ICS 65.040.20  
 
Reference number  
DRS 231: 2019  
© RSB 2019  
  
 RWANDA 
STANDARD  DRS 
231  
 
Second edition  
2019 -mm-dd 
(Reaffirmed 2019 -mm-dd) 
Storage of grains and pulses — Practical 
recommendations  
   
DRS 231: 2019  
©RSB 2019  - All rights reserved                  ii 
 In order to match with technological development and to keep continuous progress in industries, 
standards are subject to periodic review. Users shall ascertain that they are in possession of the latest 
edition  
 
 
 
 
 
 
 
 
© RSB 2019  
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized 
in any form or by any means, electronic or mechanical, including photocopying and microfilm, without 
prior written permission from R SB. 
Requests for permission to reprodu ce this document should be addressed to : 
Rwanda Standards  Board  
P.O Box 7099 Kigali -Rwanda  
KK 15 Rd, 49  
Tel. +250 788303492  
Toll Free: 3250  
E-mail: info@rsb.gov.rw  
Website: www.rsb.gov.rw  
ePortal: www.portal.rsb.gov.rw   DRS 231: 2019  
iii                ©RSB 2019  - All rights reserved  
 Contents  Page 
Foreword  ................................ ................................ ................................ ................................ ............................  iv 
1 Scope  ................................ ................................ ................................ ................................ ......................  6 
2 Normative references  ................................ ................................ ................................ ............................  6 
3 Terms and definitions  ................................ ................................ ................................ ...........................  6 
4 Requirements  ................................ ................................ ............................. Error! Bookmark not defined.  
5 Specific guidelines  ................................ ................................ ................................ ................................  8 
5.1 Pre-harvesting  ................................ ................................ ................................ ................................ ...... 8 
5.2 Harvesting  ................................ ................................ ................................ .Error! Bookmark not defined.  
5.3 Threshing  ................................ ................................ ................................ ...Error! Bookmark not defined.  
5.4 Drying of  grain  ................................ ................................ .......................... Error! Bookmark not  defined.  
5.5 Cleaning a nd sorting  ................................ ................................ ............... Error! Bookmark not defined.  
5.6 Packaging  ................................ ................................ ................................ ..Error! Bookmark not defined.  
5.7 Transportation  ................................ ................................ .......................... Error! Bookmark not defined.  
5.8 Storage  ................................ ................................ ................................ .......Error! Bookmark not defined.  
6 Major storage pests and their control  measures  ................................ ...Error! Bookmark not defined.  
Annex  A (normative)  Major storage pests and their c ontrol measures for milled rice  ..............................  9 
. DRS 231: 2019  
©RSB 2019  - All rights reserved                  iv 
 Foreword   
Rwanda Standard s are prepared  by Technical Committees and approved by R wanda Standards Board (RSB) 
Board  of Directors in  accordance with the procedures of RSB, in compliance with Annex 3 of the WTO/TBT 
agreement on the preparation, adoption and application of standards.  
The main task of technical committees is to prepare national standards.  Final Draft Rwanda S tandards 
adopted by Technical committ ees are ratified  by me mbers of RSB Board of Director s for publication and 
gazettment as Rwanda Standards.  
DRS 23 1 was prepared by Technical Committee RBS/TC 3, Cereals, pulses, legumes and cereal products.  
Committee memb ership  
The following organizations were represented on the Technical Committee on Cereals, pulses, legumes and 
cereal products (RBS/TC 3 )  in the preparation of this standard.  
Paragraph of p articipants  
: 
Rwanda  Standards Board  (RSB) – Secretariat  DRS 231: 2019  
v                ©RSB 2019  - All rights reserved  
 Introduction   
In Rwanda, grains particularly maize, rice, wheat and sorghum and pulses are among the most important 
staple foods. These grains and pulses are produced on a seasonal basis and in many places there are two 
harvests a year. In such a case, most of the production of grains and pulses must be held in storage for 
periods varying from one month up to a year in order to play a role of food security. Storage is an important 
component within a farming system to fetch better pricing for the farming com munity. The desire to stabilize 
prices  of basic  food stuffs  is one of the  major reasons  why governments  try to influence  the amount of storage 
occurring and often undertake storage  themselves.  
Grain storage facilities take many forms depending on the quantity of grain to be stored, the purpose of 
storage, and the location of the store. In general, it is recommended that grains for food purposes be stored 
with whole grains rather than milled grains as the husk provides some protection against insects an d helps 
prevent quality deterioration. However, when grains can be stored as milled grains, 20 % less storage 
capacity will be needed.  
Grains  are hygroscopic  and in open  storage  systems  the grain  moisture  content  will eventually  equilibrate  with 
the surrounding air. High relative humidity and high temperatures contribute to high equilibrium or final 
moisture content. In many tropical countries, the equilibrium moisture content is above safe storage moisture 
levels. Hence, there is a strong ne ed to manage the stored products by controlling the weather parameters 
and also from pests. This led RBS to take up the consultancy to develop standards and regulations in PHHS 
of storage of grains and  pulses.  
This standard has been developed to take into account:  
a) the needs of the market for the quality wheat  grains;  
b) the need  to facilitate  fair domestic,  regional  and international  trade  and prevent  technical barriers  to trade 
by establishing a common trading language for crop producers, buyers and  sellers;  
c) the needs of the producers in gaining knowledge of market standards, conformity assessment and post 
harvest  handling;  
d) the need to transport the product in a manner that ensures keeping of quality until it reaches the 
consumer;  
e) the quality  and safety  parame ters of the product  should  meet before  they are put on  the market;  and 
f) the need to promote good agricultural practices that will enhance wider market access, involvement of 
small -scale  traders  and hence  making  farming  a viable  means  of generating  income.  DRS 231: 2019  
©RSB 2019  - All rights reserved                  6 
 Storage of grains and pulses — Practical recommendations  
1 Scope   
This Draft Rwanda Standard specifies the guidelines and practical recommendations for establishment, maintenance 
and management of storage facilities for grains and pulses.  
2 Normative references   
The following documents are referred to in the text in such a  way that some or all of  their content constitutes 
requirements of this document.  For dated references, only the edition  cited applies. For undated references, the 
latest edition of the referenced document (including any amendments) applies.  
1) ISO 6322 -1, Storage of cereals and pulses — Part 1: General recommendations for the keeping of 
cereals.  
2) ISO 6322 -3, Storage of cereals and pulses — Part 3: Control of attack by pests . 
3 Terms and definitions   
For the purposes of this standard, the follow ing terms and de finitions apply.  
3.1 
warehouse  
building for storage of grains and pulses  
3.2 
storage  
process of maintaining the quality of grains and pulses after harvests for a given period  
3.3 
storage structure  
facilities made to store the grains and pulses  DRS 231: 2019  
7                ©RSB 2019  - All rights reserved  
 3.4 
fumigati on 
process of using chemicals in the form of fumes to control pest or to disinfect  
3.5 
moisture content  
amount of moisture in the grain expressed as percentage of the total weight of the sample on wet basis  
3.6 
pallet  
frames  used on floors for stacking bags to prevent direct contact between the grains and the floor  
3.7 
aeration  
moving of air through stored grains at low airflow rates (generally between 0.07 m3 – 0.28 m3 per minute per ton) 
for purposes other than drying, to m aintain or improve its quality  
4 Location of the storage  facility  
4.1 It shall be accessible to transport. There should be ample space to facilitate movement and parking of transport.  
4.2 The site shall be dry and located at relatively high elevation to avoi d water logging and safe from natural 
flooding  calamities.  
4.2 It shall not be near any facility where the danger of fire is constantly  present.  
4.3 Warehouses should preferably be protected by trees or other surroundings from steady direct sunlight. The 
building should be situated where it will not be exposed to afternoon  sunlight.  
4.4 Warehouses, which will be fumigated from time to time, shall not be located near busy public facilities such as 
schools, hospitals,  etc. 
4.5 The structure shall be on solid ground and not in swampy  areas.  DRS 231: 2019  
©RSB 2019  - All rights reserved                  8 
 4.6 Surroundings  in the vicinity  shall be satisfactorily  sanitary.  
4.7 Electricity,  water  services  and drains  should  be available.  
4.8 Areas adjacent to warehouse should be clear of forests, bushes, refuse, equipment and  machinery.  
5 Storage in the  open  
5.1 General  
 
Storage in the open is the cheapest but the least satisfactory method. There is high risk of attack by birds, r odents, 
insects and mites, development of fungi, damage by bad weather, theft and other mishaps. Generally, such storage 
should be for short periods only. Open storage may be used for bumper harvests when other stores are full. It 
should be in a dry, cool place.  
5.2 Uncovered  storage  
5.2.1  Uncovered storage is less undesirable in dry countries, where a short, sharp shower shall only affect the 
surface (to a depth of up to 5 cm) and subsequent sunshine shall dry out the grain again. Such exposure, however, 
may result in damage by  bleaching.  
5.2.2 Storage in the open should, if possible, be on a "hard standing" surface or another prepared smooth  surface,  
preferably  raised  0.5 m above  ground  level,  and featuring  an insulating  system  giving protection against running 
water and moisture rising from the ground, and allowing a complete removal of the grain.  
5.3 Covered  storage  
5.3.1  Sometimes, a temporary roof, for example of corrugated iron on a wooden frame, may be erected over a 
stack of b agged grain or a heap of bulk grain; "walls" of hessian curtains or tarpaulins may be used to give 
additional protection against the  weather.  
5.3.2 Alternatively heaps of grain (bulk or bagged) may be covered with waterproof tarpaulins provided  that suitab le 
precautions  are taken  against sun and consequent  sweating. It is good  practice to fold back these tarpaulins on dry 
days to allow any condensed moisture to evaporate. The cover should be tightened by heavy objects (tyre, 
sandbag, breeze -block, etc.) placed at the foot of heaps. The cover should  overlap  at least  50 cm taking  account  of 
the direction  of the dominant  wind.  
5.3.3 Unthreshed maize shall be commonly stored in open -sided cribs, for example wi th wire-mesh sides, to allow 
drying to take place where atmospheric conditions are favourable. Maize on the cob can  be stored  relatively  easily  
and safely,  as it has not suffered  mechanical  damage  due to threshing. It is essential to cover the open -sided cribs, DRS 231: 2019  
9                ©RSB 2019  - All rights reserved  
 to avoid rain getting into the cereals and to restrict mould growth. Special attention should be given to protecting 
maize from birds and rodents (see ISO 6322 - 
6 Storage in specially constructed buil dings other than silos (flat  stores)  
6.1 General  requirements  
Grains should be put in buildings in order to protect them from weather, prevent from entry of pests, and security. 
The following requirements should be taken care of:  
a) the construction of the buildi ng shall conform with the National Building  Standards;  
b) storage  should  permit  some  control  of temperature  and humidity,  to keep  the grain  as cool,  as dry and at as 
uniform a temperature as possible;  
c) the structure shall be properly built to provide good storage conditions, easy access and safe working 
conditions, and shall not provide harbourage for  pests;  
d) the physical dimension of the storage facilities should take into consideration of the availability of land, quantity 
of the material and bulk density of the material  stored;  
e) there shall be provisions for water tightness, to prevent liquid water from reaching  the grain;  
f) the building shall have a walking space made of cement concrete all around the  warehouse;  
g) efficient drainage shall be provided to preven t flooding;  
h) provisions for drying facilities that may be necessary in conjunction with the storage facilities to assure that food 
grains are in the range of safe moisture content should be  there;  
i) illuminations should be sufficient that lights shall not be too bright or too dim; 
j) artificial lighting by fluorescent lamp is preferable for the interior of the warehouse;  and 
k) the minimum requirement for illumination is 3 watts per  square meter.  
 DRS 231: 2019  
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 6.2 Construction of the  building  
6.2.1  Site and  foundations  
The orientation should be such that radiant heat gain from the sun is minimal; i.e. with the long axis north - south in 
the temperate zone, and with the long axis east -west in the tropics. The foundations should be of adequate strength 
to take the weight of the building an d of the grain filling, and should be termite proofed where necessary.  
6.2.2  Floor  
6.2.2.1  The floor should be sound, smooth, hard and waterproof. "Tamped" earth is not recommended. A wooden 
floor has cracks and crevices which can harbour rubbish, insects and mi tes. A smooth and hard surface  usually  
means  concrete  of good  quality  treated  with a hardener  to prevent  dust.  A walled  construction joined to the ground 
by a curved, smooth profile without projections eases cleaning. The water barrier should be carried th rough to the 
damp proof course in the walls; usually it is "sandwiched" in the concrete. The foundation of the floor should be 
constructed above ground  level.  
6.2.2. 2 The floor should be adequately strong and capable of withstanding heavy loads and  vibrati ons. 
6.2.2. 3 The floor shall be elevated or constructed higher than the existing  ground.  
6.2.2. 4 The floor should be 1m above the ground to permit easy loading or unloading into trucks at the sides of the  
warehouse.  
6.2.2. 5 If loading  and unloading  of trucks  will be permitted  inside  the warehouse  the floor shall be 0.3m  above the  
ground.  
6.2.2. 6 There shall be provisions for wear resistance and safety (refractoriness and elimination of skidding risks).  
6.2.2. 7 The floor should be smooth and easy to  clean. 
6.2.2. 8It should  be free from cracks  where  moisture  from the ground  may affect  the stored  grain. Moisture  sealing  
compound  or asphalt  should  be provided  to fill the floor cracks  against  moisture.  
6.2.3  Walls  
6.2.3.1  The walls should be sound and smooth, and, if permitted by local regulations, light in colour  (usually  white)  
on the outside  to reduce  the absorption  of heat and painted  light colour  inside  to detect the insect pests. The 
construction should avoid having "dead spaces", and the interior plastering should be free from  cracks.  DRS 231: 2019  
11                ©RSB 2019  - All rights reserved  
 6.2.3. 2 Walls of buildings may be constructed of different materials in accordance with local availability and 
practice: timber (not recommended), clay bricks or blocks, bricks or mason ry. They should be covered on the 
interior with a  coating.  
6.2.3. 3 It is important that the construction be strong enough to withstand the pressure exerted on the walls by the  
grain.  
6.2.3. 4 There shall be no opening between wall and  roof. 
6.2.3. 5 A water/damp -proof barrier should be incorporated into the base of the walls. Water proofing compound 
should be incorporated during the plastering and finishing of the walls.  
6.2.3. 6 A concrete strip about one (1) meter wide shall be laid around the wareho use to prevent rain from eroding 
the base of the walls below the damp  course.  
6.2.4  Roof  
6.2.4.1  The roof should  be sound,  waterproof  and, if permitted  by local regulations,  light in colour (usually white) on 
the outside. Girders and supporting pillars should be avoided as far as possible. Supporting pillars do not present 
problems on the side of stores, however in the middle they cause obstructions to the loading/unloading, cause grain 
stacking and decrease the store capacity. If the roof is flat, it should have a slight slope so that rainwater runs off. 
The inclination of the roofs should be sufficient to drain rainwater quickly, taking into account that the water may be 
forced up by the wind. A inclined roof with wide roof space (eaves) helps insulation of the warehouse. The roof 
should be  a good  thermal  insulator,  not affected  by condensation,  and give protection  against  attack  by pests and 
moulds. It should be designed so as not to provide harbourage for insects and mites. These  measures require 
attention to the sealing between walls and roof, and protection of all possible openings  with a finely  meshed  grill. An 
internal  ceiling  is not advised,  as it may provide  harbourage  for predators. Roofing materials include tiles, slates, 
bituminized felt and galvanized iron or aluminium sheet.  
6.2.4. 2 All drain pipes from roof gutters should be external. Rainwater drain pipes should be closely spaced, of 
sufficient size and installed without bends. It is bad practice to have pipes running down the insides of buildings, as 
they act as a harbourage for insects and mites and as runways for rodents, and, if defective, can allow rainwater to 
damage the grain. All external water and drain pipes should be fitted with sheet metal rat guards  to prevent rats 
gaining access to the store eaves. Pipes should also have mesh baffles fitted inside their lower open  ends.  
6.2.4. 3 Internal  pillars  supporting  roof frames  should  be avoided  because  it can interfere  with the pest control and 
other stock ma nagement  procedures.  
6.2.4. 4 Roof frames made of wood or steel shall be designed so that they transfer the weight of the roof to the 
supporting columns or to the  walls.  
6.2.4. 5 It shall be provided with the necessary lateral and vertical wind brace to resist forces due to strong winds 
and earthquakes.  DRS 231: 2019  
©RSB 2019  - All rights reserved                  12 
 6.2.4. 6 The strength  of the roof construction  should  be sufficient  to handle  the weight  of the strongest  winds that can 
be expected  
6.2.4. 7 The roofing  materials  made  of galvanized  iron sheets  shall be in light colours  (white  or beige).  
6.2.4. 8 The chosen materials may be fire proof and highly  refractory.  
6.2.4. 9 The height of the interior between the beam and floor is 6.0 m (minimum) for normal temperature 
warehouse.  
6.2.5  Doors and  windows  
6.2.5.1  Roof lights a nd windows should be kept to a minimum or avoided. They should be left open as little as  
possible.  
6.2.5. 2 Windows  should  be protected  by mesh  grilles  to keep  birds  out when  the windows  are open.  
6.2.5. 3 Store doors should close tightly and should, if possible, be made of metal. If they are made of timber, the 
lower part of both the door and the frame should be covered by a steel strip protecting them against attack by 
rodents. It is useful to protect them  with a canopy against  rain. 
6.2.5. 4 The number of doors will depend on the required frequency of access to the stored product but at least two 
doors are needed to rotate stocks on a “first in, first out” basis. The size of the doors depends on loading/unl oading 
operations (e.g. if trucks need to enter the store). The design of loading doors should take into account the 
difficulties in maintaining them rodent -proof in service. The door shall be fit tightly for insect control  and fumigation. 
Roll up doors  are generally  used  because  of its capability  to close  tightly.  
6.2.5. 5 Preferably  the door should  be made  of steel  or at least  reinforced  along  their lower  edges  with metal plate as 
protection against  rodents.  
6.2.5. 6 If swing doors are fitted, they should be open outwards in order not to reduce the storage capacity of the  
warehouse.  
6.2.5. 7 Preferably  the door should  be made  of thick metal sheet  and fitted  with heavy -duty hinges,  so that it can 
absorb  shock.  
6.2.5. 8 The door should be provided with a locking system not too sensitive to  dust.  
6.2.5. 9 A canopy should be constructed over every entry door to allow continuous loading and unloading even 
when it rains.  DRS 231: 2019  
1                ©RSB 2019  - All rights reserved  
 6.2.6  Proofing  
6.2.6.1  In so far as possible, every precaution should be taken against the entry of insects, rodents, birds and 
bats to the  building.  
6.2.6 .2 When the building needs to be fumigated, it shall be sealed. Sealing shall be complete before the store 
is filled. If the building cannot be sealed, fumigation can be carried out under gas - proof  sheeting.  
6.2.7  Ventilation  
6.2.7.1  Ventilation should be controllable. In a nearly full building, the grain itself largely controls the conditions 
in the store. Permanent natural ventilation is not desirable, as it may let in moist air. However,  a certain  degree  
of ventilation  shall be possible  at certain  times  of day to obtain  the required coolness.  This coolness  can be 
helped  by shading  by slatted  windows,  provision  of wide eaves,  etc. 
6.2.7. 2 Ventilation apertures are vital for the circulation of air. They should be small but suitable for the size of 
the building and positioned in the lower and upper part of the walls and on the roof. These  apertures  should  be 
fitted  with an anti-bird grille  on the outside  and with a mesh  on the inside.  
6.2.7. 3 A suitable meshed ventilation duct should be placed in each gable so that warm air accumulating 
under the roof can  escape.  
6.2.7. 4 Ventilation openings such as louvers shall be fitted on the outside with anti-bird grills (20mm  mesh) 
and on the inside  (10 cm behind  the grills)  with insect  screens  (removable  for cleaning), which will deter most  
insects.  
6.2.7. 5 Rotary ventilators or continuous ridge vent are built on the roof top which provide good ventilation in 
addition to the louvers all over the warehouse.  
6.2.7. 6 In addition  to natural  ventilation  exhaust  fans should  be introduced  for forced  ventilation.  
6.2.8  Electrical  works  
Receptacles for transporting equipment and wire distribution for electric fans shall be provided. If possible, 
conduit tubes shall be used. Electrical works should conform to the National Electrical Standards.  
6.3 Storage of bagged produce in  buildings  
6.3.1  Cleanliness  
Cleanliness and good hygiene are essen tial. The store shall be perfectly sound, i.e. cleaned and treated prior 
to any storage operation. During storage, frequent cleaning is absolutely essential.  
  
©RSB 2019  - All rights reserved                  2 
 6.3.2  Provision of spacers  
6.3.2.1  The use of spacers (pallets) avoids the sacks being in direct contac t with the ground, and is therefore 
vital when the ground is not perfectly watertight. As a  precaution, it is recommended that  spacers be used in 
humid areas. Their main role is to enable ventilation, avoid localized falling temperatures, prevent humidity 
leading to condensation or rising from a badly constructed floor. It is desirable  that spacers  be standard  
pallets,  of manageable  size, and therefore  easy to lift. They  should be treated with pesticides and stacked 
neatly when not in  use. 
6.3.2. 2 However, in dry areas and when stores have been correctly constructed, it is not essential to use 
palettes.  
6.3.3  Stacking  
6.3.3.1  Stacking should be such that the stacks are geometric, well constructed and mechanically strong so 
that handling  and pesticide  treatment  are possible.  The use of a regular  stacking  pattern  will allow  the bags  to 
be easily  counted.  
6.3.3. 2 Stacking around pillars or against walls should be avoided, as this makes inspection and fumigation 
difficult, and it can damage the  building.  
6.3.3. 3 Gangwa ys should be wide enough (at least 1 m) to allow proper inspection and spraying. An 
inspection walkway shall always be left between stacks and the store  walls.  
6.3.3. 4 The retention of impermeable sheets after fumigation prevents re -infestation but is not advised 
because of condensation  problems.  
6.3.3. 5 Bags of grain in each lot should be stacked in basic patterns of cluster formation so that bags can be 
easily counted, and quality  maintained.  
6.3.3.1  Stack  height  
6.3.3.3.1 The height  of stacks  should  be at least  1 meter  between  the top of the stack  and the roof frames.  
6.3.3.3.2 The size of the piles should conform to fumigating sheets in situations where warehouses cannot be 
made  airtight.  
6.3.3.2  Pathways  
When the loading and unloading take place within the warehouse, a  central pathway of width not less than 1m 
wider than the width of the widest vehicle loaded should be designed . 
6.3.3.3  Side  spacing  
Side spacing shall be done as follows:  
a) space between piles shall be 1 meter wide;  and 
b) a minimum of 1 meter space between the edge of the pile and the wall shall  be provided.  DRS 231: 2019  
3                ©RSB 2019  - All rights reserved  
 6.4 Bulk storage in “flat stores”  
6.4.1  Cleanliness  
It is essential that the interior of premises and their surroundings and all handling equipment be cleaned and 
treated with pesticide.  
6.4.2  Equipment  
6.4.2.1  Bulk flat storage  is a cost -effective solution, but difficulties in handling and application of pest control 
measures may arise. It is therefore important to provide all equipment required for good storage and quality 
control: an appropriate handling system, fumigation equ ipment, sampling equipment, and means for testing 
the grain  temperature.  
6.4.2. 2 In all bulks there is the danger of temperature gradients developing, leading to moisture migration and 
possible mould growth, especially in the 5 cm - 20 cm layer at the surf ace and against the walls and floor. This 
can be overcome by a ventilation system providing artificial ventilation through the  bulk.  
6.4.3  Disposition of the  grain  
6.4.3.1  In some stores, grain is stored against the walls of the building. Walls should be strong enough to 
resist lateral pressure exerted by the grain.  
6.4.3. 2 Storage may be between partitions which compensate for the lack of wall strength and enable  batches  
to be separated.  These  partitions  may be made  of concrete  elements,  timber  or metal partitions. However, 
grain handling becomes much more  difficult.  
6.4.3. 3 It is recommended that a space between the top of the bulk and the roof be left to  enable a  person  to 
pass  to test the grain,  if the original  construction  of the storage  does  not permit  this. 
6.4.3. 4 Where a ventilation system can be used, it is recommended that the surface of the bulk be levelled to 
improve the air  movement.  
7 Silo storage  
7.1 The storage units may vary in size from small bins with a capacity of a few kilograms to l arge installations 
of many silo bins holding in excess of 1 000 tonnes each. These large grain silos are convenient but costly to 
install. The large terminal silos are really handling installations and are not cost -effective  for long-term storage.  
For storage, the main  requirement  is simplicity,  with the minimum of mechanization, and mass -produced unit 
systems are  ideal.  
7.2 Materials used should be appropriate to the size of the silo, for  example:  
a) a) locally available "holding" bins may be made of clay, basketwork, etc.; old oil drums may be used, after 
thorough cleaning   
©RSB 2019  - All rights reserved                  4 
 b) larger bins (more than 10 t) may be made of wood (plain or plywood), brick or concrete (slabs or cast), sheet 
metal (steel, corrugated iron, aluminium) or metal mesh (lined with hessian, bituminized paper, polyethylene, 
polyvinyl chloride, butyl rubber,  etc.);  
c) very large silos may be made of corrugated steel or cast  concrete.  
7.3 As with other buildings, silos should be designed to be of adequate strength, and should be constructed so 
that there are no cracks or  crevices.  
7.4 Fumigation facilities, and cleaning, sampling, temperature control and monitoring, as well as ventilation 
systems, should be included when the installation plans are drawn up. Some of the bin constructions  in large  
bins cannot  be fumigated  unless  completely  covered  by gas-proof  sheeting.  
7.5 Whilst automatic mechanized handling arrangements are not necessary for small silos, they are desirable 
for large  installations.  
8 Special systems of  storage  
8.1 Airtight  storage  
8.1.1  General  
8.1.1.1  Storage in airtight structures may be used to control and to prevent insect and mite infestations in dry 
grain and prevent development of moulds in grain which is too damp. The principle of the method is the same 
in both applications, namely elimin ation of the oxygen that insects or moulds require for their growth. This 
result is achieved mainly by allowing the respiration of the grain and of other organisms. Purging with 
nitrogen, carbon dioxide or other inert gases can speed up the process but is not essential, and can be difficult 
in practice.  
NOTE Killing insects in a commodity depends on the concentration of carbon dioxide (CO 2) used. With a 
concentration of CO 2 of more than 35 % over a period of 10 days, 99 % mortality of insects has been  achieved.  
8.1.1.2  In practice, grain respiration will reduce the concentration of oxygen in an airtight container to about  2 
8.1.1. 3 % over a period of between several days and 3 weeks, thus killing insects which were initially present. 
If the re -entry of oxygen per day, through slight leakage, is less than about 0.5 % of the free space above the 
grain, including  inter granular  air, any second  generation  insects  will be unable  to survive,  and the infestation  
will die out.  If the leak is greater  than this, insects  may be able to grow and  build  up a slight  infestation.  
8.1.1. 4 Airtight storage is ideally suited to control insect and mite infestations in dry grain, without the use of 
pesticides. It is particularly suitable for long -term famine reserves in warm  regions, where an upper limit for 
moisture  content  of 13.5 % (wet mass  basis)  is recommended.  Under  these  conditions  there  is little change  in 
the properties of the grain, which may be used for almost all purposes, including human consumption. It is 
advisable,  however,  not to store  seed grain  in a sealed  container  for more  than one cropping  season.  
8.1.2  Airtight storage to prevent mould  growth  
8.1.2.1  During airtight storage of grain with moisture content greater than 16 % (wet mass basis), changes 
resulti ng from the activity of naturally occurring enzymes may occur. There may also be activity of semi - DRS 231: 2019  
5                ©RSB 2019  - All rights reserved  
 anaerobic microorganisms. The grain undergoes certain changes which affect its milling and baking 
properties, leading to subsequent unsuitability for these c ommercial  uses.  
8.1.2. 2 If humid grain is stored in an airtight artificial housing, the moisture content of the grain should ideally 
be in the range 18 % - 22 % (wet mass basis). Above this value, problems relating to grain coagulation and 
compaction arise to such a point that at moisture contents above 25 % (wet mass basis) a special unloading 
system is required.  
8.1.2. 3 If air tightness is not sufficient, harmful microorganisms may dev elop, especially if suitable means are 
not available to reduce the entry of oxygen to a  minimum.  
8.1.3  Types of airtight  stores  
8.1.3.1  Underground  stores  
8.1.3.1.1  These  have  the advantage  of ensuring  a relatively  stable  temperature,  thus avoiding  risks of moisture  
migration.  
8.1.3.1. 2 The site should be chosen with care in order that the water table is below the bottom of the store. 
Ground water and rain water should not be able to enter. The walls of the store should be watertight;  the 
concrete  may be covered  with a protective  layer.  The roof should  be watertight.  
8.1.3.2  Above -ground  structures  
8.1.3.2.1  Above -ground silos are also used for the storage of high-moisture grain. The silos may be 
constructed of sheet steel, which may be painted, galvanized or vitreous -enamelle d. The plates are usually 
bolted together against special mastic. Arrangements for the release of pressure and for emptying with 
minimum entry of air are essential. Emptying should be carried out at a predetermined rate to minimize the 
development of toxic microorganisms in the surface  grain.  
8.1.3.2. 2 Airtight silos generally of up to 500 t capacity may be made by supporting a bag in a metal mesh 
cage. The bag is commonly  made of butyl rubber, but polyethylene and polyvinyl chloride of adequate 
thickness, or similar materials, may be  used.  
8.2 Cool  storage  
8.2.1  General  
Ventilation may be used to keep grain at temperatures lower than the limits of development of insects and 
spread of moulds. To avoid warming during the day, stores should be insulated.  
8.2.2  Cooling ventilation with ambient  air 
8.2.2.1  By lowering the temperature to below 12 °C, the development of most insects becomes so slow that 
infestations are prevented. Grain cooling by ventilation with ambient air is now a well established  practice.   
©RSB 2019  - All rights reserved                  6 
 8.2.2. 2 When  the moisture content exceeds the safety level, cooling ventilation prevents the growth of moulds 
as it lowers the temperature of the grain. When the moisture content exceeds 18 % (wet mass basis), a musty 
smell appears after 2 - 6 months depending on the temperature, and moulds  develop.  
8.2.3  Chilling with refrigerated  air 
The grain has to be brought to the desired temperature rapidly if mould and insect damage are to be 
prevented. This temperature depends on the moisture content and should not be higher than 10  °C for grain 
with a moisture content of 15 % (wet mass basis). Artificially refrigerated air ventilation enables rapid cooling 
which  will limit the development  of insects.  This may even  kill them  and will, in particular,  enable  grain  to keep 
very well. 
9 Warehouse  management  
9.1 Fumigation  
9.1.1  In cases of insect infestation and presence of mould, fumigation should be carried out. The appropriate 
chemical should be used for insect pests or microorganisms. During fumigation dosage of the chemical and 
airtight conditions shall be carefully  observed.  
9.1.2 Fumigants are effective against storage pests because as gases they can reach the pests in the most 
remote hiding place. The range of safe fumigant chemicals that can be used is now restricted to phosphine 
and carbon  dioxide.  
9.2 Phosphine  fumigation  
9.2.1  Phosphine fumigation is undertaken using tablets and pellets. These tablets and pellets release 
phosphine  gas when  they come  into contact  with humid  air. Phosphine  is toxic to all insects.  When  insects  are 
exposed to fumigation in a sealed environment all stages of development from the eggs, larvae, pupae to 
adults are  killed.  
9.2.2 Phosphine does not impair the grain nor leave residues that could be hazardous to the consumer when 
correctly applied and t he grain aerated. Care shall be taken when using phosphine as a gas as it is very toxic 
to humans. Fumigation shall take place in an enclosure that can be tightly sealed. Once the exposure time is 
ended,  the grain  shall be aerated  and the bin checked  for residual  phosphine  gas before  entry.  
Table 1 – Minimum exposure times at 60 % relative humidity  
 
Temperature (oC)  Tablets (days)  Pellets (days)  
Under 5  No fumigation  No fumigation  
5 -10 10 8 
11-15 5 4 
16-25 4 3 
Over 25  3 3 DRS 231: 2019  
7                ©RSB 2019  - All rights reserved  
 Example: at 25 oC, the minimum exposure time for tablets and/or pellets is 3 days  
9.3 Carbon dioxide  fumigation  
9.3.1  Insects need oxygen for respiration. With carbon dioxide fumigation, much of the oxygen in the storage 
bin is replaced by carbon dioxide that suffocates, dehydrates and also produces toxic chemicals in the blood 
of the  
9.32 insects. To be effective, elevated carbon dioxide levels shall be maintained until all insects die. The 
required exposure time depends on the percentage of carbon dioxide and the temperature of the  grain. The 
cost of CO fumigation is high.  
Table 2 — Guidelines for carbon dioxide application  
 
O Grain Temperature ( C)  Minimum CO Levels (%)  2 Days for control  
25 - 30 80 8.5 
25 - 30 60 11 
25 - 30 40 17 
25 - 30 20 Weeks to months  
 
10 Facility  requirement  
1) A warehouse shall contain the following facility requirements:  
a) quality control  laboratories;  
b) workshops;  
c) garage for  vehicles;  
d) dead stock  store;  
e) bag stacker;  
f) poisonous chemicals  store;  
g) isolation  shed;  
h) toilets and washing  facilities;  
i) personnel  office; 
j) safety:   
©RSB 2019  - All rights reserved                  8 
 1) guards for machines should be  provided;  
2) warning signs or boards shall be fixed in hazardous/dangerous  places;  
3) fire fighting equipment (fire extinguishers, fire hydrants) shall be provided and be installed in a 
conspicuous and accessible  location;  
4) there shall be provisions for first aid facilities;  and 
5) all utilized warehouses shall be identified by their warehouse name, code and location printed on a 
standard billboard exhibited outside the warehouse. A daily Stock Status Board for purpose s of 
continuously  monitoring  stock  inventories  within  the warehouse  shall be maintained  
 
 
 
 
 
 DRS 231: 2019  
9                ©RSB 2019  - All rights reserved  
 Annex  A 
(normative)  
 
Typical grain warehouse  
Figure 1 — Typical grain warehouse  
 
 
 
 
 
 
 
 
 
 
 
 
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 Annex  B 
(normative)  
 
Sample design of a typical warehouse  
B.1 Floor plan (1:100)  
 
 
 
 
 
 
 
 
 
 
DRS 231: 2019  
11                ©RSB 2019  - All rights reserved  
 B.2 Elevation  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 B.3 Right side  elevation  
 
 
 
DRS 231: 2019  
13                ©RSB 2019  - All rights reserved  
 Bibliography  
 DRS 231: 2019  
 
Price based on  12 pages  
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