POST-HARVEST TECHNOLOGY OF AGRICULTURAL PRODUCT (Lecture 04)

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Tempering of parboiled paddy
• Paddy is stored in a bin during two successive during process to equilibrium the moisture content
through out the grains, this process termed as tempering of paddy.
• Purpose
1. Shorten the total drying time: During the falling rate period of drying moisture
migration becomes slower than the rate of evaporation of moisture from surface
due to dry surface. During tempering the mc equalizes in the kernel and the
evaporation on the surface becomes rapid as starting.
2. Tempering helps to prevent breakage during milling: During fast drying a
moisture gradient develops, the inner and outer sections of the kernel shrink at
different rates and internal stresses are set up. On the other hand during
tempering inner and outer layers of the kernel are allowed to equalize moisture
content and internal stresses and breaking becomes minimized.
The sequential operations of parboiled paddy
1. Receiving paddy
2. Weighing and recording
3. Dumping into the pit
4. Elevating to the cleaner
5. Cleaning
6. Elevating to the dryer
7. Drying (1st pass): In 30 min of drying 3% moisture reduced.
8. Tempering: 8 hrs.
9. Drying (2nd pass): In 30 min, 2-2.5% mc reduction.
10. Tempering: 8 hrs.
11. Drying (3rd pass): In 30 min 2 % mc reduced.
12. Either godown or to the rice milling.

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Dying of parboiled paddy
• The drying of parboiled paddy differ from raw paddy due to-
i. Moisture content of parboiled paddy considerably higher than
raw paddy ( dry basis mc 55%).
ii. Grain texture becomes change due to starch gelatinization.
iii. Drying of parboiled paddy begin at 100 oC during steaming, while
raw paddy at ambient temperature.
iv. In some cases, the husks of parboiled paddy splited up.
Drying of parboiled paddy
• Sun-drying: 1-3 cm layer on straw mats, hard soil or cemented floor.
Spread usually in the morning and continued about four hours. It is
the heaped and covered by mats for tempering 2-3 hrs. then again
spread for 2 hrs in the sun till desirable moisture content. Total time
8-10 hrs during summer and 13-15 hrs during winter.
• Mechanical drying: No breakage occur when drying slowly in the
shade. During rapid drying a steep moisture gradient develops
between the surface of the grain and its center which is more wet,
this put up stresses which relieves by cracking at a certain stage.

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Measure suggested to avoid breakage
• Conditioning: This is to keep paddy hot for sometime after
drying, i.e. prevent cooling which develops cracks due to
moisture gradient. Conditioning period is varied with variety,
usually about 2 hrs is mostly counted.
• Tempering: Stop first passes of drying at 16-17% moisture and
temper. Then dry again till moisture content 12.5 %. It was
found that tempering for about 8 hrs at RT and 2 hrs at 06 oC
was optimal.
Method of drying
1. Drying with high or medium temperature air:
In the Schule process high temperture air is used in a pre-drying
process where most of the moisture content is reduced. This is
followed by two or more column dryers where drying with
medium heat until moisture content 13 %.
2. Cooling before drying
To stop gelatinization parboiled paddy is cooled to 60 oC by
ventilation process and then followed by hot/medium air drying
process.
3. Vacuum drying
This is usually done inside the contained in which the rice is
steamed and thereby eliminates difficulties with loading and
transporting hot paddy. Final drying is sometimes done with
medium temp air. Advantages: save fuel cost, removal off flavor,
better operational control and possible recovery of condensate
water.

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Milling of Paddy
• Milling consists series of operation:
1. Cleaning of paddy to remove straw, chaff, dirt, etc. : Cleaner.
2. Shelling of husk: paddy separator and/or husk aspirator
3. Polishing to remove outer bran layer: Polishing cone.
4. Grading: rice grader.
5. Separator of husks, un-shelled paddy, bran and broken at
appropriate stages: rice cleaner.
Traditional methods
• Home pounding
1. Mortar and pestle: mortar made of wood or stone and pestle
made of wood and is fitted with an iron hub at one end and an
iron ring at the other, so that it can be used for dehusking.
2. Dhenki: Improve system of mortar pestle and operated with the
help of lever system. Here the pestle is attached to a wooden bar
1.83-2.44 m long and the mortar made of wood and fixed in the
ground.
3. Hand Chakki: Consisting two grinding discs, made of stone or
wood and placed one over another. When the upper disc is
rotated with handle over the lower one, which is fixed, paddy is
dehusked. Husked rice is polished with dhenki.

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Huller Mill
• An iron ribbed cylinder, having helical ribs upto ½ to 1/3 of the
length and straight ribs in the remaining portion, rotates in a
cylindrical housing made of sieves.
• A straight edge or knife is inserted into the chamber to control the
husking and polishing operation by adjusting the clearance between
rotating cylinder and the knife.
• Three consecutive huller operation is done varying clearance
• Gyratory seive shifter use to separate each portions.
• Cylinder rotates at a speed 800-900 rpm by an electric or diesel
motor by a flat felt or pulley drive.
• Capacity: 25-500 Kg/hr.
• Disadvantages: low outturn of rice (63% avg.), little or no control on
polishing, and rice bran get mixed with husk, resulting loss of
valuable byproducts.

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Sheller Mill
• Improved type of huller mill where husking, polishing,
separation and grading are performed by separate
machines.
• An under-runner disc sheller used to remove husk, which
contain two discs where upper one fixed and lower one
rotates.
• Adjusting the clearance helps to degree of husk removal.
• After that huller mill is used for subsequent steps.

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Modern Rice Mill
1) Paddy cleaning
 Carried out to remove bulk impurities like straw, chaffs,
stones and dust by using screening and air-blast aspiration.
 Scalping: rotating sieve first throws off the straw and other
impurities larger than paddy.
 Stone removes by gravity separation and
 Finally air-blast separation to remove dusk and lighter
impurities.

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Modern Rice Mill
2) Shelling
 The rubber-roll sheller consists of two rubber rolls, one
fixed and another one adjustable.
 The husking action is provided by contact pressure caused
by the deformation of rubber at the contact point between
the rolls and sharing force of the roll friction.
 Clearance between the rolls suggested ½ of paddy thickness
and spring pressure per unit width of roll over4.75 kg/cm2.

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Modern Rice Mill
3) Separation of husk and paddy
 First husk is removed by aspiration with air suction created
by fan.
 Next paddy is removed from the shelled rice (brown rice) by
using vibrating inclined tray where there difference in size,
density and coefficient of friction is used.
 Paddy and shelled rice stratify in where shelled rice makes
its way to the upper side and paddy moves to lower sides to
the outlet.
Modern Rice Mill
4) Whitening and polishing
 By whitening process the brown layer is removed from the brown rice
(Pearling process).
 The polisher removes any bran layer remaining and gives a shiny
appearance.
 Two types of machine used : Abrasion and friction type.
 Abrasive type: Consists cone shaped cast iron cylinder coated with an
abrasive and mounted horizontally or vertically in a perforated screen
shell. Rotation speed: 800-1000 m/min and pressure 200 g/cm2.
 Friction type polisher: Rubbing action where 26 ribs on the periphery.
Rotating speed: 300 m/min and pressure 500 gm/cm2.
 Usually the whole process is done in a series of machines to avoid too
much pressure on grain. First abrasive type and later friction type
machines are used.

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Modern Rice Mill
5) Separation for bran and polished rice
 The bran adhered to the polished rice is removed by air-
aspiration or by shifting.
 Cooling of grains heated to avoid lumping in the bags.
 In shifting, a set of gyrating sieves is used for removal of
broken rice.
Modern Rice Mill
6) Rice graders
 Gyroshifter, grading cylinder and disc separator.
 Grains above ¾ grain are “head rice”, remaining broken
further graded into coarse brokens (1/2 to ¾), fine brokens
(1/4 to ½) and less than ¼ size.

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Problems of Modern Rice Mills
• A modern rice mill involves with following process:
a) Procurement prog.
b) Transportation
c) Drying and storage
d) Milling facilities and
e) Rice marketing prog.
• Essential part for the success
1) Modern equipment and methods
2) Trained operators and technicians, and
3) Trained management.

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Problems of Modern Rice Mills
1) Quality control in the procurement paddy
2) Mechanical drying of paddy:
care should be taken on some steps like at very high mc (23-30%
db) higher air temp (60-70 oC) and mc (14-18% db) air temp less
than 60 oC.
3) Proper selection of components of the milling equipment: use of
feed rolls, elevators, unloading devices etc.
4) Rice variety selection for modern milling rice process.
5) Lastly maintenance of machinery should be done trained
engineers.
Some important terms
 Products of rice milling: Sheller bran, husks, bran, polish,
milled rice (head, broken rice)
 Byproducts are: germ, bran, husks, brokens
 Degree of milling
• Defines as the percentage of powdered materials removed
from the brown rice by milling. In our countries around 3-
4%. For highly milled rice it may up to 7%.