Oxidation processes and chemical reactions

Oxidation-reduction-reactions are a series of chemical reactions characterized by electron transfer from one molecular species to another (Kotz, Treichel, & Townsend 141). Specifically, oxidation describes the loss of electrons, while reduction refers to the gain of electrons (Kotz, Treichel, & Townsend 142). The molecular species which removes, or accepts, electrons during an oxidation-reduction-reaction is known as an oxidizing agent, or an electron acceptor (Kotz, Treichel, & Townsend 142). Conversely, molecules which donate electrons are termed reducing agents, or electron donors (Kotz, Treichel, & Townsend 142).

While oxidizing agents frequently contain oxygen, its presence is not a strict requirement for oxidation (Kotz, Treichel, & Townsend 141-143). Oxidizing molecules usually possess a high electronegativity, resulting in a strong attraction toward electrons (Kotz, Treichel, & Townsend 142). In addition to oxygen, the halogens fluorine, chlorine and bromine are common oxidizers that are strongly electronegative (Kotz, Treichel, & Townsend 141-143). In the case of the browning apple, however, the oxidizer is the oxygen present within the air (Nicolas et al.).

Reducing agents are molecular species which reduce other molecules, i.e. they donate electrons to another molecule (Kotz, Treichel, & Townsend 142). Reducing agents tend to be extremely varied and often highly electropositive (Kotz, Treichel, & Townsend 142-143). The high electropositivity accounts for a reducing agents predilection to donate electrons.

The browning of an apple occurs when the skin is broken and cell walls of the apple are compromised (Nicolas et al.). Ruptured cell walls allow the cellular contents to be exposed to the oxygen present within the air. In apples, an enzyme known as polyphenol oxidase facilitates the oxidation process (Nicolas et al.). Polyphenol oxidase, also known as tyrosinase, is an enzymatic protein found within the chloroplast of the apple cell (Nicolas et al.). Polyphenol oxidases function to drive o-hydroxylation of monophenolic compounds to produce o-diphenols and further catalyze the oxidative conversion of o-diphenols to o-quinones (Nicolas et al.). When the apple cells are broken, an abundance of oxygen and polyphenols are introduced to the polyphenol oxidases, which begin rapidly generating o-quinones. The o-quinones, themselves, are colorless, however they react with other phenolic compounds and can self-polymerize to form compounds which produce the brown color associated with apple oxidation (Nicolas et al.). This oxidation process forms a thin brown layer on the exposed surface of the apple.

The rate of browning in different apple types is dependent on both the concentration of polyphenol oxidase and polyphenols within the apple (Nicolas et al.). Decreased levels of either component within the oxidation reaction can slow the browning process. Apples which exhibit slow browning are often specifically chosen by farmers and the food industry, leading to a genetic selection of fruits with lower or less active polyphenol oxidase and polyphenols (Nicolas et al.). The activity of polyphenol oxidase can also be perturbed by thermal denaturation via heating or by coating the exposed apple with ascorbic acid, thus lowering the pH (Nicolas et al.). Ascorbic acid is also an effective antioxidant, which may slow the oxidation reaction (Nicolas et al.). The enzymatic oxidation can similarly be slowed by placing the apple within a colder environment.