🏠Grain Storage and Processing

India loses an estimated 7-10% of its food grain production to post-harvest spoilage — primarily due to improper drying and inadequate storage. From the humble mud-walled Pusa bin to modern reinforced concrete silos, the right storage structure at the right moisture content can prevent millions of tonnes of grain from going to waste. This chapter covers the complete chain from drying to storage.

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• Concrete is usually defined as a mixture of cement, sand, coarse aggregate (gravel) and water in correct proportions and thoroughly mixed so that upon setting, it forms a rock-like mass. Concrete is the most widely used construction material for storage structures due to its strength, durability, and resistance to pests and moisture.

• Slump test is performed to measure the plasticity of concrete. A higher slump value indicates a more workable (fluid) concrete mix, while a lower value indicates a stiffer mix. The test helps ensure the concrete has the right consistency for the intended application — for example, foundations need a lower slump than columns.

• Curing of concrete is the process of strengthening it through hydration (7 days). During curing, the concrete surface is kept moist so that the cement continues to react with water, gaining strength over time. Proper curing for at least 7 days is essential for achieving the desired compressive strength — inadequate curing can reduce strength by up to 30%.

• Mortar: Mixture of cement, sand and water, or lime, sand and water. Mortar is used as a binding material in brick and stone masonry for constructing storage bins and godowns.

• Compressive strength of concrete ranges between 300 to 700 kg/cm2. The actual strength depends on the mix ratio, quality of materials, and curing conditions.

• Seasoning of wood is primarily to rid of all the surplus sap and moisture, to avoid decay or being affected by fungus. Properly seasoned wood is stronger, lighter, and more resistant to insect attack — making it suitable for constructing traditional storage structures and dunnage in godowns.

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• Rankine formula/Airy formula is used for determination of lateral pressure in shallow bins. In shallow bins, the grain exerts pressure on the walls similar to a fluid, and these formulas help engineers design walls strong enough to withstand that pressure.

• Janssen formula is used for determination of lateral pressure in deep bins. In deep bins, friction between the grain and the bin wall supports part of the grain weight, so the lateral pressure does not increase indefinitely with depth — Janssen’s formula accounts for this important friction effect, making it the standard for silo design worldwide.

• LSU continuous flow dryer is most commonly used in India, developed at Louisiana State University, Baton Rouge, USA in 1949. It remains popular because of its simple design and effectiveness in reducing grain moisture to safe storage levels.

• Aero and hydrodynamic properties such as Drag Coefficient and Terminal Velocity of agricultural products are important and required for air and water conveying systems and separation equipment. Terminal velocity is the speed at which a grain particle falls through air at a constant rate — it is used to separate lighter impurities from heavier grain in cleaning machines.

• Fourier law is used for heat transfer through conduction. Understanding heat transfer is important in storage engineering to predict how temperature changes inside a storage structure, which affects grain quality and insect activity. Hot spots in stored grain promote insect breeding and mold growth.

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• Moisture content (M.C.) of some of the crops at harvest and for safe storage:

Maintaining the correct moisture content is critical — too much moisture promotes mold growth and insect infestation, while too little can cause grain cracking and weight loss. Grain must be dried to the safe moisture level before storage to prevent spoilage.

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• Angle of repose is the angle between the base and the slope of the cone formed on a free vertical fall of the granular material to a horizontal plane. A tilting top drafting table and circular platform are used for measurement of angle of repose. This property is important for designing storage bins, hoppers, and conveying systems — the bin walls and hopper slopes must be steeper than the angle of repose to ensure the grain flows out freely under gravity.

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• Refrigeration — the process of removal of heat from a space at a temperature lower than the surrounding temperature. The principle of refrigeration is based on the second law of thermodynamics. Refrigeration is used in cold storage of perishable agricultural produce like fruits, vegetables, and dairy products to extend their shelf life by slowing down biological processes.

• Latent heat of fusion of water at atmospheric condition is 80 kcal and Latent heat of vaporisation of water at atmospheric condition is 540 kcal. These values are fundamental in calculating the energy required for freezing and evaporative drying processes. Remember: it takes much more energy to vaporize water (540 kcal) than to freeze it (80 kcal).

• Scalping refers to the removal of a few large particles in an initial process, e.g. scalper. Scalping is the first step in grain cleaning, removing large foreign materials like straw, stones, and sticks before further processing.

• Cleaning process generally means the removal of foreign and undesirable matter from desired grain/products as much as possible to ensure its purity, e.g. cleaner. Cleaning is done on the basis of size, weight and length of impurities. Clean grain fetches better market prices and stores more safely.

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Types of Impurities

• Large impurities — straw, panicles, soil clumps, stones, bag strings, iron parts
• Small impurities — dust, sand, soil particles, weed seeds, insects and small stones
• Same size as grain — empty grains, broken grains, shriveled seeds

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Cleaning of impurities

• Light impurities (in weight) are removed by Aspiration (air blowing) or by sieving. The air stream blows away lighter materials while heavier grain falls through.

• Small, large & heavy impurities are removed by Sieving. Different screen sizes separate impurities based on their dimensions.

• Same size but heavier impurities are removed by Gravity separation. This method exploits differences in density between grain and impurities.

• Same size & weight impurities are not easily removed — these require specialized techniques or may not be separable with standard equipment.

• Weed seeds are removed by sieving based on their size differences from the main grain.

• Iron particles are removed by Gravity separation or Electromagnetic separation method. Magnets or electromagnets attract and remove metallic contaminants effectively.

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Air Cleaning Machine (ASM)

• Cleans the grain based on size and weight:
  • ASMs perform absorption, main scalping and grading.
  • Absorption — Light weight materials are removed by air suction.
  • Main scalping — Grains of other crops are removed by screen due to grain shape differences.
  • ASMs have 2-8 screens.
  • 3 screens clean small impurities, but 4 screens are used for desired and more purity. More screens mean a cleaner final product.

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Types of ASM

• Open double sieve cleaner — open type, 2 oscillating sieves, 300-400 rpm of transmission shaft. Simple and widely used for basic cleaning.

• Self-cleaning sieve — wooden frame, compartment (20cm x 20cm or more), 2 rubber balls of 1.9 cm and 3.2 cm. The rubber balls bounce during operation and knock grain particles out of the sieve openings, preventing clogging — a clever mechanical solution to a common problem.

• Single action sieve — a basic sieve-type cleaner.

• Single action Aspirator cleaner — one fan or blower, closed type of cleaner. Combines air cleaning with sieving for better results.

• Double action Aspirator cleaner — two stages of air cleaning for more thorough impurity removal.

• Single Drum type cleaner — developed by Japan, for stone separation by gravity. Particularly effective at removing stones that are similar in size to grains — a problem that sieving alone cannot solve.

• Double Drum type cleaner — provides two stages of drum cleaning for higher purity.

• Separation refers to the sorting of cleaned products (grain) according to various physical characteristics i.e. on the basis of size, shape, weight, electrical conductivity, density, texture and colour, from inert material and other crop seeds, e.g. separator.

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Principle of Separation of Grain/Seed

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Grading

• Grading refers to the removal or separation of smaller (under size), over size (bigger) and shriveled grains from main/pure/well fitted (healthy) grain of the same crop depending upon the various commercial values and other uses, e.g. grader. Grading adds value by sorting grain into uniform quality categories that command better market prices.

• More effective in mixture of grains or mixture of same crop varieties.

• Based on length and shape both (Cleaning + Separation).

• ASM used for grading of grains due to air current by fan.

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GRADERS

• Same length but different thickness — Rotating graders are used. They separate grains based on their cross-sectional size.

• Different length but same thickness — Trieurs are used. They exploit length differences to separate short grains from long ones.

• Different length and different thickness both — Rotating graders + Trieurs combination provides complete separation.

• Mixture of different varieties with same length & thickness — Grading Impossible. When grains are identical in both dimensions, mechanical separation is not feasible.

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Rotating Grader

• Also called “Grading reels”.
• It grades on the basis of thickness. The rotating cylinder has perforations of specific sizes — grains smaller than the perforation pass through while larger grains travel forward along the cylinder.

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Trieurs

• A rotating indented steel cylinder machine for grading of grains on the basis of length.

• It has thousands of pockets (2-8mm) for uploading mixed grain (short & long). Each pocket is sized to pick up grains of a specific length.

• During rotation of cylinder (50 rpm), each pocket uploads with grains and short grains fall from a higher point and long grains fall first from pockets. This is because short grains fit entirely within the pocket and are carried higher before falling out, while long grains protrude and drop out sooner due to gravity.

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Sieves

• Small broken grains are separated when mixed with short, medium but rounded variety. Sieves separate based on the width or cross-sectional dimensions of the grain particles.

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Dryers

• Drying refers to removal of moisture from grains and other products to a predetermined level in equilibrium with the surrounding air (10 to 12 per cent normally). Proper drying is the single most important step in ensuring safe, long-term grain storage. Inadequately dried grain is the leading cause of post-harvest losses in developing countries.

• Recirculatory Batch Dryer (PHTC dryer, developed from IIT Kharagpur, 2 tons capacity). In this dryer, grain is circulated repeatedly through the drying zone until the desired moisture level is reached, ensuring uniform drying.

• Louisiana State University Dryer (popular in India & USA)

• Size of rectangular bins: (1) 1.2m x 1.2m (2) 1.5m x 1.5m (3) 1.8m x 1.8m (4) 2.1m x 2.1m
• Air distribution from ‘V’ shaped channels. These channels ensure uniform airflow through the grain bed for even drying across the entire bin.

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• Conditioning is the process of maintaining uniform moisture and temperature of stored agriculture produce by moving a small amount of air through it. Conditioning prevents moisture migration within stored grain that could lead to hot spots and spoilage, especially during seasonal temperature changes.

• Moisture measurement method:

  • Oven dry method — 130 degrees C +/- 2 degrees C for 1 hour. This is the most accurate standard method used in laboratories.
  • Chemical method — use of Calcium Carbide. When calcium carbide reacts with moisture in the grain, it produces acetylene gas — the pressure of this gas indicates the moisture content.

• Equilibrium Moisture Content (EMC): When a grain (product) is exposed to a continuous supply of air at a constant temperature and humidity under partial pressure, grain loses or gains moisture from the surrounding air, until the vapour pressure of grain and air come to equal. At this point, the grain is in equilibrium with its environment and will neither gain nor lose moisture. EMC is a critical concept for determining safe storage conditions.

• Henderson equation is the most popular equation for determining EMC.

• EMC curve or Isotherm: a plot of EMC and ERH (Equilibrium Relative Humidity) at a particular temperature (25 degrees C). This curve is essential for predicting the safe storage conditions for any grain.

• Grain isotherm is commonly in S-shaped (sigmoid). The S-shape reflects different moisture binding mechanisms at different humidity levels — tightly bound water at low humidity, capillary water at medium humidity, and free water at high humidity.

• Henderson equation of grain isotherm:

  1-RH = e^-CTMn

• Where,

  • RH = Relative humidity
  • C = grain constant (Kelvin)
  • T = temperature of air (degrees K)
  • M = EMC of grain on dry basis (per cent)
  • n = grain constant (dimensionless)

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• Psychrometric chart is a graphical representation of thermodynamic properties of air at 1 atm pressure. It shows the relationships between temperature, humidity, dew point, and enthalpy — all crucial for designing drying and storage systems. Engineers use this chart to determine the drying potential of air at any given condition.

• The thickness of grain in thin-layer drying is normally up to 15 cm.

• The deep bed drying grain depth ranges from >15 to < 45 cm. Deep bed drying is more common in practice, where air is forced through a thicker layer of grain using fans.

• Grain handling mechanical devices:

  • Belt conveyor — used for longer distance horizontal transport of grain. Belts are gentle on the grain and can handle large volumes.
  • Bucket elevator — very efficient for vertical conveyance (<50 meter). Buckets scoop grain at the bottom and discharge at the top, making it ideal for filling tall storage bins and silos.
  • Screw conveyor — used for short distance transport. A rotating helical screw moves grain through a trough. Simple and compact but can cause some grain damage at high speeds.

• Triple point of water lies at 0 degrees C and 4.59 mm of mercury. At this point three phases of water viz. solid (ice), liquid water and water vapour coexist at equilibrium. This concept is fundamental in understanding phase changes during drying and freezing processes.

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Storage

• Grain is generally stored either in bags or in bulk. Bag storage allows easy identification and handling of individual lots, while bulk storage is more space-efficient for large quantities.

• A combined system of bag-cum-bulk storage is also practiced in some parts of the country.

• In villages the bulk storage system is more common than the storage in bags which is considered to be a practicable method of storing grain in the government godowns as well as in trade.

• Main three types of storage structures for storage of grains:

  • Traditional storage structures — locally made, small capacity, using indigenous materials.
  • Improved storage structures — upgraded versions of traditional structures with better materials and techniques.
  • Modern storage structures — engineered facilities like silos and warehouses.
  • Farm Silos — specialized structures for storing animal fodder (silage).

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Traditional Storage Structures

• In these types of storage structures the grain is generally stored in bulk.

• These types of storage structures generally have capacities between 1 to 50 tonnes.

• The storage of grain is generally done in one of the following storage structures in the different rural and urban regions of India in bulk as well as in bag storage:

  • Morai (Eastern & Southern India) — an elevated bamboo or wooden structure that keeps grain above ground, protecting it from moisture and rodents.
  • Bukhari — a cylindrical mud structure.
  • Kothar — a large mud-walled structure.
  • Mud kothi — made of clay and straw.
  • Kuthla, Muda (Bihar) — traditional Bihar grain stores.
  • Kanaj (Karnataka, Maharashtra) — bamboo or cane basket structures.

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Improved Storage Structures

• Improved storage structures are the storage structures for storage of food grains.

• In this type of storage structures there are some improvements made in traditional storage structures, such as better materials, moisture-proofing, and insect-proofing.

• This type of storage structures have a higher storage capacity and allow long-term storage of food grains compared to traditional storage structures.

• Improved type of storage structures have capacities generally 1.5 to 150 tonnes.

• The storage of grain is generally done in one of the following storage structures in the different rural and urban regions of India in bulk, bag as well as bag and bulk storage.

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Pusa bin

• Pusa bin is like other traditional storage structures made of mud.

• To make the storage structure moisture proof, a plastic film is used in all the inner sides of the bin. This simple modification dramatically improves the storage life by preventing moisture ingress and insect entry. The Pusa bin was developed by the Indian Agricultural Research Institute (IARI), New Delhi and is one of the most cost-effective improved storage solutions for farmers.

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Brick and cement bin

• These type of storage structures are very strong and the effect of seasons on these is minimum. They provide excellent protection against moisture, pests, and rodents due to the durability of brick and cement construction. They are more permanent and require less maintenance than mud-based structures.

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Bunker Storage

• This type of storage structure is used for long term storage and a larger volume of grains storage. Bunker storage is particularly useful for government and cooperative agencies managing large grain surpluses during procurement seasons.

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‘CAP’ Storage structures

• The word ‘CAP’ is used for Cover and Plinth — plinth from the bottom and cover from the top.

• This type of open storage is considered as transit storage and serves the purpose of storage of food grains in bags for a short period. CAP storage is widely used by the Food Corporation of India (FCI) when godown capacity is insufficient to accommodate the entire procurement.

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Modern Storage Structures

• In India, for larger volume of food grains to be stored in bulk, ‘silo’ and conventional godowns (Shed) designed for bagged storage are used.

• The godowns have side walls of brick or stone masonry and sloped roofing in asbestos or Corrugated Galvanized Iron (CGI) sheets over steel trusses.

• Silos are constructed from steel or reinforced concrete. They offer superior protection against moisture, pests, and rodents, and allow mechanized loading and unloading. Silos are the gold standard for grain storage in modern agriculture.

• There are a cluster of adjoining silos in any modern large-capacity processing plant.

• The modern permanent storage system should be selected for the safe keeping of stored grains and other products.

• The modern storage structures should be selected on the basis of first on quality and then on cost considerations. This principle ensures that grain safety is never compromised for short-term savings.

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Types of modern storage structures

• Silo type of storage structures
  • Silos/bins are classified into two groups depending upon the relative dimensions of the container. These are classified as:
    • Deep bins
    • Shallow bins

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Shallow bins

• A bin whose relative dimensions are such that the plane of rupture meets the grain surface before it strikes the opposite side. In simple terms, the bin is wide relative to its height.

• Squat silos come under shallow bins. A squat silo has a wall height to diameter ratio of 0.5 or even less. Squat silos can compete with sheds for low-cost quality storage because they require less wall material per unit of stored grain.

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Deep bins

• A bin whose relative dimensions are such that the plane of rupture meets the opposite side before it emerges from the grain. These bins are tall relative to their width and store grain more compactly.

• Vertical Silos come under this type of storage structures.

• There are two types of vertical silos:

  • Flat bottom vertical silo — requires mechanical sweeping equipment to empty completely because grain does not flow out from flat surfaces on its own.
  • Hopper bottom vertical silo — has a conical bottom that allows grain to flow out by gravity, making it easier to empty completely. The hopper angle must be steeper than the grain’s angle of repose.

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Shed

• Generally, horizontal sheds have been used to provide low-cost, large volume storage.

• For storing grains and other products, very large volume sheds have also been constructed by Central Warehousing Corporation. These sheds can store thousands of tonnes of bagged grain and are the backbone of India’s public grain storage system.

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Farm Silos

• Farm silos are farm structures used to store and protect animal fodder so that it is preserved in an ideal condition for farm animals. They are different from grain silos — farm silos are specifically designed for preserving green fodder.

• Animal fodder is cut and packed in the air tight silo to allow a partial fermentation to occur. This controlled anaerobic fermentation preserves the nutritional value of the fodder by lowering the pH through lactic acid production.

• The stored fodder is known as silage. Good quality silage retains much of the green fodder’s protein and energy content and can be fed to animals during lean periods when green fodder is not available.

• There are two types of farm silos:

  • Tower silos — vertical cylindrical structures.
  • Horizontal silos — trench, pit, or bunker type structures.

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Tower silos

• Cylindrical shape and made of masonry, wood or metal.
• Cost of construction is comparatively much higher than that of horizontal type.
• Loading of animal fodder is difficult due to the height.
• Mechanical loader or a large capacity blower is essential for filling. However, tower silos offer better preservation due to lower surface-to-volume ratio, which means less silage is exposed to air and less spoilage occurs at the top.

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Horizontal silos

• In horizontal silos, pit type, bunker type and trench or stake type of storage structures are used for storage of animal fodder.

• There are surface as well as below ground (underground) types of storage structures used on most dairy farms as temporary and permanent storage structures for silage.

• The spoilage of silage and dry matter losses of these silos ranges between 20 to 30 percent. Proper sealing and compaction can significantly reduce these losses. The key to minimizing spoilage is to exclude air as much as possible during filling and sealing.

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Pit Silos

• Permanent pit silo is a circular deep well which is lined all around the side, and sealed from bottom, so that water may not rise into it.

• Made in areas where the soil is deep and the water table is very low. High water tables would flood the pit, spoiling the silage.

• Made of bricks, stones or concrete, and either cement or lime can be used as a binding material.

• A 22.5 cm thick wall will be used satisfactorily up to 15 meter depth.

• The entire surface which is coming in contact with the silage should be plastered to make it smooth, air tight and water tight. This prevents air infiltration that would cause aerobic spoilage.

• A simple roof is made over the silo to protect the silage from sun and rain.

• Corrugated metal sheet dome or half pitch roof with ample overhang on all the sides are most economical and provide more space for filling.

• Stairs may be built along with the wall for removing silage from the silo.

• The diameter of a silo is usually limited to 6 m and its depth is kept 2 to 3 times that of diameter.

• When the silo is opened for removing the silage, nobody should enter till the gases are removed. Fermentation produces carbon dioxide and other toxic gases that can be lethal in an enclosed space. Always ventilate the silo thoroughly before entry.

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Trench Silos

• Unlined trench silo can be made easily without involving any investment on building materials such as brick, cement and sand. This makes it the most economical option for small farmers who want to preserve green fodder.

• Unlined silos give more spoilage and are likely to have caved side walls due to excessive rain and tend to become muddy at the bottom. So, lined trench silos have therefore become popular for better preservation.

• The walls of the trench silos can be lined with brick, concrete or cement plaster with reinforcing wire mesh. Lining greatly reduces wall collapse and moisture problems.

• If possible, the silo should be roofed.

• Drains should be made around the trench to intercept surface water and prevent flooding.

• To facilitate drainage, it is desirable to locate the trench silo on sloping ground.

• Capacity depends on size of herd and number of days the silage is fed in a year.

• It is always economical to construct only one trench silo, even if it is quite large. This minimizes the total surface area exposed to spoilage.

• Sidewalls are given generally 33 per cent slope to prevent collapse and facilitate compaction during filling.

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Modified Storage Systems

• MAP (or MAS) is Modified Atmosphere Packaging in which we can change the composition of gas in a container with a fixed gas mixture. After doing so we do not have any control over the process and the gas composition changes inevitably in the container due to activities such as product respiration. MAP is simpler and cheaper but less precise than controlled atmosphere systems.

• Controlled Atmosphere Packaging (CAP) (or CAS) — we have control over the storage atmosphere (which is in fact air tight). As the desired gas composition (i.e. high carbon dioxide level and low oxygen level) changes due to metabolic activity of fruits and vegetables, it is possible to add fresh air or nitrogen to achieve pre-determined (desired) gaseous composition. CAP is more sophisticated and is used for high-value produce like apples and pears that need to be stored for months.

• The normal composition of air is 78% nitrogen and 21% oxygen, with the balance made up of carbon dioxide (0.035%), other gases and water vapour.

• An increase in the proportion of carbon dioxide and/or a reduction in the proportion of oxygen within specified limits maintains the original product quality and extends the product shelf life. Low oxygen suppresses respiration and microbial growth, while elevated CO2 further inhibits spoilage organisms.

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• The plate type heat exchanger is mostly used in large size dairies. Its compact design provides efficient heat transfer for pasteurization and cooling of milk with minimal space requirement.

• Brine — Concentrated CaCl or NaCl solution — is used for cooling water or milk directly. Brine has a lower freezing point than water, allowing it to remain liquid at sub-zero temperatures for effective cooling of dairy products and other perishables.

• Solid CO2 is called Dry ice. Dry ice sublimes directly from solid to gas at -78.5 degrees C and is used for rapid cooling and freezing of perishable products during transport where mechanical refrigeration is not available.

• Two most commonly used refrigerants are Ammonia and Freon-10. Ammonia is preferred in large industrial systems for its efficiency, while Freon variants have been commonly used in smaller units (though many are being phased out due to their ozone-depleting potential under international agreements like the Montreal Protocol).

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Summary Cheat Sheet