The fruits of rangpur red lime (Citrus × limonia), Tahitian lime (Citrus × latifolia) and sweet orange (Citrus sinensis) cv. ‘Sweety’ were obtained from local farms in the municipality of Caicedonia, Valle del Cauca, Colombia. Approximately 4, 3.8, and 10 kg of rangpur red lime, Tahitian lime, and sweet orange, respectively, were taken for analysis after being collected in initial stages of maturation.

The fruits of each species were divided into three lots. The first lot (control) consisted of fruits without treatment. The second lot was immersed for 5 min in a 150 mg/L sodium hypochlorite solution, drained and left to dry for 10 min (Sapers, 2014). UV-C radiation was applied to the third lot (Garzón-García et al., 2023). Groups of six fruit were kept for 5 min under a 20 W UV lamp (UVC20W, ANTIVID19, Ecolite S.A.S, Cali, VAC, CO). The radiation source was located 11 cm above the work area (Figure 1). After treatments, the fruits were distributed in nine containers divided into species and treatments. These were stored under ambient conditions (26 °C with a relative humidity of 70 %) for 21 d, simulating storage at local shops.

Figure 1

pH of samples was measured with a pH meter (HI 991001, HANNA Instruments, Woonsocket, RI, US). Titratable acidity (TA) was measured according to the AOAC 942.15 method, utilizing phenolphthalein as an indicator, and titrating with 0.1 N NaOH until a pH of 8.1 was achieved. Citric acid was considered as the primary organic acid (AOAC, 2012). Total soluble solid content (TSS) was assessed by using a digital refractometer (PAL-1, ATAGO CO., TYO, JP).

Equatorial diameter (ED or width) and polar diameter (PD or length) were determined using a caliper, following the methodology of Kadluczka and Grzebelus (2022). Fresh weight of samples was measured with a precision balance (Xtar, Lexus, UP, IN). Fruits were weighed at day 0, 7, 14, and 21 of storage. The difference between the initial and final weight was expressed as weight loss percentage (WL %) during a storage interval (Peralta-Ruiz et al., 2020).

W L % = I i t i a l   w e i g h t - W e i g h t   d u r i n g   s a m p l i n g I n i t i a l   w e i g h t (1)

The peel color of fruits was assessed using a 3NS800 spectrophotometer (3NH, Zengcheng District, GZ, CN), calibrated with a D65 illuminant and a 10° observer angle. Measurements were presented in CIELab coordinates (L* or lightness: white(+)/black(-), a*: red(+)/green(-), b*: yellow(+)/blue(-)) and hue (h*; 0° or 360°: red, 90°: yellow, 180°: green, and 270°: blue). Hue and yellow index (YI) were calculated according to Pathare et al. (2013).

h * = t a n - 1 b * a * (2)

Y I = 142.86 b * L * , (3)

A completely randomized design with two factors and a 3×4 factorial arrangement was used for each species. The first factor was the postharvest treatment: Control (C), immersion in a sodium hypochlorite solution (SH), and UV-C exposition (UV). The second factor corresponded to the sampling time during storage (0, 7, 14, and 21 d). A two-way ANOVA and a Tukey’s test were performed to assess differences in mean values between treatments and the effect of factors on pH, TA, TSS, width, length, percentage of weight loss and color parameters of rangpur red lime, Tahitian lime, and sweet orange (p < 0.05). The results of these tests were presented as the mean of triplicate measurements ± standard deviation in the tables, and as the mean ± standard error in the figures. The relationship between TA, TSS, width, length, and percentage of weight loss was established by means of a correlation matrix and the calculation of the Pearson correlation coefficient (ρ). All analyses were performed in Minitab 17 (Minitab Inc, State College, PA, US).

Figure 2 illustrates the results of the determination of pH, titratable acidity, and total soluble solid content of rangpur red lime, Tahitian lime, and sweet orange subjected to different postharvest treatments. According to the two-way ANOVA, the postharvest treatment, the storage time, and the interaction of these factors had a significant effect on physicochemical properties of rangpur red lime and Tahitian lime (p < 0.05). Values of pH corresponding to rangpur red lime (C: 2.45 - 2.33, SH: 2.45 - 2.25, UV: 2.45 - 2.32) and Tahitian lime (C: 2.67 - 2.42, SH: 2.67 - 2.28, UV: 2.67 - 2.41) decreased slightly during storage. However, these values were within the ranges reported for citrus fruits. Mohd-Hanif et al. (2016) noticed no significant changes in pH of lime (Citrus aurantifolia) juice irradiated with different UV dosage (from 22.76-44.24 mJ/ cm²). This last finding might be attributed to the fact that lime juice was not stored for an extended period, as opposed to the whole fruit storage duration in the current study.

Figure 2

TA tended to decrease until day 14 of storage, while TSS content increased throughout the storage period. Concentration of organic acids and sugars in citrus fruits depend on ripening stage, environmental conditions, and agronomical practices (Asencio et al., 2018). Generally, organic acids accumulate during early stages of fruit ripening and storage, while sucrose levels are increasing (Liao et al., 2019). In a separate study, sweet oranges (Citrus sinensis (L.) Osbeck) were irradiated using ultraviolet light for six d after harvest, causing a remarkable increase in sucrose, fructose, and glucose content (Hu et al., 2019). On the other hand, any factor had effect on physicochemical properties of sweet orange (p>0.05). Considering that pH (C: 3.47 - 3.48, SH: 3.47 - 3.59, UV: 3.47 - 3.59) and TA (C: 4.58 - 4.45, SH: 4.58 - 4.58, UV: 4.58 - 4.42) did not vary significantly, the treatments were efficient in reducing microbe during the storage time (Pan and Zu, 2012). TSS also remained almost constant during the storage period (C: 10.43 - 12.87, SH: 10.43 - 10.60, UV: 10.43 - 11.27), indicating that the orange did not undergo rapid ripening or senescence. Pristijono et al. (2019) irradiated Tahitian limes with different doses (0, 3.4, 7.2, and 10.5 kJ/m²) and stored at room temperature (10 °C and a relative humidity of 80 %). Similarly, TSS and TA of Tahitian limes did not change, suggesting that pre-storage treatments did not affect these quality parameters may be due to distinct responses of defense mechanism.

The measurements of length and width confirmed that the fruits possessed an ovoid shape. This holds significance as it enables the design of processes and equipment tailored to the morphology of the fruits. It was established that postharvest treatment, storage time, and interaction between the factors had significant effects on width, length, and weight of rangpur red lime, Tahitian lime, and sweet orange (p < 0.05). According to Figure 3, length and width tended to decrease during the initial days of storage. Weight loss is associated with the water loss by transpiration and respiration during ripening and senescence stages (Barreto et al., 2016). Moreover, when the relative humidity falls below 90 % and the ambient temperature increases, metabolic activity intensifies, leading to greater water loss and a decline in fruit quality. At the end of the storage, rangpur red limes exhibited average weight loss percentages of 30.5 %, 39.1 %, and 44.2 % for the C, SH, and UV treatments, respectively. Tahitian lime (C: 27.4 %, SH: 34.4 %, UV: 9.6 %) and sweet orange (C: 21.9 %, SH: 11.5 %, UV: 11.3 %) presented greater average weight loss percentage without treatment because of their size. Lufu et al. (2020) reported that larger fruits exhibit a lower rate of moisture loss compared to smaller fruits because smaller fruits have a higher surface area to volume ratio.

Figure 3

SH and UV samples reduced weigh loss by decreasing metabolic activity and delaying respiration and transpiration rate. Pristijono et al. (2019) irradiated Tahitian limes with distinct doses (0, 3.4, 7.2, and 10.5 kJ/m²) and observed that the highest dose resulted in the most significant reduction in weight loss (1.5 %) during storage for up to 28 d in air at 10 °C. Mandarin fruits cv. ‘Ortanique’ were subjected to disinfection by immersion in a 10 % sodium hypochlorite solution for 2 minutes. Although this treatment did not result in a significant reduction of weight loss, by the end of a 28-day storage period, the percentage of weight loss reached approximately 5 % (El Guilli et al., 2016). Variations were attributed to differences in temperature and relative humidity. In the present study, higher temperatures, and a relative humidity of 70 % were considered. These conditions facilitated accelerated weight loss compared to the investigations referenced.

As Table 1 shows, there was a strong inverse correlation between weight loss and equatorial diameter. It suggested that weight loss primarily led to a decrease in the width of citrus fruits during storage. For rangpur red lime, a moderate and inverse correlation was observed between the percentage of weight loss and titratable acidity. This could be attributed to the decrease in titratable acidity being associated with a reduction in organic acid content during respiration (Burdon et al., 2007). Concurrently, there is a release of CO₂ and water, contributing to weight loss. In the case of Tahitian lime, there was a moderate correlation between TA and pH, as these parameters consistently remained stable during storage. This suggests that the constancy of organic acids contributed to acidity and a low pH (Miller et al., 2022). A low correlation between TA and TSS was found. As reported by Batista-Silva et al. (2018), during ripening of non-climacteric citrus fruits, there is a decline in titratable acidity attributed to the catabolism of citrate. Furthermore, granulation might occur as a natural process in citrus species, wherein sugar levels increase during ripening and subsequently stabilize or decrease, leading to a deterioration in the quality of citrus fruits (Johari et al., 2023).

Table 1

Correlation	Citrus fruit
	Rangpur red lime		Tahitian lime		Sweet Orange
	Coefficient p-value	Coefficient p-value	Coefficient p-value
TA-pH	0.317 0.060	0.538 0.001	0.308 0.067
TA-TSS	-0.012 0.950	-0.358 0.032	-0.091 0.597
WL %-TA	-0.465 0.004	-0.547 0.001	0.110 0.522
WL %-ED	-0.901 0.000	-0.928 0.000	-0.792 0.000
WL %-PD	-0.879 0.000	-0.801 0.000	-0.649 0.000

In accordance with the two-way ANOVA, it was found that postharvest treatment, storage time, and the interaction between these factors had a significant effect on all surface color parameters of rangpur red lime, Tahitian lime, and sweet orange (p < 0.05). The use of UV-C and NaClO enabled the increase of L* values for all citrus fruits throughout and at the end of the storage period, respectively (rangpur red lime: 45.610 - 59.391, Tahitian lime: 47.170 - 60.308, sweet orange: 59.643 - 61.366). Control samples exhibited accelerated ripening and granulation, leading to a dry appearance and a reduction in luminosity (rangpur red lime: 45.610 - 40.628, Tahitian lime: 47.170 - 37.281, sweet orange: 59.643 - 50.710). Conversely, SH treatment induced browning on the surfaces of the fruits (rangpur red lime: 45.610 - 51.468, Tahitian lime: 47.170 - 57.272, sweet orange: 59.643 - 54.049). Garzón-García et al. (2023) noted that immersing fresh-cut mango in a 10 mg/L sodium hypochlorite solution led to a reduction in L* values. This effect occurred because NaClO can act as an oxidizing agent. Consequently, UV-C treatment resulted in the inhibition of polyphenol oxidase (PPO) activity.

According to Figure 4, the results of a* coordinate showed that the rangpur red lime reached full ripeness by day 14 of storage. Subsequently, it began to exhibit a loss of red color which may be due to carotenoids oxidation and isomerization promoted by light, temperature, oxygen, and abiotic stress during storage (Rodriguez-Amaya, 2019). Nevertheless, UV-C treatment delayed significantly changes in a* coordinate compared to the other treatments by mediating the activity of some enzymes such as carotene isomerase (Sonntag et al., 2023). On the other hand, values of a* coordinate corresponding to Tahitian lime showed slightly differences during storage, but these remained in the green scale. Pristijono et al. (2019) also found that Tahitian limes irradiated with 3.4, 7.2, and 10.5 kJ/m² retained the green peel color during storage. This implied that the selected UV-C treatment delayed chlorophyll degradation and the increase in phaeophytin content. However, higher UV-C doses may lead to nonenzymatic browning and a reduction in brightness and green color (Sonntag et al., 2023). For sweet orange, sodium hypochlorite and UV-C treatments were found to regulate ripeness and inhibited browning. Untreated samples exhibited a decrease in a* values after day 14 of storage, possibly for the increment of metabolism activity and thus, the enzymes activity responsible for carotenoid degradation.

Figure 4

The b* coordinate also indicated the apparition of red tones and the accumulation carotenoids in citrus fruits (Lado et al., 2019), as shown in Figure 4. For rangpur red lime, untreated fruits and samples treated with NaClO showed a rapid increase in b* values until day 14 of storage. Afterward, these values tended to stabilize and decrease, respectively. Conversely, the UV-C treatment demonstrated an elicitor effect, as the values of a* increased in irradiated samples during storage. As reported by Castillejo et al. (2022), UVR8 protein is a UV-B and UV-C specific photoreceptor that interacts with COP1 and regulates HY5 gene expression, which increases the production of carotenoids for photoprotection. For Tahitian lime, the b* values of untreated samples declined after day 14 of storage, possibly due to granulation and the fruit taking on grayish hues (Sharma et al., 2006). In contrast, samples treated with sodium hypochlorite and UV-C exhibited a tendency to increase the a* coordinate until the end of the storage period. Sweet oranges showed an increase in the a* coordinate until day 14 of storage, which indicate fruit maturation. Following that, a* values for untreated samples and those treated with NaClO showed a tendency to decrease. UV-C treatment could preserve the yellow by reducing ethylene production (Lacerna et al., 2018).

In Table 2, the values of h* and YI of samples are presented with the results of two-way ANOVA and Tukey post-hoc test. The calculation of YI and h* allowed to establish that untreated samples experienced accelerated ripening at the start of storage, and granulation towards the end of the storage period, respectively. The treatments effectively prevented accelerated ripening. Nevertheless, UV-C irradiation induced uniform coloration on the citrus fruits peel, as evidenced by the rise in YI. According to Pristijono et al. (2019), the green color is a crucial factor for marketing Tahitian limes. Typically, lower hue values are exhibited during storage, accompanied by the emergence of yellow tones due to maturation.

Table 2

Color parameter	Treatment	Storage time (d)
			0	7	14	21
Rangpur red lime
	C	113.367±0.081a	93.048±0.026e	83.764±0.005g	104.408±0.003b
h*	SH	113.367±0.081a	98.224±0.000c	68.808±0.002j	74.547±0.008h
	UV	113.367±0.081a	95.940±0.008d	70.256±0.023i	89.141±0.003f
	C	55.347±0.521i	89.705±0.365h	99.149±0.008g	115.846±0.316f
YI	SH	55.347±0.521i	124.757±0.044d	147.438±0.034a	125.649±0.024d
	UV	55.347±0.521i	118.809±0.212e	136.352±0.207b	131.475±0.061c
Tahitian lime
	C	112.983±0.071c	106.865±0.003e	103.754±0.010g	110.122±0.005d
h*	SH	112.983±0.071c	114.938±0.061b	115.301±0.015a	96.396±0.003j
	UV	112.983±0.071c	106.315±0.021f	98.643±0.003h	98.025±0.007i
	C	42.583±0.557i	112.721±0.227b	119.760±0.132a	86.368±0.076f
YI	SH	42.583±0.557i	82.521±0.507g	76.047±0.127h	111.874±0.126b
	UV	42.583±0.557i	93.932±0.048e	105.209±0.039d	110.107±0.060c
Sweet orange
	C	88.923±0.021b	79.5853±0.037f	77.763±0.014g	90.710±0.011ª
h*	SH	88.923±0.021b	87.242±0.004c	77.327±0.003h	72.057±0.005j
	UV	88.923±0.021b	86.714±0.010d	84.132±0.006e	73.885±0.002i
	C	138.564±0.037e	147.920±0.824c	157.004±0.276a	129.798±0.054g
YI	SH	138.564±0.037e	150.205±0.149b	131.721±0.109f	124.561±0.044h
	UV	138.564±0.037e	139.977±0.039d	156.934±0.239a	150.420±0.016b