The study was conducted to evaluate data on washing methods of fresh produce using chlorinated water to establish a standard method for the small or medium sized food industry for ensuring microbiological safety of fresh produce. To evaluate the appropriate concentration of chlorine water and treatment time, cut produce of lettuce, cabbage and cucumber was inoculated with Escherichia coli O157: H7 and submerged in chlorine water (100, 200 and 400 ppm of free available chlorine) for 1, 3, 5 and 10 min with gentle agitation at room temperature. Population of E. coli O157: H7 on lettuce, cabbage and cucumber decreased significantly (1-1.5 log10 cfu/g) within 1 min, regardless of available chlorine concentration. However, the population of E. coli O157: H7 did not decrease much (<0.5 log10 cfu/g) when the treatment time was extended to 10 min. Normal microflora also decreased significantly (0.5-1.9 log10 cfu/g) within 1 min, regardless of available chlorine concentration and fresh produce used. Extended treatment time was not found to be effective. Reduction of E. coli O157: H7 on the surface and the cut edge of lettuce, cabbage and cucumber was also evaluated. It was found that the number of E. coli O157: H7 attached at the cut edge was consistently higher than at the surface, regardless of vegetable used. Therefore, cutting process of fresh produce should be taken into account and a standard process must be developed to ensure microbiological safety of fresh cut vegetables and fruits.
To estimate the risk of microbial hazard in foods packaged in hermetically sealed containers, forty-nine such foods, purchased at markets in Fukuoka, were tested for pH and water activity. The pH and water activity of eighteen foods were in excess of 4.6 and 0.94, respectively. For five out of the eighteen foods the sterilizing or heating method was not indicated. Bacterial contamination of food packaged in hermetically sealed containers produced in Fukuoka was examined from the perspective of food hygiene. Five types of foods designated A to E, each type consisting of 15 samples (3lots×5samples), were produced in Fukuoka. In each sample, aerobic bacteria, aerobic spore-formers, growth of clostridia organisms, mouse lethality with botulinum toxin, water activity and pH were examined. Bacteria grown on standard agar medium were used for homologous search for 16S rDNA base sequence by blast research. The pH and water activity of four food types (A, B, C and D) were in excess of 4.6 and 0.94, respectively. In food A, all 15 samples demonstrated growth of bacteria on standard agar plates, and 8 samples on clostridia medium. Bacteria detected on standard agar medium were identified as aerobic spore-forming bacteria (Bacillus, Paenibacillus, Virgibacillus, Brevibacillus), Pseudomonas, Staphylococcus, Kocuria, Methylobacterium, and Microbacterium. In food C, each sample in three lots was contaminated with the organisms of Genus Bacillus (B. subtilis, B. firmus, or B. subtilis), respectively. In food E, the water activity of all 15 samples was below 0.85. All of them, however, were also contaminated with the bacteria of Genus Bacillus (B. subtilis, B. simplex, or Bacillus spp.). A label saying that “the contents were heated under pressure to sterilize the food after packaging in a hermetically sealed container” was put on two types of foods, but one of them was contaminated with bacteria. No sterilization methods were described in the labels of the other three food types, and two of them demonstrated bacterial contamination. In order to prevent food poisoning, the makers of these foods should describe the proper conditions for sterilization. In cases of insufficient sterilization, the foods must be kept under cool conditions and the period of safe consumption should be shown in a label under the Food Hygiene Act.
In this study, we examined the bacteria of fermented food produced in North Vietnam. The samples produced in Hanoi city were collected in March 2003. Salt concentrations, pH and microflora of 5 kinds of 37 samples were analyzed. The salt concentrations tend to be lower than that of Japanese lightly fermented vegetables and some samples did not ferment enough to increase the acidity. The aerobic plate count varied between 5 to 8 log CFU/g. Two to five log CFU/g of coliform bacteria were also detected; all identified strains were similar to as known natural flora in fresh vegetables however. Lactic acid bacteria (LAB), including Lactobacillus spp., Leuconostoc spp., Lactococcus spp. and Pediococcus spp., were isolated from samples. Bacteriocin producing strains effective for Listeria monocytogenes were screened from the isolated 200 LABs. Three Lactococcus lactis spp. lactis strains producing nisin were obtained. The DNA sequences of nisA gene (structural gene of nisin A) were completely the same as that of previously reported ones. A Bacilus subtilis strain that exhibited antimicrobial activity against Gram-positive bacteria was also isolated from a lightly fermented eggplant.
Bacillus cereus shows two distinct types of food poisoning, recognized as the diarrhoeal syndrome caused by enterotoxin and the emetic type caused by heat-stable emetic toxin. Since the latter accounts for the majority of B. cereus foodborne illness in our country, it is necessary to examine the emetic toxin producing activities as well as biochemical confirmations and serotyping tests of isolated organisms. Standard methods for detection of B. cereus emetic toxin are oral challenge in mammals or microscopic assessment of vacuole formation in HEp-2 cells. Because of their requiring specific facilities or skilled techniques both methods create difficulties in some laboratories. In place of them, evaluation of tetrazolium salt MTT assay has previously been described. We studied colorimetric cell metabolic assay in HEp-2 cells by tetrazolium salt WST-8 which produces a water-soluble formazan. This method resulted in equal or higher sensitivity than vacuolation assay. We suggest the application of WST-8 assay as a method for detection of B. cereus emetic toxin.