How Degreening of Citrus Fruits Enhances Appearance and Quality

• Artificial degreening changes only citrus peel color and does not affect other quality parameters.
• Several citrus factors, like maturity at harvest and cultivar-specific ethylene sensitivity, will influence degreening success.
• Postharvest degreening is the standard procedure and requires careful consideration of cultivars to determine atmosphere conditions, ethylene concentrations, and exposure duration to achieve the desired results.
• Precision measurement of ethylene is necessary, as higher than recommended ethylene can harm the fruit’s quality and reduce shelf life.

Ethylene degreening is a standard procedure for citrus fruits, where exposure to the gas results in desirable changes in appearance to increase consumer acceptance. However, ethylene gas, a natural phytohormone, also causes fruit senescence and decay. Therefore, the degreening process must be precise and conducted in the correct environment. Learn more about the degreening process and how to avoid its pitfalls to improve citrus sales.

Degreening

Citrus fruits are non-climacteric and do not require ethylene for ripening. However, exogenous ethylene application can degreen the peel, as it activates ripening processes in the peel that degrade chlorophyll and start the biosynthesis of yellow or orange carotenoids.

Degreening is necessary because color development requires low temperatures after citrus reaches maturity. In the world’s tropical regions, these low temperatures may not be available at fruit maturity, and fruit at commercial maturity will retain some green color in the outer peel or flavedo, see Figure 1. So, artificial and external ethylene applications are used to degrade chlorophyll.

Figure 1: Parts of a citrus peel, Moreno & Rodríguez Mellado 2022. (Image credits: https://doi.org/10.3390/oxygen2020009)

Stage of Degreening

Since degreening changes only the peel color, it is crucial to harvest citrus when fully mature and ripe. Immature citrus with high acidity and low sugar content will not ripen further or meet consumer satisfaction and national standard requirements. Artificial degreening is suitable only for mature, edible citrus fruit.

Degreening can be done as a preharvest or postharvest procedure.

• Preharvest degreening: Most citrus responds well to preharvest ethylene application on the tree. The advantages are less fruit injury and postharvest deterioration. However, ethylene also affects other plant parts and leads to senescence, so the concentration required for degreening causes defoliation.
• Postharvest degreening: Postharvest degreening is commonly used for early varieties of citrus that mature before temperatures fall enough for a color break. It is more viable than preharvest methods. It doesn’t cause defoliation, and fruits can be washed and waxed. 

Degreening Methods

Degreening is usually done using external ethylene application, though artificial colorings with permitted dyes are also practiced. However, these dyes leave a chemical residue, so this method is not advisable. On the other hand, ethylene is a phytohormone produced naturally by fruits and plants. Artificial ethylene produced by chemicals like Ethephon is harmless and even suitable for degreening organic citrus fruits.

Degreening processes are complex and depend on several external and internal factors.

The typical internal factors influencing degreening in all methods are fruit maturity level and sensitivity to ethylene. The external factors will vary depending on the procedure.

Preharvest Degreening

Preharvest sprays of 200 ppm (parts per million) ethylene are necessary for degreening most mandarin cultivars, and some varieties of tangerines (‘Dancy’) and sweet oranges (‘Hamlin’). While cultivars like ‘Satsuma’ mandarin and ‘Washington Navel oranges require 500-1000 ppm. Lemon varieties like ‘Bearss’ need over 1000 ppm.

Factors affecting preharvest degreening are as follows:

• Temperatures: Low air night temperatures of 7–13°C, day temperatures of 20°C, and soil temperatures of 12°C favor color development in citrus.
• Tree vigor: Citrus harvested from high-vigor trees are challenging to degreen.
• Rootstock: Grafts with vigorous rootstocks that improve tree vigor also affect degreening.
• Orchard management: Applying high-nitrogen fertilizers or insecticides can delay color change.

Postharvest Degreening

Postharvest ethylene degreening produces uniform color development in citrus and is possible in the following ways:

Dip

The method is considered better than commercial gas. Fruits can be dipped in ethephon

solutions of 500 to 12000 ppm for 1 to 10 minutes. For most cases, such as lemon, grapefruits, and some mandarin (‘Szinkom’), tangerine (‘Clementine’), and sweet orange (‘Valencia’) varieties, 1000 ppm is enough. Using ethylene concentrations above 5000 ppm causes rind injury in ‘Eureka’ lemons and delayed degreening in ‘Clementine’ tangerines.’

Fruit stalk application

Ethephon is administered through the fruit stalk. This method uses fewer chemicals but is not practical on a commercial scale.

Ethylene Gas

Citrus exposure to ethylene gas is the standard practice in commercial units. Citrus fruit is exposed to 5-10 ppm ethylene gas for 1-3 days, depending on the extent of greenness in rooms with a controlled atmosphere at 20-26^oC and 90-95% RH. Conditions for degreening are specific to the production area. Ethylene gas can be introduced into the room through the trickle or batch method.

Figure 2: “Visual appearance of ‘Satsuma’ mandarins after degreening with various ethylene concentrations. Pictures were taken after five days of postharvest degreening in the presence of ethylene at 0 (left), 0.1 (middle), and one ppm (right) at 20°C,” Porat 2008. (Image credits: http://www.globalsciencebooks.info/Online/GSBOnline/images/0812/ TFSB_2(SI1)/TFSB_2(SI1)71-76o.pdf).

Factors Affecting Postharvest Degreening

Ethylene concentrations, the duration of the degreening process, air temperature, relative humidity, ventilation, and air circulation are factors influencing degreening.

Ethylene concentrations and duration

The ethylene threshold level for degreening is between 5 and 10 ppm, depending on the cultivar, see Figure 2. Higher concentrations do not accelerate degreening but can cause several problems in citrus:

• It leads to senescence and decay processes, such as fruit calyx dryness and browning.
• Other harmful effects are rind defects like peel bruising and green islands, see Figure 3.
• Too much ethylene, longer treatment times, and high temperatures can induce several postharvest diseases, such as anthracnose or ‘degreening burn.’

These problems can be solved using the minimum required amounts of ethylene for the shortest duration.

Phenomena such as the “ethylene carryover effect” can be avoided and also exploited to reduce degreening time. Fruits should be exposed to ethylene for only 1-2 days, and the rest of the color development occurs during storage at 15-20°C through carotenoid synthesis and accumulation in citrus flavedo.

Figure 3: “Development of peel disorders and defects in citrus fruit after degreening. Pictures were taken after 5 days of degreening with ethylene at 1.5 ppm, at 20°C, followed by cold storage for 10 days at 5°C,” Porat 2008. (Image credits: http://www.globalsciencebooks.info/ Online/GSBOnline/images/0812/TFSB_2(SI1)/TFSB_2(SI1)71-76o.pdf).

Atmospheric conditions

Temperature and relative humidity are crucial factors in the degreening process.

High temperatures up to 30-35°C favor chlorophyll breakdown but can reduce carotenoid formation. Temperatures of 15-20°C can slow degreen but do produce color change. The ideal temperatures are between 20-25°C and cultivar-specific.

A high relative humidity of 90-95% is used during degreening to prevent fruit transpiration due to the warm temperatures used in degreening. Lower relative humidity of 70-90% doesn’t affect degreening but can cause shriveling, softening, and weight loss.

Ethylene degreen can enhance color development, but since using excess gas is harmful, its levels must be monitored carefully, along with temperature and relative humidity throughout the process.

Degreening Impact on Fruit Quality

Ethylene’s effect on fruit quality is limited to changing peel color. Research on several citrus fruits has shown that ethylene doesn’t affect ripening processes in the fruit flesh to improve other quality parameters.

However, some minor negative impacts of using ethylene can occur, as the phytohormone is also responsible for senescence and decay in all plant parts.

• The ethylene degreening process can increase fruit respiration rate and intrinsic ethylene production.
• Ethylene triggers the synthesis of odor-producing chemicals to change the composition of aroma volatiles, marginally affecting mandarin and orange flavor. However, flavor was not affected in grapefruit.
• The sugar content and titrable acidity are not affected except to a small extent in mandarins and oranges.
• The nutraceutical value of citrus from health-promoting compounds like vitamin C, total phenols, and flavonoids remains unchanged.

Besides ethylene, high temperature and relative humidity during degreening can decrease the quality of organoleptic fruit. Temperature can produce off-odors to change the flavor of oranges and mandarins. High temperature and relative humidity can increase Penicillium digitatum infection in oranges, causing fruit decay.

Degreening improved fruit appearance, boosting consumer acceptance of all citrus fruits. Moreover, the minor changes in internal fruit quality didn’t negatively affect consumer satisfaction, so postharvest degreening can be a valuable tool in increasing citrus sales.

Best Practices for Degreening

Degreening is a complex process. To increase its benefits and avoid pitfalls, growers and packers should follow the recommended best practices listed below.

1. Harvest fruits that are not entirely green but show some color to ensure the internal maturity of citrus. Also, greener fruits require longer degreening time, which can reduce shelf life.
2. The degreening process time, conditions, and success are cultivar-specific.
3. Trials can be conducted with small batches of fruits to determine ethylene sensitivity and degreening duration.
4. Fruits should be color-sorted to produce batches at similar maturity. Subjecting sorted batches to different concentrations and durations of ethylene treatment reduces calyx senescence associated with exposure to high postharvest ethylene concentrations.
5. Ensuring good circulation with air flowing through and not around the fruit load is vital.
6. Adequate ventilation prevents carbon dioxide accumulation due to respiration during degreening.
7. Use pre-greening fungicides to preempt postharvest diseases due to high temperatures and relative humidity. However, ensure fruits are dry after fungicide application before degreening.
8. Citrus should not be waxed before degreen as it will hinder degreening.
9. Use the lowest ethylene concentrations quickly to prevent rind disorder and other problems.
10. Stakeholders should know that specific preharvest treatments can slow or reduce degreening. Citrus oil sprayed 2-4 weeks before harvest will not have full-color development. Similarly, citrus getting late gibberellic acid sprays will need longer degreening times and still not develop colorfully.

Degreening success will depend on a combination of cultivar, initial flavedo color, ethylene concentration, and the duration of exposure.

Measure Ethylene

While ethylene degreening is valuable, more is not better. Stakeholders will need precision tools to monitor ethylene in minute quantities of ppm. Felix Instruments Applied Food Science has custom-made devices, such as the portable F-960 Ripen It! Gas Analyzer that measures ethylene between 0-1000 ppm, and the fixed F-901 AccuRipe & AccuStore, which can remotely monitor and control ethylene levels. Both tools also measure carbon dioxide levels that must be kept low during degreening, and the F-901 AccuRipe also measures temperature and relative humidity.

Calculating the correct gas amounts based on each cultivar’s ethylene sensitivity is necessary to improve shelf-life and prevent disorders and postharvest diseases in citrus after degreening.

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