ANTIMICROBIAL ACTIVITY OF AVOCADO SEED EXTRACTS AGAINST BIOETHANOL YEAST STRAINS

Antimicrobial compounds extracted from natural sources, especially from the fruit-processing industries, offer a novel and safer option against microorganisms contaminating the fermentation industry, replacing the chemical treatments based on the utilization of sulfuric acid and antibiotics, which are sometimes not efficient and can lead to high production costs and directly effect the environment. The avocado fruit (Persea americana) is utilized in natura or to produce oil, leaving the epicarp and seed as byproducts. Bioactive compounds with antibacterial activity are frequently found in avocado residues but a little is known about the effects of seed extracts on yeasts from the bioethanol industry. This work aimed to evaluate two extracts of avocado seeds (variety Breda) obtained with the solvents ethanol and methanol (concentration of 80% in water), on the growth of two yeasts: an industrial strain of Saccharomyces cerevisiae (PE-2) and a strain of Dekkera bruxellensis, a contaminant of the bioethanol process. A portion of 20 g of avocado seeds were triturated, dried in full sun and placed in 200 mL of the solvent and left shaking for 48 hours at room temperature. The solution was filtered, the solvent was evaporated and concentrations ranging from 100 to 4000 mg/L were prepared in sterile water. The experiments were carried out in a 96-well cell culture plate filled with YPD broth, the yeast inoculum (106 cells/mL) and the extracts on triplicate. The plate was incubated at 30oC, 24 hours, with shaking, in a Plate reader Infinite 200 PRO equipment, with absorbance readings at 600 nm. The inhibitory effect was considerably high for D. bruxellensis for both extracts from the concentration of 100 mg/L when compared to the control (without extract). On S. cerevisiae, a decrease in the specific growth rate and an increase in lag phase when compared to the control (without extract) were observed but not as extensive as with D. bruxellensis, which did not vary the optical density within 9 hours of cultivation. These results showed that ethanol and methanol extracts may be promising antimicrobial for alcoholic fermentation contaminating microorganisms, that encourage further experiments to confirm the selectivity in inhibiting the yeasts and evaluate how this inhibition occurs.