Solid-state fermentation process optimization of palm kernel meal and effects on release of available nutrients

【Objective】 This study aimed to optimize the solid-state fermentation conditions of palm kernel meal to enhance the contents of available nutrients， providing technical foundation for fertilizer utilization of palm kernel meal. 【Method】 Contents of available nutrients， available nitrogen （N）， available phosphorus （P）， and available potassium （K） were indicators to evaluate the effects of single-strain， dual-strain （inoculation ratio of 1∶1）， and triple-strain （inoculation ratio of 1∶1∶1） mixed fermentation using Aspergillus niger， Bacillus subtilis， and Streptomyces microflavus on palm kernel meal （with an inoculation rate of 0.30% for all） for screening the optimal inoculant that could effectively enhance the contents of available nutrients in fermented products of palm kernel meal. The L₉（3³） orthogonal experimental design was employed to optimize fermentation conditions （water content of 40%-70%， brown sugar addition amount of 0-30%， and inoculation amount of 0.15%-0.45%） to determine and validate the optimal conditions. Simultaneously， fermentation time was extended to find the best fermentation cycle. 【Result】 In the inoculant screening trial for solid-state fermentation of palm kernel meal， the contents of available nutrients in single-strain fermentation of Aspergillus niger for 16 d were higher than those under other treatments， as the contents of available N， available P， and available K were 3.645， 9.040， and 8.812 g/kg respectively， with the organic matter degradation rate of 38.78%， thus， Aspergillus niger was selected for orthogonal optimization. Orthogonal experiment indicated that： the factors influencing comprehensive nutrient evaluation value followed the order of brown sugar addition amount > water content > inoculation amount； the inoculation amount had no significant influence on any nutrient （P>0.05）， while the water content significantly influenced available potassium （P<0.05， the same below）； brown sugar addition amount significantly influenced contents of available N， available P， and available K. Dynamic analysis of nutrients during orthogonal experimental fermentation demonstrated that the contents of available nutrients generally increased with prolonged fermentation time， with treatments without brown sugar addition exhibiting more pronounced enhancement in contents of nutrients， with higher nutrient release efficiency observed under higher water condition； for the available N under treatment 7 （water content of 70% without brown sugar，inoculation amount of 0.30%） showed rapid early accumulation and continuous growth； available P exhibited the most pronounced increase， while available K increased steadily. From orthogonal optimization， the optimal fermentation conditions were water content of 70%， brown sugar addition amount of 0， and inoculation amount of 0.45%. Under validation experiment under optimal fermentation conditions， the contents of available nitrogen， available phosphorus， and available potassium in final fermented products were 4.421， 9.866， and 8.173 g/kg， respectively， and the optimal fermentation cycle was 16-21 d； in final fermented products， organic matter content was 69.01%， total nutrient （N， P， K） content was 5.87%， fresh sample water content was 23.07%， pH was 6.98， and a seed germination index was 88.40%， all meeting the requirements of China’s industrial standard NY 525-2021 Organic Fertilizer. 【Conclusion】 Aspergillus niger can be used as the solid-state fermentation inoculant for palm kernel meal， with fermentation parameters set at water content of 70% and inoculation amount of 0.45%， which enhances the available nutrients in the fermented product， and no nutrients are degraded within 21 d.