CHAPTER XVIII MILLET: Post-harvest Operations

1.1 Economic and Social Impact of Millet

1.2 World Trade

1.3 Primary Products

1.4 Secondary and derived product

1.5 Requirements for Export and Quality Assurance

1. Introduction

Preface

Millets represent a collective term referring to a number of small-seeded annual grasses that are cultivated as grain crops, primarily on marginal lands in dry areas of temperate, subtropical and tropical regions. They are regarded as a subsistence product and generally looked upon as a famine crop for the poor.

The statistical documentation for millet is generally poorer and more fragmented than that for rice and wheat despite its nutritional superiority. It may not come as a surprise to note that some scientists may not differentiate millets from sorghum. Some may be even astonished to learn how vast and variable are the millet species and varieties.

Postharvest operations, such as threshing, drying, cleaning, packaging, storage, processing and transportation are as significant. This comprises the second half of activities following pre-harvest operations. Therefore, these are crucial processes in the whole food chain for millet production.

As stated before, documentation of the post-harvest operations for millet is not as rich in content as other cereals. The purpose of this chapter is to add to the scanty information on millets with the eventual aim of providing facts on this often neglected yet consequential cereal crop.

1. Introduction

Millet is a cereal crop plant belonging to the grass family, Graminae (FAO, 1972). The term "millet" is used loosely to refer to several types of small seeded annual grasses (FAO and ICRISAT, 1996), belonging to species under the five genera in the tribe Paniceae, namely Panicum, Setaria, Echinochloa, Pennisetum and Paspalum, and one genus, Eleusine, in the tribe Chlorideae (FAO, 1972). Most of the genera are widely distributed throughout the tropics and subtropics of the world (de Wet, 1987). The genus Pennisetum for example, includes about 140 species, some of which are domesticated and some are growing in the wilderness.

Millets are small grained cereals, the smallest of which include kodo, foxtail, proso (common millet), little and barnyard millets (Rao, 1989). They are the staple food of the millions inhabiting the arid and semi-arid tropics of the world, and are distributed in most of the Asian and African countries and parts of Europe.

According to Dendy (1995) the most important millets are pearl millet (Pennisetum glaucum), finger millet (Eleusine coracona), proso millet (Panicum miliaceum) and foxtail millet (Stalia italica). But according to FAO (1972) and Hulse et al., (1980), the most important cultivated millet species are: pearl Millet (Pennisetum typhoides), also known as bulrush millet; proso millet (Panicum miliaceum), also known as common millet; foxtail millet (Setaria italica); Japanese barnyard millet (Echinochloa crusgalli var. Frumentacea or E. colona (Sawa)); finger millet (Eleusine coracona) also known as birds food millet or African millet; and kodo millet of India (Paspalum scorbiculatum).

Other millets include little millet (Panicum sumatrense), tef millet (Eargrostis tef) and Fonio millet (Digitaria exilis and D. iburua) (Dogget, 1989). The various types of millets, their food values and comparison are shown in Tables 1 and 2.

 Table 1. Various types of millets and their food values (%) compared to rice

Source: Ponnuthurai, 1989.  

Table 2. Structural comparison of some millets

Source: Compiled by Saldivar and Rooney (1995) from various sources.         

Table 3. Chemical composition of pearl millet and its anatomical parts.

Source: Saldivar and Rooney, 1995.

The development and structure of pearl millet is similar to that of sorghum, with some exceptions. Pearl millet is an erect, short-day (Dave, 1987) annual grass that tillers more profusely than sorghum. Tillering is primarily from basal nodes, and the number of tillers varies considerably in different varieties. Branches may also arise from other nodes, each branch terminating in to an inflorescence. The inflorescence of pearl millet is a panicle, cylindrical in nature, broadest at the middle and slightly tapering towards both apex and base. The length as well as thickness of the panicle varies between 0.20 to 0.45 m long.

The leaves of pearl millet are generally smaller than those of sorghum, 90 to 100 cm in length, and 5 to 8 cm in width. The stem, on the average is thinner than that of sorghum (1 to 3 cm in diameter). The panicles are long and cylindrical, ranging from 15 cm to 1 m in length and 2 to 5 cm in width. At maturity, the stem tends to be woodier than that of sorghum (House et al., 1995).

Plant heights range from 1.2 to 3.5 m, depending on dwarfing genes (House et al., 1995) and on the environmental factors (such as climate, soil type, etc). Roots are fibrous with the main root being thin, small and quickly replaced in function by adventitious roots. The stem is round to oval, with flat, cordate leaf blades, of 0.75 m or more in length and 0.05 to 0.07 m wide (FAO, 1972).

Pearl millet is adapted to hot climates, and is even more resistant to drought than sorghum (Dave, 1987). No other cereal crop grows so well in hot dry regions like pearl millet. It yields reasonably well on poor sandy soils on which most other crops fail.

Pearl millet takes about 80 to 100 days to mature, but some varieties may mature earlier. It is primarily cross-pollinating, as stigmas emerge one to three days before the anthers (protogynous) and therefore have opportunity to be cross-pollinated. The seeds are one third to one half the size of that of sorghum, and tend to be more elongated and tear shaped. Spikelets can be bristled (awned) but in most varieties, there are no awns (House et al., 1995).

Pearl millet is the most widely grown of all the millet species (FAO, 1972). It is grown on an estimated 27 million hectares, followed by finger millet (Singh et al., 1987, quoted by Saldivar and Rooney1995). World production of millets has been stable during the last "decade", because it is mainly cultivated as a subsistence crop. In 1991, 37.1 million ha were sown, with an average yield of 781 kg/ha, totalling 28.9 million metric tonnes (FAO, 1992, quoted by Saldivar and Rooney, 1995). Thirty percent of the production was harvested in Africa, 56 percent in Asia and 13 percent in the former USSR. Major producers were India (31.1 percent), China (15.8) percent and the former USSR (12.4 percent) (Saldivar and Rooney, 1995)

Pearl millet is the major millet grown in Africa (Spencer and Sivakumar, 1987) and it is the fourth most important cereal food crop grown in India (Harinarayana, 1987).

Table 4. Synonyms of pearl millet as known in different countries of the world.

Source: FAO, 1972.

It is believed that pearl millet was first domesticated in India and then moved to Africa, but most of the earlier scientists believed that Africa is the centre of origin of pearl millet and that it was introduced to India from Africa (Vavilov, 1951 quoted by FAO, 1972).

In India, pearl millet is the fourth most important food crop (Harinarayana, 1986)

Pearl millet is the most important cereal crop in arid and semi-arid regions of the world, and is grown most widely in India (Dave, 1987). As a semi-arid and arid crop, pearl millet is traditionally a component of the dryland system, usually grown in soils with depleted fertility which receive 150 - 750 mm of rainfall per year (Harinarayana, 1987).

Nutritionally superior to rice and wheat (Harinarayana, 1987, Seetharam et al., 1989) pearl millet is commonly used to make unleavened bread, thin or thick porridge or may be cooked like rice. The chemical composition of pearl millet and its anatomical parts are shown in Table 3.

The major factors that restrict the production potential of pearl millet are low hybrid coverage, slow varietal spread, poor plant establishment, no fertilizer, weeds (such as Striga spp) and diseases (such as downy mildew) and pests (such as grain-eating birds) (Harinarayana, 1987).

Pearl millet has been given different names in various countries of the world (Table 4).

The structure of a mature caryopsis of pearl millet is shown in Figure 1. It is very small, weighing between 3-15 g/1 000 kernels (as compared to 5-80 g/1 000 kernels in sorghum).

 Figure 1: Schema of Pearl Millet Seed Structure. (Source: Rooney and McDonough, 1987)

The typical seed (kernel) of pearl millet is a caryopsis similar in structural components to sorghum. Kernel shape, size and appearance (colour) vary significantly among pearl millet varieties, and within a sample, kernels vary significantly in size and shape (Rooney and McDonough, 1987). They are 3-4 mm long and 2.25 mm wide, usually yellowish-grey or steel-grey in colour, but varieties are known which have pearly-white or yellow grains (FAO, 1972) The appearance of pearl millet inflorescence is shown in Figure 2.

 Figure 2:Pearl millet inflorescence.

Among the millets of the world, finger millet (Eleusine coracona) ranks fourth, after pearl millet (Pennisetum americanum L.), foxtail millet (Setaria italica), and proso millet (Panicum miliaceum) (Gupta, et al., 1989). Finger millet is the most widely grown small millet in the world (see definition of small millets under Section 2.8.3).

Finger millet, also known as ragi in India (Hulse et al., 1980) is a highly tillering annual grass, whose average height is a little over 1.0 m, but can reach as high as 1.6 m. Tillers come from the base of the plant and axillary buds along the stem (House et al., 1995). Each tiller produces a panicle. Leaves are generally 30 to 40 cm long, but can reach 70 cm and are narrow (1.5 to 3 cm). Panicle branches commonly come from the same place giving a finger like appearance (Figure 3). The number of branches ranges from 4 to 19; they can be straight (3 to 10 cm long) or they can be curved like a hand with fingers partially closed (hence the name finger millet). Seeds are formed in florets generally arranged in two rows along the panicle branch. The seeds are generally dark brown, red brown or purple, although light brown and cream coloured seeds are found. Seeds are hard and very small, up to 2 mm in diameter.

There are two groups of cultivars of finger millet, namely (a) the African highland types with grains enclosed within the florets and (b) Afro-Asiatic types with mature grains exposed out of the florets (Hulse et al., 1980). The cultivars vary in many characters, including height, ranging from dwarf-types (40 cm) to tall types of about 1 m. The colour of the vegetative organs varies from green to purple. The inflorescence may have straight and open spikes, incurved or closed spikes and branched spikes, resembling a cock. s comb. The length of spikes may range from 3 to 13 cm, the colour of grains may vary from white through orange red, deep brown, purple to almost black.

 Finger millet is generally grown in higher rainfall areas (600-1200 mm) and is one of the better crops for acid soils. It matures within 100 to 130 days. Finger millet is an important staple food in East and Central Africa and in India (Hulse et al., 1980).

In Uganda, Finger millet is the second most important cereal after maize (Esele, 1989). Uganda is regarded as the centre of its origin, and was probably taken to India some 3000 years back (Hulse, 1980).

 Figure 3: Finger millet

Foxtail millet

Foxtail, Figure 4, is an annual grass that is variable in its morphology (House et al., 1995). It is an important crop grow for food and feed in China. It is one of the main cereal crops grown in Northern China, where the other important crops are wheat and corn (Jiaju, 1989).

Plants range from single-stemmed to highly tillered. The highly tillering types range from 30 cm to almost 1 m, each tiller having a panicle pretty well described by its name - foxtail. Panicles are frequently small, 10 to 15 cm in length and 1.5 to 3 cm in diameter. The central rachis is frequently lax, so the panicle droops. By contrast, some varieties, especially those common in China, are single-stemmed, and range from 60-70 cm up to 150 cm high. Leaves are 30 to 35 cm long and 1.5 to 3 cm wide. The panicles have short branches, the central rachis is frequently stiff, but heads on some varieties droop. Seeds are small (2 to 3 mm) and are generally light cream in colour. Foxtail is adapted to temperate regions although found in the tropics. It has a broad range of maturity, from 70 to 120 days.

 Figure 4: Foxtail millet

Proso, also known as common millet (Figure 5), is an annual grass adapted to temperate parts of the world. It is suited to a dry continental climate, and grows further north than most other millets (Hulse et al., 1980). It is mainly cultivated in Eastern Asia, including Mongolia, Manchuria, Japan, India, Eastern and Central Russia. It is also found in Arabia, Syria, Iran, Iraq, and Afghanistan. Common millet is reported to be the largest millet crop in USA, and one of the most popular cereals in Northern China, commanding a price equal to that of wheat (Hulse et al., 1980)

Proso is a shallow-rooted plant, which is highly variable in morphology, ranging from as short as 25 cm to 230 cm. Many tillers are formed, each with an open panicle with drooping branches. Panicle branches range from 4-5 cm up to 15 cm in length. Individual seeds weigh from 5 to 9 mg. Its inflorescence consists of a slender panicle up to 45 cm long, which may be open or compact. The caryopsis is generally white, oval and smooth.

Common millet displays possibly the lowest water requirement of any cereal. It matures in 60 to 90 days and may be grown where the climate is too hot, the rainy season too short or the soil too poor for any other cereal (Hulse et al., 1980).

 Figure 5: Proso millet

Kodo millet (Figure 6) is grown commercially in India (House et al., 1995), although the wild grass is a widespread tropical weed (Dogget, 1989). It is an annual millet that varies in height from 30 to 90 cm and has many basal tillers, between 10 and 48. The inflorescence is small, 2 to 12 cm. It matures late compared to other small millets. It is self-pollinating, florets generally remain closed during the flowering period. The grain occurs in a hard husk, making de-branning difficult. The crop is drought resistant, hardy and frequently grown on poor soils.

Kodo millet is reported to be poisonous after the rain, perhaps due to fungal infection (Dogget, 1989), but clean healthy grain seems to pose no health problem.

 Figure 6:Kodo millet

Japanese barnyard millet (Echinochloa frumentacea), Figure 7, is a short plant, frequently grown in Egypt as a reclamation crop on land, which is too saline for rice (Arnon, 1972, quoted by Hulse et al., 1980). It is the fastest growing of all the millets, and, under favourable moisture and temperature conditions, the grains would mature in 45 days after sowing (Hulse et al., 1980). However, Rachie, 1975, quoted by Hulse et al., (1980) reported that Echinocloa millets grow well in different seasons and at high elevations, but may require 3 - 4 months to mature.

Others (Pursegrove, 1972, quoted by Hulse et al., 1980) also reported that Japanese barnyard millet is a fast growing crop, and may take only 6 weeks to mature. Though not of importance, it is grown in the Orient and India, and as a forage crop in USA where it is reported to produce eight harvests per year.

The height of Japanese barnyard millet varies between 50 and 100 cm, the inflorescence consists of a panicle frequently tinged with purple, bearing up to 15 lateral branches. It is normally grown as a rainfed crop, but may be cultivated in waterlogged conditions and survive submersion. It may yield between 700 and 800 kg of grain per ha.

 Figure 7:Japanese barnyard millet

There are two races of little millet (Panicum miliare), namely, nana and robusta (House et al., 1995). Plants in race nana vary from 60 to 170 cm in height. The inflorescence is 14 to 15 cm long, erect, open, and highly branched. These branches sometimes droop at maturity. Plants in the race robusta are 120 to 190 cm tall. The inflorescence is 20 to 45 cm long, open or compact, and highly branched. It is primarily a self-pollinated crop, with up to 3.5 percent cross pollination.

Little millet (Figure 8) is grown throughout India up to altitudes of 2100 m, but is of little importance elsewhere (Hulse et al., 1980). Although little millet has received comparatively little attention from plant breeders, it appears to thrive under conditions where no other edible plant will survive. It matures between 2.5 to 5 months. The yields are generally less than 0.5 tonnes/ ha, but under favourable conditions, may reach close to 1 t / ha. Little millet tends be confused with common millet, but it is generally shorter, and has smaller panicles and seeds than common millets.

 Figure 8:Little millet

Tef

Tef in the most important cereal crop in Ethiopia, particularly in the poor drained heavy soils that predominate in the central plateau. Nevertheless, the crop has not become important outside Ethiopia (Dogget, 1989)

Fonio, also known as hungry rice, is grown as a cereal crop throughout the savanna zone of West Africa. In parts of Guinea and Nigeria, it is the staple crop. It is considered to be the oldest West African cereal, and its cultivation is thought to date back to 5000 BC. The crop can grow on poor soils, but it is not grown outside Africa (Dogget, 1989).

Small millets (also referred to as minor millets) may be defined as millets cultivated for their grains which are borne on short, slender grassy plants, but pearl millets (Pennisetum) are excluded (Doggett, 1989). Small millets include Finger millet, Proso millet, foxtail millet, little millet, Baranyard (Sawa) millet, kodo millet, Tef millet and Fonio millet. The grains of small millets are nutritionally superior to rice and wheat, providing minerals and vitamins (Rao, 1989).

In India, small millets occupy 4.5 percent of the cultivated area and are confined to vast stretches of dryland and hilly tracks (Rao, 1989). The productivity of finger millet in India is the highest among the small millets, at 1 150 kg/ha during 1983-84. The productivity of other small millets remained at 450 kg/ha (Rao, 1989).

A structural comparison of the most important millets has already been presented in Table 2.

1.1 Economic and social impact of millets

Millets, in most cases, have been grown in difficult conditions, and it is scarcely surprising that they involve high production risks (Dogget, 1989). They have always been crops for situation where there is a risk of famine, as well as offering a low but more reliable harvest relative to other crops.

Although it is found in other countries, finger millet has gained little importance outside Africa and India. Equally important to note is that, common millet has received little attention from plant breeders (Hulse et al., 1980).

In most parts of the world, millet is grown as a subsistence crop for local consumption. Commercial millet production is risky, especially in Africa because the absence of large market outlets means that fluctuations in output cause significant price fluctuations, particularly in areas where millet is the main food crop (FAO and ICRISAT, 1996). Apart from grain production, millet is also cultivated for grazing, green fodder or silage.

1.2 World trade

According to Spencer and Sivakumar (1987) world production of all millets is about 29 million tonnes, of which 35 percent is produced in Africa (Tables 5 and 6). This represents about one third of all the world millets, 70 percent of which is grown in West Africa. According to FAO and ICRISAT (1996), developing countries, mainly Asia and Africa, account for about 94 percent of global output of millet, estimated at some 28 million tonnes (according to the 1992-1994 average) (Table 7).

Table 5. World distribution of sorghum and major types of millets.

2. Shown as zero for trade less that 50 tonnes 

Source: FAO & ICRISAT, 1996.