Wine Composition and Chemistry

Question 1

What is wine?

Answer 1

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.

Question 2

From a scientist’s perspective what gives wine its unique aromatic, flavor and tactile characteristics?

Answer 2

Chemical compounds present in the grapes, created by fermentation or that occur along the way.

Question 3

What are the primary components of wine?

Answer 3

Water
Alcohol
Acid
Sugar
Phenolic Compounds

Question 4

What percentage of wine is comprised of water?

Answer 4

80-90%

Question 5

What percentage of wine is comprised of alcohol?

Answer 5

10%-15%

Question 6

What does alcohol contribute to our perception of the wine?

Answer 6

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”

Question 7

What types of alcohol can be found in wine?

Answer 7

Ethyl alcohol or ethanol;

Small amounts of glycerol, methanol, fusel alcohols/higher alcohols

Question 8

What percentage of wine is comprised of acids?

Answer 8

.5% - .75%

Question 9

What acids are most commonly found in wine?

Answer 9

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.

Question 10

Which acids are found in grapes and which are produced by fermentation?

Answer 10

Grape - tartaric, malic, citric, succinic

Fermentation - lactic, acetic, succinic

Question 11

What two measures are used to describe a wine’s acidity?

Answer 11

Total acidity which measure volume;

Ph which measures the chemical strength of the acid

Question 12

What kind of scale is used to measure Ph

Answer 12

A logarithmic scale, so the lower the Ph, the stronger the acid

Question 13

Why do winemakers place a particular emphasis on Ph?

Answer 13

It gives an indication of stability and plays a role in sulfur additions.

Question 14

What is the Ph of most wines?

Answer 14

2.9 - 3.4

Question 15

What percentage of sugar do grapes contain at harvest?

Answer 15

15% - 28%

Question 16

What two types of sugars are found in grapes?

Answer 16

Glucose and Fructose, two high fermentable monoaccharides, or “simple sugars.”

Question 17

What does residual sugar contribute to the final wine?

Answer 17

In addition to sweetness, the mouthfeel, body and texture of the wine.

Question 18

Give three reasons a winemaker may choose to leave a small or significant amount of residual sugar in a finished wine?

Answer 18

To create a dessert wine, up to 24% rs;
To balance acidity;
To disguise minor flaws in a lower quality wine

Question 19

What are phenolic compounds and what other names to they go by?

Answer 19

Molecules that can impact the sensory profile of the wine;

Phenolyics, polyphenolics, polyphenals

Question 20

Names five categories of phenolic compounds.

Answer 20

Anthocyanins, Flavonols, Tannins, Vanillin, Reservatol

Question 21

What phenolic compound determines the color of a red wine?

Answer 21

Anthocyanins, together with acidity. ranging from blue to purple to red. More acidic wines generally appear redder.

Question 22

What phenolic compound determines the color of a white wine?

Answer 22

Flavonols, increase with exposure to sunlight and give wines their golden color.

Question 23

What are tannins?

Answer 23

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.

Question 24

Where does vanillin come from?

Answer 24

Vanillin is present in oak and imparts a vanilla aromas to barrel-aged wines.

Question 25

What is reservatrol?

Answer 25

A phenolic primarily found in the skins and seeds of grapes that is believed to have beneficial health effects in humans.

Question 26

What effect does the polymerization of phenolic compounds have on wines?

Answer 26

It creates a sediment;
Makes the wine less astringent;
Makes the wine lighter in color

Question 27

What other components are present in wine?

Answer 27

Aldehydes, esters, gases and sulfites.

Question 28

What are aldehydes and how are they formed?

Answer 28

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.

Question 29

What are esters and how are they formed?

Answer 29

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.

Question 30

What are the two most common dissolved gases found in wine?

Answer 30

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.

Question 31

What are sulfites?

Answer 31

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.