Practical Applications for Aroma Development in Wine
By: Denise M. Gardner
This post is a follow up from a previous Pennsylvania Quality Alliance (PQA) meeting featuring Alain Razungles. An introduction to wine aroma and sensory can be found here: http://bit.ly/WineAromaSensory This is part two of a two-part series. Part one focused on specific points and studies that Alain Razungles, French enology professor, discussed during his visit to the eastern U.S. You can review Alain’s talk by visiting the Penn State Extension Enology website here. Part two emphasizes pratical applications of those points and how to utilize the information from Alain’s talk in a vineyard or winery.
Picking Grapes for Flavor
A good quality measure that a winemaker can utilize in the winery is learning to pick grapes for flavor. More often than not, grapes in the east are picked on Brix and pH readings over a progression of time. In the Mid-Atlantic, sometimes the need to pick due to weather restrictions also occurs. For those years in which growers can hold grapes on the vine and wait, a greater flavor development of that variety may occur (Coombe and McCarthy 1997).
As stated in Alain’s presentation, aroma/flavor development is one of the last ripening stages to occur in the grape berry. Coombe and McCarthy (1997) labeled the aroma/flavor ripening stage as engustment, which occurs after sugar (Brix) accumulation has stopped or severely slowed down. This ripening stage is illustrated in Figure 1.
Although difficult to detect aroma/flavor development through most analytical means, growers and winemakers can train themselves in [grape] Berry Sensory Analysis, as outlined by the Institut Cooperatif du Vin (ICV) or by using the Winegrape Berry Sensory Assessment in Australia guide by E. Winter, J. Whiting, and J. Rousseau (ISBN: 9781875130412). Grape berry sensory allows an individual to better evaluate wine grapes using visual, tactile, and taste sensations that change as a grape ripens. One of those key attributes is flavor.
Overall, berry assessment is not very difficult, but it does require adequate training and practice. Berries are analyzed on a 4 point “maturity-indication” scale for several attributes that indicate pulp, skin, and seed ripeness. Sensory analysis is completed with a standardized record system that allows winemakers to easily document each grape tasting. This visual representation reflects when a grape variety comes into its engustment stage. Generally, the winemaker would choose to pick at this stage, as engustment remains active for a short length of time, usually a few days. Harvesting grapes within this engustment stage allows for the greatest free aromatic potential of the future wine.
The Role of [Aroma] Precursors in Wine – Protect Wine Aroma/Flavor
Although many people came away from Alain’s talk wanting to measure aroma precursor concentrations in berries and juice, as these precursors indicate the aroma potential of the wine after fermentation is complete, the analysis is quite complicated and time consuming. Dr. Bruce Zoecklein from Virginia Tech has featured many articles on aroma-precursor and glycoside development, concentration, and analysis for wine grape varieties (http://www.vtwines.info/).
Although difficult to analyze in a precursor form, these non-aromatic compounds are present in grapes and juice, and are converted to aroma/flavor compounds during fermentation. Therefore, conservation of aroma/flavor precursors in grapes and wines is related to the production of volatile aroma/flavor compounds during fermentation and through wine aging. Retention of these precursors and their corresponding volatile compounds should remain an emphasis during wine production from harvest through retail.
Ensuring that grapes have properly ripened beyond sugar accumulation and into engustment will allow the fermentation to produce a varietal wine. Consequently, berry ripeness will determine the starting material for the future wine and dictate the quality of future aroma/development through the winemaking process. If the grapes are in poor condition, the chances of producing a poor quality wine are high. Rots, mildews, molds, bacteria, and immature grapes all contribute to off-flavors in the wine. Additionally, lack of crop maintenance can produce or retain off-flavors that may mask any varietal characteristics.
A lot of aroma/flavor development is determined by yeast and bacteria selection for primary and secondary fermentation. Yeast and bacteria (such as malolactic bacteria) utilize some aroma-precursors during fermentation, which contributes to the final aroma/flavor of the wine. Additionally, other secondary metabolic pathways in yeast and bacteria will produce aromas/flavors indirectly (Swiegers et al. 2005).
Aroma/flavor compounds are generally volatile, meaning that they can easily escape from the wine in a gas phase, and are sensed by the nose or mouth of a human. During fermentation, some aroma/flavor is lost due to the rapid production of carbon dioxide. This phenomenon is quite noticeable with early production of hydrogen sulfide, which blows of during rapid stages of fermentation and is then lost by the time fermentation is complete.
However, temperature also influences the volatility of aroma/flavor compounds. The higher the temperature, the more aroma/flavor compounds escape into the gas phase. White wines are especially susceptible to aroma/flavor loss during and after fermentation as their volatile aroma/flavor compounds are easily blown off with higher temperatures. Small changes to white wine production can help enhance wine quality with aroma/flavor retention through bottling and retail:
- The utilization of temperature controlled fermentations is an option for retaining aroma/flavor of the finished wine. Whites should be fermented below 65°F(18°C) to retain their aroma/flavor. Reducing the ambient temperature surrounding the fermentation vessel slows down the fermentation, which inhibits the fermentation from reaching high internal temperatures and retains aroma/flavor of the final wine.
- Keep white wines fairly unexposed to oxygen helps preserve aroma/flavor in the wine. Oxygen depletes aroma/flavor in two main ways. First, it contributes to the depletion of varietal aroma/flavor compounds that blow off into the gas phase. Second, oxygen is a key requirement for oxidation and other spoilage processes. These processes contribute their own aroma-active compounds that can compete with and mask the varietal aroma/flavor compounds that remain intact. Production causes of oxygen ingress include pumps, storage (headspace in tanks or barrels), racking, fining, and bottling.
- Maintaining proper sulfur dioxide levels through longer term wine storage will stabilize the wine chemically, which will protect the aroma/flavor composition of that wine as well. Free sulfur dioxide levels should be monitored and analyzed on an every-other-week to every-three-weeks basis, and more frequently if tanks contain headspace.
The Role of Dimethyl Sulfide (DMS) in Red Wines
Although DMS is a fruit flavor-enhancing compound at very low concentrations, it can also be incredibly detrimental to wine quality in “higher” concentrations, depending on the wine variety and its chemical composition. Alain demonstrated that Syrah, a variety that naturally contains DMS as part of its varietal aroma/flavor composition, when first sniffed had an earthy-based aroma, but with incorporation of oxygen (i.e. swirling in the glass), it became very fruity and jammy (Segurel et al. 2004).
Although DMS is appears to be a prevalent aroma compound in Syrah, much like 3-mercaptohexan-1-ol (3MH) is in Sauvignon Blanc, it is not a realistic additive for wines. It is, after all, a sulfur-containing compound that can contribute to a reduced or stinky nose in wine. Most winemaking practices are designed to avoid accumulation of sulfur-containing compounds like DMS because these compounds are difficult to control and often mask fruitiness of wines.
Even though sulfur-containing compounds are essential for yeast metabolism, if produced in excess they do cause a relatively undesired aroma/flavor in wine. Humans vary greatly in their ability to sense these compounds. While one individual many not smell the sulfur-containing compound, another may be overwhelmingly appalled by its aroma at the same concentration. Some of the more common sulfur-containing compounds that are aroma-active are listed below.
These compounds have very low detection thresholds (<30 ppb), which means it does not take a lot of a given compound to be sensed by most people (Table 1). It has been proposed that hydrogen sulfide can give rise to many sulfur-containing compounds (Sweigers et al. 2005). Additionally, many are present in the grapes as cysteine-conjugates, a form of sulfur-containing non-aromatic precursors (Sweigers et al. 2005). The formation of these compounds is somewhat debateable in the research literature. However, it is generally acceptable to assume that winemakers do not want to accumulate sulfur-containing compounds in their odor-active form.
How does one avoid the over production of odor-active sulfur-containing compounds?
- Proper YAN management during primary fermentation to reduce potential hydrogen sulfide formation
- Inoculate for MLF immediately following primary fermentation to avoid native MLF associated off-flavors
- Avoid long term lees contact with fruity wines
When it comes to wine, flavor development starts in the vineyard. All viticultural decisions affect the final flavor profile of the grapes, which are the starting product for the wine. For example, research has shown that over-cropping, especially Vitis vinifera varieties, can lead to increased prevalence of green flavors. In excess, these flavors are undesirable in wine as they mask the fruit aromas/flavors. Management decisions in the vineyard can ultimately lead to lesser production of green flavors through strict crop thinning practices. These decisions must start in the vineyard because the flavor problems they cause are not always easily manipulated in the wine.
Many wine processing decisions can also alter wine flavor. Whether the flavor alteration is small or large will depend greatly on the processing decision and when it is implemented. For example, improper management of headspace in tanks or barrels will have a large effect on the wine’s flavor. In comparison, altering yeast selections may have a small effect on the final wine flavor. Such decisions require a comfort level with winemaking and a firm understanding of flavor nuance in wine.
Although these two papers touch upon specific flavors in wine, there are a series of other resources available to winemakers that discuss wine flavor development:
- WineFlavor 101 Workshop Series through UC Davis: http://wineserver.ucdavis.edu/
- Bruce Zoecklein’s Enology Grape Chemistry Group: http://www.vtwines.info/
- Pennsylvania Wine Quality Initiative (WQI) workshops through Penn State Extension Enology: http://extension.psu.edu/enology
Coombe, B.G. and M.G. McCarthy. 1997. Identification and naming of the inception of aroma development in ripening grape berries. Aust J Grape Wine Res. 3:18-20.
Darriet, P. 1999.
Segurel, M.A., A.J. Razungles, C. Riou, M. Salles, and R.L. Baumes. 2004. Contribution of dimethyl sulfide to the aroma of Syrah and Grenache Noir wines and estimation of its potential in grapes of these varieties. J. Agric. Food Chem. 52: 7084-7093.
Swiegers, J.H., E.J. Bartowsky, P.A. Henschke, and I.S. Pretorius. 2005. Yeast and bacteria modulation of wine aroma and flavour. Aust J Grape Wine Res. 11:139-165.