ORIGINAL ARTICLES
Dry Matter, Adhesiveness and Their Relationship with Other Attributes as Quality Indicators for Pumpkin Consumption
2023 Volume 92 Issue 4 Pages 464-475
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2023 Volume 92 Issue 4 Pages 464-475
This study investigated the role of dry matter (DM) and adhesiveness (or stickiness) and their relationship with the physicochemical and antioxidant properties of raw and steamed pumpkins. Through path analysis, raw pumpkin with a high DM value yielded a positive correlation with total flavonoid contents (TFC) and Ferric Reducing Antioxidant Power (FRAP) value in its raw form, and contained high total soluble solids and 2,2-diphenyl-2-picryl-hydrazyl (DPPH) activity in its steamed form. DM showed a correlation with total phenolic compounds (TPC) and DPPH activity. A sticky pulp generated high DM, TPC, TFC, and antioxidant capacity. The TPC and TFC were positively correlated with the antioxidant capacity. There was no relationship between cell structure, and starch granule characteristics of the raw pulp and cooked DM or stickiness. Therefore, DM and stickiness may be used as quality indicators of chemical contents and antioxidant capacity in pumpkin and could support pumpkin improvement programs.
Global recommendations and demand for organically grown food products are constantly increasing, driven by people’s desire for a healthier way of living. Fresh produce offers considerable nutritional benefits that have the potential to sustain and/or improve overall health.
Specifically, pumpkin fruit is a functional food replete with bioactive components such as carotenoids, phenolic compounds, vitamins, minerals, and antioxidant capacity (Amin et al., 2019) that contribute to the prevention of diseases and promotion of good health. While for many kinds of fruits and fruit vegetables, simple chemical analysis may not necessarily result in the efficient determination of their nutritional value and antioxidant capacity after cooking and processing, this may be different for pumpkin fruits.
This study advances the idea that when it comes to pumpkin cultivars, general quality can be simply determined by examining the dry matter content and other physicochemical characteristics of raw fruit that correlate with cooking quality based on the adhesiveness or stickiness of cooked pumpkins. Using these indicators to predict the chemical characteristics and quality after cooking among pumpkin cultivars, a selection process to satisfy market preferences could be more manageable and effective.
A number of studies reported that dry matter content is an essential indicator in the horticultural sector due to its important relationship with the physical and chemical composition of fruits and vegetables. In pumpkin with a high dry matter content, there were high levels of vitamin C and sugars (Medelyaeva et al., 2021). In a study by Javaherdashti et al. (2012), a pumpkin with high dry matter content had high total phenolics and total flavonoids. There was no relationship between the dry matter and flesh (pulp) firmness in apple fruit (Palmer et al., 2010), while dry matter had a relationship with total soluble solids (TSS) in cucumber (Valverde-Miranda et al., 2021). However, stickiness is preferred by pumpkin consumers because of the sticky feeling in the mouth. However, there is a lack of information showing the relationship among stickiness and physicochemical and antioxidant properties before and after cooking; more detailed information on the relationship between the dry matter and stickiness and the physicochemical properties of cooked pumpkin is required. Taking these insights and findings into account, this study aimed to determine the relationship between the dry matter and stickiness and the physicochemical and antioxidant properties of raw and steamed pumpkin. The results can be used to improve breeding programs and thus adjust pumpkin to reflect to meet both global and local market requirements. In addition, the starch morphology of raw pumpkins was analyzed to determine if there is a relationship with the other attributes.
Five pumpkins (Cucurbita moschata Duchesne ex Poir.) were used in this study. There was one commercial cultivar (control), an F1 hybrid pumpkin. The other four were inbred pumpkins (Thong Lanna (TL) 3, TL4, TL5, and TL6) and were not similar to each other. They were certified and registered plant varieties under the Plant Varieties Act B.E. 2518 in Thailand on December 25, 2014. Also, they were registered to protect new plant varieties. According to the Plant Variety Protection Act B.E. 2542 on January 8, 2019, these pumpkins can be used until January 7, 2031. These five pumpkins had the same maturity at 35 days after flowering (DAF). The organically grown pumpkin fruits were harvested from Rajamangala University of Technology Lanna, Lampang, and were delivered immediately to King Mongkut’s University of Technology Thonburi, Bangkok. To prepare the raw and steamed samples, the fruits were washed then weighed (after wiping with cloth to dry) and cut to remove the seeds. The raw fruit samples were peeled and chopped into small pieces, and liquid nitrogen (N2) was applied. For the cooked samples, 1-cm3 (length × width × diameter) of pulp was steamed in boiling water for ten minutes. All the samples were stored at −20°C for further analysis (chemical and antioxidant properties).
Fruit weight and pulp thicknessThe pumpkin fruits were weighed using a 4,000-g weighing scale (Model P4102; OHAUS Corporation, NJ, USA). The broadest part of the pulp at the seed cavity level (The International Union for the Protection of New Varieties of Plants, 2007) was measured to assess pulp thickness, and was expressed in centimeters.
Pulp firmness, adhesiveness (stickiness), and pulp colorThe firmness and adhesiveness of raw and steamed samples were analyzed using a Texture Analyzer (TA.XT plus Texture Analyser; Stable Micro Systems, Godalming, United Kingdom). The firmness and adhesiveness of the raw samples were assessed under the following conditions: 5 mm·s−1 pre-test speed, test speed 1 mm·s−1, and 1 mm·s−1 post-test speed using a 5-mm probe diameter.
Twenty pieces (cm3 size) of steamed pulp for every experimental unit were used to analyze firmness and adhesiveness. The following details conditions were used for the steamed pulp: 1 mm·s−1 pre-test speed, 1 m·s−1 test speed, and 5 mm·s−1 post-test speed using a 35 mm probe diameter.
The Commission International del’ Eclariage (CIE) color system (L*, a*, and b*) was used to measure the color of raw and steamed samples via a colorimeter (Model CR-400; Minolta, Tokyo, Japan). The study used five sliced raw fruits having 2-cm thickness, and eight pieces of steamed samples with 0.2-cm thickness and 2-cm width. The color change was calculated by applying the following formula:
Dry matter
The dry matter of samples was determined using Association of Official Analytical Chemists (2000). Raw and steamed samples (10 g each) were dried at 70°C for 48 h using an oven (Models 30-1060; Memmert, Schwabach, Germany).
Total soluble solids and total soluble proteinsTSS were recorded using a digital refractometer. Equal parts of samples and distilled water were used to allow easy juice extraction.
The Bradford (1976) method was used to analyze the total soluble proteins of pumpkin with Bovine Serum Albumin as a standard. Briefly, 10 mL of phosphate buffer (7.0 pH) was added to 2.5 g of sample and it was homogenized. Centrifugation was done at 10,000 rpm at 4°C for 20 min. Four mL of Coomassie Brilliant Blue was added to one mL supernatant. After 10 min of incubation, the absorbance of the blank, standard, and samples was recorded at 595 nm using a spectrophotometer (Hanon i5; Jinan Hanon Instruments Co., Ltd., Jinan, China).
Presence of starch using an iodine test and scanning electron microscopy (SEM)Sliced (crosswise cuts) raw pumpkins were dipped in 0.2% iodine solution for 10 s with minor modifications (Simantara et al., 2018).
The method of Singh et al. (2007) was followed for starch isolation. Scanning electron micrographs were obtained using a scanning electron microscope (JSM6610LV; JEOL, Tokyo, Japan). The pumpkin powder was sprinkled on a carbon-tape stub and coated with gold. An accelerating potential of 10 kV was used for micrography.
Sample extraction and chemical analysisA 5-g sample with 10 mL of 80% ethanol was homogenized. Centrifugation at 12,000 rpm was done for 20 min at 4°C and afterward the supernatants were collected to analyze total sugar, total phenolic compounds (TPC), total flavonoid contents (TFC), and antioxidant activity using 2,2-diphenyl-1-picryl-hydrazyl (DPPH) radical scavenging activity and Ferric Reducing Antioxidant Power (FRAP) assays. The absorbance of these parameters was recorded using a spectrophotometer (Hanon i5; Jinan Hanon Instruments Co., Ltd.).
Total sugarThe method of Dubois et al. (1956) with minor modifications was used to determine the total sugar content of the samples. The mixtures (0.5 mL sample extract + 1.0 mL 5% phenol + 5.0 mL sulfuric acid) were incubated at 60°C for 60 min. Afterward, the absorbance of samples and glucose (standard) were recorded at 490 nm.
Total phenolic compounds and total flavonoid contentsTPC was determined using the Folin-Ciocalteu method (Singleton and Rossi, 1965) with minor modification. Briefly, 0.02 mL sample extract was mixed with 1.6 mL distilled water, 0.1 mL Folin-Ciocalteu phenol reagent, and 0.2 mL 20% sodium carbonate solution. Then, the mixture was incubated at 40°C for 20 min. The absorbance was measured at 765 nm after incubation.
The TFC analysis used a procedure as previously reported (Chang et al., 2002). A 0.5 mL sample was added to 10% aluminum chloride (1 mL), 1 M sodium acetate (1 mL), distilled water (2.8 mL), and kept for 40 min at room temperature. A UV-VIS spectrophotometer set to 415 nm was used to record the sample absorbance and quercetin as the standard. The TFC was expressed as mg quercetin equivalents (QE)·g−1 of each fresh sample.
Antioxidant capacity DPPH radical scavenging assayDPPH radical scavenging activity was determined following the method of Brand-Williams et al. (1995) with minor modifications. A 0.5 mL supernatant was mixed with 2.85 mL DPPH working solution. Then, the mixture was incubated for 30 min at room temperature (28 ± 2°C) in the dark. The absorbance of the samples was observed at 515 nm using a spectrophotometer. DPPH radical scavenging activity was calculated using the following formula:
Ferric reducing antioxidant power (FRAP) assay
The antioxidant capacity of the samples used was measured with Benzie and Strain’s (1996) method with minor modifications. Mixtures (0.5 mL sample extract + 2.85 mL freshly made FRAP working solution) were incubated for 30 min under dark conditions. The absorbance of the samples, including the Trolox (standard), was recorded at 593 nm using a spectrophotometer.
Fixation, embedding and pumpkin fruit cross-sectionsTransversely cut 10 to 15 mm sections of each pumpkin were immediately soaked in formalin acetic alcoholic solution and left for more than 18 h. All fixed samples were washed with alcohol solutions (30% and 50%) twice every hour and soaked in 70% ethanol overnight. Samples were dehydrated by soaking them in 50%, 70%, 85%, and 100% alcohol solutions every hour. Afterward, dehydrated samples were infiltrated three times using paraffin wax at 60°C and immediately embedded. Obtained 35 μm thick cross-sections of the samples were put in a sliding microtome (HM 450; MICROM International GmbH, Walldorf, Germany). A stereomicroscope (Eclipse E200; Nikon Corporation, Tokyo, Japan) was used to observe the cellular structure of the samples. The procedures were slightly modified from the method of Hawes and Beatrice (2001).
Statistical analysisPumpkin fruits were arranged in a Complete Randomized Design, and statistically analyzed using SPSS (SPSS for Windows Version 17.0, Released 2008, SPSS Inc., Chicago, IL, USA) program. Mean standard errors (SE) were used to present the data. One-way analysis of variance and means difference were measured by Tukey’s Honestly Significant Difference (HSD) test. The means before and after steaming were subjected to an independent samples t-test. Prior to performing path analysis, stepwise multiple regression was also conducted using SPSS to determine the predictors that would affect the dependent variable.
Various fruit weights and pulp thickness were observed among the five pumpkins (Fig. 1). The commercial cultivar (CC) was the heaviest fruit and TL3 was the lightest. The pulp of TL4 was thicker than that of other cultivars.
The fruit weight (A) and pulp thickness (B) of pumpkins, and dry matter (C), pulp firmness (D) and adhesiveness (E) of pumpkins before and after steaming. TL–Thong Lanna; CC–Commercial Cultivar. The pulp firmness and adhesiveness were measured using a 5-mm diameter probe for raw pulp, and a 35-mm diameter probe for steamed pulp. For details see Materials and Methods. Values are means ± standard error (SE); Means followed by different lowercase letters (a–c) for raw pumpkins and uppercase letters (A–C) for steamed pumpkins in a graph are significantly different at P ≤ 0.05 by using Tukey’s Honestly Significant Difference (HSD) test.
The dry matter (raw) percentage of TL3, TL4, and CC were not significantly different, but they were significantly higher than TL5 and TL6 (Fig. 1C). In contrast with the steamed form, TL3 and TL4 had a significantly higher dry matter percentage than the other cultivars.
Pulp firmness and adhesiveness or stickinessCC and TL3 had significantly firmer pulp. After steaming, TL3 produced a much higher value compared to the other cultivars, but with no significant difference (Fig. 1D).
Regarding adhesiveness or stickiness, TL3 had high values in the raw and steamed forms as compared to the other pumpkins (Fig. 1E).
Pulp color and color changeThe raw pumpkin color based on the CIE system revealed that the L* values of TL4, TL5, and TL6 were similar to CC. However, they were significantly higher than TL3 (Fig. 2A). The a* values were higher in TL3 and TL4 compared to CC (Fig. 2B). TL3 yielded the highest b* value (Fig. 2C).
L* (A), a* (B), b* (C) values, and color change (D) of pumpkins before and after steaming. TL–Thong Lanna; CC–Commercial Cultivar. Values are means ± standard error (SE); Means followed by different lowercase letters (a–c) for raw pumpkins and uppercase letters (A–B) for steamed pumpkins in a graph are significantly different at P ≤ 0.05 by using Tukey’s Honestly Significant Difference (HSD) test.
The a*, b*, and L* values decreased in all pumpkins. A marked color change in CC was observed when compared with the TL pumpkins (Fig. 2D).
Presence of starch using an iodine test, cell shape and structure, and starch morphologyThe presence of starch indicated by dark staining was different among TL pumpkins (Fig. 3A). TL4 had the lightest appearance, while CC had the darkest.
Stained pulp after dipping in 0.2% iodine solution (A), raw pulp (B), cell structure (C), and scanning electron microscopy (SEM) images of the starch (D) of raw pumpkins. TL–Thong Lanna; CC–Commercial Cultivar.
The cell shape and structure were different among the pumpkins. CC, TL3, and TL4 had bigger sizes cells, whereas TL5 had smaller ones. Cell shapes varied from ovoid to circular and these shapes were both present in all pumpkins.
A variation in the starch granule shape and size distribution was evident (Fig. 3D). Bigger starch granules were observed in TL3, while CC and TL5 granules were smaller. The shapes of starch granules were spherical, oval, polyhedral, and domed (Fig. 3D). The spherical shape was observed in all pumpkins.
Total soluble solids (TSS), total sugar, and total soluble proteins (TSP)TL5 had the highest TSS, while TL6 had the lowest, similar to the other raw pumpkins. TSS levels were similar in all raw pumpkins. On the other hand, TL3 exhibited higher TSS than TL6 after steaming, but the values were comparable to the other pumpkins (Fig. 4A).
The total soluble solids (A), total sugar (B), and total soluble proteins (C) of pumpkins before and after steaming. TL–Thong Lanna; CC–Commercial Cultivar. Values are means ± standard error (SE); Means followed by different lowercase letters (a–d) for raw pumpkins and uppercase letters (A–E) for steamed pumpkins in a graph are significantly different at P ≤ 0.05 by using Tukey’s Honestly Significant Difference (HSD) test.
The total sugar contents in raw and steamed pumpkins differed markedly (Fig. 4B). Raw CC had a significantly higher total sugar content than all the TL pumpkins. After steaming, total sugar contents increased by more than five-fold when compared to raw pumpkins. CC was significantly the highest among the steamed pumpkins.
The raw and steamed pumpkins clearly manifested significant differences in their TSP (Fig. 4C). Raw TL3 and TL5 produced the higher TSP compared to the CC. After steaming, a TSP reduction was observed.
Total phenolic compounds and total flavonoid contentsRaw TL3 contained the highest TPC, and this was consistently significantly higher than CC and relatively higher than the other pumpkins (Fig. 5A). TL4, TL5, and CC showed significantly reduced contents after steaming and CC was the lowest among all pumpkins.
The total phenolic compounds (A), total flavonoid contents (B), DPPH radical scavenging capacity (C), and FRAP value (D) of pumpkins before and after steaming. TL–Thong Lanna; CC–Commercial Cultivar. Values are means ± standard error (SE); Means followed by different lowercase letters (a–e) for raw pumpkins and uppercase letters (A–E) for steamed pumpkins in a graph are significantly different at P ≤ 0.05 by using Tukey’s Honestly Significant Difference (HSD) test.
Flavonoid content in raw TL3 was higher than in CC and other pumpkins (Fig. 5B). TL5 and TL6 were comparable and were the lowest among the pumpkins. Even after steaming, the TFC of TL3 was still the highest and it was followed by TL4, TL6 and CC.
Antioxidant capacityVarying antioxidant capacities among the pumpkins were observed (Fig. 5C and 5D). For raw samples, TL3 had the highest DPPH while TL5 had the lowest. After steaming, the DPPH values of the pumpkins almost doubled. TL3, TL4, and CC recorded the highest DPPH, whereas TL5 and TL6 were significantly lower than these pumpkins.
In raw form, the highest FRAP value among the pumpkins was observed in TL3, followed by TL4 (Fig. 5D). TL5 and TL6 generated similar FRAP values to CC and were lower than TL4. When steamed, the five pumpkin cultivars showed an increase in FRAP values of more than seven-fold. Surprisingly, TL pumpkins had higher FRAP values than CC. TL3 outranked the four pumpkins with CC being the lowest.
Relationships of adhesiveness and dry matter to other attributesNo raw attributes had any relationship with TSS (Table 1a), whereas a very high and positive correlation between pulp firmness and TFC was observed. The dry matter showed a strong positive relationship with the FRAP value, TFC, pulp firmness, and adhesiveness or stickiness. In contrast, the L* value showed a high negative association with the dry matter. Adhesiveness also had a very high association with TPC, FRAP and the b* value, whereas L* showed a very strong negative relationship. In addition, the DPPH and TFC showed a strong positive correlation with adhesiveness.
Pearson correlation of raw attributes (vertical) to raw attributes (horizontal).
For steamed samples, a very strong positive relationship was observed for TPC with TFC and FRAP (Table 1b). A strong positive relationship between the b* value and TFC with FRAP was observed. The TFC and dry matter showed a very strong correlation. FRAP, a* value, and b* values had a strong positive relationship with the dry matter of the steamed pumpkins. At the same time, TPC and dry matter had a strong relationship. The adhesiveness or stickiness was very high and positively associated with the TPC, TFC, FRAP, b* value, and pulp firmness. A strong correlation between the adhesiveness, dry matter and the a* value was observed.
Pearson correlation of steamed attributes (vertical) to steamed attributes (horizontal).
Only L* and TFC showed a direct effect on the adhesiveness of raw and steamed pumpkins, respectively. On the other hand, there was a strong positive relationship between the raw dry matter and steamed attributes such as TSS and DPPH (Table 1c). A moderate positive relationship among dry matter and steamed attributes, pulp firmness, TFC, TPC, and FRAP was also observed. Raw adhesiveness was observed to have a very strong positive correlation with the steamed attributes of pumpkins, such as the TFC and FRAP and it was also positively correlated with the pulp firmness and dry matter. The TPC, DPPH, and TSS were moderately associated with raw adhesiveness.
Pearson correlation of raw pumpkins (vertical) to steamed attributes (horizontal).
Regarding the path analysis, FRAP and TFC characteristics had a very significant direct effect on dry matter at 0.47 and 0.43, respectively, when the other characteristics were constant (Table 2a). FRAP and TFC had a very significant total effect with values 0.69 and 0.68, and TSS had no effect. A significant indirect effect of FRAP and TFC characteristics of −0.10 and −0.09 was observed. The dry matter trait was inversely related to the FRAP and TFC. The coefficient of determination was 56.1%, indicating that there were other factors influencing dry matter trait.
The direct (bold-faced), indirect and total effects of dry matter (raw) to the raw attributes.
TSS and DPPH characteristics had a highly significant direct effect on the steamed dry weight at 0.44 and 0.41, respectively when the other characteristics were constant (Table 2b). The total effects of TSS and DPPH were highly significant at 0.67 and 0.65, respectively. TSS had an indirect effect on DPPH at 0.23 and DPPH was influenced indirectly through TSS at 0.25, indicating that the steamed dry weight was positively related to TSS and DPPH. The coefficient of determination was 65.9%, indicating that there were other factors influencing dry matter (steamed).
The direct (bold-faced), indirect, and total effects of dry matter (raw) to steamed attributes.
Pumpkins are among many vegetables that offer innumerable health benefits. Each pumpkin has advantages over others, especially in terms of physicochemical and antioxidant properties; these vary depending on the cultivar, maturity, pre-harvest environment, and postharvest-cooking methods, which are factors that affect attributes and cooking quality.
Regarding the quality, dry matter is used as an indicator of the fruit quality (Stoyanova et al., 2018). A dry matter increase is associated with fruit development, and is therefore a harvest determinant. This study, which maturity was the same (35 days after flowering (DAF); using CC as the reference) among the five pumpkins, produced scientific information about their differences and correlations among raw and steamed attributes. TL3 can be harvested the same as CC because of its attributes. Early harvesting at 35 DAF was the same for all pumpkins as this is the reference for home consumption in Thailand. Therefore, this harvesting time was used in this study. Also, early harvesting was done to avoid the adverse effect of unpredictable weather conditions, pest infestation and disease infection. According to the path analysis, raw pumpkins with a high dry matter had a high TFC and FRAP value, which was observed in TL3. This concurred with the results of Javaherdashti et al. (2012). In terms of the cooking quality, there was a higher DPPH and TSS if the dry matter value was high. This indicates that dry matter as a quality indicator is very helpful for plant breeders, farmers, and consumers. This is because a large dry matter content indicates strong antioxidant properties and capacity due to the direct effect of these attributes. In a previous study, a type of pumpkin with a large dry matter content had a lower fruit yield, while lower dry matter yielded higher fruits (Haytova et al., 2020). In contrast, as the fruit became mature and bigger, the dry matter increased (Muenmanee et al., 2016). This indicates that there are complex factors that affect the dry matter of pumpkin such as the fruit maturity and type. Thus, pumpkins with lower dry matter may need longer than 35 DAF. However, with regards to the fruit maturity at 35 DAF as in the study, the dry matter can be used to predict the quality of the pumpkins with a higher TSS and DPPH in cooked pulp, as observed in TL3. Although that the dry matter of TL3 was the same as CC, the attributes were better in the former than the latter. Moreover, in the Pearson’s correlation, dry matter was positively correlated with pulp firmness, which was in contrast to apple fruit (Palmer et al., 2010). Pumpkins with a large dry matter contained firmer pulp, as observed in TL3 and CC. It was reported that as the fruit matured, the firmness decreased due to cell wall degrading enzymes (Sharma and Ramana Rao, 2013). As for pulp firmness, a previous study showed that pumpkins with firmer pulp had larger cellular structures in mature fruit than younger smaller fruit (Figueiredo Neto et al., 2013). Our results showed that the cell structures of pumpkins differed. This concurred with a previous study in which bigger cell structures as in CC (Fig. 3C) were almost the same in TL4 and TL3, which showed firmer pulp compared to TL5. TL6, however, had a low firmness value even though its cell structure was nearly the same size as TL3. This indicates that the growth of cells from pumpkins other than TL5 was faster, and that there is a relationship between the size of cells and firmness, that is, larger cells contributed to a firmer pulp, as shown in CC, TL3, and TL4. In contrast, TL5 may need more time to mature and produce bigger cells and stronger cell integrity.
Further, it was observed that pumpkins with a large amount of dry matter such as TL3, TL4, and CC had firmer pulps and larger cellular structures. Therefore, TL3 and TL4 could be harvested the same as CC, whereas TL5 should be harvested later than 35 DAF. It also suggests that TL3 could be one variety for long-term storage, the same as CC due to its high firmness value. It was also observed that a sticky pumpkin, TL3 had the same cellular structure as CC, TL4, and TL6, indicating no relationship between pumpkin cell structures and stickiness.
Starch is composed of amylose and amylopectin. One method to identify the presence of starch in pumpkins is to use an iodine solution. Results showed that TL4 produced a lighter stain, while CC had darker stain. Dark staining was also observed in TL3, TL5, and TL6 indicating a higher starch content, the same as CC. Related studies showed that a dark stain indicated a higher starch content (Simantara et al., 2018). Thus, plant breeders and producers may use this method to determine the starch content of pumpkins. However, the starch content had no relationship with the dry matter or adhesiveness. This indicates that a large amount of dry matter does not indicate a high starch content (darker stain). The commercial cultivar produced a darker stain followed by TL3, TL5, and TL6, for which the dry matter and adhesiveness were significantly different.
Therefore, there was no relationship with the starch content using iodine solution. All the pumpkin samples were treated equally. The pulp of each pumpkin was soaked in 0.2% iodine solution for 10 s. The conflicting findings in this study compared with other studies may be due to the fruit maturity (35 days after flowering, which is relatively early), which was not the same as in other studies. In addition, the varieties were different from the previous studies. It was also reported in another study that an iodine test can predict the starch content in pumpkin to an accuracy of 80–85% (Simantara et al., 2018), which can be used as an effective low-cost method for starch determination. Moreover, they measured the chroma and b* value to assess the color of fresh and stained pumpkins, and a relationship with starch content was reported. A low chroma and b* value (darker stain in pumpkin pulp) indicates a high starch content. The dark staining using iodine was caused by a reaction of iodine and amylose; this probably caused an error in this case and the pumpkins in this study have contained more raffinose and stachyose (Terazawa et al., 2001). Therefore, another measurement method is recommended.
A large amount of dry matter content did not indicate larger or smaller starch granules. Results showed that pumpkins with a large amount of dry matter, i.e., TL3 and CC, had bigger and smaller starch granules, respectively (Fig. 3D). This means that the starch granule size or shape did not affect the dry matter content of the pumpkins. The pumpkin starch granule shapes included a mixture of spherical, oval, polyhedral, and domed cells, which was also observed in other fruits (Singh et al., 2007). The starch granule size or shape may not relate to the adhesiveness or stickiness, but a size variation among pumpkins was observed. TL3, which is sticky, was bigger in terms of granule size, whereas CC (non-sticky) was smaller. According to previous reports, starch granule size varies between waxy and non-waxy rice starches and among cultivars (No et al., 2019; Wani et al., 2012), the same as observed in this study. The stickiness may be related to the amylose content and may be affected by other factors; this requires further investigation.
The antioxidant properties of pumpkins with a large dry matter content revealed a high TPC and TFC and strong antioxidant activity (Fig. 5). This was observed in the raw and steamed pulp of TL3. Strong antioxidant capacity is related to high TPC and TFC, as shown by their positive relationships (Table 1). Moreover, the relationship of the raw attributes to steamed attributes, particularly the antioxidant properties and antioxidant capacity, showed a positive association. Thus, dry matter can be used as a quality indicator of pumpkins with the same fruit maturity as in this study due to its positive connection to other attributes (Table 1). In connection with the increase in the TPC and TFC, steaming helps release flavonoids in pumpkins, which softens the cellular structures and extracts more TPC, accounting for the increased content. Variations in the increase among the pumpkins was caused by the temperature-dependent stability of the specific flavonoid class, which ultimately affected the TFC after cooking. However, the conversion of bound flavonoids to a free form may have occurred, significantly increasing the TFC in TL3, TL4, TL5, and TL6 after steaming. CC may have required a longer steaming time to release the bound flavonoids. TL3 contained the highest TPC and TFC, possibly due to free forms of phenolic compounds and flavonoids, and possession of specific compound classes resistant to heat. However, this phenomenon requires further investigation. In a previous study, pumpkins with a lot of high dry matter had high total phenolic and antioxidant activity (Dinu et al., 2016). That study concurred with the present study; that is TL3 with a lot of dry matter contained high TPC and TFC, and DPPH and FRAP values were 2–7% higher than the other pumpkins. This was probably caused by enzymatic activity, which may be higher in this pumpkin than in other pumpkins.
The antioxidant capacity increased as the TPC and TFC increased because these could donate their hydrogen or electrons. Our results showed that the increase in TPC and TFC after steaming intensified the antioxidant capacity in the assay, FRAP. Also, the DPPH values doubled after steaming the samples, while in FRAP, there was an up to 14-fold increase. TL pumpkins are health-promoting vegetables and a potential source of antioxidants, especially steamed TL3, which had the highest DPPH radical scavenging capacity and FRAP values linked to its high TPC and TFC. Results of this study corroborate findings from previous studies, which confirmed that cooking leads to higher antioxidant capacity due to the release and extractability of certain compounds, i.e., TPC (Carvalho et al., 2014). Moreover, our results agree with a previous report about the positive correlation between total phenolic contents and antioxidant activity (Dinu et al., 2016).
TL3, which had a large amount of dry matter had a sticky pulp due to high adhesiveness or stickiness (Fig. 1E with red box) and contained a greater number of larger starch granules than CC, as well as significantly different sweetness. Steamed pulp of all pumpkins synergistically increased the total sugar content, and CC was the sweetest. However, even if the total sugar content increased, it was not reflected in the TSP due to the denaturation of proteins after steaming, which adversely decreased the TSP content of all pumpkins.
The increase in total sugar, besides from the hydrolysis of starch with heat (Wei et al., 2017), may be due to raffinose and stachyose which are broken down after steaming and has also been observed in beans (Messina, 2014). Terazawa et al. (2001) found that raffinose oligosaccharides, monosaccharides, and sucrose are the major soluble sugars present in raw pumpkin. Raffinose comprises galactose, glucose, and fructose, while stachyose contains two galactose molecules, one molecule of glucose and one of fructose (Pessarakli, 2016; Yahia et al., 2019). The content of raffinose and stachyose decreased after cooking (Messina, 2014; Oboh et al., 2000). Therefore, steaming may break down these oligosaccharides into simple sugars, leading to an increase in the total sugar content of the tested pumpkins, leaving a sweeter pulp. This indicates that the high level of total sugar in steamed CC may contain high levels of oligosaccharides that were converted into more simple sugars, making it sweeter, followed by TL6. TL5 contained lower levels of simple sugars and was the lowest among the pumpkins. Additionally, this pumpkin can be used by consumers who prefer the pulp tastes to not be too sweet. Therefore, a further investigation of the oligosaccharides of the studied pumpkins is needed.
In this study, it was hypothesized that soluble solids and sugar content would have positive correlation, but our results showed that there was no relationship between these two attributes. This was probably caused by soluble solids being made up of sugars, organic acids, soluble pectin and anthocyanins. However, this method is very simple for determination in a raw pumpkin. While total sugar focuses of the analysis of sugar content. This gives a sweetness determination, which is important for consumers who prefer sweet pulp and for plant breeders. In addition, our study only used five cultivars, which may be too small number.
The relationship among adhesiveness and other attributes (raw form) were positive. A sticky pumpkin had a small fruit size, a more dry matter, TPC, TFC, and antioxidant activity, as observed in TL3. The adhesiveness of raw pumpkins can be correlated to the cooking quality based on the Pearson’s correlation. Results showed that it was positively correlated with the steamed attributes in the following order: TFC, FRAP, dry matter, TPC, DPPH, and TSS. This indicates that a sticky pulp with a lot of dry matter would have higher levels of antioxidant compounds, antioxidant capacity, and TSS; consumers may enjoy the sticky feeling of these pumpkins along with positive nutritional benefits.
In the study of Crisosto et al. (2012) on kiwifruit, soluble solids and firmness were needed along with the dry matter. Our results agree with the previous report, which showed a positive correlation between TSS and pulp firmness. Therefore, dry matter could be used in the selection of cooked pumpkin.
ConclusionFindings in this study substantiate and verify the correlation between dry matter and adhesiveness with the physicochemical and antioxidant properties of raw and steamed pumpkins. It was found that raw pumpkins with large dry matter content had higher TPC, TFC, and antioxidant capacity (DPPH and FRAP values). A sticky pumpkin with a large dry matter content had the same attributes. For the nutritional composition of steamed pulp, a lot of dry matter and sticky pumpkin in raw form were predictors of the positive attributes mentioned above. Thus, the results can benefit plant breeders, consumers, and the food industry. Generally, Southeast Asian (SEA) countries prefer a sticky pulp; therefore, pumpkins with a sticky pulp could be further commercialized and to meet the consumer demand. Consequently, plant breeders may use the results of this study in crop improvement programs for pumpkin and to grow pumpkin with superior traits (a lot of dry matter, high levels of antioxidants and sticky pulp) would be used as parent lines for hybrid production.
Consumers can have the option to choose from different pumpkins. If they prefer a sticky and not-so sweet pulp, along with high levels of antioxidants. TL3 is ideal for them. Whereas for those who prefer a sweeter pulp that is not so sticky, but still with high levels of antioxidants (second rank after TL3), TL6 is recommended. Likewise, these pumpkins may be used by consumers around the world depending on their preference.
The food industry will benefit from the results of the study. Pumpkins with sticky pulp, and high in antioxidants and antioxidant capacity can be promoted.
The general public may also find the results of this study beneficial, particularly in their selection of pumpkin fruits for consumption. An investigation of the physicochemical and antioxidant properties of the same pumpkins with different fruit maturity is recommended to determine the peak of starch and total sugar contents. This will also support pumpkin improvement programs.
The authors would like to thank Dr. Chalermchai Wongs-Aree and Dr. Panida Boonyaritthongchai for their suggestions to improve the study.
