UC Davis

Because of laboratory constraints posed by the pandemic, two projects are included as part of this thesis, a theoretical prediction of barrel sanitization and an experimental exploration of phenolic extraction in columns.Oak barrels are often used to give wine, beer, and distilled beverages flavors and colors. However, microorganisms can penetrate into the wood, making cleaning and sterilization difficult. This can lead to undesired microbial growth in the contents of the barrel during subsequent use. Information on heat penetration into barrel staves during steam treatment and associated thermal inactivation rates is scarce. Therefore, we incorporated heat transfer and Arrhenius-type death kinetics to build a mathematical model to predict the killing rate of the microorganisms when steam sterilizing a wooden barrel. First, we use this model to predict temperature profiles in barrel staves as a function of distance from the steamed surface and time of steaming. Next, we evaluated the thermal inactivation of microorganisms at 0.8 cm into the stave (the maximum wine penetration depth into a stave) to calculate the time needed to achieve 5-log reduction in live cells. Using this approach, we found that the required sterilization times for Brettanomyces bruxellensis, Saccharomyces cerevisiae, and Leuconostoc mesenteroides are 9 minutes, 12 minutes, and 200 minutes, respectively. This result is useful for winemakers to determine how long they desire to steam a barrel to prevent growth and contamination of key microbes in their wine. Phenolic extraction is essential for red winemaking. Fermentation along with phenolic extraction usually takes 7 to 14 days, although the tannin extraction can continue for weeks until an equilibrium is reached. Yet process intensification would allow winery equipment to be used multiple times during harvest, thus decreasing capital investment. While previous work showed the potential to speed up the conversion of sugar to alcohol considerably, this methodology will only be truly useful for red winemaking if phenolic extraction can be completed more rapidly and with more control. In this work, we performed phenolic extraction from grape solids in a column setup and let juice or wine flow through the column to model a continuous extraction. The effect of four different factors was tested: column temperature, liquid ethanol concentration, flow rate through the column, and column diameter using a design of experiments approach. We found that liquid flow rate is the most significant factor, and the temperature and ethanol concentration had minor effects but not statistically significant. With all optimized factors, we were able to reach the same concentration as the bucket control for the final extraction in 86 hours on the bench scale compared to 144 hours for the control process. Both the pigment concentration and tannin concentration were similar. This work shows the potential to achieve process intensification and more control of the phenolic values in the finished wine.