Keep your apples in the pink

Strategies to mitigate the risk of internal browning. By Anna Mouton.

“Internal browning touches people’s pockets,” says Dr Elke Crouch, a post-harvest physiologist with the Department of Horticultural Sciences at Stellenbosch University. “Particularly in Cripps Pink, because it’s a high value crop.” Crouch is part of a group investigating the impact of harvest maturity and storage conditions on internal browning in Cripps Pink apples. The team is led by Heleen Bergman from ExperiCo.

The researchers sampled apples from five farms each in Ceres and Grabouw. They collected fruit during three harvest seasons from 2017 to 2019. The apples were evaluated for maturity using a range of measurements including firmness, colour, total acidity, total soluble solids, and starch breakdown. The aim was to harvest at optimum maturity which is characterised by starch breakdown of 30%–40%.

Apples were harvested in April and kept in controlled atmosphere storage. Five treatments were used — see table 1. Fruit were then subjected to a simulated shipping period under regular atmosphere for six weeks followed by a ten-day shelf life.

Table 1 Five different storage protocols were tested. Treatment 2 delivered the fewest defects and most consistent results.

“The first season we stored for about nine months, and we didn’t get any internal browning,” says Crouch. “This season we realised, if you have a high-risk browning year, no matter what you do, don’t push it.” Stored fruit showed a high prevalence of internal browning and superficial scald leading Crouch to caution against attempting to store Cripps Pink apples for as long as nine months.

Don’t store up trouble

Apples can suffer from diffuse browning, radial browning, or a combination of these. Diffuse browning has historically been more common in South Africa — but radial browning caused significant losses last season.

“Diffuse browning is managed by having the right harvest maturity,” says Crouch. “If you harvest anything after 40%–50% starch breakdown, that’s high risk and you shouldn’t store longer than four months.” This can be difficult in practice especially in trees with a large canopy.

“One should harvest the inside and outside fruit separately, because we know their starch breakdown levels are completely different,” says Crouch. “Unfortunately the fruit that hangs longest is also the ripest.” Crouch recommends measuring starch breakdown in apples from the inside of the canopy to avert long-term storage of fruit with starch breakdown of greater than 50%.

Radial browning is highly seasonal because it is strongly influenced by pre-harvest factors. Radial browning is more common in years with lower temperatures during the period in spring when fruit undergo early cell division and expansion.

Harvest maturity is not a predictor of radial browning. Crouch notes that their most recent data indicated that fruit harvested early were more susceptible to radial browning than fruit harvested later. More research is needed to clarify the relationship.

The risk of both radial and diffuse browning is reduced by using step-down cooling. “The message here is, don’t store at a single temperature of 1°C. When we did the long-term step-down cooling, the firmness was better, the quality of the fruit was better — step-down cooling is a definite must,” emphasises Crouch.

Step down the browning

Results so far suggest that the best protocol for step-down cooling starts at 3°C for 30 days, followed by 2°C for 30 days, followed by 1°C for the remainder of storage. Crouch points out that any step-down cooling treatment is better than taking fruit directly to 1°C.

All the apples undergoing step-down cooling as well as one group stored directly at 1°C were treated with 1-MCP — 1-methylcyclopropene (marketed as SmartFresh) — within seven days of storage. The application of 1-MCP to maintain fruit quality is a standard industry practice. A control group was stored directly at 1°C and not treated with 1-MCP.

Both groups stored at 1°C had high levels of internal browning. Treatment with 1-MCP reduced diffuse browning — as expected. Diffuse browning is related to increased maturity and 1-MCP retards maturity.

This research was conducted by a team from the Department of Horticultural Sciences at the University of Stellenbosch in collaboration with ExperiCo. The work is funded by Hortgro.