Wine Phenols - White Wine Processing Considerations
Posted June 2013; updated November 2015
Winemaking methods can vary quite significantly depending on the wine style. The goal of most processes involving white grape varieties is to minimize extraction of phenols.
This begins with grape harvesting. There is a high correlation between soluble solids and phenol concentration. The best method for minimizing phenol extraction is hand harvesting and whole cluster pressing because berry structure is maintained for as long as possible, thus keeping soluble solids at a minimum. Machine harvested fruit will have greater berry breakage and increase skin/seed/stem contact, but pressing as soon as possible will help limit phenols.
Unfortunately, skin contact not only promotes phenolic concentration, but also extraction of aromatic compounds and aromatic precursors. These are highly desired for aromatic grape varieties such as Gewurztraminer, Albarino, and Sauvignon Blanc, and many winemakers choose to allow short-term skin contact from 2-12 hours prior to pressing. To limit phenol extraction, sulfur dioxide and carbon dioxide are added to limit oxidation.
Most wineries will separate juice at the press into 2 batches. Commonly referred to as free-run and press-fraction juices, the two batches will often have significantly different compositions (this will be discussed in depth in an upcoming article). In regards to phenolics, press-fraction juice will have higher concentrations of flavonoid phenols due to higher soluble solids and increased contact with stems and seeds, and higher concentrations of hydrolyzable tannins due to oxidation.
Additives during pressing and solids separation are commonly used to decrease phenolic extraction. Sulfur dioxide is added to machine harvested grapes pre-pressing to limit oxidation. Various commercially-produced enzymes, primarily pectinase, aid with the removal of soluble solids during solids separation activities (enzymes and solids separation will be discussed in depth in a following post). Bentonite, PVPP, and gelatine are all applicable for fining pre-fermentation and post-fermentation. Bentonite does not directly react with phenols but does help precipitate protein-phenol complexes; it is often used in conjunction with enzymes during cold settling to decrease soluble solids and create more compact lees. PVPP (polyvinylpolypyrolidone) is used specifically for removal of flavonoid phenols, while gelatine is more effective for removal of nonflavonoids. All fining agents can have a negative effect on aromatic compounds, so they are often considered a double-edged sword and application must be done carefully.
The infrequently used method of hyper-oxidation can remove phenols in white grape juice by promoting oxidation and sedimentation of phenol compounds. Since it can have a fairly detrimental effect on aromatics, this technique is usually reserved for lower price-point products.
Oak may also be used during fermentation and maturation with various white varieties, most notably Chardonnay. Oak will increase the wine's phenolic concentration, but usually these phenols are considered positives.
When Phenols are Wanted
Phenolic extraction with white grape varieties is desired by an increasing number of winemakers in order to produce desired wine styles with more body and mouthfeel.
The increasing popularity of 'Orange wine' in the modern market is just one example. Of course, this is nothing new since on-skins fermented whites have been produced for thousands of years. Some winemakers are beginning to think differently, using on-skin ferments and high-solids ferments to produce blending components. These wines often lack desired aromatics and flavors but have structure. Integrated into blends, they produce wines of character that have the best of both worlds.
