Acidity

Fruit Acids

Fruit acid is not a scientific term, but it is a term that has practical use. Chemically speaking, most fruit acids can be α-hydroxy-acids (AHA), dicarbonic acids, often both. It is important to understand that not all acids are created equal when it comes to taste. In fact, besides having an acidic taste, they have a very different mouth-feel.

In wine-making, the most important fruit acids are (in alphabetical sequence) citric acid, fumaric acid, gluconic acid, glycolic acid (metabolises to oxalic acid!), α-hydroxycaprylic acid, lactic acid, malic acid, mandelic acid (from amygdalin in many fruits like bitter almonds, chokecherries, black cherries, apricots), oxalic acid, salicylic acid, and tartaric acid.

There are additional acids that can develop during fermentation and these acids are undesirable, for example acetic acid, butyric acid, or propionic acid. These are products that develop only when fermentation goes wrong, meaning that micro-organisms other than the wine yeast are involved, or that oxygen (from air) could get to the wine, leading to the oxidation of ethanol to acetic acid. In alcohol-rich wine, the latter occurrence can build up some ethyl acetate, which gives a flavour of nail polish to the wine – obviously not what we want.

Although present only in small amounts in fruit, ascorbic acid (vitamin C) deserves special mention for two reasons: it is a vitamin, and synthetic ascorbic acid is sometimes added as an anti-oxidant to wine. The regulations of the EU only allow for a maximum concentration of 150 mg/l of ascorbic acid.

Succinic acid should also be mentioned as a naturally occurring acid in grapes, but it plays no significant role among the fruit acids outside of the creation of the ester mono-ethyl succinate, which adds a fruity aroma to wine. However, it can be present in wine in higher concentrations than in fruit, this is because it is produced in small quantities by yeast as a side-product of the yeast’s natural metabolic activity.

Please follow link to find information about other Fruit Acids.

Acidity

As the diagram below shows, the acid content of fruits can vary over a wide range.

The acidity of wine is one of the crucial factors that gives body and balance to finished wine. For this reason, acidity control must be done properly – just right before start of fermentation, with little room for errors. Unfortunately, there is no simple way to measure acidity (which is normally done by titration). However, it is easy to estimate the amount of acid that has to be added in wine-making; this is described in the recipes provided in the Recipes section. All fruit species have “acid signatures,” they operate based on the types and concentrations of acids, with little variation in them.

Acidity can be measured as “total titratable acidity” (TTA), which is the determination of acidity by titration with a volumetric solution of sodium hydroxide. Titratable acidity is commonly expressed as tartaric acid equivalents (TAE), since tartaric acid is the most significant single acid in wine-making. Osorno lab performs TTA analyses. The minimum sample volume required for TTA analyses is 100 ml. More information about wine analyses can be found in Osorno Store.

Table 1 of TAE Conversion Factors of Fruit Acids

Acid Conversion Factor
Acetic 0.20
Citric 1.92
Fumaric 0.77
Lactic 0.39
Malic 0.89
Oxalic 0.60
Tartaric 1.00

As one can see in the Table 1, 1 g of citric acid corresponds to about the double amount of tartaric acid.

Within the TTA of wine, one can distinguish between volatile acids (example: acetic acid) and non-volatile acids (example: tartaric acid). Obviously, non-volatile acids are by far dominant in wine.

Acidity and pH

Despite common belief, pH and acidity do not have a linear relationship, because the pH scale is logarithmic, so that statements of the kind “10 grams of citric acid will increase pH by 0.2” are nonsense. With the necessary knowledge of chemistry, one can certainly calculate the pH value, however, the question will still be what the benefit might be. After all, pH in itself is a lot less significant than the total acidity, and what chemists call “buffer capacity” of the wine. Without going too deep into chemistry, let’s just say that buffer capacity relates to the mineral content, which is essentially determined by the amount of fruit used, or more precisely the fruit to water ratio. This also relates to the mouth-feel of wine, because the buffer capacity influences the body of the finished wine. If need be, the buffer capacity can be increased by adding certain minerals. Some wines such as mead and dandelion wine are notoriously mineral deficient. This cannot be compensated by using mineral-rich water, because these waters will most likely still be lacking a sufficient potassium concentration. Potassium hydrogen tartrate, commonly known in the English-speaking world as “cream of tartar”, will serve this purpose.

The pH value is largely influenced by the nature of the fruit acid; some acids are “more acidic” than others. The “more acidic” is chemically expressed by what is called pKa value (acid dissociation constant); the table shows this for some select acids.

Table 2 of pKa of Fruit Acids

Acid

pKa

Acetic

4.76

Citric

3.13

Fumaric

3.03

Lactic

3.86

Malic

3.40

Oxalic

1.25

Tartaric

2.89

The lower the value in the table, the “stronger” the acid. For dibasic and tribasic acids, the given value refers to the first stage only.

Acidity Correction

AIf acidity needs correction, which is always the case for fruit wine, and occasionally needed for grape wine, one adds either tartaric acid, lactic acid, or citric acid. These acids are listed in the sequence of preference in which they should be used. Unfortunately for one’s wallet, this is the reverse sequence of price, with tartaric acid being the most expensive, and citric acid the cheapest. Undoubtedly, tartaric acid is best for the taste of the finished wine.

Naturally, most grape wines contains about 5 – 6 g/l acidity as tartaric acid equivalents (TAE). It is important to understand and remember that the acidity of wine is not expressed as TAE in all countries. Most notably, France uses an alternate scale. In France, wine acidity is given as sulfuric acid equivalents. In order to convert to TAE, the French acidity value should be multiplied by 1.53. So a wine with an acidity of 3.9 g/l in France does in fact have 6.0 g/l as TAE.

In the author’s experience, the minimum acidity value should be 5 g/l, and definitely not lower than 4 g/l. The risk of wine defects increases significantly when the acidity is too low.

Hardly ever will one see the need to decrease acidity when making wine; although if needed, this is typically done by using a sufficient amount of water in the recipe to dilute the original acidity of the fruit. As examples can be named wines from tamarind or lemon. Should the necessity arise, acidity can be reduced to the benefit of taste by adding a calculated amount of Acidity Neutralizer (usually potassium carbonate); this correction can also be done with finished wine. Unfortunately, potassium carbonate is hygroscopic and therefore not easily handled.

Example

Keeping with the theme of this book, we will do the example calculation based on a target value of 10 l of finished wine. Let us assume that we want to produce kiwi wine, starting with 2.5 kg fruit. Kiwi contains typically 500 mg malic acid and 990 mg citric acid per 100 g of fruit; those are the dominating acids in kiwi so that we can ignore others. This means that we have 5 g of malic acid and 9.9 g of citric acid per kg of fruit, which leads us to the 12.5 g of malic acid and 25.5 g of citric acid in the 2.5 kg of fruit that we intend to use in this batch.

In using the Table 1 of TAE equivalents provided above, we calculate that 12.5 g of malic acid is the equivalent of 11.1 g of tartaric acid, and that 25.5 g of citric acid is the equivalent of 49.0 g of tartaric acid, resulting in a total acidity of 60.1 g (as TAE) for the 10 l batch. This is perfectly within the range of an alcohol-rich wine with residual sweetness, in other words a dessert type wine. If we allow this wine to age properly, we have to take into account that it will lose its acidity through the loss of malic acid, owing to malolactic fermentation. In the end the aged wine will still be above the recommended threshold of 50 g TAE acidity.

Exercise

Calculate for the situation where 4 kg kiwi were used for a 10 l batch instead of 2.5 kg. This will show that the acidity increases well above a comfortable value, thus demonstrating the significance of the dilution effect of water. As this exercise shows, a calculation of the correct fruit to water ratio always has to be the first step before calculating the amount of sugar that has to be added.

Acidity and Colour

Obviously, nearly uncoloured fruit (for example, white grapes) do not give a coloured wine. It may be surprising that juice pressed from red grapes is coloured almost like juice from white grapes. As in grapes, the colour is often mainly in the skin of fruit, and can be released only when the initial fermentation takes place on the crushed fruit (maceration).

Almost all natural fruit colours, and most plant colours, are in a chemical group that is called anthocyanin (anthocyanins are the pigments that give red, purple, blue and black colours to plants and fruits). They exist in nature not just to enhance beauty, but to also serve a purpose, for example as the protection from the degenerating effect of the sun’s UV light on leaves and fruit. Their main stability range on the pH scale is between 3 and 4, which nicely matches the conditions that we have in wine. Evidently, the best colouration of wine can be reached when there is sufficient acidity during the maceration period.

It should be pointed out that the colour of all anthocyanins depends strongly on pH, which makes them suitable to be used as pH indicators. The colour changes from red to purple, then blue and black when pH goes up from 1 to 13.his sequence also explains why there are no true blue roses. This is because the pH of the petals is within the acidic range. Needless to say that blueberry skin is slightly alkaline, otherwise they would be redberries. From the information provided above, it should be clear why blueberry wine is dark red, not blue.

It must be mentioned that anthocyanins are largely not bio-available for the human body. It is commonly reported that about 99% are not resorbed in the human intestines, but stay with their anti-oxidant properties in the intestines. Despite the plethora of studies about the health effects of eating berries, it seems that only in recent years the bio-availability of anthocyanins has become a subject of research (Iva Fernandes, Frederico Nave, Rui Gonçalves, Victor de Freitas, Nuno Mateus; Food Chem. 135,2 812 (2012).

Prices and Availability

The prices of fruit acids vary widely; some of them are produced on a large industrial scale, for example citric acid, and some have as their sole source what can be found in nature, for example tartaric acid, for which the sole industrial source is wine. Osorno offers certain organic acids for wine making. To see availability please visit our Chemicals Section. If you would like to purchase wine acids or perform various wine analyses or to get advise about calculations of wine ingredients for wine start up, please visit Osorno Store.