Physical Properties of Canadian Grown Flaxseed in Relation to Its Processing

Abstract

The average flaxseed length, width, thickness and 1000 seed mass were 4.64 mm, 2.37 mm, 1.0 mm and 5.77 g, respectively. The average surface area was 15.49 mm² while the sphericity and aspect ratio were 0.48 and 51.04%, respectively. The average bulk density was 652.16 kg/m³ while the true density was 784.36 kg/m³, and the corresponding porosity was 16.83%. The angle of repose for flaxseed was 24.40 while the terminal velocity was 2.9 m/s. The results showed that the mean value of static coefficient friction was least in case of plastic sheet while it is highest for rubber sheet.

Keywords:

• Physical properties
• Flaxseed
• Moisture content

INTRODUCTION

Flax (Linum usitatissimum) is the Canada's third major oilseed crop after canola and soybean. Flax is an economically important oilseed crop, especially for Canada, which produced about 40% of the world's flaxseed and is the world's largest exporter, representing about 75% of the global flax trade.[1] The world demand for flaxseed is currently dominated by the industrial use of flaxseed oil. However, flaxseed is making great strides in the world's food supply, and demand for human food and livestock markets is expected to increase owing to the unique properties of this ancient crop.[2] Flaxseed is the most prominent oilseed studied to date as a functional food, since it is a leading source of the omega-3 fatty acid α-linolenic acid (ALA) (52% of total fatty acids) and of phenolic compounds known as lignans (>500μg g⁻¹, as is basis).[3]

Flaxseed is rich in fat, protein, and dietary fibre. The compositions of flaxseed averaged 30–40% fat, 20–25% protein, 20–28% total dietary fibre, 4–8% moisture and 3–4% ash, and the oil contains vitamins A, B, D and E, minerals, and amino acids.[4] The composition of flaxseed can vary with genetics, growing environment, seed processing and method of analysis. Generally, the protein content of the seed decrease as the oil content increases.[5,6] Flaxseeds also contain substantial amounts (5–8%) of soluble fibre mucilaginous material and it has a high water holding capacity and shows similar functional properties to those of gum arabic.[7,8]

Flaxseed, immediately after removal from the straw, usually has moisture regain between 11% and 16%. It is usual to dry the seed in a stream of hot air at the time of cleaning, so that the moisture is reduced to between 5 % and 8%. The flaxseed is flat and oval with a pointed tip. It is a little larger than a sesame seed and a smooth-glossy surface. The color of flaxseeds ranges from medium to reddish-brown to a light yellow. The dimensions of the seed vary approximately 3.0–6.4 mm in length, 1.8–3.4 mm in width, and 0.5–1.6 mm in thickness.[9] Generally, oily flaxseeds have greater dimensions than fibre flaxseeds.[10] The seeds have a crisp and chewy texture and a pleasant, nutty taste.[5]

Flaxseed is an important raw material to obtain functional foods due to its special characteristics, and it offers advantages over other available ω–3 sources.[11] Thus, in view of its considerable economic potential in foods and chemical industries, it is important to determine the physical properties of flaxseeds. These properties are often essential to develop equipment for handling, transportation, drying, storage, and other processes, such as oil extraction. In the process of extracting flaxseed oil and its derivatives, the seeds undergo a series of unit operations. Commercial processing of flaxseed and other oilseeds is carried out to extract the oil and to produce a residual meal. The primary processing steps include seed cleaning, flaking, cooking, pressing, solvent extraction, and solvent removal.[12] Thus, this knowledge is essential for the design of harvest and post harvest equipment (e.g., mechanical harvesters, driers, graders, decorticators, oil expeller, storage bins, etc.).[13]

Researchers have made significant efforts in evaluating basic physical properties of agricultural materials and have pointed out their practical utility in machine and process design.[14] Although the recent scientific development have improved the handling and processing of bio-materials through mechanical, thermal, electrical, optical and other techniques, a little is known about the basic physical characteristics of bio-materials. Such basic information is important to not only engineers but also to food scientists, processors, plant breeders, and other researchers who may find new uses.[15] There are many studied focusing on determining physical properties of plant materials such as ackee apple seeds [16] locust bean seed,[17] millet,[18] chillies,[19] hemp seed,[20] groundnut kernel,[21] almond nut and kernel,[22] lentil grains,[23] edible squash seed,[24] sheanut,[25] peanut,[26] white speckled red kidney bean grains,[27] watermelon seed,[28] orange,[29] African nutmeg,[30] jatropha seed,[31] jatropha fruit,[32] mung bean grain,[33] simarouba fruit, and kernel.[34] However, as Canada is the leading exporter of flaxseed, there is no information on physical characteristics in relation to processing, available about, the Canadian grown flaxseed. Therefore, this study was undertaken to determine the physical properties of flaxseed, grown in Canada.

Thus, the knowledge on the morphology and size distribution of flaxseeds is essential for an adequate design of the materials equipment for cleaning, grading, and separation. The size (such as length, breadth, thickness, arithmetic mean diameter, and geometric mean diameter) and shape are important in designing of separating, harvesting, sizing, and grinding machines. The product shape can be determined in terms of its sphericity and aspect ratio, which affect the flow ability characteristics of the products. Moisture content is useful information for the development of the drying process and gravimetric properties like bulk density; true density and 1000-seed mass are use for the design of equipment related to aeration, drying, storage, transport and also affect the structural loads.[34,35] Porosity (calculated from bulk density and true density) of the mass of seeds determines the resistance to airflow during aeration and drying where as surface area affect the resistance to airflow through the bulk material bed and data on them are necessary in designing the drying process. Frictional properties, such as the angle of repose and the coefficient of friction, are important for conveying systems, the design of grain bins, and other storage structures whose operation is influenced by the compressibility and flow behavior of materials.[35] The objective of this study was to determine the various physical properties of flaxseeds, (namely: axial dimensions, average diameters, shape, mass, density, terminal velocity, and frictional properties) pertaining to milling and processing.

MATERIALS AND METHODS

Sample

Flaxseed used in this study was a commercial seed, procured from agricultural firm, Saskatoon, Canada. The sample is cleaned manually to remove all foreign materials such as dust, stones, dirt, immature seed, etc., and pooled together to obtain approximately 10 kg of seed materials. The seeds are kept in an airtight plastic vessel and stored at 5°C before use. Before starting a test, the seeds were allowed to warm up under ambient room conditions (22–25°C, 30–40% RH) to the equilibrium moisture.

Moisture and Oil Contents

The samples were prepared for analysis by grinding about 50 g of seeds to pass through a sieve with circular openings of 1 mm diameter and mixed thoroughly. The moisture content of the sample was determined by oven drying method. Two grams of the finely ground material were dried in a mechanical convection oven (GCA Corporation; USA) at 103°C for 6 h. Weight loss on drying to a final constant weight is recorded as moisture content of the sample. Moisture free sample was used to extract oil in a soxhlet-type extractor and n-hexane (boiling point 40–50°C) was used as solvent.[36] Reported values are means of five determinations.

Physical Characteristics

The seed material is divided into 5 lots each and 20 samples are selected at random from each lot of flaxseed to obtain 100 samples each for conducting the experiment. Hence, measurement of all size and shape are replicated one hundred times. The fruit seed, in terms of the three principal axial dimensions, that is length (L), width (W) and thickness (T) are measured using the digital vernier with an accuracy of 0.01 mm. Since seed size was considered an important parameter in processing bulk samples were classified into three categories namely small, medium and large based on their length.[37]

The basis on which dimensional classification is to be based is set up by calculating the average dimension (X) and the associated standard deviation (σ _x ). Then, small, medium, and large size flaxseeds are so defined that their specific X dimension satisfies the following inequalities[26]:

The arithmetic mean diameter, D_a and geometric mean diameter, D_g of the seeds were calculated by using the following relationships[15]:

(1)

(2)

The sphericity, φ of flaxseeds were calculated by using the following relationship[15]:

(3)

where, L is the length, W is the width and T is the thickness (all in mm). The aspect ratio, R_a is calculated[38] as:

(4)

The 1000 seed mass was estimated by selecting 50 random samples and weighed in an electronic balance (Ohaus Corporation; USA; precision of 0.001g). Finally, the obtained results were multiplied by 20. The reported value is mean of 20 replicates. The surface area of seed sample was calculated from the following relationship[20,31,39,40] considering a sphere of the same geometric mean diameter.

(5)

where, S is the surface area (mm²). The standard bulk density of flaxseed was measured by a 500 ml capacity metallic container (90 mm diameter and 79 mm height) and a metallic cone (225 mm top diameter, 38 mm bottom diameter, and 160 mm height). The cone was placed on top of the container with its bottom 45 mm above the container. The cone had a flat slide gate at the bottom, which was used to allow flaxseed to flow into the container.

Initially, the mass of the empty container was measured. The container was then filled with flaxseeds by opening the slide gate at the bottom of the cone. Excess seed was leveled off by smoothening with a wooden rod and the mass of the filled container was then measured. Bulk densities were calculated as the ratio of the masses of samples to the volume of the container and reported in kg/m³.[41] The true density defined as the ratio between the mass and the true volume of the bulk material, was determined by air displacement with an air comparison multi pycnometer (Quantachrome, Model MVP-2, Made in USA).[42,43] Porosity is the property of the grain which depends on its bulk and true densities. The porosity, ϵ, in % is the parameter indicating the amount of pores in the bulk materials. It is calculated from bulk and a true density using the relationship given as follows[15]:

(6)

where, ϵ is the porosity (%); ρ_b is the bulk density (kg/m³); and ρ_t is the true density (kg/m³). Reported values of all density characteristics are means of 10 replications. The angle of repose was determined by allowing the seeds to fall on a circular plate of diameter 13 cm from a height of 15 cm to form a natural heap. The angle of repose was calculated from the diameter and height of heap and the average value of the angle of repose was recorded.[40]

Terminal velocity was measured by using an air column. For each test, a sample was dropped into the air stream from the top of the air column, up which air was blown to suspend the material in the air stream. The air velocity near the location of the fruit suspension was measured by an electronic anemometer having a least count of ms⁻¹.[44]

The coefficients of friction between the seeds and the materials that are used commonly in the flaxseed post-harvest processing such as stainless steel, galvanized sheet iron, aluminum sheet, plywood, rubber sheet, plastic sheet were determined. The surfaces were attached to a tilting table, one each during the experiment. A wooden frame (150 mm long, 100 mm wide, and 40 mm height), was placed lengthwise on the tested surface. It was filled with the sample and leveled. The table had a manually operated handle, which allowed the surface to tilt until the sample started to slide. An angle meter attached to the tilting table measured the angle at which the grain started to slide down the surface. The static coefficient of friction was calculated as the tangent of the angle measured.[41] The static coefficient of friction was calculated from the following relationship:

(7)

where, μ is the coefficient of friction and α is the angle of tilt in degrees.

RESULTS AND DISCUSSION

The average seed moisture at the time of experiment was 6.85 (±0.14)% dry basis. This moisture content of the seed increases the storage stability, and also helps in easy dehulling process. The yield of oil was found to be 39.05%. These values are within the range as reported in the literature for flaxseeds.[4,45] These seeds presented three unequal semi-axes, and they may, therefore, be described as being flat ellipsoid in shape. Table 1 shows the size distribution of the flaxseed. Longitudinal dimension (L) for the seed ranged from 4.29 to 4.90 mm with the mean value as 4.64 ± 0.14 mm. About 62% of the seeds were of medium size with length ranging from 4.49 to 4.78 mm, while about 20 and 18% were large size (L > 4.78 mm) and small size ( L < 4.49 mm) seeds respectively. The frequency distribution curves of length, width, and thickness of the flaxseeds are shown in Fig. 1.

Table 1 Size distribution of flaxseeds at the moisture content of 6.85% (d.b.)

		Size category
Particulars	Ungraded	Large	Medium	Small
Length of seed, mm	4.29–4.90	>4.78	4.49–4.78	<4.49
Percentage of sample (by number)	100	20	62	18
Average dimensions
Length (L), mm	4.64 ± 0.14	4.83 ± 0.03	4.63 ± 0.08	4.43 ± 0.05
Width (W), mm	2.37 ± 0.10	2.52 ± 0.04	2.36 ± 0.05	2.22 ± 0.05
Thickness (T), mm	1.0 ± 0.09	1.12 ± 0.05	0.99 ± 0.04	0.87 ± 0.04
Arithmetic mean diameter (mm)	2.67 ± 0.08	2.82 ± 0.04	2.66 ± 0.06	2.51 ± 0.04
Geometric mean diameter (mm)	2.21 ± 0.08	2.38 ± 0.05	2.21 ± 0.06	2.04 ± 0.05

Figure 1 Frequency distribution curve of flaxseed dimension at 6.85% moisture content (d.b.).

The average seed width and thickness were 2.37 and 1.0 mm, respectively (Table 1). These measurements were in the same range than those reported by Freeman[9] for flaxseeds. The longitudinal dimensions of flaxseed were observed to lie within the same range to those of fenugreek seeds[39] and green gram,[46] higher than those of sesame seed,[40] Locust bean seeds,[47] tef seed,[48] and lower than those of peanut,[26] gram,[49] chickpea split,[50] groundnuts from Botswana,[51] and soybean.[52] Although Mohsenin[15] had effectively highlight the importance of these and other characteristic axial dimensions for determining aperture size and other parameters in machine design, the comparison of the data with existing work on the other seeds can be sufficient in making symmetrical projections towards process equipment adaptation.

The geometric mean diameter is found to be 2.21 ± 0.08 mm, being this value lower than the length and width, and higher than thickness (Table 1). L/T ratio exhibited the highest value, while the L/D_g and L/W ratio presented close values. The coefficients of correlation (r) showed that all the ratios were found to be highly significant (Table 2). This fact indicates that the length of the seed is positively related to its width, thickness, and geometric mean diameter. The following general expression can be used to describe the relationship among length, width, and thickness of flaxseed:

Table 2 Correlation of seed dimension, with 6.85% (d.b.) average moisture content

Particulars	Ratio	Degree of freedom	Correlation coefficient (r)	p-Value
L/W	1.96	99	0.980	0.0000***
L/T	4.68	99	0.972	0.0000***
L/Dg	2.09	99	0.986	0.0000***
W/T	2.39	99	0.982	0.0000***
W/Dg	1.07	99	0.993	0.0000***
T/Dg	0.45	99	0.996	0.0000***
***Significant at the level 0.1%.

The seed shape is determined in terms of its sphericity and aspect ratio.[32] It is seen from the Table 3 that the sphericity and aspect ratio of the flaxseed is found to be 0.48 and 51.04%, respectively. The experimental values of sphericity and aspect ratio for flaxseed resulted to be lower than that of fenugreek seed,[39] sesame seed,[40] green gram,[46] tef seed,[48] groundnut from Botswana.[51] As per the investigations by Garnayak et al.[31] and Bal and Mishra,[53] it is reported that the grain is spherical when the sphericity value is more than 0.70. In this study, flaxseed should not be treated as an shape equivalent to sphere. Considering the low aspect ratio (which relates the seeds width to length) and sphericity, it may be deduced that flaxseeds would slide on their flat surfaces rather than roll. This tendency to either roll or slide is very important in the design of hoppers and dehulling equipment for the seed because most flat seeds slide easier than spherical seeds, which roll on structural surfaces. Furthermore, the shape indices indicated that the flaxseed might be treated as a flat ellipsoid for an analytical prediction of its drying behaviour. However, the surface area is found to be 15.49 (±1.69) cm². The surface area is a relevant tool in determining the shape of the seeds. This will actually be an indication of the way the seeds will behave on oscillating surfaces during processing.[54]

Table 3 Physical properties of Canadian flaxseed

Physical properties	N	Unit of measurement	Mean	Minimum value	Maximum value	Standard deviation
Moisture content (d.b.)	5	%	6.85	6.72	7.02	0.14
Oil content	5	%	39.05	38.10	39.75	0.63
Sphericity	100	decimal	0.48	0.45	0.51	0.01
Aspect ratio	100	%	51.04	49.42	52.88	0.68
1000 seed mass	20	g	5.77	5.58	5.89	0.09
Surface area	100	mm^2	15.49	11.68	19.75	1.69
Bulk density	10	kg/m^3	652.16	648.12	659.84	14.23
True density	10	kg/m^3	784.36	758.0	798.25	15.70
Porosity	10	%	16.83	14.32	18.81	1.73
Angle of repose	10	degree	24.4	23.5	25	0.65
Terminal velocity	10	m/s	2.90	2.78	2.96	0.10
Co-efficient of friction
stainless steel sheet	10		0.26	0.25	0.28	0.01
galvanized iron sheet	10		0.32	0.30	0.34	0.02
aluminum sheet	10		0.27	0.27	0.28	0.01
plywood	10		0.26	0.25	0.27	0.01
rubber sheet	10		0.33	0.32	0.34	0.02
plastic sheet	10		0.23	0.22	0.24	0.01
N is the number of samples.

The one thousand seed weight values obtained for flaxseeds were 5.77 ± 0.09 g (Table 3). This value is lower than those reported for fenugreek seed,[39] green gram,[46] chik pea split,[50] groundnut from Botswana[51] but higher than those of sesame seed,[40] tef seed.[48] This parameter is useful in determining the equivalent diameter, which can be used in the theoretical estimation of seed volume and in cleaning using aerodynamic forces.

The bulk density of flaxseed was found to be 652.16 kg/m³ (Table 3). The bulk density was thus, higher than that of sesame seed,[40] locust bean seed [47] cumin seed,[55] sunflower seed,[56] and lower than that of green gram,[46] tef seed,[48] groundnuts from Botswana,[51] cowpea.[57] The lower value of bulk density applied to the flaxseed may be attributed to its larger in size.

The true density and porosity of flaxseed is found to be 784.36 ± 15.70 kg/m³ and 16.83 ± 1.73%, respectively. The true density value of flaxseed is lower than that of sesame seed,[40] locust bean seed,[47] groundnuts from Botswana,[51] cumin seed,[55] and higher than that of jatropha seed,[31] sunflower seed.[56] The true density of the flaxseed is found to be lighter than water and, therefore, flaxseed can easily be separated from other heavier foreign materials. It is worth to mention that porosity of the seed mass determines the resistance to airflow during aeration and drying process.

The average value of angle of repose of the flaxseed is 24.4 ± 0.65⁰. This is a result of the flat shape of the seeds, which facilitates sliding. It is important to note that this frictional property for the flaxseed is higher than that of pigeon pea, [60] oil bean seed,[58] locust bean seed,[47] and African star apple[59]; it is in the same range to that of green gram,[46] tef seed,[48] and pearl millet[61] but lower than those of sesame seed,[40] cumin seed,[55] and sunflower seed.[56] The smoother outer surface and the shape of the seeds are apparently responsible for the relatively lower values of repose angle, and thus the easiness of the seeds to slide on each other.

The terminal velocity of the flaxseed is found to be 2.9 ± 0.1 m/s. This value is lower than those reported for karingda seed,[43] green gram,[46] sunflower seed,[56] and pigeon pea.[60] This is because of size differences between the seeds on one hand and probable differences in the gravimetric composition of the seeds on the other. The data on terminal velocity will be useful in the cleaning of the flaxseeds to remove impurities like dry leaves and dust, which are lighter than the seed, as well as in the separation of the seeds from heavier impurities, such as stones and pebbles. Seed cleaning unit operation is an important pre requisite for the grain industry.

The results of the static coefficient of friction, which will directly and indirectly affect the design of the processing machine, was determined on six different contacting materials (stainless steel sheet, galvanized iron sheet, aluminum sheet, plywood, rubber sheet and plastic sheet) have been tabulated in Table 3. It is observed that the static coefficient of friction is highest (0.33) for rubber sheet and lowest (0.23) for plastic sheet among all the other contacting materials. Also, it was reported that the static coefficient friction was reached its maximum value on rubber surface in different studies.[44,57] The least static coefficient of friction may be owing to the smoother and more polished surface of the plastic sheet than the other materials used. It must be noted that the static coefficient of friction is important for designing of storage bins, hoppers, pneumatic conveying system, screw conveyors, forage harvesters, threshers, etc.[13,62]

CONCLUSIONS

The results obtained from the study show that the average characteristic dimensions; length, width, and thickness were 4.64, 2.37, and 1.0 mm for flaxseed, respectively. The frequency distribution curves of the axial dimensions tend a normal distribution. The arithmetic mean diameter and geometric mean diameter were 2.67 and 2.21 mm, respectively. This parameter can be used for the theoretical determination of seed volume and sphericity. The mean value of sphericity for flaxseed was 0.48 while the average aspect ratio was 51.04%. The shape of the grain was confirmed to be flat ellipsoid. The averaged one thousand seed mass and surface area for flaxseeds was 5.77 g and 15.49 mm², respectively. For flaxseed the bulk density and true density was 652.16 and 784.36 kg/m³ respectively while the porosity was 16.83%. Flaxseeds showed a low values of angle of repose of 24.4⁰ and terminal velocity of 2.9 m/s. The static coefficient of friction was lowest (0.23) on plastic sheet and highest (0.33) on rubber sheet among six different contacting surfaces. In summary, this paper deals with the physical properties of flaxseeds, enlarging the knowledge about this species and providing useful data for its post-harvest handling and further industrial processing.

NOMENCLATURE

X	=	Average dimension
Da	=	Arithmetic mean, mm
Dg	=	Geometric mean, mm
L	=	Length of seed, mm
r	=	Coefficient of correlation
Ra	=	Aspect ratio, %
S	=	Surface area, mm2
φ	=	Sphericity of seed, decimal
T	=	Thickness of seed, mm
W	=	Width of seed, mm
α	=	Angle of tilt, ( 0 )
ϵ	=	Porosity, %
μ	=	Static coefficient of friction, decimal
ρb	=	Bulk density, kg/m3
ρt	=	True density, kg/m3
σx	=	Standard deviation

ACKNOWLEDGMENTS

This work was supported by a grant from Graduate Student Exchange Program (GSEP) Fellowship by the Canadian Bureau for International Education (CBIE) on behalf of the Dept. of Foreign Affairs and International Trade Canada (DFAIT) and Dr V. Meda's research grants. The authors are thankful to Mr. R. Welford for providing flaxseed for this study.