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PE9 - Wilting

Definition

In the case of peaches, excessive moisture loss tends to lead to wilting rather than shrivel and this is characterized wilted or rubbery fruit.

General information

Moisture loss from fruit is governed by forces that promote the movement of water out of fruit, such as high ambient and pulp temperatures and low relative humidity, and the forces that retard the movement of moisture out of fruit such as low temperature and high relative humidity. Another force that restricts moisture loss from the fruit is the skin of the fruit itself.   This is one of the factors that differentiate cultivars with a low potential for moisture loss and from those with a high potential for moisture loss. The skin properties are affected by the maturity of the fruit at harvest. Normally, the wax cuticles in immature fruit are not fully developed and in over-mature fruit, cracks occur in the wax cuticle. For this reason immature and over-mature fruit is more susceptible to moisture loss than fruit harvested in the optimum maturity window.

The speed at which water vapour dissipates from fruit depends on the vapour pressure deficit (VPD). The air velocity around the fruit also plays a role. As indicated in the book, Postharvest (Wills et al., 2007), VPD is the difference between the vapour pressure of the fruit (a function of temperature and equilibrium humidity) and the vapour pressure of the surrounding air (a function of temperature and relative humidity). This means the bigger the difference between the fruit temperature and the temperature surrounding the fruit the higher the moisture loss.

Causes and remedies

Harvesting

To reduce wilting, harvest fruit early in the morning when temperatures are cooler, and if applicable, as soon as fruit are dry. Lower fruit pulp temperatures lead to a lower VPD during cooling and moisture loss will be reduced.

 

Harvest maturity

Harvest fruit at the optimum maturity specified for the cultivar since the potential for excessive moisture loss from optimum maturity fruit will be lower than in fruit harvested too immature or too mature.

  

Packing and cooling

Woolliness susceptible cultivars (see cultivar specific information)

While moisture loss can be limited by packing and cooling peaches as soon as possible after harvest, this may not be possible if pre-conditioning is required. Fruit that must be pre-conditioned must still be packed as soon as possible after harvest. If fruit cannot be packed on the day of harvest, do not cool the fruit to -0.5 °C before packing as this will extend the pre-conditioning period. In this case fruit can be stored at 15 °C prior to packing the following day. Force-air cool fruit immediately after pre-ripening to -0.5 °C. Fruit must be cooled to -0.5 °C within 24 hours after commencement of cooling.

Cultivars not susceptible to woolliness

Pack peaches as soon as possible after harvest. If fruit cannot be packed on the day of harvest, the fruit can be placed at -0.5 °C prior to packing. Fruit should be packed as soon as possible regardless if the target temperature has been reached. Shrivel sheets should be used on nectarines but not on peaches. Fruit must be forced-air cooled immediately after packing. Fruit must be cooled within 24 hours after commencement of cooling.

Storage

Moisture loss occurs throughout the handling chain. It is therefore important to limit the period at high temperatures before and after packing and to cool the fruit as soon as possible. Relative humidity in cold stores should be 85% or higher, especially during forced-air cooling, since moisture loss from fruit during forced-air cooling is high. Forced-air cooling fans must be switched off as soon as the target cold storage temperature of -0.5 °C is reached. Because fruit could be cold stored for relatively long periods, moisture loss can be significant if not well managed.  During cold storage and shipping humidity cannot always be regulated, so it is best to store peaches for as short a time possible to limit wilting.

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