Solid-state fermentation is experiencing renewed interest for industrial enzyme production. Previous studies on the effects of fungal inoculum size on product yield have focused on spore inoculum. However, some organisms require vegetative inocula. This study investigated the effects of initial inoculum colony age, vegetative inoculum size, and duration of fermentation on the production of fungal biomass and phytase in solid-state fermentation using Aspergillus niger grown on wheat bran and soy meal. Initial inocula from 7- and 14-day-old potato dextrose agar (PDA) plates were used to study the effect of inoculum colony age in liquid culture and its further influence on fermentation. The liquid inoculum size of (60-480 mg of biomass/5 g substrate) and the duration of fermentation were studied for 10 days with periodic sampling every 48 hours. The study was conducted as a replicated full factorial experiment. Statistical analysis showed that phytase production and growth were not affected by inoculum size (over 60 mg/5 g substrate) after 48 hours of fermentation. The duration of frrmentation was highly significant with a maximal phytase (1001 ± 94 U/g substrate) and biomass (0.032 ± 0.007 g glucosamine/g substrate) formation at 192 h of SSF. Phytase yield and biomass formation were strongly correlated for the 7- and 14-day plate cultures (the estimated correlation coefficients being 0.87 and 0.73), indicating that phytase production is strongly growth-associated.
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