Normal acidity (pH: 4.11, acetic acid: 0.64%), low acidity (pH: 4.75, acetic acid: 0 .10%) and very low acidity (pH: 5.82, acetic acid: 0.01%) mayonnaises were inoculated with Salmomella Enteritidis (SE) (1.5×107 cfu/g), and stored at 25°C, 2°C and -20°C for 14 days. Normal pH (4.95) and low pH (4.39) cucumber salads were inoculated with SE (1.5×106cfu/g) and stored at 25°C, 2°C and -20°C. The number of SE and the presence of SE in 25g samples were determined periodically. In the normal acidity mayonnaise, SE decreased rapidly at 25°C and became negative in a 25g sample after 3 days, and at 2°C and -20° decreased gradually but was positive in a 25g sample after 14 days. In the low acidity mayonnaise, SE decreased more slowly, compared with that in the normal acidity mayonnaise. SE was<10cfu/g at 25°C after 14 days, but positive in a 25g sample. It was 102cfu/g at 2°C after 14 days, and it was 103cfu/g at -20°C after 14 days . In the very low acidity mayonnaise, at 25°C SE increased rapidly (1010cfu/g after 3 days) and maintained high numbers for 14days. At 2° and -20°C the number of SE remained constant for 14 days. In the normal pH salad, at 25°C, SE increased rapidly (109cfu/g after 1 day) and maintained high numbers for 14 days. At 2°C and -20°C, SE numbers remained constant for 14 days. In the low pH salad, at 25°C, SE decreased gradually and became negative in a 25g sample after 14 days. At -20° C, the decrease of SE was slower (<10cfu/g, positive in 25g sample) and at 2°C the decrease of SE was the slowest (103cfu/g after 14 days). It was noted that, in storage at -20°C, SE in some low pH (high acidity) foods decreased and sometimes became negative in using a 25g enrichment method.
We developed a rapidSalmonelladetection kit using the colorimetric DNA/rRNA solid phase sandwich hybridization method in microtiter wells. This sandwich hybridization requires two adjacent, non-overlapping probes, one fixed to a well capture probe (Capture) and another to a labeled detection probe (Detector). The DNA sequence in the 23S rRNA gene of Salmonella Typhimurium was determined. By comparison of this sequence with those ofEscherichia coli, Citrobacter freundii and Enterobacter cloacae, two oligonucleotide probes (Capture and Detector) for the detection kit were selected. The OD value of the substrate reaction was highest when the non-target region between Capture and Detector wasO base. When HTT broth was used for the selective enrichment broth, sandwich hybridization reaction was inhibited by the deposited calcium on the microtiter well bottom . EDTA (final concentration 133mM) was the most effective chelate agent which inhibited deposition of calcium to the micro-titer plate bottom but not the hybridization reaction forSalmonellaTyphimurium. The kit effectively identified all 12 serovars ofSalmonellaspp . tested, and yielded no false-positive reactions in the examination of 15 pure cultures of non-salmonella.