Turning up the heat on shrivel

How humidity and temperature can be used to limit moisture loss in plums. By Grethe Bestbier.

Moisture loss of as little as one percent of their total mass can cause some plum cultivars to shrivel. Although shrivel is a post-harvest defect that affects a variety of fruit, plums are particularly sensitive. According to Arrie de Kock, senior researcher at ExperiCo, shrivel in plums is a major problem in the industry that needs serious attention, starting with humidity control.

The connection between shrivel and moisture loss

Shrivel is a post-harvest condition caused by moisture loss in fruit, manifesting as wrinkles on especially plums and nectarines. The degree of shrivel depends on various fruit and environmental characteristics – some encouraging moisture loss and others opposing it.

Different fruit types react differently to moisture loss. Peaches and apricots, for example, do not show symptoms of shrivel as easily as plums. Some cultivars, like African Delight and Laetitia, are also more sensitive to moisture loss than others. This is mainly influenced by peel properties – small cracks in the peel, the characteristics of the lenticels and the thickness of the wax. A thicker wax layer on plums will generally decrease moisture loss, while a high number of open lenticels will increase moisture loss. For more on this, read our article on shrivel in plums.

Environmental factors such as temperature and humidity also affect moisture loss. Higher fruit temperature increases the vapour pressure deficit between the fruit flesh and the immediate environment, resulting in greater moisture loss. Humidity, not only in the orchard, but throughout the production chain, plays a crucial role in moisture loss.

While fruit have a relative humidity of nearly 100%, the relative humidity of the environment is much lower than this. Storing fruit at a relative humidity below 100% will result in moisture loss.

This movement of water from a fruit to its environment is driven by the vapour pressure deficit. This is the difference between the relative humidity inside the fruit and that of the environment. A large difference increases moisture loss. The vapour pressure deficit is increased by increasing the temperature of the product and decreasing the relative humidity of the environment. Simply put, the larger the temperature and humidity difference between the fruit and the environment, the greater the vapour pressure deficit, and the more moisture will be lost.

Managing the vapour pressure deficit is very important for managing shrivel. This can be achieved by controlling the humidity around the fruit.

Managing moisture loss from harvest to storage

According to De Kock, it is best to combine various techniques to limit moisture loss in plums. Firstly, it is important to harvest at optimum maturity, when moisture loss is at its lowest. If harvested while too green, the fruit’s wax layer is not completely formed, and if harvested when too ripe, this layer is often cracked, both leading to increased moisture loss.

On the tree, the sun can increase the fruit’s temperature drastically. When the fruit is harvested, there is a large vapour pressure deficit, and moisture loss is common. In the harvest bins, covering the fruit with a wet blanket will avoid sun damage and increase humidity around the warm fruit, limiting moisture loss. Moisture loss increases again during transport to the packhouse and when fruit is exposed to high temperatures upon reaching the packhouse.

During storage, moisture loss in plums is reduced using specific packaging. Fruit are either packed in perforated plastic bags or placed in a plastic wrapper. The packaging increases the humidity around the fruit, while the perforations regulate oxygen and carbon dioxide levels. During respiration, the fruit uses oxygen and releases carbon dioxide – the perforations in the bags or wrappers keep carbon dioxide levels low and oxygen levels high enough to avoid anaerobic reactions that produce unwanted odours and tastes.

After packaging, fruit must be cooled rapidly. Surprisingly, moisture loss often peaks during the forced-air cooling process. Air is forced over the fruit and removes the more humid layer in contact with the fruit peel. This layer would otherwise protect the fruit against moisture loss. Cooling the fruit too quickly can also place the fruit under stress and lead to internal damage. There are guidelines to follow, says De Kock. “We usually recommend a period of between 24 and 48 hours from when the warm fruit enter the cold room, up to when it is cooled”

Finally, as the fruit temperature decreases in the cold room, so does the vapour pressure deficit. However, despite a relatively low vapour pressure deficit, this is one of the stages with high moisture loss, because of the long time the fruit spends in cold storage. According to De Kock, regulating the humidity in cold rooms is an aspect that needs more focus. “People do not pay much attention to this,” says De Kock. “We think that might be a void.”

Relative humidity in the cold room has a massive impact on the vapour pressure deficit. Increasing the relative humidity from 85% to 95%, for example, decreases the vapour pressure deficit as much as threefold, reducing moisture loss significantly. Humidifiers can be installed to regulate cold rooms’ humidity. “We recently recommended that we do a project to see what exactly the best relative humidity is to operate cold rooms at,” says De Kock.

Image supplied by Imke Kritzinger | Stellenbosch University.