“Nowadays, more and more rice mills are installing rice polishers that subject rice to multiple polishing passes. The degree of polishing has reached an all-time high”

1. R
ice is a vital staple food, feeding half
of the world’s people and, according
to the Food and Agricultural
Organisation, providing as much as
20 percent of the global population’s
dietary energy supply.
In 2015, global rice farming
produced 743 million tonnes of
paddy rice – which yielded 493
million tonnes of white rice. Of this, an estimated 300 million
tonnes of rice is polished.
Originally, rice was consumed as unrefined, whole grain brown
rice. The evolution of polished rice has changed our relationship
with this staple food and with it, consumer tastes and demands.
Today there are more than 40 000 different varieties of rice,
each with their own characteristics, and each forming an integral
part of the culinary traditions of many different regions and
cultures. For instance, sushi and biryani from Asia, paella and
risotto from Europe, as well as rice pudding - a British classic.
In demand
Until the late 20th century, rice mills around the world,
including top-quality millers, did not integrate polishing into the
production process. However, owing to the steadily increasing
demand for whiter, silkier rice, the polishing process is now
considered to be a crucial stage in the milling process.
Although it is widely accepted that brown rice has a much
higher nutritional value than white rice, many consumers prefer
the taste of the polished white alternative. Furthermore, it easier
to digest, needs no pre-soaking, cooks quicker and uses less
water.
It should be noted that the cooking process might cause the rice
to burst, making it look coarser, which consumers can find off-
putting. However, this can be reduced, if the degree of polishing
is adapted to suit the particular variety of rice.
Polished rice has benefits for food producers and retailers too.
It improves the appearance of the grain, making it more visually
appealing at the point of sale, meaning it can command a higher
price. It also removes traces of bran left after the whitening
process. This is particularly important, as glycerides in the bran
turn rancid when exposed to oxygen. If they are not removed,
they reduce the shelf life and eventually result in a product that is
unfit for consumption.
However, the demand for high-gloss, transparent-looking rice in
some parts of the world has been so high that unsafe, unapproved
methods have been used to give the desired result. For instance,
glazing the rice with non-food-safe additives, such as oil or
talcum powder.
Fortunately, an increasing number of rice mills are turning to
innovative rice polishing technology to deliver new standards,
improve quality, enhance food safety and deliver the degree of
whiteness and silkiness that consumers demand.
Humble beginnings
Prior to the advent of modern polishers, several simpler
methods were used including pounding the rice using a pestle and
mortar, rubbing it on the floor, beating gently with clubs in jute
bags and treading by humans and animals.
These makeshift means, often carried out in poor hygiene
conditions, not only required significant time and energy, but
usually resulted in a poorly-finished and significantly damaged
rice, with high levels of wastage.
The first commercial rice polisher is widely believed to have
been patented by the British engineer Sampson Moore. The
inventor, a prominent engineer during the British Industrial
Revolution, was credited in the London Gazette for his invention
on June 21st 1861, for “improvements in the machinery or
apparatus for dressing and polishing rice”.
Since those early days, a range of machines have improved the
efficiency and quality of milled, polished rice.
Rice polisher development
Unpolished rice naturally has a coarse surface, with ridges
that protect individual grooves, where the bran sits. Prior to
polishing, the rice must go through a whitening process, designed
to level out undulations naturally found in the caryopses of all
rice varieties and this helps to remove the majority of the bran.
However, the abrasive elements used cannot be made fine enough
to remove all of the bran without damaging the grain, which is
why polishing is required.
This gentler process, which removes dust, flour and bran
residues, uses a pressing and rubbing technique to create friction.
As the grains rub against each other, their surfaces are smoothed,
removing the remainder of the bran, allowing more light to be
reflected, which in turn makes the rice appear whiter and glossier.
The first generation of polishers were adapted from whitening
machines. They featured a similar vertical cone design but had
two basic differences. The first was that the cone was made of a
simplified steel wire construction and covered with wood, on to
“Nowadays, more and more rice mills are
installing rice polishers that subject rice to
multiple polishing passes. The degree of
polishing has reached an all-time high”
- Sujit Pande, Rice expert, Buhler
Rice Polishing
150 years of innovation
58 | March 2016 - Milling and Grain
F

2. which leather strips were nailed. The
second was that the cone rotated at a
speed that was 25 percent slower than
the vertical cone design.
The process began with the rice
entering the space between the cone
and the wire screen, it was then
gripped by the leather strips that rolled
the grains over each other and against
the leather and wire screen. Then, with
the application of a small amount of
pressure, the remaining bran particles
were removed and the rice became
shinier or more transparent.
Unfortunately, this method caused
breakages, particularly in long
grain rice, thus reducing its value.
Furthermore, the leather strap needed
to be replaced periodically, increasing maintenance costs.
These issues drove further innovations, including the creation
of the horizontal polisher, which consisted of three principal
parts: a feed screw, a cam roll and screen.
Rice is fed into the machine by gravity, while both the feed
screw and cam rolls rotate. The feed screw pushes the rice into
the working chamber where a retainer plate, with an adjustable
counterweight, creates a controlled pressure on the rice kernels.
The fractioning effect is achieved by rolling or rubbing the rice
and through displacement. A screen basket covers the cam rolls,
allowing a pre-determined distance between them. Air suction
enhances the compactness and cools the rice while sucking away
the residue.
Performance matters
Modern day polishing machines now offer a significant
improvement in efficiency and wastage reduction. However, there
are a number of factors that impact performance.
Broken grains of rice can very much impair the polishing
process because surface fractioning cannot be achieved if the
grains are sandwiched between broken kernels. The polishing
effects are improved if ‘brokens’ are sifted out beforehand.
Polishing time is a deciding factor for both silkiness and
breakage. A shorter polishing time results in less silkiness.
A longer time improves silkiness but increases the risk of
breakages.
Screen basket and cam distance influences the degree of
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Diagram of a horizontal polisher
Milling and Grain - March 2016 | 59
F

3. polishing. The screen basket covering the cam rolls
allows a pre-determined distance between them, with
a greater distance (i.e. a wider chamber) reducing the
degree of polishing.
Rotational speed determines the degree of polishing.
A higher rotational speed increases the number of
individual impacts, which results in a higher degree
of polishing. However, it also increases the amount of
rice breakages.
Air suction is used to compact and cool the rice. If
the volume of air used is excessive, it can reduce the
compactness and the effectiveness of the polishing
process, thus reducing the shine on the rice.
The importance of water
Older machines that relied solely on friction to
achieve a lustrous look and feel, generated a lot
of heat, causing a large proportion of the rice to
break. Manufacturers tried to counteract this by
incorporating a hollow shaft, through which air was
blown to cool the rice. Unfortunately this proved
ineffective.
One solution is the use of atomised water, to
humidify the rice grains and thereby increase friction.
In modern water mist polishers, temperatures remain
lower, to prevent the rice surface from drying out.
The addition of water also helps to create a slip layer
between the bran fragments and the rice kernel, improving the
removal of bran, resulting in a smoother appearance, longer shelf
life and a higher yield of unbroken rice.
However, if the rice still contains bran particles, the polishing
effect can be reduced when a water jet polisher is used because
the fat in the bran solidifies at low temperatures. Any bran
particles that come into contact with water disintegrate, triggering
enzymatic activities, which turn the rice yellow and release a
rancid smell. It is therefore essential to the remove the bran
before water polishing.
Bühler: Planning today for the demands of tomorrow
Integrating food safety into every aspect of rice production is
vital for each player in the supply chain.
Bühler, the global leader in rice processing solutions, always
adheres to good machine and engineering practices to ensure
food safety can be maintained easily, throughout the lifespan of
its products, which include the range of rice UltraPoly™ and
SuperPoly™ polishers.
A polisher must have little or no residue left in the machine
during and after operation, and be easy to clean at regular
intervals. Ideally, there should be no crevices or sharp edges
on the housing to avoid accumulation of dust. Bühler designs
its polishers to enhance food safety - a key feature in Bühler’s
UltraPoly™ is a unique replaceable tooth design on the cam,
which provides a slot for cleaning inside the cam, thus improving
hygiene standards.
It is also important that equipment is designed to have the best
balance between polish performance, energy consumption and
yield. Bühler is committed to advancing its knowledge and
developing the relevant technology to help resolve these issues.
One such development is Bühler’s design to optimize the entire
polishing chamber - including the cam roll, sieve geometry, water
addition system and aspiration.
Traditionally, aspiration was only understood as a means of
removing bran during polishing. However, research reveals
that the air flow within the polishing chamber has a significant
influence on the finished result of the polishing. Using
computational fluid dynamic analysis (CFD), Bühler has
optimized the air flow, ensuring that even with a relatively short
polishing chamber, the desired polish can be achieved.
Bühler has successfully incorporated this innovation in its
SuperPoly™ horizontal polisher, which enables processors to
deliver a highly polished and shiny rice kernel without increasing
wastage or energy consumption.
Furthermore, Bühler has developed a high-capacity polisher
that is able to match the polishing performance achieved by small
capacity polishers; something the industry has required for a
while.
The challenge has always been the distance between the
screen basket and cam –a wider chamber increases capacity, but
diminishes polishing. To overcome this, many polishers contain
two chambers within the same frame. However, these have the
inherent disadvantage of higher power consumption per ton of
rice processed.
Bühler’s UltraPoly™ range of polishers takes a more innovative
approach. Patented screen design based on years of research,
gives an excellent and efficient polishing performance, even at
high capacity.
While these innovations are a step change in rice polishing,
measuring the reflective quality of the rice, often referred to as its
silkiness, is still a challenge for rice millers.
There is no current measuring unit, or equipment, for
determining the level of polish achieved, meaning the degree to
which the grain reflects light is based purely on an expert’s sight
evaluation. This subjective method can be a source of dispute,
which in turn can impact the selling price.
While rice polishing technology has clearly made great
headway, there is still room for improvement.
With its 150 years of expertise, Bühler will continue to lead the
way in developing new technologies to counter this issue and
be at the forefront of other future innovations in rice processing
– from handling, storage and milling solutions, supporting and
working in partnership with rice processors to overcome future
challenges.
Diagram of a vertical polisher
Rice inlet
Cone covered with wood
Pulley
Shaft
Leather
Screen
Rice
outlet
60 | March 2016 - Milling and Grain
F

4. GROWING INTO
THE FUTURE
TAKING CARE
ADDING VALUE
SOLUTIONS FOR HANDLING
AND STORAGE OF
GRAIN AND SEED
Cimbria develops and manufactures an
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Praestejorden 6 | DK-7700 Thisted
Phone: +45 96 17 90 00
E-mail: unigrain@cimbria.com
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