The Fermentation of Fresh Grape
The fermentation process will slow as the sugar is consumed and in two or three weeks will be essentially complete but depending on the temperature at which the fermentation takes place, the lower temperature the longer to process which means it can take up to a month for the fermentation to finish. You can also stop the progress too soon, known as “stuck fermentation”, leaving residual sugar in the wine.
Here is a list of thing you should have before the fermentation process begins:
Potassium Metabisulphite or Campden Tablets: These tablets are added before fermentation and again before bottling.
Pectic Enzyme: This is added to help break down the fruit during fermentation.
Hydrometer: This allows you to determine the alcohol level of your wine and it will help you to track the progress of your wine’s fermentation.
Yeast Energizer: Provides essential minerals, trace nutrients and vitamins for yeast growth and metabolism during fermentation.
Yeast Nutrient: Add to fermentation to increase yeast activity.
Yeast: Use the proper yeast for the wine you’re making – don’t guess or use a packet of yeast just because it’s handy.
Stage 1 – Crushing and primary fermentation
Crushing is the process of gently squeezing the berries and breaking the skins to start to liberate the contents of the berries.
During the primary fermentation, the yeast cells feed on the sugars in the must and multiply, producing carbon dioxide gas and alcohol. The temperature during the fermentation affects both the taste of the end product, as well as the speed of the fermentation.
For red wines, the temperature is typically 22 to 25 °C, and for white wines 15 to 18 °C.
For every gram of sugar that is converted, about half a gram of alcohol is produced, so to achieve a 12% alcohol concentration, the must should contain about 24% sugars.
Yeast is normally already present on the grapes, often visible as a powdery appearance of the grapes. The fermentation can be done with this natural yeast, but since this can give unpredictable results depending on the exact types of yeast that are present, cultured yeast is often added to the must. One of the main problems with the use of wild ferments is the failure for the fermentation to go to completion, that is some sugar remains unfermented.This can make the wine sweet when a dry wine is desired. Frequently wild ferments lead to the production of unpleasant acetic acid (vinegar) production as a by product.
Once fermentation begins, the grape skins are pushed to the surface by carbon dioxide gases released in the fermentation process. This layer of skins and other solids is known as the cap. As the skins are the source of the tannins, the cap needs to be mixed through the liquid each day, or “punched”.
Malolactic fermentation is carried out by bacteria which metabolize malic acid and produce lactic acid and carbon dioxide. The process is used in most red wines and is discretionary for white wines.This fermentation is often initiated by inoculation with desired bacteria.
Stage 2 – Pressing
Pressing is the act of applying pressure to grapes or pomace in order to separate juice or wine from grapes and grape skins. Pressing is not always a necessary act in winemaking; if grapes are crushed there is a considerable amount of juice immediately liberated (called free-run juice) that can be used for vinification. Typically this free-run juice is of a higher quality than the press juice. However, most wineries do use presses in order to increase their production (gallons) per ton, as pressed juice can represent between 15%-30% of the total juice volume from the grape.
With red wines, the must is pressed after primary fermentation, which separates the skins and other solid matter from the liquid. With white wine, the liquid is separated from the must before fermentation . With rose, the skins may be kept in contact for a shorter period to give color to the wine, in that case the must may be pressed as well.
Stage 3 – Secondary Fermentation and aging
After a period in which the wine stands or ages, the wine is separated from the dead yeast and any solids that remained (called lees), and transferred to a new container where any additional fermentation may take place.
Whether the wine is aging in carboys,tanks or barrels, tests are run periodically in a laboratory to check the status of the wine. Common tests include °Brix, pH, titratable acidity, residual sugar, free or available sulfur, total sulfur, volatile acidity and percent alcohol. These tests are often performed throughout the making of the wine as well as prior to bottling. In response to the results, a winemaker can then decide if more sulfur needs to be added or other slight adjustments before it is bottled.
During the secondary fermentation and aging process, which takes three(3) to six(6) months, the fermentation continues very slowly. The wine is kept under an airlock to protect the wine from oxidation. Proteins from the grape are broken down and the remaining yeast cells and other fine particles from the grapes are allowed to settle. Potassium bitartrate will also precipitate, a process which can be enhanced by cold stabilization to prevent the appearance of (harmless) tartrate crystals after bottling. The result of these processes is that the originally cloudy wine becomes clear. The wine can be racked during this process to remove the lees.
Stage 4 – Bottling and Filtration
Filtration in winemaking is used to accomplish two objectives, clarification and microbial stabilization. In clarification, large particles that affect the visual appearance of the wine are removed. In microbial stabilization, organisms that affect the stability of the wine are removed therefore reducing the likelihood of re-fermentation or spoilage.
The process of clarification is concerned with the removal of particles; those larger than 5 -10 micrometers for coarse polishing, particles larger than 1 – 4 micrometers for clarifying or polishing. Microbial stabilization requires a filtration of at least 0.65 micrometers. However, filtration at this level may lighten a wines color and body. Microbial stabilization does not imply sterility. It simply means that a significant amount of yeast and bacteria have been removed. A final dose of sulfite is added to help preserve the wine and prevent unwanted fermentation in the bottle. The wine bottles then are traditionally sealed with a cork, although alternative wine closures such as synthetic corks and screw-caps, which are less subject to cork taint, are becoming increasingly popular. The final step is adding a capsule to the top of the bottle which is then heated for a tight seal.