Module 7 - Phenolic compounds Flashcards
How are phenolic compounds quantitatively important in wine?
As a group they are often the next most abundant components in wine after ethanol and the carboxylic acids.
How do phenolic compounds contribute to the sensory character of wine?
They provide the red pigment of red wine, the yellow colours in white wine and the brown colours of oxidised wine.
They may influence aroma, and they certainly are the dominant contributors of the taste sensations of bitterness and astringency in wine.
How do phenolic compounds influence the winemaking process?
Major differences in red and white wine production result from the need to extract phenolic pigments from skins to gain red colour, and from the effect that red pigments have upon sulfur dioxide activity.
What are three other contributions of phenolic compounds to wine?
- influence the protein stability of wine
- play an important role in wood maturation
- participate in two important processes that are encountered in winemaking: ageing and oxidation.
Are phenols volatile?
Most phenolic compounds are non-volatile but a few volatile odorous phenols exist.
What do the terms phenols and phenolics encompass?
What are polyphenols?
All compounds with hydroxyl groups attached to aromatic rings.
Polyphenols are a class of compounds with multiple phenol rings within a single structure.
What is the term tannin used to describe?
Tannin is used to describe all polymeric polyphenols. However, from a strict chemical perspective, the term tannin does not define a chemical structure but instead declares a specific function for a substance - the ability to tan animal hide.
Structures with four or more phenolic components joined together are commonly called tannins although as described in Chapter 11, this terminology derives from the ability of the specific phenolic polymers to react with proteins in animal hide to form leather.
What factors alter phenolic content?
- Grape variety (genetics)
- Site, particularly climate, soil, aspect, sun exposure, elevation (environment)
- Vine, cluster, berry (within and between plants on the same site, there is high variability).
Where are grape phenolics largely found?
In the skin and the seed of the berry, while the juice and the pulp have much lower concentration.
Why doe red grapes have higher total phenolic concentrations than white grapes?
Due to the presence of the red anthocyanin in the skin.
Why and how are the amount and relative extraction of different phenolic classes controlled during winemaking? How is control exercised.
Due to the sensory impact of phenolics.
Control is exercised by manipulation of extraction, called maceration, as well as additions of protein and other agents to bind and precipitate tannins, a process called fining.
What are two categories are wine phenolics grouped into?
Flavonoids and non-flavonoids.
What are the general phenolic content of wines?
It varies widely, but is generally about 200 mg/L of gallic acid equivalents (GAEs) in white wines and 2000 mg/L in reds that are ready to drink.
What are the major phenols in grape juice and the major class of phenolics in white wine?
Where do they originate?
Hydroxycinnamates.
Hydroxycinnamates originate in the pulp.
What characterises flavonoids?
A three-ring system with a central oxygen containing ring (C ring) that has different oxidation states defining the flavanoid class.
What are the different classes of flavonoids and where do they originate?
What are the most abundant class of flavonoids?
- Flavan-3-ols (skins and seeds)
- Flavonols (skin)
- Anthocyanins (skin)
Flavan-3-ols are the most abundant class.
How are flavonoids derived in wine?
Derived from extraction of grape skins and seeds during fermentation and other maceration steps, and comprise the majority of the phenols in red wine.
Ethanol is a good solvent for polyphenol extraction, and over a typical 4-10 day fermentation a good portion of polyphenols are extracted into red wine.
What favours the passage of red pigment and tannins into the juice?
- breakdown of the tissue structure,
- the presence of ethanol,
- presence of sulfur dioxide,
- absence of oxygen,
- high temperatures,
- prolonged contact time of juice with the grape solids,
- agitation to ensure mixing of juice that is intimately in contact with grape solids with juice that is not.
Why is the phenolic content of wine significantly less that the levels of phenolic material in grapes?
Even if extraction of skin and seed phenolic material is encouraged, the phenolic content of wine is always
significantly less than the levels in the grape.
Extraction of phenolic material into the juice is incomplete, and some material is lost from the juice by reaction with proteins, yeast cells and fining agents.
Consequently juice and wine phenolic levels are usually 25‑50% of the grape berry levels.
What is the difference between
flavonoids and the nonflavonoids?
The flavonoids are the phenolic compounds of grape skins and seeds and include the red pigments and grape tannins; the non-flavonoids are the phenolic compounds of juice
Is the aim of extraction to maximise the extraction of phenolic material?
- No
- to be selective, and to extract colour, desirable tannin, balanced bitterness and attractive flavours with minimal extraction of less satisfactory components.
- Aspects such as time, temperature, alcohol extraction and maceration should be controlled to obtain the best character from the quality of fruit available.
What will a short time of contact of fermenting juice with skins and seeds provide a wine?
Adequate colour but with low tannin content and only slight astringency. Such wines are usually suitable for drinking at a young age, having less phenolic character to mask their freshness and fruit flavours.
This is because monomeric phenolic compounds, such as anthocyanins, are more rapidly extracted from grape solids than tannins.
However, they are generally not suited to ageing, and lose their colour and flavour structure on the palate relatively quickly.
What will increased contact time of fermenting juice with skins and seeds provide a wine?
With increased contact time, there is an increase in tannin content and, up to a point, an increase in flavour richness.
The wines age more apparently slowly, and improve in character with age. Increased contact time is most successful with those grape varieties that have the flavour richness to balance the extracted tannin, and in cool climates that allow good flavour development.
During wine ageing, the skin and seed derived (flavonoid) phenolic compounds do not remain unchanged.
What happens to them?
- Slow reactions occur, including the monomers, dimers, and more polymerised material (tannins) reacting together to form larger molecules.
- This behaviour is reflected in an increase, with age, of the average molecular weight of flavonoid material in wine.
- It is also evident as a gradual decrease of the concentration of monomeric flavonoid compounds, including the red pigments, as they become progressively incorporated into polymeric material.
What is one reason anthocyanin levels decline during the late stages of fermentation on skins?
Ageing process commences as soon as flavonoid phenolic compounds become extracted into the juice, i.e. during fermentation in the case of red wine production by fermentation on skins.
Through the process, red
pigments become progressively incorporated into polymeric material producing polymeric pigment
What happens to red pigment colour in wine with polymerisation?
- the vivid crimson of pigments in very young wine gives way to the duller brick-red colour of polymeric pigment as the wine ages.
- Such polymeric pigment usually
contributes half the colour of one year-old wines and is the dominant source of colour in aged red wines. If there is extensive polymerisation, the polymers become insoluble and precipitate leading to colour loss.
Why is protein instability less of a problem in red wines?
Through hydrogen‑bonding, tannins are capable of combining with proteins to give protein-tannin complexes.
Although the proteins and the tannins themselves are soluble, the complexes often readily precipitate and become removed from the wine in racking or filtration.
In red wines, where tannin material is abundant, a significant amount of grape protein is usually precipitated in this way, reducing protein levels below the point at which later protein instability might be a problem.
In white wines where tannins are almost absent, reduction of protein concentrations by this interaction is not possible, and bentonite fining is usually necessary to avoid later protein instability.
How can tannin levels be reduced in a red wine?
Addition of a carefully selected quantity of a protein, such as gelatine, isinglass or albumin (egg white) can result in precipitation of a protein-tannin complex with a consequent reduction of tannin concentration, a process called
fining.
