Evolution of the physicochemical properties of oil-free sweet potato chips during microwave vacuum drying

Journal Pre-proof Evolution of the physicochemical properties of oil-free sweet potato chips during microwave vacuum drying

Ricardo Lemos Monteiro, Jaqueline Oliveira de Moraes, Jessica Daiane Domingos, Bruno Augusto Mattar Carciofi, João Borges Laurindo PII:

S1466-8564(19)31027-6

DOI:

https://doi.org/10.1016/j.ifset.2020.102317

Reference:

INNFOO 102317

To appear in:

Innovative Food Science and Emerging Technologies

Received date:

15 August 2019

Revised date:

31 January 2020

Accepted date:

23 February 2020

Please cite this article as: R.L. Monteiro, J.O. de Moraes, J.D. Domingos, et al., Evolution of the physicochemical properties of oil-free sweet potato chips during microwave vacuum drying, Innovative Food Science and Emerging Technologies(2020), https://doi.org/ 10.1016/j.ifset.2020.102317

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© 2020 Published by Elsevier.

Journal Pre-proof Evolution of the physicochemical properties of oil-free sweet potato chips during microwave vacuum drying Ricardo Lemos Monteiroa, Jaqueline Oliveira de Moraesa Jessica Daiane Domingosa, Bruno Augusto Mattar Carciofia, João Borges Laurindoa* a

Department of Chemical and Food Engineering, Federal University of Santa Catarina,

EQA/CTC/UFSC, 88040-900, Florianópolis - SC, Brazil. *Corresponding author: Tel.: +55 48 3721.6402; Fax: +55 48 3721.9687. e-mail:

ABSTRACT

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[email protected]

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The demand for healthy and convenient foods is a worldwide trend. Sweet potato attracted great attention due to its carbohydrates with a low glycemic index.

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Dehydrated sweet potatoes can be an excellent alternative for using and adding value

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to this raw material. The objective of this study was to evaluate the physicochemical properties of sliced sweet potato during the microwave vacuum drying (MWVD) for

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producing crispy oil-free chips. Fresh sweet potato samples were selected, peeled, sliced, blanched, and then dehydrated using a microwave oven adapted with a vacuum chamber and a rotation system to operate under vacuum. It was measured the

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evolution of moisture, water activity, temperature, color, apparent specific mass, porosity, and acoustic/mechanical analysis of the texture during the MWVD. Crispy

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sweet potato chips were obtained in less than 30 min, presenting low moisture (0.028 g g-1 db) and water activity (0.262). The dehydrated samples showed high porosity (67.5%) and a low apparent density (0.456 g cm-3). Optical micrographs and acoustic/mechanical properties showed an expanded (puffed) product structure with large pores, which resulted in irregular acoustic/mechanical signals, characteristics of a crispy food matrix. Colorimetric analyses indicated a little change between fresh and dried samples, with an absence of burnt spots. In conclusion, MWVD is a suitable process to produce highly porous sweet potato chips, adding value, and extending the vegetable’s shelf life. Keywords: sweet potato snacks; free of oil; crispness; microwaves; dehydration.

Journal Pre-proof 1. INTRODUCTION Vegetables are produced all over the world and consumed raw, minimally processed, cooked, dehydrated, or fried. These raw materials can also be processed to serve as industrial ingredients (B to B products). Sweet potato is outstanding among other vegetables due to its excellent nutritional composition, presenting starch, dietary fibers, minerals, vitamins, and antioxidants (Ishida et al., 2000). Also, it has been highlighted as an excellent carbohydrate option, once it presents starch with a low

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glycemic index. Controlled consumption of sweet potatoes is beneficial for diabetic patients and may help them to regulate blood glucose levels (Allen, Corbitt, Maloney,

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Butt, & Truong, 2012).

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Harvest, post-harvest, and storage losses of fruits and vegetables can reach

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40% of the production in developing countries (Jayaraman & Gupta, 2014). In this context, dehydration is an excellent alternative for extending the shelf life of fruits and

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vegetables by reducing the moisture

content

and water

activity,

inhibiting

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microbiological growth and reducing enzymatic activities (Aguilera, Chiralt, & Fito, 2003; Fellows, 2009; Ratti, 2001; Singh & Heldman, 2009; Van Arsdel, Copley, &

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Morgan, 1963). Healthy, natural, and convenient food snacks are a trend in the world market. Then, sweet potato chips free of oil is an interesting ready-to-eat product, which is a health-promoting food without requiring preparation time. Sun drying, air drying, vacuum drying, and freeze-drying are traditional drying methods that have been applied industrially in the dehydration of fruits and vegetables (Fellows, 2009; Incropera, Lavine, Bergman, & DeWitt, 2007; Link, Tribuzi, & Laurindo, 2017; Sablani, 2006; Tribuzi & Laurindo, 2016; Zotarelli, Porciuncula, & Laurindo, 2012). The food industry has invested in drying technologies with shorter processing times that can minimize nutritional losses during processing. In this context, studies on the drying of fruits and vegetables coupling microwave and vacuum deserve attention. This drying technique is a viable alternative for the production of high-quality

Journal Pre-proof dehydrated products with high drying rates and competitive costs. Literature reported that microwave vacuum drying results in products of high nutritional and sensory quality, with reduced shrinkage (Barreto, Tribuzi, Marsaioli Junior, Carciofi, & Laurindo, 2019; Datta & Anantheswaran, 2001; Drouzas & Schubert, 1996; Gunasekaran, 1999; Hu, Zhang, Mujumdar, Xiao, & Jin-cai, 2006; Krokida & Maroulis, 1999; Lin, Durance, & Scaman, 1998; Monteiro, Carciofi, & Laurindo, 2016; Monteiro, Link, Tribuzi, Carciofi, & Laurindo, 2018b, 2018a; Mousa & Farid, 2002; Scaman & Durance, 2014; Zhang,

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Tang, Mujumdar, & Wang, 2006). Besides, the utilization of microwave under vacuum reduces sensory

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degradation (color, taste) and results in products of easy rehydration when compared

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to those dried by air drying (Clary, Mejia-Meza, Wang, & Petrucci, 2007; Lin et al.,

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1998; Monteiro et al., 2018b). Low values of moisture content, loss constant, and attenuation factor are responsible for causing standing waves inside the food material,

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which in turn cause rapid heating and may lead to hot spots and burning during drying

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(Datta & Anantheswaran, 2001). Thus, the main disadvantage of microwave drying is the lack of uniformity inherent in the electromagnetic field inside the cavity, which can

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generate problems of nonuniform heating, impacting on the quality of the final product and leading to higher consumption of energy (Cohen & Yang, 1995; Datta & Anantheswaran, 2001; Sebera, Nasswettrová, & Nikl, 2012). The uniformity of the microwave drying can be improved by moving the product into the cavity, such as using a turntable or a rotating drum (Cohen & Yang, 1995; Domínguez-Tortajada, PlazaGonzález, Díaz-Morcillo, & Balbastre, 2007; Monteiro, Carciofi, Marsaioli, & Laurindo, 2015; Raaholt & Isaksson, 2017; Sebera et al., 2012). The best microwave drying parameters can be determined from the evolution of the temperature, moisture content, and water activity during drying at different conditions. The drying conditions also influence the physicochemical, nutritional, and sensory properties of the dried product. The changes of apparent density, porosity,

Journal Pre-proof texture, and color during drying have an impact on the product quality and its acceptance by consumers (Madiouli, Sghaier, Lecomte, & Sammouda, 2012). The objective of this study was to produce sweet potato chips by microwave vacuum drying (MWVD), obtaining a crispy snack, oil-free, and without preservatives. The

product

quality

was

evaluated

during

processing

by

determining

the

physicochemical properties, such as moisture content, 𝑎𝑤 , color, apparent density,

2. MATERIALS AND METHODS Selection and preparation of samples

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2.1.

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porosity, microstructure, and texture.

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Fresh sweet potatoes (Ipomoea batatas L. Lam.) were obtained at the local

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market (Florianópolis, SC, Brazil) and kept under refrigeration (8 ± 2 °C) until their use. Samples were selected by visual analysis for avoiding the presence of cuts, holes, and

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microbial deterioration by mold. The selected sweet potatoes had an average length of

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232.0 ± 38.0 mm, and an average diameter measured at the right end, center and left end of 51.5 ± 11.2, 71.0 ± 8.6, and 54.5 ± 11.8 mm, respectively. Selected sweet

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potato samples were washed, peeled, and cut into slices of 4.2 ± 0.5 mm thick using a professional mandolin (Progressive, Model - PL8 ®, USA). For better sample uniformity, the slices were cut into a cylindrical shape using a stainless-steel mold with 41.7 mm internal diameter, which resulted in samples with a diameter of 41.5 ± 0.2 mm. The slices were blanched (97 ± 2 °C for 5 min) and cooled by immersion in cold water (6 ± 2 °C for 3 min) with a sample/water ratio of 1:20 (g: mL). After blanching, the samples were placed for 1 min on a filter paper to remove the excess of water adhered to their surfaces.

2.2.

Drying procedure

The drying of the samples by MWVD was carried out in a microwave oven adapted to operate with a rotary system under vacuum. The microwave oven

Journal Pre-proof (Electrolux, Model MEX55, Brazil) had 45 liters internally, a maximum output power of 1000 W, and microwaves frequency of 2450 MHz. A cylindrical polypropylene container (30.0 cm diameter and 20.2 cm high), connected to the turntable mechanism, was placed inside the oven cavity to allow vacuum application. The rotation of the container results in a more homogeneous absorption of the microwaves by the food during drying. Full details about the adaptation of the microwave vacuum dryer can be found in Monteiro et al., (2015).

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For the drying procedure, the samples (16 slices, 97.04 ± 0.04 g) were uniformly distributed around the edges of a polypropylene circular tray (27.0 cm in

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diameter) and inserted into the vacuum container. Then, the chamber pressure was

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reduced to approximately 4 kPa (measured by a transducer Warme, Model –

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WTP4010, Brazil), when the microwave generator was turned on, keeping the vacuum pump (D.V.P, Vacuum Technology, Model - LC.305, Italy) switched on until the end of

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the drying process. During drying, three different powers were used: 1000 W for the

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first 6 min, 400 W for the next 6 min, and 200 W until the end of the process (15 min). This power manipulation was used to obtain a high drying rate while preventing

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overheating and carbonization of the samples. Traditional microwave ovens operate using the so-called duty cycle, switching from maximum power to zero power (on-off). In the present study, the microwave oven has a cycle period of 29 s. In this way, when set up at 1000 W, the magnetron continually produced microwaves, i.e., it was full time “on.” On the other hand, at 400 and 200 W, it was "on" for 11 and 5 s, respectively, and "off" for 18 and 24 s, respectively. Samples were taken to determine their physicochemical properties during drying. The process was carried out destructively, i.e., each experimental data was determined from a new drying experiment. This procedure was used to avoid the distortion of the results due to the interruption of the drying process (Monteiro et al., 2018b)

Journal Pre-proof

2.3.

Dried samples characterization

The samples were characterized during MWVD by moisture content, 𝑎𝑤 , temperature,

color,

apparent

density,

porosity,

acoustic/mechanical

analysis.

Micrographs of the dehydrated product were taken with an optical microscope, and the moisture sorption isotherm was also determined.

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2.3.1. Moisture The water content was determined by the gravimetric method using a vacuum

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oven (TECNAL, Modelos - TE-395, Brazil) at 70 °C, according to A.O.A.C.

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methodology (2005). The analyses were performed in triplicate.

2.3.2. Water activity

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The samples were ground, and the water activity determined by a digital

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hygrometer (Aqualab Model - Series 3 TE, Decagon Devices, Inc., Pullman, United

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States) by measuring the dew point at 25 C. The analyses were performed in triplicate.

2.3.3. Temperature measurements The temperature of the samples during drying was measured immediately after the oven has been switched off (off-line). It was using a T-type thermocouple (Iope, Model-A-TX-TF-TF-R30AWG, Brazil) connected to a data acquisition system (Agilent Technologies, Model - 34970A, United States). The measurements were performed in triplicate.

2.3.4. Apparent density and porosity The porosity (𝜀) of the samples was estimated from the values of the apparent (𝑉ap ) and true (𝑉𝑡 ) volumes, according to Equation 1.

Journal Pre-proof 𝑉

𝜀 = (1- 𝑉 𝑡 ) .100

(1)

ap

in which 𝑉𝑡 was determined using a compressed air pycnometer (Sereno, Silva, & Mayor, 2007), and 𝑉ap was determined indirectly by the measurement of the buoyant force using n-heptane (Lozano, Rotstein, & Urbicain, 1980; Yan, Sousa-Gallagher, & Oliveira, 2008). The apparent density (𝜌ap ) of the samples was determined by the ratio between the mass of the sample and the 𝑉ap .

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2.3.5. Color measurements

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A Minolta Chroma Meter portable colorimeter (Konica Minolta, Model - CR-400,

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Japan), adjusted to operate with a standard lighting system D65 and 10° viewing angle, was used to determine the color parameters of fresh and dried samples. The

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measurements were performed on the surface (both sides) of four slices of sweet

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potato. The global color was evaluated by the parameter 𝛥E*, using as reference a

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fresh sample (L0 , a0 , and b0 ), defined according to Equation 2. (2)

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𝛥E*= √(𝐿∗ -L0 )2 + (𝑎∗ -a0 )2 + (𝑏 ∗ - b0 )2

2.3.6. Mechanical properties The instrumental texture of the sweet potato slices was determined from the puncture test. The experiments were performed with a 2 mm diameter cylindrical probe in a texture analyzer (Stable Micro System, Model - TA-XT2i, UK) with a Texture Expert Exceed 2.61 program (Stable Micro Systems, UK). The speed test was 3 mm s-1 until penetrating 70% of the sample thickness. The analysis was performed in triplicate, with four perforations in each sample in different spots. The force oscillations during the penetration test are related to potentially crispy samples (Laurindo & Peleg, 2007, 2008; Seymour & Ann, 1988; Van Vliet, Visser, & Luyten, 2007). Thus, the experimental penetration force data were smoothed by a non-

Journal Pre-proof parametric adjustment based in a second-order polynomial approximation, as described by Savitzky & Golay (1964). The normalized residuals were calculated by the difference between the recorded and the adjusted force for the same penetration (Laurindo & Peleg, 2007, 2008). All the calculations were performed using the Matlab® software (Math Works Inc., Model-R2011b, USA), using the sgolayfilt function. Smoothing was performed considering the overall shape of the curve. The secondorder polynomial function and a window with a size of 49 experimental points were

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chosen in the sgolayfilt function for all the smoothing procedures. The residual pattern does not depend on the quality of fit when it only captures the general shape of the

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force-displacement curve (Laurindo & Peleg, 2008).

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The parameters obtained by the mechanical properties were: i) mean area

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below the force versus strain curve, which is related to the total work involved in the test; ii) average of the maximum forces; (iii) average number of peaks (force drop

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below 0.049 N) and (iv) average peaks at each peak.

2.3.7. Acoustic properties

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The acoustic properties during the penetration test were determined using a signal acquisition device, the National Instruments 9234 (NI 9234) AD converter with 24-bit resolution and 102 dB dynamic range. The signals were captured by a microphone (GRAS Sound & Vibration, Model GRAS 46AE ½ "CCP Free-field Microphone Set, Denmark), with a sensitivity of 52.27 mV Pa-1, connected to an IEPE preamplifier connected to the acquisition system. The sampling rate was defined as 51.2 kHz, which covers the human hearing frequency (20 Hz to 20 kHz). The microphone was positioned 5 cm apart and at a 45° angle to the sample. The microphone and the texturometer were placed inside a semi-anechoic chamber to attenuate ambient noise. The acoustic data were treated using a bandpass FIR filter with an attenuation of 60 dB to eliminate the noise produced by the motor of the texturometer. The settings for this filter are the lower 1 kHz band cutoff frequency,

Journal Pre-proof the lower bandpass frequency of 3.125 kHz, the higher 22 kHz band cutoff frequency, and the higher 20 kHz bandpass frequency. The parameters obtained in the analysis of acoustic properties were: i) an average number of peaks (drop in sound pressure level higher than 10 dB); ii) SPL10 (mean of the sound pressure level of the ten largest acoustic peaks) and iii) SPL𝑚𝑎𝑥 (mean of the sound pressure value of the maximum acoustic peak). The analysis was performed in triplicate with four acoustic measurements per

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sample. Full details of the equipment used can be found in Andreani et al., 2020 and

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Murta et al., 2017.

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2.3.8. Optical micrographs

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Micrographs of surfaces and fractures of dried samples were captured using an optical microscope (Meiji, Model - RZ, Japan) with a digital camera (OptiCam, Model -

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(Tucsen, V, 7.3.1.7, China).

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OPT 10000, Brazil). The captured images were analyzed using TSview software

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2.3.9. Moisture Sorption isotherms Moisture sorption isotherms were determined by the gravimetric-static method. Sweet potato samples were dehydrated for 27 min, ground in a knife mill (TECNAL, Model - TE 631 / 2, Brazil), and stored in plastic pots. Next, the powders were dehydrated with phosphorus pentoxide for 60 days at room temperature. Water sorption isotherms were determined from the equilibrium moistures reached by the powder samples submitted to different relative humidity (RH) achieved with saturated saline solutions at 25 ± 1 °C (RH of 11.7% - LiCl, 32.8% - MgCl2, 43.8% - K2CO3, 52.9% Mg(NO3)2, 64.3% - NaNO2, 75.3% - NaCl, 80.4% - (NH4)2SO4, 84.3% - KCl, 90.7% - BaCl2) (Labuza & Ball, 2000).

Journal Pre-proof After reaching equilibrium with saturated salt solutions (9 weeks), the sample's moisture was determined by the gravimetric method. The GAB model (GuggenheimAnderson-de Boer), Equation 3, was fitted to the experimental data.

𝑋𝑒𝑞 =

(𝐶−1).𝐾.𝑎𝑤 .𝑋𝑚 1+(𝐶−1).𝐾.𝑎𝑤

+

𝐾.𝑎𝑤 .𝑋𝑚 1−𝐾.𝑎𝑤

(3) in which 𝑋𝑒𝑞 is the equilibrium moisture, expressed in dry basis, 𝑋𝑚 is the moisture

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absorbed in the monolayer, 𝑎𝑤 is the water activity, 𝐶 is a constant related to the heat

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of sorption of the monolayer, 𝐾 is the constant associated with the heat of sorption of

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the multilayer. The parameters of the GAB model were estimated using nonlinear regression through the least-squares determination with the aid of MATLAB software

2.3.10. Statistical analysis

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(R2011b).

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The results obtained during the color analysis and acoustic/mechanical properties were analyzed statistically with the program Statistica 7.0 (StatSoft, Tulsa,

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USA) using analysis of variance (ANOVA) and Tukey test at 95% confidence.

3. RESULTS AND DISCUSSION 3.1.

Drying kinetics

Fresh-sweet potato samples used in this study had a moisture content (𝑋𝑑𝑏 ) of 3.559 ± 0.240 g g-1 on dry basis (db) (78.07 ± 1.2% in wet basis, wb) and aw of 0.993 ± 0.002. Similar values, between 63% and 83% of moisture (wb), were reported by Aina, Falade, Akingbala, & Titus (2009), Falade & Solademi (2010) and Osundahunsi, Fagbemi, Kesselman, & Shimoni (2003). After the blanching process, the samples had an 𝑋𝑑𝑏 of 4.277 ± 0.380 g g-1 (db), or 81.12 ± 1.4% (wb), and 𝑎𝑤 of 0.994 ± 0.002.

Journal Pre-proof Figure 1 presents the drying curves, water activity (𝑎𝑤 ), samples’ temperature, chamber pressure, and the power applied during MWVD drying. The evolution of the water activity was used as a quick and straightforward way to estimate the approximate water vapor pressure of the product during drying. The blanched sweet potato was dried by MWVD for 27 min and reached 𝑋𝑑𝑏 = 0.028 ± 0.006 g g-1 (db) or 2.7 ± 0.6% (wb), and 𝑎𝑤 = 0.262 ± 0.041. The triplicates of the drying process showed good reproducibility. The 𝑎𝑤 decreased slowly until the first five minutes of drying. However,

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these 𝑎𝑤 values are related to the whole sample, suggesting that the surface water

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activities could be lower. After that, the 𝑎𝑤 values decreased linearly over time. During the whole drying process, the temperature of the samples was below 42 ºC because of

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the vacuum application and the reduction of microwave power (Figure 1).

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The drying rate curves (𝑑𝑋𝑑𝑏 /𝑑𝑡) shown in Figure 1a indicated that the heating period and a possible constant drying rate period was very short. The water activity and

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temperature did not change appreciably between the heating period and the fourth

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minute of drying (Figure 1b and 1c). These results suggest the presence of an almost constant-short drying period that could not be evidenced by the drying curve. In this

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inferred constant period, the drying rate was approximately 0.78 g g-1 min-1. According to Zhang et al. (2006), microwave drying has three periods; a heating period, a constant drying rate period, and a falling drying rate period. The period of constant drying rate is noticeable by the conversion of the microwaves into the latent heat of vaporization of the free water. In the present experiments, the temperature of the sample increased to 30 °C and remained constant until the fourth minute of drying. This temperature is near the boiling temperature of the water at 4 kPa (29 °C), corroborating the existence of a constant (despite short) drying rate period. During the falling rate period the samples’ moisture showed slowly and continuous reduction, while their temperatures have risen above the water boiling point (at 4 kPa) once the rate of microwaves absorption by the samples was higher than that

Journal Pre-proof required to evaporate the water (Monteiro et al., 2016, 2015; Scaman & Durance, 2014).

3.2.

CHARACTERIZATION OF THE SAMPLES DURING DRYING

Micrographs Figure 2 shows images of surfaces and fractures of sweet potato dehydrated by

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MWVD, obtained with the optical microscope. MWVD samples show a porous structure due to the volumetric heating caused by the microwaves, which caused evaporation at

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vacuum pressures. Besides, the vapor flow from the interior to the surface is

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considered the primary mechanism that causes the puffing effect and the formation of

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new porous space (Monteiro et al., 2016, 2018b, 2018a; Zhang et al., 2006). Pictures

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of expanded samples can be seen in Figure 2.

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Acoustic/mechanical properties of samples during drying Food texture is one of the attributes that most influence the choice of

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consumers. Texture can be evaluated through sensory or, indirectly, from instrumental analyzes. Sensory evaluation of the texture is the most used method by industries, but it is expensive and time-consuming. Instrumental methods can be used and correlated with sensory analyses. When this is possible, they can be reliable and practical to suitable to predict sensory texture attributes (L. Chen & Opara, 2013; Kim et al., 2009, 2012). Crispy food is a material that breaks rather than deforms (Fillion & Kilcast, 2002; Vickers, 1983), as is the case of dehydrated sweet potatoes investigated in this study. In this way, mechanical/acoustic behavior of crispy foods during puncture and compression tests can be a technique to evaluate their texture indirectly. Figure 3 shows representative puncture test curves, i.e., strength data as a function of the relative deformation of the samples. The irregularity of the force-deformation curves is

Journal Pre-proof explained from the crackling of the dried-samples structure during the probe penetration. The number and the values of the force peaks can be determined and used to correlate with the product crispness (Laurindo & Peleg, 2007, 2008). The experimental force-deformation data were smoothed, to estimate the crispness of the samples from the irregularities of the curves, and the normalized residuals were calculated by the difference between the recorded force and the adjusted force (Figure 3). The smoothed curves represent the global behavior of the

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force-deformation, whereas the residuals represent the instantaneous oscillations of the force of penetration (González Martínez, Corradini, & Peleg, 2003; Laurindo &

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Peleg, 2007, 2008).

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Blanching modifies the fresh potato structure, with a reduction of both the

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maximum strength and the irregularity of the force-deformation curves (Figure 3a,b). The samples removed from the 4th to the 12th min of drying showed low initial

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resistance to the probe penetration, with a gradual increase of this resistance due to

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the material densification under the probe. This force-penetration pattern is characteristic of soft (non-crispy) materials. It is well known that water has a plasticizing

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effect on foods, which explain the increase of the penetration force during drying until reaching its maximum value (Table 1). According to Peleg (2006), when some cereals and snacks absorb moderately moisture, there is a simultaneous loss of brittleness and a measurable increase of their stiffness and toughness, perceived as hardness by not trained panelists. The force-deformation curves show small irregularities, which is represented by the differences between the smoothed curve and the force-deformation experimental data (residues) from the 12th min of drying (𝑋𝑑𝑏 = 0.139 ± 0.032 g g-1). Only at the end of the drying process (27 min, 𝑋𝑑𝑏 = 0.028 ± 0.006 g g-1) the samples resulted in jagged force-deformation curves, with force oscillations of larger amplitude. This curve pattern is characteristic of crispy foods (Andreani et al., 2020). It is important to comment that sugar and fibers present in sweet potato can delay the crispy texture development during drying (Oh, Lee, & Hong, 2018).

Journal Pre-proof The crispness of foodstuffs can also be assessed by simultaneous analysis of acoustics and mechanical results from puncture or compression tests (Çarşanba, Duerrschmid, & Schleining, 2018; Paula & Conti-Silva, 2014; Philipp, Buckow, Silcock, & Oey, 2017; Saeleaw & Schleining, 2011). This combination provides more information on product crispness than either method separately (Arimi, Duggan, O’Sullivan, Lyng, & O’Riordan, 2010; Vickers, 1987). The sound evaluation of crunchy food texture has been shown to correlate with the acceptance or rejection of crispy

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products by consumers (Saeleaw & Schleining, 2011). The samples were taken from the dryer at predetermined times and subjected

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to acoustic/mechanical tests, and the developed force and the sound pressure level

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(𝑆𝑃𝐿) emitted during penetration of the probe were recorded (Figure 4). The

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synchronized signals allowed the comparison of important mechanical and sound

Table 1.

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events. The parameters obtained from the acoustic/mechanical tests are presented in

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MWVD samples presented the greatest acoustic irregularities after 12 min of drying. However, a low number of force peaks was observed at this drying time, which

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may be related to the lower crispness of the product. Sweet potatoes dehydrated for 27 min presented higher numbers of force and acoustic peaks, as well as higher SPL10 and SPL𝑚𝑎𝑥 . The higher number of force peaks was directly related to the higher number of acoustic peaks, SPL10 and SPL𝑚𝑎𝑥 . The intensity and the high number of acoustic peaks are directly related to the product crispness (Arimi et al., 2010; Çarşanba et al., 2018; Castro-Prada, Luyten, Lichtendonk, Hamer, & Van Vliet, 2007; J. Chen, Karlsson, & Povey, 2005; Duizer, 2001; Piazza & Giovenzana, 2015; Saeleaw & Schleining, 2011; Salvador, Varela, Sanz, & Fiszman, 2009; Sanz, Primo-Martín, & Van Vliet, 2007; Van Hecke, Allaf, & Bouvier, 1998). According to Van Hecke et al. (1998), dry crispy/crunch products show as many peaks as the number of ruptures of the cell walls, without accumulating force. In noncrunchy moist products, there are no abrupt ruptures of the cell walls due to their

Journal Pre-proof viscoelasticity, and a continuous increase of the force developed during the penetration of the probe is observed. In this case, the peaks of force are not present. According to Chauvin, Younce, Ross, & Swanson (2008), cellular foods that contain only air within intercellular spaces, such as potato chips, are called dried-and-crispy foods. On the other hand, foods that contain fluids inside their cells, such as fruits and vegetables, are called crunchy foods. Thus, fresh sweet potato can be considered a moist crunchy food, which presents small irregularities in the acoustic-mechanical curves,

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characteristic of fresh and crunchy vegetables. The work required to deform the sample is related to the area under the force-

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deformation curve. This area, the maximum force, and the number of force peaks

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increased until the 12th min of drying. The increase of both the area and the maximum

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force are associated with the hardness of the material felt by panelists (Chanvrier, Jakubczyk, Gondek, & Gumy, 2014). After 27 min of drying the area under the curve,

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and the mean value of the force peaks decreased. It is related to the formation of a

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porous and fragile structure. The expansion of the food matrix (puffing effect), along with the reduction of the sample moisture, produced a crispy structure that shows many

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peaks of force during the puncture test, as observed in Table 1.

Porosity and apparent density of samples during drying: Figure 5 shows the apparent density (𝜌ap ) and porosity (𝜀) of sweet potato slices during drying by MWVD, which showed that these physical properties change appreciably during drying. This influence has been reported in the literature by several studies (Krokida & Maroulis, 1997, 1999; Madiouli et al., 2007; Porciuncula, Segura, & Laurindo, 2016). Fresh sweet potatoes presented 𝜌ap of 1.077 ± 0.007 g cm-3 and 𝜀 of 4.3 ± 2.1%. The blanching process caused small water absorption and reduced the porosity to 2.2 ± 0.7%. The water absorption and the temperature of 97 ± 2 °C caused starch gelatinization, though partially because of the short blanching time. During

Journal Pre-proof drying, ε increased and 𝜌ap decreased, as already reported by Porciuncula et al. (2016) for conductive multi-flash drying (KMFD), convective drying, and vacuum drying of banana samples. At the end of the drying process (27 min), the bulk density of the samples was reduced to 0.456 ± 0.119 g cm-3, while the porosity increased to 67.5 ± 8.6%. During the MWVD, the increase of the capillary pressure, which causes shrinkage, is compensated by forces related to gas expansion caused by volumetric heating under

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vacuum (Segura, Badillo, & Alves-Filho, 2014). Thus, drying by MWVD can produce

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Color evolution during drying

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highly porous dehydrated sweet potatoes.

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Figure 6 shows the photographs of samples of fresh, blanched sweet potatoes and samples during drying by MWVD. The mean values (± standard deviation) of the

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CIELab color scale parameters (𝐿∗, 𝑎∗ , 𝑏 ∗, and 𝛥E*) measured in the samples taken

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during drying are shown in Table 2.

Through Figure 6 and the parameter 𝐿∗, which indicates the lightness, it is

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observed that blanched samples darkened to the fresh samples. However, MWVD samples presented lighter staining compared to blanched samples, demonstrated by the higher values of 𝐿∗. After blanching, the 𝑎∗ and 𝑏 ∗ parameters decreased as compared to the values determined from fresh samples. Subsequently to drying, there was an increase in both parameters when compared to those measured in the blanched samples, indicating that drying modified the sample's color for shades closer to red (higher parameter 𝑎∗ ) and yellow (higher 𝑏 ∗). Overall, dry samples presented low values of 𝛥E* (≈10), indicating few color changes when compared to fresh samples. Similar results on the behavior of color parameters were reported by Setiady et al. (2009) for microwave vacuum drying, convective drying, and freeze-drying of potato

Journal Pre-proof samples. Potato samples did not present significant differences (p ≤ 0.05) of the parameters 𝑎∗ , 𝑏 ∗, 𝛥E*, during drying. The results obtained in this study show that MWVD produces dehydrated sweet potatoes with uniform coloration, like fresh samples, and the absence of burned parts, thus being good-looking to consumers.

Sorption Isotherms

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Figure 7 shows the experimental moisture sorption isotherms of MWVD samples, with the GAB model fitted to the experimental data. The GAB model

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represented well the experimental data, with a determination coefficient of 0.999 and a

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root-mean-square error of 0.003. The model parameters are 𝑋𝑚 = 0.049 g g-1 (db),

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associated with the monolayer moisture, 𝐾 = 0.941, and 𝐶 = 38.09. The curve is a type II isotherm (Brunauer, Emmett, & Teller, 1938). After aw of 0.64, there is a rapid

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increase in the amount of adsorbed water, due to condensed water (water films) and

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the formation of sugar solution. Then, samples with aw higher than 0.64 presented force-deformation curves of soft materials; only after 27 min of drying the crispy texture

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was observed for all the samples of dried sweet potato, when their water activities ranged between 0.2 and 0.3. Therefore, to stay crispy during storage, these chips of sweet potatoes (aw =0.2-0.3) must be stored in packages with very low water vapor permeability.

4. CONCLUSIONS Microwave vacuum drying of blanched sweet potatoes can be done at very high drying rates, with a drying time of 30 minutes. These chips present low moisture content (approximately 0.03 g g-1) and water activity near 0.2-0.3. Moreover, the sweet potato chips have a very porous structure and color close to the fresh samples. The puncture test performed with the chips resulted in jagged force-penetration curves and simultaneous sound-pressure-level curves with a huge number of acoustic peaks and

Journal Pre-proof high sound pressures level, characteristic of crispy products. Therefore, microwave vacuum drying has great potential for producing attractive-and-crispy sweet potato chips. Future works should correlate sensory to the instrumental texture of sweet potato chips, as well as their acceptance by consumers.

ACKNOWLEDGEMENTS Authors thanks to the National Council for Scientific and Technological Development

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(CNPq-Brazil) for financial support and Coordination for the Improvement of Higher

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Education Personnel (CAPES-Brazil) for the scholarship.

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REFERENCES

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A.O.A.C. (2005). Association of Official Analytical. Chemists. Official methods of analysis (18th ed.). Gaithersburg, Maryland: A.O.A.C. INTERNATIONAL. Aguilera, J. M., Chiralt, A., & Fito, P. (2003). Food dehydration and product structure. Trends in Food Science and Technology, 14(10), 432–437. https://doi.org/10.1016/S0924-2244(03)00122-5 Aina, A. J., Falade, K. O., Akingbala, J. O., & Titus, P. (2009). Physicochemical properties of twenty-one Caribbean sweet potato cultivars. International Journal of Food Science and Technology, 44(9), 1696–1704. https://doi.org/10.1111/j.13652621.2009.01941.x Allen, J. C., Corbitt, A. D., Maloney, K. P., Butt, M. S., & Truong, V. (2012). Glycemic index of sweet potato as affected by cooking methods. The Open Nutrition Journal, 6(1). Andreani, P., de Moraes, J. O., Murta, B. H. P., Link, J. V., Tribuzi, G., Laurindo, J. B., … Carciofi, B. A. M. (2020). Spectrum crispness sensory scale correlation with instrumental acoustic high-sampling rate and mechanical analyses. Food Research International, 129(December 2019). https://doi.org/10.1016/j.foodres.2019.108886 Arimi, J. M., Duggan, E., O’Sullivan, M., Lyng, J. G., & O’Riordan, E. D. (2010). Effect of water activity on the crispiness of a biscuit (Crackerbread): Mechanical and acoustic evaluation. Food Research International, 43(6), 1650–1655. https://doi.org/10.1016/j.foodres.2010.05.004 Barreto, I. M. A., Tribuzi, G., Marsaioli Junior, A., Carciofi, B. A. M., & Laurindo, J. B. (2019). Oil–free potato chips produced by microwave multiflash drying. Journal of Food Engineering, 261(May), 133–139. https://doi.org/10.1016/j.jfoodeng.2019.05.033 Brunauer, S., Emmett, P. H., & Teller, E. (1938). Adsorption of Gases in Multimolecular Layers. Journal of the American Chemical Society, 60(2), 309–319. https://doi.org/10.1021/ja01269a023 Çarşanba, E., Duerrschmid, K., & Schleining, G. (2018). Assessment of acousticmechanical measurements for crispness of wafer products. Journal of Food Engineering, 229, 93–101. https://doi.org/10.1016/j.jfoodeng.2017.11.006 Castro-Prada, E. M., Luyten, H., Lichtendonk, W., Hamer, R. J., & Van Vliet, T. (2007). An improved instrumental characterization of mechanical and acoustic properties

Journal Pre-proof

Jo ur

na

lP

re

-p

ro

of

of crispy cellular solid food. Journal of Texture Studies, 38(6), 698–724. https://doi.org/10.1111/j.1745-4603.2007.00121.x Chanvrier, H., Jakubczyk, E., Gondek, E., & Gumy, J. C. (2014). Insights into the texture of extruded cereals: Structure and acoustic properties. Innovative Food Science and Emerging Technologies, 24, 61–68. https://doi.org/10.1016/j.ifset.2013.11.013 Chauvin, M. A., Younce, F., Ross, C., & Swanson, B. (2008). Standard scales for crispness, crackliness and crunchiness in dry and wet foods: Relationship with acoustical determinations. Journal of Texture Studies, 39(4), 345–368. https://doi.org/10.1111/j.1745-4603.2008.00147.x Chen, J., Karlsson, C., & Povey, M. (2005). Assessment of Biscuits. Journal of Texture Studies, 36(00), 139–156. Chen, L., & Opara, U. L. (2013). Approaches to analysis and modeling texture in fresh and processed foods - A review. Journal of Food Engineering, 119(3), 497–507. https://doi.org/10.1016/j.jfoodeng.2013.06.028 Clary, C. D., Mejia-Meza, E., Wang, S., & Petrucci, V. E. (2007). Improving grape quality using microwave vacuum drying associated with temperature control. Journal of Food Science, 72(1), 23–28. https://doi.org/10.1111/j.17503841.2006.00234.x Cohen, J. S., & Yang, T. C. S. (1995). Progress in food dehydration. Trends in Food Science & Technology, 61(January). Datta, A. K. ., & Anantheswaran, R. C. (2001). Handbook of Microwave Technology for Food Applications. New York: Marcel Dekker Inc. Domínguez-Tortajada, E., Plaza-González, P., Díaz-Morcillo, A., & Balbastre, J. V. (2007). Optimisation of electric field uniformity in microwave heating systems by means of multi-feeding and genetic algorithms. International Journal of Materials and Product Technology, 29(1–4), 149–162. Drouzas, A. E., & Schubert, H. (1996). Microwave application in vacuum drying of fruits. Journal of Food Engineering, 28(2), 203–209. https://doi.org/10.1016/02608774(95)00040-2 Duizer, L. (2001). A review of acoustic research for studying the sensory perception of crisp, crunchy and crackly textures. Trends in Food Science and Technology, 12(1), 17–24. https://doi.org/10.1016/S0924-2244(01)00050-4 Falade, K. O., & Solademi, O. J. (2010). Modelling of air drying of fresh and blanched sweet potato slices. International Journal of Food Science and Technology, 45(2), 278–288. https://doi.org/10.1111/j.1365-2621.2009.02133.x Fellows, P. J. (2009). Food processing technology: principles and practice. Elsevier. Fillion, L., & Kilcast, D. (2002). Consumer perception of crispness and crunchiness in fruits and vegetables. Food Quality and Preference, 13(1), 23–29. https://doi.org/10.1016/S0950-3293(01)00053-2 González Martínez, C., Corradini, M. G., & Peleg, M. (2003). Effect of Moisture on the Mechanical Properties of Pork Rind (Chicharrón). Food Science and Technology International, 9(4), 249–255. https://doi.org/10.1177/108201303031118 Gunasekaran, S. (1999). Pulsed microwave-vacuum drying of food materials. Drying Technology, 17(3), 395–412. https://doi.org/10.1080/07373939908917542 Hu, Q. G., Zhang, M., Mujumdar, A. S., Xiao, G. N., & Jin-cai, S. (2006). Drying of edamames by hot air and vacuum microwave combination. Journal of Food Engineering, 77(4), 977–982. https://doi.org/10.1016/j.jfoodeng.2005.08.025 Incropera, F. P., Lavine, A. S., Bergman, T. L., & DeWitt, D. P. (2007). Fundamentals of heat and mass transfer. Wiley. Ishida, H., Suzuno, H., Sugiyama, N., Innami, S., Tadokoro, T., & Maekawa, A. (2000). Nutritive evaluation on chemical components of leaves, stalks and stems of sweet potatoes (Ipomoea batatas poir). Food Chemistry, 68(3), 359–367. https://doi.org/10.1016/S0308-8146(99)00206-X Jayaraman, K. S. ., & Gupta, D. K. (2014). Handbook of Industrial Drying. Cap. 25:

Journal Pre-proof

Jo ur

na

lP

re

-p

ro

of

Drying of Fruit and Vegetables (A. S. MUJUMDAR, Ed.). Taylor & Francis Group. Kim, E. H. J., Corrigan, V. K., Hedderley, D. I., Motoi, L., Wilson, A. J., & Morgenstern, M. P. (2009). Predicting the sensory texture of cereal snack bars using instrumental measurements. Journal of Texture Studies, 40(4), 457–481. Kim, E. H. J., Corrigan, V. K., Wilson, A., Waters, I. R., Hedderley, D. I., & Morgenstern, M. P. (2012). Fundamental fracture properties associated with sensory hardness of brittle solid foods. Journal of Texture Studies, 43(1), 49–62. https://doi.org/10.1111/j.1745-4603.2011.00316.x Krokida, M. K., & Maroulis, Z. B. (1997). Effect of drying method on shrinkage and porosity. Drying Technology, 15(10), 2441–2458. https://doi.org/10.1080/07373939708917369 Krokida, M. K., & Maroulis, Z. B. (1999). Effect of microwave drying on some quality properties of dehydrated products. Drying Technology, 17(3), 449–466. https://doi.org/10.1080/07373939908917545 Labuza, T. P., & Ball, L. N. (2000). Moisture Sorption – Practical aspects of isotherm measurement and use (2nd ed.; A. A. of C. Chemists, Ed.). Laurindo, J. B., & Peleg, M. (2007). Mechanical measurements in puffed rice cakes. Journal of Texture Studies, 38(5), 619–634. https://doi.org/10.1111/j.17454603.2007.00116.x Laurindo, J. B., & Peleg, M. (2008). Mechanical characterization of shredded wheat. Journal of Texture Studies, 39(5), 444–459. https://doi.org/10.1111/j.17454603.2008.00153.x Lin, T. M. ., Durance, T. D. ., & Scaman, C. H. (1998). Characterization of vacuum microwave, air and freeze dried carrot slices. Food Research International, 31(2), 111–117. https://doi.org/10.1016/S0963-9969(98)00070-2 Link, J. V., Tribuzi, G., & Laurindo, J. B. (2017). Improving quality of dried fruits: A comparison between conductive multi-flash and traditional drying methods. LWT Food Science and Technology, 84, 717–725. https://doi.org/10.1016/j.lwt.2017.06.045 Lozano, J. E., Rotstein, E., & Urbicain, M. J. (1980). Total Porosity and Open‐Pore Porosity in the Drying of Fruits. Journal of Food Science, 45(5), 1403–1407. https://doi.org/10.1111/j.1365-2621.1980.tb06564.x Madiouli, J., Lecomte, D., Nganya, T., Chavez, S., Sghaier, J., & Sammouda, H. (2007). A method for determination of porosity change from shrinkage curves of deformable materials. Drying Technology, 25(4), 621–628. https://doi.org/10.1080/07373930701227185 Madiouli, J., Sghaier, J., Lecomte, D., & Sammouda, H. (2012). Determination of porosity change from shrinkage curves during drying of food material. Food and Bioproducts Processing, 90(1), 43–51. Monteiro, R. L., Carciofi, B. A. M., & Laurindo, J. B. (2016). A microwave multi-flash drying process for producing crispy bananas. Journal of Food Engineering, 178, 1–11. https://doi.org/10.1016/j.jfoodeng.2015.12.024 Monteiro, R. L., Carciofi, B. M., Marsaioli, A., & Laurindo, J. B. (2015). How to make a microwave vacuum dryer with turntable. Journal of Food Engineering, 166, 276– 284. https://doi.org/10.1016/j.jfoodeng.2015.06.029 Monteiro, R. L., Link, J. V, Tribuzi, G., Carciofi, B. A. M., & Laurindo, J. B. (2018a). Effect of multi-flash drying and microwave vacuum drying on the microstructure and texture of pumpkin slices. Lwt, 96(February), 612–619. https://doi.org/10.1016/j.lwt.2018.06.023 Monteiro, R. L., Link, J. V, Tribuzi, G., Carciofi, B. A. M., & Laurindo, J. B. (2018b). Microwave vacuum drying and multi-flash drying of pumpkin slices. Journal of Food Engineering, 232, 1–10. https://doi.org/10.1016/j.jfoodeng.2018.03.015 Mousa, N., & Farid, M. (2002). Microwave vacuum drying of banana slices. Drying Technology, 20(10), 2055–2066. https://doi.org/10.1081/DRT-120015584 Murta, B. H. P. ., Andreani, P. ., Carciofi, B. A. M., Tribuzi, G., Moraes, J. O. ., & Paul,

Journal Pre-proof

Jo ur

na

lP

re

-p

ro

of

S. (2017). Challenges on developing an acoustical measurement system for applications in food engineering. Anais Do 15o Congresso de Engenharia de Áudio Da AES Brasil. Oh, S., Lee, E. J., & Hong, G. P. (2018). Quality characteristics and moisture sorption isotherm of three varieties of dried sweet potato manufactured by hot air semidrying followed by hot-pressing. Lwt, 94(February), 73–78. https://doi.org/10.1016/j.lwt.2018.04.044 Osundahunsi, O. F., Fagbemi, T. N., Kesselman, E., & Shimoni, E. (2003). Comparison of the physicochemical properties and pasting characteristics of flour and starch from red and white sweet potato cultivars. Journal of Agricultural and Food Chemistry, 51(8), 2232–2236. https://doi.org/10.1021/jf0260139 Paula, A. M., & Conti-Silva, A. C. (2014). Texture profile and correlation between sensory and instrumental analyses on extruded snacks. Journal of Food Engineering, 121, 9–14. Peleg, M. (2006). On fundamental issues in texture evaluation and texturization - A view. Food Hydrocolloids, 20(4), 405–414. https://doi.org/10.1016/j.foodhyd.2005.10.008 Philipp, C., Buckow, R., Silcock, P., & Oey, I. (2017). Instrumental and sensory properties of pea protein-fortified extruded rice snacks. Food Research International, 102(May), 658–665. https://doi.org/10.1016/j.foodres.2017.09.048 Piazza, L., & Giovenzana, V. (2015). Instrumental acoustic-mechanical measures of crispness in apples. Food Research International, 69, 209–215. Porciuncula, B. D. A., Segura, L. A., & Laurindo, J. B. (2016). Processes for controlling the structure and texture of dehydrated banana. Drying Technology, 34(2), 167– 176. https://doi.org/10.1080/07373937.2015.1014911 Raaholt, B. W., & Isaksson, S. (2017). Improving the heating uniformity in microwave processing. In The microwave processing of foods (pp. 381–406). Elsevier. Ratti, C. (2001). Hot air and freeze-drying of high-value foods: A review. Journal of Food Engineering, 49(4), 311–319. https://doi.org/10.1016/S0260-8774(00)002284 Sablani, S. S. (2006). Drying of fruits and vegetables: Retention of nutritional/functional quality. Drying Technology, 24(2), 123–135. https://doi.org/10.1080/07373930600558904 Saeleaw, M., & Schleining, G. (2011). A review: Crispness in dry foods and quality measurements based on acoustic–mechanical destructive techniques. Journal of Food Engineering, 105(3), 387–399. Salvador, A., Varela, P., Sanz, T., & Fiszman, S. M. (2009). Understanding potato chips crispy texture by simultaneous fracture and acoustic measurements, and sensory analysis. LWT-Food Science and Technology, 42(3), 763–767. Sanz, T., Primo-Martín, C., & Van Vliet, T. (2007). Characterization of crispness of French fries by fracture and acoustic measurements, effect of pre-frying and final frying times. Food Research International, 40(1), 63–70. Savitzky, A., & Golay, M. J. E. (1964). Smoothing and Differentiation of Data by Simplified Least Squares Procedures. Analytical Chemistry, 36(8), 1627–1639. https://doi.org/10.1021/ac60214a047 Scaman, C. H. ., & Durance, T. . (2014). Combined Microwave Vacuum-drying. In D. W. Sun (Ed.), Emerging Technologies for Food Processing. London: Elsevier Academic Press. Sebera, V., Nasswettrová, A., & Nikl, K. (2012). Finite Element Analysis of Mode Stirrer Impact on Electric Field Uniformity in a Microwave Applicator. Drying Technology, 30(13), 1388–1396. https://doi.org/10.1080/07373937.2012.664800 Segura, L. A., Badillo, G. M., & Alves-Filho, O. (2014). Microstructural Changes of Apples (Granny Smith) During Drying: Visual Microstructural Changes and Possible Explanation from Capillary Pressure Data. Drying Technology, 32(14), 1692–1698. https://doi.org/10.1080/07373937.2014.919001

Journal Pre-proof

Jo ur

na

lP

re

-p

ro

of

Sereno, A. M., Silva, M. A., & Mayor, L. (2007). Determination of particle density and porosity in foods and porous materials with high moisture content. International Journal of Food Properties, 10(3), 455–469. https://doi.org/10.1080/10942910600880736 Setiady, D., Tang, J., Younce, F., Swanson, B. A., Rasco, B. A., & Clary, C. A. (2009). Porosity, color, texture and microscopic structure of Russet potatoes dried using microwave vacuum, heated air, and freeze drying. Applied Engineering in Agriculture, 25(5), 719–724. Seymour, S. K., & Ann, D. D. H. A. M. (1988). Crispness and crunchiness of selected low moisture foods. Journal of Texture Studies, 19(1), 79–95. Singh, R. P., & Heldman, D. R. (2009). Introduction to food engineering (4th ed.). California: Elsevier Inc. Tribuzi, G., & Laurindo, J. B. (2016). Dehydration and Rehydration of Cooked Mussels. International Journal of Food Engineering, 12(2), 173–180. https://doi.org/10.1515/ijfe-2015-0275 Van Arsdel, W. B., Copley, M. J., & Morgan, A. I. (1963). Food Dehydration: Principles. AVI Publishing Company, Incorporated. Van Hecke, E., Allaf, K., & Bouvier, J. M. (1998). Texture and structure of crispy‐puffed food products part II: Mechanical properties in puncture. Journal of Texture Studies, 29(6), 617–632. Van Vliet, T., Visser, J. E., & Luyten, H. (2007). On the mechanism by which oil uptake decreases crispy/crunchy behaviour of fried products. Food Research International, 40(9), 1122–1128. https://doi.org/10.1016/j.foodres.2007.06.007 Vickers, Z. M. (1983). Pleasantness of food sounds. Journal of Food Science, 48(3), 783–786. Vickers, Z. M. (1987). Sensory, acoustical, and force‐deformation measurements of potato chip crispness. Journal of Food Science, 52(1), 138–140. Yan, Z., Sousa-Gallagher, M. J., & Oliveira, F. A. R. (2008). Shrinkage and porosity of banana, pineapple and mango slices during air-drying. Journal of Food Engineering, 84(3), 430–440. https://doi.org/10.1016/j.jfoodeng.2007.06.004 Zhang, M., Tang, J., Mujumdar, A. S., & Wang, S. (2006). Trends in microwave-related drying of fruits and vegetables. Trends in Food Science and Technology, 17(10), 524–534. https://doi.org/10.1016/j.tifs.2006.04.011 Zotarelli, M. F., Porciuncula, B. D. A., & Laurindo, J. B. (2012). A convective multi-flash drying process for producing dehydrated crispy fruits. Journal of Food Engineering, 108(4), 523–531. https://doi.org/10.1016/j.jfoodeng.2011.09.014

Journal Pre-proof Table 1 - Mean values and standard deviations of the parameters obtained in acousticmechanical tests. Acoustic instrumental parameters Averag Numbe SPL1 Sample Area Number of Maximum force e Peak r of SPLmá 0 s (N.mm) peaks (N) Forces acousti x (dB) (dB) (N) c peaks 74,3 79,5 ± 575 ± 11,4 ± b* c ab b a Fresh 30,9 ± 4,0 13 ± 3 13,0 ± 1,2 ± 9,5 441 b 2,4 b 8,7 a 61,8 66,5 ± 15 ± 9 1,1± a ab a a 0 min 2,6 ± 0,7 3±1 1,7 ± 0,6 ± 9,8 a 0,7 a 7,1 a 59,3 64,0 ± 12 ± 6 0,9 ± a ab a a 2 min 1,9 ± 1,1 2±1 1,4 ± 1,2 ± 5,8 a 1,1 a 3,3 a 60,4 63,8 ± 14 ± 6 1,6 ± a ab a a 4 min 2,0 ± 0,6 2±0 2,7 ± 1,6 ± 4,9 a 1,8 a 3,8 a 59,5 63,7 ± 12 ± 7 3,6 ± a a a a 6 min 3,0 ± 1,6 2±1 6,0 ± 3,2 ± 6,1 ab 3,8 a 3,8 a 59,4 63,4 ± 8 ± 5 4,4 ± a a a a 8 min 4,2 ± 3,9 2±0 6,3 ± 6,0 ± 6,4 ab 6,0 a 3,5 a 62,5 67,4 ± 19 ± 16 29,1 ± b ab b ab 10 min 46,8 ± 43,3 3±1 32,9 ±19,1 ± 9,7 c 20,8 a 7,3 89,6 97,1 ± 606 ± 47,6 ± c b c c a 12 min 82,2 ± 36,0 6±4 66,8 ± 25,2 ± 16,5 568 d 24,0 b 16,6 107, 112,3 3389 ± 12,2± b c c d b 27 min 35,3 ± 18,8 16 ± 6 55,1 ± 38,4 1 ± ± 7,2 1454 b 18,6 c 6,3 *Means followed by different letters in the same column represent significant differences (p≤0.05) according to Tukey's test.

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Mechanical instrumental parameters

Journal Pre-proof Table 2 - Color parameters (𝐿∗ , 𝑎∗, 𝑏∗ , and 𝛥E*) of fresh, blanched and dried sweet potatoes by MWVD.

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Samples 𝜟E* 𝑳∗ 𝒂∗ 𝒃∗ d c b Fresh 83,45 ± 1,36 ** -2,24 ± 0,94 30,65 ± 3,00 a a a 0 min 57,09 ± 2,17 -7,21 ± 0,74 24,16 ± 1,60 27,66 ± 2,05b 2 min 73,98 ± 3,83b -5,21 ± 0,57b 36,16 ± 2,75c 12,02 ± 2,36a cb b cd 4 min 77,28 ± 2,38 -4,66 ± 0,43 37,54 ± 2,12 9,74 ± 2,56a cd b d 6 min 80,49 ± 2,33 -4,94 ± 0,64 39,70 ± 2,41 10,15 ± 2,48a 8 min 80,17 ± 3,79cd -4,85 ± 0,36b 39,59 ± 2,29d 10,58 ± 2,00a d b d 10 min 81,12 ± 1,3 -4,74 ± 0,49 40,41 ± 1,81 10,46 ± 1,61a cd b d 12 min 80,24 ± 1,73 -4,52 ± 0,40 39,66 ± 1,94 9,94 ± 2,11a 27 min 77,67 ± 3,09c -4,51 ± 0,63b 38,03 ± 1,95cd 10,04 ± 2,26a **Means followed by different letters in the same column represent significant

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differences (p≤0.05) according to Tukey's test.

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Journal Pre-proof Figure 1 - a) Curves of sweet potato drying by MWVD (triplicate) and temporal evolution of the drying rate; b) Time evolution of samples water activity (aw) (triplicate) and power (▬) applied during drying; c) Temporal evolution of samples temperature (triplicate and standard deviation) and time evolution of the pressure in the system (▬). Figure 2 – a) Images obtained by optical microscopy of fracture (A) and surface (B) of sweet potato samples dehydrated by MWVD. Figure 3 - Amplitude of force oscillation from representative results of penetration tests of dehydrated sweet potato samples. Left column: Strength-deformation data (○) and

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non-parametric adjustment (▬); Right column: curves of residues. (a) fresh samples,

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(b) bleached, (c) 2 min, (d) 4 min, (e) 6 min, (f) 8 min, (f) 8 min, (g) 10 min, (h) 12 min,

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(i) 27 min.

Figure 4 - Evolution of applied force (▬) and sound pressure level (SPL) (▬) versus

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the sample penetration time: (a) fresh, (b) bleached, (c) 2 min, (d) 4 min, (e) 6 min,

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(f) 8 min, (g)10 min, (h) 12 min, (i) 27 min.

Figure 5 - Evolution of porosity (a) and apparent density (b) with time and moisture

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content in wet bases (Xwb) of sweet potato slices during drying by MWVD. Figure 6 - Photograph of the potatoes taken in different stages of drying by MWVD.

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Samples: (1) Fresh, (2) blanched - 0 min, (3) 2 min, (4) 4 min, (5) 6 min, (6) 8 min, (7) 10 min, (8) 12 min, 9) 27 min.

Figure 7 - Sorption isotherms of sweet potatoes dehydrated by MWVD. Experimental data (●). The curve-adjusted GAB model is represented by continuous line.

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Journal Pre-proof Conflict of interest The authors do not have conflict of interest concerning the subject of this manuscript

Author Statement

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R. L. Monteiro: Conceptualization; methodology; software; validation; formal analysis; investigation; data curation; writing - original draft; writing - review & editing; visualization.

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J. O. de Moraes: Methodology; software; formal analysis; writing - review & editing.

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J. Domingos: Validation; investigation; writing - review & editing.

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B. A. M. Carciofi: Conceptualization; methodology; resources; writing - review & editing; supervision; funding acquisition.

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J. B. Laurindo: Conceptualization; methodology; resources; writing - review & editing; supervision; project administration; funding acquisition.

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Journal Pre-proof Highlights:

Dried-and-crisp sweet potato slices are attractive snacks.

Microwave vacuum drying is a suitable process for the production of sweet potato crispy chips, oil-free, and without preservatives

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Microwave vacuum drying allows producing dried sweet potato chips in short times, with affordable costs

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