General Winemaking Options

Question 1

Explain the effects and use of oxygen in winemaking?

Answer 1

Oxygen is responsible for a number of reactions that occur between the compounds in grape must or wine and for this reason can have a significant effect on wine style and quality. Although oxygen itself is not very reactive with many compounds in the must and wine, the reactions it does take part in create products that then go on to react with many must and wine compounds. These reactions are oxidation reactions. The timing and amount of oxygen exposure is key, making the difference between a positive or negative effect, and therefore an understanding of the role of oxygen is essential.

Oxygen is generally threatening for the production of fresh, fruity wines. Many of the aroma compounds that give these wines their fruity style, for example the thiols found in Sauvignon Blanc, break down in the presence of oxygen, and this can lead to a loss of fruitiness. Furthermore, the products of oxidation reactions may contribute to unwanted aromas to the wine; for example, acetaldehyde (from the oxidation of ethanol) can give a nutty, apple aroma. The colour of white wines can also turn darker, becoming gold and then brown with increased oxidation, and therefore white wines tend to need greater protection. Phenolic compounds in red wines have an anti-oxidative effect, which means that they can absorb more oxygen before such effects are perceptible.

The practice of minimising oxygen exposure during the winemaking process is sometimes called reductive or protective winemaking.

The effect of oxygen on the must or wine can be limited by:

• avoiding ullage in vessels. Ullage is the headspace of air between the wine and the top of the container. It can be avoided by ensuring vessels are filled up to the top. In vessels that are not completely airtight, such as those made of wood, there may be a gradual loss of liquid through evaporation. Therefore, these vessels should be topped up regularly with more wine to avoid ullage.
• use of ‘inert’ gases. Gases such as nitrogen, carbon dioxide and argon can be used to flush out oxygen from vessels, pipes and machinery (such as presses) because these gases do not react with compounds in the wine. Inert gases can also be used to fill the empty headspace of any containers where the wine does not reach the top to prevent oxygen coming into contact with the wine.
• addition of sulfur dioxide.
• use of impermeable containers. Stainless steel and thick concrete vessels are impermeable to oxygen, whereas wooden vessels allow gentle ingress of oxygen. The use of glass bottles with screwcap can also minimise exposure to oxygen during storage in bottle.
• cool, constant temperatures. Cool temperatures slow the rate of oxidation reactions; hence the reason for maturing wines in relatively cool cellars or picking grapes early in the morning so that the fruit is not warm.

However, controlled exposure to oxygen can be positive for many wines. In fact, oxygen is required at the start of fermentation of all wines to promote growth of a healthy yeast population and, in some cases, lack of enough oxygen in winemaking or storage can lead to reductive off- flavours. In the production of some white wines, exposure to oxygen before fermentation is thought to lead to greater oxidation stability in the wine, increasing age potential. In red wines, oxygen is essential in the reaction between anthocyanins and tannins that leads to greater colour stability.

Exposure to oxygen over time also leads to changes in the aromas/flavours of wine, which can give a greater range and diversity of characteristics; fresh fruits become dried fruits and notes such as honey, caramel, coffee, leather and mushroom can develop. A high level of oxidation is vital in some wine styles, such as Oloroso Sherry, Madeira and Tawny Port, but less extreme oxidation also contributes to the complexity of many matured white and red wines.

Oxygen exposure can be increased by:

• cap management
• use of small wooden barrels
• increasing the number of rackings or amount of lees stirring
• allowing ullage in wine containers
• use of techniques that involve pumping oxygen through the must or wine. (e.g. hyperoxidation / micro-oxygenation)

Question 2

Explain the effects and use of sulfur dioxide in winemaking?

Answer 2

Sulfur dioxide is a preservative that is almost universally used in winemaking, where it has the following properties:

• Anti-oxidant – SO2 only reacts with oxygen itself very slowly; it reduces the effects of oxidation by reacting with the products of oxidation reactions, so they cannot oxidise further compounds in the wine. It also inhibits oxidative enzymes.
• Anti-microbial – It inhibits the development of microbes such as yeast and bacteria. Different species of yeast and bacteria can vary in their tolerance to SO2.

SO2 can be applied in various forms: gas, liquid or solid, as sulfur dioxide, potassium metabisulfite or potassium bisulfite. A small amount of SO2 (10 mg/l or less) is also naturally produced during fermentation. Maximum concentration levels of SO2 are defined by local laws as it is a toxic substance. For example, in the EU 160 mg/l SO2 is the maximum permitted for red wines and 210 mg/l is the maximum permitted for white wines. Sweet wines are permitted to contain higher levels. The maximum permitted SO2 levels are lower for organic wines than non-organic wines (and in the USA SO2 additions are not permitted for organic wines). Producers of natural wines may choose not to add any SO2 at all or use only a very small amount. The concentrations of SO2 found in wine are far below toxic levels; however, even at these very low levels some people can experience an allergic reaction. If a wine contains over 10 mg/l of SO2, the label must state that the wine contains sulfites.

SO2 is generally added soon after the grapes are picked and/or reach the winery. It may then be added at various points during the winemaking process and usually at bottling. When SO2 is added to must or wine, it dissolves and some of it reacts with compounds in the liquid. This proportion is called ‘bound SO2’ and it is ineffective against oxidation and microbes. The proportion that is not bound is called ‘free SO2’. The vast majority of the free SO2 exists in a relatively inactive form and a small proportion exists as molecular SO2, which is the most effective against oxidation and microbes.

The pH level of the must or wine has a key effect on the efficacy of SO2 in that a greater proportion of free SO2 is in the molecular form at lower pH levels. This means that a greater amount of SO2 needs to be added to musts and wines with relatively high pH to protect them from oxidation and microbes.
The timing and size of SO2 additions also influences the effectiveness of the added SO2. Adding a larger amount when the grapes are crushed, at the end of malolactic conversion and at bottling is considered as more effective than adding smaller amounts throughout the winemaking process.

Judicious additions of SO2 are beneficial and often necessary to produce unfaulty wines that remain unfaulty once packaged. However, where possible, quality-conscious winemakers will aim to limit additions of SO2 both because of the legal restrictions listed above and also because high levels of SO2 can dull wine aromas/flavours and sometimes cause the wine to taste harsh. Good winery hygiene and effective grape sorting can limit the amount of harmful microbes in the wine and the winery. Limiting oxygen exposure and keeping grapes/must/wine at cool temperatures can also reduce the amount of SO2 needed to protect from oxidation and microbial spoilage.

Question 3

Explain the transportation process to the winery?

Answer 3

Once harvested, grapes will be transported to the winery. In the vineyard, hand-harvested grapes are typically put in small crates that the pickers can carry. Depending on the scale of operation, the options are:

• To transport the grapes in small crates to the winery. This may be for quality purposes or simply because of small-scale
  grape growing. For example, in many Italian regions many grape growers will only have one hectare to harvest and will use their own tractor to transport the grapes. Small crates mean minimal crushing of grapes and therefore limited oxidation and microbial spoilage.
• The small crates are tipped into larger hoppers (large bins) for transport to the winery. Without protective measures, this would involve some crushing of grapes and therefore oxidation and potentially microbial spoilage. Some grape growers will add SO2, generally in the form of potassium metabisulfite, at this point to minimise this (see Sulfur Dioxide in Oxygen and Sulfur Dioxide).

Machine-harvested fruit has already been destemmed and is therefore transported as grapes in larger containers, with some release of juice. Again, some grape growers will choose to add SO2 at this point. This is typical of larger estates around the world.

Once grapes have been picked, they are vulnerable to oxidation and to ambient yeasts and acetic acid bacteria (turns alcohol to acetic acid (vinegar)). All of these threats to quality rise with higher temperatures. Black grapes are less vulnerable to oxidation because they contain more phenolic compounds that have anti-oxidative properties. Measures can be taken to minimise these threats:

• Harvesting grapes at night when temperatures are lower (see Harvesting Options) or harvesting at sunrise if harvesting by hand
• Addition of SO2 for its anti-oxidant and anti-microbial properties at the time of harvesting (see Sulfur Dioxide in Oxygen and Sulfur Dioxide)
• Reduction of the grapes’ temperature by putting them in a cold storage room once received at the winery
• Sanitising harvesting equipment/bins. (reduces chance of microbial infection only_
• Collecting and transporting the grapes in small crates to minimise crushing.

Question 4

Explain the process of grape reception?

Answer 4

CHILLING
If the grapes are warm when they reach the winery (e.g. they have been picked on a sunny, warm afternoon), the winemaker may choose to chill them to a lower temperature before crushing and pressing begins. Warm temperatures increase the rate of oxidation and therefore chilling can help preserve fruity aromas. Chilling also helps to reduce microbial spoilage.

Chilling of whole bunches usually takes place in a refrigeration unit. The chilling takes time, which may slow the processing of the grapes (but can be a useful place to store grapes if all the sorting tables, presses and other equipment are already in use).

A heat exchanger can also be used for chilling (or heating in cool climates) if the grapes are in a more fluid format (e.g. fruit that has been machine picked, grapes that are destemmed and possibly crushed). Heat exchangers can work very quickly.

Both of these options incur costs in terms of equipment and energy. Where possible, harvesting at night or early in the morning will be encouraged in warm climates to bring in cool grapes and avoid these costs.

SORTING

The level of grape sorting (also known by the French word triage) that is required, and indeed whether sorting takes place at all, depends on a number of factors including the ripeness and health of the fruit arriving at the winery, the intended final wine quality and price, whether any sorting has been carried out in the vineyard (e.g. by skilled hand-pickers) and the physical state of the grapes (if grapes arrive in large containers, the bottom grapes will have crushed and released juice; this is too liquid to sort).

The more sorting that is carried out, the higher the cost. This is both due to the labour requirement and time taken for meticulous hand sorting and because greater scrutiny often results in less yield. A judgement has to be made as the level of sorting justified in relation to the return expected from the sale of the wine. In poor years and in cool climates a greater of level of sorting may simply be required (for all but the most basic quality wines) to remove mouldy and under-ripe grapes. In very good years, fruit may arrive in near perfect condition and require little sorting (MOG (material other than grapes) may still need to be removed – leaves, twigs, insects, etc.). The grapes for inexpensive wines may not be sorted at all (as sorting costs money and time and requires either expensive machinery or labour). The key drivers are the health of the grapes on arrival at the winery and then the quality of the wine to be made in relation to the price that can be gained for the wine.

For quality wines there are a range of sorting options:

• Removing unwanted grapes/bunches before picking or during hand-harvesting
• Sorting by hand on a table or a moving or vibrating belt (the latter also removes MOG); this can take place before or after destemming, or occasionally both before and after destemming
• Optical sorting, which is a high-tech, high-cost option that uses digital imaging and software technology to scan individual grapes. The machine scans a 100-grape sample chosen by the grape grower as a reference. The full load of grapes to be sorted are then passed through the machine and it rejects individual grapes that do not conform to the sample and any MOG. This can be done either in a harvesting machine or at reception in the winery. This option is typical of high value grapes, for example Grand Cru Classé estates in Bordeaux.

DESTEMMING

Hand-harvested grapes for most white wines and many red wines are destemmed on arrival at the winery. (Machine-harvested grapes are already destemmed because the grapes are shaken from their stems during harvesting.) Destemmers generally work by a series of blades within a rotating drum that remove the grapes from the stems. Destemming is common in wineries around the world.

Stems contain tannins, which can be extracted if the stems are left in contact with the wine. These tannins are not wanted in white wines and are additional to skin/seed tannins in red wines, so can be desirable in some wines and not in others (see Whole Berry/Bunch Fermentations). If stems are not ripe, they can convey unwanted green flavours and bitter tannins to wine.

Grapes are not destemmed for wines made in certain ways. Examples include:

• red wine fermentations that use some whole bunches (e.g. with Pinot Noir in Burgundy or Sonoma)
• carbonic maceration (e.g. with Gamay in Beaujolais)
• whole bunch pressing for some white wines (e.g. common for high-quality sparkling wine).

CRUSHING

Crushing grapes, which happens at the beginning of the winemaking process, is the application of sufficient pressure to the grapes to break the skins and release the juice, making it available for fermentation. (It is not to be confused with pressing, which is the separation of the juice or wine from the skins and seeds.) It is important that the pressure applied is gentle enough not to crush the seeds, which would add bitterness. Traditionally, crushing was done by the pressure of human feet.

Many wineries combine these two last processes with a combined destemmer–crusher machine. Using this machine means that sorting can only be done at the level of whole bunches.

The mixture of grape juice, pulp, skins, seeds that comes from the crusher is commonly termed ‘must’. For white wines, must may also refer to the grape juice that is fermented (pressing and clarification means pulp, skins and seeds have been removed). Hence, in winemaking, ‘must’ typically refers to the substance that is being fermented.

Question 5

Explain the process of pressing and the various presses?

Answer 5

In white winemaking, the grapes are almost always pressed to extract the juice from the grapes and to separate the skins from the juice before fermentation. In red winemaking, the grapes are typically crushed before fermentation and pressed after the desired number of days on the skins or at the end of fermentation.

An important choice is the type of press to use:

• Pneumatic presses –These are currently the most popular type of press used in wineries throughout the world. They are also called ‘air bag presses’. The press is made up of a cylindrical cage with a bladder that runs down the side or middle of it. Grapes are loaded into the tank (on one side of the bladder). The other side of the cage is filled with air and, as the bladder inflates, the grapes are gradually pushed against grates on the side of the cage, separating the juice or wine from the skins. The advantages of the pneumatic press are that it can be programmed to exert different amounts of pressure (light pressure for less extraction, harder pressure for greater extraction, which can provide different blending components if needed) and that it can be flushed with inert gas before use to protect the juice or wine from oxidation. Pneumatic presses are common in wine regions around the world in medium to large-scale wineries that can afford the initial investment.

Basket presses – These are a more traditional form of press, but are still used by some winemakers. They are also called ‘vertical presses’ or ‘champagne presses’. A ‘basket’ is filled with grapes and pressure is applied from above. The juice or wine runs through gaps or holes in the side of the basket and is collected by a tray at the bottom of the press. A pipe transfers the juice or wine to another vessel. Basket presses are not sealed vessels, and therefore cannot be flushed with inert gases to avoid oxygen exposure. Some winemakers believe these to be gentler than pneumatic presses. However, they generally hold a smaller press load, are much more labour intensive and are therefore most suited to small wineries making premium wines. They are to be found in wine regions around the world.

Though less widely used, other types of presses include the horizontal screw press and the continuous press. The horizontal screw press is similar to a basket press mounted horizontally above a rectangular draining tray. It is less gentle than many other types of press and therefore less popular. This press, as well as pneumatic and basket presses, all require batch processing. A volume of grapes is loaded into the press, they are pressed, the skins are removed, the press may be cleaned and the next batch is then loaded. This can take a lot of time.

The continuous press allows grapes to be continually loaded into the press as it works by using a screw mechanism; this allows for quicker pressing of large volumes of grapes. However, it is also less gentle than pneumatic and basket presses and therefore best suited to producing high volumes of inexpensive wines. Consequently, they tend only to be used to produce some inexpensive, high volume wines.

Most modern presses are computerised. The winemaker can program the pressure and length of the press cycle to obtain the desired results. Applying less pressure will extract less tannin and colour from the skins, but will result in a smaller volume of juice/wine. There can therefore be a compromise between the quality of the juice/wine and volume of wine that can be made. A longer press cycle extends the contact between the skins and the juice/wine, which extracts more aroma/flavour and tannin.

The solid remains of the grapes left after pressing is called pomace.

Question 6

What must adjustments can a winemaker make?

Answer 6

Winemakers can make a number of adjustments to the must. The general aim is to create a more balanced wine, especially if there has been a compromise in achieving optimum ripeness of sugars, acids, tannins and flavours. Adjustments to the must are generally made after must clarification for white wines. (Adjustments can also be made after fermentation.)

ENRICHMENT

It is common practice for winemakers in cooler climates to enrich the must either before or during fermentation to increase the alcoholic content of the final wine. The general EU term ‘enrichment’ refers to a range of practices: adding dry sugar, grape must, grape concentrate or rectified concentrated grape must (RCGM – manufactured, flavourless syrup from grapes) and the processes of concentration (reverse osmosis, vacuum extraction, chilling).

The common practice of adding dry sugar is also known as chaptalisation (after Jean-Antoine Chaptal, whose name came to be associated with the practice). The source of the sugar can be beet or cane sugar. In the EU this is allowed within limits in the cooler parts of Europe. Warmer areas (broadly southern Europe) are not permitted to add sugar, but they can add grape concentrate or RCGM, again within limits. With rising average temperatures in Europe during the growing season, there may be less need for enrichment.

In practice, adding sugar is done when fermentation is underway because the yeasts are already active and can therefore cope better with the additional sugar in the must.

Sugar levels in musts can also be concentrated by technological means of removing water: reverse osmosis, vacuum evaporation or cryoextraction (freezing the must, or even the final wine, and removing ice from it). The first two of these options are expensive because of the initial outlay on the machines used and therefore are limited to wines that will have a high return or wineries that produce high volumes of wine. Cryoextraction tends to cost less and so may be used more widely. In all cases, the costs must take into account that after these concentration processes there will be less wine to sell.

REDUCING ALCOHOL

In warm or hot regions where sugar can accumulate in the grapes quickly, it may be desirable to lower the potential alcohol of the wine slightly by adding water to the grape must. This is only legal within some countries or regions (e.g. in California water may be used within the addition of other wine processing additives). However, adding water also dilutes the grapes’ aromas/flavours and acids. Other ways of reducing the alcohol content involve removing alcohol from the wine (see Post-fermentation Adjustments).

ACIDIFICATION

In warm climates without any cooling influences, the malic acid in grapes tends drops dramatically as the grapes ripen. If the wine is not acidified, it could lack freshness. Acidification can also be used to lower pH (the benefits of low pH levels are covered in Wine Components). Acidification is routine in most warmer parts of the world for inexpensive and mid-priced wines and many premium wines.

Acidification is typically carried out by the addition of tartaric acid, the acid characteristic of grapes. Other options are:

• citric acid (though not permitted in the EU for acidification)
• malic acid (less used as it could be turned into lactic acid by malolactic conversion)
• lactic acid (may be used if adjustments need to be made after malolactic conversion; it tends to taste less harsh than the other acids).

Acidification can take place before, during or after fermentation. However, winemakers typically prefer to acidify before fermentation starts to benefit from the effects of a lower pH and because they believe that the acidity added at this stage integrates better within the profile of the wine as a whole. However, total acidity and pH are affected during the various winemaking processes, including malolactic conversion (if allowed to occur) and tartrate stabilisation (see Stabilisation in Finishing and Packaging); therefore, the winemaker must take this into account when deciding the amount of acid to add.

Within the EU, winemakers are not allowed both to chaptalize and to acidify musts. This is to prevent wines being ‘stretched’ by the two additions.

DEACIDIFICATION

In cool climates where grapes may have to be picked before they are fully ripe (e.g. due to the threat of poor weather), it may be necessary to deacidify the must or wine. Any calculation of the desired final level of acidity will need to take account of the lowering of acidity brought about by malolactic conversion. Deacidification is carried out by adding calcium carbonate (chalk) or potassium carbonate, and it lowers acidity by the formation and precipitation of tartrates. A high-tech option is deacidification by ion exchange. This last option requires considerable investment or hiring expensive machinery. The producer will need to check that this option is legal in the intended country of sale. Producers in the EU will need to ensure that they only deacidify within the legal limits set by EU law.

ADDING TANNINS

Powdered tannins may be added to help to clarify musts and, in the case of red wines, to help to stabilise the colour of musts and improve mouthfeel. Tannins may either be added to the must before fermentation or to the wine before maturation.

Question 7

Explain the process of alcoholic fermentation?

Answer 7

Alcoholic fermentation is the conversion of sugar into ethanol (also known as ethyl alcohol) and carbon dioxide carried out by yeast in the absence of oxygen (‘anaerobically’). This conversion also produces heat, which has to be managed.

YEAST

Yeast is the collective term given to the group of microscopic fungi that convert sugar into alcohol and affect the aroma/flavour characteristics of wines. Initially, yeast need oxygen to multiply quickly, but once any oxygen is used up by the yeast (in aerobic respiration), they switch to fermentation. The yeast species that are most often used in winemaking convert the sugars in the must to produce alcohol if given the right conditions: a viable temperature range, access to yeast nutrients, especially nitrogen, and the absence of oxygen. As well as alcohol, carbon dioxide and heat, the process also produces:

• volatile acidity (vinegar and nail polish remover smell); in standard fermentation, not enough is produced to be perceptible
• very small amounts of naturally-produced SO2
• wine aromatics
  + from aroma precursors: aroma precursors are compounds that have no flavour in the must, but are released by the action of yeast and create aromas in wine. Examples include thiols (e.g. 4MMP, which gives aromas of boxwood/gooseberry in Sauvignon Blanc), and many terpenes (e.g. linalool and geraniol, which give Muscat its floral, grapey aroma)

+ created by yeast: for example, esters, which give many fruity flavours (e.g. banana flavour in wines made with carbonic maceration and recently released – for instance, Beaujolais Nouveau). The action of some species or strains may also produce detectable levels of undesirable reductive sulfur compounds (rotten eggs, rotten cabbage) and acetaldehyde (bruised apple, paint thinner).

• glycerol, which increases the body of the wine.

Saccharomyces cerevisiae is the most common species of yeast used in winemaking. It can withstand well the high acidity and increasingly high alcohol level of the must as it ferments. It is fairly resistant to SO2 in comparison to other yeast species. It reliably ferments musts to dryness. There are many strains within the species, which gives rise to the option to choose a strain (known as a selection) for particular outcomes. For example, some winemakers will select a strain of yeast to boost the aromatic character in Sauvignon Blanc (e.g. mid-priced Marlborough Sauvignon Blanc) while other winemakers may choose either ambient or cultured yeasts that produce a more restrained fruit profile (e.g. in Sancerre). Other species of yeast are used for particular wines, for example, Saccharomyces bayanus for must with high potential alcohol or for re-fermenting sparkling wine.

For any one fermentation vessel, the winemaker must choose between using ambient or cultured yeast.

Question 8

What is ambient yeast and what are its advantages and disadvantages?

Answer 8

Ambient yeasts (also called wild yeasts) are present in the vineyard and the winery. They will include a range of yeast species (e.g. Kloeckera and Candida), most of which will die out as the alcohol rises past 5 per cent. Typically, Saccharomyces cerevisiae quickly becomes the dominant yeast, even in ‘wild fermentations’.

Advantages:

• Ambient yeast can add complexity resulting from the presence of a number of yeast species producing different aroma compounds.
• It costs nothing to use.
• Recent studies have shown that the dominant yeast population in a must is unique to a place or region, thus supporting the idea that yeast strain contributes to the individuality of wines or even the terroir of a wine.
• Using ambient yeast may also be used as part of the marketing of the wine.

Disadvantages:

• Fermentation may start slowly. This can be dangerous for the build-up of unwanted volatile acidity and the growth of spoilage yeasts (such as Brettanomyces) and bacteria, potentially leading to off-flavours.
• Fermentation to dryness may take longer, which may not be desirable in a high volume winery. There is also increased risk of a stuck fermentation (fermentation ceases or slows) leaving the wine in a vulnerable state to spoilage organisms.
• A consistent product cannot be guaranteed, which can be a drawback, especially for producers looking for consistency over many large vessels or across vintages.

Question 9

What is cultured yeast and what are its advantages and disadvantages?

Answer 9

Cultured yeast (also sometimes called selected yeast or commercial yeast) are yeast strains that are selected in a laboratory and then grown in volumes suitable for sale. Commercially available cultured yeasts are often single strains of Saccharomyces cerevisiae.

To use cultured yeast, the must may be cooled down to prevent fermentation by ambient yeast and then the cultured yeast added, which quickly overwhelm the natural yeast population. Another option is to add SO2 to the must to suppress ambient yeasts. A starter batch, made up of fermenting grape must activated with the cultured yeast, is then added to the tank of must to be fermented.

Advantages:

• Cultured yeast produces reliable, fast fermentation to dryness.
• Cultured yeast produces low levels of volatile acidity and, given its speed and reliability, there is less danger from spoilage yeasts and bacteria. Many winemakers report that the reason they use cultured yeast is for the security of a clean, completed ferment.
• Cultured yeast also helps to produce a consistent product from one vintage to another.
• With a large selection of cultured yeast strains available commercially, the winemaker’s choice can also affect the style of wine created. This ranges from the choice of neutral yeasts for a sparkling wine base to enhancing the floral or fruity characteristics of aromatic varieties.

Disadvantages:

• Some believe that using cultured yeast leads to a certain similarity of fruit expression (and hence the charge of ‘industrial wine’).
• Using cultured yeast adds the cost of using a commercial product.

While the fermentation itself does not require exposure to oxygen, this is needed at the early stages to enable yeast to multiply rapidly at the beginning of fermentation. Winemakers may also add yeast nutrients, particularly if nitrogen levels in the must are low. Low levels of nitrogen in the must can lead to a stuck ferment and ‘rotten egg’ smells from undesirable sulfur compounds created by yeast that are stressed by the low nitrogen levels. Diammonium phosphate (known as DAP) or thiamine (vitamin B1) can be added as yeast nutrients.

Question 10

Explain the effects of temperature

Answer 10

The speed of fermentation is related to the temperature of the must, which in turn affects the style of wine being produced. Winemakers may favour a relatively warm start to fermentation (e.g. 25°C / 77°F), and then monitor it regularly and cool or warm the must as required.

As stated, the fermentation temperature helps to define the wine style:

Cool: 12–16°C / 54–61°F
Fresher, fruitier white wines and rosé

Cool temperatures promote the production and retention of fruity aromas and flavours

Mid-range: 17–25°C / 63–77°F
Easy-drinking fruity red wines to retain fruit aromas and for low tannin extraction

Middle of this temperature range for less fruity white wines, top of this range for barrel-fermented white wines (to reduce formation of fruity esters e.g. isoamyl acetate)

Warm: 26–32°C / 79–90°F

Used for powerful red wines

Maximum extraction of colour and tannins, but can result in some loss of fruity flavours

Above 35°C (95°F) the fermentation may slow down and stop as yeasts struggle to survive, with risk of a stuck fermentation. Hence, the temperature must be controlled to prevent this from happening.

Question 11

Explain the different fermentation vessels?

Answer 11

STAINLESS STEEL
– This is the modern standard as it is easy to clean, comes in a large range of sizes and enables a high degree of control over the temperature of the must or wine. These are neutral vessels and so are very good at protecting the wine from oxygen, and they also do not add any flavours. Stainless steel tanks are the most common type of vessel used in modern, high- volume wineries due to price, hygiene and a very high level of mechanisation possible (automatic pump-over, temperature control, automatic emptying, etc.). They can require substantial initial financial investment in the tanks themselves and in computerised temperature-control systems.

CONCRETE
– Concrete vats were an inexpensive option in the last century, with the vats being built in situ on a large scale. They are now coming back into fashion because of their high thermal inertia: they maintain an even temperature much more efficiently than stainless steel. Smaller, egg-shaped vessels in concrete, which are very expensive, are said to set up convection currents that mix the fermenting must and mix the lees during maturation (see The Role of Lees in Still Wine Maturation).

WOOD
– Some areas of Europe have retained their traditional large wooden fermentation casks (e.g. 1000 litres or above in Alsace, Germany or Italy). Wood retains heat well. However, great care has to be taken with hygiene as the pores in wood can harbour bacteria and spoilage organisms. Some winemakers value the small amount of oxygen that fermenting red wine in oak provides. They can be reused many times and so are inexpensive over the long term. However, they require capital investment when new large oak casks are bought. White wines may also be fermented in small wooden barrels. This is relatively rare for red wines due to the need to manage the cap of skins. For further details on wooden vessels, see The Role of Wood in Maturation.

ALTERNATIVES
There are also a number of alternative options. Plastic vessels are light, versatile and useful for small-batch fermentations. However, plastic is permeable to oxygen and it can be difficult to control the temperature in plastic vessels. Terracotta has been used historically (e.g. amphorae in Georgia) and is in use in small-scale production today.

Question 12

Explain malolactic conversion?

Answer 12

Malolactic conversion, often called malolactic fermentation (MLF, ‘malo’), is the result of lactic acid bacteria converting malic acid into lactic acid and carbon dioxide, and it produces heat. It typically happens after alcoholic fermentation and occasionally during it. Certain conditions encourage it to happen: 18–22°C (64–72°F), a moderate pH (3.3–3.5) and low total SO2. Historically, it often happened spontaneously in the spring following harvest as temperatures rose in the cellar. Now the process can be started by adding (‘inoculating with’) cultured lactic acid bacteria and making sure that the optimum conditions are available.

Certain conditions inhibit or prevent malolactic conversion taking place: temperature below 15°C (59°F), a low pH and moderate levels of SO2. If winemakers want to ensure that it is less likely to happen, they can add the enzyme lysozyme, which kills lactic acid bacteria, or move any batch of wine going through malolactic conversion to another part of the winery to avoid the spread of lactic acid bacteria. Alternatively, lactic acid bacteria can be filtered out to avoid malolactic conversion taking place.

Red wines routinely go through malolactic conversion. It is a winemaker’s choice for white wines. The outcomes of malolactic conversion are:

• Reduction in acidity and rise in pH
• Some colour loss in red wines
• Greater microbial stability
• Modification of the flavour

Some winemakers choose to conduct malolactic conversion in barrels for both white and red wines rather than in larger batches in tanks. The advantages are the ability to be able to stir the lees at the same time and promote better integration of the flavours. However, this is more work because barrels may be at different temperatures and so will need monitoring individually. Some winemakers prefer to promote malolactic conversion at the same time as alcoholic fermentation. Some studies have shown that this can increase fruity characteristics (or alternately not detract from the final wines) and reduce production times, saving money as wines can be finished and sold earlier.

Question 13

Explain post-fermentation adjustments?

Answer 13

REMOVAL OF ALCOHOL

It may be desirable to remove alcohol either to produce a reduced alcohol wine (e.g. below 5.5 per cent) or to adjust the level of alcohol marginally. The simplest solution for marginal adjustment (where permitted) is to add water to the must. However, adding water also reduces the intensity of flavour. High-tech solutions include:

Reverse osmosis – A form of cross-flow filtration that removes a flavourless permeate of alcohol and water, which can be distilled to remove the alcohol. The permeate is then blended back to recreate the wine. This is the most common high-tech option. The equipment can be rented or bought, but is generally costly.

Spinning cone – A device that first extracts volatile aroma compounds from wine and then removes the alcohol. The flavour components are then blended back into the wine of the desired alcohol level. This technology is only financially viable for large volumes of wine.

Within the EU, alcohol reduction by these last two means is legal, but within specified limits.

COLOUR

Winemakers may wish to reduce unwanted colour tints, for example by fining the wine (see Fining in Post-fermentation Clarification) or, in volume wines, enhance colour intensity in a red wine by adding very small amounts of the grape-derived colouring agent, MegaPurple. This is not permitted within the legislation of some regions (e.g. Ribera del Duero).