1. Introduction
Tomato (Solanum lycopersicum L.) is one of the most popular fruit vegetables worldwide, rich in functional compounds offering a plethora of health benefits to consumers [1]. Indeed, the specific compositional traits of tomato, together with its common consumption in modern diets, make it one of the primary dietary sources of phytochemicals such as carotenoids, polyphenols, and vitamins involved in reducing the risks of prostate cancer, cardiovascular diseases, and neurodegenerative diseases [2,3]. The global cultivation area of this species is ~5 Mha [4], a feature arising from its adaptability to different growth conditions, extending well beyond its center of origin [5,6]. In the Mediterranean basin, tomato serves as the predominant field and greenhouse vegetable crop, playing a significant role in shaping the Mediterranean diet. As a result, its consumption is typical throughout this region [7]. From a biological viewpoint, the tomato supply chain is marked by a continuous introduction of new genotypes on the market, making tomatoes one of the most dynamically evolving crops among vegetables [8]. Specifically, certain types of tomatoes, such as mini plum tomatoes intended for fresh consumption, are especially characterized by this phenomenon. Indeed, new mini plum tomato cultivars with diverse fruit pigmentation (yellow, orange, brownish) are frequently introduced to the market, resulting in significant product diversification [9]. While this provides an opportunity to tailor products to the diverse preferences of the European market, it also poses significant challenges in optimizing the supply chains intended for the premium tomato market. Indeed, these novel tomato genotypes are typically marketed in specialized niches where flawless quality is essential [9]. As a climacteric fruit, tomatoes exhibit a relatively short postharvest life due to their high respiration rates and fast ripening and senescence processes. These characteristics can significantly reduce the time span during which tomatoes are deemed acceptable by consumers, leading to substantial economic losses during storage and shelf life [10]. Indeed, the metabolic processes of tomatoes persist even after detachment from the plant, causing them to continue ripening and even deteriorating to a state where they become valueless [11]. For this reason, low-temperature storage (typically between 8 and 13 °C, depending on fruit type, ripening stage, and storage duration) is widely regarded as an effective approach to mitigate water loss and ethylene biosynthesis in tomatoes, helping to bridge the spatial and temporal gaps between production and consumption [12]. However, the behavior of tomatoes, even under standard refrigerated storage conditions, can vary depending on the genotype, resulting in alterations to the tomatoes’ keeping quality (reduced shelf life), potentially leading to increased economic costs (loss of capital, fuel, and labor) and environmental impacts (due to landfilling), coupled with losses of valuable phytonutrients [13]. Indeed, modern tomato cultivars exhibit significant qualitative and quantitative differences in their carotenoid (the main pigments responsible for tomato color) and phenolic profiles [14,15,16]. Each of these components undergoes specific changes during refrigerated storage, thereby differentially affecting the evolution of tomatoes’ postharvest quality [11,13]. Hence, to address the growing demands for efficient agri-food supply chains, along with for tomatoes with high quality and functional profiles, it is appropriate to have in-depth knowledge about whole patterns of postharvest change in these properties, as a function of the storage conditions applied to the emerging germplasm.
For the abovementioned reasons, the objective of this study was to examine the postharvest modifications in key quality and functional variables of two recently popular mini plum tomato cultivars in a Mediterranean environment, one with orange fruit (‘Santy Naranja’) and one with dark red, green-striped fruit (‘Melange’), compared to a well-established cultivar (‘Angelle’, red fruit), induced by refrigerated storage (10 °C) for up to 14 days.
2. Materials and Methods
2.1. Experimental Site, Plant Material, and Growth Conditions
The experiment was conducted in 2023 (from 27 February to 16 July) in a greenhouse sited in Sicily (South Italy; 36°73′ N, 15°06′ E, 60 m a.s.l.), i.e., one of most representative areas of greenhouse tomato cultivation in Italy. The local climate is semi-arid Mediterranean, with mild winters and warm, rainless summers. The trial was conducted in the center of an East–West oriented, multi-span cold greenhouse (100 × 50 m) having a steel tubular frame, lateral windows along the sides, and covered with an ethylene vinyl acetate film (200 μm thick, total visible transmission > 84%).
Three mini plum tomato cultivars were selected on the basis of their distinct carpometric traits, including epicarp color (Table 1): among these, ‘Santy Naranja’ and ‘Melange’ are recently gaining popularity, while ‘Angelle’ (introduced as the reference cultivar) already has an established cultivation presence in the reference area. On February 27th, tomato seedlings (3 true leaves), selected for uniformity and health status, were transplanted in North–South oriented rows (0.30 × 2.00 m, 1.67 plant m−2) and trained at two stems up to the 8th cluster. Each experimental unit included 18 plants (3 rows each including 6 plants, net of borders). Drip irrigation (two drippers plant−1, 2.4 L h−1) was performed up to field capacity when the external accumulated evapotranspiration (calculated through the Penman–Monteith equation) reached 40 mm. Fertigation was performed by managing a complete nutrient solution [9], whereas pollination was promoted by using bumblebee hives. Further crop practices included the manual removal of lateral stems and the application of Beauveria bassiana and Azadirachtin (when needed). All clusters were hand-harvested between 30 May and 16 July.
2.2. Fruit Sampling and Storage Conditions
For the postharvest experiment, tomatoes belonging to the 4th cluster (i.e., those harvested between 23 and 28 June 2023) were used. Specifically, collected clusters were transported to the laboratory soon after harvesting. Overall, 162 clusters (18 clusters × 3 cultivars × 3 replicates) were divided into 3 main batches for the characterization of fruits after 0 (harvest date), 7, and 14 days of storage at 10.0 ± 0.5 °C (90 ± 5% RH) (hereafter referred to as S0, S7, and S14, respectively). To this end, fruits were detached from the rachis, selected for the absence of defects and uniformity (within each cultivar), washed with deionized water, and dried with paper towels. For characterization at S7 and S14, twelve to twenty-three fruits per replicate were placed in commercial, biodegradable, and compostable polylactic acid (PLA) trays measuring 13.5 × 12.5 × 5.0 cm (Ingeo Ecozema®, Ecozema s.r.l., Santorso, Vicenza, Italy) for a final net weight of 250 ± 9 g, after which they were stored under the abovementioned conditions. Each experimental unit included 6 trays.
2.3. Determination of Main Carpometric Traits
Fruit fresh weight (g) was recorded at each time point through an electronic scale (0.01 g accuracy) to determine the fresh weight loss at S7 and S14, whereas fruit water content (WTc, expressed on a percentage basis) was determined by reweighing tomatoes after complete desiccation in a thermoventilated oven (Binder, Milan, Italy) at 105 °C (~72 h). Fruit firmness was measured by using a digital texture analyzer (mod. TA-XT2, Stable Micro Systems, Godalming, UK) with a 490 N nominal force load cell and a stainless steel plate as a probe (100 × 85 × 6 mm). All the tests were performed by applying a deformation force (12 fruits per replicate) for up to 2 mm fruit deformation along the equatorial axis, with a test speed of 10 mm s−1. The chromatic coordinates of the fruits were measured on the equatorial region of eight whole fruits (2 readings per fruit) through a tristimulus Minolta Chroma meter (model CR-200, Minolta Corp., Osaka, Japan) calibrated with a standard white tile (UE certificated) with illuminant D65/10°. The epicarp color at S0 was expressed in terms of lightness, hue angle, and chroma [17]. At S7 and S14, the modification of the epicarp color was expressed in terms of total color difference (∆E*ab = [(∆L*)2 + (∆a*)2 + (∆b*)2]1/2), where ∆L*, ∆a*, and ∆b* represent the differences in the chromatic coordinates recorded at either S7 or S14 compared to S0 [13].
2.4. Headspace Gas Composition, Total Soluble Solids, and Titratable Acidity
The oxygen (O2) and carbon dioxide (CO2) percentages were monitored during the shelf life through a Dansensor Checkpoint portable gas analyzer (Dansensor, Ringsted, Denmark) on three replicates. Total soluble solids (TSS) and titratable acidity (TA) were determined on previously pureed and centrifuged fruits (5 min at 2376 g) to obtain a clear juice. Total soluble solids (%) were determined via a digital refractometer (DBX-55, Atago, Tokyo, Japan) equipped with an automatic temperature compensation system. For the analysis of TA, 10 milliliters of clear tomato juice were titrated with 0.1 M NaOH to pH 8.2 by using phenolphthalein as an indicator until the pink end-point. Titratable acidity was calculated according to the following equation [18]:
TA (% citric acid) = 0.0064 × (volume of NaOH in mL) × 10
The abovementioned determinations allowed for the calculation of the TSS/TA ratio (adimensional).
2.5. Functional Variables
For the biochemical analyses, twenty-five fruits per replicate, selected for uniform size and color within each cultivar, were flash-frozen and lyophilized in a freeze-dryer (mod. Alpha 1–4 LD plus, Martin Christ, Osterode am Harz, Germany). The freeze-dried samples were then stored at −80 °C before being ground using a mill (A11 basic, IKA, Konigswinter, Germany) for subsequent analyses.
2.5.1. Total Phenolic Content
The total phenolic content (TPC) of tomato extracts was determined according to Buturi et al. [19] with minor modifications. To this end, 200 mg of freeze-dried, pulverized samples per replicate were extracted with 1.5 mL of methanol (80%) for 15 min at 70 °C; each extract was then cooled on ice and centrifuged at 5345 g for 5 min (25 °C). An aliquot of 100 µL of supernatant was mixed with 100 µL of Folin–Ciocâlteu reagent (1:10 v/v in water); after 2 min, the solution was mixed with 800 µL of Na2CO3 solution (6% w/v). Samples were agitated and incubated for 60 min at room temperature. The absorbance was read at 760 nm by using a Jenway 7315 UV–Vis spectrophotometer (Cole-Parmer, Stone, UK). The TPC values were obtained from a standard curve (r = 0.999 ***) obtained by plotting the absorbances of known concentrations of gallic acid. The results were expressed as mg of gallic acid equivalents (GAE) kg−1 dry weight (DW).
2.5.2. Total Carotenoid Content
The total carotenoid content (TCC, including xanthophylls) of tomato extracts was determined according to Lichtenthaler and Wellburn [20] with modifications. Briefly, 50 mg of lyophilized tomato powder was extracted with 5 mL acetone, vortexed for 1 min, sonicated for 20 min in an ultrasonic bath (~6 °C), and centrifuged for 10 min (5345× g at 6 °C). The supernatant was collected, and the solid phase was re-extracted twice (2 mL acetone) until it was colorless. Samples were read using acetone as a blank. Readings were performed at 470, 644.8, and 661.6 nm, and the absorbance values were substituted in the standard equations [21]. The results were expressed as mg kg−1 DW.
2.5.3. Determination of the Ascorbic Acid Content
The ascorbic acid (AsA) of the fruits was extracted and quantified following the HPLC method described by Nisperos-Carriedo et al. [22] and Sánchez-Mata et al. [23]. HPLC separation was effected through a Nextera XR LC-20AD XR liquid chromatograph pump (Shimadzu Italia S.r.l., Milano, Italy) equipped with a Diode Array Detector SPD-M40 and a SunShell C-18 (3.0 id × 100, 2.6 µm p.s.) column, set at 25 °C. For the extraction, 0.1 g of dried samples were added with 2 mL of H3PO4 0.05 N, then centrifuged at 15,000 rpm at 4 °C for 15 min. The supernatants were filtered and used for analysis. The ascorbic acid was separated through a 2% KH2PO4 buffer solution at pH 2.3 (B) and acetonitrile 100% (A) as mobile phases at a flow rate of 0.5 mL min−1 in a linear gradient. B concentration decreases to 98% in 1 min and to 88% in 8 min. The mobile phases returned back to the initial concentration in 2 min. The spectra were recorded in the range 190–400 nm, where the maximum absorbance for ascorbic acid is at 245 nm (Supplementary Figure S1). The identification and quantification were made using the external standard method, using a pure commercial standard (Thermo Scientific Chemicals, Milan, Italy). A double injection of 10 µL of each sample was analyzed. The results were expressed as mg kg−1 DW.
2.5.4. 2,2-Diphenyl-1-picrylhydrazyl Assay
The 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity of tomato extracts was determined according to Brand-Williams et al. [24] with modifications. To this end, 100 mg of lyophilized tomato powder was mixed with 5 mL methanol (80%) and vortexed for 1 min. Samples were then placed into an ultrasonic bath for 10 min (~6 °C) and centrifuged for 15 min at 2236× g (6 °C). For the reaction, 150 µL of supernatant was mixed with 1350 µL of DPPH solution (150 µmol), after which the samples were vortexed and placed in the dark (30 min). The decrease in absorbance of the methanolic solution of DPPH was read at 515 nm, and values were obtained from a standard curve (r = 0.999 ***) obtained via the change in absorbance against different Trolox concentrations. The results were expressed as mg Trolox equivalents (TE) kg−1 dry weight.
2.5.5. Ferric Reducing Antioxidant Power Assay
For the ferric reducing antioxidant power (FRAP) assay [25], 200 mg of lyophilized tomato powder was extracted with 10 mL methanol (80%), vortexed for 1 min, and placed in the dark for 30 min. Samples were then centrifuged for 10 min at 4000× g (6 °C). The FRAP reagent consisted of 10 mL of acetate buffer (300 mmol, pH 3.1) mixed with 1 mL of 2,4,6-tris(2-pyridyl)-s-triazine (TPTZ) solution (10 mmol in 40 mmol HCl) and 1 mL of ferric chloride (20 mmol). For the reaction, 150 µL of supernatant was mixed with 300 µL of ultrapure water, vortexed, and added to 3 mL of FRAP reagent. Samples were placed in the dark for 10 min (20 °C). The FRAP readings were conducted following the shift in absorbance at 593 nm upon the formation of the blue compound Fe(II)-tripyridyltriazine from the colorless oxidized Fe(III) form in the presence of a specific concentration of the sample. The FRAP values were calculated from a standard curve (r = 0.999 ***) obtained by plotting the change in absorbance against different Trolox concentrations. The results were expressed as mmol TE kg−1 dry weight.
2.6. Reagents and Solvents
All reagents and solvents were purchased from Merck KGaA (Darmstadt, Germany) and were of analytical or HPLC grade. Bidistilled water was used throughout the analytical trial, while all chemical analyses were performed in triplicate.
2.7. Statistical Procedures
All collected and calculated data were firstly subjected to Shapiro–Wilk’s and Levene’s test to check for normal distribution and homoscedasticity, respectively, then to a factorial ‘genotype × storage time’ analysis of variance (ANOVA), according to the randomized blocks design adopted in the experiment. Percentage data were Bliss transformed before the ANOVA (untransformed data are reported), whereas means were compared through Tukey’s HSD test (p ≤ 0.05). A correlation analysis was also performed to check for any significant relationship among variables. All calculations were performed using Excel® version 2016 (Microsoft Corporation, Redmond, WA, USA) and Minitab® version 16.1.1 (Minitab Inc., State College, PA, USA).
3. Results
3.1. Carpometric Traits
At harvest, the three cultivars displayed significant differences in fruit FW and chromatic coordinates. Specifically, ‘Melange’ showed the highest fruit FW (19.63 g) along with the lowest values of L* (25.39) and chroma (32.09) (Figure 1a–d). In contrast, ‘Santy Naranja’ and ‘Angelle’ showed lighter fruits (15.80 and 10.96 g, respectively) along with the highest L* (31.23, on average), with ‘Angelle’ also exhibiting the lowest hue angle (37.42) (Figure 1a–d). Regardless of the storage time, ‘Santy Naranja’ and ‘Melange’ maximized the differences in terms of fruit WTc (90.27 vs. 88.94 g 100 g−1 FW, respectively) and firmness (6.13 vs. 4.69 N, respectively) (Table 2). On the other hand, refrigerated storage significantly reduced fruit firmness in all three cultivars (up to 4.72 N at S14, −23%) (Table 2). Moreover, the significant ‘cultivar × storage time’ interaction revealed that passing from S7 to S14, stronger increases were recorded for FWL in ‘Melange’ (from 5.93 to 11.27 g kg−1, +90%) and for ΔE*ab in ‘Santy Naranja’ (from 2.29 to 3.96 g kg−1, +73%) (Figure 2a,b).
3.2. Headspace Gas Composition and Taste-Related Variables
As shown in Table 3, the headspace O2 and CO2 concentrations were affected by the cultivar, with ‘Santy Naranja’ proving a higher O2 concentration than ‘Melange’, along with the lowest CO2 concentration among the studied genotypes. Both gases displayed opposing trends along the storage period, with O2 decreasing from S0 to S14, while CO2 increased during the same interval. The ANOVA revealed that over the period S0-S7, both ‘Angelle’ and ‘Melange’ had the strongest decrease in O2 concentration (which passed, on average, from 20.65 to 17.30%) along with the sharpest increase in CO2 concentration (which passed, on average, from 0.03 to 4.77%) (Figure 3a,b). Contrastingly, between S7 and S14, ‘Santy Naranja’ showed the widest variations in terms of O2 (19.77 vs. 16.23%, respectively) and CO2 (2.90 vs. 5.10%, respectively) (Figure 3a,b). On the other hand, ‘Melange’ and ‘Angelle’ proved the highest TSS and TA values, respectively, while ‘Melange’ also showed the highest TSS/TA ratio (Table 3). Both TSS and TA demonstrated a significant decrease throughout the storage period; specifically, TSS exhibited a significant decrease at S7, remaining constant afterward. Simultaneously, TA decreased only in ‘Angelle’ and ‘Melange’, demonstrating the most marked declines passing from S7 to S14 (from 0.33 to 0.23% and from 0.29 to 0.25%, respectively) (Figure 3c). Additionally, these two cultivars experienced a significant increase in the TSS/TA ratio over the S7–S14 period, rising from 21.77 to 30.52 in ‘Angelle’ and from 31.46 to 35.22 in ‘Melange’ (Figure 3d).
3.3. Functional Variables
The studied cultivars have significant differences in terms of functional variables, with ‘Santy Naranja’ displaying the lowest TPC, AsA, and DPPH values, while ‘Angelle’ outperformed the other cultivars for TCC, AsA, and FRAP values (Table 4). Over the S0-S14 period, all cultivars experienced a similar increase in FRAP, rising from 64 to 74 mg TE kg−1 DW (+16%) (Table 4). Differently, the ANOVA revealed that the response to refrigerated storage was genotype-dependent for all the remaining variables. Indeed, considering S0 and S14, ‘Angelle’ and ‘Melange’ experienced the sharpest decreases in TPC (−2556 and −2870 mg GAE kg−1 DW, respectively), together with a significant rise in TCC (+258 and +158 mg kg−1 DW, respectively) (Figure 4a,b). On the contrary, ‘Santy Naranja’ showed a significant TCC decline between S7 and S14, dropping from 398 to 210 mg kg−1 DW (−47%) (Figure 4b). Moreover, more pronounced decreases in AsA were noticed in ‘Santy Naranja’ and ‘Angelle’ between S0 and S7 (−1162 and −1477 mg kg−1 DW) and in ‘Melange’ between S7 and S14 (−1629 mg kg−1 DW) (Figure 4c), with ‘Santy Naranja’ also profiling the strongest DPPH decrease passing from S7 to S14 (−23 mg TE kg−1 DW) (Figure 4d).
3.4. Correlation among Variables
Overall, 91 correlations were analyzed, of which 50 (55% of the total) were significant, revealing 26 negative and 24 positive relationships (Table 5). Among the negative correlations, the highest significance was found between O2 and CO2 concentrations in the headspace (−0.944 ***), FWL and O2 concentration (−0.862 ***), ΔE*ab and O2 concentration (−0.745 ***), ΔE*ab and AsA (−0.760 ***), and between TA and the TSS/TA ratio (−0.693 ***) (Table 5). Conversely, the strongest relationships among the positive correlations were found between FWL and CO2 concentration (0.908 ***), FWL and ΔE*ab (0.782 ***), ΔE*ab and CO2 concentration (0.778 ***), TSS and TPC (0.758 ***), and between fruit firmness and O2 concentration (0.714 ***) (Table 5).
4. Discussion
4.1. Carpometric Traits
In the current experiment, none of the studied cultivars showed signs of postharvest spoilage; however, it was possible to observe different behaviors among them during shelf life. In this regard, ‘Melange’ proved to have the highest average fruit weight at harvest, along with the poorest ability to maintain this weight during postharvest, as evidenced also by its more pronounced decline in fruit water content during storage. As also revealed by our correlation analysis, this inevitably resulted in a faster softening of its fruits, given the close relationship existing between fruit firmness and tissue water potential [26]. Indeed, fruit transpiration (e.g., through the stem scar, cuticle wax imperfections, or lenticels) is among the main drivers of fruit softening during postharvest, overall impacting the consumers’ perception of fruit preservation and, consequently, purchasing behavior [13,27]. Therefore, these drawbacks of ‘Melange’ are likely to shorten the shelf life of its fruits, potentially leading to significant waste generation over a large-scale supply chain.
The color of the epicarp is another key trait that influences the quality of tomatoes, having a pivotal role in defining the period during which tomatoes remain acceptable for consumers [28]. According to the different fruits’ colors, the S0 hue angle was the most discriminating variable among the studied cultivars, while for the other two chromatic variables, the fruits of ‘Melange’ had the most distinct color properties since these fruits were detected as darker (lower L*) and duller (lower chroma). To characterize the postharvest behavior of the studied tomatoes, we employed ΔE*ab, a chromatic variable that describes the overall color alterations in tomato fruits throughout storage [13] and is influenced by several (unspecified) factors [29]. According to Dattner and Bohn [30], two colors can be optically distinguished if the overall color variation is greater than 1. The fruit belonging to all three cultivars displayed a significant color variation at S7, which likely indicates the persistence of the ripening processes during postharvest. In our experiment, a highly significant positive correlation between ΔE*ab and FWL was observed (0.782 ***), indicating that color variation was associated with the deterioration of the qualitative characteristics of tomatoes. However, in the orange-fruited ‘Santy Naranja’, these chromatic variations were more intense and prolonged over time; this suggests poor stability over time of its fruits’ appearance, a feature posing a risk of diminished product reliability at the sight of consumers during shelf life. Indeed, many studies have highlighted the role of food color in impacting the consumers’ perception of other sensory characteristics (e.g., sweetness, saltiness, flavor); this particularly applies to the orange color, whose chromatic alterations (even when moderate) usually greatly impact consumers’ acceptance [31,32,33].
4.2. Headspace Gas Composition and Taste-Related Variables
The freshness of vegetables is generally regarded as also dependent on the extent of their metabolic activity [34]. In this sense, it is widely accepted that respiration rate can be considered as an expression of the metabolic activity of plant tissues, being inversely correlated to postharvest life of fresh vegetables [35]. As typically occurs in climacteric fruits, tomato ripening is accompanied by a peak in metabolic activity, but the resulting respiration rate can vary according to several factors, such as tomato types and cultivars. Thus, to effectively manage tomatoes postharvest, it is crucial to understand the ripening characteristics of different genotypes [36]. Under our experimental conditions, the O2 concentration in the headspace was negatively correlated with FWL (−0.862 ***) and ΔE*ab (−0.745 ***) and positively correlated with firmness (0.714 ***). Conversely, the CO2 concentration showed opposite trends, highlighting a strong relationship between fruit senescence and respiration rates. All three cultivars reached similar CO2 concentrations at the end of the experiment but displayed different respiration trends throughout the storage period. Specifically, ‘Angelle’ and ‘Melange’ reached a stable state in terms of O2 and CO2 concentrations after 7 days of storage and maintained this condition until the end of the experiment. This highlights the establishment of an equilibrium between the O2 and CO2 transmission rate through the container and the fruits’ respiration rate. In contrast, ‘Santy Naranja’ exhibited a slower variation, reaching a CO2 concentration comparable to the other cultivars only at S14. This slower variation can be attributed to lower respiratory activity during the initial 7 days of storage, implying possible slower ripening and senescence processes during the initial postharvest period.
In tomatoes, total soluble solids are a common measure of the fruit concentration of soluble sugars (glucose, fructose, and sucrose), while titratable acidity is related to the fruit concentration of organic acids, mainly malic and citric acids [37]. Their absolute values, along with their ratio, are responsible for the perceived equilibrium between sweetness (TSS) and sourness (TA) during mastication, contributing to the overall tomato taste and flavor [38]. In this experiment, all three cultivars exhibited a similar temporal decrease in total soluble solids, consistent with the catabolism of carbon substrates necessary to drive the ripening and senescence processes occurring during postharvest [39]. Accordingly, we found significant correlations linking TSS and headspace gas composition (i.e., positive with O2 and negative with CO2), demonstrating that the deterioration of the tomatoes’ taste characteristics was largely due to the fruits’ aerobic metabolism. ‘Angelle’ showed a more pronounced decline in fruit titratable acidity between S7 and S14, indicating a stronger intensification of energy consumption for its autocatalytic metabolism. Indeed, it has been reported that organic acids, particularly citrate, tend to substitute sugars as a respiratory substrate in case of depleted cytosolic carbohydrate reserves within the fruit [40]. Overall, this led to increased variability in TSS/TA in ‘Angelle’ throughout the S0-S14 period, contrasting with the comparatively stable TSS/TA observed in ‘Santy Naranja’ and ‘Melange’. Such lower temporal oscillations in TSS/TA observed in the latter cultivars indicated their superior ability to maintain a consistent taste profile during postharvest.
4.3. Functional Variables
Where the fruits’ functional composition was concerned, total phenols and carotenoid contents were substantially in line with those previously reported by Cannata et al. [9] in a set of 15 mini plum tomato cultivars. Polyphenols represent a wide group of secondary metabolites regulating plant growth, reproduction, and adaptation to environmental cues [41]. As a general trend in our experiment, TPC was highly correlated with CO2 headspace concentration (−0.658 ***) and with TSS (0.758 ***) and TA (0.599 ***), confirming that phenolic compounds are closely linked to the fruit’s senescence stage [42]. The studied cultivars markedly differed in their polyphenols content, with the dark red-fruited ‘Melange’ being particularly rich at S0 (with a TPC more than double that of ‘Santy Naranja’) and at S14 (when compared to the reference cultivar Angelle). This can be regarded as a strong nutraceutical advantage of ‘Melange’ since polyphenols are powerful antioxidants likely implicated in reduced incidence of numerous chronic and neoplastic diseases, but overall neglected in tomato breeding programs [13]. During storage, ‘Santy Naranja’ showed a stable fruit TPC between S0 and S7, with a significant decrease observed only at S14. Differently, both ‘Angelle’ and ‘Melange’ exhibited a comparable trend of continuous decrease in fruit TPC between S0 and S14, suggesting an ordinary temporal progression of the ripening processes. Indeed, continuous depletion of polyphenols is a well-documented phenomenon in tomato fruits during ripening [39]. In contrast, their sustained synthesis during refrigerated storage, albeit transient, has been linked to a stress response induced by the low surrounding temperatures. Cold stress prompts the accumulation of several phenolics in plant cells, which helps maintain redox balance, thereby preventing lipid peroxidation and the collapse of fruit cells [41]. Hence, the distinct trend in TPC observed in ‘Santy Naranja’ between S0 and S7 suggests that, especially during the initial transition from field to storage conditions, these tomatoes exhibit greater susceptibility to the typical storage temperatures for tomatoes, possibly due to chill stress.
Carotenoids are chill-susceptible C40 isoprenoids associated with thylakoid membranes, where they contribute to light harvesting and protection of photosystems. In human nutrition, they provide a wide array of biological benefits, contributing to free radical scavenging or modulating the pathogenesis of cancers and coronary heart diseases [43,44]. The studied cultivars differed highly in terms of carotenoid contents and trends during storage. ‘Angelle’ was the richest cultivar in terms of carotenoids, likely as a consequence of its strongest accumulation of all-trans-lycopene, i.e., the prevailing pigment of red-fruited tomatoes [14]. Moreover, it also proved the strongest increase in TCC between S0 and S7, thereby demonstrating a highly positive balance between carotenoid neosynthesis and degradation during storage. Accordingly, it has been reported that carotenoid accumulation can continue during postharvest storage as long as the integrity of the fruit is maintained, thus preserving the enzymatic activity responsible for carotenogenesis [45]. This pattern was also observed between S0 and S7 in ‘Santy Naranja’ and ‘Melange’. However, while predominant carotenogenesis was observed in ‘Melange’ up to S14, the drastic reduction of carotenoids recorded in ‘Santy Naranja’ at S14 suggests, for this cultivar, the occurrence of metabolic modifications favoring their degradation over neosynthesis. Since carotenoid content (quantitative and qualitative) is the primary determinant of tomato fruit color [14], such reduction observed in ‘Santy Naranja’ appears to explain, at least partially, the broader chromatic fluctuations of its fruits recorded over time.
Except for ‘Melange’ at S7, both novel cultivars exhibited significantly lower levels of AsA compared to ‘Angelle’, suggesting their lower favorability concerning this important nutraceutical trait. Indeed, in the human body, ascorbate is one of the major soluble redox molecules, protecting cellular structures and enzymes against irreversible oxidation. Additionally, it plays a role in epigenetic control over gene expression and enhances iron absorption [46]. After synthesis, AsA in plants can rapidly undergo oxidation due to its ROS-scavenging function. Indeed, the balance between its oxidation and recycling processes significantly contributes to maintaining AsA levels and redox status in plant cells [47]. Therefore, the overall AsA reduction observed at the end of the storage period in all studied cultivars suggests a gradual predominance of fruit senescence phenomena, albeit with timing and intensity strongly genotype-dependent. Accordingly, in our experiment, AsA was negatively correlated with FWL (−0.616 ***), ΔE*ab (−0.706 ***), and CO2 concentration in the headspace (−0.485 *), i.e., with those variables more typically linked to fruit aging.
DPPH and FRAP assays are widely used to assess the free radical scavenging activity in vegetables. DPPH activity is based on the stable free radical’s ability to decolorize in the presence of antioxidants, while the FRAP assay relies on the reduction of ferric to ferrous ions at low pH, resulting in the formation of a colored ferrous–tripyridyltriazine complex [25,48]. In the current experiment, DPPH was positively correlated with TPC (0.520 **) and AsA (0.618 ***), whereas FRAP was positively correlated with TCC (0.612 ***). Accordingly, all the aforementioned alterations in the secondary metabolites composition resulted in a temporal variation of the antioxidant capacity of tomatoes determined with the DPPH and FRAP assay, with a particularly pronounced DPPH decrease recorded in ‘Santy Naranja’, probably as a consequence of its stronger temporal declines in its TCC and AsA contents. This underscores its lesser resilience over time regarding its nutraceutical characteristics during postharvest.
5. Conclusions
The results of the present experiment have highlighted a highly diversified postharvest behavior among the studied cultivars. Compared to the reference cultivar Angelle, the novel genotypes were primarily characterized by higher fruit transpiration and softening over time (‘Melange’), as well as more pronounced and prolonged changes in fruit color characteristics, accompanied by a more marked reduction in carotenoid content and fruit antioxidant capacity (‘Santy Naranja’). Despite their more consistent TSS/TA ratio during postharvest, these responses clearly indicate a lower capacity of these cultivars to maintain their peculiar traits over time under refrigerated conditions, with potentially negative effects on their ability to positively influence the consumers’ purchasing behavior. Hence, in order to promote greater diversity in tomato typologies available on the market, it seems appropriate to set up additional postharvest strategies (other than simply refrigerated storage), preserving the sensory and nutraceutical characteristics of these novel cultivars throughout the supply chain.
C.C.: investigation, methodology, formal analysis, writing—original draft; R.P.M.: conceptualization, methodology, supervision, writing—original draft; C.A.C.R.: investigation, methodology, writing—original draft; F.B.: investigation, formal analysis; G.M.: conceptualization, supervision, writing—review and editing; C.R.: supervision, visualization, writing—review and editing; L.S.: methodology, formal analysis, writing—review and editing; C.L.: project administration. All authors have read and agreed to the published version of the manuscript.
The data presented in this study are available on request from the corresponding author.
The authors declare no conflicts of interest.
Footnotes
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Enlarge this image.Figure 1. Fruit weight (a) and CIELab chromatic coordinates L* (b), hue angle (c), and chroma (d) at S0 of the studied mini plum tomatoes (mean ± standard deviation). For each variable, different letters between histograms indicate significance at Tukey’s HSD test (p ≤ 0.05).
Enlarge this image.Figure 2. Fruit fresh weight loss (a) and overall color variation (b) of the studied mini plum tomatoes as affected by ‘cultivar × storage time’ interaction (mean ± standard deviation). Light bars: S7; dark bars: S14.
Enlarge this image.Figure 3. Headspace O2 (a) and CO2 (b) concentration, titratable acidity (c), and TSS/TA ratio (d) of the studied mini plum tomatoes as affected by ‘cultivar × storage time’ interaction (mean ± standard deviation). White bars: S0; grey bars: S7; black bars: S14.
Enlarge this image.Figure 4. Total phenolic (a), carotenoids (b), and ascorbic acid (c) content and DPPH (d) of the studied mini plum tomatoes as affected by ‘cultivar × storage time’ interaction (mean ± standard deviation). White bars: S0; grey bars: S7; black bars: S14.
Main information related to the studied genotypes.
| ‘Santy Naranja’ | ‘Angelle’ | ‘Melange’ | |
|---|---|---|---|
| Seed company | Enza Zaden Italia srl, Viterbo (VT), Italy | Syngenta Italia, Milano (MI), Italy | ISI sementi S.p.A., Fidenza (PR), Italy |
| Fruit color | Orange | Red | Dark red, green stripes |
| Fruit size | Medium to small | Small | Large |
| Fruit shape index (L/D) | 1.44 | 1.47 | 1.16 |
| Yield potential | High | High | Medium |
Main carpometric traits of mini plum tomatoes as affected by the main factors (mean ± standard deviation).
| Source of Variation | Fruit WTc | Fruit FWL | Fruit Firmness | ΔE*ab |
|---|---|---|---|---|
| Cultivar (C) | ||||
| ‘Santy Naranja’ | 90.27 ± 0.44 a | 6.79 ± 2.91 b | 6.13 ± 0.90 a | 3.13 ± 0.98 a |
| ‘Angelle’ | 90.19 ± 0.54 a | 6.70 ± 1.63 b | 5.40 ± 0.70 b | 1.53 ± 0.26 b |
| ‘Melange’ | 88.94 ± 0.48 b | 8.60 ± 3.01 a | 4.69 ± 0.65 c | 1.91 ± 0.30 c |
| Storage time (S) | ||||
| S0 | 90.01 ± 0.69 a | - | 6.13 ± 0.81 a | - |
| S7 | 89.83 ± 0.89 a | 5.12 ± 1.01 b | 5.37 ± 0.70 b | 1.89 ± 0.42 b |
| S14 | 89.57 ± 0.77 a | 9.61 ± 1.41 a | 4.72 ± 0.79 c | 2.48 ± 1.17 a |
| C × S | NS | * | NS | *** |
| Overall mean | 89.8 ± 0.78 | 7.36 ± 2.60 | 5.41 ± 0.94 | 2.19 ± 0.90 |
Different letters within each column’s factor indicate significance at Tukey’s HSD test (p < 0.05). NS: not significant; *, ***: significant at p ≤ 0.05 and 0.001, respectively; WTc: water content; FWL: fresh weight loss; ΔE*ab: total color difference.
Headspace O2 and CO2 concentrations, total soluble solids (TSS), titratable acidity (TA), and their ratio in mini plum tomatoes as affected by the main factors (mean ± standard deviation).
| Source of Variation | O2 | CO2 | TSS | TA | TSS/TA |
|---|---|---|---|---|---|
| Cultivar (C) | |||||
| ‘Santy Naranja’ | 18.90 ± 2.13 a | 2.68 ± 2.22 b | 7.83 ± 0.58 b | 0.26 ± 0.03 b | 30.0 ± 3.7 b |
| ‘Angelle’ | 18.53 ± 1.77 ab | 3.28 ± 2.50 a | 7.59 ± 0.94 b | 0.30 ± 0.06 a | 26.3 ± 4.0 c |
| ‘Melange’ | 18.29 ± 1.76 b | 3.34 ± 2.50 a | 9.28 ± 0.53 a | 0.28 ± 0.03 ab | 32.8 ± 2.6 a |
| Storage time (S) | |||||
| S0 | 20.67 ± 0.31 a | 0.03 ± 0.01 c | 9.05 ± 0.65 a | 0.30 ± 0.04 a | 30.5 ± 3.6 a |
| S7 | 18.12 ± 1.39 b | 4.15 ± 0.97 b | 7.99 ± 0.97 b | 0.30 ± 0.04 a | 27.0 ± 4.9 b |
| S14 | 16.93 ± 0.99 c | 5.12 ± 0.52 a | 7.65 ± 0.89 b | 0.24 ± 0.02 b | 31.7 ± 3.3 a |
| C × S | ** | ** | NS | * | * |
| Overall mean | 18.61 ± 1.86 | 3.10 ± 2.35 | 8.23 ± 1.02 | 0.28 ± 0.04 | 29.7 ± 4.3 |
Different letters within each column’s factor indicate significance at Tukey’s HSD test (p < 0.05). NS: not significant; *, **: significant at p ≤ 0.05 and 0.01, respectively.
Functional variables of mini plum tomatoes as affected by the main factors (mean ± standard deviation).
| Source of Variation | TPC | TCC | AsA | DPPH | FRAP |
|---|---|---|---|---|---|
| Cultivar (C) | |||||
| ‘Santy Naranja’ | 1501 ± 298 c | 301 ± 84 b | 1530 ± 640 c | 97 ± 11 b | 65 ± 9 b |
| ‘Angelle’ | 2101 ± 545 b | 505 ± 139 a | 2717 ± 846 a | 108 ± 7 a | 77 ± 6 a |
| ‘Melange’ | 2872 ± 580 a | 332 ± 74 b | 2264 ± 724 b | 109 ± 6 a | 68 ± 8 b |
| Storage time (S) | |||||
| S0 | 3206 ± 667 a | 293 ± 41 b | 2871 ± 711 a | 108 ± 8 a | 64 ± 9 b |
| S7 | 2035 ± 431 b | 440 ± 129 a | 2167 ± 801 b | 109 ± 4 a | 73 ± 8 a |
| S14 | 1233 ± 308 c | 404 ± 166 a | 1472 ± 458 c | 96 ± 10 b | 74 ± 7 a |
| C × S | *** | ** | * | * | NS |
| Overall mean | 2158 ± 855 | 379 ± 141 | 2170 ± 875 | 104 ± 10 | 70 ± 9 |
Different letters within each column’s factor indicate significance at Tukey’s HSD test (p ≤ 0.05). NS: not significant; *, **, and ***: significant at p ≤ 0.05, 0.01, and 0.001, respectively.
Pearson’s product-moment correlation coefficients (r) among variables (n = 27).
| Fruit | Fruit | Fruit | ΔE*ab | O2 | CO2 | TSS | TA | TSS/TA | TPC | TCC | AsA | DPPH | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Fruit FWL | NS | - | |||||||||||
| Fruit firmness | 0.637 *** | −0.690 *** | - | ||||||||||
| ΔE*ab | NS | 0.782 *** | NS | - | |||||||||
| O2 | NS | −0.862 *** | 0.714 *** | −0.745 *** | - | ||||||||
| CO2 | NS | 0.908 *** | −0.678 *** | 0.778 *** | −0.944 *** | - | |||||||
| TSS | −0.479 * | −0.431 * | NS | −0.521 ** | 0.430 * | −0.526 ** | - | ||||||
| TA | NS | −0.504 ** | NS | NS | 0.381 * | NS | NS | - | |||||
| TSS/TA | NS | NS | NS | NS | NS | NS | 0.425 * | −0.693 *** | - | ||||
| TPC | NS | −0.670 *** | NS | −0.626 *** | 0.576 ** | −0.658 *** | 0.758 *** | 0.599 *** | NS | - | |||
| TCC | NS | NS | NS | NS | NS | 0.437 * | −0.550 ** | NS | −0.409 * | −0.385 * | - | ||
| AsA | NS | −0.616 *** | NS | −0.706 *** | 0.420 *** | −0.485 * | 0.464 * | 0.583 ** | NS | 0.615 *** | NS | - | |
| DPPH | NS | −0.438 * | NS | −0.566 ** | 0.396 *** | NS | 0.436 * | 0.564 ** | NS | 0.520 ** | NS | 0.618 *** | - |
| FRAP | NS | 0.479 * | −0.435 * | NS | −0.414 *** | 0.513 *** | −0.408 * | NS | −0.392 * | NS | 0.612 *** | NS | NS |
NS: not significant; *, **, and ***: significant at p ≤ 0.05, 0.01, and 0.001, respectively.
Supplementary Materials
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Details
; Mauro, Rosario Paolo 1
; Chiara Alessandra Carmen Rutigliano 1
; Basile, Federico 1
; Muratore, Giuseppe 1
; Restuccia, Cristina 1
; Sabatino, Leo 2
; Leonardi, Cherubino 1
1 Dipartimento di Agricoltura, Alimentazione e Ambiente (Di3A), University of Catania, Via S. Sofia, 100, 95123 Catania, Italy;
2 Dipartimento di Scienze Agrarie, Alimentari e Forestali, University of Palermo, Viale delle Scienze, Ed. 4, 90128 Palermo, Italy;