Internal Flesh Browning in Apple and Its Predisposing Factors—A Review

This review article is focused on internal flesh browning (IFB)-related physiological disorders affecting apple (Malus domestica Borkh.) fruit. The expression of different physiological and metabolic IFB-related disorders during post-harvest storage are investigated along with the pre-harvest factors contributing to development. The effectiveness of commercially available pre-harvest technologies for preventing IFB-related disorders are also examined. Internal flesh browning-related disorders are erratic and devastating disorders that result in post-harvest deterioration of fruit quality in apples. Internal flesh browning-related disorders can result in severe economic losses to the apple industry through reduced consumer trust and market acceptability of susceptible cultivars. There are several IFB-related disorders and incidence can range from 0 to 100% of a crop, with severity ranging from no brown flesh to browning of the entire fruit flesh. While IFB-related disorders are found in several apple cultivars, some cultivars are more prone than others. The development of IFB-related disorders involve complex mechanisms depending upon the different types and causes, or factors involved in loss of structural integrity and functional stability of the cell membranes and cell components. Membrane disruption followed by enzymatic oxidation of fruit phenolic compounds by polyphenol oxidases and the production of brown polymers is considered to be the general underlying mechanism causing the browning of flesh tissue. It can be observed in different patterns based on the injured portion of the fruit flesh and the cause of membrane disruption. Three broad categories of IFB-related disorders, including chilling injury, internal CO2 injury, and senescent-related browning disorders, are discussed along with their sub-types. The development of IFB-related disorders can be influenced by both pre-harvest factors and post-harvest conditions and their interactions. Although commonly associated with storage, IFB can also be found immediately after harvest and sometimes in unharvested fruit prior to full maturity. As pre-harvest conditions are a strong contributor to IFB-related disorders, the influence of several pre-harvest orchard conditions, including fruit size, crop load, maturity at harvest, cultivar, climatic conditions, seasonal temperatures, growing degree days, and major mineral nutrients, such as nitrogen (N), phosphorus (P), potassium (K), and calcium (Ca) are reported. Although there are contradictory findings in the studies reported, in general, factors such as larger fruit size, light crop load and delayed harvesting, along with cool temperatures after bloom and warmer temperatures before harvest, increase the risk of IFB-related disorders. In relation to fruit mineral concentrations, high N and low Ca have been associated with increasing IFB, while there is conflicting evidence in relation to the impact of both P and K. This review also examines the effectiveness of commercial pre-harvest technologies such as 1-methylcyclopropene, aminoethoxyvinylglycine and diphenylamine in the prevention of IFB-related disorders, but none of these technologies were found promising due to varied and contradictory results.