Microfloral and chemical changes during the processing of heshiko produced by aging salted mackerel with rice bran through conventional practices in the Wakasa Bay area,Fukui, Japan

Microbiological profiles during the processing of heshiko, produced by aging salted mackerel with rice bran for over seven months, were investigated in connection with the production of organic acids and volatile basic nitrogen (VBN). Viable counts in commercial heshiko samples were in the range of 10⁴–10⁷ cfu/g in 2.5% NaCl-GYP agar medium, and over 80% of them were identified as Tetragenococcus halophilus via 16S rRNA gene sequence analysis. When experimentally processing heshiko using conventional practices, viable counts in fish flesh increased to 10⁶ cfu/g during the aging process with rice bran, which was accompanied by a marked increase in lactic acid but only slight production of VBN. Although the dominant species among the microflora in raw mackerel was identified as a Staphylococcus sp., the microflora diversified during the salting process. T. halophilus was prominent during the early stage of aging. The microflora gradually simplified with aging, and eventually consisted of only T. halophilus after five months of aging. These results suggest that a simple microflora consisting of T. halophilus forms when processing heshiko using conventional practices because of the stable environment present, which contains carbohydrates as fermentation materials in rice bran, along with high salinity (around 10%).