Effect of pulsed electric fields on the structure and frying quality of “kumara” sweet potato tubers

Accepted Manuscript Effect of pulsed electric fields on the structure and frying quality of “kumara” sweet potato tubers

Tingting Liu, Ethan Dodds, Sze Ying Leong, Graham T. Eyres, David John Burritt, Indrawati Oey PII: DOI: Reference:

S1466-8564(16)30412-X doi: 10.1016/j.ifset.2016.12.010 INNFOO 1694

To appear in:

Innovative Food Science and Emerging Technologies

Received date: Revised date: Accepted date:

4 October 2016 9 December 2016 18 December 2016

Please cite this article as: Tingting Liu, Ethan Dodds, Sze Ying Leong, Graham T. Eyres, David John Burritt, Indrawati Oey , Effect of pulsed electric fields on the structure and frying quality of “kumara” sweet potato tubers. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Innfoo(2016), doi: 10.1016/j.ifset.2016.12.010

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ACCEPTED MANUSCRIPT Effect of Pulsed Electric Fields on the structure and frying quality of “kumara” sweet potato tubers

Tingting Liu1,2,3, Ethan Dodds1, Sze Ying Leong1, Graham T. Eyres1, David John Burritt2 and

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Indrawati Oey1,3* Department of Food Science, University of Otago, PO Box 56, Dunedin 9054, New Zealand

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Department of Botany, University of Otago, PO Box 56, Dunedin 9054, New Zealand

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Riddet Institute, Palmerston North, New Zealand

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*Corresponding author: Professor Dr. Indrawati Oey

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Department of Food Science, University of Otago PO BOX 56, Dunedin 9054, New Zealand

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Phone: +64-3-479-8735

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Fax: +64-3-479-7567

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Email: [email protected]

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ACCEPTED MANUSCRIPT Research Highlights:

1. PEF induced cell electroporation is not homogenous across the whole sweet potato tuber. 2. Softening due to PEF was more pronounced on the flesh than on the skin.

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3. PEF treatment prior to frying reduced the oil content of fried sweet potato chips. 4. PEF reduced the temperature dependence of the darkening rate of sweet potato chips

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during frying.

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ACCEPTED MANUSCRIPT Abstract The aim of this research was to study the effect of pulsed electric fields (PEF) on the microstructure of “kumara” sweet potato (Ipomoea batatas cv. Owairaka) and its quality after frying. Whole sweet potato tubers were treated at different electric field strengths ranging from 0.3 and 1.2 kV/cm with specific energy levels between 0.5 and 22 kJ/kg. Cell viability

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was determined using tetrazolium staining to investigate the uniformity of the PEF effect across tubers. Based on the patterns of viable cells it was observed that the effect of PEF was

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not homogeneous across the tuber. This result was also supported by the pattern of enzymatic browning due to PEF facilitating the reaction of polyphenoloxidase and phenols. PEF

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treatment resulted in significant softening of the ground tissues, but not on the dermal tissues, as determined by texture analysis. With respect to frying quality, tubers pre-treated with PEF

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at electric field strength of 1.2 kV/cm and fried at 190 °C had an 18% lower oil content than non-PEF treated samples. The kinetics of browning as a function of frying time could be described by a fractional conversion model. The activation energy (Ea) of the browning rate

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during frying increased (more temperature sensitive) due to PEF pretreatment at 0.5 kV/cm and 1.2 kV/cm. It implies that PEF pretreatment allows frying the potato chips at lower

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temperature in order to achieve the same brown colour intensity as the non-PEF treated tubers.

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This study shows clearly that PEF could reduce the energy required for cutting and frying of kumara.

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Industrial Relevance: This study provides evidence that the effect of PEF processing on whole kumara tubers is not uniform, demonstrating heterogenous distribution. These findings provide important information for food industry to design appropriate PEF processing

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conditions for solid materials. More importantly, PEF treatment reduced the energy required for cutting and frying of kumara, and reduced the oil content in the fried kumara chips.

Keywords Pulsed electric fields; sweet potato; structure; texture; frying; kumara

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ACCEPTED MANUSCRIPT Introduction Sweet potatoes (Ipomoea batatas), also known as “kumara” in New Zealand, are starchy, sweet-tasting, tuberous roots that were one of the most important crops and a staple food in the diets of the Maori people (Cambie & Ferguson, 2003). Sweet potatoes are rich in ascorbic acid, thiamine, riboflavin, niacin, phosphorus, iron, calcium, and dietary fiber (Farinu & Baik, 2007). The major storage proteins of kumara are sporamins A and B, which act as proteinase

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inhibitors and may have anti-cancer properties (Scott & Symes, 1996). Kumara also contains

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coumarins, which are implicated to be anti-coagulants (Cambie & Ferguson, 2003). In New Zealand, there are three main varieties of sweet potatoes commercially available,

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which can be differentiated by the colour of their skin and flesh. i.e. Toka Toka Gold or gold sweet potatoes (sweet potatoes with golden skin and flesh); Beauregard or orange sweet

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potatoes (sweet potatoes with a rich orange flesh and sweeter than gold) and Owairaka Red or red kumara (the most common sweet potatoes variety with red skin and creamy white flesh).

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Owairaka Red sweet potatoes have a very firm texture and require considerable energy either to be cut or for other forms of physical disintegration for processing. This represents a significant challenge for processing industries where large quantities of sweet potatoes are

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cut/sliced on a daily basis. Different processing methods have been investigated to reduce the

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firmness of potatoes or sweet potatoes, such as heat blanching (Nourian & Ramaswamy, 2003), steaming, hot air and microwave softening (Alvarez & Canet, 2001). However, thermal treatment has a high energy cost and may also induce undesired quality changes such

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as the loss of natural color and flavor. In the last decade, the use of non-thermal processing such as pulsed electric fields (PEF) technology has been studied and the research findings show that PEF can soften the texture of apples and potatoes (Lebovka, Praporscic, &

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Vorobiev, 2004), sugar beet (Lebovka, Shynkaryk, & Vorobiev, 2007) and carrot (Lebovka et al., 2004; Leong, Richter, Knorr, & Oey, 2014). So far, only limited studies have been carried out investigating the application of PEF as a means to soften sweet potatoes. Pulsed electric field processing (PEF) applies very short voltage pulses (in μs) with a high electric field strength to a product placed between two electrodes. PEF processing at low electric field strengths between 0.2 and 1 kV/cm for 0.1 to 10 ms can disrupt plant tissues without a significant increase in temperature (Faridnia, Burritt, Bremer, & Oey, 2015; Fincan & Dejmek, 2002; Lebovka, Bazhal, & Vorobiev, 2002). Electric fields between parallel plate electrodes are quasi homogenous. However, parallel plate electrodes do not produce a

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ACCEPTED MANUSCRIPT homogenous electric field when the sample to be treated has a heterogeneous composition and an irregular shape (Ivorra & Rubinsky, 2007). The electric field distribution across tissues has been studied using mammalian tissues to treat cancer or tumours. However, there is little information on the effect of electric fields across plant tissues in the literature. Faridnia et al. (2015) showed for the first time the microstructure of different parts of whole potato tubers after PEF treatment and provided evidence that the distribution of the PEF

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effect across the tubers is not uniform. However, there is still a lack of understanding whether this knowledge can be directly translated to other plant tubers/roots with different tissue structures, such as kumara. The tuberous roots of sweet potatoes (I. batatas) are structurally

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different (Figure 1) to the tuberous stems of the common potato (Solanum tuberosum), with

potatoes may respond differently to PEF treatment.

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differences in cell size, tissue arrangement and vascular architecture and hence sweet

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Sweet potatoes are widely cooked using deep-frying and consumed in the forms of French fries and chips (in current study, “chips’ are referred as fried thin slices). Since fried kumara

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or potato chips typically contain more than 30% fat (Dana & Saguy, 2006) and there is currently a demand for healthier foods, different processing techniques have been applied to reduce the fat content in fried chips. Recent research on the deep-fat frying quality of PEF

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pre-treated potato cubes (e.g. processing with specific energy input of 18.9 kJ/kg and electric

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field strength of 0.75 kV/cm) showed a lower oil uptake upon frying than untreated potato cubes (Ignat, Manzocco, Brunton, Nicoli, & Lyng, 2015). Hence, PEF could be used as a pre-

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treatment before frying to reduce the oil uptake of fried potato chips. The main objective of the present study was to investigate the effect of PEF on the

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microstructure and texture of sweet potatoes (I. batatas cv. Owairaka) and quality (i.e. color, oil content) after frying. Whole unpeeled and uncut sweet potatoes were used in this study as Faridnia et al. (2015) showed that cutting or peeling increased the effects of PEF on potatoes. The distribution of the electric field effects and the sensitivity of different cell types to the pulsed electric field treatment were investigated in sweet potatoes using tetrazolium salt viability staining (Faridnia et al., 2015). PEF induced membrane permeabilization determined by conductivity measurements (Faridnia et al., 2015) could not be used for this study because sweet potatoes have a thick resilient skin that hinders the leakage of ions, from the more easily damaged internal tissues, into the surrounding media. As sweet potatoes contain moderate levels of polyphenoloxidase (PPO) and phenols, which are located in separate

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ACCEPTED MANUSCRIPT compartments and only react to form brown polyphenolics when they come into contact with each other, enzymatic browning was used to assess the impacts of PEF on cell damage. In addition, firmness and cutting force of sweet potatoes after PEF treatment were measured and scanning electron microscopy (Cryo-SEM) was used to visualize the effect of PEF on cellular microstructure, hence enabling changes in physical properties to be related to structural changes. Finally, the frying quality (i.e. browning and oil content) of PEF pretreated sweet potato chips was studied and compared with non-PEF pretreated (further coded as

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“untreated”) chips. The kinetics of browning during frying were studied at different

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temperatures. In addition, the fat content of the chips was determined.

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1. Material and methods

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2.1 Chemicals and reagents

Sodium dihydrogen phosphate (NaH2PO4), disodium phosphate (Na2HPO4), ethylenediamine

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tetra-acetic acid (EDTA), sodium chloride (NaCl) were purchased from BDH chemicals (Poole, UK). 2,3,5-triphenyl tetrazolium chloride and 1,2-dihydroxybenzene (or catechol)

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2.2 Sweet potatoes sample

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were purchased from Sigma Aldrich (St. Louis, USA).

Sweet potatoes (Ipomoea batatas cv. Owairaka) harvested between May and August 2015

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were used in this study. This cultivar has red skin with white flesh and sourced from Kaipara Kumara (Northland, New Zealand). Upon arrival, the sweet potatoes were visually inspected and any tubers with cuts or bruises or damages were discarded. Tubers were sorted based on

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shape, size and weight, with tubers in the small size range (~50g, 50 mm length and 35-42 mm width) used for this study. Sweet potatoes were stored in the dark at 15-17oC and used within seven days of harvest.

2.3 Pulsed Electric Fields (PEF) treatment Whole unpeeled sweet potatoes were PEF treated, using an ELCRACK® HVP 5 PEF system (German Institute of Food Technologies, Quakenbruck, Germany) in batch treatment configuration. The treatment chamber consisted of two parallel stainless steel electrodes with a distance of 80 mm and had a dimension of 100 mm length × 80 mm width × 50 mm depth

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ACCEPTED MANUSCRIPT (total volume of 400 mL). The pulse shape (square wave bipolar) was monitored on-line using an oscilloscope (Model UT2025C, Uni-Trend Group Ltd, China). The specific energy input was calculated using Eq. (1).

Specific energy input, Wspec (kJ/kg) =

(1)

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V is the voltage peak value of pulses (kV), n is the number of pulses applied (dimensionless), m is the width of the square-wave pulses (in µs), R is the effective load resistance (in ohm)

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calculated based on the cross sectional area of chamber, pulse current and the conductivity of the sample and the buffer to be treated, and W is the total weight of sample (sweet potato and

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buffer). For each PEF treatment, one whole unpeeled sweet potato tuber was used. The tuber was first washed with running tap water, weighed, and then placed in the PEF chamber with

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the proximal-distal axis parallel to the electrodes as shown in Figure 2. The chamber was then filled with phosphate buffer (10 mM, pH 7; conductivity of 1400 μS/cm), ensuring that there

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were no air bubbles present to guarantee smooth delivery of electric current across the two electrodes, until the tuber was completely immersed in the buffer. The conductivity of the buffer was matched to the average conductivity of the kumara tuber (1400 μS/cm), which was

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measured using a hand held conductivity meter (LF-STAR, R. Mathäus, Germany). The total

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weight of the tuber and buffer solution was monitored and standardized throughout the study. Different electric field strengths were used, i.e. 0.3, 0.5, 0.8 and 1.2 kV/cm with a total pulse number of 540, pulse width of 20 μs and frequency of 50 Hz. These parameters were selected

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to achieve moderate to severe cell damage in kumara tubers based upon our preliminary studies. Different specific energy levels 0 (untreated), 0.47±0.00, 2.76±0.07, 6.95±0.07 and

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21.18±0.13 kJ/kg were used. The temperature and electrical conductivity of the buffer were measured before and after PEF treatment using a temperature/conductivity meter (CyberScan CON 11, Eutech Instruments, Singapore). The initial temperature of the phosphate buffer was maintained at 20oC ± 1 using a water bath and each PEF treatment was conducted in triplicate. The maximum temperature increase after PEF treatment was 0.8 °C, when samples were treated at the highest electric field strength of 1.2 kV/cm. After PEF treatment, the tuber was removed from the chamber and subjected to analysis.

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ACCEPTED MANUSCRIPT 2.4 Study on the effect of PEF on the microstructure of sweet potato 2.4.1. Determination of vascular bundle arrangement Whole untreated or PEF-treated (electric field strengths of 0.3, 0.5, 0.8 and 1.2 kV/cm) sweet potato tubers were sliced with using a mandoline to achieve a thickness between 1-1.5mm. Transverse slices from the proximal ends the middle regions and distal ends, as illustrated in

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Figure 2B, were placed on a light box and images were taken using a Canon PowerShot SX50 digital camera (Tokyo, Japan), to record the arrangement of the vascular tissues in the

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tuber prior to conducting cell viability staining, microstructure analysis and the enzymatic

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browning assay, as detailed below.

2.4.2. Determination of cell viability using tetrazolium salt staining

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Tetrazolium salt staining was used to evaluate the effect of PEF on cell/tissue viability within the slices and to determine the proportion of viable cells, as previously described by Faridnia

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et al (2015). The principle of tetrazolium salt staining is based on the formation of an insoluble red formazan from the reduction of the salt by oxidoreductase enzymes. The formation of the insoluble red formazan is directly proportional to the number of living cells

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as they contain oxidoreductase enzyme (Berridge, Herst, & Tan, 2005). A 0.5% w/v solution

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of the tetrazolium salt (2,3,5-triphenyltetrazolium chloride, Sigma Aldrich, St Louis, USA) was prepared in Milli-Q water on the same day as the PEF treatment and stored in a dark bottle to avoid precipitation and pH changes. Immediately after PEF treatment, each tuber

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was sliced, as detailed above. Each slice was placed in a petri dish and fully immersed in the tetrazolium solution. The petri dishes were then covered in tin foil to protect the samples

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from light and left for 24 hours at 18-20 oC. After 24 hours the tuber slices were rinsed with water, blotted dry with a paper towel and photographed as detailed above. Image analysis using colour threshold method (ImageJ software, ImageJ1.47e, Abramoff, 2004) was applied to measure the red and white areas on each slice, indicating living and dead cells respectively. 2.4.3. Evaluation of changes in cell permeability based on enzymatic browning Tubers were sliced as detailed above and the slices next to those used for the cell viability assay were used to evaluate enzymatic browning. Slices from three different regions of each sweet potato tuber were used for the enzymatic browning tests, i.e. the proximal end, the middle region and distal end, as illustrated in Figure 2. These slices were held at room

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ACCEPTED MANUSCRIPT temperature and photographed immediately after PEF treatment. The preliminary kinetic analysis showed that enzymatic browning of untreated and PEF treated kumara had developed by 30 min (data not presented). Therefore, another set of images was taken 30 min after PEF treatment. Samples without PEF treatment were used as untreated samples. The activity of polyphenoloxidase (PPO) was measured immediately after PEF treatment. Tuber slices were frozen in liquid nitrogen and ground to a fine powder using a cryogrinder

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(IKA®A11B, Rawang, Malaysia). The frozen powder was placed in 2 mL centrifuge tubes (Labcon, Hannover, Germany) and stored at -80°C until further analysis. Enzyme extracts

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were prepared by mixing 0.5 g frozen tuber powder with 1 mL of extraction buffer (50 mM

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NaH2PO4 containing 0.5 mM EDTA, pH 6.5), followed by centrifugation at 9400g for 2 min at 4°C. The resultant supernatant was used to determine PPO enzyme activity by measuring the initial rate of quinone formation as indicated by an increase in absorbance at 410 nm at 6

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second intervals, using a spectrophotometer (Ultrospec 3300 pro, Amersham Biosciences, Sweden). Substrate solution (50 mM NaH2PO4 containing 0.1 M catechol and 0.5 mM EDTA,

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pH 6.5) was prepared prior to conducting the enzymatic assay and was maintained on ice. The reaction mixture contained 1 mL of extraction buffer, 0.07 mL of enzyme extract and 0.4 mL substrate solution. PPO activity for each enzyme extract was assayed in triplicate at 20°C

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and each tuber slice was sampled 3 times. The enzyme velocity was calculated from the slope

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of the absorbance versus reaction time and the increase in absorbance was proportional with the formation of quinone. One unit of PPO activity was defined as the decrease of one

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absorbance unit per minute at pH 6.5 and 20°C.

2.4.4. Microstructure analysis using Cryo-SEM

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Changes in the cell microstructure of untreated and PEF-treated (0.5 and 1.2 kV/cm) sweet potato tubers were investigated using cryo-scanning electron microscopy (cryo-SEM). Samples of tissue, 10mm×10mm x 1-1.5mm, were cut from the mid-tuber ground and subdermal ground regions of tubers (Figure 2A) and were attached to aluminium discs with Tissue Tek OCT, frozen using liquid nitrogen and stored in liquid nitrogen until they were imaged. For imaging samples were transferred to a Gatan Alto 2500 cryo preparation chamber/cryo stage (Gatan Inc, Pleanton, California, USA), maintained at a constant temperature of -135°C. Samples were sublimed at a temperature of -85°C for 2-5 min to remove ice build up and then viewed using a JEOL JSM 6700F field emission scanning electron microscope (JEOL Ltd, Tokyo, Japan). 9

ACCEPTED MANUSCRIPT 2.5. Texture analysis of sweet potato 2.5.1 Firmness measurement using penetration testing Sweet potato tubers with diameters between 35 to 42 mm were selected for this test, with measurements conducted on 5 different tubers for untreated sample and for each PEF treatment. The penetration test was conducted using a texture analyser (TA. HDplus, Stable

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Micro Systems Ltd., Surrey, England) with a spherical probe (Part Code P/5S), a slotted blade insert and a heavy-duty platform.

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Each tuber was placed in the center of the blade insert, the probe was set to 20 mm above the

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tuber and the penetration test was started with a constant speed of 1.5 mm/s. Following each penetration test, the tuber was repositioned for the next test, with a total of three tests

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conducted per tuber. The maximum force for the first peak represents the rupture force for the

2.5.2 Measurement of cutting force

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skin of the tuber and the average force after the first peak represents flesh firmness.

Sweet potato tubers with diameters between 35 to 42 mm were selected for this test, with

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measurements conducted on 5 different tubers for untreated sample and for each PEF

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treatment. Cutting force was measured using a texture analyser (TA.HDplus, Stable Micro Systems Ltd., England), blade set (Part Code HDP/BS) including a blade holder, a slotted blade insert, a heavy duty platform, and blade with guillotine edge (Blade 11054, 90 mm

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height, 70 mm width and 3 mm thickness). The instrument settings were: test speed 1.5 mm/s; distance: 35 mm; and trigger force: 40 N. Each tuber was placed in the center of the blade

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insert and the knife set to 10 mm above the tuber and to cut the tuber transversely.

2.6. Evaluation of frying quality of sweet potato slices Untreated and PEF treated (0.5 kV/cm and 1.2 kV/cm) tubers were sliced as section 2.4.2 and fried at 150 ± 4°C, 170 ± 2°C or 190 ± 2°C using a deep fryer (Breville, BDF 300 deep fryer, Sydney, Australia) for one minute. Canola oil (Sunfield, New Zealand) was used as the frying medium. The oil was pre-heated for 1 h prior to frying and discarded after 6 h of use (Blumenthal, 1991). Three slices of sweet potato from each treatment were placed in a small frying net. Five frying nets were placed into the fryer at the same time. Immediately after frying for different predefined treatment times, each frying net was removed from the fryer,

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ACCEPTED MANUSCRIPT and the tuber slices were placed on metal mesh racks for 5 minutes to cool down and allow any excess oil to drip off.

2.6.1. Determination of oil content The fried chips were stored in a desiccator with silica beads to ensure that there would be no moisture uptake. The moisture content of the untreated, 0.5 kV/cm and 1.2 kV/cm samples, at

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each frying temperature, was then determined by placing them in a drying oven for 12 hours at 60°C. The total lipid content of each sample was carried out using Soxhlet extraction

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2.6.2. Colour determination of fried sweet potato chips

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according to AOAC (2000).

The colour of fried sweet potato chips was measured using a HunterLab Colorimeter

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(Miniscan XE Plus, HunterLab, Reston Virginia, USA). Each chip was placed in a white ceramic bowl so that the chip did not break while the colour was being analysed. L, a and b

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values were measured five times at different places on each sample and then the browning index (Davalos, Rubinsky, & Mir) was calculated according to Eq (2). Browning index represents the purity of brown colour and is considered as an important parameter associated

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with browning (Mohammadi, Rafiee, Emam-Djomeh, & Keyhani, 2008). BI =

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X=

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Where

2.6.3. Kinetic study on the colour changes during frying This test was conducted to investigate the effect of PEF treatments (i.e. 0.5 kV/cm and 1.2 kV/cm) on the kinetics of brown colour formation during frying and compared with the untreated samples. The frying conditions, including temperature, oil, frying, slicing condition, were the same as described above. Sweet potato tuber slices from the same sample treated under the same PEF treatment condition were used for the frying test at each frying

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ACCEPTED MANUSCRIPT temperature. Three replicates for each PEF pre-treatment condition were used. At 150°C, 18 slices from each tuber were put inside the metal mesh basket at the same time. At every minute of frying, one chip was taken out of the fryer and afterwards cooled on a metal mesh rack to allow excess oil to drip off. The last chip was taken out at 18 min. At 170°C, the time interval decreased from one min to 40 seconds, and the last chip was taken out from the fryer at 12 min. At 190°C, one chip was taken out of the fryer every 30 seconds, and the last chip

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was taken out of the fryer at 9 min. The colour of fried chips was measured using a HunterLab Colorimeter (Miniscan XE Plus, HunterLab, Reston Virginia, USA).

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The rate constant for changes in the L (lightness) value during frying was estimated using a

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first-order model (Eq. 3), as previously used to describe the colour changes during deep frying (Baik & Mittal, 2003).

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ln(L) = ln(L0) – kt

(3)

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Where L is the lightness of fried chips at time t, L0 is the lightness of fried chips at time t=0 (the lightness of untreated chips); k is the rate constant (min -1) of L value changes; and t is time in min. For a first-order reaction, the rate constant (k) can be estimated using a linear

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regression analysis (SAS 9.4) of the natural logarithm of L value versus frying time. In case

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of prolonged frying treatment, a fraction conversion model (Eq. 4), a modified first-order model was used, which takes into account the non-zero L value (Marc, Indrawati, Ann Van, & Chantal, 2002).

(4)

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L/L0 = Ls + Ll ▪ exp (-kt)

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Where L is the lightness at time t, L0 is the lightness of the fried chips at time t=0, Ll is the labile fraction at time t, Ls is the non-zero fraction after prolonged frying. The temperature dependence of the rate constant k is estimated using Arrhenius equation (Eq. 5) k = kref ▪exp {

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Where kref is the rate constant at the reference frying temperature (Tref), Ea is the activation energy, and R is the universal gas constant rate (R=8.314 J/mol·K).

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ACCEPTED MANUSCRIPT 2.7. Data analysis A one way analysis of variance was used to study the effect of PEF treatment on the cell viabilities, PPO activities, and texture properties of sweet potato tubers. To study the effect of PEF and sampling position on cell viability and PPO, a two-way analysis of variance was conducted using R software (version 3.2.2). The effect of PEF treatment on the frying quality of kumara chips including the oil content and browning index was studied using a one way

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analysis of variance. A two way analysis of variance of PEF treatments (n=3 or 3 independent replicates) and frying temperatures (n=3 or 3 independent replicates) was used to

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study the effect of PEF treatment and frying temperature on the oil content and browning index of fried chips. Significant differences (p<0.05) between means for the experimental

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data were determined using the Tukey’s post hoc multiple comparison test.

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The kinetic parameters of k, Ls and Ll were estimated using non-linear regression analysis (SAS 9.4). The quality of the fitting between the experimental data and the predicted data to

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estimate the aforementioned kinetic parameters was evaluated based on visual inspection of the fitting, residual analysis and coefficient of determination as corrected r2 based on Eq (6)

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(Hendrickx, 2002)

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Corrected r2 = 1-

(6)

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Where m is the sum of observations, SSQReg is sum of squares of the model, SSQTotal is sum of squares of total observations and j is sum of parameters. The Ea value was estimated using a linear regression analysis by plotting the natural logarithm of the rate constant (k) as a

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function of the reciprocal of absolute temperature (SAS 9.4, 2013).

3. Results and discussion

3.1 Effect of PEF on the structure of sweet potato tubers This is the first study to investigate the effect of PEF treatments on whole unpeeled “kumara” sweet potato tubers. Several techniques were used to investigate the influence of PEF treatment on the cells that make up the tuber tissues, including cell viability, microstructure and textural properties. 13

ACCEPTED MANUSCRIPT 3.1.1. Evaluation of the influence of PEF on cell viability Tetrazolium staining was used to evaluate the distribution of living and dead cells following PEF treatment. Figure 3 shows representative images of untreated and PEF treated slices from the proximal ends, the middle regions and distal ends of tubers. Increasing PEF electric field strengths resulted in increasing cell death in all three regions of the tuber, with no apparent region specific differences (F

(2, 175) =

0.05, p >0.1) observed. Tubers treated at 0.3

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kV/cm were similar to untreated tubers, containing mostly viable red stained cells (Figure 3) with 99.8-100% viable cells (Table 1). Hence electric field strength of 0.3 kV/cm is not high

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enough to cause enough cellular damage to be fatal and any electroporation due to PEF appears reversible, with cells able to repair themselves (Angersbach, Heinz, & Knorr, 2000).

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However, when electric field strength was increased to 0.5 kV/cm and greater, an increase in cell death was observed in all three regions of the tuber (Figure 3). These findings are

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consistent with a previous study on potato (Faridnia et al., 2015), where PEF treatment with higher electric field strengths (above 0.5 kV/cm) caused more cell death, as shown by

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tetrazolium salt staining. This trend of higher electric field strengths causing more cell death is also supported by quantitative data, where image analysis was used to determine the percentage of living cells in each slice. No significant differences in cell viability were

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observed when tuber slices from the proximal ends, the middle regions and distal ends of

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tubers were compared (data not presented). Hence Table 1 shows the estimated mean viability of whole tuber tissues. Analysis of variance showed a significant effect of treatment on cell viability (F

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= 880, p < 0.001). Tubers treated at an electric field strength of 0.3

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kV/cm showed no significant difference in cell viability when compared to untreated tubers (Table 1). When electric field strengths were increased to 0.5 kV/cm, 0.8 kV/cm or 1.2

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kV/cm, estimated tuber cell viability significantly decreased to approximately 65%, 41% and 18%, respectively (Table 1). Mohammadi et al. (2008) observed that when electric field strengths were increased, the transmembrane potential of a cell increases accordingly, which can then lead to electroporation and in some instances cell death. The transmembrane potential is proportional to the electric field strength and cell size, and is dependent on the electrical properties of the cell (Schwan, 1983). Differences in these properties would lead to an inhomogenous PEF effect. Faridnia et al. (2015) showed that PEF treatments had a greater effect upon the inner medulla of potato (Solanum tuberosum L.) tubers than other tissue types, with cells located in the inner medulla showing more damage compared to cells located in the outer medulla. 14

ACCEPTED MANUSCRIPT However, in the current study, no clear patterns for the tissue distribution of living or dead cells could be observed in sweet potato tubers following PEF treatment. The most likely explanation for this is the structural differences between S. tuberosum tubers, which are formed from stolons and have a more uniform tissue structure, with more evenly distributed vascular tissues, than the root tubers of I. batatas, which have a more irregular tissue structure, especially with respect to vascularization. Davalos et al. (2003) state that when a

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current move across a non-homogenous material made up of different cell types, the cells that have a higher resistivity will be more prone to higher electric field strengths, electroporation and cellular damage than the other cells. In relatively organised structures, such as S.

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tuberosum tubers, the impact of PEF treatment on different cell/tissue types can clearly be

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observed, due to the organisation of cell types into relatively easily distinguished tissue patterns during tuber development. In contrast, the impact of PEF treatment on the different

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cell/tissue types in I. batatas cannot be easily distinguished due to the less uniform cell/tissue patterns established during tuber development.

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3.1.2. Evaluation of cell permeability based on browning and PPO activities In plant tissues most PPO is restricted to the plastids, whereas the phenolic substrates are

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located in the vacuoles. The reaction between PPO and phenols occurs only when these

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compartments are disrupted after cell damage and the enzyme and substrate come into contact (Mayer, 1986; Queiroz, da Silva, Lopes, Fialho, & Valente-Mesquita, 2011). Since PPO catalyses the oxidation of polyphenols to quinones to produce brown pigments, the

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effect of PEF on cellular damage in complex plant organs, such as tubers, can be evaluated based on the degree and location of PPO induced enzymatic browning that occurs after PEF

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treatment.

Increasing electric field strengths facilitates an increase in the occurrence of enzymatic browning in tuber slices from all regions of the tuber (Figure 4). Jung, Lee, Kozukue, Levin, and Friedman (2011) found that the proximal end of sweet potato tubers had significantly more phenolic compounds than other regions of the tuber, while no difference in the distribution of phenolics within sweet potato tubers was observed by Walter & Schadel (1981), hence cultivar and/or batch variation was likely. In the present study, tubers treated at 0.3 kV/cm showed minimal enzymatic browning and were similar to untreated tubers (Figure 4), providing further evidence that electric field strength of 0.3 kV/cm is not high enough to cause a significant cellular damage in the sweet potato tubers. However, when electric field 15

ACCEPTED MANUSCRIPT strength was increased to 0.5 kV/cm and greater, increased enzymatic browning was observed in all three tuber regions, with browning most obvious in the dermal and vascular tissues (Figure 4). The above results could be explained by a combination of PEF enhanced release of phenolics from the vacuole and PPO from plastids. While there are no reports on the PPO distribution in sweet potato tubers, a study on the distribution of PPO in Phalaenopsis leaf explant found that the PPO was mainly distributed in vascular tissues and

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the epidermis (Ru, Lai, Xu, & Li, 2013). Analysis of PPO enzyme activity supported the above observations, with no significant

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difference in PPO activities in tuber slices taken from the proximal ends, the middle regions

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and distal ends of sampled tubers (F (2, 70) = 2.24, p > 0.1). Analysis of variance demonstrated significant differences in PPO activity between the five treatments (F (4, 70) = 13.8, p < 0.001) (Table 1). Tubers treated at an electric field strength of 0.3 kV/cm showed a significant lower

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PPO activity when compared to untreated tubers (Table 1). There is no significant difference in PPO activities when the electric field strength increased to 0.5 kV/cm and 0.8 kV/cm.

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When the level of electric field strengths were increased to 1.2 kV/cm, a significant increase in PPO activity was achieved, with a maximal value of 14.3 units per gram tuber tissue (Table 1). This increase, of approximately 10% compared to untreated tubers, was most likely

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due to wound induced increases in total tissue PPO activity. PEF treatment at low electric

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field strengths (less than 12 kV/cm) have been reported to increase the activities of several native enzymes (e.g. peroxidases and invertase) and it has been suggested that a moderate PEF intensity can activate some enzymes (Ohshima, Tamura, &Sato, 2007). This provides

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further evidence that the PEF induced increase in browning observed in the tissue sections is the result of the cellular disruption caused by PEF rather that PEF stimulating large increases

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in PPO activity.

Enzymatic browning is a surface phenomenon resulting from the conversion of phenolic compounds to brown melanins, a process that requires PPO and oxygen. The untreated sweet potato tuber shows minimal enzymatic browning, with the browning that did occur most likely resulting from the cutting of slices from the tubers. During cutting, cells are damaged releasing PPO and phenolics from their intracellular compartments at the cut surface, which results in enzymatic browning. SEM images (Figure 5) clearly show that slicing damages the outer most layers of a tuber slice, but that most of the underlying cells remain intact. According to Garcia and Barrett (2002), the sharpness of the blade used for cutting during

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ACCEPTED MANUSCRIPT sample preparation can affect the browning of a product. If a dull blade is used, more cellular damage can occur and consequently there is more enzymatic browning. In the present study, a new sharp blade was used that resulted in minimal cutting induced cellular damage/browning; this is an important consideration when browning is used as a marker to evaluate PEF induced cellular/tissue damage.

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3.1.3 Microstructural changes evaluation of tuber tissues using Cryo-SEM Microstructural changes in the cells making up the mid-tuber ground and sub-dermal ground

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tissues of tuber samples were evaluated using cryo-SEM (Figure 5). One of the advantages of using cryo-SEM is that samples are in a near native state (Thompson, Walker, Siebert,

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Muench, & Ranson, 2016). Thus the damage caused by sample preparation is minimized. Cutting the tubers into slices exposed the internal contents of the outer layer of cells,

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including large numbers of starch grains. The underlying cells making up the mid-tuber ground and sub-dermal ground tissue of untreated tubers remained largely intact and turgid,

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during cryo-SEM processing, resulting in good preservation of general tissue structure (Figure 5). However, PEF treatments of 0.5 kV/cm or greater resulted in the loss of cellular integrity in the ground tissues, with an almost complete loss of tissue structure occurring after

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PEF treatments of 1.2 kV/cm (Figure 5). Unlike cells of the mid-tuber ground tissues, the

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cells making up the sub-dermal ground tissues appeared to retain greater structural integrity after PEF treatments of tubers than those of the mid-tuber ground tissues, with a clear tissue structure still visible after PEF treatment at 1.2 kV/cm (Figure 5). In plant organs, including

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tubers, the cells that make up the dermal layers are structurally stronger, often with stronger, thicker cell walls than the cells that make up the bulk of the ground tissues. This results in dermal tissues being more robust than ground tissues and hence dermal tissues and their

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underlying cells often retain a higher degree of structural integrity than ground tissues even after cell death. The above results illustrate the importance of using a range of techniques, including microscopy and biochemical analyses to assess the impacts of PEF treatments on complex plant tissues.

3.2. Effect of PEF on the textural properties of sweet potato tubers As shown above, PEF treatment of complex plant tissues such as tubers causes substantial changes in cell and tissue properties that are not always uniform across the organ in question. From a food processing perspective, this results in an end product that could have very

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ACCEPTED MANUSCRIPT different properties than the starting material. Hence, the influence of PEF treatments on the textural properties of sweet potato tubers was investigated. Penetration tests, using a small probe, were used to study the firmness of the skin (dermal tissues) and flesh (ground and vascular tissues) of sweet potato tubers. Figure 6A shows the mean puncture force profiles of PEF treated sweet potato tubers. After the probe had come into contact with the skin of the tuber, force began to accumulate, and the sample began to deform to resist the force. Once

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sufficient force was applied, the skin ruptured resulting in a release of force, seen as a dip in the force curve. When the force profiles of untreated tubers and PEF treated tubers are compared (Table 1), it is clear that the amount of force required to rupture the skin

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(maximum force) of untreated and PEF treated tubers does not differ (F

(4,70)

= 1.8, p > 0.1).

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The results of the present study differ from a study on apple fruits, which found the PEF treatments reduced the force required to rupture the fruit skins (Bazhal, Ngadi, Raghavan, &

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Nguyen, 2003). However, the skin of an apple fruit is structurally very different and so is likely to be much weaker than the dermal tissues of sweet potato tubers.

significantly (F

(4,70)

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In contrast, there were significant differences in flesh firmness between treatments = 7.7, p < 0.001) where the force required to penetrate the flesh of the

tubers was reduced following PEF treatment with electric field strengths of 0.5 kV/cm or

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higher (Table 1). The force required to penetrate the flesh reduced from 56 N for the

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untreated samples to 40 N for the 1.2 kV/cm PEF treated samples, indicating an almost 30% reduction in flesh firmness. These results are not surprising given the fact that the sweet potato tuber dermal tissues appear to retain greater structural integrity following PEF

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treatment than the ground tissues (Figure 5). A similar softening effect on carrot tissues following PEF treatment has previously been reported by Leong, Richter, Knorr, & Oey

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(2014). The cutting force required for a blade to cut transversely through sweet potato tubers was measured, with trends similar to those detailed above for the skin penetration tests (Table 1). Figure 6B shows the cutting force required and the distance (depth) required for the blade to move until the skin was cut for untreated and PEF treated tubers. For untreated and 0.3 kV/cm PEF treated tubers, the distances required before skin rupture (maximum force) were reached were 8 mm and 6.2 mm, respectively. While for tubers treated with PEF field strengths of 0.8 kV/cm and 1.2 kV/cm a depth of approximately 11 mm was reached before the skin ruptured. However, it is important to note that no significant difference in maximal cutting force or total work was observed between untreated and PEF treated tubers (Table 1). These results can also be explained by the fact that the sweet potato tuber dermal tissues are

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ACCEPTED MANUSCRIPT generally more robust than the ground tissues (Javelle, Vernoud, Rogowsky, & Ingram, 2011), and would appear not to be damaged by PEF treatment. In contrast, the underlying cells in the ground tissues are much less robust than the dermal cells and are damaged by PEF treatment. As a result, the force required to penetrate the epidermis does not change, but the force required to penetrate the ground tissue is significantly reduced (Table 1). This could allow the ground tissues to collapse somewhat more in tubers treated with PEF electric field strengths

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above 0.3 kV/cm and hence increase the blade travel distance prior to the skin giving way

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and the cut being made.

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3.3. Frying quality of the PEF treated sweet potato chips 3.3.1. Evaluation on oil uptake

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From the previous sections, it is clear that PEF treatment modifies the structural properties of

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sweet potato tubers, which consequently could impact the subsequent processing, e.g. frying. Table 2 presents the effect of PEF treatment and frying temperature on oil uptake and browning of sweet potato chips. Analysis of variance demonstrated a significant effect of

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PEF treatment (F (2, 72) = 137.7, p < 0.001) where PEF decreased the oil content in fried chips.

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Chips obtained from tubers PEF-treated at 1.2 kV/cm and fried at 190°C showed significantly lower oil uptake (18%) than the untreated samples (22-23%).

No significant effect of

temperature was observed (F(2, 18) = 0.4, p = 0.69). There was also no significant interaction

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effect between temperature and PEF treatment on the oil content of fried chips. This result is in line with the finding of Janositz, Noack, and Knorr (2011), where PEF

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treatment (1.8 kV/cm) reduced the oil content by 38.66% for fried potato strips. The reduction in oil content in PEF treated samples is due to the higher vapour pressure difference between untreated and PEF treated samples, as PEF treatment increases the porosity of samples allowing within sample vapour to move to the surface of the sample at a much higher rate. Higher vapour flow rates result in fewer voids within the sample for oil to enter. Ignat et al. (2015) also found that PEF treatments (18.9 kJ/kg, 9000 pulses at 0.75 kV/cm and 810 pulses at 2.5 kV/cm) reduced the uptake of oil when compared to water dipped and blanched samples. The increased smoothness of cut surfaces, caused by PEF treatment, may also lead to a decrease in oil content due to better draining of the oil after frying. In addition, PEFinduced electroporation might also cause the loss of cytoplasm from cells into the

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ACCEPTED MANUSCRIPT extracellular space, resulting in more water being located outside the cells, creating a barrier to oil uptake and hence reduce oil uptake during frying (Ignat et al., 2015). Bouchon, Hollins, Pearson, Pyle, and Tobin (2001) used infrared microspectroscopy to study the absorption of oil in fried potato tubers and found that pore size had a significant impact upon the absorption of oil by fries. Therefore, structural changes that occur during processing, at the microscopic level, clearly have an impact upon the final characteristics of the fried food products.

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The majority of PEF research has been carried out on the deep-frying of potato fries, and not chips. Chips are made from a thin slice of the tuber and therefore have a much larger surface

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area to volume ratio compared to potato fries. Therefore, there may be differences in PEF

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effects on the oil uptake in the samples.

As shown in Figure 5, a large amount of cellular damage occurred when sweet potato is

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exposed to electric field strength of 1.2 kV/cm. The intracellular components, such as water, are no longer bound to the cellular structure and the water movement can occur at a faster

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rate (Janositz et al., 2011). When the sweet potato samples were placed into deep fryer at 190 °C, the water evaporation occurred faster compared to 150 °C and 170 °C. This means that there is less moisture in the higher frying temperature of 190 °C and the oil is able to

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drain more freely.

3.3.2. Evaluation on browning index of fried chips The extent of browning during frying can be affected by various factors including the content

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of reducing sugars, amino acids and proteins at the surface and the frying temperature and time (Marquez & Anon, 1986). In this study, the browning index increased with higher

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temperatures (Table 2). It has been reported that non-enzymatic browning develops faster at higher frying temperatures (above 150°C) than lower temperatures (<150°C) (Bordin, Kunitake, Aracava, & Trindade, 2013). Therefore, frying at a higher temperature results in chips with darker colour when the frying duration is kept the same (Pedreschi, Moyano, Kaack, & Granby, 2005). Analysis of variance demonstrated a significant effect of PEF treatment (F

(2, 72)

= 5.1, p <

0.05) where PEF increased browning index of fried chips. As shown in Table 2, when the sweet potato chips were fried at 150°C for 1 min, samples treated by PEF at an electric field strength of 1.2 kV/cm showed a significantly higher browning index than the untreated

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ACCEPTED MANUSCRIPT sample. It was observed that an electric field strength of 0.5 kV/cm was not high enough to significantly affect the development of browning at 150oC. The increase in cell permeability due to PEF treatment might result in more available reducing sugars on the surface of sweet potato slices (López, Puértolas, Condón, Raso, & Ignacio Álvarez, 2009). Besides frying temperature and duration, the superficial reducing sugar will determine the intensity of the brown colour of fried chips. A previous study showed that blanching was used to remove the

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superficial reducing sugar and the intensity of the brown colour/darkness of chips was reduced (Reis, Masson, & Waszczynskyj, 2008). In addition, the same study observed that

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PEF pre-treatment could lead to more uniform browning of potato chips during frying.

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3.3.3. Frying kinetics

A detailed kinetic study on the changes in the L* value (lightness/darkness) of sweet potato

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during frying was conducted. The data were fitted into a fraction conversion model. Figure 7 illustrates the darkening curves of untreated (a), 0.5 kV/cm PEF treated (b) and 1.2 kV/cm

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PEF treated (c) sweet potato chips after frying at 150, 170 and 190°C. For each treatment group, the relative lightness decreased during frying. When the chips were fried for the same duration, the colour becomes darker at higher temperature. The estimated rate constant k,

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labile fraction Ll, stable fraction Ls and activation energy Ea are summarised in Table 3. A

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fractional conversion model could be used to describe the kinetics of decreasing lightness during frying, indicating non-zero fraction after prolonged frying at the same frying temperature. The non-zero resistant fraction Ls decreased with increasing frying temperature

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(Table 3). The stable fraction Ls at the same frying temperature was decreased by PEF treatment. For example, at 170°C, the Ls was 0.51 (51%) in the untreated samples, while the

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Ls was 0.38 (38%) in the 1.2 kV/cm PEF treated group. The reduction of the stable fraction term by PEF at an electric field strength of 1.2 kV/cm combined with frying at 170°C was almost equal to that when increasing frying temperature to 190°C, where the Ls was 0.35 (35%) for the untreated sample at 190°C. After prolonged frying time, the lightness of PEF treated chips fried at 170 °C could be similar to an untreated sample fried at 190°C. As expected, the rate constant k values for darkening during frying increased with elevating temperatures for each treatment group (Table 3). To describe the temperature dependence of the k values of the labile fractions, the Arrhenius equation was used. The activation energy for untreated sweet potato chips was 54.4 kJ/mol, while the activation energy was 66 kJ/mol

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ACCEPTED MANUSCRIPT for 1.2 kV/cm PEF treated chips and 85 kJ/mol for 0.5 kV/cm treated chips. This means that PEF treatment decreases the temperature dependence of the k value, or in other words, PEF treatment increases the temperature sensitivity towards browning. This phenomenon can be explained by PEF induced cell permeability that could facilitate the evaporation of water and oil entering the chips and the release of sugar. In addition to the PEF effect, it should be taken into account that during deep fat frying of starchy foods, two mass transfers take place in two

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opposite directions i.e. water and some soluble materials escape from the food products and

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oil enters (Blumenthal, 1991).

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Conclusions

This study clearly shows the effect of PEF across an unpeeled whole sweet potato is not homogenous and the flow of electric current depends on the pattern of vascular bundles.

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Increasing the electric field strength enhances changes in cell membrane permeability, which eases the flow of materials from the intracellular to the extracellular space and vice versa.

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This phenomenon was clearly observed from the pattern and distribution of enzymatic browning. Structural modifications, due to PEF treatment, influence the textural properties of sweet potato tubers and the processing conditions following PEF treatment. The softening

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effect, due to PEF treatment, was more pronounced on the ground tissues than on the skin.

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This implies that removing the skin of sweet potato tubers prior to slicing is critical to take maximal advantage of PEF induced softening of the ground tissues. Structural modification of sweet potato tubers caused by PEF also affects its frying quality. This study found that

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PEF treatment of sweet potato tubers at 1.2 kV/cm significantly reduced the oil content of chip fried at 190°C. More importantly, based on the estimated kinetic parameters, PEF

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reduced the temperature dependence of the darkening rate of sweet potato chips during frying. This means that PEF pretreated sweet potato chips require a lower temperature to achieve a similar level of darkness to that obtained by frying untreated chips at higher temperatures. This study provides evidence to show that PEF can decrease the energy required for subsequent processes such as cutting and frying.

Acknowledgement This research was carried out as part of the Food Industry Enabling Technologies programme funded by the New Zealand Ministry of Business, Innovation and Employment (contract

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ACCEPTED MANUSCRIPT MAUX1402). The authors also thank Kaipara Kumara for kindly providing fresh sweet potato during the study and Riddet Institute CoRE for providing PhD scholarship to Tingting Liu.

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Caption of figures Figure 1. Transverse section (a) and longitudinal section (b) slice of a sweet potato tuber showing the structure of the tuber Figure 2. Experimental design and sampling scheme.

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Figure 3. Representative images of tetrazolium staining of tuber slices, taken from different tuber regions to evaluate the effects of electric field strengths. Red and white staining indicates viable and dead cells, respectively. Parts a, b, c indicate slices from the proximal end the middle region and distal end, respectively.

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Figure 4. The representative images of tuber slices treated by various electric field strengths.

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Figure 5. The SEM micrographs for tuber ground and dermal tissues treated with different PEF conditions (0 kV/cm, 0.5 kV/cm and 1.2 kV/cm). “es” refers to the extracellular space.

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Figure 6. Penetration test (A) and cutting force profile (B) of tubers through the middle region (5 independent replicates) with various PEF treatments.

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Figure 7. Fitting of fractional conversion model and the experimental lightness value of sweet potato chips fried at 150°C, 170°C and 190°C for untreated (a) and PEF pretreated sweet potato chips at 0.5 kV/cm (b) and 1.2 kV/cm (c). L0 are the initial lightness at time 0; L are the lightness at time t.

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ACCEPTED MANUSCRIPT Table 1. Effect of PEF on estimated tuber cell viability, polyphenoloxidase (PPO) activity, and cutting and penetration tests of sweet potato tubers (expressed as mean ± standard error). Treatment

% of viable cells r

PPO activity (units/g of sweet potato)s

Penetration test t

Cutting test q

Flesh firmness (N)

Skin strength (N)

Maximum Force (N) 172±52

0.0 kV/cm

100±0.0 a

12.9±0.3 b

56.2±9.6 a

66.2±9.9

0.3 kV/cm

99.8±0.2 a

11.2±0.3 c

54.7±3.9 ab

69.6±13

0.5 kV/cm

65.4±3.2 b

13.4±0.3 ab

46.7±5.1 bc

63.2±8.5

0.8 kV/cm

41.1±4.0 c

13.4±0.3 ab

37.9±5.2 d

1.2 kV/cm

18.6±7.0 d

14.3±0.3 a

40.4±6.0 cd

F value†

F(4, 175)= 879.8

F (4, 70) = 13.8

F(4,70) = 7.7

P value†

p < 0.001

p < 0.001

p < 0.001

D E

SC

2311±470

130±14

2046±299

175±58

2195±227

59.1±9.1

203±84

2282±438

69.8±8.4

225±58

2381±254

F(4, 70) = 1.8

F(4,20) = 1.9

F(4,20) = 0.7

p =0.14

p = 0.14

p = 0.58

U N

A M

T P E

†

I R

T P

Total work of strength (N·s)

ANOVA results for Sample effect (n=5). Different letters (a, b..) in the same column indicate significant differences (p<0.05) according to Tukey’s HSD multiple comparison test.

C C

r

Results were obtained on 180 observations (5 treatments × 4 slices per position × 3 positions per tuber × 3 replicates ).

s

Results were obtained on 75 observations (5 treatments × 3 positions per tuber × 5 replicates).

t

Results were obtained on 75 observations (5 treatments × 3 positions per tuber × 3 replicates ).

q

A

Results were obtained on 25 observations (5 treatments × 5 replicates).

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Table 2. Oil uptake and browning index of sweet potato chips after frying as a function of PEF treatment and frying temperature. Treatment

Temperature (°C)

Oil content* (g*g-1 dried chips)

Browning index#i

Untreated

150

0.23±0.01ab

78.4±13.5 e

0.5kV/cm

170 190 150 170

0.23±0.01ab 0.22±0.01ab 0.23±0.01ab 0.22±0.01ab

113.8±25.5 bc 152.4±7.1 a 86.1±13.4 de 133.9±9.2 ab

1.2 kV/cm

190 150 170

0.24±0.01a 0.20±0.01bc 0.20±0.01bc

149.6±14.3 a 100.5±12.4 cd 125.3±20.9 b

190

0.18±0.01c

T P

I R

C S U

N A

155.3±6.3 a

ANOVA Results

M

Sample†

F(8,18) = 2.6, p< 0.05

PEF‡

F(2,18) = 9.0, p<0.05

Temperature ‡

F(2,18) = 0.4, p = 0.69

F(2, 72) = 137.7, p<0.001

PEF*Temperature Interaction ‡

F(2,18) = 0.5, p = 0.75

F(4,72) = 2.7, p<0.05

D E

PT

F(8, 72) = 37.1, p<0.001 F(2, 72) = 5.1, p<0.05

E C

*

Values are reported as mean ± standard error of 3 observations (3 replicates).

#

C A

Values are reported as mean ± standard error of 9 observations (3 slices × 3 replicates)

† Results of one-way ANOVA (Sample, n=9). ‡ Results of two-way ANOVA (PEF treatment; Temperature). Different letters (a, b..) in the same column indicate significant differences (p<0.05) according to Tukey’s HSD multiple comparison test. i

Browning index is measured when chips are fried for 1 min at each frying temperature.

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ACCEPTED MANUSCRIPT Table 3. Kinetic parameters for Lightness (L value) changes of sweet potato chips during frying (mean ± standard error) Treatment

Temperature (°C)

Ls

Ll

k value (min-1)

Corrected r2

Ea (kJ/mol)

Corrected r2

Untreated

150

0.53±0.02

0.47±0.01

0.13±0.01

0.839

54.4±6.5

0.986

0.5kV/cm

170 190 150

0.51±0.01 0.35±0.00 0.51±0.09

0.52±0.02 0.70±0.02 0.55±0.08

0.30±0.03 0.49±0.03 0.06±0.01

0.825 0.865 0.779

1.2 kV/cm

170 190 150

0.45±0.03 0.33±0.01 0.46±0.03

0.58±0.02 0.80±0.02 0.50±0.03

0.19±0.02 0.49±0.03 0.11±0.02

0.843 0.904 0.721

170 190

0.38±0.02 0.33±0.01

0.59±0.02 0.77±0.02

0.22±0.02 0.48±0.03

D E

U N

A M

T P E

C C

A

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I R

SC

0.805 0.874

T P

85.0±4.5

0.997

66.0±3.7

0.996

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