In The Winery • May - June 2013
Oxidation Phenomena in Grapes, Musts and Wines Managing Oxygen From Vine To Bottle By Daniel Pambianchi
I
t is often said that wine is made in the vineyard. Mother Nature oversees grape berry development throughout the growing season and then delivers a healthy harvest perfect for making great wine. But harvest to the winemaker is the beginning of a challenging road in managing oxidative effects towards that lustful bottle of wine. These effects start right in the vineyard with diseased or damaged grapes, if any, and with how grapes are harvested and transported back to the winery. Even sound grapes subjected to rough handling or crushing under the weight of the harvest will set enzymatic and chemical (non-enzymatic) oxidative mechanisms in motion. Although there are overlaps between these mechanisms, enzymatic oxidation mainly occurs in grapes and juice (must) until the end of fermentation, and, while chemical oxidation can also occur in must, it primarily affects wine from fermentation right to bottling (and even serving).
bate oxidation. Laccases are relatively immune to sulfur dioxide (SO2) and have longer-lasting activity, well into the wine aging phase. Moldy or damaged berries and bunches should be removed to the extent possible. Must also contains glutathione (GSH), a tripeptide that acts as a natural antioxidant by reacting with quinones to form colorless glutathione-containing complexes known as grape reaction products (GRP). However, when GSH becomes depleted, any remaining quinones undergo a coupled oxidation that exacerbates browning by forming secondary quinones that can go on to react with other polyphenols to form aggregate complexes and impact color intensity and stability. Some polyphenols are also regenerated during this coupled oxidation therefore renewing the potential for phenolic browning if PPOs and O2 are still available.
OXIDATION IN WINE
OXIDATION IN MUST
Flavanols/tannins are the main substrates of chemical oxidation in wine, but here, oxidation is catalyzed by iron and copper metal ions, which occur naturally in tiny quantities in grapes but can become significant if, for example, brass or copper equipment is used or if vines and wine are treated excessively with copper.
Polyphenols are the main substrates of both enzymatic and chemical oxidation in must. These substrates include flavanols (the building blocks of tannins), anthocyanins (color pigments), phenolic acids, and tannins.
But bisulfite ions (HSO3–) from SO2 additions can also bind directly with quinones (instead of reducing these to colorless polyphenols) to form bisulfite addition products that can no longer be reversed back to colorless forms.
Enzymatic oxidation is activated by naturally occurring polyphenoloxidase enzymes (PPOs) in the presence of oxygen (O2) as soon as grapes are crushed or pressed and cause phenolic acids in grape pulp and flavanols/tannins extracted from grape skins during maceration to oxidize to brown-colored quinones in what is known as phenolic browning. PPOs become inactive by the end of fermentation. Quinones can be reduced back to their polyphenols and reverse browning effects through sulfite or ascorbic acid additions.
In the presence of metal ions, O2 can become reduced into hydrogen peroxide (H2O2), which can oxidize bisulfite ions into sulfates (SO42–) and divert free SO2 from performing its protective tasks. H2O2 can also oxidize ethanol into smelly acetaldehyde. If caught quickly, sulfite can be added to get bisulfite ions to bind and deal with the acetaldehyde; otherwise, acetaldehyde will bind with tannins and anthocyanins and even cause tannins to bind with anthocyanins to form complex pigmented polymers that will alter wine color and stability.
Let’s review these mechanisms to understand how they affect must and wine quality; we’ll also look at the effects of microbiological oxidation.
Phenolic browning is particularly problematic at higher pH because of the higher concentration of oxidizable polyphenol ions, and at higher temperatures, which tend to accelerate oxidative effects. This can begin in ruptured grape berries hanging on vines or in grapes damaged from handling or during transportation. And moldy B. cinerea-affected grapes contain naturally occurring laccase enzymes that further exacer-
877-892-5332
If quinones are left untreated or cannot be reduced back to their polyphenols, they too can bind with tannins and anthocyanins to form very complex pigmented polymers that will aggravate color problems. Adding ascorbic acid can become risky if there are no more
The Grapevine • May - June 2013
Page 15