Not much information can be found o

Not much information can be found on the correlation between measurements
conducted on all of the types of equipment mentioned, except for the Holmen durability
tester and Pfost tumbling Can. Data from McKee (19901, indicated that for the Holmen
pellet tester there was a linear decrease in durability, ranging from approximately 95%
to 60% with testing time, up to 5 min. The F’fost tumbling can showed a curvilinear
decrease, ranging from approximately 98 to 91% with time, up to 20 min. From these
results it can be concluded that the Holmen pellet tester gives results in a wider range
and in a shorter timespan than Pfosts tumbling can.
Wood (1987) found in his research a log-linear relationship (R = 0.94) between
hardness tested with the Kahl device and durability tested with a Holmen pellet tester.
These relations may be found within certain diet formulations. However, because of the
fact that hardness and durability are two distinct quantities, differences in relationships
may be found between different diet formulations. These differences may become more
distinct when new types of equipment are used to manufacture pelleted feed, e.g.
expanders.
3.3. Other criteria
In the former paragraphs some of the most widespread measures of physical quality
from a manufacturers point of view are evaluated. However, other criteria exist to
evaluate feed physical properties. These criteria are for instance water absorption index
(WAI), water solubility index (WSI), and sinking velocity (SV), all criteria being useful
in e.g. fish feeding. Other criteria well related to properties of the raw materials from
which feeds are manufactured are the angle of repose, bulk density and specific weight.
These criteria can be used to characterise the different feeds or feedstuffs. Some of these
criteria can be related to complete feeds as well. For a more fundamental description of
both methods and theories on physical properties related to characterising plant and
animal materials, the reader is referred to Mohsenin (1986). Since these criteria are not
specific for pellets as such, and thus not within the scope of this study, they are not
discussed.
4. Discussion
Different methods for the evaluation of the attrition behaviour of pellets supply
different information. It is obvious that it is not possible to use each type of device with
every type of feed pellets. The choice for a certain method therefore is partly determined
by the objective of the measurement, related to handling or nutritional purposes or to
study simulated production of fines caused by either fragmentation or abrasive stres.ses.
The described methods for the evaluation of physical quality of pellets have been
shown to be tests that use stresses brought about by bending/tension or compression. In
the context of animal feed pellets, bending tests and tensile tests in general may give
more information compared with compression tests (Luyten et al., 1992) but are of less
value due to the type of attrition that feed pellets undergo between manufacturing in the
feed mill and the animal feeding trough.