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3. Post-harvest losses

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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:

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:

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:

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:

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:

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

Effects

Countermeasures

3.3.2 Heat

Causes

Effects

Countermeasures

3.3.3 Moisture

Causes

Effects

Countermeasures

3.3.4 insect Pests

Causes of infestation

Effects

Countermeasures

3.3.5 Microorganisms

Causes of infestation

Effects

Countermeasures

3.3.6 Rodents

Causes of infestation

Effects

Countermeasures

3.3.7 Birds

Causes of infestation

Effects

Countermeasures

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 systmes de stockage du mas 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.


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