Biogas production from fruit wastes: fruit flavors as booster or inhibitors?

Abstract

In order to improve biogas production from fruit waste, investigation of effect of fruit flavors on biogas production was carried out. The work was performed in batch anaerobic digestion using synthetic medium and thermophilic bacteria for 30 days. Twelve different flavors belonging to aldehydes (hexanal, nonanal, and E-2- hexenal), terpenes (carene, α-pinene, and myrcene), alcohol (octanol), lactone (furanone), and esters (methyl butyrate, ethyl butyrate, ethyl hexanoate, and hexyl acetate) were tested. Anaerobic digestion without addition of flavors was used as reference. For aldehyde groups, hexanal and nonanal showed inhibition effect at concentration of 0.005% whereas E-2-hexenal required higher concentration i.e. 0.05% to show its effect. Addition of 0.5% of hexanal, nonanal, and E-2-hexenal resulted in lower biogas production than that of the corresponding reference by factor of 276, 317 and 434%, respectively. Similar to aldehydes, all terpenes group showed inhibition at concentration of 0.005%. Addition of 0.5% of carene, α-pinene, and myrcene decreased biogas production by factor of 300, 255, and 330%, respectively compared to the reference. Moreover, in the presence of 0.5% of octanol and furanone reduced biogas production by factor of 433 and 183%, respectively compared to the reference. On the contrary, higher biogas production was obtained by addition of all esters tested. Methyl butyrate, ethyl butyrate, ethyl hexanoate, and hexyl acetate at concentration of 0.5% increased biogas production by factor of 291, 717, 542, and 640%, respectively compared to the reference. In conclusion, most of flavor compounds are inhibitor for biogas production for the exception of ester groups.