Sugar in grape juice interacting with yeast to produce alcohol is the principle of winemaking. Below I’m explaining to you the whole winemaking process from grape harvesting to wine bottling, using still red and white as examples.
Table of Contents
1. Grape Harvesting
Every autumn (September-November in the Northern Hemisphere; March-May in the Southern Hemisphere; due to global warming, August in the Northern Hemisphere and February in the Southern Hemisphere are not uncommon now), ripe grapes are to be harvested. There are two ways of harvesting: machine and manual. Machine harvesting is usually used in larger, flatter vineyards. The arch-shaped harvester picks grapes as it drives over a row of vines. In the case of manual harvesting, the grapes are picked by workers with scissors.
2. Sorting
After the grapes are picked and delivered to the winery, they are first weighed and then sent for sorting. The conveyor belt sends down the grapes to the sorting table whose both sides stand people sorting out the crushed or rotten ones. Sorting is time-consuming and labour-intensive, and usually only quality wineries do it. In other words, the grapes used to make a bottle of wine that costs £3 to £5 in a supermarket are usually not sorted, meaning some of the grapes used in winemaking are inevitably rotten or oxidised. If there is only a small amount of spoilage in the whole batch of wine grapes, the wine can still pass the chemical analysis and be OK to drink, but the quality is no doubt inferior to the wine made from sorted grapes.
3. Crushing & Destemming
Now grapes are sent to a crusher-destemmer, a machine that uses a spirally rotating blade to remove grape stems and simultaneously crush the berries. When grapes are crushed, the seeds fall out and are removed. A small amount of juice flowing out in this process is called free-run juice, pure and free of pulp impurities. There are wines made from free-run juice only. You can imagine how pure the flavour and how high the price can be!
Some harvesters come with crushing and destemming functions. In this case, grapes are destemmed and deseeded as they’re harvested.
Certain wineries use optical sorters to further sort the grapes after destemming and deseeding to make sure every grape meets the winery’s winemaking standard. Only very well-capitalised wineries, such as Opus One in California, can afford optical sorters.
4. Pressing & Skin Removal
Green grapes for white winemaking: grapes are pressed and skins are removed at this stage. Grape skins contain tannins, whose astringency is not desirable in white wines, so the skins must be removed before fermentation.
Black grapes for red winemaking: Tannins and dark pigments (anthocyanins) on the skins of black grapes are, on the other hand, characteristic of red wines, so the skins must be kept. Pressing takes place after fermentation.
5. Fermentation
White wine: the skinless grape juice is put into a vat, where yeast is added. Fermentation begins.
The vats are usually made of stainless steel and the juice must be kept at a constant temperature so that the yeast works and the fermentation proceeds smoothly. There is usually no air-conditioning in the fermentation area, so the vats are fitted with thermostatic coating.
Fermentation can last from a few days to a few weeks.
Red wine: The skin-on black grape juice is put into a vat, with yeast added. Fermentation also take anywhere from a few days to a few weeks.
The skins would float to the surface of the fermenting juice and form a floating skin layer called “cap”. In order to evenly distribute grape skins’ tannins and anthocyanins into the juice, winemakers usually have two “cap management” options:
- Pump-over: Connect a tube to the bottom of the vat and let the wine flow into the tube, then put the other end of the tube into the uncovered vat from above. This pump-over process continuously breaks the cap, allow tannins and anthocyanins to be more evenly distributed and better extracted
- Punch-down: The winemaker presses the cap down from the top of an uncovered vat with a long-handled tool
Both options can now be automated, suitable for mass-production wineries.
6. Malolactic Conversion
Now the grape juice has been made into wine, which contains sharp malic acid that may affect the balance of flavours. The winemaker may add lactic acid bacteria to the wine to initiate malolactic conversion, converting malic acid into smoother lactic acid. This process is more used in red wine because red wine is tannin-rich and astringent. If it also contains a large amount of malic acid, it’ll be too astringent to be undrinkable. Moreover, malic acid usually gives the wine a green apple aroma and flavour, which is not characteristic of red wine.
As for white wine, the lack of tannic astringency (because of skin-off fermentation) mades a higher level of acidity more bearable. Also, the green apple flavour imparted by malic acid is very common in white wines, so the more the merrier. But there are always exceptions. Some whites do undergo malolactic conversion at the end of fermentation. It all depends on what style of wine the winemaker wants to make.
The skins in red wine are now removed.
7. Post-fermentation Maturation
Depending on the desired style, the winemaker would choose whether or not to carry out post-fermentation maturation (if not, the wine will go straight to the next step: blending). Post-fermentation maturation usually takes place in oak barrels or concrete vats. Newer oak barrels release vanillin and tannins into the wine, while older barrels do not, but both old and new barrels are permeable to air, allowing the wine to oxidise slightly. While concrete vats are impermeable to air and “flavour-neutral”, not releasing any flavours into the wine. I always find concrete vat maturation having a lovely Taoist feel attached to it, not forcing anything into the wine, just letting it grow freely.
8. Blending
For a wine made from multiple grape varieties, the winemaker would blend the wine at this stage after each grape variety has been made into wine separately. In the case of a single varietal wine, batches of wine may have been made differently and are blended now. For example, if a red wine is made from 100% Pinot Noir grapes, but part of the wine is placed in new oak barrels and the other part is placed in old oak barrels, at this stage, the winemaker blends the wine according to the style desired.
9. Clarifying & Filtering
Before bottling, it is necessary to remove impurities from the wine:
- Clarifying: Add a certain substance to the wine, by the principle of positive and negative charges attracting each other, to attach to the impurities
- Filtering: After the attachment, larger and heavier compounds are formed, which sink to the bottom of the container and are removed by filtering
Red wine contains phenolic impurities that can be attached to proteins of gelatin, isinglass, eggs, milk, potatoes or legumes and removed. Wines clarified with gelatin and isinglass proteins are carnivorous. Wines clarified with egg and milk proteins are vegetarian, and those clarified with potato and legume proteins are vegan.
For white wine, PVPP (a chemical substance) or activated carbon, is often used to attach to the brown pigment in oxidising-styled white wines to make the colour brighter and more appealing.
10. Adding Sulfur Dioxidie
The winemaker now adds sulphur dioxide into the wine, to prevent the wine from oxidising and going bad.
The addition of sulfur dioxide can be off-putting for many people. but in fact, sulfur dioxide is widely use as an antioxidant in food, such as nuts and dried fruits. Use within the legal limit does not cause harm to the human body. However, it must be noted that sulphur dioxide is an allergen that can trigger asthma. People allergic to sulphur dioxide must be careful.
11. Bottling (bottle maturation)
After bottling, some wines are put on the market directly, while others will be kept in the cellar of the winery for a period of time before being available on the market.
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