What causes green ripe mangoes?

Summary

How to increase yellow skin colour

Pre-harvest:

  • avoid excessive nitrogen fertilisation
  • ensure that trees receive adequate water
  • maintain an open tree canopy to improve light penetration
  • avoid crowding trees.

Post-harvest:

  • harvest only mature fruit and delay harvesting if green fruit is known to be a problem
  • minimise the risk of fruit injury, especially during de-sapping and heat treatment
  • maintain optimum temperatures by:
    • pre-cooling fruit using forced air cooling
    • transporting the fruit at 12 °C if transport times exceed 2-3 days
    • ripening fruit between 18 and 22 °C using a forced air system or air stacking
    • using ethylene see the Mango Ripening Guide.

Introduction

Green ripe fruit is a major quality problem that reduces the saleability of  mangoes.

Consumers and retailers expect ripe mangoes to have a yellow background skin colour, preferably with some pink/red blush. Green ripe fruit softens but the skin remains green or appears motley green/yellow. There is little demand for this fruit.

The change in skin colour from green to yellow is a process that typically occurs during ripening of mangoes. The other ripening processes are softening, conversion of starch to sugars, loss of acidity, and the development of flavours and aromas. Both orchard management and post-harvest handling practices can affect the change in skin colour.

A project funded by the Australian Centre for International Agricultural Research, 'Disease Control and Storage Life Extension in Tropical Fruit', investigated orchard management factors affecting mango quality, examined reasons for green ripe fruit and the results of this work are discussed below.

Production location

Fruit from 12 growers in the Burdekin district  found large variations in the amount of green skin on the fruit when ripe. Fruit from four growers in each of four production locations: Northern Territory (NT), Atherton Tablelands, Burdekin and the Sunshine Coast (SC) was also examined. Figure 1 shows the large differences between growers within each district in the percentage of the skin with green colour at ripe, but no consistent effect of production district was found. Results were similar over two seasons.

Nutrition

The South African mango industry has problems with mangoes ripening with too much green skin colour for the export market. By following the history of some of these export lines, the industry identified that orchards receiving high nitrogen applications often produced fruit with greener skin at ripening. Similar results were obtained with other fruits.

We also investigated the effect of nutrition on skin colour by measuring the mineral concentration of fruit from the investigations mentioned above. Fruit with higher nitrogen concentrations can have more green colour on the skin at ripening. Some growers have observed that 'starving' their trees of nitrogen produced fruit with good colour. This is a double edged sword however as trees require sufficient nitrogen to produce a marketable yield.

Irrigation

Fruit from two irrigation trials over four years - one in the Northern Territory and one in the Burdekin - was also assessed. In both trials, fruit from trees suffering water stress had more green colour on the skin at ripening.

Shade

The affect of fruit position on the tree was assessed. In most instances, factors that contributed to the fruit getting less sun (fruit growing on the southern side of the tree, or shaded by leaves of the same tree or adjacent trees) resulted in fruit with more green colour at ripening. This fruit also had less red colour and tended to be less mature. This pattern was observed over two seasons and has also been noted with other fruits.

Maturity

As part of the irrigation trial in the Northern Territory, fruit was harvested five times at approximately weekly intervals. The amount of green colour on the skin when ripe decreased in the more mature fruit Figure 2. This has been confirmed in other investigations. Fruit harvested after 80 days from panicle emergence were considered commercially mature.

Ripening temperature

Temperature during post-harvest ripening has the greatest effect on the amount of green on the skin when ripe. The optimum range for the development of yellow colour is 18-22 °C. If fruit ripens at higher temperatures above 22 °C (Figure 3) or at lower temperatures (11-14 °C) in cool storage, the amount of green on the skin will be higher.

We did not notice any effect of ripening or storage temperature on the red blush. However, it appears that the red colour can be improved by exposing the fruit to light after harvest, and there may also be a positive effect on yellow colour. This requires further investigation.

Ethylene

It has been well demonstrated that a 24-hour treatment with ethylene will trigger mango ripening. Controlled exposure to ethylene at the optimum temperature of 18-22 °C will enhance the appearance of the fruit in the carton by synchronising softening and skin colour. Skin colour was more uniform between fruit within the carton, but we have not yet investigated whether this treatment will reduce the green skin problem.

Skin injury

Factors that cause 'stress' to the fruit will increase the risk of green colour on the skin at ripening. For example, excessive heat from disease control or disinfestation treatments can increase the green skin colour. Physical injury during harvesting and handling can also cause localised green areas on the skin at ripening.