Statistical Optimization of Solid‐State Fermentation by <i>Aspergillus oryzae</i> for Valorization of Olive Cake and Its Application as a Poultry Feed

The agro‐industrial wastes gained much attention in recent years as an alternative source of animal feed because of the accelerated increase in the prices of feed and the need for the safe disposal of these wastes. Olive cake (OC), as one of the supplementary olive oil extraction by‐products, is distinguished by its excellent nutritional value. However, the crude untreated OC is fibrous with low protein content which makes it unsuitable for animal feed. In this investigation, four indigenous microorganisms (two bacteria, one yeast, and one filamentous fungus) were isolated from OC. These isolates were evaluated for their ability to valorize OC as poultry feed by reducing fiber content and increasing protein content under solid‐state fermentation. The filamentous fungal isolate NFAO4 demonstrated the best performance for these objectives. NFAO 4 fungal isolate was selected as the best isolate to valorize OC. It was identified as Aspergillus oryzae based on ITS sequencing with 97.35% similarity. The conditions of solid‐state fermentation were optimized using one‐factor‐at‐a‐time approach to be beef extract, pH 6, incubation period of 14 days at 28°C, and 3% inoculum size. The nutritional value of OC was enhanced by decreasing the crude fiber percentage to 29.01 and the crude protein was increased by 1.67‐fold. Statistical optimization determined the optimum factors to be 19.48, 7, 30°C, 2%, and 30% for fermentation time, pH, fermentation temperature, inoculum size, and moisture content, respectively, which decreased the fiber content to 28.1%. The treated OC was utilized alone without any additives as a rooster’s feedstuff for 48 h. Its gross energy and the apparent metabolizable energy were remarkably increased compared with untreated OC. These results demonstrated the ability of Aspergillus oryzae to increase the nutritional value and the digestibility of OC to be used as poultry feed without any additives.