The importance of harvesting and postharvest management is underscored by the fact that large losses of crop yield occur due to poor handling techniques. Therefore, harvesting, drying, milling and storage should receive serious attention.
i. Harvesting and Threshing:
In tropical Asia, rice harvesting is carried out by manual labour. Rice plant is cut with a sickle leaving 10-15 cm stubble and dried in the field for 2-3 days or carried away from main field to threshing yard. In some areas, only panicles are removed. On the other hand, combine harvesters are used in some countries with mechanised agriculture.
Time of Harvest:
Many factors must be considered to obtain optimum rice harvest. The grain must be mature, high in quality and have proper moisture content. Field should be sufficiently dry to support harvesting and transport. It is essential to harvest the crop in time to minimisse the losses due to rats, birds and from shattering and lodging.
The losses may be between 5 and 10 per cent if not harvested until the crop dry to 14-16 per cent moisture before harvest. Timely harvesting ensures good grain quality, high market value and improved consumer acceptance.
Farmers judge harvest time by examining the percentage of ripened grains in the panicle. The crop should be ready for harvest when 80 per cent of the panicles are straw coloured and the grains in lower portion of panicles are in hard dough stage. If the crop remains in the field too long and dries to less than 18 per cent moisture, the cyclic moisture content due to day-night humidity differences will cause fine cracks called sun cracks or stress cracks that will reduce head rice yield and increase broken kernels at milling.
Optimum time of harvest for lowland rice is between 28 and 34 days after heading during dry season and 34 and 38 days after heading during wet season. From India, Govindaswami and Ghosh (1968) reported that harvesting between 27 and 39 days after flowering at high moisture context (18-23%) gave maximum head nee recovery.
In general, rice should be harvested at 18-22 per cent moisture or when grains on lower part of the panicle are in hard dough stage. Crop on sandy loan soils may be drained 10 days ahead of targeted harvest date, while on clay and clay loam soils 15 days ahead of harvest.
Threshing:
Methods for rice threshing vary from region to region. Commonly used methods are grouped into three: manual, animal and mechanical.
Common method of grain separation from panicle is by impact, which may be by hand beating, treading or mechanically by a rotating drum with spikes or rasp bars. Hand threshing is usually by beating small bundles of harvested crop on hard platform. Occasionally, grain is spared from straw by having animals tread on the harvested crop.
Mechanical threshers have gained prominence in many countries in the recent past due to the problems associated with timely availability of labour and wages to the labour. Many big farmers use combine harvesters owned by individuals who provide custom threshing services to the fanners.
In India, the hire charges range between 1,200 and 1,500 hr-1, depending on the demand for the combiners. Power operated threshers are commercially available in the country for small and marginal fanners. However, they are not economical to such farms, since threshing is usually by the family members.
Cleaning and Grading:
Cleaning the threshed produce (unhusked rice) is necessary to remove foreign seed, chaffy grains and trash for improving storability and milling quality. Cleaning is mostly by hand winnower, which takes advantage of wind. Other cleaners include winnowing basket, wooden or metal boxes with perforations and a combination of the above.
Winnowing fans have become popular in rice regions. The produce can be cleaned after threshing of crop without depending on the wind. Winnower employing screen separation and air are used to separate the light seed. The Hoshangabad winnower developed in Madhya Pradesh is the oldest design of manually operated machine used in central and eastern India. The Central Institute of Agricultural Engineering (CIAE) developed the pedal operated cleaner, which was later on motorised. The winnowers have 100-150 kg hr-1 capacity and are low cost machines.
For seed processing, cleaners and graders based on western designs are used on large scale. Quality cleaning involves removal of trash, foreign material, broken and shrivelled grains. Seeds of uniform size are obtained with graders of oscillating screen type, disc type or indented cylinder type. The Pektus and Crippen type oscillating screen cleaner-graders are most popular.
Several cleaners and graders indicated below have been developed in the country:
1. CIAE hand operated double screen grain cleaner with a capacity of 150 kg hr-1.
2. CIAE pedal operated air screen grain cleaner with a capacity of 350 kg hr-1.
3. TNAU paddy winnower with a capacity of 750 kg hr-1.
4. TNAU power operated seed cleaner and grader with a capacity of 2.0 t hr-1.
5. CFTRI stone separator with a capacity of 400 kg hr-1.
Drying:
Rice is, normally, harvested at moisture content around 20 per cent. Harvested produce should be dried to 14 per cent or lower moisture content within three or four days to prevent mold and fungi growth. Milled grain discoloured by mold or fungus is called heat damaged or stack burn grain, which is obviously of low quality. Rice dried too quickly will fissure and thus be more likely to break during milling, hence reducing head rice yield.
In most of Asia, rice grain is commonly dried in the sun. In general, 4-5 days sun drying is necessary to reduce the grain moisture content to an acceptable level. Two methods of sun drying are common. In the first, the harvested rice crop is left in the field as loose bundles to dry for 4-6 days depending upon the weather conditions.
In the second method, immediately after harvesting or the next day it is threshed and the wet grain spread on dry ground, concrete drying yard, or canvas. Repeated stirring is necessary to obtain uniform drying.
Sun drying is a slow process since it takes 4-6 days depending on the weather. Crop dryers have been developed to dry the produce quickly. Dryers are designed based on producing hot air by burning wood, charcoal, husks, shells, oil, gas or electricity.
Various types of rice dryers have been developed. However, sun drying is still the most popular method. In order to make sun drying easier, the CRRI, Cuttack developed 1.0 m2 wire mesh type racks for drying the grain in sun. These racks can be stacked one over the other to reduce the space requirements.
Grain Storage:
Rice grain absorbs moisture from surroundings till its moisture content is in equilibrium with surrounding air temperature and humidity. It is, generally, considered that 13 per cent (wet basis) moisture content is acceptable for rice storage. The storage structure should keep the grain moisture content and relative humidity of the stored grain stable throughout the storage period.
Low temperature and low moisture content are necessary for storing rice grain for long period. Rice grain with 10-14 per cent moisture content can be stored in good condition around 18°C for more than two years. If the moisture content is higher, germination decreases after about an year.
Rice is stored as rough rice (paddy) or milled rice. Rough rice is stored for home consumption in sacks, metal or wooden containers, bamboo baskets, drums or small granaries. For storing large quantities, a special room in the house is used. It is desirable to store rice in gunny sacks on wooden platforms that allow free passage of air between grain sacks and the floor.
Scientific storage of grain is recommended to avoid losses and to maintain the quality. Traditional storage bins of the country have been improved. Many state agroindustrial corporations and small-scale fabricators are producing grain storage bins for use at farm and urban household level. For extended period of storage, grain fumigation is necessary to control insect pests.
ii. Milling:
About 85 per cent of the rough rice produced in the country is converted in to rice. A fraction of rough rice (10%) is used for making rice products (puffed and flaked rice) and about 5 per cent of the rice production is used as seed for the next crop. Milling ranges from hand pounding with mortar and pestle in many remote areas of Asia to modern equipment in developed countries.
Milling is the process of converting rough rice (paddy) into milled or brown rice. The purpose of milling is to remove the hulls and bran from harvested dried rough rice and to produce milled, polished or white rice with minimum breakage and with minimum impurities in the final product.
The following terminology is somewhat specific to the rice industry:
Rough rice:
Rough rice, which is also known as paddy rice or unhusked rice is the harvested, unshelled rice as it comes from the field after drying.
Milling:
The process of converting rough rice into milled or brown rice.
Brown rice:
It is the rice with hull removed and bran and embryo remaining.
Milled rice:
Rice grain with husks, bran and germ removed.
Germ:
Microscopically small plant (embrio) attached to endosperm of the seed.
Hulling:
The process of removing the lemma and palea (husks or hulls) from rough rice.
Hulls:
Lemmas and paleas of rough rice that are usually a waste product of milling.
Brokens:
Milled rice kernels that are less than three-fourths of whole kernels.
Head rice:
Milled rice kernel that is more than three-fourths of the whole kernel.
White rice:
Kernel remaining after hulls, bran layer and germ are removed. Includes head rice and brokens.
Parboiled rice:
Rough rice steeped in warm water under pressure, steamed, dried and milled.
Rice bran:
Outer layer of the caryopsis just beneath the hull containing the outer bran layer and parts of embryo. It is rich in protein and vitamin B.
Polishing:
Removed of aleurone layer of brown rice kernel to produce white rice.
Grading:
Separation of brokens from unbroken rice. Broken rice is further graded into different size fractions for different uses.
Rice Milling:
Rice milling is a gigantic industry in India. Milling of rice is accomplished by various systems suitable for small-scale industry to very large units. These include huller, sheller, huller cum sheller mill, modern rice mill and rice mill complexes. Total capacity of rice mills in India is 300 M t yr -1 on 300 working days. The recovery of rice from huller type mill is usually below 65 per cent and from modern rice mills 66-70 per cent of rice.
The huller technology is simple and low cost but the rice recovery is low especially for raw rice. It gives byproducts: mixture of husk, bran and broken, used as animal feed. These cannot be further processed.
Rice Shelters:
They consist of disc type sheller to remove the husk and a vertical cone type polisher to remove the bran, germ etc. Head rice recovery is 4-5 per cent higher than that of hullers. The mini rice mills employ disc or centrifuged shellers and screen separators and a polishing unit.
Modern Rice Mills:
It is a technology developed in Japan where in rough rice is passed between two closely spaced rubber rollers and rotate in opposite direction at different speeds. Milling is completed with two other units at a paddy separator and a polisher.
Parboiling:
Parboiling is a premilling hydrothermal treatment given to unhulled rice (rough rice) to improve its milling quality, nutritive value, cooking quality and storability. The process is usually accomplished in three steps: soaking, steaming and drying. Incidental cracks in the rice grain get fused and nutritive substances on rice kernel get fused with the kernel and conserved, besides improving head rice recovery.
Parboiled rice stores better than raw rice. The bran from parboiled rice contains more oil than that from raw rice. The only disadvantage is that parboiled rice is harder than raw rice, takes more time and energy in milling as well as in cooking. Parboiling may also impart some undesirable odour to rice.
Traditionally, parboiling was either by single or double boiling method. Single boiling method involves soaking the unhulled rice in water at room temperature followed by open steaming for 20- 30 minutes using iron kettles and then sun drying.
In double steaming method, the unhusked rice is first steamed to raise its temperature and then soaked in cold water for 24-36 hrs. After soaking, it is steamed second time for 20-30 minutes followed by sun drying.
Major drawback of the conventional parboiling is that the rice is of dark colour with off smell due to long soaking time. These drawbacks are substantially removed with the introduction of modern methods of parboiling developed at CFTRI, Mysore, IIT, Kharagpur and CRRI, Cuttack.
Byproducts of Milling:
One hundred kg of clean unhusked rice would yield 20-22 kg husk, 2 kg bran, 2 kg germ and 72 kg rice, assuming that there is no loss in milling. Therefore, the two major byproducts are husk and bran.
Paddy husk:
One-fifth of unhusked rice by weight consists of husk. It has about 20 per cent ash, 90 per cent of which is silica. The heat value of husk is 3000 K cal kg-1. Husk is used as fuel in rice milling industry. Husk ash is used in the manufacture of pazzolan cement, extraction of silicon and as building material. Husk is also used as soil conditioner, insulating material and as raw material for extracting many chemicals such as furfural, sodium silicate, silica gel and charcoal.
Bran:
Rice bran is the most valuable byproduct of rice milling industry. It contains 12-15 per cent protein, 14-25 per cent oil and is a rich source of vitamin B-complex. Bran can be utilised as animal feed and is a source of oil extraction. Rice bran oil is of two grades: edible grade and industrial grade. Edible grade oil goes into production of vanaspati.
The nonedible grade oil is used for making soaps and several other detergents. Deoild cake is used as poultry and cattle feed. For edible oil extraction, rice bran has to be stabilised as soon as it comes out of rice mill.
There are three methods of bran stabilisation dry heat treatment, wet heat treatment and mineral acid treatment. Rice bran stabilisers have been developed by GBPUAT, Pantnagar; CRRI, Cuttack; CFTRI, Mysore and OTRI, Anantapur. Rice bran oil is high in unsaturated poly fatty acids having high medicinal value.
iii. Quality Considerations:
Milled rice is graded based on quality of the kernels. Standards of milled rice permit a more precise measure of value. Pricing of rice is based on a milled sample or on grade of the entire lot after milling. Rice grain is divided into four classes based on length and shape of the milled rice.
Quality in rice may be categorised into four broad areas: milling quality, cooking and eating quality, nutritive quality and specific standards for cleanliness and purity. All these are important in judging suitability for a particular quality. However, rice has different uses. Hence, the quality characteristics desired vary considerably, being ultimately related to the final consumer acceptance.
Milling Quality:
Medium slender, medium bold, short bold and round type grain without belly when milled at 14 per cent moisture do not easily break and have high head rice recovery. Extra-long and excessively slender grains with white belly are prone to break. Extra-long or long slender grain with elliptical ends, high density and minimum white belly will have high head recovery.
Endosperm Appearance:
Appearance of milled rice is important to consumer, producer and miller. Milled rice with complete translucent grain is preferred. Buyers in recent years are preferring an uniform tan or brown colour of the kernel.
Opaque areas in a translucent endosperm are known as while belly (white area towards embryo side), white center (central portion of endosperm white) or white back (white areas in the opposite side of embryo). All the three are, usually considered as white belly. Temperature, humidity and moisture conditions at grain development stage affect white belly development.
Grain Length and Shape:
Preference for grain length and shape vary from place to place. In general, long slender grains are preferred in the international market. Bright straw colour husk is an indicator of healthy grain development, which in turn will lead to high head recovery.
Aroma:
The characteristic pleasant aroma is evident at all stages of growth in almost all the aerial parts of the plant. The principal aroma compound responsible for the unique flour of basmati rice was identified as 2-acetyl-l-pyrroline, which is present in volatile oil fraction of cooked rice. It is widely preferred and highly priced in the rice trade.
Cooking and Eating Quality:
Grain cooking and eating standards vary considerably from region to region. Milled rice with long slender translucent grain with or without aroma, high volume expansion during cooking (linear kernel elongation), fluffiness (non-sticky and tender), firm texture of cooked kernel, appealing taste and longer self-life are more preferred in domestic and international market, except the Japonica growing regions, where there is preference for sticky rice. Fluffiness and texture of cooked rice are mainly conditioned by amylose content and gelatinisation temperature of milled rice.
Nutritional Quality:
Rice is a nutritious cereal crop, used mainly for human consumption. It is the main source of energy and is an important source of protein providing substantial amounts of the recommended nutrient intake of zinc and niacin. However, rice is very low in calcium, iron, thiamine and riboflavin and nearly devoid of beta-carotene.
Rice protein is biologically the richest by virtue of its high true digestibility (88%) among cereal proteins and also provides minerals and fiber. Rice can also be found in cereals, snack foods, brewed beverages, flour, oil, syrup and religious ceremonies to name a few other uses. Rice is also believed to have medicinal properties. Although, this is not scientifically proven, it has been used in many countries for medicinal purposes.
Composition per 100 g of edible portion of milled rice is given below:
Consumer Demand:
Rice grain quality connotes different things to different people because quality preferences vary widely between countries, regions and communities within the country, each having specific liking to a particular variety/type of rice.
For consumers, quality encompasses complete range of visual, sensory and palatability criteria, which include impressive appearance of raw as well as cooked rice texture in terms of stickiness/flakiness, appealing aroma or no aroma and desirable palatability score.
However, the criteria for price and market quality of rice like physical and visual characteristics are not always related criteria for cooking and eating quality and for nutritional quality of the cooked rice.
Domestic Preference:
India being the home for thousands of quality rices presents a picture of wide differences in preferred quality attributes. Typical Indian rice is of non-sticky but soft after cooking and has intermediate gelatinisation temperature (GT) with long or medium slender grains. Many of the south Indian varieties have high amylose, medium gel consistency and intermediate GT giving a harder texture to the cooked rice, where as those from Kerala are, generally, coarse and heavy red rices.
The small grained rices from Gujarat and Maharashtra have high amylose and intermediate GT. Northwest India has fine grained scented varieties. Sticky and glutinous types are in great demand in northwestern states, while 60 per cent of rice eaters in India prefer parboiled to raw rice.
Quality Preference in Different Countries:
Rice consuming countries present a mosaic pattern in their quality preference. Appreciation of aging is considered a desired phenomenon in tropical Asia but the same is undesirable Japan, Australia, Korea, parts of China and Italy, which consume soft and relatively sticky Japonica rice.
Strong preference exists for long grain aromatic basmati rice in the Middle East. Bangladesh, Nigeria and Liberia consume parboiled rice while glutinous rice is the staple food in parts of Thailand and Laos. Thus, historical and sociocultural factors of a particular region play an important role in defining what consumer consider as good quality rices.
It is the chemical properties of starch (amylose) content (AC), gelatinisation temperature (GT), gel consistency (GC), grain elongation and aroma which influence the cooking quality, while milling recovery, grain shape and appearance largely determine the market acceptability of the variety.
Glutinous rice (AC 1 -2%) cook glossy and sticky and are extensively utilised in rice based products while high AC (more than 25%) types become hard on cooking, hence less preferred may be better used for parboiling. The most preferred amylose is that of intermediate range which cooks flaky, soft and tender on cooling. Such types are more popular in India and most of the rice consuming countries.
Quality Grades in International Market:
Exports of rice are limited compared to other food grains.
Six basic types dominate trade scenario:
(1) High quality,
(2) Medium quality long grain,
(3) Short grain,
(4) Parboiled,
(5) Aromatic, and
(6) Glutinous rice.
Among these, non-aromatic high and medium quality long grain and short-grained Japonicas comprise more than 90 per cent of the volume.
Rice for export is graded based on physical properties such as length of grain, degree of milling, endosperm translucency, percentage of broken, damaged, coloured or chalky grains, moisture level and cleanliness. Australia, Surinam, Uruguay, Thailand, and USA are the main exporters of high standard rice with the latter two countries being the major suppliers.
As for aromatic or fragrant rices, which form less than 10 per cent of world trade, Indian subcontinent is the only best source. Basmati rices of India and Pakistan, noted for their distinctive aroma, command an excellent price (US $ 300-450 t-1) depending on the quality.
Basmati Quality Features:
Basmati rices are prised for three distinct quality features: pleasant aroma, superfine grain (more than 6.5 mm long) and extreme grain elongation which is more than double its original kernel length and soft texture of cooked rice. The overall productivity of basmati rices in traditional basmati growing areas is less than 2.0 t ha-1.
Although, scores of long and short-grained scented varieties are grown, many fulfill one or two but not all characteristics of basmati rices. Only few types like Basmati 370 (Punjab basmati) Karnal local (Travadi basmati), Type 3 (Dehradun basmati) and Basmati 217 (Pakistan basmati) confirm to the norms of export.
Main characteristics of Indian basmati rice:
Origin:
Authentic basmati rice is sourced from northern India at the foothills of the Himalayas. Whilst basmati rice can be sourced from India and Pakistan, Indian basmati is traditionally considered premium.
Colour:
The colour of a basmati is translucent, creamy white. Brown basmati rice is also available but the most commonly used is white basmati.
Grain:
Long grain. The grain is long (6.61 – 7.5 mm) or very long (more than 7.50 mm and 2 mm breadth).
Shape:
Shape or length-to-width ratio is another criterion to identify basmati rice. This needs to be over 3.0 in order to qualify as basmati.
Texture:
Dry, firm, separate grains. Upon cooking, the texture is firm and tender without splitting and it is non-sticky. (This quality is derived from the amylose content in the rice. If this value is 20-22 per cent, the cooked rice does not stick. The glutinous, sticky variety preferred by the chopsticks users has 0-19 per cent amylose).
Elongation:
The rice elongates almost twice upon cooking but does not fatten much. When cooked the grains elongate (70-120 per cent over the pre-cooked grain) more than other varieties.
Flavour:
Distinctive fragrance. The most important characteristic of them all is the aroma. Incidentally, the aroma in basmati arises from a cocktail of 100 compounds — hydrocarbons, alcohols, aldehydes and esters. A particular molecule of note is 2-acetyl-1-pyrroline.
Uses:
Flavour and texture complements curries because it is drier rice and the grains stay separate. Also suits biryani and pilaf (where saffron is added to provide extra colour and flavour). Great for Indian and Middle Eastern dishes.
Main benefits:
Aromatic fragrance and dry texture.
Basmati rices are predominantly cultivated in the Punjab province of undivided India. In India, basmati rices are mainly confined to northwest states of Punjab, Haryana, western Uttar Pradesh and to a limited area in Himachal Pradesh, Jammu and Kashmir and Rajasthan. India produces about 0.6-0.7 M t of milled basmati rice from an area of about 0.7-0.8 M ha.
Haryana share accounts for 27 per cent of total basmati area and 37 per cent of production in the three major basmati-producing states of Haryana, Punjab and Uttar Pradesh. Around 70 per cent of basmati rice production in the country is exported to Saudi Arabia, (68%), UAE, UK, Kuwait, Bahrain and other countries.
Several basmati cultivars have been released for general cultivation in basmati growing areas (Table 1.23).
Non-basmati Rice:
Non-basmati rice of Indica type constitutes over 80 per cent of world trade. Of the various quality types traded, long grain of high and medium quality have substantial markets with Thailand, USA, China and Vietnam being the main exporters. India is exporting around 350,000 t of non-basmati annually to Saudi Arabia, Iran, UAE and Bahrain.
India exports mainly PR 106, Panthahan 4, Haryana Gaurau, IR 64 among long grain and IR 8 among short grain types. Network Programme on Genetic Enhancement of Quality Rices for higher productivity and export identified varieties for export quality.
Long slender groups – Divya, Prakash, Saleem, Mandya, Vani, NP 130.
Medium slander group – Sonamashuri, Sambamashuri, Padmini, Kavya, White ponne, Kalinga III.
Parboiled rice is a major item in the international trade, as high volume of it is consumed in Bangladesh, India, Sri Lanka, Pakistan, South and West African countries, Saudi Arabia and Nigeria. There are two types of parboiled rice based on quality: low grade, medium and short-grained type produced by conventional methods and high grade produced mainly by US and Thailand.
Rice Food Products:
Utilisation of rice functional properties to produce rice food products both traditional and new products are a vast untouched area as far as India is concerned. Rice processed products industry once developed would generate more income and employment in rural areas.
In India, rice is consumed either cooked as white or flavoured biryani and as fermented rice flour products, puffed, parched and beaten rice is also used in many traditional preparations. However, Japan, China and USA developed an ultimate range of rice based products starting from instant easy cooking rices to cakes, crackers, noodles, weaning baby foods, fermented rice wines etc.
Major processed foods are indicated below:
Easy cooking and instant rices – Retort rice, Canned rice, Alpha rice, Frozen rice, Easy- to-cook brown rice, Boil-in-bag rice.
Resorted and puffed rices – Rice flakes, puffed rice, Shredded rice.
Convenience foods – Canned soups with rice, meat and rice dinner, Casseroles, Poultry and rice products.
Rice flour-based foods – Rice noodles, Rice bread, Rice cakes and Crackers.
Instant foods – First solid food, Genmi meal.
Fermented foods/drinks – Rice wines (Sake, Japan, Shoshinchu, China).
This vast untapped reservoir of developing rice processed products for the changing needs of rice consumers both within India and abroad needs due importance.