Reducing Bitterness in Wines

As reported previously (Noble, final report, AVF 91), astringency and bitterness of 18 wines, varying in ethanol (1, 8, 14{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23}), pH (3.0 and 3.6) and added phenolics (none or 1500 mg/L catechin or tannic acid) were evaluated by time-intensity (Tl) methods. Using this procedure, the wine is ingested and perceived intensity is rated continuously Increasing ethanol concentration dramatically increased both the intensity and duration of bitterness and area under the Tl curve, while having very little effect on any astringency. Addition of tannic acid dramatically increased bitterness and astringency intensity and duration, more than addition of an equivalent amount of catechin. Raising pH from 3.0 to 3.6 had virtually no effect on bitterness but decreased astringency intensity and duration. As reported herein, salivary flow was collected in response to the same wines. The largest increase in production of saliva was elicited by decrease in pH (increase in TA); tannic acid, which elicited intense astringency, had the next largest effect, followed by increase in ethanol. Examining the taste responses of the judges, grouped by salivary flow rate, showed that (pooled over all samples) low-flow judges perceived bitterness and astringency more intensely and longer than high-flow individuals. The effect of method of sample evaluation on sourness, bitterness and astringency was preliminarily explored in water and in wine. The temporal responses using “sip and spit” procedure (the normal lab protocol) was compared to the Tl responses when samples were sipped and swallowed, more similar to actual consumption method. With the exception of astringency in wine, no differences between the two methods were found. For evaluation of astringency, when samples were swallowed, astringency intensity was slightly lower and the total duration was slightly shorter, than when the samples were expectorated. However, considerably variation among judges occurred , thus this effect is not be concluded to significantly affect perception based on this preliminary study. To explore the effect of rate of salivary flow on Tl perception of these three attributes in water and in wine, data for the expectorated samples was analyzed for low, medium and high flow subjects for each samples. Inconsistent differences were seen across flow groups, further, by analysis of variance problems with reproducibility were encountered. In contrast to the previous study, here high flow judges tended to have a longer persistence of bitterness or astringency than low flow judges. These low-flow-judges rated the samples higher in both attributes, consistent with the above study. Because of the inconsistent responses of judges, we have repeated the experiment in wine to determine if any real effects of salivary flow can be determined. Twenty judges rated the intensity of sourness, bitterness and astringency in wines by time-intensity methods following extended training. These results, while not demonstrating a large influence of salivary flow on perception were more consistent with the first study. Although the results are still being analyzed statistically, preliminary analysis across all judges indicates that the data are reproducible across replications and that significant differences among samples for each attribute were found for all Tl parameters. While neither concnetration nor salivary flow rate affected the time-to maximum, maximum intensity and total duration increased significantly between samples. Although very small differences in maximum intensities were found among flow groups, the subjects with the highest flow rates, perceived all three attributes for the shortest duration.

Determination of long term normal storage effects on urea and ethyl carbamate in wine.

The prediction of the changes in ethyl carbamate are directly proportional to the ethanol concentration and to the urea concentrations. The rate at which the formation occurs follows the Arrhenius equation. It is simply a chemical reaction (no enzymes involved). The higher the storage temperature the faster the reactions proceed. Essentially, only a small percentage of the urea is lost with time. At the temperatures of normal storage, the hydrolysis rate is very, very slow. Citrulline also reacts, but much more slowly with ethanol to form ethyl carbamate.

Factors Affecting Sugar Utilization and Rate of Fermentation During Vinification

Adenine supplementation stimulated fermentation rate in Montrachet at low (15°C) temperature by reducing the lag time to onset of fermentation, and by shortening the time to dryness. There appeared to be little impact on maximum fermentation rate. Prise de Mousse and Pasteur champagne were unresponsive to adenine supplementation. The effect of adenine was greatest at an intermediate concentration and not affected by nitrogen supplementation. Thus, the stimulation appears to be adenine-specific, not a simple consequence of the presence of extra nitrogen. Loss of two glucose transporter proteins SNF3 and HXT2, reduced rate of fermentation at the end of fermentation. The HXT1 glucose transporter does not play a major role during vinifcation. Interestingly, fermentations conducted by mutants lacking HXT2 were always over run with bacteria. Loss of this gene affected competitiveness of this yeast strain.

Investigation of pattern of utilization of nitrogen compounds and urea

A field experiment, fertilized at seven different nitrogen levels was tested. All the samples responded in a similar manner with no significant effect on rates of fermentation or amounts of residual urea. This was the first year of the treatments and for all practical purposes was base line information. No differences were noted between on skin or off skin fermentations due to treatments. The results of ten dessert grape fermentations made from Davis grapes showed that two of the varieties ended up with extremely high urea values of 98 and 134 mg/L. This is due to high nitrogen values in the grape and failure to use all the assimilable amino acids.

Volatile Sulfur Compounds: Incidence and factors affecting their formation in Californian Wines.

Research Accomplishments: As previously suggested, nitrogen-deficient grape juices produced higher concentration of hydrogen sulfide. However, despite the examination of musts reported to have produced serious sulfur spoilage problems and which were shown to be nitrogen deficient, no wines were produced which contained serious volatile sulfur spoilage problems as shown in these two studies by the absence of mercaptans. The role of nitrogen deficiency in musts or of different yeast strains in the production of sulfur compounds, despite very rigorous analytical analyses of precisely controlled fermentations is still unclear. As demonstrated by the results of the last two years, no simple relationship between must composition and production of sulfur off-flavors exists. Complex factors which require painstaking and systematic study appear to regulate the production of these compounds.

Winemaking Without Sulfur Dioxide

Spoilage organisms: Zygosaccharomvces and Kloeckera, whose presence in wines seems to be increasing as use of sulfur dioxide is decreasing, were found to be reticent to control at levels which controlled all other spoilage organisms. Studies carried out during this year show that both of these organisms can be efficiently controlled by carbon monoxide but at much higher levels than those employed to control Brettanomyces, Dekkera, and Hansenula.