1. Harvesting the Grapes
Before the grapes are harvested, they are tested for sugar content by measuring the Brix levels using a refractometer. This is done regularly in the days and weeks before harvest. It is very important to choose the right moment to harvest the grapes and this is a decision that is made by the grower together with the producer, if they are not the same person.
Harvesting can be done either by hand picking the grapes or using a mechanical harvester.
Hand picking is sometimes a requirement for some appellations and represents a mark of quality although the modern machine harvesters can be almost just as good but for a fraction of the time spent hand picking. After hand picking is done, grapes are then collected into a container and transported to the winery.
- Only the best grapes are picked while damaged bunches and rotten berries are avoided
- Grapes can also be picked anywhere and on any terrain.
- Preferable for thin skinned grapes that break easily
- Very time consuming
- Labor is much more expensive
- Since it takes longer, there is always the risk that grapes will become hot and oxidation will begin in the time between the grapes being harvested and transported to the winery
Mechanical harvesters, unlike hand pickers, can only be done on either flat terrains or slopes that are not too steep. Mechanical harvesters surround a row of vines and shake the vines so that the grapes fall off. This is done rather fast in comparison to hand pickers but can however cause some damage to the grapes in the process. Having said that, modern mechanical harvesters have come a long way in minimising the damage caused to the vines and can even de-stem the grapes and filter from MOG.
- Time efficient
- Much cheaper
- Grapes can be picked any time, even in the morning or at night when the temperatures are cooler. This decreases the risk of damaging the grapes in the process and is especially important in hotter climates
- Cannot distinguish between good and damaged grapes
- Can only pick grapes and not bunches
- Will pick material other than grapes (MOG) meaning everything that comes along with grapes like bugs, snails, leaves
2. Crushing and De-stemming
After harvesting the grapes are delivered to the winery where they are crushed and de-stemmed. Destemming can be done with a crusher-destemmer machine. The machine can either be manual for small volumes or motorised for larger volumes. The machines removes the stems and crushes the grapes open without breaking the seeds. For red wines, producers sometimes choose to keep some stems for fermentation which adds structure and texture to the wine.
At this stage it is common to add to the must additives like Sulfur dioxide (SO2, also referred to as Potassium metabisulfite) which acts as an antioxidant and prevents spoilage and wild yeast fermentation.
Sulfites in Wine
Sulfur dioxide (SO2) acts as an antioxidant by absorbing O2 in wine and stops spoilage, bad bacteria and wild yeast fermentation. Enzymatic oxidation starts to occur as a result of polyphenoloxidases (PPO) being released during the crushing of the grapes which will then oxidise the hydroxycinnamic acids (hydroxycinnamates) into benzoquinone. Hydroxycinnamates are responsible for the browning of the must and SO2 acts as an inhibitor in this process, stopping the wine from oxidising and losing its colour.
Once sulfites get inside the wild yeast cell, they bind with enzymes and proteins and destroy the cell. There are 2 states in which sulfites exist in wine – free and bound.
- Free: molecular SO2 + bisulfite HSO3 + Sulfite SO3
- Bound: when sulfites attach to sugars and phenols
- Total SO2 = Free SO2 + Bound SO2
Only molecular SO2 is antibacterial and represents a small percentage (around 5%) of the free SO2 in wine. The amount of molecular SO2 depends on the pH level in wine, since the proportion of molecular SO2 decreases the higher the pH.
Potassium metabisulfite is a form of SO2 (KMB, KMBS) that comes as a powder containing SO2 and potassium.
Note: SO2 occurs in all wines as a byproduct of fermentation which is usually less than 10 PPM (parts per million or milligrams per liter). Wines containing more than 10 PPM must be labeled “contain sulfites”. Organic wine must not contain any added sulfites in addition to having grapes that have been organically grown. Wines with no added SO2 can present some economical disadvantages for producers since they normally have a shorter shelf life.
3. Testing the Must
At this point the must is pumped into fermentation tanks where it is cooled and the juice is tested for sugar, acidity and pH. The fermentation tanks can be stainless steel, wooden, or plastic (mainly for home use).
Testing the sugar or Brix
Brix (°Bx) measures the sugar content of grapes (must and wine) at harvest by indicating the number of grams of sucrose per 100 grams of liquid. It is measured on a scale from 1 to 100 and is used to estimate the potential alcohol level of the wine. Most table wine grapes are harvested at 22 – 26 Brix.
Some of the most common ways of measuring Brix is done by using either a refractometer or a hydrometer. A refractometer measures the refraction of light passing through the liquid. The higher the sugar levels, the denser the liquid and the higher the refraction. Refractometers can measure sugar levels even in very small samples like the juice of a grape. Hydrometers also work based on the density of the liquid, the higher the density the more sugar there is and the higher the hydrometer will float.
Measuring Brix is useful for winemakers to identify how ripe the grapes are and consequently the optimal time to harvest them but also to get an estimation of the potential alcohol content. This will then determine wether the must will need added sugar in order to achieve the desired alcohol levels or comply with the appellation’s regulations in some cases.
When it comes to the appropriate level of Brix, this really depends on the type of grapes and style of wine. Some grapes like Primitivo and Barbera can have a higher Brix of around 30° at harvest. This can be corrected by adding chlorine-free water to the must, a process which is also called rehydrating the grapes.
- Brix < 22° then sugar can be added
- Brix > 25° then water can be added otherwise there is risk of ending up with a wine that is not dry
Note: Estimating potential alcohol content can be done using the following formula: Brix × 0.59
Acidity: pH and TA
Wine has 3 main types of organic acids: tartaric, malic and citric with the first 2 comprising around 90% of the total acid. The riper the grapes become, the amount of malic and tartaric acid declines and the pH increases. The acidity level of a must can be measured either by pH or titration. Having said that, measuring pH and TA will not tell us the same thing, many times they are not even correlated. For example 2 different musts can have the same pH level while having different TA levels. This is because acids have different levels of tartness, with some tasting tarter or stronger than other.
- TA vs pH – Titratable acid will measure the strength of an acid while pH will measure the amount of acid in a wine irrespective of its strength.
- Titratable acid is commonly reported as grams of tartaric acid in 1 later of wine and is measured in either grams per liter or percentage. For example 6.1 g/l = 0.61% TA. TA is measured using an acid test kit or titration kit.
- pH – can be measured using a digital pH meter and will show anything between 0 and 14, however the lower the pH the stronger the acidity so a pH of 2 will be much more acidic than a pH of 5. Anything below a pH of 7 is classified as acidic. Usual levels of pH in wine are between 3.4 and 4.0. A wine that has a pH of for example 3.0 is 10 times more acidic than a wine that has a pH of 4.0.
Knowing the pH levels in wine is important since it can tell us a couple of things about how the wine will behave. A high pH may cause the wine to go bad after a few years and results in poor ageing potential. It also makes the wine more susceptible to oxidation and may cause red wines to lose their colour intensity and to become bricky in colour.
The pH level can also conceal or enhance fruit flavours however this is where balance is really key. A wine that has a low pH may have more fruit flavours but may also end up tasting too aggressive and acidic while a wine with a higher pH may lack complexity and become plane.
- If pH is too high then tartaric acid is added
- If pH is too low then calcium carbonate/ potassium tartrate or some other substances could be added
It is important to measure both TA and pH and adjust the must if necessary otherwise you can end up with a wine that is either too tart or has a high pH and runs the risk of spoilage (or vice versa).
4. Adding Yeast
Fermentation will naturally start in wine with the presence of wild yeast which can be found anywhere, in the air, on surrounding surfaces, in the vineyard etc. Wild yeast is however unpredictable and may lead to an incomplete fermentation and/or unpleasant flavours being developed in the wine. This is why a lot of winemakers will often add strains of cultured yeast which is carefully selected depending on the grape varietal and the desired outcome (body, aroma, complexity). Because wild yeast is found in vineyards and all around us, this is why SO2 is added after crushing and de-stemming in order to kill any undesirable wild yeast. Most commercial wines today use cultured (also called inoculated) yeast which ensures that all wines are consistent. Cultured yeast can also survive different temperatures and alcohol levels which means that there are greater chances of the must undergoing a complete fermentation leaving no unfermented sugar in the must.
Having said that, there still are many producers (especially in the Old World) who have been using wild yeast successfully for a very long time and this is where skill is the main driver for success. Cultured yeast strains only started being sold in the 60s, up until then there was no other option than wild fermentation. Although wild yeast is very unpredictable and can lead to stuck fermentation, bad flavours and odder, when done right, it can produce great results.
Wild yeast takes longer to ferment which means that the grapes will be in contact with the skins for a longer time resulting in added complexity, body and colour. Wild yeast will stop fermenting when the alcohol levels reach around 3% – 4% leaving unfermented sugar in the wine. This is when things could either go wrong or surprisingly well. Among the yeast that is found on the grapes, besides wild yeast there may also be Saccharomyces cerevisiae (also used in baking) which is a desirable cultured yeast that will take the wine to a complete fermentation after the wild yeast dies off and the resulting wine will be complex and aromatic. The downside is that the presence of Saccharomyces cerevisiae is not guaranteed and even when it is naturally found on grapes, it is in very small amounts which may not be enough to take the must to a complete fermentation.
Once the yeast is added to the must, depending on the temperature of your must, fermentation will start within the first 3 days. During fermentation, the yeast will consume the sugar releasing alcohol and CO2, the latter pushing the grape skins to the top of the must creating a so called cap. The cap will have to be broken a couple of times during the day, a process also called “punching the cap”.
Punching the cap will ensure that:
- The skins have enough contact with the must, which maximises the extraction of colour, tannin and flavour
- Any excess heat which might cause the fermentation to reach temperatures that are too high, is released
- Bacteria that can form on top of the cap and can therefor spoil the wine, is destroyed when mixing it with the rest of the must that has high alcohol levels
Temperature is another important factor that winemaker needs to take into account as fermentation usually occurs between 5 and 35 °C. Lower temperatures bring out the fruit flavours in wine and are common for white wines while higher temperatures result in wines with more tannin and darker colour hence preferred for red wines.
Some winemakers may add yeast nutrients during fermentation in order to prevent the presence of Hydrogen Sulfide (H2S), Volatile Acidity (VA) or other issues which will spoil the wine. This may occur once the alcohol levels rise and the yeast requires more nutrients in order to complete the fermentation.
Once the yeast has consumes most of the sugar in the must, which normally takes around a week, the next step is to press. Pressing is done when the optimal sugar levels are reached by monitoring the Brix levels using a refractometer. Depending on the style of wine the winemaker might choose to press either at 0° Brix or slightly above. Wines that undergo extended maceration will reach 0° Brix and will result in more intensity and tannins while a lighter style of wine with a fruitier profile might press when Brix levels are around 3° – 5°.
Note: Extended maceration is when the must is in contact with the skins and the seeds for a longer period of time after the alcoholic fermentation has completed. This can take 1 to 2 weeks and is done in order to maximise flavour and add tannin.
Pressing is basically filtering the wine from the all the skins, stems and seeds so that you are left with the clear juice. This is done by pumping the fermented wine into a wine press such as a bladder press. The bladder press has an inflatable rubber bladder inside which expands depending on the amount of pressure you want to apply, pushing the grapes to the side and gently extracting the juice. The bladder press is a cylinder which will sometimes rotate in order to mix all the remaining wine in the press.
When the wine is pumped into the wine press, before any pressure is applied, there will be juice that will freely start running through the press which is called free run juice. As the pressing of the remaining mass of skins and grapes continues, the winemakers will usually decide how firmly they want to press. The higher the pressure, the higher the amount of phenolics are extracted which may result in a harsher wine. Winemakers will often keep the juices they obtain from each press separately and test them first before deciding to blend them since free run wine is usually not as tannic and intense as the press wine.
7. Malolactic Fermentation
Malolactic or Secondary fermentation (MLF) is a process in which malic acid which is naturally found in grapes, is transformed into lactic acid and CO2. Malic acid is one of the 3 acids that is commonly present in grapes alongside tartaric and citric acid, and unlike the primary fermentation, MLF does not use yeast to start the fermentation but Oenococcus oeni which is a bacteria that is part of the lactic acid family. Wine producers can also choose to inoculate the wine with strains of malolactic bacteria.
Although there is no alcohol release during MLF, the purpose of the secondary fermentation is to:
- Lower the acidity levels
- Decrease the harshness of the wine, lactic acid is softer than malic acid
- Add body and complexity
- Add buttery flavours especially in white wines
This step is optional although common in the production of most red wines in order to add complexity and balance out the harshness and acidity levels. Malolactic fermentation may also be stopped with the addition of sulfur dioxide.
8. Aging / Maturation
After fermentation, depending on the style of the wine, most red wines will undergo some sort of ageing wether that is a couple of months for lighter wines or years for bigger, heavier reds. Aging is done to improve the quality of the wine by adding complexity and decreasing the level of acidity and harshness especially with wines that are high in tannins, creating rounder wines. Aging can take place in different types of vessels some of which are wooden, concrete and stainless steel vessels. Quality red wines are usually aged in wooden barrels made from oak for a minimum of 9 months during which they absorb wood tannins and vanillin.
Types of vessels
These vessels do not allow any oxygen inside the tank which preserves the aromatic, fruity and fresh aromas of the wine. They also do not add any extra flavour to the wine as for example oak or wooden barrels do.
Add complexity and aroma because of the compounds found in wood which give the wine smoky, spicy and woody aromas. The type of oak – American, French, Hungarian, Slovenian are some of the most widely used types of oak – and wether it is new or used, will also have a big impact on the final outcome. American oak tends to add sweeter notes like vanilla and coconut while European oak will add more spicy, savoury and elegant notes. New oak is generally more potent in its compounds and will have a bigger impact on the aroma of the wine than used oak.
Lately concrete vessels in the shape of eggs have become a popular choice for winemakers. These types of vessels are somewhere between the stainless steel and oak barrels. Their egg shape allows for the wine to breath just like in oak barrels, adding depth and texture to the wine while at the same time creating a neutral and temperature stable environment same as the stainless steel tanks.
After maturation, the wine can be blended either with wines from different grape varieties or from the same variety but different vessels. Sometimes there can be variations between different vessels of the same wine during aging. Blending ensures that the same consistent style is maintained.
The next step after fermentation is to clear the wine from lees (dead yeast) and other sediment that occurs in wine and makes it cloudy. There are a three main clarification methods that winemakers can choose from: racking, fining and filtration. The first one is not as harsh as the other 2 ones and removes the bulk of the sediment. Fining and filtration remove any excess tannins, bacteria or leftover yeast ensuring that the wine is stable and that there is no risk of yeast reacting with sugar resulting in unwanted fermentation. At the same time, filtration and fining can also remove particles that add complexity and flavour to the wine. Winemakers that only use racking to clarify their wine will usually label the wines as unfiltered or mention it in some way.
This is done by leaving the wine to settle so that the sediment accumulates at the bottom of the container after which the wine is pumped into another vessel leaving behind the remaining of the dead lees and sediment. This can be done a few times to ensure proper clarification.
This method includes the addition of a fining agent (gelatine, bentonite, egg whites, casein etc) which has the ability to bind with suspended compounds in wine such as protein and yeast cells (tannin and phenolic molecules) and form bigger particles which can then be removed.
Depending on the style of wine, different fining agents are used although today there is a lot of preference for vegan fining agents such as bentonite (a type of clay derived from volcanic ash). Egg whites and gelatine for example are mainly used to remove excess harsh tannins in red wines while bentonite can be used to remove excess proteins which cloud the wine.
This is the last clarification step and consists of removing any leftover particles from the wine. There are a couple of systems which can filter the wine, one of which is a Plate and frame filter system. In this method the wine is pumped through the filter system which basically consists of a series of sheets/plates with a rough and smooth side that trap any particles left in the wine and pump it out filtered.
Before bottling, the wine will undergo another round of filtration and will be adjusted with sulfur dioxide in order to inhibit any potential fermentation and prevent undesirable bacteria. Based on how susceptible the wine is to light but also the regional requirements and traditions, the wine can be bottled in see-through or tinted green/brown glass bottles. Depending on wether the wine will age in the bottle or has the potential to do so, the wine will have a screw cap or a cork with the latter indicating that the wine can age.