Enology Notes

Enology Notes #54, July 18, 2002

To: Regional Wine Producers

From: Bruce Zoecklein

Subject: Uninoculated Yeast Fermentations

Uninoculated 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. 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.

Various enzyme activities can improve processing and wine quality. Yeasts are important sources of enzymes. Many yeast genera and species, including Saccharomyces spp., possess glucosidase activity which is capable of influencing wine aroma as a result of the hydrolysis. However, most strains of Saccharomyces cerevisiae are not recognized as significant producers of extracellular hydrolytic enzymes.

Some producers are interested in 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, Metschenickowia pulcherrima, Candida pulcherrima, Candida stellata, Pichia membranaefaciens, Hansenula anomala, as well as Cryptocccus, Rhodotorula and Saccharomyces spp.

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 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).

An 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 have confirmed the diversity of strains with respect to higher alcohol and ester production.

The information below (adapted from Zoecklein et al., 1997) shows an evaluation of the influence of several commercial yeasts and an uninoculated fermentation on grape glycosides. The lower the glycoside concentration, the greater the concentration of free volatiles. The lowest concentration of glycosides was noted for the uninoculated fermentations. Thus, the potential for indigenous wine yeasts to produce extracellular enzymes of enological importance is present.

Effect of four strains of Saccharomyces cerevisiae and uninoculated fermentation on the glycoside content of Riesling wines. Prise de Mousse 369 micromol glycosyl-glucose per liter D47 374 micromol glycosyl-glucose per liter Fermiblanc 352 micromol glycosyl-glucose per liter VL1 379 micromol glycosyl-glucose per liter Uninoculated 294 micromol glycosyl-glucose per liter

Part of the interest in uninoculated yeast fermentations stems from the perception of enhanced complexity. The increase in aroma and flavor from some uninoculated 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. We have produced a number of wines by uninoculated or mixed fermentations which have been made available for tasting at winemakers roundtable meetings.

Over the years, significant differences were noted in the alcohol content, extract, VA, and several of the color parameters in uninoculated vs. inoculated wines. The sugar to alcohol conversion of most uninoculated fermentations is not as efficient as that of cultured wine yeasts. This helps explain both the differences in alcohol and extract. Many uninoculated 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 uninoculated fermentations. As a result, the uninoculated fermentations have greater color strength and purity (less tawniness) and appear brighter. The aroma and aroma intensities are different between control and uninoculated yeast fermentations, with the uninoculated fermentation frequently providing a broader, less one dimensional 'nose'.

Uninoculated fermentations are conducted either with or without the aid of a starter. Creating a starter helps to assure that Saccharomyces spp. are in relatively high concentrations.

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Dr. Bruce Zoecklein
Associate Professor and Enology Specialist
Head Enology-Grape Chemistry Group
Department of Food Science and Technology
Virginia Tech
Blacksburg VA 24061
Enology-Grape Chemistry Group Web address: www.vtwines.info or www.fst.vt.edu/zoecklein/index.html
Phone: (540) 231-5325
Fax: (540) 231-9293
Email: bzoeckle@vt.edu