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ENCYCLOPEDIA OF GRAIN SCIENCE CONTRIBUTORS’ INSTRUCTIONS PROOFREADING

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Enc Grain Sciences, article id: GRNS-00167

Manuscript queries and/or remarks - please respond to these points on returning your proofs Please check the XREF as this article title is not given in the list provided. Please provide approximately 15 suggested terms for the glossary. Please check - the article (00166) is missing from the list provided. Please make sure that the urls appearing in the ‘‘Further Reading’’ section are still valid. 200pp. or p. 200? Please check all other references for this usage. Please provide in full ‘‘Marketing Serv., Res. Div. Marketing . . . ’’. Please supply year of publication. Please supply year of publication. Please provide journal name in full. Please provide a list of relevant websites. Please confirm that all web links provided are still valid. Deep-link urls cannot be accepted. Hence, please provide a link to the main site only. Links to commercial organizations, if any, should be placed in a separate section Commercial Websites. (MQ11) Please make sure that the permission to use figures has been granted. (MQ12) Please include this in the Further Reading section.

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(MQ1) (MQ2) (MQ3) (MQ4) (MQ5) (MQ6) (MQ7) (MQ8) (MQ9) (MQ10)


Please check this information is correct

Fleurat-Lessard, F INRA Post-Harvest Biology and Technology Laboratory BP No. 81 F-33883 Villenave d’Ornon France

Key Words: Bulk storage, cooling aeration, in-bin drying, grain cleaning, storage condition, physico-chemical management, quality retention, deterioration rate, grain respiration, microbiological spoilage, preventive treatment, seed viability retention, predictive modeling, storage strategy, intervention means, safe storage time

Glossary

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MQ2

Enc Grain Sciences, article id: GRNS-00167 STORED GRAIN/Physico-Chemical Treatment

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STORED GRAIN a0005

Physico-Chemical Treatment F Fleurat-Lessard, INRA Post-Harvest Biology and Technology Laboratory, Villenave d’Ornon, France ª 2004, Elsevier Ltd. All Rights Reserved.

Nomenclature

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aw rh M T Mt

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Activity of water Relative humidity (%) Moisture content (% wet basis) Temperature ( C) Safe storage time (days)

by the farmers in order to identify quality grades; (2) the preservation of original properties and composition of freshly harvested grain; and (3) the permanent supply of the first processing industries with homogeneous grain batches of a specific quality grade. Postharvest losses of grain are a significant factor in the world food supply and may represent as much as 510% of the world production of cereal grains and oilseeds. On a qualitative basis, there is a constant risk of grain quality deterioration in storage. The quality of grain is specified with different attributes closely related to the making of a specific end product. It can be defined as the combination of an intrinsic and an extrinsic component with complex relationships. The intrinsic component of grain quality may be considered as both the initial condition and the biochemical composition of grain at the harvest (Figure 1). The extrinsic component, which includes the soundness, the purity, the sanitary, and the safety condition, is dependent on the action of deterioration factors. The two major biological causes of deterioration of properties and quality of stored grain are microorganisms (storage fungi) and invertebrate pests (insects and mites).

Introduction

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The storage of cereal grains is achieved for extended periods of time in order to supply the domestic and export needs. The storage period may last several months to a year or even more when grain is stored for market regulation objectives or as strategic reserve. At a central position along the processing chain, the storage stage has to play three essential roles: (1) the assessment of the quality of each grain load delivered

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Grain quality

Composition and fitness for processing

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Initial condition

Wholesomeness, soundness and safety

Impurities

Grain defects

Intrinsic condition

Contaminants

Physico-chemical criteria

protein content gluten quality

broken kernels

moldy grains

sprouted kernels

insect damaged

foreign matter impurities

sprout damaged shrivelled kernels

inorganic matters weed seeds other cereals dead insects

moisture

starch content

test weight

starch quality

pesticide residues

1000-kernel weight

lipids

mycotoxins

kernel size

fiber (cellulosic)

diseased kernels

heavy metals

color

heat damaged

noxious seeds

visual defects

radioactivity

hardness flour properties

minerals (ash) enzymes activity other criteria for processing

Figure 1 Basic criteria and parameters involved in the definition of grain commercial quality four components are identified: intrinsic MQ11 and physico-chemical condition; sanitary and safety condition; biochemical composition and nutritional value; and properties for procesMQ12 sing. (Adapted from Fleurat-Lessard 2002.)

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Grain Cleaning

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Cleaning grain is usually done before storage of grain lots in bins or granaries. Freshly harvested grain contains a small proportion of impurities, also called ‘‘dockage.’’ Dockage components include small or large seeds of weeds or other grains, chaff, straw, dust, soil pellets, small stones and boulders, cracked kernels, and other trash (see Contaminants of Grain (00038)). In other respects, harvested grain may contain shrivelled kernels, smaller than those of healthy grain and being in most cases heavily contaminated by pathogen fungi and their related mycotoxins. A maximum percentage of impurities is acceptable in grain trade, according to fluctuations of the respective limits for each kind of impurities fixed by each country regulations (see Defects of Cereal Grain (00042)). When this limit is exceeded, some quality criteria are affected (e.g., the test weight). Moreover, the presence of a significant amount of larger-sized impurities such as ear fragments, plant stalks, green weeds parts, grain husks, and chaff, etc., represent a serious risk for safe storage because they are a source of microbiological spoilage and lead to favorable conditions of multiplication for insects and mites. In the case of high percentage of wet impurities, grain heating may start in some parts of the grain bulk a short

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time after binning (humid hot spots). In addition, these regions with heated grains are liable to become compact enough to disrupt airflow distribution and hinder an eventual beneficial effect of drying or cooling later operations. Thus, the grain cleaning through the separation of a significant amount of impurities may improve grain condition before storage. Grain cleaning operation can be carried out with a large range of equipment, ranging from the simple sieves used in developing countries to remove light impurities (thanks to the wind) to very sophisticated high-speed industrial equipment found in grain terminal elevators. Some of these high-flow-rate cleaners may clean 1012 t of grain per hour with a work input of 1 kW h. Some other cleaning equipment combines grain cleaning and calibrating operations. It has a much lower output than specific cleaning machines and it is mainly used by the seed-producing companies before conditioning perfectly cleaned and calibrated seed grains. Winnowing machines can also be included in grain cleaning equipment even if they are mainly used to clean the part of the grain harvest kept by small farmers as their own seed reserves. From an economical point of view, the financial profitability of removing dockage and foreign material of grain devoted to export depends on the initial dockage level. Some export markets will pay a premium for dockage-free grain while other markets do not. Consequently, grain cleaning is generally not practiced in all cereal producing and exporting countries. In USA and in France, grain operators have little economic incentive to provide dockage-free grain. The delivery of cleaned grain is marginally profitable in Australia, a cereal exporting country that supports a policy of exporting high quality grain. In Australia, the grain cleaning costs are deducted by the grain handling company from the value of the raw grain delivered by the farmers to commercial grain stores.

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The deterioration of stored grain quality being irreversible, the prevention of quality losses is of prime importance for any grain store manager. To face the storage issues, he disposes off a range of equipment, tools, materials, and techniques to prevent (or reduce) the grain quality deterioration process. The stored grain treatment includes the elimination or the inhibition of the main causes of loss such as infestation by insects or mites, and contamination by microorganisms. But, minor causes of quality losses such as grain respiration, gradual deterioration of viability, nutritive quality, and end-use properties are also concerned by preventive actions. The preservation of stored grain from adverse storage conditions that may endanger its marketing value depends on different means that can be considered either preventative or corrective. The preventive means include cleaning, drying, aeration, cooling, pest control treatment, kernel breakage prevention, and controlled atmosphere storage. The corrective actions have rather recourse to quick-action treatments such as high flow-rate cleaning, fluidized-bed drying, temperature shock, fumigation, and some other grain sanitation treatments. This review of the means of stored grain preservation complements the previous works dealing with good storage practices of grain stocks that have been published in books listed at the end of this article.

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STORED GRAIN/Physico-Chemical Treatment

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In-Bin Drying

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Preliminary Considerations

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Seeds being hygroscopic, grain either absorbs moisture from the environment (under high relative humidity (rh) conditions) or looses moisture (under low rh conditions). This relationship is generally represented graphically by two typical curves, respectively, when grain is adsorbing or desorbing moisture. These curves represent the correlation between the surrounding air rh and the water activity (aw) potential in the kernel at a constant temperature (so-called isothermal sorptiondesorptionequilibrium curves). When this thermodynamic equilibrium is established, the

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equilibrium rh ð%Þ 100

½1

Available water 20 Critical point for water activity (aw) C

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Bacteria Yeasts Molds

10

Enzymes Maillard reactions

5

0 0

0.25

0.50 0.65 0.75

Oxidation Denaturation of proteins 1.0 0.93 0.85

aw

Figure 2 Grain desorption curve (wheat at 25 C) focusing on the f0010 zone in which water adsorbed in grain becomes available to support various processes of quality deterioration within specific limits of grain moisture content and water activity level. (Redrawn from Multon JL (ed.) (1988) Preservation and Storage of Grains, Seeds and their By-Products, 1096p. New York: Lavoisier Tec and Doc.)

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In this equilibrium condition, there is a close relation between the moisture content of the grain, and the aw (or the rh inside the grain bulk). Consequently, grain aw can be related to the moisture content of grain by mathematical models. Numerous mathematical models of these sorptiondesorption equilibrium curves have been built up in dependence with the temperature level for almost any type of seed or food matrix. Several researchers have regularly refined the equations that are now available for the majority of cereals and are accurate enough to be used for the monitoring of aeration or moisture migration changes in a stored grain bulk through rh sensors (eqns [2a] and [2b]). C1 rh ¼ exp expðC3MÞ ½2a T þ C2

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aw ¼

Moisture content (% w.b.)

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temperature and pressure being constant and equal in the two phases (gaseous atmosphere and grain), the grain aw can be numerically identified with the ratio of the partial pressure of water in grain surrounding atmosphere (in ‘‘empty’’ space of the grain mass) (eqn [1]):

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1 ðT þ C2Þ lnðrhÞ M¼ ln C3 C1

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where rh is the relative humidity, M the moisture content, T the temperature, and C1, C2, C3 the coefficients. The modified ChungPfost eqn [2a] gives a good fit with experimental sorption curve of cereal grain, especially for wheat and it can be used for the accurate conversion of moisture content into grain aw for several cereals.

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½2b

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Moisture Content and Allowable Safe Storage Times

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The importance of water in grain can be deduced directly from the sorption curve. From a critical aw corresponding to the end of the linear portion of the sorption curve (Figure 2), a less tightly bound water appears that becomes available for the feeding biodeterioration processes. Thus, above a limit of aw in grain the respiration becomes active. Consequently, the grain moisture content (or the aw) is the most important factor determining the intensity of grain respiration. The heat produced by the respiration of organisms living in the grain bulk increases the temperature of the grain that indirectly favored the fungal growth. Grain respiration results from the aerobic consumption of complex carbohydrates (starch) by

living organisms. This oxidation of energetic nutrients of grain also generates liquid water (eqn [3]): C6 H12 O6 þ 6O2 ! 6CO2 þ 6H2 O þ 2817kJ ½3

In airtight storage, when carbon dioxide level exceeds 10% and the oxygen levels falls down below 1%, the respiration process is inhibited and the anaerobic fermentation that occurs produces less heat (eqn [4]): C6 H12 O6 ! 2C2 H5 OH þ 2CO2 þ 209kJ

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In addition, nonuniform temperatures in the grain bulk generate air convection currents and leads to moisture migration, especially when large changes in external air temperature occur. These moisture migrations may induce the ‘‘top crusting’’ phenomenon that develops at or near the grain surface in metallic bins or in flat storage during the grain natural cooling (Figure 3). Among the various living organisms in the stored grain ecosystem, the storage fungi represent the major causes of deterioration in grain and seeds. The main deleterious effects of fungi on stored grain quality are: (1) decrease in germinability; (2) induction of changes in kernel color and external aspect; (3) induction of hot spots where grain is heating; (4) Inducing a mustiness odor that may be detectable by smelling; (5) induction of various biochemical changes; (6) production of mycotoxins that, if consumed, may be

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Enc Grain Sciences, article id: GRNS-00167 4

STORED GRAIN/Physico-Chemical Treatment Cold air

(a)

Crusting Moisture content increase

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Convection air currents

21–25°C

25–30°C

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(b)

Figure 3 (a) Moisture migration pattern in freshly harvested grain stored during several months in a nonaerated metallic bin when outdoor temperature is lower than grain temperature showing the ‘‘surface crusting’’ phenomenon (adapted from various sources). (b) Grain cooling of a flat-bed grain bulk by a blowing system of aeration with multiple longitudinal air ducts progress in cooling and observed distribution of cooled and less-cooled regions of the bulk.

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Regions with a low air flow rate

Aeration fan (blowing)

30–35°C

injurious to man and to animals; and (7) loss in weight and decrease of specific weight. p0060 The water activity (aw) of grains is closely related with the growth and the metabolic rate of storage molds. Therefore, the safe storage life of a grain bulk is dependent on the level of moisture content of grain at the beginning of storage. However, below the aw threshold of ‘‘available’’ water presence, both the germination of fungal spores and the growth of storage fungi are inhibited. Today, the different thresholds of aw allowing the development of fungal species that may grow in stored grain are well known, especially for the species that are potential mycotoxin producers such as Aspergillus flavus, A. ochraceus, Penicillium cyclopium, P. verrucosum, and P. viridiMQ1 catum (Table 1) (see Mycotoxins). This knowledge

has been regularly refined, permitting the development of predictive models of ‘‘safe storage life’’ of stored grain from indexes of deterioration such as the decrease in germinability or from composite indexes combining relevant quality criteria. These quantitative models take into account the original quality of grain and the expected storage conditions (temperature and moisture). These models permit the prediction of the quality status of stored grain at any stage or the determination of the time left before to reach the minimum acceptable level of quality. The provision of models to predict safe storage life of stored grain offers the possibility of maximizing and guaranteeing market value in terms of grain-specific utilization by the processor or the consumer. As an example, an equation applicable to the determination of the safe storage



Table 1 List of storage (xerophilic) fungi harmful for stored cereal grain quality and the associated mycotoxin production (when it exists) Mycotoxins

Scopulariopsis brevicaulis Aspergillus parasiticus Penicillium brevicompactum Aspergillus ochraceus

Aflatoxin

Aspergillus flavus Aspergillus fumigatus Penicillium aurantiogriseum

Ochratoxin Penicillic acid Aflatoxin Gliotoxin Penicillic acid

Penicillium verrucosum Aspergillus versicolor Wallemia sebi Aspergillus restrictus Aspergillus candidus Eurotium amstelodami Eurotium chevalieri

Ochratoxin Citrinin Sterigmatocystin Walleminol — Kojic acid — —

Grains more often affected

Minimal aw for growth

Corresponding grain moisture at 20 C

Oilseeds/rice/peanuts Corn/peanuts/cottonseed Peanuts/rice/corn Corn/wheat/barley/oats

0.90 0.86 0.86 0.84

22.5 19.8 19.8 19.0

Corn/peanuts/cottonseed Oilseeds/cereals Cereals/cereal products Corn/peanuts/rice Corn/wheat/barley/oats Wheat/barley/corn Pulses/cereals/by-products Wheat/rice/corn/beans Wheat/corn/by-products All cereals/by-products Pulses/wheat/corn

0.82 0.82 0.81

18.0 18.0 17.3

0.80 0.80 0.78 0.75 0.75 0.75 0.74

16.8 16.8 15.8 15.5 15.5 15.5 15.1

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Fungus species

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Lower limit of moisture content and aw enabling germination and growth at different storage temperature levels (adapted from several sources).

period for malting barley, before the limit of 95% viability level is reached, was recently validated (eqn [5]):

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Safe storage time Mt (malting barley): Mt = ln(35/T )/exp[–21.22 + 20.33 . aw]

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Safe storage time (days)

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where Mt is the safe storage time, T the temperature, and C4 and C5 the coefficients of the equation, variable with the type of cereal (e.g., for malting barley: C4 = 21.22 and C5 = 20.33). In generalizing the previous equation for malting barley, a model of calculation of sorption equilibrium was derived. It can be useful in monitoring moisture content changes during drying or aeration operations. This predictive model may be included in the knowledge base of software packages aimed at the management of stored grain treatment or they can also be used as prediction tools (Figure 4). To properly stored grain, both the grain moisture content and the temperature level must be compatible with the expected period of time the grain will be stored and with the grain intended use. The commercial upper limit of moisture content corresponds generally to critical moisture level for grain respiration activity, lowered with a safety margin of 2%. Thus, the lower limit of grain moisture content allowing the growth of storage molds at 20 C in malting barley is 16.2% and the limit recommended for safe storage conditions was recently fixed at 14% in Europe. The recommended moisture contents for stored grain are listed in Table 2 in the two common situations: local short-term storage (less than 6 months) and regional or terminal long-term storage (for more than 6 months).

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lnð35=T Þ expðC4 þ C5 aw Þ

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Mt ¼

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T = Stored-grain temperature: 10°C

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20°C 25°C

4000

30°C

2000 365 d 0 12

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Moisture content at 20°C (% w.b.) Figure 4 Quantitative model predicting the safe storage life of f0020 malting barley (95% germinability) in relation to moisture content and temperature in stored grain (constant conditions).

The Main Objectives of Drying

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In regions where the climate is humid, mature grain is harvested at moisture content levels incompatible with safe storage for a long period of time. The removal of the excess of moisture from grain can be achieved by grain drying. Thus, the prime objective of grain drying involves reducing the moisture content in harvested grain in order to minimize mold-spoilage hazard during long-term storage. Thus, drying grain to the equilibrium with an air rh of less than 70% is a necessary prerequisite for safe storage. Additionally,

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Table 2 Maximum recommended moisture contents of various grains for short-term (6 months) or long-term (more than 6 months) storage Short-term storage (less than 6 months)

Long-term storage more than 6 months)

Corn Durum wheat Edible pulse seeds Malting barley Oats Rape seeds (canola) Rye Sorghum Soybeans Sunflower seeds Wheat

15.5 14 16 14.5 14.5 9.5 14 14.5 13 10 14.5

13.5 13 13.5 13 13 8 12.5 13 11 8 13

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Grain species

grain temperature is related to the rate of moisture diffusion and therefore can be monitored and controlled in order not to exceed a maximum rate of diffusion that could produce grain damage. The rate of evaporation of external moisture is also dependent on the air temperature (at a fixed airflow). These two stages are in constant interaction and in unstable equilibrium. The drying rate is also affected by atmospheric conditions. In this situation of complex interactions between several variables, the initial setting of the parameters of the drying system cannot be predicted, and it is necessary to test the drying process with several batches of grain before determining the exact setting of the dryer on a specific storage site. The main parameters that must be taken into account in determining a grain dryer settings are given as follows:

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Adapted from several sources.

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low moisture content levels are less favorable conditions for the growth and buildup of populations of insects and mites.

1. the evaporation capacity, which is a fundamental characteristic of any dryer (expressed in kilogram of water evaporated per hour: kg h1); 2. the heating capacity, which is the quantity of heat produced by the dryer per hour (kJ h1); 3. the drying yield, which is the percentage of effectively used drying power (corresponding to the amount of water removed from grain) on the delivered power (energy consumed by the dryer); 4. the net calorific power of the dryer burner fuel, which is the heat quantity effectively delivered by the combustion of fuel (in kJ m3); 5. the specific airflow rate through the grain mass, which is the rate of the airflow per unit of grain mass (in m3 h1 of air per m3 of grain); and 6. the specific heat consumption, which represents the heat quantity required for the evaporation of 1 kg of grain water. This value is closely related to the fuel consumption and is generally minimized by the grain-dryer manufacturer.

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Grain Drying Technologies

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There are two different grain drying techniques.

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1. High-temperature drying which is performed from specialized equipment with a high grain-flow rate and mainly devoted to the drying of wet grain that cannot be stored in its initial condition at the harvest (maize, sorghum, paddy, oilseeds, and all small-grain cereals grown under wet climates). This method involves an initial high capital investment and high running costs for a drying operation achieved in a few hours in continuous-flow grain dryers. 2. Low-temperature or natural air drying, which concerns grain batches needing a moderate reduction of moisture content at the harvest. This kind of drying can be carried out with minimal specific equipment in grain stored in bins specifically equipped for drying purpose. The complete drying can last over a period of several weeks. p0085

Only the second type of drying corresponding to ‘‘in-bin’’ grain drying will be described below.

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In-Bin Grain Drying in Practice

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Drying has two basic stages: (1) diffusion of internal moisture to the surface of the kernel and (2) removal of external moisture by an airflow around the kernel. Water vapor pressure is increased inside the kernel which causes moisture to diffuse through the micropores of the seed pericarp layers. The

In addition, the evaporation capacity and the rate of dried grain produced per hour in defined conditions of initial moisture content of grain and blown air temperature level in the dryer are also taken into account. In static dryers operating in a bin equipped with air ducts in the bottom or with a perforated floor, a close relation between the temperature of the air blown through the grain mass, and the renewal rate of the air per unit of grain bulk volume must be respected in order to obtain a homogeneous profile of moisture content throughout the dried grain bulk. Different types of static driers for stored grain are used in developed countries, especially at the farm level. The original in-bin high-temperature drying system is initially loaded with a batch of grain, which remains in the bin until the drying operation is achieved. During the first phase of drying, grain is heated and in the second phase grain is cooled by

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Preliminary Considerations

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The stored grain bulk is slowly influenced by environmental temperature due to its low thermal conductivity. Although temperature has little direct influence on grain condition, it greatly influences the development of insect and mites and microorganisms, and it affects the viability of seeds. The first objective of aeration is to reduce the temperature of grain at the start of the storage period. Harvested grain typically comes into store at average outside temperature in summer, i.e., often above 25 C or more in Mediterranean or subtropical cropping areas. At such temperature levels even dry grain is at risk from insects and moisture migration within regions of the grain bulk. Grains harvested or dried at commercial moisture levels (1214% moisture) and at temperature levels as high as 2535 C cannot be preserved during a long period of time if they are not cooled at lower temperature levels, thereby inhibiting quality-deterioration processes. Moreover, the viability of seeds decreases rapidly in a few weeks in grain stored at 30 C or more.

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Purpose and Benefit of Aeration

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Aeration is the process of forcing air through grain to reduce its temperature. The main beneficial effects of grain cooling are given in the following:

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Aeration and Cooling

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limit the development of fungal microflora on humid grain (especially maize and sorghum) and the consecutive production of various mycotoxins, either produced by field fungi (when growing conditions remaining favorable for the field microflora) or by the most competitive strains of storage molds (on partially dried grain).

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ambient air aeration in order to recover its initial temperature. During the drying/hot-air stage, the grain mass is crossed from bottom to top or from side to side by the ‘‘drying front.’’ The eventual heterogeneity that may occur at the end of the drying process, when the drying front comes out the grain mass, is generally limited by mechanical stirring-up devices. In these systems, the air temperature is 5060 C with a specific flow rate of 140 m3 h1 of air per m3 of grain. During the drying operation, the grain of the bottom of the bulk is systematically drawn up at the top of the bulk by several vertical augers (Archimedean screw). Another type of dryer built on the same principle provides a continuous grain flow within the drying bin. The dried layer of grain at the bottom is regularly extracted and moist grain is automatically added above the previous batch. In this configuration, the maximum thickness of the grain layer should be limited at 24 m, and the specific airflow rate at 300 m3 h1 of air per m3 of grain, and air temperature can be set in the range 4080 C, depending on the sensibility to heat injury of the processed grain. Storage bins equipped with aeration systems can also been used in a specific process of drying called ‘‘dryeration.’’ In this process, the grain is partly dried in a conventional continuous-flow dryer and then the partially dried batch of warm grain is aerated by ambient air in a resting bin during 2 weeks. This type of process is more economical and involves less labor input compared to a continuous flow drier. However, the aeration of semi-dry grain requires specific aeration equipment. The specific flow rate must be maintained between 40 and 60 m3 h1 of air per m3 of grain. Nevertheless, after the cooling and the end of the drying operation, the grain must be transferred in a long-term storage bin. These additional handlings of the grain batch may increase the amount of broken grain. The dryeration system is used only to dry wet grain at the harvest such as maize or sunflower seeds. There is a risk for a slight loss of dry matter during the aeration of warm grain (coming from the drier at a temperature levels of 5060 C) and the cooling bin must be equipped with air extraction fans in the headspace above the grain mass in order to minimize the water condensation problems that may occur in any metallic bin. The temperature level reached by grain during conventional or dryeration drying, which is more than 60 C, is lethal for most of the hidden stages of insect primary feeders that may be present inside maize, rice or sorghum kernels at harvest. This disinfestation effect of heating grain during the drying process will be developed further with the heat shock treatment. The drying of maize cobs in a bin equipped with an inclined perforated floor has also been used in order to

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1. preservation of the technological properties of stored grain at a grade level as close as possible as its initial grade at the harvest time (e.g., baking quality of wheat, viability of malting barley); 2. limitation of the moisture migration phenomenon in bulked grain storage and related mould development, which is the principal cause of damage to several grain quality parameters (e.g., reduction of seeds viability and increase in fat acidity); 3. reduction of the dry matter loss consecutive to the natural respiration of grains remaining active in high temperature conditions. The associated risks of release of metabolic water and of hot spot forming are also reduced; and 4. prevention of the insect multiplication at a high rate of increase when temperature can be lowered below 1214 C, the level at which the rate of


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Stored grain aeration requires the provision of an airexhaust ventilator associated to an adequate storage bin design. Grain bins devoted to aeration have to be equipped with perforated ducting on the floor through which air is blown into (or sucked through) the grain. The design and the dimension of the ducts, as well as the optimal characteristics of the ventilator are described in previous reviews. The air-feeding network (pipes and ducts) has to be properly calculated to minimize the pressure drop for an appropriate airflow rate (515 m3 h1 m3). A centrifugal fan is not appropriate for aeration of bin higher than 15 m, when fan static pressure exceeds 2 kPa (e.g., for wheat aerated in a 15 m-high bin with an airflow rate of 10 m3 h1 m3). When air is blown, the fan compresses and heats the cooling air (e.g., 2 C in the conditions of the previous example). This disadvantage is overcome when air is sucked from top to bottom by a fan at ground level. When air is forced through the grain mass, it carries both a ‘‘cooling front’’ and a ‘‘moisture front.’’ The temperature front moves rapidly, this speed being governed by the rate of airflow and the temperature of the aeration air. The cooling power of this front is rather independent of the initial temperature of the grain. In well-designed aeration installations, the speed of the moisture front is so slow that wetting problems seldom

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Other Miscellaneous Treatments

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Practical Implementation of Cooling Aeration

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The use of cooling aeration was first developed to reduce problems of moisture translocation with warm grain when stored in metallic bins. But, the potential of cooling grain by forced air aeration is currently used to combat arthropods as pest and to unify temperatures, thus preventing moisture migration and hot spot formation. The theoretical basis of temperature transfer to grain by aeration either with ambient or with refrigerated air has been investigated by a large number of authors since the 1980s that lead to the production of many reference reviews. More recently, several simulation models for temperature migration during a cooling/aeration process have been produced and they may become useful supports for automatic control of aeration and for an adequate design of air ducts and ventilator characteristics. Heat and moisture migration within regions of an aerated grain bulk can now be visualized in real time for each distance element of the bulk and at each time increment using finite element/difference methods. With modern computers, real-time calculation can be carried out fast enough to produce moving images.

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occur or they are localized in very small regions of the grain bulk. There are many storage situations where ambient air conditions are not sufficient to cool grain. Nevertheless, aeration with refrigerated air achieves much lower temperatures when ambient conditions are warm. In warm climates, or when very warm grain (3540 C) is stored immediately after the harvest, aeration with ambient air may not be sufficient to control fungi on moist grain, or to preserve the germination capacity and quality of stored grain. Grain chilling through refrigerated air ventilation is regarded as an expensive method if used only for insect control purposes, but it can be justified for storage of fragile grains such as malting barley and seed grains in hot conditions, when retention of viability is required. Refrigerated air units for chilling grain have been developed to enable aeration to be carried out in summer for temperate regions of Europe and North America and in tropical climates where aeration with ambient air is totally impractical.

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increase of the most cold-tolerant species falls to zero.

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Mechanical Impact, Turning, and Pneumatic Conveying

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When grain is warming in a bin, it can be turned (e.g., out-loaded and returned to storage) to help maintain a homogeneous temperature and, if any, to eliminate the ‘‘hot spots’’ or ‘‘caked grain’’ during turning bulk grain from one bin to another. However, at each time grain is moved, there may be a loss due to additional breakage. In addition, when ‘‘hot spots’’ are effectively formed, the fungal spoilage and contaminated grain with mycotoxins can be mixed with the sound part of the grain batch during moving. There is only a slight decrease in the average temperature of grain during turning from bin to bin.

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Limitation of Kernel Breakage

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The free-fall of grain during empty bin filling or throwing grain through grain thrower in loading vessel holds or flat-bed storage compartments presents some serious risks of kernel breakage. This increase of the percentage of broken kernels, a part of the dockage, can lower the grade and the commercial value of an entire grain batch. The maximum breakage is obtained from drops on a concrete surface inclined 45 , and minimum breakage is observed from drops onto a grain surface. Grain handling through a bucket elevator is also a source of significant breakage. Reducing grain velocity and impact by different systems reduces physical damage. Drops of less than 12 m, or on a layer of grain reduces the breakage. Increasing the size of the

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Deteriorative forces

Initial condition Variety Grade

Mites Insects

Defects

Storage molds

Impurities

Grain respiration

Physico-chemical condition

Small vertebrate pests

Safe storage life and issues

Insecticide application

Storage bin structure

Fumigation

Sanitation practices

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Drying

Handling facilities

Cooling

Local climate

Cleaning

Intervention means

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Storage plant environment

Figure 5 Cause-and-effect diagram (Ishikawa fishbone chart) applied to the stored-grain ecosystem: description of the changes with time and storage conditions of quality criteria of malting barley.

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Further Reading

Christensen CM (ed.) (1982) Storage of Cereal Grains and their Products, 544p. St. Paul, MN: American Association of Cereal Chemists. FAO (1985) Manual of Pest Control for Food Security Reserve Grain Stocks, 200p. Plant Production and Protection Paper No. 63. Rome: FAO. Highley E, Wright EJ, Banks HJ, and Champ BR (eds.) (1994) Proceedings of the 6th International Working Conference on Stored Product Protection, vol 2, 1274p. Wallingford: CAB International. Holman LE (1960) Aeration of Grain in Commercial Storage. US Dept. Marketing Serv., Res. Div. Marketing Research Report No. 178 (http://www.ext.nodak.edu). Jayas DS, White NDG, and Muir WE (eds.) (1995) StoredGrain Ecosystems, 757p. New York: Marcel Dekker. La Documentation Française (1988) Conservation des grains en re´gions chaudes, 546p. Paris, France: La Documentation Française. Manalabe RE (xxxx) Grain Aeration (http://www.fao.org). Mekvanich KK (xxxx) Mobile Maize Dryer Development at Farm and Cooperative/Collector Levels (http:// www.fao.org). Multon JL (ed.) (1988) Preservation and Storage of Grains, Seeds and their By-Products, 1096p. New York: Lavoisier Tec and Doc. Navarro S and Noyes R (eds.) (2001) The Mechanics and Physics of Modern Grain Aeration Management, 647p. Boca Raton, FL: CRC Press. Niquet G and Lasseran JC (1991) Guide pratique stockage et conservation des grains a` la ferme (http:// www.fao.org/inpho/vlibrary/x0163f/X0163f08.htm). Pfost HB, Maurer SG, Chung DS, and Milliken GA (1976) Summarizing and reporting equilibrium moisture data for grains. ASAE Paper No. 76-3520.

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grain stream is also beneficial. Reduced bin height, additional equipment to reduce velocity, and slower handling of grain can reduce kernel breakage hazards.

Future Prospects

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The grain bulk is an ecosystem and, in most cases, stored grain treatments are used as preventive methods in order to prevent the occurrence of biodeterioration processes during the expected storage time (Figure 5). The stored grain quality management more often requires the combination of different methods and approaches to give optimal results. This combination needs to take into account the wholeness of the user’s requirements in order to meet his demand, e.g., for residue-free grain or viability-guaranteed seeds after a long storage time. Recently, a qualitative reasoning approach was developed to support storage treatments combination and their logical chaining (see Stored Grain: Pest Management (00210)). This new approach should allow the grain handlers to more easily fulfill the buyer’s quality expectations and to minimize the costs of stored grain preservation.

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See also: Chemicals for Grain Production and Protection (00032). Contaminants of Grain (00038). Defects of Cereal Grain (00042). Diseases and Pests of Plants (00044). Food Safety Through the Production Chain (00055). Organic Growing of Grains (00120). Stored Grain: Handling from Farm to Storage Terminal (00164); Non-Vertebrate Pests (00165); Pest Management (00210); MQ3 Vertebrate Pests (00166).

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STORED GRAIN/Physico-Chemical Treatment Sun Da-Wen and Woods JL (1994) The selection of sorption isotherm equations for wheat based on the fitting of available data. J. Stored Prod. Res. 30: MQ9 2743p.

Relevant Websites

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Sauer DB (ed.) (1992) Storage of Cereal Grains and their Products, 4th edn., 630p. St. Paul, MN: American Association of Cereal Chemists. Shejbal J (ed.) (1980) Controlled Atmosphere Storage of Grains, 608p. Amsterdam, The Netherlands: Elsevier. Sinha RN (ed.) (1973) Grain Storage: Part of a System, 482p. Westport, NY: The Avi Publishing.

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