Vintner's Corner



Vol. 13, No. 3 May - June, 1998

Bruce Zoecklein

Department of Food Science and Technology

VPI & SU

Blacksburg, VA 24061-0418, E-mail: bzoeckle@vt.edu




Table of Contents



I. Grape-Derived Aroma and Native Yeast Fermentations 1



II. ASEV-ES Annual Meeting and Sparkling Wine Symposium 3




I. Grape-Derived Aroma and Native Yeast Fermentations

The concentration of aroma components found in wine can be influenced by environmental factors (such as climate and soil), cultivar, fruit condition (maturity, rot), conditions during fermentation (pH, temperature, juice nutrients, microflora) and by post-fermentation treatments. As discussed in previous issues, grape-derived aroma compounds are present as free volatiles, which may contribute directly to odor, or as sugar-bound conjugates (mainly glycosides). Glycosides are, in part, aroma precursors. Glycoside hydrolysis may occur via acid or enzymes resulting in the release of free volatiles, thus enhancing potential aroma. The mechanisms by which free volatiles are released is receiving considerable attention. Initial research focused on monoterpene glycosides and their hydrolysis products which contribute to the floral character of varieties such as Riesling. Subsequent research has demonstrated the involvement of the products of glycoside hydrolysis in varietal aroma in a wide range of non-floral varieties (Williams et al., 1989). Thus, the sensory significance of glycoside hydrolysis to varietal grape aroma has been established providing justification and rationalization for their quantification.

As discussed in previous additions, various enzyme activities can improve processing and wine quality. Yeasts are important sources of enzymes. Many yeast genera and species, including Saccharomyces sp., possess glucosidase activity which is capable of influencing wine aroma as a result of the hydrolysis of glycosidically bound aroma precursors (Gunata et al., 1994). However, most strains of Saccharomyces cerevisiae are not recognized as significant producers of extracellular hydrolytic enzymes. This was confirmed by research we conducted evaluating the effects of selected commercial strains of Saccharomyces cerevisiae on grape glycosides ( Zoecklein et al.,1997). In our study, the ability of commercial strains to hydrolyze glycosides varied by only about 7%. In conventional winemaking using cultured strains of Saccharomyces cerevisiae, it would appear that a large percentage of the grape aroma potential remains in the sugar-bound, non-volatile form.

Some producers are interested in native, non-inoculated yeast fermentations, partly due to the perception of enhanced aroma complexity. A wide range of yeasts has been found on grapes and in wines due to variations in vine age, geography, variety, harvest and winemaking methods. In addition to Saccharomyces cerevisiae, it is known that other yeasts can grow during the early stages of fermentation. Frequently isolated native organisms include: Hanseniaspora uvarum, Kloeckera apiculata, Metschnickowa pulcherrima, Candida pulcherrima, Candida stellata, Pichia membranaefaciens, Hansenula anomala as well as Cryptocccus, Rhodotorula and Saccharomyces sp. (Redd and Nagodawithana, 1991). Non-inoculated fermentations occur as a succession of yeast populations beginning with relatively weak, although numerically superior species present on the fruit. These organisms are susceptible to increasing alcohol levels and are not as alcohol tolerant as strains of Saccharomyces cerevisiae. Over time the activity of native, non-Saccharomyces species declines and indigenous populations of Saccharomyces cerevisiae are established and finish the fermentation. Such fermentations may be completed by as many as ten strains of Saccharomyces cerevisiae (Mortimer, 1995). A explanation for the possible enhancement of aroma in wines produced by indigenous yeasts is that many different organisms are involved. Investigations of the effects of yeasts on wine aroma has confirmed the diversity of strains with respect to higher alcohol and ester production. However, there is limited information on the production of grape-derived aroma from non-Saccharomyces yeasts.

Table 1 (adapted from Zoecklein et al., 1997) shows the results of an evaluation of the influence of several commercial yeasts and a native fermentation on grape glycosides. The lower the glycoside concentration, the greater the degree of hydrolysis. Enhanced hydrolysis can result in the release of aroma volatiles. The lowest concentration was noted for the native, uninoculated fermentations. Extracellular hydrolytic enzyme production from native organisms may be the source of reduced glycoside concentrations. Thus, the potential for indigenous wine yeasts to produce extracellular enzymes of enological importance may be present. In a research program in cooperation with Fresno State University, we are currently conducting screening tests on a number of indigenous wine yeasts for the production of -glucosidases.



Table 1. Effect of four strains of Saccharomyces cerevisiae and native yeasts on the glycoside content (expressed as Ámol glycosyl-glucose) of Riesling wines.



Yeasts
Prise de Mousse D47 Fermiblanc VL1 Native
Post-Fermentation 369a ▒ 76.5 374a ▒ 47.4 352b ▒ 6.9 379a ▒ 48.0 294c ▒ 35.48

Significance of LSD test of treatment means at P 0.05 and standard deviation. Means with the same letter are not significantly different.



Part of the interest in native yeast fermentations stems from the perception of enhanced complexity. The increase in aroma and flavor from some native fermentations may stem from the fact that "native" yeasts may excrete hydrolyzing enzymes that may be more active during fermentation than those produced by cultured wine yeasts (see Table 1). We produced a number of wines by native or mixed fermentations which are available for tasting at winemakers roundtable meetings. The differences in wine chemistry between native and cultured fermentations of several Virginia grown cultivars are illustrated in Figure 1. The abbreviations used in the figure are as follows: Al = alcohol, Ex = extract, In = intensity (A420 nm + A520 nm), Hu = hue (A420nm/A520nm), Tp = total phenols, An = total anthocyanins, %C = the % of the anthocyanins in the colored form, and Pol = the relative degree of phenol polymerization.

Significant differences were noted in the alcohol content, extract, VA, and several of the color parameters. The sugar to alcohol conversion of most native yeasts is not as efficient as that of cultured wine yeasts. This helps explain both the differences in alcohol and extract. Many native fermentations have better palate structures with more depth as a result of small but subthreshold levels of residual sugar which frequently remain. Differences in total phenols are the result of both the differences in alcohol and the time needed to complete fermentation.

While the cultured fermentations generally had a higher concentration of anthocyanin pigments, the percentage of pigments in the colored form was frequently greater in the native fermentations. As a result, the native fermentations have greater color strength and purity (less tawniness) and appear brighter. The aroma and aroma intensities are different between control and native yeast fermentations with the native yeast frequently providing a broader, less one dimensional 'nose'.

Native fermentations are conducted either with or without the aid of a native starter. Creating a native starter helps to assure that Saccharomyce sp. are in it relatively high concentrations.



II. ASEV-ES Annual Meeting and Sparkling Wine Symposium



The annual meeting of the American Society for Enology and Viticulture, Eastern Section is scheduled for July 22-24, 1998 at the Crowne Plaza Hotel in Grand Rapids, Michigan. An international symposium on Sparkling wines - Issues in Sparkling Wine Production will be held in conjunction with the meeting (see attachment).



References



Gunata, Y.Z. Dugelay, L., Sapis, J.C., Baumes, R., and Bayonove, C. (1994). Role of enzymes in the use of the flavor potential from grape glycosides in winemaking. In Progress in Flavor Precursor Studies. Proceedings of the International Conference. (P. Schreier and P. Winter-Halter, Eds.), Wurzburg, Germany.



Mortimer, R.K. (1995). Yeast isolation from spontaneous fermentations of grape musts in California and Italy. Pract. Wine. Vin. May/June, 7-10.



Reed, G., and T.W. Nagodawithana. (1991). Yeast Technology, 2nd ed., p. 454. Van Nostrand Reinhold, New York.



Williams, P.J., M.A. Sefton and B. Wilson. In Flavor Chemistry, Trends and Developments. R. Teranishi, R.G. Buttery and F. Shahidi, Eds. ACS Symposium Series No. 388. American Chemical Society, Washington, D.C. pp. 35-48 (1989).



Zoecklein, B.W., J.E. Marcy, J.M. Williams and Y. Jasinski. Effect of native yeasts and selected strains of Saccharomyces cerevisiae on glycosyl glucose, potential volatile terpenes and selected aglycones of White Riesling (Vitis vinifera L.) Wines. J. Food Comp. Anal. 10:55-65 (1997).