lunes, 18 de junio de 2012

Estrategias para reducir los niveles de alcohol en el vino


ESTRATEGIAS PARA REDUCIR LOS NIVELES DE ALCOHOL EN EL VINO

El AWRI (The Australian Wine Research Institute) ha desarrollado una ficha técnica que proporciona informaciones prácticas sobre las diversas medidas que pueden adoptar viticultores y enólogos para regular la concentración de alcohol en los vinos. Se trata de informaciones obtenidas a partir de datos provenientes de la experiencia colectiva de viticultores, enólogos e investigadores.

La ficha analiza prácticas vitícolas, de vinificación, fermentativas y post-fermentativas, además de tecnologías de producción que pueden influir sobre la concentración de alcohol. demás presenta una síntesis del efecto sensorial y las preferencias del consumidor de los vinos con elevado contenido de alcohol.

REDUCING ALCOHOL LEVELS IN WINE

There has been a trend over recent decades towards wines having a higher concentration of alcohol and this trend can be linked to greater consumer acceptance for certain wine styles.

Growing quality grapes in warmer and drier climates has led to wines that are rich and full-bodied with ripe fruit flavour profiles. Warm, dry climates minimise fruit deterioration and allow winemakers greater flexibility in choosing when to harvest. This flexibility facilitates the production of wines with a greater range of flavour styles for many of our popular wine types. The option to extend the time before harvest, aiming to reduce green characters and to enhance the preferred flavour profile of wines, also might lead to higher sugar concentrations (expressed as TSS total soluble solids, ºBrix, or Baumé). This, in turn, leads to wines with elevated levels of alcohol. High alcohol levels can negatively affect wine flavour (e.g. hotness, lack of balance), and also increase costs in countries where taxes are levied according to alcohol concentration. Increasingly, health concerns linked to alcohol consumption shape both national and international public health recommendations to lower the alcohol content of alcoholic beverages such as wine.

The aim of this Fact Sheet is to provide practical information on a range of approaches that may be used to adjust alcohol concentration. The information is gathered from the collective expertise of practical grapegrowers, viticulturists, winemakers and scientists, and is based on experience and review of current scientific data. Please note that considerations of wine quality and wine style are beyond the scope of this Fact Sheet.

Sensory impacts and consumer preferences:

Elevated alcohol levels clearly increase the perceived warmth or hotness of wine. Ethanol increases the perception of bitterness in both white and red wines. High levels of alcohol are sometimes associated with increased apparent sweetness, which may be inappropriate for the intended style of the wine. Higher alcohol levels might mask some wine aroma and flavour attributes. In a recent study, ethanol did not cause any noticeable changes in the intensity of white wine aroma when the ethanol concentration was varied within 11.6 % v/v to 13.6 % v/v. In the same study, however, elevated ethanol levels increased the hotness, bitterness, drying, roughing and metallic sensations on the palate while it did not consistently alter fullness or viscosity.

The link between alcohol and consumer preference varies across consumer groups. In recent studies, winemaker preference did not relate to alcohol concentration, while almost 40% of wine consumers in Australia and more than 50% of wine consumers in China reported lower levels of liking for higher alcohol wines, with reference to hotness and bitterness. Similar trends were found for Riesling and Chardonnay, where the preferences of a sizable proportion of consumers tested were correlated negatively with alcohol levels. Hence, it is important for wineries to consider market demands and their market segment when adjusting alcohol levels in wine.

Viticultural practices:

- Reducing leaf area: The rate of sugar accumulation in berries is largely determined by the ratio of leaf area to fruit weight (LA/FW). However, flavour and phenolic ripeness may be independent of LA/FW. A relatively high LA/FW may cause sugar concentration to reach unacceptably-high levels by the time that flavour or phenolic ripeness is judged to be optimal for a particular wine style. Therefore, reduction of leaf area after fruitset may lead to better synchronisation of sugar and flavour/phenolic ripening, and thus lower alcohol concentration in wine without any detrimental effect on flavour/phenolic profile. This can be achieved by shoot topping to reduce leaf number per shoot or by leaf removal above the bunch zone. However, these practices should be trialled with caution because they may cause excessively-delayed ripening at high crop loads or excessive bunch exposure.

- Pre-harvest irrigation: Contrary to the views of some in the industry, substantial increase in irrigation volume during the pre-harvest period does not appear to have any significant effect on sugar accumulation or alcohol concentration in the resulting wine. Adverse risks associated with this practice include a significant delay in the ripening of high crop loads and, in some regions, a prolonged maturation period might extend beyond the onset of autumn-winter rains.

- Maturity and flavour profile: Delaying harvest produces grapes with fuller flavour and reduced green characters, especially for Cabernet Sauvignon grapes. It is commonly assumed, therefore, that wines made from such grapes will be preferred by consumers. However, recent studies have shown that this is not necessarily the case. A series of wines made from sequentially-harvested Cabernet Sauvignon grapes showed a range of alcohol contents between 11.8 % v/v to 15.5 % v/v. These wines were subjected to extensive chemical and sensory analyses and consumer preference tasting. Although fruit flavours, viscosity and hotness increased with maturity and fresh green characters decreased with grape ripeness, consumer preferences did not increase with maturity. In fact, consumer liking was similar for wines containing 13.6 % v/v to 15.5 % v/v, indicating that harvesting earlier could deliver a wine that consumers prefer and that contains less alcohol.

Opportunities:
- Consider whether there is a relationship in your particular vineyard between leaf area and yields in lowering sugar concentration in berries and, if so, evaluate methods to decrease the leaf area/yield ratio in order to lower grape sugar concentration.
- Understand the relationship between grape flavour and final wine flavour, and consider if your berry sensory assessment is providing an accurate guide to the final flavour profile in wine.
- Consider whether harvesting earlier can deliver wines containing lower ethanol content without impacting negatively on wine style.

Fermentation and winemaking practices:

- Blending: Technologies exist which now make it possible to fractionate grape musts and wines. Those fractions can later be recombined in various proportions or combinations to achieve optimal wine quality.

Standard winemaking practices, which ensure the efficacy of any wine additions, necessitate the use of water, e.g. to dissolve tartaric acid or to convert bentonite into a usable slurry. These practices are regulated in most countries with the total amount of water not to exceed certain limits. In Australia, water added as part of permitted wine additions, such as fining agents, is limited to 7% v/v (70 mL of water per litre of wine). Winemakers normally limit any dilution factor due to the resultant effect of diluting wine flavour. However, if the total permissible addition of 7% were made, it could potentially lower the final alcohol concentration by almost 1% v/v. Again, it is of key importance that the effects of dilution on other must parameters (e.g. titratable acidity) and wine flavour be considered.

- Enzyme additions: The enzyme glucose oxidase (GOX) from the fungus Aspergillus niger catalyses the conversion of glucose into gluconic acid and hydrogen peroxide. Addition of commercial preparations of the enzyme to grape juice prior to fermentation has been shown to decrease ethanol concentration in the resulting wine by 0.7% v/v compared to untreated wines. Since GOX works more efficiently when oxygen is supplied this figure could be improved by aeration. GOX activity also causes an increase in total acidity, a slight decrease in pH and generates hydrogen peroxide. The effect of hydrogen peroxide production on wine colour or phenolic compounds has not been investigated.

- Fermenter design: There is some evidence that aeration and higher fermentation temperatures can decrease alcohol levels. Tank type and design have also been indicated as important factors, with open top fermenters reported to give lower alcohol concentrations after fermentation. However, scientific studies have yet to establish the degree to which these factors modify alcohol levels and wine flavour.

- Choice of wine yeast: Generally, commercial wine yeasts do not show much variation in the amount of ethanol yielded in wine following fermentation. However, the yeast strain AWRI 796 has been shown, in some laboratory-scale trials, to give lower ethanol concentrations than several other commercial wine strains. For example, compared to EC1118, AWRI 796 delivered a reduction of ethanol as high as approximately 0.4% v/v. In addition, AWRI 796 is able to complete fermentation of musts prepared from high Baumé fruit, although attention to YAN (yeast assimilable nitrogen) and aeration are important in clarified musts. This yeast also produces higher titratable acidity (TA) than others.

Opportunities:
- Assess winemaking practices for fermentation of early-harvested low Brix grapes. - Assess the use of yeast strain AWRI 796 or equivalent for your particular grape must and wine style, and confirm the quantum of reduction in wine alcohol.
- Understand how alcohol might be lost during fermentation due to factors such as open versus closed fermentors, and active pump-overs versus plunging or heading down.

Post-fermentation practices and processing technologies:

Ensure that application of the following methods complies with wine regulations of your destination market.

- Physical removal of grape sugar or wine alcohol: Engineering options for reducing sugar content of juice and alcohol concentration in wine include membrane-based systems (such as reverse osmosis and evaporative perstraction), vacuum distillation and spinning cone distillation. These techniques allow for effective and precise control of alcohol reduction, and have seen widespread evaluation across the industry. However, in some circumstances, other sensory compounds might also be removed which could impact on wine quality; scientific evidence for establishing and minimising the impact of these approaches on wine flavour is currently limited. A pragmatic approach to address this is to totally de-alcoholise a small parcel of wine which is then back blended to achieve the desired overall alcohol concentration whilst minimising quality losses.

- Loss of alcohol by evaporation: During barrel maturation, both water and ethanol evaporate. Ethanol concentration slowly increases in dry cellars as water evaporates faster than ethanol in this environment. Conversely, in cellars with a relative humidity over 70%, ethanol concentration slowly decreases over time. Alcohol was reported to drop by 0.2% v/v when barrels were stored for 12 months at 15°C with relative humidity over 90%. Mould development is a risk when barrels are exposed to high humidity for prolonged periods without appropriate monitoring.

Opportunities:
- Where appropriate, utilise blending of high and low alcohol wine to achieve the desired final alcohol concentration, balance and quality.
- Consider whether engineering techniques to decrease alcohol in wine are appropriate for your production scale and wine style.
-  If wine is to be kept in barrels for long periods of time, pay attention to relative humidity and mould development in cellar.

For more information: www.awri.com.au

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