Biomedical Engineering Reference
In-Depth Information
that are often wheat-based as well. Isoflavones, plant-based
phytoestrogen compounds capable of eliciting estrogenic
or anti-estrogenic effects, are found in soybeans, wheat,
alfalfa, flax, and other plant-based ingredients (
Mortensen
et al., 2009
). Isoflavones may be important to consider
when conducting endocrine, bone metabolism, and
cardiovascular trials, although little research has been done
with primate models to determine the isoflavone level
appropriate for these types of trials (
Gu et al., 2006
).
a specific size, and the resulting drop in pressure as the diet
leaves the extruder die allows for expansion of the diet,
which then hardens upon cooling (
Cheftel, 1986
). Various
combinations of moisture, pressure, residence time, and
temperature can be utilized to achieve different product
characteristics (
Tran et al., 2008
). Extrusion offers the
potential to create diets in various shapes and sizes, which
can allow for optimization of particle size to the species
being fed (
Harper, 1979
). Also, extruded diets are generally
more durable than pelleted diets; a typical corn-soybean
meal ration had ~ 94% durability when extruded and 50%
durability when pelleted (
Jones et al., 1995
). The process of
extrusion does add volume to the diet ingredients in the
form of air that is incorporated as the product expands when
leaving the extruder die. As a result, the density of an
extruded diet is less than that of a pelleted diet.
The heat associated with the extrusion process can
cause physical and chemical changes to the diet compo-
nents. Most notably, extrusion can significantly impact the
availability of starch. Extrusion causes swelling and rupture
of starch granules, increase in cold water solubility, and
partial to complete release of amylose and amylopectin
(
Bjork and Asp, 1983
). Some of this starch (particularly
amylose) may be crystallized into an amorphous matrix
(retrogradation), which may result in fiber-like properties in
the lower intestine (fermentation and/or excretion) (
Tran
et al., 2008
). The degree of chemical and physical modi-
fication of starch in a diet depends on the source of the
starch and the extrusion conditions. In general, extrusion of
dietary starch sources increased the digestible starch
content by ~ twofold (
Tran et al., 2008
). Lipid components
can also be affected by extrusion conditions; extrusion of
wheat bran under different conditions resulted in varying
levels of cholesterol digestibility, and sterol excretion
(
Kahlon et al., 2006
). Analysis of dietary lipids post-
extrusion requires an acid hydrolysis step prior to ether
extraction. This is likely due to formation of fatty acid-
amylose complexes, which will be less extractable with
ether but will not impair lipid utilization by the animal
(
Bjork and Asp, 1983
).
Additional benefits of extrusion include some protein
denaturation which can increase protein digestibility,
inactivation of raw food enzymes (e.g. protease inhibitors),
and significant destruction of microorganisms, including
bacteria and yeast (
Harper, 1979
). Potentially undesirable
effects of extrusion can include destruction of amino acids
and loss of vitamin activity (
Bjork and Asp, 1983
). In
particular, the water soluble B vitamins folate, thiamin,
pyridoxine, and cyanocobalamin (B
12
) tend to be most
heat-labile (
Cheftel, 1986
). However, losses of amino acids
and vitamins are dependent on the amount and source of
substrate and the extrusion parameters (
Tran et al., 2008
).
For example, vitamin C losses during the extrusion process
can be significant, although more stable sources of vitamin
Choosing Appropriate Physical Form
In addition to choosing a diet with the appropriate nutrient
levels for the species being fed, the physical form of the diet
will be an important consideration. Physical forms avail-
able in commercial-type feeds include meal, pellets,
extruded particles, moist diets, and gels. In general, the
more physical processing required, the more costly the diet
will be, such that extruded and moist diets tend to be more
expensive than meal-type products. Gel diets generally
require some processing by the end user.
Meal diets consist of ground ingredients that are mixed
together with no further processing. Meal diets are not
commonly fed to primates, as the physical properties of
a meal diet are not conducive to most primates' habitual
manners of food ingestion, usually resulting in excessive
waste. Meal diets can be gelled, which ameliorates this
drawback. Similarly, pelleted diets are not commonly fed to
primates, except perhaps as part of research protocols
utilizing feed additives where extrusion technology is not
practical due to the amount of feed produced or due to
temperature sensitivity of the feed additives. Pelleting diets
involves mixing the ground ingredients, then subjecting
that mix to moisture (usually steam) and mechanical
pressure to compress the ingredient mix into solid particles
of a known shape and size. Pellets are generally quite hard
and relatively durable and the nutritional value of many
ingredients are improved by pelleting although not to the
extent seen with extrusion. For example, poultry feeds that
were steam pelleted had a 30% increase in starch gelati-
nization compared to a meal (unprocessed) diet. When the
same diet components were extruded, the starch gelatini-
zation was increased 150% compared to the control meal
(
Jones et al., 1995
). However, the heat of pelleting (up to
180
F) may cause some vitamin loss; vitamin A levels were
reduced by 6.5% by pelleting at 176
F(
Jones, 1986
).
Extruded diets are very commonly fed to primates.
Extruded diets are produced by mixing the ground ingre-
dients, often ground to a uniform particle size, then sub-
jecting that mix to steam conditioning, followed by
physical pressure as the mix is forced through a long
extruder barrel where some cooking takes place due to the
pressure generated by the force of the feed material through
the barrel. Finally, the product is forced through a die of
