Solid State Fermentation as a Tool to Stabilize and Improve Nutritive Value of Fruit and Vegetable Discards: Effect on Nutritional Composition, In Vitro Ruminal Fermentation and Organic Matter Digestibility

This research aimed to evaluate in vitro organic matter digestibility, fermentation characteristics and methane production of fruit and vegetable discards processed by solid state fermentation (SSF) by <i>Rhizopus</i> sp. Mixtures were composed of approximately 28% citric fruits, 35% other fruits and 37% vegetables. Fruit and vegetables were processed and fermented to obtain a stabilized product. Nutritional characterization and in vitro ruminal fermentation tests were performed to determine the effect of fungal bioconversion on digestibility, end products and gas production kinetics. Results indicate that SSF reduced organic matter and reducing sugars, while it increased crude protein and neutral detergent fiber, acid detergent fiber and neutral detergent insoluble protein. The in vitro gas production showed that SSF led to a reduction of the organic matter digestibility (<i>p</i> < 0.001), short chain fatty acids (SCFA; <i>p</i> = 0.003) and CH₄ (<i>p</i> = 0.002). SSF reduced the gas production from the insoluble fraction (<i>p</i> = 0.001), without modifying the production rate (<i>p</i> = 0.676) or the lag time (<i>p</i> = 0.574). Regarding SCFA profile, SSF increased acetic (<i>p</i> = 0.020) and decreased propionic (<i>p</i> = 0.004) and butyric (<i>p</i> = 0.006) acids proportions, increasing acetic to propionic (<i>p</i> = 0.008) and acetic plus butyric to propionic (<i>p</i> = 0.011) ratios. SSF succeeded in obtaining a stabilized material enriched in protein, but at the expense of a reduction of protein availability and organic matter digestibility. These changes should be considered before including them in a ruminant’s rations.