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Effect of moisture content on some physicalproperties of green wheat

Majdi A. Al-Mahasneh a,*, Taha M. Rababah b

a Department of Biosystems Engineering, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordanb Department of Food Science and Human Nutrition, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan

Received 28 December 2005; received in revised form 19 March 2006; accepted 12 April 2006Available online 12 May 2006

Abstract

Determination of physical characteristics of grain and agricultural commodities is important in the design of harvesting, handling,and processing equipment. Physical properties of green wheat (frikeh) kernels were established over moisture content range from 9.3to 41.5% (w.b.) which covers the moisture range from harvesting to storage. Increasing of moisture content was found to increase axialdimensions, mass of 1000 seeds kernel surface area, kernel volume, and static friction coefficient, while decreasing bulk density, true den-sity, and porosity. Kernel length, width, thickness, and effective mean diameter increased from 6.24 to 6.66 mm, 3.65 to 4.22 mm, 3.43 to3.85 mm, and 4.19 to 4.75 mm, respectively. Mass of 1000 seeds was found to increase from 32.57 to 51.95 g. Kernel surface area, vol-ume, and sphericity were found to increase from 47.07 to 61.02 mm2, 28.84 to 41.95 mm3, and 0.685 to 0.721, respectively. Bulk density,true density, and porosity were found to decrease with moisture content increase from 711.3 to 675.4 kg/m 3, 1333.2 to 1241 kg/m3, and46.65 to 45.59, respectively. Static friction coefficients were found to increase as moisture content increases. Static friction coefficient waslargest between green wheat kernels and plywood surface ranging from 0.41 to 0.62, while the largest increase in static friction coefficientwith moisture content was observed for stainless steel and ranged from 0.22 to 0.64. 2006 Elsevier Ltd. All rights reserved.

Keywords: Physical properties; Frikeh; Green wheat; Moisture content

1. Introduction

Green wheat, commonly known as frikeh (also knownas frekeh or freekah) is a product of green (under-mature)durum wheat with a distinctive smoked flavor. Greenwheat is becoming an increasingly popular food in the

Middle Eastern countries and throughout the world. Ithas been reported that 200300 thousand tons are pro-duced in Middle East every year (Williams & El-Haramein,1985). Green wheat (frikeh), is made when culms andspikes are green. The good time for harvesting wheat formaking green wheat is when some milky endosperm exudesfrom a grain bent sharply between thumb and forefinger.The proper stage for making green wheat is critical to qual-

ity characteristics. If wheat is harvested and roasted tooearly, the grain will collapse, while too late harvestingand roasting will not provide the distinctive green colorand flavor of green wheat (Musselman & Mouslem, 2001).

In the traditional methods of green wheat making,wheat sheaves are cut and collected in the morning, then

allowed to dry for 24 h in an open place. After drying,sheaves are placed on clean bare ground or pieces of sheetmetals and burned over using the dried remains of othercrops such as barley straw. Some farmers nowadays usedirect flame throwers which use butane fuel instead. Dur-ing burning, sheaves are turned frequently with a pitch forkto expose the green leaves and glumes. When the glumesare charred and the tips of some of the grains are slightlyblackened, burning is stopped and the spikes are left tocool. Few hours later, the heads are threshed by feedingthem into a threshing machine that is usually brought to

0260-8774/$ - see front matter 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2006.04.045

* Corresponding author.E-mail address: [email protected] (M.A. Al-Mahasneh).

www.elsevier.com/locate/jfoodeng

Journal of Food Engineering 79 (2007) 14671473
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the site. The fresh green wheat is slightly sweet, chewy, witha desirable smoked flavor. Green wheat is then sold freshor dried in the shade to avoid bleaching. Green wheathas a high initial moisture content (4045% w.b.), but dur-ing the drying process, it loses about 40% of its weight.Good quality green wheat is plump, slightly charred, firm

when fresh, green when dried, contains few remains ofthe pleas, lemmas, glumes, and free from stones and debris(Musselman & Mouslem, 2001; Williams & El-Haramein,1985). Nutritional composition of green wheat shows thatit has 77% carbohydrates, 12.7% protein, 16.5% dietaryfiber, small amounts of vitamin A, B1, B2, C, and E. Min-eral composition shows high amounts of potassium, mag-nesium, and calcium (Bird & Mular, 2003).

Traditional methods that are currently in use for greenwheat production have two major disadvantages: firstly,they are slow and cannot be suited for large scale produc-tion, and secondly they cannot be guaranteed to provide a

consistent good quality product. Mechanization of thegreen wheat production may involve three main steps: inthe first step, the cut sheaves are roasted by exposing themto direct fire while being conveyed on a metal conveyorbelt, in the second step, the roasted sheaves are threshedto separate green wheat kernels from other residues, andin the third step, the threshed kernels are dried to safe stor-age moisture content. Therefore, a good knowledge of thephysical properties of the crop can greatly facilitate thedesign of the required machinery for this process.

Principal axial dimensions of seeds are useful in selectingsieve separators, in calculating grinding power during sizereduction. They can also be used to calculate surface areaand volume of kernels which are important during model-ing of grain drying, aeration, heating, and cooling. Severalresearchers reported the use of principle axial dimensionsto measure physical dimensions of seeds and kernels(Amin, Hossain, & Roy, 2004; Aviara, Gwandzang, &Haque, 1999; Calsr, Marakoglu, Ogut, & Ozturk, 2005;Deshpande, Bal, & Ojha, 1993; Oje, 1993; Omobuwajo,Akande, & Sanni, 1999). Simple correlations were used torelate the dimensions of several seeds and kernels to eachother, such as watermelon, pumpkin, karingda, lentils,and guna (Aviara & Haque, 2000; Carman, 1996; Joshi,Das, & Mukherjee, 1993; Makanjuola, 1972; Suthar &

Das, 1996). The effect of moisture content on kernel dimen-

sions of soybeans, rapeseeds, guna seed, and groundnutwere reported by Aviara and Haque (2000), Calsr et al.(2005), Deshpande et al. (1993) and Kaleemullah (1992),respectively. Volume of gram and guna seeds were pre-dicted using the arithmetic mean of the three principal axis,effective diameter, and average geometric diameter of the

seeds (Aviara et al., 1999; Dutta, Nema, & Bhardwaj,1988).

Bulk density, true density, and porosity can be useful insizing grain hoppers and storage facilities; they can affectthe rate of heat and mass transfer of moisture during aer-ation and drying process. Grain beds with low porosity willhave greater resistance to water vapor escape during thedrying process, which may lead to higher power to drivethe aeration fans. Bulk density was measured by usingAssociation of Official Analytical Chemists procedure(AOAC, 1980); while true density, was measured by eitherwater displacement method (Aviara et al., 1999; Ogunjimi,

Aviara, & Aregbesola, 2002; Omobuwajo et al., 1999) orgas displacement method (Joshi et al., 1993; Suthar &Das, 1996). Porosity can be calculated from bulk densityand true density.

Design of grain harvesting and handling system requiresthe determination of friction coefficient between grain andstructural surface in contact with the grain. Such informa-tion is useful in sizing motor requirements for grain trans-portation and handling. Friction coefficients are dividedinto static and dynamic friction coefficient. Static coeffi-cient of friction has been measured by placing seeds on aspecific structural surface and tilting the surface until seedsstart to slide (Aviara et al., 1999; Deshpande et al., 1993;Ogunjimi et al., 2002). Dynamic coefficient of friction forseeds and agricultural materials has been measured bymoving seed bulks against structural surfaces such as gal-vanized steel sheet, wood, and plywood, at a constantvelocity (Lawton, 1980). Flowability of grain and otherpowdery materials is usually measured using angle ofrepose. Angle of repose has been measured using a spe-cially structured box which has a removable front panel(Dutta et al., 1988; Fraser, Verma, & Muir, 1978; Ogunjimiet al., 2002). Terminal velocity of seeds and grains is usefulin design of pneumatic conveying systems, and grain clean-ing facilities (Baryeh, 2002; Omobuwajo, Sanni, & Balami,

2000; Ozarslan, 2002; Sacilik, Ozturk, & Keskin, 2003).

Nomenclature

L length (mm)W width (mm)T thickness (mm)

B diameter of the spherical part of the kernel (mm)De effective mean diameter (mm)U sphericity (%)V kernel volume (mm3)S kernel surface area (mm2)

e porosity (%)qb bulk density (kg/m

3)qt true density (kg/m

3)l

static friction coefficientM1000 mass of 1000 seeds (g)Mc moisture content (%)R2 coefficient of determination

1468 M.A. Al-Mahasneh, T.M. Rababah / Journal of Food Engineering 79 (2007) 14671473


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Green wheat production provides an economic advan-tage for local farmers, since it is commonly sold at a pricethat is three times as high as wheat. In an effort to improvethe competitiveness of this industry, and improve the socio-economic status of the local farmers, the Jordanian govern-ment has encouraged research on developing an effective

mechanization for this growing industry. Therefore, theobjective of this study was to explore some of the physicalproperties of green wheat and their dependence on mois-ture content, which can help out in the design of handling,processing, and packaging machinery for green wheat pro-duction. The following properties were investigated: princi-ple dimensions, effective diameter, mass of 1000 kernels,sphericity, bulk density, true density (particle density),porosity, static coefficient of friction.

2. Material and methods

2.1. Green wheat (frikeh) making

Green wheat (frikeh) was made according to the tradi-tional procedure in late April 2005 at a nearby wheat fieldabout 15 km east of Irbid city, Northern Jordan. Wheatsheaves were cut and collected in the morning, thenallowed to dry for 3 h in an open place. After drying,sheaves were placed on clean pieces of metal sheets andburned by the dried remains of barley straw. Sheaves wereturned frequently with a pitch fork to expose the greenleaves and glumes to direct fire. Burning stopped after15 min when the glumes were charred and the tips of someof the grains were slightly blackened, the spikes were left to

cool. Two hours later, the heads were threshed by feedingthem into a threshing machine that was brought to the site.A bulk amount of green wheat was filled in polyethyleneplastic bags directly after threshing until use.

2.2. Moisture content

Twenty kilograms of green wheat were collected directlyafter preparation and placed in plastic bags. Moisture con-tent was measured according to ASAE standard (ASAE,1983) as follows: triplicates of samples were obtained usinga Boerner grain divider (Kartar Scientific Industries, India).The samples were oven dried at 103 C for 72 h, then trans-ferred into a desiccator with a desiccant and allowed to cooldown. The sample weights were then recorded using a dig-ital balance to 0.005 g (Denver Instruments, USA). Aver-age moisture content was obtained and was found to be41.5% (w.b.). Five kg of green wheat (frikeh) at this mois-ture content were kept in sealed plastic bags in refrigeratorat 2 C for further use. The remaining quantity was dividedinto three 5-kg samples and were left to dry in a convectionoven at 100 C for different times to provide three othermoisture contents, 29.2%, 17.1%, and 9.3%. The four mois-ture contents selected were to cover the whole moisture con-tent range of the green wheat from harvesting to storage.

All measurements (except axial dimensions) were obtained

in 10 replicates at the four moisture contents prepared,while axial dimensions were obtained at fifty replicates ateach moisture content.

2.3. Physical properties

Fifty seeds were randomly selected using Boerner graindivider, at each moisture content and according to the pro-cedure described by Dutta et al. (1988). For each seed, thethree principal dimensions; length, width, and thicknesswere measured using a digital vernier caliper to an accuracyof 0.1 mm (ZZW Precision Tool Supply, China). Effectivediameter (De), sphericity (U), kernel volume (V), and sur-face area (S) were calculated from the three principaldimensions according to the following relationships (Jain& Bal, 1997):

De LWT1=3 1

U

LWT1=3

L

2

VpB

2L

2

62L B

3

SpBL

2

2L B

4

B WT1=2

5

True density of seeds was measured using water displace-ment method. Five hundred milliliters of water were placedin a 1000 ml graduated measuring cylinder then 30 g seedswere immersed in water. The amount of displacement in

water was recorded quickly. The short duration of measure-ment did not allow water to be absorbed by the kernels eas-ily. True density was obtained as the ratio of kernels weightto the volume of displaced water (Aviara et al., 1999; Ogunj-imi et al., 2002; Omobuwajo et al., 1999). Bulk density wasmeasured using AOAC method, in which a 500 ml cylinderwas filled with seeds from a height of 15 cm. The excessseeds were removed by sweeping the surface of the cylinderand the seeds were not compressed. Bulk density was thencalculated as the ratio between the kernels weight and thevolume of the cylinder (Aviara et al., 1999; Ogunjimiet al., 2002; Omobuwajo et al., 1999). Porosity was calcu-lated using the relationship between bulk and true densities

according to Mohsenin (1986) as follows:

e 1 qb

qt

100 6

Static coefficient of friction for seeds on different struc-tural surfaces including: mild steel, stainless steel, alumi-num, and plywood was measured by applying the inclinedplane method which involves using a hollow metal cylinder(50 mm diameter and 50 mm high) open at both ends andfilled with seeds at the specific moisture content. The cylin-der was then placed on an adjustable tilting plate withoutallowing the metal cylinder to touch the inclined surface.

The tilting surface was then raised slowly and gradually

M.A. Al-Mahasneh, T.M. Rababah / Journal of Food Engineering 79 (2007) 14671473 1469


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using a screw mechanism until the cylinder started to slidedown. At this point, the angle of tilt was measured andthe friction coefficient was calculated as the tangent ofthat specific tilt angle (Dutta et al., 1988; Suthar & Das,1996).

3. Results and discussion

3.1. Axial dimensions

The axial dimensions, including length (L), width (W),thickness (T), and effective mean diameter (De) in mm ofgreen wheat (frikeh) kernels over the moisture contentrange from 9.3% to 41.5% (w.b.) are shown in Table 1. Alinear relationship was observed between moisture contentand axial dimensions as follows:

L 0:0075Mc 6:157 R2 0:941

W

0:0101M

c 3:535 R

2

0:955T 0:0078Mc 3:304 R

2 0:919

De 0:0196Mc 3:98 R2 0:916

As moisture content increased from 9.3% to 41.5% (w.b.);kernel length increased from 6.24 to 6.66 mm (6.7% in-crease), kernel width increased from 3.65 to 4.22 mm(16.5% increase), kernel thickness increased from 3.43 to3.85 mm (15.6% increase), and effective mean diameter in-creased from 4.19 to 4.75 mm (13.3% increase). The in-crease in width and thickness was about twice as much asincrease in length. Axial dimensions are important in deter-

mining aperture size in design of grain handling machinery.Similar findings were reported for other grains hazelnuts(Aydin, 2002), rapeseeds (Calsr et al., 2005), and soy-beans (Deshpande et al., 1993).

3.2. Mass of 1000 kernels

Mass of 1000 kernels was found to increase from 32 to52 g as moisture content increased from 9.3% to 41.5% asshown in Fig. 1, as follows:

M1000 0:592Mc 26:648 R2 0:990

The results showed that mass of 1000 seeds were similar toTurkey okra (Calsr, Ozcan, Hacseferogullar, & Yldz,2005), lentils (Makanjuola, 1972), but larger thanmillet (Baryeh, 2002), and karingda seeds (Suthar & Das,1996).

3.3. Kernel volume and surface area

The moisture dependence of kernel volume and surface

area for the moisture content ranging from 9.3% to 41.5%w.b. is shown in Fig. 2. These relationships were best fittedusing a second-order polynomial as follows:

V 0:0055Mc2 0:242Mc 30:76 R

2 0:998

S 0:0049Mc2 0:1768Mc 48:52 R

2 0:995

Kernel volume increased by 44%, and surface area 29.2%with moisture content increase from 9.3% to 41.5%. Thesecond-order polynomial relationship might be due to thewide range of moisture content used (9.341.5%) whichwas selected to simulate the moisture content range forthe green wheat kernels from harvest to storage. Similar

studies, however, with smaller moisture content rangesshowed that such nonlinear relationships may be neededto adequately predict volume and surface area using mois-ture content. Some of those studies were reported for millet(Baryeh, 2002), gram (Dutta et al., 1988), soybean(Deshpande et al., 1993), and popcorn (Karababa, 2006).The ratio between volume and surface area is usually calledthe characteristic length. Characteristic length has impor-tant role in the case of irregularly shaped objects. Some

Table 1Axial dimensions of green wheat (frikeh) kernels with moisture content

Property(mm) 9.3% 17% 29% 41.5%

Length 6.24 0.14a 6.26 0.21 6.53 0.15 6.66 0.09Width 3.65 0.18 3.68 0.17 4.02 0.20 4.22 0.13Thickness 3.43 0.21 3.38 0.18 3.67 0.11 3.85 0.17Effective diameter 4.19 0.15 4.23 0.14 4.67 0.20 4.75 0.11

a Average standard deviation.

20

25

30

35

40

45

50

55

60

0 5 10 15 20 25 30 35 40 45

Moisture content (% w.b.)

M

1000(g)

Fig. 1. Variation of mass of 1000 kernels with moisture content.

20

30

40

50

60

70

0 5 10 15 20 25 30 35 40 45


Volume(mm3),Surfacearea(mm

2)

surface area volume

Fig. 2. Variation of kernel volume and surface area with moisture

content.



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of its applications include determination of projected areaof particles moving in turbulent air stream, which can beuseful in designing grain cleaners, separators, and pneu-matic conveyors. As the ratio between surface area and vol-ume increases, the rate of heat and mass transfer fromkernel increases, which affects several unit operations such

as drying, cooling, and heating.

3.4. Sphericity

Sphericity was found to increase with the increase inmoisture content as shown in Fig. 3, as follows:

U 0:001Mc 0:673 R2 0:986

The sphericity of green wheat kernels increased slightlyfrom 68.5% to 71.8% with moisture content increase from9.3% to 41.5%. Such high sphericity values suggest that ker-nels may be expected to roll rather than slide on their sur-face, which is a property that is quite important in designof grain hoppers. It also indicates that green wheat kernelscan be treated as spheres for an analytical prediction ofthere drying behavior. These results were found to be lowerthan those of popcorn kernels (Karababa, 2006), cowpeas(Olapade, Okafor, Ozumba, & Olatunji, 2002), but higherthan those of sunflower kernels (Gupta & Das, 1997).

3.5. Bulk density, and true density

Green wheat bulk density (qb) and true density (qt) werefound to decrease with the increase in moisture content,Fig. 4, as follows:

qb 1:0574Mc 720:17 R2 0:980

qt 2:746Mc 1359:4 R2 0:979

Bulk density and true density were found to decrease from708.2 to 675.2 kg/m3 and from 1332.5 to 1241.3 kg/m3,respectively, as moisture content increased from 9.3% to41.5%. The decrease in true density with moisture contentincrease was greater than the decrease in bulk density. Thedecrease in bulk and true density was mainly due to the lar-ger increase in kernel volume compared to the increase in

kernels mass. Similar results were reported for rapeseed(Calsr etal., 2005), soybeans (Deshpande et al., 1993), sun-

flower seeds (Gupta & Das, 1997), popcorn kernels (Karab-aba, 2006), and karingda seeds (Suthar & Das, 1996).

3.6. Porosity

Porosity was calculated from bulk density and true den-sity of green wheat kernels. Porosity was found to decreasefrom 46.66% to 45.61% with the moisture content increasefrom 9.3% to 41.5% as shown in Fig. 5. A second-orderpolynomial relationship was obtained as follows:

e 0:0011Mc2 0:021Mc 46:535 R

2 0:999

This relationship showed a slight decrease of porosity withmoisture content increase. Higher porosity provides betteraeration and water vapor diffusion during deep bed drying.Similar trend was reported for hazel nuts (Aydin, 2002),gram (Dutta et al., 1988), sunflower (Gupta & Das,1997), popcorn (Karababa, 2006). However, porosity wasreported to increase with moisture content increase for mil-let (Baryeh, 2002), rapeseed (Calsr et al., 2005), soybeans(Deshpande et al., 1993), and sunflower (Gupta & Das,1997).

0.68

0.685

0.69

0.695

0.7

0.705

0.71

0.715

0.72

0.725

0 5 10 15 20 25 30 35 40 45


Sphericity%

Fig. 3. Variation of kernel sphericity with moisture content.

400

500

600

700

800

900

1000

1100

1200

1300

1400

1500

0 5 10 15 20 25 30 35 40 45


Density(kg/m3)

true density bulk density

Fig. 4. Variation of bulk and true density with moisture content.

45.4

45.6

45.8

46

46.2

46.4

46.6

46.8

0 5 10 15 20 25 30 35 40 45


Porosity()

Fig. 5. Variation of porosity with moisture content.



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3.7. Static friction coefficient

Static friction coefficient of green wheat kernels on ply-wood, mild steel, stainless steel, and aluminum surfaces forthe moisture content range from 9.3% to 41.5% wasobtained by the following equations:

Ply-wood: ls 0:0032Mc 0:414; R2 0:873

Mild steel: ls 0:0032Mc 0:396; R2 0:988

Stainless steel: ls 0:0067Mc 0:174; R2 0:989

Aluminum: ls 0:0052Mc 0:243; R2 0:950

Fig. 6 shows a linear increase in static friction coefficientwith increase in moisture content for the moisture range

from 9.3% to 41.5%. Plywood showed the highest frictioncoefficient followed by mild steel sheet, then aluminum,and finally stainless steel. This might be due to the surfaceroughness, which is largest in the case of plywood andprobably least for stainless steel. The increase in frictioncoefficient with moisture content increase was largest forstainless steel followed by aluminum then both mild steeland plywood as shown by the slope (Fig. 6). It can be no-ticed also that friction coefficients converged at high mois-ture content (41.5%), which indicates that friction existedbetween a relatively thick water layer and it is independentof structural surfaces. Increase in friction coefficient with

moisture content increase may be explained by the in-creased cohesive force of the wet seeds with the structuralsurface, since the surface becomes stickier as moisture con-tent increases. Similar findings were reported for millet(Baryeh, 2002), rapeseed (Calsr et al., 2005), pumpkinseeds (Joshi et al., 1993), and karingda seeds (Suthar &Das, 1996).

4. Conclusions

1. Axial dimensions, mass of 1000 seeds, sphericity, staticfriction coefficient of green wheat kernel increased line-

arly with increase in moisture content.

2. Kernel volume, and surface area were found to followsecond-order increasing polynomial with increase inmoisture content.

3. Bulk density and true density were found to decrease lin-early with moisture content increase.

4. Porosity was found to follow a second-order decreasing

polynomial with moisture content increase.

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0.15

0.25

0.35

0.45

0.55

0.65

0.75

0 5 10 15 20 25 30 35 40 45


Staticfrictioncoefficient

Wood Aluminum Stainless steel mild steel

Fig. 6. Variation of static friction coefficient with moisture content.



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