The phenolic component of wine is responsible for the color and taste of wine and plays an important role in the quality of the finished product and associated health benefits. Winemaking processes are essential for controlling the phenolic potential of wine; however, the fundamental mechanisms and kinetics of extraction of phenolics from grape skins during red wine fermentations are poorly understood, making informed manipulation of the phenolic composition difficult. From our previous studies, temperature seems to have a significant effect on phenolic extraction. The formation of a grape skin "cap" in the upper half of the fermentor during a red wine fermentation creates a two-phase extraction system, as well as a temperature gradient as great as 12-14°C. Two experimental objectives were evaluated in order to further study cap extraction and its relation to cap temperatures.
First, a fermentation of Cabernet Sauvignon was performed and evaluated with a grid of 63 temperature sensors that formed a cross-section of a 2000 L cylindrical tank. Samples were drawn from 15 different sampling-points near these sensors to measure the gradients in phenolic composition associated with physical matrix differences and temperature gradients. Temperature data were taken every five minutes throughout the fermentation, and demonstrated a maximum temperature gradient of 12°C which gradually decreased in magnitude throughout the fermentation. A full set of spatial samples were taken for chemical analysis twice per day before pumping-over, and then every hour for six hours after two key pumpover events. Samples were analyzed using RP-HPLC, phloroglucinolysis, and micro path length UV-Vis spectrophotometry. The resulting data demonstrated the existence of chemical gradients for many important phenolic compounds. Extraction of compounds located in the skins appears to occur early in the fermentation while those located in the seeds occurs later. Large gradients in tannin concentrations were observed, along with gradients in the mean degree of polymerization, percent galloyation, and percent seed tannin contributions. The various gradient plots visually reinforced the importance of cap management events for the homogenization of gradients in temperature and chemical concentrations.
In the second experiment, 15 research-scale fermentation treatments of Cabernet Sauvignon were performed in triplicate with varying pump-over volumes, temperature gradients, and submerged caps. Samples were taken throughout the fermentations and analyzed with RP-HPLC and phloroglucinolysis. For the pump-over volumes evaluated in this study, no significant differences in phenolic extraction were observed, although the fermentation lengths were decreased for the treatments with increased pump-over volumes. Overall, a trend of increased extraction with increased temperature was observed, which is consistent with several published studies. Compounds predominately located in the seeds were extracted to a much greater extent with increased temperature than those located in the skins. Comparison of the phenolic extraction profiles of the various cap and liquid temperature treatments suggests that must temperature has a much greater effect on phenolic extraction than cap temperature; however phloroglucinolysis of the finished wines after 6 months in bottle demonstrated a relationship between increased seed tannin extraction and increased cap temperatures.
These two studies increase our understanding of cap extraction in red wine fermentations and contribute important information for future research aimed at optimizing color and phenolic extraction in red wine fermentations. It is hoped that the present work involved with cap extraction will contribute to the understanding of cap management, and improve the ability to manage key phenolic compounds during winemaking.
|Advisor:||Block, David E.|
|Commitee:||Boulton, Roger B., Oberholster, Anita|
|School:||University of California, Davis|
|Department:||Viticulture and Enology|
|School Location:||United States -- California|
|Source:||MAI 52/01M(E), Masters Abstracts International|
|Subjects:||Food Science, Agriculture, Chemical engineering|
|Keywords:||Cap management, Cap temperature, Enology, Extraction, Phenolics, Wine|
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