John Golding of NSW DPI discusses why the use and selection of wax is critical when it comes to meeting market demands for citrus.
While waxing citrus can help maintain quality and enhance the fruit’s appearance, using the wrong type of wax or applying an excessively thick layer can overly restrict gas exchange between the fruit’s skin and the air.
This lowers the oxygen levels and increases the level of carbon dioxide within the fruit.
This then triggers the accumulation of fermentation off-flavours in the fruit. These off-flavours can make the fruit taste old or stale.
Therefore, the selection and use of wax is critical to meeting market demands.
Why wax?
When a piece of fruit grows in an orchard it has a natural wax, which protects its skin from damage and water loss.
However, when this fruit is harvested, then handled and cleaned in the packhouse, this natural wax layer is damaged or removed.
Therefore, it is essential to replace this wax with another natural food-grade wax in the packhouse to maintain the quality of the fresh citrus through the supply chain.
The use of wax on fresh citrus fruit helps reduce water loss, improves appearances, increases freshness, protects the fruit’s surface, slows down the development of some rind disorders, and reduces wastage.
The most important of these benefits is the reduction in water loss from the fruit. Stopping water loss from the fruit after harvest is critical to maintaining fruit firmness and quality.
The application of wax provides a barrier to water loss and slows down the evapotranspiration of the water from the fruit.
In addition, the wax barrier also reduces air permeability of the peel, thus creating a unique environment to extend shelf life.
The application of wax also gives the fruit a strong, shiny appearance, which appeals to many consumers worldwide.
Types of wax
All commercially available citrus waxesfood-grade
The main formulations used on citrus are carnauba and shellac-based waxes, or a combination of both. Older formulations, such as polyethylene and resin-based waxes, are still used by packers but less frequently.
Carnauba is a natural wax extracted from the leaves of the carnauba palm, which is grown in Brazil.
Carnauba is widely used as a component in citrus waxes due to its attractive lustre (shine).
However, it has relatively poor control of water loss and is relatively permeable to gas exchange.
Shellac-based waxes are derived from shellac, which is a resin secreted by the female lac bug, which lives on forest trees in India and Thailand.
Shellac waxes impart high shine and help to maintain moisture within the fruit and reduce shrinkage.
However, due to the nature of shellac waxes, they are more susceptible to the development of off-flavours than carnauba-based wax coatings.
Commercially available citrus waxes have been developed to optimise fruit quality during storage by combining different waxes and adding substances like emulsifiers to them.
It is best for each packer to test which wax is best for their waxing operations, their fruit and their customers. As with all chemicals, it is important to read and follow the label on the wax to ensure best results.
It is also important to check what is in the wax, to ensure it complies with market requirements.
Morpholine is sometimes used as an emulsifier in some waxes, which helps make the wax glossy.
However, some importing countries, including the European Union, have banned the use of morpholine. Morpholine-free waxes are available if targeting these markets.
Application
Waxwater
Fruit should be damp prior to application. Wax is normally sprayed or dripped onto pieces of fruit while they are being rotated on a bed of brushes. The revolving brushes help to spread the wax evenly over the fruit’s surface.
It is important that the wax film is thin, otherwise the gas exchange may be overly restricted, causing off-flavours in the fruit during storage.
Combining wax with other chemicals
In some instances, the wax formulation may be used as a carrier of post-harvest fungicides and inhibitors of senescence.
For example, the post-harvest fungicide Imazalil can be applied in the wax to suppress sporulation of green and blue mould within the packed fruit.
This application of Imazalil in the wax is not the primary method of decay control and other post-harvest fungicide dips are still necessary to control decay.
This article was written by John Golding, a Research Horticulturist with the New South Wales Department of Primary Industries.
The article is from the Citrus Postharvest Program (CT19003). This project has been funded by Hort Innovation, using the citrus research and development levy and contributions from the Australian Government.
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