Quality of Postharvest Degreened Citrus Fruit

*Julia Morales, Lourdes Cervera, Pilar Navarro and Alejandra Salvador*

## **Abstract**

External color is a key factor that defines external citrus fruit quality. Degreening with exogenous ethylene exposure is a widely used postharvest treatment applied to promote external citrus fruit color development, mainly with those cultivars that reach internal maturity while their external peel color is still green. Ethylene plays a crucial role in the color change of citrus fruit because it induces two simultaneous, but independent, processes—chlorophyll degradation and carotenoid synthesis. However, it is important to know, in addition to the effect on skin color development, whether this treatment can negatively affect other fruit quality parameters. This chapter addresses the influence of postharvest degreening treatment on the physicochemical, nutritional, and sensory quality of citrus fruit.

**Keywords:** ethylene, color, biocomponents, antioxidant, disorders, flavor

## **1. Introduction**

External citrus fruit color is an important quality attribute to be considered for the fresh market. Consumers associate high-quality fruit with the typical bright orange skin color, while they associate greenish skins with unripe fruit [1].

In citrus fruit, color change requires not only night temperatures close to 12°C but also marked differences between day and night temperatures. At 12°C, the expression of the key genes of the carotenoid biosynthesis pathway is stimulated, which increases concentrations of carotenoids and xanthophylls [2]. However, in tropical countries without cold night temperatures, citrus fruit reaches acceptable internal quality without displaying the characteristic orange color, and fruit is consumed when the skin is yellowish-green.

In Mediterranean citrus production areas, early citrus varieties, especially mandarins and oranges, reach internal maturity before full external coloration. In these cases, a degreening treatment, with the application of exogenous ethylene, is a common postharvest practice followed to improve external fruit color, which contributes to their market acceptance and extends their marketing season. Moreover, in mature citrus fruit harvested after the color break, it has been reported that exogenous ethylene can reduce the incidence of important postharvest physiological disorders, such as non-chilling peel pitting or chilling injury [3, 4]. These disorders are manifested

as peel damage, as well as the incidence of the disease caused by *Penicillium digitatum* without negatively affecting fruit organoleptic properties [3, 5].

The degreening treatment applied to enhance external citrus fruit color involves exposing the fruit to low ethylene concentrations [6]. However, in addition to ethylene concentration, other important factors, such as temperature, humidity or carbon dioxide (CO2), and oxygen in the atmosphere, are implied during color change. The exhaustive control of all these factors, as well as the process duration, are requirements to achieve the desired external color without promoting undesirable reactions related to physiological disorders or sensory changes, which harm fruit quality during their posterior shelf-life.

This chapter describes the parameters involved in the degreening process, and its impact on external or internal fruit quality.

## **2. Factors involved in the postharvest degreening process**

In climacteric fruit, ethylene plays a key role in governing physiological and biochemical changes that occur during ripening, including the color break, softening, and accumulation of sugars, acids, aroma volatiles, and vitamins [7]. In contrast, citrus fruit is non-climacteric, and their natural ripening is not accompanied by rises in respiration and ethylene production rates [8]. However, exposure to exogenous ethylene has been shown to stimulate various ripening-related processes, such as destruction of the green chlorophyll pigments and accumulation of orange/yellow carotenoids, in citrus peel tissue. Exogenous ethylene increases chlorophyllase activity and gene expression as well as other genes involved in chlorophyll breakdown [9, 10]. Ethylene also downregulates chlorophyll biosynthesis, by repressing the gene expression of Mg-chelatase and most genes involved in photosynthesis and chloroplast biogenesis [11]. Ethylene also stimulates the transcription of carotenoid biosynthetic genes in citrus fruit peel, which is concomitant to both the transformation from chloroplast into chromoplast and the accumulation of xanthophylls and carotenoids [11, 12]. Therefore, degreening treatment with exogenous ethylene exposure is a widely used postharvest treatment to promote external color development in citrus, especially in those cultivars that reach internal maturity while external peel color is still green [12, 13].

Commercial postharvest degreening is usually carried out in packinghouses, specifically in temperature-controlled chambers equipped with automatic injectors to provide the appropriate ethylene concentration. The applied ethylene concentration is low, close to 1–5 ppm [14, 15]. This concentration suffices to cause a color change, and it has been reported that increasing ethylene concentration has no significant effect on improving peel color or reducing degreening times [16]. Exposing citrus fruit to higher ethylene concentrations can cause undesirable effects related to accelerated fruit senescence. It is, therefore, necessary to monitor ethylene levels constantly throughout the process to ensure that its concentration is sufficient for proper degreening without detriment to quality.

Temperature is a determinant of color evolution during the ethylene degreening process. Temperature strongly influences chlorophyll degradation and carotenoid synthesis. High temperatures close to 30°C lead to rapid chlorophyll loss but delay carotenoid accumulation. However, the temperature within the 18–20°C range allows greater carotenoid accumulation, although chlorophyll degradation is slower. Very low temperatures (close to 5°C) during the process can repress carotenoid accumulation and affect carotenoid composition in flavedo [17]. In Spain, mandarins are

#### *Quality of Postharvest Degreened Citrus Fruit DOI: http://dx.doi.org/10.5772/intechopen.105119*

subjected to degreening treatment at 18–21°C, while oranges are exposed to a slightly higher temperature of 20–22°C [18]. However, the degreening of lemons is carried out at 25–30°C [19]. Similarly in Israel and California, mandarins and oranges are exposed to 20–25°C [20–22]. In Florida, most citrus fruits are commercially degreened at a relatively high temperature of 28–29°C [23].

Another important factor to consider in the degreening treatment is the time required to reach the desired fruit color, which very much depends on both the cultivar and the initial fruit color which, in turn, are controlled by fruit maturity and orchard conditions [18]. Citrus peel color increases with the ethylene exposure time during the degreening process. However, the negative effects induced by ethylene are also stronger the longer the exposure time is. Therefore, the time during which fruit is exposed to ethylene should be as short as possible, and optimal temperature, ethylene concentration, humidity, and aeration condition should apply. Not exceeding 72–96 h of treatment is advisable to avoid peel disorders during posterior commercialization [24]. Current color sorters, which work with photoelectric cells on handling lines, allow the fruit to be selected according to their initial color and to adjust degreening treatment duration to avoid overexposing fruit to ethylene. It should be noted that fruit color evolution continues once the fruit is transferred from ethylene chambers to marketable conditions [6]. However, the temperature to which fruit is subjected after the degreening treatment is key for posterior color evolution. Color development can be limited if temperatures are low while shipping citrus fruit [25]. The combination of periods with and without ethylene exposure has successfully achieved the desired change in mandarins and oranges, and optimal ethylene exposure duration has been estimated by considering color at harvest and subsequent marketing conditions [6]. In addition, the color change that fruit undergoes during degreening treatment very much depends on the variety [6, 24, 26].

Exposing fruit to ethylene increases the fruit respiration rate inside degreening chambers [27]. CO2 is known as an ethylene antagonist which, at high concentrations, inhibits the action of ethylene and delays the color change process. Therefore, the chambers in which degreening is carried out must be equipped with specific CO2 sensors to continuously monitor its concentration. CO2 must be kept below 0.15–0.2% to allow proper degreening. High CO2 levels in the atmosphere may induce acetaldehyde and ethanol production with the consequent risk of off-flavors in fruit [20]. The oxygen concentration has to remain above 20% because, apart from its role in respiration activity, oxygen is necessary for chlorophyll degradation and carotenoid biosynthesis. Therefore, adequate ventilation is essential to supply oxygen and remove CO2 accumulation in degreening chambers. Moreover, during degreening treatments, keeping relative humidity at around 95% is desirable to obtain satisfactory peel color change results and to avoid fruit dehydration and skin alterations [28].
