**4. Effect of degreening on bioactive compounds**

Nowadays, consumers demand fruit with, in addition to an attractive appearance, high sensory and nutritional quality. Citrus fruits are known for their health-promoting benefits due to their high content of bioactive compounds with antioxidant properties. The antioxidant activity of citrus fruit is due to the water-soluble fraction, including vitamin C and polyphenols, and also to the apolar fraction that contains carotenoids. Despite the common dogma that ethylene has a minor effect on the internal ripening processes of citrus fruit, it has not been systematically examined [55].

About the physicochemical parameters of firmness, total soluble solids (TSS), and acidity, there are many reports that ethylene application does not modify them in different citrus cultivars [27, 55–57]. Nevertheless, the effect of degreening on these attributes can differ depending on the citrus cultivar. In a study that compared three early citrus cultivars, only the 'Owari' mandarin had higher softening and higher TSS in degreened fruit (2 ppm, 2–8 days) than in non-degreened ones [58].

In citrus fruit, vitamin C is widely regarded as the most important water-soluble antioxidant compound and an excellent reducing agent [59]. Vitamin C content in Citrus sp. depends on both the maturity stage and other pre- and postharvest factors. Many studies have focused on evaluating the effect of degreening treatment on vitamin C content. Sdiri et al. [6] did not observe any significant reduction in vitamin C in mandarins 'Clemenules' and 'Clemenpons' after exposing them to ethylene (2 ppm) for 48 h, 72 h, or 120 h. Mayuoni et al. [55] performed a study on different citrus cultivars and found no differences in the vitamin C content of 'Star Ruby' grapefruit and 'Satsuma' mandarins after degreening treatment lasting up to 72 h. Only 'Navel' oranges presented a slight detriment after 72 h, which was attributed to the storage period after the degreening process as no differences were found between degreened and non-degreened fruit. In orange 'Navelina' and seven early mandarins ('Basol', 'Clemenrubí', 'Clemenpons', 'Clemenules', 'Orogros', 'Oronules', 'Prenules'), no differences were observed between the degreened fruit with ethylene (2 ppm, 120 h) and the untreated fruit after cold storage to simulate quarantine conditions (1°C, 16 days), plus a shelf-life period (20°C, 7 days) [24]. In tangerine 'Batu-55', a 24-hour degreening treatment at 1, 3, or 5 ppm did not influence vitamin C content [60]. About grapefruit, no differences have been reported in 'Star Ruby' between non-degreened and degreened fruit with ethylene (2 ppm, 60 h) after a 21-day storage period at 10°C (to simulate the shipment period) and followed up to 14 days at 20°C (to simulate retail store conditions) [56]. However, in other studies ethylene has been reported to induce an increase in vitamin C. Chaudhary et al. [57] detected more vitamin C in degreened 'Rio Red' grapefruit (3.5 ppm ethylene, 72 h) after 35 storage days at 11°C than in non-degreened fruit. Sdiri et al. [61] also observed a slight increase in L-dehydroascorbic acid in degreened 'Clemenules' and 'Clemenpons' mandarins after simulating a shelflife period (20°C, 7 days). This increased vitamin C content could be due to the dominant expression of the gene encoding L-galactose-1-phosphatephosphatase (GPP), an enzyme that is related to vitamin C biosynthesis, after finding rising GPP transcript levels in ethylene-treated tomatoes [59].

The exogenous ethylene effect can affect the enzymes involved in plant metabolic pathways, such as phenylalanine ammonia-lyase (PAL), the first enzyme in the phenylpropanoid pathway, or chalcone synthase (CHS) [62]. Regarding phenolic compounds (flavanones, flavones, polymethaoxy flavones, flavanols, hydroxybenzoic acids, and hydroxycinnamic acids), no ethylene effect has been observed in oranges 'Navel' and 'Valencia Delta', 'Batu-55' tangerine and 'Star Ruby' grapefruit when submitted to degreening [55, 56, 60, 63]. Sdiri et al. [24] studied the influence of ethylene exposure on the phenolic profile of eight early-season commercial citrus varieties during degreening treatment (2 ppm, 120 h), plus the simulation of quarantine conditions (1°C, 16 days). Only the flavanones profile was modified in the 'Clemenrubi' and 'Clemenpons' mandarins, whose fruit had been exposed to ethylene and showed the highest total flavanones content after the shelf-life period. An increase in phenolic compounds has also been reported in other studies. Higher total phenolic content has been found in the juice of lemon 'Fino' obtained from degreened fruit compared to non-degreened fruit [19]. In 'Rio Red' grapefruit, Chaudhary et al. [57] showed that degreened fruit exhibited higher limonin and flavonoids contents and lower furocoumarin levels (mainly 6–7-dihydroxybergamotin) than non-degreened fruit after 7 days of cold storage at 11°C. Nevertheless, in grapefruit 'Rio Red', no differences were detected between degreened and non-degreened fruit at the end of the study period (21 days at 11°C, plus 14 days at 21°C). Chaudhary et al. [16] evaluated the effect of ethylene concentration on the flavonoid profile in the same cultivar and found that the 10 ppm-degreened fruit had significantly higher contents for most of the phytochemicals measured compared to the 5 ppm-treated fruit.

Although exposing citrus fruit to ethylene induces carotenoid accumulation in peel [12], scarce information is available about the effect of degreening treatment on the carotenoid content in juice sacs. Matsumoto et al. [17] studied the effect of fruit exposure to different concentrations and temperatures on the carotenoid accumulation in the flavedo and juice vesicles of satsuma mandarins. The results of their study revealed that carotenoid synthesis in citrus was temperature-sensitive, and this effect was tissue-dependent. Storage at 20°C increased carotenoid accumulation in flavedo and maintained carotenoid content in juice sacs. However, storage at 5°C and 30°C slightly increased carotenoid content in flavedo and decreased contents in juice sacs. No exogenous ethylene effect on carotenoid content in the juice sacs of the fruit stored at 20°C and 5°C was observed. Chaudhary et al. [16] also reported that ethylene exposure did not affect the β-carotene and lycopene contents in 'Star Ruby' grapefruit juice during degreening treatment.

Therefore, by considering these recent findings, we conclude that degreening treatment can be used to enhance early citrus fruit peel color with minimal effects on nutritional quality.

## **5. Sensorial quality of degreened citrus fruit**

Some studies have pointed out that degreening can impair the taste quality of citrus fruit [55, 64, 65]. Sometimes not using degreening has even been the objective to differentiate citrus fruit by assuming a superior organoleptic quality as in Protected Geographical Indication (PGI) 'Clémentine de Corse' [66]. Nevertheless, other studies report that degreening treatments performed under standard conditions do not affect sensory citrus fruit quality [6, 18, 67]. Alteration to the organoleptic quality of degreened fruit has been mainly related to low temperature during their posterior storage period [68].

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

There are many volatiles responsible for a flavor or aroma sensation in citrus fruit. Moreover, combinations of volatiles yield different flavors than those expected from individual compounds [69]. Citrus fruit presents a complex profile of volatile organic compounds (VOCs), and it is possible to find complex combinations of a subset of up to 300 compounds [70]. Of these, aldehydes and esters are the compounds with the strongest impact on citrus aroma. Changes in the aroma-active compounds of degreened citrus fruit have been reported as being extremely variety-dependent. The changes in the volatile profile of several citrus fruit cultivars submitted to degreening treatment, plus 16-day storage at 1°C and shelf-life, have been studied [71]. While the aroma active compounds of 'Clemenules' and 'Navelina' are not influenced by ethylene exposure, 'Oronules' and 'Clemenrubí' presented higher levels of some esters, such as ethyl propionate and ethyl octanoate, in degreened fruit than in nondegreened fruit. Other cultivars, such as 'Basol' and 'Prenules' mandarins have shown few changes caused by ethylene. It should be noted that despite the effect of degreening on certain volatile compounds in some of these varieties, no differences in sensory quality were observed. Mayuoni et al. [55] detected a minor effect of degreening treatment (4 ppm, 72 h) on volatiles content and composition in the juice of 'Navel' oranges, 'Star Ruby' grapefruit, and 'Satsuma' mandarins. While ethylene exposure did not affect the flavor of oranges and grapefruit, a slight detriment to sensory acceptability was observed in mandarins.

In recent years, several studies have reported no negative degreening effects on sensory citrus fruit properties. In 'Owari', 'Clemenules' and 'Oronules' mandarins and in 'Navelina' oranges, Morales et al. [58] have shown that exposure to ethylene (2 ppm for 2–8 days) does not bring about any significant changes in internal sensory characteristics. In fact, consumers were unable to detect the effect of ethylene degreening on physicochemical parameters, although slight differences in soluble solids and acidity were noted in some cultivars. The ethylene degreening treatment has been shown to increase consumer purchase intention. In 'Fino' lemons, a sensory analysis determined that degreened fruit subsequently stored for up 28 days at 10°C obtained a similar overall liking to non-degreened fruit. In addition, degreened lemons were perceived as having a better typical lemon aroma than non-degreened lemons [19].
