Quercetin modulates stress tolerance in saccharomyces cerevisiae under oxidative and weak acid stress : A dissertation submitted in partial fulfilment of the requirements for the Degree of Master of Science in Food innovation at Lincoln University

Abstract

Quercetin is a naturally occurring dietary flavonoid abundant in fruits and vegetables and is widely recognised for antioxidant and cytoprotective activity. Extensively studied in mammalian systems, its dose dependent effects in microbial cells under fermentation-relevant stress remain less defined. The aim of this investigated how quercetin modulates growth, viability and survival rate in Saccharomyces cerevisiae BY4741 when exposed to oxidative and organic acid stressors. Using quercetin as an epigenetic associated modulator is non-GMO approach and relevant in New Zealand, where the use and release of GMO requires regulatory approval. Genetically modified foods are subjected to critical assessment and approval which is a limitation in the routine industry uptake of GMO strains. Exponentially growing cultures were challenged with hydrogen peroxide (H₂O₂) to induce oxidative stress or acetic acid to simulate weak-acid stress encountered during fermentation. Responses were quantified using OD₆₀₀ growth kinetics, CFU-based viability/spot assays, and fluorescence microscopy with CellROX Green to visualise intracellular reactive oxygen species (ROS) at the single-cell level. Quercetin produced a non-linear, dose dependent response in yeast physiology. Under H₂O₂ challenge, quercetin pre-treatment partially improved CFU survival and reduced CellROX Green fluorescence relative to stressed controls, indicating reduced ROS-associated burden and enhanced oxidative-stress tolerance. In severe acetic acid stress caused near-complete loss of colony formation across treatments, and CellROX fluorescence progressively faded, consistent with membrane damage and metabolic collapse limiting probe oxidation rather than true ROS quenching. Overall, quercetin can enhance tolerance to moderate oxidative stress but is insufficient under extreme weak-acid stress. Photodegradability is the major limitation of the bioavailable quercetin, it is recommended use emulsifiers-based delivery strategies to strengthen dose–response interpretation for functional fermentation applications.