Abstract
The effect of X-ray irradiation on Campylobacter jejuni, Campylobacter coli, Arcobacter butzleri and Salmonella Typhimurium was examined by measuring the recovery on different media. Irradiation doses (1.0-7.0 kGy) caused a significant reduction in microbial counts irrespective of the media used for recovery. Recovery was highest on Blood Mueller Hinton Agar (D-value being 3.30, 3.32, 3.56 and 6.86 kGy for C. jejuni, C. coli, A. butzleri and S. Typhimurium respectively) and lowest on selective media (D-value being 3.20, 3.08, 3.36 and 3.20 kGy for C. jejuni, C. coli, A. butzleri, and S. Typhimurium respectively). To investigate possible mechanisms leading to the inactivation of C. jejuni, a range of mutants defective in the oxidative stress defence were assessed for tolerance to irradiation. Irrespective of the media used for recovery a sodB mutant was more sensitive to irradiation, suggesting that oxidative strain contributes to cell killing (D-value being 2.98 kGy for sodB mutant). The survival of Campylobacter and Salmonella in the presence of nitrite, nitrate, other salts and solutes during irradiation was also carried out to determine whether ionic solutes enhanced or limited the lethal affects of irradiation. Nitrite, nitrate, NaCl and KCl provided protection against irradiation, and nitrite was by far the most protective agent. In the presence of 20 mM nitrite the D-value of C. jejuni was 24.45 kGy compared to D-values of 10.69 in the presence of 100 mM nitrate and 3. 63 in the absence of any salt. When ionic salts other than nitrites and nitrates were used a protective effect was seen but this was less than that seen for nitrite/nitrate. For example the D-values in the presence of 100 mM NaCl and 50mM KCl were 9.11 and 7.27 respectively. Glucose had no protective effect as had been seen for the ionic salts. To determine whether or not the protective effect of ionic salts was due to their effect on 'OH formation, 'OH production was measured at high and low ionic strength using terephthalic acid (TA). However, 'OH production as measured by this method was unaffected by salt concentration. Direct solvent extraction method (DSE) was used to identify 2- dodecylcyclobutanone (DCB) in irradiated chicken. Isolation of the DCB was successfully achieved from irradiated chicken samples at (1, 3, 5 and 7kGy). The concentration of the extracted DCB in the irradiated chicken samples varied from 0.57 to 2.9 mug/g lipid following exposure to 1kGy and 7kGy, respectively. DCB levels in irradiated chicken samples stored under chilled conditions for 4 weeks and frozen conditions for 12 months were reduced by 5.3% and 73%, respectively. The use of microbiological and chemical methods to identify shelf-life of irradiated frozen chicken samples stored for up to twelve months was carried out by determining the Total Plate Count (TPC) and DCB. The total aerobic bacteria counts increased by 0.82 and 0.18 log cycle for unirradiated, irradiated frozen chicken respectively stored up to 12 months. This was unexpected and may reflect a slow recovery from injury at -18°C.