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. 2021 Jan 6;58(12):4616–4625. doi: 10.1007/s13197-020-04949-7

Effect of heating methods on astringency recurrence, syneresis, and physical properties of persimmon paste

Yoko Tsurunaga 1,✉, Misaki Onda 1, Tetsuya Takahashi 1
PMCID: PMC8479057  PMID: 34629526

Abstract

The soluble tannins in Japanese persimmon gives it an unpleasant astringent taste, which can be removed using certain conventional methods. However, these methods are not very efficient and lead to the recurrence of astringency upon heating. Thus, the use of persimmon in processed food is severely limited. Although the effects of heating time and temperature on astringency recurrence have been explored in detail, the effect of the heating method used has not yet been clarified. Considering this, we subjected an astringency-removed paste to various heat treatments, namely, stir frying, boiling, and pressurization, and examined their effect on astringency recurrence. Soluble tannin contents were also determined, and sensory evaluation was conducted. It was observed that stir frying, which is accompanied by moisture evaporation, is superior with respect to the suppression of astringency recurrence and the prevention of syneresis compared with the other two methods. Moreover, the bright orange color and the gummy texture obtained upon stir frying are favorable for commercial purposes. Therefore, it is expected that these findings will lead to the significant improvement of astringent persimmon processing.

Keywords: Persimmon paste, Heating methods, Astringency recurrence, Syneresis, Physical properties

Introduction

Japanese persimmon contains soluble tannins, which cause organoleptic astringency. This astringency is often removed via treatment with carbon dioxide, dry ice, alcohol, or other compounds before the fruit is sold or used in the form of pastes in the processing of various foods (Matsuo and Itoo 1982; Salvador et al. 2008; Yamada et al. 2002; Zhu et al. 2016). The typical methods that are used in persimmon astringency removal have limited efficiency and often lead to the recurrence of astringency upon heating (Taira and Takabayashi 2006; Ben‐Arie and Sonego 1993). This limits the use of persimmon in processed foods. Even though the phenomenon of astringency recurrence owing to heating is well-known, data on the extent of astringency recurrence with respect to the heating temperature, heating time, and heating method is scarce. Kitagawa (1969) reported that the heat treatment of immature and intact ‘Hiratanenashi’ persimmon in boiling water for 0–150 min led to the loss of astringency, which was regained when the heating time was extended (Kitagawa 1969). Lee and Lee (2012) also reported that operational parameters, such as heating conditions, temperature, and heating time have a significant effect on astringency recurrence in persimmon juice (Lee and Lee 2012). However, research on the effects of heating temperature and time on astringency recurrence in persimmon is limited and there are no detailed studies explaining the effects of different heating methods on the degree of astringency recurrence. Thus, to further expand the applications of astringent persimmon in processed food that require heat treatment, we examined the effect of the heating method as well as the temperature and time on astringency recurrence, syneresis, and the physical properties of persimmon using astringency-removed paste (ARP). We hypothesized that the application of a suitable heating method could reduce or completely prevent the recurrence of persimmon astringency in the ARP during further processing. Three ARP heating methods, namely, stir frying, boiling, and pressurization, were employed. Soluble tannin contents were measured, and a sensory evaluation by skilled panelists was conducted. In addition, the effect of heating on external appearance, syneresis, and physical properties of the paste was examined. The results suggest that a specific heating method can suppress heat-induced astringency recurrence, and it is expected that the applications of these experimental results will significantly improve the processing of astringent persimmon.

Materials and methods

Materials

Saijo persimmons (20 kg), a variety of persimmons that require astringency removal treatment, were harvested at the Shimane Agricultural Technology Center (Izumo City, Shimane Prefecture, Japan) in November 2017 and used as the test material. Immediately after harvesting, the fruits were placed in a 1 mm-thick polyethylene bag and enclosed in 100 g of dry ice (1% by weight), and astringency was removed at room temperature (20 ± 1.0° C) in a period of 4 days. The stalks and seeds of the astringency-removed fruits were then removed, and the fruits were cut into four pieces with the peel. Thereafter, they were ground for 2 min to obtain a smooth paste using an Osterizer 16-speed blender with a 1 L capacity (Sunbeam Oster, Florida, USA). This process was repeated until the 20 kg of harvested fruits was transformed into a paste. The paste was then stirred thoroughly for homogenization. After degassing, the homogenized paste was divided into 1 kg portions and stored in plastic bags (B-1318, Meiwa Pax Co., Ltd., Osaka, Japan) at − 25 °C until further analysis.

Heating method

Given that such a study has not been reported thus far, heating conditions similar to those employed in food processing were considered. A stir fry area at 100 °C was set on a hot plate, assuming that the paste would be heated as is the case during jam production. In addition, for cookies and cakes, a stir-fry area at 150 °C was set to simulate baking at approximately 150 °C. The 80, 90 and 100 °C sections were set assuming heat sterilization at normal pressure in the paste (De Roeck et al. 2009; Sandoval et al. 1994). Furthermore, a 121 °C section was set assuming the high-temperature sterilization of the retort pouches (Sevenich et al., 2013, 2014). The standard temperature and time for heating retort food are 121 °C and 4 min, respectively, and to sterilize the heat-resistant spores, heating at 121 °C for 20 min is typically performed. The procedure used in this study was as follows: first, the ARP (50 g) was fried on a hot plate (HI-1000, AS ONE Co., Osaka, Japan) with constant stirring to ensure uniformity. Thereafter, it was heated to 100 and 150 °C for 1, 5, and 10 min. At both 100 and 150 °C, considerable evaporation of water occurred after frying for 10 min, and further treatment was difficult. Thus, a treatment time of 10 min was considered. To boil the samples, the ARP (50 g) was put in a heat-resistant bag (B-1318, Meiwa Pax Co., Ltd., Osaka, Japan) and heated to 80 or 90 °C using a water bath (BS400, Yamato Scientific Co., Ltd., Tokyo, Japan), and to 100 °C using an induction heater (IH) (MR-B20, TOSHIBA Co., Tokyo, Japan) for 40 and 60 min, respectively. For the pressurization of the sample, ARP (50 g) was enclosed in an aluminum pouch (AL14, SEISANNIPPONSHA Ltd., Tokyo, Japan) and heated at 121 °C in an autoclave (BS-245, TOMY SEIKO Co., Ltd., Tokyo, Japan) for 4 and 20 min. In this study, non-heated samples were denoted as NH. This was repeated 3 times for 1 treatment area (Fig. 1).

Fig. 1.

Fig. 1

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Effect of different heating methods on the external appearance of astringency-removed persimmon paste. NH: Non-heated

External appearance and syneresis

A digital camera was used to observe the appearance of the heat-treated pastes (Takahashi et al., 2017; Tsurunaga et al., 2013). To observe the changes in the external appearance of the non-heated and heat-treated ARP samples, the ARP samples were placed in a stainless-steel dish (φ40 mm × H15 mm) and photographed using a digital camera (WG-40 W, Ricoh Co., Ltd., Tokyo, Japan). To examine the degree of syneresis, the centrifugation method employed in the preparation of yogurt, starch gel, etc. was adopted (Nguyen et al. 2017; Pongsawatmanit et al. 2006; Sanchez et al. 2010; Yarnpakdee et al. 2015). ARP (15 g) (W0) was placed in a 50 mL tube for the NH and heat treatment processes and was photographed using a digital camera (WG-40 W, Ricoh Co., Ltd., Tokyo, Japan) after centrifugation (50B-7, Sakuma Manufacturing Co., Ltd., Tokyo, Japan) at 3000 rpm for 5 min (Fig. 2). The supernatant was carefully collected using a Pasteur pipette, and its weight (W1) was measured for the determination of syneresis, which was calculated as W1 / W0 × 100 (Sanchez et al. 2010).

Fig. 2.

Fig. 2

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Effect of different heating methods on a the syneresis, and b syneresis rate of astringency-removed persimmon paste. NH: Non-heated. Means are shown, with vertical bars indicating standard error (n = 3). The different letters indicate statistical differences (P < 0.05)

Physicochemical evaluation of astringency

The soluble tannin content of the samples was measured and used as an index of astringency because of the high correlation between tannin content and organoleptic astringency in humans (Amorim et al. 2020). To determine the soluble tannin content, 80% methanol was added to 5 g of ARP, and the mixture was ground. Each sample solution was appropriately diluted, and the soluble tannin content was measured using the Folin method (Goldstein and Swain, 1965) and analyzed by partially modifying the method proposed by (Chung et al. 2015). The extract (90 μL), the Folin-Ciocalteu reagent (90 μL), and 10% sodium carbonate (90 μL) were mixed in a 96-well microplate. After incubation for 60 min at room temperature (20 ± 1.0 °C), the reaction color was measured using a microplate-reader (SH-9000Lab, CORONA ELECTRIC Co., Ltd., Ibaragi, Japan) at 690 nm. The soluble tannin content was expressed proportionally as mg of ( +)-catechin per 100 g of fresh fruit (mg/100 g FW).

Sensory evaluation of astringency

Fifteen members of Shimane University participated in the sensory evaluation. The Kallithraka's method (Kallithraka et al. 2011), which is related to the sensory evaluation of wine astringency, was partially modified and employed in this study. All the members who participated in the sensory evaluation are experienced in ARP astringency evaluation and are familiar with the evaluation procedure. Each sample was placed in the mouth for 30 s and the astringency was evaluated. Before moving on to the next sample, the mouth was rinsed thoroughly with water and a 5 min break was observed. With this break, it was possible to minimize the impact of the previous evaluation on the following sample. Astringency was scored on a 4 (0–4) point scale, with 4 indicating the highest level of astringency and 0 indicating the absence of astringency.

Physical properties (hardness and gumminess)

Hardness and gumminess were measured using a creep meter (RE2-33005B, YAMADEN Co., Ltd., Tokyo, Japan). The samples were placed in a stainless-steel dish (40 mm diameter), and each sample (with a height of ~ 15 mm) was compressed twice with a resin plunger (20 mm diameter) at a speed of 10 mm/s. The distortion rate was determined to be 66.67%, and the firmness and gumminess were calculated. Measurements were obtained using a 20 N load cell at 20 ± 1 °C (Takei et al. 2016; Watanabe et al. 2018). This test sequence was performed on 5 samples from each heat treatment.

Soluble solid content

A digital refractometer (Model PEN-SW, Atago Co. Ltd., Tokyo, Japan) was used to measure the soluble solid content of the pastes. The refractometer readings are expressed as percentage total soluble solids by weight (Guo et al. 2015).

Statistical analysis

Statistical analyses were performed using SPSS software version 25.0 (IBM Inc.), and the results were expressed as mean ± standard error. The data were subjected to one-way analysis of variance (ANOVA) to determine the differences between samples. Significant differences were compared followed by Tukey’s test at a significance level of P < 0.05 (Donmez et al. 2017; Sanli et al. 2011).

Results and discussion

External appearances and syneresis

The external appearances of the different sample are shown in Fig. 1. The NH samples and the stir-fried samples were bright orange in color. However, the color changed to dark orange after boiling and pressurization. Pressurization at 121 °C for 4 and 20 min imparted a dark color to the samples. The orange color of persimmons is due to the presence of carotene (Homnava et al. 1990; Zhao et al. 2011). However, as carotene is known to be relatively heat-resistant (Ghosh et al. 2019), other factors were presumed to be responsible for the color change to dark orange. Persimmon fruits accumulate high-molecular-weight proanthocyanidins in their "tannin cells" during their growth (Ikegami et al. 2007). These proanthocyanins turn dark when oxidized (Ikegami et al. 2009). Thus, the dark color resulting from pressurization at 121 °C for 4 and 20 min may be due to the solubilization of tannins and the oxidation of proanthocyanidins during heating. The external appearance and syneresis of each sample after centrifugal separation are shown in Fig. 2a and b. The sample pressurized at 121 °C for 4 min exhibited syneresis. A considerable degree of syneresis was also observed in the sample pressurized for 20 min. However, the stir-fried sample showed no syneresis. Edible persimmon fruit paste is widely used, and its commodity value depends on factors such as the retention of its bright color and the absence of syneresis in the product after processing. Pectin in fruits greatly affects the their physical properties.; it forms a gel that holds water (Willats et al. 2006). However, it is also known that heating significantly increases its solubilization and depolymerization (Van Buggenhout et al. 2009). In this study, the syneresis of samples pressurized at 121 °C for 4 and 20 min with a high degree of heating may be due to the decomposition of pectin. These results suggest that stir frying is the most suitable heating method for the prevention of syneresis.

Recurrence of persimmon astringency

Results of the physicochemical evaluation of astringency using the Folin method and the results of the sensory evaluations are shown in Fig. 3 and Fig. 4, respectively. Human sensory perception allows the recognition of astringency based on taste at soluble tannin contents above 100 mg/100 gFW (0.1%). In this study, the samples that were boiled at 100 °C for 40 min 60 min showed soluble tannin contents above 100 mg/100 gFW (i.e., 111.33 ± 1.39 mg/100 and 178.30 ± 4.59 mg/100 gFW, respectively), as shown in Fig. 3. Soluble tannin contents above 100 mg/100 gFW were also observed in the samples pressurized at 121 °C for 4 min (295.44 ± 3.93 mg/100 gFW) and 20 min (465.99 ± 3.64 mg/100 gFW) (Fig. 4). Based on the sensory evaluation, stir-boiling (at 90 °C for 60 min, 100 °C for 40 min, and 100 °C for 60 min) and pressurization (at 121 °C for 4 and 20 min) were statistically significant compared to NH. The results of the sensory evaluation were mostly consistent with the physicochemical astringency data acquired using the Folin method. However, the results of the sensory evaluation of the stir-boiling experiment at 90 °C for 60 min differed from the results obtained by the Folin method. The soluble tannin content obtained using the Folin method after boiling at 90 °C for 60 min was below 100 mg/100 gFW. Although the astringency should be negligible at such a soluble tannin content in the sensory evaluation, a slight astringency was felt. This discrepancy between the Folin method and the sensory evaluation results at 90 °C for 60 min may be because of panelists who are significantly sensitive to astringency. For treatments other than that at 90 °C for 60 min, the results of the sensory evaluation were consistent with those obtained using the Folin method. Thus, the soluble tannin content estimated using the Folin method is a suitable index of astringent taste in humans. There have been no reports revealing the recurrence of persimmon astringency due to heat treatment using anything other form of persimmons other than intact persimmons (Kitagawa, 1969) or persimmon juice (Lee and Lee, 2012). Studies on the evaluation of the efficiency of heating methods, such as stir frying, boiling, and pressurization, on the degree of persimmon astringency recurrence using persimmon paste, which is widely used commercially, are relatively scarce. Thus, the processing of persimmon paste is potentially more useful than the processing of persimmon juice or intact persimmon. Considering its practicality for food processing, this experiment shows that boiling at temperatures below 90 °C is preferable at normal pressure. Furthermore, stir frying (at maximum 150 °C) showed no recurrence of persimmon astringency at any temperature or for any heating duration, which was in contrast to the observations made after boiling (100 °C) and pressurization (121 °C). The key differences among stir frying, boiling, or pressurization were heating time, heating temperature, and evaporation. No evaporation was observed when the ARP was boiled or pressurized during heating, including when the process was conducted in the bag. However, the ARP was not stir-fried in a bag, and this led to a decrease in water content due to evaporation. The water contents of the ARP samples that were stir fried for 1, 5, and 10 min at 100 °C were 76.2 ± 0.1%, 54.6 ± 0.5%, and 28.7 ± 1.4%, respectively; the corresponding values at 150 °C were 64.0 ± 0.9%, 50.6 ± 1.7%, and 25.3 ± 0.5%, respectively (n = 3). These values were lower than the water content of the NH sample (78.3 ± 0.2%), indicating that heating for 5 min or more at 100 °C and for 1 min or more at 150 °C are the conditions required to evaporate moisture during stir frying. Additionally, it was observed that limiting the stir-frying time to 10 min was the only way to prevent evaporation and burn-deposit formation. Assuming that persimmon astringency recurrence is dependent on the heating time and temperature, pressurization at 121 °C for 4 min should not lead to astringency recurrence, considering that the degree of heating is less than that used for stir frying (150 °C for 5 and 10 min). However, pressurization at 121 °C for 4 min resulted in astringency recurrence, indicating that persimmon astringency recurrence is independent of the heating time and temperature. The mechanism of persimmon astringency recurrence involves the solubilization of insoluble tannins upon heating, which is believed to occur even after the initial removal of astringency by treating the fruits with carbon dioxide, dry ice, and alcohol, or by other related methods (Ben‐Arie and Sonego, 1993; Taira and Takabayashi, 2006). Some studies have also claimed that astringency removal via the treatment of persimmon fruits with carbon dioxide, dry ice, alcohol, or using other related methods results in the chemical bridging of insoluble tannins with acetaldehyde (Matsuo and Itoo, 1982) and complexation between pectin in the cell walls of the fruits and insoluble tannins (Taira et al., 1997). Thus, we reasoned that stir frying had no effect on astringency regardless of regardless of the heating time and temperature. This is because the pectin content increased due to evaporation during stir frying, resulting in an immediate complexation between pectin in the cell wall and the insoluble tannins. However, further studies are necessary to further elucidate this mechanism, which involves the removal of astringency regardless of the heat treatment condition. Overall, these experiments allowed us to elucidate the impact of heating methods on persimmon astringency recurrence and led us to conclude that stir frying can remove astringency regardless of the high-heat treatment method employed, in contrast to what was observed for boiling and pressurization.

Fig. 3.

Fig. 3

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Effect of different heating methods on the soluble tannin content of persimmon paste. Means are shown, with vertical bars indicating standard error (n = 12). The different letters indicate statistical differences (P < 0.05)

Fig. 4.

Fig. 4

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Effect of different heating methods on the sensory evaluation of astringency-removed persimmon paste. Means are shown, with vertical bars indicating standard error (n = 15). The different letters indicate statistical differences (P < 0.05)

Physical properties and Brix

The impact of the heating method on the hardness and gumminess of ARP are shown in Fig. 5a and B, respectively, while Fig. 6 shows the Brix values, which are indicators of soluble solid contents. The hardness did not vary significantly with boiling and pressurization, and no significant difference was observed with respect to the NH sample (P < 0.05). On the other hand, the hardness of the sample obtained after stir frying at 100 °C for 5 and 10 min were 11,416.7 ± 409.3 and 43,184.0 ± 4918.9 N/m2, respectively, while at 150 °C, the corresponding hardness values following stir frying for 5 and 10 min were 28,510.0 ± 4077.6 and 40,096.4 ± 7427.6 N/m2 respectively. These values were significantly different compared with that of NH (P < 0.05). In addition, the changes in the gumminess value of the samples following boiling and pressurization were not significant compared with that of NH (P < 0.05). However, the values obtained following stir frying at 100 °C for 5 and 10 min and at 150 °C for 1, 5, and 10 min were 1758.7 ± 1008.0, 4369.10 ± 216.1, 1458.2 ± 210.9, 2678.0 ± 455.9, and 4365.8 ± 898.1 Pa, respectively. These values are remarkably high and are significantly different compared with that of NH (P < 0.05). Further, the hardness and gumminess of the stir fried samples were higher as the heating time was extended and as the heating temperature was increased. However, the experiments conducted at 100 or 150 °C for 10 min showed no significant differences. The key difference between stir frying and boiling or pressurization was that stir frying was accompanied by evaporation during the heat treatment. Additionally, during boiling or pressurization, there was no evaporation because the paste was heated in a bag, while during stir-frying, the paste was heated in an open state; this evaporation resulted in a significant decrease in water content with time. Soluble solid contents (Brix) were measured to corroborate this observation. Generally, Brix increases as moisture evaporation increases (Hurtado et al. 2002). Figure 6 indicates that the Brix value did not change during boiling and pressurization, nor did it change for NH. However, it increased with increasing stir frying time, and as the heating time and temperature increased, the highest value (42.0 ± 0.75) was attained following stir frying at 150 °C for 10 min. The Brix values correlated well with the hardness and gumminess values. The high hardness and gumminess values of the stir-fried sample could be attributed to moisture evaporation from ARP during the heat treatment. (Vieira et al., 2008) reported that the decrease in water content during candy production brings about an increase the solid content, resulting in changes physical properties. Thus, it could be inferred that in this experiment, the water content decreased or the Brix increased only in the stir-fried area, resulting in the significant changes in the observed hardness and gumminess of the stir-fried sample. It can be further concluded that persimmon pastes with low water contents might be not suitable for the preparation of high moisture containing jellies or jams. However, the paste obtained via stir frying is highly suitable for ice creams, cookies, and breads, which preferably require low moistures. Thus, as hypothesized in the introduction, stir frying can significantly improve the quality of processed persimmon, and considering the recurrence of persimmon astringency during heating, it is the best approach for manufacturing high-quality persimmon paste, with respect to external appearance, syneresis, and physical properties. The study further highlights that it is possible to expand these applications of astringent persimmon to food processing.

Fig. 5.

Fig. 5

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Effect of different heating methods on a Hardness and b Gumminess of astringency-removed persimmon paste. Means are shown, with vertical bars indicating standard error (n = 5). The different letters indicate statistical differences (P < 0.05)

Fig. 6.

Fig. 6

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Effect of different heating methods on the Brix of astringency-removed persimmon paste. Means are shown, with vertical bars indicating standard error (n = 5). The different letters indicate statistical differences (P < 0.05)

Conclusion

The effects of different heating methods (stir frying, double boiling, and pressurization) on astringency recurrence (soluble polyphenol content, sensory evaluation) in ARP samples were examined. Additionally, the effects of heating on the external appearance, syneresis, and the physicochemical properties of ARP were also examined. The recurrence of astringency upon heating of persimmons that were already treated for astringency was considered a normal phenomenon. However, this study revealed that astringency recurrence and syneresis did not occur when stir frying (which involves moisture evaporation) was used as a heating method. Further, this heating process resulted in the formation of a firm, gummy paste with a favorable color. Moreover, the study revealed that for a heating temperature below 90 °C, even boiling resulted in the formation of a paste with a favorable color. However, the paste obtained after boiling was not as firm or gummy as that produced via stir frying. Therefore, this study reveals a process that can suppress astringency recurrence due to heating. We expect that these findings will significantly improve astringent persimmon processing.

Acknowledgments

This work was supported by JSPS KAKENHI Grant Number JP 16K00814, the Shimane University Grant for Joint Research Project led by Female Researchers under the MEXT Initiative for Realizing Diversity in the Research Environment (Collaboration Type) and FY2020 Shimane University Internal Competitive Grants.

Compliance with ethical standards

Conflicts of Interest Statement

There are no conflicts of interest to declare.

Informed consent

All the study participants provided informed consent.

Footnotes

Publisher's Note

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Contributor Information

Yoko Tsurunaga, Email: ytsurunaga@hmn.shimane-u.ac.jp.

Misaki Onda, Email: m.scheff-120@yahoo.ne.jp.

Tetsuya Takahashi, Email: takahashi@hmn.shimane-u.ac.jp.

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