2020 年 26 巻 3 号 p. 339-350
We applied a vacuum microwave drying (VMD) process to shiitake mushrooms under different pressure conditions below 20 kPa, and investigated drying time, hardness and rehydration characteristics, extracted guanylic acid, color, and sensory evaluation. The samples were subjected to different microwave treatments at different levels of power (25 W/g dry matter, 50 W/g dry matter, and 75 W/g dry matter) and absolute pressures (3 kPa, 10 kPa, and 20 kPa). VMD reduced drying time by approximately 13 to 70 times compared with hot air drying (HAD). Decreasing the pressure tended to soften the rehydrated samples, whereas the amount of guanylic acid extracted from samples treated at 20 kPa was the largest. The sensory evaluation score of VMD-treated shiitake mushrooms was larger than HAD-treated mushrooms. Collectively, these results suggest that VMD is more suitable and useful than HAD in producing high quality dried shiitake mushrooms.
Shiitake mushrooms (Lentinula edodes), which grow naturally throughout the year, have been cultivated as a food source for over 600 years. It is currently the second most widely cultivated edible mushroom worldwide, representing about 25% of global mushroom production, and has a production expansion exceeding that of any other cultivated mushroom species (Boa, 2004; Jiang et al., 2015). In Japan, where shiitake mushrooms have an annual production of approximately 70 000 t, it is the third most cultivated edible mushroom (i). Shiitake mushrooms contain significant amounts of nutrients, such as bioactive polysaccharides (lentinan), dietary fiber, ergosterol, vitamins B1, B2, and C, and minerals (Choi et al., 2006). Several studies have shown it to also have useful medical attributes, including antitumor and antimicrobial properties, and the ability to improve liver functioning, lower cholesterol (Takehara et al., 1979; Mizuno et al., 1995; Fukushima et al., 2001), lower blood pressure, and strengthen the immune system against disease, including viral infections (Regula and Siwulski, 2007).
The high moisture content of fresh shiitake mushroom (85% water) makes it highly perishable; degradation starts within a few days after harvesting. It is therefore dried to extend its shelf life (Wu and Wang, 2000; Kimura and Kamewada, 2008; Therdthai and Zhou, 2009; Kantrong et al., 2014). The drying process enables water to be removed, which halts or impedes not only the growth of microorganisms that cause spoilage, but also the occurrence of biochemical and chemical reactions (Venkatachalapathy and Raghavan, 2000; Vega-Mercado et al., 2001; García-Segovia et al., 2011; Tian et al., 2016). Drying also improves some of the edibility and nutrient characteristics of shiitake mushrooms compared with the fresh product; the dried product contains greater amounts of some nutrients, such as vitamin D2 (Kawano, 1992; Mau et al., 1998; Jasinghe and Perera, 2006), and also has a superior umami and flavor due to the presence of guanylic acid and lenthionine (Mouri et al., 1966; Morita, 1967; Hiraide et al., 2004; Yang et al., 2019).
Hot air drying (HAD), a common method used in preparing shiitake mushrooms, generally involves drying the mushrooms at 40 °C to 60 °C for 14 h to 20 h (Yoshida et al., 1987; ii). However, HAD has some disadvantages, such as its low energy efficiency and lengthy drying time; there are also concerns about quality degradation of shiitake mushrooms dried using this method. Traditional drying methods, including HAD, also cause many unfavorable changes in plant products, such as excessive shrinkage, discoloration, oxidation of functional ingredients, and severe degradation of nutritional and sensorial properties. Therefore, the application of new drying techniques using low temperatures and a reduced drying time have been proposed (Dehnad et al., 2016; Orikasa et al., 2018).
Microwave drying (MD) is one such alternative drying method. It has several advantages compared with HAD, including the availability of uniform energy to the inner sides of the material, inhibition of enzymatic degradation, a shorter drying time and greater energy efficiency, reduced floor space requirements, and faster startup and shutdown conditions (Yongsawatdigul and Gunasekara, 1996; Bal et al., 2010; Hirun et al., 2014; Kantrong et al., 2014; Sorour and El-Mesery, 2014; Dehand et al., 2016; Orikasa et al., 2018). A second alternative is vacuum microwave drying (VMD), which has been the subject of research due to its potential for providing high quality dried food products, such as fruits, vegetables, grains, and mushrooms. Processing times are reduced in VMD. This is because of the large vapor pressure difference between the core and surface of the dried material, which enables rapid removal of internal moisture, while penetration of the entire sample by microwaves stimulates water vibrations, resulting in the generation of internal heat (Chong et al., 2014). Decreasing pressure reduces the water boiling point, therefore lowering the drying temperature (Durance and Wang, 2002). This low drying temperature prevents or slows chemical reactions associated with reduced food quality, and reduces thermal oxidation of the product (Orikasa et al., 2014; Wojdylo et al., 2014; Orikasa et al., 2018). In a previous study, VMD was used to process shiitake mushrooms and the physical parameters, such as the drying time, hardness, and moisture ratio were evaluated; however, the pressure condition applied was above 20 kPa. In addition, there have been no reports providing a sensory evaluation of shiitake mushrooms dried using VMD, including the umami component (guanylic acid) (Kantrong et al., 2014). Our objective was therefore to apply the VMD process to shiitake mushrooms at different pressure conditions below 20 kPa, and to evaluate not only physical, but also chemical parameters.
Materials Shiitake mushrooms (L. edodes) were purchased from a local market in Morioka City and stored in a refrigerator at 4 °C for a maximum of 3 days. Stems were removed by cutting prior to drying. The initial moisture content of the shiitake mushrooms was obtained according to the AOAC method (AOAC, 1995) using a hot air oven at 105 °C for 24 h to calculate the dry basis (db) and wet basis (wb) content, and was determined to be 10.74 ± 0.45 g water/g dry matter [db] (=0.9147 ± 0.0034[wb]). For comparison, shiitake mushrooms dried using HAD were also purchased from some local markets in Morioka City. We used some HAD-treated shiitake mushrooms (Donko in Iwate; Japan Agricultural Cooperatives-Iwate-Hanamaki, Japan; Dried shiitake mushroom in Takachihokyo; Sugimoto Co., Ltd., Japan; Koshin in Oita; Ohsho Shiitake Co., Ltd., Japan; Dried shiitake mushroom; Ohsho Shiitake Co., Ltd., Japan; Dried shiitake mushroom in Iwate; Hop Step Inc., Japan) due to sample availability.
Drying method The VMD experiments were conducted in a vacuum microwave instrument (Fig. 1). The system consisted of an oil rotary vacuum pump (TSW-100; Sato Vac Inc, Japan), a cold trap (UT-3000; EYELA, Japan), a vacuum control unit (NVC-2100L; EYELA, Japan) and a microwave oven (µReactor Ex; Shikoku Instrumentation Co., Ltd., Japan). The temperature within samples during the drying process was measured using a thermometer (FL-2000; Anritsu Meter Co., Ltd., Japan) with a 1.0-mm diameter fiber optic thermosensor (FS-100; Anritsu Meter Co., Ltd., Japan). Three samples of shiitake mushroom, weighing a total of 50 g, were placed in the vacuum chamber, which was constructed of borosilicate glass and contained within the microwave oven. The samples were subjected to different microwave treatments at different levels of power (25 W/g dry matter, 50 W/g dry matter, and 75 W/g dry matter) and absolute pressures (3 kPa, 10 kPa, and 20 kPa). Microwave was irradiated after the pressure reached set values, and the target pressure was reached within 2 minutes. Drying was carried out under the following conditions: 3 kPa, 25 W/g dry matter (3–25); 3 kPa, 50 W/g dry matter (3–50); 3 kPa, 75 W/g dry matter (3–75); 10 kPa, 25 W/g dry matter (10–25); 10 kPa, 50 W/g dry matter (10–50); 10 kPa, 75 W/g dry matter (10–75); 20 kPa, 25 W/g dry matter (20–25); 20 kPa, 50 W/g dry matter (20–50); and 20 kPa, 75 W/g dry matter (20–75). The samples were dried until a final moisture content of 13% wb was achieved (Xue et al., 1996; Tian et al., 2016). For each measurement, we compared the samples and results of our experimental drying conditions with the HAD-treated shiitake mushrooms purchased from a local market.
Schematic diagram of the experimental apparatus used for vacuum microwave drying.
Hardness measurement The hardness of the rehydrated samples was measured using a hardness tester (TPU-2C; Yamaden Co., Ltd., Japan) at three different areas on a single lamella from each sample. In this study, the mean values of the measured data were described as hardness values. The hardness analysis was conducted following the methods described by Kotwaliwale et al. (2007), Giri and Prasad (2007), and Kantrong et al. (2014), with the following instrument parameters: speed, 10 mm/s; strain, 30% of sample height; diameter of cylinder probe, 3 mm. The tester was linked to a computer that recorded and analyzed the data using TPU Analysis Windows Ver. 2 (TA-TPU2; Yamaden Co., Ltd., Japan). Measurements were made in triplicate for each sample.
Rehydration ratio The rehydration ratio was determined by immersing the dried samples into distilled water (50 mL/g dry sample) at 4 °C for 5 h. The dried mushroom rehydration ratio was calculated according to Equation 1:
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Microstructure analysis The microstructure of the dried samples was observed using scanning electron microscopy (SEM) (JSM-7800; JEOL Ltd., Japan) to formulate a backscattered electron image at an accelerating voltage of 5 kV and magnification of ×500. Cross sections of the caps of dried samples were used for this procedure. Samples were prepared as follows. Specimens were cut into squares, with each side measuring 5 mm. These samples were soaked in 2.5% glutaraldehyde fixing solution overnight to achieve prefixation, and then treated with 1% osmic acid fixing solution for 1 h to achieve postfixation, after washing with phosphate buffer. Following postfixation, alcohol dehydration was undertaken, and the resin was embedded in an epoxy resin. An ultramicrotome was used to prepare thin sections (about 1 µm) from the samples, which were mounted on glass slides and were electron-stained with lead and uranium, after which the SEM observation was performed.
Guanylic acid extraction measurement The method for measuring the guanylic acid extraction was adapted from Kurosu and Iwaguro (2008), using high performance liquid chromatography (HPLC) equipped with a degasser (DGU-20A3; Shimadzu Co., Ltd., Japan), pump (LC-20AD; Shimadzu Co., Ltd., Japan), and detector (SPD-20A; Shimadzu Co., Ltd., Japan). The following instrument parameters were used in the HPLC analysis: mobile phase acetonitrile (ACN), 5 mM TBA-Br/20 mM (NH4)2HPO4 = 20:80; UV wavelength, 225 nm; flow rate, 1 mL/min; column (Cadenza CD-C18, 4.6 × 250 nm; Imtakt Corp., Japan). The method for creating samples was also adopted from Kurosu and Iwaguro (2008). Guanylic acid was extracted by immersing dried shiitake mushroom into distilled water (50 mL/g dry sample) at 4 °C for 5 h supposing shiitake mushroom broth, and then filtrated with a 0.45-µm filter and injected with 10-µL amounts into the HPLC. Each measurement was made in triplicate.
Sample temperature measurement Changes in temperature within samples during the drying process were measured at 1 s intervals using the FL-2000 thermometer with the 1.00 mm diameter FS-100 fiber optic thermosensor. In order to measure continuous changes in temperature within the sample, a drying end time was set based on measurements of the drying time required to achieve a final moisture content of 13% wb or less, with the average value taken as the drying end time. Measurements of each sample were repeated five times.
Color measurement L*, a* and b* coordinates were used to determine the color of the samples using a colorimeter (CR-13 Chroma Meter; Konica-Minolta Inc., Japan) at three different areas on a single lamella from each sample before and after drying treatments. The mean values of the measured data were designated as color values in this study, and expressed as L* (whiteness/darkness), a* (redness/greenness), b* (yellowness/blueness). The total color difference (ΔE) was calculated according to Equation 2:
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Sensory evaluation The sensory evaluation was undertaken using a panel of six participants (three males and three females; mean age, 22.5 years) recruited for this study from Department of Agriculture students attending Iwate University. The participants were selected based on the results of a taste sensitivity test (Furukawa, 2012) that determines whether a subject is able to distinguish between the five tastes (sweetness, bitterness, sourness, umami, and saltiness). Informed consent to participate in the sensory evaluation was obtained from all participants. The sensory evaluation of this study was approved by the Research Ethics Committee of Iwate University (No. 201915). The panel agreed to evaluate the sensory attributes of the samples, including softness, umami, color and appearance, flavor, and total satisfaction. The samples used were caps and soups of dried shiitake mushrooms produced by the following drying conditions: 3–25, 3–75, 20–25, and 20–75. The cap samples were immersed into distilled water (50 mL/g dry sample) at 4 °C for 5 h and cooked with superheated steam at 120 °C for 15 min in a water oven (AX-PX3; Sharp Co., Japan); the soups were the extraction liquid. The five sensory attributes of each dried shiitake mushroom were scored on a five-point scale: +2 (soft, strong or good), +1 (slightly soft, strong or good), 0 (normal), −1 (slightly hard, weak or bad), and −2 (hard, weak or bad). The score results were used for comparison with the purchased HAD-treated shiitake mushrooms (score 0). The scores allocated by each subject to each attribute were summed, and the results averaged as the total scores for each of the five attributes. During the evaluation, distilled water was provided to participants to cleanse their palate between sample tastings.
Statistics Quantitative data are presented as mean ± SE. Tukey-Kramer tests with a significance level of p < 0.05 were applied for the statistical analyses, using the Excel statistical software package (BellCurve for Excel version 2.11; Social Survey Research Information Co., Ltd., Tokyo, Japan).
Drying time The drying times determined for VMD-treated shiitake mushrooms are shown in Fig. 2. HAD-treated shiitake mushrooms have a drying time ranging from approximately 14 h to 20 h (Yoshida et al., 1987; Xue et al., 1996; Zhao et al., 2019; ii), whereas that for VMD-treated shitake mushrooms was approximately 0.2 h to 1.1 h, and drastically shortened by approximately 13 to over 70 times compared to HAD. Drying time shortened with decreasing pressure and increasing microwave power, with the latter being particularly influential. During VMD under low pressure conditions with microwave heating moisture transport was substantially accelerated by the increase in vapor pressure difference between the sample surface and core of the dried material (Lin et al., 1998; Chong et al., 2014; Orikasa et al., 2018). Therefore, VMD treatment shortened the drying time compared with HAD.
Drying time for shiitake mushrooms treated by vacuum microwave drying (VMD) at different pressures (n = 3–4). The bars show mean ± SE. Lower case letters above each bar represent significant differences (p < 0.05).
Hardness and rehydration characteristics The results for the hardness of rehydrated VMD-treated shiitake mushrooms are shown in Fig. 3. When pressure was fixed, there was no significant difference in hardness detected between different microwave power conditions; therefore, microwave power had very little effect on hardness. In contrast, changes in pressure had a large effect, with hardness decreasing as pressure decreased.
Hardness of shiitake mushrooms rehydrated after treatment with vacuum microwave drying (VMD) at different pressures (n = 3). The bars show mean ± SE. Lower case letters above each bar represent significant differences (p < 0.05).
In order to investigate the reasons for the influence of pressure on hardness, we measured rehydration characteristics, such as the rehydration ratio and microstructure (Fig. 4). Under fixed pressure conditions, rehydration was hardly affected by microwave power, similar to the results for hardness. However, a large effect of changes in pressure on rehydration was observed; rehydration increased as pressure decreased, with a significant difference shown between 3 kPa and 20 kPa (p < 0.05).
Rehydration of shiitake mushrooms treated with vacuum microwave drying (VMD) at different pressures (n = 3). The bars show mean ± SE. Lower case letters above each bar represent significant differences (p < 0.05).
Scanning electron micrographs of VMD-treated shiitake mushrooms are shown in Fig. 5. Cells were scattered and pore sizes were large under conditions of lower pressure and higher microwave power; however, the cells aggregated with increasing pressure and microwave power. These results suggested that a decrease in pressure and microwave power prevented cell aggregation, with the latter being particularly influential. By reducing chamber pressure, it is possible that the pressure differential between the core and surface of the dried material was increased, and the outward force also increased. Therefore, this produced puffing characteristics (Lin et al., 1998; Sham et al., 2001; Durance and Wang, 2002; Zhang et al., 2007; Bai-Ngew et al., 2011). Sham et al. (2001) and Durance and Wang (2002) reported that low density samples (i.e., with a large pore size) were produced by the puffing effect. Puffing characteristics were produced with reduced chamber pressure, and prevented cell aggregation and caused enlarged pore size. Consequently, the rehydration ratio increased with decreased pressure because of increased water absorption due to the larger pore size.
Scanning electron micrographs of VMD shiitake mushrooms treated with vacuum microwave drying (VMD). Scale bar 50 µm. Samples are shown at: (a) 3 kPa-25 W (pressure and microwave power, respectively); (b) 3 kPa-50 W; (c) 3 kPa-75 W; (d) 10 kPa-25 W; (e) 10 kPa-50 W; (f) 10 kPa-75 W; (g) 20 kPa-25 W; (h) 20 kPa-50 W; and (i) 20 kPa-75 W.
These results show that decreasing the pressure produces puffing characteristics in the samples, prevents cell aggregation and enlarges the pores, and promotes rehydration characteristics, therefore tending to soften the rehydrated samples.
Extracted guanylic acid The guanylic acid extraction amounts for the VMD-treated shiitake mushrooms are shown in Fig. 6. The extraction amounts increased with increasing pressure and microwave power, with pressure being particularly influential; however, there were no significant differences in the guanylic acid extraction amount. The guanylic acid extraction amount for HAD-treated shiitake mushrooms was 142.02 ± 7.27 mg/100 g dry shiitake mushroom. There were no significant differences between HAD and VMD. However, the mean values of the extraction amount from the VMD samples treated at 20 kPa tended to be larger than those HAD treatment, probably due to the increase in guanylic acid production.
Guanylic acid extraction amount in shiitake mushrooms treated with vacuum microwave drying (VMD) at different pressures (n = 3). The bars show mean ± SE. Lower case letters above each bar represent significant differences (p < 0.05).
In shiitake mushrooms, guanylic acid production is related to ribonuclease (RNase) activity; guanylic acid is produced by the degradation of RNA, with the optimal temperature for RNase being approximately 60 °C (Mouri et al., 1966; Endo et al., 1980; Sawada, 1998). Sawada (1998) reported that the quantity of nucleotides, including guanylic acid, began to increase inside shiitake tissue at temperatures above 50 °C, increased significantly from 60 °C to 70 °C, and then decreased above 70 °C. It has also been reported that the degradation of RNA began above 50 °C, and almost all of the RNA was degraded during incubation at 60 °C, with 70% to 80% or more of RNA being degraded in the first 10 min of incubation. For this reason, we measured temperature changes within samples during the drying process because of the significant effect of temperature on guanylic acid production.
The temperature results for VMD-treated shiitake mushroom samples are shown in Fig. 7. The inner temperatures rose to approximately 40 °C at 3 kPa, 50 °C at 10 kPa, and 60 °C at 20 kPa after microwave irradiation, and remained at an almost constant temperature during drying. These temperatures were close to the boiling point of water relevant to each pressure. The temperature (T), taken to represent saturated vapor pressure (esat) at 3 kPa, 10 kPa, or 20 kPa, was 29.0 °C, 45.8 °C, and 60.1 °C, respectively, based on Equation 3 (Tetens, 1930).
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Temperature of shiitake mushroom samples treated with vacuum microwave drying (VMD) at different pressures. The samples are shown at pressures of (a) 25 W; (b) 50 W; and (c) 75 W (n = 5).
Although the temperatures calculated from Eq. 3 were close to the measured inner temperature, there were some differences, probably due to thermal conduction from the chamber and heat transfer from water vapor in the chamber. We used the inner sample temperature data to calculate the cumulative time by adding the time spent in the temperature zone from 60 °C to 70 °C, which is observed as the largest increase in guanylic acid production.
The cumulative time results for VMD-treated shiitake mushrooms in conditions between 60 °C and 70 °C are shown in Fig. 8. There was a long cumulative time observed for 20 kPa, with a significant difference for the 20–25 and 20–50 treatments (p < 0.05). The long cumulative time from 60 °C to 70 °C suggests that RNase can be activated and guanylic acid production can increase. Therefore, the reason for the largest guanylic acid extraction amounts being observed at 20 kPa was presumed to be the increase in guanylic acid production.
Cumulative time from 60 °C to 70 °C for shiitake mushrooms treated with vacuum microwave drying (VMD) under different pressures (n = 5). The bars show mean ± SE. Lower case letters above each bar represent significant differences (p < 0.05).
These results show that the reason for the largest guanylic acid extraction amounts being found for the VMD-treated shiitake mushroom at 20 kPa was the long cumulative time observed from 60 °C to 70 °C, under such temperature conditions RNase can be activated and guanylic acid production increase. However, RNase activity might not be able to be maintained due to reduced water activity during drying. In addition, the degradation of guanylic acid due to longer drying time and high drying temperature might occur. Further study about changes in RNase activity of the VMD-treated shiitake mushrooms should be required and the causes of the changes in the amount of guanylic acid during drying should be cleared.
Color The results of the color determination of VMD-treated shiitake mushrooms are shown in Fig. 9. The lamellae of dried shiitake mushrooms treated at 3 kPa and 10 kPa showed yellow and bright colors, while those treated at 20 kPa had dark colors, including brown and black, and were burned as microwave power increased. The color parameter results (ΔL*, Δa*, Δb*, and ΔE) are described in Fig. 10. Yellowness increased and total color difference decreased with decreasing pressure and microwave power. Blackness, redness, and total color difference increased with increasing pressure and microwave power. Argryopoulos et al. (2008) reported that the main cause of color changes during drying was enzymatic and non-enzymatic browning reactions. The Δa* and Δb* values were positive and ΔL* values were negative in all VMD-treated shiitake mushrooms, because browning reactions were activated during drying and the lamellas became yellower, redder, and darker compared with fresh mushrooms. In addition, the ΔE values were ≥ 10, and samples treated at 20 kPa had an ΔE value of ≥ 20. These results indicated that color change progressed as a result of drying. The increase in redness at 20 kPa was considered to be due to browning reactions as a result of burning. In contrast, the increase in yellowness, especially at 3 kPa and 10 kPa, was considered to be due to the tyrosinase reaction; this reaction is representative of enzymatic browning reactions, in which tyrosinase oxidizes tyrosine and melanin is produced (Aoyama et al., 2010). There are two types of melanin; eumelanin (dark brown) and pheomelanin (yellow and reddish) (Ito et al., 2000; Adachi, 2011). The tyrosinase reaction rate increase as temperature increases, reaching its maximum at from 45 °C to 55 °C, however, irreversible deactivation started to occur suddenly after passing optimal temperature, and tyrosinase was finally deactivated at over 60 °C (Yang and Wu, 2006; Aoyama et al., 2010). The reason that yellowness increased at 3 kPa and 10 kPa is considered to be the long drying time at ≤ 50 °C, because the tyrosinase reaction was able to progress and yellow pheomelanin was produced.
Shiitake mushroom samples after treatment by vacuum microwave drying (VMD) at different pressures (kPa) and microwave power (W). Scale bar 2 cm. Samples are shown at: (a) 3 kPa-25 W (pressure and microwave power, respectively); (b) 3 kPa-50 W; (c) 3 kPa-75 W; (d) 10 kPa-25 W; (e) 10 kPa-50 W; (f) 10 kPa-75 W; (g) 20 kPa-25 W; (h) 20 kPa-50 W; and (i) 20 kPa-75 W.
Color parameters of shiitake mushrooms treated with vacuum microwave drying (VMD) at different pressures. (a) ΔL* (whiteness/darkness); (b) Δa* (redness/greenness); (c) Δb* (yellowness/blueness); and (d) ΔE (color difference) (n = 3). The bars show mean ± SE. Lower case letters above each bar represent significant differences (p < 0.05).
Sensory evaluation The sensory evaluation results for rehydrated VMD-treated shiitake mushroom caps are shown in Fig. 11. The score for the sample treated under the 3–25 condition was the highest of the four sets of conditions examined in terms of umami, color, flavor, and total satisfaction. Moreover, it had a higher score for all sensory attributes than HAD-treated mushrooms. There were significant differences shown between 3–25 and three other conditions (p < 0.05) in total satisfaction: between 3 kPa and 20 kPa (p < 0.05) in softness, and between 3–25 and 20–75 (p < 0.05) in color and appearance. The sensory evaluation of the rehydrated VMD-treated shiitake soups revealed the highest score under the 3–25 condition for umami and total satisfaction, but the highest score for flavor was achieved under the 20–75 condition (Fig. 12). However, there was no significant difference in soups.
Sensory evaluation of the caps of shiitake mushrooms rehydrated after treatment with vacuum microwave drying (VMD) under four different conditions of pressure and microwave power: 3 kPa-20 W (3–25); 3 kPa-75 W (3–75); 20 kPa-25 W (20–25); and 20 kPa-75 W (20–75). The scores of HAD-treated shiitake mushrooms were 0. The asterisks indicate significant differences (p < 0.05) between 3–25 and 3 other conditions in total satisfaction, between 3 kPa and 20 kPa in softness, and between 3–25 and 20–75 in color and appearance.
Sensory evaluation of the soups of shiitake mushrooms rehydrated after treatment with vacuum microwave drying (VMD) under four different conditions of pressure and microwave power. 3 kPa-20 W (3–25); 3 kPa-75 W (3–75); 20 kPa-25 W (20–25); and 20 kPa-75 W (20–75). The scores of HAD-treated shiitake mushrooms were 0. There was no significant difference.
The scores of samples treated under the 3–25 and 3–75 conditions were higher for softness than the standard (score 0; HAD), which coincided with hardness results for both drying treatments. Therefore, the softness sensory evaluation matched the physical measurements of low hardness.
The evaluation scores for umami of the caps were higher for samples treated under the 3–25 and 3–75 conditions than for the 20–25 or 20–75 conditions; however, the guanylic acid extraction amounts of samples treated at 20 kPa were larger than at 3 kPa. When considering umami characteristics for the soups, the score for soups derived from caps treated at 20–25 and 20–75 was high. Therefore, at 20 kPa, guanylic acid was extracted to the soup and the umami component consequently reduced in the caps. The evaluation scores for umami of the soups were high for samples treated under the 20–25 and 20–75 conditions. These results showed a trend of correlation with the extracted guanylic acid.
In terms of color and appearance, the score of samples treated under the 3–25, 3–75, and 20–25 conditions were higher than the standard (score 0; HAD). Dried shiitake mushroom, which shows little cap deformation of cap and has yellow and bright coloration, is high quality (ii); the sample treated at 3 kPa, which had strong yellowness and bright colors, was in particular evaluated highly. Burning was the main factor causing the low evaluation for the sample treated under the 20–75 condition, affecting the evaluation of both the cap and soup, as well as lowering overall satisfaction with the product.
Our results show that VMD-treated shiitake mushrooms subjected to the 3–25 condition are of high quality and more highly evaluated for both cap and soup when compared with HAD-treated mushrooms.
We applied VMD to the production process of dried shiitake mushrooms, and investigated the resultant physical and chemical parameters. Under a vacuum environment of ≤ 20 kPa, drying time was reduced as pressure decreased and microwave power increased; compared with HAD, VMD reduced drying time by approximately 13 to 70 times. Decreasing the pressure produced puffing characteristics in the samples, prevented cell aggregation and enlarged the pores, and increased rehydration characteristics, thus tending to soften the rehydrated samples. The amount of guanylic acid extracted from shiitake mushrooms treated at 20 kPa was largest. Shiitake mushrooms treated with VMD at 3 kPa and 10 kPa showed yellow and bright colors, while mushrooms treated at 20 kPa showed dark colors, including brown and black. The total sensory evaluation score for the cap and the soup of rehydrated VMD-treated shiitake mushrooms at 3 kPa and 25 W/g dry matter was the highest of the four VMD treatments evaluated, and higher than the HAD-treated. This high score was attributed to the results for hardness and rehydration characteristics, extracted guanylic acid, and color. Our results suggest that VMD is more suitable and useful than HAD in producing high quality dried shiitake mushrooms. However, the results for hardness and rehydration characteristics and that for extracted guanylic acid contradict each other. This leads to the finding that shiitake mushrooms treated with VMD at 3 kPa and 10 kPa are more suitable for producing a quality shiitake cap product, whereas VMD treatment at 20 kPa produces a highly quality umami component (guanylic acid). Nonetheless, our findings support the applicability of VMD as a desirable and effective alternative drying method for shiitake mushrooms.
Acknowledgments This work was supported by JSPS KAKENHI, Grant Number JP17K08015 (Grant-in-Aid for Scientific Research(C)) and JP16H05001 (Grant-in-Aid for Scientific Research(B)).
