Malic acids

Malic acid has a smooth lingering taste. Although it also has a tart taste, this is not as sharp as that of citric acid, yet it is longer lasting. It has also been found to mask the bitter aftertaste of synthetic sweeteners, yet when using sweeteners such as aspartame, it was found that the slower increase to peak tartness does not overpower the sweeteners and as such, less sweetener is needed. However, in some end products such as citrus-flavored drinks, the initial sharpness associated with citric acid is preferable (Fowlds, 2002). 

Water used in many manufacturing processes often contains small amounts of hard water salts. However, malic acid is highly soluble, and the solubility of its calcium salt prevents cloudiness of end products containing malic acid as an acidulant. Malic acid and its salts have been confirmed by the FDA as GRAS to be applied as a flavor enhancer and acidulant as well as a pH control agent at levels varying from 6.9% for hard candy to 0.7% for miscellaneous uses (Fowlds, 2002). 

Malic acid is known in two active isomeric forms the naturally occurring isomer has the L-(-)-configuration. This dibasic acid as a member of the TCA cycle is formed by hydration of fumaric acid catalyzed by fumarase. In the glyoxylate 

Malic acid synthesis, which provides the racemic D,L-form, is achieved by addition of water to maleic/fumaric acid. L-Mahc acid can be synthesized enzymatically from fumaric acid with fumarase (Lactobacillis brevis, Paracolobactrum spp.) and from other C sources (paraffins) by fermentation with Candida spp. 

L- or L-lactic acid (pK = 3.86) is utilized as an 80% solution. A specific property of the acid is its formation of intermolecular esters, providing oligomers or a dimer lactide  

Such intermolecular esters are present in all lactic acid solutions with an acid concentration higher than 18%. More dilute solutions favor complete hydrolysis to lactic acid. The lactide can be utilized as an acid generator. Lactic acid is used for improving egg white whippability (pH adjustment to 4.8-5.1), flavor improvement of beverages and vinegarpickled vegetables, prevention of discoloration of fruits and vegetables and, in the form of calcium lactate, as an additive in milk powders. 

Lactic acid production is based on synthesis starting from ethanal, leading to racemic D,L-lactic acid (Formula 8.25) or on homofermentation Lactobacillus delbruckii, L. bulgaricus, L. leichmannii) of carbohydrate-containing raw material, which generally provides L-but also D,L-lactic acids under the conditions of fermentation. 

The L-( —)-form of malic acid occurs naturally in apples and other fhiits hence its German name Apfelsdure ( apple acid ). It is an extremely versatile 4-carbon building block possessing an additional carboxyl group at the 4-position that serves as a useful handle easily manipulated to provide a variety of synthetically useful functionalities. 

Although L-malic acid (1) can be isolated from a variety of fruits, it is manufactured industrially by the fermentation of fumaric acid with immobilized fumarase as the biocatalyst [1]. This process is capable of producing up to 30 tons of the acid per month. L-Malic acid is commercially available from many suppliers, and is relatively inexpensive. The current price is approximately 208/Kg. 

The cost of L-malic acid can be dramatically reduced by preparing it from L-aspartic acid, the current price of which is only 53.70 for 2 Kg. This process requires only a single step, in which the amino group of aspartic acid is converted to the desired hydroxyl function under nitrous acid deamination conditions [2]. The configuration of the chiral center is retained, affording 1 with 97% ee. 

The present chapter concludes with a table of physical data associated with the common derivatives of malic acid discussed throughout this part of the book. 

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Colourless prisms m.p. 130 C. Manufactured by treating maleic anhydride with water. It is converted to the anhydride by heating at By prolonged heating at 150 "C or by heating with water under pressure at 200 C, it is converted to the isomeric (trans) fumaric acid. Reduced by hydrogen to succinic acid. Oxidized by alkaline solutions of potassium permanganate to mesotartaric acid. When heated with solutions of sodium hydroxide at 100 C, sodium( )-malate is formed. Used in the preparation of ( )-malic acid and in some polymer formulations. 

Malic acid crystallizes in colourless needles m.p. lOO C. It o- curs in many acid fruits, such as grapes, apples and gooseberries. It can be prepared by microbiological processes using various moulds or from ( + )-bromosuccinic acid by the action of NaOH. 

Acetophenone similarly gives an oxime, CHjCCgHjlCtNOH, of m.p. 59° owing to its lower m.p. and its greater solubility in most liquids, it is not as suitable as the phenylhydrazone for characterising the ketone. Its chief use is for the preparation of 1-phenyl-ethylamine, CHjCCgHslCHNHj, which can be readily obtained by the reduction of the oxime or by the Leuckart reaction (p. 223), and which can then be resolved by d-tartaric acid and /-malic acid into optically active forms. The optically active amine is frequently used in turn for the resolution of racemic acids. 

Maleic acid may be prepared by warming malic acid with acetyl chloride, distilling the mixture under atmospheric pressure to isolate maleic anhydride, and hydrolysing the latter by boding with water. 

A. Maleic acid. Assemble the apparatus shown in Fig. Ill, 28, 1. Place 45 g. of dry mahc acid in the 200-250 ml. distilling flask and cautiously add 63 g. (57 ml.) of pure acetyl chloride. Warm the flask gently on a water bath to start the reaction, which then proceeds exothermically. Hydrogen chloride is evolved and the malic acid passes into solution. When the evolution of gas subsides, heat the flask on a water bath for 1-2 hours. Rearrange the apparatus and distil. A fraction of low boiling point passes over first and the temperature rises rapidly to 190° at this point run out the water from the condenser. Continue the distillation and collect the maleic anhydride at 195-200°. Recrystallise the crude maleic anhydride from chloroform (compare Section 111,93) 22 g. of pure maleic anhydride, m.p. 54°, are obtained. 

Optically inactive starting materials can give optically active products only if they are treated with an optically active reagent or if the reaction is catalyzed by an optically active substance The best examples are found m biochemical processes Most bio chemical reactions are catalyzed by enzymes Enzymes are chiral and enantiomerically homogeneous they provide an asymmetric environment m which chemical reaction can take place Ordinarily enzyme catalyzed reactions occur with such a high level of stereo selectivity that one enantiomer of a substance is formed exclusively even when the sub strate is achiral The enzyme fumarase for example catalyzes hydration of the double bond of fumaric acid to malic acid m apples and other fruits Only the S enantiomer of malic acid is formed m this reaction... 

The reaction is reversible and its stereochemical requirements are so pronounced that neither the cis isomer of fumaric acid (maleic acid) nor the R enantiomer of malic acid can serve as a substrate for the fumarase catalyzed hydration-dehydration equilibrium... 

In the resolution of 1 phenylethylamine using (-) malic acid the compound obtained by recrystallization of the mixture of diastereomeric salts is (/ )... 

Malic acid [97-d7-d] Malnutrition Malodor evaluation Malolacticfermentation Malonate [1797-75-7] Malonate esters Malonates... 

Acid content calculated as tartaric acid is about 6—7 g/L for best flavor and stabiUty. It is higher for tart low Brix musts and less important for sweet high Brix musts. High acid levels coiacide with a higher level of the second acid of grapes, malic acid. 

In addition to alcohoHc fermentation, a malolactic fermentation by certain desirable strains of lactic acid bacteria needs to be considered. Occasionally, wild strains produce off-flavors. Malolactic fermentation is desirable in many red table wines for increased stabiUty, more complex flavor, and sometimes for decreased acidity. Selected strains are often added toward the end of alcohoHc fermentation. AH the malic acid present is converted into lactic acid, with the resultant decrease of acidity and Hberation of carbon dioxide. Obviously this has more effect on the acidity the more malic acid is present, and this is the case in wine from underripe, too-tart grapes. Once malolactic fermentation has occurred, it does not recur unless another susceptible wine is blended. 

Ergonovine (100, R = NHCH(CH3)CH2 0H) was found to yield lysergic acid (100, R = OH) and (+)-2-aminopropanol on alkaline hydrolysis during the early analysis of its stmcture (66) and these two components can be recombined to regenerate the alkaloid. Salts of ergonovine with, for example, malic acid are apparently the dmgs of choice in the control and treatment of postpartum hemorrhage. 

Many natural and synthetic organic compounds are hydroxy dicarboxyhc acids (see also Hydroxycarboxylic acids). This article discusses mainly malic and tartaric acids thiomalic acid is included because of its stmctural similarity to malic acid. 

Physical Properties. Mahc acid crystallines from aqueous solutions as white, translucent, anhydrous crystal. The S(—) isomer melts at 100-103°C (1) and the R(+) isomer at 98-99°C (2). On heating, D,L-mahc acid decomposes at ca 180°C by forming fumaric acid and maleic anhydride. Under normal conditions, malic acid is stable under conditions of high humidity, it is hygroscopic. 

Fig. 1. R,i (zt)-Malic acid solubility in water, showing maximum solubilities vs temperature (4).

Fig. 2. Malic acid pH values vs concentration for R,S(zt)-malic acid (4).

Chemical Properties. Because of its chiral center, malic acid is optically active. In 1896, when tartaric acid was first reduced to malic acid, the levorotatory enantiomer, S(—), was confirmed as having the spatial configuration (1) (5,6). The other enantiomer (2) has the R configuration. A detailed discussion of configuration assignment by the sequence rule or the R and S system is available (7). 

The optical activity of malic acid changes with dilution (8). The naturally occurring, levorotatory acid shows a most peculiar behavior in this respect a 34% solution at 20°C is optically inactive. Dilution results in increasing levo rotation, whereas more concentrated solutions show dextro rotation. The effects of dilution are explained by the postulation that an additional form, the epoxide (3), occurs in solution and that the direction of rotation of the normal (open-chain) and epoxide forms is reversed (8). Synthetic (racemic) R,.9-ma1ic acid can be resolved into the two enantiomers by crystallisation of its cinchonine salts. 

As a dibasic acid, malic acid forms the usual salts, esters, amides, and acyl chlorides. Monoesters can be prepared easily by refluxing malic acid, an alcohol, and boron trifluoride as a catalyst (9). With polyhydric alcohols and polycarboxyUc aromatic acids, malic acid yields alkyd polyester resins (10) (see Alcohols, polyhydric Alkyd resins). Complete esterification results from the reaction of the diester of maUc acid with an acid chloride, eg, acetyl or stearoyl chloride (11). 

Alkyl hahdes in the presence of silver oxide react with alkyl malates to yield alkoxy derivatives of succinic acid, eg, 2-ethoxysuccinic acid, H00CCH2CH(0C2H )C00H (12,13). A synthetic approach to produce ethers of malic acid is the reaction of malic esters and sodium alkoxides which affords 2-alkoxysuccinic esters (14). 

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