The physical dimensions of rice
kernels are of vital interest to those engaged in the rice industry (Amos,
2007). These dimensions are important in marketing and grading, in developing
new rice varieties, in cleaning and grading equipments, in drying operations,
and in processing. These include the seed/ and grain size, shape and weight.
These can be determined by careful measurement of the seed and grain of the
kernels (Rickman et al., 2006; Slaton
et al., 2000). Appearance is also
another critical quality attribute for rice. Rice buyers, millers, and
consumers judge the quality of the rice on the uniformity of its size and shape
as well as the appearance of its overall size-shape relationship (Armstrong et al., 2005). Grouping of varieties is
made on the basis of sizes: long, medium or short (Belsnio, 1980). Rice of
different sizes adversely affects the milling quality and yield; therefore
proper segregation of grain according to sizes is absolutely necessary to
improve the milling quality of rice (Belsnio, 1980, Mahadavappa and Nandisha,
1987).
Grain Quality
Li
(2003) reported that there are more than 1,200 varieties of rice under
cultivation throughout the world. Nguyen (2001) indicated that the differences
in varieties were related to morphology of the plants and grains, resistance to
falling, precocity, ramification, productivity, as well as resistance and
tolerance to biotic and a biotic factor. According to Slaton et al. (2000) rice
is marketed under three market types designed as long-grain, medium-grain, and
short grain. Varieties of each grain type must conform within narrow limits to
the size and shape specifications established for that type. Thus, grain size
and shape are among the first criteria of rice quality that breeders consider
in developing new varieties for release in commercial production (mutters,
1998). If the variety does not conform to recognized standards for grain size,
shape, weight, and uniformity, it is simply not considered for release (Mutters
1998; Anon, 2007).
In early developmental stages, close
visual examination of the grain is made to ascertain that its configuration
conforms to that of other commercially acceptable varieties of the same grain
type. Inherent grain defects, such as irregularly shaped grains, sharp pointed
extremities, over-sized germs and deep creases which cause bran streaks in
milled rice, must also be eliminated in early developmental work, as these
defects reduce the milling yields of rice and detract from the general
appearance of the processed product (Mutters, 1998).
To the farmer, according to Juliano
(1993), grain quality refers to quality of seed for planting and dry grain for
consumption, with minimum moisture, microbial deterioration and spoilage. The
miller or trader looks for low moisture, variety integrity and high total and
head milled rice yield. Market quality is mainly determined by physical
properties and variety name; whereas cooking and eating quality is determined
by physicochemical properties. Martinez et
al. (2005) reported that demand by consumers for rice of better quality can
also influence its production. Different characteristics of grain quality of
rice largely determine the products’ market price and acceptability. If the
consumer does not like the flavour, texture, aroma, appearance or ease of
cooking and processing in a new variety, whatever other outstanding traits it
may possess loses its value. The quality of rice is closely related to the
quality of its milled whole kernel, since all the domestic crop is milled to a
high degree (Anon, 2007).
Hammermeister (2008), suggested that
knowing about grain quality starts with knowing the anatomy of a single grain,
whether the grain is to be used for feed or for human consumption, the key
characteristics of a grain still apply. Research from IRRI (2009) further
indicated that rice grain quality was not solely a varietal characteristic but
also depended on the crop production environment, harvesting, processing and
handling system. Irshad (2001) categorized the quality characteristics in rice
into 3 broad areas:
1. Physical characteristics which include
moisture content, shape, size and milling.
2. The analysis of physico-chemical
characteristics of rice including amylose content, protein content, gel
consistency, volume of expansion of cooked rice, and cooking time and
3. The organoleptic quality of cooked rice which
include colour, aroma, hardness, stickiness, and consistency.
GRAIN DIMENSION
The
length and width of the rice grain are important attributes that determine the
classes of rice. Rice grains may be objectively classified into grain-type
categories based on three physical qualities; length, shape and weight. Length
is a measure of milled rice kernel in its greatest dimension. The shape is
determined by a ratio of two of the three dimensions-length, width, and
thickness. Richman et al. (2006)
classified milled rice grain based on the length-width ratio as slender
(>3.0), medium (>2.1<3.0), bold (>1.1<2.0) and round
(<1.1). Belsnio (1980) was able to
determine the grain class of rice by measuring the length of the whole grain.
He classified the type of milled rice according to the length of the whole
grain as: Extra long-milled rice of which 80% of the whole milled rice kernels
have a length of 7.0mm or more, long-milled rice kernels have a length of 5.0mm
or more but shorter than 6.0mm, and short-milled rice of which 80% of the whole
milled rice kernels are shorter than 5.0mm. Some attempts have been made to sub
divide the types into finer gradations, such as long-slender and
medium-slender. However, these distinctions may not be justified, since there
is considerable overlap between long and long-slender and between medium and
medium slender types (Belsnio, 1980s).
Thousand Grain Weight
The
1,000 grain weight provides information about the size and density of the
grain. Grain of different density mill differently and are likely to retain
moisture differently and cook differently (Richman et al., 2006). Uniform grain weight is important for consistent grain
quality. The weight of rice kernel can vary considerably with moisture content,
the type of soil where the rice is grown, fertilizer treatment, and weather
conditions.
Shape
Shape
is also important in heat and mass transfer calculations, screening solids to
separate foreign materials, grading of fruits and vegetables and evaluating the
quality of food materials. The shape of a food material is usually expressed in
terms of its sphericity and aspect ratio.
Sphericty
is an important parameter used in fluid flow and heat and mass transfer
calculations (Serpil and Servent, 2006). The sphericity and aspect ratio is
used to describe the shape of the seed. The geometry foundation of the concept
of sphericity is said to rest on the isoperimetric property of a sphere. The
sphericity of solid and material could be measured, after the object has been
traced on paper, using the formular below (Figure 2.5).
Sphericity = Di
Dc
Where
Di
= diameter of largest inscribed circle
Dc
= diameter of smallest circumscribed circle.
Assuming
that the volume of the solid is equal to the volume of a triaxial ellipsoid with
intercepts a, b, c and that the diameter of the ellipsoid, the degree of
sphericity can also be expressed as follow:
Sphericity
= (volume
of solid) 1/3
Volume of circumscribed sphere
= (pie/ 6 a b c)1/3 = (
b c )1/3
pie/ 6 a 3 a2
= Geometric mean diameter = (
a b c )1/3
Major diamet a
Where:
a =
longest intercept
b =
longest intercept normal to a and
b
Density
Quality
of food material can be assessed by measuring their densities. Density data of
foods are required in separation processes, such as centrifugation and in
pneumatic and hydraulic transport of powders and particulates. In addition
measuring the density of liquid is required to determine the power required for
pumping.
The
grain density or true density is the density of a pure substance or a composite
material calculated from the densities of its components considering
conservation of mass and volume (Serpil and Servent, 2006). According to Deshphande et al; (1993) the
true density is defined as the ratio of mass of seed to the solid volume
occupied.
Bulk
density is the density of a material when packed or stacked in bulk. Bulk
density of particulate solids is measured by allowing the sample to pour into
container of known dimensions.
Porosity
Porosity
is an important physical property characterizing the texture and the quality of
dry and intermediate moisture foods. Porosity data is required in modeling and
design of various heat and mass transfer processes such as drying, frying,
baking, heating, cooling and extrusion. It is an important parameter predicting
diffusional properties of cellular foods. In addition to that, porosity is used
for studying the storage of agricultural products and to preview the quality
loss of the material until it’s marking time (Ghadge and Prasad, 2012).
According
to Mohsenin (1986), porosity is defined as the ratio of the intergranular void
space volume and the volume of the bulk grain.
Angle of
Repose
Angle
of repose is another important physical property used in particulate foods such
as seeds, grains, and fruits. When granular solids are piled on a flat surface,
the sides of the pile are at a definite reproducible angle with the horizontal.
This angle is called the angle of repose of the material.
The
angle of repose is important for the design of processing, storage and
conveying systems of particulate material. When the grains are smooth and
rounded, the angle of repose is low. For very fine and sticky materials the
angle of repose is high (Serpil and Servent, 2006).
Note
that generally
When
angle of repose < 35o : there is free flow
35 < Angle of repose < 450 : cohesiveness
angle of repose > 550 limited flow
Coefficient of Friction
This
property develops an important role on silo wall pressure and grain flowing
behaviours. The friction coefficient is defined as the ratio of the friction
forces (force due to the resistance of movement) to the normal force on surface
of the material used in the wall. For biological products, according to
Mohsenin (1986), two types of friction coefficient are considered, the static
coefficient determined by the force capable to initiate the movement and the
dynamic coefficient determined by the force
needed to maintain the movement of the grains in contact with the wall
surface which depends on the type and nature of the material in contact.