Improving consumer appeal of Honey Gold mango by reducing under skin browning and red lenticel discolouration

Abstract

‘Honey Gold’ is a relatively new Australian mango variety that is gaining in popularity among consumers. The fruit are, however, susceptible to developing under-skin browning (USB) and red lenticel spots. These superficial quality defects result in downgrading of fruit consignments and financial loss.
USB can develop in response to a combination of physical (e.g. abrasion) stress and low (e.g. <14°C) temperature exposure during postharvest handling. Fruit produced in hot tropical environments, such as the Northern Territory, are particularly susceptible for reasons that are unresolved. Development of red halos around lenticels is often associated with heavy rainfall prior to harvest. Entry of rain water into lenticels is proposed to trigger a wound response leading to localised red pigment production.
The current project aimed to improve the consumer appeal and profitability of ‘Honey Gold’ mango by:
• Reducing the occurrence of USB by exploring the influence of pre-harvest and harvest practices.
• Identifying strategies to minimise red lenticels using inhibitors and promoters of red skin colour.
Harvesting ‘Honey Gold’ mangoes from the Northern Territory at night was the most effective practice for reducing the sensitivity of fruit to developing USB. Fruit harvested between 10 PM and 10 AM displayed 50-75% reductions in USB incidence compared to those picked at 2 PM. The higher level of USB in the afternoon harvest was related to relative sap phytotoxicity. These findings prompted ‘Honey Gold’ growers in the Northern Territory to harvest fruit at night. Piñata Farms, who market ‘Honey Gold’, reported that since adopting night harvesting in 2014/15, USB has decreased from 20% of the Northern Territory crop to <1% in 2016/17. Also, harvesting at night facilitated a 20% increase in picking speed.
Pre-harvest bagging of fruit was the most reliable technique for reducing red lenticels. Applying brown paper or Tyvek® plastic bags at 4 weeks before harvest reduced this skin defect by 67-85% relative to non-bagged fruit. The bags blocked up to 90% of sunlight and, in turn, reduced red pigment production, including skin blush. In contrast, postharvest exposure of fruit to blue LED light for 3-4 weeks at 22°C enhanced skin blush. By increasing blush, this treatment suggests the potential to mask red lenticels.
The current project also aimed to verify and optimise ‘Honey Gold’ crop forecasting and downgrade analysis procedures developed in a previous project, MG10009. Accumulation of ≥1500 heat sums above a 12°C base from full flowering was confirmed to predict the earliest harvest date to provide fruit with good eating quality. In-market fruit quality downgrade analysis with improved recording and report back systems was well received by growers. The feedback stimulated practice change that translated into increased fruit yield and quality. Formal downgrade analysis at pack sheds is also recommended.
For fruit that were downgraded, alternative products and markets were identified to improve returns. A non-premium fruit line was developed and marketed successfully. Juice grade fruit were trialled as a new ingredient in an existing mango ice block product. Two major retailers relaxed grading standards and rationalised fruit size by state markets to accept premium fruit with minor defects, and of all sizes.
Implementation of these key project outcomes has contributed to significantly improving the commercial viability of ‘Honey Gold’ mango production, particularly in the Northern Territory.