Acids in citrus fruits

Fruit compresses have been applied to the face as beauty aids for millennia. The alpha-hydroxy acids contained in fruit extracts, tartaric acid in grapes, citric acid in citrus fruits, malic acid in apples, mandelic acid in almond blossoms and apricots are thought to be active principles for skin rejuvenation. Such alpha-hydroxy acids do stimulate HA production in cultured dermal fibroblasts (unpubl. exp.). The results of such alkaline preparations may depend more on their peeling effects rather than on the ability of alpha-hydroxy acids to stimulate HA deposition. 

Tasting chemical solutions is not a good practice. However, we have all had acids such as ascorbic acid (vita- response to exposure tO either acids or min C), acetylsalicylic acid (aspirin), and citric acid (in citrus fruits) in our mouths, and we are familiar with bases their characteristic sour taste. Soaps, which are basic, have the characteristic bitter taste of bases. 

For routine analysis a measurement of titratable acidity is usually sufficient, with the acidity of the fruit/fruit product calculated as the predominant acid, e.g., as citric acid in citrus fruits, as malic acid in apples. The sample is diluted in distilled water and titrated against dilute sodium hydroxide to either a phenolphthalein endpoint or to pH 8.10. Volatile acidity, generally expressed as acetic acid, can be measured by distilling the sample using a steam distillation apparatus, with titration of the distillate as above. Individual acids can be determined using a... 

Downer, A. W. E. 1942. The stability of ascorbic acid in citrus fruit juice products. J. hoc. Chem. Ind. 61, 80-82. 

Several of the TCA cycle acids are so named because they accumulate to high concentrations in certain plant tissues— malic acid in apple Pyrus malm), citric acid in citrus fruits and fumaric acid in fumitory [Fumaria officinalis). Accumulations of this sort (there are a number of other examples) were initially taken as evidence that the TCA cycle was not operating rapidly in these plants. However when C-labelled TCA acids are fed to such tissues and then the specific activities (radioactivities per unit amount of carbon) of the evolved carbon dioxide and of the TCA acids within the tissue are measured it is found that whereas the specific activity of the carbon dioxide may be the same from two different acids, the specific activities of these acids in the cells may be very different indeed. This indicates that the whole cellular content of the acids does not participate in the TCA cycle, part of the acid is participating in the TCA cycle, part is remote from the site of this cycle. In certain crassulacean plants which contain high concentrations of malic acid it can be shown that there is a small metabolically active pool associated ivith the TCA cycle and a large pool which is relatively metabolically inert. Equilibration of label between these pools is slow and it is probable that the inert pool corresponds to the vacuole and the active pool with an intramitochondrial compartment. 

Carboxylic acids with one acid group are known as monobasic acids while those with two acid groups are dibasic acids. All acids with more than one acid group are in the class of polybasic acids. The simplest organic acid, formic acid, is responsible for the irritation of bee and ant stings. Vinegar is a 5% solution of acetic acid in water. The acetic acid is responsible for the characteristic sour taste. Citric acid, found in citrus fruits and used in soft drinks, is a tribasic acid with three carboxylic acid groups. The dibasic acid, adipic acid, is a major component of nylon. 

Alpha hydroxy acids (AHAs) are water-soluble substances and thereby penetrate the outermost epidermal skin layers. In contrast, beta hydroxy acids (BHAs) are lipid (fat) soluble and are capable of penetrating to the underlying layers of skin (the dermis) located 1-5 mm below the surface of the skinJ2 Most AHAs are derived from plant materials and marine sources. Commonly used AHAs include malic acid (found in apples), ascorbic acid (a common ingredient in numerous fruits), glycolic acid (a constituent of sugar cane), lactic acid (a component of milk), citric acid (naturally abundant in citrus fruits), and tartatic acid (found in red wine). A common BHA is salicylic acid (an ingredient in aspirin). 

You find them in citrus fruits (citric acid), vinegar (acetic acid), aspirin (acetylsalicylic acid), and numerous other natural and synthetic compounds, as well on numerous organic exams. In this chapter you explore the structure, synthesis, and reactions of these acids and acids like them. 

The sour taste of fruit is due to the fruit s own brand of acid. "Citrus fruits, for example, contain citric acid. In other fruits the sour taste is often disguised by the sweetness of fruit sugars. 

Ascorbic acid is widely distributed in nature, but it occurs in especially high concentration in citrus fruits and green plants such as green peppers and spinach. Ascorbic acid can be synthesized by all plants and animals with the exception of humans, other primates, and guinea pigs. Therefore, vitamin C must be present in our dietary substances. 

B Risch, K Herrmann. Hydroxycinnamic acid derivatives in citrus fruit. Z Lebensm Unters Forsch 187 530-534, 1988. 

Occurring widely in nature, malic acid is closely associated with apples. It is the second major acid, after citric, found in citrus fruits and it is present in most berry fruits. Malic acid is slightly stronger than citric in perceived acidity, imparting a fuller, smoother fruity flavour. 

The organic acids of citrus fruit include a group of carboxylic acids (23) with different acids predominant in various component parts of the fruit. Citric acid is the main acid in the juice, representing from 80 percent of the total acidity in juice from ripened oranges, about 90 percent of that of grapefruit and nearly all of that of lemon. The pH of the juice greatly affects the sourness of the product (24) and is, in turn, affected by the cations, especially potassium. The major acids in citrus peel are malic, oxalic (25), malonic (26), and quinic (27). 

The amount of protein in citrus fruit is relatively low (Table II), and the juice and peel have about the same amount (29). Much of the value that is considered as protein is either free amino acids or non-protein constituents which contain nitrogen. 

Nutritionally, the most important water-soluble vitamins in citrus fruits are ascorbic acid, folic acid and pyridoxine. Clinical studies on the bioavailability of these vitamins, as well as basic research on the absorption and chemistry of these vitamins, have yielded valuable information adding to our overall understanding of the nutritional quality and bioavailability of these vitamins found in citrus fruits. 

Versteeg (8) speculated on the function of PE in vivo. He noted the high activity of PE in citrus fruit compared to the amount of available pectin. The fruit contain sufficient activity to deesterify the pectin to low methoxy pectin in 10 min at optimum pH. He suggested that the methyl transferase found by Kauss and Hassid (39) to esterify pectic acid to pectin in mung bean shoots and to be located in a lipid-membrane complex (31) functioned as pectinesterase after the lipid membranes were destroyed and the environment changed. However, no definitive experiments to establish the role of PE in fruits were reported. 

Although limonoic acid A-ring lactone has been shown to be a substrate of a lactonase that catalyzes the formation of limonin in citrus fruit, several other enzymes in citrus can also use the A-ring lactone as substrate, however, the products are not bitter products. These enzymes and their reactions are reviewed in the next section. 

Fuzfai Z and Molnar-Perl I, Gas chromatographic-mass spectrometric fragmentation study of flavonoids as their trimethylsilyl derivatives Analysis of flavonoids, sugars, carboxylic and amino acids in model systems and in citrus fruits. J Chromatogr A 1149 88-101 (2007). 

Citric acid is present in citrus fruits. It is composed of carbon, hydrogen, and oxygen. When a 0.5000 g sample of citric acid was subjected to carbon-hydrogen combustion analysis, 0.6871 g of carbon dioxide and 0.1874 g of water were produced. Using a mass spectrometer, the molar mass of citric acid was determined to be 192 g/mol. 

Write the equation for the reaction of citric acid, HgC HgO (the acid present in citrus fruits), ahd potassium hydroxide to form water and potassium citrate, Is citric acid a monoprotic acid, a diprotic acid, or... 

Vitamin C is used to metabolize carbohydrates, for tissue repair and capillary endothelium, and for synthesis of protein, lipids, and collagen. Vitamin C is also needed for absorption of iron and folic acid metabolism. Vitamin C is found in citrus fruits, tomatoes, leafy green vegetables, and potatoes. Excess serum levels of vitamin C are excreted without any negative effects. Vitamin C is commercially available as Ascorbicap, Cecon, Cevalin, and SoluCap C. 

---