General Process of How White Wine is Made

For the purposes of simplicity, we will take the process from the harvest of the grapes.  The elements of Viticulture (growing the grapes) is an entire separate and complex series of decisions that will be the subject of another discussion.
 

1. Harvest

• Occurs when grapes are optimally ripe and display “varietal characteristics”
• Can also be determined by weather (picked early if threatening rains or frost)
• Can be done by machine or hand (depends on how close the vines are and the variety of grape)

2. Crushing the grapes

• Whites are usually pressed upon arrival (Increaded risk of oxidation in white grapes)
• May have 2-3 hours of skin contatc at low temperature (imparts more extraction of characteristics)
• Ideally breaking the skin to allow juice to flow without damage to seeds, stems or skin (these contain high amount of tannin that can add astringency and bitterness)
• Must is the mixture of juice only in white wine (red wine is fermentated with skins)
• SO2 typically added at this point to help prevent spoilage and off-odors from fermentation of wild yeast and bacteria found on the skins
• SO2 used as little as possible due to cause of pungent odor and some sulfite-sensitive consumers
• SO2 can also be used as an antioxidant (preventing oxidation of the wine)

3. Pressing

• The must is squeezed to extract more juice (different fractions of juice are extracted at
   increasing pressures)
• Free-run juice is extracted with gentle pressures and press juices with higher pressures
• Free-run juice has more sugar and less acid and tannin than press juice
• All or part of the press juice can be blended back with the free-run juice to improve balance
• Free-run juice has a much higher yield than press juice
• After pressing, the pomace, or press cake (must solids) are put back into the soil
• Pressing done before fermentation in whites and after fermentation in reds

4. Alcoholic Fermentation

• Can be done in stainless steel tanks or barrels
• Yeast feed on Glucose(sugar) in the grape juice and produce ethyl alcohol and carbon dioxide (and heat)
• Sugar content in grapes measured in Brix (pronounced bricks)
• For every 2 degrees Brix (2% sugar) you yield about  1% alcohol
• Yeast can only do this conversion in the absence of oxygen
• Heat is produced during the transformation, which can be an enemy to wine
• High temperatures can cause desired aromatic and flavor compounds to be lost from the wine
• High temps can also kill yeast and produce off odors
• Tanks have heat exchangers and cooling jackets to keep optimum temp (15 – 20° C for white)
• Fermentation tanks have a one way valve to release the carbon dioxide without letting in oxygen called “fermentation locks”
• Alcoholic fermentation is over when the appropriate amount of sugar has been consumed for the style of wine being made
• Dry wines usually allow fermentation to go to completion, yeast consumes all the sugar and turns it to alcohol
• Sweet tasting wines are removed with some residual sugar still unconsumed in the wine

Malolactic Fermentation (MLF):

• Sometimes the tanks will be inoculated with bacteria that catalyze a second fermentation (Seen in Chardonnay and sometimes Sauvignon Blanc)
• In this process, malic acid in the juice is converted to lactic acid and carbon dioxide
• Acidity in the juice is lower as a result; lactic acid (found in milk) is less acidic than malic acid
• Different grape varieties and different vintages have different levels of malic acid
• MLF softens the fruitiness of the wine but increases the complexity (giving a buttery and hazelnut flavor and a more full-bodied mouth feel)
• For some wine makers this is undesirable, due to the decrease in fruit flavors
• Sometimes MLF can be as difficult to prevent as it is to encourage

5. Racking

• Basically when wines are transferred from tank to tank
• After fermentation, the yeast cells settle out and form a sediment on the bottom of the tank
• These sediments at the bottom of wine tanks are called lees
• Typically the new wine is transferred off of the lees and to another tank for clarification and stabilization
• Some wines are left on the lees for some time (sur lie) to give a yeast character and complexity

6. Clarification

• Done to convert a cloudy, yeasty liquid to a clear finished product
• Done via racking, centrifugation, filtering and fining
• Centrifugation is using high pressures to remove particles; much faster than racking
• Filtering involves forcing the wine through a medium(filter) that removes particles
• Fining is done to alter properties of the wine and clarify them
• Fining involves mixing certain fining agents to the wine, allowing them to react with components in the wine, this forms particles that settle out and are removed by filtration or centrifugation
• Fining agents include gelatin, bentonite, tannin, casein, and even egg albumen.

7. Stabilization

• Heat-stabilization: when a wine is exposed to high temperatures and doesn’t become cloudy
• Fining with bentonite, a clay that sticks to certain proteins that can cause haziness
• Cold-stabilization: when a wine gets close to freezing and doesn’t produce crystals
• These crystals are potassium bitartrate, and are readily found in most fruit juices
• Wineries chill the wine to produce these crystals and filter them out
• Microbiological stability: when the wine will not undergo any other fermentations or bacterial growth causing cloudiness or off aromas and flavors
• Sterile filtration and aseptic bottling helps along with addition of preservatives

8. Aging the Wine

• Can be done in stainless steel tanks or oak barrels, depending on the desired result
• Few whites are aged for long periods of time

9. Blending, Finishing, and Bottling

• Blending is sometimes done to improve wine complexity or remedy any defects
• Blending can occur at any time during the process
• Different rules apply when blending wine, depending on the country and appellation
• Final filtrations and fining occur here and wine is tested for stability and SO2 levels
• Bottling done with strictest aseptic technique in sterilized bottles