A high-level map of the sources of wine aroma

A while ago I had set out on a journey aimed at comprehensively describing all of the attributes of a quality wine and had developed the below framework as a series of guideposts towards this end. I had covered most of the intrinsic factors -- with the exception of aroma/flavors -- when I got sidetracked by items which piqued my interest along the way. I am now ready to return to this task, prompted in large part by some excellent research work on aromas that has come to my attention recently.

According to Robinson et al., (Origins of Grape and Wine Aroma. Part 1. Chemical Components and Viticultural Impacts, Am. J. Enol. Vitic, 65:1 (2014)), "The sensations of flavor occurs when odor-active molecules stimulate sensors in the mouth and nose, which the brain collates to produce a flavor perception." Olfactory, gustatory, and trigeminal assets cooperate in order to produce the perception of flavor but, according to the authors, smell plays an outsized role "in the overall perception of the product." Smell, as defined by the authors, "is a biological and electrophysiological process that converts the molecular information of an odorant into a perceptual response."

We continue with some baseline definitions. An odor is a volatile compound, or combination of volatile compounds, that stimulates the olfactory organ to register a smell. The odor threshold of a compound is the lowest concentration at which its smell can be detected. The perception threshold is the minimum detectable concentration for 50% of a group of tasters while the recognition threshold is the minimum concentration of that compound necessary for identification of the odor.

If we relate the foregoing to wine, quality wine can be characterized as having complex associations of aroma compounds that exceed the odor threshold; and, for a subset of tasters, exceed the recognition threshold. But what are the sources of these odor? The figure below captures those sources at a high level.

As the figure shows, wine odors are the sum of the odors from the grape, maceration, yeasts, alcoholic fermentation, malolactic fermentation, and aging. Additionally (discussed as an odor source by Robinson et al., but not described graphically above), "chemical changes associated with acid and enzyme-catalyzed modification of both non-aroma-active and aroma-active grape constituents" are also sources of wine odor. The Wine Institute characterizes these odors as shown in the table below.

Over the course of a number of future posts I will delve deeper into these sources and attempt to show how the interaction of the odor compounds aid in the perception of a quality wine.


©Wine -- Mise en abyme

Diurnal temperature shifts and wine quality

Diurnal temperature range (DTR) refers to the difference between the highest daytime temperature and the lowest nighttime temperature.

Source: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter3/daily_trend4.html

As it relates to wine, many estates/regional promoters tout the fact that their estates are located in areas of high diurnal variation because of, as they see it, the benefits conferred on grapes grown in the region.  For example, washingtonwine.org states "One of the greatest natural phenomena for growing grapes which end up balanced between ripe sugars (which will equate to alcohol in the wine) and crisp acidity is a difference between day time and night time temperatures ..."  The site goes on to say that Washington State has some of the most dramatic fluctuations in the world with differences of as much as 40℉ between the high day and low night temperatures.  The organization sees the cool evenings as preserving malic acid in the grape which "translates through fermentation to wine and adds freshness and balance."  It should be noted here that the organization sees wine balance as a mediation between sweetness and acidity, a position that runs counter to the definition that has been established on this blog.

Concha y Toro, one of the leading Chilean producers, sources grapes for its Don Melchor label from the Puente Alto vineyard in Maipo Valley.  According to the winery's website (conchaytoro.com), "It is the sharp diurnal temperature differences that assist tannin ripening and the development of aromas as well as fixing exceptionally high quantities of polyphenol compounds in the grape."  In their discourse on the benefits of high diurnal variation, Concha y Toro focuses exclusively on its perceived benefits for the secondary metabolites versus the primary metabolite arguments of washingtonwine.org but, in both cases, the argument is "high DTR is beneficial for the grape and its end product, the wine."  That is not a universally held position.

There is a second school of thought which decries significant DTRs and views balanced wines as a product of balanced temperatures.  The principal of this school is John Gladstone (Wine, Terroir, and Climate Change) but philosophical adherents are sprinkled throughout the industry (see, for example, Climate and the ripening process, wine business.com, August 2007; What makes LI wines "cool", Suffolk Times, March 31, 2011).  The basic tenet of this school is that a low temperature range during the growing season creates wines of the best quality and that sites with this characteristic tend to be in maritime or high-latitude locations.

According to adherents, the factory that is the vine operates on all cylinders during the daytime with the presence of light allowing for photosynthesis and the accumulation of sugars into the berry.  Photosynthesis ceases with the onset of darkness, but, according to this school, if the nighttime temperature does not fall below a certain level, respiration and flavor and tannin synthesis will continue, resulting in more rapid and complete phenolic ripening of the fruit at lower sugar levels.  Diurnal variation in arid regions, according to this school, allows production of high levels of sugar during the daylight hours and the cool nights take the vines out of the effective metabolic range.  This slows the phenolic ripening process thus allowing the accumulation of higher sugar levels over the longer ripening period and an unbalanced wine.

At this moment let us take a step back and look at what is known regarding temperature effects on berry development:

Higher temperatures

• increased rate of sugar accumulation
• increased rate of organic acid degradation
• inhibition of anthocyanin development

Lower temperatures

• reduction in the rate of sugar accumulation 
• reduction in the rate of organic acid degradation.

We also know that phenolic ripeness is highly temperature dependent (Mark Greenspan, wine business.com) and that phenolic ripening processes will operate sub-optimally or shut down as you move further away from the optimal (Mark Greenspan; Louisa Hargrove, Suffolk Times).

Based on the foregoing, the following Greenspan adages still hold true:

• In arid regions, cool nights are essential for the prevention of rapid acid metabolism
• Different wine styles are produced in high-DTR versus narrow-DTR areas
• Areas with narrow-DTRs generally gain phenolic ripeness at lower sugar levels than do their high-DTR counterparts

© Wine -- Mise en abyme

Oak: Its contribution to wine aroma and overall quality

Wine odor is one of the key markers of wine quality and, as a part of my series on wine quality, I have set myself the task of identifying and characterizing the sources of wine odor and showing how the interactions of these odor components aid in the perception of wine quality.

Wine is aged in wooden barrels to: (i) enhance its flavor, aroma, and complexity through transfer of substances from the wood to the wine; and (ii) allow gradual oxidation to occur.  As a result of its "strength, resilience, workability, and lack of undesirable flavor," oak is the wood of choice for most wine cooperage applications.

The oak used in the maturation of alcoholic beverages fall into one of three species: Quercus alba, Quercus robur, and Quercus sessilis.  Q. robur and Q. sessilis, and their respective subspecies, are European white oaks while Q. alba is the source of 45% of the white oak lumber produced in the US.  American oak used in barrel production is sourced from Kentucky, Missouri, Arkansas, and Michigan but there is no apparent regional distinction.  European oak, on the other hand, may have designations which reach all the way to the forest from which the oak originated.  For example, French oak from the department of Alliers may be sourced from a forest named Tronçais.

Sources: enologyinternational.com; Dr. Murli Dharmadikari; Principles and Applications in Wine Science

Oak wood used in the production of wine barrels must possess the following characteristics (Ronald S. Jackson, Principles and Applications in Wine Science): straight-grained; strong; resilient; fault -free; and free of undesirable odors that could taint the wine.  Once the tree is harvested, the resulting log is first cut into tubular sections and they are, in turn, split into halves, quarters, eighths, and then the final stave size.

The rough-hewn staves are now ready to be seasoned.  The cut staves are stacked in the open air in alternating east-west, north-south rows for a period of about three years.  Open-air drying exposes the staves to wind, rain and UV rays.  The stacks are dismantled and randomly re-assembled annually in order to ensure that a single barrel does not gain an inordinate number of staves from a single tree.  Some producers choose to kiln-dry the staves but, while attainment of the desired humidity levels occurs earlier, it can result in the loss of as much as 70% of desirable compounds.

The oak staves undergo a number of chemical changes as a result of seasoning: (i) ellagitannins are polymerized and become less soluble; (ii) there is an increase/decrease of lignin degradation products (such as eugenol, vanillin, syringaldehyde) in the outer portion of the staves; and (iii) the leaching and degradation of phenolic compounds (such as tannin) by oxygen, rain, and UV radiation.

After seasoning is completed, the staves are cut to their final sizes and then assembled into a shape approximating that of the final product except that one end is open and there are spaces between the staves (these spaces widen as one travels closer to the open end of the assembly).  A heat source is introduced into each proto-barrel through the open end and heat is applied to the inner surface for a duration, and at an intensity, consistent with the level of toast desired for the finished barrel.  The table below shows the chemical changes to the oak that result from varying toast levels.

As stated previously, wine is aged in wooden barrels to: (i) enhance its flavor, aroma, and complexity through transfer of substances from the wood to the wine; and (ii) allow gradual oxidation of the wine. In the first instance, many of the wood's native aromatic compounds, as well as the aromatic compounds created during seasoning and toasting, are absorbed, and integrated, into the wine, thus contributing to wine richness and aromatic complexity.  For example, hemicellulose will hydrolyze upon exposure to wine, creating, as a result, sugars and acetyl groups.  The sugars are further converted to furanaldehydes and ketones while the acetyl groups are converted to acetic acid during maturation.  A small proportion of lignin will dissolve in wine (these are called native lignins) while some undergo ethanolysis and are oxidized to aromatic compounds.  These compounds have low olfactory thresholds and will, therefore, impact the wine's aromatic profile. As noted by Dr. Murli Dharmadikari, common descriptors of oak-aged wines are oaky, vanilla, smoky, toasty, spicy, and coconut.

In terms of gradual oxidation, wine loss from barrels amount to approximately 2% per year, resulting from the fact that water and ethanol are smaller molecules and will diffuse into the wood and, ultimately, escape as vapor.  If the air in the cellar is dry, more water is lost and the wine is more concentrated in terms of alcohol.  If the environment is too humid then more alcohol is lost, reducing the ethanol content in the remaining wine.  This loss of liquid opens up a space between the wine surface and the barrel which the winemaker generally "tops up" in order to prevent oxidation and acetic spoilage.  During this "topping-up" process, small amounts of oxygen are dissolved in the wine.  Oxygen is also introduced into the wine during winery operations such as filtering and racking.

The oxygen which is now in the wine reacts with resident phenolic compounds (pigments and tannin) in a manner such that: (i) the red color in wine is stabilized and enhanced (monomeric anthocyanins combine with tannins to form stable polymeric pigments); (ii) tannins are softened (polymerization and precipitation as well as tannin-polysaccharide combinations); (iii) complex aromas develop; and (iv) there is improvement in the wine's body and mouthfeel.  It should be noted here that the tannin resident in the wine at this time is a combination of grape tannin plus the oak tannin absorbed from the barrel (30% from the innermost four millimeters of wood).

While the discussion above has been limited to oak cooperage, winemakers have been seeking similar benefits -- at significantly reduced cost  -- by using cooperage substitutes.  Using substitutes such as oak chips and staves allow for the transfer of oak flavors to the wine while supplemental techniques such as micro-oxygenation and the use of neutral barrels will facilitate controlled oxidation, the other significant benefit of oak barrels.

For wines with the appropriate phenolic structure, oak maturation can be especially beneficial to the quality of the finished product.


© Wine -- Mise en abyme

The science of viticulture: Climate and site selection

Quality grapes are a precursor of quality wine and the science of viticulture has developed and evolved with a single goal in mind: the delivery of high-quality wine grapes to the winery.  The quality of wine grapes produced in a specific harvest is not only a function of that year's harvest conditions.  Rather, it is the result of a combination of factors which, together, represent the full scope of viticultural science.  I will cover most of the inherent elements of viticultural science in a series of posts as a part of my investigation of the source of grape-derived odors.

There are three major areas of concern for the viticulturist as he/she sets about the task of delivering quality grapes to the winery door: (i) selecting the site which will best ensure goal attainment; (ii) setting up the vineyard with the appropriate elements such that cost-effective goal-attainment is promoted; and (iii) implementing a cost-effective, repeatable vineyard management regime which is reflective of the operating conditions.  Within these major considerations there are a number of sub-elements where the rubber really meets the road and I will focus our coverage of the topic on these areas beginning with today's writeup on climatic considerations in site selection.

The site selected for a new vineyard will determine the amount and quality of fruit produced, the resources required to manage the vineyard, and, ultimately, the profitability of the vineyard.  Selecting a site for a new vineyard is generally a compromise between a number of factors.  For example, most of the exceptional vineyard sites in the world have been under vine for many a year, leaving less-than-perfect options available for the aspiring vineyard owner.  Second, most prospective vineyard owners are drawn to sites that are readily accessible to them and this limiting facor comes with a given climate.  And so on.  Site selection is thus the process of making an optimal choice within the bounds provided bythe needs of the wine grapes, the available site options, and associated limiting factors. 

Source: Compiled from arcserver2.iagt.org

The key site-selection factors for consideration are climate and site physical characteristics.  Climate, according to Dr. Tony Wolff (Lecturer and Viticulturist, Virginia Tech) and John D. Boyer, is the average course of weather in a region over an extended period as measured by temperature, precipitation, and wind speed, among other variables (Vineyard Site Selection, Virginia Cooperative Extension).  Weather is itself defined as the state of the atmosphere at a specific point in time using the same variables as referenced in the climate definition above.  The climate of a grape-growing region will determine, to a large extent -- and all things being equal -- both the grape varieties that can be grown and the styles of wine that can be produced.  For example, Syrah appears to flourish in warm climates while Riesling does best in cold.  That is not to say that these varieties cannot be grown outside of these environments; that is to say, however, that varietal typicity is compromised when these varieties are grown outside of their "zones."

As it relates to the wine regions of the world, the ideal climates for vitis vinifera are Mediterranean and marine west-coast climates which are both characterized by mild, wet winters and warm, dry summers.  The mild winters promote long-term survivability of the vines (and increased quality of the juice as the vines age) and the wetness provides a reservoir of water that the vine roots can tap into during the grape maturation cycle.  The warm, dry summers provide the heat and light that are the engines of vegetative and crop growth while keeping at bay the threat of rot and flavor dilution that would accompany summer/fall rains.

Source:www.buywineonline.co.uk

In viticulture, three separate aspects of climate are normally considered: macro-climate, meso-climate, and micro-climate.  Macro-climate refers to climatic effects over large (hundreds to thousands of miles) geographic areas and are either continental or the aforementioned maritime.  Continental climates are modified by large land masses and are characterized by hot summers and cold winters.  Maritime climates, on the other hand, are modified by proximate large bodies of water which heat up and cool down at a slower rate than does the adjoining land mass.  This scientific fact results in the warming of winter winds as they blow over a warmer body of water and the warming of landside vineyards as the winds make landfall.  This warming could act to extend the growing season and minimize the potential vine impact of winter low-temperature events. On the other side of the coin, warm spring air blowing in over the still-cold water will be cooled down and will retard the development of landside vineyards, minimizing their potential for damage from spring frosts.

Meso-climate covers a much smaller area than does macro-climate and is generally the scale at which site decisions are made.  It is at this level that that the physical aspects of the surroundings -- elevation, slope, aspect -- can temper broader macro-climatic effects.  The climatic effects of these physical elements will be covered when they are discussed individually.

Micro-climate are the conditions that exist in the vineyard from the soil upward into the vine canopy and, as such, is more relevant when the land is under vine than in the site-selection phase.

One of the key grape needs is adequate sunlight and heat to allow both the fruit and the vegetative aspects of the plant to mature.  The progression of the grape through its various stages of maturity is influenced by the ambient temperature with research indicating that growth of the grapevine begins when temperature exceeds 10℃.  A measure -- growing degree days (GDD) -- has been developed to measure the accumulation of heat (as measured by temperature) in excess of 10℃ over a growing season.  Extensive research has yielded the following GDD parameters which can be used as input in the site-selection dialogue.

Source: Compiled from oregonviticulture.net

These then are the broader climatic considerations for the viticulturist in selecting a site for a new vineyard for the production of quality wine grapes.  The physical characteristics that should be evaluated will be covered in the next post on viticultural science.


© Wine -- Mise en abyme

Odor as an indicator of wine quality: Mapping the sources

Most of the stock photographs of wine notables that you encounter show them holding a glass of wine to their noses.  This is no accident as wine odor is one of the key markers of wine quality and they want to be shown either assessing the quality of the wine or wallowing in the enjoyment that accrues to a person who is fortunate enough to be sniffing a high-quality wine.  According to Vincent Ferreira (Laboratory for Flavor Analysis and Enology, University of Zaragoza), "The most relevant notes of great wines are caused by complex associations of aroma compounds playing different notes ..."  And it is these compounds and their notes that will be the focus of a series of posts (beginning with this one on the sources) on odor as an indicator of wine quality.  The arrow on the figure below shows our current position on the Wine Quality Assessment Framework.

We begin with some baseline definitions.  An odor is a volatile compound, or combination of volatile compounds, that stimulates the olfactory organ to register a smell.  The odor threshold of a compound is the lowest concentration at which its smell can be detected.  The perception threshold is the minimum detectable concentration for 50% of a group of tasters while the recognition threshold is the minimum concentration of that compound necessary for identification of the odor.

If we relate the foregoing to wine, quality wine can be characterized as having complex associations of aroma compounds that exceed the odor threshold; and, for a subset of tasters, exceed the recognition threshold.  But what are the sources of these odor?  The figure below shows the sources of wine odors.

As the figure shows, wine odors are a sum of the odors from the grape, maceration, yeasts, alcoholic fermentation, malolactic fermentation, and aging.  The Wine Institute characterizes these odors as shown in the table below.

Over the course of the next few posts I will detail the elements that ensure the production of quality wine grapes, characterize the sources of wine odor, and, finally, show how the interaction of these odor compounds aid in the perception of a quality wine.


© Wine -- The View From Orlando

The role of tannins in wine balance

Wine quality, as perceived by the consumer, has a number of elements, an important one of which is wine balance.  I have defined wine balance in a previous post but will reproduce Dr. Bruce Zoecklein's (Virginia Tech Enologist) Palate Balance Equation here to highlight the included elements.

              Sweet ⇄ Acid + Phenolics (Astringency and Bitterness).

I have treated the sweet (alcohol) and acid elements of this equation in prior posts and will focus on the phenolics element in this post.  The placement of phenolics (tannin) and wine balance in the quality assessment framework is indicated by the arrow in the figure below.

So what are tannins?  According to the indefatigable Dr. Zoecklein (Enology Note #116), tannins are "a heterogeneous group of phenolic compounds" with properties to include: astringency (caused when the tannin binds with protein in saliva; evidenced by mouth pucker and a bitter aftertaste); bitterness; the ability to react with ferric chloride; and the ability to bind with proteins.  A key characteristic of phenols, according to Dr. Zoecklein, is the ability to associate with (polymerize, in scientific lingo) themselves and other compounds thus yielding larger molecules.  The degree of polymerization (an actual metric) tends to increase with the passage of time.

There are two main types of tannins: hydrolizable and condensed (more properly called proanthocyanidins).  Hydrolizable tannins are found in the bark of oak and other plants and are formed in the growing tree for the purpose of food storage. These tannins are called hydrolizable because they can be broken down into smaller components in the presence of an acid or water.  Condensed tannins are insoluble and are found in tea, pomegranates, and the seed, skins, and stems of grapes.

Grape tannins are a combination of compounds (cathecin, epicathecin, epigallocathecin) which link up in chains and of which at least two need to be present for the compound to be termed a tannin. Seed tannins weigh, on average, 3.5 - 5mg per berry while skin tannins weigh in between 0.5 and 0.9 mg.  Seed tannin polymers are shorter than skin tannin polymers (the longer the tannin chain the higher the astringency) yet seed tannins are perceived by winemakers to be harsher, greener, and more astringent than skin tannins and that is evidenced in the way that the berry is handled once it enters the winery.  Oak tannins are astringent in tree matter and need to be seasoned and toasted -- as a part of the barrel treatment -- in order to increase their usefulness.

Grape tannins accumulate during the first period of berry growth with skin tannin synthesis beginning earlier than seed tannin synthesis and then ending with the conclusion of the first phase of growth. Seed tannin synthesis continues into the early period of berry ripening before concluding.  Both skin and seed tannins continue to mature during the berry ripening phase.

Tannins release is a function of how the grapes are handled in the winery.  Tannins are not desirable in white wines so white wine grapes are pressed lightly and there is no contact between the juice and skin.  In the case of red wines, tannins are desirable for color, mouthfeel, and aging and there is extensive skin contact. The amount of tannins released are a function of skin thickness (Cabernet Sauvignon, Nebbiolo, and Syrah are thick-skinned and thus release more tannins than do thin-skinned varieties like Pinot Noir, Gamay, and Cabernet Franc), the length of maceration, and the number of cap punch-downs to which the must is subjected.  Skin tannins release early and easily (as they are water soluble) but then plateau.  Seed tannin release is slow, steady, and long and requires alcohol as a solvent.  Once grape tannins are in solution, 80% of them undergo one or the other of the structural changes listed below:

• Tannin-tannin linkages
• Oxidative change
• Acid-catalyzed reactions
• Bind with anthocyanins (color compounds) to form polymeric pigments that ensure long-term color stability
• Formations that provide structure and mouthfeel to wines.

Grape tannins provide color, flavor, structure, and texture to the wine and serves a preservative function.  Oak tannins play an essential role in wine maturation in that they: (i) promote oxidation products (react with oxygen in the presence of a transitional metal to release activated oxygen which, in turn, oxidizes alcohol to acetaldehyde);  (ii) produce astringency; and (iii) aid in the removal of off-notes.  Tannin-anthocyanin complexes sediment out of wines as they age resulting in browner, less tannic wines.

Tannin affects wine balance in the following ways:

• The lower the tannin levels, the greater the amount of acidity the wine can support; conversely, the higher the tannin levels, the lower should be the acidity
• High-acid, high-tannin wines tend towards astringency
• Too much tannin results in wines that are heavy on the palate, lacking in finesse, and possessing a rough finish
• Increasing alcohol content increases the intensity of bitterness and decreases the sensation of astringency
• Low alcohol levels will result in dominant acidity and astringency and harsh, thin wines
• Lowering wine pH increases the astringency of the tannins.

The winemaker has a number of tools at his/her disposal to increase/reduce tannin levels as required and I will discuss those tools in a follow-up post.

© Wine -- The View From Orlando

The role of acidity in wine balance

Now that I have provided a contextual framework for wine balance, I can return to the promised discussion of acidity, one of its constituent elements.  As a refresher on acid's relationship to balance, Dr. Bruce Zoecklein's (Virginia Tech oenologist) Palate Balance Equation is reproduced below. The arrow on the figure following the equation illustrates where balance and acidity fall within the wine quality assessment framework.

              Sweet ⇄ Acid + Phenolics (Astringency and Bitterness).

Acids play an important role in the cellular and metabolic functions of the grape berry and in the color and texture of the fermented wine.  The precursors of acid are formed in the leaves of the grape plant and are transported to the berries where they are synthesized to acids.  Acid accumulation begins at the start of berry development and continues unabated until the beginning of the ripening process.  Acid levels tend to vary acording to the controlling temperatures of the growing region; in warmer regions acid is used up during respiration, resulting in lower acidity levels in the fruit at harvest.  Conversely, acid levels are higher and sugar levels lower in cooler-climate growing regions.

The primary acids found in grapes and fermented wine are tartaric, malic, and citric acids as well as the tartaric and malic derivatives potassium hydrogen tartrate (cream of tartar) and potassium hydrogen malate.  Tartaric acid -- which occurs in nature in fruits such as grapes, bananas, and tamarinds -- represents between 50% and 66% of the acid content in a ripe berry and, as such, controls the acid content in the finished wine.  The tartaric acid level falls off as the grape ripens but not as much as in the case of malic acid.  Crystallized tartaric acid precipitates out of the wine during fermentation and can form crystals on the underside of the wine bottle cork if the wine is stored below 50ºF.  Tartaric acid is resistant to attack by wine microbes (and thus lends ageability with lower spoilage risk to the finished product) and is the winemaker's material of choice if/when a decision is made to add acid to a wine.

Malic acid is the second most important contributor to grape acid levels with amounts ranging between 23% and 40% of the total acid content.  The grape utilizes malic acid during respiration at a rate higher than for tartaric acid, leading to a higher ratio of tartaric-to-malic acid at harvest than at the earlier stages of fruiting.  Unlike tartaric acid, malic acid can be metabolized by a number of organisms and winemakers take advantage of this fact to to reduce wine acidity through malolactic fermentation, a process wherein the bacteria convert the hard malic acid to the softer lactic acid and carbon dioxide.  Malolactic fermentation increases the wines aging potential as the bacteria that metabolize the malic acid also scavenge remaining nutrients and, in so doing, reduce the potential for future microbial spoilage.  Malolactic fermentation occurs post-alcoholic-fermentation and is automatic for most red wines and selected whites.

Acetic acid is produced during fermentation by the conversion of ethanol to acetic acid by a species of  Acetobacter or from the actions on glucose by selected anaerobic bacteria.  The acid is present in most wines at levels of approximately 0.5 g/L and is detectable by humans as a pungent odor at levels of 1.0 g/L and above.  The legal limit for acetic acid in wine is 1.2 g/L in California and 1.4 g/L elsewhere in the U.S.  Acetic acid boils off when heated and as such is referred to as volatile acidity.

The winemaker needs to know the acid content of the grape and must in order to: decide when to harvest; determine pre-fermentation must treatment; monitor wine stability; and comply with TTB requirements of 0.5% minimum acid levels.  Total acidity is the sum of the hydrogen ions of both fixed and volatile acids that are present in the wine and, as such, is the most accurate representation of acid concentration.  Total acidity is difficult to measure accurately, however, and so the more easily measurable titratable acidity (TA) is used as its proxy.  Acids and bases neutralize each other to water so the acidity of a liquid can be approximated by determining the amount of an alkaline solution that is required to neutralize it to water.  The acidity level revealed in this manner is called the substance's titratable acidity.  Red table wines generally range between 0.6% and 0.7% TA as levels below 0.4% render the wine susceptible to infection and spoilage.

A second method for measuring the acidity of a wine is through observation of its pH (potential of hydrogen) level.  The higher the number of hydrogen ions (H+) in a liquid, the more acidic it is while the higher the number of hydroxide ions (formed when an oxygen ion bonds to a hydrogen ion and represented as OH-) in the liquid, the higher its alkalinity. The pH scale (illustrated below) runs from 0-14 with acidic solutions falling below 7,

Source: epa.gov

7 as a point of neutrality, and alkaline solutions falling between 7 and 14.  A change of 1 unit on the scale represents a 10-fold change in pH.

The pH level of a wine affects the way it is perceived by the wine drinker as well as its reaction to micro-organisms.  Low-pH wines are generally viewed as sour and render tannins more astringent but they also  limit micro-organism growth.  Higher pH provides a more favorable environment for micro-organism growth and reduces the functionality of sulfur application.  White wine pH ranges between 3.0 and 3.3 while red wine pH falls between 3.3 and 3.5.  Low pH values are often correlated with high TAs and vice versa.

To summarize, acid gives wine a tartness and freshness while countering the effect of sweetness and magnifying the astringency of tannins.  If a wine has too much acid it will be puckery and sour; too little and it will be flat, flabby, and dull.  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 de-acidify using potassium bicarbonate or calcium carbonate or dilute the wine with water or a low-acid wine.  If the wine has too much sugar, it will be flabby and cloying and will not refresh the palate.  In such a case the winemaker may choose to acidify the wine by adding tartaric, malic, or citric acid.

Whether naturally obtained or engineered, appropriate acidity is a key element of wine balance.

© Wine -- The View From Orlando

Development of A Framework for the Assessment of Wine Quality

My current objective is to develop of a framework for assessing wine quality and that effort was launched with yesterday's post on the definition and high-level scope of wine quality.  I herein examine three efforts which seek to define the elements of wine quality and then use the learnings resulting from these explorations to construct a broad-based framework for the assessment of wine quality.

The first study considered was Stephen Charters study titled The Intrinsic Dimensions of Wine Quality: An Exploratory Investigation.  In this study the author conducted interviews and focus groups with 105 participants and, as a result of that effort, developed the quality dimensions contained in the figure below.

The Dimensions of Wine Quality (Source: Figure 1 of The Intrinsic Dimensions of Wine Quality)

The study did uncover some extrinsic wine quality factors (appellation systems, classification systems) but the focus was on development of intrinsic factors -- those identified in the glass when the wine is consumed.  It should be noted that this study was conducted in Australia using Australian participants and Australian wines.

I have a number of concerns with this study.  In my earlier post on defining the wine quality space, I indicated that customers with objective knowledge use objective cues to define quality.  In this study, the author used 105 individuals (60 consumers, with the remainder divided between wine producers and other wine industry players) with the consumer participants categorized as low-involvement (24), medium-involvement (25), and high-involvement (11).  With this obvious disparity in objective knowledge, the likelihood of getting solid input on intrinsic factors is low.  And the results bear this out.  As can be seen in the figure, a number of the components appear to me to be non-objective (drinkability and pleasure, for example) and most lack a sense of dimension (How do you measure drinkability?).  So, while the model is good for directionality, the questions raised render it unsuitable for our purposes.

The second model explored was Richard Leahy's Components of Wine Quality which appeared in a December 19, 2011 post on crushpadwine.com/blog.  Leahy divides wine quality factors into two camps: primary (sensory) parameters of wine quality and secondary elements of wine quality.  In that the scenario he describes entails judging a large group of wines, I will assume that these are intrinsic wine quality factors.  The components are, according to Leahy:

Primary Elements                  Secondary Elements
Visual                                    Balance
Aroma                                   Intensity
Taste                                     Length
Texture

My issues with this list is that: (i) it only considers intrinsic quality elements; (ii) it is presented at too high a level; and (iii) no potential values are presented for the identified elements.  Also, no material is presented which allows one to assess whether a wine is of low or high quality.

The final model evaluated was the WIne and Spirits Education Trust (WSET) Systematic Approach to Tasting Wine.  A copy of the tasting sheet, showing the categories, and potential values associated with each category, is presented below.

A few points of note: (i) This mechanism is clearly targeted at the objective individual; (ii) it clearly identifies each of the categories that exists in its model and provides potential values for each; and (iii) it posits that an assessment of quality is a journey rather than a eureka moment.  In the case where a product is being assessed for quality, the metrics associated with each parameter is provided  so thatthe customer can assess that parameter against his or her requirements.  For example, in the case of a car, the parameter miles per gallon will come with a number, 26 let's say.  In the case of wine quality assessment using this tool, we know the parameter but the value or metric associated with that parameter has to be teased out through tasting; metrics cannot be assigned to flavors or flavor intensity until the wine is tasted. For someone who is appropriately trained in the method, a quality bottle of wine would have the following characteristics:

• Appropriate quality
• Intensity and color befitting its age and variety
• Clean nose
• Intensity, development, and aroma characteristics befitting its age and variety, as perceived by the taster
• Sweetness level appropriate for the wine style
• Balance between variety, tannin, and alcohol
• Body appropriate for the varietal and wine style as perceived by the taster
• Medium-to-pronounced flavor intensity
• Flavor characteristics that are appropriate to the variety, style, and age of the wine.

Based on how well the taster feels that the wine addresses the above characteristics, it can be rated as having poor, acceptable, good, very good, or outstanding quality.

The WSET approach is comprehensive and allows an objective taster to arrive at a quality conclusion about a specific bottle of wine at a specific time.  It is conceivable that someone else tasting that same bottle may arrive at a different conclusion but that illustrates human differences (number of taste buds, for example) rather than being an inherent weakness in the approach.  The shortcoming of this approach is that it does not take account of the vast majority of consumers who will never use it.  That is to say, its lack of extrinsic quality components renders it unusable by the wine drinker who is equipped with subjective knowledge.

Let's stop at this point and take stock of the situation.  I have shown that the quality of a wine can be derived using the WSET Systematic Approach to Tasting Wine.  That tool is only suitable, however, for individuals with pertinent, objective knowledge, leaving the large mass of the wine drinking public to its own devices as it relates to assessing the quality of wine.  And assessing the quality of wine is not a parlor game; it is more often than not a precursor to a purchase decision.  A comprehensive wine quality assessment has to address the unaddressed issue in the WSET tool and provide a mechanism for the assessment of wine quality by those with a subjective bent.  We have identified a number of these subjective cues and have coalesced them with the WSET tool to create a comprehensive wine quality assessment framework.  The proposed framework is illustrated in the figure below.

The extrinsic cues can be used either singly or in combination by the customer in order to arrive at quality decisions.  For example, the customer may use the objective opinion of the wine critic as an indicator of quality and use that as a basis for a purchase decision.  Or, on the other hand, the customer may use pricing and vintage cues in order to make that decision.

In closing, I would like to note that quality in the world of wine differs from quality in most other products and services because of the variability in terroir, production methods, and, most importantly, the differences in perception that accrue to us a result of our human condition.  Wine quality, at the end of the day, is a truly personal assessment.

I will be reviewing the various elements of the wine quality assessment framework in future posts.

© Wine -- The View From Orlando

A Framework for the Assessment of Wine Quality: Defining the Space

In my most recent post I wrote about balance as a measure of wine quality and promised to detail its constituent components in follow-up posts.  As I was mulling over my approach to fulfilling this promise, it occured to me that I had not provided a context for the quality discussion.  I will correct that oversight in this blog post by developing a wine-quality framework and will discuss the wine balance components in subsequent posts.

Before we begin depiction of the wine-quality framework, I will explore some definitions of the quality concept.  ISO, the Geneva-based, international standards-setting organization, famed for development of the ISO series of quality standards, avers that "The quality of something can be determined by comparing a set of inherent characteristics with a set of requirements.  If those inherent characteristics meet all requirements, high to excellent quality is achieved."  If the requirements are not met, ISO stipulates that the product or service provided is of low or poor quality.

According to what-is-quality.com, the formal definition of quality in the U.S. is "the characteristics of a product or service that bear on its ability to satisfy stated or implied needs.  A product or service that is free of deficiencies."

The two definitions are congruent in that they both address quality levels (high and low quality in the case of ISO and quality/no quality in the case of the U.S.) but they appear to differ in the degree of rigor associated with requirements-identification.  ISO refers to a set of requirements while the U.S. standard refers to stated needs, which gives the impression of customer-driven requirements.  But the U.S. standard further refers to implied needs inferring, in my opinion, that by buying a product or service, a customer is admitting to a set of needs, regardless of whether those needs are formally stated or not.

According to Clodfelter and Fowler (Do Consumers' Perceptions of Product Quality Differ from Objective Measures of Product Quality?), "Quality is a multi-dimensional construct that cannot be equated with or measured by a single cue or attribute." In assessing quality, customers call on objective (accurate and current information on relevant measurable and verifiable standards) or subjective (their judgement about a product's/service excellence) knowledge.  Individuals with objective knowledge will respond to intrinsic -- cannot be changed without changing the nature of the product or service -- cues while the consumer with subjective knowledge will respond to extrinsic -- related to, but outside of, the offered product or service. In the wine world we can think of minimum alcohol level as an intrinsic cue and price as an extrinsic cue.

With quality defined in the foregoing, we should be pivoting to an identification of the elements of wine quality.  But it is not that easy.  The first level of difficulty is encountered when seeking a consensus on the elements that should be included in a wine quality framework; or whether quality should be broadly discussed at all.  The great French oenologist Emile Peynaud, for example, felt that quality was such a personal thing that it only existed in relation to an individual.  In the cases where components of quality are identified, no qualitative or quantitative values are associated with the components.  And, finally, no attempt is made to differentiate between the relative contribution of each component to the overall quality measure.

I will examine a number of wine quality schemas and propose a comprehensive framework in my next post.

© Wine -- The View From Orlando

Wine balance as a quality measure

According to Dr. Bruce Zoecklein, Head of the Enology/Wine Chemistry Group at Virginia Tech, wine can be broken down into the three sensory categories indicated in the table below.

                              Wine Sensory Components

Structure                                  Texture                               Flavor
Sweet                                       Light (delicate)                   Nutty
Acid                                         Rich (dense)                       Earthy
Astringency                                                                         Herbal
Bitterness                                                                            Smoky
                                                                                            Spicy
                                                                                            Berry
                                                                                            Tropical Fruit
                                                                                            Apple*
                                                                                            Citric*
                                                                                            Pear*
* Found mainly in white wines

Source: Derived from Figure 1 of Matching Food and Wine.

As regards the structural components, Dr. Zoecklein argues that a balanced relationship must exist between the tastes of sweetness, on the one hand, and acid, astringency and bitterness on the other, in order to yield the perception of a quality wine to the taster.  The preferred relationship is captured in his Palate Balance Equation (Zoecklein: Components of Red Wine Mouthfeel):

           Sweet ⇄ Acid + Phenolics (Astringency and Bitterness),

where

          Sweet = Carbohydrates + Polysaccharides + Ethanol,
          Acid = Population of organic acids, and
          Phenolics = Skin, seed, and stem phenols + barrel phenols + enological tannins +             volatile phenols.

Based on the foregoing equation, an increase in the sweetness element of the equation will lead to a reduction of the taster's perception of acidity and phenolics; and the reverse is also true.  Dr. Zoecklein sees this balance, or harmony, as a key indicator of wine quality and in that he is joined by Wines and Vines and Crushpad Blog among others.  According to Chris Stamp, writing in Wines and Vines, "... a balanced wine is a wine in which the various components work together to provide a pleasing taste."  According to Richard Leahy, writing in Crushpad Blog, wines that are in balance tend to stay that way while wines that are out of balance tend to grow moreso over time.

How does balance relate to quality?  If, for example, 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 of the taster.  A wine with too much sugar, on the other hand, will be flabby and cloying and will not refresh the palate.   Such wines will not be perceived as quality wines by the taster.

Over the course of future blog posts I will be examining the components of the balance equation in greater detail in order to provide the reader with an understanding of the factors that the winemaker has to consider in his/her quest for balance, one of the key measures of a quality wine.