Shiga toxin production by sausage-borne Escherichia coli O157:H7 in response to a post-processing in vitro digestion challenge.

In order to study the effects of the fermentation-drying procedure and subsequent in vitro digestion on Shiga toxins (Stx) production by Escherichia coli O157:H7, dry sausages were inoculated during the formulation step with pure cultures of strains 5-1 and ATCC 43895. The inoculated sausages were submitted to a minimum (30 min, pH between 3.1 and 3.5) or a maximum (120 min at stepwise adjusting the pH downward) gastric challenge followed by a 240-min pancreatic challenge at pH 8.0 and 37 degrees C. Production of toxins by the overnight culture controls, assessed using the Vero cell assay, was dependent on the pathogen cell concentration. The effect of cell concentration was not relevant in sausage samples and data showed: (a) higher Stx production in contaminated sausage samples than in overnight cultures; (b) the lowest Stx levels were detected with undigested sausage samples; (c) the maximum gastric challenge enhanced Stx production, compared to minimally digested and undigested samples. Reverse transcriptase polymerase chain reaction (RT-PCR) performed on extracts from inoculated, digested (4.5-6 h process) and undigested sausages produced amplicons for both stx1 and stx2 mRNA, suggesting that post-stress expression of stx genes had occurred. Our data suggest that sub-lethal stresses imposed by the fermentation-drying procedure and subsequent digestion of ingested food (i.e. contaminated sausages) may affect the degree to which the surviving E. coli O157:H7 cells express their virulence in vivo.

Influence of extraction temperature on amounts and quality of rendered chicken fat recovered from ground or homogenized skin.

A study was undertaken to determine the influence of extraction process on recovery and quality of fat from chicken skin. In order to do so, 20-kg batches of skin were ground (9.5 mm plate) or homogenized in a colloidal mill (0.2-mm knife set), then heated to 50 or 80 C to rupture fat cells. The fat was recovered by centrifugation and was evaluated for composition, appearance, and stability. A maximum amount of fat (89.6% of the fat initially contained in skin) was recovered from homogenized skin heated to 80 C, whereas heating ground skin to 50 C yielded the lowest fat recovery (51.5% of skin fat content). In general, fat composition and appearance were little affected by extraction conditions, with the exception that the fat extracted from homogenized skin contained more (P < or = 0.001) unsaponifiable cell membrane constituents (0.17 to 0.20%), including antioxidant tocopherol fractions (10.3 microg/ml), than the fat extracted from ground skin (0.08% and 7.5 to 8.3 microg/ml, respectively). This difference likely contributed to the greater oxidative stability of the fat extracted from homogenized skin, which was observed in Schaal oven tests.

Effect of low extraction temperatures on microbiological quality of rendered chicken fat recovered from skin.

Ground or finely homogenized skin, inoculated with circa 7 log10 cfu/g of an Acinetobacter sp., Brochothrix thermosphacta, Candida tropicalis, Debaryomyces hansenii, Enterobacter agglomerans, Enterococcus faecalis, a Lactobacillus sp., or Pseudomonas fluorescens, or not inoculated, was heated to 50 or 80 C to release fat from adipocytes, and the released fat was separated by centrifugation. Extraction at 80 C resulted in nearly complete inactivation of indigenous and inoculated flora, resulting in microbiological counts generally below detection level in skin residue and rendered fat. In contrast, large numbers of organisms (3.69 to 7.28 log10 cfu/g) survived the 50 C extraction process. Even though the majority (91.5 to 99.9%) of these organisms remained in the residual skin at the time of fat separation, some organisms were also found occasionally in fat at concentrations of 2.85 to 3.74 log10 cfu/g. Therefore, an additional step such as flash pasteurization is recommended for safety, should extraction temperatures below 80 C be selected.

Antibacterial activity of selected fatty acids and essential oils against six meat spoilage organisms.

The antibacterial activity of selected fatty acids and essential oils was examined against two gram-negative (Pseudomonas fluorescens and Serratia liquefaciens) and four gram-positive (Brochothrix thermosphacta, Carnobacterium piscicola, Lactobacillus curvatus, and Lactobacillus sake) bacteria involved in meat spoilage. Various amounts of each preservative were added to brain heart infusion or MRS (deMan, Rogosa and Sharpe) agars, and the minimum inhibitory concentration was determined for each organism. Essential oils were analysed by gas-liquid chromatography to determine the concentration of selected components commonly found in spices. B. thermosphacta, P. fluorescens and S. liquefaciens were not affected by fatty acids, and generally overcame the inhibitory effect of essential oils after 24 h of exposure. Among the fatty acids, lauric and palmitoleic acids exhibited the greatest inhibitory effect with minimum inhibitory concentrations of 250 to 500 micrograms/ml, while myristic, palmitic, stearic and oleic acids were completely ineffective. For essential oils, clove, cinnamon, pimento, and rosemary were found to be the most active. The 1/100 dilution of those oils inhibited at least five of the six tested organisms. A relationship was found between the inhibitory effect of essential oils and the presence of eugenol and cinnamaldehyde.