ABSTRACT
References ...................................................................................................................................... 335
The term “organic acid” refers to organic compounds with acidic properties. The acidity of organic acids is associated with their carboxyl group (C(=O)OH), and therefore they are called carboxylic acids. Carboxylic acids undergo a substitution reaction in which the –OH groups are replaced. Organic acids can be classi ed according to the type of carbon chain (aliphatic, alicyclic, aromatic, and heterocyclic), their extent of saturation and substitution, and the number of carboxyl groups (mono-, di-, tri-carboxylic). Monocarboxylic acids, such as formic (1C), acetic (2C), propionic (3C), and butyric (4C) acids, are highly volatile liquids with a pungent taste. Butyric acids and others that have an aliphatic chain with more than four atoms are oily compounds, and hence considered as fatty acids. Lactic acid (3C) is a monocarboxylic acid; however, it contains a hydroxyl group and is considered as an alpha hydroxyl acid. Succinic acid (4C) is a dicarboxylic acid. Lactic and succinic acids have low volatility and thus are not considered as volatile acids. Oxalic (2C), malic (4C), and tartaric (4C) acids are also dicarboxylic acids; however, the last two contain one and two hydroxyl groups, respectively. Citric acid (6C) is the best-known tricarboxylic acid with one hydroxyl group
that is found in foods. Its isomer, isocitric acid, is used for authenticity determination in several juices. Pyruvic acid is a keto acid, produced in biochemical processes and is found in small amounts in microorganism-involved products, such as in yogurt and wine. Fumaric acid, which is the isomer of maleic acid, is an intermediate in the Krebs cycle and is found naturally in several plants and mushrooms. Some basic properties of organic acids found in foods are given in Table 10.1 and their chemical structures are shown in Figure 10.1. Organic acids that are present in foods come are either intrinsic, or added to foods to carry out some function (acidi er, chelator, antioxidant synergist, and antimicrobial), or produced by microorganisms. Organic acids are not considered as nutrients but they give a characteristic taste to food, increase palatability, and promote digestion. Therefore, they
TABLE 10.1 Some Properties of Organic Acids in Foods
are one of the major contributors to the avor, besides sugars and volatile compounds. A signi cant number of organic acids are involved in acid metabolism. Some organic acids decrease during ripening. In plants, a small part of them bind to cations to form salts. The amount of these salts can vary in the range of 3% in lemons or 20%–30% in pears [1-6].
