Introduction

The Annonaceae family is widely distributed in tropical and subtropical regions (Chidinma et al., 2024). Within this family, the genus Annona is the most representative, including notable species such as soursop (Annona muricata L.), cherimoya (Annona cherimola Mill.), sugar apple (Annona squamosa L.), and custard apple (Annona reticulata L.). The custard apple tree is commonly found in the wild, in backyard orchards, and along rural roadsides (Andrés & Segura-Ledesma, 2014). Various morphological studies conducted on Annonaceae species have helped identify both quantitative and qualitative traits useful for characterizing materials employed in genetic improvement programs, aiming to select those with the highest productive potential and fruit quality (Castañeda-Garzón et al., 2016). However, despite its potential, A. reticulata L. remains an underutilized and undervalued species, especially when compared to soursop, cherimoya, and sugar apple, which have achieved a greater degree of domestication and commercialization (Hernández-Fuentes et al., 2016).

Regarding the fruit morphology of custard appel, Handique et al. (2022) reported the existence of fruits with round, oval, and cordiform shapes, and exocarp colors ranging from reddish to green-yellow and red-yellow. They also documented fruit weights ranging from 145 to 443.3 g, lengths between 6.1 and 8.9 cm, and diameters from 5.8 to 8.6 cm. Similarly, Vidal-Hernández et al. (2015) observed red-colored fruits with white pulp, as well as variants with red or pink pulp and exocarp, with weights between 356 and 450 g, lengths from 9.0 to 12.9 cm, and diameters ranging from 8.5 to 9.7 cm.

Physicochemical analyses of custard apple pulp have revealed a high content of dietary fiber, vitamins A and C, calcium, magnesium, and iron, sometimes in higher concentrations than those found in soursop (Ananthavalli & Karpagam, 2023; Panda et al., 2023). Moo-Huchin et al. (2014) reported vitamin C levels of 23 mg∙100 g^-1 of fresh weight in custard apple pulp, as well as the presence of bioactive compounds such as anthocyanins, phenolic compounds, flavonoids, and carotenoids.

Consumer interest in foods with high nutritional value has grown significantly (Moreno et al., 2014), prompting the development of preservation and processing methods that maintain the physicochemical and sensory properties of raw materials to produce high-quality food products (Pérez-Flores et al., 2023). In this regard, Senadeera et al. (2018) incorporated custard apple pulp into yogurt, successfully enhancing its antioxidant capacity. These findings support the potential of Annona reticulata not only as a fresh-consumption fruit but also as a valuable ingredient in the food and pharmaceutical industries, due to its nutraceutical potential to contribute to consumer health and well-being.

Based on the above, the aim of this study was to assess the morphological and physicochemical characteristics of Annona reticulata fruits collected from four localities in the municipality of Tepic, Nayarit, Mexico. The information collected will contribute to the scientific documentation of this species’ traits and may ultimately support the development of a germplasm bank focused on the conservation and utilization of fruits with specific characteristics, depending on their intended application, whether for fresh consumption, food industry processing, or pharmaceutical use.

Materials and methods

Plant material and experimental design

In April 2021, custard apple fruits were collected from wild trees (Table 1). Trees were selected based on their local representation, prioritizing those with higher abundance and commercial importance in the municipality of Tepic, Nayarit. From each selected tree, physiologically mature fruits were harvested manually using visual criteria focused on optimal size and quality. The fruits were then transported in plastic crates to the Food Technology Unit at the Universidad Autónoma de Nayarit, where they were stored at room temperature (28 ± 2 °C and 95 % relative humidity) until they reached ripeness suitable for consumption (from 4 to 7 days). The experimental design employed a completely randomized model, with individual fruits serving as the experimental units and three replications for each variable.

Table 1. Sampling sites and sample size of custard apple collections in Tepic, Nayarit, Mexico.

Morphological and physicochemical characterization of fruits

The morphological characterization of the fruits was carried out following the methodology used by Bioversity International and CHERLA (2008) for cherimoya. The shape of the fruits, the shape of the areoles, and the type of reticulation were described. Length (mm) and diameter (mm) were measured with a digital caliper (420-150 mm, Caliper, China), while the weight of each fruit (g) was determined using a digital balance (Scout-pro, OHAUS, USA). The color components of the exocarp of physiologically mature fruits were analyzed using a colorimeter (CR-400, Konica Minolta, Japan): brightness or luminosity (L) (0 = pure black, 100 = pure white), hue angle (h) (0 = purple-red, 90 = yellow), and chromaticity (C) (intensity ranging from gray to pure chromatic) (Solorzano-Morán et al., 2015).

At the stage of consumption maturity, fruit firmness (N) was determined using a texture analyzer (CT3-50kg, Brookfield, USA). The fruits were then sliced to separate and weigh the exocarp (g), mesocarp (g), and fresh seeds (g); the number of seeds per fruit was also recorded. The pulp was analyzed for total soluble solids (TSS; °Brix), pH (Jiménez-Zurita et al., 2016), and titratable acidity (TA; % citric acid) following the guidelines of the Association of Official Analytical Chemists (AOAC, 2005). Finally, the fruit's maturity index (MI) was calculated based on the ratio of TSS to TA (TSS/TA).

Statistical analysis

The data was analyzed using a one-way analysis of variance based on locality, followed by Tukey’s test for mean comparisons (P ≤ 0.05). Statistical analysis was performed using SAS® software version 9.0 (SAS Institute Inc., 2000).

Results and discussion

Morphological evaluation

Fruit and areole shape, type of reticulation, and exocarp color

The fruits from all four locations had three shapes: cordiform, oval, and irregular (Figure 1a, 1b, and 1c, respectively). These results are consistent with those reported by Pathak and Zaman (2014), who noted oval, globose, irregular, heart-shaped, and nearly round fruit forms, similar to those observed in Annona squamosa by Hasan et al. (2024). Differences in fruit shape may stem from pollination and fertilization challenges, which are influenced by flower morphology, the type of pollinators, and genetic factors (Jiménez-Zurita et al., 2016; Zhao et al., 2021). Fruit development can be shaped by the species’ adaptation to biotic and abiotic stress factors in the environment (Morales-Santos & Sánchez-Hernández, 2022), as well as by alterations in phytohormones, which have a direct impact on the fruit's shape, structure, and overall quality (Yahia, 2019).

Regarding the areoles, four main shapes were identified in all sites: water drop or spheroidal, rhomboidal, pentagonal, and hexagonal (Figure 2). As for the type of reticulation, the predominant form was imprinted or marked (Figure 3b); however, in the fruits from Jicote, smooth reticulation was observed, with areoles shaped like water drops and rhomboids (Figure 3a). Cruz and Deras (2000) described smooth exocarps and non-prominent carpels in custard apple fruits, while Bioversity International and CHERLA (2008) observed that reticulation in cherimoya could be smooth, imprinted, umbonate, mamillate, or tuberculate. In this study, only the smooth and imprinted forms were observed, which agrees with the findings of Andrés and Andrés-Hernández (2011), who noted that the other variants are not characteristic of A. reticulata L.

The color change in the exocarp during ripening is attributed to chlorophyll degradation, associated with the synthesis of carotenoids (Benito-Bautista et al., 2015). In Annona muricata, this transition involves a shift from green to green-yellow as the fruit reaches consumption maturity, which is linked to the predominant presence of carotenoids following chlorophyll degradation (Badrie & Schauss, 2010). Additionally, color variations can be connected to the phenotypic plasticity of plant species, a key trait for adaptation and evolution in response to environmental conditions (Hernández-Verdugo et al., 2015).

The exocarps of the fruits from Trapichillo, 14 de Marzo, and Tepic were dull green (h = 106.90, C = 27.39 and L = 46.51) with yellow-orange hues (Figures 4a and 4b); however, in 14 de Marzo, some fruits were brown (h = 84.34, C = 27.10 and L = 47.04) with orange-red tones (Figure 4c). Meanwhile, the fruits from Jicote were brown (h = 95.51, C = 27.21 and L = 55.34) with yellow shades (Figure 4d), and some were purple (h = 84.34, C = 27.10 and L = 47.07) (Figure 2e).

These findings are consistent with those reported by Vidal-Hernández et al. (2015), who evaluated custard apple fruits and identified four exocarp colors: red, yellow, purple, and pink, attributing these variations to genetic differences. The green exocarp with yellow hues and purple coloration matches the reports by Handique et al. (2022) and Vidal-Hernández et al. (2015). Yellow and orange fruits have also been documented (Cruz & Deras, 2000), as well as red fruits with pink tones (Hasan et al., 2024).

Weight, length, diameter, and firmness

In this study, the fruits from the Jicote and Tepic had the highest weights (P ≤ 0.05), with 367 and 369 g, respectively. In contrast, the fruits from Trapichillo and 14 de Marzo weighed 184.9 and 226.4 g, respectively (P ≤ 0.05) (Table 2). Similarly, the fruits from Jicote and Tepic also exhibited significantly larger dimensions (P ≤ 0.05) compared to those from Trapichillo and 14 de Marzo (Table 2). Hasan et al. (2024) reported fruit weights ranging from 105.25 to 137.8 g in Annona squamosa, which are lower than the weights observed in custard apple, likely due to species-specific differences.

Table 2. Comparison of means for weight, length, diameter, and firmness of custard apple fruits from four sites in Tepic, Nayarit, Mexico.

The higher weight observed in fruits could be related to the canopy size, as it enhances light capture, which in turn promotes the accumulation of photosynthates for optimal fruit development (Poyam et al., 2022). Additionally, variations in fruit weight and size may be influenced by factors such as tree age and vigor, the number of fruits per tree, soil and climate conditions (like soil moisture and fertility), and the wild nature of the trees (Handique et al., 2022; Moreira-Macías et al., 2016). In general, morphological variations serve as indicators of the expression of genetic components in plants as they respond to environmental factors (Hasan et al., 2024).

Regarding firmness, the fruits from Jicote and Trapichillo had the highest values, while those from 14 de Marzo and Tepic were the least firm (Table 2). These differences are likely due to the heterogeneity in the thickness of the exocarp in custard apple, as firmness values ranged from 2 to 19 N within the same plant material.

González-Agüero et al. (2016) reported that firmness in cherimoya fruits decreased from 16 to 4.9 N after 5 and 8 days of storage at 20 °C, respectively. These findings are consistent with the results of this study, as custard apple and cherimoya, although distinct species, both belong to the Annonaceae family and share similar characteristics, particularly in exocarp reticulation. It is important to note that firmness loss during ripening is a genetically regulated process involving biochemical and physiological changes that affect firmness, color, flavor, and texture (Martínez-González et al., 2017). The reduction in firmness is primarily due to modifications in the polymer networks of the primary cell wall. These changes are driven by a coordinated and interdependent action of enzymes and proteins, such as polygalacturonase, pectin methylesterase, and β-galactosidase, among others (Martínez-González et al., 2017).

Pulp weight, exocarp weight, number of seeds, and fresh seed weight

The pulp weight of fruits from Jicote was statistically higher than that of fruits from the other sites, which showed no significant differences among them (Table 3). These variations may be attributed to the presence of firm, non-softened sections in the pulp of certain fruits, which made extraction and, consequently, quantification more difficult. Additionally, the amount of pulp is directly related to the final fruit size, and its variability may be influenced by factors such as metabolite accumulation, light exposure, water relations between the plant and the fruit, phytohormonal responses to environmental and endogenous stimuli, as well as genetic factors (Zhao et al., 2021).

Table 3. Mean comparison of pulp weight, exocarp weight, number of seeds, and fresh seed weight in custard apple fruits from four sites in Tepic, Nayarit, Mexico

Moo-Huchin et al. (2014) reported a pulp yield of 44 % in custard apple fruits from Yucatán. In the present study, fruits from Jicote reached a pulp yield of 62.8 %, while yields in the other sites ranged between 20.5 and 38.7 %. These differences may be related to contrasting soil and climate conditions between the two regions, including altitude (286 m a. s. l. in Yucatán vs. 915 m a. s. l. in Nayarit), soil type (Luvisol vs. Umbrisol), average temperature (27 °C vs. 25 °C), bedrock type (limestone vs. neovolcanic), and relative humidity (88 vs. 95 %) (Instituto Nacional de Estadística y Geografía [INEGI], 2010; 2019). Ávila-de Hernández et al. (2012) and Jiménez-Zurita et al. (2016) reported pulp yields of 62 and 71 % in soursop, which agrees with the values in this study for the fruits from Jicote.

Regarding exocarp weight, fruits from Tepic had the highest values (P ≤ 0.05), followed by those from Jicote, 14 de Marzo, and Trapichillo (Table 3). Differences in exocarp weight may be attributed to variations in exocarp thickness, fruit size, and the environmental conditions in which the trees developed. Moreover, the loss of firmness and changes in exocarp texture may be linked to the action of hydrolytic enzymes and other mechanisms involved in the fruit ripening process (Martínez-González et al., 2017).

Fruits from Jicote and Trapichillo had a significantly higher number of seeds (P ≤ 0.05) compared to those from 14 de Marzo (Table 3). The seed counts recorded in this study exceed those reported by Hasan et al. (2024) for sugar apple, which ranged from 35 to 50 seeds per fruit; however, it is important to note that these are different species within the same genus. Variability in seed number may be influenced by the morphological characteristics of the inflorescence and its interaction with pollinators (Jiménez-Zurita et al., 2016). Additionally, seed size may depend on both the genotype and the environmental conditions present during plant development (García-Rodríguez et al., 2018).

Seed weight showed significant differences (P ≤ 0.05) among sites, with seeds from fruits collected in Jicote and Tepic having the highest weights (Table 3). These differences may be partially attributed to the presence of damaged or hollow seeds, often resulting from insect infestations such as wasps or borers (Bephratelloides cubensis) (Vidal-Hernández et al., 2014), as well as variations in seed shape and size. Additionally, lower seed weights may be associated with reduced accumulation of photosynthates, genetic variability, or smaller fruit size (Handique et al., 2022).

Total soluble solids (TSS), pH, titratable acidity (TA), and maturity index (MI)

Significant differences (P ≤ 0.05) in TSS concentration were observed among the fruits from the different sites. The highest values were recorded in fruits from 14 de Marzo and Jicote, while Trapichillo and Tepic showed lower concentrations (Table 4). Sugar content in fruit depends on the stage of ripeness, climate conditions, and genotype. Soluble sugars primarily result from the enzymatic conversion of starch reserves and are directly associated with the increase in sweetness during ripening (Africano et al., 2015). In climacteric fruits, TSS consists mainly of sugars, particularly glucose and fructose, which together account for approximately 80 % of the total sugar content (Vázquez-Cuecuecha et al., 2023).

Table 4. Mean comparison of the physicochemical characteristics of custard apple pulp from four sites in Tepic, Nayarit, Mexico.

Moo-Huchin et al. (2014) reported an average value of 17.75 °Brix in custard apples, which is higher than the values observed in fruits from Trapichillo and Tepic but lower than those recorded in fruits from Jicote and 14 de Marzo. González-Agüero et al. (2016) reported 18.2 °Brix in cherimoyas at the edible ripeness stage when stored at 20 °C, a value that falls within the range recorded in this study. Discrepancies in TSS values -both within custard apples and among other species of the same genus- can be attributed to the fact that these fruits are aggregate, meaning each individual may not be pollinated uniformly, which can lead to uneven ripening (de los Santos-Santos et al., 2020). Additionally, variation may be influenced by genetic and environmental factors such as temperature, solar radiation, and irrigation management (Fernandes do- Nascimento et al., 2022; Hasan et al., 2024).

Regarding pH, fruits from Tepic had the highest values, followed by those from Jicote, Trapichillo, and 14 de Marzo (P ≤ 0.05) (Table 4). In most tropical fruits, approximately 90 % of the cellular volume is occupied by the vacuole, which typically has a pH between 5 and 5.5. During the formation of sugars such as sucrose and glucose, slight pH changes occur, acidity decreases, and fruit flavor is modified (Moreno & Deaquiz-Oyola, 2016). The pH values recorded in this study are consistent with those reported by González-Pariona and Cornejo-y Maldonado (2014) for certain cherimoya ecotypes (pH of 4.71). The differences observed between studies may be attributed to genetic and agroecological factors, as significant variation can occur even among cultivars of the same species (Villalba et al., 2006).

During ripening, fruits typically accumulate sugars and experience a decline in acidity, mainly due to the degradation of organic acids such as malic, citric, tartaric, oxalic, fumaric, and succinic acids, which contribute to flavor and vary in dominance depending on the species (Valero & Serrano, 2010). Titratable acidity (TA) was significantly higher in fruits from Trapichillo (P ≤ 0.05), followed by those from 14 de Marzo, Jicote, and Tepic, which showed the lowest TA values (Table 4). Moo-Huchin et al. (2014) reported an average TA of 0.66 % in custard apple fruits, which is higher than the values reported in this study. Nolasco-González et al. (2023) found TA levels ranging from 0.59 to 0.86 % in soursops, which also exceeded the values recorded in fruits from 14 de Marzo, Tepic, and Jicote. These variations are likely influenced by environmental factors such as harvest season and location (Villalba et al., 2006).

MI was significantly higher (P ≤ 0.05) in fruits from Jicote, followed by Tepic, 14 de Marzo, and Trapichillo (Table 4). This index is directly associated with the sensory perception of flavor in fruits (Vázquez-Cuecuecha et al., 2023) and is more closely linked to fruit palatability than sugar or acidity levels alone (Wills & Golding, 2016). The increase in MI is attributed to the reduction in both acidity and pH (Fernandes do-Nascimento et al., 2022), which enhances the organoleptic quality of the fruit.

Villalba et al. (2006) reported MI values of 9.66, 11.8, and 12.53 for Colombian fruits of custard apples, cherimoyas, and soursops, respectively. The values observed in this study exceeded those reported by these authors, which could be attributed to the contrasting soil and climate conditions between Colombia and Mexico, potentially influencing the genotypic and phenotypic characteristics of the fruits. Nolasco-González et al. (2019) recorded MI values for soursop ranging from 17 to 27.8, which are lower than those observed in this study, as well as the values reported by Nascimento et al. (2019) and Paitan-Anticona et al. (2022) for soursop (ranging from 5.68 to 30). The differences across these studies could be due to the distinct species (soursop and cherimoya) involved, though both belong to the same family. The content of sugars, organic acids, and volatile compounds are associated with the flavor of fruits and are measured as TSS and TA (Pérez-Díaz et al., 2020). In climacteric fruits, the increase in the MI typically coincides with the stage where they reach their peak respiration rate and rapidly mobilize their reserves (mainly organic acids) as part of the ripening process (Vázquez-Cuecuecha et al., 2023).

Conclusions

The soursop fruits collected from the localities in the municipality of Tepic, Nayarit, showed significant differences in their morphological and physicochemical characteristics. In particular, the fruits from Jicote stood out for having a higher pulp weight, a greater number of seeds, and a higher content of total soluble solids. These characteristics suggest that the material from this locality has high potential, which could be utilized for cultivation in orchards with agronomic management, commercial propagation, and the establishment of a germplasm bank for the species.