Sensory interactions. Yet wine is more than the sum of its sensory parts. The characteristics we perceive must work in balance and harmony in order to be appealing in a wine. Too much sugar without acid or alcohol to balance makes the wine fat, flabby and uninteresting. Too much acid without some fruity, sweet or tannic, mouthfeeling qualities will leave the wine simply sour. Also tastes will work to suppress or partially mask one another, so that the sourness of a high acid wine is mollified by the balancing sweetness of residual sugar. Complex aroma qualities blend, mask and harmonize is a good wine. They stick out of the complex odor mixture in poor balance in low quality wines.

Wine quality assessment. A number of tools are available to the wine scientist to quantify the sensory attributes and measure the consumer appeal of wines. Sensory analysis methods using trained panels can characterize the strengths of individual wine characteristics of appearance, taste, aroma and feeling. Consumer appeal can be measured by hedonic scaling of likes and dislikes, as done with other processed foods. Quality assessments are also done in terms of identification of defects, where points are subtracted from a maximum score for sensory problems. Quality can also be assessed in ratings by small panels of experienced tasters in order to separate the merely good from the truly excellent, thus providing the consuming public with useful information about a product that has yearly, regional and stylistic variation.

A wine tasting log is used to record sensory preceptions. Print it and make copies to record your perceptions of the wines you taste at home.

Chemistry
A glass of wine is a mixture of chemicals and drinking it stimulates your body and mind. The phenomenon is a chemical reaction between an organism and a complex set of stimulants in the wine. Certainly the process of perception is mediated and modulated by brain functions (cognition, memory, emotion, etc.,) and social processes but the entire reaction is initiated by chemicals. One nontrivial exception to this description of the tasting experience would be the effect of seeing the label on a wine bottle before you drink it and the power prejudice has on human experiences. However, the physical properties of wine that sustain human interest and agricultural profitability come from the chemicals it contains. In this class we will investigate the chemicals in wine and discuss their origin and effects.

We can begin by asking the question: what are the chemicals in grape juice, what are their biologibal activities and how are they modified by winemaking? I have organized the chemistry of grape juice into levels of decreasing chemical concentration. The levels range from the "macrogram" or % level at the top to the femtogram level or 10^-15 g/g. We know a lot about the chemistry of grape juice at the macro level (water, sugar, acid and tannin) but very little at the femtogram level; however, there are chemicals that function at this level.

When we look at the chemistry of wine we see that the number of chemicals have increased and that all of the biological activities have changed. Sweetness is gone, the alcohol taste is present, new odors are present and a large amount of toxic substance has formed: ethyl alcohol.

Microbiology
Microorganisms on grapes - During ripening in the vineyard grapes can become infected by molds, yeasts, and bacteria. These microbial infections generally destroy desired flavors and color and add undesired acetic acid and oxidized flavors. The big exception is the infection of white grapes with the noble rot fungus (mold) Botrytis cinerea. Infection of white grapes with this mold leads to the concentration of the juice in the berry and adds characteristic aromas to the wine.

Fermentation - Grape must is converted into wine by a succession of various yeasts and lactic acid bacteria. To avoid spoilage and to enhance desired flavors the winemaker must understand how various yeasts and lactic acid bacteria respond to various vinification techniques. Growth of desired yeasts can be promoted, others suppressed. Physiological requirements of the winemaking yeasts must be respected so that they grow satisfactorily and without producing undesirable flavors.

Yeast - Grape must can be fermented spontaneously by the yeasts which enter the must with the grapes and from winery equipment. A winemaker can also choose to add selected yeast cultures which reliably complete the fermentation with little risk of off-flavors and in many cases with predictable flavor characteristics. Added starter culture yeasts must compete with indigenous yeasts.

Bacteria - Malolactic fermentation is a second fermentation of wine which follows alcoholic (yeast) fermentation. Typically lactic acid bacteria of the type Leuconostoc oenos grow in wine and convert malic acid to lactic acid and CO2. This decarboxylation reduces the acidity of the wine. The bacteria also modify the fruit flavor of wine and add some flavor compounds from their metabolism. Malolactic fermentation is an example of how the detailed understanding of the biochemical pathway of the conversion of malic to lactic acid and CO2 helps to produce starter cultures with desired activity and helps the winemaker to guide the fermentation. Temperature, pH, and availability of other sources of energy affect the rate of malic acid utilization. The malolactic activity of a starter culture is determined by strain characteristics, growth conditions, and the method of preservation.

Wine aging - During aging of wine in tanks and bottles, the wine flavor might be further modified by various yeasts and bacteria. These yeasts are generally considered spoilage yeasts. Depending on the type of microorganism and on the extent of growth, desirable fruit flavors can be lost or masked by unpleasant aromas and taste. Under good aging conditions, without spoilage microorganisms the wine continues to change due to chemical and biochemical (enzymes still active in the wine) conversions. Depending on wine style and personal preference an optimum combination of 1) fresh fruit flavors, 2) fermentation flavors, and 3) aging flavors are reached after as little as half a year and more than 10 years.

Winemaking
Climate & Vineyard Management: Winemaking must be understood as a continuum from site selection, cultivar selection, vineyard management, harvest practices (date and fruit handling), and vinification methods. Ian Merwin and Tim Martinson will give an overview of viticulture. Visits to New York wineries and a Adjunct Lecturer Peter Bell (Fox Run Vineyards) will demonstrate winemaking practices. The winemaker translates desired fruit flavors, color, and texture into wine. The mesoclimate of a vineyard and vineyard management practices determine which cultivars can be grown in a given site. Crop load, fruit exposure, vine nutrition, and protection against fungal infection are important parameters which determine grape ripening and final grape quality.

Juice Extraction: At different stages of grape ripening, the grapes contain different flavors. Flavor and color compounds are also located in different areas of the grape berry. With the selection of harvest date and conditions of juice extraction and skin contact the winemaker can select certain fruit qualities.

Fermentation: The grape must is fermented with various yeasts and lactic acid bacteria. A winemaker can add selected wine yeast to dominate the indigenous yeast which derive from the vineyard (grape surface, leaves, and stems) and the winery environment (press, tanks, barrels, hoses). The addition of a selected yeast starter culture ensures complete fermentation without off-odors and produces a wine of consistent flavor quality. Fermentation temperature and characteristics of the selected yeast strain largely determine the amount and type of flavors produced. During spontaneous fermentation a range of different yeasts grow at different stages of fermentation. A winemaker must carefully guide spontaneous fermentations to minimize the risk of spoilage by unwanted microorganisms. Successful spontaneous fermentations can produce very flavorful wines with a range of aromas and texture derived from juice flavors and modification by the metabolism of various yeasts. Following alcoholic fermentation by yeast the wine can undergo a second fermentation by lactic acid bacteria. During this so-called malolactic fermentation lactic acid bacteria convert malic acid to lactic acid which results in a lowering of the acidity of a wine. The bacteria also modify some other flavor characteristics of the wine.

Wine aging: After completion of fermentation wine is clarified and stabilized. Wine flavors continue to change while the wine is stored in wooden barrels, stainless steel tanks, and in glass bottles. During this maturation of wine various yeasts and bacteria can be present and further modify wine flavors. Different wines benefit from longer or only short aging. The aging potential of a wine is determined by the grape cultivar and vinification practices.