Detection of internal heat damage and identification of techniques to prevent the expression of the disorder in Laetitia plums

1. To determine if internal heat damage in Laetitia plums can be expressed within 3 to 5 days after harvest using rapid cooling as a risk assessment method.
2. To determine if the expression of internal heat damage can be reduced or eliminated in Laetitia plums by 1-MCP and/or slower cooling rates.

Objective 1

Laetitia plums exposed to heat waves to the extent that external heat damage was visible were harvested. Fruit was cooled rapidly (within 6 hours) or slowly (within 24 hours) to -0.5 °C. Fruit were then stored for 3 and 2 days, respectively, for each cooling rate and then examined for internal heat damage. A second examination was conducted after 2 days ripening at 20 °C.

Objective 2

Fruit from the same source used for objective 1 were cooled within 6, 24, 48 or 72 hours. Half of the fruit of each treatment, except for the 6 hour cooling treatment, was treated with 1-MCP. Fruit cooled within 6 hours served as reference treatment. After cooling the fruit was cold stored at a 42 day PD

7 dual-temperature regime and subsequently quality examined before and after 5 days shelf life.

Objective 1

In the 2^nd and 3^rd year of study, internal heat damage was detected in fruit after the 2 days ripening at 20°C that followed the cooling treatments and the further 2/3 days at -0.5°C. The levels were significantly higher in plums cooled within 6 hours, compared to 24 hours. The heat damage levels were also significantly higher after the 2 days ripening at 20°C, compared to before ripening.

Objective 2

▪▪ Fruit cooled for shorter periods and those treated with 1-MCP tended to be firmer than fruit from other treatments after cold storage preceded by longer cooling and without 1-MCP treatments.

▪▪ Laetitia plums cooled within 6 and 24 hours developed significantly more internal heat damage compared to fruit cooled slower.

▪▪ 1-MCP did not reduce the expression of internal heat damage after cold storage, but did reduce gel breakdown, skin cracks (2020 season) and gave firmer fruit.

▪▪ 48 hours slow cooling plus 1-MCP gave best overall fruit quality after cold storage, but special attention is needed to ensure good (high) relative humidity control during forced-air cooling to reduce possible moisture loss and shrivel.

The results confirmed that rapid cooling can exacerbate internal heat damage in pre-disposed Laetitia plums. Cooling Laetitia plums using 48 and 72 hours cooling rates resulted in lowest internal heat damage. 1-MCP assisted flesh firmness maintenance in slow cooled fruit. The rapid cooling risk assessment method can be used as a quality management tool for Laetitia. This is: 6 hours rapid cooling, 3 days storage at -0.5 °C, followed by 2 days at 20 °C.

Cooling Laetitia plums using a 48 hours cooling rate resulted in lowest internal heat damage. Moisture control during forced-air cooling is critical to reduce possible higher shrivel levels after cold storage and shelf life, due to the extension of the forced-air cooling process.