See previous posts on Wine Phenols: Flavonoids & Nonflavonoids, Tannins & Anthocyanins.
White varietal processing is largely aimed at minimizing phenolic extraction (except with the use of oak addtivies). This starts with harvesting. Machine harvesting grapes leads to greater berry breakage and increased skin/seed/stem contact compared to hand picking. Hand-picked grapes will usually undergo whole cluster pressing because maintaining the berry structure for as long as possible will help keep soluble solids to a minimum. With machine harvested fruit, pressing grapes as soon as possible will help limit phenolic extraction by decreasing skin contact.
Short-term skin contact (up to 12 hours) is particularly common with
aromatic varietals such as Gewurztraminer, Albarino, and Sauvignon
Blanc; this is typically desired for increased extraction of aromatic
compounds and aromatic precursors rather than phenols. During these
processes, phenolic extraction is kept to a minimum through the use of carbon dioxide and sulfur dioxide (limit oxidation) and during solids separation procedures.
Separation of free-run and press-fraction juice is commonly practiced in wineries (this will be discussed in more depth in a separate post). 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 commonly 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.
Hyperoxidation can also be used to remove phenols in white grape juice. This technique is primarily used on press-fraction juice and requires minimal sulfite levels. The juice is saturated with oxygen, leading to oxidation and sedimentation of phenolic compounds. Just like using fining agents, this technique will often lead to significant losses of aromatic compounds.
Oak may also be used during fermentation and maturation with various white varietals, most notably Chardonnay (oak and oak additives will be discussed in a following post).
Phenolic extraction with white varietals 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 (no, this is nothing new; 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 flavours but have structure; integrated into blends, they produce wines of character that have the best of both worlds.