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Winemaking at High pH

by Clark Smith

In this section, we consider the winemaking terrain above pH 3.6. Since the standards for most California wines that exist today (though not necessarily tomorrow) make it difficult to produce commercially acceptable white wines, this is essentially a discussion of red wine production. Consumer expectations for red wine differ from whites in several salient ways:

  • More tolerance of browning

  • Less emphasis on clarity

  • Less emphasis on fresh fruitiness

  • More value placed on complexity, less on cleanliness of aroma

  • Greater ageability requirements

The dominant theme of low pH winemaking is focused on prevention and control. In high pH winemaking, we often acknowledge that we have given up on prevention, and try instead to direct the inevitable to a stable and agreeable outcome.

SECTION 1: STABILITY

The microbiological triangle considers the three requirements of Inoculum, Nutrients, and Inhibitors. Any one of these elements may be addressed to control an organism's growth, as long as the ecology of a specific organism is understood. When blending, it is important to consider that two stable wines, when combined, may supply missing elements required for spoilage. The utility of these elements in the high pH realm is entirely different from the low pH environment.

I. Inhibitors:

SO2

In low pH winemaking, we have stressed the role of molecular sulfur dioxide to control the growth of microorganisms. Since its effectiveness as a inhibitor is greatly lessened at high pH, it is more sensible to speak in terms of free and total SO2.

Free SO2, which is substantially all bisulfite, should be maintained to inhibit oxidation and to combine with aldehyde as it is formed through chemical oxidation of EtOH and by the action of film yeasts. When oxygen enters a red wine, it reacts with phenolics and promotes their polymerization. A side product of this reaction is hydrogen peroxide, which may be damaging to wine flavor and can oxidize ethanol. Its reaction with SO2, however, occurs much faster, so a small amount (say 10 to 30 ppm) bisulfite is extremely effective in scavenging peroxide. Since SO2 is depleted by this action and by aldehyde binding, it must be measured by aeration/oxidation and maintained at a reasonable level throughout ageing.

A desirable consequence of sulfite oxidation to sulfate is the liberation of a titratable proton, with the effect that over time, very high pH wines tend to experience increased TA and decreased pH. This effect is proportional to the sulfite concentration, which is ten times higher at pH 4.0 than at pH 3.0, and can thus be ignored below 3.6, but can, during extended barrel age at pH 3.9, result in an increase of as much as 1 g/L in TA and in sulfate concentration (see below for flavor effects) and a reduction of 0.1 or more in pH.

Total SO2 should be measured to assess flavor impact (a soapy finish can be detected at about 200 ppm) and because of its inhibitory effect on malolactic bacteria at about 100 ppm.

Addition of SO2 to young wine will be observed to bleach monomeric pigment. This is a reversible reaction, and actually stabilizes red color by maintaining a pool of monomeric pigment, which is restored to the wine as it ages and the pigments polymerize.

Temperature

Since at high pH, SO2 is not very effective in inhibiting spoilage organisms, other means must be found to prevent spoilage. A low temperature cellar is one possibility, as many types of organisms such as film yeasts and Brettanomyces do not grow well at 55F. Cool cellars are not, however, a good strategy if the intention is not to sterile filter prior to bottling, since activity in the bottle may ensue, resulting in unpredictable results and bottle variation.

II. Nutrients:

Oxygen

Organisms which require oxygen for respiratory metabolism may be controlled by eliminating it from their environment through the use of inert gas and, preferably, by maintaining wine in full containers. These include film yeasts and Acetobacter. Frequent topping, however, has been argued to introduce more oxygen than it prevents, since it breaks the vacuum which builds in a well made barrel. Many winemakers therefore prefer to bung and roll wines which have completed fermentative activity.

Cool cellars, while they inhibit the growth of many organisms, also increase the solubility of oxygen in wine. Oxygen control is not effective against fermentative organisms, including Saccharomyces, Brettanomyces/Dekkera, and malolactic bacteria.

Oxygen is not the enemy of wine. Low oxygen winemaking, while useful for microbial control, is not always desirable from the point of view of phenolic management. Young reds often benefit from large amounts of oxygen to facilitate polymerization and complexation of harsh tannins. Older reds are generally fed oxygen in much lower levels to assist textural and aromatic development without encouraging oxidative microbiology or tannic dryness.

Fermentable sugar

Complete fermentation is an aid in the suppression of fermentative organisms such as Brettanomyces/Dekkera. Dextrocheck or Lane Eynon analysis for residual sugar is not sensitive enough for this determination, and enzymatic glucose + fructose should be run.

Malic acid

Malic and lactic acids can be tracked non-quantitatively (present/not present) by paper chromatography to monitor the onset and finish of malolactic by the simple and inexpensive method of paper chromatography. Enzymatic malic analysis requires a spectrophotometer, but is a wonderful method to monitor progress of malolactic, and is one of many reasons for even small wineries to consider the purchase of an inexpensive "spec," (around $1500). Please refer to the conclusion of the article on urease for a more complete discussion of this notion.

Because it is not microbially stable, I cannot conceive of a situation in which one would consider addition of malic acid to high pH table wine.

III. Inoculum:

Introduction of commercial organisms

We have already discussed the introduction of malolactic bacteria, so I want to confine my comments here to choices about yeast.

Inoculation for primary fermentation with commercial organisms is widely but not universally practiced. Commercial yeasts differ greatly in their fermentation vigor, and slow fermenters such as Pasteur red are popular for red wines since they allow prolonged fermentation on the skins at elevated temperature.

"Wild" or "natural" yeast fermentation simply means that we have not inoculated and do not know what characteristics the fermenting yeast may possess. Some wild fermentations are prone to foaming, sulfide production, vinegar production, high total SO2 metabolism, and sticking. Advocates point to the advantages of sluggish fermentation and the complexity of flavors which occur in these wines.

An idea which has gained recent credibility is that the advantages of both methods may be obtained by reducing the size of inoculum of commercial yeasts. The resulting delayed onset of fermentation allows natural non-alcohol tolerant yeasts such as Hansenula and Kloeckera to add complexity at the beginning of fermentation, still allowing the commercial yeast, with its predictable characteristics, to finish the fermentation. This strategy may be of particular interest to organic winemakers.

Sanitation

Wineries are not sterile environments. Spoilage cannot be prevented by maintaining an environment free of microbes. Most wineries of any age have indigenous populations of Saccharomyces, Brettanomyces, film yeast and malolactic bacteria. Care to minimize populations by reasonable sanitation procedures is nevertheless the policy of most California wineries. Less so in European cellars, where indigenous microbial populations have stabilized and a "gout de maison" or house flavor profile, which has developed over centuries and whose evolution and control has been empirically incorporated into cellar practices, may be a valued element of the wine.

Since no human pathogen can live in wine, winemakers can afford to be much more blase about sanitation than other food producers. In high pH winemaking, the goal is often to allow the wine to undergo all potential microbial activity at the winery , and sanitation should be considered in this context. For unfiltered products, a warm cellar (60-65F) which promotes potential activity, may be a wise choice.

Because of its alcohol and acidity, even high pH wine is an unusual environment, and most organisms cannot grow in it. Contact with sewage, for example, is less of a threat to wine spoilage than contact with pockets of spoiled wine. The importation of bulk wine, used barrels, and wine containing equipment should always be focused on as an event which might introduce a virulent strain of spoilage organism such as a particularly active film yeast or Lactobacillus.

Filtration

I like to define winemaking as "the art of intelligent compromise." Filtration decisions balance flavor loss against in-bottle spoilage in the context of a particular wine. Some wineries filter their cabernet but not their pinot noir, for example. Some decisions, such as sweet wine bottling, are pretty straightforward: filters are better than sorbate and cheaper than high-tech pasteurization.

In high pH winemaking, the key to bottling without filtration is to eliminate the nutrients listed above from the equation by provoking microbial activity at the winery.

SECTION 2: PHENOLIC CHEMISTRY

Red wines are distinguished from whites, more or less, by the introduction of several hundred species of phenols during primary fermentation on the skins.

Monomeric phenols contribute to aroma, while polymeric phenols (tannins) are responsible for astringency.

Winemaking philosophy has progressed considerably over the last twenty years regarding texture goals. It is now clear that gross amounts of tannin per se do not promote long ageing, since the reaction of tannin with oxygen is very slow. Wines thought to be soft and wimpy in the 70's, such as the early Jordan and Veedercrest cabernets, have aged magnificently, while their "bigger" counterparts remain harsh and unappealing. At the same time, the expanding red market has reflected its desire for less tannin in burgeoning sales of merlot over cabernet. California winemakers are now less obsessed with trying to extract "guts, feathers and everything," paying more attention to the French notions of quality. At the same time, full extraction strategies are learning to incorporate sophisticated cellaring techniques to manage texture.

The Bordolais speak in terms of ripe, short chain "soft" tannins as a goal for red wine texture, and also assert that these should form complexes with anthocyanin pigments to produce stable color and soft rich mouthfeel. Adequate tannin extraction to promote this complexing is thought to require fermentation at elevated temperature (over 80F), and may be enhanced by extended maceration for up to several weeks post fermentation.

Ripe fruit is essential for this kind of wine, and in many California vineyards may only be maximized at elevated brix as in France, and green tannins are a particular problem for uneven ripening varieties such as zinfandel. For this reason, many wineries are now allowing extra ripening, then removing excess alcohol with spinning cone and (here comes the Vinovation plug!) reverse osmosis technologies.

Layers of soft tannins may be encouraged by blending and even by co-fermentation of complementary varietals. Selective extraction strategies, those that attempt to extract the maximum monomer and leave behind the gross tannins, can be implemented at several production stages. Choices include crusher design, cap management technique, yeast selection, fermentation temperature and duration, press type and method of operation, fining regimen, and barrel manufacture.

The French notion of elevage (to "raise" the wine as one raises children) includes many approaches to enhancing tannin structure which include micro-oxygenation (to build structure, round tannins, integrate aromatics and quench reductive proclivities), periodic macro- oxygenation (to open and advance the wine's ageing) and lees contact. The timing and levels employed are critical to success with these techniques.

The rich finish of a classic bordeaux is not entirely attributable to phenols. Potassium and sulfate both contribute the back palate mouthfeel, and are natural by- products of high pH winemaking. Sulfate is the product of sulfite oxidation, which occurs much more rapidly at high pH. Potassium is the counter-ion in the highly buffered solutions typical of this genre. High pH wines are typically not cold stabilized since this results in lowered potassium and higher pH.

Cold stabilization may be useful, however, to reduce high TA in wines of the central coast, whose acid taste creates the impression of harsh mouthfeel. It should be kept in mind that while cold stabilization lowers pH if it takes place below pH 3.6, in the high pH realm it always raises pH.   

 

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