 |  | Manual on the Prevention of Post-harvest Grain Losses (GTZ) | ||
 |  | 3. Post - harvest losses | ||
|  | (introduction...) | ||
| | 3.1 Losses in quantity | ||
| | 3.2 Losses in quality | ||
| | 3.3 Sources of losses | ||
| | 3.4 Further literature | ||
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Causes, Effects and Countermeasures
Post-harvest losses may occur in the following areas:
· during harvesting
· during
transportation
· during drying
· during threshing
·
during processing
· during storage
This manual is concerned primarily with losses which occur during storage. Such losses do not only result from the effects of moisture, heat and pests. The following factors are also of importance:
· The previous history of the stored produce as well as the
growing conditions before harvesting, any field infestation with pests or fungi
or any heat damage which may have occurred during the drying process.
·
Genetic differences, i.e. differences specific to certain varieties and species
with regard to tolerance against storage
pests.
Losses in quantity of the stored produce result from grain being spilt or running out from damaged bags, from theft or from the grain being damaged by pest organisms. Losses in weight may also result from changes in the grain moisture content during the storage period. Due to the following reasons it is generally difficult to evaluate the exact extent of losses in quantity:
· There is no method of calculating losses which is simple,
quick, reliable and generally applicable at the same time.
· The exact
amount of harvested produce is often not known, particularly in small farm
storage, so that losses may be registered at a later date but not
quantified.
· in the case of infestation with insects, the loss in
weight in no way corresponds to the difference in weight before and after
infestation. When weighing the produce, left-overs, frass, webbing, pest
carcasses and rodent droppings are also weighed. Assuming that this filth cannot
be separated from the produce, the actual losses are higher than those
calculated.
3.1.1 Estimating Losses
The most simple method of establishing losses in the store is to record the amounts entering and leaving the store (weigh-in, weigh-out method), even though the results achieved using this method are not always satisfactory for the reasons and shortcomings mentioned above.
It is also possible to make use of other methods of estimating losses, out of which the count and weigh method (C&W) is fairly easy to apply in small farm storage.
By establishing the number and weight of damaged and undamaged grains of a composite sample (e.g. 1000 grains) at monthly intervals, changes in the weight of stored produce can be determined over a period of storage.
The loss in weight in per cent is calculated using the following equation:
Wu = weight of
undamaged grains
Nu = number of undamaged grains
Wd = weight of damaged
grains
Nd = number of damaged grains
Shortcomings in this count and weigh method become apparent particularly:
· when there are large variations in grain size
·
when grain is so heavily infested, that kernels cannot be counted any more
because of complete destruction
· when infestation inside the grains
occurs which cannot be detected so that at tacked grains are classified as
"undamaged".
Other applicable methods for the estimation of storage losses
are the Thousand
Grain Mass Method (TGM) and the Standard Volume Weight
Method
(SVM)
Losses in occur in various forms:
· changes in colour (e.g. yellowing of rice)
·
changes in smell
· changes in taste
· loss in nutritional value
(degradation of proteins and vitamins)
· loss in cooking, milling or
baking quality
· contamination of stored produce with mycotoxins or
pathogenic agents
· loss of germination power in seeds
Often several qualitative changes occur at the same time, usually also in connection with weight losses. Losses in quality are much more difficult to assess than losses in quantity, as they cannot always be easily recognised (e.g. loss in nutritional value). Additionally in many countries there is a lack of quality standards and quality changes may be assessed differently by individual consumers.
3.3.1 Mechanical Damage
Causes
- incorrect harvesting methods
- Poor handling, threshing,
shelling, cleaning, sorting or drying
- Bad transport and loading practices
(e.g. use of hooks)
Effects
- Losses in weight
- Losses in quality (germination power,
nutritional value)
- increased vulnerability to infestation from insect
pests, fungi and rodents
Countermeasures
- Pay attention to maximum temperatures when drying
- Use
safe techniques in harvesting, transport, processing and storage
- Take care
when handling bags
- Repair or replace damaged bags
- Do not use hooks
to carry bags
- Repair pallets (e.g. protruding nails!)
3.3.2 Heat
Causes
- Unsuitable storage structures (false location, insufficient
shade and ventilation facilities, lack of heat insulation)
- Mass
reproduction of storage pests and fungi
- Lack of aeration of store
- High
moisture content of the grain
Effects
- Losses in weight
- Losses in quality (nutritional value,
germination power)
- Good conditions for pest development
- Condensation
with subsequent development of fungi
Countermeasures
- Build suitable storage structures (see section 5.1.1)
-
Provide shade for stores or silos (e.g. by means of wide eaves or shading
trees)
- Keep temperatures as low as possible (aerate storage facility)
-
Conduct treatments for pest control
- Store bags on pallets in order to
improve aeration
- Maintain spaces of I m around all bag stacks
3.3.3 Moisture
Causes
- insufficient drying before storage
- High
relative humidity
- Constructional faults and damage to the store (unsuitable
materials, unsealed floor, walls and roof, holes, gaps, etc.)
- imbalances in
temperature (e.g. day/night) in storage facility with subsequent
condensation
- Produce stored on the floor or touching the walls
- Mass
reproduction of pests
Effects
- Losses in quality
- Losses in weight
- Development of
fungi and formation of mycotoxins
- improved conditions for the development
of pests
- Swelling and germination of seeds
- Damage to storage
structures
Countermeasures
- Dry produce sufficiently before storage
- Repair and seal
storage facility
- Keep relative humidity as low as possible in storage
facility (perform con trolled ventilation)
- Store bags on pallets
-
Maintain spaces of I m around all bag stacks
- Conduct pest control
treatments
- Avoid temperature fluctuations (day/night) in store by means of
shade and ventilation
3.3.4 insect Pests
Causes of infestation
- introduction of infested lots
- Cross infestation from
neighbouring lots or stores
- Migration from waste or rubbish
- Hiding
places in stores (cracks, fissures)
- Use of infested bags
Effects
- Losses in weight
- Losses in quality (impurities such as
droppings, cocoons and parts of insects, reduction of nutritional value,
reduction in germination power)
- increase of temperature and moisture
Countermeasures
- Harvest at the right time
- Choose tolerant varieties
-
Keep means of transportation clean
- Remove infested cobs, panicles or pods
before storage
- Ensure that produce is dry before storing
- Prevent pest
introduction by checking for infestation before storing
- Clean the store
daily
- Keep the temperature and relative humidity as low as possible
(perform controlled ventilation)
- Prevent any pest infiltration by sealing
the store (windows, doors, ventilation facilities; e.g. with the use of insect
gauze)
- Repair any damage to the store immediately
- Store old and new
lots separately
- Clean empty bags thoroughly and treat them against insects
if necessary
- Perform pest control treatments
- Rotate stocks: 'first in
- first out'
3.3.5 Micro-organisms
Causes of infestation
- High moisture content of stored produce
- High relative
humidity in store
- Condensation
- Humidity and moisture produced by
insects
Effects
- Loss of quality (smell, taste, colour, nutritional value,
germination power)
- Formation of mycotoxins
- Slight loss of weight
(mould)
- Further increase in temperature and moisture
- Further
condensation
Countermeasures
- Dry produce sufficiently before storage
- Keep relative
humidity as low as possible in storage facility (perform controlled
ventilation)
- Store bags on pallets
- Maintain spaces of I m around all
stacks
- Conduct pest control treatments
3.3.6 Rodents
Causes of infestation
- Penetration through badly closing doors, windows, ventilation
openings, holes
- Lack of barriers
- Lack of hygiene in store and
surrounding area (possible hiding and breeding places)
Effects
- Loss of weight
- High losses in quality due to
contamination of produce with faeces and urine
- Contamination of produce
with pathogenic agents (typhoid, rabies, hepatitis, plague, etc.)
- Damage of
material and facilities (bags, doors, electric cables)
Countermeasures
- Prevent entry of rodents by sealing store rat-proof
- Keep
store and surrounding area clean
- Place traps
- Carry out rodent control
measures
3.3.7 Birds
Causes of infestation
- Open or broken doors, windows, ventilation openings or roofs
Effects
- Losses in weight
- Damage to bags
- Contamination of
stored produce with droppings and pathogenic agents
Countermeasures
- Bird-proof stores (carry out repair work, fit grilles or
nets)
- Remove any nests of granivore birds from the store and surrounding
area
ANONYMOUS (1985)
Prevention of Post-Harvest Food Losses, FAO,
Rome, 121 pp.
BOXALL, R.A. (1986)
A critical review of the methodology for
assessing farm-level grain losses after harvest, TDRI, Slough, 139 pp.
HALL, D.W. (1970)
Handling and Storage of Food Grains in
Tropical and Subtropical Areas, FAO, Rome, 350 pp.
HARRIS, K.L. & C.J. LINDBLAD (1978)
Postharvest Grain
Loss Assessment Methods, American Association of Cereal Chemists, St. Paul,
Minnesota, 193 pp.
PANTENIUS, C.U. (1988)
Etat des pertes dans les systèmes de
stockage du maïs au niveau des petites paysans de la region maritime du Togo,
GTZ, Hamburg, 83 pp.
PROCTOR, D.L. & J.G. ROWLEY (1983)
The Thousand Grain
Mass Method (TGM): A basis for better assessment of weight losses in stored
grain, Tropical Stored Product information 45, 19-23, TDRI, Slough
REED, C. (1986)
Characteristics and limitations of methods
to estimate losses in stored grain, Special Report No. 16, Kansas State
University, Food and Feed Grain institute, Manhattan, Kansas, 23
pp.
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