Grapes are picked when they reach ripeness. In the weeks and days preceding harvest, the grapes are tested regularly for sugar content (brix). This is usually carried out in the vineyard. The grower, often working closely with the winemaker (unless they are the same person), walks down rows of vines taking grape samples from clusters at various locations to assure an overall representative sampling. The grape samples are crushed and the juice tested using a refractometer.
Other factors affecting the decision to pick include the possibility of rain or hail which can cause damage or delay, and the availability of labor in the case of hand-picking. Also grape varieties ripen at different times, and in hotter climates ripening can occur at a faster rate, while in cooler climates at a slower rate.
On arrival at the winery, stalks and stems are removed to prevent any bitterness tainting the juice. Following this, the grapes are lightly crushed.
Both these tasks can be performed by the same machine, a crusher-destemmer. Grapes are fed via a hopper into a rotating slotted cylinder. As this rotates, the berries pass through slots, leaving the stems behind. The grape berries are then passed through a series of rollers to release the juice.
The resulting mixture of grape juice, skins, seeds and pulp known as “must” is prepared for fermentation. Sulfur dioxide (SO2), the winemaker’s universal antioxidant and disinfectant, may be added to inhibit bacteria and oxidation and prevent to premature fermentation by the action of natural or ambient yeasts.
The must is tested for pH (the lower the pH, the higher the acid content). Desirable ranges are 3.4 – 3.6 for red wine and slightly lower for white wine. If the pH of the must is too high (that is, the acidity is too low) tartaric acid is added. If the pH of the must is too low, calcium carbonate (chalk) or other commercially available products may be added to achieve to the desired level.
Cultured yeasts are most often added, as they are much more reliable that natural yeasts present on the skins. Criteria used to select a cultured yeast include the particular varietal, brix, fermentation temperature (the hotter the temperature, the faster the fermentation) and the nutrients to be added. The winemaker often wants to control the speed of fermentation, as a faster fermentation has greater risks than a slow one.
Yeasts are living organisms and need nutrients to be effective. B group vitamins including Thiamine (B1) may be added to promote their growth. Diammonium phosphate (DAP) may also be added to ensure that all sugars are fermented out and to inhibit the formation of hydrogen sulfide.
The process of fermentation results in the conversion of sugar by the enzymes of yeast into ethyl alcohol and carbon dioxide.
The fermentation of red wine takes place with the grape solids present in order to extract color from the skins. In the majority of cases fermentation is continued until the wine is dry, and depending on the richness of the must, the final alcohol concentration is generally in the range of 11% to 14.5% by volume.
Fermentation creates heat. Good color extraction requires warm fermentation. On the other hand, cooler fermentation aids in the growing of yeast colonies and produces a higher alcoholic content. Therefore, monitoring and controlling temperature may be necessary, through the use of heat exchangers, stainless steel tanks or other cooling devices.
The traditional process for red wines is for the must to be fermented in open vats. The solids and skins rise to the surface with the CO2 and create a floating cap. This presents a challenge as the skins need to be in contact with the juice to obtain good extraction of color and tannins. To meet the challenge, juice may be mechanically pumped up and sprayed over the cap (pump-over) or alternatively the cap may be manually punched down (punch-down).
After fermentation is complete, depending on the style, the wine may be left to soak on the skins (maceration) until sufficient color, flavor and tannins are extracted.
After fermentation the wine is transferred to a press, where the skins and other solid matter of the grapes are gently squeezed or pressed to extract more juice, in addition to the free run juice. The juice released from the skins as a result of pressing will naturally be higher in tannins and coloring pigments than free run juice.
Various types of presses are available; including a pneumatic press, horizontal plate (Vaselin) press, and vertical basket (which we used in our project).
After pressing, the combined free run juice and pressed juice is transferred to a vat or other container to begin the process of racking.
Malolactic or “secondary” fermentation usually follows primary fermentation. Yeast is not involved. Harsh malic acid (as found in apples) is converted into softer tasting lactic acid (as found in milk) by action of certain bacteria. Almost without exception malolactic fermentation is desirable in red wine, giving the wine a slight buttery and/or toasty nose and sometimes a certain degree of complexity.
Malolactic fermentation can be induced by warming the vats or inoculating with strains of lactic acid bacteria; alternatively it can be prevented or stopped by treating the wine with sulfur dioxide and/or keeping the wine cool.
After fermentation, wines may taste rough. And a period of aging is required during which the tannins soften and acidity levels fall. Wine may be aged in stainless steel vats and oak barrels. Some wines are intended for early drinking and require little aging, while most high quality red wines require a period of barrel aging, usually between 9 -22 months. During the time in the barrels, the wine will absorb some oak products including wood tannins and vanillin.
During time in the barrel the wine will be racked several times to aid clarification, and topped up to replace wine lost in evaporation.
Before bottling, a number of operations may be undertaken to ensure the clarity of wine.
Fining is a process for the removal of microscopic troublesome matter (colloids) from the wine before bottling, by introducing materials such as bentonite, egg whites and gelatin, to which the colloids bind.
Filtration is a process in which the wine passes through a medium (filter) to remove bacteria and solids.
Cold stabilization may be carried out to prevent tartrate crystals from forming after the wine has been bottled. To inhibit the precipitation of tartrate crystals in the bottle, the wine is chilled until the crystals are formed and the cleared wine can be bottled.
Before bottling the winemaker may also adjust the level of SO2 for its antioxidant and disinfectant benefits. Additionally, a number of treatments may be carried immediately before bottling to ensure the wine’s final stability, such as cold sterile filtration, and pasteurization.
The winemaker must decide the shape and color of the bottle and the closure device to be used. Corks from oak trees are the traditional way of closing wine bottles. Plastic stoppers, artificial corks and the metal screw cap (Stelvin) closure have been in use for years.