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The direct method based on inlet- and outlet-gas concentrations in submerged fermentation (SmF) was adopted for estimating overall oxygen-transfer coefficients (K L a) in a packed-bed solid-state fermenter. The variation in K L a during fermentation at different air-flow rates was studied. The values of K L a were higher than those found in SmF. The moisture content was found to affect oxygen-transfer rates and coefficients. The value of K L a increased with an increase in air-flow rate in the range 0-15 litre/min but decreased at higher air-flow rates (15-25 litre/min). This could be attributed to the decrease in the moisture content of the substrate at high flow rates.