Laetitia is the second-largest plum cultivar in South Africa according to 2020 figures for area planted and cartons exported. Unfortunately, it is also at high risk of heat damage because it matures late in the season. A recent Hortgro-funded project has been looking into post-harvest conditions that promote the expression of signs of internal heat damage. We spoke to stone-fruit researcher Handré Viljoen of ExperiCo Agri-Research Solutions about the latest results.
Browning and burning
“Heat damage occurs before harvest,” says Viljoen, “when a heat wave comes through while the fruit is still on the tree. We think the critical period is around 7–10 days before harvest.” Temperatures above 40°C for a single day, above 38°C for two days, or above 35°C for three days are thought to lead to internal heat damage.
Although the damage is already present at harvest, affected plums only show signs after 7–10 days in cold storage. “But by then the fruit is already on the water,” says Viljoen. “You don’t know about the damage — the buyer discovers it when the fruit arrives and is cut open for inspection.”
The main signs of internal heat damage are pit burn and internal browning. Pit burn occurs when the fruit becomes so hot that the flesh literally cooks around the pit. Affected flesh darkens and then softens.
“When the environmental temperature is 40°C, the temperature inside the fruit can be nearly 60°C,” explains Viljoen. “During the two years I’ve been conducting trials, we didn’t experience those extreme heat waves, so we didn’t see pit burn. We saw internal browning.”
Internal browning from heat damage tends to occur in the flesh of fruit midway between peel and stone, but it can affect the shoulders of the fruit as well.
Extreme heat and direct sunlight can also cause external heat damage. External heat damage causes discoloured areas on the fruit surface. These areas tend to become sunken during cold storage. The silver lining with external damage is that affected fruit are easily identified and removed during packing.
Unmasking internal heat damage
Identifying fruit that will develop internal quality disorders due to heat damage before shipping would go some way to mitigating losses. “If you see that fruit has internal heat damage, you can decide where to send it,” says Viljoen. “For example, you can market it locally or send it to a market it will reach quickly and where it will sell quickly. Because the longer the fruit is stored, the worse the problem gets.”
Viljoen looked at methods to accelerate the development of signs of internal heat damage, so that producers could have better information on which to base decisions on the fate of their fruit. He collected fruit with signs of external heat damage and subjected them to different storage regimes before examining them for internal heat damage.
Plums were cooled to -0.5°C over a period of either 6 or 24 hours. Fruit cooled for 24 hours were subsequently kept at -0.5°C for two days, and fruit cooled for 6 hours were subsequently kept at -0.5°C for three days. The plums were examined for internal damage immediately after the storage period and again following a ripening period of two days at 20°C.
In the first year, signs of internal heat damage were visible in more fruit after two days of ripening at 20°C than immediately after removal from storage. Internal browning also developed in more fruit cooled within 6 hours than in fruit cooled within 24 hours.
Less internal heat damage was observed when the trial was repeated in the following year, probably because environmental temperatures before harvest were lower. However, fruit cooled within 6 hours, stored for three days at -0.5°C, and ripened for two days still developed significantly more browning than fruit in the other treatments.
Viljoen believes that rapid cooling followed by three days at -0.5 in fruit exposed to heat waves. “We can see within 5–6 days what the chances are of damage,” he says. This is fast enough that problem fruit can be routed away from long sea voyages.
Optimal cooling for heat-damaged fruit
The other aspect of the project on internal heat damage in Laetitia plums was finding ways to reduce or eliminate the expression of signs of heat damage. Plums used for these trials were selected from orchards that had experienced heatwaves 4–8 days prior to harvest.
Plums were cooled to -0.5°C using one of the following treatments:
- cooling within 6 hours
- cooling within 24 hours
- step-down cooling within 48 hours with cooling to 10°C in the first 24-hour period
- step-down cooling within 72 hours with cooling to 10°C in the first and to 3°C in the second 24-hour period.
Each of these treatments was conducted with and without application of 1-MCP [1-methylcyclopropene] for 24 hours on the day of intake.
After cooling, fruit were subject to PD7 by storing at -0.5°C for 10 days, followed by 7.5°C for seven days, followed by -0.5°C for 25 days. Fruit quality was assessed after cold storage and again after a shelf life of five days at 10°C. Flesh firmness, internal quality, shrivel, and decay were recorded.
The results showed that the optimal cooling rate is one that gives the best balance between the different quality characteristics. For example, cooling within 6 or 24 hours can yield better firmness but tends to exacerbate signs of internal heat damage. On the other hand, cooling for up to 72 hours can minimize signs of internal heat damage, but worsen shrivel.
“This is why we said that 48 hours is the ideal cooling period,” states Viljoen. “You might be able to cool other cultivars faster, but with Laetitia you definitely want to step-down cool over 48 hours, especially if you’re expecting problems with internal heat damage.” He warns that slower cooling promotes the development of shrivel, so humidity control remains key.
Addition of 1-MCP did not affect the expression of signs of heat damage, but it did improve firmness and reduce shrivel and internal defects in some cases. Viljoen thinks that there are benefits to using 1–MCP. “1-MCP can almost act as a safety net, especially with the container issues these days, and with the uncertainty around logistics. If your fruit are delayed somewhere you’ll have more assurance that their quality will be maintained.”
Going forward, Viljoen would like to look more closely at the conditions which give rise to heat damage. “We have an idea of the critical period when heat damage occurs, and an indication of the temperatures that lead to damage, but we don’t know how best to manage the risk from a pre-harvest perspective.”