Research progress in the mechanisms of flesh browning and its prevention of apple fruit during cold storage

Abstract: Flesh browning is a common disorder of apple fruit during cold storage. It has different characteristics and complex causes and is hard to be detected, which seriously affects the fruit cold storage and distribution. Therefore, it is of great theoretical value and practical significance to study the causes, mechanism and preventive technology of apple fruit flesh browning during cold storage. According to its different characteristics, flesh browning can be specifically divided into four types: radial flesh browning, diffusion flesh browning, flesh cavities and internal breakdown. The radial flesh browning is mainly manifested in the browning of fruit vascular bundles and the degree of flesh browning at the petiole end is slightly severer than that at the calyx end generally. The vascular bundles of fruit with diffusion flesh browning tend to turn brown slightly, but turn brown more seriously at both ends of the fruit, and there are obvious boundaries alongside the browning areas. The characteristics of flesh cavities are that the browning tissue of flesh is distributed in spots or patches, forming browning circles between the flesh tissue, and the tissue develops into lenticular pits or cavities in severe cases. The internal breakdown can be divided into low-temperature breakdown, senescence breakdown and water core breakdown. Among them, the low-temperature breakdown develops from the inner flesh layer to the outward gradually, and finally develops into a large-area browning with the core of the kernel. The common senescence includes powder-soft disorder and rubber disorder. For sake of the powder-soft disorder, the flesh becomes soft and then dry to form fragile powder in the early stage, and subsequently the flesh browning occurs in the late stage. Rubber disorder often occurs in vascular bundles and tissues close to the peel, accompanied by a slight outline of the disordered area, which is easy to sink when pressed byfingers. Water core breakdown shows that part of the flesh tissue looks like water immersion. In general, factors causing flesh browning mainly include varieties, pre-harvest climate, mineral elements, fruit position, tree load, harvest time, storage temperature, gas composition and treatment of exogenous substances. For example, low temperature breakdown often occurs in Fuji, Venus Gold and other varieties owing to the low temperature during storage; Flesh cavities and water core breakdown often occur in delaying-harvest apple fruit; A higher CO2 concentration in storage will lead to flesh cavities and diffusion browning and an improper 1-MCP treatment will increase the risk of flesh browning in different types. The enzymatic browning caused by the loss of cell membrane compartmentalization is the common mechanism that many different factors cause apple fruit flesh browning during cold storage by different ways. In this article, these different ways are summarized, including the changes of Ca2+ concentration, ethanol and acetaldehyde toxicity, the imbalance of free radical and scavenging system and energy deficit. In generally, flesh browning will be led by too low or too high Ca2+ concentration: A lower Ca2+ concentration is detrimental to the function of cell membrane, while a higher Ca2 + concentration will also destroy the calcium homeostasis, thus showing a toxic effect on cells. The imbalance of reactive oxygen species metabolism and the loss of antioxidants resulted from adversity stress will increase the risk of cell membrane peroxidation. The flesh tissue will accumulate too much ethanol and acetaldehyde owing to the change of cellular respiration pattern, which will cause toxic effects on the cell membrane. And the energy deficit caused by decreased ATP synthesis will also reduce the stability of cell membrane. In conclusion, these processes eventually lead to flesh browning through reducing the stability of cell membrane and increasing membrane permeability, enhancing the risk of internal solute leakage as well as promoting the reaction of substances with polyphenol oxidase to synthetize quinones. In the end, the measures to prevent flesh browning after harvest, including specific temperature of cold storage, gas composition and 1-MCP treatment, are summarized in this paper. Simultaneously, the essential aspects of future research are put forward, including more in-depth researches on the causes and influencing factors of different types of flesh browning, on the technologies and measures to prevent flesh browning of apples with special needs during cold storage, on the screening and identification of flesh browning biomarkers, and on the flesh browning prediction and early warning technology system.