Ascorbic enzymatic browning

Salads. A combination of citric acid and ascorbic acid is used as an alternative to sulfites in prevention of enzymatic browning in fresh prepared vegetables (56). 

Luo Y and Barbosa-Canovas GV. 1997. Enzymatic browning and its inhibition in new apple cultivar slices using 4-hexylresorcinol in combination with ascorbic acid. Food Sci Technol Int 3 195-201. 

Rodriguez-Saona, L., Wrolstad, R., Pereira, C. (1997). Modeling the contribution of sugar, ascorbic acid, chloro-genic acid and amino acids to non-enzymatic browning of potato chips. Journal of Food Science, 62, 1001-1005. 

Organic acids are added to some foods in order to prevent sedimentation or darkening thus, citric, malic, phosphoric, and ascorbic acid are used to reduce or delay enzymatic browning. Melanin formation following production of a quinone is a pH-dependent process (the rate of the former process increases with increasing pH) (8). Polyhydroxyacids such as citric and malic acid... 

During the crushing process, ascorbic acid can be added to protect against oxidation. This prevents non-enzymatic browning until the juice is pasteurized. 

To prevent non-enzymatic browning, the fruit can be dipped in a 5% solution of citric acid (or in lemon juice). This is especially advisable in the case of sliced and diced apples where the acid content is low. The acid allows the light colour to be retained, while at the same time improving the taste of the pieces of apple. The colour of the fruit can be made even lighter by adding ascorbic acid to the dip. 

Enzymes known as polyphenol oxidases cause enzymatic browning. Other names of the enzyme include phenolases and tyrosinases. The enzymes catalyze the conversion of monophenols and diphenols to quinones. The quinones can undergo a series of non-enzymatic reactions to produce brown, gray and black colored pigments, collectively known as melanins (11). Maillard reactions, caramelizations and ascorbic acid oxidations can produce similar types of colored compounds (12). For some food processing... 

Sulfites and Ascorbic Acid as Browning Inhibitors. Historically, enzymatic browning was controlled by the application of sulfites... 

L-ascorbic acid (AA) and its isomer D-erythorbic acid (EA) (also called D-isoascorbic acid) have been used as inhibitors of enzymatic browning in fruit and vegetable products for at least 50 years, (15-17). These compounds prevent quinone accumulation and subsequent pigment formation by reducing the 0-quinones generated from the phenolic substrates of PPO back to O-dihydroxyphenolic compounds (17-18). AA also can act as a PPO inhibitor (19-20). AA and EA are used interchangeably although there are indications that AA is more effective in some systems (21-22). 

In order to better understand the effect of ascorbic acid on non-enzymatic browning of citrus juices, we oxidized endogenous ascorbic acid in grapefruit with ascorbic acid oxidase. The enzyme oxidized ascorbic acid to dehydroascorbic acid and H2O without forming H2O2 (19). 

Although the inhibition of ascorbic acid oxidation in citrus juices (22,23) and other foods by EDTA was reported by several researchers (22-24), the possibility that EDTA could inhibit non-enzymatic browning in grapefruit or other citrus juices has not been explored. 

Combined treatments of HHP with some chemicals were also reported to prevent enzymatic browning during the storage of foods. Prestamo et al. [118] revealed that treatment of apple fruit after HHP treatment (400 MPa at 5°C for 30 min) with 20 mM ascorbic acid prevents the formation of brown color after 2 months of storage at 5°C. 

The major pathway leading to the formation of carbonyls is the Strecker degradation. This reaction occurs between dicarbonyls and free amino acids. The dicarbonyls involved have vicinal carbonyls (carbonyl groups separated by one double bond) or conjugated double bonds [41], While these carbonyls typically are intermediates in the Maillard reaction, they may also be normal constituents of the food (e.g., ascorbic acid), be end products of enzymatic browning (e.g., quinones), or be products of lipid oxidation[42]. 

Polyphenol oxidases and ascorbic acid oxidase, which occur in food, are known to have a Cu /Cu redox system as a prosthetic group. Polyphenol oxidases play an important role in the quality of food of plant origin because they cause the enzymatic browning for example in potatoes, apples and mushrooms. Tyrosinases, catecholases, phenolases or cresolases are enzymes that react with oxygen and a large range of mono and diphenols. 

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