Abstract The cladodes, or ‘leaves’, of a cactus, Opuntia ficus‐indica , are a traditional human food in central Mexico, with an annual production of 900 000 tons. Approximately 5% of the crop is lost during manual harvesting. This research deals with the fermentation of O. ficus‐indica and subsequent methane production. A semicontinuous fermenter was operated at 35 °C with solid retention times (SRTs) of 15, 10, and 5 days. The fermentation products were identified through gas chromatography. The digestates from the different fermentations were used to determine the specific methane production (SMP). As the primary product of fermentation (accounting for 50% of total metabolites), ethanol was identified during the first 14 days, followed by acetic acid. Ethanolic fermentation was caused by naturally occurring yeast. Butyric acid was predominant for a 5 day SRT. During fermentation, carbohydrate removal ranged from 73 to 86%. Solid retention time is crucial in O. ficus‐indica pre‐fermentation since under‐ or over‐fermentation can reduce specific methane production compared with unfermented O. ficus‐indica . The highest SMP was observed in the digestates from the 10 day SRT, at 928 NmL g VS −1 , followed by unfermented O. ficus‐indica , at 866 NmL g VS −1 . Lower SMP was achieved with SRTs of 5 and 15 days, at 580 and 525 NmL g VS −1 , respectively. Endogenous methane production increased with initial substrate concentration.
Téllez‐Flórez et al. (Mon,) studied this question.