Effects of microwave assisted pulse fluidized bed freeze-drying (MPFFD) on quality attributes of Cordyceps militaris

Food Bioscience 28 (2019) 7–14

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Food Bioscience journal homepage: www.elsevier.com/locate/fbio

Effects of microwave assisted pulse fluidized bed freeze-drying (MPFFD) on quality attributes of Cordyceps militaris

T

⁎

Xiao-fei Wua,b, Min Zhanga,c, , Bhesh Bhandarid, Zhongqin Lie a

State Key Laboratory of Food Science and Technology, Jiangnan University, Wuxi 214122, Jiangsu, China International Joint Laboratory on Food Safety, Jiangnan University, Wuxi 214122, Jiangsu, China c Jiangsu Province Key Laboratory of Advanced Food Manufacturing Equipment and Technology, Jiangnan University, Wuxi 214122, Jiangsu, China d School of Agriculture and Food Sciences, University of Queensland, Brisbane, QLD 4072, Australia e Yandi Biological Engineering Co., Ltd., Changde 415000, Hunan, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Cordyceps militaris Microwave assisted pulse fluidized bed freezedrying Hot air drying

The effect of microwave assisted pulse fluidized bed freeze-drying (MPFFD) on the quality of dried Cordyceps militaris was studied. To highlight the effect, the freeze-drying (recognized for its high-quality products) and hot air dying (the most commonly used) were also evaluated to compare the drying based on the drying characteristics, sensory qualities, volatile compounds and antioxidant activities of dried products. Results indicated that MPFFD helped to lower the rehydration time and shrinkage of dried samples. Furthermore, the rehydration ratio, color attributes, crispness, volatile compounds and antioxidant activities of dried samples were also conserved as compared to those obtained using hot air drying. Not only were the effects of MPFFD on the quality of C. militaris similar to those of freeze-dried samples, but the drying time of MPFFD was reduced by 71.9% when compared to freeze-drying. Therefore, MPFFD provides a potential method for achieving high-quality dried C. militaris with a faster drying time.

1. Introduction People in China have been using Cordyceps militaris, a member of the fungus or mushroom family, as both a nourishing food and as a traditional drug (Xiao et al., 2014). Nevertheless, freshly harvested C. militaris can deteriorate immediately, resulting in the loss of quality and nutrients. Drying is often used to preserve C. militaris and can effectively extend its shelf life. In the food industry, traditional hot air drying (HD) is the most commonly used and easily controlled method to dry a wide range of foods. Nevertheless, HD may result in inadequate sensory quality and nutrient reduction. Although freeze-drying (FD) is a wellaccepted drying method for producing high-quality dried products through sublimation under vacuum conditions, the high energy consumption because of its long drying time limits its wider application (Zhang et al., 2017). To overcome the shortcomings of FD, microwave energy is being used as the heating source (microwave freeze drying (MFD)). Compared to the conventional FD process, MFD is able to reduce drying time by 50% (Jiang, Zhang, Mujumdar, & Lim, 2014). Despite its unique advantages, ice melting and overheating may occur due to the corona discharge and non-uniform heating (Wang, Zhang,

⁎

Mujumdar & Mothibe, 2013). Wang et al. (2013) developed microwave assisted pulse fluidized bed freeze-drying (MPFFD), which uses microwaves in a pulsed while fluidized mode to obtain rapid heating and higher quality product. MPFFD uses microwave heating to substitute for the customary conduction heating during FD and its pulsed while fluidized mode can guarantee the uniformity of microwave heating, resulting in high-quality dried samples with short drying times. It combines the benefits of microwave drying (MD) and FD. The schematic diagram for MPFFD is shown in Fig. 1. The aim of this study was to estimate the effects of MPFFD on the drying characteristics, sensory quality, volatile compounds and antioxidant activities of C. militaris. The drying time, water content, rehydration time, rehydration ratio, water activity, color, crispness, shrinkage ratio and microstructure of the dried C. militaris were measured. 2. Materials and methods 2.1. Materials Fresh C. militaris (cultivated indoor) were obtained from Yandi

Corresponding author at: State Key Laboratory of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu Province, China. E-mail address: [email protected] (M. Zhang).

https://doi.org/10.1016/j.fbio.2019.01.001 Received 9 May 2018; Received in revised form 1 January 2019; Accepted 1 January 2019 Available online 02 January 2019 2212-4292/ © 2019 Elsevier Ltd. All rights reserved.

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In the fluidizing mode

Before or after the fluidizing mode Microwave heating cavity

Cordyceps militaris Cordyceps militaris

Drying chamber Gas flow

Fluidized bed Fig. 1. Schematic diagram of MPFFD.

9205A, Shanghai Hecheng Instrument Manufacturing Co., Ltd., Shanghai, China). The drying air temperatures were 40, 50 and 60 °C at an air flow rate of 1 m s−1. About 300 g of C. militaris was spread evenly in a single layer. When the moisture content of C. militaris reached < 5.0% (wet basis), the drying process was terminated.

Biological Engineering Co., Ltd. (Changde, Hunan Province, China). They were cultured in rice. The fruiting bodies were about 8–10 cm in length and about 3 mm in diameter. The color was orange (Fig. 3). After harvest, C. militaris was transported to the laboratory with refrigeration (4 °C) within 48 h. They were washed with tap water, drained and stored at 4 °C (95% relative humidity) and used within 3 days. All experiments were replicated three times.

2.3. Analytical methods 2.3.1. Color measurements The surface color of C. militaris was determined using a CR-400 Chroma Meter (Konica Minolta Sensing Inc., Osaka, Japan). Samples were closely put in two layers in a cylindrical laboratory designed mold (depth 1 cm × diameter 5 cm, used to hold samples for color measurement), and the Chroma Meter was placed vertically on the surface of the sample. The lightness (L*), redness or greenness (a*) and yellowness or blueness (b*) were used to describe the color attributes of the fresh and dried samples. Before use, the Chroma Meter was calibrated with a white Chroma Meter standard (CR-A43).

2.2. Drying methods 2.2.1. FD Samples were freeze-dried following the methods of Wang et al. (2013) using the laboratory designed freeze dryer. The laboratory designed freeze dryer (length/width/height, 1.2 × 0.9 × 1.5 m; 2700 W) consisted of a PC-controlled electric heating system, a PC-controlled trap system and a vacuum system. Before drying, the samples were frozen from 4 to −40 °C. The pressure inside the freeze-drying chamber was set at 80 Pa. About 300 g of frozen samples was put into the chamber when the temperature of the vapor condenser reached −40 °C. The maximum temperature of the heating plate was set at 50 °C, and the heating power was 100, 300 and 500 W. The moisture content during drying was obtained based on the weight loss of C. militaris using an electronic balance (ES3200, Shanghai Liheng Instrumentation Ltd., Shanghai, China). The drying process lasted until the moisture content of the sample reached < 5.0% (wet basis).

2.3.2. Shrinkage ratio A displacement method (Tian, Zhao, Huang, Zeng, & Zheng, 2016) was used to measure the shrinkage ratio of C. militaris after drying. Glass beads with a diameter of 0.1 mm (Boshan High-strength Microbeads Factory, Zibo, Shandong Province, China) served as the replacement medium. About 3 g of fresh C. militaris were marked (painted in red with a red marker) and used to measure the shrinkage ratio. Samples were shaken after beads were added to minimize air voids. The following formula was used to calculate the shrinkage ratio:

2.2.2. MPFFD Samples (300 g) were frozen to −40 °C and put into the cylindrical drying chamber (stainless steel, 40 cm o.d. and 200 cm height). The MPFFD device (Fig. 1) was designed in the laboratory (manufactured by Nanjing Yatai Microwave Power Technology Research Institute, Nanjing, Jiangsu Province, China). MPFFD was operated at −40 °C (cold trap temperature) and 80 Pa (absolute pressure). The microwave power was set at 660, 760 and 860 W, respectively. The microwave power was regulated using a microwave power controller (Gospell Electric Technology Co., Ltd., Shenzhen, Guangdong Province, China). The samples were fluidized every 10 min for 0.1 s to allow nitrogen gas (97%) to flow into the chamber. Nitrogen gas was regulated using a fine flux adjusting valve (Zhentai Instruments Co., Ltd., Shanghai, China) to ensure the pressure after fluidizing can drop back to 80 Pa within 30 s (Wang et al., 2013).

Shrinkage ratio =

Vf − Vd Vf

× 100%

(1)

Where Vf is the volume of the fresh samples and Vd is the volume of the dried samples. 2.3.3. Moisture content The final moisture content was determined using an oven method. Samples were dried at 105 °C in an oven until constant weight (Huang, Zhang, Mujumdar & Lim, 2011). 2.3.4. Rehydration ratio and time Dried C. militaris samples (1 g) were immersed into 100 mL distilled water at 25 °C. Every 30 s samples were taken out and the surfaces were gently dried with absorbent paper. Then the samples were weighed. The time taken for equilibrium was an estimate of the rehydration time. The

2.2.3. HD C. militaris was dried using a conventional hot air dryer (DHG8

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Force (g)

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fruity odor, grassy odor and nutty odor. Five g of each sample was cut into small pieces with a knife and served in odor-free plastic cups, which were coded with three-digit numbers and were randomly presented to the panelists together with an appropriate questionnaire (at room temperature, within 30 min of cutting). The panelists were in individual booths and there was no communication between panelists. Continuous non-structured scales were used for the evaluation. A 5point intensity scale anchored with the words “very low” (0−1), “low” (1−2), “medium” (2−3), “high” (3−4), “very high” (4−5) was used. Each panelist went out of the sensory laboratory to breathe fresh air for 10 min between samples.

Hardness

2.3.9. Volatile compounds analysis using GC-MS Fresh and dried samples (0.5 g, dry basis) were cut into small pieces with a knife and placed into a 15 mL glass vial. After 5 mL of distilled water and 25 μL n-nonane (as an internal standard) were added, the headspace vial was sealed. A triplus RSH Autosampler-solid phase microextraction (SPME) device (TSQ Quantum XLS, Thermo Scientific, Waltham, MA, USA) containing a fused-silica fiber (65 µm, PDMS/DVB; Supelco, Bellefonte, PA, USA) was used. The parameters were set as following: agitator temperature: 50 °C, incubation time: 2.0 min, needle speed in vial: 20 mm s−1, extraction time: 30 min, sample desorption time: 5 min. Gas chromatography–mass spectrophotometry (GC-MS) analyses were done using a TSQ Quantum XLS (Thermo Scientific). A capillary column (DB-WAX, 30 m × 0.25 mm × 0.25 µm, J & W Scientific, Folsom, CA, USA) was used. Helium was used as the carrier gas and the flow rate was 1.0 mL min−1. The initial temperature was held for 3 min, increased to 70 °C at 2 °C min−1, to 180 °C at 5 °C min−1, to 240 °C at 10 °C min−1 and held at 240 °C for 7 min. MS conditions were as follow: Interface temperature: 250 °C; source temperature: 200 °C; ionization mode: EI+; electron energy: 70 eV; scan range: 30–500 m z−1. Compounds were identified by comparing the retention times of the chromatographic peaks with those of authentic standards analyzed with the same conditions and by comparison of the retention indices (as Kovats indices) with literature, NIST 2005 (PerkinElmer Inc., Salem, MA, USA) and Willey 7 libraries (John Wiley & Sons Ltd., Hoboken, NJ, USA). The volatile compounds concentration (%) was determined based on comparison of the peak area of the sample compounds to that of the internal standard on the assumption that the instrumental response was the same for all compounds.

Crispness Time (s) Fig. 2. Schematic diagram of the textural properties of dried C. militaris obtained using the HDP/3PB.

rehydration ratio was calculated as:

Rehydration ratio =

Wa × 100% Wr

(2)

Where Wa is the weight of water absorbed during rehydration and Wr is the weight of water removed during drying. 2.3.5. Water activity A water activity meter (FA-ST lab; GBX Instrumentation Scientifique, Bourg de Peage, France) was used. Samples were cut into small pieces with a knife and evenly spread in the sample dish. SALTT75 (75% saturated sodium chloride solution) was used as a humidity standard. 2.3.6. Texture analysis The hardness and crispness of dried samples were determined using the TA.XTPlus Texture Analyzer (Stable Micro Systems, Surrey, UK) equipped with a three point bend rig (HDP/3PB). It consisted of three parts: a compressing blade, two horizontal supporting arms and a ruler. One dried C. militaris was horizontally placed onto the supporting blades (separated at a distance of 20 mm) of the 3PB rig (Fig. 2). The probe acted as a third contact point, which exerted an increasing pressure until the structure of the product broke. The probe of the rig passed through the C. militaris samples at 2 mm s−1. Trigger force and deformation strain were set at 5 g and 50%, respectively. The crispness was evaluated by calculating the beginning of oscillating breaks on the curve of the breaking force (Laurindo & Peleg, 2007). The hardness was evaluated by calculating the maximum values of the breaking force curve (Fig. 2)

2.3.10. Antioxidant activity Different dried samples (5 g) were finely ground using a grinder (Baijie Industrial Co., Ltd., Shanghai, China) and added to 100 mL of distilled water. The mixture was centrifuged at 1.57 × 103 g for 10 min at 4 °C (2–16KL, Sigma Laborzentrifugen GmbH, Osterode am Harz, Germany). The supernatant was collected for antioxidant activity testing. All the analyses were done in triplicates. 2.3.10.1. Hydroxyl radicals scavenging activity. Hydroxyl radicals scavenging activity was measured according to the method of Gu et al. (2008) with minor modifications. Briefly, 1 mL of sample extracts, 1 mL of FeSO4 (9 mM), 1 mL of salicylic acid-ethanol solution (95% ethanol, 9 mM) and 1 mL of hydrogen peroxide (8.8 mM, fresh daily) were added into a closeable tube. Then, distilled water was added to make a final volume of 15 mL. The mixture was incubated at 37 °C for 30 min. After that, the absorbance was determined at 510 nm using a spectrophotometer (DR 2800; Hach Co., Loveland, CO, USA). The reaction mixture without hydrogen peroxide was used as the control. Distilled water instead of the test sample was used as a blank sample. The hydroxyl radicals scavenging effect was calculated as:

2.3.7. Microstructure Cross-sections with a thin cut of 0.2–0.3 mm (using a sharp razor blade) of the central part of C. militaris samples after rehydration were observed directly using the inverted fluorescence microscope (Axio Vert A1, Carl Zeiss, Oberkochen, Germany). Images (400 × magnification) were obtained with a color digital camera (Carl Zeiss). 2.3.8. Odor evaluation of C. militaris Sensory analyses of the volatile compounds were carried out using 15 trained panelists (aged 20–55, 8 females and 7 males) according to the method described by Du, Plotto, Baldwin, and Rouseff (2011) with some modifications. The sensory panel was trained following ISO standard 8586-1 (ISO, 1993). Assessors were provided with all C. militaris samples and were asked to describe their odor followed with discussion. In further sessions, the odor of C. militaris was agreed on, including the following descriptors: mushroom-like odor, soil-like odor,

A − Ax0 ⎞ Scavenging activity(%) = ⎛1 − x × 100% A0 ⎝ ⎠ ⎜

⎟

(3)

Where Ax is the absorbance of the sample, Ax0 is the absorbance of the 9

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ratio and the closest color to the fresh samples. For HD, no significant difference was found in shrinkage ratio for samples dried at 40, 50 and 60 °C. However, the greatest color deterioration occurred when the temperature was set at 60 °C. The low L* value of HD dried samples may be caused by biochemical reactions in the food matrix (Chuyen, Roach, Golding, Parks, & Nguyen, 2016). According to the report by Perera (2005), enzymatic reactions, caramelization and Maillard reactions can occur during the drying process. Although a* values of all dried samples increased when compared to that of fresh ones, the redness of HD samples was greater intense. This may be caused by the “browning reaction” during the drying process. Considering the drying time and the retention of color, the temperature of 50 °C is the best choice for C. militaris in HD. Therefore, drying at 300 W in FD, 660 W in MPFFD and 50 °C in HD were selected for subsequent experimental studies.

control and A0 is the absorbance of the blank sample. 2.3.10.2. Superoxide anion radical scavenging activity. The 1,2,3trihydroxybenzene method (Gu et al., 2008) with some modifications was used to measure the superoxide anion radical scavenging activity of sample extracts. Tris-HCl buffer (4.5 mL, 0.05 M, pH 8.2) was incubated at 25 °C for 20 min. Then 1 mL of sample and 0.4 mL of 1,2,3trihydroxybenzene (2.5 mM, Xibao Biotechnology Co., Ltd., Shanghai, China) were added to the Tris-HCl buffer. After incubating at 25 °C for 5 min, the reaction was terminated by the addition of 0.1 mL of HCl (8 M). One mL of mixture diluted with 2 mL of distilled water was used to determine the absorbance at 299 nm. The reaction mixture without 1,2,3-trihydroxybenzene was used as the control. Distilled water instead of the test sample was used as a blank sample. The superoxide anion radical scavenging activity was expressed as:

A − A1 ⎞ Scavenging activity(%) = ⎛1 − 2 × 100% A3 ⎠ ⎝ ⎜

3.2. Effects of MPFFD on moisture content, rehydration time, rehydration ratio water activity and textural properties of dried C. militaris

⎟

(4) The final moisture content and water activity of the dried products are shown in Table 2. The final moisture content influences the storage stability of the dried product by affecting the extent of water diffusion. The water activity values for FD and MPFFD samples were found to be lower than that for HD, ranging from 0.35 to 0.36. All the dried samples are considered to be suitable for long-term storage in terms of water activity and among which FD and MPFFD samples showed better stability potential based on the water activity values. For dried food products, rehydration characteristics are often used to evaluate the structural quality. The rehydration times, shorter to longer for FD, MPFFD and HD were 90, 94 and 1200 s, respectively. Thus, the FD and MPFFD samples rehydrated much faster than HD samples. It is a complex process of physical and chemical changes that influences the rehydration. To statistically evaluate the rehydration capacity of different dried samples, rehydration ratio was calculated. Obviously, FD and MPFFD samples exhibited better rehydration probability than that of HD samples. During freezing process before drying, ice crystals are formed. This special form of water is removed through sublimation during FD and MPFFD, retaining the original shape of the C. militaris and generating a porous structure. This porosity contributed to FD and MPFFD samples possessing faster rehydration rates and higher rehydration ratios than those of HD samples. This result agreed well with the previous report by Rajkumar et al. (2017). Texture is identified as one of the most important criteria affecting the consumer acceptance of dried products (Huang et al., 2011). It can be seen from Table 2 that FD and MPFFD samples had similar hardness and crispness, which were much higher than those of HD samples. Crust formation is one of the reasons that lower the hardness and crispness of HD samples. Low chamber pressure during MPFFD led to a high internal vapor pressure, resulting in an expansion of structure to create pores (Jiang et al., 2014). This helped to form a crisp texture of the MPFFD samples. Using heating plate as the heating source in FD is a relatively gentle method compared with microwave, and FD samples showed high crispness because of the formation of the porous structure. Normally, HD samples shrank significantly (Fig. 3) due to the cell dehydration, which in turn made the sample structure more compact and reduced its crispness.

Where A2 is the absorbance of the sample, A1 is the absorbance of the control and A3 is the absorbance of the blank sample. 2.3.10.3. Determination of reducing power. The reducing power was determined using the method of Ahn, Kim, and Je (2014) with some modifications. Sample extracts (1 mL) in sodium phosphate buffer (2.5 mL, 0.2 M, pH 6.6) were mixed with potassium ferricyanide (2.5 mL, 1%, w v−1) and the mixture was heated in a water bath (50 °C) for 20 min. Trichloroacetic acid (2.5 mL, 10%) was added and then centrifuged at 1.04 × 103 g for 10 min. The supernatant (2.5 mL) was added to 10 mL of distilled water and ferric chloride (0.5 mL, 0.1%, w v−1), and the absorbance was read at 700 nm. The higher the absorbance, the stronger the reducing power. 2.4. Statistical analysis Analysis of variance (ANOVA) was used to obtain the mean value of the results and the standard deviation using the IBM Statistical Package for the Social Sciences (SPSS, IBM Inc., Chicago, IL, USA) software, version 21.0. The significant difference between two mean values was calculated using Duncan's multiple range tests at the 5% significance level. Three replicates were used for each treatment. 3. 3. Results and discussion 3.1. The optimal conditions of various drying methods The optimal condition of each drying method was based on the color, shrinkage ratio and drying time. Color and shrinkage ratio are the most commonly used indicators for evaluating dry products of C. militaris, which directly affect consumer acceptance. Shrinkage leads to a reduction in size and shape of dried products. Compared with other parameters such as texture, volatile compounds and antioxidant activity, the drying effects can be observed more directly and clearly using the color and shrinkage. Drying time is another important factor that should be taken into consideration because long drying time usually results in higher energy consumption. Therefore, color, shrinkage ratio and drying time were chosen for optimization. The color, shrinkage ratio and drying time of C. militaris dried using different methods are shown in Table 1. Freeze drying is known to maintain the original shape of the product. The effect of freeze drying can be directly evaluated through the shrinkage ratio of the dried products. FD samples with different heating powers showed low shrinkage ratio, the color (L* and b*) of FD samples dried at 500 W were significantly different (P < 0.05) from fresh samples. Moreover, the heating power of 100 W resulted in longer drying times. While in MPFFD samples, those dried at 660 W showed the lowest shrinkage

3.3. Effects of MPFFD on microstructure of dried C. militaris Fig. 4 shows the micrographs of fresh samples and rehydrated ones of different drying methods. The organizational structure of fresh samples showed more stereoscopic and more multiple channels compared with the dried products, which helped to maintain the moisture content and full shape of the material. Many pores were observed and the drying methods had a strong effect on the microstructure of C. militaris. The fiber structure of HD samples was arranged tightly. Moreover, the microstructure of HD samples was more compact and 10

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Table 1 The color, shrinkage ration and drying time of C. militaris dried using FD, MPFFD and HD. Drying method Fresh FD 100 W FD 300 W FD 500 W MPFFD 660 W MPFFD 760 W MPFFD 860 W HD 40 ℃ HD 50 ℃ HD 60 ℃

L* 59 57 57 55 58 54 51 43 42 37

a* ± ± ± ± ± ± ± ± ± ±

a

1 0.3ab 4ab 0.2bc 1a 1c 1d 1e 2e 1f

12 12 12 13 13 14 15 16 15 17

b* ± ± ± ± ± ± ± ± ± ±

d

1 1d 1d 1 cd 1 cd 1bc 1bc 0.4b 2b 1a

27 25 27 29 27 28 29 21 19 16

Shrinkage ratio (%) ± ± ± ± ± ± ± ± ± ±

b

2 1c 2b 1a 1ab 1ab 1a 1d 1e 1f

15 14 21 27 93 94 94

± ± ± ± ± ± ±

Drying time（min）

– 5.4 ± 0.2e 9.8 ± 0.8de 2d 3d 4c 5b 4a 5a 2a

– 1560 ± 40a 640 ± 10b 500 ± 20d 180 ± 10 f 160 ± 4fg 150 ± 7 g 560 ± 20c 480 ± 5d 440 ± 10e

Different letters (a, b, c, d, e, f and g) within the same column indicate a significant difference (P < 0.05). Table 2 The water content, rehydration time, rehydration ratio, water activity and textural properties of C. militaris dried using FD, MPFFD and HD. Drying methods FD MPFFD HD

Water content（%） a

4.9 ± 0.5 4.9 ± 0.4a 5.0 ± 1.4a

Rehydration time（s） b

90 ± 7 94 ± 8b 1200 ± 30a

Rehydration ratio (%) a

54 ± 6 50 ± 3a 34 ± 2b

Water activity a

0.35 ± 0.01 0.36 ± 0.01a 0.37 ± 0.02a

Crispness (g) a

21 ± 1 21 ± 1a 16 ± 1b

Hardness (g) 210 ± 10a 209 ± 5a 86 ± 5b

Different letters (a and b) within the same line indicate a significant difference (P < 0.05).

3.4. Effects of MPFFD on volatile compounds in dried C. militaris

exhibited more shrinkage of cellular tissue than those of FD and MPFFD samples. The loss of structured appearance of hyphae in HD samples was observed, resulting in a flattened microstructure. Same phenomenon was found during the drying of shiitake mushroom reported by Garciasegovia, Andrésbello, and Martinezmonzo (2011). FD and MPFFD samples displayed an obvious porous structure, which led to less shrinkage and promoted their ability to absorb water faster during rehydration process.

Fig. 5 shows the spider diagram obtained for the flavor attributes of fresh and different dried samples. Fresh C. militaris showed the highest flavors except nutty odor. The flavor of dried samples was lower than that of fresh ones, indicating the aroma loss during the drying process. There was no evident difference between FD and MPFFD samples. Both of them showed relatively high mushroom-like flavor. In terms of soillike odor, fruity odor, grassy odor and nutty odor, FD and MPFFD samples exhibited medium values. The lowest scores of HD samples demonstrated the weakest flavor. However, the nutty odor was the

Fig. 3. The appearance of fresh and dried C. militaris using FD, MPFFD and HD. 11

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Fig. 4. The microstructure of fresh and dried C. militaris using FD, MPFFD and HD. Mushroom-like odor

Fresh MPFFD

5

FD HD

4 3

Nutty odor

2

Soil-like odor

1 0

Grassy odor

Fruity odor

Fig. 5. Spider diagram of sensory evaluation obtained for flavor attributes of the fresh and dried C. militaris using FD, MPFFD and HD.

highest in HD samples. This might be due to the Maillard reaction during heating process (Herchi et al., 2014), which contributed to the formation of heterocyclic compounds such as furans and pyrazines. As shown in Fig. 6, the content of 1-octen-3-ol in fresh ones was much higher than that of FD, MPFFD and HD samples. 1-Octen-3-ol, which belongs to the unsaturated aliphatic alcohol, features a typical odor of mushroom (Maggi, Papa, Cristalli, Sagratini, & Vittori, 2011). Politowicz, Lech, Sánchez-Rodríguez, Szumny, and CarbonellBarrachina (2017) reported that in the formation of fresh chanterelle flavor, 1-octen-3-ol played a dominant role. The content of 3-octanone was the highest in fresh C. militaris. In the research of Costa, Tedone, De, Dugo, and Mondello (2013), 3-octanone was found to be the main volatile in chopped Agaricus bisporus samples, which is consistent with our result. 3-Octanone offers a fruity, sweet aroma and a mushroom taste. After drying, the concentration of 3-octanone decreased. 2-

Fig. 6. Key volatile compounds of fresh and dried C. militaris using FD, MPFFD and HD.

Octenal, which offers a fruity odor, was also reduced after FD and MPFFD processes. However, this compound was not detected in HD samples. In the study on cepe and oyster mushrooms, Misharina, Mukhutdinova, Zharikova, Terenina, and Krikunova (2009) reported that 1-octen-3-one was the product of the oxidation of 1-octen-3-ol. 3Octanol contributes to a strong oily, nutty, and herbaceous odor. In general, drying processes decreased the amounts of key volatile compounds in C. militaris, especially by HD that had the lowest content of 12

110

FD HD

Hydroxyl radical scavenging activity (%)

Fresh MPFFD

70

a

Relative content (%)

60

b

A

50

c

40 B

B B

30 20

d

10

110

105

Hydroxyl radical scavenging activity Superoxide anion radical scavenging activity

100

100 A

95

a

a

90

90

A

85 80

80

75 70

70

b

B

65 60

60

55

0 Al

FD

co

ls ho Al

deh

y

des

Ke

to

nes

Al

k

s ene

Al

k

s ane

Ar

om

cs ati

E

rs ste

ids Ac

MPFFD

Superoxide anion radical scavenging activity (%)

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HD

Drying methods

rs the

O

Fig. 8. Scavenging activities of hydroxyl radicals and superoxide anion radicals of dried C. militaris using FD, MPFFD and HD.

Volatile compounds Fig. 7. Proportion of the different chemical groups of volatile compounds of fresh and dried C. militaris using FD, MPFFD and HD.

hydroxyl radicals scavenging ability of Ganoderma lucidum samples dried by HD was much lower than that of FD, which was in accordance with our study. They deemed that higher antioxidant activity of FD samples can be attributed to the higher retention of uronic acid, which can chelate ferrous ion and reduce the generation of hydroxyl radicals. The hydroxyl radical scavenging activities of different dried C. militaris samples are shown in Fig. 8. The scavenging activity obtained in this work was inferior to that reported for polysaccharides from cultured C. militaris, whose scavenging rate achieved almost 100% at the concentration of 0.1 mg mL−1 (Chen, Wu, & Huang, 2013). As shown in Fig. 8, the hydroxyl radical scavenging activities of different dried products was in the following decreasing order: FD (90.4%) > MPFFD (85.7%) > HD (66.8%). The inhibition effect of MPFFD samples was slightly lower than that of FD samples, but much higher than that of HD samples. It was clearly indicated that FD and MPFFD processes were much better to sustain the scavenging activity of C. militaris compared to HD process. The lower scavenging ratio of HD samples might be attributed to the long exposure to the thermal environment (50 °C) during the drying process, which could result in the degradation of compounds related to the scavenging activity. Reductive capabilities of different dried C. militaris samples are shown in Fig. 9. The increase of absorbance indicates an increasing reducing power. As shown in Fig. 9, the reducing power of HD samples was much lower than those of MPFFD and FD samples at the same concentration. Generally, reducing capacities of C. militaris samples were in good correlation with their scavenging activities, suggesting that the reducing power contributed to the scavenging of hydroxyl radicals and superoxide anion radicals. Similar results were previously

typical flavor compounds. This may be due to the air stream that flows through the sample, resulting in volatile evaporation out of the dryer. Ketones accounted for at least 58.6% of the total concentration of aroma compounds in fresh samples, followed by alcohols and alkenes with 31.5% and 6.0%, respectively (Fig. 7). Other compounds were present in minor amounts. All drying processes were observed to have a significant influence on the volatile components. Compared with volatile compounds of fresh C. militaris, the relative contents of ketones were decreased by the drying process. They were 51.5%, 41.4% and 11.7% in FD, MPFFD and HD samples, respectively. Similar changes were also found in alcohols, which were 29.0% and 32.3% of the total volatile compounds in FD and MPFFD samples. However, the relative alcohol content increased significantly from 31.5% to 53.6% after HD. This might be caused by the large loss of ketones in HD samples, resulting in an increase of relative alcohol content. Ketones and alcohols were the major chemical groups after all drying processes, representing more than 65% of the total volatile compounds. In terms of alkanes, aromatics, esters, acids and others (mainly heterocyclic compounds), the relative contents of these chemical groups were the highest in HD samples. These heterocyclic volatile compounds, including benzothiazole, Pyran, Pyrazine, etc. were major products generated by Maillard reaction (Lee, Chung, & Kim, 2012). The mild temperature (50 °C), coupled with the hot air flows and compact structure provided favorable conditions for Maillard reaction. This could explain why the relative content of this chemical group in HD samples was the highest.

3.5. Effects of MPFFD on antioxidant activities of dried C. militaris The hydroxyl radical scavenging activity of MPFFD samples compared to those of FD and HD samples is evaluated in Fig. 8. Dore, Santos, Souza, Baseia, and Leite (2014) reported the scavenging activity of Polyporus dermoporus extracts was 96% at a concentration of 267 μg mL−1, achieving the similar scavenging effect with a lower concentration. Wu et al. (2014) found that the power of Agaricus blazei Murrill to scavenge hydroxyl radical correlated well with the increasing concentrations of the polysaccharides. As is shown in Fig. 8, both FD and MPFFD samples had better scavenging effect than that of HD samples. Generally, the antioxidation mechanism in terms of hydroxyl radical scavenging can be explained by two ways: one is the inhibition of the hydroxyl radical generation and the other is the removal of the hydroxyl radicals generated (Wang, Zhang, Liu, Sun, & Zha, 2015). In living systems, the formation of hydroxyl radicals requires the existence of iron or transition metals instead of enzyme catalysis (Kozarski et al., 2011). Research did by Fan, Li, Deng, and Ai (2012) showed that the

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Fig. 9. Reducing power of dried C. militaris using FD, MPFFD and HD. 13

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reported by Fan et al. (2012). They stated that the reductive capability was associated with the scavenging ability. Reducing power of MPFFD samples indicated that some biologically active substances in C. militaris could donate a hydrogen atom and exert antioxidant action by breaking the free radical chain. As a result, the radical chain reactions could be terminated.

subtropical strawberry cultivars using GC-olfactometry, GC-MS odor activity values, and sensory analysis. Journal of Agricultural and Food Chemistry, 59(23), 12569–12577. Fan, L., Li, J., Deng, K., & Ai, L. (2012). Effects of drying methods on the antioxidant activities of polysaccharides extracted from Ganoderma lucidum. Carbohydrate Polymers, 87(2), 1849–1854. Garciasegovia, P., Andrésbello, A., & Martinezmonzo, J. (2011). Rehydration of air-dried shiitake mushroom (Lentinus edodes) caps: Comparison of conventional and vacuum water immersion processes. LWT – Food Science and Technology, 44(2), 480–488. Gu, H.-F., Li, C.-M., Xu, Y.-J., Hu, W.-F., Chen, M.-H., & Wan, Q.-H. (2008). Structural features and antioxidant activity of tannin from persimmon pulp. Food Research International, 41(2), 208–217. Herchi, W., Arráez-Román, D., Boukhchina, S., Kallel, H., Carretero, A. S., & FernándezGutiérrez, A. (2014). A review of the methods used in the determination of flaxseed components. African Journal of Biotechnology, 11(4), 724–731. Huang, L. L., Zhang, M., Mujumdar, A. S., & Lim, R. X. (2011). Comparison of four drying methods for re-structured mixed potato with apple chips. Journal of Food Engineering, 103(3), 279–284. International Organization for Standardization (ISO) (1993). Sensory analysis: General guidance for the selection, training and monitoring of assessors. Part 1: Selected assessors. Geneva: ISO (ISO8686-1:1993). Jiang, H., Zhang, M., Mujumdar, A. S., & Lim, R. X. (2014). Comparison of drying characteristic and uniformity of banana cubes dried by pulse‐spouted microwave vacuum drying, freeze drying and microwave freeze drying. Journal of the Science of Food and Agriculture, 94(9), 1827–1834. Kozarski, M., Klaus, A., Niksic, M., Jakovljevic, D., Helsper, J. P., & Van Griensven, L. J. (2011). Antioxidative and immunomodulating activities of polysaccharide extracts of the medicinal mushrooms Agaricus bisporus, Agaricus brasiliensis, Ganoderma lucidum and Phellinus linteus. Food Chemistry, 129(4), 1667–1675. Laurindo, J. B., & Peleg, M. (2007). Mechanical measurements in puffed rice cakes. Journal of Texture Studies, 38, 619–634. Lee, S. E., Chung, H., & Kim, Y. S. (2012). Effects of enzymatic modification of wheat protein on the formation of pyrazines and other volatile components in the Maillard reaction. Food Chemistry, 131(4), 1248–1254. Maggi, F., Papa, F., Cristalli, G., Sagratini, G., & Vittori, S. (2011). Characterisation of the mushroom-like flavour of Melittis melissophyllum L. subsp. melissophyllum by headspace solid-phase microextraction (HS-SPME) coupled with gas chromatography (GCFID) and gas chromatography-mass spectrometry (GC-MS). Food Chemistry, 123(4), 983–992. Misharina, T. A., Mukhutdinova, S. M., Zharikova, G. G., Terenina, M. B., & Krikunova, N. I. (2009). The composition of volatile components of cepe (Boletus edulis) and oyster mushrooms (Pleurotus ostreatus). Applied Biochemistry & Microbiology, 45(2), 187–193. Perera, C. O. (2005). Selected quality attributes of dried foods. Drying Technology, 23(4), 717–730. Politowicz, J., Lech, K., Sánchez-Rodríguez, L., Szumny, A., & Carbonell-Barrachina, Á. A. (2017). Volatile composition and sensory profile of Cantharellus cibarius Fr. as affected by drying method. Journal of the Science of Food and Agriculture, 97(15), 5223–5232. Rajkumar, G., Shanmugam, S., Galvâo, M. D. S., Sandes, R. D. D., Neta, M. T. S. L., Narain, N., & Mujumdar, A. S. (2017). Comparative evaluation of physical properties and volatiles profile of cabbages subjected to hot air and freeze drying. LWT – Food Science and Technology, 80, 501–509. Tian, Y., Zhao, Y., Huang, J., Zeng, H., & Zheng, B. (2016). Effects of different drying methods on the product quality and volatile compounds of whole shiitake mushrooms. Food Chemistry, 197(Pt A), 714. Wang, Zhang, M., Mujumdar, A. S., & Mothibe, K. J. (2013). Microwave-assisted pulsespouted bed freeze-drying of stem lettuce slices—Effect on product quality. Food and Bioprocess Technology, 6(12), 3530–3543. Wang, X. J. L., Zhang, J. C., Liu, Y., Sun, H. J., & Zha, X. (2015). Physicochemical properties and antioxidant activities of polysaccharide from floral mushroom cultivated in Huangshan Mountain. Carbohydrate Polymers, 131(25), 240–247. Wu, S., Li, F., Jia, S., Ren, H., Gong, G., Wang, Y., & Lv, Z. (2014). Drying effects on the antioxidant properties of polysaccharides obtained from Agaricus blazei Murrill. Carbohydrate Polymers, 103, 414–417. Xiao, Y., Xing, G., Rui, X., Li, W., Chen, X., Jiang, M., & Dong, M. (2014). Enhancement of the antioxidant capacity of chickpeas by solid state fermentation with Cordyceps militaris SN-18. Journal of Functional Foods, 10, 210–222. Zhang, M., Chen, H., Mujumdar, A. S., Tang, J., Miao, S., & Wang, Y. (2017). Recent developments in high-quality drying of vegetables, fruits, and aquatic products. Critical Reviews in Food Science and Nutrition, 57(6), 1239–1255.

4. Conclusions MPFFD resulted in a similar effect as FD process on dried C. militaris. The final water content, water activity, rehydration time and rehydration ratio of MPFFD samples were very close to those of FD samples. However, the total drying time required for MPFFD was remarkably reduced when compared to FD. In terms of color, crispness and shrinkage, MPFFD samples were quite similar to those of FD samples. In comparison with HD samples, FD and MPFFD samples presented much higher contents of key volatile compounds. Furthermore, antioxidant activities of C. militaris dried by MPFFD were analogous to those of FD samples, which were significantly higher than those of HD samples. Therefore, taking into account the drying characteristics, the final dried product quality, the volatile compounds and the antioxidant activities, MPFFD provides an appropriate and potential treatment for achieving high-quality dried C. militaris with high energy efficiency as the drying time is significantly reduced. Acknowledgments We acknowledge the financial support from the National Key R&D Program of China (Contract no. 2017YFD0400901), the Jiangsu Province (China) Agricultural Innovation Project (Contract no. CX(17) 2017), the Jiangsu Province Key Laboratory Project of Advanced Food Manufacturing Equipment and Technology (No. FMZ201803) and the National First-Class Discipline Program of Food Science and Technology (No. JUFSTR20180205), all of which enabled us to carry out this study. Conflict of interest The authors confirm that they have no conflicts of interest with respect to the work described in this manuscript. References Ahn, C. B., Kim, J. G., & Je, J. Y. (2014). Purification and antioxidant properties of octapeptide from salmon byproduct protein hydrolysate by gastrointestinal digestion. Food Chemistry, 147(6), 78–83. Chen, X., Wu, G., & Huang, Z. (2013). Structural analysis and antioxidant activities of polysaccharides from cultured Cordyceps militaris. International Journal of Biological Macromolecules, 58(7), 18–22. Chuyen, H. V., Roach, P. D., Golding, J. B., Parks, S. E., & Nguyen, M. H. (2016). Effects of four different drying methods on the carotenoid composition and antioxidant capacity of dried Gac peel. Journal of the Science of Food and Agriculture. Costa, R., Tedone, L., De, G. S., Dugo, P., & Mondello, L. (2013). Multiple headspace-solidphase microextraction: An application to quantification of mushroom volatiles. Analytica Chimica Acta, 770(7), 1–6. Dore, C. M. P. G., Santos, M. D. G. L., Souza, L. A. R. D., Baseia, I. G., & Leite, E. L. (2014). Antioxidant and anti-inflammatory properties of an extract rich in polysaccharides of the mushroom Polyporus dermoporus. Antioxidants, 3(4), 730–744. Du, X., Plotto, A., Baldwin, E., & Rouseff, R. (2011). Evaluation of volatiles from two

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