Embed Size (px)
Citation preview
processes
Article
Drying Characteristics and Quality Attributes Affected by aFluidized-Bed Drying Assisted with Swirling Compressed-Airfor Preparing Instant Red Jasmine Rice
Prarin Chupawa 1 , Tiwanat Gaewsondee 2 and Wasan Duangkhamchan 3,*
�����������������
Citation: Chupawa, P.; Gaewsondee,
T.; Duangkhamchan, W. Drying
Characteristics and Quality
Attributes Affected by a Fluidized-
Bed Drying Assisted with Swirling
Compressed-Air for Preparing Instant
Red Jasmine Rice. Processes 2021, 9,
1738. https://doi.org/10.3390/
pr9101738
Academic Editor:
Jean-Louis Lanoiselle
Received: 17 August 2021
Accepted: 26 September 2021
Published: 28 September 2021
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1 Mechatronics Research Unit, Faculty of Engineering, Mahasarakham University,Maha Sarakham 44150, Thailand; [email protected]
2 Faculty of Engineering, Nakhon Phanom University, Nong Yat, Mueang Nakhon Phanom,Nakhon Phanom 48000, Thailand; [email protected]
3 Research Unit of Process Design and Automation, Faculty of Engineering, Mahasarakham University,Maha Sarakham 44150, Thailand
* Correspondence: [email protected]; Tel.: +66-4375-4316
Abstract: A new process for the production of instant red jasmine rice was investigated usingfluidized bed drying with the aid of swirling compressed air. Drying characteristics were evaluatedusing the operating parameters of fluidizing air temperature (90–120 ◦C) and pressure of swirlingcompressed air (4–6 bar). Appropriate air pressure was determined based on the highest valueof model parameters from the semi-empirical Page equation and effective diffusivity. Influencesof supply time of swirling compressed air (2–10 min) and drying temperature of 90–120 ◦C wereinvestigated and optimized based on the quality attributes using response surface methodology.Drying at 120 ◦C and compressed air pressure of 6 bar gave the highest rate constant and effectivediffusion coefficient. Drying at 120 ◦C combined with injecting swirling air for 2 min was the mostsuitable approach, while drying at 90 ◦C and supplying compressed air for 10 min was the bestchoice to preserve antioxidant properties. Air temperature of 98.5 ◦C with 2 min supply of swirlingcompressed air suitably provided high physical and rehydration properties and retained high healthbenefits of antioxidant compounds. Finally, after rehydration in warm water at 70 ◦C for 10 min, thetextural properties of the rehydrated rice sample were comparable to conventionally cooked rice.
Keywords: red jasmine rice; Mali dang; quick-cooking rice; drying characteristics; antioxidants
1. Introduction
Pigmented rice is now increasingly consumed as a source of daily calories and bioac-tive compounds, such as polyphenols, that reduce or prevent cell damage caused by freeradicals. After harvesting, its high moisture content (18–22%wb) is normally reduced toapproximately 14%wb by means of drying for safe storage [1–3]. Among pigmented ricevarieties, red jasmine rice in Northeastern Thailand (also called ‘Mali dang’) possesses thehighest total phenolic content [4]. Red jasmine rice is consumed for its health benefits andimpressive aroma. However, the long cooking time and gummy texture of the cooked riceare not preferred by consumers [5]. Therefore, instant red jasmine rice would be a popularalternative choice as a convenient product to match modern consumer lifestyles [6,7].
Drying plays the most important role in producing instant rice, as this affects re-hydration and organoleptic properties [6–9]. Copious research has investigated qualityimprovements of instant rice products [5,7,8]. Freeze drying, known as an ideal dryingprocess, was used to produce porous structural instant rice. Due to its high porosity with re-maining high nutrients, rehydrated freeze-dried rice were comparable with freshly cookedrice [9–11]. However, alternative drying techniques needed to be explored due to the highoperation cost and long drying time of the freeze-drying method [11]. The conventionalconvective hot-air drying method was employed as it was simple and cheap operation
Processes 2021, 9, 1738. https://doi.org/10.3390/pr9101738 https://www.mdpi.com/journal/processes
Processes 2021, 9, 1738 2 of 21
compared with freeze drying [7,12]. Even though this method could be improved withthe aim of providing high drying rate by using high air temperatures, sample pretreat-ments were needed to individually separate the dried kernels [9,12,13]. Hot-air dryingcombined with microwave heating has emerged as a viable method, as it gave the highestdrying rate and most porous structure [13,14]. However, several problematic steps ofpretreatments and uneven heat inside ovens are still challenges which are encountered.Fluidized-bed drying was applied to produce instant rice as a result of its high heat andmass transfer mechanisms, leading to high drying rate and short drying time comparablewith microwave drying. However, this technique was not suitable for drying cookedrice due to surface adhesion. Several steps of pretreatments were needed before beingsubjected to drying. Commonly, the rice sample is washed with cold tap water before dry-ing [9,14,15], while more complicated steps involve freezing cooked rice to below −18 ◦C,germination [16], enzymatic treatment [17,18], and low-pressure plasma treatment [5] toreduce agglomeration [6,9,13].
Due to versatility and simplicity compared to microwave drying, a fluidized-bedsystem was combined with swirling compressed air streams to avoid kernel agglomerationduring drying with no pretreatment steps. This process was successively applied to pro-duce instant riceberry [19]. The findings showed that drying temperature (70–90 ◦C) andswirling compressed air pressure (2–4 bar) significantly affected drying characteristics anddegradation kinetics of anthocyanins [19]. This process for producing instant pigmentedrice requires further research to assess the kinetic aspects for feasibility under extremelyhigh drying temperatures and swirling compressed air pressures. Evaluation of how bioac-tive compound retention is affected by operating conditions as well as optimal parametersto maximize quality attributes is also required.
Therefore, here, a process of preparing instant red jasmine rice was presented with dry-ing temperatures of 90–120 ◦C and swirling compressed air pressures of 4–6 bar. Influencesof these factors on drying kinetics were investigated, with optimal values subsequentlyused to study the effect of swirling air supply time on the rice’s physical, rehydration,and antioxidant properties. Operating parameters were optimized using response surfacemethodology.
2. Materials and Methods2.1. Preparation of Cooked Red Jasmine Rice Sample
Red jasmine rice was purchased from the community enterprise Gudkhaedon, LoengNok Tha, Yasothon Province, Thailand. The rice was sorted and cooked at a rice-to-waterratio of 1:3 by a domestic electric rice cooker and used as the control sample. To retain theiroriginal shape after cooking, the rice grains were cooked for 30 min and subsequently keptin the cooker for 10 min to ensure complete gelatinization. Moisture content was measuredbased on the standard AOAC method [20] and 200 g of cooked rice was subjected to thedrying process. Textural and antioxidant properties of the control were also examined.
2.2. A Process of Fluidized Bed Drying Assisted with Swirling Compressed Air Stream
As shown in Figure 1, a lab-scale fluidized bed dryer assisted with swirling com-pressed air consisted of three main compartments as the air heating system, cylindricaldrying chamber and swirling compressed air system. Ambient air was drawn by a 3-phase1-hp blower (Mitsubishi Electric Automation, Co., Ltd., BangKok, Thailand), with velocitycontrolled by an inverter (Model H-3200 Series, Haitec Transmission Equipment Co., Ltd.,Guangzhou, China). The air was drawn through a heating box equipped with ten 1-kWfinned heaters. Fluidizing air temperature was controlled using a PID controller (ModelMAC-3D, Shimax Co., Ltd., Akita, Japan). The hot fluidizing air was blown through aperforated plate from the bottom of a stainless-steel tube with diameter of 0.1 m and heightof 1 m (see Figure 2).
The important part of the proposed process was the swirling compressed air system(shown in Figure 3), comprising the air stream controller and air compressor. This system
Processes 2021, 9, 1738 3 of 21
successfully restricted the agglomeration of cooked rice kernels during the drying processusing swirling air streams released from two air tubes inserted at the bed bottom withoutlet size of 1 mm. The air was compressed by a 2-hp air compressor (Model PP-22, PumaIndustrial Co., Ltd., Taiwan) equipped with a pressure regulator. The compressed air wassupplied intermittently with time controlled by a PLC controller (Model Siemens Logo6ED1052-1FB08-0BA0 Logic Module, Siemens, Munich, Germany).
Processes 2021, 9, x FOR PEER REVIEW 3 of 22
was controlled using a PID controller (Model MAC-3D, Shimax Co., Ltd., Japan). The hot fluidizing air was blown through a perforated plate from the bottom of a stainless-steel tube with diameter of 0.1 m and height of 1 m (see Figure 2).
The important part of the proposed process was the swirling compressed air system (shown in Figure 3), comprising the air stream controller and air compressor. This system successfully restricted the agglomeration of cooked rice kernels during the drying process using swirling air streams released from two air tubes inserted at the bed bottom with outlet size of 1 mm. The air was compressed by a 2-hp air compressor (Model PP-22, Puma Industrial Co., Ltd., Taiwan) equipped with a pressure regulator. The compressed air was supplied intermittently with time controlled by a PLC controller (Model Siemens Logo 6ED1052-1FB08-0BA0 Logic Module, Siemens, Germany).
Figure 1. A lab-scale fluidized bed dryer assisted with swirling compressed air.
Figure 2. Dimension of a lab-scale fluidized bed dryer.
Figure 1. A lab-scale fluidized bed dryer assisted with swirling compressed air.
Processes 2021, 9, x FOR PEER REVIEW 3 of 22
was controlled using a PID controller (Model MAC-3D, Shimax Co., Ltd., Japan). The hot fluidizing air was blown through a perforated plate from the bottom of a stainless-steel tube with diameter of 0.1 m and height of 1 m (see Figure 2).
The important part of the proposed process was the swirling compressed air system (shown in Figure 3), comprising the air stream controller and air compressor. This system successfully restricted the agglomeration of cooked rice kernels during the drying process using swirling air streams released from two air tubes inserted at the bed bottom with outlet size of 1 mm. The air was compressed by a 2-hp air compressor (Model PP-22, Puma Industrial Co., Ltd., Taiwan) equipped with a pressure regulator. The compressed air was supplied intermittently with time controlled by a PLC controller (Model Siemens Logo 6ED1052-1FB08-0BA0 Logic Module, Siemens, Germany).
Figure 1. A lab-scale fluidized bed dryer assisted with swirling compressed air.
Figure 2. Dimension of a lab-scale fluidized bed dryer. Figure 2. Dimension of a lab-scale fluidized bed dryer.
Before drying, fluidizing air temperature was set and the apparatus was operated for30 min to ensure that the condition was in a steady state. Two hundred grams of cookedred jasmine rice, corresponding to a static bed height of approximately 4 cm were loadedinto a cylindrical drying chamber. All experiments were assessed under full factorialdesign with two factors of drying temperature (90, 105, and 120 ◦C) and compressed airpressure (4, 5, and 6 bar) to clarify drying kinetics. The optimal compressed air pressurewas subsequently used with variations of drying temperature (90–120 ◦C) and swirlingcompressed air supply times of 2, 4, and 6 min to investigate the influence on instant redjasmine rice quality attributes.
Processes 2021, 9, 1738 4 of 21Processes 2021, 9, x FOR PEER REVIEW 4 of 22
Figure 3. Schematic diagram of the swirling compressed air system [19].
Before drying, fluidizing air temperature was set and the apparatus was operated for 30 min to ensure that the condition was in a steady state. Two hundred grams of cooked red jasmine rice, corresponding to a static bed height of approximately 4 cm were loaded into a cylindrical drying chamber. All experiments were assessed under full factorial de-sign with two factors of drying temperature (90, 105, and 120 °C) and compressed air pressure (4, 5, and 6 bar) to clarify drying kinetics. The optimal compressed air pressure was subsequently used with variations of drying temperature (90–120 °C) and swirling compressed air supply times of 2, 4, and 6 min to investigate the influence on instant red jasmine rice quality attributes.
2.3. Analysis of Drying Characteristics Moisture content of the sample was normalized using a moisture ratio (MR) and ex-
pressed as Equation (1):
ei
etMMMMMR
−−= (1)
where M denotes moisture content and subscripts i, t and e represent moisture content at initial drying time, at a certain time and at equilibrium, respectively. All values of mois-ture content were expressed as wet basis (%wb) throughout this paper. Data of MR as a function of drying time, obtained from nine experimental runs with variations of drying temperature (90–120 °C) and swirling air pressure (4–6 bar), were fitted to an empirical Page equation, as shown in Equation (2):
)ktexp(MR n−= (2)
where k and n denote drying rate constant and power constant, respectively. Model pa-rameters appearing in Equation (2) were evaluated using a non-linear regression tech-nique.
For most biological materials, drying takes place at a falling-rate with moisture move-ment from inside to surface depending mainly on liquid diffusion [21]. Here, effective diffusivity was calculated in accordance with the first term of Fick’s second law, devel-oped for materials in a finite circular cylinder geometry, as expressed below.
( )
+−= t
rDπβλexp
πλ32MR 2
eff2
21
2122
1
(3)
Equation (3) is a liquid diffusion model governed from Fick’s law, where Deff is the effective diffusivity (m2 s−1), λ1 is first root of the Bessel function (2.4048), β1 is shape ratio (=πr/2l), r is the radius, l is the length of a rice grain (m) and t is drying time (min). The nonlinear function of Equation (3) was converted into a natural logarithm, resulting in a linear relationship between ln(MR) and t. Deff was then approximated from the slope
Figure 3. Schematic diagram of the swirling compressed air system [19].
2.3. Analysis of Drying Characteristics
Moisture content of the sample was normalized using a moisture ratio (MR) andexpressed as Equation (1):
MR =Mt − Me
Mi − Me(1)
where M denotes moisture content and subscripts i, t and e represent moisture contentat initial drying time, at a certain time and at equilibrium, respectively. All values ofmoisture content were expressed as wet basis (%wb) throughout this paper. Data of MR asa function of drying time, obtained from nine experimental runs with variations of dryingtemperature (90–120 ◦C) and swirling air pressure (4–6 bar), were fitted to an empiricalPage equation, as shown in Equation (2):
MR = exp(−ktn) (2)
where k and n denote drying rate constant and power constant, respectively. Model param-eters appearing in Equation (2) were evaluated using a non-linear regression technique.
For most biological materials, drying takes place at a falling-rate with moisture move-ment from inside to surface depending mainly on liquid diffusion [21]. Here, effectivediffusivity was calculated in accordance with the first term of Fick’s second law, developedfor materials in a finite circular cylinder geometry, as expressed below.
MR =32λ2
1π2
exp(−(λ2
1 + β21
)π2Deff
r2 t)
(3)
Equation (3) is a liquid diffusion model governed from Fick’s law, where Deff is theeffective diffusivity (m2 s−1), λ1 is first root of the Bessel function (2.4048), β1 is shaperatio (=πr/2l), r is the radius, l is the length of a rice grain (m) and t is drying time (min).The nonlinear function of Equation (3) was converted into a natural logarithm, resultingin a linear relationship between ln(MR) and t. Deff was then approximated from theslope ((5.7831 + β1)Deff/r2) of a function. Because Deff varied with different drying airtemperatures, the Arrhenius equation was used to describe effective diffusivity affected byfluidizing air temperature as:
Deff = D0 exp(− Ea
RT
)(4)
where D0 is a pre-exponential factor (m2 s−1), Ea is the activation energy (kJ kmol−1), R isthe universal gas constant (8.314 kJ kmol−1 K−1) and T is the absolute temperature ofdrying air (K).
Processes 2021, 9, 1738 5 of 21
2.4. Quality Attributes of Instant and Rehydrated Jasmine Rice2.4.1. Physical Properties
The proposed process was successfully employed to overcome the kernel agglomera-tion during drying; however, additional compressed air with high shear force increasedthe percentage of broken rice [19]. Therefore, physical properties including breakage andagglomerate percentages were evaluated using the following equations.
%Breakage =Total weight of broken rice
Total weight× 100 (5)
%Agglomerate =Total weight of agglomerates
Total weight× 100 (6)
2.4.2. Color in CIE System
The color of instant red jasmine rice was measured using a Minolta Colorimeterwith color system of CIE (L* a* b*), where L*, a* and b* represent lightness, redness andyellowness, respectively. Color difference (∆E) was evaluated against uncooked rice grains
and expressed as ∆E =√(∆a∗)2 + (b∗)2, where ∆a* and ∆b* were a color difference in
redness and yellowness, respectively.
2.4.3. Shrinkage and Rehydration Ratio
Shrinkage of instant red jasmine rice, related to the rehydration properties of driedproducts was measured by the volume change between cooked and dried kernels. A samplevolume was measured based on liquid replacement (Archimedes’ theorem). Shrinkage waspresented as a percentage and expressed by:
%Shrinkage =vi − vf
vi× 100 (7)
where vi and vf represent sample volume before and after drying (m3). Water is absorbedduring rehydration, consequently, rehydrated materials swell. Ten grams of instant redjasmine rice was immersed in 100 mL of water at temperatures of 70 ◦C and 100 ◦C for30 min. The water was rinsed and the sample was spread on a cotton cloth for 1 min toabsorb excess water on the kernel surfaces. Rehydration ratio was calculated as the weightof the rehydrated sample (wr) divided by the weight of instant rice (wi) as shown below.
Rehydration ratio =wr
wi(8)
2.4.4. Microstructure of Instant Red Jasmine Rice
Porosity of the sample is normally related to rehydration quality. Therefore, theporous structure of instant red jasmine rice prepared by different drying conditions wasexamined using scanning electron microscopy (SEM). Following the method proposedby Prasert and Suwannaporn [7] with slight modifications, the rice kernels were brokentransversely and placed on a sputtering device. Before scanning under voltage of 15 kVand ×30 magnification, the sample was coated with gold in a vacuum chamber using asputter coater.
2.4.5. Analysis of Textural Properties
Hardness and stickiness are commonly reported as they impact consumer acceptability.The ISO11747 standard protocol was used with a texture analyzer (Stable Micro System,Ta-XT2i, Surrey, UK). Seventeen grams of cooked or rehydrated rice were placed in anextrusion chamber. A plunger initially situated 55 mm above a perforated base moveddownward at a test speed of 1 mm s−1 until reaching the 3 mm gap between the probesurface and the base. The sample was extruded through a perforated plate and compressive
Processes 2021, 9, 1738 6 of 21
force was plotted against time. Mean force (kg) over the plateau region of the curve wasdetermined and its value divided by the testing area of the extrusion plate was consideredas the mean force required to extrude the sample (kg cm−2).
2.4.6. Antioxidant PropertiesSample Preparation
Dried red jasmine rice was ground and then sorted using a wired mesh size of 150 mi-crons. Extraction was conducted in accordance with the method proposed by Sutharut andSudarat [22] with slight modifications. Briefly, 0.25 g of ground sample was placed into a1.5 mL microtube. After adding 1 mL of methanol, the mixture was shaken continuouslyfor 30 s, and then heated to 60 ◦C in a water bath for 20 min with shaking at 10 min intervals.The mixture was then centrifuged at 13,000 rpm for 10 min, and the supernatant volumewas adjusted to 5 mL with methanol. The resulting extract was kept in a fridge under 4 ◦C.
Total Anthocyanin Content Analysis
The method proposed by Martynenko and Chen [23] was modified and used todetermine the total anthocyanin content (TAC) of the rice samples. Briefly, the extractwas diluted with pH 1.0 and pH 4.5 buffer solutions and then left for 20 min to reachequilibrium. Absorbance of both buffers at 510 and 700 nm was measured by a UV-VISspectrophotometer (HITACHI Model U1900) with the use of distilled water as a blank.TAC was calculated in terms of cyanidin-3-glucoside (Equation (9)) and expressed as unitsof mg cyanidin-3-glucoside per 100 g dry matter.
TAC =Adiff × MW × DF × 1000
ε× l(9)
where Adiff is the difference of absorbance between two wavelengths (510 and 700 nm)for pH 1.0 and pH 4.5, MW is molecular weight of cyanidin-3-glucoside (449.2 g mol−1),ε is the molar absorbability (26,000 L mol−1 cm−1), l is cuvette width (1 cm) and DF is adilution factor.
Total Phenolic Content Analysis
The protocol described by Butsat and Siriamornpun [24] was slightly modified toanalyze total phenolic content of instant red jasmine rice dried under different conditions.The extract was mixed with 10% Folin-Ciocalteu reagent in portions of 0.1 mL and 0.5 mL,respectively. After shaking and then storing in a dark container for 1 min, sodium carbonate(7.5%, 1.5 mL) was added to the mixture and the volume was adjusted with distilledwater to 4 mL. Before measuring the absorbance at wavelength 765 nm using a UV-VISspectrophotometer, the mixture was kept under ambient conditions for 30 min. Thestandard curve of gallic acid was used to calculate total phenolic content (TPC), presentedin terms of mg gallic acid equivalent (GAE) per 100 g dry sample.
Total Flavonoid Content Analysis
Total flavonoid content (TFC) was determined according to the method reported byChang et al. [25]. The protocol with slight modification was as follows. A mixture wasfirst prepared using distilled water (2.8 mL), methanol (1.5 mL), 10% aluminum chloride(0.1 mL), and 1M potassium acetate (0.1 mL), and then added with 0.5 mL of the extract. Theresultant solution was kept at room temperature for 30 min before measuring absorbanceat 415 nm. TFC (mg QE per 100 g dry matter) was determined using the standard curve ofquercetin solution as a function of absorbance.
Analysis of DPPH Radical Scavenging Activity
Antioxidant activity was analyzed by the DPPH radical scavenging activity methodfollowing the protocol reported by Loypimai et al. [26]. DPPH solution at a concentrationof 0.1 mmol was first prepared under darkness, and then 3.0 mL of this solution was added
Processes 2021, 9, 1738 7 of 21
into 1 mL of the extract. After shaking and storing in a dark container for 30 min, themixture was subjected to absorbance measurement at 517 nm using a spectrophotometer.Antioxidant activity of instant red jasmine rice affected by the drying process was reportedas scavenging percentage and calculated by Equation (10):
%scavenging =
(Acontrol − Asample
)Acontrol
× 100 (10)
where Acontrol and Asample are the absorbance values of the control and sample at wave-length of 517 nm, respectively.
2.5. Response Surface Methodology
To measure all responses (y), a full factorial design with two factors: fluidizing airtemperature (T, ◦C) and swirling air supply time (ST, min) with three levels was employed.Responses tested included percentage agglomerate (%agglomerate), percentage breakage(%break), percentage shrinkage (%shrink), rehydration ratio (RR), total phenolic content(TPC), total flavonoid content (TFC), total anthocyanin content (TAC), and antioxidantactivity expressed by DPPH radical scavenging (DPPH). All variables and their corre-sponding coded values are presented in Table 1. Quadratic regression, as expressed inEquation (11), was employed to predict the response variables [27].
y = β0 +k
∑i=1βixi +
k
∑i=1βiix
2i +
k
∑1≤i≤j
βijxixj (11)
Table 1. Drying variables and their corresponding coded values used in the experiment.
Variable UnitCoded Values and Ranges
−1 0 1
x1: fluidizing air temperature ◦C 90 105 120x2: swirling air supply time min 2 6 10
In Equation (11), β0, βi and βii represent the constant terms of intercept, the linear,quadratic and interaction term, respectively, while βij is the effect of the ij interactionbetween factors xi and xj.
The desirability function was used to evaluate the responses by considering the follow-ing constraints: (a) minimum values of agglomerate, breakage and shrinkage percentages,and (b) maximum values of rehydration ratio and retention of all antioxidant properties.
In order to compare the mean values of the results presented in this work, one-wayANOVA was used with Tukey’s test at 5% probability.
3. Results and Discussion3.1. Analysis of Drying Characteristics3.1.1. Drying Model
The semi-empirical Page model, widely used to describe the drying behavior ofbiological materials [28], was fitted to experimental moisture ratio (MR) with variations ofswirling compressed air pressure and fluidizing air temperature. Important characteristicparameters of drying, including drying rate constant (k), power constant (n), effectivediffusivity (Deff) and activation energy (Ea) were estimated, as shown in Table 2.
The rate constant (k) indicates the drying performance, where a high drying rate isrepresented by a high value. This parameter was used to determine the most suitablecondition, based on the highest k value, for further investigation. As shown in Table 2, thek values varied with different drying conditions and ranged from 0.09745 (at a swirlingcompressed air pressure of 5 bar and an air temperature of 90 ◦C) to 0.15072 (at a swirling
Processes 2021, 9, 1738 8 of 21
compressed air pressure of 6 bar and an air temperature of 120 ◦C). At each drying tem-perature, the k values decreased when using higher pressure (from 4 bar to 5 bar). Thisfinding concurred with our results reported previously [19], stating that decrease in thedrying rate constant was attributed to interference of additional compressed air. However,at 6 bar the k values increased at all temperatures, possibly attributed to higher turbulenceintensity and partial impingement scheme caused by the high velocity air jet releasing fromtwo nozzles located at the bottom bed.
Table 2. Drying characteristic parameters, effective diffusivity, and activation energy affected byswirling compressed air pressure and fluidizing air temperature.
SCAP(bar)
FAT(◦C) k n Deff
(×10−8 m2s−1)Ea
(kJ mol−1)
490 0.10248 ± 0.00175 de 0.83519 ± 0.00812cd 3.25 ± 0.16 de
15.45 ± 0.17 ns105 0.11983 ± 0.00651 c 0.86152 ± 0.01815 bc 4.22 ± 0.53 bc
120 0.13104 ± 0.00242 bc 0.87113 ± 0.00618 ab 4.79 ± 0.21 bc
590 0.09745 ± 0.00246 e 0.82108 ± 0.00723 d 2.92 ± 0.16 e
13.66 ± 2.28 ns105 0.10887 ± 0.00693 d 0.86671 ± 0.01079 ab 3.91 ± 0.42 cd
120 0.12217 ± 0.00639 c 0.85033 ± 0.01475 bc 4.11 ± 0.45 bcd
690 0.12511 ± 0.00748 bc 0.84925 ± 0.01741 bc 4.19 ± 0.54 bc
13.81 ± 0.17 ns105 0.13544 ± 0.00815 b 0.87502 ± 0.02087 ab 5.03 ± 0.72 b
120 0.15072 ± 0.00784 a 0.89014 ± 0.02048 a 5.94 ± 0.79 a
SCAP and FAT stand for swirling compressed air pressure and fluidizing air temperature, respectively. ns denotes‘not significant difference (p > 0.05)’, while superscripts represent significant difference (p < 0.05) among values inthe same column.
In a fluidized bed system, convection plays an important role by influencing heattransfer performance. The hydrodynamics of gas-solid phases are closely related to heattransfer phenomena and can be enhanced by turbulence due to increased flow homogeneityand mixing efficiency [29,30]. At the high pressure of 6 bar, increased turbulence intensitywas dominant and affected heat transfer phenomena compared to interference effects ofadditional ambient air streams at pressures of 4 and 5 bar. In this fluidization regime,solid circulation and contact with a gas phase could be improved, resulting in higherefficiency. Heat transfer enhancement due to increased turbulence flow field intensityhas been achieved in many applications [29–31]. Another plausible explanation was theimpingement effect from high pressure air streams at the dense bottom bed. Tangential gasjets removed thermal boundary layers on the grain surface, resulting in higher heat andmass transfer coefficients [32–34].
The Page model parameter influencing the drying mechanism depends on the materialtype and drying conditions [14]. Table 2 shows this exponential constant varying from0.82108 to 0.89014, tending to increase with higher air temperature but not definitelyobserved with respect to compressed air pressure. Correlations of the constants (k and n)obtained from the Page drying model were evaluated. Both model constants correlatedquadratically with fluidizing air temperature (T) and pressure of swirling compressedair (p), with R2 values of 0.9059 and 0.6730 for the rate constant k and power constant n,respectively. Thus, the Page equation used to describe the drying characteristics of theproposed process was expressed as:
MR = exp(−ktn) (12)
where k = 0.39744 + 1.01 × 10−3T − 0.16458P − 4.92 × 10−5TP + 5.11 × 10−7T2 + 0.01794P2
(R2 = 0.9059), and n = 0.44849 + 0.01467T − 0.17758P + 8.25 × 10−5TP − 6.62 × 10−5T2 +0.017668P2 (R2 = 0.6730).
The Page equation (Equation (12)) and its model parameters were used to predict thedrying time to obtain the desired moisture content (MC) of 10% (wb). Five drying runs withcorresponding estimated drying times (see Table 3) were conducted in triplicate for modelvalidation. The percentage of mean relative error (%error) between experimental moisturecontent (MCexp) and desired moisture content (MCdesir) for each run showed reasonable
Processes 2021, 9, 1738 9 of 21
discrepancy ranging 10.79–23.72%. The modulus of mean relative deviation (%p) was alsocalculated as the summation of all relative errors divided by the number of experimentalruns. The %p of 17.89%, calculated by Equation (13), indicated good agreement betweenthe experimental data and the predicted model results.
%p =100N
N
∑i=1
∣∣MCexp − MCdesir∣∣
MCexp(13)
Table 3. Validation results and percentage error.
Conditions Model ParametersPredicted Drying
Time (min)Actual Drying
Time (min) MC (%wb) %ErrorFAT(◦C)
SCAP(bar) k n
90 4 0.10349 0.83464 28.98 29 11.21 ± 0.93 10.7998 4.5 0.10231 0.84541 28.51 28.5 13.11 ± 1.02 23.72105 5 0.10889 0.85610 25.59 26 12.46 ± 1.32 19.74113 5.5 0.12501 0.86993 20.92 21 12.75 ± 2.19 21.57120 6 0.14893 0.88558 15.85 16 11.58 ± 1.22 13.64
FAT and SCAP stand for ‘fluidizing air temperature’ and ‘compressed air pressure’, respectively.
3.1.2. Effective Diffusivity
In a falling-rate drying process, moisture in food materials transfers to the surfaceand subsequently to ambient air by means of internal diffusion, called effective diffusivity(Deff) [21]. Table 2 shows how this drying characteristic was affected by operating param-eters. Effective diffusivity varied in the range 2.92–5.94 × 10−8 m2 s−1, higher than thenormally expected range of 10−11–10−9 m2 s−1 for food materials [35]. Deff is commonlyrelated to the drying rate constant; higher Deff means faster drying. At constant com-pressed air pressure, Deff tended to increase with higher air temperature. This is commonlyobserved in the drying process since heat and mass transfer are enhanced when higherdriving force is provided by increasing thermal energy [36]. Zielinska and Michalska [21]reported that the Deff value increased with higher air temperature in convective hot air dry-ing of blueberries. A similar result was also observed when mortiño (Vaccinium meridionaleSwartz) was dried at 40–60 ◦C [36].
With variation of swirling air pressure, a decreasing trend was found when increasingpressure from 4 bar to 5 bar at each air temperature. This was consistent with our previousstudy [19] when interference of additional ambient compressed air led to reducing dryingrate and subsequently reduced effective diffusivity. However, Deff slightly increased atthe same temperature when compressed air pressure increased to 6 bar. As previouslyexplained, high pressure air resulted in more turbulence intensity that enhanced heattransfer in the system [29–31]. Air impingement was also a plausible explanation for thisobservation. Tangential air jets with high velocity may remove or reduce the thermalboundary layers of tangentially fluidizing grains when passing nozzle outlets, and thisenhances heat and mass transfer. At high pressure, high velocity air streams avoid casehardening, resulting in increased moisture evaporation from grain surfaces [32–34].
3.1.3. Activation Energy
In the drying process, moisture is transported from a solid matrix using activationenergy. Table 2 also shows activation energy (Ea) values of 15.45, 13.66 and 13.81 kJ mol−1
at compressed air pressure of 4, 5, and 6 bar, respectively. These values were in the commonrange for dried food materials of 12.7 to 110 kJ mol−1 [37]. However, activation energyobtained here was not significantly different at varied pressures of swirling compressed air.
Based on the highest drying model parameters (k and n) and effective diffusivity (Deff),swirling compressed air pressure of 6 bar was selected to prepare instant red jasmine rice.
Processes 2021, 9, 1738 10 of 21
3.2. Influence of Fluidizing Air Temperature Combined with Supply of Swirling Compressed Air
The influences of fluidizing air temperature and supply time of swirling compressedair were investigated using response surface methodology (RSM). Responses of these twoinput parameters were categorized into two groups including physical and rehydrationproperties and antioxidant properties.
3.2.1. Physical Properties and Rehydration Ratio
Experimental design and the corresponding response values evaluated using ANOVAare presented in Table 4. During the optimization process, response values of physical andrehydration properties were predicted by estimated coefficients for the actual functionalcomponents, as shown in Table 5. All responses were extremely significant compared tothe model (p < 0.01) with regression coefficients higher than 0.7.
Table 4. Experimental design and values of physical and rehydration properties.
No. X1 X2Percentage
AgglomeratePercentageBreakage
PercentageShrinkage
RehydrationRatio
1 90 2 3.67 17.33 42.00 0.922 90 6 2.33 18.33 44.67 0.943 90 10 2.33 24.00 47.67 0.904 105 2 0.00 18.67 37.67 1.215 105 6 0.00 26.67 46.33 1.086 105 10 0.00 39.33 46.33 1.097 120 2 0.00 13.00 33.67 1.358 120 6 0.00 62.33 42.00 1.149 120 10 0.00 72.00 44.33 1.17
Table 5. ANOVA results of the fitted model for physical and rehydration properties in fluidized-beddrying assisted with swirling compressed air of instant red jasmine rice.
Source
Estimated Coefficient
PercentageAgglomerate
PercentageBreakage
PercentageShrinkage
RehydrationRatio
Intercept(a0) −0.15 31.59 45.04 1.09
Linear terms(a1) X1 −1.39 a 14.61 a −2.39 a 0.15 a
(a2) X2 −0.22 ns 14.39 a 4.17 a 0.053 b
Interaction terms(a12) X1X2 0.34 ns 13.08 a 1.25 ns 0.040 ns
Quadratic terms(a11) X2
1 1.39 a 6.27 ns −1.05 ns −0.057 ns
(a11) X22 0.22 ns −5.05 ns −2.39 b 0.053 ns
Lack-of-fit (pvalue) 0.3976 ns 0.0026 a 0.2375 ns 0.5095 ns
Model (p value) <0.0001 a <0.0001 a <0.0001 a <0.0001 a
F valueX1 102.98 54.11 20.48 42.20X2 2.66 52.47 62.21 5.34
X1X2 4.00 28.92 3.72 2.00X2
1 34.33 3.33 1.33 2.01X2
2 0.89 2.16 6.82 1.78R2 0.8734 0.8704 0.8183 0.7175
Adj.R2 0.8432 0.8395 0.7750 0.6502Predicted R2 0.7725 0.7841 0.6921 0.5417
C.V. (%) 62.72 26.00 5.24 9.00Std. Dev. 0.58 8.43 2.24 0.098
ns Not significant. a Significant at 1% (p < 0.01) (extremely significant), b Significant at 5% (p < 0.05).
Results in Table 5 show that the effects of monomial fluidizing air temperature and itsquadratic term on percentage agglomeration were extremely significant at p < 0.01, while
Processes 2021, 9, 1738 11 of 21
the other terms were insignificant (p > 0.05). The F value in Table 5 indicated fluidizingair temperature as the most important, while swirling time did not significantly affectthe agglomerate percentage. For percentage breakage, the effects of monomial fluidizingair temperature and swirling time, as well as their interaction were extremely significantat p < 0.01, while all quadratic terms were insignificant (p > 0.05). Among these twofactors, the higher F value showed that fluidizing air temperature was the most important.Similarly, the linear term of both input parameters greatly affected percentage shrinkage atp < 0.01, whereas the interaction term and quadratic fluidizing air temperature were notsignificant (p > 0.05). Results in Table 5 also showed that the quadratic swirling time wassignificant at the p < 0.05 level, and this input parameter was more important, as confirmedby the higher F value. Effects of monomial fluidizing air temperature and swirling time onrehydration ratio were also significant at p < 0.01 and p < 0.05, respectively, while the otherterms were insignificant. According to the F value in Table 5, fluidizing air temperaturewas the most important, followed by the swirling time.
Figure 4 shows three-dimensional plots of all responses regarding physical and rehy-dration properties as a function of fluidizing air temperature and swirling time. Fluidizingair temperature (T) had a negative influence on percentage agglomerate of the dried cookedrice sample (Figure 4a), while agglomeration did not change significantly with increasingswirling time (ST) at temperature higher than 90 ◦C. In our proposed process, cooked ricewas prepared before subjection to the drying step. Starch gelatinization resulted in surfacestickiness among cooked rice kernels, leading to problematic agglomeration during drying.At 90 ◦C, swirling compressed air time significantly affected the agglomeration percentageof the kernels (∼2.3–3.5%); the longer the lower. This result slightly differed from ourprevious study where percentage agglomeration of riceberry rice kernels was ∼0.8% at4 bar [19]. The difference was attributed to longer time from the onset till the end of thedrying process, while time spent supplying high pressure air streams here ranged between2 and 10 min. At temperatures higher than 90 ◦C, agglomerates were not observed and therice kernels dried quickly and were completely separated by the high shear force exertedby compressed air streams. Therefore, high fluidizing air temperature with the shortestswirling air time was the optimal condition with respect to minimal agglomeration.
Figure 4b shows the influence of fluidizing air temperature and swirling time on per-centage breakage. An obvious trend appeared when using higher temperature and longertime. Maximum value of percentage breakage at 75% was found when drying at 120 ◦Cand supplying swirling air for 10 min. Swirling compressed air reduced surface stickinessduring drying but also resulted in kernel breakage due to high shear force. Figure 4b showsthe breakage percentage of rice kernels at varying fluidizing air temperatures and swirlingcompressed air times. At a fixed temperature, breakage increased with time of supplyingswirling air, except at 90 ◦C when breakage slightly increased but not statistically signif-icantly (p > 0.05). Fluidizing air temperature strongly affected kernel breakage, ranging17.33–24.00%, 18.67–39.33% and 13.00–72.00% at 90 ◦C, 105 ◦C and 120 ◦C, respectively.Increased fragrance at higher temperature could be a possible explanation for this trend,while at the same temperature longer compressed air supply resulted in higher breakage.Consequently, 120 ◦C with compressed air supplied for 2 min was a suitable condition thatretained perfectly shaped kernels without agglomerates.
During drying, solid shrinkage generally occurs due to collapse and disruption of thecells inside a material, especially at higher temperatures [38]. Solid shrinkage is normallyinfluenced by variations of drying factors, as shown in Figure 4c. Percentage shrinkageranged from 33.67 to 47.67%, depending on the fluidizing air temperature and compressedair time. Figure 4c shows that the swirling time had positive effect on percentage shrinkage,while negative influence was found for fluidizing air temperature. Percentage shrinkageslightly decreased with higher temperatures ranging 42.00–47.67%, 37.67–46.33% and33.67–44.33% at 90 ◦C, 105 ◦C and 120 ◦C, respectively. During drying, water (moisture)evaporated and then moved from the inside toward the material surface due to the pressureimbalance resulting in rice kernel shrinkage. At higher temperatures, higher drying rate
Processes 2021, 9, 1738 12 of 21
caused rapid heating and accumulation of vapor pressure in the rice kernels. This resultedin less volume shrinkage or even volume expansion. This so-called puffing effect impactedthe microstructure, normally required to facilitate the rehydration process [7,13]. A slightdecrease in shrinkage with higher drying temperature obtained here contradicted theresults of Le and Jittanit [13] who used lower temperatures of 50–90 ◦C.
Processes 2021, 9, x FOR PEER REVIEW 12 of 22
cooked rice was prepared before subjection to the drying step. Starch gelatinization re-sulted in surface stickiness among cooked rice kernels, leading to problematic agglomer-ation during drying. At 90 °C, swirling compressed air time significantly affected the ag-glomeration percentage of the kernels (∼2.3–3.5%); the longer the lower. This result slightly differed from our previous study where percentage agglomeration of riceberry rice kernels was ∼0.8% at 4 bar [19]. The difference was attributed to longer time from the onset till the end of the drying process, while time spent supplying high pressure air streams here ranged between 2 and 10 min. At temperatures higher than 90 °C, agglom-erates were not observed and the rice kernels dried quickly and were completely sepa-rated by the high shear force exerted by compressed air streams. Therefore, high fluidiz-ing air temperature with the shortest swirling air time was the optimal condition with respect to minimal agglomeration.
(a) (b)
(c) (d)
Figure 4. Three-dimensional plots of (a) %agglomerate, (b) %breakage, (c) %shrinkage and (d) rehydration ratio as a func-tion of fluidizing air temperature (T, °C) and swirling time (ST, min).
Figure 4b shows the influence of fluidizing air temperature and swirling time on percentage breakage. An obvious trend appeared when using higher temperature and longer time. Maximum value of percentage breakage at 75% was found when drying at 120 °C and supplying swirling air for 10 min. Swirling compressed air reduced surface stickiness during drying but also resulted in kernel breakage due to high shear force. Fig-ure 4b shows the breakage percentage of rice kernels at varying fluidizing air tempera-tures and swirling compressed air times. At a fixed temperature, breakage increased with time of supplying swirling air, except at 90 °C when breakage slightly increased but not statistically significantly (p > 0.05). Fluidizing air temperature strongly affected kernel breakage, ranging 17.33–24.00%, 18.67–39.33% and 13.00–72.00% at 90 °C, 105 °C and 120
Figure 4. Three-dimensional plots of (a) %agglomerate, (b) %breakage, (c) %shrinkage and (d) rehydration ratio as afunction of fluidizing air temperature (T, ◦C) and swirling time (ST, min).
Shrinkage increased with compressed air time as a result of system interference byadditional ambient air, consistent with our results reported previously [19]. Interestingly,interference of swirling time on shrinkage percentage dramatically increased with longertime due to additional ambient swirling air interfering in the drying system, resultingin lower drying rate. The moisture was removed slowly, and resultant cell collapse anddisruption led to more solid shrinkage [38]. Therefore, in addition to the lowest agglom-eration percentage and breakage percentage, fluidizing air temperature of 120 ◦C andswirling air time of 2 min were considered as the optimal drying condition that providedthe lowest shrinkage.
Processes 2021, 9, 1738 13 of 21
Rehydration is a crucial characteristic of instant products that is normally related tomaterial shrinkage [12]. Samples with less shrinkage have more pore structure and thisfacilitates water absorption during rehydration. Figure 4d shows the rehydration ratio (RR)of instant red jasmine rice after rehydration in boiling water for 30 min. Figure 4d showsthat fluidizing air temperature had a positive influence on rehydration ratio, especiallyat the shortest swirling time, while the swirling time slightly affected rice rehydration.The RR varied with different fluidizing air temperatures and swirling air times, rangingfrom 0.90 to 1.35 (Table 4). High values indicated high capacity of water absorption duringthe rehydration process. At fixed air temperature, RR tended to decrease with longersupply of swirling air, possibly caused by more interference of lower temperature airsupplied to the system with longer compressed air time. The colder fluidizing air led toslower drying, and subsequently compacted the microstructure inside the dried kernels.At each swirling air time, drying temperature significantly affected RR and increased withhigher temperature. The porous structure could be a plausible explanation for higherRR. Maximum value of RR was obtained at 120 ◦C for 2 min. This condition providedhigh drying rate as moisture inside the cooked kernels moved quickly to the surface, andsubsequently less cell shrinkage. These results were confirmed by SEM images as shownin Figure 5a–j. It is shown from this figure that porosity in the rice kernel dried underfluidizing air temperature of 120 ◦C and supply time of 2 min (Figure 5g) was reasonablycomparable with that of freeze-dried sample, as shown in Figure 5j.
3.2.2. Antioxidant Properties
Table 6 shows values of responses, including total phenolic content (TPC), totalflavonoid content (TFC), total anthocyanin content (TAC) and antioxidant activity (DPPH)of red jasmine rice. Antioxidant properties changed with varied drying conditions. In thedrying process, temperature generally plays the most important role as it acceleratesreactions that cause oxidative decomposition or activities of bioactive compounds. Changesin antioxidant properties with various drying temperatures found here were consistentwith previous studies but in different ways, decreasing [39–42] or increasing [43,44] athigher operating temperatures.
Table 6. Experimental design and values of antioxidant properties.
No. X1 X2 TPC TFC TAC DPPH
1 90 2 483.26 26.73 35.24 53.712 90 6 484.95 27.18 35.22 53.753 90 10 488.07 27.11 34.52 53.734 105 2 475.55 26.29 34.68 51.195 105 6 474.81 26.67 34.35 51.816 105 10 479.60 26.86 34.29 52.217 120 2 471.97 24.85 34.31 50.508 120 6 471.22 24.84 34.08 50.589 120 10 478.06 25.22 33.76 51.09
Cooked sample 530.30 35.10 58.23 62.58FD sample 515.82 31.75 55.16 60.50
TPC, TFC, TAC, and DPPH is expressed in mg GAE/100g, mg QE/100g, mg cy-3-glu/100g and % scavenging,respectively. FD stands for ‘freeze dried’.
Antioxidant compounds such as phenolics, flavonoids and anthocyanins are com-monly abundant in pigmented rice as they are mainly linked with the grain pericarp [45].The influence of heat in the proposed drying method on such properties is presented inTable 6. Freeze-drying is an ideal process that provides dry materials with high physicaland chemical qualities [46]. Compared to freshly cooked rice, all antioxidant properties offreeze-dried samples slightly decreased. The highest reduction of 9.5% was determinedfor TFC, while TPC, TAC and DPPH decreased by only 2.7%, 5.3% and 3.3%, respectively.
Processes 2021, 9, 1738 14 of 21
These slight reductions in antioxidant properties were consistent with previous researchfocusing on the influence of freeze-drying [47].
Processes 2021, 9, x FOR PEER REVIEW 14 of 22
(a) (b)
(c) (d)
(e) (f)
(g) (h)
Figure 5. Cont.
Processes 2021, 9, 1738 15 of 21Processes 2021, 9, x FOR PEER REVIEW 15 of 22
(i) (j)
Figure 5. Cross sections of red jasmine rice dried under various drying conditions: (a) 90 °C 2 min, (b) 90 °C 6 min, (c) 90 °C 10 min, (d) 105 °C 2 min, (e) 105 °C 6 min, (f) 105 °C 10 min, (g) 120 °C 2 min, (h) 120 °C 6 min, (i) 120 °C 10 min, and (j) freeze-dried sample.
3.2.2. Antioxidant Properties Table 6 shows values of responses, including total phenolic content (TPC), total fla-
vonoid content (TFC), total anthocyanin content (TAC) and antioxidant activity (DPPH) of red jasmine rice. Antioxidant properties changed with varied drying conditions. In the drying process, temperature generally plays the most important role as it accelerates re-actions that cause oxidative decomposition or activities of bioactive compounds. Changes in antioxidant properties with various drying temperatures found here were consistent with previous studies but in different ways, decreasing [39–42] or increasing [43,44] at higher operating temperatures.
Table 6. Experimental design and values of antioxidant properties.
No. X1 X2 TPC TFC TAC DPPH 1 90 2 483.26 26.73 35.24 53.71
2 90 6 484.95 27.18 35.22 53.75
3 90 10 488.07 27.11 34.52 53.73
4 105 2 475.55 26.29 34.68 51.19
5 105 6 474.81 26.67 34.35 51.81
6 105 10 479.60 26.86 34.29 52.21
7 120 2 471.97 24.85 34.31 50.50
8 120 6 471.22 24.84 34.08 50.58
9 120 10 478.06 25.22 33.76 51.09
Cooked sample 530.30 35.10 58.23 62.58
FD sample 515.82 31.75 55.16 60.50
TPC, TFC, TAC, and DPPH is expressed in mg GAE/100g, mg QE/100g, mg cy-3-glu/100g and % scavenging, respectively. FD stands for ‘freeze dried’.
Antioxidant compounds such as phenolics, flavonoids and anthocyanins are com-monly abundant in pigmented rice as they are mainly linked with the grain pericarp [45]. The influence of heat in the proposed drying method on such properties is presented in Table 6. Freeze-drying is an ideal process that provides dry materials with high physical and chemical qualities [46]. Compared to freshly cooked rice, all antioxidant properties of freeze-dried samples slightly decreased. The highest reduction of 9.5% was determined for TFC, while TPC, TAC and DPPH decreased by only 2.7%, 5.3% and 3.3%, respectively. These slight reductions in antioxidant properties were consistent with previous research focusing on the influence of freeze-drying [47].
In addition to values of physical and rehydration responses, Table 7 shows ANOVA results of the fitted model for antioxidant properties in fluidized-bed drying assisted with
Figure 5. Cross sections of red jasmine rice dried under various drying conditions: (a) 90 ◦C 2 min,(b) 90 ◦C 6 min, (c) 90 ◦C 10 min, (d) 105 ◦C 2 min, (e) 105 ◦C 6 min, (f) 105 ◦C 10 min, (g) 120 ◦C2 min, (h) 120 ◦C 6 min, (i) 120 ◦C 10 min, and (j) freeze-dried sample.
In addition to values of physical and rehydration responses, Table 7 shows ANOVAresults of the fitted model for antioxidant properties in fluidized-bed drying assisted withswirling compressed air of instant red jasmine rice. Results in Table 7 show that theeffects of monomial fluidizing air temperature on TPC were significant at the p < 0.01level, while the other terms were insignificant. The F value confirmed that fluidizingair temperature was the most important input parameter. Considering the responseof TFC, influences of monomial fluidizing air temperature and its quadratic term weresignificant at the p < 0.01 level and p < 0.05 level, respectively, whereas the other termswere insignificant. The F value of fluidizing air temperature was extremely higher thanswirling time, implying a more dominant input parameter of the drying system. TACwas affected by fluidizing air temperature and swirling time at significance levels of 0.01and 0.05, respectively, while the former was a more important factor as it gave a higherF value. Estimated coefficients of DPPH radical scavenging showed that only monomialfluidizing air temperature significantly (p < 0.01) affected the response, corresponding tohigher F value and indicating that drying air temperature was more important (Table 7).
Figure 6a–d shows three-dimensional plots of TPC, TFC, TAC, and DPPH as a functionof both operating parameters. Fluidizing air temperature had a dominantly negative effecton antioxidant properties tested here, while swirling time had an insignificant influence.In Figure 6a, maximum retention of TPC at 485–489 mg GAE per 100 g was found at90 ◦C when supplying swirling times in a test range. However, at the highest temperature,influence of swirling time became more significant as TPC increased at longer time dueto interference of additional ambient air for a longer time, resulting in reduced thermaldegradation of TPC. Similarly, fluidizing air temperature had a strong effect on TFC ofdried cooked jasmine rice (Figure 6b). At each swirling time, TFC quadratically decreasedwith higher temperature. This observation was consistent with the ANOVA results oflinear and quadratic term fluidizing air temperature (X1 and X2
1, respectively), as shown inTable 7. Maximum TFC of approximately 27 mg QE per 100 g was found at 90 ◦C. TACdecreased with higher fluidizing air temperature in the same manner as found in TPC andTFC. Maximum TAC of 35 mg cy-3-glu per 100 g was obtained at 90 ◦C with swirling timeof 2 min. Antioxidant activity represented by DPPH radical scavenging (%inhibition) asaffected by input parameters is shown in Figure 6d. Again, fluidizing air temperature hada dominant influence on DPPH of the dried rice sample, while less significance was foundfor swirling time. Maximum value of 54% inhibition was obtained when drying at 90 ◦C.
After drying using the fluidized bed equipment assisted with compressed swirling airstreams, all antioxidant properties tested significantly decreased with variations of oper-ating parameters. Slight decrease was found for TPC (8–11%), TFC (23–29%), and DPPH(14–19%), while TAC dramatically decreased by 40–42%. Most antioxidant compoundsare susceptible to environmental factors such as temperature, oxygen, and humidity [43],
Processes 2021, 9, 1738 16 of 21
explaining these observations. Ratseewo et al. [48] reported that TPC, TFC, TAC, and DPPHreduced by 21%, 25%, 53% and 27.5%, respectively after red jasmine rice was subjected tohot air drying at 60 ◦C. These findings concurred with other applications, for instance, ricebran and husk [49], mulberry leaves [50], blueberry leather [51] and mortiño [36]. A dryingtemperature of 90 ◦C and all times of swirling air supply was the most suitable condition,giving the lowest reduction levels of all antioxidant properties, as shown in Figure 6. Con-versely, severe operating temperature of 120 ◦C and 2 min supply of compressed air gavethe highest percentage of reduction compared to the freshly cooked sample.
Table 7. ANOVA results of fitted model antioxidant properties in fluidized-bed drying assisted withswirling compressed air of instant red jasmine rice.
SourceEstimated Coefficient
TPC TFC TAC DPPH
Intercept(a0) 475.04 26.64 34.50 51.72
Linear terms(a1) X1 −5.84 a −1.02 a −0.47 a −1.50 a
(a2) X2 2.49 ns 0.22 ns −0.28 b 0.27 ns
Interaction terms(a12) X1X2 0.32 ns −0.002 ns 0.043 ns 0.14 ns
Quadratic terms(a11) X2
1 2.94 ns −0.62 b 0.082 ns 0.49 ns
(a11) X22 2.43 ns −0.053 ns −0.083 ns 0.025 ns
Lack-of-fit (pvalue) 0.9452 ns 0.8650 ns 0.6877 ns 0.7101 ns
Model (p value) 0.0016 a 0.0001 a 0.0012 a 0.0001 a
F valueX1 21.99 61.03 22.25 98.21X2 4.00 2.85 7.66 3.21
X1X2 0.044 0.000 0.12 0.59X2
1 1.85 7.50 0.22 3.48X2
2 1.26 0.056 0.23 0.009R2 0.5813 0.7728 0.5921 0.8340
Adj.R2 0.4816 0.7187 0.4950 0.7945Predicted R2 0.2928 0.6095 0.3246 0.7177
C.V. (%) 1.10 2.11 1.23 1.24Std. Dev. 5.28 0.55 0.42 0.64
ns Not significant. a Significant at 1% (p < 0.01) (extremely significant), b Significant at 5% (p < 0.05).
3.3. Optimization
Problems encountered with surface stickiness and preparation steps were reduced byour novel proposed drying technique to produce instant red jasmine rice. As describedin previous sections, all operating parameters affected both physical and antioxidantproperties in different ways. Therefore, correlations between drying factors (fluidizing airtemperature, T and swirling air supply time, ST) and their responses (physical, rehydrationand all antioxidant properties) were determined. Drying conditions were subsequentlyoptimized based on criteria of minimum values of agglomerate, breakage and shrinkagepercentages, maximum values of rehydration ratio, and all antioxidant properties.
Figure 7 shows a contour plot of desirability function obtained from correlationsbe-tween input parameters and response of percentage agglomerate, percentage breakage,percentage shrinkage, rehydration ratio, TPC, TFC, TAC and DPPH as response functions.Table 8 indicates all optimal process conditions with desirability values ranging from 0.532to 0.603. Based on the highest desirability value of 0.603, fluidizing air temperature andcompressed air time at 98.5 ◦C and 2 min, respectively, were recommended as a guide forprocess optimization.
Processes 2021, 9, 1738 17 of 21
Processes 2021, 9, x FOR PEER REVIEW 17 of 22
due to interference of additional ambient air for a longer time, resulting in reduced ther-mal degradation of TPC. Similarly, fluidizing air temperature had a strong effect on TFC of dried cooked jasmine rice (Figure 6b). At each swirling time, TFC quadratically de-creased with higher temperature. This observation was consistent with the ANOVA re-sults of linear and quadratic term fluidizing air temperature (X1 and 2
1X , respectively), as shown in Table 7. Maximum TFC of approximately 27 mg QE per 100 g was found at 90 °C. TAC decreased with higher fluidizing air temperature in the same manner as found in TPC and TFC. Maximum TAC of 35 mg cy-3-glu per 100 g was obtained at 90 °C with swirling time of 2 min. Antioxidant activity represented by DPPH radical scavenging (%inhibition) as affected by input parameters is shown in Figure 6d. Again, fluidizing air temperature had a dominant influence on DPPH of the dried rice sample, while less sig-nificance was found for swirling time. Maximum value of 54% inhibition was obtained when drying at 90 °C.
(a) (b)
(c) (d)
Figure 6. Three-dimensional plot of (a) TPC, (b) TFC, (c) TAC, and (d) DPPH as a function of fluidizing air temperature (T, °C) and swirling time (ST, min).
After drying using the fluidized bed equipment assisted with compressed swirling air streams, all antioxidant properties tested significantly decreased with variations of op-erating parameters. Slight decrease was found for TPC (8–11%), TFC (23–29%), and DPPH (14–19%), while TAC dramatically decreased by 40–42%. Most antioxidant compounds are susceptible to environmental factors such as temperature, oxygen, and humidity [43], explaining these observations. Ratseewo et al. [48] reported that TPC, TFC, TAC, and DPPH reduced by 21%, 25%, 53% and 27.5%, respectively after red jasmine rice was sub-jected to hot air drying at 60 °C. These findings concurred with other applications, for instance, rice bran and husk [49], mulberry leaves [50], blueberry leather [51] and mortiño [36]. A drying temperature of 90 °C and all times of swirling air supply was the most
Figure 6. Three-dimensional plot of (a) TPC, (b) TFC, (c) TAC, and (d) DPPH as a function of fluidizing air temperature(T, ◦C) and swirling time (ST, min).
Processes 2021, 9, x FOR PEER REVIEW 18 of 22
suitable condition, giving the lowest reduction levels of all antioxidant properties, as shown in Figure 6. Conversely, severe operating temperature of 120 °C and 2 min supply of compressed air gave the highest percentage of reduction compared to the freshly cooked sample.
3.3. Optimization Problems encountered with surface stickiness and preparation steps were reduced
by our novel proposed drying technique to produce instant red jasmine rice. As described in previous sections, all operating parameters affected both physical and antioxidant properties in different ways. Therefore, correlations between drying factors (fluidizing air temperature, T and swirling air supply time, ST) and their responses (physical, rehydra-tion and all antioxidant properties) were determined. Drying conditions were subse-quently optimized based on criteria of minimum values of agglomerate, breakage and shrinkage percentages, maximum values of rehydration ratio, and all antioxidant proper-ties.
Figure 7 shows a contour plot of desirability function obtained from correlations be-tween input parameters and response of percentage agglomerate, percentage breakage, percentage shrinkage, rehydration ratio, TPC, TFC, TAC and DPPH as response functions. Table 8 indicates all optimal process conditions with desirability values ranging from 0.532 to 0.603. Based on the highest desirability value of 0.603, fluidizing air temperature and compressed air time at 98.5 °C and 2 min, respectively, were recommended as a guide for process optimization.
Figure 7. Contour plot of desirability function.
Table 8. Optimal drying conditions and their corresponding output parameters.
T ST %Agglo %Broke %Shrink RR TPC TFC TAC DPPH Desirability 98.48 2 1.31 12.67 39.86 1.10 478.20 26.69 34.93 52.28 0.603 98.23 2 1.36 12.74 39.91 1.10 478.35 26.70 34.94 52.31 0.603 98.13 2 1.18 12.51 39.74 1.11 478.83 26.67 34.90 52.19 0.603
102.75 2 0.59 12.06 39.00 1.17 475.96 26.51 34.77 51.73 0.593 94.71 10 1.23 24.88 47.10 0.99 485.12 27.22 34.47 53.18 0.532
T = fluidizing air temperature (°C), ST = swirling air time (min), %Agglo = %agglomerate, %Broke = %breakage, %Shrink = %shrinkage, RR = rehydration ratio, TPC = total phenolic content (mg GAE/100g), TFC = total flavonoid content (mg QE/100g), TAC = total anthocyanin content (mg cy-3-glu/100g), DPPH = DPPH radical scavenging (%inhibition).
Figure 7. Contour plot of desirability function.
Processes 2021, 9, 1738 18 of 21
Table 8. Optimal drying conditions and their corresponding output parameters.
T ST %Agglo %Broke %Shrink RR TPC TFC TAC DPPH Desirability
98.48 2 1.31 12.67 39.86 1.10 478.20 26.69 34.93 52.28 0.60398.48 2 1.36 12.74 39.91 1.10 478.35 26.70 34.94 52.31 0.60398.48 2 1.18 12.51 39.74 1.11 478.83 26.67 34.90 52.19 0.603102.75 2 0.59 12.06 39.00 1.17 475.96 26.51 34.77 51.73 0.59394.71 10 1.23 24.88 47.10 0.99 485.12 27.22 34.47 53.18 0.532
T = fluidizing air temperature (◦C), ST = swirling air time (min), %Agglo = %agglomerate, %Broke = %breakage, %Shrink = %shrinkage,RR = rehydration ratio, TPC = total phenolic content (mg GAE/100g), TFC = total flavonoid content (mg QE/100g), TAC = total anthocyanincontent (mg cy-3-glu/100g), DPPH = DPPH radical scavenging (%inhibition).
3.4. Suitable Rehydration Condition
Surface stickiness of rice kernels can be avoided using our proposed drying method,with high quality in terms of physical, rehydration and antioxidant properties. However,textural characteristics of rehydrated rice are important for consumer preference. Therefore,instant red jasmine rice prepared by the aforementioned optimal drying condition (98.5 ◦Cwith swirling air time 2 min) was rehydrated under different rehydration conditions andtextural properties were compared with the control sample conventionally cooked by adomestic rice cooker, as shown in Table 9. The instant rice sample was suitably rehydratedin 70 ◦C water for 10 min. Under this condition, mean force of 13.18 ± 0.79 kg andmean extrusion force of 1.76 ± 0.10 kg cm−2 were reasonably comparable with the control(16.42 ± 0.36 kg and 2.19 ± 0.04 kg cm−2, respectively), while the freeze-dried sample wasextremely soft with low values of 6.60 ± 0.23 kg and 0.88 ± 0.03 kg cm−2, respectively.Therefore, instant red jasmine rice prepared by our proposed novel drying method underoptimal conditions should be rehydrated in water at 70 ◦C for 10 min.
Table 9. Textural properties of rehydrated instant red jasmine rice.
Temperature(◦C)
Rehydration Time(min)
Mean Force(kg)
Mean Extrusion Force(kg cm−2)
705 22.12 ± 3.25 a 2.95 ± 0.43 a
10 13.18 ± 0.79 c 1.76 ± 0.10 c
15 9.77 ± 1.28 d 1.30 ± 0.17 d
1005 12.77 ± 0.22 c 1.71 ± 0.03 c
10 9.11 ± 0.41 de 1.21 ± 0.06 de
15 7.37 ± 0.45 ef 0.98 ± 0.06 ef
Control 16.42 ± 0.36 b 2.19 ± 0.04 b
Freeze-dried sample 6.60 ± 0.23 f 0.88 ± 0.03 f
Values are means ± standard deviation of determinations for triplicate samples. Values with different superscriptsin each column are significantly different (p < 0.05).
4. Conclusions
Fluidized bed drying associated with swirling compressed air was proposed as analternative process to produce instant red jasmine rice. The rate constant obtained fromthe drying Page equation increased with higher temperature but decreased with thepressure of compressed air up to 5 bar. Interestingly, a drying temperature of 120 ◦Cassociated with 6 bar compressed air pressure was the most suitable, probably due tohigher turbulence intensity and/or partial impingement. Extending the time for supplyinghigh velocity air jets resulted in less kernel agglomeration but an increase in grain breakageand shrinkage. With increasing temperature and shorter time for compressed air streams, ahigher degree of rehydration was observed, as confirmed by the more porous structureshown by scanning electron microscopy. However, at more severe drying temperatures,deterioration of antioxidant properties dominated. The optimal operating parameters weredetermined based on a compromise between the physical properties, rehydration ratio, and
Processes 2021, 9, 1738 19 of 21
all antioxidant properties. Response surface methodology provided the optimal fluidizingair temperature and compressed air duration of 98.5 ◦C and 2 min, respectively. Textualanalysis of the rehydrated sample suggested that soaking instant red jasmine rice in warmwater at 70 ◦C for 10 min was preferable compared to conventional cooked rice.
Author Contributions: Conceptualization, W.D., P.C.; methodology, W.D., P.C., T.G.; software,P.C.; validation, P.C., T.G.; formal analysis, W.D., P.C.; investigation, all authors; resources, W.D.;writing—original draft preparation, P.C., T.G.; writing—review and editing, W.D.; visualization, P.C.;supervision, W.D., project administration, W.D.; funding acquisition, W.D. All authors have read andagreed to the published version of the manuscript.
Funding: This research was funded by Faculty of Engineering, Mahasarakham University (Grantyear 2020), grant number ENGEN 01/2563.
Acknowledgments: This research was financially supported by Faculty of Engineering, MahasarakhamUniversity (Grant year 2020).
Conflicts of Interest: The authors declare no conflict of interest.
References1. Chen, X.Q.; Nagao, N.; Itani, T.; Irifune, K. Anti-oxidative analysis, and identification and quantification of anthocyanin pigments
in different colored rice. Food Chem. 2012, 135, 2783–2788. [CrossRef] [PubMed]2. Goufo, P.; Trindade, H. Rice antioxidant: Phenolic acids, flavonoids, anthocyanins, proanthocyanins, tocopherols, tocotrienols,
γ-oryzanol, and phytic acid. Food Sci. Nutr. 2014, 2, 75–104. [CrossRef] [PubMed]3. Ratseewo, J.; Warren, F.J.; Siriamornpun, S. The influence of starch structure and anthocyanin content on the digestibility of Thai
pigmented rice. Food Chem. 2019, 298, 124949. [CrossRef] [PubMed]4. Poosri, S.; Thilavech, T.; Pasukamonset, P.; Suparpprom, C.; Adisakwattana, S. Studies on riceberry rice (Oryza sativa L.) extract
on the key steps related to carbohydrate and lipid digestion and absorption: A new source of natural bioactive substances. NFS J.2019, 17, 17–23. [CrossRef]
5. Chen, H.H. Investigation of properties of long-grain brown rice treated by low-pressure plasma. Food Bioprocess Tech. 2014, 7,2484–2491. [CrossRef]
6. Le, T.Q.; Jittanit, W. Optimization of operating process parameters for instant brown rice production with microwave-followed byconvective hot air drying. J. Stored Prod. Res. 2015, 61, 1–8. [CrossRef]
7. Prasert, W.; Suwannaporn, P. Optimization of instant jasmine rice process and its physicochemical properties. J. Food Eng. 2009,95, 54–61. [CrossRef]
8. Dutta, H.; Mahanta, C.L. Laboratory process development and physicochemical characterization of a low amylose and hydrother-mally treated ready-to-eat rice product requiring no cooking. Food Bioprocess Tech. 2014, 7, 212–223. [CrossRef]
9. Rewthong, O.; Soponronnarit, S.; Taechapairoj, C.; Tungtrakul, P.; Prachayawarakorn, S. Effects of cooking, drying and pretreat-ment methods on texture and starch digestibility of instant rice. J. Food Eng. 2011, 103, 258–264. [CrossRef]
10. Pei, F.; Shi, Y.; Mariga, A.M.; Yang, W.-J.; Tang, X.-Z.; Zhao, L.-Y.; An, X.-X.; Hu, Q.-H. Comparison of freeze-drying andfreeze-drying combined with microwave vacuum drying methods on drying kinetics and rehydration characteristics of buttonmushroom (Agaricus bisporus) slices. Food Bioprocess Tech. 2014, 7, 1629–1639. [CrossRef]
11. Yu, L.; Turner, M.S.; Fitzgerald, M.; Stokes, J.R.; Witt, T. Review of the effects of different processing technologies on cooked andconvenience rice quality. Trends Food Sci. Technol. 2017, 59, 124–138. [CrossRef]
12. Luangmalawat, P.; Prachayawarakorn, S.; Nathakaranakule, A.; Soponronnarit, S. Effect of temperature on drying characteristicsand quality of cooked rice. LWT 2008, 41, 716–723. [CrossRef]
13. Le, T.Q.; Jittanit, W. Drying characteristics of cooked jasmine brown rice and true densities of dried products. Kasetsart J. (Nat. Sci.)2012, 46, 256–271.
14. Jiao, A.; Xu, X.; Jin, Z. Modelling of dehydration-rehydration of instant rice in combined microwave-hot air drying. Food Bioprod.Process. 2014, 92, 259–265. [CrossRef]
15. Chen, X.; Qian, P.; Zhang, X.J.; Liu, F.N.; Lu, R.R. Improving instant rice quality by novel combined drying. Dry Technol. 2014, 32,1448–1456. [CrossRef]
16. Patil, S.B.; Khan, M.K. Germinated brown rice as a value added rice product: A review. J. Food Sci. Technol. 2011, 48, 661–667.[CrossRef] [PubMed]
17. Das, M.; Banerjee, R.; Bal, S. Evaluation of physicochemical properties of enzyme treated brown rice (Part B). LWT 2008, 41,2092–2096. [CrossRef]
18. Das, M.; Gupta, S.; Kapoor, V.; Banerjee, R.; Bal, S. Enzymatic polishing of rice—a new processing technology. LWT 2008, 41,2079–2084. [CrossRef]
19. Gaewsondee, T.; Duangkhamchan, W. A novel process for preparing instant riceberry using fluidized bed drying assisted withswirling compressed-air: Kinetic aspects. Food Bioprocess Tech. 2019, 12, 1422–1434. [CrossRef]
Processes 2021, 9, 1738 20 of 21
20. AOAC. Official Methods of Analysis, 16th ed.; Association of Official Agricultural Chemists: Washington, DC, USA, 1995.21. Zielinska, M.; Michalska, A. Microwave-assisted drying of blueberry (Vaccinium corrymbosum L.) fruit: Drying kinetics, polyphe-
nols, anthocyanins, antioxidant capacity, color and texture. Food Chem. 2016, 212, 671–680. [CrossRef] [PubMed]22. Sutharut, J.; Sudarat, J. Total anthocyanin content and antioxidant activity of germinated colored rice. Int. Food Res. J. 2012, 19,
215–221.23. Martynenko, A.; Chen, Y. Degradation kinetics of total anthocyanins and formation of polymeric color in blueberry hydrothermo-
dynamic (HTD) processing. J. Food Eng. 2016, 171, 44–51. [CrossRef]24. Butsat, S.; Siriamornpun, S. Antioxidant capacities and phenolic compounds of the husk, bran and endosperm of Thai rice. Food
Chem. 2010, 119, 606–613. [CrossRef]25. Chang, C.C.; Yang, M.H.; Wen, H.M.; Chern, J.C. Estimation of total flavonoid content in propolis by two complementary
colorimetric methods. J. Food Drug Anal. 2002, 10, 178–182. [CrossRef]26. Loypimai, P.; Moongngarm, A.; Chottanom, P. Effects of ohmic heating on lipase activity, bioactive compounds and antioxidant
activity of rice bran. Aust. J. Basic Appl. Sci. 2009, 3, 3642–3652.27. Candioti, L.V.; de Zan, M.M.; Cámara, M.S.; Goicoechea, H.C. Experimental design and multiple response optimization. Using
the desirability function in analytical methods development. Talanta 2014, 124, 123–138. [CrossRef] [PubMed]28. Onwude, D.I.; Hashim, N.; Janius, R.B.; Mat Nawi, N.; Abdan, K. Modeling the thin-layer drying of fruits and vegetables:
A review. Compr. Rev. Food Sci. Food Saf. 2016, 15, 599–618. [CrossRef]29. Ahn, J.; Sparrow, E.M.; Gorman, J.M. Turbulence intensity effects on heat transfer and fluid-flow for a circular cylinder in
crossflow. Int. J. Heat Mass Transf. 2017, 113, 613–621. [CrossRef]30. Lee, J.M.; Lim, E.W.C. Heat transfer in a pulsating turbulent fluidized bed. Appl. Therm. Eng. 2020, 174, 115321. [CrossRef]31. Stefanova, A.; Bi, H.T.; Lim, C.J.; Grace, J.R. Heat transfer from immersed vertical tube in a fluidized bed of group A particles
near the transition to the turbulent fluidization flow regime. Int. J. Heat Mass Transf. 2008, 51, 2020–2028. [CrossRef]32. Qiu, G.; Wang, D.; Song, X.; Deng, Y.; Zhao, Y. Degradation kinetics and antioxidant capacity of anthocyanins in air impingement
jet dried purple potato slices. Food Res. Int. 2018, 105, 121–128. [CrossRef]33. Li, W.; Pang, X.; Xiao, J.; Wang, X.; He, R.; Zhao, X. Degradation kinetics of pelargonidin-3-(p-coumaroyl)diglucoside-5-
(malonyl)glucoside and pelargonidin-3-(feruloyl)diglucoside-5-(malonyll)glucoside in red radish during air-impingement jetdrying. LWT 2020, 127, 109390. [CrossRef]
34. Zhang, Y.; Zhu, G.; Li, X.; Zhao, Y.; Lei, D.; Ding, G.; Ambrose, K.; Liu, Y. Combined medium- and short-wave infrared and hotair impingement drying of sponge gourd (Luffa cylindrical) slices. J. Food Eng. 2020, 284, 110043. [CrossRef]
35. Babalis, S.J.; Belessiotis, V.G. Influence of the drying conditions on the drying constants and moisture diffusivity during the thinlayer drying of figs. J. Food Eng. 2004, 65, 449–458. [CrossRef]
36. López-Vidaña, E.C.; Figueroa, I.P.; Cortés, F.B.; Rojano, B.A.; Ocaña, A.N. Effect of temperature on antioxidant capacity duringdrying process of mortiño (Vaccinium meridionale Swartz). Int. J. Food Prop. 2017, 20, 294–305. [CrossRef]
37. Mirzaee, E.; Rafiee, S.; Keyhani, A.; Eman-Djomeh, Z. Determining of moisture diffusivity and activation energy in drying ofapricots. Res. Agric. Eng. 2009, 55, 114–120. [CrossRef]
38. Rodríguez, Ó.; Santacatalina, J.V.; Simal, S.; Garcia-Perez, J.V.; Femenia, A.; Rosselló, C. Influence of power ultrasound applicationon drying kinetics of apple and its antioxidant and microstructural properties. J. Food Eng. 2014, 129, 21–29. [CrossRef]
39. Demarchi, S.M.; Quintero Ruiz, N.A.; Concellón, A.; Ginera, S.A. Effect of temperature on hot-air drying rate and on retention ofantioxidant capacity in apple leathers. Food Bioprod. Process. 2010, 91, 310–318. [CrossRef]
40. Miranda, M.; Maureira, H.; Rodríguez, K.; Vega-Gálvez, A. Influence of temperature on the drying kinetics, physicochemicalproperties, and antioxidant capacity of Aloe Vera (Aloe Barbadensis Miller) gel. J. Food Eng. 2009, 91, 297–304. [CrossRef]
41. Katsube, T.; Tsurunaga, Y.; Sugiyama, M.; Furuno, T.; Yamasaki, Y. Effect of air-drying temperature on antioxidant capacity andstability of polyphenolic compounds in mulberry (Morus Alba L.) leaves. Food Chem. 2009, 113, 964–969. [CrossRef]
42. Kuljarachanan, T.; Devahastin, S.; Chiewchan, N. Evolution of antioxidant compounds in lime residues during drying. Food Chem.2009, 113, 944–949. [CrossRef]
43. López-Vidaña, E.C.; Rojano, B.; Pilatowsky, F.I.; Zapata, K.; Cortés, F. Evaluation of the sorption equilibrium and effect ofdrying temperature on the antioxidant capacity of the Jaboticaba (Myrciaria Cauliflora). Chem. Eng. Commun. 2016, 203, 809–821.[CrossRef]
44. Piga, A.; Del Caro, A.; Corda, G. From plums to prunes: Influence of drying parameters on polyphenols and antioxidant activity.J. Agric. Food Chem. 2003, 51, 3675–3681. [CrossRef]
45. Walter, M.; Marchesan, E.; Massoni, P.F.S.; da Silva, L.P.; Sartori, G.M.S.; Ferreira, R.B. Antioxidant properties of rice grains withlight brown, red, and black pericarp colors and the effect of processing. Food Res. Int. 2013, 50, 698–703. [CrossRef]
46. Nowak, D.; Jakubczyk, E. The freeze-drying of foods—The characteristic of the process course and the effect of its parameters onthe physical properties of food materials. Foods 2020, 9, 1488. [CrossRef]
47. Shofian, N.M.; Hamid, A.A.; Osman, A.; Saari, N.; Anwar, F.; Dek, M.S.P.; Hairuddin, M.R. Effect of freeze-drying on theantioxidant compounds and antioxidant activity of selected tropical fruits. Int. J. Mol. Sci. 2011, 12, 4678–4692. [CrossRef][PubMed]
48. Ratseewo, J.; Meeso, N.; Siriamornpun, S. Changes in amino acids and bioactive compounds of pigmented rice as affected byfar-infrared radiation and hot air drying. Food Chem. 2020, 306, 125644. [CrossRef]
Processes 2021, 9, 1738 21 of 21
49. Wanyo, P.; Meeso, N.; Siriamornpun, S. Effects of different treatments on the antioxidant properties and phenolic compounds ofrice bran and rice husk. Food Chem. 2014, 157, 457–463. [CrossRef]
50. Wanyo, P.; Siriamornpun, S.; Meeso, N. Improvement of quality and antioxidant properties of dried mulberry leaves withcombined far-infrared radiation and air convection in Thai tea process. Food Bioprod Process 2011, 89, 22–30. [CrossRef]
51. Chen, Y.; Martynenko, A. Combination of hydrothermodynamic (HTD) processing and different drying methods for naturalblueberry leather. LWT 2018, 87, 470–477. [CrossRef]
