cereal and their processing for industry LECTURE 2.pptx

2. Primary processing of cereals

Total post-harvest cereal system
 Each type of cereal requires a specific post-harvest treatment, however, there
are certain general principles that apply to most of them.
 Cereals undergo a number of processing stages between harvest and
consumption. This chain of processes is often referred to as the total post-
harvest system.
 The post-harvest system can be split into three distinct areas.
 First is the preparation of harvested grain for storage.
 Second, which is referred to as primary processing (cleaning removing husk or
reduce the size).
 Third stage is secondary processing transforms the grains into edible products.

Total post-harvest cereal system
 Primary processing involves several different processes, designed to clean,
sort and remove the inedible fractions from the grains
 Primary processing of cereals includes cleaning, grading, hulling, milling,
pounding, grinding, tempering, soaking, drying, sieving
 Secondary processing of cereals (or 'adding value' to cereals) is the utilisation
of the primary products (whole grains, flakes or flour) to make more interesting
products and add variety to the diet
 Secondary processing of cereals includes the following processes: fermentation,
baking, puffing, flaking, frying and extrusion

Cleaning and Grading
 Before further processing, grains are cleaned and graded according to size
 Winnowing machines can be used to separate out the chaff, soil and dirt
 Some machines have integral sieves that combine cleaning with grading

2.Hulling
 Several grains have an unpalatable husk or shell that needs to be removed
by a decorticator
 A range of specialized machines are available for this task. A range of
small rice hullers (both manual and powered) are available.
 Less rice is broken during hulling if the rice is parboiled first.

3.Pounding/Milling
 Three main types of grain mill are available: Plate mill; Hammer mill;
Roller mill
 The choice of mill depends on the raw material and the scale of
production
 Hammer mills are almost universally used throughout the developing
world

4.Paraboiling
Parboiling rice is an optional step, but one that improves the
quality of hulling as it results in fewer broken grains
About 50% of all rice grown is parboiled
Parboiling involves soaking and heating the rice which pre-
cooks the grains, loosens the hull, sterilises and preserves the
rice

5.Drying
Prior to storage or further processing, cereal grains need to be
dried
The most cost-effective method is to spread out in the sun to dry
 In humid climates it may be necessary to use an artificial dryer
Cereal grains should be dried to 10-15% moisture before storage

6.Storage
Dried grains are stored in bulk until required for
processing
The grains should be inspected regularly for signs of
spoilage and the moisture content tested
If the grain has picked up moisture it should be re-dried
Grains are often protected with insecticides and must be
stored in rodent-proof containers

Grain Storage and Management
Grain storage is a component in the grain marketing supply chain
that evens out fluctuations in the supply of grain from one season,
usually the harvest season to other seasons, and from one year of
abundant supply and releasing to lean years.
Grain storage may be at farm, trader, and commercial or at
government levels.
At the farm level, storage is normally inter-seasonal and helps
household to ensure food supplies for the farmer and the family,
cash or barter exchange and for seed
11
AASTU, Food Eng. Dept.

At trader level, grain storage is for very limited time –
over a period of few days or weeks. The traders buy
and sell quickly to make a profit. Grain storage at this
level is not inter-seasonal.
Commercial storage is used by millers and co-
operatives to hold stock for limited periods of time to
meet demands for their needs or for their urban clients
12

Government involvement in grain storage, through its
own special departments, agencies or government grain
marketing boards focuses on the intervention in the staple
grain market to balance national supply and demand over
a time.
The purposes being to create a national food reserve
especially for
the urban population,
national food security reserves,
stimulation of productivity,
price stabilization
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Grain must be stored safely to meet the quality and
quantity requirements for the various end uses for which
it is intended.
Grain uses
 milling and baking for human food
 production of oil
 confectionary and breakfast cereals
 malting and beer production
 production of industrial products such as alcohol, starch and in pharmaceuticals
 manufacturer of animal, aquatic and pet feeds
 for production of seed for propagation
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The length of time grain can be held in storage
depends on
the moisture content of the grain,
the temperature of the grain, and
whether the grain can be kept away from heating by
means of aeration
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The primary aim of storage is to prevent deterioration of the quality
of the grain. This is achieved through control of moisture and air
movement, by preventing infestation of microorganisms, and attacks
of insects and rodents.
Food grains can be stored for relatively longer periods of time under
proper storage conditions (low temperature, inert atmosphere, etc.),
with little or no detectable loss of quality.
16

Grain spoilage is the result of microorganisms (bacteria, yeast,
and fungi) using grain nutrients for growth and reproductive
processes.
Microorganisms also produce heat during growth that can
increase the temperature of stored grains.
Under proper environmental conditions, certain
microorganisms can produce toxins or other products that can
cause serious illness and even death when consumed by
livestock or humans.
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 Losses during storage may occur in many ways:
 losses in weight due to insects, rodents or birds eating the grain
 deterioration through fungus growth and rotting
 loss in quality through biting damage, insect and rodent excrement and
fungus growth
 loss of motivation in the farmer to grow more, because he is not able
to store his harvest or part thereof in a safe way for any long period of
time
 damage to sacks, which causes waste during transportation
 decline in germination capacity of stored seeds
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Grain quality can start to deteriorate in the field prior to
harvest. Rainfall prior and during the harvest period can
encourage diseases and premature sprouting, whilst high grain
moisture levels will necessitate increased drying costs.
For example, delaying the wheat harvest can result in grain
with high -amylase content, lower specific weight and protein
contents, factors that will seriously reduce the quality of the
grain for bread making.
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Safe storage means protecting grain from weather, molds and
other microorganisms, addition of moisture, destructively high
temperatures, insects, rodents, birds, objectionable odors, and
contamination.
For safe storage the grain has to be dried to within the range 13
to 15% moisture.
Farmers themselves have often developed methods for storing
their products. Some of these traditional methods protect the
product reasonably well and need at most slight improvements.
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 On the other hand, it is possible that some traditional methods are
unsatisfactory, and lead to high losses.
 Alternatively, the introduction of a new variety may alter the storage or
drying requirements. A change in the market situation may also increase
the need for other storage methods.
 The following two factors determine the choice for the best storage
method:
 the moisture content of the product when it comes from the field
 the relative humidity of the outside air during the storage period
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Depending on the weather during the harvesting period and the
duration of storage there are four possible combinations:
Dry harvesting period and dry storage period. There are no
problems with storage, provided that the general conditions are
fulfilled.
Dry harvesting and wet storage period. The dried product will
take up moisture from the more humid air during storage,
unless it is stored in airtight and waterproof conditions. This
makes storage more expensive.
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Wet harvesting and dry storage period. During storage
the still moist product has to dry and therefore it should
be ventilated as much as possible.
Wet harvesting and wet storage period. The product
should be dried artificially and then, after threshing, be
stored in an airtight and waterproof container.
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Types of Storage Facilities: Storage facilities take
many forms, ranging from piles of unprotected grains
on the ground, underground pits or containers, and
piles of bagged grains, to storage of bins of many sizes,
shapes, and types of construction
On the Ground
Underground
Bagged Storage
Bulk Storage
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The stored products, as well as the organisms attacking stored
products are living things: they breathe.
During respiration ("breathing"), oxygen is used up and carbon
dioxide, water and heat are produced.
The rate of respiration, and thus the amount of carbon dioxide,
water and heat that are produced is strongly dependent on the
temperature and the moisture content of the product. The rate
of respiration is reduced approximately by one half for each
10°C reduction in temperature.
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Biological activity occurs only when moisture is present.
Therefore the moisture content of the product itself, as well as
the moisture content of the surrounding air, is important for
safe storage.
Each product has its own characteristic balance (or
equilibrium) between the moisture it contains and the water
vapor in the air surrounding it. This equilibrium is known as
the moisture content/relative humidity pattern.
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The moisture content of a product is expressed as a percentage
of the wet weight:
The relative humidity is a percentage measurement of the
amount of moisture (water vapor) actually in the air as
compared to the maximum amount of moisture which air could
hold at that temperature.
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In formula, for a certain temperature:
Warm air can contain more moisture. Therefore, if the amount
of moisture in the air is constant and the temperature increases,
the relative humidity will decrease.
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In general, the higher the temperature the lower the
moisture level must be in order to reduce deterioration.
The lower the temperature, the higher the permissible
moisture level for safe storage.
In fact, respiration is a self-accelerating process. The
moisture produced during respiration can increase the
moisture content of the product which in turn will
create favorable conditions for fungus growth.
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Temperature variation inside stored products should be
avoided. Locally higher temperatures may occur in the center
of grain stored in bulk in a silo, or be caused by big differences
in day and nighttime temperatures, especially in metal silos.
These so-called "hot spots" can also be caused by insects. If the
product is uniformly dry when put into storage, and is kept dry
and at a constant temperature, damage due to condensation and
transference of moisture will be minimal.
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The safe moisture content for any particular grain may
vary slightly depending on the variety. The above
mentioned safe moisture contents are valid for
temperatures up to about 27 °C.
Cereals dried to 12 - 14% moisture are free from
fungal growth, but still can be very attractive to insects.
In order to slow down insect development the moisture
content should be 9% or less
32

When air is heated it can contain more water vapor, resulting
in a decrease in its relative humidity: with a temperature
increase of 1 °C the relative humidity of the heated air will
decrease by about 4%. Therefore a product dries better in
hotter air.
Warm air is lighter than cold air. So heated air rises by itself
(vertical air current)
A faster air current through the products causes the moisture
equilibrium between product and drying air to be reached
sooner. Remark: when a product has been dried by heating,
after drying it should be allowed to cool to ambient
temperature before putting it into storage.
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Drying systems either rely on near ambient air
temperatures, or high temperature driers. The former is a
relatively slow process whereby the grain is stored in
bins or on the floor and is dried by forcing ambient, or
slightly warmer air through the grain.
Batch and continuous flow high temperature driers rely
on air temperatures of between 40 and 120ºC with a
necessity to cool the grain before storage. Drying
temperatures of grain destined for milling, malting or for
seed are more critical than for other uses.
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A product loses water (it dries out) when the
relative humidity of the drying air is lower than the
equilibrium relative humidity that corresponds with
the moisture content of the product. The larger the
difference between these two relative humidities,
the faster the drying process goes.
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Structural Considerations: Warehouse and Silo
Warehouses
 Bags which may vary in size from 22.7 kg to 1 ton are used.
 Warehouses are much less efficient than bins and higher costs
associated with operating storage, but warehouses are very
useful for higher-valued products.
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SILOS
The design of bins and silos involves bulk materials, and geometric
and structural considerations. The frictional and cohesive properties
of bulk grains vary from one grain to another.
In addition, a given bulk grain’s flow properties can vary
dramatically with changes in numerous parameters such as particle
size, moisture, temperature, and consolidating pressure.
Grain properties such as specific weight, angle of internal friction,
and grain-to-wall friction coefficient should be taken into account.
37

Storage Structure Design
The silo design procedures include selection of the optimum
hopper angles and minimum outlet dimensions
38

The structural design of a silo requires knowledge
of the distribution of pressures and shear stresses
on its walls (caused by the stored grain) and how
that distribution varies during charging, storage at
rest, discharging, and recharging.
Grain settles or packs during storage. Lightweight
grain such as oats may pack to lose as much as
28% of its volume.
39

Grain-storage bins are generally designed as thin-walled cylindrical
shells and typically loaded and unloaded along the line of their
central axis.
The loads exerted by grain on silo structures can be grouped into two
categories: those because of initial fill and those that are due to flow.
When grain is poured into a bin, it forms an angle from the
horizontal called the angle of repose. The outflow hopper at the
bottom of a bin must be cone shaped and have a slope greater than
the angle of repose, or the grain will not flow out. Smaller bins
require a steeper slope because of the greater friction on the sides of
the hopper
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However, during the operation of a grain
facility, certain loading and unloading
conditions can create a non-uniform
distribution of pressure within a grain bin.
The highest asymmetry of bin load is thought
to occur during eccentric (unusual) unloading.
Nonsymmetrical bin loads, which occur during
eccentric discharge, are considered a major
cause of bin failure.
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Grain Handling Systems
en-masse conveyor maximum operating angle is often
considered to be 7°. En-masse conveyors do less damage
to grain than augers and are often recommended in high-
capacity handling situations because of lower power
requirements.
A shrouded conveyor acts more like a drag conveyor,
pulling grain along the inside of an enclosed chamber.
Shrouded conveyors are typically used with operating
angles up to 45°.
42

U-trough conveyors are often used for
horizontal applications for reclaim from bins or
across the roofs of bins. Both tube augers and
U-troughs will do less damage to grain if run
full and at low speeds.
Bucket elevators provide an effective way to
distribute and automate handling of grain. The
power requirement of a bucket elevator depends
on the capacity and product-elevating height.
43

Pneumatic conveyors are used for transporting dry grain away from grain
dryers to storage bins.
Some advantages of pneumatic conveyors are:
 they are self-cleaning,
 they allow transport of different products without contamination;
 dust is totally enclosed except at discharge;
 they are safe (no moving parts other than blower and airlock).
Their disadvantage:
 power requirements greater than other methods,
 noisy to operate;
 and grain damage can occur if the system is not operated properly, (so grain
velocities remain around 762–914 m/min)
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Controlled Atmospheric Storage of Grain
Controlled atmospheric (CA) storage of grains includes
 commodity-modified CA storage and
 artificially modified CA storage.
In case of commodity-modified storage, respiration of the grain and the
microorganisms reduce the O2 and increases the CO2.
The atmosphere in a modified storage is changed by injecting N2 or CO2 into
the system. Nitrogen-producing exothermic generators are available
commercially for altering the intra-granular gas composition in a grain
storage system. Carbon dioxide is another gas that can also be used for CA
storage of grains.
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Grain Inspection
The grain surface should be inspected at least every
other week throughout the storage period. Evidences of
hot spots, insect infestations, or other problems that
start in the grain mass soon migrate to the surface.
Hot spots will be seen as damp, warm, and musty
areas. Insects and mold growth are more likely to show
up where broken corn has accumulated.
46

Aeration
 Aeration reduces or inhibits biological activity by cooling the grain and
preventing moisture migration by maintaining a relatively uniform
temperature throughout the grain mass.
 Power requirements for aeration vary with airflow rate, type of grain, and
the distance the air must travel through the grain. The airflow rate should be
adequate to cool the entire grain mass before deterioration begins.
 The amount of time required for cooling the grain is a function of the
 airflow rate (fan size),
 air temperature, and
 RH of the air.
47

Normal aeration airflow rates range from 1
to 2 L of air per second per cubic meter of
grain (1–2 L/s-m3)
Higher rates should be used (2–6 L/s-m3) if
grain is stored at higher moisture levels or if
a large variance in incoming moisture levels
exists
48

In bins over 900 kg capacity, the grain bulk or mass is
so large that it fails to cool uniformly enough to avoid
storage problems as outdoor temperatures change with
the seasons
The unequal temperature in the grain mass then causes
air current to circulate from warm to cold grain. Since
warm air holds more moisture than cold air, the air
moving up through the warm grain center picks up a
full load of moisture, depositing some as it moves
through the cold grain in the top layer. This causes
moisture buildup, molding, and crusting.
49

Chemical Methods
 Insect infestation in stored grain and grain products can be controlled
effectively by fumigation. Up to nine different chemicals have been used
as fumigants, but currently, only chlorpyrifos-methyl (Reldan) and
phosphine are considered safe.
 Currently, the main chemical option for controlling insects in stored
commodities is the fumigant phosphine. The fumigant phosphine has
provided an important replacement to methyl bromide in several
situations, but phosphine has one important disadvantage
 it requires an exposure period of 5 days or longer, which makes it unsuitable for
quarantine fumigation.
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Fumigants. These are chemicals which are active in
their gas form for killing insects. These are harmful to
seed viability, especially when the moisture content of
the seed is high. Particularly methylbromide and
ethylenedibromide are known for this. Phostoxin is
harmless for sowing seed and also is easier to use in its
tablet form. Because of the toxicity of fumigants
(very poisonous to man and animals), it is advisable
that only experts use them.
51

Rodents Prevention
Rodents: Rats and mice may cause considerable damage to crops
in the field and products in storage. This can occur in various
ways:
consumption of part of the product
contamination of part of the product with their excrement
damage to buildings, storage containers and packing
material
they are also carriers of diseases which are harmful to
people
52

Trapping is the most common method of rat killing.
Mousetraps should be placed at intervals of about 1 m;
rattraps should be set 4.5–9 m apart. Traps are an
alternative to rodenticides, especially where chemicals
cannot be used; however, the use of traps is more labor
intensive than chemicals.
53

Prevention of Damage by Insects
Insects and molds damage the quality of grain directly by their
feeding and development, and indirectly through generation of heat
and moisture. High temperatures and moistures favor development
of insects and molds.
Development of insects is limited by temperatures below 15°C and
by moistures below 9% in cereal grains, whereas development of
molds is limited by temperatures below 10°C and by moistures
below 13% in cereal grains.
Spraying with insecticides or fumigating minimizes insect
problems but leaves chemical residues in grain
54

Insects that can live on grain can be divided into
those that develop within grain kernels (granary weevils, rice weevils,
corn weevils, lesser and larger grain borers, and Angoumois grain moths)
and
those that develop outside the kernels (red flour and rusty grain beetles,
sawtoothed grain beetles, cadelles, khapra beetles, and Indian-meal moths)
Grain-damaging insects multiply slowly or not at all below 16°C,
and they cannot survive in temperatures of 42°C or above. Stored-
grain insects develop well at 27°C–34°C and thrive best at about
29.5°C.
55

Moisture is another important factor in controlling grain
infestation. Generally, moisture contents of 9% or lower
restrict infestation. Even though the grain is stored at relatively
safe storage moisture of 11%–14%, in the presence of insects,
the grain often “heats.”
The heat is caused by the metabolic heat of the insects.
Because of the increased temperature, moisture migration
occurs and results in increased moisture in pockets of grain.
This leads to the growth of microorganisms.
56

Insecticides. Chemicals that kill insects are in
general more harmful; especially lindane may
affect the seed viability.
If one wants to store disinfected seeds, the
moisture content of the seeds must be at the
safe level before disinfection, because
disinfectants in powder form absorb moisture.
57

Prevention of Fungi growth
Fungi can damage the product in a number of ways:
they can produce chemicals called enzymes which may stop
seeds from germinating.
they decrease the quality of the products for food, through
discoloration or change in taste (bad flavor or smell), and
they decrease the nutritive value.
some fungi produce substances which are poisonous to
people and animals.
58

Fungicides: These are chemicals that kill fungi.
Those such as mercury compounds and
dithiocarbamates do not harm the seeds at all.
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References:
 Gavin Owens, (2001). Cereals Processing Technology; Woodhead Publishing Limited and CRC
Press LLC, Boca Raton, pp 25-26.
 Jelle Hayma, (2003), The storage of tropical agricultural products Agromisa Foundation,
Wageningen.
 Rick Hodges and Graham Farrell (2004). Crop Post-Harvest Science and Technology Volume 2,
Durables Case studies in the handling and storage of durable commodities; Blackwell Science Ltd.
 M. Avung`ana Mushira. Manual on Grain Management & Equipment Maintenance in Silos, pp 6 –
64
 Rahman, M.S.(2007). Handbook of Food Preservation. Taylor & Francis Group. pp 74 – 129
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Reading Assignment
1. SILO COMPLEX OPERATIONAL SAFETY
2. SILO OPERATIONAL HAZARDS
Dust Explosion in Silos ???
61

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