Tsuyu is a cooking sauce made from soy sauce and seasonings traditionally used in Japanese foods. The amount of ingredients of the product, such as quantity of soy sauce and the types of seasonings depend on the product's intended use. Therefore, different formulations will have various concentration of hydrogen ion (pH) and water activity (A_w), which are critical factors influencing the growth of the microorganisms. The commercial product generally has a pH of 4.6–5.4 and an A_w of 0.92–0.97. Tsuyu is usually processed by the Hot Fill Hold (HFH) technique, where the product is heated to a temperature of 85–95℃, and immediately packed in a container, and held for 2–10 min after closing. Since some heat-resistant spore-forming bacteria cannot be destroyed by the HFH technique, these microorganisms may survive and grow in the product if the ingredients and/or containers are contaminated with the organisms. Clostridium botulinum is a public health significant and heat-resistant bacterium that may be able to grow and produce neurotoxins in foods having higher pH and A_w values like Tsuyu, although microbial food poisoning due to this product has never occurred in Japan. The product must be prepared to have any of the following combinations of pH and concentration of NaCl to prevent spore outgrowth: pH<4.8 at 1% NaCl (A_w 0.97), pH<5.2 at 2% NaCl (A_w 0.96), pH<5.3 at 3–4% NaCl (A_w 0.95), and pH<6.2 at 6% NaCl (A_w 0.95). At NaCl≧7% (A_w≦0.94), pH conditioning was not necessary since the organism was not able to grow and produce toxin in the product at such conditions.

Spore-forming bacteria Paenibacillus spp. were isolated from spoiled corn paste stored below 10℃. Spoilage associated with pH decrease was reproduced by inoculating isolated strains to normal corn paste. To obtain insights to prevent recurrence of Paenibacillus-mediated spoilage, we further characterized the growth temperature, drug sensitivity, and heat resistance of the isolated strains. The isolated Paenibacillus strains could grow at as low as 2.5℃, suggesting that contamination by these strains can cause spoilage during cold chain transport and storage. We also report that these strains can be controlled by heat treatment such as 140℃ for 8 sec, and by treatment with cleaning reagent containing sodium dichloroisocyanurate.

Food borne outbreaks by norovirus caused more than 10,000 acute gastroenteritis patients every year in Japan. Food handler with norovirus infection often contaminates food with the virus. Monitoring norovirus infection in food handlers is thus critical. A small number of norovirus particles are sufficient to establish infection in humans. If a food handler is infected with norovirus, co-workers are at high risk of norovirus infection. Norovirus establishes asymptomatic infection. It is recommended to implement a highly sensitive norovirus testing to every food handlers regardless of gastrointestinal symptoms to reduce the risk of norovirus contamination in food. To do this, the cost and the rapidness of the laboratory test are concerned. In this study, we evaluated a novel nucleic acid-based diagnostic kit for norovirus infection free from standard nucleic acid purification. This was shown to have sensitivity and specificity comparable to the conventional diagnostic procedures employing two-step real-time PCR, suggesting that the new kit offers a rapid and inexpensive laboratory test of norovirus infection. This kit should be able to contribute to the periodic norovirus monitoring of food handlers, leading to the prevention of food poisoning by norovirus.