Rice Quality Training Manual 09

Rice Quality Training Manual
5.0 Maintaining Quality
The quality and yield of rice is determined by the variety, the crop management, harvesting, processing and handling.
Variety
Different varieties have different physical and chemical characteristics that affect grain quality and yields. The dimension, shape, weight, volume and density of grains determine the physical characteristics of rice and in turn influence head rice yield.
Varieties that:
· Have short and medium type grains, which are more rounded, thicker and harder than long grains produce higher head yield.
· Mature earlier tend to produce less head rice than late maturing varieties.
· Fill uniformly have higher grain density and less chalkiness
· Flower unevenly also ripen unevenly. Non synchronous varieties can have a variation of up to 10% in moisture content and take 5 days longer for the grain to mature at the bottom of the panicle, when compared to the grain in the top of the panicle.
Crop management
The management of the crop will influence the time and uniformity of crop maturity. Basic requirements of good crop management include good water, nutrient, pest and harvest management.
Water management
To be able to manage water, the fields must be level and the bunds or levees well maintained. Uniform water depth across the field will contribute to more uniform crop, higher grain yields and consistent moisture content in the grain sample. Reducing the variation in moisture content at harvest reduces grain fissuring (cracking) and also reduces the chance of spoilage through yellowing and of odors.
Good water management helps reduce weed competition, which not only increases yields but also improves grain quality by reducing dockage levels and reducing moisture
differentials between weed seeds and grain. Wet spots in the grain due to uneven drying or weed seeds can lead to off odors and discoloration of the grain.
Nutrient Management
The application of the correct level and type of fertilizer for the variety and growing conditions is essential. The prudent application of nitrogen is essential to get an even maturing crop with full grain size and high protein levels. Excessive and uneven application of N can stimulate late tiller production, which results in heads on the main culm ripening a number of days faster than the tillers. This will result in more immature and green heads in the sample as well as higher moisture content that increasing the chance of fissuring and spoilage. Conversely insufficient nitrogen can lead to reduced grain size and protein content.
Plant Population
Establishing the correct number of plants is essential to maximize water and nutrient use. A target population that results in 400-500 panicles per m-2 is desirable. This means establishing at least 70-100 seedling m-2 when transplanting, or broadcasting 80-120 kg seed ha-1 when direct seeding.
Low populations may result in:
· increased tillering, which creates more variation in panicle maturity,
· increased weed populations and
· reduces the yield potential of the variety.
High plant populations may reduce yield and quality by:
· competing for water and nutrients,
· mutual shading,
· lodging and
· reduced grain size.
Harvesting
Timeliness
The optimal stage to harvest grain is between 20-25% grain moisture or when 80-85% of the grains are straw colored and the grains in the lower part of the panicle are in the hard doe stage. This is about 30 days after flowering. If the crop is harvested too late, many grains are lost through shattering or drying out and are cracked during threshing. Cracked grains break during milling. If harvested too early, there will be many immature paddy grains and this will reduce head rice yield and quality. The immature rice kernels are very slender and chalky and this results in excessive amount of bran and broken grains. Threshing should occur immediately after cutting as the longer the cut panicles remain in a stack the higher the chance of discoloration or yellowing.
Thresher Machine settings
The correct drum speed settings are needed if grain quality is to be maintained and threshing losses minimized. Drum tip speeds for a peg tooth thresher should be between 12-16 m/sec or approximately 600rpm. Higher speeds result in higher levels of grain damage while lower speeds increase the amount grain retained in the panicle. The fan speed and oscillation speed should be between 800-850 rpm. The clearance between the peg teeth and concave should be about 25 mm.
Processing
Drying
Paddy or rough rice should be dried to 14% moisture as soon as possible after threshing.
Quality can be maintained by drying and tempering the grain a number of times or in stages during the drying process. This means drying the grain on a drying pad for a number of hours or in the grain dryer and then tempering the grain by allowing it to cool down for a number of hours in a bin or in the bag. This process should be repeated at least a couple of times until the grain is at 14% moisture.
When sun drying the grain should be spread in thin layers (50-100mm) on the pad or floor and be turned and stirred 7-8 times per day. This will even up moisture distribution and increase the rate of drying. Temperatures on drying pads can exceed 508C during the day and this can cause fissuring. To reduce fissuring the paddy may need to be covered during the very hot times of the day or dried over a couple of days.
Grain is also dried in mechanical batch dryers. The most common smaller dryers have a capacity of 1-3 tons per day with drying times of 6-12 hours. For drying of paddy in tropical areas an air temperature of 40-458C is normally used with a heater capable of raising the air temperature 10-158C. An air velocity 0.15-0.25m/s is required and typical
power requirements are 1.5-2.5 kW /ton of paddy. The efficiency of these dryers is also improved by stirring the grain.
Cleaning
Threshed grain contains all kinds of trash. This trash can be vegetable, such as chaff, straw, empty grains and foreign seed as well as mineral materials such as earth and stones. Grain should be cleaned as soon as possible after harvesting and certainly before storage. The simple traditional cleaning method is winnowing, which uses wind or a fan to remove the light elements from the grain. Mechanical winnowers that incorporate a fan and several superimposed reciprocating sieves or screens are now used in many countries. These can be manually powered or motorized and have capacities from 100 kg to 2-3 tons per hour. Where combine harvesters are use, there is a trend towards using large capacity centralized seed cleaners. These are normally equipped with a series of vibrating sieves and are capable of 10-30 tons per hour.
Storage
If grain is to be stored safely for extended periods it must
· have less than 13- 14% moisture,
· be protected from insects and rodents and
· be protected from absorbing moisture either through rain or the surrounding air.
Grain is traditionally stored in 40-50kg sacks, which are made from jute or woven plastic. These bags are stacked under a roof or in a shed and must be periodically fumigated to control insects. Some farmers use granaries, which are made from timber or mud/cement or large woven baskets and these also suffer from insect and rodent damage.
Sealed storage is an option that has a lot of potential in the tropics. If grain is dried to 14% and stored in a sealed storage it reduces the risk of insect and rodent damage and the grain should not absorb moisture from the atmosphere or be damaged by rain. Sealed storages come in all shapes and sizes. They may range from a sealed 200-liter drum to the more complex and costly sealed plastic commercial storages. Most large commercial steel and concrete silos being used in western countries can be sealed for fumigation.
Rice Quality Training Manual
Equilibrium moisture content
In storage, the final moisture content of seed depends on the temperature and relative humidity of the air that surrounds the grain. The final grain moisture content resulting from storage is called the ‘equilibrium moisture content’ or EMC. The following table shows the EMC of paddy under different storage conditions. The underlined & colored areas represent the desirable environmental conditions for storage of paddy for food purposes in the tropics. If grain is not protected against humidity in the air, in particular in the rainy season when the relative humidity may reach 95%-100%, grain moisture content will rise leading to quality deterioration.
Milling
It is often said that milling is more an art than a science. Using good quality paddy, in a well-maintained mill, operated by a skilled miller produces high quality head rice. Poor quality paddy will always result in poor quality milled rice irrespective of the type of rice mill and the skill of the miller. Similarly, the use of good milling equipment and good quality paddy will not ensure a high quality product. The best quality milled rice will be attained from a mill that has
· a system of pre cleaning the paddy,
· uses a rubber roller to remove the husk,
· has two separate whiteners and one polisher and
· grades the polished white rice.
Having at least two stages in the whiting process and a separate polisher will reduce over heating the rice during milling and should allow the miller to set the individual machines to give the highest head rice and milling yield. Continual maintenance, monitoring, adjustment and replacement of components such as rubber rollers and sieves is essential to maintain high quality rice.