Microbial and chemical properties of aji-no-susu, a traditional fermented fish with rice product in the Noto Peninsula, Japan

Aji-no-susu is a Japanese fermented fish product prepared from salted horse mackerel Trachurus japonicus, and cooked rice. We studied the organic acid and free amino acid contents and microflora in 12 aji-no-susu products to clarify their features as a lactic-acid-fermented food. Salinity of the samples was approximately 7.0% (rice portion) and 6.0% (fish portion) (w/w). Water activity was approximately 0.9, and pH was approximately 4.4 and lower. In the rice portions, lactic acid content was very high (57 mg/g sample). The predominant amino acids were alanine (2.3 mg/g rice portion) and lysine (2.1 mg/g). In the case of long-fermented (4 and 12 months) aji-no-susu, a high content of gamma-aminobutyric acid (GABA, 1.5 and 1.4 mg/g) was detected. Total viable counts in rice and fish portions were 7.7 and 7.4 log colony-forming units (cfu)/g, respectively. The number of lactobacilli in the rice and fish portions was 7.3 and 7.1 log cfu/g, respectively. Yeasts were detected in eight samples. Furthermore, acid-tolerant lactic acid bacteria (LAB) (Lactobacillus plantarum), GABA-producing LAB (Lactobacillus sp.), and halophilic or halo-tolerant yeast (Debaryomyces hansenii) were isolated and identified. Results in this study indicate that aji-no-susu is a typical traditional lactic-acid-fermented fish product.