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Rice Processing and Properties
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4
Rice Processing and Properties
Priya Dangi, Ayushi Gupta and Isha Garg
University of Delhi, New Delhi, India
Nisha Chaudhary
College of Agriculture, Nagaur, Agriculture University, Jodhpur, India
CONTENTS
4.1
4.2
4.3
4.4
Introduction..................................................................................................................................... 70
Rice Properties................................................................................................................................ 71
4.2.1 Physical Properties of Rice................................................................................................ 71
4.2.1.1 Grain Dimensions, Weight and Uniformity....................................................... 71
4.2.1.2 Color................................................................................................................... 71
4.2.1.3 Bulk density, true density and porosity.............................................................. 71
4.2.1.4 Coefficient of Friction and Angle of Repose...................................................... 72
4.2.2 Mechanical Properties........................................................................................................ 73
4.2.3 Thermal Properties............................................................................................................. 73
4.2.3.1 Specific Heat....................................................................................................... 73
4.2.3.2 Thermal Conductivity......................................................................................... 73
4.2.3.3 Thermal Diffusivity............................................................................................ 74
4.2.3.4 Coefficient of Thermal Expansion...................................................................... 74
4.2.4 Biochemical Properties...................................................................................................... 74
Rice Processing............................................................................................................................... 75
4.3.1 Drying................................................................................................................................. 75
4.3.2 Cleaning.............................................................................................................................. 77
4.3.3 De-husking......................................................................................................................... 77
4.3.4 Paddy Separation................................................................................................................ 77
4.3.5 De-branning/Whitening/Polishing.................................................................................... 78
4.3.6 Grading............................................................................................................................... 78
4.3.7 Color Sorting...................................................................................................................... 78
Parboiling........................................................................................................................................ 78
4.4.1 Parboiling Process.............................................................................................................. 78
4.4.1.1 Soaking............................................................................................................... 78
4.4.1.2 Steaming or Heating........................................................................................... 79
4.4.1.3 Drying................................................................................................................. 79
4.4.2 Methods of Parboiling........................................................................................................ 79
4.4.2.1 Noncommercial Methods.................................................................................... 79
4.4.2.2 Commercial Method: CFTRI (Hot-Soaking) Process........................................ 80
4.4.3 Effects of Parboiling........................................................................................................... 80
4.4.3.1 Changes During Soaking.................................................................................... 80
4.4.3.2 Changes During Steaming.................................................................................. 81
69
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4.4.4 Effect of Parboiling on Milling Quality............................................................................. 81
4.4.5 Advantages and Disadvantages of Parboiling.................................................................... 81
4.5 Quality Analysis of Rice................................................................................................................. 82
4.5.1 Cooking Quality................................................................................................................. 82
4.5.1.1 Changes Taking Place During Cooking Process............................................... 82
4.5.1.2 Factors Affecting Cooking Quality.................................................................... 82
4.5.2 Eating Quality.................................................................................................................... 83
4.5.2.1 Grain Quality...................................................................................................... 83
4.5.2.2 Amylose Content (AC)......................................................................................... 84
4.5.2.3 Gel Consistency (GC)......................................................................................... 84
4.5.2.4 Aroma.................................................................................................................. 84
4.6 Conclusion....................................................................................................................................... 84
References������������������������������������������������������������������������������������������������������������������������������������������������ 85
4.1 Introduction
Rice (Oryza sativa L.) is considered as a semiaquatic annual grass chiefly grown as a kharif season crop.
The majority of the rice cultivated in the world belongs to two main species: Oryza sativa (Asian rice)
and Oryza glaberrima (African rice). The former species is produced and utilized on a larger scale in
comparison to the Oryza glaberrima species, which is restricted to the region of West Africa. The widely
cultivated Asian rice includes three subspecies: indica, javonica, and japonica, which vary in kernel
size and dimensions.
According to FAO (2020), the global rice production reached 501.3 million tons in the marketing
year 2019–20 with India, Pakistan, Thailand, Vietnam, and the United States as leading exporters.
India contributed roughly 112 million tons of rice to the global production in this marketing year
(Singh, 2019). Rice production is further expected to rise to 550 million tons worldwide by the year
2035 (You, You, Yue, Mun, & Lee, 2017). Around two-third of the world’s population eat rice as their
staple food. The consumption of rice in some Asian countries (Bangladesh, Burma, Cambodia, China,
India, Indonesia, Korea, Laos, the Philippines, Sri Lanka, Thailand, and Vietnam) is relatively high,
contributing about 75% of their daily calorie intake (FAO, 2001). Rice is predominantly consumed in
the form of fully milled white rice, which comprises regularly milled as well as parboiled rice and
only a small fraction is consumed as brown rice. It is also used in the preparation of processed foods,
such as noodles, puffed rice, fermented sweet rice, snack foods, and beverages (such as beer, wine,
sake, and vinegar).
The process of converting paddy into well-milled, edible, silky-white rice, involves a series of
steps such as parboiling, drying, and milling that must be carried out with utmost care in order
to produce high-quality rice. The market value of rice depends largely on its physical (moisture
content, grain dimensions, weight, density, and color), mechanical (grain strength and elasticity),
thermal properties (specific heat, thermal conductivity, thermal diffusivity, coefficient of expansion), and biochemical properties. Understanding the physical characteristics of a rice kernel is of
paramount importance as it influences the design and dimensions of the operating equipment and
helps in optimizing the storage and processing conditions. In cases, where the machineries are
improperly designed and operations are executed inappropriately, cracked and broken rice kernels
are produced, which fetches a low marketing price. Substantially, the percentage of whole grain rice
obtained is one critical parameter, since long whole grains command higher market prices than the
broken ones. The cooking and eating quality of rice is essential for establishing the economic value
of rice. The preference of rice varies greatly among people; some people prefer long and flaky rice,
others prefer short and sticky rice. The texture, aroma, flavor and a number of other parameters
play a crucial role in visualizing consumer acceptability and deciding the cost of the rice (Ghadge
& Prasad, 2012).
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4.2 Rice Properties
4.2.1 Physical Properties of Rice
The physical properties are the properties of the grain either individually or in mass that are physical in
nature but are not specifically included among other categories such as cooking, chemical, and physicochemical properties. Unlike other cereals, rice is consumed as whole grain, therefore, knowledge of the
physical properties, such as grain dimensions, hardness, grain friction, size, bulk density, true density,
angle of internal friction and static coefficient of friction, shape, uniformity and general appearance is
essential in handling, storage, and processing of rice. Moisture content, variety of rice and degree of
milling often leads to difference in the physical properties (Bhattacharya, 2011).
4.2.1.1 Grain Dimensions, Weight and Uniformity
Since rice is produced and marketed according to its size and shape, the physical dimensions, weight and
uniformity are of prime importance. The shape of rice is observed to be cylindrical with three dimensions: length (L), width (W) and thickness (T). USDA (2020) recognized seven classes of milled rice
based on length/width ratios and termed them as long-grain, medium-grain, short-grain, mixed, second
head, screenings, and brewers. Rice was classified on the basis of grain length as extra-long (>7.5 mm),
long (6.61–7.5 mm), medium (5.51–6.60 mm), or short (<5.50 mm) and on the basis of grain shape
depending upon the length-width ratio as slender (>3), medium (2–3), or bold (<1) (B.O Juliano, 1993).
Kernel dimensions are primary quality factors in many areas of processing, drying, handling equipment,
breeding, marketing, and grading.
The shape of rice can be expressed in the terms of sphericity (Φ). Reddy and Chakraverty (2004)
defined sphericity as the ratio of the surface area of a sphere (having the same volume as that of rice) to
the surface area of rice. Ghadge and Prasad (2012) computed sphericity (Sp) as
Sp =
( LWT )1/3
L
× 100
Where
L is length, W is width, and T is thickness.
The aspect ratio (Ra) is used for the classification of the shape of rice and is calculated as:
Ra =
W
× 100
L
Where
L is length and W is width.
4.2.1.2 Color
Color is used as one of the important criteria of quality in all varieties of rice. The assessment, however, is
performed on well-milled head rice. Color is influenced by such things as smut, which imparts a grey or red
color to rice seed, giving the milled rice a rosy color (Luh, 1991). Rice varieties are classed as either light
(straw) or dark (gold) hulled. Although hull color is not of major importance in the production of regular
white milled rice, it is influential in the production of parboiled rice. Varieties with light colored hulls are
generally preferred by parboilers as they tend to produce a lighter colored product than dark hulled varieties
processed under similar parboiling conditions (Bett-Garber, Champagne, Thomson, & Lea, 2012).
4.2.1.3 Bulk density, true density and porosity
Knowledge of grain’s bulk density, true density and porosity is useful in sizing grain hoppers and making
correct facilities for storage. These properties of the grain can affect the rate of heat and mass transfer of
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moisture during the aeration and drying processes (Jouki, Emam-Djomeh, & Khazaei, 2012). Varieties
differ in their fraction of high density grain due to differences, both in the degree of filling of the hull by
caryopsis and in actual caryopsis density.
Reddy and Chakraverty (2004) defined bulk density as the ratio of mass of the paddy to its total (bulk)
volume. It is determined by filling a circular container of known volume with paddy and calculated by
the given formula:
ρb =
M
V
Where
ρb [kgm−3] is bulk density
M [kg] is mass of the paddy sample and
V [m3] is volume of the container.
The true density (tρ) is the ratio of mass of the paddy to its true volume. It is determined using the
Toluene displacement method.
The porosity (ε) of the paddy is the ratio of the volume of internal pores in between the paddy to its
bulk volume. It is determined using following relationship:
ε=
(1 − ρb ) × 100
ρb
Where
ε [%] is porosity
ρb [kgm−3] is bulk density and
ρ t [kgm−3] is true density.
A grain bed with low porosity will have greater resistance to water vapor escape during the drying process, which may lead to the need for higher power to drive the aeration fans.
A study of rice properties done at CFTRI summarized that density remains constant at 1.452 g/ml in
all rice varieties. Bulk density varies appreciably in rice (0.777–0.847) and angle of repose is relatively
constant in different varieties of rice (average 37.5 degrees). With increasing moisture content, density
decreases linearly. However, bulk density decreases twice as fast and the porosity increases, owing to a
concurrent progressive increase in the frictional property, which decreases the degree of grain packing.
Density of rice increases slightly with milling, but bulk density, porosity, and angle of repose is markedly
affected by the degree of milling.
4.2.1.4 Coefficient of Friction and Angle of Repose
Ghadge and Prasad (2012) defined the angle of repose (φ) as the angle in degrees with at which the material will stand forming a heap, which can be determined using relationship:
ϕ=
tan −1 ( 2 H )
D
Where
H is height of material and
D is distance between material and horizontal.
The friction coefficient is defined as the ratio between the friction force (force due to the resistance of
movement) and the normal force on the surface of the material used in the wall. For biological products,
two types of friction coefficients 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
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Rice Processing and Properties
material in contact. The static coefficient of friction is used to determine the angle at which chutes must
be positioned to achieve consistent flow of materials through the chute. Such information is useful in sizing motor requirements for grain transportation and handling (Jouki, Emam-Djomeh, & Khazaei, 2012).
4.2.2 Mechanical Properties
Mechanical properties of grains such as rice have a significant impact on the energy demand of grinding
mills. Some rice varieties observe severe breakage losses during processing, making it quite important
to determine their engineering properties in order to optimize the design of machinery used for milling these varieties (Kruszelnicka, Marczuk, Kasner, Bałdowska-Witos, Piotrowska, Flizikowski, et al.,
2020). Considering the process of grinding, e.g., by means of grinding machines or roller mills, permanent deformation (fragmentation) occurs after exceeding the load value corresponding to the compressive strength limit. Strength is closely related to the power necessary to cause the strain and the grinded
material cross-section field (hence being dependent on its geometric features). Thus, material fragmentation occurs upon application of appropriate forces, which in the system of grinding machines and roller
mills, is performed by the rotary motion of rollers (Kunze & Calderwood, 2004). In such a case, the
force is a direct effect of torque, which in turn is related to the power of the devices affecting the energy
demand for grain processing. In general, the higher force applied corresponds to higher power, that is,
the machine energy is needed.
Grain strength depends on the type of material, especially its internal structure (porosity), moisture,
components of the grain, and biological properties. The internal structure of thegrain endosperm and
tegmen have an impact on the strength properties and the energy needed for grinding. The endosperm,
which is characterized by higher glassiness, is usually harder, thus, for permanent deformation, it is
necessary to use higher forces, which result in increased energy demands as compared to materials
whose endosperm is less glassy. The glassiness of the endosperm also has an influence on the material
fragmentation efficiency and the size of particles after division; the higher the glassiness, the easier it is
to separate the endosperm from bran, breaking the grain into smaller parts (Kruszelnicka, et al., 2020).
4.2.3 Thermal Properties
Knowing the thermal properties of rice is important for rice processing or storage. The information
about these properties plays an essential role in order to yield high-quality rice.
4.2.3.1 Specific Heat
Specific heat refers to the heat required by a substance of unit mass to raise its temperature by 1°C. The
specific heat, Cp, is given by the formula:
Cp =
Q
m ∆T
Where
Q is amount of heat,
m is mass of substance, and
ΔT is the change in temperature.
The specific heat of rice in the temperature range of 10-28°C, as reported by Kunze and Calderwood
(2004) is 1.84 J/g°C. Generally, an increase in the moisture content of cereal grains results in increase in
its specific heat (Chakraverty, Mujumdar, & Ramaswamy, 2003).
4.2.3.2 Thermal Conductivity
Thermal conductivity refers to the rate at which heat gets transferred through a unit cross-sectional area
of material by conduction when the temperature gradient is perpendicular to area.
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Handbook of Cereals, Pulses, Roots
The steady-state method is based on Fourier’s law of heat conduction:
q = − kA
dT
dt
Where
q is the rate of heat transfer,
k is thermal conductivity,
A is area of cross-sectional surface,
∆T is temperature difference, and
∆x is the distance between the two ends.
Thermal conductivity increases with increase in temperature, moisture content, and bulk density of rice.
Yang, Siebenmorgen, Thielen, and Cnossen (2003) reported that an increase in the moisture content of
a sample at a given temperature increases its thermal conductivity. As an example, the average thermal conductivity of Bengal rice at 61°C and 9.2% moisture content was reported to be 0.112 W/(mK),
which rose to 0.126 W/(mK) when the moisture content was increased to 17% at the same temperature.
Furthermore, the average thermal conductivity at 12.1% moisture content and 24°C (at glassy state) was
reported to be 0.102 W/(mK), which rose to 0.111 W/(mK) when the temperature was increased to 61°C
at the same moisture content (rubbery state).
4.2.3.3 Thermal Diffusivity
Thermal diffusivity refers to how quickly the heat can diffuse through a substance under transient conduction of heat transfer. It can be computed by the formula:
α=
k
(ρ C p )
Where
α is thermal diffusivity,
k is thermal conductivity,
ρ is density, and
Cp is specific heat.
4.2.3.4 Coefficient of Thermal Expansion
The rice grains expand and shrink during heating and cooling, respectively, although the changes are
minute. The coefficient of thermal expansion was reported to be 4.62 × 10 −4 for rubbery state, which was
much more than 0.87 × 10 −4 for glassy state. This is of importance for rubbery-glassy state transition,
which has an effect on rice fissuring (Perdon, Siebenmorgen, & Mauromoustakos, 2000).
4.2.4 Biochemical Properties
The composition of grain makes it a palatable food of high energy value, which leads nutritionists to
have a major interest in the composition of the kernel. Moisture content is one of the critical parameters
that affect shelf life of rice. If maintained and stored properly, dry rice can be maintained for years
while only a few days are required for wet rice to spoil. Rough rice moisture content of 13% is commonly accepted as a safe level for storage for less than 6 months, while 12% or less moisture is recommended for long-term storage.
The majority of fat is centralized in the bran layer of rice, which is removed during the milling process
(Punia et al., 2021a, b). Lipids are also known to influence viscoelastic properties by forming inclusion
complexes with the helical structure of amylose. It was observed that defatting rice starch reduced both
gelatinization temperature and gel viscosity of starch (B. O Juliano & Tuaño, 2018)(
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Rice Processing and Properties
75
The protein content of milled rice is low in comparison with other cereals, although the whole rice
grain content ranged from 7.0% to 10.8% of which 70–80% is the glutelin. Protein, as the other major
constituent of rice, has not been thought to strongly influence cooking and eating qualities. When differences in gross protein content were examined in relation to texture of cooked rice, only a weak relationship was found; the higher protein rice was somewhat less tender than low-protein rice because
commonly eaten rice generally contain about 7% protein and does not fluctuate widely from this level.
Rough rice has higher fiber and ash content, but lower protein and available carbohydrates than brown
rice. The important B-vitamins in rice are thiamine, riboflavin, and niacin. Modern milling removes
most of these vitamins because they are found largely in the bran and germ. The mineral composition
of the rice grain depends considerably on the availability of soil nutrients during crop growth and on the
diverse sampling, preparation, and analytical methods used by various investigators. Minerals are generally present in higher levels in brown than in milled rice. A considerable portion of the rice caryopsis
ash is accounted for by phosphorus. Potassium, magnesium and silicon are also present in large amounts
in brown and milled rice. By contrast, silica is the major element in hull ash (Bett-Garber, Champagne,
Thomson, & Lea, 2012).
4.3 Rice Processing
The harvested paddy contains undesirable, inedible portions that need to be separated to obtain edible
white rice. Husk or hull, which comprises approximately one-fifth weight of the paddy, is a woody, siliceous, inedible covering that, when removed, results in brown rice. This brown rice is further enclosed
by a fibrous and fatty bran layer that poses difficulty in cooking, causing a demand to remove this layer
by the process of abrasion. The paddy undergoes a series of steps in the milling process that affects the
quality of the edible rice produced in terms of its cooking, nutritional, and eating quality. Milling is
termed as a process that removes foreign material, husk, bran, germ, and broken kernels to yield highquality white rice grains. The milling yield of rice is affected by the variety of rice, degree of milling
(quantity of bran that is removed in the rice), and grain breakage (Bhattacharya, 2011).
Bhattacharya and Ali (2015) described the unit operations and various types of equipment involved in
rice milling in Table 4.1.
4.3.1 Drying
When rice is harvested, it contains a high level of moisture that is not suitable for storage purposes.
Drying is one essential process that must be performed as soon as rice is harvested to prevent its spoilage during storage. The ideal moisture content for storage is 12% for long-term and 14% for short-term.
Rice grains are hygroscopic in nature and exchange water with air depending upon the vapor pressure
of both the grain and air (Bhattacharya & Ali, 2015). During drying, a moisture gradient develops in the
rice grain where the center has comparatively higher moisture content than the surface. The drying rate
at the surface of the grain is initially rapid, but later slows because of the rate of moisture travelling from
center to surface. If the surrounding air is heated, the vapor pressure between grain and air increases,
which in turn increases the rate of internal moisture content. One should be very careful while drying
rice grains to achieve a minimum of broken kernels and a high-quality white rice after milling (Kunze
& Calderwood, 2004).
Drying of rice kernels can be done either by natural or an in-storage drying process. In natural drying,
the just-harvested wet paddy is left on the field for several days to expose it to natural air and sun, drying
it to a level of 15–18%. This method is rather inexpensive, but cracking of grain and breakage during
milling are the major drawbacks. Grains should be dried in thin layers and tossed regularly to prevent
uneven drying. The grain should not be over-dried, i.e., it should not be dried to less than 13–14%.
In-storage drying makes use of forced natural air or slightly heated air to dry the grains in a storage bin,
which consists of a ducting system from where forced air is passed by a fan. If the weather is humid
and the relative humidity is higher than 70%, the air should be slightly heated to reduce its relative
humidity for effective drying. This heat is called supplemental heat. The heat provided should raise the
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TABLE 4.1
Equipment and Their Principle Use in Milling of Rice
Milling Stage
Equipment
Cleaning
Scalper
De-husking
Husk
separation
Paddy
separation
Principle of Equipment
The paddy is placed in a rotating drum made of perforated metal sieve at an elevated
height. The paddy falls through the perforated openings and the larger particles, like
straw, remain on the sieve and are discharged at the end.
Paddy cleaner The paddy is fed into a column where it is aspirated as it goes through a series of
decks covered with perforated steel sheets. Objects larger or wider than the paddy are
removed on the top deck, whereas the objects of the same size or shorter than the
paddy are removed on the lower deck.
Drum-type
The paddy is first passed through a scalper to remove larger particles, after which the
cleaner
paddy is put on vibrating sieves. A fan pulls an air stream through the paddy to
remove chaff and dust. Larger particles are removed on the top sieve due to the
presence of the large perforations.
Destoner
The paddy is separated from stones and other impurities on the basis of density
differences between them. Air is pulled from under the perforated metal sheet deck
through rice. Dense objects remain on deck while the vibrations of the deck cause
lighter rice to fall under force of gravity and discharge.
Magnetic
The paddy is made to move under permanent magnets such that the metal particles get
separator
attached to it. A magnetic drum separator can also be used where ceramic magnets
produce strong metallic fields, which attach the iron particles to the drum’s surface.
Thickness
The grains are put in a cylinder consisting of rectangular slots. These grains are
grader
continuously allowed to tumble. Thin grains fall in this slot and get discharged,
whereas the normal grains remaing on the top are discharged separately after being
conveyed to the end of the cylinder.
Disc sheller
The disc sheller consists of two iron discs placed horizontally, whose inner surface is
coated with a layer of abrasive emery. The paddy gets evenly distributed over the
disc surface during rotation and gets aligned vertically due to centrifugal force and
friction of disc. The length-wise caught grains get de-husked by pressure and
shearing action.
Centrifugal
High speed impeller discs are placed vertically with a radial blade in a metallic
sheller
casing along with a hard rubber ring fixed on the inner side. The laterally fed paddy
is rotated by blades moving outward in radial direction at high speed by centrifugal
force. Frictional force causes the tip of grains to collide with the hard rubber ring at
an angle, resulting in de-husking due to the impact force.
Rubber roll
The sheller consists of two rubber-covered rollers rotating in opposite directions,
sheller
where one roll moves at about 25% higher speed. The paddy grains that fall
between the two rolls get de-husked as a result of shearing and frictional forces due
to differences in the peripheral speed of rolls.
Oscillating
Two self-cleaning sieves are equipped in a plansifter, wherein one is finely
sieves
perforated for bran and dust removal, while the other is largely perforated for
broken grain collection. The sieve overflow contains husk, brown rice, and
unshelled paddy.
Husk aspirator Air is forced through a mixture of husk, brown rice, and unshelled paddy, whereby
air lifts the husks and discharges it.
Compartment
The oscillating compartment assembly consists of one or several stacked decks in a
type separator zig-zag channel. The surface of the table is made to oscillate in a perpendicular
direction to the grain feed, which throws grain sideways, causing the unshelled/
unhusked paddy to move up to the inclined slope and brown rice to move down the
slope. In this manner, streams of pure paddy and pure brown rice are separated.
The indented trays are made to oscillate at an incline. Brown rice, though having a
Tray-type
separator
higher density, ends up getting caught in indents and eventually is moved to the
higher end by the upward oscillation motion. Due to the repelling effect of the
smooth surface of brown rice, the paddy floats on top of brown rice and then slides
down toward the lower end of tray. This is where it is sent back to the sheller.
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Rice Processing and Properties
77
Milling Stage
Equipment
Principle of Equipment
De-branning/
whitening/
polishing
Abrasive
polishers
The abrasive polisher removes bran by the swirling motion of grains between cone
and screen, which is enhanced by rubber brakes. The grains are made to come in
contact with a rotating emery surface that cuts and abrades bran from the surface of
the rice grain.
The rice is fed into the equipment under slight pressure. A strong air stream is blown
through a hollow shaft and slit of the cylinder that separates bran due to friction.
Almost completely milled rice is fed into the water jet polisher. A strong air flow and
water mist enters the chamber where rice moves in circular motion, rubbing against
each other at high speed and under high pressure. The humidification softens the
grain surface and the frictional force and pressure from the air removes dust, bran,
and aleurone layer.
Single or double perforated steel screens kept horizontally at an angle of 4° to 12° is
made to vibrate to and fro by vertical eccentric drive. A single screen separates the
rice into head rice and large broken rice at the upper, and small broken rice at the
lower section. Whereas milled rice is separated into head rice, large and small
broken rice through individual screens when double screens are used.
Steel cables suspend the perforated screens (single or double) and give a swinging
motion by eccentric drive. The different sized openings separate the milled rice into
head, large broken rice, small broken rice, or more fractions on the basis of number
of screens and openings.
Rice is fed at the raised end into the revolving cylinder consisting of indentations.
While moving downward, the broken grains fall into indents and get trapped, while
the head rice slides down. The broken grains fall from the indents when they are
inverted while rotating at a higher point and are collected in the collecting tray.
Cast iron discs with indentations on both sides are arranged radially. These rotate in
a bed of rice and the broken grains get trapped in the indentations depending on
their size; these fall out when inverted down and get collected in a collecting tray.
The rice grains pass through a photo-detector system, i.e., the sensor, which detects
the light deflected by rice and compares with the standard color. The rice with color
different from standard color is blown out by a strong stream of pressurized air.
Friction
polishers
Water jet
polishers
Grading
Oscillating
sieve
Plansifter
Indented
cylinder
Indented disc
separator
Color sorting
Optical sorter
temperature maximum by 5–8°C, reducing the relative humidity to about 60%. Care must be taken while
dealing with supplemental heat because if the air becomes too dry, i.e., if the relative humidity becomes
less than 40%, then the grain would become over-dried resulting in cracks in the grain. The grain must
not be dried below 12% moisture (Bhattacharya & Ali, 2015).
4.3.2 Cleaning
Once the paddy is harvested, it undergoes a cleaning process to remove any impurities present like sand,
stones, straw, weed seeds, and foreign materials like metal or glass pieces. The importance of this step
lies in obtaining cleaned rice, which can improve the efficiency of the milling process.
4.3.3 De-husking
After the cleaning process, the paddy is de-husked ensuring the least damage possible to the bran and
brown rice grain. This process marks the splitting of the paddy into brown rice and husk, in addition, a
number of other materials like unshelled paddy, broken rice, bran, and germ are also obtained depending
on the type of sheller used. Along with husk, these by-products are separated from the brown rice on
the basis of their size, density, and frictional properties by using oscillating sieves and husk aspirators.
4.3.4 Paddy Separation
This step ensures the separation of the unshelled paddy from the brown rice and paddy mixture, which
is then again sent back to the sheller for de-husking. Compartment-type and tray-type separators are
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the two types of paddy separators that work on the principle of density and coefficient of friction of
constituents.
4.3.5 De-branning/Whitening/Polishing
The outer (and also sometimes inner) bran layers are removed from the surface of brown rice to yield
de-branned white rice, which is essential for easy cooking and better digestion. However, if the bran is
excessively removed, it leads to the reduction of nutritional quality of rice. The germ and fine broken
grains are also removed in this step.
4.3.6 Grading
Different fractions of rice grains differ in their length and are thus graded for determining their market
value. The whole unbroken grains and the broken grains with at least three-fourth length compared to
the whole grains are graded as head rice. Broken rice that is one-eighth or less in length of whole rice is
graded as fine broken and the remaining broken grains are classified as either large broken or medium
broken.
4.3.7 Color Sorting
The discolored grains are sorted from the milled rice grains in this step. An optical sorter consisting of
a photo-detector is used to complete this process.
4.4 Parboiling
The term par-boiling refers to partially boiled (or partially cooked) rice. This method of treatment of rice
originated in India for storage and conservation purposes and has been widely practiced since ancient
times. About a fifth of all rice is parboiled before milling, and 90% of all parboiled rice is produced in
South Asia (Bhullar & Bhullar, 2013).
The parboiling process refers to the hydrothermal treatment given to the paddy where it is allowed to
be precooked within the husk without affecting its size and shape. During this process, a crystalline form
of starch present in the paddy is changed into an amorphous state as a result of the gelatinization process.
This treatment is beneficial for coarse and medium rice with a soft structure as it is prone to breakage
during the milling process. The major objectives of parboiling are to increase the total and head yield
of the paddy, prevent nutrient loss during milling, salvage wet or damaged paddy, and prepare the rice
according to the requirements of consumers (Bhattacharya, 2011).
4.4.1 Parboiling Process
The process of parboiling has three basic steps: soaking, steaming, and drying. In general, the paddy is
soaked in water for a short period of time, then heated once or twice by steam, and lastly dried before
milling.
4.4.1.1 Soaking
The fundamental aim of this step is to hydrate the paddy enough to promote the gelatinization process
upon heating. This can be achieved simply by soaking the paddy in water for a defined period. The duration
of soaking is dependent upon the temperature of the water. Low temperature conditions (<60–65°C) require
very little attention to the soaking time as the equilibrium moisture level will never exceed 30–32%
(wet basis) thereby avoiding the risk of oversoaking of paddy. However, under these conditions, it will
take longer to achieve hydration, which can lead to fermentation and the development of off-flavors. At
high temperatures, the rate of hydration increases exponentially, which promotes the soaking process
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and minimizes the risk of fermentation—both of which help bring about the gelatinization of starch. As
hydration continues, the moisture content of the kernel exceeds 30–32% (wet basis) and the husk is no
longer able to hold the expanded structure and bursts, resulting in leaching and deformation of the grain.
Practically, the process of soaking can be accomplished in three ways:
1. Soaking can be done at a temperature greater than 75°C, where the soaking time is carefully
monitored. Once the moisture content of grain reaches 30–32%, the water is drained off and the
paddy undergoes a tempering stage to equalize its moisture content.
2. Soaking the paddy at ˜ 70°C with strict regulation over time.
3. Soaking begins at ˜ 75°C and the batch is gradually allowed to cool naturally during soaking.
4.4.1.2 Steaming or Heating
Steaming is done to gelatinize the hydrated starch. Adequately and uniformly hydrated starch can
undergo the gelatinization in as little as 2 minutes of steaming at atmospheric pressure. In cases where
overimbibition may occur, chances of splitting of the husk during steaming are very common. Besides
steaming, other methods such as mild heating with hot water or sand and ohmic and microwave heating
can be successfully used for carrying out the gelatinization process.
4.4.1.3 Drying
During the soaking stage, the water uptake by paddy increases the moisture level to approximately
35–38%. For safe storage and efficient milling process, this moisture content needs to be lowered significantly to 12–14% moisture. Drying the paddy in an effective manner is of paramount importance as if
it is not done properly, it may lead to cracks after the milling process and head rice yield will decrease
considerably. Drying can be accomplished either in the sun or with hot air by following a two-stage
process. In the first stage, the paddy is dried to a moisture content of 16% instantaneously, followed by
its tempering for about 4 or 8 hours in the second stage, which will relax the steep moisture gradient
developed in the paddy (Bhattacharya, 2011).
4.4.2 Methods of Parboiling
Depending upon the scalability of the process, traditional and modern methods are employed to obtain
parboiled rice. The traditional method makes use of pottery or a boiler for direct or indirect heating
and practices either a single or double steaming process. Agricultural residues serve as the main
energy sources for carrying out local parboiling processes. Nontraditional/modern methods are highly
energy and capital intensive, and applicable only to large scale operations (Roy, Shimizu, Shiina, &
Kimura, 2006).
4.4.2.1 Noncommercial Methods
4.4.2.1.1 Soak-Drain-Cook Process
The most common and widely used process includes soaking, draining, cooking, and drying. The paddy
is soaked in water at a suitable temperature, varying from ambient (2–3 days) to about 70°C (3–4 hours)
to confer saturation (approximately 30% moisture, wet basis). The water is drained off and the paddy is
then steamed, or otherwise heated by infrared or microwave (rarely) or some other form of heating, to
cook (or to gelatinize) the starch. The steaming can either be carried out at atmospheric pressure, i.e.,
open steaming, or under elevated pressure (0.5–2.0 kg/cm2 gauge pressure).
4.4.2.1.2 Low-Moisture Parboiling
Low-moisture parboiling characterized by partially soaking the paddy followed by high-pressure steaming. In this process, the paddy is not soaked to saturation, but only partially soaked or even simply
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wetted and later exposed to high-pressure steaming (1–3 kg/cm2 gauge) that brings about the desired
gelatinization.
4.4.2.1.3 Dry-Heat Parboiling
The initial step of soaking follows the same pathway as in the soak-drain-cook process to attain the
saturation level. Afterwards, instead of being steamed, the paddy is subjected to conduction heating. As
a result, the soaked rice is gelatinized as well as dried simultaneously.
4.4.2.2 Commercial Method: CFTRI (Hot-Soaking) Process
The commercial process was developed by Central Food Technological Research Institute (CFTRI),
Mysore. The process exposes the wet paddy to reduced pressure conditions for a short time initially,
followed by steeping the paddy in 75–85°C water for 2–3 hours. The water is then drained off and the
paddy is heated under reduced pressure in a steam-jacketed vessel with live injected steam. The water
discharge valve is kept open in order to remove condensed water during steaming. The parboiled paddy
is dried either under sun or by mechanical driers and taken for further processing (Bhattacharya, 2011).
4.4.3 Effects of Parboiling
As a consequence of the parboiling process, numerous physical, chemical, and sensorial changes appear
in the raw grain and its constituents as presented in Table 4.2. The typical changes taking place during
different stages of parboiling are described further in detail below.
4.4.3.1 Changes During Soaking
4.4.3.1.1 Enzymic Activity
The soaking stage marks the initiation of the germination process in the grain and its extent relies greatly
upon temperature and presence of air and light. Soaking the paddy at room temperature for extended
periods creates an anaerobic condition, and as a result, seeds die out. However, some of the enzymes
remain active to convert sucrose into reducing sugar and assist in de novo production of sugars and
amino acids that are partly responsible for the discoloration of parboiled rice. This can be prevented by
soaking the paddy at a high temperature (>70°C), which would kill the seed quickly but simultaneously
inactivate the enzymes.
4.4.3.1.2 Migration of Water-Soluble Molecules from Outer Regions to Endosperm
During soaking, the water-soluble components present in the husk and bran layers solubilize in the water
and enter the endosperm region. As a result, milled parboiled rice is enriched with B-vitamins, sugars,
and certain minerals as compared with milled raw rice.
TABLE 4.2
Comparison of Quality Characteristics of Raw Milled Rice and Parboiled Milled Rice
Property of Rice Grains
Raw Rice
Appearance
Opaque and white
Parboiled Rice
Translucent with faint amber (yellowbrown) color
Dimensions
Comparatively longer rice kernels
Slightly shorter but broader than raw rice
grains
Milling efficiency
Yield of head rice is low with a larger
Increased yield with less number of broken
proportion broken
pieces
Nutritional profile
Vitamin-B content is comparatively low High vitamin-B content
Cooking behavior and eating quality Soft, rough, and sticky
Firmer, fluffier, and less sticky
Recovery of bran oil
Comparatively low
High
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4.4.3.2 Changes During Steaming
The gelatinization process that takes place during steaming is pivotal for improved head rice recovery
and enhanced retention of thiamine after the milling process. Interestingly, these changes are brought
about by disruption of starch, protein, and fat in the rice.
4.4.3.2.1 Changes in Starch
Soaking the paddy at high temperature exclusively, or at an ambient temperature followed by steaming,
induces swelling of the starch molecules as a result of water uptake (gelatinization). This gelatinization
results in loss of birefringence and the destruction of A-type starch granules. A proportion of gelatinized starch undergoes reassociation with lipid molecules, which explains the quintessential properties
of parboiled rice: Its firm texture in comparison to raw milled rice due to the diminished hydration at
high-temperature conditions.
4.4.3.2.2 Changes in Fat
Rice bran oil is one of the valuable by-products obtained during milling of rice. In raw rice, the oil is in
the form of distinct globules centered below the grain surface in the aleurone layer. During the parboiling process, these fat globules are disrupted and oil is released that penetrates easily into the soft bran
tissue, forming a band on the surface. The oil does not seep into the tough endosperm. This phenomenon
clearly explains the higher oil content of parboiled bran in comparison to raw bran.
4.4.3.2.3 Changes in Protein
Rao and Juliano (1970) reported the rupturing of protein bodies in rice grains as a result of the steaming
process. This is due to the reduced solubility of protein and its extent is determined by the severity of
the process.
4.4.4 Effect of Parboiling on Milling Quality
Amongst the several factors, the cracking of the kernel is one of the main factors for breakage during raw
rice milling. Cracks may develop as a result of delayed harvesting, threshing, or a rapid drying process.
Rice breakage is related to milling conditions, particularly by the relative humidity, temperature, and
extent of milling. Parboiling aids in starch gelatinization, which fills up the void spaces in the rice, reducing the breakage of kernels during milling. The most advantageous aspects of parboiling are the increase
in the head yield of rice during polishing, the polish percentage, and breakage over time; parboiled rice
takes longer than raw rice to attain the same degree of polishing. Parboiled rice requires three to four
times as much abrasive load as raw rice for the same level of polishing. Regarding the color of rice, the
polishing need for parboiled rice is less as compared to raw rice. For example, if consumers favor 80%
bran removal to achieve the parboiled rice, this rice would require polishing of 3% whereas raw rice
would need to be polished to 4% for the same quantity of bran removal (Bhattacharya, 2011).
4.4.5 Advantages and Disadvantages of Parboiling
The process offers numerous advantages and disadvantages, which are described below:
Advantages:
• The de-husking of rice becomes easier and the grain becomes tougher, reducing the losses in milling.
• The nutritive value of parboiled rice increases as the water-soluble vitamins and mineral salts,
present in the hull and bran coat, are solubilized in water and transported to the endosperm.
The riboflavin and thiamine content is four times higher in parboiled rice than in milled rice.
• Parboiled rice has comparatively lower moisture content (10–11%) than milled rice, which corresponds to its better storage.
• The starch grains embedded in a proteinaceous matrix are gelatinized and expanded until they fill up
the surrounding void spaces, thereby minimizing the occurrence of cracks and fissures on milling.
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Disadvantages:
•
•
•
•
•
•
Most of the naturally occurring enzymes in the grain are inactivated during the steaming stage.
Natural antioxidants present in grain are also destroyed.
The process may affect the quality of milled rice and bran by promoting oxidative rancidity.
The heating step causes discoloration of grains.
Due to defective steeping, parboiling sometimes causes an unpleasant smell in rice.
Sometimes during the treatment, overimbibition and deformation of the grain may occur, which
can be restricted by keeping the water and heat separated.
4.5 Quality Analysis of Rice
4.5.1 Cooking Quality
The major proportion of milled rice is consumed as table rice, and only a slight part is converted into
various other rice products. Table rice is specifically cooked in water until it turns soft by utilizing different modes, such as using a pressure cooker, double boilers, or microwave. Overall, the cooking quality of rice is mainly affected by the rice variety and its milling quality, ageing, and the cooking method
employed, which in turn influences the sensory and economic considerations of the grain.
4.5.1.1 Changes Taking Place During Cooking Process
•
•
•
•
•
•
•
•
Rice grains are hydrated readily by absorbing roughly twice its weight of water.
Hydrated starch granules in the presence of heat undergo the gelatinization process.
Rice grains become soft, easily digestible, and fit for consumption.
Expansion of the grain takes place in all the directions, chiefly longitudinally.
Inflation of the grain volume (both true and bulk volume).
Semitranslucent or translucent uncooked grains change to an opaque color.
Dissolved solids and suspended particles leach from the grain into the excess of water during cooking.
Considerable sections of grains burst either along the ventral or dorsal edge of the grain.
4.5.1.2 Factors Affecting Cooking Quality
Numerous internal factors of rice (moisture content, amylose and amylopectin ratio, and type of starch
granules), processing parameters (degree of milling, precooking, water/rice ratio during cooking, cooking methods, gel consistency, and gelatinization temperature, cooking losses and cooking time) and
postprocessing conditions (drying and storage conditions) are the factors of great importance that influence the cooking and textural characteristics of rice. Soft-textured cooked rice is obtained from those
having high water-binding capacity, which is greatly influenced by degree of milling and is indirectly
associated with the profit gained by the farmers and rice milling industry. Consequently, it is obligatory
to select a suitable degree of milling that depreciates the levels of losses and boosts cooking/eating qualities (Mohapatra & Bal, 2006).
4.5.1.2.1 Amylose to Amylopectin Ratio
The texture of rice is determined by its amylose content. A fluffy, nonsticky, flaky, and dry texture is
obtained by cooking rice varieties whose amylose content is more than 25%, which makes it capable of
absorbing more water. Amylose content of rice is related to the water absorption and volume expansion
of rice. To cook high-amylose rice, more water is needed to obtain the optimum texture as compared to
low-amylose varieties. Waxy starches being devoid in amylose and rich in amylopectin absorb less water
and cook to a sticky and pasty texture (Frei, Siddhuraju, & Becker, 2003). Furthermore, Perez, Juliano,
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Bourne, and Morales (1993) reported that amylose content is correlated positively to hardness whilst
negatively to the stickiness value of cooked milled rice.
4.5.1.2.2 Ageing
Rice undergoes a change in its cooking behavior as it ages; there is a remarkable difference between
recently harvested rice and rice that has been stored for some time. Just after harvesting, rice (commonly
known as new rice in South Asia) cooks to a somewhat lumpy, sticky, and moist mass. However, as it
ages, it cooks comparatively drier, fluffier, and free-flowing (Bhattacharya & Ali, 2015). The former is
preferred by the people who eat rice with chopsticks since the rice grains cling together. The texture of
rice can be characterized as sticky/adhesive or firm/hard/tender. Soaking is an important process, which
if not done, can lead to difficulty in cooking the rice. It is a common observation that when rice is cooked
in excess water, starch solids leach out of the rice grains, however, the case is different when they are
cooked in limited water, where these solids get redeposited. These solids, called gruel, make the cooked
rice sticky. The parboiled rice is hard, so usually less starch is leached into the cooking water, thus preventing the problem of sticky or pasty rice. Short rice grains have a stickier texture upon cooking than
medium or long rice grains (Elbashir, 2005).
4.5.1.2.3 Gelatinization Temperature
The gelatinization temperature is directly related to the cooking time; rice takes longer to cook when its
gelatinization temperature is high (Bhattacharya & Sowbhagya, 1971).
4.5.1.2.4 Water Uptake
Water uptake is related to the surface area of rice grains per unit weight; small and slender grains cook
comparatively quicker than large and round grains. At ambient temperature, the water uptake is inversely
related to amylose content and gelatinization temperature of the rice. However, chalkiness of the rice
grain promotes water uptake, which reduces the palatability of cooked rice (Bhattacharya, 2011).
4.5.1.2.5 Cooking Losses
Depletion of nutrients takes place when rice is washed in excess water prior to cooking due to discarding
the gruel, which is also resultant of cooking rice in excess water. Parboiled rice has less vitamin loss than
raw milled rice. Elbashir (2005) observed a significant loss in starch, amylose, and amylopectin content
in cooked rice.
4.5.1.2.6 Grain Elongation
Linear elongation in rice grains without significant increase in girth is considered a desirable characteristic of high-quality cooking rice. The gelatinization temperature is positively related to grain elongation.
Nonetheless, more elongation in rice would likely indicate low amylose content (Perez, Juliano, Bourne,
& Morales, 1993).
4.5.2 Eating Quality
Eating quality is defined as the sensory perception of the aroma, whiteness, gloss, flavor, tenderness
(or hardness), and cohesiveness (or stickiness) of the cooked rice. It is a function of milling quality;
adequately milled rice has more consumer acceptance for its eating quality than brown and undermilled
rice. Eating quality is evaluated by three major physicochemical characteristics of the starch— amylose
content, gel consistency, and gelatinization temperature— and can also be evaluated on the basis of grain
quality considering its size, appearance, and shape (Ahmed, Tanweer, Kabir, & Latif, 2020).
4.5.2.1 Grain Quality
The chalkiness and translucency of rice is influenced by the blurriness or opacity of its endosperm. Cruz,
Kumar, Kaushik, and Khush (1989) mentioned that customer acceptability decreases with the increase
of chalkiness in the grain.
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4.5.2.2 Amylose Content (AC)
Amylose content (AC) is considered to be a major predictor of rice eating quality as it has been associated with mechanical textural attributes such as hardness and stickiness (Custodio, Cuevas, Ynion,
Laborte, Velasco, & Demont, 2019). Waxy rice has near zero amylose, and is used for special foods such
as desserts and snacks. High amylose cultivars (>25%) are common in Indica rice, and are dry and fluffy
on cooking, often becoming hard after cooling. Low amylose cultivars (15–20%) are soft and sticky,
and include nearly all-temperate japonica cultivars. Intermediate amylose (20–25%) rice is soft but not
sticky, and is widely preferred by most consumers (Phing Lau, Latif, Rafii, Ismail, & Puteh, 2016).
4.5.2.3 Gel Consistency (GC)
Gel consistency is a measure of firmness of cooked rice as it tempers the tendency of the cooked rice
to harden after cooling. Within the same amylose group, varieties with a softer gel consistency are preferred where the cooked rice has a higher degree of tenderness. Harder gel consistency is associated with
harder cooked rice and this feature is particularly evident in high amylose rice. Hard cooked rice also
tends to be less sticky. Rice varieties can be grouped into three GC classes: high (hard and very flaky
texture), medium (flaky but softer rice), and low (soft and nonflaky rice). Rice with the same amylose
content can be classified as hard gel consistency (26–40 mm), medium gel consistency (41–60 mm), or
soft gel consistency (61–100 mm) (de Oliveira, Pegoraro, & Viana, 2020). GC is reported to be affected
by milling (lipid content), protein content, ageing of milled rice (fat oxidation), and rice flour particle size
(efficiency of dispersion) (Perez, Juliano, Bourne, & Morales, 1993).
4.5.2.4 Aroma
Aroma is a value-added character to rice, since it is a preferred trait by consumers. Rarely, however, do
consumers describe the aroma of rice beyond the subjectivity of “with fragrance”, “no fragrance”, or
“bad or unpleasant smell”. Good aroma tends to be associated with pleasant aromatics found in Jasmine
and Basmati rice types. Most often, the volatile compound 2-acetyl-1-pyrroline (2-AP) is found in relatively high concentrations in these aromatic rice varieties, lending a popcorn-like roasted smell. The
aroma of 2-AP is also associated with a milky and sweet nutty smell and exhibits a low threshold value
in water (Custodio, Cuevas, Ynion, Laborte, Velasco, & Demont, 2019). However, how these volatiles
contribute to the aroma of rice is unclear; hence defining aromatic rice is still incomplete.
4.6 Conclusion
With world rice production expecting to increase, there is a need to pay attention to its various properties
and processing techniques to ensure high quality. Physical properties of rice are important for efficient
handling, storage, and processing of rice grains. The physical properties, namely, bulk density, true
density, and porosity are used to determine the size of hoppers and the design of storage facilities. Low
porosity grains require higher power to drive aeration fans for drying. The static coefficient of friction
is used in sizing motor requirements for rice grain handling and transportation. Mechanical properties
of rice have a significant impact on the energy demand used in designing machinery and, optimizing
them to prevent grain breakage. High glassiness corresponds to easier separation of endosperm. The
coefficient of thermal expansion is an important property with respect to rice fissuring. Milling of paddy
is a very essential process, which renders the rice edible, giving it a characteristic texture, flavor, and
color—all important for consumer acceptability. Head rice recovery and number of broken grains determines the efficiency of milling process. A hydrothermal treatment called parboiling is given to the paddy
to prevent grain breakage and increase the head rice yield, along with making the de-husking process
easier, enhancing the nutritional content, and making the rice less sticky during cooking. Consumers
judge the quality of rice based on its cooking and eating ability—the better the final rice quality, the
higher its market value. The cooking quality of rice is improved by high amylose content, ageing, and
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low gelatinization temperature. The eating quality of rice is determined by sensory perceptions, such as
aroma, tenderness, cohesiveness, flavor etc., which are affected majorly by grain chalkiness, amylose
content, and gelatinization temperature. Since rice is consumed as a whole grain and in variety of processed products, understanding its key attributes and milling process is of prime importance to obtain a
product of excellent cooking and eating quality with wide consumer acceptability.
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