Formation Mechanisms of Losses in Grain Storage and Basic Principles of Storage
11 October 201323 min reading
Doç. Dr. Mustafa ERBAŞ, A. Nur DURAK, Sultan ARSLAN
Akdeniz University Faculty of Engineering, Department of Food Engineering
As grains are harvested at a certain period of the year and used in human and animal nutrition all year round, it is an important issue that they should be stored well without spoiling. Storing various products in a way to be able to protect the existing amount and quality for a certain period of time is defined as storing. Within this concept; storing in a way to be able to protect grains’ amount and biological, technological, nutritional and economic quality is defined as grain storage (TSE, 2003).
With grain storage; protection of grain’s food, feed and seed quality until the next harvest period or longer is aimed. Grains can be stored for a long time without losing their quality by their nature when they are stored in good conditions (Loewer et al. 1994).
Around the world; grain farming is mostly done in the climate conditions that are not appropriate for storage in terms of temperature and humidity values. Grains can also be spoiled when they are stored in high temperature and relative humidity conditions. Because of this situation; besides the losses of amount and quality of the grain, major risks can also be formed in terms of consumer health. Furthermore, using spoiled grains in animal nutrition create risks on human health indirectly (Brooker and et al., 1992; Posner and Hibbs, 2005; Weinberg and et al., 2007).
Damage and losses in grain storage may vary depending on the development levels of the countries. These losses are around 10 % in Turkey and in the world annually (Ekmekçi and Ferizli, 2000; Işıkber and et al., 2005; Dizlek and et al., 2008; Tunç and Erler 2008; FAO, 2011). In some conditions; some grains spoiled as a result of molding, can be put in the market after some processes like washing and husking.
As grain-based products are consumed widely by the entire society, carcinogenic mycotoxins such as aflatoxin and ochratoxin create extremely important risk on public health and cause dangerous exposures.
Cereal grain basically consists of replacement food store called as endosperm, a live plant draft that is called as embryo and respiring and a multi-layered husk protecting the others. Storage losses arise from the structure of the respiring live grain and common and complex interactions of the variable environmental conditions. Improper storage conditions both accelerate the respiratory activities grains and increase microbiological activity and pest activities.
The most important factors of the losses occurred during grain storage are grain respiration, microbial activity and pest activities. The size of these losses arising from those factors can be controlled with basically grain water content and silo conditions like relative humidity, temperature and composition of the atmosphere (Elgün and Ertugay, 2000; Posner and Hibbs, 2005; Özkaya and Özkaya, 2005).
In order to store grains without spoiling; the water content of the grain should be low and this low amount of water content should be maintained during storage. For a good grain storage and storage continuity; grains should be ripped enough, undamaged-solid; grain water content should be lower than 14 %, busk temperature lower than 15°C and the relative humidity of the storage atmosphere lower than 65 % (Lower and et al., 1994; Elgün and Ertugay, 2000).
In an explanation by Turkish Grain Board (TMO) in 2011; it was reported that ın Turkey there is 16 million-ton storing capacity 4 million of which belongs to TMO and 12 million of which belongs to private sector, there is need of another 7 million storing capacity and licensed storage operations are also encouraged (TMO, 2011). Nearly half (8 million tons) of the total 16 million-ton storing capacity consists of metal silos having various equipment, the remaining half consists of various storages in more primitive conditions.
1.FACTORS THAT FORM THE STORAGE LOSSES AND EFFECT MEHCANISMS
Cereal grains get the energy they need in order to maintain their viability through respiration. Respiration is defined as the mechanism of obtaining energy by lysing the oxygen in its structure with the help of the grain’s enzymes. As it is seen in the reaction below while carbohydrate content of the grain is consumed via respiration; carbon dioxide, water vapor and heat energy are formed during this process. While 60 % of the 690 kcal energy obtained by using one mole glucose in the respiration is used for viability, the rest 40 % (276 kcal) is radiated to the environment as heat and causes increase in the internal temperature of the storage (Elgün and Ertugay 2000; Posner and Hibbs, 2005). The increase in internal silo temperature due to respiration and/or atmospheric reasons increases the microbial activity and respiration and pest activities. These increasing activities cause the increase in the internal storage temperature and relative humidity cyclically.
C6H12O6 + 6O2 - 6CO2 + 6H2O + 690 kcal (Respiration reaction)
With a change in the storage atmosphere due to the respiration activity of the grains, while the oxygen value tends to decrease; carbon dioxide, water vapor and temperature values tend to increase (Elgün and Ertugay, 2000; Özkaya and Özkaya, 2005; Posner and Hibbs, 2005). Besides the grain respiration, microorganism and pest activities also contribute a lot to the change of the storage atmosphere gas composition.
Water vapor formed during the respiration activity causes the relative humidity inside the storage to increase; relative humidity inside the storage causes the grain water content to increase by affecting the balance between the grain and humidity that is hygroscopic.
Increased grain water content accelerates the respiration activity of the grain. If an intervention from the outside is not made, this situation continues cyclically in an extension that can be dangerous for the storage (Elgün and Ertugay, 2000; Posner and Hibbs, 2005).
Besides; the condensation of the increased relative humidity of the storage on cold surfaces in droplets causes the formation of a quite appropriate environment for microorganism activities and especially switching of the mold spores to the vegetative form.
There is an important relation between the grain water content and respiration speed. After passing the critical water content level that is 14 %, the respiration speed of the grains accelerates fast. Carbon dioxide, water vapor and heat generation increase together with the respiration speed.
As the water content of the grain increases, mold respiration increases together with the respiration speed of the grain. In a wheat storage research; it was determined that carbon dioxide generation arises only from the grain respiration at a 14.3 % water content rate and it was taken as 1 unit. In the same research, while grain respiration is 1.25 unit when the water content rate is 14.6 %, respiration as a result of the mold spores is 5 units; when the water content is 16 %, grain water content is 2.5 units and the mold respiration is 75 units (Elgün and Ertugay, 2000). In another research on the relation between water content of corn grain and storage quality; the water content of the samples was set as 14, 16, 18, 20 and 22 % and these samples were stored for 75 days 30°C. It was determined that corn grains with high water content have high dry matter loss, low germinating rate and high mold and bacteria content (Weinberg and et al., 2007).
As the heat energy that causes the temperature to increase by being released during respiration is obtained from carbohydrates, grain has losses in terms of dry matter (Pomeranz 1988). These dry matter losses that are at acceptable levels in good storage conditions may increase to quite high levels in bad storage conditions (Pomeranz, 1988; Elgün and Ertugay, 2000; Cauvain and Young, 2009). In a research, it was reported that visible mold formation emerges only after loss of dry matter at a 0.2-0.4 % rate is occurred (Magan and Aldred, 2007).
1.2 Microbial Activity
Grains are accessible products for microorganism and pest contamination due to production, harvesting and transport conditions. There are significant amounts of bacteria, yeast and mold on grains due to these contaminations. These microorganisms cannot show any activities on dry grains due to low water activity and protective husk and be inactive on the grain, they cause spoiling of the grains by getting active with appropriate moisture and temperature conditions during storage operations.
However; as molds can show activity in lower water activity and temperature conditions than bacteria and yeasts due to their physiology, they have much more important part in spoilage of the grains than other microorganisms (Loewer and et al., 1994; Dendy and Dobraszczyk, 2001; Magan and Aldred, 2007).
Mold growth depends on grain water content and storage conditions like temperature, relative humidity and oxygen content. After the grain water exceeds 14 % critical storing water content value, mold activity in the storage environment increases in parallel with the increasing temperature. Mold activity that is seen first in the places where the water droplets are continues on the grains that lost its mechanical integrity and embryo parts of the grains (Elgün and Ertugay, 2000; Özkaya and Özkaya, 2005; Posner and Hibbs, 2005).
Storage molds generally belong to Aspergillus, Penicillum and Fusarium species. These species develop mostly after the storage relative humidity exceeds 70 %. These types of molds are really dangerous as they breed toxic metabolites called as mycotoxin like aflatoxin, ochratoxin, zearalenone and patulin.
These mycotoxins cause various damages on human and animal vital systems and cancers. As a mycotoxin bred by Aspergillus molds, aflatoxin is defined as 1st degree carcinogenic by International Agency for Research on Cancer (IARC). As the temperature and humidity conditions of the environment become suitable for bacteria and yeast activity as a result of mold activity, these microorganisms cause the spoilage of the product more (Kent and Evers, 1994; Binder, 2007; Gregori and et al., 2012).
While molds can grow at optimum 25-30°C, some molds can grow at temperatures from 0°C to 55°C. Molds can grow fast in foods that are rich in carbohydrate and lipid whose physical integrity is damaged.
For the mold growth the relative humidity of the environment should be over 62 % and the material water content should be over 10 %. As molds are aerobic livings, their growth fairly slows down in the environments with low oxygen (O2) or with carbon dioxide rate (CO2) higher than 12 %. It is detected in a study on gas composition of the storage and the formation of ochratoxin that ochratoxin production in an environment with a water activity value of 0.80 and CO2 level over 30 % completely stops. It is detected by another study that the toxin produced by Fusarium type of molds, is not formed in a storage environment with 50 % CO2 and 20 % O2 (Magan and Aldred 2007).
As the grain and mold respiration increases after the grain water content and storage relative humidity exceed critical levels, the grain starts to spoil. The releases heat and water encourages grain and mold respiration.
Even if the carbon dioxide rate in the storage atmosphere exceeds 12 % and the respiration is pressured, grains continue spoiling due to hydrolytic enzyme activities (Loewer et al., 1994; Dendy and Dobraszczyk, 2001; Magan and Aldred, 2007). As the bacterial spoilage in the grains start after the storage relative humidity exceed 90 %, it emerges in the spoiled grains that are already spoiled by mold activities (Posner and Hibbs, 2005).
1.3 Pest Activity
Livings like insect, mite, rat and bird are called as pests. Because of their numbers and breeding speed insects and mites constitute the most important pest group that gives damages to the grains. These insects and mites give damages to the grains in order to feed and lay eggs. Besides grain losses in the storage due to the activities of these pests, significant technological and sensory quality losses are occurred for the products and they become risky for the health (Posner and Hibbs, 2005; Tunç and Erler 2008).
Storage pests such as Sitophilus granarius L. (weevil), S. oryzae L. (rice louse), S. zeamais Motschulsky (corn borer) (Col: Curculionidae), Trogoderma granarium (Everts) (Khapra beetle) (Col.: Dermestidae), Rhizopertha dominica F. (crop hump beetle) (Col.: Bostrychidae), Tribolium confusum Jac. Du Val. and T. castaneum (Herbst) (half-blood lice) (Col.: Tenebrionidae), Ptinus fur L. (white-spotted spider mite) (Col.: Ptinidae), Nemapogon granellus L. (=Tinea granella L.) (crop warehouse moth) (Lepidoptera: Tineidae), Ephestia kuehniella Zeller (mill moth) (Lepidoptera: Pyralidae), Acarus siro L. (flour mite) (Acari: Acaridae) are the important insect and mite species that give damage to the grains (Kırtok, 1988; Özer and et al., 1989; Loewer and et al., 1994; Dendy and Dobraszczyk 2001; Işıkber, 2005; Tunç and Erler 2008).
Significant grain losses are occurred when the temperature and humidity values appropriate for insect and mite activities are formed in storage conditions during long storing durations. Besides, local temperature and humidity increase as a result of this pest activity, increases the respiration and mold activity.
While the insect activity is scarcely any low in the approximately 10 % grain water content and at 5°C, it can reach to the highest level in 13 % grain water content and at 21°C. as the temperature increases, insects continue to give damage by migrating to the cooler areas of the storage. In a research made on crop hump beetle (R. dominica) as one of the wheat pests; it was determined that this pest can reach its activities that are started on the surface of the storage to 15 m depth in 4-month duration Flinn and et al., 2004).
When the temperature is increased over 50°C, the insects and their eggs can be inactivated but the germination power of the grain is also damaged over 43°C. . When the oxygen rate of the atmosphere inside the storage is lower than 2 % and/or the carbon dioxide rate is higher than 12 %, the insect and mite activities are minimized (Loewer and et al., 1994; Elgün and Ertugay, 2000; Flinn and ark., 2004; Posner and Hibbs, 2005).
Being milled together with the wheat or formation of these insects themselves and their body rash (larval sloughs, pupa coverings and body residues of deceased persons), feces and nets made by the larval of especially Lepidoptera insects in the flour can cause important quality losses in the flour and health problems of the consumer.
Besides causing bad smells in the flour, mite feces cause health problems like allergic skin and lung diseases (asthma) for the consumer (Kent and Evers, 1994).
Known also as vertebrate pests, rats and birds give significant damages to the grains. Rats that hive damages to the grains are split into three groups as Rattus norveginucus (Norway rat, brown rat), Rattus rattus (roof or ship rat, black rat) and Mus musculus (home rat); birds are mainly defined as local bird species living in the silo area (Loewer and et al., 1994).
Besides consuming the grains, these pests give harms by damaging the grain integrity and contaminating their feces, which pathogen microorganisms for humans, to the grains. As a result of these pest activities, serious diseases such as typhoid fever and salmonellosis can be occurred in humans besides rats can damage the storage system by gnawing the various equipment such as conveyor and automatic control systems.
For the struggle with the pests; preventing product scattering in the environment, closing the storage entrances, eliminating the dead spots inside the storage, pre-cleaning of the grain before storing, taking sanitation precautions like entolation and aspiration have an important role. Residuals of the various chemicals like pesticide used widely in struggle with the pests on products can cause serious health problems (Kent and Evers, 1994). Today phosphine gas (PH3) gas and fumigation are applied widely against pests at grain facilities (Ridley and et al., 2011).
1.4 Mechanical effects and foreign substance damages
Grains lose their grain integrity by being cracked due to mechanical reasons during harvest, transportation and transfer activities. Enzymes and substrates in separate sections in a solid grain can accelerate grain spoilage by gathering together after the elimination of grain integrity as a result of cracking.
Especially the lipids cause the grains go bitter and deteriorate fast by being effected from the lipolytic enzymes even under the safe storage heat. Besides, as the disintegrated grains lose the protection of the husk, they form a good initial environment for the formation of insect and mold damage (Loewer and et al., 1994).
As organic impurities provide mold and insect contamination and environment for them to feed; inorganic impurities cause damages to the grain integrity; the grains that are not ripped enough have high grain water content, they all have negative impacts on storage (Cauvain and Young, 2009).
1.5 Enzymatic activity
Enzymatic activities continue in the ripped dry grains even in low levels. However when the grain water content exceeds 15 % or the grain starts to spoil due to mold activity; grain’s amylolytic, proteolytic and lipolytic enzymes accelerate the spoilage of the grain. Especially in the grains whose integrity is damaged; rancidity, oxidative products and free radicals can be formed as a result of lipolytic activity (Loewer and et al., 1994; Elgün and Ertugay 2000).
2. FACTORS THAT ARE EFECTIVE ON CONTROLLING STORAGE LOSSES AND EFFECT MEHCANISMS
Today grain storage methods vary from underground wells that have been used since the ancient times, above-ground stacks and storage in the sacks to high technology methods like silos. As well as silos can be constructed from concrete or metal; the installation can be moved to another place and can be made of metal due to some advantages (despite its high heat transmission disadvantage) like high technology ease of application. Metal silos can be constructed in different sizes to 10-30 m diameter and height (Loewer and et al., 1994).
When grains are stored in bad conditions, they begin to smell fusty, their acid content increases, their quality decreases and germination power decreases due to mechanical, biochemical, microbiological and pest reasons. If this situation continuous, grain spoilages called as heating and silo burn besides mold and insect damages.
These damages can be kept within acceptable limits by storing the grains in good conditions. These damages of the grains can be kept at the lowest levels by controlling various factors like storages and grain cleaning, the use of chemical pesticides, grain water content, bulk temperature, relative humidity in the storage and controlled atmosphere inside the storage.
Cleaning inside of the storage before storing, closure of openings that can cause pest entrances to the storage and pre-cleaning of the grain to be stored with some processes like screening, entolation and aspiration are necessary sanitation applications that should be done for a good storing (Elgün and Ertugay, 2000; Tunç and Erler, 2008).
2.2 Grain water content and relative humidity of the storage
Grains are hygroscopic materials that exchange water with its environment. Until the hygroscopic balance between the hygroscopic material and environment relative humidity is set, water is transferred from the point where chemical potential is high to the one where chemical potential is low (Atkins, 1988; Sarıkaya, 1993). While the relation between the grain water content and environment relative humidity shows a linear increase to 65 % relative humidity value, this increase is much faster for higher humidity values.
When the grains are kept in 65 % relative humidity for a long time, grain water content get balanced at 14 % level (Loewer and et al., 1994). In a research; the wheat with 8 % grain water content reaches to hygroscopic balance in 14 % water content after it is kept in 75 % relative humidity for 8 days; it reaches to balance in 16 % water content when it is kept in 80 % relative humidity and reaches to balance in 23 % water content when kept in 90 % relative humidity (Elgün and Ertugay, 2000).
Grain water content is the most important factor that should be kept under control in order to make storage in a secure way. For a safe storing; grain water content should be lower than 14 % and storage relative humidity lower than 65 % depending on the type of grain (Kent and Evers, 1994). In the situations where storage relative humidity is higher than 65 %, grain water content increases over 14 % and spoils due to their hygroscopic features. (Owens, 2011; Posner and Hibbs, 2005; Elgün and Ertugay, 2000). Thus the basic precaution for a good storage of the solid grains is ensuring that the grain water content is kept under 14 % and storage relative humidity under 65 %.
As these two factors are affected from heat fast, storage temperature should be kept under control well in grain storage.
According to a wheat storage research; it was determined that grains with 20 % water content at 15°C storage temperature can be stored for 40 days; grains with 16 % water content for 160 days and when the storage temperature is increased to 25°C, grains with the same water contents can be stored for 25 and 85 days respectively (Kent and Evers, 1994).
In another research in which some wheat with 16 % and 20 % grain water content is stored for 20 weeks; bulk temperature, ergosterol, mycotoxins, mold, and carbon dioxide values remain constant in low grain water content and show increase consistently in high grain water content (Abramson and et al., 2005). As it is understood from the researches, grain water content and storage temperature should be kept as low as possible in order to increase storage duration.
According to another research; grains can be stored well in the environments where grain water content is under 14 % and storage temperature is under 18°C; however in the environments where grain water content and storage temperature increase over these values, it was determined that molds, germination and insect damages are formed respectively (Kent and Evers, 1994).
In a research; it was determined that molds breed so fast that they become visible in 100 weeks when the storage temperature is higher than 10°C and grain water content is higher than 17 % (Nithya and et al., 2011).
Grain water content and temperature of the stores grains always tend to increase due to respiration, mold and pest activities. In controlling this situation, ventilation method is used widely. In ventilation, temperature and relative humidity values of the air to be used should be lower than the values of temperature and relative humidity in principal.
Due to its dependence on atmospheric conditions, ventilation cannot be used effectively all the time and the product under risk cannot be intervened when necessary. Besides; as ventilation activity increases the oxygen rate inside the storage, it should be taken into consideration that grain respiration encourages mold and insect activities and rancidity.
Besides natural outdoor air, air whose temperature is decreased with coolers can be used as well. In order to keep the grain temperature under 15°C, ventilation should be done. Grain temperature should be kept at a temperature 5°C lower than the outside atmosphere temperature with ventilation. In the storages made with critical grain water content (14 %); precautions should be taken in conditions where storage temperature exceeds 15°C (Kırtok, 1988; Loewer and et al., 1994).
2.3 Atmosphere temperature
There is a linear relation between the air temperature and the amount of moisture the air can hold. While the air temperature increases, the amount of moisture that the air can hold increases; as the air temperature decreases, the amount of moisture that the air can hold decreases as well (Atkins, 1988; Sarıkaya, 1993). While unsaturated air, in other words low vapor pressure air enables the material to dry, saturated, in other words high vapor pressure air puts condensed water droplets to the environment when it hits on cold surfaces.
During storage; storage air exists on a dynamic balance by obtaining water from the wheat and environment as the air temperature increases or by eliminating excess water in water droplets in the environment. With the increase in temperature as a result of this relation, air increases its relative humidity by obtaining water from the wheat. When the temperature decreases it leaves its excess humidity on wheat and storage wall in droplets. These droplets become important points for mold activity.
This situation causes the formation of moldy grain layers that are moulded on the walls, top or bottom of the storage especially depending on the temperature differences of summer-winter and day-night (Loewer and et al., 1994). In a research; it was determined that grain temperature on the wall, top and bottom of the storage changes all year around (Flinn and et al 2004).
During the winter months; temperature of the grain mass outside of the storage is lower than the ones in the central areas. Constituting nearly half of the total grain in the storage and being relatively warmer, in the central area relative humidity of the air is higher.
Due to the heat and mass transfer; air in other words water vapor is transferred to the area with lower chemical potential (moisture migration) and causes the increase in grain water content by condensing in droplets on that area and storage wall and also cause an appropriate environment for mold activity. Known as moisture migration, this situation can be formed due to heating in the storage not only during winter months, but also at the beginning of the summer (Posner and Hibbs, 1997; Kırtok, 1998).
As it is seen on Figure 1, the air inside the storage during winter months sinks to the bottom of the storage from the areas close to the storage wall by cooling from the storage environment. The air increases its temperature by getting the heat from the grains during this movement and leaves its moisture in the form of dew on the top of the storage by encountering with the cool air on the top of the storage after rising from the central storage. Thus a molded, caking and impermeable area is formed due to the increase in grain water content on the upper area of the storage.
During summer months; the air inside the storage causes the formation of a zone that is dump and suitable for spoilage on the bottom of the storage by reversing and also causes the growth of mold on the grain that is on the bottom of the storage (Loewer and et al., 1994; Kırtok, 1998; Dendy and Dobraszczyk, 2001; Özkaya and Özkaya, 2005). In order to prevent these spoilages; besides chemical substances, ventilation with the cool air in winter and cooling operations in summer can be used as well.
Besides their economic and amount losses in bad storing conditions; grains incur physical, chemical, biological, technological and organoleptic losses. As a result of the spoilage of grains that have an important place on the nutrition of the entire society; mycotoxins caused by mold activities increase the risk of cancer and body parts of the pests increase the risk of allergic diseases in the society.
For a good grain storage; the factors that affect each other in the grain silo like respiration, bulk temperature, grain water content and storage relative humidity should be kept under control. Besides the respiration activities of the grains; these factors are also affected from microorganism and pest activities and atmospheric conditions. To conclude, in order to prevent economic losses and health risks; sanitation of the grains should be provided by pre-cleaning before storing, during storage process grain water content should be kept under 14 %, bulk temperature under 15°C and storage relative humidity under 65 %.
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