite Ingram's (1960) suggestion, vacuum packaging per se has been shown by Christiansen & Foster (1965) to be inhibitory. They reported that Staphylococcus aureus increased about six generations on sliced ham at both 200 and 150 C. under vacuum, but went through [11][12] generations in the absence of vacuum and reached numbers which were 20-fold greater than the maximum populations which developed under the anaerobic conditions of a vacuum pack.…”