Abstract

LIU, XIN. Extraction and Anti-Bacterial Effects of Edible Brown Algae Extracts. (Under the direction of Dr. Wenqiao Yuan). The desire to obtain natural phytochemicals with promising bioactivity and few or no side effects has been the motivation for investigating extraction processes for plants. These bioactivities make phytochemicals as potential food-preservation agents due to their effects of inhibiting lipid oxidation and spoilage microorganism growth, thus preventing food deterioration. In this study, extraction optimization of bioactive compounds from edible brown algae and their food preservation effects were investigated via the five objectives below. The first objective was to study the effects of extraction temperature (30 to 70°C), liquid-to-solid ratio (10 to 90 mL/g), and ethanol concentration (20 to 100%) of an ethanol-water binary solvent system on extracts from edible brown algae Ascophyllum nodosum. Most extracts showed higher total phenol content (TPC), total carbohydrate content (TCC) and antioxidant activity before rotary evaporation. The strongest antioxidant activity was observed at low temperature (30 and 40°C), low liquid-to-solid ratio (30 mL/g), and high ethanol concentration (80 and 100%), suggesting that the antioxidants of A. nodosum were thermal sensitive and had low polarity. The second objective was to optimize the extraction process using Box-Benhken Design (BBD) and Response Surface Methodology (RSM). The conditions for maximum antioxidant activity of the crude extract were found at 70 mL/g, 80% ethanol, and 20°C, while the conditions for the highest crude extract yield were at 60°C, 50.02 mL/g, and 45.65% ethanol. The modelpredicted antioxidant activity and yield were 72.75 mL/mg and 55.68 mg/g, respectively, which were in close agreement with the experimental results of 74.05±0.51 mL/mg and 56.41±2.59 mg/g, respectively, suggesting that the models could accurately predict and improve the extraction of antioxidants from A. nodosum. The third objective was to evaluate stability of A. nodosum extract. The TPC and antioxidant activity decreased by approximately 5% and 10%, respectively, after 6 hours of thermal treatment (40-90°C). Extract exposed to air and 25°C for 108 h showed lower TPC than those stored in closed containers and 0°C. Antioxidant activity of extracts remained constant at both 0 and 25°C, regardless of exposure to air. The results suggested that the extract of A. nodosum was stable under various conditions. The fourth objective was to evaluate the in vitro anti-bacterial effects of the crude extract against Pseudomonas fluorescens and Shewanella putrefaciens, specific spoilage organisms (SSOs) of tilapia. The Lambert-Pearson model was applied to determine minimum inhibitory concentration (MIC) and non-inhibitory concentration (NIC). The best-fit MIC and NIC for P. fluorescens was 1.145 and 0.036 mg/mL, and 0.947 and 0.106 mg/mL for S. putrefaciens, respectively. Algae extract (10μg/disc) showed larger inhibition zones than penicillin (42 μg/disc) against both bacteria strains. All results suggested that crude extract of A. nodousm has a promising anti-bacteria effect and could be applied as potential food preservation agent. The fifth objective was to investigate possible anti-bacterial mechanisms of A. nodosum extract against P. fluorescens and S. putrefaciens. Extracellular enzyme activity was inhibited and exopolysaccharide content was reduced by 44.82 and 64.67% in P. fluorescens and S. putrefaciens, respectively, treated with algae extract. Number of live cells in biofilms attached to petri dishes decreased after co-culturing with the extract. Significant increase of 280-nm observing material, protein content and conductance in the cell-free media was observed during co-culturing with the algae extract, suggesting enhanced membrane permeability. This project was the first to optimize the extraction process to obtain extracts from A. nodosum, an underexplored seaweed, and evaluated stability of the extract. It was also the first to evaluate the in-vitro antibacterial effects against the food spoilage bacteria P. fluorescens and S. putrefaciens, and to illustrate the anti-bacterial mechanisms of the extract. Findings of this project suggested that the extract could be used as novel food preservation agents. © Copyright 2020 by Xin Liu