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Milling behavior and microstructure of rice dried using microwave set at 915 MHz frequency
Accepted Manuscript Milling behavior and microstructure of rice dried using microwave set at 915�MHz frequency Gbenga A. Olatunde, Griffiths G. Atungulu PII:
S0733-5210(17)30743-9
DOI:
10.1016/j.jcs.2018.02.008
Reference:
YJCRS 2534
To appear in:
Journal of Cereal Science
Received Date: 19 September 2017 Revised Date:
22 December 2017
Accepted Date: 20 February 2018
Please cite this article as: Olatunde, G.A., Atungulu, G.G., Milling behavior and microstructure of rice dried using microwave set at 915�MHz frequency, Journal of Cereal Science (2018), doi: 10.1016/ j.jcs.2018.02.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Milling behavior and microstructure of rice dried using microwave set at 915 MHz frequency
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Gbenga A. Olatunde and Griffiths G. Atungulu* Department of Food Science, University of Arkansas Division of Agriculture 2650 N Young Avenue, Fayetteville, AR 72704, USA *Corresponding author: email: [email protected]; Tel. +14795756843
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Abstract
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primarily depend on end-use requirement. Unlike convective heated air (CHA) drying,
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volumetric heating phenomenon which is associated with industrial microwave (MW) drying of
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rice at 915 MHz frequency could induce unique changes in rice microstructure; this is likely to
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impact the rice milling characteristics, especially the DOM. Medium grain rough rice at 24%
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moisture content (wet basis) at bed thicknesses of 0.01 and 0.05 m was dried in a one-pass,
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continuous drying operation for 8 minutes using pilot scale MW set at specific energy of 450,
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600 and 750 kJ/kg. Samples from each treatment were milled for durations of 0, 15, 30, 45 and
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60 s. The result shows that, more than 80% of the SLC were removed by 30 s of milling duration
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for both MW and CHA dried samples. Irregular and hexagonal structure typically alluded to
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starch granules reduced as the specific energy increased. At specific energy of 750 kJ/kg, the
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core of the MW dried rice kernel indicated features related to thermal decomposition. The study
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findings are crucial to determining MW drying condition that maintain the rice milling
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characteristics.
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Keywords: Rough rice, drying, 915 MHz microwave, surface lipid content, degree of milling,
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In industrial milling of rice, the practice is to target certain degree of milling (DOM) which
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Introduction
To obtain optimal head rice yield (HRY) recovery and long term storability of rice dried using
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convective heated air (CHA) methods, the milling operation is set such that about 90% of the
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surface lipid content (SLC) of brown rice is removed after milling operation; this results to about
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0.4% rice SLC (Fujino, 1978).
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instrumentation have been fine-tuned to handle the variability in rice hardness or morphological
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features by milling rice for certain duration within which the desired SLC is achieved. It is
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expected that such milling durations would also be a function of the drying conditions as that
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will likely modify the rice kernel mechanical strength. Using microwaves (MWs) for drying rice,
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might induce a shift in the structure of the rice kernel. Unique to MWs is the associated
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volumetric heating phenomena which may impact the rice kernel hardness or softness resulting
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in significant impact on the milling duration required to achieve a certain SLC.
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Most reported exploratory works utilize McGill No. 2 equipment to investigate optimal rice
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milling duration. For instance, Perdon et al. (2001) investigated impacts of various milling
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durations (15, 30, 45, or 60 s) on rice pasting properties and found that the peak viscosity of rice
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increased with increasing degree of milling. Saleh and Meullenet (2007) also milled the Francis
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and Wells long-grain rice cultivars for 0, 20, 30, 40, or 50 s with the aim to achieve SLC of 0.2,
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0.3, 0.4, 0.5, and 0.6% (as-is basis). The authors underscored the need not to remove the SLC
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completely from milled rice because of decrease in cooked rice firmness as milling duration
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increased. Also, Rodríguez‐Arzuaga et al. (2016) investigated the impact of SLCs (0.64, 0.59,
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0.42 and 0.25%) on the appearance and aroma characteristics of raw rice. The authors reported
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that consumers were unable to detect appearance among samples with SLC between 0.64% to
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0.25%. Siebenmorgen et al. (2006) milled different rice cultivars for 0, 20, 30, 40, 50, 60, and 70
Over the years, rice milling equipment and processing
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s and quantified remaining SLC after the milling. The authors stated that careful determination of
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permissible SLC is the key to prevent reduction in head rice yield (HRY). Also, attainment of the
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desired SLC in milled rice depends on the cultivar, milling duration and the equipment used.
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Most studies that have investigated the impact of milling duration on SLC utilized rough rice that
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were dried using convective heated air drying techniques (CHA). In CHA, heat is applied to the
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surface of the grain resulting in both temperature and moisture differential between the surface
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and the core (Atungulu et al., 2016). The gradient formation determines the rate of moisture
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removal and duration of drying. For grains, such as rice, rapid moisture removal bears negatively
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on the mechanical properties of the rice during subsequent processing steps; hence, the grain is
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carefully dried in multiple passes with tempering. Rahman and Perera (2007) reported that rapid
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grain drying in CHA result in case-hardening at the grain surface and dry sealing of the moisture
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within the kernels resulting in tensile stress at the surface and compressive stress at the core.
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When there is formation of case hardening at the bran layer of the rice kernel (Wilhelm et al.,
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2005), then the milling duration required to achieved the desired SLC may vary.
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Theoretically, MW penetrates the grain and deliver the energy in a way that heating is uniform
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and the temperature gradient between the surface and the core of the grain is minimized
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(Olatunde et al., 2017). The benefit of the volumetric heating method in rice drying is evidenced
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from improved results of rice milling yield, milled rice quality and drying efficiency compared
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with CHA techniques (Atungulu et al., 2016; Le et al., 2014; Olatunde et al., 2017).
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Most previous studies using MW for rice drying have been mainly bench-top as scaling up of the
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MW dryers operated at 2450 MHz remained a challenge (Chandrasekaran et al., 2013; Kumar et
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al., 2014). Recent advancement in MW dryer development such as the one reported by Atungulu
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et al. (2016) and Olatunde et al. (2016) has demonstrated the potential for industrial scale drying
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of rough rice with the use of 915 MHz microwave dryer. The industrial scale MW dryer can
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generate up to 100 kW of power from a single magnetron with high thermal efficiencies to
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significantly reduce drying durations (Chandrasekaran et al., 2013; Kumar et al., 2014; Zhang et
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al., 2006). If this technology is to be recommended to the industry for adoption, there is a need to
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characterize the impact of the 915 MHz microwave dryer process on milled rice characteristics.
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Hence, the objective of this study was to investigate if the DOM of the MW dried rice is
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impacted in a manner that would require providing new recommendations on new milling
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duration to achieve targeted SLCs.
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Materials and Microwave drying
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Methods
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Freshly harvested medium grain rough rice (cultivar CL 721) with initial moisture content
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(MC) ranging between 23% and 25% (wet basis) were procured from a commercial farm in Cash
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Arkansas for this study. Moisture content measurement are reported in wet basis (w.b.). The rice
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lots were immediately cleaned upon delivery (MCi Kicker Dockage Tester, Mid-Continent
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Industries Inc., Newton, KS), and placed in sealed tub, inside a conditioned room of 4°C
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(Koolco, Hialeah, FL, US). Samples were retrieved from the conditioned room at 24 h before
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experiment starts to allow for equilibration.
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2.2
Experimental setup
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2.3
Drying test and procedure
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The description of the 915 MHz industrial MW dryer (AMTek Microwaves, Cedar Rapids,
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IA) used for the experiment is defined in Olatunde et al. (2017). In summary, the equipment
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comprises of a (i) high-powered vacuum tube with ability to self-excite and convert electric
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energy to MW radiation, (ii) heating cavity (this is the location where the drying takes place),
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(iii) system waveguide that comprises of a rectangular pipe through which the electromagnetic
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field propagated lengthwise. The pipe also serves as a means to convey the generated MW from
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the magnetron into the heating cavity, (iv) the belt conveys the material to be dried from the
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hopper through the heating cavity and discharge at the outlet and, (v) the entire system is
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controlled through an interactive touch screen control panel.
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The experiments were conducted such that the MW power applied is proportional to the weight of sample exposed to the MW radiation per unit treatment time. Hence, the power
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applied and the mass of the sample were related using equation 1. The sample dwelled inside the
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heating cavity for a total duration of 480 s; this was achieved by setting the conveyor belt speed
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to 0.00254 ms-1. The three specific energy levels of 450, 600 and 750 kJ kg-1 and bed thicknesses
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of 0.01 and 0.05 m were investigated (Table 1). The choice of the specific energy range was
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based on our previous finding that demonstrated that the optimum HRY and moisture reduction
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occur at 600 kJ kg-1 (Atungulu et al., 2016).
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(1)
where, P: power applied (kJ s-1), M: mass (kg), E: Specific energy (kJ kg-1) and t: duration of heating (s)
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After the MW drying operations, the samples were tempered according to the recommendation
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of Atungulu et al. (2016). The rice tempering operations involves collecting the dried sample as
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they were being discharged from the outlet of the MW dryer into a glass jar of 0.0095 m3 volume
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until full. The glass jar was then sealed with polyethylene wrapper (to prevent change in grain
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humidity during tempering) and set in an incubator (VWR, Radnor, Pa.) for 4 h and at 60oC.
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Immediately after the tempering, the samples were transferred to a conditioned room (5580A,
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Parameter Generation and Control, Black Mountain, NC, U.S.A) of 26oC and 65% relative
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humidity (RH, %) to allow the sample to cool down to ambient condition gradually. After the
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sample temperature stabilized with the ambient condition, the MCs was determined through a
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moisture tester (AM 5200, Perten, Kurva, Sweden). In a situation when the MC was greater than
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12.5% wet basis (w.b), the samples were allowed to remain for longer drying duration in the
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conditioned room. However, when the sample MC attained 12.5%, the sample were placed
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inside a polyethylene zip-lock bag for further processing. The steps are described in the next
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section. The control sample for this study were the samples that were not dried using MW. About
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500 g of the rough rice were measured from the original lot and placed in the conditioned room
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until the MC of 12.5% was achieved.
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2.4
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A 150 g dried rough rice samples were measured from each of the treatment (three replicates)
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as described in Table 1. The samples were dehulled using a laboratory sheller (THU 35B, Satake
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Engineering Co., Tokyo Japan). The brown rice obtained was milled with a McGill #2 (Rapsco,
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Inc., Brookshire, TX) for the desired duration (Table 1). The weight of the milled rice was
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recorded after the milling. The broken grains were sorted out of the head rice using grain sizer
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(Grain Machinery Manufacturing Corp., Miami FL, U.S. A). Head rice were regarded as milled
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kernel that has at least three-quarter size of a whole kernel. The weight of the head rice and the
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broken were also recorded and separately kept for subsequent analysis. The head rice yield
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(HRY) was then calculated as the weight of the head rice to the rough rice (Cooper and
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Siebenmorgen, 2007; Fan et al., 2000).
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2.5
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Surface lipid and protein content determination The SLC of the head rice samples were measured by using a near infrared reflectance (NIR,
DA7200, Perten Instrument, Hagersten, Sweden). A 50 g of the head rice samples were
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measured from each of the treatments into the equipment holding cup. The cup was placed under
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the focus opening of the equipment where the near infrared radiation was beamed on it. The
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algorithm provided by the manufacturer of the equipment converts the sensors data to the SLC
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data through calibration. The earlier developed and reported calibration curve of SLC in equation
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2 shows how the NIR measurement was converted to AACCI Approved Method 30-25.01
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(Matsler and Siebenmorgen, 2005; Saleh et al., 2008).
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SLC = 0.871× SLCNIR − 0.092
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(2)
SLC: surface lipid content (approved method) (%).
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SLCNIR: surface lipid content (NIR method)
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Similarly, the same data was also converted to crude protein based on AACCI Approved Method
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(46-16.01) through the earlier developed and reported calibration curve in equation 3
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Crude Pr otein = 0.747 × CPNIR + 1.893
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CPNIR is crude protein determined using NIR method.
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2.6
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Five rice kernels from each of the MW treated samples were randomly selected. The kernels
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were cut along the top and bottom side of the major axis using a metal blade (cross -section). The
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middle section of the cut kernel was selected and processed for the Environmental Scanning
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Electron Microscope (Philips XL30) (ESEM) microstructural evaluation. The specimens (cut
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kernels) were sputter coated with gold with a JSM 5410LV (JEOL) Scanning Electron
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Microscope at 15 kV. The gold coated specimens were then analyzed by the ESEM. The
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resolution of the equipment is 2.0 nm at 30kV and 10 mbar water vapor with ability to rotate at n
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× 360o, forward tilting angle of -15o to 75o and multiple differential vacuum system.
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Scanning electron microscopy (SEM)
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2.7
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Statistical Analysis The statistical analysis was conducted using SAS statistical software (SAS Institute, Cary,
NC). The variance was determined following the generalized linear model procedure. Test of
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significance was determined when p≤ 0.05. Graphs were plotted using Microsoft Excel®. All
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experiments were replicated and the result presented as mean and standard deviation.
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3
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3.1
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The impact of the milling duration on the MRY of rough rice dried using MW dryer with rice
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conveyed at 0.01 m and 0.05 m bed thicknesses are shown in figures 1 (a) and (b), respectively.
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The MRY of control samples reduced from 72.7% to 67.9% as milling duration increased from 0
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s to 60 s, respectively. However, for MW dried rice, using 0.01 m bed thickness (figure 1a), the
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MRY reduced as milling duration increased from 0 s to 60 s by 15%, 23% and 27% at specific
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energy of 450, 600 and 700 kJ/kg, respectively. Also, for MW dried sample at 0.05 m bed
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thickness (figure 1b), the reductions were 15%, 19% and 29% at specific energy of 450, 600 and
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700 kJ/kg, respectively. Since MRY is on weight basis, as milling progresses into the white rice,
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milled rice yield values are expected to decrease. In addition, MRY could reduce due to kernel
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inability to withstand the milling pressure (structural failure). The differences between the MRY
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obtained at different bed thicknesses were not significant (p<0.05). This demonstrated that the
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industrial application or scale up of the MW operation to handle high through-put required in the
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industry is feasible.
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3.2
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The impact of the duration of milling on the HRY of medium grain rice dried at various MW
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energy with rice conveyed at bed thicknesses of 0.01 m and 0.05 m are shown in figures 2 (a)
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Result and Discussion
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Effect of milling duration on milled rice yield (MRY) of MW dried rough rice
Effect of Duration of milling on head rice (HRY) of MW dried rough rice
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and (b), respectively. The HRY of control samples decreased from 75.2% to 61.7% as milling
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duration increased from 0 to 60 s, respectively. However, for MW dried rice, using 0.01 m bed
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thickness (figure 2a), the HRY reduced as milling duration increased from 0s to 60 s by 30%,
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57% and 70% at specific energy of 450, 600 and 700 kJ/kg, respectively. Also, for MW dried
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sample at 0.05 m bed thickness (figure 2b), the reductions were 34%, 31% and 82% at specific
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energy of 450, 600 and 700 kJ/kg, respectively. In general, the HRY obtained from MW dried
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rough rice were lower than the control sample irrespective of the milling duration but the
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differences between bed thickness were not statistically significant (p<0.05). Similar reduction in
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HRY obtained while using CHA methods were observed by several authors. For example,
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Perdon et al. (2001) also found that HRY for medium-grain (cv. Bengal and Orion) and long-
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grain rice (cv. Cypress and Kaybonnet) decreased from an average of 68% after 15 s of milling
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to 53% after 60 s. Also, Saleh and Meullenet (2007) found that the harvest MC could impact the
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HRY of long grain rice cultivar. The authors obtained wells rice cultivars harvested at 13.7 and
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21.4%, milled from 15 s to 45 s, and found that the HRY reduced from 69% to 64% and 71% to
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67%, respectively. Siebenmorgen et al. (1992) opined that elevated proportion of immature
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kernels in rice harvested at high MC causes reduction in HRY while delayed harvesting for low
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MC rice was linked to fissure formation due to moisture adsorption process in the field.
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Statistical analysis of the impact of milling duration and microwave treatments on the HRY of
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medium grain rice shows that bed thickness had no significant effect on the HRY (p>0.05). This
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is expected because the energy injected into the grain were a function of mass (specific energy,
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kJ/kg) within the drying chamber. However, the effect of other factors (specific energy and
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milling duration) were significant (p<0.05). One clear advantage that MW has is the one pass
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drying process that is achievable whereas in CHA, multiple passes are needed with the
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corresponding increase in energy consumption. Hence, the marginal reduction in HRY between
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the control and the MW sample may pay off by reduction in energy cost.
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Effect of duration of milling on Surface lipid content (SLC) of MW dried rough rice
The effect of the milling duration on the SLC of medium grain rice subjected to different
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microwave treatment with rice conveyed at 0.01 m and 0.05 m bed thickness is shown in figure 3
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(a) and (b), respectively. More than 80% of the SLC was removed within the first 30 s of milling
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for most of the treatments; beyond this milling duration the SLC reduction marginally varies
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with treatments level. It is important to state that most studies using CHA drying methods
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utilizes milling duration of 30 s. This implies that if microwave drying is adopted by the
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industry, there will be no need to change their existing infrastructures to accommodate the MW
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treated sample downstream.
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Furthermore, it could be seen that at 700 kJ/kg samples (figure 3b), after milling for the entire
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duration, the SLC stabilizes at 0.8%. This may be explained as, first, the effect of the bed
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thickness. If the removal of steam within the deep bed as MW drying progresses were slower,
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this may result into heat trapping within sample. Additional heating of the surface of the rice
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kernel from the vapor may result in rice kernel surface baking and the resultant case-hardening.
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Secondly, since elevated energy is applied on the rice kernel, rapid moisture removal resulted in
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overheating and fissuring of the rice. Hence, MW drying rice at powers greater than 750 kJ/ kg
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may not be appropriate. The statistical analyses of the impact of microwave treatments and the
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milling duration on the SLC shows that all the factors applied were significant on achieving the
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SLC of 0.4% (p<0.05).
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3.4
Effect of degree of milling on Protein content of MW dried rough rice
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The impact of the milling on the protein content is presented in figure 4 (a) and (b) for 0.01 and
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0.05 m bed thickness, respectively. The protein content gradually decreased with increase in
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milling duration. For instance, at MW specific energy of 600 kJ/kg, the protein content was
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found to be 8.3% and 5.9% for 0 s and 60 s milling duration, respectively. Rice bran exclusively
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comprises of 14.2 % N×5.95 protein content (Resurrection et al., 1979) hence, increase in
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milling duration or DOM would have depletive effect on the protein content. Also rice kernels
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has about 7% total protein content (Juliano, 1993; Verma and Srivastav, 2017) and 80% of the
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protein content are located in the starchy endosperm and the remaining 20% are exposed to
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removal based on DOM. Reduction in protein content was also reported for jasmine rice by
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Payakapol et al. (2011) who found a reduction from 8.9% to 6.9% as DOM increased from 0%
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and 11%. Similarly, Puri et al. (2015) reported about 20% to 25% reduction in protein content
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with increase in DOM from 0 to 10%. Liu et al. (2017) also reported reduction in protein content
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of 9.6% to 8.2% as DOM increased for 0% to 15.3%. It can be seen in figure 4 (a) and (b) that by
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15 s of milling the protein content of MW dried rice and control were essentially the same. This
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implies that the protein content cannot be used as a measure of milling degree. The statistical
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analyses of the effect of milling duration on protein content shows that the specific energy
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applied had no significant effect on the protein content (p>0.05), but the bed thickness and
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duration effects were significant (p<0.05).
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3.5
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In order to understand how the microwave treatment impacted the rice microstructure, rough rice
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subjected to specific energy of 400kJ/kg, 600 kJ/ kg and 750 kJ/kg at heating duration of 6
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minutes and bed thickness of 0.05 m were utilized for this study. The choice of this level was
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Impact of treatments on microstructure of MW dried rice kernels
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based on our earlier studies where the optimum range for head rice recovery was found to be 600
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kJ/kg at 6 min heating duration and milling duration of 30 s (Atungulu et al., 2016; Olatunde et
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al., 2017). Figures 5 (a & b), (c & d) and (e & f) show the impact of microwave specific
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energies of 450, 600 and 750 kJ/kg on rough rice microstructure after drying for 6 mins,
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respectively. For each specific energy levels, each of the figures shows the general profile of the
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cross-section and the detailed view along the edge or important features. The figures show that
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the irregular and hexagonal shapes typically alluded to starch granules could not be seen and the
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roughness associated with starch surface (Kumar et al., 2016) reduced as the specific energy
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increased. The disappearance of the starch granules could be attributed to the heating process
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caused during microwave drying. The shape of the starch disappeared gradually as Kumar et al.
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(2016) subjected rice to soaking, roasting and flaking process. Mahadevamma and Tharanathan
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(2007) also reported amorphous and semi crystalline starch granule after parboiled rice process.
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Similarly, Li et al. (2014) reported an increase in crystallinity of starch granules of presoaked
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rice subjected to microwave cooking and attributed it to rapid increase in the interior temperature
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of rice. The author stated that as the interior temperature increase, so also the interior pressure
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resulting in starch swelling and shrinking of gaps between granules. Furthermore, when the
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drying temperature exceeded the gelatinization temperature, starch granule loose it structures and
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integrity.
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In figure 5f, the rice kernels subjected to 750 kJ/kg for 6 min heating duration have holes at the
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center. This may imply that the core of the microwave dried rice was over heated resulting in
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either shrinkage of the endosperm or thermal decomposition. This could be the principal reason
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why the rice subjected to 750 kJ/kg has weak mechanical strength to survive the milling process
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without reduction in head rice as evidenced in the HRY and MRY result discussed earlier.
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4
Conclusion
The impact of using MW set at 915 MHz frequency to dry rice kernel and the degree of milling
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operation on rice milling quality and microstructure was investigated. The study found that 1. The HRY of MW dried samples were lower than control samples and reduced as milling
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duration increased from 0s to 60 s irrespective of the specific energy used during MW
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drying.
2. Protein content decreased with an increase in milling duration with no significant
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difference between MW dried rice and control sample
3. More than 80% of the SLC was removed by 30 s of milling durations for both CHA and
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MW dried rice except for samples dried at 750 kJ/kg at 0.05 m grain bed thickness
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4. The study shows that the structure of the MW dried kernel were impacted during drying
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compared to that of CHA; this may impact milling characteristics of rice downstream.
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The authors acknowledge Arkansas Nano & Bio Materials Characterization Facility, Institute for
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Nano Science and Engineering University of Arkansas for providing access to use their Scanning
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Electronics Microscope. The authors thank Dr. Elizabeth M. Martin, for assistance with electron-
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optics instrumentation and AMTek Inc., for supporting this research.
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Acknowledgements
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Atungulu, G., Smith, D., Wilson, S., Zhong, H., Sadaka, S., Rogers, S., 2016. Assessment of one-pass drying of rough rice with an industrial microwave system on milling quality. Applied Engineering in Agriculture 32, 417-429.
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Chandrasekaran, S., Ramanathan, S., Basak, T., 2013. Microwave food processing—A review. Food Research International 52, 243-261.
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Li, J., Han, W., Xu, J., Xiong, S., Zhao, S., 2014. Comparison of morphological changes and in vitro starch digestibility of rice cooked by microwave and conductive heating. Starch Stärke 66, 549-557.
334 335 336
Liu, K.-l., Zheng, J.-b., Chen, F.-s., 2017. Relationships between degree of milling and loss of Vitamin B, minerals, and change in amino acid composition of brown rice. LWT-Food Science and Technology 82, 429-436.
337 338
Mahadevamma, S., Tharanathan, R.N., 2007. Processed rice starch characteristics and morphology. European Food Research and Technology 225, 603-612.
339 340
Matsler, A., Siebenmorgen, T., 2005. Evaluation of operating conditions for surface lipid extraction from rice using a Soxtec system. Cereal chemistry 82, 282-286.
341 342 343
Olatunde, G., Atungulu, G.G., Sadaka, S., 2016. CFD modeling of air flow distribution in rice bin storage system with different grain mass configurations. Biosystems Engineering 151, 286297.
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Olatunde, G.A., Atungulu, G.G., Smith, D., Sadaka, S., Rogers, S., 2017. One-pass drying of rough rice with an industrial 915 MHz microwave dryer: Quality and energy use consideration. Biosystems Engineering 155, 33-43.
347 348
Payakapol, L., Moongngarm, A., Daomukda, N., Noisuwan, A., 2011. Influence of degree of milling on chemical compositions and physicochemical properties of jasmine rice, 2010
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International Conference on Biology, Environment and Chemistry. Singapore (SG). IPCBEE, pp. 84-86.
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Perdon, A., Siebenmorgen, T., Mauromoustakos, A., Griffin, V., Johnson, E., 2001. Degree of milling effects on rice pasting properties. Cereal chemistry 78, 205-209.
353 354 355
Puri, S., Dhillon, B., Sodhi, N.S., 2015. A Study on the effect of degree of milling (DOM) on color and physicochemical properties of different rice cultivars grown in Punjab. International Journal Of Advanced Biotechnology And Research 16, 310-319.
356 357
Rahman, M.S., Perera, C.O., 2007. Drying and food preservation, in: Rahman, M.S. (Ed.), Handbook of food preservation. CRC Press, New York, pp. 404 - 427.
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Resurrection, A.P., Juliano, B.O., Tanaka, Y., 1979. Nutrient content and distribution in milling fractions of rice grain. Journal of the Science of Food and Agriculture 30, 475-481.
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Rodríguez‐Arzuaga, M., Cho, S., Billiris, M.A., Siebenmorgen, T., Seo, H.S., 2016. Impacts of degree of milling on the appearance and aroma characteristics of raw rice. Journal of the Science of Food and Agriculture 96, 3017-3022.
363 364 365
Saleh, M., Meullenet, J., Siebenmorgen, T., 2008. Development and validation of prediction models for rice surface lipid content and color parameters using near-infrared spectroscopy: a basis for predicting rice degree of milling. Cereal chemistry 85, 787-791.
366 367 368
Saleh, M.I., Meullenet, J.-F., 2007. Effect of Moisture Content at Harvest and Degree of Milling (Based on Surface Lipid Content) on the Texture Properties of Cooked Long-Grain Rice. Cereal Chemistry Journal 84, 119-124.
369 370 371
Siebenmorgen, T., Counce, P., Lu, R., Kocher, M., 1992. Correlation of head rice yield with individual kernel moisture content distribution at harvest. Transactions of the ASAE 35, 18791884.
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Siebenmorgen, T.J., Matsler, A.L., Earp, C.F., 2006. Milling Characteristics of Rice Cultivars and Hybrids. Cereal Chemistry Journal 83, 169-172.
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Verma, D.K., Srivastav, P.P., 2017. Proximate Composition, Mineral Content and Fatty Acids Analyses of Aromatic and Non-Aromatic Indian Rice. Rice Science 24, 21-31.
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Wilhelm, L.R., Suter, D.A., Brusewitz, G.H., 2005. Drying and dehydration, Food & Process Engineering Technology. ASABE, St. Joseph Michigan, pp. 259-284.
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Zhang, M., Tang, J., Mujumdar, A., Wang, S., 2006. Trends in microwave-related drying of fruits and vegetables. Trends in Food Science & Technology 17, 524-534.
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Table 1. Experimental layout to investigate the effect of milling duration on surface lipid content of medium grain rough rice Specific energy (kJ/kg)
Bed thickness (m)
Power (kW)
Milling duration (s)
0.01
9
0.05
15 4
600
0.01
12
0.05
20 5
750
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15
15 30
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0
25
45
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90
[a]
80
600 kJ/kg
700 kJ/kg
Control
60 50 40 30
[b]
80 70 Milled rice yeild (%)
Milled rice yeild (%)
70
450 kJ/kg
60 50 40 30
20
20
10
10 0
0 15
30 45 Milling duration (s)
0
60
15
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450 kJ/kg
600 kJ/kg
700 kJ/kg
Control
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.
30 Milling duration (s)
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Figure 1. Effect of duration of milling on the milled rice yield of rough rice subjected to different microwave heating. [a]: bed thickness of 0.01 m. [b]: bed thickness of 0.05 m. Milling at 0 s duration is typically regarded as brown rice since only the hull was removed
Head rice yeild (%)
70 60 50 40 30 20 10 0 15
600 kJ/kg
700 kJ/kg
control
30 Milling duration (s)
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450 kJ/kg
45
Head rice yeild (%)
[a] 80
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[b]
450 kJ/kg
600 kJ/kg
700 kJ/kg
Control
80 70 60 50 40 30 20 10 0
60
0
15
30 Milling duration (s)
45
60
Figure 2. Effect of duration of milling on the head rice yield of rough rice subjected to different microwave heating. [a]: bed thickness of 0.01 m. [b]: bed thickness of 0.05 m. Milling at 0 s duration is typically regarded as brown rice since only the hull was removed
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[a]
450 kJ/kg
1.6
600 kJ/kg
1.4
750 kJ/kg
1.2
Control
1.0 0.8 0.6 0.4
Surface lipid content (%)
Surface lipd content (%)
1.8
0.2 0.0 0
15
30 Milling duration (s)
45
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
[b]
0
60
450 kJ/kg 600 kJ/kg 750 kJ/kg Control
15
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30 Milling duration (s)
45
60
450 kJ/kg 600 kJ/kg 750 kJ/kg Control
[a]
7 6 5
9
2 1 0 15
30 Milling duration (s)
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450 kJ/kg 600 kJ/kg 750 kJ/kg Control
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[b]
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Protein content (%)
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60
Protein content (%)
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Figure 3. Effect of duration of milling on the surface lipid content (SLC) of rough rice subjected to different microwave heating. [a]: bed thickness of 0.01 m. [b]: bed thickness of 0.05 m. Milling at 0 s duration is typically regarded as brown rice since only the hull was removed
6 5 4 3 2 1 0
0
15
30 Milling duration (s)
45
60
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Figure 4. Effect of duration of milling on the protein content of rough rice subjected to different microwave heating. [a]: bed thickness of 0.01 m. [b]: bed thickness of 0.05 m. Milling at 0 s duration is typically regarded as brown rice since only the hull was removed
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Figure 5: Cross-sectioned grains of milled subjected to microwave drying energies of 450 kJ/kg [a & b], 600 kJ/kg [c & d] and 750 kJ/kg [e & f]. For each specific energy level, each of the figure shows the general profile of the cross-section and a detailed view along the edge or important features
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Highlights
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Milling and microstructural properties of microwave dried rough rice was investigated
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More than 80% of the surface lipid content was removed by 30 s of milling durations
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Protein content was not significantly affected by the drying process
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Microwave intensity has an effect on microstructure of the dried rice
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S0733-5210(17)30743-9
DOI:
10.1016/j.jcs.2018.02.008
Reference:
YJCRS 2534
To appear in:
Journal of Cereal Science
Received Date: 19 September 2017 Revised Date:
22 December 2017
Accepted Date: 20 February 2018
Please cite this article as: Olatunde, G.A., Atungulu, G.G., Milling behavior and microstructure of rice dried using microwave set at 915�MHz frequency, Journal of Cereal Science (2018), doi: 10.1016/ j.jcs.2018.02.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Milling behavior and microstructure of rice dried using microwave set at 915 MHz frequency
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Gbenga A. Olatunde and Griffiths G. Atungulu* Department of Food Science, University of Arkansas Division of Agriculture 2650 N Young Avenue, Fayetteville, AR 72704, USA *Corresponding author: email: [email protected]; Tel. +14795756843
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Abstract
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primarily depend on end-use requirement. Unlike convective heated air (CHA) drying,
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volumetric heating phenomenon which is associated with industrial microwave (MW) drying of
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rice at 915 MHz frequency could induce unique changes in rice microstructure; this is likely to
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impact the rice milling characteristics, especially the DOM. Medium grain rough rice at 24%
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moisture content (wet basis) at bed thicknesses of 0.01 and 0.05 m was dried in a one-pass,
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continuous drying operation for 8 minutes using pilot scale MW set at specific energy of 450,
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600 and 750 kJ/kg. Samples from each treatment were milled for durations of 0, 15, 30, 45 and
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60 s. The result shows that, more than 80% of the SLC were removed by 30 s of milling duration
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for both MW and CHA dried samples. Irregular and hexagonal structure typically alluded to
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starch granules reduced as the specific energy increased. At specific energy of 750 kJ/kg, the
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core of the MW dried rice kernel indicated features related to thermal decomposition. The study
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findings are crucial to determining MW drying condition that maintain the rice milling
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characteristics.
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Keywords: Rough rice, drying, 915 MHz microwave, surface lipid content, degree of milling,
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In industrial milling of rice, the practice is to target certain degree of milling (DOM) which
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Introduction
To obtain optimal head rice yield (HRY) recovery and long term storability of rice dried using
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convective heated air (CHA) methods, the milling operation is set such that about 90% of the
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surface lipid content (SLC) of brown rice is removed after milling operation; this results to about
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0.4% rice SLC (Fujino, 1978).
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instrumentation have been fine-tuned to handle the variability in rice hardness or morphological
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features by milling rice for certain duration within which the desired SLC is achieved. It is
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expected that such milling durations would also be a function of the drying conditions as that
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will likely modify the rice kernel mechanical strength. Using microwaves (MWs) for drying rice,
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might induce a shift in the structure of the rice kernel. Unique to MWs is the associated
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volumetric heating phenomena which may impact the rice kernel hardness or softness resulting
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in significant impact on the milling duration required to achieve a certain SLC.
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Most reported exploratory works utilize McGill No. 2 equipment to investigate optimal rice
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milling duration. For instance, Perdon et al. (2001) investigated impacts of various milling
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durations (15, 30, 45, or 60 s) on rice pasting properties and found that the peak viscosity of rice
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increased with increasing degree of milling. Saleh and Meullenet (2007) also milled the Francis
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and Wells long-grain rice cultivars for 0, 20, 30, 40, or 50 s with the aim to achieve SLC of 0.2,
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0.3, 0.4, 0.5, and 0.6% (as-is basis). The authors underscored the need not to remove the SLC
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completely from milled rice because of decrease in cooked rice firmness as milling duration
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increased. Also, Rodríguez‐Arzuaga et al. (2016) investigated the impact of SLCs (0.64, 0.59,
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0.42 and 0.25%) on the appearance and aroma characteristics of raw rice. The authors reported
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that consumers were unable to detect appearance among samples with SLC between 0.64% to
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0.25%. Siebenmorgen et al. (2006) milled different rice cultivars for 0, 20, 30, 40, 50, 60, and 70
Over the years, rice milling equipment and processing
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s and quantified remaining SLC after the milling. The authors stated that careful determination of
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permissible SLC is the key to prevent reduction in head rice yield (HRY). Also, attainment of the
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desired SLC in milled rice depends on the cultivar, milling duration and the equipment used.
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Most studies that have investigated the impact of milling duration on SLC utilized rough rice that
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were dried using convective heated air drying techniques (CHA). In CHA, heat is applied to the
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surface of the grain resulting in both temperature and moisture differential between the surface
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and the core (Atungulu et al., 2016). The gradient formation determines the rate of moisture
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removal and duration of drying. For grains, such as rice, rapid moisture removal bears negatively
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on the mechanical properties of the rice during subsequent processing steps; hence, the grain is
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carefully dried in multiple passes with tempering. Rahman and Perera (2007) reported that rapid
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grain drying in CHA result in case-hardening at the grain surface and dry sealing of the moisture
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within the kernels resulting in tensile stress at the surface and compressive stress at the core.
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When there is formation of case hardening at the bran layer of the rice kernel (Wilhelm et al.,
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2005), then the milling duration required to achieved the desired SLC may vary.
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Theoretically, MW penetrates the grain and deliver the energy in a way that heating is uniform
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and the temperature gradient between the surface and the core of the grain is minimized
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(Olatunde et al., 2017). The benefit of the volumetric heating method in rice drying is evidenced
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from improved results of rice milling yield, milled rice quality and drying efficiency compared
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with CHA techniques (Atungulu et al., 2016; Le et al., 2014; Olatunde et al., 2017).
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Most previous studies using MW for rice drying have been mainly bench-top as scaling up of the
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MW dryers operated at 2450 MHz remained a challenge (Chandrasekaran et al., 2013; Kumar et
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al., 2014). Recent advancement in MW dryer development such as the one reported by Atungulu
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et al. (2016) and Olatunde et al. (2016) has demonstrated the potential for industrial scale drying
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of rough rice with the use of 915 MHz microwave dryer. The industrial scale MW dryer can
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generate up to 100 kW of power from a single magnetron with high thermal efficiencies to
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significantly reduce drying durations (Chandrasekaran et al., 2013; Kumar et al., 2014; Zhang et
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al., 2006). If this technology is to be recommended to the industry for adoption, there is a need to
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characterize the impact of the 915 MHz microwave dryer process on milled rice characteristics.
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Hence, the objective of this study was to investigate if the DOM of the MW dried rice is
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impacted in a manner that would require providing new recommendations on new milling
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duration to achieve targeted SLCs.
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2
Materials and Microwave drying
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Methods
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Freshly harvested medium grain rough rice (cultivar CL 721) with initial moisture content
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(MC) ranging between 23% and 25% (wet basis) were procured from a commercial farm in Cash
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Arkansas for this study. Moisture content measurement are reported in wet basis (w.b.). The rice
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lots were immediately cleaned upon delivery (MCi Kicker Dockage Tester, Mid-Continent
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Industries Inc., Newton, KS), and placed in sealed tub, inside a conditioned room of 4°C
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(Koolco, Hialeah, FL, US). Samples were retrieved from the conditioned room at 24 h before
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experiment starts to allow for equilibration.
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2.2
Experimental setup
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2.3
Drying test and procedure
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The description of the 915 MHz industrial MW dryer (AMTek Microwaves, Cedar Rapids,
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IA) used for the experiment is defined in Olatunde et al. (2017). In summary, the equipment
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comprises of a (i) high-powered vacuum tube with ability to self-excite and convert electric
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energy to MW radiation, (ii) heating cavity (this is the location where the drying takes place),
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(iii) system waveguide that comprises of a rectangular pipe through which the electromagnetic
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field propagated lengthwise. The pipe also serves as a means to convey the generated MW from
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the magnetron into the heating cavity, (iv) the belt conveys the material to be dried from the
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hopper through the heating cavity and discharge at the outlet and, (v) the entire system is
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controlled through an interactive touch screen control panel.
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The experiments were conducted such that the MW power applied is proportional to the weight of sample exposed to the MW radiation per unit treatment time. Hence, the power
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applied and the mass of the sample were related using equation 1. The sample dwelled inside the
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heating cavity for a total duration of 480 s; this was achieved by setting the conveyor belt speed
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to 0.00254 ms-1. The three specific energy levels of 450, 600 and 750 kJ kg-1 and bed thicknesses
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of 0.01 and 0.05 m were investigated (Table 1). The choice of the specific energy range was
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based on our previous finding that demonstrated that the optimum HRY and moisture reduction
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occur at 600 kJ kg-1 (Atungulu et al., 2016).
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where, P: power applied (kJ s-1), M: mass (kg), E: Specific energy (kJ kg-1) and t: duration of heating (s)
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After the MW drying operations, the samples were tempered according to the recommendation
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of Atungulu et al. (2016). The rice tempering operations involves collecting the dried sample as
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they were being discharged from the outlet of the MW dryer into a glass jar of 0.0095 m3 volume
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until full. The glass jar was then sealed with polyethylene wrapper (to prevent change in grain
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humidity during tempering) and set in an incubator (VWR, Radnor, Pa.) for 4 h and at 60oC.
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Immediately after the tempering, the samples were transferred to a conditioned room (5580A,
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Parameter Generation and Control, Black Mountain, NC, U.S.A) of 26oC and 65% relative
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humidity (RH, %) to allow the sample to cool down to ambient condition gradually. After the
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sample temperature stabilized with the ambient condition, the MCs was determined through a
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moisture tester (AM 5200, Perten, Kurva, Sweden). In a situation when the MC was greater than
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12.5% wet basis (w.b), the samples were allowed to remain for longer drying duration in the
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conditioned room. However, when the sample MC attained 12.5%, the sample were placed
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inside a polyethylene zip-lock bag for further processing. The steps are described in the next
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section. The control sample for this study were the samples that were not dried using MW. About
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500 g of the rough rice were measured from the original lot and placed in the conditioned room
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until the MC of 12.5% was achieved.
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A 150 g dried rough rice samples were measured from each of the treatment (three replicates)
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as described in Table 1. The samples were dehulled using a laboratory sheller (THU 35B, Satake
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Engineering Co., Tokyo Japan). The brown rice obtained was milled with a McGill #2 (Rapsco,
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Inc., Brookshire, TX) for the desired duration (Table 1). The weight of the milled rice was
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recorded after the milling. The broken grains were sorted out of the head rice using grain sizer
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(Grain Machinery Manufacturing Corp., Miami FL, U.S. A). Head rice were regarded as milled
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kernel that has at least three-quarter size of a whole kernel. The weight of the head rice and the
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broken were also recorded and separately kept for subsequent analysis. The head rice yield
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(HRY) was then calculated as the weight of the head rice to the rough rice (Cooper and
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Siebenmorgen, 2007; Fan et al., 2000).
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2.5
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Surface lipid and protein content determination The SLC of the head rice samples were measured by using a near infrared reflectance (NIR,
DA7200, Perten Instrument, Hagersten, Sweden). A 50 g of the head rice samples were
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measured from each of the treatments into the equipment holding cup. The cup was placed under
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the focus opening of the equipment where the near infrared radiation was beamed on it. The
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algorithm provided by the manufacturer of the equipment converts the sensors data to the SLC
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data through calibration. The earlier developed and reported calibration curve of SLC in equation
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2 shows how the NIR measurement was converted to AACCI Approved Method 30-25.01
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(Matsler and Siebenmorgen, 2005; Saleh et al., 2008).
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(2)
SLC: surface lipid content (approved method) (%).
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SLCNIR: surface lipid content (NIR method)
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Similarly, the same data was also converted to crude protein based on AACCI Approved Method
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(46-16.01) through the earlier developed and reported calibration curve in equation 3
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Crude Pr otein = 0.747 × CPNIR + 1.893
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CPNIR is crude protein determined using NIR method.
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2.6
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Five rice kernels from each of the MW treated samples were randomly selected. The kernels
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were cut along the top and bottom side of the major axis using a metal blade (cross -section). The
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middle section of the cut kernel was selected and processed for the Environmental Scanning
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Electron Microscope (Philips XL30) (ESEM) microstructural evaluation. The specimens (cut
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kernels) were sputter coated with gold with a JSM 5410LV (JEOL) Scanning Electron
169
Microscope at 15 kV. The gold coated specimens were then analyzed by the ESEM. The
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resolution of the equipment is 2.0 nm at 30kV and 10 mbar water vapor with ability to rotate at n
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× 360o, forward tilting angle of -15o to 75o and multiple differential vacuum system.
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2.7
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Statistical Analysis The statistical analysis was conducted using SAS statistical software (SAS Institute, Cary,
NC). The variance was determined following the generalized linear model procedure. Test of
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significance was determined when p≤ 0.05. Graphs were plotted using Microsoft Excel®. All
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experiments were replicated and the result presented as mean and standard deviation.
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3
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3.1
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The impact of the milling duration on the MRY of rough rice dried using MW dryer with rice
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conveyed at 0.01 m and 0.05 m bed thicknesses are shown in figures 1 (a) and (b), respectively.
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The MRY of control samples reduced from 72.7% to 67.9% as milling duration increased from 0
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s to 60 s, respectively. However, for MW dried rice, using 0.01 m bed thickness (figure 1a), the
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MRY reduced as milling duration increased from 0 s to 60 s by 15%, 23% and 27% at specific
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energy of 450, 600 and 700 kJ/kg, respectively. Also, for MW dried sample at 0.05 m bed
185
thickness (figure 1b), the reductions were 15%, 19% and 29% at specific energy of 450, 600 and
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700 kJ/kg, respectively. Since MRY is on weight basis, as milling progresses into the white rice,
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milled rice yield values are expected to decrease. In addition, MRY could reduce due to kernel
188
inability to withstand the milling pressure (structural failure). The differences between the MRY
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obtained at different bed thicknesses were not significant (p<0.05). This demonstrated that the
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industrial application or scale up of the MW operation to handle high through-put required in the
191
industry is feasible.
192
3.2
193
The impact of the duration of milling on the HRY of medium grain rice dried at various MW
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energy with rice conveyed at bed thicknesses of 0.01 m and 0.05 m are shown in figures 2 (a)
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Effect of Duration of milling on head rice (HRY) of MW dried rough rice
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and (b), respectively. The HRY of control samples decreased from 75.2% to 61.7% as milling
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duration increased from 0 to 60 s, respectively. However, for MW dried rice, using 0.01 m bed
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thickness (figure 2a), the HRY reduced as milling duration increased from 0s to 60 s by 30%,
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57% and 70% at specific energy of 450, 600 and 700 kJ/kg, respectively. Also, for MW dried
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sample at 0.05 m bed thickness (figure 2b), the reductions were 34%, 31% and 82% at specific
200
energy of 450, 600 and 700 kJ/kg, respectively. In general, the HRY obtained from MW dried
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rough rice were lower than the control sample irrespective of the milling duration but the
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differences between bed thickness were not statistically significant (p<0.05). Similar reduction in
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HRY obtained while using CHA methods were observed by several authors. For example,
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Perdon et al. (2001) also found that HRY for medium-grain (cv. Bengal and Orion) and long-
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grain rice (cv. Cypress and Kaybonnet) decreased from an average of 68% after 15 s of milling
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to 53% after 60 s. Also, Saleh and Meullenet (2007) found that the harvest MC could impact the
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HRY of long grain rice cultivar. The authors obtained wells rice cultivars harvested at 13.7 and
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21.4%, milled from 15 s to 45 s, and found that the HRY reduced from 69% to 64% and 71% to
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67%, respectively. Siebenmorgen et al. (1992) opined that elevated proportion of immature
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kernels in rice harvested at high MC causes reduction in HRY while delayed harvesting for low
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MC rice was linked to fissure formation due to moisture adsorption process in the field.
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Statistical analysis of the impact of milling duration and microwave treatments on the HRY of
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medium grain rice shows that bed thickness had no significant effect on the HRY (p>0.05). This
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is expected because the energy injected into the grain were a function of mass (specific energy,
215
kJ/kg) within the drying chamber. However, the effect of other factors (specific energy and
216
milling duration) were significant (p<0.05). One clear advantage that MW has is the one pass
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drying process that is achievable whereas in CHA, multiple passes are needed with the
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corresponding increase in energy consumption. Hence, the marginal reduction in HRY between
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the control and the MW sample may pay off by reduction in energy cost.
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Effect of duration of milling on Surface lipid content (SLC) of MW dried rough rice
The effect of the milling duration on the SLC of medium grain rice subjected to different
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microwave treatment with rice conveyed at 0.01 m and 0.05 m bed thickness is shown in figure 3
225
(a) and (b), respectively. More than 80% of the SLC was removed within the first 30 s of milling
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for most of the treatments; beyond this milling duration the SLC reduction marginally varies
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with treatments level. It is important to state that most studies using CHA drying methods
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utilizes milling duration of 30 s. This implies that if microwave drying is adopted by the
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industry, there will be no need to change their existing infrastructures to accommodate the MW
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treated sample downstream.
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Furthermore, it could be seen that at 700 kJ/kg samples (figure 3b), after milling for the entire
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duration, the SLC stabilizes at 0.8%. This may be explained as, first, the effect of the bed
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thickness. If the removal of steam within the deep bed as MW drying progresses were slower,
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this may result into heat trapping within sample. Additional heating of the surface of the rice
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kernel from the vapor may result in rice kernel surface baking and the resultant case-hardening.
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Secondly, since elevated energy is applied on the rice kernel, rapid moisture removal resulted in
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overheating and fissuring of the rice. Hence, MW drying rice at powers greater than 750 kJ/ kg
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may not be appropriate. The statistical analyses of the impact of microwave treatments and the
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milling duration on the SLC shows that all the factors applied were significant on achieving the
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SLC of 0.4% (p<0.05).
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3.4
Effect of degree of milling on Protein content of MW dried rough rice
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The impact of the milling on the protein content is presented in figure 4 (a) and (b) for 0.01 and
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0.05 m bed thickness, respectively. The protein content gradually decreased with increase in
244
milling duration. For instance, at MW specific energy of 600 kJ/kg, the protein content was
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found to be 8.3% and 5.9% for 0 s and 60 s milling duration, respectively. Rice bran exclusively
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comprises of 14.2 % N×5.95 protein content (Resurrection et al., 1979) hence, increase in
247
milling duration or DOM would have depletive effect on the protein content. Also rice kernels
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has about 7% total protein content (Juliano, 1993; Verma and Srivastav, 2017) and 80% of the
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protein content are located in the starchy endosperm and the remaining 20% are exposed to
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removal based on DOM. Reduction in protein content was also reported for jasmine rice by
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Payakapol et al. (2011) who found a reduction from 8.9% to 6.9% as DOM increased from 0%
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and 11%. Similarly, Puri et al. (2015) reported about 20% to 25% reduction in protein content
253
with increase in DOM from 0 to 10%. Liu et al. (2017) also reported reduction in protein content
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of 9.6% to 8.2% as DOM increased for 0% to 15.3%. It can be seen in figure 4 (a) and (b) that by
255
15 s of milling the protein content of MW dried rice and control were essentially the same. This
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implies that the protein content cannot be used as a measure of milling degree. The statistical
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analyses of the effect of milling duration on protein content shows that the specific energy
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applied had no significant effect on the protein content (p>0.05), but the bed thickness and
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duration effects were significant (p<0.05).
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3.5
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In order to understand how the microwave treatment impacted the rice microstructure, rough rice
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subjected to specific energy of 400kJ/kg, 600 kJ/ kg and 750 kJ/kg at heating duration of 6
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minutes and bed thickness of 0.05 m were utilized for this study. The choice of this level was
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Impact of treatments on microstructure of MW dried rice kernels
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based on our earlier studies where the optimum range for head rice recovery was found to be 600
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kJ/kg at 6 min heating duration and milling duration of 30 s (Atungulu et al., 2016; Olatunde et
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al., 2017). Figures 5 (a & b), (c & d) and (e & f) show the impact of microwave specific
267
energies of 450, 600 and 750 kJ/kg on rough rice microstructure after drying for 6 mins,
268
respectively. For each specific energy levels, each of the figures shows the general profile of the
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cross-section and the detailed view along the edge or important features. The figures show that
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the irregular and hexagonal shapes typically alluded to starch granules could not be seen and the
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roughness associated with starch surface (Kumar et al., 2016) reduced as the specific energy
272
increased. The disappearance of the starch granules could be attributed to the heating process
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caused during microwave drying. The shape of the starch disappeared gradually as Kumar et al.
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(2016) subjected rice to soaking, roasting and flaking process. Mahadevamma and Tharanathan
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(2007) also reported amorphous and semi crystalline starch granule after parboiled rice process.
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Similarly, Li et al. (2014) reported an increase in crystallinity of starch granules of presoaked
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rice subjected to microwave cooking and attributed it to rapid increase in the interior temperature
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of rice. The author stated that as the interior temperature increase, so also the interior pressure
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resulting in starch swelling and shrinking of gaps between granules. Furthermore, when the
280
drying temperature exceeded the gelatinization temperature, starch granule loose it structures and
281
integrity.
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In figure 5f, the rice kernels subjected to 750 kJ/kg for 6 min heating duration have holes at the
283
center. This may imply that the core of the microwave dried rice was over heated resulting in
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either shrinkage of the endosperm or thermal decomposition. This could be the principal reason
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why the rice subjected to 750 kJ/kg has weak mechanical strength to survive the milling process
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without reduction in head rice as evidenced in the HRY and MRY result discussed earlier.
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4
Conclusion
The impact of using MW set at 915 MHz frequency to dry rice kernel and the degree of milling
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operation on rice milling quality and microstructure was investigated. The study found that 1. The HRY of MW dried samples were lower than control samples and reduced as milling
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duration increased from 0s to 60 s irrespective of the specific energy used during MW
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drying.
2. Protein content decreased with an increase in milling duration with no significant
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difference between MW dried rice and control sample
3. More than 80% of the SLC was removed by 30 s of milling durations for both CHA and
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MW dried rice except for samples dried at 750 kJ/kg at 0.05 m grain bed thickness
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4. The study shows that the structure of the MW dried kernel were impacted during drying
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compared to that of CHA; this may impact milling characteristics of rice downstream.
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5
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The authors acknowledge Arkansas Nano & Bio Materials Characterization Facility, Institute for
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Nano Science and Engineering University of Arkansas for providing access to use their Scanning
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Electronics Microscope. The authors thank Dr. Elizabeth M. Martin, for assistance with electron-
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optics instrumentation and AMTek Inc., for supporting this research.
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Atungulu, G., Smith, D., Wilson, S., Zhong, H., Sadaka, S., Rogers, S., 2016. Assessment of one-pass drying of rough rice with an industrial microwave system on milling quality. Applied Engineering in Agriculture 32, 417-429.
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Chandrasekaran, S., Ramanathan, S., Basak, T., 2013. Microwave food processing—A review. Food Research International 52, 243-261.
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References
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Fan, J., Siebenmorgen, T., Yang, W., 2000. A study of head rice yield reduction of long-and medium-grain rice varieties in relation to various harvest and drying conditions. Transactions of the ASAE 43, 1709.
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Fujino, Y., 1978. Rice lipids. Cereal Chem 55, 559-571.
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Juliano, B.O., 1993. Rice in human nutrition. Int. Rice Res. Inst.
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Kumar, D., Prasad, S., Murthy, G.S., 2014. Optimization of microwave-assisted hot air drying conditions of okra using response surface methodology. Journal of food science and technology 51, 221-232.
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Kumar, S., Haq, R.-u., Prasad, K., 2016. Studies on physico-chemical, functional, pasting and morphological characteristics of developed extra thin flaked rice. Journal of the Saudi Society of Agricultural Sciences.
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Le, Q., Songsermpong, S., Rumpagaporn, P., Suwanagul, A., Wallapa, S., 2014. Microwave heating for accelerated aging of paddy and white rice. Australian Journal of Crop Science 8, 1348.
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Li, J., Han, W., Xu, J., Xiong, S., Zhao, S., 2014. Comparison of morphological changes and in vitro starch digestibility of rice cooked by microwave and conductive heating. Starch Stärke 66, 549-557.
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Liu, K.-l., Zheng, J.-b., Chen, F.-s., 2017. Relationships between degree of milling and loss of Vitamin B, minerals, and change in amino acid composition of brown rice. LWT-Food Science and Technology 82, 429-436.
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Mahadevamma, S., Tharanathan, R.N., 2007. Processed rice starch characteristics and morphology. European Food Research and Technology 225, 603-612.
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Matsler, A., Siebenmorgen, T., 2005. Evaluation of operating conditions for surface lipid extraction from rice using a Soxtec system. Cereal chemistry 82, 282-286.
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Olatunde, G., Atungulu, G.G., Sadaka, S., 2016. CFD modeling of air flow distribution in rice bin storage system with different grain mass configurations. Biosystems Engineering 151, 286297.
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Olatunde, G.A., Atungulu, G.G., Smith, D., Sadaka, S., Rogers, S., 2017. One-pass drying of rough rice with an industrial 915 MHz microwave dryer: Quality and energy use consideration. Biosystems Engineering 155, 33-43.
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Payakapol, L., Moongngarm, A., Daomukda, N., Noisuwan, A., 2011. Influence of degree of milling on chemical compositions and physicochemical properties of jasmine rice, 2010
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Puri, S., Dhillon, B., Sodhi, N.S., 2015. A Study on the effect of degree of milling (DOM) on color and physicochemical properties of different rice cultivars grown in Punjab. International Journal Of Advanced Biotechnology And Research 16, 310-319.
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Rahman, M.S., Perera, C.O., 2007. Drying and food preservation, in: Rahman, M.S. (Ed.), Handbook of food preservation. CRC Press, New York, pp. 404 - 427.
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Resurrection, A.P., Juliano, B.O., Tanaka, Y., 1979. Nutrient content and distribution in milling fractions of rice grain. Journal of the Science of Food and Agriculture 30, 475-481.
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Rodríguez‐Arzuaga, M., Cho, S., Billiris, M.A., Siebenmorgen, T., Seo, H.S., 2016. Impacts of degree of milling on the appearance and aroma characteristics of raw rice. Journal of the Science of Food and Agriculture 96, 3017-3022.
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Saleh, M., Meullenet, J., Siebenmorgen, T., 2008. Development and validation of prediction models for rice surface lipid content and color parameters using near-infrared spectroscopy: a basis for predicting rice degree of milling. Cereal chemistry 85, 787-791.
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Saleh, M.I., Meullenet, J.-F., 2007. Effect of Moisture Content at Harvest and Degree of Milling (Based on Surface Lipid Content) on the Texture Properties of Cooked Long-Grain Rice. Cereal Chemistry Journal 84, 119-124.
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Table 1. Experimental layout to investigate the effect of milling duration on surface lipid content of medium grain rough rice Specific energy (kJ/kg)
Bed thickness (m)
Power (kW)
Milling duration (s)
0.01
9
0.05
15 4
600
0.01
12
0.05
20 5
750
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15
15 30
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0
25
45
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60
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90
[a]
80
600 kJ/kg
700 kJ/kg
Control
60 50 40 30
[b]
80 70 Milled rice yeild (%)
Milled rice yeild (%)
70
450 kJ/kg
60 50 40 30
20
20
10
10 0
0 15
30 45 Milling duration (s)
0
60
15
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450 kJ/kg
600 kJ/kg
700 kJ/kg
Control
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30 Milling duration (s)
45
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Figure 1. Effect of duration of milling on the milled rice yield of rough rice subjected to different microwave heating. [a]: bed thickness of 0.01 m. [b]: bed thickness of 0.05 m. Milling at 0 s duration is typically regarded as brown rice since only the hull was removed
Head rice yeild (%)
70 60 50 40 30 20 10 0 15
600 kJ/kg
700 kJ/kg
control
30 Milling duration (s)
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450 kJ/kg
45
Head rice yeild (%)
[a] 80
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90
[b]
450 kJ/kg
600 kJ/kg
700 kJ/kg
Control
80 70 60 50 40 30 20 10 0
60
0
15
30 Milling duration (s)
45
60
Figure 2. Effect of duration of milling on the head rice yield of rough rice subjected to different microwave heating. [a]: bed thickness of 0.01 m. [b]: bed thickness of 0.05 m. Milling at 0 s duration is typically regarded as brown rice since only the hull was removed
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[a]
450 kJ/kg
1.6
600 kJ/kg
1.4
750 kJ/kg
1.2
Control
1.0 0.8 0.6 0.4
Surface lipid content (%)
Surface lipd content (%)
1.8
0.2 0.0 0
15
30 Milling duration (s)
45
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
[b]
0
60
450 kJ/kg 600 kJ/kg 750 kJ/kg Control
15
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2.0
30 Milling duration (s)
45
60
450 kJ/kg 600 kJ/kg 750 kJ/kg Control
[a]
7 6 5
9
2 1 0 15
30 Milling duration (s)
45
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0
450 kJ/kg 600 kJ/kg 750 kJ/kg Control
7
4 3
[b]
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60
Protein content (%)
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Figure 3. Effect of duration of milling on the surface lipid content (SLC) of rough rice subjected to different microwave heating. [a]: bed thickness of 0.01 m. [b]: bed thickness of 0.05 m. Milling at 0 s duration is typically regarded as brown rice since only the hull was removed
6 5 4 3 2 1 0
0
15
30 Milling duration (s)
45
60
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Figure 4. Effect of duration of milling on the protein content of rough rice subjected to different microwave heating. [a]: bed thickness of 0.01 m. [b]: bed thickness of 0.05 m. Milling at 0 s duration is typically regarded as brown rice since only the hull was removed
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Figure 5: Cross-sectioned grains of milled subjected to microwave drying energies of 450 kJ/kg [a & b], 600 kJ/kg [c & d] and 750 kJ/kg [e & f]. For each specific energy level, each of the figure shows the general profile of the cross-section and a detailed view along the edge or important features
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Highlights
2
Milling and microstructural properties of microwave dried rough rice was investigated
3
More than 80% of the surface lipid content was removed by 30 s of milling durations
4
Protein content was not significantly affected by the drying process
5
Microwave intensity has an effect on microstructure of the dried rice
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1