Effects of drying methods on drying characteristics, physicochemical properties and antioxidant capacity of okra

Accepted Manuscript Effects of drying methods on drying characteristics, physicochemical properties and antioxidant capacity of okra Hongyan Li, Long Xie, Yue Ma, Min Zhang, Yuwei Zhao, Xiaoyan Zhao PII:

S0023-6438(18)31037-5

DOI:

https://doi.org/10.1016/j.lwt.2018.11.076

Reference:

YFSTL 7640

To appear in:

LWT - Food Science and Technology

Received Date: 2 September 2018 Revised Date:

22 November 2018

Accepted Date: 23 November 2018

Please cite this article as: Li, H., Xie, L., Ma, Y., Zhang, M., Zhao, Y., Zhao, X., Effects of drying methods on drying characteristics, physicochemical properties and antioxidant capacity of okra, LWT Food Science and Technology (2018), doi: https://doi.org/10.1016/j.lwt.2018.11.076. 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 Effects of drying methods on drying characteristics, physicochemical properties and

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antioxidant capacity of okra

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Hongyan Lia, b, c, d, #, Long Xiea, b, c, #, Yue Maa, b, c, Min Zhange, Yuwei Zhaoe, Xiaoyan Zhaoa, b, c, d, *

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a

100097, China

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Research Center, Beijing Academy of Agriculture and Forestry Sciences, Beijing 100097, China

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Key Laboratory of the Vegetable Postharvest Treatment of Ministry of Agriculture, Beijing Vegetable

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Beijing Vegetable Research Center, Beijing Academy of Agriculture and Forestry Sciences, Beijing

Beijing Key Laboratory of Fruits and Vegetable Storage and Processing, Beijing Vegetable Research Center, Beijing Academy of Agriculture and Forestry Sciences, Beijing 100097, China

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d

Shenyang Agriculture University, Shenyang 110866, China

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LongDa Foodstuff Group Co., LTD., Laiyang 265231, China

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# These authors contributed equally to this work.

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* Corresponding author.

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Tel.: +86 010 51503053; Fax: +86 010 51503053.

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Email: [email protected] (X. Y. Zhao)

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ACCEPTED MANUSCRIPT Abstract: Results of an experimental study were presented and discussed for hot air drying with

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horizontal cut (HC-HA) or vertical cut (VC-HA) at 55, 60, 65, 70, 75, and 80 oC, respectively, and heat

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pump drying with HC (HC-HP) or VC (VC-HP) at 40, 50, 60, and 70 oC, respectively, on drying

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characteristic, colour quality, chemical compositions (vitamin C, chlorophyll, total phenolic and

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polysaccharide) and antioxidant capacity (2,2-diphenyl-1-picrylhydrazyl (DPPH), hydroxyl radical

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scavenging capacity and oxygen radical absorbance capacity (ORAC)) of okra samples. Results

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showed VC had shorter drying time than that of HC both during HA and HP. The effective moisture

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diffusivity determined by Weibull distribution function ranged from 0.992×10-9 m2/s to 4.409 ×10-9

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m2/s for HA, and 1.915×10-9 m2/s to 5.291×10-9 m2/s for HP. The drying activation was 37.00, and

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35.91 kJ/mol for the samples of HC-HA and VC-HA, 29.25, and 21.40 kJ/mol for HC-HP and VC-HP,

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respectively. A comprehensive evaluation of quality attributes indicated that HP was more suitable for

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okra drying as it improved drying rate and drying quality compared with HA. The optimal sample

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quality (higher contents of chemical compositions and better colour quality) was found in okra treated

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with VC-HP at 50 oC.

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Keywords: Okra; Drying methods; Drying characteristics; Physicochemical properties; Antioxidant

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capacity

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1. Introduction

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Okra (Abelmoschus esculentus L.) is a type of annual vegetable crop herbaceous

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plant, belonging to the Malvaceae family. It probably originates in African region and

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widely been grown in Asia, southern Europe and American region (Yuan, Ritzoulis,

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ACCEPTED MANUSCRIPT & Chen, 2018). Okra is a widely accepted vegetable in subtropical and tropical

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regions due to the good palatability. Okra pod has been a good source of nutrient and

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antioxidant content such as vitamin C, polyphenols, polysaccharides and mineral

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elements (Yuan, Ritzoulis, & Chen, 2018). Young pod as edible part of fresh okra

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with high moisture content (approach to 90%, w.b.), whereas the safe moisture level

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of store okra has been reported as 10% (w.b.) (Shivhare, Gupta, Bawa, & Gupta,

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2000). It can be stored only 2-3 days at room temperature. The fresh pods will wilting,

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aging and nutrients loss subsequently, which result in the low edibility due to fibrosis

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eventually (Rai & Balasubramanian, 2009).

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Drying has been one of the most widely methods of food preservation. The major

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objective of drying is to reduce water activity of raw materials and extend the shelf-

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life. Dried okra powder can be used as raw material or auxiliary material for further

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processing and stored for subsequent use. Drying not only reduces the moisture

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content of products but also alters other physical, chemical, and biological properties,

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such as antioxidant capacity, enzymatic activity, aroma, flavour and so on (Jiang et al.,

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2017). The most conventional method used for drying okra is hot air drying (HA),

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which is low-cost and easily controlled but low-energy efficiency and lengthy drying

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time during the last drying stage. In addition, the high temperature used in HA usually

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cause deterioration of important nutrition compositions and colour (An et al., 2016).

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Vacuum freeze drying (VF) has been researched and applied to okra drying (Huang &

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Zhang, 2016). VF can preserve the original nutrition compositions, active ingredients

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and colour, but it is undesirable for food industry because of higher costs (Huang &

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ACCEPTED MANUSCRIPT Zhang, 2016). A lot of work has been done to minimise the side effects of above

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techniques in agricultural products drying, such as combined infrared and hot-air

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drying technique (Onwude, Hashim, Abdan, Janius, & Chen, 2019), combined radio

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frequency vacuum and osmotic drying technique (Zhou, Li, Lyng, & Wang, 2018),

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sequential infrared blanching and hot air drying technique (Chen et al., 2018),

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combining ultrasound pre-treatment and ultrasound-assisted air drying technique

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(Magalhães et al., 2017) and so on. Heat pump drying (HP), as a new drying

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technology, expected to be an ideal technology for drying okra because it offers

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advantages such as higher energy efficiency and better product quality due to accurate

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control of drying conditions (Duan, Zhang, Li, Tian, & Wang, 2018). And low-

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temperature dehumidified air used in heat pump assisted drying is suitable for heat-

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sensitive products (Li, 2018). According to the drying characteristics of okra,

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appropriate pretreatment and drying methods are crucial for saving drying time and

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improving drying quality in actual production. The cutting pretreatment can change

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the shape parameters of materials, so it may also affect drying rate and retention of

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organic matter and nutrients. The kinetics of ascorbic acid degradation during air-

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drying of whole rosehip has indicated that cutting pretreatment could accelerate the

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drying process and improve the retention of vitamin C (Erenturk, Gulaboglu, &

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Gultekin, 2005).

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The drying time and drying quality of agricultural products were significantly

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influenced by the cutting pretreatment and drying method. The objective of this study

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was to explore the effects of different cutting types and drying methods on drying 4

ACCEPTED MANUSCRIPT characteristics and drying quality of okra. The quality attributes were analysed with

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the comprehensive weighted scoring method to determine the optimum drying

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conditions. Meanwhile, the antioxidant capacity of okras dried by HA and HP were

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evaluated. The findings will provide reference for the further processing of okra.

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2. Materials and methods

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2.1 Materials

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The okras were obtained from Longda Food Group Co., LTD. and stored in

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refrigerator at 4 oC. The initial moisture content of okra samples were 87.96±1.94%

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on wet basis (w.b.), as determined by vacuum drying at 70 °C for 24 h (AOAC, 1990).

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The okra samples with uniform sizes were manually selected, and average length and

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diameter of selected samples were about 8.0±1.0 cm, and 2.0±0.3 cm, respectively.

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Prior to experiments, okra samples were taken from refrigerator and brought up to the

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room temperature (25 °C). Then tap water was used to remove the surface dirt. Before

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drying, okras were blanched in boiling water for 3min to inactivate enzyme activity

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and immediately cooled in chilled water to avoid over-processing (Jiang et al., 2017).

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Excess water from surface was eliminated with sterile tissue paper, and then the okra

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samples were segmented with HC and VC.

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2.2 Drying methods

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About 200 g okra samples was distributed evenly and subjected to HA, HP, and

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VF with different drying conditions until the final moisture level was below 5

ACCEPTED MANUSCRIPT 7.00±1.00% (w.b.). According to the actual situation and previous studies on hot air

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drying and heat pump drying of okra (Afolabi & Agarry, 2014; Daghigh, Ruslan,

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Sulaiman, & Sopian, 2010; Wankhade, Sapkal, & Sapkal, 2013), suitable temperature

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was adjusted from 55 to 80 oC for HA and from 40 to 70 oC for HP. The HA of okra

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was significantly affected by temperature interval of 5 oC, while the HP of okra was

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observably influenced by temperature interval of 10 oC. Therefore, hot air temperature

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in a lab-scale oven (DGH-9240A, China) used in experiments was adjusted to 55, 60,

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65, 70, 75, and 80 oC with the air velocity of 1.5 m/s. The heat pump dryer,

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manufactured by Technical Institute of Physics and Chemistry, Chinese Academy of

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Sciences, was operated at drying temperature of 40, 50, 60, and 70 oC, respectively.

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Samples were placed in single layer on stainless steel wire grid. Samples were cut into

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3.0 cm length and prefrozen for 6 h in a freezer at -80 oC before being dried in a

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laboratory freeze dryer (ALPHA2-4, Germany). The operating conditions were set at -

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40 oC and 0.010 kPa for 20 h. Freeze-dried okra samples were treated as control for

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colour, nutritional components and antioxidant activity, compared with hot air dried

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samples and heat pump dried samples. All drying experiments were performed in

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triplicate. All dried samples were packaged with vacuum packaging bag and reserved

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at -20 oC avoiding light for further analysis.

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2.3 Drying characteristics analysis

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2.3.1 Moisture ratio

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The Weibull distribution function as following (Xie et al., 2018): 6

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−( )

(1)

Where MR is moisture ratio of okra, α is the scale parameter (min), which

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represents drying rate constant, β is the shape parameter, which relates to the drying

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rate and moisture transfer mechanism in the drying process.

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The MR of okra during the single-layer drying experiments were calculated using the following equation:

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(2)

Where Mt is moisture content at drying time of t (kg/kg, dry basis, d.b.), M0 is the

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initial moisture content (%, d.b.), Me is the equilibrium moisture content (kg/kg, d.b.).

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The value of the equilibrium moisture content (Me) is relatively small compared

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with M0 or Mt. Thus, Eq. (2) can be simplified as Eq. (3) (Xie et al., 2017a).

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(3)

The drying rate (g water/(g dry solid·h)) is usually calculated using Eq. (4) (Xie,

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2.3.2 The drying rate

Gao, Liu, & Xiao, 2017).

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=

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Where, t1 and t2 are the different drying time in hours during drying, respectively;

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Mt1 and Mt2 are the moisture contents of okra samples on dry basis at t1 and t2,

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respectively.

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2.3.3 Effective moisture diffusivity (Deff) and activation energy (Ea)

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The migration characteristics of material moisture can be measured by effective 7

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water diffusion coefficient. The estimate formula of moisture diffusion coefficient

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Dcal is given by Eq. (5) (Zhang et al., 2015):

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=

(5)

Where Dcal is the water diffusion coefficient estimated in drying process (m2/s), r

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represents the volume equivalent radius of the okra samples, α is the scale parameter

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of Weibull distribution model.

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The Deff of okra samples could be calculated with Weibull distribution model as

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Eq. (6) (Dai et al., 2015) :

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Where Deff is effective moisture diffusivity (m2/s), Rg is the physical dimension

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constant. For agricultural products with a shape as flat or globular shape, Rg usually

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ranges from 13.1 to 18.6 m2/s (Xie et al., 2017b).

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The dependent relationship of Deff on temperature generally described by Arrhenius-type relationship (Sehrawat, Nema, & Kaur, 2018).

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=

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=

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(

*

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-.. 0)

(7)

Where D0 is the Deff when temperature tend to infinity (m2/s), Ea is the active

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energy (kJ/mol), R is the universal gas constant (8.314 J/mol · K) and T is the drying

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temperature (oC).

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2.4 Determination of colour, vitamin C content, chlorophyll content, total phenolic

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content, water-soluble polysaccharide content, DPPH radical scavenging ability,

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hydroxyl free radicals scavenging ability and oxygen radical absorbance capacity

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ACCEPTED MANUSCRIPT Dried okra samples were ground into powder by the high-speed grinder three

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times and 40 s once. The okra powders were harvested by screening through 100

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mesh Tyler stainless steel sieve for further analysis. The physicochemical properties

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were analysed according to the previous references with some modifications as

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follows: colour attributes (Haggard et al., 2018), vitamin C content (Tan, Song, Zheng,

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Cui, & Zong, 2015), chlorophyll content (Pan, Wang, & Liu, 2004), total phenolic

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content (Limmongkon et al., 2017), water-soluble polysaccharide content (Cui, Xu,

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Sun, & Chen, 2006), DPPH-radical scavenging ability (Li, Lin, Gao, Han, & Chen,

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2012), Hydroxyl free radicals scavenging assay (Leung, Venus, Zeng, & Tsopmo,

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2018) and ORAC assay (Huang, Ou, Hampsch-Woodill, Flanagan, & Prior, 2002).

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The detail methods were described in supplemental files.

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2.5 Comprehensive evaluation method

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In order to unify the data, the evaluation indexes should be normalized.

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According to relevant literatures (Zhang et al., 2015), the normalization formula of

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negative indexes (∆E) and positive direction indexes (vitamin C content, chlorophyll

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content, total phenols content and soluble polysaccharide content) are as follows,

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respectively.

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23 = 4

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(8) (9)

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Where Yi is the normalized value of negative indexes, Yii is the normalized value

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of positive indexes, Xi is the actual value of indexes, Xmax and Xmin respectively is the 9

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maximum and minimum value of indicators. Samples comprehensive score gained by weighting according to the following formula: K=y1×l1+y2×l2+y3×l3+y4×l4+y5×l5

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Where y1, y2, y3, y4, y5 is the normalization result of ∆E, vitamin C content,

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chlorophyll content, total phenols content, soluble polysaccharide content,

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respectively; l1, l2, l3, l4, l5 is the corresponding weight of above various parameters.

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2.6 Statistical analysis

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All experiments were run at least in triplicate, with the data expressed as the

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mean and standard deviation (SD). Microcal Origin 8.0 (Microcal Software, Inc.,

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Northampton, USA) software was employed for statistical analyses. Analysis of

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variance (ANOVA) and Duncan’s multiple-range test (p<0.05) were performed to

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evaluate differences between samples.

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3. Results and discussion

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3.1 The drying characteristics of okra dried by different methods

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To compare the effect of different drying conditions on the drying kinetics of

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okra, the curves of MR versus drying time under different drying process are shown

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in Fig.1. The curves of MR dropped rapidly during the initial stage but it became

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gently at the last period during the drying process. The MR was significantly

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influenced by cutting types for HA and HP. It was apparent that drying time of VC 10

ACCEPTED MANUSCRIPT samples was shorter than that of HC samples at the same drying temperature. The

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drying time was dependent on the drying temperature and the material shapes, and

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this result was consistent with previous studies (Chungcharoen, Prachayawarakorn,

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Tungtrakul, & Soponronnarit, 2014).

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The effect of different drying conditions on drying rate is shown in Fig.2. All

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drying rate reduced with the decreased moisture content in the drying process. The

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increasing drying temperature and VC were conducive to increase the drying rate and

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effectively shorten drying time. Geometrically, VC samples had a larger specific

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surface area than that of HC samples. This could be attributed to the fact that the

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water vapor pressure inside were enhanced, which conformed to the study by

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researchers on red jujube drying (Niu, Gao, Wang, & Di, 2014). To compare HA with

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HP, it could be found that the samples dried by HP had higher drying rate than that of

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those dried by HA at the same drying temperature.

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The Weibull distribution function was used to fit the drying curve in this

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research. As shown in Table 1, the smaller value of α was, the higher drying rate

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could be attained correspondingly. It can be found that the value of α decreased with

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the elevated temperature, and the values of different drying conditions were also

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significant differences. Almost all the values of shape parameter β were more than 1,

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which indicated that there was a rising velocity period in the early of drying. The

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elevated temperature had no significant effect on the β value (p<0.05), but different

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drying methods did. This indicated that the two methods might affect the shrinkage

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ratio and integrity of okra samples and other parameters related to shape, which

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needed to be further studied. The reason for the fluctuation of Rg was also related to

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the change of okra sample shape. In Fig.2, the entire drying process occurred in the falling rate period, which

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indicated that the drying process was controlled by internal moisture diffusion. As

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shown in Table 1, the Deff and Dcal increased with elevated temperature as expected. It

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was due to the fact that the movement of water molecules could be accelerated with

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higher temperature, which leading to higher moisture diffusivity (Xie et al., 2017a).

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The difference of Deff values with various cutting types might be due to the fact that

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the migration rate of water molecules could be sped up with increased specific surface

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area. And more effective water diffusion channels for the internal water of okra

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achieved with VC. The Deff of okra dried by HP was higher than HA, which agreed

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with the fact HP could enhance the drying rate and shorten the drying time previously

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mentioned. The Deff values in this research lied within the general range from 10-11 to

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10-9 m2/s for agricultural products (Xie et al., 2017a), higher than that of okra in

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previous studies (Doymaz, 2005). This phenomenon might be due to that Deff mainly

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depended on varieties, maturation status, physical-chemical properties, cut types and

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drying condition of okra. Activation energy (Ea) is the threshold energy, or the energy

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barrier must be overcome to initiate mass diffusion from the wet material. The Ea of

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okra by different drying conditions varied from 21.40 to 37.00 kJ/mol. VC samples

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had lower Ea values than HC both HA and HP, which indicated the Ea was influenced

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significantly by characteristic surface area and tissue structures of the material. To

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compare with HC samples, VC samples had bigger evaporation area which was

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helpful to migrate moisture from okra to surrounding.

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3.2 The physicochemical properties of okra dried by different conditions

Colour is an important quality attribute that influences consumer acceptance of

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agriculture product (Gordillo et al., 2018). The △E, L*, a*, b*, C*, h* values of okra

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samples are given in Table 2. The values of a*, and b* separately represent redness (+)

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to greenness (-), and yellowness (+) to blueness (-). In HA, a* value increased firstly

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(from 55 oC to 70 oC) and then decreased (from 70 oC to 80 oC) with the elevated

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temperature. In HP, the a* values were significantly higher under low drying

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temperature (40 oC to 50 oC) than those under high drying temperature (60 oC to 70

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drying time and decreased under high drying temperature with short drying time. It

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might be due to the fact that a* value would be more significantly enhanced by drying

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time than by drying temperature. The b* values of all HA and HP samples were higher

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than VF samples in different degrees, and there was no significant influence on b*

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values among samples dried under different conditions. The trend of chroma (C*) was

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similar to that of b*. L* and h* were selected to be the index of browning, and high

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values indicated less browning. VF sample got the highest L* and h* values. The

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values of L* and h* decreased with the elevated temperature might be due to the fact

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that okra browning was more likely caused by high drying temperature. Low

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temperature, short time and suitable drying method could maintain the colour and

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improve the appearance. As shown in Table 2, it can be found that the △E values of

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ACCEPTED MANUSCRIPT VC samples were always less than that of HC samples. Those results could be

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resulted from the fact that okra browning was aggravated with increased drying time.

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The cutting types with larger specific surface area (VC samples) were also easier to

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get a smaller △E values in HP, but not completely, which might be affected by other

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factors, such as oxygen and individual difference (Jiang et al., 2017).

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Vitamin C (Vc) is a key indicator of vegetable products quality. The loss of Vc

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might be attributed to high temperature or long drying time during drying process. Vc

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was highly sensitive to oxygen, light, temperature, and moisture content than most of

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other nutritional ingredients (Cui, Li, Song, & Song, 2008). As shown in Table 3, the

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relative high temperatures, such as 60 oC and 70 oC, were adverse to the retention of

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Vc in HP. An obvious trend could be seen in HA was that the Vc content of HC

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samples was higher than that of VC samples at the same temperature. This was related

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to the excessive exposure of okras to high temperature air with VC samples. However,

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in terms of HP, samples with the cutting types for lager specific surface area were

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superior to the opposite. This could be resulted from the reason that the decreased

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drying time was beneficial to retention of Vc. For HA, Vc content varied from 23.79%

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to 46.25% compared with VF samples. For HP, Vc content ranged from 29.52% to

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72.23%. These values were in agreement with those of previous studies (Liu et al.,

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2016).

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Chlorophyll content is an important parameter for colour attribute of dried

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products. Drying method had an important impact on the degradation of chlorophyll

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during drying, resulting in colour changes in final products (Huang & Zhang, 2016). 14

ACCEPTED MANUSCRIPT As show in Table 3, it was observed that the chlorophyll content showed correlation

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with colour values as affected by drying conditions. Therefore, it could be inferred

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that the chlorophyll content was largely responsible for the composition of green for

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okra. Finding a balance between low temperature and short time was an effective

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mean to improve chlorophyll retention. The similar phenomenon as well observed by

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previous research for toona sinensis dying (Ren et al., 2016).

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Total phenolic compounds (TPC), commonly known as secondary metabolites

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play various medicinal and physiological functions in living nature and were highly

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significant in scavenging free radicals (Sultana, 2012). As shown in Table 3, there

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was no obvious difference of TPC between HA and HP. For HA, there was no

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significant influence of cutting types on TPC. However, the TPC increased and then

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decreased with elevated drying temperature. This might be due to the fact that both

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the high drying temperature and long drying time would cause the degradation of

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phenolic compounds. For HP, the TPC of HC samples was higher than that of VC

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samples. Generally, the TPC increased with elevated drying temperature. This might

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be attributed to the fact that the high drying temperature reduced the drying time,

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which decreased the thermal degradation of phenolic compounds. It has been reported

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that the TPC might be effected by processing methods, as well as environmental

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factors such as drying temperature during drying process (Shah, Shamsuddin, Rahman,

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& Adzahan, 2014).

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Okra polysaccharide is one of the most important nutritional indicator for okra

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due to the potential medical value (Liu et al., 2018). As shown in Table 3, no 15

ACCEPTED MANUSCRIPT significant differences were observed between HA and HP in polysaccharide content,

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which ranged from 2.00 to 2.41 g/100 g (d.b.) for HA and 2.14 to 2.57 g/100 g (d.b.)

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for HP, respectively. For HA, there was significantly negative correlation between

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polysaccharide content and drying temperature. In terms of cutting types,

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polysaccharide content of HC samples was higher than that of VC samples. This

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might be duo to the fact that VC caused the loss of okra mucilage due to larger cross-

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sectional area. For HP, the optimal drying temperature to get the highest

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polysaccharide content was 50 oC, followed by 60 oC. In particular, the highest

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retention rate of polysaccharide was 94.14% revealed in sample treated with VC-HA-

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50 oC.

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Different drying methods and cutting types had complicated and interactive

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effects on different indicators of okra samples. Therefore, it was necessary and

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effective to apply weighted synthetic analysis method in this study to select the

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optimal drying condition of okra. According to the indicator’s contribution to the final

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quality of okra powders products, five indicators with △E, Vc, chlorophyll content,

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TPC, and polysaccharide content were assigned to the proportion of 0.15, 0.15, 0.15,

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0.15, and 0.4, respectively. The quality attributes and comprehensive score of okra

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samples are shown in Fig.3. The colour changes, from blue to green, to yellow and

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then red, indicated an increasing comprehensive score (from lowest to highest). It was

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obvious to know that the drying condition to obtain highest comprehensive score was

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HC-HA-60 oC in HA and VC-HP-50 oC in HP, and VC-HP-50 oC in all the conditions.

340

Therefore, the VC pretreatment with HP at 50 oC was considered as the optimal

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16

ACCEPTED MANUSCRIPT 341

process to achieve better drying quality attributes.

342

3.3 The antioxidant capacity of okra dried by different conditions

The antioxidant capacities of okra samples measured by DPPH, hydroxyl radical,

344

ORAC assays are shown in Fig.4. DPPH was a stable free radical, which commonly

345

used as an indicator that could receive a hydrogen radical or an electron to become a

346

stable diamagnetic molecule in radical scavenging activity (Liu, Cai, Lu, Han, & Ying,

347

2012). The okra samples dried by different conditions had good potential to scavenge

348

the DPPH free radicals. The effect of temperature on DPPH IC50 was obvious. The

349

DPPH IC50 at 55 oC in HC-HA was higher significantly than that of other drying

350

temperatures, and a similar regularity was observed in VC-HA. This was not

351

consistent with previous studies about citrus fruit peels, which revealed that the DPPH

352

radical scavenging activity was enhanced with increased drying time (Chen, Yang, &

353

Liu, 2011). The DPPH scavenging activity of okra samples significantly affected by

354

HP. Presumably, it was a consequence that VC-HP could reduce the drying time. The

355

results were coincident with the previous research for okra subjected to drying

356

treatment (Xu & Du, 2015).

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The hydroxyl radical scavenging activity was significantly enhanced with the o

358

elevated temperature in HA (except HC-HA-80

359

consequence of excessive temperature. The hydroxyl radical scavenging activity of

360

okra powders dried at 50, 60, and 70 oC was obvious higher than that of at 40 oC in

361

HP. There was the negative correlation between drying temperature and hydroxyl 17

C). It was just likely to a

ACCEPTED MANUSCRIPT radical scavenging ability. Nevertheless, the drying methods (HA and HP) had little

363

effect on it. Similar behaviour was observed in the ORAC of the samples. Therefore,

364

the decreased antioxidant activity could be explained by the samples were

365

overexposed to high temperature and oxygen environments. However, this trend was

366

not obvious in VC-HA, which could be explained by the fact that VC samples caused

367

a great deal of loss for okra mucus and okra seeds. Results of ORAC test varied from

368

3482.77 to 4158.98 µmol Trolox/g, were relatively stable in HP and obviously higher

369

than that of HA, which ranged from 2674.34 to 3485.68 µmol Trolox/g.

370

4. Conclusion

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The effects of different cutting types and drying methods (HC-HA, VC-HA, HC-

372

HP, VC-HP) on drying characteristics, physicochemical properties and antioxidant

373

capacity of okra were examined experimentally. As expected, the drying time

374

decreased with increased drying temperature, and VC samples had shorter drying time

375

than that of HC samples both HA and HP. The Deff determined by Weibull

376

distribution model ranged from 0.992×10-9 m2/s to 4.409×10-9 m2/s for HA and

377

1.915×10-9 m2/s to 5.291×10-9 m2/s for HP. The Ea was 37.00, and 35.91 kJ/mol for

378

the samples dried by HC-HA and VC-HA, 29.25, and 21.40 kJ/mol for HC-HP and

379

VC-HP, respectively. The overall quality of dried okra samples depended on cutting

380

types and drying methods. HP samples had higher Vc retention and antioxidant

381

activity compared with HA. This study indicated that HP was a promising drying

382

method for okra quality preservation, especially at VC-HP-50oC, as it allowed better

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18

ACCEPTED MANUSCRIPT reservation of chemical compositions to obtain higher comprehensive score. Through

384

comprehensive analysis, the best drying process of okra was VC-HP-50 oC. The

385

findings in current work could provide necessary information and theoretical

386

foundation for okra drying industrially.

387

Acknowledgements

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383

The authors are grateful to financial support of China Agricultural Research

389

System, China (CARS-23-E01), the Beijing Academy of Agricultural and Forestry

390

Sciences Special Fund, China (KJCX20170205), the Special Fund for leading talent

391

in Mount Tai of Shandong Province, China (LJNY201705), the Project supported by

392

Beijing Postdoctoral Research Foundation (2018-ZZ-063).

393

Reference

394

Afolabi, T. J., & Agarry, S. E. (2014). Thin layer drying kinetics and modelling of

395

Okra (Abelmoschus esculentus (L.) moench) slices under natural and forced

396

convective air drying. Food Science & Quality Management, 28, 41-49.

AC C

EP

TE D

M AN U

SC

388

397

An, K., Zhao, D., Wang, Z., Wu, J., Xu, Y., & Xiao, G. (2016). Comparison of

398

different drying methods on chinese ginger (Zingiber officinale Roscoe):

399 400 401 402

Changes in volatiles, chemical profile, antioxidant properties, and microstructure. Food Chemistry, 197, 1292-1300.

AOAC. (1990). Official method of analysis. Association of official analytical chemists (No. 934.06). Arlington, VA. 19

ACCEPTED MANUSCRIPT Chen, J., Venkitasamy, C., Shen, Q., McHugh, T. H., Zhang, R., & Pan, Z. (2018).

404

Development of healthy crispy carrot snacks using sequential infrared blanching

405

and hot air drying method. LWT - Food Science and Technology, 97, 469-475.

406

Chen, M. L., Yang, D. J., & Liu, S. C. (2011). Effects of drying temperature on the

407

flavonoid, phenolic acid and antioxidative capacities of the methanol extract of

408

citrus fruit (Citrus sinensis (L.) Osbeck) peels. International Journal of Food

409

Science & Technology, 46(6), 1179-1185.

SC

RI PT

403

Chungcharoen, T., Prachayawarakorn, S., Tungtrakul, P., & Soponronnarit, S. (2014).

411

Effects of germination time and drying temperature on drying characteristics and

412

quality of germinated paddy. Food & Bioproducts Processing, 94, 707-716.

413

Cui, Z., Li, C., Song, C., & Song, Y. (2008). Combined microwave-vacuum and freeze drying of carrot and apple chips. Drying Technology, 26(12), 1517-1523.

TE D

414

M AN U

410

Cui, Z., Xu, S., Sun, D., & Chen, W. (2006). Dehydration of concentrated extraction

416

by combined microwave-vacuum and conventional vacuum drying. Drying

417

Technology, 24(5), 595-599.

419 420

Daghigh, R., Ruslan, M. H., Sulaiman, M. Y., & Sopian, K. (2010). Review of solar

AC C

418

EP

415

assisted heat pump drying systems for agricultural and marine products. Renewable & Sustainable Energy Reviews, 14(9), 2564-2579.

421

Dai, J. W., Rao, J. Q., Wang, D., Xie, L., Xiao, H. W., Liu, Y. H., & Gao, Z. J. (2015).

422

Process-based drying temperature and humidity integration control enhances

423

drying kinetics of apricot halves. Drying Technology, 33(3), 365-376.

424

Doymaz, İ. (2005). Drying characteristics and kinetics of okra. Journal of Food 20

ACCEPTED MANUSCRIPT 425 426 427

Engineering, 69(3), 275-279. Duan, Q., Zhang, S., Li, X., Tian, X., & Wang, D. (2018). Experimental study of heat pump dryer for apple chips drying. Journal of Qingdao University, 33(1), 80-86. Erenturk, S., Gulaboglu, M. S., & Gultekin, S. (2005). The effects of cutting and

429

drying medium on the vitamin C content of rosehip during drying. Journal of

430

Food Engineering, 68(4), 513-518.

RI PT

428

Gordillo, B., Sigurdson, G. T., Fei, L., González-Miret, M. L., Heredia, F. J., & Giusti,

432

M. M. (2018). Assessment of the color modulation and stability of naturally

433

copigmented anthocyanin-grape colorants with different levels of purification.

434

Food Research International, 106, 791-799.

M AN U

SC

431

Haggard, S., Luna-Vital, D., West, L., Juvik, J. A., Chatham, L., Paulsmeyer, M., &

436

Gonzalez, d. M. E. (2018). Comparison of chemical, color stability, and phenolic

437

composition from pericarp of nine colored corn unique varieties in a beverage

438

model. Food Research International, 105, 286-297.

TE D

435

Huang, D., Ou, B., Hampsch-Woodill, M., Flanagan, J. A., & Prior, R. L. (2002).

440

High-throughput assay of oxygen radical absorbance capacity (ORAC) using a

442 443

AC C

441

EP

439

multichannel liquid handling system coupled with a microplate fluorescence reader in 96-well format. Journal of Agricultural & Food Chemistry, 50(16), 4437-4444.

444

Huang, J., & Zhang, M. (2016). Effect of three drying methods on the drying

445

characteristics and quality of okra. Drying Technology, 34(8). DOI:

446

10.1080/07373937.2015.1086367. 21

ACCEPTED MANUSCRIPT Jiang, N., Zhang, Z., Li, D., Liu, C., Zhang, M., Liu, C., . . . Niu, L. (2017).

448

Evaluation of freeze drying combined with microwave vacuum drying for

449

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

450

consumption. LWT - Food Science and Technology, 82(1), 216-226.

RI PT

447

Leung, R., Venus, C., Zeng, T., & Tsopmo, A. (2018). Structure-function relationships

452

of hydroxyl radical scavenging and chromium-VI reducing cysteine-tripeptides

453

derived from rye secalin. Food Chemistry, 254, 165.

455

Li, J. (2018). Research on heat pump drying equipment for practical heat-sensitive

M AN U

454

SC

451

materials. Journal of Chinese Agricultural Mechanization(2), 46-48. Li, X. C., Lin, J., Gao, Y. X., Han, W. J., & Chen, D. F. (2012). Antioxidant activity

457

and mechanism of Rhizoma Cimicifugae. Chemistry Central Journal, 6(1), 1-10.

458

Limmongkon, A., Janhom, P., Amthong, A., Kawpanuk, M., Nopprang, P.,

459

Poohadsuan, J., . . . Srikummool, M. (2017). Antioxidant activity,total

460

phenolic,and resveratrol content in five cultivars of peanut sprouts. Asian Pacific

461

Journal of Tropical Biomedicine, 7(4), 332-338.

463 464 465

EP

Liu, C. H., Cai, L. Y., Lu, X. Y., Han, X. X., & Ying, T. J. (2012). Effect of

AC C

462

TE D

456

postharvest UV-C irradiation on phenolic compound content and antioxidant activity of tomato fruit during storage. Journal of Integrative Agriculture, 11(1), 159-165.

466

Liu, J., Zhao, Y. P., Wu, Q. X., John, A., Jiang, Y. M., Yang, J. L., . . . Yang, B. (2018).

467

Structure characterisation of polysaccharides in vegetable "okra" and evaluation

468

of hypoglycemic activity. Food Chemistry, 242, 211-216. 22

ACCEPTED MANUSCRIPT 469

Liu, W., Duan, X., Ren, G., Yajing, X. U., Pang, Y., & Tao, Z. (2016). Drying

470

behavior and optimization of vacuum drying parameters of okras (Abelmoschus

471

esculentus (L.) Moench). Food Science, 37(24), 29-39. Magalhães, M. L., Cartaxo, S. J. M., Gallão, M. I., García-Pérez, J. V., Cárcel, J. A.,

473

Rodrigues, S., & Fernandes, F. A. N. (2017). Drying intensification combining

474

ultrasound pre-treatment and ultrasound-assisted air drying. Journal of Food

475

Engineering, 215, 72-77.

SC

RI PT

472

Niu, R., Gao, F., Wang, Y., & Di, J. B. (2014). Effects of the pieces size on the quality

477

of dried dates. Academic Periodical of Farm Products Processing(15), 35-37.

478

Onwude, D. I., Hashim, N., Abdan, K., Janius, R., & Chen, G. (2019). The

479

effectiveness of combined infrared and hot-air drying strategies for sweet potato.

480

Journal of Food Engineering, 241, 75-87.

482

TE D

481

M AN U

476

Pan, S. Y., Wang, K. X., & Liu, Q. (2004). Study on physical and chemical properties of different sizes rice powder. Food Science, 25(5), 58-62. Rai, D. R., & Balasubramanian, S. (2009). Qualitative and textural changes in fresh

484

okra pods (Hibiscus esculentus L.) under modified atmosphere packaging in

486

AC C

485

EP

483

perforated film packages. Food Science & Technology International, 15(15), 131-138.

487

Ren, G., Liu, J., Liu, W., Qiao, X., Luo, L., & Duan, X. (2016). Drying modelling and

488

quality of toona sinensis subjected to heat pump cold air drying. Food Science,

489

37(23), 13-19.

490

Sehrawat, R., Nema, P. K., & Kaur, B. P. (2018). Quality evaluation and drying 23

ACCEPTED MANUSCRIPT 491

characteristics of mango cubes dried using low-pressure superheated steam,

492

vacuum and hot air drying methods. LWT - Food Science and Technology, 92,

493

548-555. Shah, N. N. A. K., Shamsuddin, R., Rahman, R. A., & Adzahan, N. M. (2014). Effects

495

of physicochemical characteristics of pummelo fruit juice towards UV

496

inactivation of salmonella typhimurium ☆. Agriculture & Agricultural Science

497

Procedia, 2, 43-52.

499

SC

Shivhare, U. S., Gupta, A., Bawa, A. S., & Gupta, P. (2000). Drying characteristics

M AN U

498

RI PT

494

and product quality of okra. Drying Technology, 18(1-2), 409-419. Sultana, B. (2012). Effect of drying techniques on the total phenolic contents and

501

antioxidant activity of selected fruits. Journal of Medicinal Plant Research, 6(1),

502

161-167.

TE D

500

Tan, Z. W., Song, C. Y., Zheng, M. X., Cui, B. H., & Zong, C. W. (2015).

504

Optimization and determination of the content of reduction-type VC in ripe fruit

505

of vaccinium µliginosum with phosphomolybdate-blue spectrophotometry. Hubei

506

Agricultural Sciences(7), 1713-1716.

508 509

AC C

507

EP

503

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.

Xie, L., Mujumdar, A. S., Fang, X. M., Wang, J., Dai, J. W., Du, Z. L., . . . Gao, Z. J.

510

(2017a). Far-infrared radiation heating assisted pulsed vacuum drying (FIR-PVD)

511

of wolfberry (Lycium barbarum L.): Effects on drying kinetics and quality

512

attributes. Food & Bioproducts Processing, 102, 320-331. 24

ACCEPTED MANUSCRIPT Xie, L., Mujumdar, A. S., Zhang, Q., Wang, J., Liu, S., Deng, L., . . . Gao, Z. J.

514

(2017b). Pulsed vacuum drying of wolfberry: Effects of infrared radiation

515

heating and electronic panel contact heating methods on drying kinetics, color

516

profile, and volatile compounds. Drying Technology, 35(11), 1312-1326.

RI PT

513

Xie, L., Zheng, Z. A., Mujumdar, A. S., Fang, X. M., Wang, J., Zhang, Q., . . . Gao, Z.

518

J. (2018). Pulsed vacuum drying (PVD) of wolfberry: Drying kinetics and

519

quality attributes. Drying Technology(11), 1-14.

522 523 524 525

M AN U

521

Xie, Y., Gao, Z., Liu, Y., & Xiao, H. (2017). Pulsed vacuum drying of rhizoma dioscoreae slices. LWT - Food Science and Technology, 80, 237-249. Xu, K., & Du, J. H. (2015). Effects of drying methods on antioxidant capacities of okra fruit. Food and Fermentation Industries, 42(5), 120-125. Yuan, B., Ritzoulis, C., & Chen, J. (2018). Extensional and shear rheology of okra

TE D

520

SC

517

hydrocolloid-saliva mixtures. Food Research International, 106, 204-212. Zhang, W., Gao, Z., Xiao, H., Zheng, Z., Ju, H., Long, X., . . . University, C. A. (2015).

527

Drying characteristics of poria cocos with different drying methods based on

528

Weibull distribution. Nongye Gongcheng Xuebao/transactions of the Chinese

AC C

529

EP

526

Society of Agricultural Engineering, 31(5), 317-324.

530

Zhou, X., Li, R., Lyng, J. G., & Wang, S. (2018). Dielectric properties of kiwifruit

531

associated with a combined radio frequency vacuum and osmotic drying. Journal

532

of Food Engineering, 239, 72-82.

533 534

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ACCEPTED MANUSCRIPT Table 1 Drying data of okra samples dried by different drying condition. Drying condition

Ea (KJ/mol)

Deff Dcal (10-9 m2/s) (10-8 m2/s)

Rg

α （min）

R2

β

RMSE

X2

0.992

1.007

10.153 277.043 1.132

0.9998 5.55×10-4 1.850×10-5

HC-HA-60 oC

1.068

1.067

9.986

277.968 1.321

0.9979 5.16×10-3 2.148×10-4

1.261

1.273

10.097 225.081 1.148

0.9961 7.76×10-3 3.695×10-4

1.615

1.650

10.215 178.204 1.238

0.9969 5.27×10-3 3.292×10-4

HC-HA-75 oC

1.984

2.022

10.193 147.436 1.261

HC-HA-80 oC

2.488

2.505

10.068 123.125 1.284

VC-HA-55 oC

1.764

1.940

10.998 178.712 1.317

VC-HA-60 oC

2.089

2.530

12.113 164.915 1.440

VC-HA-65 oC

2.334

2.657

11.384 136.211 1.305

3.175

3.710

11.687 109.949 1.312

0.9989 1.25×10-3 1.386×10-4

VC-HA-75 oC

3.608

4.288

11.885

91.857

1.434

0.9986 1.48×10-3 2.118×10-4

VC-HA-80 oC

4.409

5.421

12.295

84.724

1.500

0.9982 1.78×10-3 2.969×10-4

HC-HP-40 oC

1.915

2.023

10.564 212.863 0.939

0.9992 9.16×10-4 7.048×10-5

3.458

3.677

10.633 155.952 1.063

0.9983 1.66×10-3 2.069×10-5

4.129

4.219

10.217 119.994 1.084

0.9993 5.32×10-4 1.063×10-4

HC-HP-70 oC

4.502

4.545

10.095 108.453 1.041

0.9994 4.14×10-4 1.034×10-4

VC-HP-40 oC

2.480

2.919

11.768 120.504 1.051

0.9997 1.91×10-4 3.828×10-5

3.968

4.775

12.034

75.554

1.068

0.9994 3.56×10-4 1.187×10-4

4.409

5.726

12.987

70.322

1.081

0.9997 1.87×10-4 9.372×10-5

5.291

6.661

12.590

58.143

0.996

0.9999 6.92×10-4 6.938×10-5

HC-HP-50 oC HC-HP-60 oC

VC-HP-50 oC VC-HP-60 oC

29.25

21.40

AC C

EP

VC-HP-70 oC

35.91

1

0.9960 5.51×10-3 4.594×10-4 0.9988 1.66×10-3 1.510×10-4 0.9984 2.92×10-3 1.716×10-4 0.9969 4.65×10-3 3.872×10-4 0.9968 4.22×10-3 3.835×10-4

SC

VC-HA-70 oC

M AN U

HC-HA-70 oC

37.00

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HC-HA-65 oC

RI PT

HC-HA-55 oC

ACCEPTED MANUSCRIPT Table 2 The △E, L*, a*, b*, C*, h* values of okra samples dried by different drying condition. Drying condition

L*

a*

b*

VF

78.23±0.21a

-6.81±0.03j

20.28±0.14i

C*

h*

21.39±0.43g 108.57±0.04a

△E 0k 6.83±0.21f

HC-HA-60 oC 72.39±0.13d -3.57±0.04cde 23.17±0.05e 23.17±0.05def 98.85±0.09f

7.28±0.13e

HC-HA-65 oC 72.53±0.52d -2.96±0.14bc 23.15±0.17e 23.15±0.10def 97.34±0.28g

7.45±0.18cde

HC-HA-70 oC 72.92±0.18cd -2.13±0.02a 22.40±0.10gh 22.40±0.10f

95.45±0.08i

7.39±0.07de

HC-HA-75 oC 73.45±0.22bcd -2.68±0.03b 22.79±0.02g 22.95±0.02ef 96.71±0.07gh

6.80±0.04f

HC-HA-80 oC 72.43±0.07d -3.29±0.01cd 23.17±0.10e 23.41±0.04de 98.08±0.05fg

7.38±0.09de

VC-HA-55 oC 69.58±0.73f

9.19±2.11a

RI PT

HC-HA-55 oC 73.11±3.11cd -3.39±0.38cde 23.23±1.13de 23.48±0.14de 98.30±0.45f

SC

-4.28±0.09f 22.06±0.35ghi 22.47±1.18f 100.96±0.09cd

VC-HA-60 oC 72.55±0.04d -5.39±0.01gh 23.29±0.02de 23.29±0.17de 103.38±0.02bc 6.58±0.52fg 6.00±0.22hi

VC-HA-70 oC 74.00±0.09bc -3.41±0.04cde 22.59±0.04gh 22.85±0.36ef

98.58±0.08f

5.90±0.73i

VC-HA-75 oC 74.39±0.09b -3.79±0.04de 23.05±0.06ef 23.36±0.08d 99.35±0.06ef

5.62±0.07j

VC-HA-80 oC 72.59±0.27d -4.82±0.01fg 23.42±0.05de 23.91±0.07cd 101.63±0.05c

6.76±0.09fg

M AN U

VC-HA-65 oC 73.82±0.07bc -4.28±0.04f 23.47±0.08de 23.47±0.02de 100.52±0.10d

HC-HP-40 oC 71.75±0.07e -4.15±0.05ef 23.61±0.05d 23.97±0.18cd 99.96±0.10def 7.76±0.04bc HC-HP-50 oC 72.02±0.33de -4.43±0.03f

24.54±0.05c 24.93±0.12bc 100.24±0.08d

7.90±0.15b

HC-HP-60 oC 73.40±0.04bcd -3.26±0.11cd 23.54±0.20d 23.77±0.12d 97.89±0.34fg

6.83±0.21f

HC-HP-70 oC 74.44±0.10b

6.55±0.15fg

-2.71±0.02b 23.70±0.07d 23.86±0.18cd 96.53±0.04gh

24.56±0.15c 25.39±0.05b 104.72±0.04b 6.48±0.27gh

TE D

VC-HP-40 oC 73.38±0.07bcd -6.45±0.04i

VC-HP-50 oC 73.72±0.16bc -6.65±0.07ij 25.35±0.10a 26.21±0.08a 104.70±0.13b

6.79±0.10f

VC-HP-60 oC 71.82±0.15e

-5.05±0.06g 23.88±0.12d 24.41±0.08c 101.93±0.15c 7.57±0.33bcd

VC-HP-70 oC 72.41±0.06d

-4.44±0.07f

24.77±0.08b 25.17±0.23b 100.15±0.17de 7.72±0.06bc

EP

Note: a, b, c, d, e, f, g, h, i, j and k mean values of different indicators under different

AC C

drying conditions are statistically different at p<0.05.

2

ACCEPTED MANUSCRIPT Table 3 The Vc content, chlorophyll content, TPC and polysaccharide content of okra samples dried by different drying condition. Vc (mg/100g, d.b.)

Chlorophyll (mg/g, d.b.)

TPC (mg/g, d.b.)

Polysaccharide (g/100g, d.b.)

VF

189.04±2.17a

1.66±0.02a

10.58±0.16a

2.73±0.20a

HC-HA-55 oC

63.35±2.50i

1.33±0.01e

9.00±0.02g

2.41±0.11c

HC-HA-60 oC

87.43±4.32f

1.27±0.01efg

10.08±0.08bcd

2.34±0.09cde

HC-HA-65 oC

60.93±3.55i

1.24±0.01efgh

10.07±0.03bcd

2.30±0.11defg

HC-HA-70 oC

82.86±4.50g

1.20±0.01ghij

9.35±0.03ef

2.23±0.21ghij

HC-HA-75 oC

55.48±1.94j

1.29±0.01ef

9.20±0.04ef

2.20±0.03hij

HC-HA-80 oC

71.33±3.17h

1.28±0.02ef

9.30±0.08ef

2.08±0.12ij

VC-HA-55 oC

79.41±1.34g

1.56±0.01b

8.66±0.02g

2.39±0.04cde

VC-HA-60 oC

65.85±1.49i

1.43±0.01d

10.17±0.12bcd

2.25±0.13fghi

VC-HA-65 oC

61.83±1.33i

1.32±0.01e

10.38±0.09b

2.22±0.04ghij

VC-HA-70 oC

55.31±3.13j

1.23±0.01fghi

9.26±0.04fg

2.15±0.06ij

VC-HA-75 oC

44.97±3.91k

1.44±0.01cd

9.72±0.08de

2.10±0.09ij

VC-HA-80 oC

57.82±2.49j

1.56±0.02b

9.54±0.05de

2.00±0.10j

HC-HP-40 oC

94.89±0.35e

1.20±0.02ghij

10.22±0.07bcd

2.30±0.7defg

HC-HP-50 oC

105.89±2.34d

1.22±0.01ghi

9.49±0.11ef

2.40±0.10cd

HC-HP-60 oC

55.81±1.51j

1.11±0.01hij

10.18±0.13bcd

2.33±0.03cdef

HC-HP-70 oC

98.41±0.98e

1.14±0.01hij

10.32±0.09bc

2.30±0.01defg

VC-HP-40 oC

136.54±4.00b

1.34±0.01d

9.04±0.01fg

2.28±0.03efgh

VC-HP-50 oC

128.18±1.35c

1.47±0.01bc

9.80±0.02cd

2.57±0.02b

VC-HP-60 oC

79.34±0.97g

1.34±0.01d

9.90±0.14cd

2.33±0.11cdef

90.39±1.43ef

1.29±0.01ef

10.23±0.01bc

2.26±0.10fghi

SC

M AN U

TE D

VC-HP-70 oC

RI PT

Drying condition

EP

Note: a, b, c, d, e, f, g, h, i, j and k mean values of different indicators under different

AC C

drying conditions are statistically different at p<0.05.

3

ACCEPTED MANUSCRIPT Figure captions Fig.1. The curves of MR versus drying time under different drying process. Fig.2. The curves of drying rate versus moisture content (d.b.). Fig.3. The heat map of comprehensive evaluation to drying qualities of okra samples

RI PT

dried by different drying condition.

AC C

EP

TE D

M AN U

SC

Fig.4. The antioxidant capacity of okra sample dried by different conditions.

ACCEPTED MANUSCRIPT

Moisture ratio

60 oC 65 oC 70 oC 75 oC 80 oC

0

2

4

6

8

10

12

Drying time / (h)

14

16

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

55 oC 60 oC 65 oC 70 oC 75 oC 80 oC

0

2

60 oC 70 oC

2

4

6

8

10

Drying time / (h)

(c)

6

8

12

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

SC

Moisture ratio

40 oC 50 oC

M AN U

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

4

Drying time / (h)

10

(b)

14

16

TE D

Moisture ratio

(a)

RI PT

55 oC

Moisture ratio

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

0

2

4

6

Drying time / (h)

(d)

Fig.1. The curves of MR versus drying time under different drying process.

AC C

EP

Note: (a) HC-HA; (b) VC-HA; (c) HC-HP; (d) VC-HP.

40 oC 50 oC 60 oC 70 oC

8

10

ACCEPTED MANUSCRIPT 4.0

65 oC 70 oC

2.5

75 oC 80 oC

2.0 1.5 1.0 0.5 0.0

0

3.0

65 oC 70 oC

2.5

75 oC 80 oC

2.0 1.5 1.0 0.5 0.0

1 2 3 4 5 6 7 8 9 Moisture content / (g water / g dry solid )

55 oC 60 oC

3.5

0

1 2 3 4 5 6 7 8 9 Moisture content / (g water / g dry solid)

(a)

(b)

7.0

4.0

70 oC

3.0 2.0 1.0 0.0

0

1 2 3 4 5 6 7 8 9 Moisture content / (g water / g dry solid)

(c)

6.0 5.0 4.0

40 oC 50 oC

SC

5.0

50 oC 60 oC

Drying rate (g water / g dry solid / h)

40 oC

60 oC 70 oC

M AN U

Drying rate (g water / g dry solid / h)

7.0 6.0

RI PT

3.0

Drying rate (g water / g dry solid / h)

3.5 Drying rate (g water / g dry solid / h)

4.0

55 oC 60 oC

3.0 2.0 1.0

0.0 0

1 2 3 4 5 6 7 8 9 Moisture content / (g water / g dry solid)

TE D

Fig.2. The curves of drying rate versus moisture content (d.b.).

AC C

EP

Note: (a) HC-HA; (b) VC-HA; (c) HC-HP; (d) VC-HP.

(d)

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig.3. The heat map of comprehensive evaluation to drying qualities of okra samples dried by different drying condition. Note:

Chl=Chlorophyll,

TPC=Total

AC C

EP

TE D

CS=Comprehensive score.

phenolic

content,

PS=Polysaccharide,

(b)

AC C

EP

TE D

M AN U

SC

(a)

RI PT

ACCEPTED MANUSCRIPT

(c)

(d)

SC

RI PT

ACCEPTED MANUSCRIPT

M AN U

Fig.4. The antioxidant capacity of okra sample dried by different conditions.

AC C

EP

TE D

Note: (a) HC-HA; (b) VC-HA; (c) HC-HP; (d) VC-HP.

ACCEPTED MANUSCRIPT Highlights The optimal drying condition for okra was given. The effect of drying conditions on drying characteristics and qualities was

RI PT

investigated. The comprehensive weighted scoring method was applied to indicators

AC C

EP

TE D

M AN U

SC

evaluation.