Accepted Manuscript

A novel infrared freeze drying (IRFD) technology to lower theenergy consumption and keep the quality of Cordyceps militaris

Xiao-fei Wu, Min Zhang, Bhesh Bhandari

PII: S1466-8564(18)31571-6DOI: https://doi.org/10.1016/j.ifset.2019.03.003Reference: INNFOO 2137

To appear in: Innovative Food Science and Emerging Technologies

Received date: 21 December 2018Revised date: 19 February 2019Accepted date: 8 March 2019

Please cite this article as: X.-f. Wu, M. Zhang and B. Bhandari, A novel infrared freezedrying (IRFD) technology to lower the energy consumption and keep the quality ofCordyceps militaris, Innovative Food Science and Emerging Technologies, https://doi.org/10.1016/j.ifset.2019.03.003

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A novel infrared freeze drying (IRFD) technology to lower the energy

consumption and keep the quality of Cordyceps militaris

Xiao-fei Wua, Min Zhang a, b *, Bhesh Bhandari

c

a State Key Laboratory of Food Science and Technology, Jiangnan University, 214122 Wuxi,

Jiangsu, China

bJiangsu Province Key Laboratory of Advanced Food Manufacturing Equipment and Technology,

Jiangnan University, 14122 Wuxi, Jiangsu, China

cSchool of Agriculture and Food Sciences, University of Queensland, Brisbane, QLD, Australia

*Corresponding author: Professor Min Zhang, School of Food Science and Technology, Jiangnan

University, 214122 Wuxi, Jiangsu Province, China.

Email: min@jiangnan.edu.cn.

Tel: 0086-(0)510-85807976; Fax: 0086-(0)510-85807976

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Abstract:

A novel laboratory scale apparatus using infrared lamps replacing the electric

heating plate for the freeze drying of Cordyceps militaris was developed in this study.

The drying time, energy consumption, nutritional properties, antioxidant activities and

main volatile compounds of Cordyceps militaris dried by infrared freeze drying

(IRFD) and traditional freeze drying (TFD) at different drying temperatures (40, 50,

60 and 70 °C) were compared. Results indicated that drying at 40, 50 and 60 °C

resulted in higher retention of cordycepin, total phenolics, hydroxyl radical

scavenging activity, reducing power, 3-octanone, 3-octanol and 1,3-octadiene when

compared with those dried at 70 °C. IRFD could reduce 7.21~17.78% of the drying

time and 11.88~18.37% of the energy consumption at a constant drying temperature

in comparison of TFD. In terms of quality, IRFD and TFD had similar retention

effects on cordycepin, adenosine, hydroxyl radical scavenging activity and reducing

power. Consequently, the results suggest that IRFD should be a promising approach

for achieving high-quality dried products.

Industrial relevance: IRFD could reduce 7.21~17.78% of the drying time and

11.88~18.37% of the energy consumption at a constant drying temperature in

comparison of TFD without compromising the quality of the dried product. This study

can help food processing industries to produce high quality freeze dried Cordyceps

militaris and many other foods with an energy efficient freeze drying process.

Keywords: Cordyceps militaris; infrared freeze drying; energy consumption;

nutritional properties; antioxidant activities

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1. Introduction

Cordyceps militaris, which has been broadly used as a functional mushroom in

China and East Asia, is considered as a very promising industrial fungus (Chen

Bai-Xiong et al., 2018). It has shown anti-tumor (Lee et al., 2017), anti-inflammatory

(Hsiao et al., 2017) and antioxidant (Zhu et al., 2016) activities. Fresh Cordyceps

militaris is perishable and has a short shelf life. This will have an adverse effect on its

consumer access and industrial value. Drying is one of the preservation methods

which involves removal of moisture from the material. Some functional active

constituents like cordycepin, adenosine, ergosterol and polysaccharides are always

extracted from dried Cordyceps militaris for medical purposes. In many studies,

Cordyceps militaris were dried at 50 °C by an oven method (Hsiao et al., 2017,

Nurmamat et al., 2018). It is known that drying by oven with hot air is easier to

handle and has lower energy consumption than other drying methods (such as

microwave drying and freeze drying) (Xu et al., 2005). However, the heat-sensitive

components in Cordyceps militaris may be destroyed, leading to the reduction of

nutritional and functional properties. Commonly, hot air-dried foods have a dense

microstructure, tight cell connections and firm texture (Karam et al., 2016). In

addition, during the hot air drying process, the migration of solute in the material

surface can cause the crust formation (Zhang et al., 2017). As a result, the milling

process is prolonged, which in turn can adversely affect the powder quality of

Cordyceps militaris.

Freeze drying, which is also known as sublimation drying, removes moisture in

the product by sublimation of solid ice (Jin Jue et al., 2018). Due to the low

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temperature environment and minimum shrinkage of the structure during the drying

operation, better nutrition retention, color preservation and rehydration properties can

be achieved when compared with other drying methods (Fan et al., 2018). Therefore,

freeze drying is frequently applied to food and pharmaceutical industries to produce

high quality foods or biopharmaceutical drug products (Kasper et al., 2013).

Nevertheless, traditional freeze drying is a time-consuming and energy-intensive

process (Cao et al., 2018). It usually uses electric heating plates to provide the heat

required for ice sublimation, with low heat conductivity. So it is very critical to

improve the freeze drying technique that can lower the energy consumption and

reduce drying time. The use of infrared heating to replace the electric heating plate is

a promising method to overcome the problem.

Infrared (IR) radiation belongs to electromagnetic radiations with the wavelength

range of 0.78-1,000 μm (Pawar and Pratape 2017). Water and organic materials such

as carbohydrates, fats and proteins are the main ingredients of foodstuffs. The strong

absorption wavelengths of these constituents are mainly concentrated in 3 and 6 μm

(Pan and Atungulu 2010). Generally, IR wavelengths in the range of 2.5-200 μm are

used for drying purposes. Like other electromagnetic waves, the transmission of IR

energy does not require media. The energy emitted from the IR lamp directly reaches

the materials and does not heat the surrounding air (Pawar and Pratape 2017). When

the wavelength of the radiation source coincides with the absorption wavelength of

the irradiated object, the substance absorbs a large amount of infrared energy, thereby

altering and aggravating the movement of its molecules and achieving the effect of

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heating up. As a result, the drying process is significantly accelerated. Antal et al.

(2017) found that compared to single FD, the use of IR before FD could save over

14% drying time. Moreover, the IR-FD product had darker color, better rehydration

capacity, similar water activity, lower hardness and highest content of chemical

composition than single FD products. Infrared freeze drying (IRFD) can combine the

advantages of infrared heating and freeze drying. The energy required for sublimation

is provided by IR radiation instead of electric heating plate, which enhances the heat

transfer rates. At the same time, the final quality of the product is expected to be well

maintained by the FD process.

The aim of this study was to: (1) Assess the feasibility of IRFD process for

Cordyceps militaris; (2) Compare the drying time and energy consumption by IRFD

and TFD; (3) evaluate the quality indices of Cordyceps militaris dried by IRFD and

TFD.

2. Materials and methods

2.1. Materials and sample preparation

Fresh Cordyceps militaris were obtained directly from local Auchan supermarket

in Wuxi, China. The fruiting bodies were about 8~10 cm in length and about 3 mm in

diameter. Samples were washed, drained and stored at 4 °C until use. Before

experimentation, Cordyceps militaris were frozen to -40 °C. Then, about 300 g of

frozen samples were put on a tray and placed on the sample placement rack of the

drying chamber.

2.2. Experimental Apparatus

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The IRFD equipment (Changzhou One-Step Drying Equipment Co., Ltd.,

Changzhou, Jiangsu, China) consisted of the following 6 parts: a vacuum drying

chamber, a cold trap system, a vacuum system, an energy supply system (heating

system), a weighing sensor system (model KS-516, Jingjiu Co., Nanjing, China) and

K-type thermocouples. The drying chamber capacity was about 1 kg for one layer.

The pressure inside the drying chamber was set at 80 Pa. When the lamps were

working at full power, the heat flux value was about 0.703W/cm2. The temperature at

5 mm beneath the lamps (heating plate for TFD) was measured continuously using a

K-type thermocouple connected to an expansion board. A temperature controller

(modelWRNK-191, Xinghui Co., Shanghai, China) was used to control the

temperature by turning on/off the heater according to the set value and the actual

value measured by the thermocouple. The temperature in the chamber was set at 40,

50, 60 and 70 °C for each treatment, respectively. The initial moisture content

(636.92% d.b.) of Cordyceps militaris was determined by the oven method at 105 °C

until constant weigh obtained (Wang et al., 2010). The moisture content during drying

was obtained based on the weight loss of Cordyceps militaris using the weighing

sensor. The experiments were conducted until the final moisture content reached <

5.0% d.b. The time from the start of drying to the time when the moisture content of

the product < 5.0% was defined as the drying time. The schematic diagram of the

whole IRFD device is illustrated in Fig. 1.

The TFD equipment was same as the IRFD equipment except for point 4. The

energy was supplied by an electric heating plate.

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2.3. Analysis

2.3.1. Fourier transform infrared (FTIR) Spectroscopy

Freeze-dried samples were prepared according to the method of Wu et al. (2018).

FTIR spectra were attained utilizing A Nicolet 8210E FTIR spectrometer (Nicolet,

Madison, WI, USA). Each spectrum was obtained from 4000 to 400 cm-1

at 2 cm-1

resolution by 128 accumulated scans.

2.3.2. Temperature measurements

The surface temperature of Cordyceps militaris was determined with a K-type

thermocouple during the drying process. The temperature value was collected and

recorded every 5 minutes.

2.3.3. Energy consumption

The total energy consumption of dryer in this study mainly consisted of the

energy supply of the heating system, vacuum system and cold trap system. The energy

consumption of each part was measured by electricity meter (DTS634, CHNT, Leqing,

Zhejiang, China). Since the vacuum system and the cold trap system are connected to

the same electricity meter, the sum of the energy consumption of these two processes

is measured using this electricity meter. The energy consumption of each part was

listed in Table 1.

2.3.4. Measurement of cordycepin and adenosine

Infrared- and freeze-dried Cordyceps militaris (0.5 g) was ground and placed

into a 100 mL volumetric flask. Then 100 mL of distilled water was added. The

mixture was ultrasonically extracted in an ultrasonic cleaner for 1 h. After

centrifugation at 3000 rpm for 10 min, the supernatant was filtered through a 0.45 μm

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filter. High-performance liquid chromatography (HPLC) analysis was carried out on

Ultimate3000 HPLC (Thermo Fisher Scientific, Waltham, MA, USA) with a reverse

C18 column (250 × 4.6 mm, 5 μm; Phenomenex, Torrance, CA, USA). Standards of

cordycepin and adenosine were purchased from Sigma. The mobile phase consisted of

95% ultrapure water and 5% acetonitrile with a flow rate of 1.0 mL min-1

. The

injection volume was 10 μL and the column temperature was set at 35 °C. The eluent

was detected using a diode array detector (DAD) at 260 nm. Results were displayed

as mg g-1

(dry basis).

2.3.5. Determination of reducing sugar

The reducing sugar content was carried out by the 3,5-dinitrosalicylic acid (DNS)

method previously described by Jin et al. (2017) with minor modifications. Cordyceps

militaris extract (0.05g mL-1

, 2 mL) was incubated with 1.5 mL of DNS in boiling

water for 20 min. Then the mixture was cooled and its volume was diluted to 75 mL

with distilled water. The absorbance of the final solution was read at 540 nm.

Quantification was performed using glucose as a standard. The reducing sugar content

was expressed as milligrams of glucose equivalent per gram of dried sample (mg GE

g-1

, dry basis).

2.3.6. Analysis of total phenolic content

Total phenolic content (TPC) of different freeze-dried Cordyceps militaris was

determined by the Folin-Ciocalteu method described by Chen et al. (2018) with some

modifications. Initially, Cordyceps militaris extract (0.05g mL-1

, 1 mL) was mixed

with FolinCiocalteu’s reagent (1:10, 5 mL). After 5 min in the dark, 4 mL of Na2CO3

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solution (7.5%) was added. Then the mixture was incubated at 45 °C for 15 min. The

absorbance was measured at 765 nm. Quantification was achieved utilizing gallic acid

as a standard. The TPC were calculated as milligrams of gallic acid equivalents (GAE)

per gram of dried sample (mg GAE g-1

, dry basis).

2.3.7. Determination of hydroxyl radicals scavenging activity

Hydroxyl radical scavenging activity of different freeze-dried Cordyceps

militaris was determined based on the method described by Gu et al. (2008) with

minor modifications. The sample extract (0.05g mL-1

, 1 mL) were incubated with 9

mM FeSO4 (1 mL), 9 mM salicylic acid-ethanol (1 mL) and 8.8 mM hydrogen

peroxide (1 mL) at 37 °C for 30 min. Then the absorbance was recorded at 510 nm.

The following equation was used to calculate the hydroxyl radical scavenging effect:

The reaction mixture without hydrogen peroxide was used as control. Distilled water

instead of the test sample was used as a blank sample.

Scavenging activity (%) = (1-

𝐴𝑥−𝐴𝑥0

𝐴0 ) ×100

(1)

Where Ax, Ax0 and A0 was the absorbance of sample, control and blank,

respectively. In the control or blank, the hydrogen peroxide or sample was replaced by

distilled water.

2.3.8. Determination of reducing power

The reducing power of different freeze-dried Cordyceps militaris was evaluated

by the method of Ahn et al. (2014) with some modifications. Sample extract (0.05g

mL-1

, 1 mL) was mixed with 0.2 M sodium phosphate buffer (2.5 mL, pH 6.6) and

1% potassium ferricyanide (2.5 mL).Then the mixture was warmed at 50 °C for 20

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min. After that, 10% trichloroacetic acid (2.5 mL) was added followed by the

centrifugation (3000 rpm, 10 min). Afterwards, 2.5 mL of the supernatant was

allowed to be mixed with 10 mL of distilled water and 0.5 mL of ferric chloride

(0.1%). Then, the mixture absorbance was determined at 700 nm. A higher absorbance

represented a stronger reducing power.

2.3.8. Analysis of main volatile compounds

A total of 0.5 g (dry basis) infrared- and freeze-dried Cordyceps militaris was

ground and weighed into a 15 mL glass bottle containing distilled water (5 mL).

Before sealing, n-nonane (internal standard) was added instantly. Then a 65 μm

polydimeth ylsiloxane–divinylbenzene fiber (PDMS/DVB; Supelco, Bellefonte, PA,

USA) was inserted and exposed in the headspace of the vial at 50 °C. The volatiles

were absorbed onto the fiber for 30 min.

Volatile components were separated through a GC-MS instrument (TSQ

Quantum XLS, Thermo Fisher Scientific, Waltham, MA, USA) equipped with a

DB-WAX column (30 m×0.25 mm×0.25 µm). The mass spectra were obtained in an

electronic shock mode with a voltage of 70 eV and a scan range of 30 m z-1

. The

temperature of ion source was set at 200 °C.

After completing the extraction process, the fiber was inserted into the injection

port for desorption (250 °C for 3 min). The carrier gas was helium and the flow rate

was 1.0 mL min-1

. The oven temperature was initially stayed at 40 °C for 3 min. Then

it was raised to 70 °C at 3 °C min-1

. After that, the temperature was increased to

180 °C at a rate of 5 °C min-1

. Finally, 240 °C was reached at a rate of 10 °C min-1

for

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7 min.

Volatile compounds obtained by GC-MS analysis were identified according to

both of the NIST2005 and Willey 7.0 libraries. The amount of each volatile

constituent was calculated by comparing the peak area of this compound with that of

the internal standard.

2.4. Statistical analysis

All the measurements were conducted in triplicate. IBM SPSS Statistics 21.0

(IBM Inc., USA) was applied to obtain the mean value and the standard deviation of

triplicate observations using the one-way analysis of variance (ANOVA). A 5%

significance level (p<0.05) and Duncan’s multiple-range test was used to assess the

significant difference. Graphs were plotted using OriginPro 2016 (OriginLab,

Northampton, MA).

3. Results and discussion

3.1. Fourier transform infrared (FTIR) Spectroscopy

Fig.2 illustrates the major absorption bands of Cordyceps militaris. The strong

absorption band ranged from 3000 to 3500 cm-1

corresponding to the stretching

vibration of O-H and N-H (Radzki et al., 2016), which is linked to the water

molecules and amino groups. It was reported that the peak at 2926 cm-1

was specific

for CH2 groups in lipids (Dogan et al., 2007). C=O stretching at 1646 cm-1

suggested

the vibrations of proteins (Radzki et al., 2016). The absorption at 1407 cm-1

was

assigned to the aliphatic groups’ vibration of phenolics (Matijašević et al., 2016). The

absorption at 1247 cm-1

indicated the symmetric expansion of C-O-C and C-OH

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stretching vibration which belonged to aromatic acid ester and phenolic, respectively

(Li Yan-qun et al., 2013). Absorption peaks at 1150, 1078, 1045 and 993 cm-1

were

characteristic for β-glycosidic bonds belonging to the β-glucans (Matijašević et al.,

2016, Synytsya et al., 2009). It was reported that the IR absorption bands of water

were mainly located at 3300 and 1630 cm-1

(Sritham and Gunasekaran 2017),

indicating that the water absorption overlapped with the absorption bands of major

organic compounds in Cordyceps militaris. Water and organic matters like proteins,

lipids and polysaccharides are the key ingredients of fresh Cordyceps militaris.

Among these main components, water accounts for more than 80% of the weight of

fresh samples. Moreover, water shows intensive radiation absorption and weak

scattering of radiation (Pan and Atungulu 2010). Therefore, the absorption of infrared

energy by water is dominant. From the mechanism of infrared radiation heating, when

the wavelength of the radiation source is consistent with the absorption wavelength of

the object to be irradiated, the substance absorbs a large amount of infrared energy.

Thereby, the movement of the substance molecules are changed and aggravated,

achieving the effect of heating (Pan and Atungulu 2010). The wavelength of the

infrared lamp was in the range of 2.3~14 μm (4348~714 cm-1

), which was considered

suitable for the drying of Cordyceps militaris. During the IRFD process, IR was

absorbed by water (ice) and organic materials, leading to the increase of molecular

vibration. As a result, the ice in the sample was removed by sublimation and the

sample was finally dried.

3.2. Drying time

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The drying time of Cordyceps militaris dried by IRFD and TFD under different

temperatures are shown in Fig.3. Drying time was affected by the drying method and

temperature applied. Compared to TFD at the same drying temperature, IRFD

significantly (p<0.05) reduced 7.21%, 13.48%, 15.38% and 17.78% of the drying

time when the drying temperature was set at 40, 50, 60 and 70 °C, respectively. The

IR energy could be directly transferred to the target product and heat could be

transferred without gas resistance (Pan and Atungulu 2010). Furthermore, the other

substances (like tray and the wall of drying chamber) would be heated when IR was

transferred to their surfaces, leading to the increase of the ambient temperature (Pawar

and Pratape 2017). As a result, the freeze-drying process was facilitated. Tamás Antal

et al. (2017) used middle infrared drying as the pre-drying procedure before FD and

found that mid-infrared-freeze drying could save 14.3-42.9 % of the drying time in

comparison with single FD. Chen et al. (2015) reported that in the drying of jujube

slices, the short-and medium-wave infrared radiation could save 17~67% of the

drying time when compared to hot air drying. Venkitasamy et al. (2017) also found

that the drying time for IR drying was 65.57% of that in hot air drying. For both IRFD

and TFD, the increase of drying temperature resulted in the decrease of drying time.

The sublimation of ice is an endothermic process, and the energy required is about

2700 J when 1g of water changes from a solid to a gaseous state (Nireesha et al.,

2013). During the first stage of freeze-drying, the driving force for ice sublimation

(vapor transport) depends on vapor pressure. The rise in vapor pressure contributes to

the increase of driving force, which is conducive to the sublimation of ice (Fissore et

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al., 2018). Generally, higher temperatures increase the vapor pressure generated at the

ice interface (Ray et al., 2017). On the other hand, in the secondary stage of

freeze-drying, more energy is required to remove the remaining water in the product

which is strongly bound (Nireesha et al., 2013). Therefore, provided the collapse of

the material is avoided, a higher the drying temperature helps accelerate the drying

process.

3.3. Drying temperature

Fig.4 shows the online product temperature (surface temperature) of Cordyceps

militaris during IRFD and TFD under four different drying temperatures. The whole

freeze-drying process involved two stages, namely the primary drying and secondary

drying. In the first stage, the ice sublimation took place and the product temperature

increased from -25±1°C to 0 °C. After the removal of ice from the sample,

freeze-drying came to the secondary stage. This stage aimed at removing the

remaining moisture of the product through water desorption (Fissore et al., 2018).

Finally, the sample temperature reached its setting values and stayed constant. As the

sublimation of ice and evaporation of water are endothermic, enough energy should

be supplied. For the same drying method, increasing drying temperatures helped

accelerate the drying process, particularly in the last drying stage. However, if the

drying temperature was too high, melting would occur, resulting in the shrinkage,

collapse or puffing of the material (Duan et al., 2016a). Therefore, it was essential to

balance the heat input and the heat required.

At the same drying temperature, the sample temperature in the primary drying

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stage during IRFD was a little higher than that of TFD. Nevertheless, in the secondary

drying, the sample temperature of IRFD was much higher than that of TFD. In the

IRFD process, IR was used as the heating source under vacuum condition. The

product absorbed infrared energy as electromagnetic waves directly, without the need

for hot air as the energy transmission medium (Wang Lin et al., 2018). Compared with

the electric heating radiation, IR could promote the transportation of moisture to the

surface of the sample, which led to the overall increase of drying rates (Onwude et al.,

2018).

3.4. Energy consumption

The total energy consumption and energy consumed by each part of the

freeze-drying device are shown in Table 1. It was obvious that compared with TFD,

the IRFD could effectively lower the total energy consumption. IRFD significantly

(p<0.05) reduced 12.09% (40 °C), 11.88% (50 °C), 12.12% (60 °C) and 18.37%

(70 °C) of the total energy reduction in comparison with TFD. The energy consumed

by vacuum and refrigeration systems accounted for more than 85% of total energy

consumption. This result indicated that in freeze drying, the total energy consumption

is highly dependent on the energy consumption of the vacuum and refrigeration

systems. Similar results on energy consumption in the freeze drying of barley grass

have been reported by Cao et al. (2018). They found approximately 80% of the total

energy was expended by cold trap and vacuum system. For both IRFD and TFD, the

increase of drying temperature led to the decrease of the overall energy consumption.

The same trend was also observed in the energy consumption of vacuum and cold trap

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system. This was because the increase of drying temperature could significantly

(p<0.05) decrease the drying time (Fig.3), resulting in the reduced working hours for

vacuum and refrigeration systems. Beigi et al. (2017) also reported that in the drying

of rough rice, higher drying temperatures contributed to less energy consumption.

However, for heating system, the energy consumption did not change proportionately

with the increase of drying temperature. Although a high temperature meant an

increase in heat input, the drying time was shortened at higher drying temperatures,

which in turn caused a reduction in energy consumption.

3.5. Nutritional properties

Fig.5 displays the nutrients retention of Cordyceps militaris dried by IRFD and

TFD. The cordycepin and adenosine contents (Fig.5A and Fig.5B) of Cordyceps

militaris under different drying conditions ranged from 1.68 to 3.31 mg g-1

, and 3.42

to 4.06 mg g-1

, respectively. The cordycepin contents were similar to that reported by

Chimsook (2018) while the adenosine contents were higher than that of the report. In

their study, the contents of cordycepin and adenosine in freeze-dried Cordyceps

militaris were 2.32 and 1.10 mg g-1

. However, Li et al. (2018) found that the

cordycepin content was 0.19 mg g-1

, which was much lower than those in this study.

The cordycepin and adenosine contents in Cordyceps militaris were influenced by

several factors, such as light, grain substrate and additives (Lin et al., 2017). This

made an obvious variance between the contents of these active constituents in

different fermented Cordyceps militaris. From Fig.5A, it could be observed that

drying temperatures at 50 and 60 °C resulted in significantly (p<0.05) higher

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cordycepin contents in comparison with those of 40 and 70 °C. In terms of adenosine

contents, the opposite situation was detected. This might be due to the conversion of

adenosine to cordycepin when the freeze drying temperature was at 50 or 60 °C.

Despite the fact that the biosynthetic mechanism of cordycepin is not understood,

adenosine has been widely recognized as the direct precursor of cordycepin (Xia et

al., 2017).

Reducing sugar contents of different dried Cordyceps militaris are shown in

Fig.5C. For IRFD, the reducing sugar content decreased with increased drying

temperature. In terms of TFD, the content of reducing sugar first decreased and then

increased with increasing drying temperature. Compared with TFD, IRFD was more

conducive to the retention of reducing sugars at drying temperatures not higher than

50 °C. As one of the substrates for the Maillard reaction, reducing sugar is consumed

by participating in the Maillard reaction (Duan et al., 2016b). Furthermore, this

reaction is endothermic, and the increase of temperature as well as heating time will

promote the degree of Maillard reaction (Jiang et al., 2017).

Fig.5D presented the total phenolic contents of Cordyceps militaris dried by

IRFD and TFD. Like cordycepin, the high retention of total phenolic contents

occurred at 50 and 60°C. The temperatures at 40 and 70 °C led to relatively low

contents of total phenolics. This result indicated that either the prolonged drying time

or high drying temperature would contribute to the decomposition of phenolic

substances. Usually, the reduction of phenolic components is linked to the oxidation

reactions or to the changes of its chemical structure (Valadez-Carmona et al., 2017).

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Barroca et al. (2013) reported that the total phenolic contents of pear slices decreased

with the increase of drying time. Méndez-Lagunas et al. (2017) also found that dying

at 60 °C caused a higher loss in phenolic compounds in comparison with drying at

50 °C. The difference between samples dried by IRFD and TFD at 40 °C was not

significant (P>0.05). However, the total phenolic contents of Cordyceps militaris

dried by IRFD were lower than those by TFD when the drying temperature was above

40 °C. This might be due to the relatively high internal temperature caused by infrared

radiation, resulting in the degradation of phenolic compounds. Figures 5A, B and D

show that TFD samples have more favorable values than IRFD materials.

3.6. Antioxidant activities

Fig.6A and Fig.6B display the hydroxyl radical scavenging activity and the

reducing capacity of Cordyceps militaris under different drying conditions. Hydroxyl

radicals can pass through the cell membrane easily and react with the biological

molecules, resulting in molecular or cellular damage (Gao et al., 2013). Consequently,

eliminating hydroxyl radicals is important for protecting living systems. It can be

observed that the scavenging activities of all samples are above 88%, demonstrating

strong scavenging abilities of hydroxyl radicals by dried Cordyceps militaris. As

shown in Fig.6A, the hydroxyl radical scavenging activity was relative high when the

drying temperature was set at 50 and 60 °C. Although the difference was not

significant (P >0.05), compared with TFD, the IRFD led to a little higher scavenging

capacity at these two drying temperatures. At 40 and 70 °C, an opposite situation

occurred. It was reported that the hydroxyl radical scavenging ability was linked to

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the contents of polysaccharides (Wu et al., 2014), phenolics and flavonoids

(Valadez-Carmona et al., 2017). These compounds can react with the hydroxyl

radicals by donating a hydrogen atom or single electron, resulting in the scavenging of

hydroxyl radicals (Russo 2010). In this study, the hydroxyl radical scavenging

activities of Cordyceps militaris under different drying conditions correlated well with

each contents of cordycepin and total phenolics in Fig.5. This result indicated that

both cordycepin and phenolics contributed to the scavenging of hydroxyl radicals.

In terms of reducing power, there was no significant difference (P >0.05)

between IRFD and TFD samples at a constant drying temperature. Like hydroxyl

radical scavenging activity, reducing capacity was higher at 50 and 60 °C than that at

40 and 70 °C. This result suggested that both prolonged drying time and high drying

temperature were not conducive to the retention of reducing power. From Fig.6B, it

was clear that the absorbance ranged from 0.86 to 0.90, which was much higher than

that reported by Zhan et al. (2006). In their study, the absorbance of the solutes was

lower than 0.2. This might be due to two main reasons: one was the difference in

biologically active compounds contents caused by the various cultivating substrates;

the other was the difference in extracting methods. The authors used hot water

(95-100 °C) as the extraction medium, which might led to the degradation of the

bioactive substances. As a result, the reducing ability was reduced. The antioxidant

substances in the material could convert Fe3+

into Fe2+

via providing single electron,

resulting in the increase of absorbance (Khan et al., 2015). However, Xing et al.

(2005) attributed the reducing ability to the decomposition of the free radical chain

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through providing a hydrogen atom or to the prevention of the peroxide formation by

reacting with certain peroxide precursors.

To sum up, these results demonstrated that the bioactive substances in Cordyceps

militaris displayed operative antioxidant activities in terms of the hydroxyl radical

scavenging activity and reducing capability. Moreover, IRFD and TFD samples

exhibited similar antioxidant activities at a constant drying temperature.

3.7. Main volatile compounds

In our previous study (Wu et al., 2018), we have found that 3-octanone,

1-octen-3-ol, 3-octanol and 1,3-octadiene were the most abundant volatile compounds

in freeze-dried Cordyceps militaris. Malheiro et al. (2013) also found that 3-octanol,

1-octanol and 3-octanone were the most plentiful volatile compositions detected in six

wild mushrooms species. Fig.7 shows four major volatile compounds in Cordyceps

militaris dried under different conditions. At a constant drying temperature, IRFD

samples exhibited a higher retention of these four volatile compounds in comparison

with those of TFD samples. This indicated that short drying time facilitated retention

of volatile flavors. Pei et al. (2016) reported that in the freeze-drying of button

mushroom, the contents of 1-octen-3-ol, 3-octanol and 3-octanone decreased

remarkably with the extension of drying time. Compared to other main volatile

components, the contents of 3-octanone were the highest in freeze-dried Cordyceps

militaris within the range of 3.13~7.36 mg kg-1

. For both IRFD and TFD, drying at

70 °C resulted in the lowest 3-octanone contents. Drying at 60 °C led to the highest

contents of this compound, indicating that 60 °C was the most suitable drying

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temperature in freeze-drying process. Similar situations were also detected in

3-octanol and 1,3-octadiene. It was reported that 3-octanone contributed to an earthy,

fruity and mushroom flavor (Tian et al., 2016). It can be derived from the degradation

of polyunsaturated fatty acids and amino acids (Xu Y. et al., 2014). 3-Octanol is

associated with herbaceous and nutty aroma (Wu et al., 2018). 1,3-Octadiene, which

was reported as the most abundant volatile component of Hymenogaster luteus

(Studies on volatile organic compounds of some truffles and false truffles), is usually

linked to a fungal flavor. From Fig.7C, it is clear that the contents of 1-octen-3-ol

decreased with the increased drying temperature. When the drying temperature was

raised from 40 to 50 °C, the 1-octen-3-ol contents reduced dramatically from 2.56 to

0.95 mg kg-1

in IRFD and 1.93 to 0.81mg kg-1

in TFD, respectively. 1-Octen-3-ol,

which offers a typical mushroom flavor, is produced by the thermal decomposition of

linoleic acid (Lin et al., 2013).

In general, drying at 40, 50 and 60 °C was beneficial to the retention of

3-octanone, 3-octanol and 1,3-octadiene. Nevertheless, drying at 40 °C exhibited the

highest retention of 1-octen-3-ol. IRFD resulted in better volatile compounds

retention in comparison with TFD at a constant drying temperature, which might be

owned to the shorter drying time it consumed.

4. Conclusions

For both IRFD and TFD, the increase of drying temperature resulted in the

decrease of drying time and energy consumption. Drying at 40, 50 and 60 °C was

beneficial to the retention of cordycepin, total phenolics, hydroxyl radical scavenging

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activity, reducing power, 3-octanone, 3-octanol and 1,3-octadiene. The extension of

drying time or high drying temperature was detrimental to the retention of nutritional

properties, antioxidant activities and main volatile compounds of Cordyceps militaris

except adenosine, reducing sugar and 1-octen-3-ol. Compared with TFD, the IRFD

could effectively lower the drying time and total energy consumption. The quality of

IRFD samples was close to that of TFD samples. Considering the economic and

quality factors, IRFD offers a promising method for achieving high-quality dried

Cordyceps militaris.

Acknowledgments

We acknowledge the financial support from National Key R&D Program of

China (Contract No. 2017YFD0400901), Jiangsu Province (China) Agricultural

Innovation Project(Contract No. CX(17)2017), Jiangsu Province Key Laboratory

Project of Advanced Food Manufacturing Equipment and Technology (No.

FMZ201803),National First-class Discipline Program of Food Science and

Technology (No. JUFSTR20180205)，all of which enabled us to carry out this study.

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List of Tables

Table 1 Energy consumption of Cordyceps militaris in IRFD and TFD at different

drying temperatures (40, 50, 60 and 70 °C, respectively)

List of Figures

Fig.1. Schematic diagram of the IRFD dryer

Fig.2. Structural attributes of Cordyceps militaris in the frequency range of

4000~400 cm-1

Fig.3. Drying time of Cordyceps militaris in IRFD and TFD at different drying

temperatures (40, 50, 60 and 70 °C, respectively)

Fig.4. Surface temperature of Cordyceps militaris in IRFD and TFD at different

drying temperatures (40, 50, 60 and 70 °C, respectively)

Fig.5. Nutritional properties of Cordyceps militaris dried by IRFD and TFD at 40,

50, 60 and 70 °C, respectively (A: cordycepin contents; B: adenosine contents; C:

reducing sugar contents; D: total phenolic contents)

Fig.6. Antioxidant activities of Cordyceps militaris dried by IRFD and TFD at 40,

50, 60 and 70 °C, respectively (A: hydroxyl radical scavenging activity; B: reducing

power)

Fig.7. Four main volatile compounds of Cordyceps militaris dried by IRFD and

TFD at 40, 50, 60 and 70 °C, respectively (A: 3-octanone; B: 3-octanol; C:

1-octen-3-ol; D: 1,3-octadiene)

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Table 1 Energy consumption of Cordyceps militaris in IRFD and TFD at different

drying temperatures (40, 50, 60 and 70 °C, respectively)

Energy

consumption

(KW·h)

40 °C IRFD 40 °C TFD 50 °C IRFD 50 °C TFD 60 °C IRFD 60 °C TFD 70 °C IRFD 70 °C TFD

Total 14.76±0.11c 16.79±0.14a 13.20±0.10d 14.98±0.10b 11.31±0.12g 12.87±0.15e 9.69±0.11h 11.87±0.14f

Vacuum system

and cold trap 13.46±0.09c 15.68±0.12a 12.05±0.09d 13.78±0.07b 10.50±0.09f 11.43±0.12e 8.52±0.09h 10.22±0.12g

Heating system 1.30±0.04c 1.11±0.05e 1.15±0.02de 1.20±0.05d 0.81±0.05f 1.44±0.04b 1.17±0.06de 1.65±0.03a

Different letters within the same row indicate a significant difference (p<0.05).

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Fig.1. Schematic diagram of the IRFD dryer: 1. control and display panel; 2.

temperature sensor of lamp; 3. temperature sensor of sample; 4. infrared glass lamps;

5. sample placement rack; 6. load cell; 7. drying chamber; 8. observation window;

9.door; 10. condenser; 11. refrigeration unit; 12.vacuum pump

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4000 3500 3000 2500 2000 1500 1000 500

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8A

bso

rban

ce

Wavenumber (cm-1)

3371.55

2926.49

1646.39

1407.91

1247.99

1150.16

1078.011045.51

993.97

575.28

Fig.2. Structural attributes of Cordyceps militaris in the frequency range of 4000~400

cm-1

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40 50 60 70

0.00

250

300

350

400

450

500

550

600

650D

ryin

g t

ime

(min

)

Drying temperature (°C)

IRFD

TFDa

b

c

dd

de

f

Fig.3. Drying time of Cordyceps militaris in IRFD and TFD at different drying

temperatures (40, 50, 60 and 70 °C, respectively; Different letters indicate a

significant difference (p<0.05))

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0 50 100 150 200 250 300 350 400 450 500 550

-30

-20

-10

0

10

20

30

40

0 50 100 150 200 250 300 350 400 450

-40

-30

-20

-10

0

10

20

30

40

50

60

0 50 100 150 200 250 300 350 400

-40

-30

-20

-10

0

10

20

30

40

50

60

70

0 50 100 150 200 250 300 350

-40

-30

-20

-10

0

10

20

30

40

50

60

70

80

IRFD

TFD

Sam

ple

tem

pra

ture

(°C

)

Drying time (min)

40 °C

Primary dryingPrimary drying

IRFD

TFD

Sam

ple

tem

pra

ture

(°C

)

Drying time (min)

50 °C

Primary drying

IRFD

TFD

Sam

ple

tem

pra

ture

(°C

)

Drying time (min)

60 °C

Primary drying

IRFD

TFD

Sam

ple

tem

pra

ture

(°C

)

Drying time (min)

70 °C

Fig.4. Surface temperature of Cordyceps militaris in IRFD and TFD at different

drying temperatures (40, 50, 60 and 70 °C, respectively)

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40 50 60 70

0.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

3.2

3.6

4.0

Co

rdy

cep

in (

mg

g-1

, d

.b.)

Drying temperature (°C)

IRFD

TFD

c

b

a

cc

b b

c

40 50 60 70

0.00

3.0

3.2

3.4

3.6

3.8

4.0

4.2

Ad

eno

sin

e (m

g g

-1, d

.b.)

Drying temperature (°C)

IRFD

TFD

b

cd d

aa

cd

c

aA B

C D

40 50 60 70

0.00

20

22

24

26

28

30

Red

uci

ng

su

gar

(m

g G

E g

-1, d

.b.)

Drying temperature (°C)

IRFD

TFD

aa

b

c c c

d d

40 50 60 70

0.00

1.2

1.3

1.4

1.5

1.6

1.7

1.8

To

tal

ph

eno

lics

(m

g G

AE

g-1

, d

.b.)

Drying temperature (°C)

IRFD

TFD

cd c

b

e

d

b

a

cd

Fig.5. Nutritional properties of Cordyceps militaris dried by IRFD and TFD at 40, 50,

60 and 70 °C, respectively (A: cordycepin contents; B: adenosine contents; C:

reducing sugar contents; D: total phenolic contents; Different letters indicate a

significant difference (p<0.05))

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40 50 60 70

0.00

86

88

90

92

94

96

98

100H

ydro

xyl

radic

al s

caven

gin

g a

ctiv

ity (

%)

Drying temperature (°C)

IRFD

TFD

a

cd

abbc c

d ef

A

40 50 60 70

0.00

0.80

0.82

0.84

0.86

0.88

0.90

0.92

0.94

Abso

rban

ce

Drying temperature (°C)

IRFD

TFD

d cdbc ab a a

e e

B

Fig.6. Antioxidant activities of Cordyceps militaris dried by IRFD and TFD at 40, 50,

60 and 70 °C, respectively (A: hydroxyl radical scavenging activity; B: reducing

power; Different letters indicate a significant difference (p<0.05))

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40 50 60 70

0

2

4

6

8

10

3-O

ctan

on

e (m

g k

g-1

)

Drying temperature (°C)

IRFD

TFD

a

abab

c

cd d

e

f

40 50 60 70

0

1

2

3

4

5

6

3-O

ctan

ol

(mg

kg

-1)

Drying temperature (°C)

IRFD

TFD

a

b

bc

d

e e ee

40 50 60 70

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1-O

cten

-3-o

l (m

g k

g-1

)

Drying temperature (°C)

IRFD

TFD

a

b

c

c

cdde

c

e

40 50 60 70

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1,3

-Oct

adie

ne

(mg

kg

-1)

Drying temperature (°C)

IRFD

TFD

b

a

bbc

cdd

ee

A B

C D

Fig.7. Four main volatile compounds of Cordyceps militaris dried by IRFD and TFD

at 40, 50, 60 and 70 °C, respectively (A: 3-octanone; B: 3-octanol; C: 1-octen-3-ol; D:

1,3-octadiene; Different letters indicate a significant difference (p<0.05))

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A novel infrared freeze drying (IRFD) technology to lower the energy

consumption and keep the quality of Cordyceps militaris

Highlights:

A novel infrared freeze drying device (IRFD) was developed for the drying

of Cordyceps militaris

Compared to TFD, IRFD could reduce 7.21~17.78% of the drying time at a

constant drying temperature

IRFD could reduce 11.88~18.37% of the energy consumption compared to

TFD at the same temperature

The quality of IRFD samples was close to that of TFD samples

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