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