Paddy Drying Systems By: M Gummert J Rickman Agricultural Engineering Unit IRRI, Los Baños , Philippines

Content Why drying Drying and quality Drying methods Mechanical drying systems Drying strategies

Why Drying? Rice is harvested at high moisture contents >20% Quality deterioration starts immediately after harvest The wetter the grain the faster the loss of quality Different MC for different purposes (see Table) Purpose Required MC Potential problems 2 - 3 weeks storage 14 - 18% Molds, discoloration, respiration loss 8 - 12 months storage <= 13% Insect damage > 1 year storage <= 9 % Loss of viability Milling 14% Damaged grains, cracking

Improper drying, 1 Heat build-up from natural respiration excellent growth conditions for molds and insects Mold development propagate diseases in the grain may release toxins into the grain proper drying and storage can reduce propagation of molds Insect infestation insects are always a problem in stored grain at lower moisture content insect activities are lower proper drying helps keeping insects at acceptable level (4 insects per kg) Heat build-up in the grain: Natural respiration of stored, wet grain will generate heat, in particular when it is stored in sacks or in bulk. Heat will provide excellent growth conditions for molds and insects and thus deterioration in quality. Mold development: Molding of grain will propagate diseases in the grain and may release toxins into the grain. Although some molds may be present in the grain at harvest time, proper drying and storage measures can reduce further propagation of these molds. Insect infestation: Insect infestation is always a problem in stored grain in tropical climates, even if the grain is completely dried. However, the less moisture in the grain, the fewer the expected insect problems. A combination of proper drying procedures and storage practices, including storage hygiene, will keep insect infestation at acceptable levels.

Improper drying, 2 Discoloration/Yellowing heat build-up in the paddy grain before drying drastically reduces the market value of rice Loss of germination and vigor active respiration depletes the nutrition reserves molds and diseases can reduce the ability of the seed to germinate the lower the MC at the beginning of storage, the longer the seed remains viable Discoloration/Yellowing: A general yellowing of the rice grain is a result of heat build-up in the paddy grain before drying. Discolored grain drastically reduces the market value of rice since whiteness is an important quality characteristic for rice consumers. Although discoloration is a complex biochemical process, it can be easily avoided by timely drying of paddy after harvest. Loss of germination and vigor: Moisture in grain will gradually reduce germination ability of the seed during storage. Active respiration of the grain during storage will deplete the nutrition reserves that the seed uses to germinate or sprout. Molds and diseases can also reduce the ability of the seed to germinate. The lower the moisture content of seed at the beginning of storage, the longer the seed remains viable

Improper drying, 3 Loss of freshness/odor development Heat build up -> musty odor in rice. Reduces the market value of rice. If from mycotoxin-producing fungi rice might become unusable. Reduced head rice yield moisture adsorption of individual dry grains with moisture contents below 16% - fissuring mixing dry with wet grains Exposing dry grains to humid air Fissures cause cracking in milling process -> reduced the head rice recovery. Loss of freshness/odor development: Deterioration of quality or aging of stored rice results from a combination of a change in the chemical components (increase in fatty acids and decrease in sugars) and changes of rice kernel characteristics (such as kernel hardness). Heat build up in the grain (above 55ºC) due to insects, molds or high humidity will often lead to a musty odor in rice. Therefore, rice stored for longer periods under adverse conditions (high grain moisture content and/or high temperatures) can develop odors, which reduce the market value of rice considerably. In particular, molds (fungi) that grow on rice can produce offensive odors due to deterioration of chemical components in the rice. If the fungi are of the mycotoxin-producing family, rice is unsafe for nourishment and might be totally unusable for food or livestock feed purposes. Reduced head rice yield: A major cause for fissuring of rice kernels is the moisture adsorption of individual dry grains with moisture contents below 16%. This can happen either when wet grain is mixed with dry grain (in storage, in the dryer or after drying in a batch dryer with a resulting moisture gradient) or when dry grain is exposed to humid ambient air with a relative humidity higher than the equilibrium relative humidity at the corresponding grain moisture content. Fissures in rice kernels usually lead to cracking of the grain during the milling process and thus reduce the head rice recovery.

Equilibrium moisture content (EMC) Rice is hygroscopic Equilibrium moisture content (EMC) If grain is exposed to air for a longer time it will reach EMC Dry grain will adsorb water from humid air Wet grain will dry Grain properties Moisture Content, MC Temperature Air properties Relative Humidity, RH Purpose Required MC Potential problems 2 - 3 weeks storage 14 - 18% Molds, discoloration, respiration loss 8 - 12 months storage <= 13% Insect damage > 1 year storage <= 9 % Loss of viability Milling 14% Damaged grains, cracking

Drying methods

Field drying Why field drying? Waiting for the thresher Manual threshing How? Spreading the crop in the field Stacking/piling Advantages Can reduce MC by 1% per day Disadvantages Rapid quality deterioration Shattering Losses to bird and rodents when spread in the field Heat build up and rapid quality deterioration in piles Re-wetting from straw in piles In many traditional harvesting systems farmers leave their harvested rice in the field for extended time because they are either waiting for the thresher or because they want to pre-dry the paddy. In this practice, often referred to as field drying, the rice plants are often stacked in piles with the panicles inside to protect them from rain, birds and rodents, a practice that can lead to massive heat build up inside the stacks. As a result molds grow quickly and infest the grains and discoloration usually develops within the first day of field drying. Another unwanted effect is that the relatively dry grains often absorb water from the wetter straw, which leads to fissuring of the dryer grains and thus reduces the potential head rice recovery. It is impossible to produce good quality grains with field drying practices and field drying should therefore be avoided. It is impossible to produce good quality grains with field drying practices. Field drying should therefore be avoided.

Panicle drying Traditional method harvested with a small knife (ani ani) paddy grains that are still attached to the panicles for drying small amounts of paddy. stored in farmers’ houses, e.g. under the roof for protection from rodents. Problems low capacity grains inside the panicle dry slower than the grains that are exposed directly to the sun. Tips turning of the panicles improves the drying process Drying of paddy grains that are still attached to the panicles is a traditional method for drying small amounts of paddy. The panicles are harvested with a small knife, bound together and carried to the drying location. The dried panicles are then stored in farmers’ houses, for example by hanging them under the roof for protection from rodents. For drying the tied bundles are placed on pavements or mats or hung from frames. The grains inside the panicle dry slower than the grains that are exposed directly to the sun. Turning of the panicles improves the drying process but “mixing” of the grains is not possible.

Mat drying Paddy can be placed on nets, mats or plastic sheets (canvas). Advantages Most hygienic method. less contamination with stones and other dirt Easy collection in case of sudden rainfalls. Easy mixing Disadvantages of nets danger of re-wetting of the bottom grains from soil moisture Small to medium amounts of paddy can be placed on nets, mats or plastic sheets (canvas). Except for nets (which allow dirt to enter the grain) this is the most hygienic method. Paddy dried on mats or canvas does not contain stones and other dirt, often found in paddy that was dried on roads. When drying is finished or in case of sudden rainfalls the grains can be collected quickly by lifting the mats at the edges. Mixing can also be done easily that way. Drying of high moisture grain might initially be a bit faster on nets because the water can also migrate to the bottom instead of condensing on a waterproof plastic sheet. But at lower moisture contents there is the danger of re-wetting of the bottom grains from soil moisture when nets are used.

Pavement drying Better-off farmers, grain collectors, traders and millers use drying pavements specifically constructed for drying multi purposes (basketball court) Advantages high capacity / economics of scale can be partially mechanized tools for mixing and grain collection larger mills often use two or four wheel tractors Disadvantages capital requirements for the pavement pollution with stones and dirt Better-off farmers, grain collectors, traders and millers often dry their paddy on pavements, either specifically constructed for drying or also used for other purposes (e.g. as basketball court). Sun drying pavements have the advantage of high capacity and thus economics of scale and the drying operation can be partially mechanized. Usually manual tools are used for mixing and grain collection but larger mills often use two or four wheel tractors for this purpose. There is a significant capital requirements for the pavement and if cleanliness is not observed pollution with stones and dirt is common.

Sundrying and quality Layer thickness 74 76 78 80 82 84 86 88 90 1 2 3 4 5 6 7 Layer thickness, cm Recovery, % 3.5 4.5 5.5 6.5 7.5 8 Drying Time, h Head rice Milled rice Drying time Source: IRRI, (Gayanilo) Location: Philippines Intital M.C. = 24% Final M.C. = 14% 60 65 70 75 80 85 90 1 2 3 4 5 6 7 Mixing interval, h Recovery, % 5.25 5.5 5.75 6.25 6.5 Drying Time, h Head rice Milled rice Drying time Source: IRRI, (Gayanilo) Location: Philippines Intital M.C. = 24% Final M.C. = 14% Layer thickness Spread the grains in thin layers, ideally 2-4 cm. Too thin layers -> heat up very quickly Too thick result in a large moisture gradient Mixing interval mixing the grain is the most important activity for maintaining good quality Turn or stir the grain at least once per hour, better every 30 minutes to achieve uniform MC.

Tips for better sundrying Management Layer depth of 4cm Mixing every 30 minutes Monitor moisture content Monitor temperature Protection Cover the grain when temperature rise above 50-60ºC Cover during rain. Prevent contamination keep animals off the grain Use tools to improve sundrying

Heated air drying Components of a dryer Main components Drying bin Air distribution system Fan Air heater Optional Conveyors Accessories Moisture meter Dust separator

Options for heated air drying Fixed bed batch dryer Re-circulating batch dryer Continuous flow drying plant Tempering Section Drying Section Dryer Tempering bins In Out Air Grain

Fixed bed batch dryer (1) Flat bed dryer Key features Batch Capacity: 1-10t Drying time 6-8h Approximate prices: US$ 1000-2000 Kerosene or rice hull fired Advantages Simple and affordable Disadvantage Labor intensive Moisture gradient Temperature control

Fixed bed batch dryer (2) Low cost batch dryer Key features Batch Capacity: 0.2-2t Drying time: 1-2 days Approximate prices: US$ 100-200 Wood, coal or rice hull fired Advantages Simple and affordable Very cheap Uses local storage structures Disadvantage Labor intensive Moisture gradient Temperature control

Fixed bed batch dryer (3) Reversible air flow dryer Key features Same as flat bed dryer Airflow is reversed after ½ to ¾ of drying time was completed Advantages Minimized moisture gradient Less labor intensive since mixing is eliminated Disadvantage Additional cost

Re-circulating batch dryer Key features Batch Capacity: 4-12t Drying time: 8 hours Approximate prices: US$ 8,000-10,000 Kerosene fired Advantages Automatic operation Produces excellent quality Little floor area Disadvantage Wear of conveying elements Problems with very wet paddy

Mechanical drying Methods Heated-air drying Low-Temperature Drying Drying Zone Drying Zone Dry Grains Wet Grains Drying air temp.: 43ºC Air velocity: 0.15-0.25 m/s Airflow rate per t grain: >0.7 m³/s Power requirement: 1.5-2.5kW/t grain Layer depth: < 40 cm Drying time: 6-12 h Initial MC: up to 30%+ Drying air temperature: Δ T = 0-6 ºK Air velocity: 0.1 m/s Airflow rate per t grain: >0.05-0.4 m³/s Power requirement: 0.05-0.15 kW/t grain Layer depth: < 2 m Drying time: days to weeks Initial MC: 18% ( 28%) Advantages: Simple management Fast drying Affordable Low level of integration Disadvantages: 3-4% moisture gradient in final product, requires mixing or reduced layer depth Reduction in milling yield Danger of killing seeds Advantages: Very energy efficient Bins can be filled at harvest rate Maintains grain quality optimally Drying in storage structures Disadvantages: Increased risk with poor power supplies Requires bulk handling system (high level of integration in postharvest system) Long drying time

Other drying systems First stage dryers Fluidized bed dryer Rotary drum dryer Low-temperature dryer (often second stage dryer) In-store dryer Aeration facilities

Flash dryer Fluidized bed dryer from Thailand Principle Grains are pre-dried quickly in a fluidized bed As a first-stage dryer in a two-stage drying strategy Key features Batch Capacity: 4-12t Drying time: 10-15 minutes Air temperatures: 110-120°C Air velocity: 2.3 m/s Advantages Very fast pre-drying High capacity Disadvantage For pre-drying to 18% MC only High energy requirement Fluidized bed dryer from Thailand

In-store dryer Principle Slow EMC based drying with ambient air or slightly pre-heated air Key features Batch Capacity: 1… x.000 tons Drying time: 4 days to 2 weeks Air temperatures: ambient, 3-6°C above ambient Air velocity: 0.1 m/s Advantages Produces very high quality Low energy requirement Drying in storage bin Disadvantage High risk if MC is > 18% Long drying time Drying Zone Dry Grains Wet Grains

Drying Strategies Decentralized on-farm drying Requires quality incentive Low utilization of equipment Training and technical support service Centralized drying Contractors (service providers) Mills Economics of scale Two-stage drying Ideal process to produce best quality Two machines are needed for one operation First stage dryer dries only to 18%

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