Biomedical Engineering Reference
In-Depth Information
provide the animal the expected nutrient levels over that
time period. For example, vitamin C is historically of
concern in primate diets due to instability of vitamin C
sources. Newer sources of vitamin C (e.g.
L
-ascorbyl
polyphosphate) are much more stable and allow for
significantly enhanced shelf-life of primate diets compared
to years past (
Figure 10.1
).
Choosing Appropriate Ingredients
The choice of ingredients for a commercially manufactured
diet varies from traditional feed ingredients such as corn,
soybeans, wheat, animal meals, vegetable, and animal fats
to purified ingredients like casein, corn starch, and purified
amino and fatty acids. Traditional ingredients are generally
much less expensive than purified ingredients, but will also
have higher inherent nutrient variation.
Depending on the type of feed being manufactured,
there will be limits on the amount of a particular ingredient
that can be used. In meal-type diets (coarsely or finely
ground powders), there is virtually no limit to the level of
a specific ingredient that can be used. In extruded or pel-
leted products, there will be some maximal inclusion level
of a particular ingredient that can be used but still maintain
product quality. For example, purified ingredients generally
are not well suited for extrusion or pelleting processes as
compared to traditional ingredients. The inclusion of
animal-based ingredients versus plant-based ingredients is
generally chosen based on meeting the nutrient require-
ments of the animal and providing a mix of ingredients that
generally reflects the wild-type feeding strategy of the
animal being fed. For example, it is common to find
animal-based products included in diets designed to feed
primates which consume animal matter
FIGURE 10.2
Variation in crude protein content of soybean meal.
Primate diets were extruded, then stored at 70
F,
<
50% RH for 9 months.
Samples were analyzed for crude protein by NIR. (Data from Purina
Mills
internal research.)
feed provided to the animal prior to harvest, age at harvest,
and type of processing of the animal product post-harvest
(
Shewfelt, 1990
).
Commercial primate diets are generally formulated
using one of two principles: consistent ingredient levels or
consistent nutrient levels. This is a very important differ-
ence in how feeds are formulated. The advantage of having
consistent ingredient levels is that there may be less vari-
ation in non-nutritional factors such as palatability, particle
size, appearance, and so forth. The disadvantage of having
this type of formulation is that it does not take into account
the inherent variation in the nutrient content of those
ingredients. If a diet is formulated to have a constant level
of an ingredient, then nutrient concentration will likely vary
from batch to batch. Formulation that focuses on providing
consistent nutrient levels necessitates some variability in
ingredient levels, to ensure that nutrient levels are consis-
tent from batch to batch. The advantage of this type of
formulation is that the nutrients that are essential to the
animal are provided in the same amounts over time. The
disadvantage to this system is that slight variation in the
diet ingredients will occur in order to maintain consistent
nutrient levels. There is no perfect solution (i.e. a diet that is
consistent in both nutrient and ingredient levels over time),
so an educated decision about what is appropriate for the
particular animal being fed is critical.
Certain ingredient components have received consid-
erable attention in recent years, especially in the laboratory
animal community. The protein gluten, mentioned previ-
ously, may cause intestinal problems in some percentage of
animals. Low-gluten or gluten-free primate diets can be
produced by avoiding gliadin-containing ingredients along
with special consideration to vitamin and mineral carriers
in the wild
(e.g. many New World primates).
Ingredients can vary considerably in their nutrient
content (e.g. soybean meal,
Figure 10.2
). Plant-based
ingredients vary due to a number of factors, including
genetics, pre-harvest conditions, maturity at harvest, post-
harvest handling and processing (e.g. cooking or drying),
and storage (reviewed by
Shewfelt, 1990
). Pre-harvest
conditions that affect nutrient levels include soil fertility
and moisture, growth temperature, light conditions, and
plant spacing. The maturity of a plant at harvest affects its
nutrient content significantly; during growth and develop-
ment of a plant, the nutrient accumulation tends to be
positive, but during senescence, nutrient losses are
increased. Post-harvest handling and storage may impact
nutrient content due to mechanical damage, changes in
light, temperature, and humidity, and heat processing
which may affect nutrient availability (e.g. inactivation of
trypsin inhibitor in soybean meal by heat treatment).
Animal products also can have considerable nutrient vari-
ation due to similar factors including genetics, sex, type of
