SOLID-STATE FERMENTATION PROMOTES SUBSTRATE-DEPENDENT PHENOLIC BIOTRANSFORMATION IN AMAZONIAN AGRO-INDUSTRIAL BY-PRODUCTS

Amazonian agro-industrial by-products are promising sources of bioactive compounds; however, their chemical diversity may determine distinct responses to microbial bioprocessing. This study evaluated the effects of solid-state fermentation with Aspergillus niger and Rhizopus oryzae on the phenolic content, antioxidant capacity, and primary metabolism of Brazil nut, cupuaçu, and pracaxi processing cakes. A 3 × 2 × 8 factorial design was used, comprising three substrates, two fungal species, and eight fermentation times (0, 2, 4, 6, 8, 10, 12, and 14 days). Three independent fermentations were conducted per experimental condition. Total phenolic content, DPPH radical-scavenging capacity, reducing sugars, and total acidity were monitored throughout fermentation, whereas fungal growth was determined on days 0, 6, and 14. The biomass × fungus × time interaction significantly affected all responses (p < 0.001), demonstrating substrate-specific fungal metabolism. In Brazil nut cake fermented with A. niger, total phenolic content increased from 272.86 mg GAE/100 g dry matter at day 0 to a maximum of 797.54 mg GAE/100 g at day 6, corresponding to a 192% increase. At day 14, this treatment retained 680.21 mg GAE/100 g, whereas antioxidant capacity increased by 27.7% and reducing sugars decreased by 48.1%. Brazil nut cake fermented with R. oryzae showed a 160.7% increase in total phenolics between days 0 and 14. Cupuaçu cake fermented with A. niger reached the highest phenolic concentration among the fermented samples, increasing from 576.72 to 1,012.82 mg GAE/100 g at day 6. In contrast, pracaxi cake exhibited a high initial phenolic content but decreased by 31.5% and 48.6% after fermentation with A. niger and R. oryzae, respectively. The latter treatment also reduced DPPH antioxidant capacity by 26.3%, despite a 70.8% decrease in reducing sugars. These contrasting profiles indicate that fungal fermentation does not universally increase extractable phenolics but instead promotes substrate- and microorganism-dependent phenolic remodeling. Solid-state fermentation, particularly with A. niger, is therefore a promising strategy for enhancing the bioactive potential of Brazil nut and cupuaçu by-products, supporting their sustainable valorization as potential functional food ingredients.

Funding: This study was supported by FAPESPA, CNPq, Iniciativa Amazônia +10 (grant number 091/2023), Centro de Valorização de Compostos Bioativos da Amazônia, and Universidade Federal Rural da Amazônia.