Back to Home
Page of CD3WD Project or Back to list of CD3WD Publications
3.
Post-harvest losses
Contents
- Previous - Next
3.1
Losses in quantity
3.2
Losses in quality
3.3
Sources of losses
3.4
Further literature
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.
3.1
Losses in quantity
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,
leftovers, 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)
3.2
Losses in quality
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
Sources of losses
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 Microorganisms
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
3.4
Further literature
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)
Post-harvest 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, 1923, 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.
Contents
- Previous - Next