Fruit quality concerns to be managed if heat-wave conditions occur during the harvesting of plums includes sunburn, internal heat damage, gel breakdown, overripeness and shrivel.
Heat damage can manifest in different forms. The easiest to detect is external heat damage which is observed as discoloured areas on the surface of the fruit, typically caused by direct sun exposure. These “discolorations” tend to sink in during storage and hence fruit exhibiting these symptoms must be removed on the pack line. This is typical for cultivars such as Songold. On African Delight plums, sunburn may manifest as yellow skin discolouration which can develop into brown discolouration on the epidermis during cold storage.
The more difficult type of heat damage to detect, and the one that can cause greater losses, is internal heat damage that occurs in the flesh tissue. Internal heat damage is often caused when ambient air temperatures rise above 35 °C for 3 consecutive days, 38 °C for two days or 40 °C for a day. Under these conditions, it is believed that the tree draws water from the fruit to protect (cool) its growing tips. This in turn results in a situation where the cooling system in the fruit no longer functions effectively and the temperature in the fruit itself can sometimes reach as much as 15 °C higher then ambient. This simply “cooks” the internal tissue and the result is internal tissue damage. Fortune and Laetitia plums in particular are known to be susceptible.
Causes and remedies
Fruit at optimum or post optimum maturity is more susceptible to internal heat damage than fruit at pre-optimum maturity. For this reason, fruit picked within 7 days after a heat wave often shows more heat damage than fruit picked later. Fruit picked after extreme heat conditions also tend to ripen faster and this can lead to more gel breakdown, overripeness and shrivel.
Field heat removal:
For fruit packed on the day of harvest a delay time of between 4 and 6 hours between harvest and packing is recommended. Research has indicated that this is beneficial for internal quality maintenance. During this delay time, bins can be placed in a draft in the shade and the objective is to allow the fruit to lose field heat. Field heat can also be removed from the plums by placing the bins in a cold store at 12 to 15 °C (above dew point). This practise helps control moisture loss in fruit packed 24 to 36 hours after harvest. Fruit should not be placed at -0.5 °C for short periods (6 to 12 hours) prior to packing as this may lead to more moisture loss, softer fruit, more internal problems and higher decay. These negative effects on fruit quality were shown to be less if the period at -0.5 °C is extended to 48 hour or more. Field heat removal at -0.5 °C is therefore only recommended if fruit cannot be packed within 48 hours after harvest. However, this practise may increase decay due to condensation on the fruit during packing. The preferred options, especially for fruit picked after heat waves, in terms of internal quality and moisture loss, are therefore to pack the fruit within 4 to 6 hours after harvest followed by slow cooling, or to remove field heat at 15 °C and pack and commence slow cooling of the fruit within 48 hours after harvest. In shrivel prone cultivars it is imperative to use the correct internal packaging to limit moisture loss.
The optimal cooling time to the target storage temperature of –0.5 °C is between 24 and 48 hours. In the case of plums with suspected heat damage, research has shown that significantly lower levels of internal damage are likely to occur in plums cooled in 48 or even 72 hours, compared to plums cooled in 24 hours or shorter (Mariana Jooste & Arrie de Kock). Slower cooling rates will not reverse heat damage in plums that already exhibit internal problems at harvest, but seem to assist in controlling the problem in cases where damage is not yet visible at harvest.
Heat damage often manifests after cooling. For this reason, it is advised to cut and examine fruit harvested after or during a heat wave, after cooling, to visually inspect the internal quality before shipping. Heat damage normally occurs in the flesh tissue between the stone and the epidermis. However, it can also occur on the shoulders of the fruit. In cultivars like African Delight, internal heat damage may also occur as brown spots occurring randomly in the flesh tissue. For this reason, it is important to examine fruit cut through the equator, through the shoulders and in four quarters if temperatures preceding harvest were high enough to potentially cause heat damage.
With dual-temperature stored plum cultivars, it is important to limit the period at –0.5 °C to a maximum of 10 days after harvest. Chilling injury, typically, internal browning may develop in plums if the period at –0.5 °C is extended beyond 10 days before the plums are warmed to 7.5 °C. The tolerance of plums to –0.5 °C prior to the warming phase depends on inherent quality as well as cultivar. Because heat damage usually increases during cold storage, cold storage of plums exposed to extreme heat prior to harvest should be as short as possible.