Internal Browning in Plums: Contributing Factors and Practical Solutions
What is internal browning in plums?
Internal browning (IB) is a physiological disorder – an abnormal fruit condition that results from adverse environmental conditions – that can occur in plums as a result of chilling injury. Chilling injury (CI) is triggered by exposure to cold storage temperatures for a certain amount of time, especially when kept in the temperature range of 36-46°F, instead of 30°F-32°F. CI will only be perceived when the fruit are re-exposed to room temperature after postharvest storage; thus, when the fruit reaches consumers. In this condition, plums maintain a normal external appearance. However, on the interior, the flesh (mesocarp) of the fruit can change from light to dark brown in color and the fruit loses juiciness (Figure 1). Over the course of development, browning will start just below the skin of the plum and spread inwards toward the fruit’s flesh core. IB results from an oxidative process that causes the peroxidation of lipids and the reduction in the efficiency of the antioxidant systems, causing damage to the cellular membranes.
Factors that contribute to internal browning in plums
- Genetics-Cultivar susceptibility: Certain plum cultivars are more likely to develop IB due to genetic predispositions. Susceptible cultivars include ‘Sapphire’, ‘Sunkiss’, ‘Sundew’, and ‘Songold’. Nevertheless, most cultivars that are stored in cold temperatures, specifically in the temperature range of 36-46°F, are at risk for developing IB once transferred to ambient temperature or shelf-life (68°F).
- Environmental factors: Environmental factors such as high temperature have been associated with increased susceptibility to plum fruit IB. Weather patterns with heat waves where temperatures reach 95℉ for 3 days, 100℉ for 2 days, or 104℉ for 1 day just before harvest can increase IB development; or situations where rainy and cool conditions are followed by days in which the temperature exceeds 100℉ can also increase the presence of the disorder.
- Fruit maturity at harvest: Fruit maturity at time of harvest can influence the likelihood of IB development in plums. It has been reported that sensitivity to chilling injury, and particularly to internal breakdown, is higher in late-harvested than in early-harvested fruit. Furthermore, a delay in cooling fruit after it is harvested has also been shown to be associated with an increase in IB during storage. It has been suggested that practices that lead to an increased rate of ripening are related to an increase in disorder susceptibility.
Practices to implement that can prevent internal browning in plums
Internal browning development in plums can be minimized or prevented by strategies at the genetic level, preharvest, at harvest, and during postharvest storage.
At the genetic level, it is important to select plum cultivars that have low susceptibility to IB during cold storage. If you have already established plum cultivars in you orchard that show susceptibility to the disorder, you need to handle them with extreme care and avoid the triggering temperatures and factors mentioned in this newsletter.
Considering the preharvest level, it is important to avoid harvesting fruit at extreme temperatures. Instead, harvest should occur at temperatures < 82℉. If fruit are being harvested during or soon after a period of high-temperature climate conditions, immature fruit are less likely to be susceptible to IB development during post-harvest storage. Furthermore, preharvest management practices such as thinning, pruning, rootstock use, and nutrition, among others, are all important to consider as tools to decrease the development of the disorder postharvest.
Additionally, managing the maturity at harvest is another important consideration to reduce IB incidence. Avoiding harvesting fruits too late in the season will reduce IB susceptibility.
Finally, at the postharvest level, the use of 1-methylcyclopropene (1-MCP), commercially known as SmartFreshTM (Agrofresh), is an inhibitor of ethylene perception and has been used to control ripening (as ethylene is one of the key ripening hormones promoting plum fruit ripening). 1-MCP treatment, besides being proven to be effective in controlling loss of firmness, acidity, and changes in skin coloration of plum fruit, has also been reported to decrease the incidence of IB in different plum cultivars. The effect of 1-MCP reducing the incidence of IB has been more pronounced on plum cultivars that present higher ethylene production rates after harvest, e.g., ‘Larry Ann’ and ‘Fortune’. Furthermore, maintaining proper cold storage conditions, i.e., in the range of 30°F-32°F is key to decreasing IB and chilling injury development, as well as not delaying the cooling of the fruit after it is harvested.
This article appears in Volume 15, Issue 6 of the Vegetable and Fruit News.