Extraruminal Fermentation of Citrus, Grape and Apple Pomaces: Assessing the Potential to Serve as Feedstock for Production of Volatile Fatty acids

Characterisation of pomace from citrus fruits, grapes and apples has shown to be rich in compounds that contain untapped energetic value with potential to serve as feedstock for biochemical and biofuel production. This study aimed to investigate the potential of recovering the energy in citrus (CtP), grape (GP) and apple (AP) pomace by extraruminal fermentations to produce VFA with utility to serve as precursors for hydrocarbon fuels. Chemical composition, neutral detergent fiber digestibility (seven incubation time points) and volatile fatty acid yield (after 96 h extraruminal fermentation) were analysed and compared among the three-pomace substrates. There were no significant differences (P > 0.05) in the apparent digestibility of CtP (77.3%) and AP (75.2%) while that of GP (55.2%) differed significantly from both (P < 0.05). VFA yields corresponded to the digestibility in that GP (97.1 mM concentration, with a fractional yield 0.38 out of a possible 0.75) yielded the lowest amount of VFA compared to AP (126.4 mM concentration, with a fractional yield of 0.48 out of a possible 0.75) and CtP (136.4 mM concentration, with a fractional yield of 0.52 out of a possible 0.75). Analysis of the alkyl chain lengths of the VFA showed that both CtP (12.16 mmol alkyl g−1) and AP (11.31 mmol alkyl g−1) can serve as substrates for VFA electrochemical conversion to hydrocarbons. The results reported here have industrial implications as the produced VFA and the amount of alkyl chain lengths are the direct determinants of the mixture of hydrocarbons that can be produced.