The compression test methods more generally employed in the routine evaluation of
pellet attrition more or less reflect a mixture of measuring fines produced by fragmentation
and abrasion rather than what is normally distinguished by the terms ‘hardness’ and
‘durability’, respectively. There may be a relationship between hardness and durability
(Wood, 1987) and between the results obtained by different devices for durability
(McKee, 1990). However, these relations may only hold for a given feed composition.
The relationship is strongly influenced by the diet ingredient composition of the pellet
and the pelleting conditions used. With all devices applied, pellets are subjected to a
change in particle size distribution. Therefore, the addressed question should not be
“what is the hardness or durability of feed pellets” but from a scientific point of view
“which combination of fragmentation or abrasion reflects at best the way feed pellets
are handled, conveyed or stored”. The subsequent choice for a type of device also
depends on the outline of the factory, whether mechanical or pneumatic transport is
used, the form and shape of storage bins and, last but not least, the ultimate consumer’s
demand (whether man or animal) for a certain physical quality of pellets. For instance,
Skoch et al. (1983) found differences in feeding preference of pigs, when feeds in
different forms were given in a ‘choice-feeding’ experiment. Pigs preferred pellets over
meal but softer pellets over harder pellets.
The large variation that exists within results of e.g. hardness devices (Table 3) is
partly due to the fundamental misunderstanding of breaking behaviour of agglomerates
of biological origin. Analysis of pellet hardness figures with data from five experiments
confirmed this large variation (Table 5). As shown by Fell and Newton (19701,
empirical modification would permit the decrease of the large variation that can be
found in fragmentation-type measurements that employ diametrical compression such as
the Kahl test, Schleuniger test and (partly) the Kramer shear press. Careful application
of the standard conditions under which the device is operated is therefore recommended.
It may be relevant to adjust these standard conditions, in particular for those devices
which show a large variation in pellet quality results.
Current routine devices used for durability are a compromise between measurements
that give a practical value for pellet quality (on-line level) and measurements that have
scientific value. For example, durability measured according to the tumbling can method
(Pfost, 1963) is an actual measurement of abrasion. The pneumatic resistance test
(Holmen tester), however, is an actual measurement of both fragmentation and abrasion
(Robohm, 1987).
Since transport and handling involve both fragmentation and abrasion phenomena, it
would therefore relate more closely to quantities measured with the Holmen pellet tester
than with the tumbling can device, since the first device acts as an ‘in between’
simulator. For fundamental directed studies, some devices may give no valid estimate of
pellet quality as a function of various processing conditions (insufficient resolution) but
are still useful as a quick, routine device for both processing control and/or quality
control in the factory. The latter means that the use of a certain device method appears
suitable for process control within a certain factory when used under standard conditions.
In the authors’ point of view, it should be recognised that the time of physical quality
measurement after manufacturing of pellets took place should always be indicated, as