Investigation of the effects of different processing methods on the selected nutritional properties of pumpkin and determining the appropriate process for pumpkin yogurt

Abstract The processing methods, especially cooking, can cause quality losses, particularly in the nutritional value of the fresh product. This study investigated the effects of preprocessing methods on the nutritional properties of pumpkin and the physicochemical and sensory properties of pumpkin yogurt. Two different pumpkin varieties (Cucurbita pepo and Cucurbita maxima) were subjected to three different preprocessing methods (freeze‐drying, boiling, and baking). Boiling significantly increased antioxidant activity (p ≤ .05), followed by baking. C. maxima had higher TDF and TPC than C. pepo, but in both pumpkin varieties, TDF did not change with heat treatment (boiling and baking), while TPC decreased. Mineral contents remained the same or decreased with heat treatment, except for Mn and Fe. In particular, the addition of C. maxima significantly affected the color parameters (L*, a*, b*) of yogurt and improved WHC (from 68.9% to 91.6%) and hardness (from 58.0 to 193.5 g; p ≤ .05). The sensory evaluation concluded that heat‐treated (boiled and baked) samples were preferred more than freeze‐dried raw ones. In conclusion, the results revealed that adding boiled and baked pumpkins, especially the preference for C. maxima instead of C. pepo, improved the quality parameters of yogurt.

the effects of processing types on nutritional properties and deciding on the cooking method according to the characteristics of the final product is an important research subject in food science.
Therefore, the objective of this study was to evaluate the effects of preprocessing methods (freeze-drying, boiling, and baking) on the nutritional properties (dietary fiber, bioactive substances, and mineral content) of two pumpkin varieties (Cucurbita pepo and Cucurbita maxima) and the effects of the preprocessing on the physicochemical and sensory properties of pumpkin yogurt.

| Supply and preprocessing of pumpkins
Thirty kilogram of pumpkin for each variety was obtained from the agricultural producers of Sakarya.After washing under water, the seeds and skins were removed.The remaining flesh was cut into 2 cm 3 pieces and processed using three methods.In the freezedrying method, the cubes were freeze-dried using Labconco Freezone 6 freeze dryer at −45°C under a vacuum of 0.045 mbar for 6 days.In the boiling method, drinking water was added to pumpkins at a ratio of 1:1 (w:w), then boiled for 1.5 h on a 350°C heater.
In the baking method, the pumpkin cubes were cooked in the oven (Siemens iQ700) at 150°C for 70 min.All processed groups were then separately homogenized with a blender.The freeze-dried raw pumpkins as powder and the boiled and baked pumpkins as puree were obtained.Finally, the processed pumpkins were used in the production of flavored yogurts.

| Production of yogurts
For instance, plain yogurt was produced to be used in all sample groups.UHT whole cow's milk (3.0% fat and 3.0% protein) was inoculated with a mixed culture of S. thermophilus and L. delbrueckii subsp.bulgaricus (CHR Hansen YC-350 yogurt culture) and incubated at 43°C until the pH of the milk reached 4.6.After the incubation was completed, the plain yogurt was mixed, and the clot was broken.Then, it was split into lots, and other ingredients were added according to the formulations given in Table 1.The pumpkin and sugar concentrations in manufacturing were determined by preliminary sensory tests prepared with at least five different concentrations.Plain and flavored yogurt productions were duplicated by the procedure described in this section.The samples were kept at 4°C until the analysis.
The characteristics of pumpkins and yogurts were determined by at least two replicate analyses.

| Nutritive properties of pumpkin samples
Raw and preprocessed (boiled or baked) pumpkin samples to be used to determine nutritional components were freeze-dried before analysis.The total dietary fiber (TDF) of pumpkins was

TA B L E 1
The control and pumpkin yogurt formulations.
determined following the Sigma-Aldrich protocol provided in the TDF-100A Kit (Sigma-Aldrich Inc.), and the results were given as % in dry matter (DM).For analysis of bioactive components, extracts were prepared by mixing 0.5 g of dried sample in 10 mL of 70% methanol using a homogenizer.The mixture was kept in an ultrasonic water bath at 20°C for 15 min and then centrifuged at 1500 g at 4°C for 10 min.After separating the supernatant, the remaining part was used as an extract in the analyses.The total phenolic compound (TPC) analysis was carried out according to Cerit et al. (2016).In the method, 100 μL of the extract was mixed with 0.2 mL of Folin-Ciocalteu reagent and 2 mL of distilled water.The mixture was incubated for 3 min at room temperature.
Then, 1 mL of 20% sodium carbonate was added and kept in the dark at room temperature for 1 h.Absorbance was measured at 765 nm using a Shimadzu UV-1240 spectrophotometer.The results were given as mg of gallic acid equivalent (GAE) per 100 g of DM.The antioxidant capacity was determined in terms of DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging activity with some modifications of the method recommended by Shaterabadi et al. (2020).The DPPH solution was prepared at a final concentration of 0.05 mM by dissolving in 100% methanol.Three milliliter of DPPH solution was added to 200 μL of extract.After being kept in the dark for 30 min, absorbance was measured at 517 nm using a spectrophotometer (Schimadzu UV mini-1240).The DPPH scavenging activity was obtained as Trolox equivalent using a calibration curve.
Concentrations of eight elements (Ca, Cu, Fe, K, Mg, Mn, P, and Zn) were determined after freeze-drying the untreated and cooked pumpkin samples.0.5 g of sample was digested using a microwave (MARS-5 CEM) in the presence of nitric acid and hydrogen peroxide.
The mineral contents were analyzed using an inductively coupled plasma-atomic emission spectrometer (ICP-MS, Agilent 7700 series; Agilent Technologies) as described by Aktaş et al. (2015).

| Physicochemical properties of yogurt samples
The pH and titratable acidity of the yogurt samples were measured on 1, 7, and 14 days of storage.The pH was analyzed at 25°C using a pH meter (WTW 720).The titratable acidity was determined in terms of lactic acid percent (LA%) according to the AOAC methods (AOAC, 2000).The DM, protein, and ash content of the yogurts were analyzed after a week of storage using the official methods (AOAC, 2000).The textural properties of yogurts were observed using a texture analyzer (Brookfield CT3, Brookfield Engineering Laboratories).In the analysis, a compression test was applied using TA 4/1000 cylindrical probe to a distance of 20 mm with 4.0 g of trigger force and 1 mm/s test speed, and the results were expressed in terms of hardness (g).The water holding capacity (WHC) of yogurts was measured using the method described by Gomes et al. (2023) with some modifications: Approximately 20 g of yogurt sample was centrifuged at 1250 g at 4°C for 10 min.The WHC was expressed as the percentage ratio of the mass remaining after serum separation to the initial yogurt weight.The colors of yogurts were determined using a tintometer (Lovibond RT300) and expressed in CIELAB system where L* indicated lightness, a* redness or greenness, and b* yellowness or blueness.
The sensory properties were measured by an internationally accepted and widely used 9-point hedonic scale (Nicolas et al., 2010).The panelists were trained males and females aged between 18 and 45, consisting of 10 students and staff of Sakarya University (Turkey).For the evaluations of color, texture, taste, and general acceptance, responses were recorded, indicating 1 = dislike extremely and 9 = like extremely.The flavor intensity and sweetness parameters were asked for rating as 1 = none and 9 = over.

| Statistical analyses
Data of two production replicates with twice analysis were initially subjected to analysis of variance (ANOVA) using Minitab 16 software (Minitab Inc.).Results were compared using Tukey's test with a significance level of 5%.Multivariate functional principal component analysis (PCA) was applied for further evaluation of mineral content and sensory data.

| Total dietary fiber and bioactive contents of processed pumpkins
Raw flesh DM of C. pepo and C. maxima were 7.55% and 8.04, respectively, and increased to 94.5-95.0 by freeze-drying.According to Table 2, the baking process increased DM more than boiling, probably due to evaporation.The fiber content and bioactive materials of the processed pumpkins were measured after freeze-drying to eliminate the moisture effect, and the results are given in Table 2.
The TDF content was not affected by the process type (p > .05).
The TDF amount of C. maxima (34.4% in DM) was higher than C. pepo (24.3% in DM) in all processed groups.On the other hand, Kim et al. (2012) reported TDF amounts considerably lower than in our study and reported TDF of C. pepo as higher than C. maxima (10.94% and 6.44% in DM, respectively), contrary to our findings.Mehditabar et al. (2020) reported the water-soluble and -insoluble fiber fractions of baked pumpkin as 14.13 and 19.56 g per 100 g DM, respectively, with a higher total fiber amount than that in our study.
According to the values in the USDA National Nutrient Database (USDA, 2018), pumpkin contains 2.9% dietary fiber.When our results were reflected in the total weight (including moisture content), comparable results were observed.Our results were obtained in the range 2.4%-3.1% for C. pepo and 2.9%-4.1% for C. maxima, considering the moisture ratios.
The highest amount of antioxidant substance was determined in the boiled groups, then the baked ones, and the least was in the raw materials.In conclusion, the most effective method to increase antioxidant capacity was boiling, probably due to cell lysis.There are studies in the literature supporting that cell lysis increases DPPH scavenging activity (Barba et al., 2015).Similarly, in a study examining the physicochemical and antioxidant properties of C. moschata, the DPPH amount increased from 77 to 99 mg GAE/100 g DM after drying in a hot-air tray dryer (Promsakha na Sakon Nakhon et al., 2017).In a study investigating the effects of cooking techniques on the nutritional properties of pumpkin leaves, it was reported that steam application reduces antioxidant loss compared to the boiling process (Mashiane et al., 2021).
It was also determined in our present study that C. maxima was richer in TPC than C. pepo.However, the amount of TPC was significantly reduced by boiling and baking compared to the raw prod-

| Mineral contents of processed pumpkins
The mineral contents of pumpkins are given in Table 3.Among the mineral substances, potassium (K) was the major mineral of pumpkins, ranging from 25.13 to 29.65 g in 100 g of DM.Abbreviations: DM, dry matter%; TDF, total dietary fiber % in dry matter; DPPH scavenging activity, μmol Trolox/100 g dry matter; TPC, mg gallic acid equivalent in 100 g dry matter.

TA B L E 3
The mineral content of processed pumpkins.Especially in the amount of Zn, remarkable differences were detected in the literature.The relationship between the mineral content in the soil or fertilizer and its reflection in the product should be investigated by agricultural experts.
When the mineral contents were examined by PCA analysis, two distinct groups-PC1 and PC2-were observed (Figure 1).According to PC1, the baked products of both pumpkins were in the positive area, and the raw and boiled types were in the negative area.The baked C. maxima in the positive area were dominant in Zn, Ca, P, and Cu, while baked C. pepo could be represented by the other minerals-Mn, Mg, Fe, and K.According to PC2, all processed products of C. maxima and the raw group of C. pepo were in the positive area, while the boiled and baked versions of C. pepo were located in the negative area.

| Physicochemical properties of yogurts incorporated with processed pumpkins
The pH and acidity values of yogurt samples for 14 days of storage are given in Table 4.The addition of pumpkin increased the pH in all products compared to the control.The acidity was not affected in the freeze-dried raw pumpkin-added products, but decreased in boiled and baked groups.Freeze-dried raw pumpkin-added yogurts were also better protected against pH decrease and acidity increase during storage compared to the control and the other samples.
When evaluated together with the DM values shown in Table 5, it was determined that there might be a relationship between the progress of pH and DM.In yogurt with higher DM, the pH decrease was also slower, probably due to a decrease in free water for microbial and chemical activities.
The other analyzed physicochemical properties of yogurts are given in Table 5. Sugar addition increased the DM of control, as expected.The DM values of freeze-dried raw pumpkin-added yogurts increased with the increasing amount of pumpkin and were significantly higher than the control, regardless of the pumpkin variety.In general, the DM of C. pepo-added yogurts was higher than C. maxima-added ones.When the protein was analyzed in total mass, higher amounts were encountered in freeze-dried raw products, probably because of higher DM.It was also observed that the change in pumpkin variety did not affect the protein ratios.Total ash was high in raw material-added yogurts and was similar to the control in other products.The DM increase in freeze-dried raw materialadded yogurts also increased the hardness of the products.Sugar
addition significantly reduced the hardness from 85.5 to 58.0 g in control samples.The boiled and baked pumpkin-added groups had hardness values similar to the PC.It was also determined that the hardness of the yogurts with C. maxima was higher than those with C. pepo.Contrary to our results,Yildiz and Ozcan (2019) reported a decrease in the hardness of yogurt with the addition of pumpkin puree baked at 85-90°C for 15 min, probably as a result of the presence of higher moisture in the final product due to the lower thermal processing than applied in our study.As observed in all yogurt groups of our present study, the WHC considerably increased with the addition of C. maxima compared to C. pepo.In addition, it was determined that the WHC values were independent of the DM.Similarly,Bakirci et al. (2017) measured the lowest WHC in control yogurt and reported an increase with increasing pumpkin concentration.When the color properties were examined in terms of L*, a*, and b*, the L* value decreased in all pumpkin-added products compared to control samples.When the differences between C. maxima and C. pepo were evaluated, the L* values were measured as similar or higher in C. maxima-added products.However, the variety of pumpkins significantly affected a* and b* values.The a* values of yogurts containing C. pepo were similar to the control samples, which were measured as −1.4 (in green area), while the value was between +2.2 and +9.5 (in red area) in products containing C. maxima.In addition,F I G U R E 1The PCA of the mineral content of processed pumpkins.Variation in the pH and acidity values of different yogurt formulations on days 1, 7, and 14.Different capital letters in the same column represent significant differences between samples in the same storage time, and the small letters in the same line represent significant differences in the property during the storage time.Product codes of NC and PC: control samples.R, B, O: raw, boiled, baked, respectively.Numerical value: concentration (i.e., 35: 3.5%, 250: 25%).P and M: C. pepo and C. maxima, respectively.
Badr et al. (2011)021)itive values of pumpkin leaves, it was reported that the amount of K remained the same with boiling in water but slightly decreased with steam blanching(Sunmonu et al., 2021).The Fe and Mn contents we examined did not decrease with heat treatment; on the contrary, they remained the same or increased.While Cu was resistant to boiling in C. maxima, in C. pepo it was damaged by both of the heat treatments.The Zn decreased by boiling in C. pepo, but remained the same in C. maxima.Badr et al. (2011)reported a Zn value of 320.5 mg/100 g DM in the pumpkin they analyzed, which is con-The DM, TDF, antioxidant capacity, and TPC results of processed pumpkins.Different capital letters in the same column represent significant differences between samples (p ≤ .05).
Vidhya et al. (2022)a, P, and Mg.While the amounts of K, Ca, P, and Mg were not affected by heat treatments in C. maxima, a reduction was observed in C. pepo.siderablyhigherthan that in our study.On the other hand, Ca, in the same study, which was reported as 3.67 g, and Fe and Cu minerals, which were reported as 91.33 and 16.25 mg per 100 g dry basis, respectively, were closer to the values in our study.Vidhya et al. (2022), on the other hand, found the Zn content of C. maxima and C. pepo as 7.4 and 1.3 mg, respectively, per 100 g of dry basis of pumpkin flesh.TA B L E 2

properties of yogurts with processed pumpkins
Note: Different capital letters in the same column represent significant differences between samples (p ≤ .05).Product codes of NC and PC: control samples.R, B, O: raw, boiled, baked, respectively.Numerical value: concentration (i.e., 35: 3.5%, 250: 25%).P and M: C. pepo and C. maxima, respectively.Abbreviations: DM, Dry matter; WHC, Water holding capacity; L*, a*, b*, color properties.the a* value of C. maxima-added products increased significantly with increasing pumpkin concentration.Similarly, b* values were in the direction of yellowness in all products, but adding C. pepo revealed similar results to the control, while C. maxima raised this value significantly.Bakirci et al. (2017) obtained color values in the yogurts with C. moschata, similar to our findings with C. maxima.Yogurts were evaluated according to color, texture, taste, flavor intensity, sweetness, and general acceptance, and the results are given in Table 6.While C. pepo was not liked in the color evaluation, The sensory properties of yogurt samples.