Evaluation of freeze drying combined with microwave vacuum drying for functional okra snacks: Antioxidant properties, sensory quality, and energy consumption

Accepted Manuscript Evaluation of freeze drying combined with microwave vacuum drying for functional okra snacks: Antioxidant properties, sensory quality, and energy consumption Ning Jiang, Zhongyuan Zhang, Dajing Li, Chunquan Liu, Min Zhang, Chunju Liu, Di Wang, Liying Niu PII:

S0023-6438(17)30234-7

DOI:

10.1016/j.lwt.2017.04.015

Reference:

YFSTL 6152

To appear in:

LWT - Food Science and Technology

Received Date: 4 January 2017 Revised Date:

5 April 2017

Accepted Date: 5 April 2017

Please cite this article as: Jiang, N., Zhang, Z., Li, D., Liu, C., Zhang, M., Liu, C., Wang, D., Niu, L., Evaluation of freeze drying combined with microwave vacuum drying for functional okra snacks: Antioxidant properties, sensory quality, and energy consumption, LWT - Food Science and Technology (2017), doi: 10.1016/j.lwt.2017.04.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Evaluation of freeze drying combined with microwave vacuum drying for

2

functional okra snacks: Antioxidant properties, sensory quality, and energy

3

consumption

4

Running title: Evaluating FD-MVD used for processing of functional okra snacks

5

Ning Jianga, b*, Zhongyuan Zhanga, b, Dajing Lia, b, Chunquan Liua, b, Min Zhangc,

6

Chunju Liua, b, Di Wang a, b, Liying Niu a, b

7

a

8

Nanjing 210014, PR China

9

b

RI PT

1

SC

Institute of Farm Product Processing, Jiangsu Academy of Agricultural Sciences,

Jiangsu Key Laboratory for Horticultural Crop Genetic Improvement, Jiangsu

Academy of Agricultural Sciences, Nanjing 210014, PR China

11

c

12

214122, PR China

M AN U

10

AC C

EP

TE D

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

*Corresponding author. Tel.:+86 25 84391255; fax:+86 25 84391677. E-mail addresses:[email protected] (N. Jiang),

1

ACCEPTED MANUSCRIPT Abstract: This study evaluated freeze drying combined with microwave vacuum

14

drying (FD-MVD) for functional okra snacks. FD-MVD was compared using four

15

different drying methods: hot air drying (AD), freeze drying (FD), microwave vacuum

16

drying (MVD), and hot air drying combined with microwave vacuum drying

17

(AD-MVD). For main antioxidant components including epigallocatechin gallate

18

(EGC),

19

3-O-diglucoside, and quercetin 3-O-(malonyl) and antioxidant properties (IC50, AEAC,

20

FRAP, ABTS, TF, and TPC) and color (L*, a*, and b*), the values of FD-MVD were

21

similar to those of FD and significantly higher than those of AD, MVD, and AD-MVD

22

(P<0.05). Compared to FD, FD-MVD provided moderate hardness (22.43±1.18 N) and

23

crispness (6.74±0.87 N), and reduced the drying time and energy consumption by

24

approximately 75.36% and 71.92%, respectively. Principal component analysis (PCA)

25

of physicochemical and drying efficiency indexes indicated that FD-MVD is a

26

promising technique for the processing of functional okra snacks.

27

Key words: FD-MVD; Quercetin; Epigallocatechin; Antioxidant activity; Functional

28

okra snacks

31 32 33

epigallocatechin

(ECG),

quercetin

TE D

M AN U

SC

(EGCG),

EP

30

gallate

AC C

29

epicatechin

RI PT

13

34 35 36 37

2

ACCEPTED MANUSCRIPT 38

1. Introduction Okra (Abelmoschus esculentus [L.] Moench) is a type of one-year herb plant that is

40

widely grown in tropical and subtropical countries for its fibrous pods full of round and

41

young leaves (Kumar, Prasad, & Murthy, 2014). In recent years, it has also been

42

widely cultivated in north and south China (Huang, & Zhang, 2016). Okra can play an

43

important role in the human diet because of its characteristically high vitamin C,

44

vitamin A, dietary fiber, and calcium content, and low saturated fat content (Sobukola,

45

2009). Recently, anti-oxidizing plants with therapeutic potential in reducing free

46

radical-induced tissue injury have attracted increased attention (Shukla, Mehta, Menta,

47

& Bajpai, 2012). Okra has been found to be rich in quercetin and epigallocatechin,

48

which have strong free-radical scavenging and antioxidant capacities (Shui, & Peng,

49

2004; Arapitsas, 2008; Panat, Maurya, Ghaskadbi, & Sandur, 2016). Currently, the

50

demand for functional foods is strongly increasing because of awareness among

51

consumers of the impact of food on health. As the consumption of snacks is a

52

well-established habit around the world (Noorbakhsh, Yaghmaee, & Durance, 2013),

53

dehydrated okra snacks may represent an appropriate matrix to deliver quercetin and

54

epigallocatechin.

EP

TE D

M AN U

SC

RI PT

39

Currently, low-cost and easily controlled hot air drying (AD) is widely used for

56

dehydration of okra (Pendre, Nema, Sharma, Rathore & Kushwah, 2012; Wankhadea,

57

Sapkala, & Sapkalb, 2013). In addition, freeze drying (FD) and microwave vacuum

58

drying (MVD) have been studied and applied to okra dehydration (Huang, & Zhang,

59

2016; Li, Duan, & Liu, 2013). However, each drying technique has its own advantages

60

and drawbacks. AD is the most conventional drying method, but its long drying cycle

61

and high temperature usually cause degradation of important nutritional compounds,

62

flavors, and color (An et al., 2016). Freeze drying can maximally preserve the original

AC C

55

3

ACCEPTED MANUSCRIPT properties such as the activity, flavor, and shape, but it also consumes extensive time

64

and energy and is costly (Huang, & Zhang, 2016). MVD improves drying rates,

65

requires lower temperatures (Wojdyło, Figiel, Lech, Nowicka, & Oszmianski, 2014),

66

and can facilitate preservation of nutrient components and color (Tian, Zhao, Huang,

67

Zeng, & Zheng, 2016). To our knowledge, no investigation has established a drying

68

method for functional okra snacks. The current study focused on the FD-MVD

69

technique for not only antioxidant components conservation but also improving the

70

quality of texture and energy efficiency of drying process. It has been reported that the

71

sensory qualities of FD-MVD products are similar to those of FD products (Huang,

72

Zhang, Wang, Mujumdar & Sun, 2012). In addition, the combination AD-MVD drying

73

method was also reported to have reduced power consumption (Hu, Zhang, Mujumdar,

74

Xiao, & Sun, 2006) and to improve product quality (Argyropoulos, Heindl, & Müller,

75

2011) relative to microwave vacuum drying or hot air drying alone, respectively.

M AN U

SC

RI PT

63

The objective of this study was to explore the drying method for functional okra

77

snacks, which are a vehicle for quercetin and epigallocatechin. We investigated the

78

effects of FD-MVD on several antioxidant properties and sensory quality of okra, such

79

as quercetin and epigallocatechin compounds, total phenolic and flavonoid contents,

80

antioxidant capacity, hardness, crispness, shrinkage and color (when compared with

81

AD, FD, MVD, and AD-MVD), as well as the drying efficiency of each method.

82

2. Materials and methods

83

2.1. Raw materials

AC C

EP

TE D

76

84

Fresh okra was obtained from a local supermarket, with an initial moisture content

85

of 90.78 ± 0.09 g/100 g (wet basis, w.b.). Before drying, the samples were first

86

equilibrated by storing them under ambient conditions for 2 h, after which they were

87

blanched in boiling water for 150 s and immediately cooled in chilled water to avoid

4

ACCEPTED MANUSCRIPT 88

over-processing (Shivhare, Gupta, Bawa, & Gupta, 2000). The tops and tips of the

89

blanched okra were trimmed.

90

2.2. Chemicals High-performance liquid chromatography (HPLC)-grade methanol, formic acid,

92

and acetonitrile were purchased from American Tedia Co., Ltd. (Tedia Reagent, USA).

93

Glucose, standard of rutin, ascorbic acid, and gallic acid were obtained from

94

Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Folin-Ciocalteu,

95

2,2-diphenyl-1-picrylhydrazyl(DPPH);2,2'-azinobis-(3-ethylbenzthiazoline-6-sulphona

96

te) diammonium salt (ABTS), 2,4,6-tri (2-pyridinyl)-1, 3, 5-triazine (TPTZ), and

97

authentic epigallocatechin gallate (EGCG), epigallocatechin (EGC), epicatechin

98

gallate (ECG), and quercetin 3-O-glucoside standards were purchased from

99

Sigma-Aldrich (St. Louis, MO, USA).

SC

M AN U

100

RI PT

91

2.3. Drying methods

Okra (200 g) was distributed evenly and subjected to 5 different drying methods,

102

where drying was performed until the moisture content of the okra samples was <0.052

103

g H2O/g dry weight (d.w.). The drying time and drying rate (dehydration per unit time,

104

g/(h·kg)) required for okra dehydration by each method was compared at a moisture

105

content of 0.052 g H2O/g d.w. All parameters for different drying procedures were

106

determined by optimization in preliminary experiments.

107

2.3.1 Hot air drying (AD)

EP

AC C

108

TE D

101

Okra samples were dried in an electrically heated air blast-drying oven (Digital

109

display 101A-2; CIMO Medical Instrument Co., Ltd., Shanghai, China) at 70°C under

110

an air velocity of 1.5 m/s.

111

2.3.2 Freeze drying (FD)

112

Okra samples were first frozen at −30 ± 2°C, quickly placed into a freeze dryer

5

ACCEPTED MANUSCRIPT 113

(FD-1A-50; Beijing Boyikang Laboratory Instrument Co. Ltd., Beijing, China), and

114

dried under 20 Pa absolute pressure. The temperature of the heating plate and the cold

115

trap was 25°C and −50°C, respectively.

116

2.3.3 Microwave vacuum drying (MVD) An XMJD6SW-2 microwave vacuum drier (Nanjing Xiaoma Mechanical and

118

Electrical Equipment Factory, Nanjing, China) was used for the MVD process (2000 W,

119

2450 MHz). The power output of the MVD was measured calorimetrically, and the

120

vacuum was set to 70 kPa and maintained by controlling the vacuum pump and air inlet

121

(Jiang et al., 2016). In the present study, 80% of the maximum equipment power was

122

chosen, and the corresponding useful microwave power was equal to 1536 W, including

123

a control unit for microwave pulse-ratio (PR) regulation. Blanched samples were dried

124

at a PR of 1.5 (10 s on, 5 s off) until the completion of drying.

125

2.3.4 Hot air-microwave vacuum drying (AD-MVD)

M AN U

SC

RI PT

117

Okra samples were first dried by hot air drying at 70°C under an air velocity of 1.5

127

m/s for 2 h, until the moisture content was <60 g/100 g w.b., then dried by MVD at a

128

power of 1536 W with a PR of 1.5 (10 s on, 5 s off) and a vacuum of 70 kPa until the

129

completion of drying.

130

2.3.5 Freeze drying- microwave vacuum drying (FD-MVD)

EP

AC C

131

TE D

126

Samples were first dried by FD for 10 h (until the moisture content of the samples

132

was <80 g/100 g w.b.) and then processed by MVD at a power of 1536 W with a PR of

133

1.5 (10 s on, 5 s off) and a vacuum of 70 kPa until the completion of drying.

134

2.4. Specific energy requirements

135

An electric energy meter (DDSY666, CHNT, Leqing, Zhejiang, China) was used to

136

measure the total energy consumption in the different drying procedures. The energy

137

consumption required for drying each kilogram of okra sample was calculated using Eq.

6

ACCEPTED MANUSCRIPT (1).

Ekg =

139

140

Et ×1000 W0

(1)

where Ekg is the specific energy requirement (kWh), Et is the total energy consumed

141

(kWh), and W0 is initial weight of the okra samples (g).

142

2.5. Determination of color, texture, and shrinkage

RI PT

138

Color, texture, and shrinkage analyses were conducted on the dried okra samples.

144

Five pieces of dried sample (long pieces) for each drying method were chosen for each

145

analysis. During the analysis, one piece of dried sample was evaluated each time, and

146

the mean values of five pieces are reported.

M AN U

SC

143

A color meter (Model WSC-S, Shanghai Precision Scientific Instrument Co., Ltd.,

148

Shanghai, China) was used to measure the color of the okra samples with a white

149

reference tile used for calibration. The hunter-Lab units L*/(whiteness/brightness),

150

a*/(redness/greenness), and b*/(yellowness/blueness) were used to define the colors.

151

Color changes between the fresh and dried samples (△E) were measured in 3 replicates

152

using a previously described equation (Tian, Zhao, Huang, Zeng, & Zheng, 2016).

TE D

147

A texture analyzer (CT3; Brookfield Ltd., MA, USA) was applied to determine the

154

textural properties of dried okra samples. The test hardware included a load cell of 5

155

kg, a TA-39 blade shape probe, and a TA-JTPB micro 3-point bend fixture, with 2.0

156

mm s-1, 0.5 mm s-1, and 2.0 mm s-1 chosen as the pre-speed, test-speed, and post-speed

157

settings, respectively.

158 159

160

AC C

EP

153

The shrinkage ratio (SR) was measured to estimate the volume changes in dried samples. The SR of the dried okra samples was defined as follows:

SR =

V0 − Vd V0

7

(3)

ACCEPTED MANUSCRIPT 161

where V0 and Vd refer to the volume of the original and dried material, respectively.

162

Glass beads with a diameter of 0.107–0.211 mm served as a replacement medium.

163

2.6. Preparation of extracts for HPLC analysis and antioxidant activity The extraction procedure was performed as described by Arapitsas (2008), with

165

some modifications. The dried okra samples were beat into a powder, and 1.5 g was

166

accurately weighed and dissolved in a beaker with 30 mL of 95 mL/100 mL aqueous

167

methanol. Then, ultrasound-assisted extraction was performed for 30 min at 40°C.

168

After standing for 0.5 h, the supernatant was transferred to a 100-mL, round-bottom

169

flask. The residue was added to 30 mL of 95 mL/100 mL methanol solution, and the

170

same extraction method was applied once again. After standing for 0.5 h, both

171

supernatants were mixed and dried at 50°C in a rotary evaporator to remove methanol.

172

After concentration, the extract was transferred to a 10-mL volumetric flask and diluted

173

to the mark with 95 mL/100 mL methanol. A 0.45-µm organic membrane filter was

174

used to filter the solution before placement into an auto sampler vial for HPLC analysis.

175

Similarly, 5 g of each fresh okra sample was first homogenized in 30 mL of 95 mL/100

176

mL methanol using a breaking pulper (JYL-A110 type, Joyoung Company, Beijing,

177

China), and then the extraction procedure described above was repeated.

178

2.7. Identification of polyphenol compounds by triple TOF–LC–MS–MS analysis

SC

M AN U

TE D

EP

AC C

179

RI PT

164

Polyphenols of okra extracts were identified using a triple time-of-flight, liquid

180

chromatography, tandem mass spectrometry (TOF–LC–MS–MS) system. A 1260

181

Infinity LC system (Agilent, Waldbronn, Germany) was coupled to a Triple TOF 5600

182

(AB SCIEX) for data acquisition. Separation of individual polyphenols was performed

183

using a 300SB-C18 column (4.6 mm × 150 mm × 5 µm, Agilent) at 30°C. Acetonitrile

184

and deionized water were chosen as solvents A and B, respectively. A gradient was

185

generated under the following conditions: 0–3 min, 7% A; 3–10 min, 7–16% A; 10–15

8

ACCEPTED MANUSCRIPT 186

min, 16–20% A; 15–16 min, 20–30% A; 16–20 min, 30–95% A; 20–23 min, 40–60%

187

A; 23–24 min, 60–100% A; 24–35 min, 100% A until equilibrium was reached. The

188

flow rate was 0.6 mL/min. MS acquisition was performed by information-dependent acquisition (IDA) at

190

negative ionization, and analysis was performed using a full-scan, data-dependent

191

spectrum of m/z 100–2000. The MS parameters were set as follows: ion spray voltage,

192

5500 V; declustering potential, 80 V; collision energy, 10 V; temperature, 500°C with a

193

curtain gas of 35 (arbitrary units); ion source gas 1, 50; and ion source gas 2, 50.

194

PeakView 2.2 software with the applications XIC Manager and Formula Finder was

195

used for data acquisition and processing.

196

2.8. Quantification of polyphenols using the HPLC system

M AN U

SC

RI PT

189

HPLC was performed as described above in Section 2.7. DAD (Agilent

198

Technologies, California, USA) detection between 200 and 600 nm was performed

199

with a resolution of 2 nm. Phenolic compounds were quantified using external

200

calibration curves from the peak area at 280 nm. EGCG, EGC, and ECG were

201

quantified using their own standards. The derivative of quercetin was expressed as

202

quercetin-3-O-glucoside. Results were expressed as µg/kg d.w.

203

2.9. Determination of the total flavonoid content (TFC)

EP

AC C

204

TE D

197

TFC measurement was performed as described by An et al. (2016), with minor

205

modifications. Extracts of okra samples (1 mL) were transferred into 25-mL tubes.

206

Initially, 0.5 mL of 5% NaNO2 was added to each test tube. Subsequently, 0.5 mL of

207

10% Al(NO3) 3 was added after 6 min, and 4.0 mL of 4 g/100 mL NaOH was added

208

after another 6 min. Finally, 50% ethanol was added to bring the volume up to 10 mL,

209

mixed well, and allowed to stand for 10 min. The absorbance of the reaction mixture

210

was read at 510 nm, with 50% ethanol used as a blank. Rutin equivalents (mg/g of dry

9

ACCEPTED MANUSCRIPT 211

weight) were used to determine TFCs.

212

2.10. Determination of the total phenolic content (TPC) The Folin-Ciocalteu colorimetric method was used to determine the TPC of okra

214

samples as previously described (Abozed, El-Kalyoubi, Abdelrashid, & Salama, 2014),

215

with minor modifications. Briefly, extracts of okra samples (1 mL) were transferred to

216

25-mL volumetric flasks, after which 6 mL of distilled water was added. Then, 1 mL of

217

Folin-Ciocalteu phenol reagent (diluted 10-fold in deionized water) was added to each

218

flask, followed by 3.0 mL of sodium carbonate (7.5 g/100 mL) solution. After a 2-h

219

reaction in the dark, a spectrophotometer (TU-1810, Beijing Purkinje General

220

Instrument Co., Ltd, Beijing, China) was used to measure the absorbance at 765 nm,

221

and 50% ethanol was used as a blank. The number of gallic acid equivalents (GAE,

222

mg/g of dry sample) was used to determine TPCs.

223

2.11. DPPH radical-scavenging assay

M AN U

SC

RI PT

213

DPPH-radical scavenging ability was measured as previously described (Park, Jeon,

225

Kim, & Park, 2012) with modifications. Briefly, 2.0 mL of 0.14 mmol/L DPPH was

226

added to different dilutions of extract (2.0 mL in anhydrous ethanol) and shaken

227

uniformly. After standing for 30 min, the absorbance of the mixture was measured at

228

517 nm. Radical-scavenging activities were expressed as the IC50 and ascorbic acid

229

equivalent antioxidant capacity (AEAC). The IC50 was determined as the concentration

230

of extract when the removal rate of S was 50%. S was calculated using Eq. 4

232 233 234

EP

AC C

231

TE D

224

S=

A0 − Ai A0

(4)

where A0 is the light-absorption value in the absence of a scavenger, and Ai is the light-absorption value measured with the sample extract. The AEAC was expressed as ascorbic acid equivalents in mg ascorbic acid/g with

10

ACCEPTED MANUSCRIPT 235

236

the following equation: AEAC (mg ascorbic acid / g ) =

IC50 (ascorbic acid ) × 10 3 (5) IC50 ( sample)

2.12. Determination of ferric-reducing antioxidant power (FRAP)

238

FRAP was measured according to a previously described method (Zhang, Lu, Ye, Ye,

239

& Ren, 2010). Briefly, extracts of okra samples (0.5 mL) were transferred to a 4.5-mL

240

TPTZ working solution. The TPTZ working solution was composed of 25 mL of 0.3

241

mol/L acetate buffer pH 3.6, l0 mmol/L TPTZ solution, and 20 mmol/L FeCl3•6H2O

242

solution. Before absorbance determinations, the TPTZ solutions were mixed 1:1:1,

243

followed by a 30-min reaction at 37ºC. The absorbance at 593 nm was used to express

244

the ferric-reducing ability of the okra samples, and 50% ethanol was used as a blank.

245

The antioxidant capacity of FRAP was also expressed as ascorbic acid equivalents in

246

mg ascorbic acid/g.

247

2.13. Determination of ABTS antioxidant activity

TE D

M AN U

SC

RI PT

237

The ABTS+ radical cation decolorization assay was performed as described by

249

Choi et al. (2012) with some modifications to determine the ABTS antioxidant activity.

250

A 7 mmol/L ABTS stock solution was prepared by dissolving ABTS in sodium

251

acetate-acetic acid buffer (20 mmol/L, pH = 4.5). An ABTS+ working solution

252

reaching a stable oxidation state was prepared by mixing 7 mmol/L ABTS and 2.45

253

mmol/L potassium persulfate at a 1:1 ratio, followed by standing in the dark for 12–16

254

h. This solution was further diluted with sodium acetate-acetic acid buffer (20 mmol/L,

255

pH = 4.5) in a 1: 22.5 ratio to reach an absorbance of 0.70 ± 0.02 at 734 nm.

256

Stepwise dilution of extracts was performed by mixing 2.0 mL of the extract with 2.0

257

mL of ABTS+ working solution, followed by standing for 30 min in the dark. Finally,

258

the absorbance was measured at 734 nm, with absolute alcohol used as a blank.

AC C

EP

248

11

ACCEPTED MANUSCRIPT 259

Ascorbic acid equivalents in mg ascorbic acid/g were also used to express the

260

antioxidant capacity of ABTS.

261

2.14. Statistical analysis All experiments were run at least in triplicate, with the data expressed as the mean ±

263

standard deviation (SD). Microcal Origin 9.0 (Microcal Software, Inc., Northampton,

264

USA) software was employed for statistical analyses. Analysis of variance (ANOVA)

265

and Duncan’s multiple-range test (P < 0.05) were performed to evaluate differences

266

between samples. Statistical Program for Social Sciences software (SPSS 19.0, Chicago,

267

IL, USA) was used to perform principal component analysis (PCA).

268

3. Results and discussion

269

3.1. Comparison of efficiency of different conditions for drying okra

M AN U

SC

RI PT

262

Significant differences in drying time, drying rate, and energy consumption were

271

observed between the selected drying methods (P < 0.05). As Table 1 shows, FD had

272

the longest drying time of 41.23 ± 1.05 h, the lowest drying rate of 21.91±0.56

273

g/(h·kg), and the highest energy consumption of 75.85 ± 1.79 kWh/kg H2O. The

274

second longest drying time and lowest drying rate were found using the FD-MVD

275

method. However, the FD-MVD method had a relatively lower energy consumption of

276

21.30 ± 1.07 kW h/kg H2O. The second highest energy consumption occurred with the

277

AD process, with a drying time of 8.2 ± 0.4 h. Compared to the other drying methods,

278

MVD and AD-MVD exhibited lower drying time and energy consumption, whereas

279

MVD had a lower drying time and energy consumption than AD-MVD. In general, the

280

drying methods employing MVD had a higher drying rate and lower drying time and

281

energy consumption.

282

3.2. Effect of the drying process on shrinkage, hardness, crispness, and color

283

parameters

AC C

EP

TE D

270

12

ACCEPTED MANUSCRIPT The SR is an important index that reflects the extent of damage to the cell structure

285

of fruits and vegetables (Hu, Zhang, Mujumdar, Du, & Sun, 2006). The effects of

286

different drying methods on the SR of okra samples are shown in Fig. 1 and Table 2.

287

The AD method showed the highest SR. This behavior was in accordance with that

288

reported previously for edamame and pomegranate (Hu, Zhang, Mujumdar, Du, & Sun,

289

2006; Horuz, & Maskan, 2015). During AD, the surface temperature of the samples

290

was higher than their internal temperatures. As the moisture on the surface of sample

291

evaporates, the sample surface may easily form a hard shell that resists transfer of the

292

internal moisture to the surface. This phenomenon caused a moderate, but long

293

shrinkage time, which was considered to be the factor responsible for the higher SR of

294

AD (Wang, Zhang, & Mujumdar, 2014). In the FD process, the outer ice crystals in

295

the materials sublimated, and the space originally containing the ice was retained and

296

formed a highly porous structure in the final product. Therefore, FD samples showed

297

the lowest shrinkage (Fig. 1, c). MVD, AD-MVD, and FD-MVD samples were found

298

to have lower SR values than AD alone. These results may be ascribed to the fact that

299

the high internal vapor pressure produced by microwave heating and the low chamber

300

pressure provided by the vacuum caused expansion and puffing of the sample

301

structures, which could help to prevent tissue shrinkage.

SC

M AN U

TE D

EP

AC C

302

RI PT

284

Texture is one of the most important indexes of dehydrated snacks quality. The

303

hardness value is the maximum peak force of the first compression of the product

304

(Kotwaliwale, Bakane, & Verma, 2007), whereas the crispness is the first peak value

305

among the peaks of destructive power (Vincent, 2004). It is clear from the data

306

presented in Table 2 that the drying methods had a significant effect on the texture of

307

dried okra in terms of the hardness and crispness (P < 0.05). Both the hardness and

308

crispness values of dried okra showed the same pattern. That is, the AD samples had

13

ACCEPTED MANUSCRIPT the highest values of hardness and crispness, followed by AD-MVD, MVD, FD-MVD,

310

and FD (in that order). As shown in Fig. 2, for the FD-MVD samples, the number of

311

peaks, which is often used to indicate the crispness (Meng et al., 2007), was also

312

similar to that of FD samples and significantly higher than that of AD, AD-MVD, and

313

MVD samples (P < 0.05). These findings may depend mainly on the internal structural

314

properties of the dried samples (Fig. 3). For the AD samples, which had the largest

315

shrinkage, the tissue structure was compact and the porosity was the lowest (Fig. 3, a),

316

leading to the highest hardness and crispness peak values. These results might reflect

317

the fact that the MVD, AD-MVD, FD-MVD, and FD samples were under a vacuum

318

and could easily produce porous structures(Fig. 3, b to e), resulting in lower crispness.

319

Furthermore, FD samples had the lowest hardness and crispness values, possibly

320

because the material remained frozen during drying (i.e., no heat damage occurred),

321

thus resulting in reduced cell damage and a porous honeycomb-type structure (Fig. 3,

322

b) with poor ability to resist external forces (Liu, Zhang, & Mujumdar, 2012; Huang,

323

& Zhang, 2016). Following rapid microwave heating and cell destruction, the SRs of

324

the MVD, AD-MVD, and FD-MVD samples were more extensive, and their

325

organizational structures (hole walls) were more compact than those of the FD

326

samples (Fig. 3, b to e). AD-MVD samples, owing to the anterior segment of the AD

327

process, had a more compact structure than the MVD samples; thus, the AD-MVD

328

samples had higher hardness peak values. In contrast, FD-MVD samples had

329

a looser structure than MVD samples, corresponding to lower hardness peak values,

330

owing to the anterior segment of the FD process.

AC C

EP

TE D

M AN U

SC

RI PT

309

331

Color is a key quality factor that influences consumer acceptance and the market

332

value of products (Tian, Zhao, Huang, Zeng, & Zheng, 2016). Fig. 1 and Table 2 show

333

the surface-color parameters of AD, FD, MVD, AD-MVD, and FD-MVD okra

14

ACCEPTED MANUSCRIPT samples. Compared to fresh samples, dried okras exhibited lower lightness (except for

335

the FD samples), as well as lower redness and yellowness. In addition, significant

336

differences (P < 0.05) were observed in the L*, a*, and b* values of the 5 different

337

drying methods. FD samples exhibited the highest L* values and lowest a* and b*

338

values, followed by the FD-MVD samples. In addition, the color parameters of the FD

339

and FD-MVD samples were similar to those of fresh samples. This observation can be

340

explained by the fact that reduced oxygen and a lower temperature are present in a

341

vacuum drying chamber under low pressure. In such circumstances, the enzymatic

342

browning reaction (Artnaseaw, Theerakulpisut, & Benjapiyaporn, 2010), which is the

343

main cause of color degradation of dried samples, is relatively weak. However, for the

344

MVD samples, decreases occurred both in lightness and redness compared to samples

345

processed by the FD and FD-MVD methods. These findings might have been due to

346

the uniformity of drying. Over-heating is the most common problem in MVD and can

347

lead to heat spots (Jiang, Zhang, Mujumdar, & Lim, 2014). The combined FD-MVD

348

drying method could help shorten the drying time at the MVD stage and reduce the

349

probability of uneven drying. The lowest L* values and highest ∆E values were

350

obtained for samples dried by the AD and AD-MVD methods (Table 2). This may be

351

ascribed to the non-enzymatic Maillard reaction, which occurs between proteins or

352

amino acids and reduces saccharides during heating (Izli & Isik, 2014), leading to the

353

formation of brown compounds. In addition, the longer drying time during AD may

354

have caused the okra samples to darken. Correspondingly, the shorter drying time

355

during AD-MVD might have reduced the degradation of color. Therefore, the ∆E

356

value of the AD-MVD samples was slightly lower than that of the AD samples.

357

3.3. Effect of drying methods on the quantities of the main antioxidant

358

components

AC C

EP

TE D

M AN U

SC

RI PT

334

15

ACCEPTED MANUSCRIPT TOF–LC–MS–MS coupled with HPLC-DAD was employed for the identification

360

and quantification of polyphenolic compounds in okra samples. Several compounds

361

were identified, based on matching of the experimental mass spectra with the mass

362

spectra data from the NIST 11 bank (NIST 11, Washington DC) and comparisons of

363

fragmentation patterns with reported patterns in the literature, as well as retention

364

times and UV spectra (Table 3). In general, the main polyphenol components of okra

365

were identified as quercetin derivatives and (−)-epigallocatechin, which was in

366

agreement with previous findings (Arapitsas, 2008; Shui, & Peng, 2004). Nevertheless,

367

the catechins were found to be the most abundant polyphenol compounds, followed by

368

quercetin derivatives in this study. This finding may be related to the different

369

varieties of okra tested. Compounds 7, 8, and 9 were identified as epigallocatechin

370

(EGC), epigallocatechin gallate (EGCG), and epicatechin gallate (ECG), respectively,

371

by comparing their MS profiles with data reported in the literature (Clifford, Johnston,

372

Knight, & Kuhnert, 2003; Clifford, Knight, & Kuhnert, 2005) and by comparing the

373

UV spectra and the retention times of the authentic EGC, EGCG and ECG standards.

374

Shui & Peng (2004) isolated and characterized 4 phenolic compounds from okra by

375

NMR: quercetin 3-O-diglucoside (Compound 11), quercetin 3-O-glucose-xylosyl

376

(Compound 12), quercetin 3-O-glucoside (Compound 14), and quercetin 3-O-(malonyl)

377

glucoside (Compound 15), which were also compared with the authentic quercetin

378

3-O-glucoside standard, based on their retention times and UV–vis spectra. Arapitsas

379

(2008) reported 4 phenolic compounds that were found in the seeds and skin of okra.

380

Compound 10 was identified as quercetin 3-O-hexoside-rhamnoside isomers, based on

381

its m/z of 609.1434, which fragmented at 301 because of the loss of hexose (162 Da)

382

and rhamnose (146 Da) residues, and based on comparison with published data

383

(Oszmiański, & Wojdyło, 2014).

AC C

EP

TE D

M AN U

SC

RI PT

359

16

ACCEPTED MANUSCRIPT Five types of relatively high-proportion antioxidant components were selected (Fig.

385

4). As shown in Fig. 4, the EGCG, ECG, and EGC contents were 1395.15, 836.28, and

386

435.88 µg/g in fresh okra, respectively; after drying, all of these levels decreased

387

significantly. FD samples had the highest average contents of EGCG, ECG, and EGC

388

(1277.24, 767.44, and 399.79 µg/g, respectively), which followed by the FD-MVD,

389

AD-MVD, MVD, and AD samples. No significant differences were observed among

390

the FD and FD-MVD samples for EGCG, the FD-MVD and AD-MVD samples for

391

ECG, or the FD, FD-MVD, and AD-MVD samples for EGC. It can be inferred that

392

EGCG may be more sensitive to high temperature, whereas ECG is more sensitive to

393

microwave irradiation. In addition, both EGCG and ECG levels decreased

394

significantly in the MVD group with intense microwave irradiation. These findings are

395

consistent with previous data that microwave irradiation can decrease the content of

396

EGCG and ECG (Wang, Zhou, Mo, & Zhang, 2002). EGC was much more stable than

397

EGCG and ECG during FD, AD-MVD, and FD-MVD; however, EGC levels

398

decreased significantly following the intense microwave irradiation occurring during

399

MVD. The lowest EGCG, ECG, and EGC values were all found in the AD samples,

400

with which the higher temperatures and long drying times accelerated the

401

decomposition.

SC

M AN U

TE D

EP

AC C

402

RI PT

384

Fig. 4 also shows that the quercetin 3-O-diglucoside and quercetin 3-O-(malonyl)

403

glucose levels were 346.3 and 253.4 µg/g in fresh okra, and 325.1 and 247.78 µg/g in

404

FD samples. No significant differences were observed between the fresh and FD

405

samples. FD-MVD, MVD, AD-MVD, and AD samples had average contents of

406

287.55 and 231.23 µg/g, 273.97 and 217.84 µg/g, 240.75 and 209.32 µg/g, and 232.93

407

and 196.50 µg/g, respectively, for these quercetin derivatives. FD-MVD and MVD

408

samples showed low degradation of the quercetin derivatives, whereas the AD and

17

ACCEPTED MANUSCRIPT AD-MVD processes exhibited a strong impact. These results are in accordance with

410

data from Schulze, Hubbermann, & Schwarz (2014), which indicated that quercetin

411

3-O-diglucoside and quercetin 3-O-(malonyl) glucose were much more stable under a

412

low-oxygen environment (FD, FD-MVD, MVD). The combination of high

413

temperature and a long dehydration time, as well as the oxidation during AD,

414

accelerated the auto-oxidation of quercetin, such that the contents of AD and

415

AD-MVD samples decreased significantly. In addition, the vacuum used for MVD

416

was much lower than that used for FD, which might have resulted in the low

417

degradation of quercetin derivatives during FD-MVD and MVD compared to FD.

418

3.4. Effect of drying methods on the TFC and TPC of okra extracts

M AN U

SC

RI PT

409

Different drying treatments had significantly different effects on the TFC and TPC

420

of okra samples (P < 0.05). As shown in Table 4, FD samples had the highest TFC

421

and TPC values (17.46 ± 0.23 mg rutin/g d.w. and 11.59 ± 0.02 mg GAE/g d.w.,

422

respectively), followed by the FD-MVD, MVD, AD-MVD, and AD samples. These

423

results could be ascribed to the effects of temperature and oxidation (Lou et al., 2015;

424

Hamrouni-Sellami et al., 2013). The low-oxygen and temperature environment during

425

FD and FD-MVD could effectively minimize the losses of phenolic acids and

426

flavonoids. However, during FD-MVD treatment, the exposure of the okra samples to

427

oxygen was higher than that during FD treatment; therefore, the oxygen loss was

428

slightly higher than that with FD treatment. Similarly, MVD treatment had less drying

429

time and exposure to oxygen, followed by AD-MVD and AD. However, the heat

430

generated from the MVD process was more intense and rapidly applied than that in

431

FD-MVD, which may have made it easier to induce thermal degradation of phenolic

432

compounds. Therefore, the TFC and TPC values of MVD samples were slightly lower

433

than those observed with the FD-MVD samples. The lowest TFC and TPC values

AC C

EP

TE D

419

18

ACCEPTED MANUSCRIPT 434

were all found in the AD samples. This was due to the high temperature and long

435

drying time required during AD, which can accelerate oxidation (Hamrouni-Sellami et

436

al., 2013).

437

3.5. Effect of drying methods on the antioxidant activity of okra extracts In this study, DPPH, FRAP, and ABTS assays were performed to evaluate the

439

antioxidant activity of okra extracts. The free radical DPPH has been widely used to

440

evaluate the free radical-scavenging activity of antioxidants (Lou et al., 2015). In the

441

DPPH test, FD samples showed the highest AEAC values (lowest IC50), followed by

442

FD-MVD (7.11 ± 0.14 mg Vc/g d.w., IC50 0.40 ± 0.01 mg/mL extract), MVD (6.96 ±

443

0.01 mg Vc/g d.w., IC50 0.41± 0.01 mg/mL extract), and AD-MVD (6.10 ± 0.23 mg

444

Vc/g d.w., IC50 0.47 ± 0.02 mg/mL extract) samples, whereas the AD samples showed

445

the lowest free-radical scavenging abilities (Table 4). The DPPH-scavenging ability

446

was highly correlated with the TFC (R2 = 0.983) and TPC (R2 = 0.990). Many reports

447

have found high correlations among TFC, TPC, and antioxidant activities (An et al.,

448

2016).

TE D

M AN U

SC

RI PT

438

As shown in Table 4, FD samples also exhibited the highest FRAP values (16.76 ±

450

0.55 g Vc/ g d.w.), followed by FD-MVD (14.53 ± 0.41 Vc g/g d.w.), MVD (13.90 ±

451

1.54 Vc g/g d.w.), and AD-MVD (13.12 ± 1.50 Vc g/g d.w.) samples, whereas the AD

452

process exerted the most negative effect. FRAP values were also highly correlated

453

with TFC (R2 = 0.953) and TPC (R2 = 0.960) values.

AC C

454

EP

449

The ABTS values showed a similar overall trend compared with those obtained by

455

the FRAP assay. However, the ABTS values were much higher than the FRAP values

456

(Table 4). The highest antioxidant capacity was found in the FD samples, followed by

457

the FD-MVD, MVD, AD-MVD, and AD samples. ABTS values also exhibited high

458

correlations with the TFC (R2 = 0.979) and TPC (R2 = 0.969).

19

ACCEPTED MANUSCRIPT 459

3.6. PCA The five drying methods used for the okra samples were evaluated using the

461

exploratory PCA technique. The physicochemical and antioxidant properties were

462

taken into account to observe any possible clusters, where △E was chosen to represent

463

the color index and drying time was used to represent the drying efficiency index. As

464

shown in Fig. 5, the cumulative contribution of the first and the second principal

465

components accounted for 93.16% of the total variance (PC1 = 83.29%, PC2 = 9.87%,

466

respectively). PC1 was highly correlated with TFC (0.960), TPC (0.950), shrinkage

467

(−0.972), hardness (−0.943), crispness (−0.897), △E (−0.992), IC50 (−0.953), AEAC

468

(0.963), FRAP (0.990), ABTS (0.983), EGCG (0.872), ECG (0.924), EGC (0.830),

469

quercetin 3-O-diglucoside (0.970), and quercetin 3-O-(malonyl)glucose (0.998). PC2

470

was mainly correlated with energy consumption (0.926) and drying time (0.662).

M AN U

SC

RI PT

460

The PC1 and PC2 scores of FD samples were much higher than those of other

472

drying methods as they had higher contents of EGCG, ECG, EGC, quercetin

473

3-O-diglucoside, quercetin 3-O-(malonyl) glucose, TFC, and TPC as well as higher

474

antioxidant activity, good texture, and good color quality, at the cost of higher energy

475

consumption and a longer drying time. MVD and AD-MVD could be clustered into 1

476

group as they both were negatively correlated with both PC1 and PC2. This is because

477

that they had negative effects on active component contents and antioxidant activity

478

with less drying time and energy consumption. The AD process showed a high

479

negative correlation with PC1, whereas it was positively correlated with PC2, which

480

indicated that AD had negative effects on active component contents, antioxidant

481

activity, and texture and color quality, with a longer drying time and more energy

482

consumption. Remarkably, the FD-MVD process showed a high positive correlation

483

with PC1 and a negative correlation with PC2, indicating that it had

AC C

EP

TE D

471

20

ACCEPTED MANUSCRIPT 484

a beneficial influence on the antioxidant properties, sensory quality, and energy

485

consumption.

486

4. Conclusion FD-MVD was evaluated for use in the processing of functional okra snacks.

488

FD-MVD led to higher retention of antioxidant properties, better color and quality of

489

texture, and relatively lower drying time and energy consumption. Based on principal

490

component analysis (PCA) of antioxidant properties, sensory quality, and drying

491

efficiency for the five different drying methods, it could be concluded that FD-MVD is

492

a promising technique for the processing of functional okra snacks.

493

Acknowledgements

M AN U

SC

RI PT

487

494

The financial support provided by the Basic Business Fund for Major Achievement

495

Cultivation of the Jiangsu Academy of Agricultural Sciences (ZX[15]1008) is

496

appreciated.

EP AC C

498

TE D

497

21

ACCEPTED MANUSCRIPT 499

References

500

Abozed, S. S., El-Kalyoubi, M., Abdelrashid, A., & Salama, M. F. (2014). Total

501

phenolic contents and antioxidant activities of various solvent extracts from whole

502

wheat and bran. Annals of Agricultural Sciences, 59(1), 63-67. An, K., Zhao, D., Wang, Z., Wu, J., Xu, Y., & Xiao, G. (2016). Comparison of

504

different drying methods on chinese ginger (Zingiber officinale roscoe): changes in

505

volatiles, chemical profile, antioxidant properties, and microstructure. Food

506

Chemistry, 197, 1292-1300.

SC

508

Arapitsas, P. (2008). Identification and quantification of polyphenolic compounds from okra seeds and skins. Food Chemistry, 110(4), 1041-1045.

M AN U

507

RI PT

503

Argyropoulos, D., Heindl, A., & Müller, J. (2011). Assessment of convection, hot-air

510

combined with microwave-vacuum and freeze-drying methods for mushrooms

511

with regard to product quality. International Journal of Food Science &

512

Technology, 46(2), 333–342.

TE D

509

Artnaseaw, A., Theerakulpisut, S., & Benjapiyaporn, C. (2010). Drying characteristics

514

of shiitake mushroom and jinda chili during vacuum heat pump drying. Food &

515

Bioproducts Processing, 88(2), 105-114.

517 518 519 520

Choi, S. H., Ahn, J. B., Kim, H. J., Im, N. K., Kozukue, N., Levin, C. E., & Friedman,

AC C

516

EP

513

M. (2012). Changes in free amino acid, protein, and flavonoid content in jujube (Ziziphus jujube) fruit during eight stages of growth and antioxidative and cancer cell

inhibitory

effects

by

extracts. Journal

of

Agricultural

&

Food

Chemistry, 60(41), 10245-10255.

521

Clifford, M. N., Johnston, K. L., Knight, S., & Kuhnert, N. (2003). Hierarchical

522

scheme for LC-MSn identification of chromogenic acids. Journal of Agricultural &

523

Food Chemistry, 51(10), 2900-2911.

22

ACCEPTED MANUSCRIPT 524

Clifford, M.N., Knight, S., & Kuhnert, N. (2005). Discriminating between the six

525

isomers of dicaffeoylquinic acid by LC-MSn. Journal of Agricultural & Food

526

Chemistry, 53(10), 3821–3832. Hamrouni-Sellami, I., Rahali, F. Z., Rebey, I. B., Bourgou, S., Limam, F., & Marzouk,

528

B. (2012). Total phenolics, flavonoids, and antioxidant activity of sage (Salvia

529

officinalis L.) plants as affected by different drying methods. Food & Bioprocess

530

Technology, 6(3), 806-817.

RI PT

527

Horuz, E., & Maskan, M. (2015). Hot air and microwave drying of pomegranate

532

(Punica granatum, L.) arils. Journal of Food Science and Technology, 52(1),

533

285-293.

535

M AN U

534

SC

531

Huang, J., & Zhang, M. (2016). Effect of three drying methods on the drying characteristics and quality of okra. Drying Technology, 34(8), 900-911. Huang, L. L., Zhang, M., Wang, L. P., Mujumdar, A. S., & Sun, D. F. (2012).

537

Influence of combination drying methods on composition, texture, aroma and

538

microstructure of apple slices. LWT-Food Science and Technology, 47(1),

539

183-188.

TE D

536

Hu, Q. G., Zhang, M., Mujumdar, A. S., Xiao, G. N., & Sun, J. C. (2006). Drying of

541

edamames by hot air and vacuum microwave combination. Journal of Food

AC C

542

EP

540

Engineering, 77(4), 977-982.

543

Hu, Q. G., Zhang, M., Mujumdar, A. S., Du, W. H., & Sun, J. C. (2006). Effects of

544

different drying methods on the quality changes of granular edamame. Drying

545

Technology, 24(8), 1025-1032.

546

Izli, N., & Isik, E. (2014). Effect of different drying methods on drying characteristics,

547

color and microstructure properties of mushroom. Journal of Food & Nutrition

548

Research, 53(2), 105-116.

23

ACCEPTED MANUSCRIPT 549

Jiang, H., Zhang, M., Mujumdar, A. S., & Lim, R. X. (2014). Comparison of drying

550

characteristic and uniformity of banana cubes dried by pulse-spouted microwave

551

vacuum drying, freeze drying and microwave freeze drying. Journal of the Science

552

of Food & Agriculture, 94(9), 1827–1834. Jiang, N., Liu, C., Li, D., Zhang, Z., Zhifang, Y. U., & Zhou, Y. (2016). Effect of

554

thermosonic pretreatment on drying kinetics and energy consumption of

555

microwave

556

Engineering, 177, 21-30.

dried

Agaricus

bisporus

slices.

Journal

of

Food

SC

vacuum

RI PT

553

Kotwaliwale, N., Bakane, P., & Verma, A. (2007). Changes in textural and optical

558

properties of oyster mushroom during hot air drying. Journal of Food

559

Engineering, 78(4), 1207-1211.

M AN U

557

Kumar, D., Prasad, S., & Murthy, G. S. (2014). Optimization of microwave-assisted

561

hot air drying conditions of okra using response surface methodology. Journal of

562

food science and technology, 51(2), 221-232.

TE D

560

Li, J., Duan, Z., & Liu, Y. (2013). Study on the vacuum microwave drying

564

characteristics and dynamics of okra. Science & Technology of Food

565

Industry, 31(22), 285-289.

567

Li, J., Chen, X., Deng, J., Yue, W., Liu, L., Yuan, T. U., & Zhou, Y. (2002). Extraction

AC C

566

EP

563

and antioxidant activity in vitro of okra flavonoids. Food Science, 35(10), 121-125.

568

Liu, P., Zhang, M., & Mujumdar, A. S. (2012). Comparison of three

569

microwave-assisted drying methods on the physiochemical, nutritional and sensory

570

qualities of re-structured purple-fleshed sweet potato granules. International

571

Journal of Food Science & Technology, 47(1), 141–147.

572

Lou, S. N., Lai, Y. C., Huang, J. D., Ho, C. T., Ferng, L. H. A., & Chang, Y. C. (2015).

573

Drying effect on flavonoid composition and antioxidant activity of immature

24

ACCEPTED MANUSCRIPT 574

kumquat. Food Chemistry, 171, 356-363. Meng, X. J., Wang, D., & Jin-Feng, B. I. (2007). Explosion puffing drying of

576

hami-melon at modified temperature and pressure. Food Science, 28(12), 183-187.

577

Noorbakhsh, R., Yaghmaee, P., & Durance, T. (2013). Radiant energy under vacuum

578

(REV) technology: A novel approach for producing probiotic enriched apple

579

snacks. Journal of Functional Foods, 5(3), 1049–1056.

580

RI PT

575

Oszmiański, J., & Wojdyło, A. (2014). Influence of cherry leaf-spot on changes in the content

of

phenolic

compounds

in

sour

cherry

(Prunus

582

leaves. Physiological and Molecular Plant Pathology, 86, 28-34.

cerasus

L.)

SC

581

Panat, N. A., Maurya, D. K., Ghaskadbi, S. S., & Sandur, S. K. (2016). Troxerutin, a

584

plant flavonoid, protects cells against oxidative stress-induced cell death through

585

radical scavenging mechanism. Food Chemistry, 194, 32-45.

M AN U

583

Park, J. H., Jeon, G. I., Kim, J. M., & Park, E. (2012). Antioxidant activity and

587

antiproliferative action of methanol extracts of 4 different colored bell peppers

588

(Capsicum annuum L.). Food Science & Biotechnology, 21(2), 543-550.

TE D

586

Pei, F., Shi, Y., Gao, X., Wu, F., Mariga, A., Yang, W., …Hu, Q. (2014). Changes in

590

non-volatile taste components of button mushroom (Agaricus bisporus) during

591

different stages of freeze drying and freeze drying combined with microwave

AC C

592

EP

589

vacuum drying. Food Chemistry, 165, 547-554.

593

Pendre, N. K., Nema, P. K., Sharma, H. P., Rathore, S. S., & Kushwah, S. S. (2012).

594

Effect of drying temperature and slice size on quality of dried okra (Abelmoschus

595

esculentus, (l.) moench). Journal of Food Science & Technology, 49(3), 378-381.

596

Schulze, B., Hubbermann, E. M., & Schwarz, K. (2014). Stability of quercetin

597

derivatives in vacuum impregnated apple slices after drying (microwave vacuum

598

drying, air drying, freeze drying) and storage. LWT Food Science and

25

ACCEPTED MANUSCRIPT 599 600 601

Technology, 57(1), 426-433. Shivhare, U.S., Gupta, A., Bawa, A.S., & Gupta, P.(2000). Drying characteristics and product quality of okra. Drying Technology, 18(1-2), 409-419. Shui, G., & Peng, L. L. (2004). An improved method for the analysis of major

603

antioxidants of Hibiscus esculentus, linn. Journal of Chromatography A, 1048(1),

604

17-24.

RI PT

602

Shukla, S., Mehta, A., Mehta, P., & Bajpai, V. K. (2012). Antioxidant ability and total

606

phenolic content of aqueous leaf extract of Stevia rebaudiana, bert. Experimental

607

& Toxicologic Pathology, 64(7), 807-811.

SC

605

Sobukola, O. (2009). Effect of pre-treatment on the drying characteristics and kinetics

609

of okra (Abelmoschus esculetus (l.) moench) slices. International Journal of Food

610

Engineering, 5(2),1-20.

M AN U

608

Tian, Y., Zhao, Y., Huang, J., Zeng, H., & Zheng, B. (2016). Effects of different drying

612

methods on the product quality and volatile compounds of whole shiitake

613

mushrooms. Food Chemistry, 197, 714-722.

616 617 618

food. Engineering Failure Analysis, 11(5), 695-704.

EP

615

Vincent, J. F. V. (2004). Application of fracture mechanics to the texture of

Wang, H., Zhang, M., & Mujumdar, A. S. (2014). Comparison of three new frying

AC C

614

TE D

611

methods for drying characteristics and quality of shiitake mushroom (Lentinus edodes). Drying Technology, 32(15), 1791–1802.

619

Wang, X., Zhou, Z., Mo, K., & Zhang, J. (2002). Influence of microwave on extraction,

620

structure and catechin composition of tea-polyphenol. Transactions of the Chinese

621

Society of Agricultural Engineering, 18(2), 110-114 (In Chinese).

622 623

Wankhade, P. K., Sapkal, R. S., & Sapkal, V. S. (2013). Drying characteristics of okra slices on drying in hot air dryer. Procedia Engineering, 51, 371-374.

26

ACCEPTED MANUSCRIPT 624

Wojdyło, A., Figiel, A., Lech, K., Nowicka, P., & Oszmiański, J. (2014). Effect of

625

convective and vacuum–microwave drying on the bioactive compounds, color, and

626

antioxidant capacity of sour cherries. Food & Bioprocess Technology, 7(3),

627

829-841. Zhang, H., Jiang, L., Ye, S., Ye, Y., & Ren, F. (2010). Systematic evaluation of antioxidant

629

capacities of the ethanolic extract of different tissues of jujube (Ziziphus jujuba Mill.) from

630

China. Food and Chemical Toxicology, 48(6), 1461-1465.

AC C

EP

TE D

M AN U

SC

RI PT

628

27

ACCEPTED MANUSCRIPT Figure captions

632

Fig. 1. Images of a fresh okra sample (a) and okra samples dried by AD (b), FD(c),

633

MVD(d), FD-MVD (e), and AD-MVD (f). AD, air drying; FD, freeze drying; MVD,

634

microwave vacuum drying; FD-MVD, combined drying consisting of freeze drying and

635

microwave vacuum drying; AD-MVD, combined drying consisting of air drying and

636

microwave vacuum drying.

637

Fig. 2. Average texture profile analysis curve of okra samples dried by AD (a), FD (b),

638

MVD (c), FD-MVD (d), and AD-MVD (e). AD, air drying; FD, freeze drying; MVD,

639

microwave vacuum drying; FD-MVD, combined drying consisting of freeze drying and

640

microwave vacuum drying; AD-MVD, combined drying consisting of air drying and

641

microwave vacuum drying.

642

Fig. 3. Scanning electron micrographs of okra samples dried by AD (a), FD (b), MVD

643

(c), AD+MVD (d), FD+MVD (e). AD, air drying; FD, freeze drying; MVD, microwave

644

vacuum drying; FD-MVD, combined drying consisting of freeze drying and microwave

645

vacuum drying; AD-MVD, combined drying consisting of air drying and microwave

646

vacuum drying.

647

Fig. 4. Changes in epigallocatechin gallate (EGCG), epicatechin gallate (ECG),

648

epigallocatechin (EGC), quercetin 3-O-diglucoside, and quercetin 3-O-(malonyl)

649

glucose contents of okra extract with different drying processes. For each column,

650

values followed by the same letter (a–e) are not statistically different at P < 0.05 as

651

measured by Duncan’s test. AD, air drying; FD, freeze drying; MVD, microwave

652

vacuum drying; FD-MVD, combined drying consisting of freeze drying and

653

microwave vacuum drying; AD-MVD, combined drying consisting of air drying and

654

microwave

655

(EGCG);“

AC C

EP

TE D

M AN U

SC

RI PT

631

vacuum

drying.

“

”

represents

epigallocatechin

” represents epicatechin gallate (ECG); “

28

gallate

” represents

ACCEPTED MANUSCRIPT epigallocatechin (EGC); “

” represents quercetin 3-O-diglucoside and “

657

represents quercetin 3-O-(malonyl) glucose.

658

Fig. 5. Principal component analysis plot of data for different antioxidant properties,

659

sensory quality, and drying efficiency of okra samples prepared using the five drying

660

methods. AD, air drying; FD, freeze drying; MVD, microwave vacuum drying;

661

FD-MVD, combined drying consisting of freeze drying and microwave vacuum

662

drying; AD-MVD, combined drying consisting of air drying and microwave vacuum

663

drying

SC

RI PT

656

664

M AN U

665 666 667

671 672 673 674 675 676

EP

670

AC C

669

TE D

668

677 678 679 680

29

”

ACCEPTED MANUSCRIPT Tables

682

Table 1 Drying time, drying rate, and energy consumption of okra samples dried by

683

different drying methods.

684

Table 2 Color parameters, hardness, crispness, and shrinkage of okra samples dried by

685

different drying methods.

686

Table 3 Retention times, UV/Vis spectra and characteristic ions of phenolic

687

compounds of okra.

688

Table 4 Changes in total flavonoid and phenolic content and antioxidant activity of

689

okra dried by AD, FD, MVD, FD-MVD, and AD-MVD.

SC

M AN U

690 691 692 693

EP

697

AC C

696

TE D

694 695

RI PT

681

30

698 699 700

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 1.

701

31

703

AC C

702

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 2.

32

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

704 705

Fig. 3.

33

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

706

Fig. 4.

AC C

EP

TE D

707

708 709

Fig. 5.

34

ACCEPTED MANUSCRIPT

Table 1

710

Drying time

RI PT

Specific energy

Drying

Drying rates

Consumption

method

(h)

[g/(h·kg)]

－

－ c

AD

8.2±0.41

FD

41.23±1.05

MVD

0.25±0.02

FD-MVD

10.16±0.51

AD-MVD

2.15±0.11

M AN U

Fresh

SC

( kWh/kg)

c

49.2±1.46

e

75.85±1.79

110.39±5.52

a

3635.05±290.80

TE D

e

21.91±0.56

b

d

89.10±4.47

－

a

d

421.08±21.54

b

b

a

4.38±0.22

e

21.3±1.07

c

10.75±0.54

d

.Note: Data are shown as the mean ± SD (n = 3). Means within a column with the same letter are not significantly different as indicated by Duncan’s multiple range test

712

(P < 0.05). AD – air drying; FD – freeze drying; MVD – microwave vacuum drying; FD-MVD – combined drying consisting of freeze drying and microwave vacuum

714

AC C

713

EP

711

drying; AD-MVD – combined drying consisting of air drying and microwave vacuum drying

715

35

ACCEPTED MANUSCRIPT

Table 2 Shrinkage

method Fresh

Hardness

(%) －

Crispness

(N)

(N)

－

Surface color

RI PT

Drying

L*

－

a*

b*

△E

78.62±0.17 b -4.23±0.29a

36.34±0.38a －

66.01±0.32f

-0.82±0.09d

33.81±0.19b 13.31±0.31a

SC

716

82.72±2.65d

57.19±2.22 a

24.86±4.79a

FD

24.76±4.36a

14.95±1.05 e

3.80±0.59d

80.12±0.07a

-3.32±0.13c

30.49±0.03e 6.11±0.40e

MVD

60.55±1.58c

25.68±1.93 c

9.15±0.45b

69.89±0.14d

-3.12±0.16c

32.04±0.09cd 9.79±0.32c

FD-MVD

33.49±3.57b

22.43±1.18 d

6.74±0.87c

72.15±0.32c

-3.31±0.03b

31.80±0.05d 7.96±0.45d

AD-MVD

62.43±3.09c

31.23±3.47 b

68.07±0.08e

-1.33±0.05d

32.26±0.41c 11.68±0.26 b

TE D

M AN U

AD

10.36±1.94b

Note: Data are shown as the mean ± SD (n = 3). Means within a column with the same letter are not significantly different as indicated by Duncan’s multiple

718

range test (P < 0.05). AD – air drying; FD – freeze drying; MVD – microwave vacuum drying; FD-MVD – combined drying consisting of freeze drying and

719

microwave vacuum drying; AD-MVD – combined drying consisting of air drying and microwave vacuum drying

721

AC C

720

EP

717

722

36

ACCEPTED MANUSCRIPT

Peak No.

Rt (min)

Λ max (nm)

[M-H]-(m/z)

Proposed formula

Measured Mass

153.9783

RI PT

Table 3

723

MS/MS fragments (m/z)

Tentative identification

0.24

108.9705

2, 4-dihydroxybenzoic acid

360.1056

0.31

197.0477,153.0984

Syryngic acid galactoside

164.0473

0.26

118.9363

p-Coumaric acid

356.1107

0.30

193.0513, 175.0411, 160.0178, 132.0228

Ferulic acid hexoside

342.0951

0.27

179.0355, 161.0246, 133.0301

Caffeoyl hexose

191.0323, 179.0102

3-Caffeoylquinic acid

SC

Phenolic acids and derivatives

△m (ppm)

4.85

254

152.9588

C7H6O4

2.

7.93

274

359.0984

C15H20O10

3.

8.56

280

163.0401

C15H18O8

4.

9.55

325

355.1035

C16H20O9

5.

10.58

326

341.0878

6.

11.02

324

353.0878

C16H18O9

354.0951

0.30

210

305.0671

C15H14O7

306.07395

0.28

7.

11.82

TE D

EP

C15H18O9

AC C

Flavan-3-ols

M AN U

1.

37

139.0328； 155.0665；225.0933

Epigallocatechin (EGC)

ACCEPTED MANUSCRIPT

17.67

280

457.1399

C22H18O11

458.0849

9.

22.03

280

441.3723

C22H18O10

442.0899

10.

14.16

350

609.1461

C27H30O16

610.1534

11.

15.52

356

625.1436

C27H30O17

626.5169

12.

16.27

356

595.1324

C26H28O16

13.

17.21

356

479.0869

C21H20O13

14.

18.55

354

463.0882

C21H20O12

15.

19.37

356

550.1

C24H23O15

16.

20.03

356

609.1529

C27H30O16

17.

22.81

350

477.1039

18.

23.27

340

489.1039

0.27

305.0687, 287.0547

RI PT

8.

Epigallocatechin gallate (EGCG)

0.28

331.0735, 169.0256

Epicatechin gallate (ECG)

0.29

301.0370

Quercetin Hexoside-deoxyhexoside

0.29

177.9987, 301.0364

Quercetin 3-O-diglucoside

596.4909

0.15

301.0352

Quercetin 3-O-glucose-xylosyl

480.3757

0.26

317.0333, 271.0273

Myricetin 3-O-glucose

464.0955

0.27

301.0368, 283.0266

Quercetin 3-O-glucoside

551.1037

0.27

505.1017, 301.0352

Quercetin 3-O-(malonyl)glucose

610.5175

0.29

315.0523, 299.0221

Isorhamnetin 3-O-glucose-pentose

C22H22O12

478.1111

0.15

315.0534

Isorhamnetin 3-O-hexoside

C23H22O12

490.1111

0.30

285.0424

Kaempferol3-O-6-acetylglucosi de

AC C

EP

TE D

M AN U

SC

Flavonols

38

ACCEPTED MANUSCRIPT Table 4

724

Fresh TFC (mg Rutin/g d.w.) 18.54 ±0.27 a

AD

FD

MVD

FD-MVD

AD-MVD

10.90±0.25f

17.46 ±0.23b 14.16±0.32d

16.06±0.30c

11.38±0.47e

11.59±0.02b

10.33±0.40d

10.99±0.11c

8.51±0.34e

12.73±0.31a

7.70±0.17f

IC50(mg/mL)

0.34±0.01c

0.51±0.01e

0.37±0.01a

0.41±0.01 b

0.40±0.01b

0.47±0.02d

AEAC(mgVc/g d.w.)

7.85±0.02 a

5.67±0.03e

7.67±0.02b

6.96±0.01c

7.11±0.14c

6.10±0.23d

FRAP(mgVc/g d.w.)

17.85±0.95a

11.19±0.87d

16.76±0.55b

13.9±1.54c

14.53±0.41c

13.12±1.50c

ABTS(mgVc/g d.w.） ）

32.89±0.50a

20.70±0.78d

29.71±0.69a

27.92±0.70b

22.48±2.52c

SC

RI PT

TPC (mg GAE/g d.w.)

24.07±0.61c

Note: Values are the mean ± SD (n = 3). For each column, values followed by the same small or capital

726

superscript letter were not significantly different at P < 0.05 (Duncan’s test). AD – air drying; FD – freeze

727

drying; MVD – microwave vacuum drying; FD-MVD – combined drying consisting of freeze drying and

728

microwave vacuum drying; AD-MVD – combined drying consisting of air drying and microwave vacuum

729

drying.

AC C

EP

TE D

M AN U

725

39

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Highlights: • The main quercetin and epigallocatechin in okra fruit were identified and quantified • FD-MVD was first applied in functional okra snacks • Comprehensive evaluation of quality and energy consumption was built by PCA • FD-MVD is very promising for functional okra snacks in good quality and cost saving