J Food Process Preserv. 2019;00:e14279. wileyonlinelibrary.com/journal/jfpp  | 1 of 12https://doi.org/10.1111/jfpp.14279

© 2019 Wiley Periodicals, Inc.

1  | INTRODUCTION

Fruits and vegetables are a rich source of vitamin C, minerals, and phytochemicals. However, some of these nutrients are prone to degradation during food processing operations because of their sensitivity to temperature, light, and oxygen. Characterizing the kinetics of nutrient degradation during processing is essential for

optimizing operating parameters toward minimal nutrient degrada‐tion. Accordingly, empirical models have been previously developed (Devahastin & Niamnuy, 2010) to predict changes in nutrient con‐centrations during food processing.

Drying is an essential unit operation in the food processing in‐dustry. The objective of removing moisture from food could be to extend shelf life, reduce the cost of transportation and storage, or

Received:18May2019  |  Revised:19September2019  |  Accepted:16October2019DOI:10.1111/jfpp.14279

O R I G I N A L A R T I C L E

Kinetics of nutrient change and color retention during low‐temperature microwave‐assisted drying of bitter melon (Momordica charantia L.)

Thi‐Van‐Linh Nguyen1,2 |   Phuoc‐Bao‐Duy Nguyen3 |   Xuan‐Cuong Luu1 | Bao‐Long Huynh4 | Sitaraman Krishnan5  |   Phong T. Huynh2,5

1Faculty of Environmental and Food Engineering, Nguyen Tat Thanh University, Ho Chi Minh City, Vietnam2Faculty of Chemical Engineering, HCMC University of Technology, Vietnam National University Ho Chi Minh City (VNU‐HCM), Ho Chi Minh City, Vietnam3Faculty of Electrical and Electronics Engineering, Vietnam National University Ho Chi Minh City (VNU‐HCM), Ho Chi Minh City, Vietnam4Faculty of Chemical Engineering, Ho Chi MinhCityUniversityofFoodIndustry,HoChi Minh City, Vietnam5Department of Chemical and Biomolecular Engineering, Clarkson University, Potsdam, NY,USA

CorrespondenceSitaramanKrishnanandPhongT.Huynh,Department of Chemical and Biomolecular Engineering, Clarkson University, Potsdam, NY13699,USA.Email:skrishna@clarkson.edu(S.K.)andhuynhtienphong@hcmut.edu.vn (P. T. H.)

AbstractThe changes in nutrient (phenolics and vitamin C) and color of bitter melon slices, during microwave‐assisted drying (MWAD) at different microwave power values (1.5, 3.0, and 4.5 W/g) and air temperatures (20, 25, and 30°C) are reported. Fits of the experimental data to a semi‐empirical mathematical model were used to determine the half‐lives of the nutrients. Color change during drying was similarly characterized usingCIELABcolorspaceparameters.Theratesofnutrientdegradationandcolorchange increased with an increase in microwave power, and drying temperature, but the drying times decreased significantly. The total electrical energy consumed under various drying conditions was measured. Although a microwave power of 1.5 W/g and an air temperature of 25°C resulted in the highest levels of nutrient retention, the dryingtimeandprocessenergyconsumptionwererelativelyhigh.Incomparison,thedrying conditions of 4.5 W/g and 30°C provided an acceptable compromise.

Practical applicationsLow‐temperatureMWADisapromisingalternativetoconventionaldryingforfoodthat contains temperature‐sensitive nutrients. This article provides a methodology for analyzing the kinetics of nutrient and color change during a MWAD process. A 32 factorial design was used to derive predictive correlations for drying time, nutrient half‐life, time constants characterizing color change, and electrical energy consumed during the drying process. The relative influences of microwave power and drying temperature could be readily discerned using these correlations. The reported study will help in developing and optimizing microwave drying processes for a variety of food products.

www.wileyonlinelibrary.com/journal/jfppmailto:mailto:https://orcid.org/0000-0002-1228-8393mailto:https://orcid.org/0000-0003-2379-7107mailto:skrishna@clarkson.edumailto:huynhtienphong@hcmut.edu.vnhttp://crossmark.crossref.org/dialog/?doi=10.1111%2Fjfpp.14279&domain=pdf&date_stamp=2019-11-05

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to obtain a product that is preferred dry (e.g., dried fruit). There are many dehydration methods available for this purpose. These include drying using hot air or vacuum, freeze‐drying, and microwave drying. However, to preserve the heat‐sensitive nutrient content of foods, low‐temperature drying methods such as vacuum and freeze‐dry‐ingarepreferred (deBruijnetal.,2016;Figiel&Michalska,2017).These approaches are often associated with a low driving force for moisture removal, and high equipment maintenance and operation costs. Recently, microwave‐vacuum drying was reported as a means to enhance the drying rate (Ando et al., 2019; Das & Arora, 2018).

Inworkreportedherein,volumetricheatingusingmicrowavewasused to increase the driving force for low‐temperature air drying of bitter melon. The rapid evaporation of water due to fast microwave energy transfer and internal heat generation (which provides for the heat of vaporization of water, ≅ 2.442 kJ/g at 25°C), results in a high mass fluxof escapingwater vapor (Lyons,Hatcher,&Sunderland,1972; Prabhanjan, Ramaswamy, & Raghavan, 1995). Hence, quicker drying is possible by volumetric heating than by the conventional surface heating methods wherein temperature gradients and evap‐oration‐induced concentration gradients within a material limit its dryingrate(Lyonsetal.,1972).Microwave‐assisteddrying(MWAD)have been previously reported to reduce drying times of mate‐rials such as carrot cubes (Prabhanjan et al., 1995) and okra slices (Kumar,Prasad,&Murthy,2014)comparedwithconventionaldry‐ing. However, there are few reports investigating nutrient changes caused by the microwave power input.

Bitter melon is a highly nutritious food for culinary and therapeu‐tic purposes because of its bioactive compounds such as phenolics, inparticular,flavonoids,andvitaminC(Beheraetal.,2010;Tan,Kha,Parks,&Roach,2016).ItsvitaminCcontentofabout40mg/100goffresh fruit is comparable to those in other fruits, such as strawberry. Vitamin C is prone to degradation during processing and storage, due to various factors, including temperature, light, and air oxida‐tion(Rahmanetal.,2015;Santos&Silva,2008).Therefore,vitaminC is often used as an index of nutrient quality in food processing research. Bitter melon is also a rich source of phenolic compounds possessing antioxidant activity (Tan et al., 2016). These phenoliccompounds too are prone to degradation during processing.

Nutrient concentration and organoleptic properties such as color are critical metrics for the quality of a dried food product. Thus, a number of studies have investigated the rates of degrada‐tion of vitamin C and phenolics, and of color (Aamir & Boonsupthip,

2017; da Silva, Arévalo Pinedo, & Kieckbusch, 2005; Demirhan&Özbek, 2009;Mana, Orikasab,Muramatsuc, & Tagawaa, 2012;Marfil, Santos, & Telis, 2008;Mudgal & Vishakha, 2009;Orikasa,Wu,Shiina,&Tagawa,2008;Pirone,Ochoa,Kesseler,&DeMichelis,2007; Villota & Hawkes, 2019). However, there are no reports on the degradation of these nutrients during MWAD of bitter melon. In the present study, thin slices of bitter melon were dried in alow‐temperature air stream (at temperatures of 20, 25, or 30°C) in the presence of microwave radiation from a 2.45 GHz magnetron. Microwave radiation was used to provide the activation energy for the drying process and drive the moisture from the bulk of the slices to their surface. An excess of air was supplied to the drying chamber to remove both moisture and the heat from the slices to prevent the sample from overheating. The concentrations of vitamin C and phenolic compounds were measured at different times during the drying process. The color change was also quantified and analyzed.

2  | MATERIALS AND METHODS

2.1 | Materials

Fresh bitter melons (Momordica charantiaL.)werechosenandboughtfrom a local market at Ho Chi Minh City, Vietnam. The selected bit‐ter melons were green, 18–22 cm long, and without any visual de‐fects. Before drying, the bitter melons were washed, deseeded, and cut into slices of 5 mm thickness.

Folin–Ciocalteu's reagent (2N solution), gallic acid, and ascorbic acid (99.7% purity) were purchased from Sigma‐Aldrich (USA). Allthe chemicals, including anhydrous 2,6‐dichlorophenolindophenolsodium salt (DCPIP), metaphosphoric acid, and sodium carbonate(99.5% purity) were used as received. Distilled water was used in the preparation of aqueous solutions of reagents, and for all extractions and dilutions.

2.2 | Microwave drying apparatus

A custom‐built microwave drying equipment was used to dry the slices of bitter melon. The apparatus consisted of a drying chamber connected with an air supplier system (see Figure 1). The magne‐tronmountedinthedryingchamberoperatedat2.45GHz.Itspoweroutput was adjustable from 150 to 750 W in 150 W increments. Ambient moist air was first cooled to 15°C to remove moisture

F I G U R E 1  Schematicofthemicrowavedrying equipment: (1)—fan; (2)—water collector tray; (3)—evaporator coil; (4)—expansionvalve;(5)—compressor;(6)—electrical control panel; (7)—condenser coil; (8)—electrical heater; (9)—microwave oven; (10)—air exhaust

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through condensation by a custom‐built dehumidifier using refriger‐ant R‐134a. The partially dehumidified air, with a residual humidity of 10 g of water per kilogram of dry air, was then passed through two electrical heaters to attain the required temperature before reach‐ing the drying chamber. Thermal sensors (mounted right after the heaters) controlled the air temperature within a range of ±0.5°C of the setpoint.

The velocity of air through the drying chamber, measured using a digital anemometer (AVM‐713, Tecpel Co.), was about 1 m/s. Based on the hydraulic diameter of the square duct (4 times the cross‐sec‐tional area divided by the wetted perimeter) equal to 0.5 m, the Reynolds number of the airflow through the chamber was ≅32,000 (estimated using air kinematic viscosity of 1.57 × 10−5m2/s at 25°C), enabling high rates of heat and mass transfer between the sample and air stream. As a result, the sample temperature, measured using athermocouple,wasalwaysmaintained≤30°C.

2.3 | Microwave drying conditions

To investigate the kinetics of degradation of vitamin C and phenolic compounds during MWAD of bitter melon, a three‐level full facto‐rial experimental design with two factors, namely microwave power density (MWPD) and the air temperature, Tair, used for drying, was constructed. The low, mid‐, and high levels of MWPD were 1.5, 3.0, and 4.5 W/g, respectively, and those of Tair were 20, 25, and 30°C, respectively. For the process air, with an absolute humidity of 10 g water per kilogram of dry air, the relative humidity (RH) values at 20, 25,and30°Cwereabout69%,51%,and38%,respectively,spanninga relatively wide range of RH for a seemingly narrow range of drying temperatures.

Weighedslicesoffreshbittermelon(16pieces,6.3±0.4geach,and ≅100 g total mass), obtained from the central cylindrical part of the fruit, were placed symmetrically on a 24.5‐cm diameter glass plate (turntable), which was mounted inside the drying chamber and rotated at 4 rpm to ensure uniform microwave irradiation. The slices were removed from the chamber every 30 min of drying, and their mass was recorded. For the determination of the total phenolic con‐tent (TPC) and vitamin C concentration, an aliquot of the total mass was sampled and weighted using an analytical balance with a pre‐cision of 0.1 mg. Pieces of bitter melon, ≅ 1 g during the first hour ofdryingand0.5−0.3gsubsequently,wereexcisedfromrandomlyselected slices on the stage. Three experiments were conducted for each combination of MPWD and Tair (a total of 27 experiments). The results reported herein, for any given drying time, are averages of the results from three drying experiments at the same MPWD and Tair combination.

2.4 | Determination of the total phenolic content

Apreciselyweighedaliquot(0.50−0.15g)ofthebittermelonsliceswas crushed and ground with a few milliliters of distilled water (at room temperature) using a pestle and mortar. The aqueous extract was separated from the crushed mass by vacuum filtration through

Whatman No. 1 filter paper and washed with distilled water until an aliquot of the extract showed no color change upon adding a few drops of the Folin–Ciocalteu reagent. The extract was taken in a 50‐ml volumetric flask, diluted to the mark using distilled water, and mixed.

The total phenolic content (TPC) of the sample was determined based on the Folin–Ciocalteu assay, using gallic acid as standard (Singleton,Orthofer,&Lamuela‐Raventós,1999).To1mlofadilutedextract, 1 ml of Folin–Ciocalteu's reagent (diluted to a concentration of 0.2N with distilled water) and 1 ml of sodium carbonate (20% w/v) were added. The sample was placed in the dark for 30 min before measuringabsorbanceat765nmusingaUV–Visspectrophotometer(SHIMADZUUV‐1800).TPCwasexpressedasgallicacidequivalent(GAE) in mg/g (dry basis).

2.5 | Determination of vitamin C concentration

The vitamin C content was determined by AOACmethod 967.21(Nielsen, 2010). Bitter melon samples were extracted using a pro‐ceduresimilartothatusedforTPCdetermination(cf.Section2.4),but using aqueous metaphosphoric acid (3% w/v) instead of distilled water. A 5 ml aliquot of the metaphosphoric acid extract of the samplewas titratedwith theDCPIPsolution.DCPIP isabluedyethat turns pink in acidic conditions. It is reduced by ascorbic acidto a colorless compound. The end‐point was detected when excess DCPIP gave a light but distinct rose pink color that persisted formorethan10s.TheDCPIPsolutionwasstandardizedusingastand‐ard solution of ascorbic acid. Vitamin C content was expressed in mg/g (dry basis).

2.6 | Characterization of color change

The surface color of five samples, randomly picked from the stage at 30 min intervals, was measured with a colorimeter (NR110 Precision Colorimeter, Shenzhen 3nh Technology Co.). The instrument wasstandardized before every measurement using a ceramic plate. The reportedcolorvaluesare in theCIEL*a*b* color space, expressed in the three numerical values, L*, a* and b*, representing the light‐ness, green–red, and blue–yellow coordinates, respectively. L∗ =0 represents black and L∗ =100 denotes white. Negative a* represents green, while its positive value indicates red. The negative and posi‐tive values of b* represent blue and yellow, respectively. Values of a* and b*canrangefrom−128to+128.

2.7 | Statistical analysis of experimental data

Statisticalsignificanceofthetwoparameters,namelyMWPDandTair, were first evaluated by analysis of variance using the two‐fac‐tor ANOVA function in the Analysis ToolPak ofMicrosoft Excel(2016).Meanvaluesofobservations(suchashalf‐life,finalconcen‐tration,andtotalenergyconsumption)wereusedintheanalysis.Intwo‐wayANOVAwithoutreplication,itisassumedthatthereisnointeraction between the factors, and the F statistic is used to test

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the significance of each factor (McDonald, 2014). The mean square for each of the two factors is calculated, as also the total mean square by considering all of the observations as a single group. The remainder (or error) mean square is found by subtracting the fac‐tor mean squares from the total mean square, and the F statistic is obtained as the ratio of the factor mean square to the total mean square. The p values are based on the right‐tailed F probability dis‐tribution with the numerator degrees of freedom equal to that of the factor and the denominator degrees of freedom equal to that of the error (2 and 4, respectively, for three levels of each of the two factors). The critical F value corresponding to these degrees of freedomis6.94forasignificancelevelof.05and4.32forasignifi‐cance level of .10.

The regression function in the Analysis ToolPak of Excel was used for deriving correlations between the factors and the effects. The interaction and second‐order terms of the factors were included if found significant (p < .05). The uncertainties in the fit parameters re‐portedhereinarethestandarduncertainties(68%confidencelevels).

3  | RESULTS AND DISCUSSION

3.1 | Microwave power density

Three different levels of the input microwave power (generated by the magnetron), namely, 150, 300, and 450 W, were investigated. Based on the initial mass of 100 g, of the fresh bitter melon slices, these input power values correspond to MPWDs of 1.5, 3.0, and 4.5 W/g, respectively. The actual absorbed microwave power that is converted into heat energy (used for water evaporation) would depend on the microwave frequency, f, electric field intensity, E, and the dielectric loss factor, ε′′,accordingtoEquation(1)forthedissipatedpowerden‐sity, Pv (W/m

3) (Pitchai,Birla,Subbiah, Jones,&Thippareddi,2012;Steinetal.,1994),whereinε0 is the permittivity of free space.

Factors such as the microwave cavity design, and the number and arrangement of samples in the cavity, also affect the dissipated power density, because of their effect on E.

The mass and size of the melon slices would change throughout the drying process because of the removal of water. Besides, there is mass loss due to sampling for composition analysis. Physical proper‐ties such as ε′′andthermalconductivitywouldchangesignificantlydue to composition change (particularly the concentration of water, the primary microwave‐absorbing species, in the sample). Thus, it is not possible to maintain a constant value of Pv throughout a drying experiment.

For a meaningful comparison of the data sets from experi‐ments conducted at different microwave power and Tair, factors such as the initial mass and size (diameters and thickness) of the fresh bitter melon slices, the positioning of the slices on the mi‐crowave stage, and the amounts of samples removed for analysis during various stages of drying, were kept consistent in all of the experiments.

3.2 | Kinetics models for nutrient and color changes during drying

The change in the concentration of phenolic compounds during a conventional drying process has been variously modeled as zero‐order, first‐order, second‐order, or pseudo‐first‐order reactions (Villota &Hawkes, 2019). Similarly, vitamin C degradation duringdrying has beenmodeled as a first‐order reaction (da Silva et al.,2005;Manaetal.,2012;Orikasaetal.,2008).AnempiricalWeibullmodel was used to fit vitamin C degradation kinetics in hot‐air dry‐ingofcamucamu(daSilvaetal.,2005)andtomatoes(Marfiletal.,2008), respectively. The Weibull model was also found to be suita‐ble for describing changes in vitamin C concentration during storage (Oms‐Oliu, Odriozola‐Serrano, Soliva‐Fortuny, & Martín‐Belloso,2009; Wang et al., 2018) and for fitting kinetic data acquired dur‐ing conventional drying of mango slices (Corzo, Bracho, & Alvarez, 2010), longan (Ju et al., 2018), and pepino (Uribe et al., 2011).

Many researchers have investigated the change in color of food‐stuff during drying. The change of color with time during microwave drying of spinach (Dadali, Demirhan, & Özbek, 2007), okra (Aamir & Boonsupthip, 2017; Dadalı, Kılıç Apar, & Özbek, 2007), and basil(Demirhan & Özbek, 2009) has been previously investigated in terms of the variation of one or more of the color parameters, namely L*, a*,

b*, hue angle [=arctan (b∗∕a∗)

], and chroma

�√a∗2+b∗2

�.

3.3 | A mathematical model for the kinetics of nutrient degradation during drying

The concentration, Cs, of a substrate (phenolic compounds or vitamin C) per unit volume of water in the material will increase because of the removal of water from the material during drying and decrease becauseofconsumptionbyareaction.Ifaw is the water activity of the material at any time t, the rate of decrease in water activity (be‐cause of drying) is given by:

where kc is the mass transfer coefficient and a∗w

is the water activity of the air used for drying, which will depend on the temperature and water concentration of the incoming air stream. a∗

w will increase

as the air absorbs moisture from the material being dried. But it can be assumed to be essentially constant if the volumetric flow rate of air is large. The solution of Equation (1) using the initial condition, aw=aw0 at t=0, results in Equation (3), which predicts an exponen‐tial decrease in water activity in the material during drying.

Assuming that all of the substrate is present in the aqueous phase, and considering a binary mixture of the substrate and water, the molar concentration, Cs, of the substrate is related to water ac‐tivity, aw, by Equation (4a):

(1)Pv=2�f�0���E2

(2)dawdt

=−kc(aw−a

∗w

)

(3)aw=a∗w+(aw0−a

∗w

)e−kct

(4a)Cs=1−

(aw∕�w

)

Vw(aw∕�w

)

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where �w is the activity coefficient of water in the material, and Vw is the molar volume of water (≅18.07 cm3/mol). Generally, �w has a strong dependence on the material composition, but for the sake of deriving an analytical equation that qualitatively captures the pa‐rameters influencing the drying process, �w≅1 is assumed. Thus,

Assuming first‐order degradation kinetics, the rate of change of substrate concentration in the material can be written as:

Equation (5) can be solved analytically if it is assumed that

This approximation would be valid if the air entering the process haslowwateractivity(orhumidity).SubstitutionofEquation(6)inEquation (5) and using the initial condition, Cs=Cs0 at t=0, results in

where

Ifthemasstransfercoefficient,kc, is significantly lower than the reaction rate constant, k1, that is, k1≫kc, which seems to the case from the data of this study, then A≅0, and Equation (7) simplifies to

Itwasfoundthattheexperimentaldataonthevariationoftheconcentrations of phenolic compounds and vitamin C with time could be fitted well to a stretched exponential model given in Equation (10) [rather than the simple exponential model of Equation (9)]:

where � can be considered the reaction time constant (equal 1∕k1 for a first‐order reaction; cf. Equation (9)] and � is a fitting parameter. Equation(10)isalsocommonlyreferredtoastheWeibullmodel.Itisreadily seen from Equation (10) that the substrate half‐life, according to this model, is given by:

The parameters � and � can be determined from the experimen‐tal Cs∕Cs0 versus tdata.Linearregressionofln

[− ln

(Cs∕Cs0

)] versus

ln t would give � as the slope and −� ln � as the intercept, from which � can be calculated using e−intercept∕slope.

3.4 | Properties of fresh bitter melon slices

Table 1 summarizes the initial properties of fresh bitter melon (before drying), including water content, concentrations of phenolic compounds and vitamin C, and the color parameters. The 95% confidence intervals, based on the standard deviation of measurements, are given along with the average values for multiple samples of the bitter melon slices.

3.5 | Drying time

The equilibrium moisture content that could be achieved by drying at the lowest MWPD (1.5 W/g) and the lowest Tair of 20°C (highest RH) was about 0.3 g/g. Therefore, the time required to attain this level of drying was used as the basis for comparison of all the MWPD and Tair combinations investigated in this study.

Using linear regression of the experimental data, the time, tdry (min) required for reduction in the moisture content from the starting value of about 13.9 g/g (dry basis) to the final value of 0.3 g/g (≅97.8% reduction in moisture content) was found to be re‐lated to the microwave power and air temperature according to the following equation:

where x1 and x2 are the encoded levels of MWPD and temperature, respectively, such that the low, mid‐, and high values of MWPD, or the temperature, correspond to −1, 0, and 1, respectively. The standard error of Equation (12) was 15 min and the adjusted R2 value for the fit was .95. The average absolute deviation (AAD) of the fitted value from the experimental value was approximately 4.1%. AAD is defined as the average of [(yi−yp)∕yi] × 100, where yi is the measured value of a parameter and yp is the value predicted by the mathematical model.

3.6 | Degradation kinetics of phenolic compounds

Figure 2 shows the kinetics of degradation of phenolic compounds in bitter melon. Representative concentration versus time plots are shown

(4b)Cs=1−aw

Vwaw

(5)dCsdt

=d

dt

(1−aw

Vwaw

)−k1Cs

(6)aw≅(aw0−a

∗w

)e−kct

(7)Cs (t)

Cs0=Aekct+

(1−A

)e−k1t

(8)A=kc(

kc+k1)Cs0Vw

(aw0−a

∗w

)

(9)Cs (t)

Cs0≅e−k1t

(10)Cs (t)

Cs0≅e−(t∕τ)

α

(11)t1∕2=(ln 2

)1∕α�

(12)tdry (min)=(210±9

)−(60±6

)x1−

(45±6

)x2+

(25±11

)x22

TA B L E 1   Characteristics of fresh bitter melon samples (before drying)a

Moisture content (g/g) 13.9 (0.5)

Phenolic compounds (mg/g) 8.0 (0.4)

Vitamin C (mg/g) 5.2 (0.4)

Reducing sugar (mg/g)b 36(7)

Color parameters

L* (lightness) 55 (1)

a* (green/red) −18(1)

b* (blue/yellow) 36(1)

pHc 5.4 (0.1)

aData are expressed as mean value (standard deviation); chemical com‐positions are based on dry mass. bReducingsugarconcentrationwasdeterminedusingtheDNSmethod.cpHwasmeasuredusingHannaInstrumentsHI2211pHmeter.

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in Figure 2a for experiments conducted at 25°C. (Data were acquired at temperatures of 20 and 30°C as well.) The logarithms of normalized concentrations are plotted against time in Figure 2b. The nonlinearity of the plots indicates a slight deviation from first‐order kinetics behavior.

The exponent α [cf. Equation (10)] did not exhibit a statistically significant correlation either with MWPD or with temperature, at a significance level of .05. The model with a variable value of pa‐rameter α for different levels of MWPD and Tair was tested using F statistic, and was rejected [F(8,60)=1.6,p=.15]infavoroffixedα (M=1.18,SD=0.04)model.

Figure 2c shows the half‐lives of phenolic compounds under various processing conditions. Analysis of variance showed that at each drying air temperature, there was a significant decrease in t1∕2 with an increase in MWPD [F(2,4)=12.5,p=.02].t1∕2 decreased with an increase in Tair from 20 to 30°C when the MWPD was 1.5 or 3.0 W/g, which is expected. However, t1∕2 showed a slight in‐crease with TairwhentheMWPDwas4.5W/g.Linearregressionofthe data, considering the interaction of the two factors, resulted in the following equation for the half‐life of phenolic compounds (in minutes) under various MWPD and Tair values. As in the case of Equation (12), x1 and x2 are encoded levels of MPWD and Tair, respectively. The standard error for the fit was approximately 38 min and the adjusted R2 value was .94. The AAD was 7.5%

Figure 2d shows the residual concentration of phenolic com‐pounds in the final product, dried to the same level of moisture content of 0.30 ± 0.04 g water/g (dry basis), under different values

of MWPD and Tair. MWPD was a somewhat more significant fac‐tor [F(2,4)=6.26,p= .06] influencingtheresidualTPC;theeffectof Tair was not significant [F(2,4)=0.35,p= .7].Thedryingcondi‐tions that yielded the highest residual concentration of the phenolic compounds were the lowest microwave power (1.5 W/g) and an air temperature of 20 or 25°C. A MWPD of 4.5 W/g and Tair of 20°C resultedinadropintheTPCtolessthan30%oftheinitialvalue.Incontrast, an MWPD of 4.5 W/g and Tair of 30°C and resulted in a de‐crease in TPC to only about 55%, which is attributed to the relatively low drying time (the lowest in the set of nine experiments) of approx‐imately 2.5 hr to achieve the desired moisture content of 0.3 g/g (dry basis) under these conditions

The degradation of phenolic compounds during drying is com‐prehensively reviewed by McSweeney and Seetharaman (2015).The enzymes participating in polyphenol decomposition include polyphenol oxidase (PPO), peroxidase (POD), and lipoxygenase(LOX).LOX,however,isnotpresentinsignificantquantitiesinthefleshofthefruit.Hence,theactivitiesofPPOandPODareofpri‐mary concern. Both PPO and POD aremetalloproteins, contain‐ing metal ion cofactors. The redox‐active cations of metals such ascopper (Klabunde,Eicken,Sacchettini,&Krebs,1998)and iron(Dunford,2016)presentintheseoxidativeenzymescanactascata‐lystsfortheoxidationofpolyphenols(Nokthai,Lee,&Shank,2010).

Several researchers have investigated the impact ofmicrowaveirradiation on enzyme activity. Henderson, Hergenroeder, and Stuchly(1975)studiedmicrowaveirradiationofanaqueoussolutionofhorseradishPODat25°Candreportedinactivationoftheenzymeonly when the appliedMWPDwas greater than 60W/g (a ratherhigh value) and the exposure time was higher than 20 min. Using

(13)t1∕2 (phenolics, min) =

(303 ± 13

)−(158 ± 16

)x1

−(56 ± 16

)x2 +

(69 ± 19

)x1x2

F I G U R E 2  Kineticsofdegradationof phenolic compounds at different microwave power inputs and drying air temperatures. (a) Concentration versus time for drying at 25°C. Curves are the results of a regression analysis of the experimental data, based on Equation (10). The root mean square deviations of the experimental data from the model were approximately 0.22, 0.33, and 0.29 mg/g, for the MWPDs of 1.5, 3.0, and 4.5 W/g respectively. (b) The logarithm of normalized concentration versus time. (c) Degradation half‐life under different combinations of microwave power and drying air temperature. (d) Residual concentration of phenolic compounds after drying at 20, 25, and 30°C. The number above each bar is the processing time, in hours, to reach a moisture content of 0.3 g/g (dry basis)

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fluorescence and circular dichroism measurements, Porcelli et al. (1997) reported a nonthermal, irreversible, and time‐independent in‐activation of enzymes. The inactivation rate was found to be related to the energy absorbed and independent of the enzyme concentration. Theirexperimentswereusinga10.4GHzmicrowaveanda70−90°Ctemperature range. de Pomerai et al. (2003) found that exposure to microwave radiation enhanced the aggregation of bovine serum al‐bumin in a time‐ and temperature‐dependent manner. Amyloid fibril formationwasobservedinthecaseofbovineinsulinat60°C.

Microwave heating can result in localized high‐temperature regions, even though the bulk temperature is maintained low by convective airflow. There are many reports indicating that the heat generatedbymicrowavecaninactivatebothPPOandPOD(Chávez‐Reyes, Dorantes‐Alvarez, Arrieta‐Baez, Osorio‐Esquivel, & Ortiz‐Moreno,2013;Latorre,Bonelli,Rojas,&Gerschenson,2012;Lopeset al., 2015;Matsui, Gut, deOliveira, & Tadini, 2008; Siguemoto,Pereira,&Gut, 2018).Chávez‐Reyeset al. (2013) found thatPPOin the mesocarp of the loquat fruit was inactivated after 210 s of 478 W microwave exposure. Correspondingly, an improved phenolic extraction(anincreaseinTPC)wasobserved.Incontrast,theTPCof bitter melons of the present study was found to decrease during MWAD, which is attributed to the high convective mass transport rates of oxygen from the process airflow to the melon slices.

The Maillard reaction products generated during heating can in‐hibitenzymessuchasPPO,andtherefore,preventthedegradationof polyphenols (Tan & Harris, 1995). However, the low temperature, relatively low pH, and a low reducing sugar concentration, all of which are prevalent in the bitter melon slices of the present study (see Table 1) would not be favorable for the Maillard reaction.

VitaminCisachelatingagentthatdeactivatesPPO(McSweeney& Seetharaman, 2015), and has been widely used as an inhibitorof PPO and POD (Doğan & Salman, 2007; Landi, Degl′Innocenti,Guglielminetti, & Guidi, 2013). Bitter melon contains a significant concentration of vitamin C. As a result, when the drying tempera‐tureandtheMWPDarelow,vitaminCcaninhibitPPOandcanslowdowntherateofdegradationofthephenoliccompounds.IncreasingMWPD and increasing Tair enhanced the decomposition of vitamin C (vide infra), leading to an acceleration of the TPC decrease.

3.7 | Degradation kinetics of vitamin C

Figure 3a,b shows the kinetics of degradation of vitamin C dur‐ing MWAD of bitter melon, along with the best‐fit curves based on Equation (10). Neither Tair [F(2,4) = 0.029,p = 1.0] norMWPD[F(2,4) = 0.95,p = .5] had a significant effect onα, for which the mean and the standard deviation of the mean (based on all nine sets of experiments) were 1.27 and 0.09, respectively.

The vitamin C half‐life values are shown in Figure 3c. MWPD was found to be the dominant factor [F=15.2,p=.014].Theeffectof Tair on t1∕2 of vitamin C was not significant [F(2,4)=1.81,p=.3].Equation (14) gives the correlation between the half‐life in minutes, and the MWPD and temperature. The standard error for the fit was approximately 23 min and the adjusted R2 value was .82. The AAD was8.6%.Notethatthisequationdoesnotcapturethenonmono‐tonic trends observed in the temperature dependence of half‐life (e.g., at 1.5 W/g; cf. Figure 3c).

(14)t1∕2(vitamin C, min

)=(164 ± 8

)−(57 ± 10

)x1 −

(16 ± 10

)x2

F I G U R E 3  Kineticsofdegradationof vitamin C at different microwave power and drying air temperatures. (a) Concentration versus time for drying at 25°C. Curves are the results of a regression analysis of the experimental data, based on Equation (10). The root mean square deviations of the experimental data from the model were approximately0.16,0.24,and0.40mg/g,for the MWPDs of 1.5, 3.0, and 4.5 W/g, respectively. (b) The logarithm of normalized concentration versus time. (c) Degradation half‐life under different combinations of microwave power and drying temperature. (d) Residual concentration of vitamin C after drying at 20, 25, and 30°C. The number above each bar is the processing time, in hours, to reach a moisture content of 0.3 g/g (dry basis)

8 of 12  |     NGUYEN Et al.

The residual vitamin C, in the product dried to a moisture con‐tent of 0.3 g/g (dry basis), was lower than that of the phenolic compounds, indicating that vitamin C was more prone to microwave‐induced degradation than the phenolic compounds. The concentra‐tion of vitamin C remaining in the dried product was generally below 45% of the initial value. The product dried using MWPD of 1.5 W/g and air at 25°C had the highest residual vitamin C concentration. Here again, MWPD was the dominant factor in determining the re‐sidual vitamin C concentration [F(2,4)=6.26,p=.06],andtheeffectof Tair was not significant [F(2,4)=2.73,p=.12].

The concentration of residual vitamin C was significantly lower in the product dried using MWPD of 1.5 W/g and Tair of 20°C compared to the product obtained by drying at 25°C (cf. Figure 3d), which is at‐tributed to the differences in the drying time (to attain the final moisture contentof0.3g/g).Ittook6hrtodrythesamplesat20°C,whilethedrying time was approximately 4.5 hr at 25°C and even shorter when Tair was 30°C (4 hr; cf. Figure 3d). The residual vitamin C concentration was low when the drying time was long, and the MWPD was high.

VitaminCorL‐ascorbicacid issusceptibletoenzymaticdegrada‐tion to generate dehydroascorbic acid, which is quickly and irreversibly hydrolyzed to 2,3‐diketogulonic acid (Deutsch, 2000) that nonen‐zymatically decomposes to oxalate and L‐tartaric acid (Green& Fry,2005;Hancock&Viola,2005).Munyaka,Makule,Oey,VanLoey,andHendrickx (2010) reported that the L‐ascorbic acid present in broc‐coli readily converted to dehydroascorbic acid when the broccoli was heatedtoatemperatureintherangeof30–60°C.ThelossofvitaminC in bitter melon processed by MWAD evidently starts with the con‐versionofL‐ascorbicacidtodehydroascorbicacid,followedbythermaldecomposition of dehydroascorbic acid to produce water‐soluble C2 and C3 acids (Green & Fry, 2005) that migrate to, and concentrate at, the surface, to further accelerate the hydrolysis of dehydroascorbic acid. (The decomposition of dehydroascorbic acid is acid catalyzed). IncreasingtheMWPDcouldproducelocalhotspotsthatresultinthethermaldegradationofdehydroascorbicacid.Similarly, increasingTair would not only favor the thermal degradation of dehydroascorbic acid but also concentrate the acidic by‐products of the hydrolysis reaction at the surface, accelerating vitamin C degradation.

Ghazala, Ramaswamy, van de Voort, Prasher, and Barrington (1989) investigated the kinetics of thermal degradation of vitamin C and reported first‐order rate constants for the reaction in the temperature range of 110–150°C. The half‐lives, calculated using their data (for decomposition in distilled water), are approximately 4hrat110°Cand0.5hrat150°C.Itisinterestingtonotethatthehalf‐life determined for the MWAD process is in a similar range (cf. Figure 3c), although the drying temperature was significantly lower than those investigated by Ghazala and coworkers.

3.8 | Color change kinetics

The color change during drying can result from pigment destruc‐tion, oxidation, enzymatic browning, nonenzymatic browning, and phenol polymerization (Bahloul, Boudhrioua, Kouhila, & Kechaou,2009). The Maillard reaction (between reducing sugars and amino

compounds such as proteins, peptides, and amino acids) is an exam‐ple of a process that results in nonenzymatic browning. However, the concentration of reducing sugar in bitter melon is low (see Table 1). Therefore, Maillard reaction may not have a strong influence on the color change during MWAD of bitter melon.

Chlorophyll is the main colorant present in bitter melon. The chlorophyll in a raw fruit decomposes by enzymatic reactions during the ripening process. The decomposition starts with magnesium dechelation catalyzed by chlorophyllase enzyme, followed by ring‐openingoxidationcatalyzedbypheideaoxygenase (PAO)enzyme(Christ&Hörtensteiner,2014).ThePAOenzymecouldalsobeac‐tivated by biotic and abiotic factors, including injury (during slicing) and osmotic stresses (during water removal), leading to the degrada‐tion of chlorophyll and a color change.

Thermal degradation of chlorophyll can occur, but only at relatively high temperatures, >60°C (Lípová, Krchňák, Komenda,& Ilík, 2010;Weemaes,Ooms,VanLoey,&Hendrickx,1999).Chlorophyllwouldbeexpected to be a strong microwave absorber because of polar bonds. Nevertheless, it issufficientlystableundermicrowave irradiation. Infact, microwave has been used for the extraction of chlorophyll from microalgae (Pasquet et al., 2011). Therefore, the effect of microwave radiation on chlorophyll decomposition can be considered negligible.

The characterization of chlorophyll concentration in the drying specimenswasbeyondthescopeoftheworkreportedherein.Onlythe changes in the values of the color parameters with drying time are reported. Figure 4 shows the values of L∗, a∗, and b∗, obtained from the experimental data during drying. The magnitudes of L∗, a∗, and b∗ decreased with drying time, indicating the samples became darker (decrease in L∗) and less green (decrease in the magnitude of a∗) during drying. The data of color parameters versus time was fitted to the first‐order model reaction.

where y represents the color parameter (L∗, a∗, or b∗) at time t, y0 is the value of the corresponding color parameter at time t=0, and � is the first‐order time constant.

The curves in Figure 4a–c are based on Equation (15) and the val‐ues of the time constants, for various drying conditions, are shown in Figure4d–f.Equation(16)givesacorrelationof�L∗ (in minutes) to the MWPD and temperature values. The adjusted R2, the standard error andAADvaluesforthefitswere.87,68min,and6.9%,respectively.

Equation (17) gives a correlation of �a∗ (in minutes), for which the adjusted R2, the standard error and AAD values were .98, 17 min, and 7.1%, respectively.

Equation (18) gives a correlation of �b∗ (in minutes), for which the adjusted R2, the standard error and AAD values were .87, 78 min, and 6.6%,respectively.

(15)y=y0exp (−t∕�)

(16)�L∗ (min)=(684 ± 23

)−(167 ± 28

)x1−

(130 ± 28

)x2

(17)�a∗ (min) =

(227 ± 12

)−(95 ± 7

)x1−

(68 ± 7

)x2

+(54 ± 12

)x2

1+(44 ± 12

)x2

2+(67 ± 8

)x1x2

https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/oxygenase

     |  9 of 12NGUYEN Et al.

The time constants corresponding to temporal variations of L∗, a∗, and b∗ decreased significantly with an increase in the MWPD, in‐dicating that the color changes happened faster.

At a given MWPD, the color parameters, L∗, |a∗|, and b∗ decreased with an increase in Tair. A decrease of up to 30% in lightness (L

∗) and up to 50% in |a∗| (greenness) was observed when the specimen was dried to a moisture content of 0.30 g/g (dry basis). The darkening of the samples (see Figure 5) is attributed to the increase in the volu‐metric mass density of the color pigments because of the removal of water and shrinkage of the specimen. The decrease in greenness, quantified by the decrease in |a∗|, and the increase in yellowness,

quantified by the decrease in b∗, are most likely due to the enzymatic chlorophyll degradation mechanism discussed previously. Similarchanges were previously reported in microwave drying of okra (Dadalıetal.,2007),spinach(Dadalietal.,2007;Ozkan,Akbudak,&Akbudak,2007),coriander,mint,anddrill(Kathirvel,Naik,Gariepy,Orsat,&Raghavan,2006).

3.9 | Net energy consumption

The total electrical energy consumed in the process (by all units, including the magnetron, the air blower, the dehumidifier, and the heater), to dry the sample to a final moisture content of ≅0.3 g/g, was measured using an energy meter. The results for different drying conditionsareshowninFigure6.Asexpected,astrongdependence

(18)τb∗ (min) =

(712 ± 45

)−(140 ± 30

)x1

−(174 ± 30

)x2 +

(170 ± 55

)x2

1

F I G U R E 4   (a–c) Variation of color parameters with time during microwave‐assisted drying at different microwave power and drying air temperatures. Curves are the results of a regression analysis of the experimental data, based on Equation (15). The standard uncertainty in themeasurementofeachcolorparameterisabout1unit.(d−f)Timeconstantsofafirst‐orderrateequationdescribingthekineticsofcolorchange

F I G U R E 5   Photographs of (a) fresh bitter melon slices and (b) slices after drying at 4.5 W/g and 30°C

10 of 12  |     NGUYEN Et al.

of energy consumption on the MWPD [F(2,4) = 42.2,p = .002] isobserved. The dependence on Tair was weaker [F(2,4)=4.9,p=.08].The measured energy, E, in kilowatt hour, could be correlated to the MWPD and Tair using Equation (19). The adjusted R2, the stand‐ard error and AAD values for the fit were .93, 1.9 kW hr, and 9%, respectively.

Figure6showsthatenergyconsumptionwasthelowestwhentheMWPD was the highest (4.5 W/g). The seemingly contradictory result of lowest energy consumption at the highest microwave power can be reconciled by the fact that the total drying time was lower when the MPWD was higher. With an increase in the drying time, the energy used by other electrical components of the system, such as the fan and the dehumidifier, which draw a significant amount of energy besides the magnetron in the microwave oven, would also increase. Thus, a higher MWPD that results in a lower drying time would be favorable from the perspective of process energy consumption

When the bitter melon slices were dried using a microwave power of 1.5 W/g and an air temperature of 25°C, the drying time (required to achieve a moisture content of 0.3 g/g in the final prod‐uct) was 4.5 hr. The residual TPC and vitamin C concentrations were

(71 ± 1

) and (47 ± 3) % of the corresponding initial values,

respectively. However, process energy consumption was relatively high [(25.4 ± 0.7) kWhr]. In comparison, thedrying timewasonly2.5 hr when 4.5 W/g microwave power and 30°C air temperature were used. The TPC and vitamin C retention values were (54 ± 1) and (33 ± 2) %, respectively, and process energy consumption was only (7.7 ± 0.6)kWhr(cf.Figures2d,3d,and6).Thecolorretentionwas satisfactory for these drying conditions.

4  | CONCLUSIONS

This work analyzes the influence of radiation power and air flow temperature on the nutrient and color change of bitter melon during a MWAD process. The chemical degradation rates of phenolic com‐pounds and vitamin C and the rates of change of the color param‐eters (quantified in terms of half‐life values) generally increased with an increase in the MWPD. Plausible mechanisms for the microwave influence on color pigments and oxidative degradation of nutrients are discussed using information in the associated literature. Enzymes suchasPPOthatleadtoadecreaseintheTPCcanbeinactivatedby microwave radiation, which should result in improved phenolic extraction in a microwave drying process. However, the TPC of bitter melons decreased in the present study, because of the high oxygen concentration in the melon slices due to good convective masstransferfromtheairusedfordrying.PPOisalsoknowntobedeactivated by vitamin C, thereby, linking the kinetics of degradation of the phenolics to that of the degradation of vitamin C. Therefore, finding a suitable drying condition to retain the highest quality of the productisamultiobjectiveoptimizationproblem.SincebothvitaminC and phenolics are susceptible to enzymatic degradation, a blanch‐ing step could be added to the MWAD process, to deactivate these enzymes. Additionally, the overall energy consumed during drying is also an important consideration because it relates directly to process economics.

CONFLICT OF INTEREST

The authors have declared no conflicts of interest for this article.

ORCID

Sitaraman Krishnan https://orcid.org/0000‐0002‐1228‐8393

Phong T. Huynh https://orcid.org/0000‐0003‐2379‐7107

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