2017 Volume 34 Issue 2 Pages 118-125
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 (Aw), which are critical factors influencing the growth of the microorganisms. The commercial product generally has a pH of 4.6–5.4 and an Aw 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 Aw 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 (Aw 0.97), pH<5.2 at 2% NaCl (Aw 0.96), pH<5.3 at 3–4% NaCl (Aw 0.95), and pH<6.2 at 6% NaCl (Aw 0.95). At NaCl≧7% (Aw≦0.94), pH conditioning was not necessary since the organism was not able to grow and produce toxin in the product at such conditions.
