Trends in biochemical, anatomical mechanisms and molecular aspects in enzymatic browning of apples: a review

Apples (Malus domestica Borkh), like fresh fruits and vegetables, are susceptible to enzymatic browning (EB) and especially during handling, storage and processing, mostly as a result of phenolic compounds being enzymatically oxidized, basically, by polyphenol oxidases (PPOs) enzymes. EB reduces consumer acceptance and marketability leading to postharvest losses of apples. This review focused on EB, addressing the trends in biochemical, anatomical mechanisms, molecular aspects and the future perspectives. At least 24 phenolic compounds were detected in apples, with the peel often retaining the highest concentrations. Phenolic compounds with low molecular weight, like catechin, p-coumaric acid, chlorogenic acid and caffeic acid seem to function as a more effective substrate for PPO. The first step in EB process is cell collapse either by cutting, peeling, bruising, senescence etc. The collapsed cells enable PPO to be in direct contact with the polyphenolic substrates localized either in cell vacuole or in the cell wall in the presence of oxygen to produce melanin, the brown polymer. A precise transformation of apple genomes is a major advance in gene-editing technology focused on improving crop quality, especially in post-harvest technology. The advancement of innovative genome editing techniques to attain precise and targeted alterations in apples for the mitigation of browning activity. Innovative genome editing methods are exploited to silence genes expressing PPOs and relative genes of oxidative enzymes, as well as underscoring notable differences in the constitutive expression pattern of critical phenylalanine ammonia lyase (PAL) enzyme, constituting the most promising strategy to avert undesired browning in plants. Food scientists are seeking to select cultivars that are less sensitive to EB, in addition to using genetic engineering tools to inhibit the browning enzymes. The Arctic^® apple is a unique example of this, as researchers focused on producing an apple resistant to EB with the goal of reducing waste and limiting the use of chemicals in apple industry.