Isolation and Evaluation of Stress-Tolerant Indigenous Yeast Isolates from Naturally Fermenting Nipa (Nypa fruticans Wurmb.) Sap for Potential Bioethanol Production

Abstract

This study aimed to isolate and characterize stress-tolerant yeasts from nipa (Nypa fruticans Wurmb.) sap for bioethanol production, addressing the Philippine bioethanol industry's need for alternative feedstocks and stress-tolerant yeast strains. Two dominant putative yeast strains (NY1 and NY2) were isolated from 72-hour naturally fermenting sap collected in Calizo, Balete, Aklan. These indigenous isolates were evaluated for thermotolerance, osmotolerance, ethanol tolerance, and acid tolerance, and compared against a commercial Brewer’s yeast (Saccharomyces cerevisiae) control. The indigenous isolates demonstrated greater early-stage stress resistance, exhibiting higher osmotolerance (up to 20% sucrose) and acid tolerance (thriving at pH 2.0–4.0) than the commercial strain. Although the commercial control showed greater overall resilience to late-stage ethanol concentrations and heat (45°C), isolate NY1 exhibited adaptive thermotolerance at 40°C. Laboratory-scale batch fermentation assays (125 mL working volume) indicated the ethanologenic potential of both isolates. In a Simulated Nipa Sap (SNS) medium, NY1 and NY2 achieved mean ethanol yields of 3.66% v/v (4.57 mL) and 3.28% v/v (4.10 mL), respectively, which were higher than their observed yields in a standard YPD medium (3.36% and 3.04% v/v). Ethanol production was verified via Fourier-Transform Infrared (FTIR) spectroscopy. These preliminary findings suggest the potential of indigenous nipa sap yeasts for bioethanol applications. Future research should prioritize the molecular identification of these isolates and the scaling up of fermentation processes to evaluate their suitability for industrial application.