EFFECT OF BIOACTIVE ACETOGENINS ON YEAST GROWTH AND ETHANOL PRODUCTION DURING FERMENTATION OF SEEDS FROM ANNONACEOUS FRUITS

One of the main aspects to be considered when using agro-industrial wastes as a part of culture broth for biotechnological purposes, is the eventual presence of natural bioactive compounds that could interfere on cell growth or product formation. Previous works have reported the potential of fruit residues like seeds and peels as alternative substrates for ethanol production, but the presence and role of toxic or bioactive compounds in such materials is barely studied. Acetogenins are commonly present in the seeds of annonaceous fruits and have been reported to be potent inhibitors of the electron transport chain and cell respiration, being highly toxic to arthropods and vertebrates. These compounds are thermo-resistant and in low concentrations, they show antitumor effect. The aim of this work was to study the influence of acetogenins on cell growth and ethanol production when seeds of annonaceous fruits are used as feedstock in alcoholic fermentation. Seeds of Annona muricata (graviola), Annona squamosa (ata), Annona crassiflora (araticum) and the hybrid atemoia were pre-treated with amyloglucosidase for starch hydrolysis and then added to yeast current broth as substitutes for carbohydrate source. Fermentations were run in triplicates for each seed species, in 100 mL Erlenmeyer flasks during 48 hours, using the yeast Saccharomyces cerevisiae, strain S-05. Cell growth was evaluated by counting in Neubauer chamber, while ethanol production was calculated out of the amount of carbon dioxide released (weight loss). After fermentation process an increase of cell concentration was observed in all experiments, from 1.23xe7 cells/mL after inoculation, to 2.6xe7 cells/mL (graviola), 2.13xe7 cells/mL (ata), 3.07xe7 cells/mL (araticum) and 1.73xe7 cells/mL (atemoia), while control cultures containing glucose as carbon source showed a maximal concentration of 4.12xe7 cells/mL. Ethanol yield ranged from (1.35) to (3.66) g/100 mL in all seed fermentations. Although some growth inhibition was expected, the cell concentration increase was similar in all seed fermentations and in average, it was 18% lower when compared to glucose control. Some yeast species and plants have the capability of finding alternate routes or shortcuts in their electron transport chains when exposed to pharmaceuticals with inhibitory effect on some electron transporters and this fact could explain the expressive cell growth observed in all cultures containing annonaceous seeds. Since ethanol production is an anaerobic process, not being dependant on cell respiration, it was not apparently affected by the presence of acetogenins. These results evidence the potential of such wastes for ethanolic fermentation, with no need to previously remove or inactivate toxic acetogenins and moreover, a possibility to reduce the high volumes of residues generated from fruit consumption, both in natura or from manufacture of industrial derivatives.