Gallate, alkyl

Tammela P, Laitinen L, Galkin A et al. (2004) Permeability characteristics and membrane affinity of flavonoids and alkyl gallates in Caco-2 cells and in phospholipid vesicles. Arch Biochem Biophys 425 193-199... 

Figure 4. Chemical structures of different alkyl gallates.

Autoxidation can be inhibited or retarded by adding low concentrations of chainbreaking antioxidants that interfere with either chain propagation or initiation (286). Chain-breaking antioxidants include phenolic and aromatic compounds hindered with bulky alkyl substituents. Common synthetic chain-breaking antioxidants used in food lipids include butylated hydroxyanisole (BHA), butylated hydroxyto-luene (BHT), ferf-butyUiydroquinone (TBHQ), and propyl gallate (PG). This class of antioxidants react with peroxy free radicals to terminate reaction chains. The antioxidant radical (A ) formed in Equation 5 should be relatively stable and unable to initiate or propagate the oxidation chain reaction. 

Butylated hydroxyanisole is frequently used in combination with other antioxidants, particularly butylated hydroxytoluene and alkyl gallates, and with sequestrants or synergists such as citric acid. 

Other alkyl gallates are also used as antioxidants and have approximately equivalent antioxidant properties when used in equimolar concentration however, solubilities vary, see Section 17. 

The alkyl gallates are incompatible with metals, e.g. sodium, potassium, and iron, forming intensely colored complexes. Complex formation may be prevented, under some circumstances, by the addition of a sequestering agent, typically citric acid. Propyl gallate may also react with oxidizing materials. 

Propyl gallate is prepared by the esterification of 3,4,5-trihydroxybenzoic acid (gallic acid) with -propanol. Other alkyl gallates are prepared similarly using an appropriate alcohol of the desired alkyl chain length. 

FIGURE 12.1 Antioxidants. Top from left to right, alkyl gallates, the two isomers of BHA. Bottom fom left to right, BHT and TBHQ. 

Alkyl Gallates as Additives in Nonfood and Noncosmetic Industries 247... 

Propyl (PG, E 310), octyl (OG, E 311), and dodecyl (or lauryl) (DG, E 312) esters of gallic acid, usually named alkyl gallates, form a part of the synthetic phenolic antioxidants, mainly used as food additives, with PG being the most frequently used alkyl gallate. The molecular structure of these compounds is composed of a hydrophilic head (phenolic ring) and a hydrophobic alkyl chain (Figure 14.1). PG is slightly soluble in water but OG and DG are insoluble in water. The three compounds are soluble in ethanol, ethyl ether, and oils. 

Alkyl gallates are mainly used to protect fats, oils, and fat-containing food from rancidity that results from the formation of peroxides. Their use as antioxidant additives is regulated by legal authorities in a limited number of foods, with maximum limits in each case. The acceptable PG daily intake (ADI) recommended by the joint FAO/WHO Expert Committee on Food Additives (JECFA) is 0-1.4 mg of additive per kilogram body weight, and the maximum level recommended in foodstuffs is 200 mg/kg [1]. A recent reevaluation by the European Food Safety Authority (EFSA) of PG as food additive has concluded that it is not of safety concern at the current uses and use levels [2]. 

FIGURE 14.1 Chemical structures of gallic acid (a) and alkyl gallates (b). 

In addition to the use of alkyl gallates as food and cosmetic additives, the multifunctional action of these compounds and their potential application for other purposes has been widely investigated, as described below. Some of these applications, which are discussed below, involve their use as enzymatic inhibitors, as anticancer, antifungal and antimicrobial agents, and as additives in nonfood and noncosmetic industries (Figure 14.2). These applications are mainly based on the antioxidant and prooxidant properties of these compounds, which, together with their potential toxicity, are also discussed below. 

Antioxidant and Prooxidant Activities of Alkyl Gallates and Their Potential Toxicity as Food Additives... 

A review published in 1986 on the toxicological effects of alkyl gallates [4] described different oral short- and long-term toxicity assays in several animals using large amounts... 

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