Yeasts and lactic acid bacteria consortia associated with orange peel waste

Introduction Orange peel waste (OPW) is a residue of orange industrial transformation which accounts for 50-60% of the mass of processed fruit and that need to be adequately treated. One promising way is the anaerobic digestion for methane production. The ensilage microflora is an important factor for a successful conservation of the vegetal raw material. While lactic acid bacteria (LAB) are desirable microorganisms, yeasts are considered undesirable due to their fermentative metabolism that decreases the sugars availability for lactic fermentation and subsequent methane production. To date, little it is reported on microbial composition of ensiled OPW. This work aims at investigating the OPW yeast and LAB composition and their changes throughout the ensilage.Materials and MethodsThree type of reactors were prepared: a) fresh OPW, b) fresh OPW added with 20% of water, and c) fresh OPW with a bottom filter to separate the percolating liquid. Ensilage was carried out anaerobically and at room temperature for 28 days. Microbial counts and physico-chemical parameters were evaluated at 0, 7, 14, 21 and 28 days. Throughout the ensilage, yeasts and LAB were isolated respectively on YPD and MRS agar supplemented with antibiotics. The yeasts were screened by PCR-RFLP of the 5.8S ITS rRNA region while the LAB by PCR-ARDRA of the 16S rRNA gene. Representative isolates for each RFLP and ARDRA profiles were chosen for 26S and 16S sequence analysis.ResultsStarting from a pH value of 3.7, a decrease was observed in all the trials throughout the ensilage. The CO2 was produced mainly during the first stage of the ensilage. At 7 days, the biogas produced was 90% and the 96% of the total for trials a) and c), respectively. The a) biogas production was lower than b) and c) ones. At 0 and 7 days, all the trials showed higher yeasts loads than LAB; then, LAB became dominant in all the trials. The a) and b) trials had similar LAB trends with values up to two log units lower than the value of the c) trial at 28 days. Similar yeasts trend was observed for b) and c) trials reaching, at 28 days, lower values than a) trial. At the end of the ensilage, c) trial showed the lowest and the highest yeast and LAB counts, respectively. The yeasts belonged to Saccharomyces, Pichia, Hanseniaspora, Kazachstania, Torulaspora, and Kregervanrija genera while the LAB to Lactobacillus, Leuconostoc genera and to Lactobacillus plantarum group.ConclusionsThis study broaden the little knowledge reported until now on the OPW microflora and revealed yeasts and LAB biodiversity. Our results may be useful to optimise the treatment in order to improve further biogas production. Future research will be carried out to study the isolated strains towards characters useful in the anaerobic digestion for biogas production.