Investigation of hot-air assisted radio frequency (HARF) dielectric heating for improving drying efficiency and ensuring quality of dried hazelnuts (Corylus avellana L.)

Journal Pre-proof Investigation of Hot-Air Assisted Radio Frequency (HARF) Dielectric Heating for Improving Drying Efficiency and Ensuring Quality of Dried Hazelnuts (Corylus avellana L.) Wenjie Wang, Wenjun Wang, Jooyeoun Jung, Ren Yang, Juming Tang, Yanyun Zhao

PII:

S0960-3085(19)30834-X

DOI:

https://doi.org/10.1016/j.fbp.2020.01.006

Reference:

FBP 1209

To appear in:

Food and Bioproducts Processing

Received Date:

31 August 2019

Revised Date:

12 December 2019

Accepted Date:

11 January 2020

Please cite this article as: Wang W, Wang W, Jung J, Yang R, Tang J, Zhao Y, Investigation of Hot-Air Assisted Radio Frequency (HARF) Dielectric Heating for Improving Drying Efficiency and Ensuring Quality of Dried Hazelnuts (Corylus avellana L.), Food and Bioproducts Processing (2020), doi: https://doi.org/10.1016/j.fbp.2020.01.006

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Investigation of Hot-Air Assisted Radio Frequency (HARF) Dielectric Heating for Improving Drying Efficiency and Ensuring Quality of Dried Hazelnuts (Corylus avellana L.)

Wenjie Wang a, Wenjun Wang a, Jooyeoun Jung b, Ren Yang c, Juming Tang c, Yanyun

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a

Department of Food Science and Technology, Oregon State University, Corvallis, OR

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97331-6602, U.S.A.

Department of Food Science and Technology, University of Nebraska-Lincoln, Lincoln,

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b

NE 68583-0726, U.S.A.

Department of Biological Systems Engineering, Washington State University, Pullman,

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WA 99164-6120, U.S.A.

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c

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Zhao a

* Corresponding author: Dr. Yanyun Zhao, Professor Dept. of Food Science & Technology Oregon State University

Corvallis, OR 97331-6602

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E-mail: [email protected]

Highlights 

Hot-air assisted RF (HARF) heating provided fast dry for Oregon hazelnuts

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2-step HA-HARF at 12% intermediate MC reduced shell cracking <15% for Jefferson

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Long HA pre-drying during 2-step HA-HARF drying caused undesirable lipid

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oxidation Continuous HARF and 2-step HA-HARF led better drying performance and nut

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quality

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ABSTRACT

Hot-air assisted radio frequency (HARF) dielectric heating was utilized to dry

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inshell hazelnuts (Barcelona and Jefferson) in order to achieve rapid, uniform drying and better nut quality. The process parameters studied were electrode gap (EG) (14, 15 and 16

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cm) and sample thickness (ST) (3, 5 and 7 cm) for HARF drying, 2-step hot-air (HA)HARF drying at different intermediate moisture contents (MC) (16, 14 and 12%, wet basis), and two HARF modes (continuous and intermittent). The optimal intermittent HARF operations were identified as 15 cm EG and 5 cm or 7 cm ST, resulting in drying

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time of 100.5 min and 180.9 min for Barcelona and Jefferson, respectively, vs. 22 h using HA drying. The 2-step HA-HARF drying at 12% intermediate MC reduced shell cracking ratio < 15%, provided fast drying rate and improved heating uniformity, whereas a longer HA pre-drying time resulted in undesirable high lipid oxidation of dried kernels. The HARF and 2-step HA-HARF in the continuous mode increased drying rate and better

retention of bioactive compounds in comparison with the intermittent mode. This study generated important information of HARF technology for drying hazelnuts with improved drying performance and nut quality compared with conventional HA drying.

Abbreviations: a*, redness; AAE, ascorbic acid equivalent; ANOVA, analysis of

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variance; aw, water activity; b*, yellowness; C*, chroma; DPs, dielectric properties;

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DPPH, 2, 2-diphenyl-1-picrylhydrazyl; DW, Dry weight basis; EG, electrode gap; FA,

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free fatty acids; FC, Folin-Ciocalteu; GAE, gallic acid equivalent; H*, hue; HA, hot-air; HARF, hot-air assisted radio frequency; L*, lightness; LSD, least significant difference;

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MC, moisture contents; MR, moisture ratio; PV, peroxide value; RF, radio frequency;

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uniformity index.

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SD, standard deviation; ST, sample thickness; TPC, total phenolic content; UI,

Keywords: Postharvest hazelnuts; Hot-air assisted radio frequency drying (HARF);

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Intermittent and continuous RF modes; Heating uniformity; Nut quality.

1. Introduction Freshly harvested hazelnuts undergo immediate post-harvest processes, including washing, disinfecting, and hot-air drying at 38-49 °C for an average time of 24 hours. Industry standard and federal regulation require an upper limit on moisture content (MC, wet basis) of dried inshells for ~10% or kernels ~6-7% (USDA, 2016) and water activity

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(aw) of kernels below 0.70 for preventing mold and aflatoxins contamination (Ozay et al., 2008). Our earlier studies suggested hot-air (HA) drying at 43 °C/40% RH as appropriate

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conditions to obtain desired drying rate (9 to 22 hours to reach targeted moisture content for studied nut cultivars) and relatively high retention of bioactive compounds with low

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lipid oxidation and enzyme activity (Wang et al., 2018a). Meanwhile, the use of 38

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°C/60% RH condition could produce dried nuts with lower shell-cracks for cultivars like Barcelona and Jefferson (Wang et al., 2018b). However, conventional HA drying for

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hazelnuts is in general slow, and sometimes inner kernels are not properly dried due to the low thermal conductivity of voids between shell and kernel (Özilgen & Özdemir,

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2001). In addition, inefficient heat transfer from hot-air medium to cold wet samples could cause energy losses and lengthy exposure to elevated air temperature that may lead to over-heating of nut surface, causing shell-cracks. To improve drying efficiency and heating uniformity with a lower cracking ratio

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and high nut quality, a novel drying method, known as radio frequency (RF) dielectric heating was proposed in this study. In theory, RF waves generate heat volumetrically within the materials due to friction of rotation of dipole molecules (i.e. water) and through ionic conduction under a high-voltage alternating electric field (Jiao et al., 2018; Zhou & Wang, 2019). Since hazelnuts contain a high amount (~60%) of non-polar oil

with heat-sensitive bioactive compounds, use of RF drying may help retain nut quality with a potential differential heating effect. In a RF system, hot-air circulation at constant temperature is commonly applied to facilitate removal of moisture from the product while help maintaining a stable product surface temperature, such heating operation is known as hot-air assisted radio frequency (HARF) drying (Wang et al., 2014a). From the

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previous drying study on walnuts using different drying methods, it was found that applying HARF could reduce the drying time and retain the most antioxidant compounds

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comparing with conventional HA and vacuum drying (Zhou, Gao, Mitcham, & Wang, 2018). There could be two modes of HARF in terms of the operation, intermittent and

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continuous (with and without pausing of RF power during the HA circulation). The

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intermittent mode allows temperature and moisture redistribution within samples during the pausing (cooling) cycles, thus increasing heating uniformity in the next RF heating

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cycle (Jumah, 2005). Although an intermittent mode operation commonly takes longer drying time due to frequent heating and cooling cycles, it reduces energy consumption in

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comparison with continuous mode (Wang et al., 2011). In respect to drying high-oil nut samples, it was suggested that the temperature should not exceed 50 oC for preventing lipid oxidation (López et al., 1997a). The HARF drying studies on walnuts (Zhang, Zheng, Zhou, Huang, & Wang, 2016) and macadamia nuts (Wang, Zhang, Johnson, et al.,

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2014) reported a high equilibrium internal temperature over 80 oC under continuous HARF drying mode. However, there is limited information on comparing the difference between intermittent and continuous HARF drying in respect to lipid oxidation and drying rate on tree nuts, especially no study on hazelnuts.

In HARF processing, several factors should be considered to achieve uniform and efficient drying. The intensity of the electric field applied to the product in a RF system is influenced by the distance between two parallel electrodes, in which the smaller gap distance directly contributes to the elevated RF power (Jiao et al., 2018; Piyasena et al., 2003). Sample thickness (ST) is another potential factor determining the RF heating

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uniformity (Jiao et al., 2018; Mujumdar, 2014). As a part of the loading volume, it could influence the drying rate of the materials since both RF power absorption and penetration

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depth in the material depend on the dielectric properties (DPs) of the materials, which is mainly influenced by MC of the materials. Hence, the moisture distribution of the

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material for a fixed volume is a concern in RF operation. Under the same setting, smaller

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ST was suggested to avoid overheating of sample surface and obtain quicker cooling during pausing of RF operation (Sun, 2005a), while the thicker sample loading was

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suggested to develop a more uniform RF heating process (Wang, Zhang, Gao, Tang, & Wang, 2013). Several HARF nut drying and roasting studies, including cashew kernels

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(Liao, Zhao, Gong, Zhang, & Jiao, 2018) and peanuts (Zhang, Zhou, Ling, & Wang, 2016a), applied sample thickness as one of the treatment factors. Since the thickness of sample loads for a given product needs to be optimized for achieving the maximal RF power intensity, good product quality, and uniform heating (Tiwari et al., 2011), the

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effects of sample thickness on Oregon hazelnuts should be considered. In addition to the two factors stated above, other strategies for improving the drying efficiency and heating uniformity were also discussed, including the control of HA temperature (Wang, Zhang, Johnson, et al., 2014; Zhang et al., 2016), the movement of conveyor belt for placing the loading box (Wang, Zhang, Gao, et al., 2014), and the application of stirring for nuts

during RF pausing (Wang, Yue, Tang, & Chen, 2005). Recently, RF heating was also considered to combine with vacuum (Zhou, Xu, et al., 2018) and osmotic drying (Zhou, Li, Lyng, & Wang, 2018) for achieving fast drying and high quality of kiwifruit. HARF drying has been conducted on several types of tree nuts, including macadamia nuts (Wang et al., 2014a), walnuts (Zhang at al., 2016a), and almonds (Gao et

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al., 2010), but no study has been reported for hazelnuts. Hence, it is necessary to investigate the drying and cracking performance of HARF for developing scientific

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understanding and applicable procedures for hazelnut processing. Meanwhile, to achieve an synthetic effect of HA drying for lower shell cracking and HARF for faster drying

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rate, a 2-step HA-HARF drying using HA as a pre-drying treatment to lower MC of nuts

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to certain intermediate MC and then HARF drying to reach designated MC was conducted. This 2-step drying was considered as an effective strategy to simultaneously

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improve drying efficiency, reduce shell cracking, as well as lower energy consumption. This study was thus aimed to investigate the optimal HARF drying conditions on

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hazelnuts inshells for improving drying efficiency, heating uniformity, nut quality and reducing cracking ratio through two specific objectives: 1) to study the effect of electrode gap (EG) and sample thickness (ST) on intermittent HARF drying for two hazelnut cultivars (Barcelona and Jefferson); and 2) to identify the optimal intermediate MC (16,

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14 and 12%) of nuts in the 2-step HA-HARF drying for Jefferson inshells with two drying modes (intermittent and continuous) in consideration of drying characteristics and nut quality. It is anticipated that the results generated from this study would provide critical information on developing a practical protocol of HARF drying on hazelnuts with improved drying efficiency and preserved nut quality.

2. Material and Methods 2.1 Material Freshly harvested Oregon hazelnuts (Barcelona and Jefferson) were provided by the Oregon Hazelnut Marketing Board. Nuts were machine-harvested from early

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September to middle October 2018, washed and cleaned in a commercial processing line. They were then packed in large woven sacks and transported immediately to Oregon

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State University, Corvallis, OR, U.S.A. Samples were analyzed for MC of inshells and

kernels immediately. The sacks were then wrapped individually by a 0.127 mm thickness

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low density polyethylene bag (PlasticMill, New Jersey, U.S.A.) and stored at 1 °C

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refrigeration room till drying tests (within 6 months).

Chemical reagents were obtained from different manufacturers: Ethyl ether

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anhydrous from EMD Millipore (Billerica, MA, U.S.A.); Folin-Ciocalteu (FC) reagent, gallic acid and ammonium thiocyanate from Sigma-Aldrich (St. Louis, MO, U.S.A.);

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Chloroform, ferrous sulfate and sodium carbonate from Fisher Scientific (Hampton, NH, U.S.A.); 2, 2-diphenyl-1-picrylhydrazyl (DPPH) (95%), hexane, from Alfa Aesar (Ward Hill, MA, U.S.A.); Sodium hydroxide, L-ascorbic acid, ethanol, and methanol from MACRON, Avantor Performance Materials (Center Valley, PA, U.S.A.);

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Phenolphthalein from Mallinckrodt (Blanchardstown, Dublin, Ireland); and Cumene hydroperoxide from TCI America (Portland, OR, U.S.A.).

2.2 Drying apparatus and operation

A 6-kW, 27-MHz pilot-scale RF system (COMBI 6-S, Strayfield International, Wokingham, U.K.) equipped with a customized auxiliary hot-air system using a 5.6-kW electrical strip heater and a blower fan was used for HARF drying tests (Wang et al., 2010). The electrode gap (EG) distance was adjustable between 12 to 20 cm. Nut samples were placed in a rectangular plastic Teflon container with perforated bottom and top lid

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made of Nylon with inner dimensions (25.5 cm × 15.5 cm × 8 cm). The 600, 900 and 1200 g of fresh nuts representing approximately 3, 5 and 7 cm of sample thickness,

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respectively, were filled into the container for HARF processing. In each operation, the

sample-loaded container was placed at a fixed position above the bottom electrode, and

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constant hot-air circulation (38 °C, 1.3 m/s) was applied vertically through the container

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from the perforated bottom electrode. To achieve the stable air circulation for heating nut surface, hot air flow was conditioned in the empty RF cavity at least 1 h prior to the

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HARF treatment. A four-channel FISO fiber optic temperature measurement unit (UMI, FISO Technologies, Inc., Saint‐ Foy, Quebec, Canada) was used to monitor sample

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internal temperature distribution across the entire loads during drying, in which one sensor was hanged above the sample container for recording air temperature during heating, and other three sensors were inserted into the pre-drilled nuts which placed in the center of the top layer (Sensor 1, top-center), the center of the bottom layer (Sensor 2,

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bottom-center), and the corner of top layer (Sensor 3, top-corner), respectively (Fig. 1). Before starting the HARF, nuts were heated from ambient temperature to around 30 °C under the HA circulation, then RF power was turned-on and started counting as the first RF heating cycle. To reduce lipid oxidation during nut drying, the RF heating was immediately paused when any of the three sensors reached 49 °C. To ensure a consistent

heating cycle with 10 °C gradient, the next RF heating cycle was restarted when the internal sample temperature from all locations were all below 40 °C with the continuous air circulation. During each pause (< 10 min), nut samples were taken out from the RF unit, stirred manually for two to three times in the box right after taking the thermal image using an infrared camera from the top view of the sample box, and then measured

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MC of inshells and counted the cracking ratio. HARF drying was terminated when MC of inshells reached ~10% (wb). In terms of continuous HARF drying, the samples were

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continuously heated until nuts internal temperature reached ~80 °C, and samples were

taken out every 5 min for capturing thermal image photos on nut surface, measuring MC

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operations were conducted in duplication.

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of inshells, and counting cracking ratio (all were done within 5 min). All HARF drying

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2.3 Experimental design

Fig. 2 illustrates the scheme of this study. The first study was the optimization of

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intermittent HARF drying by considering electrode gap (EG) and sample thickness (ST) on Barcelona and Jefferson, respectively. A completely randomized design with 3 electrode gaps (14, 15 to 16 cm) and 3 sample thickness (3, 5 to 7 cm) were applied. The optimal HARF condition for each cultivar was selected considering a better drying

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performance with shorter drying time (total RF heating and total HARF time), less number of pauses, faster initial heating rate (°C/min), smaller uniformity index (UI, defined in session 2.5) and less cracking ratio (%) after HARF. In addition, quality parameters including MC, aw, color index (lightness, chroma and hue) (n =10) of kernel surface, oil recovery (%), peroxide value (PV) and free fatty acids (FA) of dried kernels

were compared among all nine intermittent HARF conditions. Significant differences among treatments were then analyzed by the least significant difference (LSD) test under one-way analysis of variance (ANOVA) analysis (P < 0.05) (n = 3). To evaluate and discuss the correlation of drying and cracking kinetics under the optimal HARF condition for each cultivar, the drying-cracking curve as a function of total HARF drying time was

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developed (session 3.1-3.4). Secondly, since Jefferson is the cultivar found mostly liable to shell cracking

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during HA drying (up to 50% cracking) in comparison with Barcelona (~10%), several drying methods and conditions were evaluated on Jefferson inshells with respect to the

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drying efficiency, heating uniformity, cracking ratio and nut quality. Specifically, HA at

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38 °C/60% RH and 43 °C/40% RH, optimized HARF from previous study, and 2-step HA-HARF, in which nuts were dried by HA first to reach a given MC of 16, 14, and

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12%, called intermediate MC, then continued for HARF using two modes (intermittent and continuous) till reaching inshell MC of ~10% were compared on their drying

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characteristics including drying time (defined in session 2.5), UI, and cracking ratio. To visualize the kinetic of drying and cracking among these drying conditions, the drying and cracking curves were provided. At the end, the nut quality attributes including MC, aw, color, oil recovery, lipid oxidation, total phenolic content (TPC) and antioxidant

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capacity (DPPH) of dried kernels were compared among the drying methods and conditions using the LSD test under one-way ANOVA analysis (P < 0.05) (session 3.5 – 3.7).

2.4 Selection of electrode gap range

Different electrode gap (EG) in the RF unit could lead to various electric current (I, A), thus influencing generated RF power (P, kW), as shown P = 5 × I − 1.5 from RF unit manufacturer (Jiao et al., 2012). For determining the appropriate electrode gap range for optimization, a method from Wang et al., (2014a) was followed. Briefly, an empty sample container or container filled with 900 g of nuts (from both cultivars) was placed

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between the two electrode plates individually. The initial electric current (I) was recorded immediately after the RF power was turned on at ambient temperature (~25 °C) without

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the air circulation. This procedure was repeated by reducing electrode gap from 20 to 12 cm, and the experiment was operated twice for each treatment, mean values were

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recorded for drafting the power (P)– electrode gap (EG) curves. Three intermediate

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electrode gaps inducing moderate RF power were selected for following optimization

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(Wang et al., 2014b).

2.5 Drying characteristics of nuts

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For each HARF operation, MC of inshells at each pausing point was analyzed using a non-destructive method by the Steinlite Moisture Tester (SB 900, SeeDWuro Equipment Co., Des Plaines, IL, U.S.A.). The time (min) of each RF heating cycle over entire HARF drying process was recorded and summed up as the total RF heating time

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(min). The total HARF time (min) included the sum of all RF heating and cooling cycles over an entire HARF drying process. Number of pauses was also counted for individual HARF operation. For each HARF condition, initial heating rate (°C/min) was defined as the mean internal sample temperature from all three nut sensors divided by the exact processing time for the first RF heating cycle.

During each HARF drying test, surface temperature of fresh nuts and the nuts at the pausing points were captured by a digital infrared camera (FLIR T400, FLIR Systems, Inc., North Billerica, MA, U.S.A.) with an accuracy of ± 2 °C. For visualizing the distribution of nut surface temperature in the container, the mean and standard deviation (SD) of the nut surface temperature before and after each HARF drying

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operation were recorded for calculating the heating uniformity index (UI). UI was defined as the ratio of the rise in SD of nut surface temperature (°C) to the rise in mean of

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nut surface temperature (°C) for the same loads during entire HARF processing, and

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expressed as (Wang et al., 2014a) :

(Eq. 1)

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where μ0 and μ was the initial and final mean of nut surface temperature for the sample

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loads (°C), respectively, and σ0 and σ was the initial and final SDs of nut surface temperatures for the sample loads (°C) over the entire HARF drying process,

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respectively. The smaller UI value commonly indicated a more uniform heating. Nut shells showing obvious opening on the surface (usually opening >0.3 mm)

was counted as a cracked nut. Cracking ratio (%) of nuts during HARF drying at each pause was calculated based on the percentage of number of cracked nuts among total 30

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randomly picked nuts. The cracking ratio (%) after drying was compared among different HARF treatments (methods and conditions). The drying and cracking kinetics of inshells under the optimal HARF condition for each cultivar were drafted in the same plot, respectively. To best interpret the drying kinetics of nuts with different MC, moisture ratio (MR) was used and calculated by the following equation (Onwude et al., 2016):

MR =

(Eq. 2)

where MCt (%,DW) indicated the MC of inshells at the time t (min or h), MCe (%, DW represented MC of inshells at the end of the drying (min or h), and MC0 (%, DW) indicated the initial MC of inshells. Both MR and cracking ratio (%) kinetics were plotted as a function of total HARF drying time (min) implying the information, such as the

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occurrence of different drying stages, the coordination of drying rate and cracking ratio and difference on drying-cracking correlation between two nut cultivars in terms of

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2.6.1. Physicochemical properties of kernels

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2.6 Quality attributes of nuts

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different MC and varied moisture distribution.

MC of kernels (%, fresh weight basis) was measured using the same non-

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destructive machine as mentioned above. aw of kernels was determined by an electronic hygrometer (Aqua Lab 3 TE, Decagon Devices Inc., Pullman, WA, U.S.A.).

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The color index of kernel surface was analyzed using ten randomly picked kernels

which was measured by a portable colorimeter (Minolta Chroma Meter CR‐ 410, Konica Minolta Holdings, Tokyo, Japan) calibrated by a white board (L* = 98.4, a* = +0.1, b* =

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+1.4). Lightness (L*), redness (a*) and yellowness (b*) were recorded, and chroma (C*, color saturation) and hue (H*, color purity) values were calculated (Drogoudi et al., 2016).

2.6.2. Oil recovery and lipid oxidation of kernels

Kernel oil was extracted by hexane (1:10, w/v) and condensed through a rotary evaporator at 40 °C (Brinkmann Instruments, Westbury, NY, U.S.A.). Oil recovery (%) from kernels was calculated as a weight percentage (w/w, %) of extracted oil (g) from a total powdered kernel weight (g) (Özkal et al., 2005). Peroxide value (PV) presents primary hydroperoxide compounds from lipid

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oxidation, and free fatty acids (FA) indicates the rancidity of oxidized products from both oxidative and enzymatic rancidity, as well as the accumulation of natural fatty acid

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composition in the kernel oils. PV and AV were both analyzed for determining the level of lipid oxidation for kernel samples. Specifically, PV was determined by a modified

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iron-based spectrophotometric method (Shantha and Decker, 1994). Briefly, 500 mg of

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oil sample was mixed with 5.5 mL of methanol-chloroform mixture solution (1:2, v/v). A 50 μL of 3.94 mol/L ammonium thiocyanate and 50 μL of 0.072 mol/L ferrous sulfate

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solution were added in order for initiating the reaction, then the mixtures were vortexed and rested for 20 min in the ambient temperature, following with the absorbance

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measurement at 510 nm using a UV/Vis spectrophotometer (UV160U, Shimadzu Corporation, Kyoto, Japan). PV was calculated using an external standard curve with cumene hydroperoxide hexane solution (0, 0.008, 0.024, 0.04 and 0.08 mg/mL), and transferred to a unit of meq O2/ kg oil with a conversion number based on Steltzer (2012).

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Free fatty acids (FA) was estimated using a modified titration method by Eddy et

al. (2011). Briefly, 500 mg of oil sample was mixed with 8 mL of ethanol-diethyl ether mixture (1:1, v/v). A few drops of 1% (w/v) phenolphthalein ethanol solution was added as a pH indicator. The mixture was titrated with 0.01 mol/L NaOH till the pink color appeared and lasted for at least 10 s with stirring. Results were reported as % oleic acid.

In addition, K232 and K270 indicating the conjugated diene and triene accumulated in oil samples during oxidation were also evaluated for comparing kernel oil stability from all drying methods. The conjugated compounds could be formed during the propagation step of lipid oxidation (intermediate products) that was able to be identified at wavelengths of 232 and 270 nm (Shahidi & Zhong, 2010). Thus, 100 mg of oil sample

the UV spectrophotometer. K values were calculated as: (Eq. 3)

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K=

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was diluted into 10 mL (1%, w/v) solution with hexane and read at two wavelengths by

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where E was the absorbance of the solution at 232 or 270 nm, [c] was the concentration

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of oil (g/100 mL), and l was the length of the quartz cuvette (cm).

2.6.3. Bioactive compounds and antioxidant activity of kernels

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Total phenolic content (TPC) was analyzed according to the modified FolinCiocalteu (FC) method from Singleton & Rossi (1965). Briefly, 0.5 mL of extracts or

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standard solution were mixed with 0.5 mL of FC reagent and 7.5 mL of distilled water. The mixtures were vortexed and rested for 20 min in the ambient temperature. After adding 3 mL of 20% (w/v) sodium carbonate solution, the mixture was further incubated

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at 40 °C water bath for 20 min. The reaction mixture was then transferred into an ice bath for 3 min. The absorbance was read at 765 nm. TPC was calculated using an external standard curve with gallic acid aqueous solution (0, 0.1, 0.3, 0.5, 0.7 and 1.0 mg/mL). Results were reported as mg gallic acid equivalent (GAE)/g samples (Dry weight basis, DW).

DPPH antioxidant activity was analyzed by a modified method from Bondet et al. (1997). Briefly, 0.5 mL of extracts or standard solution were mixed with 2 mL of DPPH reagent (9 mg of DPPH in 100 mL methanolic solution), vortexed, and reacted under dark conditions for 20 min. The absorbance was read at 517 nm. DPPH was calculated using an external standard curve with ascorbic acid aqueous solution (0, 0.01, 0.03, 0.05, 0.07

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and 0.1 mg/mL). Results were reported as mg ascorbic acid equivalent (AAE) /g samples

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(DW).

2.7 Statistical analysis

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All measurements were conducted in triplicate except the color index (n=10).

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The data were reported in mean value and SD. The difference among treatments for individual quality attribute was subjected a one-way ANOVA and means were separated

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a significance level of 0.05.

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using LSD post-hoc test with SAS v9.2 (The SAS Institute, Cary, NC, USA) at

Results and discussion

3.1 Correlation of electrode gap and generated RF power As shown in Fig. 3, the smaller electrode gap (EG) could generate higher RF

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power (P). Adding the loading of nuts elevated the RF power when EG was less than 16 cm. This was because under a more intense electrode field from smaller EG, the presence of the nuts changed the overall impedance of the RF applicator to draw more RF power from the generator (Sun, 2005b). This trend of P-EG correlation is consistent with other HARF nut optimization studies, showing that loading of samples boosted the RF power

and the RF power was significantly higher at the smaller electrode gaps (Wang et al., 2014a; Zhang et al., 2016a). In this study, a slight variation of RF power at the EG range of 13-16 cm between the two cultivars was observed, which could be caused by the different dielectric properties (DPs) and heat capacities in a AC field of the two nut cultivars due to their different chemical compositions (Wang et al., 2011). To obtain a

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stable heating rate from RF treatment, 14, 15 and 16 cm electrode gap were thus chosen

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in further tests with a moderate RF power.

3.2 Effect of electrode gap and sample thickness on drying characteristics of inshells

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The data in Table 1 summarize drying characteristics of two nut cultivars under

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nine intermittent HARF conditions with different electrode gap (EG) and sample thickness (ST). For Barcelona, total RF heating time over all cycles was 20.9-54.7 min

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with a total HARF drying time of 100.5-141.3 min. Among the treatments, the HARF drying with smaller EG (14 cm with all STs, and 15 cm EG with 5 or 7 cm ST) resulted

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in shorter total RF heating time (< 30 min). And the shortest total HARF drying time (100.5 min) was obtained at 15 cm EG/5 cm ST. On an average, over 10 pauses were necessary for drying Barcelona using intermittent HARF, and less pauses were used under HARF conditions with 16 cm EG. In addition, it was observed that the faster initial

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heating rate (> 4 °C/min) occurring at the intermittent HARF conditions with short total RF heating time. Similar findings were reported in HARF drying on inshell almonds, in which a fast heating rate and shorter drying time was found when using a smaller EG (Li et al., 2017). The short RF and HARF drying time with more pauses as well as faster initial heating under smaller EG could attribute to a greater electric field intensity (E) in

the electromagnetic field which negatively correlated with the electrical potential between the electrodes (Jiao et al., 2014; Marra et al., 2009). For almost all HARF conditions tested in this study, a consistent heating pattern was also observed: the fastest heating rate was achieved at sensor 3 (top-corner), followed by sensor 1 (top-center), while the slowest heating rate was found at sensor 2 (bottom-center) (data not shown).

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The corner hot-spots under HARF were also reported in other studies, such as drying inshell almonds (Li et al., 2017) and soybeans (Huang et al., 2015). Further studies for

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improving heating uniformity using HARF drying of nuts are necessary for further

development. Based on the UI values, the most uniform heating (UI = 0.037) was also

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achieved at 15 cm EG/5 cm ST. Meanwhile, the significantly low cracking ratio (~ 40 to

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50%) at the end of drying was obtained at 14 cm EG/3 or 5 cm ST and 15 cm EG/5 cm ST conditions (P < 0.05). Overall, 15 cm EG/5 cm ST could induce the best drying

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characteristics on Barcelona inshells.

For Jefferson, the total RF heating time under all HARF conditions ranged from

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31.0 to 87.7 min with total HARF drying time of 180.9 – 237.8 min. Among all nine HARF conditions, like Barcelona, the shorter total RF heating time (< 40 min) for drying Jefferson under intermittent HARF was achieved at all 14 cm EG group and 15 cm EG/7 cm ST condition. The least total HARF drying time (180.9 min) was obtained at 15 cm

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EG/7 cm ST. Average number of pauses was over 20 under intermittent HARF for Jefferson. Since an RF heating rate of 5–10 °C/min is desirable considering the heating efficiency and uniformity (Jiao et al., 2016), only 14 cm EG/7 cm ST (6.7 °C/ min) and 15 cm EG/7 cm ST (5.1 °C/min) conditions could reach the acceptable initial RF heating rate for Jefferson. Therefore, there was no clear coordination among EG or ST and

number of pauses and initial heating rate for drying Jefferson using HARF. The most uniform heating under intermittent HARF drying was obtained at 14 cm EG/5 or 7 cm ST and 15 cm EG/3 or 5 cm ST conditions (UI < 0.040), while the lowest cracking ratio was at 14 cm EG/3 cm ST and 15 cm EG/7 cm ST conditions (~45 %). Overall, the optimal intermittent HARF condition for Jefferson was 15 cm EG/7 cm ST.

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Generally, it was found that the application of the smaller EG and larger ST could facilitate faster RF drying due to elevated RF power and increased DPs resulted from

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more loads of the samples (Ozturk et al., 2018). Under the optimal HARF drying

conditions, it took 100.5 or 180.9 min to finish the drying process for Barcelona and

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Jefferson nuts, respectively (Table 1), similar time to the walnut study reported

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previously, in which 100 min of total drying time was necessary to dry 1.6 kg of walnuts with initial MC of 20% at 18 cm EG (Zhang, Zheng, Zhou, Huang, & Wang, 2016b).

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Between the two cultivars, Jefferson required longer RF heating and total HARF drying time, as well as a greater number of pauses than Barcelona, which was probably due to

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the higher initial MC of Jefferson nuts (21%) requiring longer time for mass and heat transfer. Previous RF heating studies also reported various nuts-dependent optimal RF conditions (Wang et al., 2010; Wang et al., 2014a), and some even provided specific RF settings (EG and ST) for drying inshells and kernels of the same nut type (Gao et al.,

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2010).

3.3 Effect of electrode gap and sample thickness on quality attributes of dried kernels

Table 2 reports the quality attributes of dried kernels from all nine intermittent HARF conditions. ANOVA analysis showed significant differences among treatments on MC, color index of kernel surface and PV of kernel oils (P < 0.05) for both cultivars. For Barcelona, significantly lower MC of kernels (<6.3%) was found on nuts dried under 14 cm EG/7 cm ST, and 15 or 16 cm EG/5 cm ST conditions. Significantly

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higher lightness (L*) and chroma (C*) of dried kernel surface were found on samples dried under 15 cm EG/7 cm ST, 16 cm EG/5 or 7 cm ST conditions (P < 0.05), while

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higher hue (H*) of dried kernel was observed on nuts from 14 cm EG/5 cm ST, 15 cm

EG/3 or 5 cm ST, and 16 cm EG/3 cm ST conditions. Significantly lower PV of kernel

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oils was observed on nuts dried under 14 cm EG/3 cm ST, 15 cm EG/3 or 5 cm ST, and

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16 cm EG/7 cm ST conditions. Hence, 15 cm EG/5 cm ST was confirmed as the optimal intermittent HARF conditions for Barcelona to retain the nut quality with lower MC,

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higher hue and lower PV.

For Jefferson nuts, significantly lower MC of dried kernels (<7.7%) was found

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under 16 cm EG/3 or 7 cm ST. Higher L* and C* values of kernels were achieved from all HARF treatments except at 15 cm EG/5 cm ST. In contrast, 15 cm EG/5 cm ST induced significantly higher H*. Significantly low PV was obtained at 15 cm EG/7 cm ST and 16 cm EG/3 cm ST conditions. Therefore, the optimal HARF condition for

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Jefferson was confirmed as 15 cm EG/7 cm ST for preserving nut quality with higher L* and C* and lower PV, the same condition for achieving better drying performance as reported in the former session. Results on HARF drying characteristics and nut quality overall demonstrated that an optimal HARF condition with faster drying rate and better heating uniformity could

also ensure nut quality with a lower MC, better color quality, and lower lipid oxidation. Between the two cultivars, although drying was both ended when MC of inshells reached ~10%, HARF dried Jefferson kernels had higher MC and aw along with less oil recovery and higher PV, which was probably caused by its higher initial kernel MC and aw facilitating oxidative (López et al., 1997a) and enzymatic rancidity (López et al., 1997b)

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during the drying process. The walnut drying study also reported significant lower oxidation level of dried kernels using HARF than that using HA due to significantly

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reduced drying time from 240 min (HARF drying) to 100 min (HR drying) (Zhang et al., 2016b). Moreover, the study on cashew nuts indicated the benefits of HARF drying for

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retaining sensory quality of dried cashew nuts (Liao et al., 2018).

3.4 Coordination of drying and cracking ratio of nuts during optimal intermittent

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HARF drying conditions

Fig. 4 shows the reduction of MC and increase in cracking rates the drying of

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hazelnuts under the optimal intermittent HARF conditions for each cultivar. To better illustrate the drying kinetics throughout the entire drying process, two drying stages were proposed depending on the drying and cracking rate, calculated from the slope of the MR and cracking ratio (%) curves. The fastest drying rate (0.248% MC (DW)/ min for

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Barcelona and 0.344% MC (DW)/min for Jefferson) was observed in stage I at the beginning of HARF process owning to the quick removal of free water from wet nuts. In stage II, reduced drying rate (0.054% MC (DW)/min for Barcelona and 0.041% MC (DW)/min for Jefferson) was found comparing with stage I as the vapor from kernels started to transfer through shells to outside. At the end of stage II, a further reduced

drying rate (0.0042% MC (DW)/min for Barcelona and 0.021% MC (DW)/min for Jefferson) was observed, presumably because the final remaining water in the dried samples was in form of bound water and became difficult to be removed (Zhang et al., 2016a). In addition, Barcelona had ~40% reduction of MR in the stage I and ~60% in the stage II, while Jefferson had ~60% reduction of MR in the stage I and ~40% in the stage

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II. The reason could be the higher initial MC and more loads of sample in HARF drying for Jefferson than Barcelona, which led to higher RF energy generated for heating nuts.

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No shell cracking was found in the stage I for Barcelona, but it rose to ~50% at the end of stage II, while Jefferson had ~10% cracking in stage I and reached to ~50% at the end of

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stage II.

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For both cultivars, most of cracks occurred during stage II when the free water of shells almost all evaporated and the moisture started to transfer through the shells. It is

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likely that during Stage II, excessive vapor pressure was formed within the voids between kernel and shell causing mechanical stresses in the shells with the removal of moisture

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from shell. There was a clear trend of the correlation between drying rate and cracking ratio, in which severer cracking (average cracking rate of 0.72%/min between 33.27 and 73.17 min for Barcelona, and 0.51%/min between 25.27 and 90.23 min for Jefferson) occurred with faster drying rate (0.079% MC (DW)/min between 33.27 and 73.17 min for

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Barcelona, and 0.066% MC (DW)/min between 25.27 and 90.23 min for Jefferson). This trend also confirmed our former hypothesis on mechanism of drying-induced cracking (Wang et al., 2018b). Although the final cracking ratio between two cultivars was similar (~50%), it was found that Jefferson started cracking earlier than Barcelona did. This phenomenon

might attribute to the varied initial MC distribution in fresh shell and kernel between the two cultivars (Data not shown). The higher MC in fresh Jefferson kernels led to higher power absorption and dissipation from RF, thus building more vapor pressure than Barcelona. Poulin et al. (1997) also reported that uneven initial moisture distribution within a mineral and cellulosic material could induce a slower drying with increased non-

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uniform heating from HARF. Hazelnut drying is a complex process involving solid, liquid and gas phases, the heat and mass transfer should be further investigated for

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precise explaining the drying phenomena under HARF processing.

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3.5 Effects of intermediate MC and drying modes in the 2-step HA-HARF drying on

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the drying characteristics of nuts

Table 3 summarizes the drying characteristics of nuts from HARF and the 2-step

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HA-HARF drying with three different intermediate MC (16, 14 and 12%) using two drying modes (intermittent and continuous). In terms of drying time, the 2-step HA-

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HARF drying at 12% intermediate MC significantly reduced total RF heating time to 0.1 h and total HARF drying time to 0.2 – 0.3 h for both drying modes. Between two drying modes, intermittent drying generally doubled the total HARF drying time plus the addition of cooling cycles with better heating uniformity (low UI), while the continuous

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mode induced faster drying and elevated the nut internal temperature to 80-100 °C within 2 min with less heating uniformity (higher UI) (Table 3). This phenomenon was also shown in another HARF drying study for coffee beans having an uneven heating while faster heating rate (Pan et al., 2012). The advantage of intermittent RF drying was discussed by Kumar et al. (2014), in which pausing the input of energy for heating allowed moisture transferring from sample center to surface during the cooling cycles,

thus improving the heating uniformity through a redistribution of heat for the next heating cycle. In terms of cracking ratio, the 2-step HA-HARF drying at 12% intermediate MC using both drying modes resulted in significantly low cracking ratio (<15%), and there was no significant difference between two drying modes. However, Jefferson inshells under two drying modes of HARF and the 2-step

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HA-HARF at 16 and 14% intermediate MC induced significant high cracking ratio (~3060%), which could be caused by the high initial MC of inshells and kernels promoting the

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cracking. The study on RF drying of peanuts also indicated an increased heating rate for kernels with higher MC (Zhang et al., 2016b). Between the two drying modes, the

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frequent heating and cooling cycles in the intermittent HARF for the moist samples with

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same MC contributed to regular formation of vapor within the voids, thus increasing

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cracking during the drying process in comparing to the continuous mode.

3.6 Comparison of drying and cracking performances among different drying

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methods

Fig. 5 shows drying and cracking curves over the entire drying processes for total

10 different drying methods/conditions (HA, HA43C, HARF and 2-step HA-HARF at 3 different intermediate MC and two modes). Note that HA43C, hot-air drying at 43

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°C/40% RH, was included in this session since it was the condition recommended from our previous study inducing the fastest drying rate for all four Oregon hazelnut cultivars without considering the cracking issue (Wang et al., 2018a). It was thus used as a negative control for comparing with HARF and 2-step HA-HARF drying methods.

Obviously, all HARF drying and 2-step HA-HARF drying conditions resulted in shorter total drying time (1.1-11 h) compared to HA drying (22 h) and HA43C drying (12 h). The application of HARF or using it in the later stage of 2-step HA-HARF drying took the drying time generally from 0.2 h to 3 h. Between the two HARF drying modes, all continuous HARF treatments had similar drying rates, represented by the slope of the

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MR curve. However, the higher MC of nuts in the intermittent HARF processing induced a slower drying rate, probably caused by reduced heating uniformity from uneven

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moisture distribution in the fresh (or HA pre-dried) nuts (Poulin et al., 1997). In addition, continuous HARF or 2-step HA-continuous HARF drying required much shorter drying

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time than that of intermittent HARF and 2-step HA-intermittent HARF since none

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cooling cycle was applied.

Unfortunately, cracking issue was not resolved by using HARF drying only and

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most of the 2-step HA-HARF drying conditions in comparison with HA (7.4%) and HA43C (29.6%) except for the 2-step HA-HARF drying at 12% intermediate MC using

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both drying modes (cracking <15%). To reduce cracking during a HARF process, low amount of MC remaining in samples from the pre-HA drying is necessary and more uniform heating could reach during the subsequent HARF (Piyasena et al., 2003). Between the two drying modes, the continuous HARF or 2-step HA-continuous HARF

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consistently induced faster drying with less cracking than that under the intermittent mode. While this finding was contradictory to the higher UI under continuous mode of processing in the former session, it could be explained as that the cracking under HARF processing might have been caused by the temperature gradient in the shells during the

frequently cooling and reheating cycles, rather than variations among nut surface temperature (i.e., hot spots on the corners of the sample containers). In general, use of 2-step HA-HARF drying method at 12% intermediate MC with both drying modes was mostly suggested for drying Jefferson inshells with low cracking. This result complemented the recommendation by Jones & Rowley (1996) to operate HA

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first, then RF for achieving a more efficient drying process. Jones & Rowley (1996) also

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suggested to control RF energy input within 10-20% of total energy input.

3.7 Comparison of dried nut quality among different drying methods

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Table 4 compares the nut quality among fresh, hot-air (HA and HA43C), HARF

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(intermittent or continuous mode), 2-step HA-HARF (16, 14 and 12% intermediate MC, intermittent or continuous mode) dried samples. Based on one-way ANOVA analysis (P

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< 0.05), significant differences were observed among effects of the different treatments on all quality parameters except for the K232 value. Specifically, application of HARF or

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2-step HA-HARF drying method resulted in significantly lower MC (< 8%) of dried kernels than that from HA (8.4%), but in same significant level with that from HA43C (~7%) drying method. In contrast, a previous study reported RF treatment resulted in a higher MC of dried almond kernels than that of HA drying (Gao et al., 2010). The

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intermittent HARF and 2-step HA-HARF with two drying modes resulted in significantly reduced aw (< 0.70) in comparison with HA drying (0.7-0.8). These results demonstrated that HARF drying of Jefferson could improve drying efficiency as kernel of this cultivar was commonly found having higher MC than requirement even after the inshell nuts already reached the targeted drying end point (~10% MC) (Table 5). The same result was

also found in our former study using HA drying of hazelnuts (Wang, Jung, McGorrin, Traber, et al., 2018). For color index, HA, intermittent HARF, and HA-intermittent HARF drying at 12 and 16% intermediate MC all retained high kernel surface L* value and was similar to that of fresh kernels. Significantly higher C* value was obtained in HA dried nuts, followed by intermittent HARF dried nuts. Significantly higher H* value

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was found on kernels dried by HA43C, continuous HARF, HA-intermittent HARF at 14% intermediate MC and all HA-continuous HARF drying conditions (P< 0.05).

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Significant high oil recovery was obtained in nuts dried by HA and all 2-step HA-HARF drying conditions. HA or 2-step HA-HARF drying at 12% intermediate MC produced

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nuts with higher PV. In other words, significantly lower PV was achieved by HA43C,

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HARF with both drying modes, HA-intermittent HARF at 16% intermediate MC, and HA-continuous HARF drying at 16 and 14% intermediate MC conditions. Meanwhile,

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significantly lower FA was found in dried nuts from all drying methods except for the intermittent HARF drying. Although K232 was not significant different among the

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treatments (P > 0.05), significantly lower K270 was found in the dried nuts from HA, continuous HARF and all 2-step HA-HARF drying. It was obvious that the continuous mode of HARF and HA-continuous HARF produced the nuts with higher oil stability, which could attribute to the shorter drying time and higher drying efficiency with the

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application of HARF. For bioactive compounds, all drying conditions, except those from HA-intermittent HARF at 14% intermediate MC and HA-continuous HARF at 14 and 12% intermediate MC, retained higher TPC. Significantly higher DPPH of dried kernels was found in nuts from continuous HARF drying followed by HA-intermittent HARF

drying at 14% intermediate MC, and all HA-continuous HARF drying conditions, which was similar to the DPPH of fresh nuts. Overall, continuous HARF and 2-step HA-HARF drying methods could retain better nut quality with high DPPH and less lipid oxidation. While the use of 12% intermediate MC for 2-step HA-HARF drying led to significantly lower MC and aw, it

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resulted in less oil stability and low phenolics and antioxidant capacity, probably due to the longer drying time from HA pre-drying process. Similarly, previous study on walnut

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found no significant difference on lipid oxidation using same time of HARF and HA

drying, but longer drying time from HA contributing to the oxidation of nuts (Zhang et

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al., 2016b).

Conclusion

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This study generated significant new information on hot-air assist RF (HARF) drying of hazelnuts. It was found that the intermittent HARF drying at 15 cm electrode

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gap and 5 cm sample thickness for Barcelona nuts and 15 cm electrode gap and 7 cm sample thickness for Jefferson nuts were the optimal conditions to obtain high drying efficiency and heating uniformity, good nut quality, and low shell cracking. Among all tested drying conditions, the 2-step HA-HARF drying using 12% MC as intermediate

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MC could reduce shell cracking to less than 15% along with improved heating uniformity for Jefferson nuts. Furthermore, continuous HARF mode resulted in fast drying rate, and the continuous HARF and 2-step HA-HARF drying produced nuts with better kernel quality. However, the longer HA pre-drying in the 2-step HA-HARF drying might lead to undesirable lipid oxidation. Therefore, different drying conditions should be considered

depending on the main purposes of drying, i.e., fast drying rate, low shell crack, or better nut quality. Studies to further improve heating uniformity and reduce shell cracking of hazelnut during HARF drying are in progress. These studies include applying different hot-air temperatures, and adjustment of electron gap distance and intermittent stirring of nuts during drying. Meanwhile, comparison on the storability of hazelnuts from different

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drying methods (HA, HARF and 2-step HA –HARF) will be studied. Furthermore,

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inactivation of concerned microorganisms in hazelnuts using HARF is investigated in the

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authors’ lab.

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Declaration of interests

The authors declare that they have no known competing financial interests or personal

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Acknowledgements

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relationships that could have appeared to influence the work reported in this paper.

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The authors expressed their gratitude for the financial support from the Oregon

Department of Agriculture Specialty Crop Block Grant and Oregon Hazelnut Marketing

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Board, and the donation of hazelnuts from Oregon hazelnut growers.

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References Bondet, V., Brand-Williams, W., & Berset, C. (1997). Kinetics and Mechanisms of Antioxidant Activity using the DPPH.Free Radical Method. LWT - Food Science and Technology, 30(6), 609–615. Drogoudi, P., Pantelidis, G. E., Goulas, V., Manganaris, G. A., Ziogas, V., & Manganaris, A. (2016). The Appraisal of Qualitative Parameters and Antioxidant Contents During Postharvest Peach Fruit Ripening Underlines the Genotype Significance. Postharvest Biology and Technology, 115, 142–150. Eddy, E. O., Ukpong, J. A., & Ebenso, E. E. (2011). Lipids Characterization and Industrial Potentials of Pumpkin Seeds (Telfairia occidentalis) and Cashew Nuts (Anacardium occidentale). Gao, M., Tang, J., Wang, Y., Powers, J., & Wang, S. (2010). Almond Quality as Influenced by Radio Frequency Heat Treatments for Disinfestation. Postharvest Biology and Technology, 58(3), 225–231. Huang, Z., Zhu, H., Yan, R., & Wang, S. (2015). Simulation and Prediction of Radio Frequency Heating in Dry Soybeans. Biosystems Engineering, 129, 34–47. Jiao, S., Johnson, J. A., Tang, J., & Wang, S. (2012). Industrial-Scale Radio Frequency Treatments for Insect Control in Lentils. Journal of Stored Products Research, 48, 143–148. Jiao, S., Zhu, D., Deng, Y., & Zhao, Y. (2016). Effects of Hot Air-assisted Radio Frequency Heating on Quality and Shelf-life of Roasted Peanuts. Food and Bioprocess Technology, 9(2), 308–319. Jiao, Y., Tang, J., & Wang, S. (2014). A New Strategy to Improve Heating Uniformity of Low Moisture Foods in Radio Frequency Treatment for Pathogen Control. Journal of Food Engineering, 141, 128–138. Jiao, Y., Tang, J., Wang, Y., & Koral, T. L. (2018). Radio-Frequency Applications for Food Processing and Safety. Annual Review of Food Science and Technology, 9(1), null. Jones, P. L., & Rowley, A. T. (1996). Dielectric Drying. Drying Technology, 14(5), 1063–1098. Jumah, R. (2005). Modelling and Simulation of Continuous and Intermittent Radio Frequency-Assisted Fluidized Bed Drying of Grains. Food and Bioproducts Processing, 83(3), 203–210. Kumar, C., Karim, M. A., & Joardder, M. U. H. (2014). Intermittent Drying of Food Products: A Critical Review. Journal of Food Engineering, 121, 48–57. Li, R., Kou, X., Cheng, T., Zheng, A., & Wang, S. (2017). Verification of Radio Frequency Pasteurization Process for In-Shell Almonds. Journal of Food Engineering, 192, 103–110. Liao, M., Zhao, Y., Gong, C., Zhang, H., & Jiao, S. (2018). Effects of Hot Air-Assisted Radio Frequency Roasting on Quality and Antioxidant Activity of Cashew Nut Kernels. LWT, 93, 274–280.

Jo

ur na

lP

re

-p

ro

of

López, A., Piqué, M. T., Boatella, J., Parcerisa, J., Romero, A., Ferrá, A., & Garcí, J. (1997a). Influence of Drying Conditions on the Hazelnut Quality. I. Lipid Oxidation. Drying Technology, 15(3–4), 965–977. López, A., Piqué, M. T., Boatella, J., Parcerisa, J., Romero, A., Ferrá, A., & Garcí, J. (1997b). Influence of Drying Conditions on the Hazelnut Quality. II. Enzymatic Activity. Drying Technology, 15(3–4), 979–988. Marra, F., Zhang, L., & Lyng, J. G. (2009). Radio Frequency Treatment of Foods: Review of Recent Advances. Journal of Food Engineering, 91(4), 497–508. Mujumdar, A. S. (2014). Microwave and Dielectric Drying. In Handbook of Industrial Drying. CRC Press. Onwude, D. I., Hashim, N., & Chen, G. (2016). Recent Advances of Novel Thermal Combined Hot Air Drying of Agricultural Crops. Trends in Food Science & Technology, 57, 132–145. Ozay, G., Seyhan, F., Pembeci, C., Saklar, S., & Yilmaz, A. (2008). Factors Influencing Fungal and Aflatoxin Levels in Turkish Hazelnuts (Corylus Avellana L.) During Growth, Harvest, Drying and Storage: A 3-Year Study. Food Additives & Contaminants: Part A, 25(2), 209–218. Özilgen, M., & Özdemir, M. (2001). A Review on Grain and Nut Deterioration and Design of the Dryers for Safe Storage with Special Reference to Turkish Hazelnuts. Critical Reviews in Food Science and Nutrition, 41(2), 95–132. Özkal, S. G., Salgın, U., & Yener, M. E. (2005). Supercritical Carbon Dioxide Extraction of Hazelnut Oil. Journal of Food Engineering, 69(2), 217–223. Ozturk, S., Kong, F., Singh, R. K., Kuzy, J. D., Li, C., & Trabelsi, S. (2018). Dielectric Properties, Heating Rate, and Heating Uniformity of Various Seasoning Spices and Their Mixtures With Radio Frequency Heating. Journal of Food Engineering, 228, 128–141. Pan, L., Jiao, S., Gautz, L., Tu, K., & Wang, S. (2012). Coffee Bean Heating Uniformity and Quality as Influenced by Radio Frequency Treatments for Postharvest Disinfestations. Transactions of the ASABE, 55(6), 2293–2300. Piyasena, P., Dussault, C., Koutchma, T., Ramaswamy, H. S., & Awuah, G. B. (2003). Radio Frequency Heating of Foods: Principles, Applications and Related Properties--A Review. Critical Reviews in Food Science and Nutrition, 43(6), 587–606. Poulin, A., Dostie, M., Proulx, P., & Kendall, J. (1997). Convective Heat and Mass Transfer and Evolution of the Moisture Distribution in Combined Convection and Radio Frequency Drying. Drying Technology, 15(6–8), 1893–1907. Shahidi, F., & Zhong, Y. (2010). Lipid Oxidation and Improving the Oxidative Stability. Chemical Society Reviews, 39(11), 4067–4079. Shantha, N. C., & Decker, E. A. (1994). Rapid, Sensitive, Iron-Based Spectrophotometric Methods for Determination of Peroxide Values of Food Lipids. Journal of AOAC International, 77(2), 421–424.

Jo

ur na

lP

re

-p

ro

of

Singleton, V. L., & Rossi, J. A. (1965). Colorimetry of Total Phenolics with Phosphomolybdic-Phosphotungstic Acid Reagents. American Journal of Enology and Viticulture, 16(3), 144–158. Steltzer, E. T. (2012). Evaluation of Chemical Assays for Determining Hydroperoxides Levels in Oxidized Lipids (Rutgers University - Graduate School - New Brunswick). Sun, D.-W. (2005a). Radio-Frequency Processing. In Emerging Technologies for Food Processing (pp. 445–468). Retrieved from http://ebookcentral.proquest.com/lib/osu/detail.action?docID=269950 Sun, D.-W. (2005b). Thermal Food Processing: New Technologies and Quality Issues. Taylor & Francis. Tiwari, G., Wang, S., Tang, J., & Birla, S. L. (2011). Analysis of Radio Frequency (RF) Power Distribution in Dry Food Materials. Journal of Food Engineering, 104(4), 548–556. USDA. (2016, August). Filbert/Hazelnut Kernels and Filberts in the Shell. Retrieved from https://www.ams.usda.gov/sites/default/files/media/FilbertHazelnut_Inspection_I nstructions%5B1%5D.pdf USDA. (n.d.). Grades and Standards. Retrieved August 5, 2018, from Oregon Hazelnut Industry website: http://oregonhazelnuts.org/buy-hazelnuts/wholesale/grades-andstandards/ Wang, S., Tiwari, G., Jiao, S., Johnson, J. A., & Tang, J. (2010). Developing Postharvest Disinfestation Treatments for Legumes Using Radio Frequency Energy. Biosystems Engineering, 105(3), 341–349. Wang, S., Yue, J., Tang, J., & Chen, B. (2005). Mathematical Modelling of Heating Uniformity for In-Shell Walnuts Subjected to Radio Frequency Treatments with Intermittent Stirrings. Postharvest Biology and Technology, 35(1), 97–107. Wang, W., Jung, J., McGorrin, R. J., Traber, M. G., Leonard, S. W., Cherian, G., & Zhao, Y. (2018). Investigation of Drying Conditions on Bioactive Compounds, Lipid Oxidation, and Enzyme Activity of Oregon Hazelnuts (Corylus avellana L.). LWT, 90, 526–534. Wang, W., Jung, J., McGorrin, R. J., & Zhao, Y. (2018). Investigation of the Mechanisms and Strategies for Reducing Shell Cracks of Hazelnut (Corylus avellana L.) in Hot-air Drying. LWT - Food Science & Technology, 98, 252–259. Wang, Y., Li, Y., Wang, S., Zhang, L., Gao, M., & Tang, J. (2011). Review of Dielectric Drying of Foods and Agricultural Products. International Journal of Agricultural and Biological Engineering, 4(1), 1–19. Wang, Y., Zhang, L., Gao, M., Tang, J., & Wang, S. (2013). Temperature- and MoistureDependent Dielectric Properties of Macadamia Nut Kernels. Food and Bioprocess Technology, 6(8), 2165–2176.

Jo

ur na

lP

re

-p

ro

of

Wang, Y., Zhang, L., Gao, M., Tang, J., & Wang, S. (2014). Pilot-Scale Radio Frequency Drying of Macadamia Nuts: Heating and Drying Uniformity. Drying Technology, 32(9), 1052–1059. Wang, Y., Zhang, L., Johnson, J., Gao, M., Tang, J., Powers, J. R., & Wang, S. (2014). Developing Hot Air-Assisted Radio Frequency Drying for In-shell Macadamia Nuts. Food and Bioprocess Technology, 7(1), 278–288. Zhang, B., Zheng, A., Zhou, L., Huang, Z., & Wang, S. (2016). Developing Hot AirAssisted Radio Frequency Drying for In-Shell Walnuts. Emirates Journal of Food and Agriculture, 459–467. Zhang, S., Zhou, L., Ling, B., & Wang, S. (2016). Dielectric Properties of Peanut Kernels Associated with Microwave and Radio Frequency Drying. Biosystems Engineering, 145, 108–117. Zhou, X., Gao, H., Mitcham, E. J., & Wang, S. (2018). Comparative Analyses of Three Dehydration Methods on Drying Characteristics and Oil Quality of In-Shell Walnuts. Drying Technology, 36(4), 477–490. Zhou, X., Li, R., Lyng, J. G., & Wang, S. (2018). Dielectric Properties of Kiwifruit Associated With A Combined Radio Frequency Vacuum and Osmotic Drying. Journal of Food Engineering, 239, 72–82. Zhou, X., Xu, R., Zhang, B., Pei, S., Liu, Q., Ramaswamy, H. S., & Wang, S. (2018). Radio Frequency-Vacuum Drying of Kiwifruits: Kinetics, Uniformity, and Product Quality. Food and Bioprocess Technology, 11(11), 2094–2109. Zhou, X., & Wang, S. (2019). Recent Developments in Radio Frequency Drying of Food and Agricultural Products: A Review. Drying Technology, 37(3), 271–286.

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Fig. 1. Schematic view on the positions of nuts in which three individual fiber optic probes were inserted, respectively in a polymer sample-loading container. Sensor 1 – Top-center layer, sensor 2 – Bottom-center layer, and sensor 3 –Top-corner layer. +

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The height of a sample load was defined as sample thickness (ST) (3, 5 or 7 cm) for HARF drying condition.

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HARF drying optimization: • Electrode gap (cm): 14, 15 and 16 • Sample thickness (cm): 3, 5 and 7

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Intermittent mode

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Fresh Oregon hazelnuts inshells (Barcelona and Jefferson)

Intermittent mode

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Continuous mode

Optimal HARF drying

Quality measurements

Optimal HARF drying 2-step hot air-HARF drying (Intermediate MC: 16%, 14%, and 12%)

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Fresh Oregon hazelnuts inshells (Jefferson)

Drying characteristics

Drying characteristics Quality measurements

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Hot air drying (38 °C/60% RH and 43 °C/40% RH)

Fig. 2. Flow diagram of the hot-air radio frequency (HARF) drying study for Oregon hazelnuts.

Comparison of different drying methods on nuts

of ro

Jo

ur na

lP

re

-p

Fig. 3. Generated RF power (P, kW) as the function of electrode gap (EG, cm) with or without loading of fresh Barcelona and Jefferson (900 g, inshells). + Barcelona had inshell MC of ~17%, and Jefferson had inshell MC of ~ 21%.

38

[Barcelona] I

II

I

II

ro

of

I

I

II II

I

I

ur na

lP

re

II

-p

[Jefferson]

Fig. 4. Drying – cracking curves of nuts as a function of total HARF drying time (min) at the optimal intermittent HARF drying conditions for Barcelona and Jefferson inshells. indicating MR and , where MCt was the MC of inshells

Jo

indicating cracking ratio. MR =

at the time (t, min), MC0 was the MC of inshells before drying (%), and MCe was the MC of inshells at the end of drying (%).

39

-p

ro

of

(a)

Jo

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(b)

Fig. 5 Drying (a) and cracking (b) curves of Jefferson inshells from different drying methods (HA, HARF and 2-step HA-HARF drying using two drying modes (intermittent or continuous). HA = drying at 38 °C/60% RH, HA43C = drying at 43 °C/40% RH, 2step HA-HARF = Combined HA and RF drying (15 cm electrode gap (EG) / 7 cm sample thickness (ST)) using intermittent or continuous mode, in which the intermediate MC of 40

Jo

ur na

lP

re

-p

ro

of

inshell nuts was 16, 14 and 12% MC, respectively.

41

of

Table 1. Effects of electrode gap (EG) and sample thickness (ST) during intermittent hot air-assisted radio frequency (HARF) drying on the drying characteristics of Barcelona and Jefferson inshells Completely randomized design Total RF heating time (min)

Total HARF time (min)

ro

Sample thickness (ST)

number of pauses

Initial heating rate (°C/min)+

Uniformity index (UI)++

Cracking ratio (%) after HARF+++

-p

Electrode gap (EG)

Drying characteristics of inshell nuts

[Barcelona] 17 4.1abc 0.060 38.9c, ++++ 14 6.5a 0.050 50.0abc 14 cm ab 18 6.2 0.075 54.4ab 14 2.5bc 0.047 63.3a abc 14 4.4 0.037 48.9bc 15 cm abc 15 5.5 0.054 56.7ab 8 2.4c 0.063 62.2ab c 12 2.3 0.045 58.9ab 16 cm 12 2.3c 0.047 62.2ab [Jefferson] 36.9 219.9 25 4.3ab 0.057 44.4b 3 cm 35.4 182.9 20 4.5ab 0.031 51.1ab 14 cm 5 cm a 31.0 237.8 28 6.7 0.031 51.1ab 7 cm 50.6 197.3 24 3.0ab 0.037 54.4ab 3 cm ab 54.0 184.0 19 3.3 0.036 57.8a 15 cm 5 cm ab 33.9 180.9 22 5.1 0.051 45.6b 7 cm 87.7 201.4 17 2.4b 0.046 46.7ab 3 cm b 58.7 198.3 22 2.8 0.042 50.0ab 16 cm 5 cm 51.4 206.2 24 3.3ab 0.047 53.3ab 7 cm + Initial heating rate (°C/min) indicated the mean of internal nut temperature from three sensors’ locations divided by the first RF heating cycle time under each HARF drying condition.

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117.5 105.6 141.3 114.0 100.5 117.1 104.2 126.0 120.2

lP

25.9 20.9 22.2 35.8 24.9 29.8 54.7 50.1 43.0

Jo

ur na

3 cm 5 cm 7 cm 3 cm 5 cm 7 cm 3 cm 5 cm 7 cm

++

Uniformity index (UI) over the entire HARF drying condition was calculated as

, defined as the ratio of the rise in

42

Jo

ur na

lP

re

-p

ro

of

standard deviation of nut surface temperature (σ, °C) to the rise in mean °C) surface temperature from the infrared images taken before and after entire HARF drying process. +++ Cracking ratio (%) after HARF drying was calculated as a number ratio of cracked nuts in total 30 randomly picked dried nuts. ++++ The mean value for initial heating rate and cracking ratio with same lower letter in the superscript indicated non-significant difference among treatments based on least significant difference (LSD) post-hoc test under one-way ANOVA analysis (P< 0.05)

43

Color index+

ro

Electrode Sample gap (EG) thickness (ST)

Moisture-related quality

Moisture Water activity Lightness Chroma content (aw) (L*) (C*) (MC, %)

-p

Completely randomized design

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Table 2. Effects of electrode gap (EG) and sample thickness (ST) during intermittent hot air-assisted radio frequency(HARF) drying on dried kernels quality for Barcelona and Jefferson inshells

Hue (H*)

[Barcelona] 47.1bc 30.3bcd 32.6de 14 cm 45.5c 28.9cd 35.5abc bc bc 46.9 30.9 33.8bcd 46.0bc 30.7bc 36.0ab c d 43.5 28.4 37.0a 15 cm 49.5ab 31.5ab 33.0cde c bcd 43.7 30.1 37.9a 49.5ab 32.9a 34.1bcd 16 cm a ab 51.3 31.6 31.1e 0.0007 0.0014 <0.0001 P value [Jefferson] 7.7b 0.68a 52.3ab 34.9ab 28.3b 3 cm a a ab a 8.0 0.69 54.0 35.5 27.1b 14 cm 5 cm 7.9a 0.65ab 54.6ab 35.1ab 27.5b 7 cm b a ab ab 7.7 0.70 54.3 35.2 27.4b 3 cm 7.9a 0.60b 47.8c 32.5c 31.8a 15 cm 5 cm a ab ab a 7.9 0.62 54.2 35.9 27.8b 7 cm 7.5c 0.65ab 55.3a 32.9bc 26.5b 3 cm a ab b abc 8.0 0.67 51.5 33.6 28.5b 16 cm 5 cm 7.6bc 0.69a 52.2ab 33.1bc 27.7b 7 cm < 0.0001 0.1342 0.0007 0.0290 0.0095 P value + Lightness (L*), redness (a*) and yellowness (b*) of dried kernel surface. Chroma (C*) =

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re

0.61abcd 0.64ab 0.58abcd 0.63abc 0.64a 0.53bcd 0.52d 0.55abcd 0.52cd 0.0983

lP

6.5ab，* 6.6a 6.2de 6.3cd 6.1e 6.3cd 6.3cd 6.2de 6.4bc < 0.0001

ur na

3 cm 5 cm 7 cm 3 cm 5 cm 7 cm 3 cm 5 cm 7 cm

Oil-related quality

Oil recovery (%)++

PV+++

FA++++

28.9ab 27.0b 28.4ab 28.1ab 28.9ab 28.1ab 33.5a 27.2b 25.8b 0.3137

0.22cdef 0.27cde 0.33bc 0.14ef 0.08f 0.86a 0.44b 0.29bcd 0.17def <0.0001

0.43b 0.55a 0.50ab 0.44ab 0.45ab 0.48ab 0.54ab 0.43b 0.47ab 0.2919

29.2a 21.5b 28.6a 25.1ab 24.5ab 24.2ab 28.7a 26.1ab 28.0ab 0.3155

0.43bc 0.83a 0.36bc 0.89a 0.36bc 0.06d 0.27cd 0.57b 0.52bc <0.0001 hue (H*) =

0.48b 0.45b 0.53b 0.48b 0.44b 0.61b 0.59b 0.62b 1.03a 0.1239

44

Jo

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lP

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-p

ro

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(n = 10). ++ Oil recovery (%) was calculated as a mass ratio of oil in total weight of samples. +++ Peroxide value (PV) representing the primary oxidation products (hydroperoxide compounds) was reported in the unit of meq O2/ kg oil. ++++ Free Fatty acids (FA) representing the rancidity of nuts was reported in the unit of % oleic acid. * The mean value of each measurement in the column with same lower letter in the superscript indicated non-significant difference among treatments based on least significant difference (LSD) post-hoc test in one-way ANOVA analysis (P< 0.05)

45

P value +

16

2.5

14 12 21

5.0 10.5 0

0.6 a,B*

3.0 a,A

3.0 d,E

0.025 a,C*

45.6 b,BC

0.4 ab,BC

1.9 b,B

4.4 c,D

0.029 a, BC

61.1 a,A

0.3 bc,DE 0.1 c,E 0.8 a,A

1.2 c,C 0.3 d,E 1.1 a,C

6.2 b,B 10.8 a,A 1.1 d,F

0.060 a, ABC 0.034 a,ABC 0.090 a, A

50.0 ab,B 8.9 c,E 43.3 a,BC

re

HARF 2-step HAHARF

0

16

2.5

0.5 b,B

0.8 b,CD

3.3 c,E

0.082 a,AB

37.8 ab,CD

14 12

5.0 10.5

0.3 c,CD 0.1 d,E 0.0004

0.5 c,DE 0.2 d,E <0.0001

5.5 b,C 10.7 a,A <0.0001

0.068 a,ABC 0.072 a,ABC 0.1617

30.0 b.D 13.3 c.E <0.0001

ur na

Continuous

21

lP

Intermittent HARF 2-stepHAHARF

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Table 3. Comparison of drying time, uniformity index and cracking ratio of nuts using different drying methods (HA, HARF and 2-step HA-HARF using two drying modes (intermittent or continuous) for Jefferson inshells Cracking Total Intermediate HA preTotal RF Total Drying Drying HARF Uniformity ratio (%) MC of nuts drying heating drying mode method drying index (UI) + after (%) time (h) time (h) time (h) drying ++ time (h)

Uniformity index (UI) over the entire HARF drying condition was calculated as

standard deviation of nut surface temperature (σ, °C) to the rise in mean before and after entire HARF drying process.

°C) surface temperature from the infrared images taken

Cracking ratio (%) in the endpoint was calculated by a ratio of cracked nuts in total 30 randomly picked dried nuts.

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++

, defined as the ratio of the rise in

*

The mean value of measurements with same lower letter in the superscript for same drying mode indicated non-significant difference among treatments based on least significant difference (LSD) post-hoc test in ANOVA analysis (P< 0.05); values for all eight drying conditions in the column were also compared for significance with same upper letter in the superscript .

46

Lipid-related quality

Oil recovery (%) ++

PV+++

FA+++

12.4c

0.10bc

28.3b

26.6ab

33.5de

31.7a

54.1a

35.9bc

47.4c

0.66c

6.0f

Intermediate MC at 12%

Treatment

K270+++

++++

+

TPC

DPPH

0.46b

0.88a

0.04a

0.20a

0.16bc

0.28a

0.44bc

0.84a

0.03abc

0.08bc

0.10d

23.5b

0.11bc

0.42bcd

0.89a

0.04ab

0.09bc

0.06e

27.8b

24.2b

0.06c

0.61a

0.84a

0.04a

0.09b

0.11d

34.1cde

32.7a

22.9b

0.00c

0.41bcd

0.94a

0.03abc

0.06bcd

0.27a

53.4ab

33.2de

27.7b

30.5a

0.11bc

0.42bcd

0.72a

0.02c

0.09bc

0.08de

0.64cd

49.9bc

33.3de

31.3a

25.0ab

0.28a

0.46bc

0.78a

0.02c

0.02e

0.18bc

6.6f

0.61d

54.7a

34.5cd

27.8b

25.6ab

0.26a

0.39bcd

0.79a

0.03abc

0.06bcd

0.05e

Intermediate MC at 16%

6.6f

0.67c

48.6c

32.7e

32.0a

28.3ab

0.00c

0.36bcd

0.80a

0.02bc

0.05cd

0.20b

Intermediate MC at 14%

6.4g

0.67c

48.3c

32.2de

32.2a

28.7ab

0.05c

0.33d

0.78a

0.02c

0.03de

0.16c

Intermediate MC at 12%

6.5f

0.64cd

49.9bc

34.5cd

31.3a

28.3ab

0.22ab

0.35cd

0.83a

0.02c

0.04de

0.17bc

< 0.0001

< 0.0001

<0.0001

<0.0001

<0.0001

0.0005

0.0006

0.0022

0.604

0.0335

<0.0001

<0.0001

L*

C*

14.7a, *

0.90a

53.7a

38.4a

38 °C/60% RH

8.4b

0.77b

52.7ab

36.8ab

43 °C/40% RH

7.1e

0.66cd

48.5c

HARF

Intermittent

7.9c

0.62cd

Continuous

8.0c

0.75b

HAintermittent HARF

Intermediate MC at 16%

7.6d

Intermediate MC at 14%

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HAcontinuous HARF

ur na

P value

27.0b

re

HA43C

H*

-p

aw

Fresh HA

Bioactive+++++

K232

MC

lP

Name

Color index +

Moisture-related quality

ro

Treatment

of

Table 4. Comparison of dried nut quality using different drying methods (HA, HARF and 2-step HA-HARF drying using two drying modes (intermittent or continuous) for Jefferson inshells

47

++

Oil recovery (%) was presented as a mass ratio of oil in total weight of samples.

+++

. Hue value (H*) =

.

of

Lightness (L*), redness (a*) and yellowness (b*) of dried kernel surface. Chroma value (C*) =

PV value representing the primary oxidation products (hydroperoxide compounds) in the HARF dried nuts was reported as meq O2/ kg oil.

ro

+

Free fatty acids (FA) value representing the accumulation of natural fatty acids and hydrolytic rancidity after HARF processing was reported as % oleic acid. ++++

-p

K232 indicating the conjugated diene and K270 indicating conjugated triene in oil samples were calculated by the absorbance of solution at 232 and 270 nm divided by the concentration of oil in solution (1%, w/v) and length of cuvette (cm) +++++

TPC (total phenolic content) was reported as mg GAE/ g samples (DW). DPPH representing antioxidant activity was reported as mg AAE/ g samples (DW). *

Jo

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The mean value of each measurement in the same column with same lower letter in the superscript indicated non-significant difference among treatments based on least significant difference (LSD) post-hoc test in one-way ANOVA analysis (P< 0.05)

48