Acids are present in all fruits in varying amounts. As the fruit is developing, the acid serves to protect the seeds until they are ready; the sour or bitter taste of unripe fruit makes them unappealing for consumption. As the fruit ripens the acid content generally decreases whilst the sugar content increases, until the fruit reaches ripeness.
The main acids measured in fruit juices are citric, tartaric and malic. Different types of fruit contain differing levels of these acids. Ascorbic acid is also present in much lower concentrations.
Present in most fruits, particularly citrus products. For example, lemon juice typically contains around 5% citric acid. Citric acid is a weak organic acid that is used in many food products, either to add a sour taste or to help preserve the product by reducing the pH. The citric acid consumed in food serves no dietary function and is neither good nor bad for the health. The body also naturally produces citric acid for use in the Krebs cycle, a process which is an essential part of the metabolism for almost all life forms.
Present in many types of fruit including apples, strawberries, plums, tomatoes and grapes. Malic acid is an organic acid that again has a sour taste, and is used in some products for this purpose.
Present in a range of plants including grape and bananas, tartaric acid is also added to foods both to give a sour taste and for antioxidant properties.
In general, acid content of fruit juice is measured as it allows some characterisation of the taste of the product. What one perceives as "sweetness" is generally due to the relative proportions of sugar and acid in the product, rather than simply the total content of either. It is therefore common, particularly with citrus products, to characterise juice in terms of the ratio of Brix to acidity, and indeed this often simply referred to as ratio within the juice industry. For example, orange juice is often sold against a specification detailing this ratio, as this has a arguably the greatest impact on the taste. The most common orange juices available typically have a ratio of around 14-16, which is largely due to the type of fruit and the growing conditions in Brazil, the largest commercial exporter of orange products. Orange Juice from some regions (such as Florida, Jamaica) may be much sweeter, with ratios over 20 available, whilst other areas (e.g. parts of Africa) grow oranges with a ratio as low as 10, which would taste much drier and sharper, approaching that of grapefruits for which very sweet examples may reach ratios as high as 10.
For general comparison purposes, very high sugar, low acid fruits such as dates may have ratios exceeding 100, whilst at the other end of the scale high-acid, low-sugar fruits such as lemons and limes may have a ratio approaching 1.
In addition to the main acids noted above, Lactic and Acetic acid are also found in many fruit juices, purees and concentrates. These are generally not naturally present in the fruit itself, but are generated as a product of bacteria digesting parts of the fruit. Levels of these acids are therefore often used as indicators of hygiene standards during storage and processing of the fruit, as high levels typically indicate that material may have been stored or processed improperly, allowing the relevant bacteria to flourish. Maximum levels are not formally defined within legislation, but many processors work according to acceptable tolerances achievable with good manufacturing practice as defined for different fruits by the AIJN (click here for more information about AJIN).
Lactic and acetic acids are not dangerous to human health and indeed are found as an intentional component of many other food products (e.g. acetic acid is a primary component of vinegar, lactic acid is present in dairy products such as yoghurt, cottage cheese). Excess levels in fruit products are therefore avoided as these can compromise the taste of the juice.