It has been suggested that cellular damage from oxygen radicals is one of the processes leading to cardio-vascular disease and cancer. Natural antioxidants prevent uncontrolled oxidative reactions by decreasing molecular oxygen levels, scavenging chain-initiating and chain-propagating free radicals, chelating metals, or decomposing peroxides. Beer is rich in antioxidants, derived both from malt and hops, consisting mostly of flavanoids and phenolic secondary plant metabolites. Much research has been conducted concerning antioxidant activity of beer in relationship to flavor stability. Yeast cells possess both enzymatic and non-enzymatic antioxidant systems to defend against oxygen radicals, in addition to scavenging and absorbing molecular oxygen for cell synthesis. It is well known that bottle-conditioned beer has a longer shelf life than conventional beer in terms of flavor stability and freshness. This is likely due to a complex relationship between the yeasts inherent ability to scavenge oxygen species, produce SO₂, chelate transition metals and employ other methods to defend against molecular oxygen. The objective of this research was to determine whether bottle-conditioned beer (which contains live yeast) has a higher antioxidant activity compared to that of conventional beer. Initial experiments were conducted to establish a baseline of antioxidant potentials. The first experiment consisted of determining the antioxidant potential of commercially available beers and how those values compared to common foods and drinks. Next, live yeast was added to commercially available artificially carbonated beer, to determine whether the presence of live yeast alone had an impact on antioxidant potential. Lastly, in the first set of beer trials wort was prepared, brewers yeast added, and then allowed to complete primary fermentation. The beer was bottle-conditioned (naturally carbonated) by the addition of a second yeast strain and either a 'high' or 'low' level of sugar, or artificially carbonated. Treatments consisted of using three different yeasts. A control was prepared by artificially carbonating beer without live yeast. Antioxidant potentials were determined using Ferric Reducing Antioxidant Potential (FRAP) analysis. Results indicated that there was an increase in the level of antioxidant activities between the bottle-conditioned beers compared to the control beer, whether or not the high or low level of yeast was present. In the second set of beer trials wort was prepared, brewers yeast added, and then allowed to complete primary fermentation, as in the first set of beer trials. The beer was bottle-conditioned by the addition of a second yeast strain and sugar, or artificially carbonated followed by the addition of a second yeast but no sugar. Treatments consisted of using three different yeasts. A control was prepared by artificially carbonating beer without live yeast. Dissolved oxygen, free and total sulfite, yeast viability, and antioxidant capacities were again determined. Results indicated that there was an increase in the level of sulfite, a decrease in dissolved oxygen, yeast remained viable for a longer period of time, and antioxidant activities were higher in the bottle-conditioned beers when compared to the control beer. Furthermore, while the differences were not as great, the same trends were observed for all parameters when comparing the artificially carbonated beers containing live yeast to the control beer. The elevation in antioxidant activities of beer with live yeast present (live beer) was significant. The third and final phase of experiments focused on the contribution of antioxidant potential specifically from the yeast. Yeast cells for each strain were cultivated in beer wort, harvested, washed, and cell extracts prepared. The crude yeast extracts were subjected to heat treatment, size fractionation followed by heat and protease treatments, glutathione determination, and lipid extraction, and then analyzed for antioxidant activity. Results indicated a complex interaction between many different yeast components that contributed to the total antioxidant activity provided specifically from yeast. Rather than one single compound, the yeast contributed heat stable components, consisting of proteins and enzymes, molecules with high, medium, and low molecular weights, and active lipid portions. The overall results suggest, that while the malt and hops components likely play the major role in antioxidant activity of beer, beer containing live yeast has a significant increase on that antioxidant activity. Consequently, the flavor stability and health benefits from beer containing live yeast would be increased. / Graduation date: 2003