Alice M. Doherty

The prevalence and spread of Escherichia coli O157:H7 at a commercial beef abattoir

Aims: To investigate the prevalence and virulence characteristics of Escherichia coli O157:H7 after a number of beef process operations at a commercial Irish abattoir.

The relationship between hide cleanliness and bacterial numbers on beef carcasses at a commercial abattoir

Cattle were visually inspected in the lairage of a commercial abattoir and assigned to a category ranging from 1 (very clean) to 5 (very dirty) depending on the observed cleanliness of the hide. Animals from categories 2, 3 and 5 were slaughtered and total viable counts (TVCs) enumerated at five sites (hock, brisket, cranial back, bung and inside round) on the subsequent carcasses. The TVCs at the hock were significantly higher on category 5 than on category 2 carcasses (P < 0·05). Similarly, TVCs at the brisket were significantly higher on categories 3 and 5 than on category 2 carcasses (P < 0·05). There were no differences in counts among the categories at any of the other sites. The TVCs averaged over the five carcass sites were higher for category 5 than for category 2 carcasses (P < 0·05). The TVCs at the brisket were significantly higher than all other sites (P < 0·01). In general, carcasses from category 2 animals slaughtered in a batch with dirtier animals (categories 3 and 5) did not have higher TVCs than carcasses of category 2 animals slaughtered at the beginning of the day in the absence of dirtier animals. The introduction of improved hygienic practices during the dehiding of category 4 animals resulted in reduced TVCs at the brisket (P < 0·001).

Thermal inactivation of Listeria monocytogenes and Yersinia enterocolitica in minced beef under laboratory conditions and in sous‐vide prepared minced and solid beef cooked in a commercial retort

D‐values were obtained for Listeria monocytogenes and Yersinia enterocolitica at 50, 55 and 60 °C in vacuum‐packed minced beef samples heated in a laboratory water‐bath. The experiment was repeated using vacutainers, which allowed heating of the beef to the desired temperature before inoculation. D‐values of between 0·15 and 36·1 min were obtained for L. monocytogenes. Pre‐heating the beef samples significantly affected (P < 0·05) the D60 value only. D‐values for Y. enterocolitica ranged from 0·55 to 21·2 min and all the D‐values were significantly different (P < 0·05) after pre‐heating. In general, the D‐values obtained for core inoculated solid beef samples were significantly higher (P < 0·05) than those generated in minced beef when heated in a Barriquand Steriflow commercial retort.

Physical characteristics of lamb primals packaged under vacuum or modified atmospheres.

Lamb primals (shoulders) were packaged under vacuum, 80% O(2)/20% CO(2), 50% CO(2)/50% N(2) or 100% CO(2) and stored at 5 or 0 °C. Pack contents were examined at 7 day intervals to determine the composition of the pack atmosphere, drip loss, colour (muscle and fat) and pH (surface and internal). The composition of the gas atmospheres changed very little during storage. The only significant differences between developed head space compositions above primals stored at the two different temperatures (5 and 0 °C) were noted in packs stored for 28 days under 80% O(2)/20% CO(2). Low levels of drip loss (<0.5%) were noted in all packs stored under the modified gas atmospheres. In contrast, significantly higher levels of drip loss (0.5-1.1%) were noted in vacuum packaged lamb stored at 5 and 0 °C. Acceptable muscle colour was observed 2 hr after opening of all packs. The only significant differences between atmospheres for lean muscle colour were noted after 28 days storage. Fat colour did not generally change during storage in any of the atmospheres, apart from a slight bleaching effect at 7 days. There were no significant differences between the surface or internal pH values noted after storage under any of the atmosphere/temperature combinations. In general, higher pH values were observed at the surface of the meat than in the interior. This pattern was noted before and after storage.

The Effect of Vacuum and Modified Atmosphere Packaging on the Shelf-life of Lamb Primals, Stored at Different Temperatures

Lamb primals (shoulders) were vacuum packaged or packaged in modified atmospheres containing 80% O(2)/20% CO(2), 50% CO(2)/50% N(2) or 100% CO(2), and stored at 5 or 0 °C. They were examined microbiologically at 7 day intervals for total counts obtained under (1) aerobic, (2) CO(2) enriched or (3) anaerobic conditions; B. thermosphacta; pseudomonad and Enterobacteriaceae counts. Off-odour assessments were also carried out at these times. In general, there were no significant differences between the total counts obtained from the different incubation conditions in any of the atmospheres. The only exception was noted in 80% O(2)/20% CO(2) at 5 °C. Significant differences between atmospheres for the total counts were observed at 0 °C only. In the case of B. thermosphacta, the pseudomonads and the Enterobacteriaceae, differences between atmospheres were noted at 5 and 0 °C. In general, vacuum packs and 80% O(2)/20% CO(2), and the two high CO(2) atmospheres fell into distinct groups. Storage temperature had a significant effect on all three counts. The relationship between bacterial counts and time was modelled using regression analysis. Data from total counts gave the equations of best fit. Significant differences between atmospheres in terms of off-odour production were observed at 5 °C only. The effect of temperature on off-odour production was significant in all atmospheres except 100% CO(2). A scheme was devised based on the growth of different groups of organisms which facilitated comparisons between studies on packaged meats. The results of the present work and that of others are discussed in relation to the different growth patterns which developed with packaging treatments and storage temperature.

Survival and growth of Aeromonas hydrophila on modified atmosphere packaged normal and high pH lamb

The growth of A. hydrophila on normal pH (5.5-5.8) and high pH ( > 6.0) lamb stored under modified atmospheres was examined. Lamb pieces and mince, inoculated with A. hydrophila were packaged in air, vacuum pack, 80% O2/20% CO2, 50% CO2/50% N2, or 100% CO2 and stored at 5 or 0 degrees C for up to 42 days. Samples were examined for the survival and/or growth of A. hydrophila by enrichment in alkaline peptone water and/or direct plating on starch ampicillin agar. The pH of each sample was estimated. On lamb pieces and mince of normal pH, A. hydrophila numbers decreased during storage at 5 and 0 degrees C under all the packaging conditions. The organism was not detected after 21 days storage. In contrast, A. hydrophila numbers were maintained or increased on high pH lamb under most storage regimes. Storage at 5 degrees C allowed significant increases in A. hydrophila numbers on high pH lamb under all the atmospheres, except 100% CO2. In lamb held at 5 degrees C under 100% CO2 for up to 42 days, A. hydrophila was recovered from most samples, although cell numbers decreased during storage. After storage at 0 degrees C, A. hydrophila was recovered from high pH packs stored under all atmospheres. Significant increases in cell numbers were only observed in minced lamb of high pH stored under air or vacuum. The pH values of lamb pieces and mince held at 5 or 0 degrees C under any of the packaging atmospheres did not change in a uniform manner during storage.

Investigations on the growth of Listeria monocytogenes on lamb packaged under modified atmospheres

The growth of Listeria monocytogenes in air and modified atmosphere packaged lamb pieces and mince, stored at 5 or 0°C was investigated at 42 days. The modified atmospheres included: (i) vacuum pack; (ii) 80% O 2 /20% CO 2 ; (iii) 50% CO 2 /50% N 2 ; and (iv) 100% CO 2 . On lamb pieces at 5°C, growth of L. monocytogenes occurred in air and all the modified atmospheres, except 100% CO 2 . L. monocytogenes growth on minced lamb at 5°C was reduced compared with lamb pieces. Growth did not occur in vacuum packaged mince or in an atmosphere containing 100% CO 2 . At 0°C, growth of the organism was completely inhibited on pieces and mince in all the modified atmospheres tested and in air. It was noted that pH did not increase or decrease in a regular manner throughout the storage period. Thus pH was eliminated as a factor controlling the growth of L. monocytogenes on modified atmosphere packaged lamb.

Growth of Yersinia enterocolitica O:3 on modified atmosphere packaged lamb

The growth of pathogenic Yersinia enterocolitica serotype 0:3 was investigated on lamb pieces and lamb mince, packaged in air and a range of modified atmospheres, at 5 or 0° C. The modified atmospheres investigated were: (i) vacuum pack; (ii) 80% O 2 /20% CO 2 ; (iii) 50% CO 2 /50% N 2 ; and (iv) 100% CO 2 . Y. enterocolitica 0:3 counts were examined at 28 days in each of the atmosphere/temperature combinations. Growth of Y. enterocolitica 0:3 occurred on lamb pieces stored at 5° C in air and all the modified atmospheres tested. On lamb pieces stored at 0° C, pathogen growth was noted in air, vacuum packs and 50% CO 2 /50% N 2 . In all atmospheres except air, growth of Y. enterocolitica 0:3 on minced lamb at 5 and 0° C was reduced compared to growth on lamb pieces. At 5° C on minced lamb, growth of the organism occurred in air, vacuum packs and 50% CO 2 /50% N 2 . However, at 0° C, growth was observed in air only. An atmosphere containing a high O 2 concentration was inhibitory to the growth of Y. enterocolitica 0:3. In addition, the combination of a high CO 2 (100%) atmosphere and low temperature (0° C) was inhibitory to growth.

Synergistic effect of heat and sodium lactate on the thermal resistance of Yersinia enterocolitica and Listeria monocytogenes in minced beef

The effect of sodium lactate (NaL) (0, 2·4 or 4·8%), in heating and recovery media, on Yersinia enterocolitica and Listeria monocytogenes numbers recovered from minced beef heated at 55 °C, was examined. Survivors were enumerated on selective media at pH 5·7/7·4 (Y. enterocolitica) or pH 5·7/7·2 (L. monocytogenes). Recovery of the organisms depended on the pH and NaL levels in the recovery medium. The heat resistance of Y. enterocolitica (P < 0·001) and L. monocytogenes (P < 0·01) decreased as the concentration of NaL in the minced beef increased from 0 to 2·4% or 4·8%. The thermal destruction of pathogens in foods processed using mild temperatures may be enhanced by the addition of 2·4% NaL.

The effects of treating bovine hide with steam at subatmospheric pressure on bacterial numbers and leather quality

Aims: To examine the effect of subatmospheric steam treatment on total viable counts (TVCs) on bovine hide and on the quality of derived leather.