To reduce the solid waste in soy sauce refuse (SSR) that is a by-product of soy sauce fermentation, both biological and chemical treatment were examined. When anaerobic digestion of SSR was carried out at the amounts of 25 g/l under 30% (v/v) inocula of thermophilic methanogenic sludge at 50°C, 120 mM of CH4 was produced and 50% (w/v) of mixed-liquor suspended solids in SSR was decreased after 35 days. Although in this culture 30 days' cultivation was required to get 100 mM of CH4, when the same amounts of SSR (25 g/l) were repeatedly added to the culture, the time requirement to get 100 mM of CH4 could be reduced to 20 days and 15 days at second and third batch cultures, respectively. When SSR was treated with 5% (w/v) NaOH for 24 h, the supernatant contained 98% (w/v) of protein that were alkaline-solubilized and insoluble in the intact refuse, and the residual pellet contained insoluble fiber.

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