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Better Winemaking through Biochemistry: Special Topic “Hang Time”

By Jim Harbertson

Introduction

The issue of extending grape ripening beyond traditional goals known commonly as “hang time” is being practiced on a global scale. Even winemakers from more traditionally based France are demanding higher sugar concentrations in their fruit from growers. This is causing a rift in the ongoing classic struggle between grower and winemaker because the fruit, when excessively ripe, no longer accumulates mass. The berries begin to dehydrate and, for growers who have contracts based on weight, this means smaller profit margins. The winemaker’s argument has essentially been that the ends justify the means. Their wines are currently seen as higher quality and garner higher ratings from wine critics who have developed a palate for “hang time” wines.

Although the battle between grower and winemaker is an interesting spectacle, this article is an attempt to dig into some of the procedures to make those high scoring wines. It will begin by discussing the basic biochemical processes that occur during extended ripening and finish by discussing how those changes mandate alterations to standard winemaking practices.

Ripening

Although the berries of Vitis vinifera have their peculiarities, the basic ripening phenomenon they undergo is not that different from other plant species that bear fruit. As the grape ripens it gets softer, accumulates sugars, dilutes and metabolizes organic acids, becomes less bitter and astringent, intensifies in flavor, and in the red and black varietals becomes enriched in pigment. In general, plants have evolved to have complex relationships with other species that they rely upon to consume their fruit and disperse their seeds for future generations (post digestion of course). Grapes are no different in this regard but because humans have cultivated them for over two thousand years they may have picked up a few variables from controlled breeding.

The accumulation of sugar begins in earnest when the berry starts to soften and increases exponentially until it reaches about 24 to 26% sugar. After this point sugar accumulation is thought to be virtual, and due to the loss of water through dehydration.

The grape organic acids (tartaric and malic), unlike sugars, are synthesized early during berry development and remain static on a per basis until the berry starts to soften. After the berry softens the berry goes through a period of engorgement and metabolism of one of its primary organic acids (malic acid) thus a decline in acidity is observed. The amount of organic acid metabolism is highly dependent upon climate, in hot climates larger losses of acidity are observed whereas in cooler climates acids are retained.

Classically grape ripeness has been selected based upon the aforementioned measurements of sugar and acidity. However, more experienced winemakers look for flavor development due to terpenoids and other aroma compounds. They also chew the skins and seeds assessing their bitterness and astringency (caused by catechins and tannins). The observation that bitterness and astringency diminish during ripening has recently been confirmed by scientists. In some varietals vintners look specifically for the loss of vegetative characters. Methoxy pyrazine (a volatile vegetative aroma) diminishes with increased sun exposure. The development of fruit aroma and desirable flavor components is less understood; it is yet unknown whether they are truly present in higher quantities or merely seem so due to dehydration effects.

High Brix Must

The main difficulty with high Brix must is that yeast cannot tolerate the ethanol concentrations that it enables them to produce and will perish before they ferment the must to dryness. The ethanol tolerance range of most wine yeast is about 14 to 15.5 % ethanol. This range is well known, but many winemakers are pushing the upper limits and beyond. The most common way of avoiding this problem is to add water to adjust the sugar concentration so that the ultimate ethanol concentration does not exceed the yeast tolerance range. Corrective water additions are most commonly done in conjunction with other solutes such as yeast, yeast nutrients, acid, fining agents or maceration enzymes to prevent multiple dilution effects. There are winemakers who prefer to live a little on the edge and just allow the component additions be the only dilution. This is commonly called the “let it ride” method that is most assuredly derived from the term commonly used when playing card games. In some cases this style of winemaking causes a fermentation to stick and the fermentation must be re-inoculated with another yeast species that is more tolerant of ethanol such as Saccharomyces bayanas. A very popular method, this is where a good portion of the highly alcoholic wines that are typical of our new era are derived.

Another problem with higher ethanol is the taxation levied on wines that are 14% or less are $1.07 per gallon and those that are between 14-21% ethanol are taxed at $1.57 per gallon. This of course provides a major economic imperative to lower the finished ethanol concentration. Alcohol reduction can of course be done with blending different lots or by dealcoholization with reverse osmosis or spinning cone technology. Each technique has its own caveats, and it is beyond the scope of this article to discuss all of them. It is commonly agreed upon by winemakers that simply targeting an ethanol concentration based on taxes does not always yield the best wine. The ethanol concentration must be in balance with other wine components such as astringency, and sourness and this must be taken into consideration when deciding what ethanol concentration you are looking for. This may be different in every situation and thus should be addressed at an empirical level. It is also interesting to note that the volatility of many different aroma compounds is affected by ethanol concentration. For instance the solubility of hydrogen sulfide (rotten egg aroma) is two and a half times greater in ethanol than water.

Low Acid Must

Another common extended berry maturation side effect is low acid, and this can be exacerbated in very warm climates where the pH can reach 4.0 or above. Typically the pH range of fruit is between 3.3 to 3.8 and more desirably around 3.5. There are numerous problems associated with high pH if not corrected early. Probably the most dangerous is infection of your wine with a spoilage organisms like Brettanomyces, Lactobacillus, Zygosaccharomyces etc. It is very common to use sulfur dioxide to inhibit spoilage organism growth. There are difficulties that arise at high pH with the use of sulfur dioxide. Essentially the efficacy of sulfur dioxide is very low at high pH because the inhibitory form of sulfur dioxide is found at low concentrations. At lower pH values the inhibitory form of sulfur dioxide is at higher concentrations and far more effective. See figure 1 for an equation to determine the sulfur dioxide addition necessary to reduce a population of microorganisms by an order of magnitude based on pH.

Normally, high pH musts are ameliorated early with tartaric acid (malic and citric acids are both unstable to microbial metabolism). About a day after crushing, after most of the potassium from the skin has leached into the must, a more accurate estimate of pH and titratable acidity can be made. Generally, this is when a corrective acid addition is made. However, if there are indications (historical or otherwise) that the incoming fruit will have acid deficiencies, a prophylactic addition can be made. Prophylactic additions are generally done conservatively and are used to simply reduce the pH enough to facilitate the reduction of wild yeast with early sulfur dioxide additions.

Another lesser-known problem has been observed in red wines with higher pHs and higher ethanol concentrations. Potassium bitartrate and potassium tartrate crystals have been observed after only a few months of bottle aging. It is well known that ethanol contributes to the instability of tartrate acid salts. Ethanol alters the dielectric constant of the wine. A two percent ethanol addition can shift the equilibrium constants of acids up to 0.2 pH units. This effect is deceivingly subtle, but it essentially leads to the precipitation of acid salts. This may not seem like an issue since higher alcohol products are sold to consumers who are not bothered by a little particulate matter at the bottom of the bottle. But, as higher alcohol red wines begin to find their way into lower tier products these instabilities may be deemed objectionable by lesser-informed consumers and salesman. The most obvious way around this is to cold stabilize these products, something that is currently done for white wines but not red wines. The main complaints with cold stabilization are that can be costly, and some wine consultants say that chilling removes colloids that contribute positively to the wine’s mouthfeel. This may provide another incentive to invent ways to quickly and cheaply stabilize wines for tartrate salt precipitation. Recently, electrodialysis has been used in a few California wineries to remove potassium and calcium ions without having to chill the wine down. The technology for the process originally comes from water desalination. The technique uses an electric field and selective membranes to remove the positively charged potassium and calcium ions and the negatively charged tartaric acid ions that are associated with tartrate acid salt precipitation. The two separate membranes are selective for the different ions, and as the ions pass through the membranes they are removed using a saline solution. An alternative to this process is to simply label your wine as unfiltered, informing consumers that a natural sediment may be present.

Color and Astringency

Typically winemakers indicate that extending the ripening period is done in an effort to reduce the astringency of both grape skins and seeds. Most studies indicate that seed tannin on a per berry basis declines until harvest. Sometimes seed tannins can be constant for up to four weeks prior to harvest. One conceivable danger (outside of adverse weather events) is that the berry weight may decline during this period, and an increase in tannin on a fresh weight basis may be observed. This may not be a problem for those who are going to water back anyway. But for those who will let it ride, they may be a caught by surprise during the fermentation when they are extracting more than they had intended.

Color is well known to increase until harvest, however, extensive studies of extended maturation trials have yet to be done. Anthocyanins, the compounds responsible for grape pigmentation, are well known for being influenced by temperature and sunlight. Thus, it follows that their accumulation will be highly dependent on these variables during the extended maturation period. However, it is known that anthocyanins are unstable in wine and it is conceivable the same could be true in the berry. Research is obviously necessary to determine this outcome.

Other Concerns

An extra caveat for grapes that are brought in at the end of the season is that in many cases, materials other than grape (MOG) end up in the crusher at very high levels because the vine becomes brittle during senescence. Grape leaves, tendrils, racchi, cane portions and even a few spiders are often seen in greater quantity than desired. This means the fruit must be sorted more carefully and can be more expensive to process. There are more automated technologies available but sorting tables and harvest labor are still the most prevalent methods for sorting used. Other problems particular to Washington is that during the later months (October, November) the fruit also is brought in at a much lower temperature than normal and must be warmed up in order for yeast inoculation with a heat exchanger.

Conclusion

“Hang time” promises to be a hot topic in both the near and distant future. While I prefer not to entangle myself with the politics and business dealings, please feel free to contact me with questions or insight gleaned from your own experience with extended ripening.