Monday, February 27, 2012

The role of alcohol in wine balance

Wine balance is one of the important measures of wine quality and attainment of said balance, according to Dr. Bruce Zoecklein (Virginia Tech oenologist), requires the harmonic functioning of each element in the Palate Balance Equation reproduced below. The arrow on the figure following the equation illustrates where balance and alcohol fall within the wine quality assessment framework.

              Sweet ⇄ Acid + Phenolics (Astringency and Bitterness).




Fortunately for wine drinkers, the structure that grape vine plants have evolved for the protection and nourishment of their genetic-material-carrying seeds, also serves as the raw material for the drink of their choice.  At harvest, the vitis vinifera berry has the following composition (Murli Dharmadhikari, Director and Enologist, Iowa State University Extension):
  • Water (70 - 80%)
  • Dissolved solids
    • Sugar (primarily glucose and fructose; 150 - 200 g/L if ripe)
    • Organic acids
    • Phenolic compounds
    • Nitrogenous compounds
    • Aroma compounds
    • Minerals
    • Pectic substances (primary role is cementing plant cells together).
The dissolved solid of interest herein is sugar so we will focus exclusively on that topic for the remainder of this post.

Sugars exist in the grape vine as sucrose (a one-to-one bond of the single-molecule sugars glucose and fructose) and are transported to the berry from the vine where it is metabolized.  As the berry develops (see figure below), the vine-sourced sucrose decreases in volume and is replaced by carbohydrates from surrounding leaves.  Once in the berry, sucrose is separated into its constituent parts by the enzyme invertase.

As the figure below shows, xylem flow (water and nutrients from the roots) to the berry stops shortly after veraison but the flow of nutrients from surrounding leaves (via the phloem) continues through the bulk of the ripening phase.  As the figure shows, the sugar, measured in °Brix (the sugar-content metric used in the US; % sugar by weight), begins to accumulate in the berry post-veraison as sugar consumption declines.  The sugar content at berry maturity can range between 12 and 28% with the optimal level for harvest falling between 19 and 25 Brix.

Grape berry development (Source:www.extension.org)

In a 1966 study, W. Mark Kliewer of UCDavis concluded that the following sugars resided in a mature grape berry: Stachyose, Manninotriose, Raffinose, Melibiose, Maltose, Sucrose, Galactose, Glucose, and Fructose.  With the exception of glucose and fructose, all of the sugars are found in minute amounts and do not contribute in any way to the sensory characteristics of fermented wine. 

Glucose is a 6-carbon-atom sugar resulting from the breakdown of sucrose by the berry and is the first sugar to be broken down by the yeasts during the fermentation process.  There is 5 times more glucose than fructose in the berry at the beginning of the ripening stage but the levels are about equal at the time of harvest due to rapid fructose accumulation in the later stages of berry development.  Through its participation in the development of glycosides, glucose plays a role in the flavors of wine based on the interaction of those glycosides with phenolic compounds such as anthocyanins and terpenoids.  Fructose is twice as sweet as glucose and is a key component in the sweet sensation of wine.  Depending on the time of harvest, the ratio of glucose to fructose in the berry will vary: In an unripe berry, glucose will be dominant; at the ripening stage, the levels will be even; if the grape is over-ripe, fructose will dominate; and if the grape is ripe, the levels will vary depending on whether the variety is high-fructose (e.g., Chardonnay and Pinot Blanc) or high-glucose (e.g., Chenin Blanc and Zinfandel).

Wine is a result of using yeasts (Saccharomyces) in an anaerobic (oxygen-free) environment to convert the sugars from the pressed grape juice into ethanol in the two-step process illustrated in the figure below.  The first step -- glycolysis -- results in the 6-carbon glucose being split into two 3-carbon pyruvate molecules.  In the next step --- alcoholic fermentation - four atoms of oxygen and two atoms of carbon leave the pyruvate, resulting in acetaldehyde which is converted into ethanol.


Alcoholic fermentation (Source: http://alcoholicfermentation.net/

Wine is never fermented completely dry as sugars such as xylose and arabinose survive the fermentation process unscathed.  The amount of alcohol produced as a result of fermentation is directly related to the amount of sugar present in the juice initially.  The conversion ratio is ºBrix x 0.55 = alcohol in wine (Dr. Dharmadhikari).  Residual sugar is the measure of sugar solids remaining after alcoholic fermentation and is stated in g/L.  For dry wines the residual sugar ranges between 0.2 and 0.3 g/L while the measure for sweet dessert wines ranges between 5 and 15 g/L.

In wine, the sweetness of sugar balances out the bitterness of phenols.  Sweetness is perceptible by humans at around 1% of volume but that perception is dependent on the size of the sugar molecule.  Both fructose and glucose are single sugar molecules and are perceived as sweet by humans because they fit neatly into the receptors on the tongue.  Carbohydrates, on the other hand, are long chains of sugars, do not fit neatly into the receptors on our tongues, and are not perceived as sweet.

As stated previously, wine balance is viewed as a key indicator of wine quality.  If a wine has insufficient sugar in relation to its acids and phenols, it will present as harsh and acidic and will retard the evolution of flavors in the mouth.  In such a case the winemaker may choose to add sugar (chaptalize) in the form of cane sugar (sucrose in cane sugar is inverted by the enzymes present in the wine and converted into glucose and fructose) or corn sugar (dextrose which will be fermented quickly by the yeasts)

Alcohol adds to the sensation of sweetness in a wine. It also adds a thickness. Too much alcohol and the wine can present as hot, lead to a reduced perception of wine aroma, and can impart a sense of intoxication.  For a winemaker doing business in the US, there is an approximately $.50 difference in the taxes paid per gallon of produced wine if the alcohol level goes beyond 14.001%.  If the winemaker believes that the wine would benefit from de-alcoholization, there are three methods available to him/her: reverse osmosis, spinning cone, and adding water.  These methods will be discussed in greater detail in a subsequent post.

Hopefully the normal grape-growing and alcoholic fermentation process would have produced a balanced wine but if an imbalance were brought about by an insufficiency or excess of alcohol, the winemaker has tools available to address the problem.

© Wine -- The View From Orlando

No comments:

Post a Comment