Structural analysis of vacuum microwave dried shiitake mushrooms based on X-ray CT images and physical properties

Abstract

This study was designed to investigate the effects of changes in vacuum microwave (VMW) drying conditions on the drying kinetics, microstructures and physical properties of shiitake mushrooms. Under the tested conditions of 3 kPa and 20 kPa, low pressure and high microwave power in particular increased the rate of moisture change of shiitake mushrooms. Morphological observations from X-ray computed tomography (CT) images showed that samples dried at 3 kPa had many more porous structures with smaller pores, while clumps of cells formed and led to the formation of larger pores in samples dried at 20 kPa. Under the 20 kPa condition, the apparent density increased, and the internal porosity decreased, immediately after drying. These results implied that shrinkage of the samples occurred in the early stages of drying. However, drying at lower pressure led to the dried samples with lower densities and higher internal porosities because of the puffing effect.

Introduction

Drying is one of the most traditional food processing methods used to extend the shelf life of fruit, vegetables and mushrooms because it is difficult to preserve these foods for long periods after harvesting (Dandamrongrak et al., 2002; Tamaki et al., 2012). Drying removes water and consequently reduces water activity, which halts or delays the growth of microorganisms and the occurrence of biochemical and chemical reactions responsible for spoilage (Venkatachalapathy and Raghava, 2000; Vega-Mercado et al., 2001; García-Segovia et al., 2011; Tian et al., 2016). In addition, drying is an efficient way to reduce costs or difficulty associated with packaging, handling, storing and transporting food because drying reduces the weight and volume (Barbosa-Cánovas and Vega-Mercado, 1996). For example, shiitake mushrooms (Lentinus edodes) are the second most widely cultivated edible mushroom in the world. They represent approximately 25 % of global production (Boa, 2004). The annual production of shiitake mushrooms in Japan is approximately 70 000 metric tons, which makes them the third most widely produced mushroom in Japanⁱ⁾. Shiitake mushrooms have been stored and commercialized as a dried food. Drying increases the contents of guanylic acid (umami component) and lanthionine (flavor) (Hiraide et al., 2004); approximately 2 000 metric tons of dried shiitake mushrooms are produced annuallyⁱ⁾.

In the manufacture of dried fruit, vegetables and mushrooms, sun drying and air drying are relatively simple and traditional methods that are generally used during processing (Sagar and Suresh, 2010; Ando et al., 2019a). However, these methods have some disadvantages, such as low energy efficiency, excessive shrinkage of food, oxidation of functional ingredients and reductions in aroma, color, and nutrient contents (Maskan, 2001a; Maskan, 2001b; Durance and Wang, 2002; Hu et al., 2006; Orikasa et al., 2008; Guiné and Barroca, 2012; Dehnad et al., 2016; Yang et al., 2019). Newer drying methods have been developed to reduce the drying time, as well as to improve the energy efficiency and the quality of dried fruit, vegetables and mushrooms (Bezyma and Kutovoy, 2005).

Vacuum microwave (VMW) drying, one of the alternative methods, rapidly dries food materials at relatively low temperatures using microwave irradiation under reduced pressure. VMW drying is an effective method that improves the drying rates, color, rehydration characteristics and sensory impressions of food, and it consumes less energy than conventional air drying (Lin et al., 1998; Durance and Wang, 2002; Hu, 2006; Bondaruk et al., 2007; Giri and Prasad, 2007; Kantrong et al., 2014; Jiang et al., 2017; Orikasa et al., 2018; Kurata et al., 2020). For dried shiitake mushrooms and other cases, food is rehydrated and cooked before eating; therefore, the response of a dried food to rehydration is important. Rehydration involves complex behavior affected by various factors, e.g., drying method and process, and physical structure and chemical composition of a food (Marabi and Saguy, 2004). Durance and Wang (2002) applied hot-air drying (HAD) and VMW drying to tomatoes, and the results showed that VMW-dried tomatoes were less dense and rehydrated more rapidly than HAD-dried tomatoes. The results of Lin et al. (1998) also showed that dried samples with lower density had higher rehydration ratios. Therefore, microstructure and physical properties, such as density and porosity, are important parameters that are closely related to the rehydration of dried food. Previous studies compared the effects of VMW drying and conventional drying methods on the microstructure and/or physical properties of food (Lin et al., 1998; Durance and Wang, 2002; Giri and Prasad, 2007; Monteiro et al., 2022). However, few studies addressed the effects of differences in VMW treatment conditions on dried food quality to determine the optimum VMW drying conditions. In our previous study (Kurata et al., 2020), we investigated the effects of VMW drying on the sensory evaluation, extraction of guanylic acid, and rehydration of dried shiitake mushrooms. However, that research did not quantitatively compare the microstructures and physical properties of the dried mushrooms, such as density and internal porosity, for various conditions (different vacuum degree and microwave powers). Additionally, that study did not measure changes in physical properties during drying or the structure of VMW-dried foods. Therefore, the objective of this study was to investigate the effects of changes in VMW drying conditions (microwave powers and vacuum degree) on the drying kinetics, microstructures and physical properties of dried shiitake mushrooms.

Materials and Methods

Materials  Shiitake mushrooms (Lentinula edodes) were purchased from a local market and stored in a refrigerator at 4 °C for a maximum of three days. Stems were removed by cutting prior to drying. The initial moisture content of the samples was 9.47 ± 0.63 g water/g dry matter (mean ± S.D.) (n = 22), as measured by the oven method at 105 °C for 24 h (Ozkan et al., 2007).

Drying method  VMW experiments were conducted in a vacuum microwave instrument (Fig. 1). The system consisted of a lubricated rotary vacuum pump (TSW-100; Sato Vac Inc, Japan), a cold trap (UT-3000; Tokyo Rikakikai Co., Ltd., Tokyo, Japan), a vacuum control unit (NVC-2100; Tokyo Rikakikai Co., Ltd., Tokyo, Japan) and a microwave oven (μReactor Ex; Shikoku Instrumentation Co., Ltd., Japan) with a rotating antenna to prevent uneven heating of food. Three samples of shiitake mushroom that weighed a total of approximately 40 g [sample weight: 14.02 ± 2.42 g (mean ± S.D.)] were placed in the vacuum chamber (size: diameter 140 mm × height 120 mm) that was constructed of borosilicate glass and contained within the microwave oven. Samples were subjected to various microwave treatments at different power levels [25 W/g dry matter (2.4 W/g fresh weight) and 75 W/g dry matter (7.2 W/g fresh weight)] and absolute pressures (3 kPa and 20 kPa). Samples were irradiated with microwaves after the pressure reached the set value; the target pressure degree was reached within 2 min. Drying was performed under the following conditions: 3 kPa, 25 W/g dry matter (3–25); 3 kPa, 75 W/g dry matter (3–75); 20 kPa, 25 W/g dry matter (20–25); and 20 kPa, 75 W/g dry matter (20–75). These conditions were determined on the basis of information from a previous report (Kurata et al., 2020). We chose the microwave power levels (25 and 75 W/g dry matter) and absolute pressures (3 and 20 kPa) that produced significant differences in the measured parameters.

Fig. 1

Schematic diagram of the vacuum microwave drying apparatus.

Moisture content analysis  The moisture contents of the samples were calculated from measurements of the masses with an electric balance (FX-120i, A&D Instruments, Ltd., Tokyo, Japan). After the specified drying times, the samples were removed from the vacuum chamber, weighed, and not returned to the chamber. The mass lost was regarded as evaporated water. The drying rate was expressed as the decrease in moisture content per unit time (−dM/dt [h⁻¹]). The moisture content of each sample was calculated from the initial moisture content and the mass. The samples were dried until a final moisture content of 0.15 g water/g dry matter was reached (Xue et al., 1996; Tian et al., 2016). The drying rates of the samples were estimated using the kinetic constant from Lewis's model shown below, and this model was used to quantify the rate of moisture change during drying under various conditions (Orikasa et al., 2008; Ando et al., 2019a; Ando et al., 2019b; Yamakage et al., 2021).

  

where MR is the moisture ratio, M, M₀, and M_e are the moisture content, initial moisture content, and equilibrium moisture content (g water/g dry matter), respectively, k is the drying rate constant (h⁻¹) and t is the drying time (h). The parameter M_e was initially obtained using the nonlinear least squares method to determine MR. The parameter k was estimated using the least squares method to evaluate the drying rate of each sample to quantify the rate of moisture change.

The changes in the moisture content were applied to a diffusion model. Equation (2) expresses the diffusion equation as follows (Wu et al., 2007):

  

where D is the diffusion coefficient (m²/s), l is the characteristic length (m) (=2.06 × 10⁻² ± 0.001 9 m.), and k = Dπ²/4l². A nonlinear least squares method was applied to Eq. 2, which are the exact solutions of the diffusion equation, to obtain the parameters M_e and k. The root mean squared error (RMSE) was calculated as an index of goodness of fit.

Sample temperature measurements  Temperature changes inside the samples during the drying process were measured at 1 s intervals using a thermometer (FL-2000, Anritsu Meter Co., Ltd., Tokyo, Japan) with a 1.00 mm diameter fiber optic thermosensor (FS-100, Anritsu Meter Co., Ltd.). The sensor was set inside the center of the cap of a shiitake mushroom. The temperature was measured from 0 to 75 min in 25 W/g dry matter and from 0 min to 25 min in 75 W/g dry matter, and measurements were made in quintuplicate for each sample.

X-ray CT observations  The microstructures of whole dried samples at their final moisture content were observed using X-ray CT at beam line BL20XU of Spring-8 (Hyogo, Japan). Samples were extracted from inside the cap of a shiitake mushroom, cut into cubes with 1 mm sides and placed in a Lindemann glass capillary (1.5 mm diameter). Samples were placed in the holder and rotated while projection images were taken by a charge-coupled device (CCD) camera. The X-ray energy was 20 keV, and 1 800 projection images were acquired while rotating each sample through 180 degrees. The measurement time was 100 ms per image. The spatial resolution of the CCD camera was 0.5 μm. The images obtained were reconstructed to obtain tomographic images by convolution backprojection method with a Ramachandran filter. Binarized tomographic images were obtained using Dragonfly software, version 4.1 (Object Research Systems, Montreal, Canada) with a machine learning algorithm that distinguished cell tissue from pores. By repeating the learning, inference, and addition of teacher images in sequence, we confirmed that efficient learning is possible and that tissue extraction from tomograms can be achieved with a high degree of accuracy.

Volume (i) Apparent density The water displacement method was used to measure the apparent density of the samples; distilled water was the displacement liquid (Chihara, 1988; Ma et al., 1998; Soma et al., 2004; Tamaki et al., 2012; Orikasa et al., 2017). Samples were completely immersed in a container of distilled water (20 °C). We measured the volume within 10 s to limit the amount of water absorption. The volume of the sample V_sample was determined as follows (Soma et al., 2004; Tamaki et al., 2012; Orikasa et al., 2017):

  

where m_cws is the mass of the container when samples are completely immersed in the container of distilled water (kg), m_cw is the mass of the container and distilled water (kg), and ρ_w is the density of water at the temperature (20 °C) at the time of measurement (kg/m³). The density of water at 20 °C is 998.2 kg/m³ (Haynes, W.M., 2014). The apparent density of a sample was calculated as follows (Soma et al., 2004; Tamaki et al., 2012; Orikasa et al., 2017):

  

where m_sample is the mass of the sample (kg). Samples for measuring the apparent density were used without air cooling after the drying treatment to measure the fluctuations of the apparent densities of samples for each drying time. All measurements were repeated three times.

(ii) Density of dry matter and true density  The density of dry matter and the true density were measured based on the methods of Madamba et al. (1994) and Soma et al. (2004). The procedure used for preparing samples of dry matter was as follows: samples were dried at 60 °C for 48 h in a constant temperature drying oven (MOV-212U, SANYO Electric Biomedical Co., Ltd. Tokyo, Japan). The dried samples were ground and further dried at 70 °C for 48 h at 0.13 kPa in a vacuum constant temperature drying oven (VOS-300VD, Tokyo Rikakikai Co., Ltd.). Powdered samples were passed through a 500 μm sieve and used as the dry matter samples in this study. Toluene was used as the displaced liquid, and the density of the dry matter (ρ_d) was calculated by the pycnometer method as follows (Sharp, 1927; Chihara, 1988):

  

where m_s is the mass of dry matter placed in the pycnometer (kg), m_t is the mass of the pycnometer and toluene when the pycnometer was filled with toluene (kg), m_st was the mass of the pycnometer, toluene and dry matter when the pycnometer was filled with toluene (kg), and ρ_toluene is the density of toluene (kg/m³). The density of toluene at 20 °C is 886.9 kg/m³ (McLinden and Splett, 2008). The true density ρ_t (kg/m³) was calculated as follows (Madamba et al., 1994):

  

where M_w is the moisture content of the samples (wet basis decimal). All measurements were repeated three times.

(iii) Internal porosity  The internal porosity ε_a was calculated from the apparent density ρ_ap and the true density ρ_t as follows (Maroulis et al., 2002; Orikasa et al., 2009):

  

Hardness measurement  The hardness of dried samples was measured with a hardness tester (TPU-2C; Yamaden Co., Ltd., Tokyo, Japan) at three different areas on a lamella from each sample. In this study, the mean values for the measured data were described as the experimental result. The load on the samples was measured, and the breaking load was defined as the sample stress, with the following instrumental parameters: speed = 10 mm/s; diameter of the cylinder probe = 3 mm. The stress σ (Pa) was calculated as follows:

  

where S is the cross-sectional area of the plunger (m²), and P is the load on the sample (N).

The tester was linked to a computer that recorded and analyzed the data using TPU Analysis Windows Ver. 2 (TA-TPU2; Yamaden Co., Ltd.). All measurements were repeated three times.

Statistics  Quantitative data are presented as the mean ± S.E. Tukey–Kramer tests with a significance level of p < 0.05 were applied for the statistical analyses, using an Excel statistical software package (BellCurve for Excel version 2.11; Social Survey Research Information Co., Ltd., Tokyo, Japan).

Results and Discussion

Drying time effect of VMW conditions on the drying kinetics of shiitake mushroom samples  Fig. 2 shows the average moisture contents of samples during VMW drying. The moisture content decreased rapidly at low pressure and high microwave power. High microwave power in particular rapidly reduced the moisture content. The moisture content of the 3-25, 3-75, 20-25, and 20-75 samples reached 0.15 g water/g dry matter after 0.75, 0.21, 1.25, and 0.29 h, respectively. Kurata et al. (2020) reported that the drying time was approximately 0.2 h to 1.1 h under the conditions (microwave powers: 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). In comparison to the drying times at the same vacuum degree, the drying times of the 75 W/g dry matter samples were 3.6–4.3 times shorter compared to those of the 25 W/g dry matter samples. Fig. 3 shows the drying characteristic curves of individual samples during VMW drying. The drying rates decreased in a convex upward curve with decreasing moisture content, and the drying rates of the 75 W/g dry matter samples were higher than those of 25 W/g dry matter samples. Fig. 4 shows the internal temperature changes during drying. The internal temperature rose to approximately 30 °C for 3–25, 45 °C for 3–75, 60 °C for 20–25, and 65 °C for 20–75 immediately after the start of drying and gradually increased thereafter. The similar trend in temperature change was confirmed in the report of Kurata et al. (2020). The temperatures, taken to represent saturated vapor pressure at 3 kPa, and 20 kPa, were 24.1 °C, and 60.1 °C, respectively; they were calculated by the Tetens equation (Tetens, 1930). Although the calculated temperatures were close to those that were measured inside the samples, there were some differences. The temperature sensor was set inside the center of a cap of a sample to measure the sample temperatures during drying. Microwave energy can rapidly penetrate the inner layer of a dried sample, and thus, water molecules in a sample vibrated more intensely near the center of the sample and generate heat. In this study, the boiling points of the samples increased even under reduced pressure because moisture was unable to escape and thus concentrated inside the samples as drying proceeded (Ando, 2019a). Furthermore, thermal conduction from the chamber and heat transfer from water vapor in the chamber affected the sample temperatures.

Fig. 2

Changes in moisture content of shiitake mushrooms treated by vacuum microwave drying.

The error bars mean S.E. (n = 6).

The dashed lines represent approximations given by the exponential model (Eq. 1).

Fig. 3

Drying characteristic curves of shiitake mushrooms treated by vacuum microwave drying. The data are mean values of 6 replicates.

Fig. 4

Temperature of shiitake mushroom treated by vacuum microwave drying.

The data are mean values of 5 replicates.

The straight dashed lines represent the temperatures calculated from the saturated vapor pressure.

The temperature, taken to represent saturated vapor pressure at 3 kPa, and 20 kPa, are 24.1 °C, and 60.1 °C, respectively.

To quantitatively evaluate the effect of changes in the VMW drying conditions (microwave powers and vacuum degree) on the moisture change rate, the drying rate constant k for each sample was calculated by using Lewis's model. The dotted line and dashed lines in Fig. 2 are moisture contents approximated by the exponential model (Eq. 1). Table 1 shows the estimated parameters for the drying rate constant k for each drying condition. The values of k cannot be compared under the same temperature conditions because the temperature changed during drying under each condition as shown in Fig. 4. In comparison to the constant k value at the same vacuum degree, the values of k for the 75 W/g dry matter samples were approximately 2.6–3.2 times greater than those of the 25 W/g dry matter samples at both 3 and 20 kPa. The greater the microwave power was, the more heat was generated within the samples, which created a larger vapor pressure difference between the core and surface of the sample (Wang and Sheng, 2006). Therefore, interior moisture migration accelerated, and the surface water evaporation increased during heating at higher power. In comparison to the constant k for the same microwave power, the values of k for the 3 kPa samples were approximately 1.1–1.4 times greater than those of the 20 kPa samples for both 25 and 75 W/g dry matter. This was attributed to the lower water boiling point caused by the lower pressure (Durance and Wang, 2002). In addition, a diffusion model was applied to the moisture content changes to determine the values of the diffusion coefficient of the dried samples under each VMW condition. The values of RMSE for 3–25, 3–75, 20–25, and 20–75 were 0.075, 0.077, 0.070, and 0.084, respectively. The diffusion coefficient D was calculated from Me, k and l, and the values of D for 3–25, 3–75, 20–25, and 20–75 were 1.57 × 10⁻⁷, 4.81 × 10⁻⁷, 1.06 × 10⁻⁷, and 4.12 × 10⁻⁷, respectively. At the same vacuum degree, the values of D for the 75 W/g dry matter samples were approximately 3.1–3.8 times greater than the values of D for the 25 W/g dry matter samples at both 3 and 20 kPa. However, at the same microwave power, the values of D for the 3 kPa samples were approximately 1.2–1.5 times greater than those of the 20 kPa samples with both 25 and 75 W/g dry matter. The higher microwave power accelerated the moisture transfer relative to the lower microwave power.

Table 1. Drying rate constant k estimated from the exponential model for drying sample.

	k (h−1)	R2 (−)
3 kPa-25 W/g dry matter	2.81	0.990
3 kPa-75 W/g dry matter	7.44	0.990
20 kPa-25 W/g dry matter	2.06	0.993
20 kPa-75 W/g dry matter	6.60	0.990

These results showed that both low pressure and high microwave power increased the moisture change rate of shiitake mushrooms. However, under the tested conditions of 3 and 20 kPa, the adjustment of the microwave power would be more important than the adjustment of the absolute pressure to shorten the drying time. To discuss the effects of drying kinetics on structure, we observed microstructures, as discussed in the following section.

Structural attributes of dried shiitake mushroom determined from X-ray CT observations  Fig. 5 shows tomograms, 3D images, and binarized tomographic images of dried shiitake mushrooms that reached a final moisture content obtained by X-ray CT. In the 3 kPa samples, many small pores measuring approximately 20 μm or less in pore size were observed, and fine internal pores were scattered throughout (Fig. 5a and 5b). In contrast, in the 20 kPa samples, the cell structure collapsed and the cells formed clumps, and mesh structures with many large pores, approximately 40-80 μm in pore size, were observed (Fig. 5c and 5d). 3D movies (Supplemental Video 1, 2, 3, and 4) also showed small pores in the 3 kPa samples and clumps of cells and large pores in the 20 kPa samples. When the microwave power increased, the cells formed clumps, but changes in the microstructure of the dried shiitake mushrooms were greater when the pressure was increased than when the microwave power was increased. According to the result discussed in the previous section, the adjustment of the microwave power would be a key to shortening the drying time under the tested conditions of 3 kPa and 20 kPa. Giri and Prasad (2007) and Kantrong et al. (2014) reported that a shorter drying time and more rapid moisture transfer from food helped to prevent structural collapse and expansion of tissue. In this study, however, the microstructure of dried shiitake mushrooms changed greatly at lower pressure, and the drying time and the moisture change rate had little effect on the microstructure.

Fig. 5

Tomograms, 3D images, and binarized tomographic images of dried shiitake mushrooms treated with vacuum microwave drying.

Dried samples reached a final moisture content were observed.

Samples are shown at:(a) 3 kPa-25 W (pressure and microwave power, respectively); (b) 3 kPa-75 W; (c) 20 kPa-25 W; and (d) 20 kPa-75 W.

(i) Tomogram of dried sample and scale bar is 200 μm (ii) 3D image of dried sample and length of a side of cubes is 0.25 mm, (i) Binarized tomographic images of dried shiitake mushrooms treated with vacuum microwave drying. Scale bar is 100 μm Red shows sample cells and blue shows pores.

The large vapor pressure difference between the core and the surface of the dried sample was produced by microwave heating and reducing the chamber pressure. Then, the outward force from the interior of the material also increased because heat was generated inside the food. This could have produced the puffing effect (Lin et al., 1998; Sham et al., 2001; Durance and Wang, 2002; Zhang et al., 2007; Bai-Ngew et al., 2011). Kurata et al. (2020) reported that puffing characteristics prevented cells from forming clumps. According to Lin et al. (1998), Sham et al. (2001), and Durance and Wang (2002), the puffing effect produced dried samples with low densities. Therefore, we considered that in this study, the puffing effect prevented the formation of clumps of cells at the low pressure of 3 kPa. Additionally, as the samples dried, the spaces among the cells were maintained, which resulted in dried samples with low densities, i.e., with many pores. However, as shown in Fig. 5a–5d, shiitake mushrooms dried at 20 kPa seemed to have many more pores than those dried at 3 kPa, and many large pores were observed in the 20 kPa samples.

According to the result of the previous section, the adjustment of the microwave power would be a key means to shorten the drying time under the tested conditions of 3 and 20 kPa. However, the microstructure of dried shiitake mushrooms changed greatly due to lower pressure.

Relationships between structural characteristics and physical properties  The apparent and true densities of raw and dried samples and the density of dry matter were measured, and the results were used to calculate the internal porosity. The density of dry matter was 1 330.05 ± 17.51 kg/m³. Fig. 6, 7, and 8 show the changes in apparent density, true density and internal porosity, respectively. Under the 3 kPa condition, the apparent density gradually decreased with decreasing moisture content. However, under the 20 kPa condition, the apparent density increased with decreasing moisture content to approximately 5 g water/g dry matter and then decreased as drying progressed, and the curve was convex upward. The difference in microwave power did not significantly affect the changes in the apparent density. The apparent densities of 25 and 75 W/g dry matter decreased as moisture contents decreased, and the changes seemed to overlap. However, the internal porosity showed the opposite behavior, as expected. The internal porosity at 3 kPa gradually increased, and that at 20 kPa showed a downward convex shape. The true density increased exponentially with decreasing moisture content. Under the 20 kPa condition, the apparent density increased, and the internal porosity decreased immediately after drying to approximately 5 g water/g dry matter. These results implied that shrinkage of the samples occurred in the early stages of drying. Table 2 shows the apparent densities, true densities and internal porosities of samples dried until a final moisture content less than 0.15 g water/g dry matter was reached. As a result, the 3 kPa samples had lower density and higher porosity than the 20 kPa samples. To discuss the changes in pores based on physical property data, the internal porosity increase/decrease ratio was calculated by dividing the internal porosity of the dried sample after each condition by that of the fresh sample. The ratios in 3–25, 3–75, 20–25, and 20–75 cases were 1.67, 1.70, 1.54, and 1.62, respectively. The internal porosities of the 3 kPa samples increased by up to 1.1 times compared to the 20 kPa samples. The densities and internal porosities of the dried samples were affected by vacuum degree and hardly affected by microwave power. In addition, these results showed that shiitake mushrooms dried at 3 kPa had low densities and many internal pores. As mentioned in the previous section, we considered that the formation of clumps of cells was prevented by the puffing effect at the lower pressure of 3 kPa, and the spaces between the cells were maintained as the samples dried, which resulted in dried samples with low densities, i.e., with many pores. In the tomograms (Fig. 5) and 3D images, although the 20 kPa samples seemed to have many more pores than the 3 kPa samples, the porosities of the entire samples cannot be compared from the images because the samples were cut into the same size when observing the structures of the samples with X-ray CT. Orikasa et al. (2005) reported that sample shrinkage was restrained by surface hardening during HAD, which produced increased pores and decreased density. Durance and Wang (2002) reported that a high vacuum degree enhanced the puffing effect, and thus led to less dense dried samples. This implied that the puffing effect prevented shrinkage during drying, and samples maintained their shapes at the end of the drying process. To discuss the changes in volume, the ratios of volume shrinkage were calculated by dividing the volume of the dry sample after each treatment by that of the fresh sample. The ratios in the cases of 3–25, 3–75, 20–25, and 20–75 were 0.43, 0.46, 0.20, and 0.24, respectively. The ratio produced at 3 kPa was 1.9–2.2 times larger than that produced at 20 kPa. These results implied that shrinkage of the samples occurred in the early stages of drying and that the puffing effect was larger in the 3 kPa samples than the 20 kPa samples, which prevented the samples from shrinking during drying. However, the effects of puffing were not sufficient at 20 kPa, which led to the formation of clumps of cells and shrinkage of the entire sample. As a result, after drying at 20 kPa, the internal pores in the entire sample were smaller than those seen after drying at 3 kPa.

Fig. 6

Apparent density of shiitake mushroom treated by vacuum microwave drying.

The error bars mean S.E. (n = 3).

Fig. 7

True density of shiitake mushroom treated by vacuum microwave drying.

The data are mean values of 3 replicates.

Fig. 8

Internal porosity of shiitake mushroom treated by vacuum microwave drying.

The error bars mean S.E. (n = 3).

Table 2. Apparent density (ρ_ap), true density (ρ_t) and internal porosity (ε_a) of the samples after each drying treatment.

	ρap (kg/m3)	ρt (kg/m3)	εa (-)
3 kPa-25 W/g dry matter	164.19 ± 14.70a	1 292.68 ± 28.43a	0.87 ± 0.014a
3 kPa-75 W/g dry matter	151.15 ± 4.78a	1 292.75 ± 26.35a	0.88 ± 0.006a
20 kPa-25 W/g dry matter	261.68 ± 24.44b	1 305.39 ± 22.52a	0.80 ± 0.017b
20 kPa-75 W/g dry matter	214.98 ± 5.62ab	1 384.12 ± 35.21a	0.84 ± 0.008ab

*Each value represents the means ± S.E. (n = 3).

**Different letters are significantly different according to Tukey–Kramer test (p < 0.05).

In measuring the hardness of dried samples, all dried samples were observed to break when stress was applied. The stress values at the breaking points for dried shiitake mushrooms for 3–25, 3–75, 20–25, and 20–75 were 1 963.62 ± 227.11, 1 628.22 ± 217.18, 4 292.33 ± 472.68, and 3 116.03 ± 333.06Pa, respectively. The stresses of the 20 kPa samples were approximately 1.9–2.2 times greater than those of the 3 kPa samples. Lin et al. (1998) applied VMW drying, HAD, and freeze drying to carrots, and the results showed that the sample hardness decreased with decreasing density. According to Orikasa et al. (2005), sample shrinkage was restrained by surface hardening during HAD, which increased porosity and decreased density. In other words, shrinkage was likely to produce samples with higher densities and fewer pores. The volumes of samples dried at 3 kPa were nearly twice those of samples dried at 20 kPa as the result of the ratios of volume shrinkage. Furthermore, Table 2 shows that the densities resulting from drying at 3 kPa were smaller than those for 20 kPa, and the internal porosities produced at 3 kPa were larger than those at 20 kPa. Puffing created porous structure in food, and it reduced the density of the material (Sham et al., 2001). Therefore, shrinkage of the samples was restrained at 3 kPa, whereas shrinkage occurred at 20 kPa, and the dense structure was attributed to hardness. Fig. 9 shows the mechanism for structure formation in VMW-dried shiitake mushrooms. Under the 3 kPa condition, sample shrinkage was restrained by the puffing effect, and the samples maintained their shapes while drying, which resulted in lower densities and many more pores. Under the 20 kPa condition, the effect of puffing was not sufficient since the pressure was not sufficiently low, which caused the formation of clumps of cells inside the samples and shrinkage of entire samples and resulted in dried samples with higher densities and larger pores.

Fig. 9

Mechanism of structural formation of dried shiitake mushrooms during vacuum microwave drying.

We investigated the effects of porosity on rehydration because dried shiitake mushrooms are rehydrated and cooked before they are eaten. Rehydration ratio data were quoted from a previous study (Kurata et al., 2020), and the rehydration ratios in 3–25, 3–75, 20–25, and 20–75 were 4.96, 5.64, 2.61, and 2.97, respectively. The rehydration ratio was determined by immersing dried samples into distilled water (50 mL/g dry sample) at 4 °C for 5 h. The rehydration ratios were calculated by dividing the mass of the rehydrated mushroom by that of the dried mushroom. Fig. 10 shows the relationship between the rehydration ratio and internal porosity. The internal porosity increased as the pressure decreased, and then the amount of rehydration increased. Therefore, the internal porosity tended to correlate with the rehydration ratio. Marabi and Saguy (2004) also pointed out that the rehydration ratios of dried foods increased with increasing the internal porosity of dried samples. Under the 3 kPa condition, small clumps of cells were scattered, and small pores were observed. In contrast, large clumps of cells and large pores were observed for the 20 kPa condition. As a result, sample shrinkage was restrained and small pores could hold water when rehydration under the 3 kPa condition, however, at 20 kPa, these clumps of cells prevented water from entering among cells and large pores could not hold water when rehydration resulting in reduced the rehydration ratio. Therefore, low pressure increased the internal porosities of the samples and created many pores that could hold water, which resulted in increased rehydration capacity.

Fig. 10

Relationship of rehydration to internal porosity of shiitake mushrooms treated by vacuum microwave drying.

The error bars mean S.E. (n = 3).

Conclusions

The present study investigated the effects of different microwave power levels and vacuum degrees during VMW drying on the drying kinetics, microstructures and physical properties of shiitake mushrooms. Under the test conditions of 3 kPa and 20 kPa, low pressure and high microwave power in particular increased the rate of moisture change of shiitake mushrooms. Furthermore, higher microwave power increased the diffusion coefficient of shiitake mushrooms and accelerated moisture transfer. These results demonstrated that higher microwave power can be used effectively to shorten the drying time under the test conditions of 3 kPa and 20 kPa. However, vacuum degree was a more important parameter than microwave power in helping to maintain structures and physical properties. The samples dried at 3 kPa had lower densities and more porous structures than those dried at 20 kPa because the puffing effect prevented shrinkage during the drying process. However, the samples dried at 20 kPa shrank because the puffing effect was smaller. These findings for the relationships between microstructures and mechanisms of structural formation during VMW drying may be valuable for manufacturing dried foods with porous structures. Future studies should be undertaken to evaluate productivity, energy costs and food qualities, such as flavor or nutrient contents, prior to practical applications of the VMW drying method. In addition, not only density and volume but also data such as pore size and form of cells using fresh and dried shiitake mushrooms that have not been molded from X-ray CT images should be investigated in the future study and used to discuss structural changes during drying to prepare for industrial applications.

Acknowledgements  This work was supported by JSPS KAKENHI, Grant-in-Aid for Scientific Research (C), Grant Numbers JP17K08015 and JP22K05901, and Grant-in Aid for JSPS Fellows, Grant Number JP23KJ0079.

Conflict of interest  There are no conflicts of interest to declare.

Supplemental Video 1

Movies of 3D image of shiitake mushrooms dried in 3 kPa and 25 W/g dry matter obtained by X-ray CT observation.

This video can be played on Adobe Acrobat.

Supplemental Video 2

Movies of 3D image of shiitake mushrooms dried in 3 kPa and 75 W/g dry matter obtained by X-ray CT observation.

This video can be played on Adobe Acrobat.

Supplemental Video 3

Movies of 3D image of shiitake mushrooms dried in 20 kPa and 25 W/g dry matter obtained by X-ray CT observation.

This video can be played on Adobe Acrobat.

Supplemental Video 4

Movies of 3D image of shiitake mushrooms dried in 20 kPa and 75 W/g dry matter obtained by X-ray CT observation.

This video can be played on Adobe Acrobat.

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