|Abstract or Summary
- 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.