Acid is an essential part of wine having three functions. Obviously acidity is an important taste and works in conjunction with the amount and style of tannins, the sweetness and alcohol content. Even when a wine is dry it can still contain some sugar content, unimplementable sugars like poly saccharides and the implied sweetness of alcohol. Acid balances the bitter and sweet tastes present and highlights fruit flavours though too much acid will start to dominate and over shadow the fruit used. Acidity is a fine balancing act and while there are no magic numbers there are generally accepted ideals. Personal taste is still the simplest determining factor for what a home wine maker wants.
As well as the “art” of acidity and taste, Acid acts biologically as a preservative. A red wine with a pH of 3.4 to 3.6 protects against most spoilage bacteria allowing a wine to age over years rather than months and allows it to develop deeper more complex flavours. In white wines there is generally stronger acidity so the pH is between 3.0 to 3.4.
Acidity is a double edged sword and while acidic wine are more protected from bacterial infection and growth they are more prone to oxidation with white wines being particularly at risk. This is because acid reacts chemically in wine. Oxidation is unwanted but usually the chemical reactions from acids are a catalyst to create tastes and mature wine. Acids bind tannins, fix colour pigments or alter ancoythanins to create ever increasing taste compounds.
TYPES OF ACID
There are many types of acid in wine but the most important is tartaric acid that is the dominant acid in grapes which are almost unique as a fruit because of this. Tartaric acid has a sharp taste giving wine its unique taste. As grapes are unique in this sense tartaric acid is often added to fruit wines via raisins or a direct tartaric additives to ape the characteristics of grape wine. One reason “banana water” is sometimes used to give body is because like grapes they are also high(ish) in tartaric acid. Tartaric acid can be seen as a basis that other combinations of acids sit upon hopefully creating a balance that please rather than pain you as you drink.
Malic acid is dominant in unripe grapes and certain ripe fruit like blackberries, cherries and apples and in a large volume can give a sharp unripe taste to wine. In moderation it is usually in wines but using the ripest fruit you can is important to bring balance. Malic acid will reduce naturally as a product of fermentation and an additional second malolactic fermentation can be done to further reduce it. Malic acid changes into the softer velety tasting Lactic acid during malolactic fermentation where an added bacteria culture ferments the acid just as yeast does with sugar. While lactic acid is a weak tasting acid it moves the wine towards the sourest profile if present in too much quantity creating a sour milk taste.
Tartaric and malic acid make up 90% of pre-fermentation grape wine acidity. Succinic acid is far less present and is created via fermentation and is important in creating esthers and complexity in wine as it reacts throughout the aging process. The taste can add a slight saltiness to a wine as well as bitterness. Levels of sucinic acid can be promoted by certain yeast selections and fermentation temperature and typical levels are 05g/L to 1.5g/L but can be higher, red wine usually has more present than whites.
Citric acid may well be the dominating acid in many fruit wines like blackcurrant, gooseberry or orange wine. It has a dominating “artificial” taste if in excess and especially if added just before bottling when it will not mellow out. Added early citric acid reduces in both taste and strength during fermentation and is seen as a very active biodynamic acid that can promote bacterial growth unlike tartaric acid that is an antioxidant and preservative.
Other acids present vary from Gallic acid added from oak aging and is as litle s 0.0001g/L, Ascorbic, Sorbic, Citramilic and many others can be present in minute amounts. The variety of acids combine to create a unique combination in your wine of sharp, velvety, mellow, sour, fresh, ripe, green or artificial sensations and even rancid or vinegar if Butyric or Acetic Acid is sadly present.
Acetic Acid is the acid present in vinegar and generally thought of as a wine fault ruining wine. A very talented chef friend and fan of the new Biodynamic Wine school likes a little VA (Volatile Acidity created by acetic acid) but anything more than a whiff of VA will make a wine taste like vinegar and probably turn it into vinegar if given enough time.
Wine acidity can be measured in two ways pH and TA. pH can be seen as a technical measurement to determine if a wine will be stable as it ages. TA measures the taste-able acidity on the tongue. The differing acids in your wine behave slightly differently to each other so one acid can affect pH more than TA and vice versa. The relationship between pH and TA is linked but not absolutely uniform.
pH is the measurement of both the total acids and alkalis present in a wine. It is based on a scale of 1 to 14. Water is neutral so sits in the middle at 7. 7 to 14 are alkaline and acids is measured “backwards” from 7 to 0. This means a high pH of say 6 is a low strength acid, while the lower the pH the stronger the acidity. Between each value the acidity increases by a factor of 10 as the scale is logarithmic. A small amount of alkalis and a lot of acid will be present in wine and that total will push it well into the mid ranges of acidity.
Most wine pH’s fall around between 3 to 4. 3.0 to 3.4 is desirable for white wines, while about 3.3 to 3.65 is best for reds. These are the pH readings pre-fermentation. At the start of fermentation acidity will have a low pH to take account of the ageing of the wine as the various acids are used as catalysts in chemical reactions creating the more complex tastes as it ages. Later when a red wine is popped open the pH will have risen to between 3.5 and 3.8. Whites will have a similar pH rise of about 0.1 to 0.3 pH.
Testing pH is easy either with a pH strip that is centred around the levels you need or using a more complex and expensive but more accurate pH meter. Beware these do need to be calibrated with a neutral solution and have a shelf life so can be expensive for an amateur.
TA – “Titratable Acidity” or “Total Acidity,” is another way of measuring acidity, this time discarding the ratio of alkalinity to acidity and measuring the taste-able acidity only by volume of the wine must. This is far more a measurement of the character of the wine being made rather than the more technical preservative pH measurement. It gives an indication of the genuine taste of the acidity and a hint of what the acids can do in terms of chemical reactions while fermenting and then bulk ageing.
Most red wines will have a total acidity of about 0.6 to 0.7% TA that converts nice and easily to 6 to 7g/L – that is 6 to 7 grams of acid per litre of wine. A white wine will have a higher TA between 6.5 to 7.5g/l and often fruit wines are thought to have slightly lower acidity of 5.5 to 6.5 g/l as they will have a more dominant fruit flavour that is intended to show through the acidity and take account of the generally less acidic nature of most non grape fruit – that is open to debate in my view and often many fruit wines are made with too little acid and taste flabby and thin in my view.
Dry White Grape Wines – 6.5 – 7.5g/L
Sweet White Grape Wine – 7 – 8.5g/L
Dry Red Grape Wine – 6 – 7g/L
Sweet Red Grape Wines – 6.5 – 8g/L
Sherry Grape Wines – 5 – 6g/L
Non-Grape White Wines – 5.5 – 6.5g/L
Non-Grape Red Wines – 5 – 6g/L
The character of a wine will be influenced by the acidity level you choose within the desirable limits of both pH and TA. A lower pH/higher TA will have a sharper acidity with more fruit forward tastes while a wine towards a higher pH/lower TA will be smoother but with less fruit evident. There is no right or wrong only personal preference for your palette. As an example Argentinian Malbecs have a lower acidity (higher pH) to a French Malbec that has a higher acidity (lower pH) Both use the same fruit but climate has determined the sugar and acidity level despite the same grapes being used to make the wine.
Testing TA is done an initially intimidating but actually simple to use kit. A sample of wine is taken. If it is at the start of making the wine and fruit will be macerating get a decent sample and use a blender to pulp the fruit then allow it to settle out – this allows the acidity of both the liquid must and the macerating skins to be tested. If an already fermenting wine is being tested the carbon dioxide must be shaken or boiled out to get an accurate result.
A 3mm sample can be diluted with pH neutral distilled water to make colour change more visible and then 3 drops of indicator solution added. After a little shake to disperse it sodium hydroxide is added drop by drop with good notes taken to remember how may millilitres are added. While the acid is active it will neutralise the sodium hydroxide solution to a clear colour, when all the acid is used the colour sticks. White wines are an easy test as the indicator is a purple colour and easily seen. A red wine requires a little more observation to see the original red wine colour turn to more brick red hue rather than a genuine colour change.
The millilitres of the sodium hydroxide indicator solution used is a direct representation of the TA present. As an example 3.5ml of sodium hydroxide added would be equal to 3.5g/L acid present or 3.5%. If you want a wine with a target of 6g/L acidity 2.5g/L would need to be added. My first use of a TA test kit I worked out that my blueberry wine needed 25grams of acid added to the 10 litres of must to get it to my desired 6g/L acidity.
Adjusting acidity should always be done before fermentation begins to give the yeast the best environment to ferment and multiply in. An unbalanced must can stress yeast and create sulphurous by products in the wine. A balanced wine also gives more time for optimal chemical processes to occur as the wine ferments as all the basic materials are already present to go on to make the more complex taste, aroma and colour elements. While acidity can be adjusted after fermentation or just before bottling this gives less time for it to react chemically and less ability to protect the wine from spoilage.
Adding acidity is as easy as adding an acid blend from a wine supply shop. A basic calculator is here: https://www.winebusiness.com/tools/?go=winemaking.calc&sid=5
Tartaric acid is an easy bench mark to use to increase acidity:
1g/L addition will increase the TA by about 1g/L and will decrease the pH by 0.1 pH units.
1g/L of Malic acid will increase TA by 1.12 g/L and will decrease the pH by 0.08 pH units.
1g/L of Citric acid will increase TA by 1.17 g/L and will decrease the pH by 0.08 pH units.
It should be notes that the larger the decrease in pH extra acid will be needed to be added as the pH scale is logarithmic rather than linear. Moving from pH 3.6 to 3.3 may need 4g/l of tartaric acid rather than just 3g/L
Using these measurements pH and TA can be adjusted at different rates if needed in extreme cases – the acids also taste different so careful consideration should be used. Citric acid may well compliment a high malic content in a plum, fig or blackberry wine. Adding tartaric acid may balance out the dominant citric acid in blueberry, strawberry or elderberry wine.
Riper fruit has less acid present so the easiest way to reduce acidity is to check the sugar content before you harvest. A refractometer is an inexpensive though certainly luxurious piece of kit to measure the sugar in fruit. The riper the fruit the more sugar will be shown on the refractometer and in all probability there will be less acid present. Obviously this is at harvest rather than actually making your wine.
If a wine tastes too acidic you may not need to actually adjust the acidity. Back-sweetening may cut through the mouth puckering acidic sensation and making wine is about a balance of flavours rather than numbers on a chart.
Personally I prefer dry wines so adding sweetness is not always an option. Cold crashing is a simple system where a wine is placed in a cold environment of only a few degrees or even 0°C so tartaric acid crystallises and precipitates out falling as wine diamonds. These can simply be left behind when the wine is racked. This methodology is not an exact science though and caution should be used to extract only the acid you need. Some wine makers choose to cold crash as standard and if tartaric acid was added it should be increased to 2g/L rather than 1g/L to take into account of the wine diamonds crystallising.
Potassium Bicarbonate or Calcium Carbonate (precipitated chalk) can actually be added to a wine to reduce acidity. Easy to use it is added as a powder and stirred into the must in several stages over an hour or so. The reaction is quick and clearly visible as a gentle fizz as the acid is neutralised and carbon dioxide is produced with calcium tartrate and calcium malate crystals eventually forming and sinking with the lees. Precipitated chalk is most reactive with Oxalic acid that is present in Apricots, figs, kiwi fruit, plums, red currants and most of all rhubarb but this is generally a rare acid in most fruit. Oxalic acid has a harsh lab made like taste and should always be managed to an acceptable level even if acidity is lowered below accepted levels and may have to be raised with tartaric additions! Precipitated chalk will de-acidify oxalic acid by choice then move onto tartaric acid before others. As it moves down this chain it becomes less efficient at de-acidification and could shift the mix of acidities to be “less wine like”
1g/L of Potassium Carbonate will reduce TA by 1.0 g/L
Calcium Carbonate / Precipitated Chalk: varies and between 0.7 and 1.5 g/L will reduce TA by 1g/L. No more than 17.5g (3 and a half tsp) of Calcium Carbonate should be used per British gallon of wine (4.5 Litres) as taste and residue can be left behind
MANAGING ACIDITY WITH MALOLACTIC FERMENTATION
Malolactic fermentation is both a way to reduce acidity and to manage the acids in the wine. Malolactic cultures are bacteria that ferment acid similarly to the way yeast ferment sugar into alcohol. Generally used in all red wines and an increasing number of white wines like Chardonnay it can also be used by home wine makers for both grape and fruit wines. Malolactic fermentation changes harsh malic acid into the softer velvety tasting Lactic acid that is less sharp on the tongue and promotes natural fruit flavours.
The culture can either be in a powder or liquid form and is easily poured into the wine. This fermentation is almost impossible to start below pH 3.2 and should ideally be done between pH3.2 to 3.4 so using Precipitated Chalk may need to be used to hit those figures. It is best to do after initial fermentation has stopped or at least radically slowed down. The wine should be around room temperature at 20°C and no sulphites like campden tablets should be used as the cultures are delicate and temperamental.
Once begun it will produce a slight barnyard/chicken coop odour and a thin brown film or small brownish clumps will probably form on the surface of the wine. This is totally different to the rotten egg smell of spoiled wine with the milky tendril Lactobacillus spoilage bacteria or white film yeast.
Malolactic fermentation can last as long as three months and in that time it should be left to its own devices – do not rack the wine during this time or add any preservatives. Keep the bungs in place and airlocks topped up. Any surface film or colonies will fall to the bottom of the demijohn with the lightest of agitation. If a few malolactic fermentations have been done over the years a culture may not be needed and spontaneous inoculation with ambient spores in the atmosphere will occur. I was lucky with an Elder and Black wine in which it happened and hope that it will occur in all my wines in the future. The fine lees left after racking from “malo” fermentation can be used to further inoculate other wines but this must be done immediately and the culture cannot be kept.
This has been my most indepth and most researched post so far but Im not a pro – any additions or corrections welcome!