Postharvest effect of gaseous ozone on physicochemical quality, carotenoid content and shelf-life of mango fruit

Abstract

This study investigated the effect of gaseous ozone (O₃) exposure time on the postharvest quality and shelf-life of mangoes. “Keitt” mango fruit harvested at physiological maturity was exposed to 0.25 mg/L of O₃ for 12, 24,36, or 48 hours, and the control fruit were untreated. Fruit were thereafter stored at 10 ℃ for three weeks and ripened at ambient temperature for one week. Postharvest parameters such as mass loss, decay incidence, firmness, total soluble solids (TSS), titratable acidity (TA), and carotenoid content were assessed at weekly intervals. The findings showed that the physiological weight loss of untreated fruit (30.92%) was significantly higher (p <0.05) compared to O₃ (12 h), O₃ (24 h), O₃ (36 h), and O₃ (48 h) treated fruit, which was 28.49%, 25.90%, 20.54%, and 20.50%, respectively. Fruit treated with O₃ (36 h) significantly maintained firmness, delayed TSS accumulation, and decreased loss of TA. The untreated fruit had a higher decay incidence compared to other treatments at the end of storage. Moreover, the total carotenoid content was notably higher in O₃ (48 h) treated fruit during storage. Overall, the results demonstrated that the shelf-life of mango fruit was longer under the 36 h and 48 h treatments. These research findings indicate that O₃ could be used effectively to maintain the postharvest quality and extend the shelf-life of mango fruit. Therefore, O₃ (36 h) is recommended as a cost-effective postharvest treatment for “Keitt” mangoes.

Keywords:

• ozone
• mango fruit
• carotenoids
• decay index
• postharvest
• shelf-life
• physicochemical quality

1. Introduction

Mango is a vital fruit due to its high nutritional properties and economic value (Hmmam et al., 2021). The fruit contains essential dietary compounds such as proteins, vitamins, flavonoids, phenolics, and carotenoids (Maldonado-Celis et al., 2019). It is highly perishable with a relatively short shelf-life due to high respiration rate and ethylene production leading to biochemical changes (Hmmam et al., 2021; Kaur et al., 2020). The increased respiration rate during mango ripening leads to enzyme activities, increased aroma, decreased firmness, and color changes (Hmmam et al., 2021). Various postharvest technologies are used to maintain fresh produce quality, including ozone (O₃), pulsed electric field, heat treatment, ultraviolet irradiation, and chemical treatment.

Ozone is produced by electric corona discharge or ultraviolet radiation (Pandiselvam et al., 2019). It has high oxidizing power and antimicrobial properties that are effective against an extensive spectrum of microorganisms (Brodowska et al., 2018; Contigiani et al., 2018). The benefit of O₃ is that it doesn’t leave chemical residues that pose threats to the environment and human health. Ozone is an unstable gas that decomposes rapidly to form highly reactive oxidative radicals such as superoxide anion and hydroxyl radical (Perry & Yousef, 2011). It is speculated that the free radicals attribute to the augmented oxidizing power of O₃. The radicals are highly unstable with a short lifespan. They react with other molecules to gain the missing electron (Brodowska et al., 2018). Therefore, the United States of America Food and Drug Administration (FDA, 2001) declared O₃ as Generally Recognized as Safe (GRAS).

Ozone extends the shelf-life of fresh produce by delaying respiration rate, ripening, and senescence. For instance, Chen et al. (2020) reported that O₃ (15.008 mg m^{− 3}) reduced the respiration rate in “the west mi 25” cantaloupes during storage at 4 ℃ for 42 days. Ozone oxidizes ethylene in fresh produce during storage (Aslam et al., 2020). The respiration rate influences biochemical changes that initiate cell wall degradation processes, resulting in loss of firmness and ripening. Previous research has shown that O₃ maintains firmness in various horticultural crops (Chen et al., 2020). For instance, Panou et al. (2021) reported that gaseous O₃ (1.0 mg/L, 40 min) maintained firmness in “Carbarosa” strawberries during storage at 1 ℃ for 16 days. Fruit softening is characterized by the modification of cell wall components, including hemicellulose, pectin, and polysaccharides leading to loss of fruit firmness (Toti et al., 2018). Enzyme activities play an essential role in cell wall degradation. For instance, Cardenas-Perez et al. (2018) reported an increase in enzyme activities of Polygalacturonase (PG) and pectin methylesterase (PME) during mango ripening. Furthermore, the cell wall pectin methyl esters are hydrolyzed by the PME enzyme (Toti et al., 2018). While, the PG enzyme degrades pectic polysaccharides into water-soluble galacturonides resulting in fruit softening (Cardenas-Perez et al., 2018).

The carotenoid content in mango fruit is a good source of pro-vitamin A (3894 IU/100 g) (Lebaka et al., 2021). During fruit ripening, mango changes color from green to yellow-orange due to carotenoid biosynthesis (Liang et al., 2020). Ali et al. (2014) reported that gaseous O₃ (5 mg/L, 96 h) enhanced the β-carotene content in “Sekaki” papaya during storage at 25 ℃ for 14 days. Ozone has been shown to be effective in the carotenoid content in fresh produce (de Almeida Monaco et al., 2016; Minas et al., 2010).

Ozone is an eco-friendly postharvest technology and could provide a potential solution in maintaining fruit quality and extending the shelf-life of mango fruit. Therefore, preserving fruit quality is one of the critical solutions in sustaining food and nutritional security. This study evaluated the influence of O₃ on the physicochemical quality attributes and carotenoid content of mango fruit (cv. Keitt) during cold storage.

2. Materials and methods

2.1. Fruit material

Mango fruit (cv. Keitt) were harvested at physiological maturity from Goedgelegen Farm (25° 778ʹS, 30° 447ʹE) of Westfalia (Pty) LTD (Tzaneen, South Africa). Fruit were couriered overnight under cold storage to the laboratory at the University of KwaZulu-Natal (Pietermaritzburg Campus, 29° 624ʹS, 30° 403ʹE). On arrival at the laboratory, fruit were graded for uniformity according to color and size (850–1000 g), and thereafter stored in carton boxes at 10 ℃ and 90 % relative humidity (RH) for three weeks mimicking shipment to European markets. The fruit were ripened at ambient temperature (25 ℃) and 60 % RH for one week.

2.2. Ozone treatment

The fruit were treated with 0.25 mg/L gaseous O₃ equipped with an ozone analyzer (Corona discharge ozone generator, Ozone Purification Technology, Johannesburg, South Africa) during cold storage. Mango fruits were intermittently treated with O₃ for 12 (day 0), 24 (day 0 & 7), 36 (day 0, 7 & 14), or 48 h (day 0, 7, 14 & 21). After each O₃ treatment, the fruit was removed from a O₃ cold room and stored without O₃ at 10 ℃, and control fruit were untreated. A screening study was conducted to evaluate the effect of various O₃ exposure times on mango fruit quality. The fruit were subjected to gaseous O₃ at various levels (i.e., Level 1: 12 h; Level 2: 24 h; Level 3: 26 h, and Level 4: 48 h) during cold storage. The exposure times to O₃ used in the current study were selected based on the biochemical parameters of the screening experiment which was conducted in 2018.

2.3. Fruit firmness and physiological weight loss (PWL)

Fruit firmness, expressed in Newton (N), was measured on the opposite side of the fruit’s equatorial region using a handheld firmness tester with tip dimension of 0.25 cm², (HP-FFF, Bareiss, Germany). Fruit were weighed on arrival at the laboratory and after every seven days. The same fruit were weighed throughout the storage period. The PWL percentage was calculated using the equation by Akhtar et al. (2010):

(1) $\mathrm{P}\mathrm{W}\mathrm{L}\left(\mathrm{\%}\right)=\left(\mathrm{A}-\mathrm{B}\right)/\mathrm{A}\mathrm{x}100$(1)

Where A indicates the fruit weight before storage and B means the fruit weight after storage.

2.4. Fruit color

Color changes in mango pulp were determined as described by González-Aguilar et al. (2001) using a chromameter (Chroma Meter, Konica Minolta Sensing, INC., Japan). They were expressed as Luminosity (L), a* (red to green), b* (yellow to blue), °Hue angle (H), and Chromaticity (c) values. In triplicates, readings were done on the surface of half-cut mango pulp. Prior to scanning fruit, the chromameter was calibrated by scanning a white reference brick, y = 0.3215, Y = 87.0, X = 0.3.

2.5. Total soluble solids (TSS) and titratable acidity (TA)

Fruit were peeled, cut into cubes, and homogenized using a blender. The puree was triple=filtered using a muslin cloth, and the juice was used for determination of TSS and TA levels. The TA was tested using automated titration (Rondolino G 20’s Compact Titrator, Mettler Toledo, Schwerzenbach, Switzerland). Prior to titration, the sensor was calibrated using buffer standards with pH 7 and 4, with the slope and zero point (offset): Slope: − 59,16 mV/pH @ 25°C and Zero point: ±0 mV or pH 7.00. Juice (8 mL) was mixed with distilled water (40 mL) and titrated with 0.1 M of sodium hydroxide (NaOH) to the endpoint (pH 8.1) and expressed as % malic acid using equation (2)

(2) $\mathrm{\%}\mathrm{T}\mathrm{A}=0.067\mathrm{x}\mathrm{T}\mathrm{i}\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{x}\mathrm{N}\left(\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}\right)\mathrm{x}100$(2)

TSS was analyzed using a digital refractometer (B+S RFM340+ refractometer, Bellingham and Stanley Ltd, Tunbridge Wells, Kent, United Kingdom) and reported as a percentage.

2.6. Decay index (DI)

For each treatment, nine fruits were evaluated for the DI on the peel. Fruits were classified into four groups according to their severity (Score 0–4). Score 0 (none) means no visible decay, score 1 means that there is few (1%) scattered decay, score 2 means that 2–20 % of fruit surface is covered by decay, score 3 means that there is 21–50 % decay incidence while score 4 means that there’s > 50% fruit decay. The DI index was calculated using equation 3 as defined by Khaliq et al. (2016)

(3) $\mathrm{D}\mathrm{I}=\sum \left(\mathrm{D}\mathrm{S}\mathrm{x}\mathrm{N}\mathrm{F}\mathrm{E}\mathrm{C}\right)/\left(\mathrm{N}\mathrm{T}\mathrm{F}\mathrm{x}\mathrm{H}\mathrm{D}\mathrm{S}\right)$(3)

Where DS, NFEC, NTF, and HDS are disease scale, number of fruit in each class, number of total fruit, and highest disease, respectively.

2.7. Total carotenoid content

Total carotenoid content was measured as described by More and Rao (2019) with modifications. In triplicates, 1 g of the sample was added to 14 mL of hexane: acetone (3:2 v/v) and incubated for 1.5 h in the dark. Thereafter, samples were centrifuged (Avanti J-265 XP, Beckaman Coulter, Indianapolis, USA) at 10 000 rpm for 10 min at 4 ℃. The extract was transferred to a volumetric flask and adjusted to 25 mL using hexane: acetone. The absorbance was measured at 450 nm using the spectrophotometer (Shimadzu Scientific Instruments INC., Columbia USA) against hexane: acetone as a blank. The total carotenoid content was calculated using equation 4, and results were expressed as µg g^{− 1} DM.

(4) $\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{o}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{o}\mathrm{i}\mathrm{d}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t}=\left(\mathrm{O}{\mathrm{D}}_{450}\mathrm{x}4/\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}\mathrm{o}\mathrm{f}\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}\left(\mathrm{g}\right)\right)\mathrm{x}100$(4)

Where OD₄₅₀ is the sample absorbance at 450 nm.

2.8. Determination of fruit shelf-life

The shelf-life of mango fruit was determined as described by Chamnan et al. (2019). Mango fruit were evaluated for firmness and decay incidence for seven days after cold storage. The fruit were deemed unmarketable when the decay incidence was more than 0.20.

2.9. Statistical analysis

Data was subjected to analysis of variance (ANOVA) using GenStat statistical software (GenStat®, 18 edition, VSN International, UK). Means were separated using using Fischer’s least significant difference (LSD) at 5 %. Pearson correlation coefficient was used to determine relationships between physicochemical parameters.

3. Results

3.1. Fruit PWL

Fruit PWL (%) increased sharply during storage in treated and untreated mango fruit (Figure 1). The results demonstrated that O₃ significantly (p < 0.05) reduced the PWL percentage. On day 14 , fruit treated with O₃ 24 h (8.45%), had a high mass loss compared to O₃ 48 h (6.98%). At the end of storage, untreated fruits had the highest PWL (30.92%) compared to O₃ (12 h), O₃ (24 h), and O₃ (36 h) which was 28.49%, 25.90%, and 20.54%, respectively. The treatment means of control and O₃ (12 h) were not significantly different at the end of storage.

Figure 1. Effect of gaseous ozone (p < 0.05; 0.25 mg/L) on fruit physiological weight loss (%) stored at 10 ℃ for three weeks and one-week shelf-life (±SE, n = 9).

3.2. Fruit firmness

Fruit firmness decreased significantly (p < 0.01) in all treatments during storage (Table 1). The fruit treated with O₃ (24 and 36 h) maintained higher firmness from day 14 till the end of storage. Compared to other treatments, the control fruit and O₃ (12 h) had low fruit firmness at the end of storage. However, no significant difference was observed between treatment means of O₃ (24 h), O₃ (36 h), and O₃ (48 h).

Table 1. Effect of gaseous ozone (0.25 mg/L) on mango fruit texture (N) stored at 10 ℃ for three weeks and one-week shelf-life at ambient temperature

Treatments	Storage time (Days)
	0	7	14	21	28
Control	62.23^ab	59.46^c	52.84^gh	47.95^jk	40.35^m
O3 (12 h)	62.47^ab	59.60^c	54.90^efg	50.02^ij	40.35^m
O3 (24 h)	63.38^a	59.41^c	56.71^de	52.27^hi	46.74^kl
O3 (36 h)	63.05^a	58.88^cd	56.12^ef	53.12^gh	45.61^kl
O3 (48 h)	63.80^a	60.46^bc	56.96^de	54.12^fgh	44.86^l

Notes: Means sharing the same letter between rows and within the columns are not significantly different (p < 0.05; LSD = 2.30; n = 9).

3.3. Fruit color

Ozone treatments significantly affected the color of mango pulp during storage. There was no significant difference between treatment means regarding the °Hue angle and b* value of mango fruit. The lightness significantly (p < 0.05) decreased in all treatments during storage (Table 2). On day 14, fruit treated with O₃ (12 h) had a high L* value compared to other treatments. There was no significant difference between the treatment means of O₃ (24 h), O₃ (36 h), and O₃ (48 h) on day 14. At the end of storage, fruit treated with O₃ (48 h) had a high L* value compared to O₃ (12 h), O₃ (36 h) and the control. The chroma values varied among treatments during storage (Table 3). Chroma significantly (p < 0.05) decreased in all treatments from day 14 till the end of storage. The chroma of O₃-treated fruit and control was significantly different at the end of cold storage. However, increasing the O₃ exposure time decreased the chroma value of mango fruit. The fruit treated with O₃ (24 h) and control had the highest chroma compared to other treatments at the end of storage. The a* value of mango fruit significantly (p < 0.05) increased in all treatments during the storage period (Table 4). There was no significant difference between the treatment means of all treatments at days zero and seven. The untreated fruit had a high a* value compared to other treatments at the end of cold storage. At the end of storage, untreated fruit had a high a* value compared to O₃ (12 h), O₃ (24 h), O₃ (48 h), and O₃ (36 h).

Table 2. Changes in the L* value of mango fruit treated with O₃ and stored at 10 ℃ for three week and one-week shelf-life at ambient temperature

Treatments	Storage Time (Days)
	0	7	14	21	28
Control	61.52^abc	57.73^cde	49.63^ghi	50.64^ghi	46.7^hi
O3 (12 h)	64.58^abc	58.54^cde	61.84^abc	47.68^hi	49.61^ghi
O3 (24 h)	67.61^a	56.24^efg	58.80^cde	48.56^hi	45.92^i
O3 (36 h)	70.65^a	54.97^efg	52.90^egh	49.80^ghi	47.09^hi
O3 (48 h)	69.21^a	49.09^fgh	53.89^efh	49.16^ghi	51.57^efg

Notes: Means sharing the same letter between rows and within the columns are not significantly different (p < 0.05; LSD = 2.65; n = 9).

Table 3. Changes in the chroma of mango fruit treated with gaseous ozone (0.25 mg/L) and stored at 10 ℃ for three weeks and one-week shelf-life at ambient temperature

Treatments	Storage time (Days)
	0	7	14	21	28
Control	44.73^ab	37.90^cd	34.14^def	32.40^efg	31.35^efg
O3 (12 h)	45.15^ab	44.82^abc	40.43^bcd	30.42^efg	25.55^fg
O3 (24 h)	47.01^ab	40.67^bcd	38.73^cdf	30.91^efg	31.56^efg
O3 (36 h)	52.41^a	36.04^def	35.31^def	35.28^def	28.84^h
O3 (48 h)	46.73ab	40.04^def	36.31^def	32.02^efg	30.13^fg

Notes: Means sharing the same letter between rows and within the columns are not significantly different (p < 0.05; LSD = 7.23; n = 9).

Table 4. Changes in the a* value of mango fruit treated with gaseous ozone (0.25 mg/L) and stored at 10 ℃ for three weeks and one-week shelf-life at ambient temperature

Treatments	Storage time (Days)
	0	7	14	21	28
Control	−4.56^j	1.81^i	6.12^fgh	12.29^bc	17.58^a
O3 (12 h)	−3.87^j	1.19^i	7.95^efg	10.20^cde	14.79^ab
O3 (24 h)	−4.22^j	0.92^i	4.56^ghi	8.08^def	11.76^cd
O3 (36 h)	−3.78^j	1.46^i	3.57^hi	6.04^fgh	9.21^cde
O3 (48 h)	−5.22^j	0.96^i	2.64^hi	5.12^ghi	10.72^bcd

Notes: Means sharing the same letter between rows and within the columns are not significantly different (p < 0.05; LSD = 3.83; n = 9).

3.4. Fruit pulp TSS and TA

The increase in TSS and a decrease in TA, play an important role in mango fruit ripening. In this study, TSS gradually increased during storage in both control and O₃-treated fruit (Table 5). The TSS of ozone-treated fruit was significantly (p < 0.05) different from that of the control. At day 14, there was no significant difference between the treatment means of O₃-treated fruit. At the end of storage, the highest TSS was observed in untreated fruit and O₃ (12 h), which was 17.38 and 17.45, respectively. The TA of untreated and treated mango fruit significantly decreased during storage. Fruit treated with O₃ (36 h) had significantly (p < 0.05) higher TA compared to other treatments on day 14.

Table 5. Effect of gaseous ozone (0.25 mg/L) on mango fruit TSS (%) and TA during storage at 10 ℃ for three weeks and one-week shelf-life at ambient temperature

Storage Time (Days)	Treatments
	TSS
	Control	O3 (12 h)	O3 (24 h)	O3 (36 h)	O3 (48 h)
0	9.85^m	10.46^lm	9.58^m	10.14^lm	9.72^m
7	13.30^fgh	12.72^hji	12.86^hij	12.17^ijk	11.81^jkl
14	15.40^cde	14.12^efg	14.38^efg	13.30f^hi	13.81^fgh
21	16.77^ab	14.92^cde	14.61^efg	14.57^efg	14.50^efg
28	17.45^a	17.52^a	15.34^cde	16.47^abc	16.23^bcd
	TA
0	1.19^i	1.06^hi	1.14^i	1.07^ghi	1.02^gh
7	0.98^g	0.92^fg	1.02^gh	1.01^gh	0.95^g
14	0.77^d	0.73^d	0.79^de	0.80^e	0.87^f
21	0.72^d	0.65^bc	0.65^bc	0.74^d	0.73^d
28	0.47^a	0.54^a	0.60^b	0.68^bc	0.63^b

Notes: Means sharing the same letter between rows and within the columns are not significantly different (p < 0.05; LSD = 1.28 and 0.06, respectively; n = 9).

3.5. Decay index

The decay index of mango fruit varied between treatments during the storage period (Figure 2). Ozone treatment significantly (p < 0.05) decreased the decay of mango fruit. The fruit showed signs of decay from day seven in control treatment and O₃ (12 h). Untreated fruit had a higher decay incidence compared to other treatments from day 14 until the end of storage. At the end of storage, untreated fruit had high decay (42.83%) compared to O₃ (12 h), O₃ (24 h), O₃ (36 h), and O₃ (48 h), which were 36.26%, 28.22%, 20.52%, and 19.29%, respectively.

Figure 2. Effect of gaseous ozone on decay index (%) of mango fruit stored at 10 ℃ for three weeks and one-week shelf-life at ambient temperature (p < 0.05; ±SE, n = 9).

3.6. Total carotenoid

The total carotenoid content of mango fruit notably increased during storage (Figure 3). Ozone treatment significantly (p < 0.05) enhanced the carotenoids of mango fruit. The highest carotenoid content was observed in O₃ (48 h) from day 14 till the end of storage. On day 14, there was no significant difference between the treatment means of control, O₃ (12 h), O₃ (24 h), and O₃ (36 h). Fruit treated with O₃ (48 h) had high carotenoid content compared to O₃ (24 h), O₃ (12 h), and control at the end of the storage period.

Figure 3. Effect of gaseous ozone (0.25 mg/L) on total carotenoid content of mango fruit stored at 10 ℃ for three weeks and one-week shelf-life at ambient temperature (p < 0.05; ±SE, n = 9).

3.7. Fruit shelf-life

Ozone treatment significantly (p < 0.05) extended fruit shelf-life of mangoes. The shelf-life of the treated fruit ranged between four and six days. Untreated fruit ripened faster compared to other treatments (Figure 4). However, no significant differences were observed between the ripening time of O₃ (36 h) and (48 h).

Figure 4. Effect of gaseous ozone (0.25 mg/L) on fruit shelf-life (p < 0.05; ±SE, n = 9).

3.8. Relationship between DI and mango fruit physicochemical parameters

Fruit firmness was negatively correlated to TSS (R² = −0.72), PWL % (R² = 0.89), carotenoids (R² = −0.63) and DI (R² = −0.67). The TSS was positively correlated to PWL % (R² = 0.73) and a* value (R² = 0.67) (Table 6). A positive correlation was observed between PWL %, carotenoids (R² = 0.76), and DI (R² = 0.66). The TA was negatively correlated to PWL % (R² = −0.73) and carotenoids (R² = −0.62). Luminosity was positively correlated to the chroma (R² = 0.86), while a weak correlation was observed between firmness and luminosity (R² = 0.51).

Table 6. Pearson correlations coefficient of firmness, TSS, TA, carotenoids, DI, L*, and chroma in mango fruit

	Firmness	TSS	PWL %	TA	Carotenoids	L*	Chroma	a*
TSS	−0.72
PWL (%)	−0.89	0.73
TA	0.64	−0.60	−0.73
Carotenoids	−0.63	0.60	0.76	−0.62
L*	0.51	−0.46	−0.52	0.45	−0.46
Chroma	0.39	−0.43	−0.44	0.38	−0.39	0.86
a*	−0.72	0.67	0.74	−0.59	0.53	−0.47	−0.39
DI (%)	−0.67	0.46	0.66	−0.39	0.51	−0.27	−0.21	0.45

4. Discussion

4.1. Fruit PWL

The mass is an essential parameter as fresh horticultural produce is sold based on weight. Thus, it is imperative that postharvest treatments minimize the loss of water and fruit mass during storage. da Silva et al. (2020) reported that aqueous O₃ (3 mg/L) decreased the weight loss in “Palmer” mango during storage at 20 ℃ for 20 days. Physiological moisture loss occurs in fresh produce due to increased transpiration and respiration rate. The decreased fruit weight loss in O₃-treated fruit could be due to retarded respiration rate (Zhang et al., 2011). In mango fruit, shriveling occurs when weight loss increases above 10 % and the marketability of the fruit decreases (Barman et al., 2014; Vázquez-Celestino et al., 2016). Current results indicate that the marketability of untreated fruit decreased from day 14 of storage. On the other hand, the marketability of fruit treated with O₃ for 36 and 48 h decreased after 21 days of cold storage.

4.2. Fruit firmness

Loss of firmness occurs in mango fruit postharvest due to biochemical modifications resulting in softening (Yashoda et al., 2006). The current study showed that O₃ delayed loss of firmness for 14 days in mango fruit. Similarly, Zhang et al. (2021) reported that O₃ (1 mg/L, for 5 h) maintained the firmness of cantaloupe melon during storage at 4 ℃ for 30 days. The retained firmness could be attributed to O₃ retarding modification of cell wall degrading enzymes such as polygalacturonase and β-galactosidase. Minas et al. (2014) reported that O₃ (0.3 µL/L⁻¹) inhibited the enzyme activities of polygalacturonase and endo-1,4-β-glucanase/1,4-β-glucosidase in “Hayward” kiwifruit during storage. Zhang et al. (2021) reported that O₃ (1 mg/L, for 5 h) down-regulated pectinesterase and polygalacturonase gene expression in cantaloupe melon. The enhanced firmness could be attributed to O₃ inhibiting cell wall modifying enzymes, thus decreasing fruit softening and extending shelf-life.

4.3. Fruit color

The color changes from green to yellow occur in mango fruit postharvest, indicating ripening. The O₃ treatment effectively maintained the interior color of mango fruit during storage. These findings corroborate those of Jia et al. (2020), who reported that O₃ (200 mg m^{− 3}) retained the L* value in “Jin Qiu hong” peach. Ozone treatment maintained a high L* value in “Carbarosa” strawberries during storage (Panou et al., 2021). The color changes from green to orange-yellow were observed in both treated and untreated fruit. However, untreated fruit showed higher a* values than ozone-treated fruit. Panou et al. (2021) reported a low a* value in “Carbarosa” strawberries treated with O₃ (1 ppm). Similarly, Bolel et al. (2019) observed decreased chroma values in “Hicaznar” pomegranates treated with O₃ (4 mg/L). The high chroma values in ozone-treated fruit could be attributed to O₃ delaying metabolic processes involved in color pigment degradation. Current results indicate that O₃ (36 h) delayed color changes in mango fruit. Chroma suggests the intensity of the color that is affected by pigment saturation (Sousa et al., 2021). The changes in color from green to yellow could be attributed to chlorophyll degradation and accumulation of pigments. Mango fruit ripening is characterized by decrease in chlorophyll content. The decline in chlorophyll could be attributed to the thylakoids collapsing inside the cloroplasts.

4.4. Fruit pulp TSS and TA

Total soluble solids (TSS) is a vital indicator of maturity in horticultural crops. In the current study, O₃ treatment delayed TSS accumulation. Similar results were reported by Paico et al. (2018) in mango fruit treated with O₃. However, Tran et al. (2015) reported that O₃ (10 µL L^{− 1}) had no significant effect on TSS of “Nam Dok Mai No.4” mango fruit. The delayed TSS accumulation could be attributed to O₃ inhibiting starch conversion into soluble sugars. Ozone causes oxidative shock to sugar metabolism, thus delaying and shifting the metabolic pathway (Panou et al., 2021). Proteins responsible for sugar metabolism, particularly sucrose synthase, are abundant in fruit during ripening (Muccilli et al., 2009). In the current study, O₃ treatment delayed the TA loss. Jia et al. (2020) reported that O₃ delayed the loss of TA in “Jin Qiu hong” peach during storage. Chen et al. (2020) reported that high O₃ (15.008 mg m^{− 3}) concentration delayed the TA decrease on the exocarp and sarcocarp of “West mi 25” cantaloupes during storage at 4 ℃ for 42 days. Intermittent gaseous O₃ treatment was ineffective in maintaining the TA of “Hicaznar” pomegranate stored at 6 ℃ for 4 months (Buluc & Koyuncu, 2022). Barboni et al. (2010) reported that O₃ delayed the decrease of malic, citric, and quinic acids in “Hayward” kiwifruit. The high TA could be attributed to O₃ modifying the metabolic processes. Buluc and Koyuncu (2022) reported that O₃ delays the respiration rate, thus maintaining the TA. The organic acids are utilized during respiration, as they donate the hydrogen ion. The TSS and organic acids affect fruit flavor and ripening.

4.5. Decay index

Postharvest decay reduces fruit quality and restricts the growth of the mango industry. The current results showed that high O₃ exposure time decreased decay in mango fruit during storage. Similarly, Bambalele et al. (2023) observed that O3 (0.25 mg/L, 36 h) decreased the mycelial growth and disease incidence of Lasiodiplodia theobromae in mango fruit stored at 10 ℃ for 21 days. Ozone (200 mg m^{− 3}) decreased the decay percentage in “Jin Qiu hong” peach during storage (Jia et al., 2020). The reduced decay in treated fruit could be attributed to the high oxidizing potential of O₃. Ozone enhances the flavonoid content and peroxidase activity in mango, thus increasing the disease resistance of the fruit (Bambalele et al., 2023). Ozone is reported to be effective against fungal diseases such as anthracnose, stem-end rot, and grey mold (Savi & Scussel, 2014; Terao et al., 2019).

4.6. Total carotenoid

The concentration of carotenoids is known to increase during mango fruit ripening. This study showed O₃ treatment increased the total carotenoid content from day 14 till the end of storage. Minas et al. (2010) reported that O₃ (0.3 µL L^{− 1}) treatment increased the carotenoid content in “Hayward” kiwifruit. In contrast, Toti et al. (2018) reported that O₃ (0.3 mg/L) was ineffective in maintaining the carotenoid content of cantaloupe melons during storage. The enhanced carotenoid content in O_3-treated mango could be attributed to O₃ altering the carotenoid biosynthesis pathway during fruit ripening. Carotenoid biosynthesis occurs through the methylerythritol phosphate pathway (Ma et al., 2018). The gene expression of phytoene desaturase, phytoene synthase, ζ-carotene desaturase, zeaxanthin epoxidase, and β-carotene hydroxylase are upregulated during mango fruit ripening and correlates with β-carotene (Ma et al., 2018). Ozone down-regulated VviZDS1, VviLBCY2, VviZISO1 and VviCISO1 genes in “ML1 microvine” berries (Campayo et al., 2021). These genes are involved in the early stages of carotenoid biosynthesis during fruit ripening. In the current study, the enhanced carotenoid content correlated with increasing orange-yellow color, indicating fruit ripening.

4.7. Fruit shelf-life

Gaseous O₃ extended the shelf-life by 25 days in “Daw” longan fruit stored at 5 ℃ for 40 days (Chamnan et al., 2022). The current study suggests that gaseous O₃ effectively extends the shelf-life of mango fruit. The extended shelf-life could be attributed to O₃-inhibiting enzyme activities, biochemical changes, and fruit ripening.

4.8. Relationship between DI and mango fruit physicochemical parameters

Current results revealed a negative correlation between TSS and firmness. Mustapha et al. (2020) showed that firmness was negatively correlated to TSS and TA. According to the correlation analysis, total carotenoid content was positively correlated to the a* value and TSS. Similarly, Hmmam et al. (2021) observed a positive correlation between carotenoids and color in mango fruit. The carotenoid content contributes to the mango fruit color and nutritional value (Hmmam et al., 2021; Maldonado-Celis et al., 2019).

5. Conclusion

This study revealed that gaseous O₃ effectively maintained the physicochemical parameters such as firmness, color, TSS, TA, and carotenoids of mango fruit during storage. The decay of mango fruit was delayed until day 14 of storage by O₃ treatment. Current results indicate the O₃ treatment should be applied at the pre-climacteric stage for 24 or 36 h to yield superior results. Superior results were observed in treatments that received O₃ for 36 and 48 h. However, increasing O₃ exposure time from 36 to 48 h yielded similar results in most physicochemical parameters. Therefore, O₃ treatment for 36 h should be considered to economize costs. Current results provide the basis for developing postharvest O₃ protocols for mango fruit. However, the mode of action for O₃ in enhancing fruit quality warrants further investigation.

Author contribution

Conceptualization, A.M., L.S.M., and S.Z.T.; laboratory analysis, N.B.; data curation, N.B., original draft preparation, N.B.; review and editing, A.M., L.S.M., and S.Z.T.; project administration, A.M. All authors have read and agreed to the published version of the manuscript.

Availability of data

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Acknowledgments

The authors are grateful to Thokozani Nkosi for technical assistance in the laboratory and for the student financial support provided by Moses Kotane Institute.

Disclosure statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Additional information

Funding

This research was funded by the National Research Foundation’s Support for Y-rated Researchers Grant [CSRP: 2205046155].

Notes on contributors

Nonjabulo L. Bambalele

Nonjabulo L. Bambalele is a Postdoctoral Researcher in Horticultural Science at the School of Agricultural Earth and Environmental Science at the University of KwaZulu-Natal, KwaZulu-Natal, Pietermaritzburg, South Africa. Her research is focused on the agronomic performance of crops and the use of ozone to maintain the quality of fresh produce.

Asanda Mditshwa

Asanda Mditshwa is an Associate Professor in the discipline of Horticultural Science at the School of Agricultural Earth and Environmental Science at the University of KwaZulu-Natal, KwaZulu-Natal, Pietermaritzburg, South Africa. His research is focused on postharvest physiology and technology with a special interest in finding effective non-chemical technologies such as dynamic controlled atmospheres, ozone, UV-C, and edible coatings for preserving quality and extending shelf-life.

Lembe S. Magwaza

Lembe S. Magwaza is an Associate professor in the discipline of Crop Science at the School of Agricultural Earth and Environmental Science at the University of KwaZulu-Natal, KwaZulu-Natal, Pietermaritzburg, South Africa. He is a C-rated researcher by the National Research Foundation of South Africa. His research is focused on non-destructive technologies of evaluating internal quality of fresh produce; as well as developing strategies to maximize the postharvest shelf life of different crops.

Samson Z. Tesfay

Samson Z. Tesfay is an Associate in the discipline of Horticultural Science at the School of Agricultural Earth and Environmental Science at the University of KwaZulu-Natal, KwaZulu-Natal, Pietermaritzburg, South Africa. His research is focused on fruit biochemistry, plant antioxidants, and climate-smart crops such as moringa.