DEGREENING

What is Degreening?

Degreening is the process of removing the green colouring (known as chlorophyll) from the skin of citrus fruit. This is achieved by introducing measured amounts of ethylene gas into a temperature and humidity controlled environment.

It is important to note that only mature fruit is suitable for degreening. The reason is two-fold; firstly, immature fruit has very high acid levels and low sugar levels which do not meet the Australian Citrus Standard (ACS), and secondly, citrus fruit is non-climacteric. This means that unlike tomatoes, peaches or pineapples, it will not continue to ripen after harvest.
 

What is actually occurring during the degreening process?

Chlorophyllase enzymes are breaking down green chlorophyll and unmasking orange or yellow carotenoid pigments within the fruit. 

It is the ethylene gas that assists with the breaking down of the chlorophyll in the skin of the fruit, allowing the yellow or orange peel to predominate, although this alone does not help the fruit to ripen.

How does temperature and humidity play a part?

Colour development of the skin is promoted by cooler temperatures (< 15 °C).

In the majority of citrus growing regions with a Mediterranean type climate, cooler temperatures in late autumn promote good colour development. However, in the warmer subtropical and tropical growing regions (such as Queensland, Australia) these cooler temperatures may not occur until much later, well after the early varieties are internally mature.

Low relative humidity levels may inhibit the process of degreening, resulting in soft fruit and loss of overall size. It may also accentuate physical blemishes and increase stem end rind breakdown. Best results for degreening can be obtained with relative humidity levels between 80-100%.

How much ethylene do I use for degreening citrus?

It is common practice to use ethylene gas to degreen early season fruit in Queensland, such as Imperial mandarins. The success of degreening can vary with variety, the initial colour of the fruit, concentration of ethylene used and the duration of exposure to ethylene.

The concentration of ethylene required is between 3 and 10 parts per million of the fresh air intake. Air intake is between 1% and 2% of the room volume per minute.

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