This paper examines the chemistry behind apple browning through the lens of oxidation-reduction reactions. It defines key concepts such as oxidizing agents, reducing agents, and electronegativity before explaining how the enzyme polyphenol oxidase catalyzes the conversion of phenolic compounds into brown-pigmented polymers when apple cells are damaged and exposed to oxygen. The paper also discusses practical and industrial methods for slowing enzymatic browning, including ascorbic acid treatment, temperature reduction, oxygen exclusion, and selective breeding of apple varieties with lower polyphenol oxidase activity.
Oxidation-reduction reactions are a series of chemical reactions characterized by the transfer of electrons 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 that 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 that 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 in the air (Nicolas et al.).
Reducing agents are molecular species that reduce other molecules — that is, they donate electrons to another molecule (Kotz, Treichel, & Townsend 142). Reducing agents tend to be extremely varied and are often highly electropositive (Kotz, Treichel, & Townsend 142–143). This high electropositivity accounts for a reducing agent's predilection to donate electrons.
The browning of an apple occurs when the skin is broken and the cell walls of the apple are compromised (Nicolas et al.). Ruptured cell walls allow the cellular contents to be exposed to the oxygen present in 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 that 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.
"How enzyme and polyphenol levels control browning speed"
"Practical techniques to inhibit apple oxidation"
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