Grape Fermentation in Winemaking

Have you ever thought about how important it is to choose the right grapes when making wine at home? This guide to grape fermentation in winemaking will help you make wine that impresses your friends and family, without spending too much on fancy equipment.

Throughout the long process of fermentation, nature takes control and gives us the perfect mix of alcohol. It is thanks to the microscopic yeast on the grape's skin that we get a potent drink that puts us in a good mood.

Grape quality: the key to fermentation

What makes a good wine? The quality of its grapes.

If you want to make a great wine, you need grapes of the highest quality. This is why you must know the best harvest period for your grape variety. Here is what to do and what to avoid:

• Avoid harvesting grapes that are wet from rain or have too much dew, as the must will be too dilute. The weather should not be too hot either, as fermentation could fail if it is not done properly.
• Do not wash the grapes before pressing, because the yeast would be lost while the pesticide would remain.
• It is essential to press quickly after the harvest to avoid acidification.
• Make sure to use open plastic trays so the stored grapes can breathe.
• The process must take place in a clean, ventilated room without strong smells, as fermentation releases carbon dioxide, which is harmful to health.

The best thing to do is find grapes from a nearby grape harvest or vineyard and check the condition of the grape branches. If you decide to buy from your local vendor, be aware that each quintal produces 70 litres of product when you use a press. Otherwise, the yield decreases.

From pressing to fermentation

The first step when making any wine is pressing and destemming, which separates the liquid from the fruit and removes the stems using a stainless steel destemmer. If you only have a crushing machine, you will have to separate the stems by hand, as this helps the pulp and skins reduce acidification. During this step, add potassium metabisulphite, as it has antiseptic and antioxidant properties. A teaspoon is enough for each quintal (do not add more, as it can stop the fermentation!). The must from this first pressing should be added directly to a fermentation tank to allow maceration. Be sure the tank has been disinfected with the right oenology cleansers.

During the fermentation

• Do not cover the tank hermetically (use only a sheet).
• Close doors and windows, but allow the room to ventilate properly.
• Keep the temperature constant, between 21 and 25 C° (if the temperature is around 18 °C, use heaters).
• Check that carbon dioxide production does not increase (you can also use a stearic candle).
• Do not fill the tank more than 75%, as the must could overflow during the tumultuous phase of fermentation.

You can ferment the must in the same tank where it was pressed. However, to avoid the acidic nature of the must when it transforms, we suggest using a wooden tank or a plastic tub suitable for food contact, and placing a weight on the must to press it down. Winemakers usually use crossed wooden poles, sized and positioned on the must, with weights to help apply pressure.

Fermentation timing

The tumultuous fermentation begins 4-5 hours after pressing. The yeasts on the skins of the berries may not be sufficient, so add selected yeasts to help the process start:

• Add the doses prescribed on the packaging.
• Dilute them in warm water.
• Add some sugar. After a while you will notice foam on the surface, which means it is fermenting. After 3-4 hours you can add it to the must.

It may take up to 24 hours for the must to start bubbling, and the duration may vary. For black grapes it is 4-5 days. During these days you need to punch down the cap, which means breaking it with a stick so that the skins release substance and colour into the must. This should be done at least every 12 hours, mixing the skins, also called marc, which form the cap on the surface of the must. During this process we also suggest making some “passages”: take the liquid from the bottom, using a pump or by hand, and pour it directly over the cap. If you want a full-bodied wine that is bitter, richly coloured and high in alcohol, slow fermentation can last 18-21 days.

Fermentation temperature and density

One of the keys to your fermentation is checking the temperature and density daily. We recommend taking notes in a notebook so you can track your fermentation performance. Measure the temperature with a thermometer, and measure the density with the right chemical glassware for food use, such as a hydrometer.

During fermentation this value decreases until it reaches a value between 0.990 and 0.995 (racking values). If the value is higher than 1, fermentation has not finished yet. If the density stabilises at a value higher than 1 during fermentation, it means the fermentation is blocked and must be restarted (possible causes include a room that is too cold or a yeast deficiency).

Warning: this value must be around 0 (zero), but it cannot go below zero, or the wine could become acidic.

White grapes’ fermentation

For white grapes, let the must ferment for only 1 day. If you would like a rosé made with the marc, we suggest stopping the fermentation after 18-24 hours at most. To obtain a healthy wine that will keep, it is necessary to reach an alcohol volume of at least 11.5-12, but the must does not always contain enough sugar. To avoid this, check with a hydrometer in a cylindrical container of must. If the alcohol volume is too low, you should add sucrose (sugar), measuring 1.7kg for each quintal of must for each grade that you want to increase.

If the fermentation does not start properly, the room temperature may be too low, in which case you need to heat the room. A yeast deficiency may also be the cause, so you should buy selected yeasts to add to the must. On the other hand, the must temperature may have exceeded 36 °C, in which case the fermentation would stop because the yeasts no longer work.

Racking, pouring and final bottling of homemade wine

The alcoholic fermentation ends after 4 to 10 days. Taste the must to check that all the sugar has been transformed into alcohol. At this point, racking starts, which is the separation of the skins from the wine. For this process you can also use a wicker sieve that will hold back the skins and the seeds. Transfer the liquid completely into a full tank, and only at this point can we lower the temperature to under 16 °C. We put the skins, which still contain liquid, in the press and perform the first pressing. This liquid is added to the rest, while the liquid from the second pressing should be mixed with other wines to be consumed daily. The wine tank (tank, demijohn or barrel) will be filled with liquid and used to remove sediment deposits from the fermentation process. It will need to be periodically drained and refilled with new wine to avoid air contact.

A useful suggestion is not to shake the wine before pouring. The sediments not only make the liquid cloudy but also carry harmful bacteria. The first pouring must be done in early March, the second in June and the third in late spring, on clear, dry days. During the pouring, take a sample of your wine of about ½ litre, seal it in a bottle of the same capacity and have it analysed by your oenologist or at a specialised shop like The Artisans Bottega, where we can give you the right advice to correct any defect.

Now you only need to choose the right time to bottle your wine, and then you must wait at least 6 months before tasting it. We have seen our grandparents make their own wine, passed down from generation to generation, a process handed down over the centuries and improved with machines and equipment that allow you to make your own delicious homemade beverage. Stick close to our blogs, and never lose sight of our expert guides. We have a lot of knowledge and can answer all of your questions to feed your passion for making your own DIY projects.