Module 8 - Oxidation Flashcards
What is oxidation? What is reduction?
Oxidation is a chemical change in which electrons are lost from a substrate.
This change increases the substrate’s oxidation state and the substrate is said to be oxidised.
Reduction is the opposite of oxidation; it is a chemical change with gain of electrons by a substrate.
This change decreases the substrate’s oxidation state.
What is an oxidising agent?
A substance that causes oxidation is called an oxidising agent.
What are the components of wine that most readily undergo oxidation?
Phenolic compounds, ascorbic acid, sulfur dioxide and/or ethanol.
What are the components of wine that most readily undergo reduction?
Oxygen (which is
dissolved in the wine) and hydrogen peroxide.
The reduced form of these reagents will generally have an increase in the number of bonds to hydrogen in the product.
In winemaking what initiates oxidation processes?
Reaction occurs through catalysts that mediate the reaction between oxygen and substrates.
In winemaking they are ions of certain heavy metals, particularly copper and iron. These may either be free or complexed ions in solution, as is the case for non‑enzymatic oxidation, or they may be present in the active site of biological catalysts called enzymes, as is the case for copper ions in enzymatic oxidation.
What is the oxidising agent in winemaking and what is it reduced to?
In winemaking, both enzymatic and non‑enzymatic oxidations use oxygen as the oxidising agent and it is
(ultimately) reduced to water.
Can oxidation progress in wine without oxygen?
Oxidation uses molecular oxygen as the primary oxidising agent. If oxygen is not available then oxidation
generally cannot proceed; if its availability is lowered then oxidation will be decreased.
Can oxidation occur in wine in the absence of catalysts?
Oxidation can only occur through the action of catalysts. If catalysts are only available at particularly low
concentrations then oxidation will proceed very slowly.
Factors that reduce the concentration or activity of these catalysts will reduce the extent of oxidation.
Is oxidation of wine usually reversible?
Much energy is released during oxidation. For this reason, if catalysts and oxygen are available the reaction
will proceed readily and not usually be reversible.
What does the sensory description of ‘reduction’ refer to?
The sensory description of ‘reduction’ refers to the accumulation of low molecular weight sulfur compounds (e.g. hydrogen sulfide, methanethiol) in a wine, while chemical reduction represents the overall gain of electrons by at least one atom in a compound.
It just so happens that the sensory ‘reduction’ character often occurs when wines are stored for long periods in conditions of low
oxygen concentration. Given that wines gained ‘oxidation’ sensory characters in the presence of oxygen, the sensory characters they gained in conditions of low oxygen became known as ‘reduced’.
What is the principal substrate of oxidation in wine or juice?
What is the main oxidation product of a phenolic compound?
The principal substrates of oxidation in wine or juice are phenolic compounds.
The main oxidation product of a phenolic compound, which is an ortho-quinone.
Why is ascorbic acid artificially to juice or wine?
Ascorbic acid is readily oxidisable.
It reacts very rapidly with dissolved oxygen, can act as an oxygen scavenger, and may be added artificially to juice or wine for this purpose.
Does sulfur dioxide react as an antioxidant by combining directly with oxygen?
Studies of sulfur dioxide reactivity do not support this.
It reacts so slowly at the pH of wine that it is not competitive with other wine substrates.
It does have other important roles, described later, but does not scavenge or combine with oxygen directly.
What are the main effects of oxidation?
- development of brown colouration, not only through colourless phenolic compounds becoming brown but
also through red phenolic pigments, if present, becoming brown - precipitation of phenolic material as a result of oxidised phenolic compounds reacting with proteins to give
- insoluble protein‑quinone products;
precipitation of phenolic material as a result of oxidation-induced polymerisation of phenolic compounds - flavour loss, or, masking of flavour by oxidation‑produced aroma substances
- premature ageing in the resulting wine
- decreases of concentration of sulfur dioxide and ascorbic acid.
What is an important effect of non-enzymatic oxidation?
Its ability to produce acetaldehyde by
oxidation of ethanol
Why is the oxidation of wine generally more detrimental than oxidation of juice?
Some effects of juice oxidation can be reversed or partially reversed by the reducing environment of primary
yeast fermentation.
Additionally, juice oxidation is principally enzymatic oxidation, which does not have some of the side‑effects that non‑enzymatic oxidation can have.
If oxygen is consumed through oxidation, its rate of replacement by oxygen from the air will depend upon a number of factors:
The surface area exposed to air.
The volume of liquid.
The flow of air and liquid at their interface.
The dissolved oxygen content of the liquid.
The oxygen content of the gas.
Oxidation can be retarded by reducing the availability of oxygen.
How can this be achieved?
- Minimising exposure to the gas phase, for example by use of closed tanks and enclosed presses and
drainers; avoidance of splashing or aeration; maintaining tanks and barrels without ullage space; use of tall, narrow containers rather than squat, broad containers, and handling large volumes in preference to small volumes; - Reducing the oxygen content of the gas phase by addition of inert gasses, e.g. inert gas blanketing in
containers, inert gas flushing of hoses, and use of inert gas flushing in bottling; - Reducing the dissolved oxygen concentration by sparging juice or wine with inert gas.
Is enzymatic or non-enzymatic processes more important in wine (vs juice)?
The catalytic effect of oxidative enzymes is strong and enzymatic oxidation is much more rapid than
non‑enzymatic oxidation. However, loss of enzymatic activity can occur with time, so although enzymatic
processes are the more important in juice, non‑enzymatic processes become important in wine
What two oxidative enzymes are of particular concern in winemaking?
What compounds do they act on?
Two oxidative enzymes are of particular concern in winemaking, catechol oxidase and laccase.
They act on a complex group of compounds called ‘polyphenolic’ compounds.
What influence do enzymes have on oxidation?
- They act catalytically to speed up the process. Under conditions favourable to the operation of an enzyme, oxidation is very much more rapid than without it;
- They are selective in the type of oxidation process that they carry out, and may bring about processes that
are not possible without them; - They are selective in the type of substrate that they oxidise. An enzyme will generally oxidise only certain types of components.
Is hydrogen peroxide produced from oxygen during enzymatic oxidation?
Note that, unlike non-enzymatic oxidation, hydrogen peroxide is not produced from oxygen during the process.
Enzymatic oxidation, therefore, does not generate a further oxidising agent such as hydrogen peroxide but converts oxygen directly to water.
Where is catechol oxidase (phenolase, polyphenol oxidase or tyrosinase) found?
How does it find its way into juice or wine?
Catechol oxidase is an oxidative enzyme that is naturally present in the grape berry, mainly associated with membranes within cells, particularly in chloroplasts.
Although this ties the enzyme to the solid parts of the grape
berry, the enzyme becomes increasingly released into the juice during berry ripening.
Membrane‑bound enzyme
can also act upon juice components and become released into juice during grape processing. The greater the cell damage and the longer the juice‑solids contact, the more these are likely to occur.
What is an important characteristic of catechol oxidase (phenolase, polyphenol oxidase or tyrosinase)?
An important characteristic of the enzyme is that it undergoes gradual inactivation during oxidations of phenolic compounds. This is due to irreversible binding of the enzyme to oxidised phenolic compounds, leading to loss of
activity
Where does laccase come from?
This enzyme is not a berry component but an enzyme of many fungi.
It is secreted into the berry through the hyphae of Botrytis cinerea infecting the berry; it is therefore an enzyme characteristic of infected fruit.
The enzyme is completely soluble and readily finds its way into the juice.
It can oxidise a wider range of phenolic substrates including red pigments and ascorbic acid. This is because as well as oxidising phenolic compounds with adjacent hydroxyl groups, as for catechol oxidase, laccase can oxidise phenolic compounds with a single hydroxyl group
The enzyme is much more resistant towards inactivation than catechol oxidase, to the point that although
catechol oxidase activity is seldom found to persist through to the wine, laccase activity can still be strong in the wine.
Why is laccase give rise to problems with wines made from Botrytis infected red grapes?
What problems arise?
The strong activity of laccase towards oxidation of red grape pigments and the persistence of its activity into the
wine gives rise to particular problems with Botrytis infected red grapes.
Oxidation of the pigments leads to loss of red colour and yellowing.
What effect does pH have on catechol oxidase?
Catechol oxidase is very active in the pH range encountered in winemaking. It has optimum activity at pH 4.8 with a broad range of activity from pH 3 to pH 5.
Activity falls to about 80% of maximum activity at pH 3. However, the stability of the enzyme with time is best close to pH 7, and in the usual winemaking pH range its activity gradually decreases. This instability contributes to the ease of avoiding catechol oxidase activity after primary fermentation.
