
==== Front
Curr Res Food Sci
Curr Res Food Sci
Current Research in Food Science
2665-9271
Elsevier

S2665-9271(24)00151-5
10.1016/j.crfs.2024.100825
100825
Research Article
Effect of heat pump drying temperature on moisture migration characteristics and quality of instant Tremella fuciformis
Wu Li abcde
Chen Shouhui a
Li Yibin abcde
Xie Wei a
Tang Baosha tbsty@126.com
abcde⁎
a Institute of Food Science and Technology, Fujian Academy of Agricultural Sciences, Fuzhou, Fujian, 350003, PR China
b Key Laboratory of Subtropical Characteristic Fruits, Vegetables and Edible Fungi Processing (Co-construction by Ministry and Province), Ministry of Agriculture and Rural Affairs, Fuzhou, Fujian, 350003, PR China
c National Edible Fungus Processing Technology Research and Development Branch, Fuzhou, Fujian, 350003, PR China
d Fujian Province Key Laboratory of Agricultural Products (Food) Processing Technology, Fuzhou, Fujian, 350003, PR China
e Fujian Characteristic Agricultural Products Processing Technology and Economic Integration Service Platform, Fuzhou, Fujian, 350003, PR China
⁎ Corresponding author. at: Institute of Food Science and Technology, Fujian Academy of Agricultural Sciences, Fuzhou, Fujian, 350003, PR China. tbsty@126.com
22 8 2024
2024
22 8 2024
9 1008254 6 2024
14 8 2024
17 8 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Low-field nuclear magnetic resonance (LF-NMR) was used to analyze the moisture migration characteristics and water distribution of instant Tremella fuciformis (ITF) during heat pump drying at 35 °C, 45 °C and 55 °C. A fuzzy mathematical evaluation method was then used to investigate the effect of heat pump drying temperatures (35 °C, 45 °C and 55 °C) on the quality of ITF. Based on the screened appropriate temperature of 45 °C, the thin layer heat pump drying curve and drying rate curve of ITF were drawn, then four mathematical models were tested based on the coefficient of determination (R2). The result showed three water components (bound water (T21), immobilized water (T22), and free water (T23)) in ITF before drying. During the drying process, the heat pump drying of ITF was carried out gradually from the outer to the inner. The sensory total score (77.66 ± 6.57), rehydration ratio (4803.00 ± 90.00%), and viscosity (45.01 ± 3.61 Pa s) of ITF in heat pump drying at 45 °C were the best. The optimal drying temperature of 45 °C was obtained by the fuzzy comprehensive evaluation with the highest comprehensive score of 77.85%. With the prolongation of drying time, the water content of ITF showed a gradual decreasing trend, and the water content was controlled to below 8.51% after 6 h. The drying curve shows that the drying phase of the ITF is divided into three phases, including preheat, constant speed and deceleration. The drying speed first increases, remains constant in the middle and then decreases. Among the four models tested, the cubic regression equation aligned with the change rule of the ITF drying curve (R2 = 0.99397). By studying the effect of drying temperature on the moisture migration characteristics and the quality of ITF, we aim to provide more data support for the production of T. fuciformis.

Graphical abstract

Image 1

Highlights

• LF-NMR was used to analyze the moisture migration characteristics of ITF during heat pump drying.

• A constant migration of moisture of ITF in the direction of the higher degree of combining (free water to combined water).

• Based on the fuzzy mathematical evaluation method, the optimal heat pump drying condition of high-quality ITF is 45℃, 7-8 h.

• The cubic regression equation [y = ax3+bx2+cx + d, (R2 = 0.99397)] achieved the best fit of ITF dried by heat pump drying.

Keywords

Instant T. fuciformis
Heat pump drying
Drying curves
Moisture migration characteristics
Handling Editor: Quancai Sun
==== Body
pmc1 Introduction

According to data (China Edible Fungi Association, 2021), China produced a total of 536,000 tons (fresh products) of Tremella fuciformis, representing over 90% of global production. Furthermore, China is a major country in the production, consumption and export of T. fuciformis. Its distinctive flavor and pharmacological properties have attracted considerable interest from researchers (Yuan et al., 2022). T. fuciformis contains multiple physiological activities that can improve immune deficiencies and possess anti-tumor, antioxidant, hypoglycemic, and cholesterol-lowering effects (Deng et al., 2023; Ma et al., 2021). Moreover, it is highly favored by consumers due to its potential as a nutritional supplement and medicinal product that can enhance body immunity (Wang et al., 2019). T. fuciformis polysaccharides have been widely applied in various fields, such as food, medicine, and cosmetics (Mineroff and Jagdeo, 2023)

As people's awareness of the nutritional value and health benefits of T. fuciformis has grown, the deep-processing industry has developed rapidly. T. fuciformis is incorporated into other products as a food excipient, to enhance the quality of the products in question (Wang et al., 2023). The study indicated that replacing 75% of pork fat with T. fuciformis significantly improved the sausage's textural properties and sensory quality and increased essential and non-essential amino acid levels (Hu et al., 2021). By incorporating soluble dietary fiber from steam explosion T. fuciformis stems into the biscuits, the gluten network structure and moisture distribution in the biscuits were significantly improved (Wang et al., 2024). Adding 3% T. fuciformis powder to bread increased the bread's hardness, chewiness, gumminess, and moisture content. When 10% T. fuciformis powder was added, it reduced the glucose release of the bread by 23.81% (Feng et al., 2023). Nevertheless, only a limited number of deep-processed products utilize T. fuciformis as the primary raw material. These include T. fuciformis beverage (Hou, 2020), freeze-dried T. fuciformis soup (Li, 2022) and ITF soup (Yang et al., 2023). The portability of freeze-dried T. fuciformis soup is a notable advantage. However, this convenience is offset by the ease with which moisture is absorbed and the high cost of freeze-drying. Although ITF soup is convenient to consume, it is large, heavy, and difficult to carry. Furthermore, it does not offer a hot drink, a significant drawback. The development of ITF has the potential to address these shortcomings. Drying is the key technology used in this product. The conventional method of hot air drying is a lengthy process that necessitates using elevated temperatures, which can result in discoloration and a suboptimal visual presentation of the sample (Sun et al., 2023a). The freeze-drying process is lengthy and energy-intensive (Li, 2022). Heat pump drying is a highly efficient and energy-saving method that utilizes the principles of the inverse Carnot cycle. It is widely applied in food drying due to its low energy consumption, precise temperature control, and broad applicability (Hao et al., 2022; Fernando et al., 2021; Zhao et al., 2022). Therefore, to address the shortcomings of existing products with dried T. fuciformis as the main ingredient, the use of heat pump drying and the development of products that simultaneously meet the requirements of portability, ready-to-eat convenience, energy saving and cost reduction, and good quality are expected to have a broad market prospect (Liu et al., 2023; Bogard et al., 2024).

This study made ITF as the research object, the moisture migration characteristics of ITF at different drying temperatures were analyzed with LF-NMR. The effects of different heat pump drying temperatures on the quality of the ITF were investigated using the fuzzy mathematical evaluation method. The heat pump drying curve and drying rate curve of ITF were plotted based on the optimal heat pump drying temperature of 45 °C, and the best heat pump drying fitting curve of ITF was screened from four classical mathematical models. This study can provide theoretical support for the processing improvement and quality enhancement of ITF products.

2 Materials and methods

2.1 Materials

T. fuciformis was purchased by Fujian Xiangyun Biotech Development Co., Ltd. After removing the inedible parts of the sample, it was stored in a −20 °C refrigerator for preservation.

2.2 The preparation of instant T. fuciformis

Took 300 g of cleaned T. fuciformis and blanched it in boiling water at 100 °C for 2 min. After blanching, remove the water from the T. fuciformis. Then, place the sample in a food blender (Midea MJ-LZ35Easy236, China) and blend at 2500 r·min−1 for 5 s. Subsequently, evenly spread in a plastic drying tray and dried to a constant weight in a heat pump dryer (XUEFENG ZWH-KFX-BT12, China) (at temperatures of 35 °C, 45 °C, 55 °C, with humidity maintained at 20%). Took 2 g of instant T. fuciformis and placed it in a thermos, added 200 mL of hot water at 100 °C, and let it stand for 5 min. They then separated the filter residue and supernatant to determine the texture and the viscosity, respectively.

2.3 Dry basis water content

The following was the formula for calculating the moisture content Fi during the heat pump drying process:(1) Fi=mi(1−F)×m0×100

The mi represented the sample mass at a specific time during the heat pump drying process, g. m0 represented the sample mass when it reached a constant weight during drying, g. Fi represented the moisture content of the sample at a specific time during drying, %. F represented the moisture content of the sample when it reached a constant weight during drying, %.

2.4 Moisture migration characteristics

The samples were prepared according to section 2.2. After placing the samples in sizes of 30 mm × 20 mm × 10 mm, they were dried in heat pump dryers set at temperatures of 35 °C, 45 °C, and 55 °C. Samples were weighed every hour, and simultaneously, the moisture migration characteristics of the samples were analyzed using an LF-NMR analyzer (Niumag NMI20-040H-I, China). The CPMG sequence had a sampling frequency of 100 kHz, a main frequency of 20 MHz, an analog gain of 20 dB, and 90° and 180° pulse of 6.6 μs and 11.6 μs, respectively. The interval between two scans was 4000 ms, the number of repetitions was 3 times, the echo time was 0.6 s, and there were 9000 echoes. The measured data was fitted using the SIRT 1000000 algorithm.

During the LF-NMR analysis, the samples were simultaneously scanned to obtain the pseudo-color T2-weighted images of the samples. The scanning parameters were as follows: the repetition time (TR) was 1000 ms, the echo time (TE) was 19.94 ms, and the slice thickness was 1 mm, with three repetitions.

2.5 Rehydration ratio

The 2 g samples dried at different heat pump temperatures were weighed to a thermos, into which 200 mL of water at 100 °C was added. After waiting for 5 min, the surface moisture of the samples was removed and weighed. The following was the formula for the rehydration ratio.(2) R=mm1×100

R represented the rehydration ratio of the sample, with m representing the weight of the sample after rehydration and m1 representing the weight of the sample before rehydration.

2.6 Hunter whiteness

The colorimeter (Threenh NS810, China) was used to measure the ITF samples prepared after drying at different heat pump temperatures and their rehydrated samples. The following was the formula for calculating the Hunter whiteness of the sample.(3) W=100−(100−L*)2+a*2+b*2

W represented the Hunter whiteness of the sample, with L*, a*, and b* representing the values for the sample's lightness, redness, and yellowness, respectively.

2.7 Texture characteristics

The TA.XT Texture Analyzer (Stable Micro Systems, UK) equipped with a P36/R probe was used to measure the texture characteristics of the rehydrated ITF. 30 g of rehydrated ITF was filled into a 100 mL beaker, trigger force = Auto, pre-test speed = 1 mm s−1, test speed = 2 mm s−1, post-test speed = 2 mm s−1, and displacement distance = 15 mm, and the repeated measurements were carried out three times.

2.8 Rheological features

The viscosities of the ITF supernatant determined after drying at different heat pump temperatures were measured using a rheometer (Anton Paar MCR 102e, China) at 36 °C and 50 °C, the shear rate of 1 s−1 and a strain of 1%.

2.9 Polysaccharide content

5 mL of water and 20 mL of ethanol were added into 2 g of sample, followed by ultrasonic extraction for 30 min. After the extraction, the sample was centrifuged to discard the supernatant. After discarding the supernatant, 50 mL of water was added to the sample, and ultrasonic extraction was performed three times for 30 min each. The supernatant was then separated, and the volume was adjusted to 200 mL. After volume adjustment, 1 mL of the sample was mixed with 1 mL of 5% phenol and 5 mL of sulfuric acid. The mixture was then reacted in the dark at 30 °C for 30 min. Finally, the absorbance of the sample was measured at 490 nm using a UV–Vis spectrophotometer (Purkinje GENERAL TU-1810, China), and the polysaccharide content was calculated.

2.10 Sensory evaluation

The sensory evaluation panel, consisting of 10 professionally trained panelists (five male and five female), evaluated the color, texture, and aroma of filter residue of ITF prepared at different heat pump temperatures.

2.11 Fuzzy comprehensive evaluation

The fuzzy mathematical evaluation method has been slightly modified based on the method reported in the literature (Peng et al., 2008). Based on the evaluation and analysis of different quality indicators of ITF. The test group to be evaluated was set as X= (x1, x2, x3), and the factor set was determined as U = (u1, u2, u3, u4), where u1 represented moisture content; u2 represented rehydration ratio; u3 represented polysaccharide content; and u4 represented sensory score. Then, the subjective experience judgment method is used to determine the weight set according to the contribution of each factor to the product quality. Weight set A = {a1 (0.2), a2 (0.2), a3 (0.2), a4 (0.4)}. The comprehensive evaluation Y was obtained by calculating the weighted sum based on the importance of moisture content, rehydration ratio, crude polysaccharide content, and sensory score. The formula for the comprehensive evaluation Y is as follows.(4) Yi=Xi1max−Xi1Xi1max×a1+Xi2Xi2max×a2+Xi3Xi3max×a3+Xi4Xi4max×a4(i=1,2,3)

The Ximax represented the maximum value among the variables Xi (i = 1, 2, 3) under the influence of factor U.

2.12 Drying curve and drying rate curve

Took cleaned and fresh T. fuciformis and blanched it in boiling water at 100 °C. After blanching, remove the water from the T. fuciformis. Then, place the sample in a food blender (Midea MJ-LZ35Easy236, China) and blend at 2500 r·min−1 for 5 s. Subsequently, evenly spread in a plastic drying tray (the thickness is about 1 cm) and dried to a constant weight in a heat pump dryer (XUEFENG ZWH-KFX-BT12, China) at temperatures of 45 °C, with humidity maintained at 20%. Finally, the weight (m) is measured every 30 min (n = 3). Calculated the moisture content (Fi) and drew the drying curve according to formula (1), and calculated the drying rate (Di) and drew the drying rate curve according to formula (5).(5) Di=Fi−Fi+1ti+1−ti

Di represented the moisture content evaporated per unit time of ITF samples during the heat pump drying process, min−1; Fi represented the moisture content of ITF samples during the heat pump drying, %; t was the heat pump drying time, min.

2.13 Statistical analysis

The images were drawn using Graphprism software and Origin 2022 software. The data was expressed as mean ± standard deviation (SD), and one-way ANOVA was employed using SPSS 22 to analyze the significance between different treatment groups. Different lowercase or uppercase letters represent statistically significant differences at p < 0.05 or p < 0.01, respectively.

3 Results and discussion

3.1 Moisture migration characteristics

3.1.1 The changes in water components during the drying process

LF-NMR is a novel non-destructive testing technique that has been widely applied in the study of moisture migration characteristics in food. The changes in the transverse relaxation time of hydrogen protons in water can reflect the water freedom in ITF (Zhu et al., 2018; Cheng et al., 2019). A shorter transverse relaxation time indicates a higher degree of water binding to the substrate in the sample. In contrast, a longer transverse relaxation time suggests higher water freedom in the sample (Jiang et al., 2021). As shown in Fig. 1, ITF had three different water components, components bound water (T21), immobilized water (T22), and free water (T23), distributed within the ranges of 0.1–10 ms, 10–100 ms, and 100–1000 ms of the relaxation times (X-axis), respectively. The water components of ITF were found to be similar under different heat pump drying temperatures. Over time (Z axis), the water in the sample evaporated, the peak areas of T22 and T23 also decreased continuously, and the product only contained T21 (located on the far left of the X axis with the relaxation time 0.1–10 ms) at the end of drying, indicating that heat pump drying can change the water composition and water content of ITF. In other words, the longer the drying time, the trend of the change of the composition of the water in the sample was to migrate towards the component T21 with a smaller relaxation time (migration leftward). The findings followed the established principles of water transfer during microwave vacuum drying vegetables (Lv et al., 2017). In addition, it had been reported that water with higher degrees of freedom (T22 and T23) was more likely to evaporate, and water with lower degrees of freedom (T21) was more difficult to evaporate. Hence, the freedom of water continued to decrease during the drying process (Sun et al., 2023a). Furthermore, the reduction of free water in the early stages of drying ITF was most pronounced. The higher the drying temperature, the faster the decrease in the signal amplitudes of free water and immobilized water (Si et al., 2023), indicating that the drying temperature of the heat pump might be an essential factor affecting the drying rate of ITF.Fig. 1 Variation of T2 inversion spectrum with drying time under at (A) 35 °C, (B) 45 °C, and (C) 55 °C heat pump drying temperature of instant T. fuciformis.

Fig. 1

3.1.2 The migration characteristics of different water components

The changes in free water, immobilized water, and bound water during the heat pump drying process of ITF under different drying temperatures were illustrated in Fig. 2. In the drying process, heat was usually first applied to the surface of the ITF, which made it easier for surface free water (T23) to come into contact with the air, obtain enough energy to convert into steam, have a higher evaporation rate and, as a result, create a moisture gradient on the surface and inside the ITF. Secondly, when the ambient air of the ITF was in an unsaturated state, water would be driven from the high-moisture area (inside the ITF) to the low-moisture area (surrounding air) under the action of the moisture gradient. Zhou et al. (2015) found that both temperature-induced water vapour pressure gradient and moisture gradient were the main driving forces for moisture movement during microwave drying of lumber. The temperature gradient and moisture gradient of corn kernels in hot air drying were symmetrically distributed around their geometrical centers, almost equal in magnitude but opposite in direction (Sun et al., 2023b). Thirdly, during the heat pump drying process, there was a rapid air flow, so the water from the inside of the ITF diffused to the surface, and then evaporated from the surface into the air. Mai et al. (2011) found that the temperature difference could directly drive the drying process of maize. Still, the water migration rate caused by air convection was much higher than that driven by temperature gradient. During the drying period of 0 h–7 h, the peak area of free water (M23, representing moisture content of free water) under different temperatures showed a decreasing trend, with the fastest decrease observed at 55 °C, which was consistent with the significant reduction in signal intensity of free water in the early stages of drying. The proportion of free water (A23) exhibited an initial increase followed by a decline. During the 0–7 h drying period, the peak areas of immobilized water (M22) under different heat pump drying temperatures all showed a decreasing trend, reaching a stable value. The proportion of immobilized water (A22) exhibited an initial decrease followed by an increase, as immobilized water continuously decreased in content during the drying process. Still, in the later drying stages, free water's content decreased rapidly while immobilized water reached a stable state. A clear reduction in T21 content (M21) was observed and a proportion of the water with a low binding degree was removed when the drying time was 0 ∼ 1h. As the drying time increased to 1 ∼ 3h, a considerable quantity of T23 was lost through evaporation, while a small proportion of T23 combined with nutrients and other substances, thereby increasing the content of T21 (M21) (Xue et al., 2020; Yu et al., 2022). When the drying time was 3 ∼ 4h, combining T23 with nutrients was less efficient than evaporating T21. Consequently, the overall T21 content (M21) was reduced, and the peak area of T23 and T22 was close to 0 between 4 and 7h. Furthermore, the water with a low binding degree was transformed into T22, which was subsequently removed. The remaining highly bound water was challenging to remove under the prevailing dry conditions, resulting in a tendency for the peak area of T21 (M21) to stabilize during this period. Nevertheless, the proportion of T21 (A21) continued to increase throughout the drying process. This might be attributed to the gradual migration of T23, which reduced the total water volume of the material. In contrast, T21 exhibited a strong binding force and high migration resistance, leading to an upward trend in its proportion (A21). The outcomes were comparable to those of previous investigations on heat pump drying of kiwifruit slices (Liu et al., 2020).Fig. 2 Changing trend of different water components at different heat pump drying temperatures.

Fig. 2

3.1.3 The pseudo-color T2-weighted images

Fig. 3 presents the pseudo-color T2-weighted images of ITF at different heat pump drying temperatures. The color of the pictures represents the H+ proton density, which can indicate changes in the sample's moisture content (Lu et al., 2016; Xu et al., 2021). Before drying, ITF had a high moisture content and uniform moisture distribution. When the drying time was 0 h, the uniform yellow color of the image indicates the uniform distribution of water in the sample. As the drying time was extended, the color distribution area of the sample gradually diminished from the outer to the interior, indicating that the outer part of the ITF first lost water. The middle water evaporated, and finally, the central part of the sample water evaporated. As the drying temperature increases, the rate of shrinkage of the water distribution area accelerates. These findings are in accordance with the results of the moisture migration analysis presented in Fig. 2. Additionally, at the same drying time, the area of the pseudo-color T2-weighted images decreased with increasing temperature (Wang et al., 2021). The higher the drying temperature, the shorter the time required for the pseudo-color T2-weighted photos to turn into the background color. By the later drying stage, the sample's external moisture was almost completely removed, leaving only a trace amount of moisture in the central part of the sample. At the endpoint of drying, the image turned entirely into the background color (Rao et al., 2020). Therefore, the moisture content in the central part of the ITF could be used to determine the endpoint of drying for the sample.Fig. 3 Effect of different heat pump drying temperature processes of instant T. fuciformis on pseudo-color T2-weighted images. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3

3.2 Dry basis water content and rehydration ratio

Fig. 4 shows that the water content of all three temperature-treated groups was below 10% after heat pump drying. The moisture content at 55 °C drying temperature of the sample was significantly lower compared to the two groups (p < 0.05), while there was no significant difference between the 35 °C and 45 °C groups. By reducing the sample's moisture content, the activity of enzymes and the growth of microorganisms within the sample would be inhibited, thereby extending the storage period of the sample (Jaworska et al., 2020). Therefore, ITF subjected to heat pump drying at 55 °C may have better quality and an extended shelf life. Rehydration is necessary to restore dried products to an edible state (Li et al., 2022). The rehydration ratio measures the degree to which a dried product can be restored to its original edible state. The rehydration ratio of ITF dried at 45 °C using heat pump drying was the highest and significantly more significant than the other two groups (p < 0.05). The rehydration rate of the 35 °C group was the lowest, which may be due to the long drying time and slow water migration rate under the condition of low drying temperature, which was not conducive to the formation of porous structures. The results were in accordance with those previously observed by Li et al. (2017) on Chinese yam utilizing heat pump drying.Fig. 4 Effect of heat pump drying temperatures on water content and rehydration ratio of instant T. fuciformis. Different lowercase letters represent statistically significant differences at p < 0.05.

Fig. 4

3.3 Hunter whiteness

Fig. 5 showed that after processing at different drying temperatures, the Hunter whiteness values of ITF ranged between 38 and 47. It is worth mentioning that the Hunter whiteness values of filter residue of ITF significantly decreased with increasing drying temperature (p < 0.05). This might be attributed to excessively high drying temperatures, causing a solid color change in the T. fuciformis, resulting in a yellowish appearance (Xu et al., 2022).Fig. 5 Effect of heat pump drying temperatures on Hunter whiteness of instant T. fuciformis. Different lowercase letters represent statistically significant differences at p < 0.05. Different uppercase letters represent statistically significant differences at p < 0.01.

Fig. 5

3.4 Texture characteristics

Texture is an important indicator affecting the sensory evaluation of food. Firmness can reflect the crispness and mouthfeel of the sample, while consistency can reflect the fluidity of the sample, and cohesiveness can reflect the compactness of the sample structure. Consistency and viscosity indicators are used to judge the viscosity of the sample. For ITF, appropriate viscosity and softness are the key factors determining its mouthfeel (Zheng et al., 2023). From Table 2, it can be seen that the firmness of the T. fuciformis particles in the 35 °C group was significantly higher than that of 45 °C and 55 °C groups (p < 0.05), indicating that the mouthfeel of ITF prepared at lower temperatures may be inferior. The consistency of the samples in the 45 °C group was the highest, significantly higher than the other two groups (p < 0.05).Table 1 Sensory evaluation index of instant T. fuciformis.

Table 1Parameter	Description	Score	
Color	Silvery white.	21–30	
Light yellow.	11–20	
Deep yellow.	1–10	
Texture	Moderate viscosity.	24–40	
Excessive or deficient viscosity.	13–24	
Unacceptable viscosity.	1–13	
Aroma	Intense characteristic fragrance.	21–30	
Faint fragrance.	11–20	
None fragrance.	1–10	

Table 2 Effect of heat pump drying temperature on the texture of filter residue and the viscosity of supernatant of instant T. fuciformis after brewing.

Table 2Drying temperature	Texture of filter residue	Viscosity of supernatant （Pa·s）	
Firmness（g）	Consistency（gsec）	Cohesiveness（g）	36 °C	50 °C	
35 °C	259.72 ± 3.87 a	939.90 ± 4.98 b	69.06 ± 0.79 a	21.05 ± 1.31 b	13.97 ± 0.77 b	
45 °C	215.73 ± 1.34 b	976.87 ± 6.22 a	66.03 ± 4.84 a	45.01 ± 3.61 a	27.16 ± 1.44 a	
55 °C	211.90 ± 5.56 b	945.11 ± 4.70 b	66.63 ± 5.38 a	5.73 ± 0.61 c	3.98 ± 0.27 c	
Note: Different lowercase letters in the same column of data represent statistically significant differences at p < 0.05, the same below.

Viscosity is one of the important criteria for sensory evaluation of ITF soup (Li et al., 2020), and inappropriate viscosity can reduce consumers' acceptance of ITF. Analyzing the viscosity of samples by observing the rheological characteristics of the samples is a common method in food processing and analysis (Huang et al., 2016). The product of the 45 °C group had the highest viscosity at the temperatures of 36 °C and 50 °C, and the viscosity was higher at the temperature of 36 °C than at 50 °C. This is because the increase in temperature promotes intermolecular activity and causes viscosity to decrease. T. fuciformis, known as the “edible bird nest of ordinary people”. T. fuciformis is also rich in natural plant gum, making viscosity an important quality indicator of ITF (Xue, 2013). A temperature of 36 °C was optimal for consumption of food and drink intake. At this temperature, the supernatant of ITF dried at 45 °C exhibited the requisite viscosity, following consumer expectations.

3.5 Polysaccharides content

Research has shown that T. fuciformis polysaccharides are a functional macromolecule in T. fuciformis, not only possessing functions such as lowering blood sugar and lipids, anti-cancer, antioxidant, anti-aging effects but also serving as an immune enhancer, effectively improving the body's immune function (Bach et al., 2015; Lee et al., 2023; Jiang et al., 2012). Therefore, using polysaccharide content as an essential indicator for developing new T. fuciformis products is important. Fig. 6 shows that the sample with a drying temperature of 35 °C had the highest polysaccharide content, significantly different from samples dried at temperatures of 45 °C and 55 °C.Fig. 6 Effect of heat pump drying temperature on polysaccharide content of instant T. fuciformis. Different lowercase letters represent statistically significant differences at p < 0.05.

Fig. 6

3.6 Sensory evaluation

According to the sensory evaluation criteria (Table 1) and combined with Table 3, it can be concluded that the color of ITF did not show significant changes with the increase in drying temperature and remained slightly white. The texture of all three treatment groups was moderate, and they all had the unique aroma of T. fuciformis. The total score of ITF dried at 45 °C by heat pump drying was the highest, and therefore, it might be selected as the optimal drying temperature.Table 3 Effect of heat pump drying temperature on the sensory score of instant T. fuciformis after brewing.

Table 3Drying temperature	Sensory score	
Color	Texture	Aroma	Sensory total score	
35 °C	23.00 ± 1.61 a	29.66 ± 3.31 a	23.20 ± 1.94 a	75.86 ± 5.99 a	
45 °C	24.00 ± 1.55 a	29.26 ± 4.94 a	24.40 ± 0.80 a	77.66 ± 6.57 a	
55 °C	22.20 ± 2.75 a	28.86 ± 1.98 a	22.10 ± 2.77 a	73.16 ± 6.04 a	

3.7 Fuzzy comprehensive evaluation

Using water content, rehydration ratio, polysaccharide content, and sensory score as indicators, a comprehensive score was obtained for ITF prepared by different heat pump drying through the fuzzy mathematical evaluation method. The ITF produced at a heat pump drying temperature of 45 °C was found to have the highest comprehensive score of 77.85 (Table 4). Therefore, the heat pump drying temperature 45 °C was selected to prepare ITF and establish subsequent drying curves and corresponding mathematical models.Table 4 Influence of heat pump drying temperature on the results of the fuzzy mathematical comprehensive evaluation of instant T. fuciformis.

Table 4Drying temperature		Water content
（%）	Rehydration ratio
（%）	Polysaccharide content
（g·100 g−1）	Sensory total score	Comprehensive score	
35 °C		8.76 ± 0.12 a	3656.00 ± 23.00 c	4.04 ± 0.08 a	75.86 ± 5.99 a	/	
45 °C		8.51 ± 0.66 a	4803.00 ± 90.00 a	3.49 ± 0.09 b	77.66 ± 6.57 a	/	
55 °C		6.08 ± 0.46 b	4331.00 ± 79.00 b	2.79 ± 0.07 c	73.16 ± 6.04 a	/	
Data normalization	Weight	0.20	0.20	0.20	0.40		
35 °C		0.00	0.76	1.00	0.98	74.30	
45 °C		0.03	1.00	0.86	1.00	77.85	
55 °C		0.31	0.90	0.69	0.94	75.65	
Note: “/” indicates no data.

3.8 Heat pump drying curve and drying fitting curve

It can be observed from Fig. 7A that under the drying temperature of 45 °C, the moisture content of ITF decreased with the increase of drying time, and the moisture content of T. fuciformis reached a stable state after drying to a certain degree. Starting from the 6th hour of heat pump drying, the moisture content of ITF reached 8.51% and remained stable. In Fig. 7B, it was evident that there are four drying inflection points in the drying process of the ITF, namely A, B, C, and D. The drying process was divided into three stages, the AB stage was the preheating period, the BC stage was the constant rate period, and the CD stage was the deceleration period.Fig. 7 Heat pump drying curve and drying rate curve of instant T. fuciformis. A ∼ D indicated the drying inflection points.

Fig. 7

Fig. 8 showed the kinetic study of drying characteristics is significant for understanding the moisture diffusion mechanism and quality control during the drying process. In this experiment, four drying models were selected for studying the drying kinetics of ITF: (1) Newton model: y = exp (-kx), (2) Page model: y = exp (1 (exp (lnk + nlnx))), (3) diffusion approximation equation: y = aexp (-kx)+(1-a)exp (-kax), and (4) cubic regression equation: y = ax3+bx2+cx + d (Kumari and Khatkar, 2018; Shi et al., 2013; Figiel, 2010). The fitting degree of these models was evaluated, and the model with the highest fitting degree was selected as the mathematical model for ITF. The results showed that the curve fitted by the cubic regression equation model was closer to the actual value, and the coefficient of determination (R2) value was 0.99397 and close to 1. The cubic regression equation can better reflect the moisture content variation of ITF with drying time. This study lays the foundation for using regression equations to calculate its drying inflection point time. The curves fitted by the other three models were not close to the actual value, and the R2 values were all less than 0.8 (R2 = −0.62372 in the Newton model, R2 = 0.3963 in the Page model, and R2 = 0.56277 in the approximate diffusion model). Therefore, the cubic regression equation model [y = ax3+bx2+cx + d, a = −2.08187 ± 0.19248, b = 73.14102 ± 4.59506, c = −817.88955 ± 29.45114, d = 2909.80904 ± 49.2892 (R2 = 0.99397)] was selected as the best model for the heat pump drying curve of ITF. In addition, the cubic regression equation model had two intersection points with x, which were x1 = 7.1979 and x2 = 10.8255. The fitting curve began to show a decreasing and then increasing trend in this interval, but the moisture content would decrease with the increase of heat pump drying time until it reached stability. Therefore, the fitting curve after x1 = 7.1979 did not have any reference significance.Fig. 8 Heat pump drying fitting curve of instant T. fuciformis.

Fig. 8

4 Conclusion

LF-NMR technology was used to analyze the moisture migration rules and distribution characteristics of bound water (T21), immobilized water (T22), and free water (T23) in ITF during heat pump drying. The research results indicated that before the drying process, ITF contained three types of moisture (The order of relaxation time from largest to smallest was as follows: T23 > T22 > T21.). During ITF heat pump drying at temperatures of 35 °C, 45 °C and 55 °C, the law of water migration indicated that the freedom of water components continued to decrease, and water components were continuously transported to the component T21 with the smallest relaxation time (migration to the left). The pseudo-color T2-weighted images revealed the water distribution characteristics of ITF's heat pump drying process. The water was evenly distributed on the ITF before drying. As the drying time was extended, the water distribution area of the ITF gradually contracted from the outer to the interior. Furthermore, the rate of contraction in the water distribution area was accelerated with the increase in drying temperature (35 °C, 45 °C, 55 °C). The fuzzy mathematics evaluation method analyzed the key quality indexes of ITF at different heat pump drying temperatures (35 °C, 45 °C, 55 °C). The optimal temperature for heat pump drying of ITF was determined to be 45 °C. The comprehensive score of the key quality indexes of ITF at 45 °C was as high as 77.85%. In detail, the total sensory score of ITF was 77.66 ± 6.57, the rehydration ratio was 4803.00 ± 90.00%, the consistency was 976.87 ± 6.22 gsec, and the viscosity was 45.01 ± 3.61 Pa s. Finally, a heat pump drying curve and a drying rate curve for ITF were established at a heat pump drying temperature of 45 °C. On this basis, a theoretical model of the drying curve was constructed. The cubic regression equation y = ax3+bx2+cx + d (R2 = 0.99397) had the best-fitting effect among the four mathematical models. Theoretical models could rapidly predict drying times, endpoints, and product moisture.

Compared with the traditional hot-air-dried ITF at 60 °C, the heat-pump-dried ITF at 45 °C has a looser apparent structure (Fig. S1), significantly improved rehydration ability by more than 2 times, increased viscosity of the supernatant at 36 °C and 50 °C after brewing by more than 1 time, and reduced the hardness of filter residue by 82.65%. However, the heat pump dried ITF retains the original bright yellow color (lower whiteness) and there is no difference in moisture content between the two products (Table S1).

This study improved the heat pump drying process of ITF products, with the optimum drying temperature of 45 °C and drying time of 7–8 h. Under the optimal drying process conditions, the product quality was significantly improved, including the rehydration rate of ITF increased by 10.90–31.37%, and the viscosity of the supernatant increased by 113.82–685.51%. This product meets the requirements of portability, instant convenience, energy saving and cost reduction, and good quality, and basically solves the shortcomings of existing ITF products. Further research into multistage drying modes (where the drying temperature varies between 35 °C and 55 °C during a single drying process) is expected to achieve more uniform moisture distribution and better results.

Fundings

This work was supported by funding from the 10.13039/501100005270 Fujian Provincial Department of Science and Technology [2022R1032005 , 2023R1099 , 2023J01201 , and 2023R1100 ], 10.13039/501100018914 Fujian Academy of Agricultural Sciences [CXTD2021018-2 , CXPT2023009 , YC20210007 and ZYTS2024017 ], Fujian Provincial People's Government-Chinese Academy of Agricultural Sciences Agricultural High-quality Development Surpasses "5511" Collaborative Innovation Project (XTCXGC2021014 ), Fujian Province Modern Edible Fungus Industry Technology System Construction Project (Min-caizhi [2019] No.897).

CRediT authorship contribution statement

Li Wu: Conceptualization, Visualization, Methodology, Data curation, Writing – original draft, Writing – review & editing. Shouhui Chen: Writing – original draft, Visualization, Software, Data curation, Supervision. Yibin Li: Supervision, Writing – review & editing, Project administration, Funding acquisition, Writing – review & editing. Wei Xie: Methodology, Data curation, Visualization. Baosha Tang: Conceptualization, Supervision, Funding acquisition, Writing – review & editing, Project administration, All authors have read and agreed to the published version of the manuscript.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A Supplementary data

The following is the Supplementary data to this article.Multimedia component 1

Multimedia component 1

Data availability

Data will be made available on request.

Appendix A Supplementary data to this article can be found online at https://doi.org/10.1016/j.crfs.2024.100825.
==== Refs
References

Bach E.E. Costa S.G. Oliveira H.A. Silva J.A. da Silva K.M. de Marco R.M. Use of polysaccharide extracted from Tremella fuciformis berk for control diabetes induced in rats Emir. J. Food Agric. 27 7 2015 585 591
Bogard J.R. Downs S. Casey E. Farrell P. Gupta A. Miachon L. Convenience as a dimension of food environments: a systematic scoping review of its definition and measurement Appetite 194 2024 107198
Cheng S. Ranran L. Yang H. Wang S. Lin R. Tan M. Characterisation of moisture migration of shiitake mushroom (Lentinula edodes) during storage and its relationship to quality deterioration International Journal of Food Science & Technology 55 5 2019 2132 2140
China Edible Fungi Association Analysis of the results of the national edible fungi statistical survey in 2021 [2023-03-13] https://mp.weixin.qq.com/s/RQ_iz6ROjGUz07ywpHTnjg 2021
Deng W. Wu L. Xiao Z. Li Y. Zheng Z. Chen S. Structural characterization and anti-inflammatory activity of polysaccharides from Tremella fuciformis on monosodium urate-stimulated RAW264.7 macrophages Foods 12 24 2023 4398 38137202
Feng H. Luo L.F. Wang L.Y. Ding Y.Y. Sun L.P. Zhuang Y.L. Effects of Tremella aurantialba on physical properties, in vitro glucose release, digesta rheology, and microstructure of bread J. Food Sci. 88 12 2023 4853 4866 37872789
Fernando A.J. Amaratunga K.S.P. Madhushanka H.T.N. Jayaweera H. Drying performance of coffee in a batch-type heat pump dryer Transactions of the Asabe 64 4 2021 1237 1245
Figiel A. Drying kinetics and quality of beetroots dehydrated by combination of convective and vacuum-microwave methods J. Food Eng. 98 4 2010 461 470
Hao W.G. Liu S.N. Lai Y.H. Wang M.T. Liu S.Z. Research on drying Lentinus edodes in a direct expansion heat pump assisted solar drying system and performance of different operating modes Renew. Energy 196 2022 638 647
Hou Y. Study on processing technology and plant design of milk beverage with tremella and lily Food Ferment. Ind. 46 1 2020 210 214
Hu H.W. Li Y. Zhang L. Tu H.J. Wang X.Y. Ren L.L. Use of tremella as fat substitute for the enhancement of physicochemical and sensory profiles of pork sausage Foods 10 9 2021 2167 34574276
Huang J.J. Zeng S.W. Xiong S.B. Huang Q.L. Steady, dynamic, and creep-recovery rheological properties of myofibrillar protein from grass carp muscle Food Hydrocolloids 61 2016 48 56
Jaworska G. Sidor A. Pycia K. Jaworska-Tomczyk K. Surówka K. Packaging method and storage temperature affects microbiological quality and content of biogenic amines in Agaricus bisporus fruiting bodies Food Biosci. 37 2020 100736
Jiang M.X. Wu P. Xing H.H. Li L. Jia C. Chen S. Water migration and diffusion mechanism in the wheat drying Dry. Technol. 39 6 2021 738 751
Jiang R.Z. Wang Y. Luo H.M. Cheng Y.Q. Chen Y.H. Gao Y. Gao Q.P. Effect of the molecular mass of Tremella polysaccharides on accelerated recovery from cyclophosphamide-induced leucopenia in rats Molecules 17 4 2012 3609 3617 22447024
Kumari P. Khatkar B.S. Nutritional composition and drying kinetics of aonla fruits Journal of Food Science and Technology-Mysore 55 8 2018 3135 3143
Lee Q.C. Han X.J. Zheng M.F. Lv F. Liu B. Zeng F. Preparation of low molecular weight polysaccharides from Tremella fuciformis by ultrasonic-assisted H2O2-Vc method: structural characteristics, in vivo antioxidant activity and stress resistance Ultrason. Sonochem. 99 2023 106555
Li S.H. Yan Z.H. Zhang Z.X. Zhang Z.H. Technology research of Chinese yam heat pump drying Grain Processing 42 6 2017 44 48
Li Y. Preparation and Recover Ability Research of Instant Tremella Fuciformis Soup 2022 Jiangnan University
Li Y.B. Chen J.C. Lai P.F. Tang B.S. Wu L. Influence of drying methods on the physicochemical properties and nutritional composition of instant Tremella fuciformis Food Sci. Technol. 40 3 2020 741 748
Li Y. Li J.W. Fan L.P. Effects of combined drying methods on physicochemical and rheological properties of instant Tremella fuciformis soup Food Chem. 396 2022 133644
Liu W.C. Zhang M. Mujumdar A.S. Chen J.J. Role of dehydration technologies in processing for advanced ready-to-eat foods: a comprehensive review Crit. Rev. Food Sci. Nutr. 63 22 2023 5506 5520 34961367
Liu Y.H. Zeng Y. Hu X.Y. Sun X. Effect of ultrasonic power on water removal kinetics and moisture migration of kiwifruit slices during contact ultrasound intensified heat pump drying Food Bioprocess Technol. 13 2020 430 441
Lu N.Y. Zhang L. Zhang X. Li J. Labuza T.P. Zhou P. Molecular migration in high-protein intermediate-moisture foods during the early stage of storage: variations between dairy and soy proteins and effects on texture Food Res. Int. 82 2016 34 43
Lv W.Q. Zhang M. Bhandari B. Li L.L. Wang Y.Q. Smart NMR method of measurement of moisture content of vegetables during microwave vacuum drying Food Bioprocess Technol. 10 2017 2251 2260
Ma X. Yang M. He Y. Zhai C.T. Li C.L. A review on the production, structure, bioactivities and applications of Tremella polysaccharides Int. J. Immunopathol. Pharmacol. 35 2021 20587384211000541
Mai Z.W. Li C.Y. Zhang Y. Xu F.Y. Lu Y.L. Research on corn drying process under temperature gradient Adv. Mater. Res. 314 2011 1502 1506
Mineroff J. Jagdeo J. The potential cutaneous benefits of Tremella fuciformis Arch. Dermatol. Res. 315 7 2023 1883 1886 36757441
Peng B.Z. Yue T.L. Yuan Y.H. A fuzzy comprehensive evaluation for selecting yeast for cider making Int. J. Food Sci. Technol. 43 1 2008 140 144
Rao W.L. Wang Z.Y. Li G.X. Meng T.T. Suleman R. Zhang D.Q. Formation of crust of dried meat and its relationship to moisture migration during air drying J. Food Process. Preserv. 44 1 2020 E14255
Shi Q.L. Zheng Y.Q. Zhao Y. Mathematical modeling on thin-layer heat pump drying of yacon (Smallanthus sonchifolius) slices Energy Convers. Manag. 71 2013 208 216
Si M.J. Zhu W.X. Bai X.T. Luo L. Ning Y.Y. Drying characteristics of tiger nut heat pump based on low-field nuclear magnetic resonance technology Journal of the Chinese Cereals and Oils Association 38 5 2023 19 26
Sun M.L. Xu Y. Ding Y.Y. Gu Y. Zhuang Y.L. Fan X.J. Effect of ultrasound pretreatment on the moisture migration and quality of Cantharellus cibarius following hot air drying Foods 12 14 2023 2705 37509798
Sun T.S. Wang J.Z. Ling F. Zhu L. Hot‐air drying of corn kernels: drying kinetics and quality improvement Cereal Chem. 100 4 2023 1015 1025
Wang C.R. Lin M.F. Li Y.B. Zhuang W.J. Guo Z.B. Effect of steam explosion modified soluble dietary fiber from Tremella fuciformis stem on the quality and digestibility of biscuits Int. J. Biol. Macromol. 265 2024 130905
Wang D.Q. Wang D.G. Yan T.X. Jiang W.F. Han X.Y. Yan J.F. Guo Y.R. Nanostructures assembly and the property of polysaccharide extracted from Tremella Fuciformis fruiting body Int. J. Biol. Macromol. 137 2019 751 760 31254577
Wang H.C. Che G. Wan L. Liu M.G. Sun W.S. Experimental study on drying characteristics of rice by low-field nuclear magnetic resonance J. Food Process. Eng. 44 6 2021 E13705
Wang L. Zhang F. Zheng B.D. Zhang Y. Pan L. Stability and flavor of set yogurt fortified with Tremella fuciformis polysaccharide during cold storage Curr. Res. Food Sci. 7 2023 100536
Xu B. Chen J. Tiliwa E.S. Yan W. Azam S.R. Yuan J. Wei B. Zhou C. Ma H. Effect of multi-mode dual-frequency ultrasound pretreatment on the vacuum freeze-drying process and quality attributes of the strawberry slices Ultrason. Sonochem. 78 2021 105714
Xu J.X. Wang D.N. Lei Y.P. Cheng L.J. Zhuang W.J. Tian Y.T. Effects of combined ultrasonic and microwave vacuum drying on drying characteristics and physicochemical properties of Tremella fuciformis Ultrason. Sonochem. 84 2022 105936
Xue L. Study on the Effect of Drying Method on Tremella Fuciformis Quality and the Processing Technology of Instant Tremella Fuciformis Food 2013 Zhejiang University of Technology
Xue G. Li M. Guan Z.Q. Yang C.L. Moisture change of tilapia fillet in microwave vacuum drying process based on low-field nuclear magnetic resonance Journal of Guangdong Ocean University 40 6 2020 123 129
Yang Y.A. Niu B. Chen H.Z. Sun P.L. Chen H.J. Optimization of processing conditions and flavor of Snow Pear, Lotus Seed and Tremella Soup based on GA -BP neural network and genetic algorithm Chinese Journal of Nuclear Agriculture 37 12 2023 2395 2407
Yu Z.H. Zhu W.X. Bai X.T. Luo L. Wei Z.J. Rule of water changes in the far-infrared drying of tiger nut based on low-field NMR and imaging technology Food and Fermentation Industry 48 16 2022 131 137
Yuan H.J. Dong L. Zhang Z.Y. He Y. Ma X. Production, structure, and bioactivity of polysaccharide isolated from Tremella fuciformis Food Sci. Hum. Wellness 11 4 2022 1010 1017
Zhao H.B. Dai J.A. Wu K. Simulation of coupled heat-mass transfer in sea cucumbers with heat pump drying Applied Sciences-Basel 12 11 2022 5508
Zheng Z.P. Wu L. Li Y.B. Deng W. Chen S.H. Song H.B. Effects of different blanching methods on the quality of Tremella fuciformis and its moisture migration characteristics Foods 12 8 2023 1669 37107464
Zhou F.Y. Chen B.W. Miao P. Analysis of the moisture movement during microwave drying of lumber using NMR technology J. Anhui Agric. Univ. 42 1 2015 45 49
Zhu D. Liang J. Liu H. Cao X. Ge Y. Li J. Sweet cherry softening accompanied with moisture migration and loss during low‐temperature storage J. Sci. Food Agric. 98 10 2018 3651 3658 29250795
