5 Storage and Pre-processing Storage relative difficulties involved in their avoidance vary with location. Successful storage methods Compared with many other fruits, cereal grains are extremely amenable to storage, principally as range from primitive to highly sophisticated their moisture content at harvest is relatively low The hazards besetting cereal grain storage are: and their composition is such that biodeteriora-- moisture content; tion is slow. Harvesting is seasonal but the need -excessive temperature; for fresh cereal products is continuous. The least Anicrobial infestation; requirement for storage, therefore, is for the -insect and arachnid infestation; period between harvests. Under appropriate con- -rodent predation; ditions this can easily be met and indeed storage bird predation; for many years without serious loss of quality is biochemical deterioration; possible. Even in biblical times long periods of -mechanical damage through handling. storage were apparently achieved In spite of the diversity of cereal grain types The complexity of storage problems results and the ambient conditions throughout the cereal from the matrix of interactions of the various producing and consuming world the hazards of hazards. They are considered separately in the storage are fundamentally similar although the text but their combined effects, some of which Increose Grain Insects, Fungi arachnids Respiration Charring Heat Dry matter Water Caking loss Increose FIG 5.1 Schematic diagram of the cumulative effects of grain storage at moisture contents above the safe levels. The effects on stored grain itself are shown in bold type. 103
Charrlng Heo t Water Caking I Dry matter loss1
104 TECHNOLOGY OF CEREALS are shown in Fig. 5. 1, should be borne in mind throughout Moisture content and storage temperature Moisture content 1s expressed as a percentage of the grains, wet weight. The safe moisture O>Eceog8 contents for storage vary according to the type of cereal but it is widely assumed that they are equivalent to the equilibrium moisture content of the respective grains at 75%Rh and 25C (Table 5.1) TABLE 5.1 Equilibrium Moisture Contents of Grains at 75%RH and 25C FiG 5 d storage time(number of Cereal Type on of temperature(C)and 14.3(25°-28℃) Guilbot,(1963)Producteur Agricole Fran 14.3 ITCF, Paris 14.9(25°-28°C) 15.3 Wheat 14.7 Based on values in Bushuk and Lee, 1978 In temperate regions the moisture contents at which grain is stored are closer to those described as wet rather than dry. The significance of moisture contents cannot be considered alone as the deleterious effects of excessive dampness are Moisture content affected critically by ambient temperature and the composition of the surrounding atmosphere The increase in relative humidity of the interseed atmosphere with temperature, is slight. It amounts to about 0.60.7% moisture increase for each Mould 10C drop in temperature temperature as they affect storage of whea aind The relationship between moisture content and Risks to which stored cereal grain is exposed as m Burges, H. D. and Burrel, M. J(1964). S shown in Fig. 5.2 The relationship depicted takes account only of the maintenance of grain quality as to sound clean samples. Broken grains are almost by grain viability. The relationship is also always present to some extent as a result of ant, however, through its effects on infesting damage during harvesting or transferring to stores. In broken grains endogenous enzymes and The values used in Figs 5.2 and 5.3 refer their substrates, kept separate in the whole gr
104 TECHNOLOGY OF CEREALS are shown in Fig. 5.1, should be borne in mind 35' 0 H r [\ throughout. Moisture content and storage E 30' 5 temperature e c7' Moisture content is expressed as a percentage of the grains' wet weight. The safe moisture contents for storage vary according to the type of cereal but it is widely assumed that they are equivalent to the equilibrium moisture content of the respective grains at 75% RH and 25°C (Table 5.1). 50 io0 I50 ?d TABLE 5.1 Equtlibnum Moisture Contents of Grains at 75% RH and 25°C Cereal Type Moisture ?LO Barley Maize 14.3 Oats 13.4 Rye 14.9 (25"-28"C) F~~ 5.2 potential storage the (number of days) of wheat grain as a function of temperature ("C) and moisture content ("h), the germination rate maintained being 70%. From 14.3 (250-280c) Guilbot, (1963) Producteur Agncole Francats, Suppl. mai ITCF, Paris. Rice 14.0 40' Sorghum 15.3 (u) h ?? red 14.7 (rt) + white 15.0 (rt) s Wheat e 30" durum 14.1 (u) 3 g 200 G 100 ?! u i Based on values in Bushuk and Lee, 1978. I In temperate regions the moisture contents at which grain is stored are closer to those described moisture contents cannot be considered alone as the deleterious effects of excessive dampness are affected critically by ambient temperature and the composition of the surrounding atmosphere. The increase in relative humidity of the interseed atmosphere with temperature, is slight. It amounts 10°C drop in temperature. temperature as they affect storage of wheat is The relationship depicted takes account only of the maintenance of grain quality as assessed by grain viability. The relationship is also important, however, through its effects on infesting organisms, as Fig. 5.3 shows. The values used in Figs 5.2 and 5.3 refer as wet rather than dry. The significance of 0" 5 io 15 20 25 0 Moisture content % Good conservation H Insects m Germination n Moulds to about 0.6-0.7% moisture increase for each FIG 5.3 Risks to which stored cereal grain is exposed as a %. From Burges, H. D. and Burrel, M. J. (1964) J. Sci The relationship between moisture content and function of grain temperature in "C and moisture content in shown in Fig. 5.2. Food AFC. 1: 32-50. to sound clean samples. Broken grains are almost always present to some extent as a result of damage during harvesting or transferring to stores. In broken grains endogenous enzymes and their substrates, kept separate in the whole grain
STORAGE AND PRE-PROCESSING can achieve contact and lead to necrotic deteriora- radicle, and leaves and stem develop from the tion. Further, the most nutritious elements of the plumule(see Ch. 2). Hydrolytic enzymes are grain, endosperm and embryo, are exposed released into the starchy endosperm, and these to moisture, micro-organisms and animal pests catalyze the breakdown of stored nutrients into whereas in the whole grain they are protected a soluble form available to the developing plantlet by fruit coat, seed coats and possibly husks. The conditions required for germination are Impurities can also reduce storage time in that also conducive to other, more serious, hazards weeds present in the crop ripen and dry at a such as excessive mould growth. They would different rate from the crop itself. Hence, still rarely occur throughout a well managed store green plant material with a relatively high but could develop in pockets due to moisture moisture content-can carry excessive moisture migration. Deterioration results from loss of into store even when mixed with dry grain weight due to enzyme activity and a loss of quality resulting from excessive enzyme activity in the Changes during storage in the grains products of processing. These problems would themselves ated through turning the grain Having germinated Respiration in store the grain would also be useless for seed purposes as the process cannot be restarted In a natural atmosphere gaseous exchange will occur in a stored cereal crop This is due to Microbial infestation respiration and it involves a depletion in atmos pheric oxygen and an increase in carbon dioxide fungal spores and mycelia, bacteria and yeasts with the liberation of water, and energy(as heat). are present on the surfaces of all cereal crops Respiration rates measured in a store normally During storage they respire and, given adequate include a major contribution from micro-organisms moisture, temperature and oxygen, they grow that are invariably present at harvest; neverthe- and reproduce, causing serious deterioration in less even ripe dry grain, suitable for storing, grains contains living tissues in which respiration takes a distinction may be drawn between those that place, albeit at a very slow rate. The attack developing and mature grain in the field and embryo are the tissues involved and those that arise during storage Field fungi other organisms present, their rate of respiration thrive in a relative humidity(rh) of 90-100% increases with moisture content and temperature while storage fungi require 70-90%RH. Several Respiration is a means of releasing energy from investigations have shown that a rh of 75% stored nutrients(mainly carbohydrates)and a is required for germination of fungal spores consequence of long storage is a loss of weight .(Pomeranz, 1974) Under conditions unfavourable to respiration this Storage fungi are predominately of the genera may, however, be insignificant and under any Aspergillus, of which there are five or six groups circumstances it is likely to be of little consequence and Penicillium, the species of which are more in relation to other storage hazards. Respiration clearly defined. Some of the more common storage can be reduced by artificially depleting the oxygen fungi and the minimum relative humidity in in the atmosphere which they can thrive are listed in Table 5.2 As with other spoilage agents deper dent upo minimum moisture content, fungi may be a problem even when the overall moisture content Germination of grain is an essential and natural in the store is below the safe level. This can phase in the development of a new generation of result from air movements leading to moisture plant. It involves the initiation of growth of the migration. Warm air moving to a cooler area will embryo into a plantlet roots develop from the give up moisture to grains, thus remaining
STORAGE AND PRE-PROCESSING 105 can achieve contact and lead to necrotic deteriora- radicle, and leaves and stem develop from the tion. Further, the most nutritious elements of the plumule (see Ch. 2). Hydrolytic enzymes are grain, endosperm and embryo, are exposed released into the starchy endosperm, and these to moisture, micro-organisms and animal pests catalyze the breakdown of stored nutrients into whereas in the whole grain they are protected a soluble form available to the developing plantlet. by fruit coat, seed coats and possibly husks. The conditions required for germination are Impurities can also reduce storage time in that also conducive to other, more serious, hazards weeds present in the crop ripen and dry at a such as excessive mould growth. They would different rate from the crop itself. Hence, still rarely occur throughout a well managed store green plant material - with a relatively high but could develop in pockets due to moisture moisture content - can carry excessive moisture migration. Deterioration results from loss of into store even when mixed with dry grain. weight due to enzyme activity and a loss of quality resulting from excessive enzyme activity in the products of processing. These problems would apply even if the germination process were termin- Changes during storage in the grains ated through turning the grain. Having germinated themselves in store the grain would also be useless for seed Respiration purposes as the process cannot be restarted. In a natural atmosphere gaseous exchange will Microbial infestation occur in a stored cereal crop. This is due to respiration and it involves a depletion in atmospheric oxygen and an increase in carbon dioxide Fungal spores and mycelia, bacteria and yeasts with the liberation of water, and energy (as heat). are present on the surfaces of all cereal crops. Respiration rates measured in a store normally During storage they respire and, given adequate include a major contribution from micro-organisms moisture, temperature and oxygen, they grow that are invariably present at harvest; neverthe- and reproduce, causing serious deterioration in less even ripe dry grain, suitable for storing, grains. contains living tissues in which respiration takes A distinction may be drawn between those that place, albeit at a very slow rate. The aleurone attack developing and mature grain in the field and embryo are the tissues involved and, like and those that arise during storage. Field fungi other organisms present, their rate of respiration thrive in a relative humidity (RH) of 90-100% increases with moisture content and temperature. while storage fungi require 70-90% RH. Several Respiration is a means of releasing energy from investigations have shown that a RH of 75% stored nutrients (mainly carbohydrates) and a is required for germination of fungal spores consequence of long storage is a loss of weight. (Pomeranz, 1974). Under conditions unfavourable to respiration this Storage fungi are predominately of the genera may, however, be insignificant and under any Aspergillus, of which there are five or six groups, circumstances it is likely to be of little consequence and Penicillium, the species of which are more in relation to other storage hazards. Respiration clearly defined. Some of the more common storage can be reduced by artificially depleting the oxygen fungi and the minimum relative humidity in in the atmosphere. which they can thrive are listed in Table 5.2. As with other spoilage agents dependent upon a minimum moisture content, fungi may be a problem even when the overall moisture content Germination Germination of grain is an essential and natural in the store is below the safe level. This can phase in the development of a new generation of result from air movements leading to moisture plant. It involves the initiation of growth of the migration. Warm air moving to a cooler area will embryo into a plantlet. Roots develop from the give up moisture to grains, thus remaining
TECHNOLOGY OF CEREALS TABLE 5.2 Thermophilic fungi die at 60C and the process Approximate Minimum Equilibruim Relative Humidity for is kept going by spore forming bacteria and thermophilic yeasts up to 70C. RHOO) limi In recent years attention has been given to the Aspergillus halophiticus toxic products of fungi such as Aspergillus flavus and Fusarium moniliforme which produce aflatoxin A. glaucus group and zearalenone(see Ch. 14) AAAAAAAAPP Insects and arachnids Insects that infest stored grains belong to the beetle or moth orders: they include those capable of attacking whole grain (primary pests)and those 80-90* that feed on grain already attacked by other pests (secondary pests). All arachnid pests belong to the order Acarina(mites)and include primary P. islandicum 83t and secondary pests. Most of the common insects Christensen,CMand Kaufmann, HH (1974)Micro- and mites are cosmopolitan species found through flora. In: Storage of Cereal Grain and their Products. Christensen, out the world where grain is harvested and stored (Storey, 1987). Insects and mites can be easily t Ayerst, G. ( 1969). The effects of moisture and temperati distinguished as arachnids have eight legs and Stored Prod.Res.5:127-141 insects, in their most conspicuous form, have six. Table adapted from Bothast, R. J (1978) Reference to the most conspicuous form is sary as some insects (including those that infest grain) develop through a series of metamorphic in equilibrium with them. Unless temperature forms. There are four stages: the egg, the larva adients are extreme the exchanges occur in the pupa and the adult or imago. Although some the vapour phase; nevertheless, variations in female insects lay eggs without mating having moisture content up to 10% within a store are occurred, this is less usual than true sexual possible. reproduction, and this and dispersion are the two If mould growth continues in the presence of principal functions of the adult. Large numbers oxygen, fungal respiration increases, producing of eggs are produced and these are very small more heat and water. If the moisture content is Those of primary pests may be deposited by the allowed to rise to 30% a succession of progressively female imago inside grains, in holes bored for the heat-tolerant micro-organisms arises. Above 40c purpose prior to the egg laying. Under suitable mesophilic organisms give way to thermophiles. conditions eggs hatch and from each a larva Alphabetical List of Primary Insect and Arachnid Pests ystematic name Common name Grain (or flour)mite Acaridae Cryptolestes ferrugineus Stephens red grain beetle coperwha dominica Lesse in borer Bostrichidae ilus granarius L. Sitotroga cerealella olivier Angoumois grain moth Gelechiidae
106 TECHNOLOGY OF CEREALS TABLE 5.2 Approximate Minimum Equilibruim Relative Humidity for Growth of Common Storage Fungz Thermophilic fungi die at 60°C and the process thermophilic yeasts up to 70°C. Mould RH(%) limit In recent years attention has been given to the Aspergillus halophiticus 68* toxic products of fungi such as Aspergillus flavus A. restictus group 70* and Fusarium miliforme which produce aflatoxin A. glaucus group 73* and zearalenone (see Ch. 14). A. chevalieri 71t A. repens 71t Insects and arachnids A. candidus group 80* A. candidus 75t Insects that infest stored grains belong to the A. ochraseus group 80* A. jlavus group 85* A. jlavus 78t beetle or moth orders: they include those capable A. nidulans 78t of attacking whole grain (primary pests) and those 80-90* 82t that feed on grain already attacked by other pests A. fumigatus Penicillium spp. P. cyclopium 82t (secondary pests). All arachnid pests belong to P. martensii 79t the order Acarina (mites) and include primary 83t and secondary pests. Most of the common insects P. islandicum and mites are cosmopolitan species found throughOUthe world where grain is harvested and stored (Storey, 1987). Insects and mites can be easily distinguished as arachnids have eight legs and insects, in their most conspicuous form, have six. Reference to the most conspicuous form is necessary as some insects (including those that infest grain) develop through a series of metamorphic forms. There are four stages: the egg, the larva, the pupa and the adult or imago. Although some female insects lay eggs without mating having occurred, this is less usual than true sexual reproduction, and this and dispersion are the two principal functions of the adult. Large numbers of eggs are produced and these are very small. Those of primary pests may be deposited by the female imago inside grains, in holes bored for the purpose prior to the egg laying. Under suitable conditions eggs hatch and from each a larva is kept going by spore forming bacteria and * Christensen, C.M. and Kaufmann, H.H. (1974) Microflora. In: Storage of Cereal Grain and thezr Products. Christensen, C. M., (Ed.) Amer. Assoc. of Cereal Chemists Inc. St. Paul, MN. U.S.A. t Ayerst, G. (1969). The effects of moisture and temperature on growth and spore germination in some fungi. J. Stored Prod. Res. 5: 127-141. Table adapted from Bothast, R. J. (1978). in equilibrium with them. Unless temperature gradients are extreme the exchanges occur in the vapour phase; nevertheless, variations in moisture content up to 10% within a store are possible. If mould growth continues in the presence of oxygen, fungal respiration increases, producing more heat and water. If the moisture content is allowed to rise to 30% a succession of progressively heat-tolerant micro-organisms arises. Above 40°C mesophilic organisms give way to thermophiles. TABLE 5.3 Alphabetical List of Primary Insect and Arachnid Pests Systematic name Common name Family Acarus siro. L Grain (or flour) mite Acarid a e Cryptolestes ferrugineus Stephens Rust red grain beetle Cucujidae Rhyzopertha dominica F. Lesser grain borer Bostrichidae Sitophilus granarius L. Grain weevil Curculionidae Stiophilus oryzea L. Rice weevil Sitophilus zeamais Motschulsky Maize weevil Sitotroga cerealella Olivier Angoumois grain moth Gelechiidae
STORAGE AND PRE-PROCESSING 107 nus Siro nite the grain weevil, and teus, the rust-red grain beetle(bottom). Crown Copyright Central Science Laboratory 1993
STORAGE AND PRE-PROCESSING 107 FIG 5.4 Acarus siro, the flour mite (top), Sitophilus granarius, the grain weevil, and Gryptolestes ferrugineus, the rust-red grain beetle (bottom). Crown Copyright Central Science Laboratory 1993
TECHNOLOGY OF CEREALS TABLE 5. 4 Alphabetical List of Most Important Secondary Insect Pests Common Name Anagasta kuehniella Zella* mediterranean flour moth Phycidae Almond moth Flour mill beetle hestia elutella huhne Oryzaephilus surinamensis L ephilus mercator Auv Merchant grain beetle Tenebriodes mauritanicus l ranidae teum Herbst Red flour beetle conidae Triboliumconfusum Duval Confused flour beetle Formerly Ephestia kuehniella. emerges. The larva is the form most damaging The time taken for development of both insects to the stored crop as it feeds voraciously. In and mites is influenced by temperature, the consequence it grows rapidly, passing through a greater the temperature the more rapid the deve series of moults during which its soft cuticle is lopment up to the maximum tolerated by the shed, thus facilitating further growth. Finally species pupation occurs; the pupa, chrysalis or cocoon The primary pests- those attacking whole does not eat and appears inactive. However, grains -are given in Table 5.3 changes continue and the final metamorphosis The three most damaging of these pests in the leads to the emergence of the adult form. The U. K. are shown in Fig. 5. 4. life cycle of mites is simpler as eggs hatch into The most important secondary insect pests nymphs which resemble the adult form, although those feeding only on damaged or previously there are only six legs present at this stage. By a attacked grains - are given in Table 5.4 series of four moults the adult form is achieved The saw-toothed grain beetle is shown in Fig. 5.5 Saw-toothed grain beetle, Crown Copyright. Central Science ratory 1993
108 TECHNOLOGY OF CEREALS TABLE 5.4 Alphabetical List of Most Important Secondary Insect Pests Systematic Name Common Name Family Anagasta kuehniella Zella* Mediterranean flour moth Phycitidae Gadra cautella Walker Almond moth " Cyptolestes pusillus Schonherr Flat grain beetle Cucu jidae Cyptolestes turacus Grouv Flour mill beetle " Ephestia elutella Hubner Tobacco moth Phycitidae Oyzaephilus surinamensis L. Saw-toothed grain beetle Cucujidea Oryzaephilus mercator Fauv. Plodia interpunctella Hubner Indian meal moth Phycitidae Tenebriodes mauritanicus L. Cadelle Ostomatidae Tribolium castaneum Herbst. Red flour beetle Tenebrionidae Tribolium confusum Duval Trogodenna granarium Everto Khapra beetle Dermestidae Merchant grain beetle " Confused flour beetle " * Formerly Ephestia kuehniella. FIG 5.5 Oryzaephilus Laboratory 1993. 108 TECHNOLOGY OF CEREALS TABLE 5.4 Alphabetical List of Most Important Secondary Insect Pests emerges. The larva is the form most damaging to the stored crop as it feeds voraciously. In consequence it grows rapidly, passing through a series of moults during which its soft cuticle is shed, thus facilitating further growth. Finally pupation occurs; the pupa, chrysalis or cocoon does not eat and appears inactive. However , changes continue and the final metamorphosis leads to the emergence of the adult form. The life cycle of mites is simpler as eggs hatch into nymphs which resemble the adult form, although there are only six legs present at this stage. By a series of four moults the adult form is achieved. The time taken for development of both insects and mites is influenced by temperature, the greater the temperature the more rapid the development up to the maximum tolerated by the species. The primary pests -those attacking whole grains- are given in Table 5.3. The three most damaging of these pests in the U.K. are shown in Fig. 5.4. The most important secondary insect pests ~ those feeding only on damaged or previously attacked grains -are given in Table 5.4. The saw-toothed grain beetle is shown in Fig. 5.5. FiG 5.5 Oryzaephilus surinamensis. Saw-toothed grain beetle. Crown Copyright. Central Science Laboratory 1993
STORAGE AND PRE-PROCESSING FiG 5. X-ray phot of wheat grains, two uninfested (bottom leff), the others showing cavities n. an insect is visible within one of the cavities. (Part of a picture in Photograph. Sci., 1954, 2: 113; reproduced by courtesy of Prof. G. A. G. Mitchell and the Editor fournal of Photographic Science. Those insects listed in the tables are considered even though only a hollow bran coat may remain major pests. They are particularly well adapted Detection by means of soft X-rays is possible to life in the grain bin and are responsible for (Fig. 5.6) most of the insect damage to stored grain and cereal products. Minor pests occur mainly in Damage caused by insects and mites stores in which grain has started to deteriorate due to other causes, while incidental pests include Serious grain losses due to consumption of those that arrive by chance and need not even be grain by insects and mites occurs only after able to feed on grains. For further information prolonged storage under suitably warm conditions on minor and incidental insect pests specialist They are most serious in hot climates. Other works such as Christensen(1974)should be problems caused by insects include creation of hot- consulted spots around insect populations where metabolic Among the major primary pests five species activity leads to local heating Moisture move- develop inside grains. Weevils (grain, rice and ments and condensation in cooler areas results in maize) lay eggs inside while lesser grain borers caking, and encourages fungal infestation(see and Angoumois grain moths deposit eggs outside Fig 5.1) but their newly hatched larvae promptly tunnel Introduction of insects and mites from wheat into grains. The presence of the insect and the stores to flour mills can cause serious deteriora- damage it causes may not be evident from outside tion in the products. Mite excreta taints flour with
STORAGE AND PRE-PROCESSING 109 FiG 5.6 X-ray photograph of wheat grains, two uninfested (bottom left), the others showing cavities caused by insect infestation. An insect is visible within one of the cavities. (Part of a picture in J . Photograph. Sci., 1954, 2: 113; reproduced by counesy of Prof. G. A. G. Mitchell and the Editor of Journal of Photographic Science.) even though only a hollow bran coat may remain. Detection by means of soft X-rays is possible (Fig. 5.6). Those insects listed in the tables are considered major pests. They are particularly well adapted to life in the grain bin and are responsible for most of the insect damage to stored grain and cereal products. Minor pests occur mainly in stores in which grain has started to deteriorate due to other causes, while incidental pests include those that arrive by chance and need not even be able to feed on grains. For further information on minor and incidental insect pests specialist works such as Christensen (1974) should be consulted. Among the major primary pests five species develop inside grains. Weevils (grain, rice and maize) lay eggs inside while lesser grain borers and Angoumois grain moths deposit eggs outside but their newly hatched larvae promptly tunnel into grains. The presence of the insect and the damage it causes may not be evident from outside Damage caused by insects and mites Serious grain losses due to consumption of grain by insects and mites occurs only after prolonged storage under suitably warm conditions. They are most serious in hot climates. Other problems caused by insects include creation ofhotspots around insect populations where metabolic activity leads to local heating. Moisture movements and condensation in cooler areas results in caking, and encourages fungal infestation (see Fig. 5.1). Introduction of insects and mites from wheat stores to flour mills can cause serious deterioration in the products. Mite excreta taints flour with
110 TECHNOLOGY OF CEREALS a minty smell and hairs from the animals bodies however, have several advantageous features can cause skin and lung disorders in workers Thus underground stores provide protection from handling infected flour. Silk from the larvae of temperature fluctuations, the most successful the Mediterranean four moth webs together simple ones being found in hot dry regions. They causing agglomeration of grains and blockages in are filled, to leave little air space, and sealed , to handling and processing equipment. In tropical approach the concept of hermetic storage under countries termites can weaken the structure of a which insects and moulds rapidly use up oxygen store, leading to its collapse giving rise to high CO2 content of the intergrain atmosphere. In more humid regions ventilation Vertebrate pests is de ble as the crop may have to be stored before reaching a safe moisture level. Such a The principal vertebrate pests in cereal stores system is suited to cob maize rather than threshed are rodents and birds. In many countries the three grains, as adequate space for air movement within main rodent species involved are: the store is essential. Clearly the requirements Rattus norvegicus -the Norway, common of ventilation and exclusion of insects are not immediately compatible and hence careful design Ratus rattus is essential Mus musculus the roof, ships or black rat; Storage of maize as cobs is practised now largely by small scale growers producing for the apart from consuming grains, particularly the requirements of the local community. It was at embryo of maize, rodents cause spoilage through one time adopted more widely even in highly their excretions which contain micro-organisms commercial practice, much as small grain cereals pathogenic to man. These include salmonellosis, were stored unthrashed in ricks murine typhus, rat-bite fever and Weils disease In the commercial context stores are needed Rodents also damage stores'structural elements, for three purposes containers, water pipes and electric cables In well-managed stores access by rodents 1. Holding stocks on the farm prior to sa denied and good housekeeping practice, such 2. Centralization before distribution or processing as removal of grain spillages, maintenance of during the year following harve uncluttered surroundings and regular inspec- 3. Storage of annual surpluses over a longer tions, prevent problems. The same is true of birds. These are serious pests only when access Farm stores may consist of any available space is easy, as for example in hot countries where that will keep out the elements. The facilities for grain may be left to dry in the sun. Damage to protection against mould and pests are very drains and blockage of pipes by nests can variable Stores range from small wooden enclos give rise to secondary storage problems through ures in the barn, to round steel bins holding 25- promoting local dampness in some stores 80 tonnes, to silos of larger capacities. Good on farm storage facilities allow farmers to choose the Design of storage facilities time to sell, to receive the best prices The degree of centralization depends upon the The requirements of long term safe storage are marketing regime within the country of produc protection against dampness caused by weather tion. In North America, Country elevators and or other sources, micro-organisms, destructively Terminal elevators with storage capacity up to high temperature, insects, rodents and birds, 500,000 tonnes exist. The country elevators provide objectionable odours and contaminants and un- a local staging en route to terminal elevators which authorized disturbance. Clearly the simplest stores include high-capacity equipment for cleaning such as piles on the ground, unprotected are suit- drying and conditioning of grain. The term able for short periods only. Other simple stores, 'elevator'is applied to the entire facility although
110 TECHNOLOGY OF CEREALS a minty smell and hairs from the animals’ bodies however, have several advantageous features. can cause skin and lung disorders in workers Thus underground stores provide protection from handling infected flour. Silk from the larvae of temperature fluctuations, the most successful the Mediterranean flour moth webs together simple ones being found in hot dry regions. They causing agglomeration of grains and blockages in are filled, to leave little air space, and sealed, to handling and processing equipment. In tropical approach the concept of hermetic storage under countries termites can weaken the structure of a which insects and moulds rapidly use up oxygen, store, leading to its collapse. giving rise to high C02 content of the intergrain atmosphere. In more humid regions ventilation is desirable as the crop may have to be stored before reaching a safe moisture level. Such a Vertebrate pests The principal vertebrate pests in cereal stores system is suited to cob maize rather than threshed are rodents and birds. In many countries the three grains, as adequate space for air movement within main rodent species involved are: the store is essential. Clearly the requirements of ventilation and exclusion of insects are not immediately compatible and hence careful design Rattus nomegzcus - the Norway, common or brown rat; is essential. Rattus rattus -the roof, ships or black rat; Storage of maize as cobs is practised now largely by small scale growers producing for the Mus musculus - the house mouse. Apart from consuming grains, particularly the requirements of the local community. It was at embryo of maize, rodents cause spoilage through one time adopted more widely even in highly their excretions which contain micro-organisms commercial practice, much as small grain cereals pathogenic to man. These include salmonellosis, were stored unthrashed in ricks. murine typhus, rat-bite fever and Weil’s disease. In the commercial context stores are needed Rodents also damage stores’ structural elements, for three purposes: 1. Holding stocks on the farm prior to sale. containers, water pipes and electric cables. 2. Centralization before distribution or processing In well-managed stores access by rodents is denied and good housekeeping practice, such during the year following harvest. as removal of grain spillages, maintenance of 3. Storage of annual surpluses over a longer uncluttered surroundings and regular inspec- period. tions, prevent problems. The same is true of birds. These are serious pests only when access Farm stores may consist of any available space is easy, as for example in hot countries where that will keep out the elements. The facilities for grain may be left to dry in the sun. Damage to protection against mould and pests are very drains and blockage of pipes by nests can variable. Stores range from small wooden enclosgive rise to secondary storage problems through ures in the barn, to round steel bins holding 25- promoting local dampness in some stores. 80 tonnes, to silos of larger capacities. Good onfarm storage facilities allow farmers to choose the time to sell, to receive the best prices. The degree of centralization depends upon the Design of storage facilities The requirements of long term safe storage are marketing regime within the country of producprotection against dampness caused by weather tion. In North America, Country elevators and or other sources, micro-organisms, destructively Terminal elevators with storage capacity up to high temperature, insects, rodents and birds, 500,000 tonnes exist. The country elevators provide objectionable odours and contaminants and un- a local staging en route to terminal elevators which authorized disturbance. Clearly the simplest stores include high-capacity equipment for cleaning, such as piles on the ground, unprotected, are suit- drying and conditioning of grain. The term able for short periods only. Other simple stores, ‘elevator’ is applied to the entire facility although
STORAGE AND PRE-PROCESSING 111 it refers literally to the mechanism(normally belt Settling is a continuous process arising in part and bucket) by which grains are raised to a from the collapse of hulls, brush hairs, embryo level from which they can be deposited into the tips etc large capacity silos invariably found on the Elevators are associated with good transport Control of pests and spoilage of grains facilities by road, rail, water or all three. Many are capable of loading grain into vessels at a rate A It is sometimes necessary to provide storage taken to ensure that the grain is in a suitable raIn e normal capacit of an condition for storing Criteria for the latter include elevator facility or elsewhere. In such conditions a suitably low moisture content, a low mould a relatively inexpensive expedient is the flat store. count and freedom from insects. Wheat contain- This is little more than a cover for a pile of dry ing live insects can be sterilized by passage grain adopting its natural form as poured Such through an entoleter(fig. 5.7), run at about 1450 a form is described by the angle of repose In the rev/min (BP 965267 recommends speeds of 3500 case of wheat the angle is 27 to the horizontal, rev/min for conditioned wheat, 1700 rev/min for hence fat stores have roofs close to this angle. dry wheat. Hollow grains and insects may be Very temporary stores may make use of inflatable broken up and can be removed by subsequent aspiration Flat stores are easy to fill but, as they have flat floors, removal of stocks is more difficult, usually requiring the use of mechanical shovels In con trast, silos usually have a floor formed like a conical hopper whose walls make an angle greater than 27 to the horizontal. Piles created by grains falling freely from a central spout are not uniform as whole grains tend to roll from the apex down the sloping surfaces. Small impurities and broken grains roll less readily and thus become trapped in the central core of the pile. Such a core is described as the spoutline. As the interstices can amount to 30% of the spoutline can reach that level. Because air irculation and hence heat loss is prevented, the spoutline can be associated with early deterioration through overheating. The diameter of the spoutline is proportional to the width of the bin Also in contrast to tall tower -like stores fat stores require little strength in the side walls. In a silo much of the pressure of the column of grain is borne not by the floor but by the side walls This is because each grain rests on several grains below it so that some of the weight is distributed laterally until it reaches the walls and, by friction and tiswunrGaceordingteche eereul ype. W,heara FiG5.7 Diagrammatic section through an En rests on them. In all stores some settling occurs ey:2.3 impeller: scnu is relatively dense and settling may be only 6% taken by air. (Reproduced from Milling, 1969 of volume but oats may pack as much as 28%
STORAGE AND PRE-PROCESSING 111 Settling is a continuous process arising in part from the collapse of hulls, brush hairs, embryo tips etc. Control of pests and spoilage of grains in store Deterioration in store is less likely if care is taken to ensure that the grain is in a suitable condition for storing. Criteria for the latter include a suitably low moisture content, a low mould count and freedom from insects. Wheat containing live insects can be sterilized by passage through an entoleter (Fig. 5.7), run at about 1450 rev/min. (BP 965267 recommends speeds of 3500 rev/min for conditioned wheat, 1700 rev/min for dry wheat.) Hollow grains and insects may be broken up and can be removed by subsequent it refers literally to the mechanism (normally belt and bucket) by which grains are raised to a level from which they can be deposited into the large capacity silos invariably found on the sites. Elevators are associated with good transport facilities by road, rail, water or all three. Many are capable of loading grain into vessels at a rate of 2,750 t/h. It is sometimes necessary to provide storage for grain beyond the normal capacity of an elevator facility or elsewhere. In such conditions a relatively inexpensive expedient is the flat store. This is little more than a cover for a pile of dry grain adopting its natural form as poured. Such a form is described by the angle of repose. In the case of wheat the angle is 27" to the horizontal, hence flat stores have roofs close to this angle. Very temporary stores may make use of inflatable covers. aspiration. Flat stores are easy to fill but, as they have flat floors, removal of stocks is more difficult, usually requiring the use of mechanical shovels. In contrast, silos usually have a floor formed like a conical hopper whose walls make an angle greater than 27" to the horizontal. Piles created by grains falling freely from a central spout are not uniform as whole grains tend to roll from the apex down the sloping surfaces. Small impurities and broken grains roll less readily and thus become trapped in the central core of the pile. Such a core is described as the spoutline. As the interstices can amount to 30% of the occupied space, fines in the spoutline can reach that level. Because air circulation and hence heat loss is prevented, the spoutline can be associated with early deterioration through overheating. The diameter of the spoutline is proportional to the width of the bin. Also in contrast to tall tower-like stores, flat stores require little strength in the side walls. In a silo much of the pressure of the column of grain is borne not by the floor but by the side walls. This is because each grain rests on several grains below it so that some of the weight is distributed laterally until it reaches the walls and, by friction, rests on them. In all stores some settling occurs ~~~~~~~~~~~~~~~ff~~~~~~~~~~. R+M is re1ative1y dense and sett1ing may be Only 6oh of volume but oats may pack as much as 28%. FIG 5.7 Diagrammatic section through an Entoleter Aspirator. 1, fixh ide~; 2, 3, e; 4< swkg mm; -5< .ph discharge over 6, cone; 7, valve controlling air flow. Arrows indicate path taken by air. (Reproduced from Milling, 1969, Oct 10, by courtesy of the Editor.)
112 TECHNOLOGY OF CEREALS The store itself should provide protection from Pesticides used to control insects, during weather(particularly wet)and intrusion by insects of cereal grains, are of two types. Those and rodents. High temperatures are undesirable designed as a respiratory poison, and are and variation should be reduced to a minimum applied as gas or volatile liquid, are described as as this can lead to local accumulation of moisture fumigants. Those designed to kill by contact or All spoilage agents depend upon respiration ingestion are described as insecticides. They may and hence a depletion of oxygen inhibits their be applied in liquid or solid form proliferation and activity. To achieve this it is Of the gaseous fumigants, methyl bromide and necessary to provide a seal around the grain and phosphine(PH3)are the main examples. Examples a minimal headspace. In a sealed store of liquid' fumigants are mixtures of 1, 2 dichloro- depletion can be achieved by natural or ethane and tetrachloromethane: although the neans Natural depletion results from respiration most effective fumigant is methyl bromide, this which in most organisms consumes oxygen and gas does not penetrate bulk grain well and the produces carbon dioxide. Artificial atmosphere use of a carrier gas such as tetrachloromethane ontrol comes about by flushing of interstitial and is an alternative to the fan-assisted circulatory head spaces with a gas other than oxygen, usually Few stores have the necessary misused alone systems required if methyl bromid nitrogen or CO as these are relatively inexpensive Complete removal of oxygen is not possible t required depends Experiments carried out in artificial conditions in upon the susceptibility of the species of insects the U. K showed that baking properties of wheats present to the fumigant. For example a three day ere maintained for eighteen years in low oxygen exposure to phosphine may eliminate the saw- conditions. At ambient temperatures germinative toothed grain beetle but six days at low temperature energy was seriously reduced and although this may be needed to kill the grain weevil reduction was prevented by storage at 5C this 'Liquid fumigants penetrate bulks well. The was the only advantage of low temperature in proportions need to be adjusted to suit the depth addition to oxygen depletion recorded(Pixton, of the grain stored Up to three metres deep a 3: 1 mixture of 1.2 dichloroethane tetrachloro Sealed conditions are unusual and prevention methane is suitable but for penetration to a depth of spoilage in many cases depends upon careful of 50 m equal proportions are needed Fumigation maintaining of the stored grains'condition, and requires the stores to be sealed to prevent escape prophylactic treatments with chemicals. Fortun- of the toxic fumes. ately, nearly all threats to grain quality cause temperature rises and monitoring of temperature, Pesticide residues through incorporation of thermocouples,can reveal a great deal about condition Some of the various types of pesticide(herbi- Forced ventilation can reduce temperatures cides, fungicides, insecticides and rodenticides) but it may be necessary to remove the cause by used in the field or in storage, may persist in use of chemical treatments. Such treatments are grains being processed or indeed into foods as relevant primarily when the problem is caused consumed In the U. K. the maximum residue limits by insects. Because of the possible persistence of (MRLs) permitted are mostly defined in EC pesticides on cereals, their use in stores is increas- Directive 86/232 and amendment 88/298, which ingly becoming regarded as a last resort. In most came into force on 29 July 1988. Additional countries strict codes of practice apply to their MRLs came into force in December 1988 refer- Ise. In the U. K. the legislation is contained in ring to pesticides that have been refused approval the Food and environment Protection Act 1985, in the u. k. but are used elsewhere; or those that the Control of Pesticides Regulations 1986 and have been consistently found in U. K. monitoring, HSE Guidance Notes(E440/85)on Occupational where the limits provide a check that good agri Exposure Limits cultural practice is being observed(see Table 5.5)
112 TECHNOLOGY OF CEREALS The store itself should provide protection from Pesticides used to control insects, during storage weather (particularly wet) and intrusion by insects of cereal grains, are of two types. Those that are and rodents. High temperatures are undesirable designed as a respiratory poison, and are hence and variation should be reduced to a minimum applied as gas or volatile liquid, are described as as this can lead to local accumulation of moisture. fumigants. Those designed to kill by contact or All spoilage agents depend upon respiration ingestion are described as insecticides. They may and hence a depletion of oxygen inhibits their be applied in liquid or solid form. proliferation and activity. To achieve this it is Of the gaseous fumigants, methyl bromide and necessary to provide a seal around the grain and phosphine (PH3) are the main examples. Examples a minimal headspace. In a sealed store oxygen of ‘liquid’ fumigants are mixtures of 1,2 dichlorodepletion can be achieved by natural or artificial ethane and tetrachloromethane: although the means. Natural depletion results from respiration most effective fumigant is methyl bromide, this which in most organisms consumes oxygen and gas does not penetrate bulk grain well and -the produces carbon dioxide. Artificial atmosphere use of a carrier gas such as tetrachloromethane control comes about by flushing of interstitial and is an alternative to the fan-assisted circulatory head spaces with a gas other than oxygen, usually systems required if methyl bromide is used alone. nitrogen or COz as these are relatively inexpensive. Few stores have the necessary fans. Complete removal of oxygen is not possible. The period of treatment required depends Experiments carried out in artificial conditions in upon the susceptibility of the species of insects the U.K. showed that baking properties of wheats present to the fumigant. For example a three day were maintained for eighteen years in low oxygen exposure to phosphine may eliminate the sawconditions. At ambient temperatures germinative toothed grain beetle but six days at low temperature energy was seriously reduced and although this may be needed to kill the grain weevil. reduction was prevented by storage at 5°C this ‘Liquid’ fumigants penetrate bulks well. The was the only advantage of low temperature in proportions need to be adjusted to suit the depth addition to oxygen depletion recorded (Pixton, of the grain stored. Up to three metres deep a 1980). 3: 1 mixture of 1,2 dich1oroethane:tetrachloroSealed conditions are unusual and prevention methane is suitable but for penetration to a depth of spoilage in many cases depends upon careful of 50 m equal proportions are needed. Fumigation maintaining of the stored grains’ condition, and requires the stores to be sealed to prevent escape prophylactic treatments with chemicals. Fortun- of the toxic fumes. ately, nearly all threats to grain quality cause Pesticide residues temperature rises and monitoring of temperature, through incorporation of thermocouples, can reveal a great deal about condition. Some of the various types of pesticide (herbiForced ventilation can reduce temperatures cides, fungicides, insecticides and rodenticides) but it may be necessary to remove the cause by used in the field or in storage, may persist in use of chemical treatments. Such treatments are grains being processed or indeed into foods as relevant primarily when the problem is caused consumed. In the U. K. the maximum residue limits by insects. Because of the possible persistence of (MRLs) permitted are mostly defined in EC pesticides on cereals, their use in stores is increas- Directive 86/232 and amendment 88/298, which ingly becoming regarded as a last resort. In most came into force on 29 July 1988. Additional countries strict codes of practice apply to their MRLs came into force in December 1988 referuse. In the U.K. the legislation is contained in ring to pesticides that have been refused approval the Food and Environment Protection Act 1985, in the U.K. but are used elsewhere; or those that the Control of Pesticides Regulations 1986 and have been consistently found in U.K. monitoring, HSE Guidance Notes (E440/85) on Occupational where the limits provide a check that good agriExposure Limits. cultural practice is being observed (see Table 5.5)
