Wine Composition and Chemistry Flashcards
What is wine?
Wine is the fermented product of grapes.
Although strawberry wine and rice wine exist, for example, the unmodified term “wine” refers to grapes in the US.
From a scientist’s perspective what gives wine its unique aromatic, flavor and tactile characteristics?
Chemical compounds present in the grapes, created by fermentation or that occur along the way.
What are the primary components of wine?
Water Alcohol Acid Sugar Phenolic Compounds
What percentage of wine is comprised of water?
80-90%
What percentage of wine is comprised of alcohol?
10%-15%
What does alcohol contribute to our perception of the wine?
It carries the wine’s aroma to our nose as it evaporates;
It contributes to the wine’s body and weight;
It can result in slow, thick-appearing “tears” or “legs”
What types of alcohol can be found in wine?
Ethyl alcohol or ethanol;
Small amounts of glycerol, methanol, fusel alcohols/higher alcohols
What percentage of wine is comprised of acids?
.5% - .75%
What acids are most commonly found in wine?
Tartaric - highest in Ph, with tendency to form crystals, or tartrates at low temperature
Malic - tart, like green apples, found in underrripe and cool-climate grapes
Citric - exists in minute quantities but can contribute to the total acidity of the wine.
Lactic - not found in grapes, created by lactic acid bacteria by a process called malolactic conversion; contribute a softer mouthfeel and creamy or buttery characteristic.
Acetic - a volatile acid which can contribute to a wine’s complexity, but higher concentrations produced by a chemical reaction between ethanol and oxygen caused by acetobacter, can be considered a fault.
Succinic - sharp, bitter or salty, a minor component of the grape and a normal byproduct of fermentation.
Which acids are found in grapes and which are produced by fermentation?
Grape - tartaric, malic, citric, succinic
Fermentation - lactic, acetic, succinic
What two measures are used to describe a wine’s acidity?
Total acidity which measure volume;
Ph which measures the chemical strength of the acid
What kind of scale is used to measure Ph
A logarithmic scale, so the lower the Ph, the stronger the acid
Why do winemakers place a particular emphasis on Ph?
It gives an indication of stability and plays a role in sulfur additions.
What is the Ph of most wines?
2.9 - 3.4
What percentage of sugar do grapes contain at harvest?
15% - 28%
What two types of sugars are found in grapes?
Glucose and Fructose, two high fermentable monoaccharides, or “simple sugars.”
What does residual sugar contribute to the final wine?
In addition to sweetness, the mouthfeel, body and texture of the wine.
Give three reasons a winemaker may choose to leave a small or significant amount of residual sugar in a finished wine?
To create a dessert wine, up to 24% rs;
To balance acidity;
To disguise minor flaws in a lower quality wine
What are phenolic compounds and what other names to they go by?
Molecules that can impact the sensory profile of the wine;
Phenolyics, polyphenolics, polyphenals
Names five categories of phenolic compounds.
Anthocyanins, Flavonols, Tannins, Vanillin, Reservatol
What phenolic compound determines the color of a red wine?
Anthocyanins, together with acidity. ranging from blue to purple to red. More acidic wines generally appear redder.
What phenolic compound determines the color of a white wine?
Flavonols, increase with exposure to sunlight and give wines their golden color.
What are tannins?
Astringent, bitter compounds found in the skins, stems and seeds of the grapes, as well as in oak. They form the structure or “backbone” of many red wines, and help to preserve and protect the wine from oxygen. Tannins can create a textural drying sensation in the mouth.
Where does vanillin come from?
Vanillin is present in oak and imparts a vanilla aromas to barrel-aged wines.
What is reservatrol?
A phenolic primarily found in the skins and seeds of grapes that is believed to have beneficial health effects in humans.
What effect does the polymerization of phenolic compounds have on wines?
It creates a sediment;
Makes the wine less astringent;
Makes the wine lighter in color
What other components are present in wine?
Aldehydes, esters, gases and sulfites.
What are aldehydes and how are they formed?
Oxidized alcohols that are formed when wine is exposed to air. All wines can take on an oxidized or materized character if exposed to excessive O2 during fermentation or maturation, but some wines, such as sherry, are encouraged to form aldehydes. The most common type is acetaldehyde, which contributes a tangy aromas.
What are esters and how are they formed?
The largest group of aroma compounds, formed by the joining of an acid and an alcohol. The most common is ethyl acetate, which imparts a fruit or flower aromas at low concentrations but can contribute a faulty varnish or glue characteristic at higher ones.
What are the two most common dissolved gases found in wine?
O2 from oxidation and CO2, a natural byproduct of fermentation.
Oxidation can be prevented by avoiding exposure to air or with sulfurs that absorb free oxygen molecules.
CO2 can add petillance to wine for a fresh, lively effect.
What are sulfites?
A glass of chemicals based on the element of sulfur, and is used as both an antioxidant and an antiseptic to stabilize the wine.
It is a natural byproduct of fermentation, so all wines contain traces of sulfites, even if none are added by the winemaker.
People can be extremely sensitive to sulfites, so in the US, wines destined for interstate commerce containing more than 10 parts per millionth of SO2 must say “Contains Sulfites” on the label.
