Technical processing is used to a great extent in feed production.
Protein-containing feedstuffs and compound feeds are treated with
steam and heat. For proteins damaged by heat during processing,
the measurement of ileal digestible amino acids gives a better estimation
of the amino acids available to the animal compared with total
content or fecal digestibility. However the thermal damage which may
occur to an amino acid during processing is not accurately measured
by the ileal digestibility technique since although the amino acid can
be absorbed it cannot contribute to metabolic reactions due to structural
damage. This loss in ability to participate in metabolism can only
be estimated by measuring the physiologically active amino acids.
Examples of different processing techniques and resulting reactions
and effects that can restrict the availability of certain amino acids are
shown in Table 6. The degree of heat used in the process is particularly
important. Individual feeds are subjected to a number of different
thermal treatments such as the toasting of soybeans and soybean
meal, rapeseed products, peas and field beans; pasteurising of fish
meal or drying of corn gluten and wet cereals. Compound feeds are
subject to intense treatments (during pelleting up to 80°C; expanding
up to 110°C; extruding up to 130°C) for technical, nutritional, physiological
and hygienic reasons.
Components – such as reducing sugars – frequently present in compound
feeds, favour the formation of Maillard products with lysine
and reduce its availability under intensive feed processing conditions.
At present the effect of these processing procedures on amino acid
availability has not been extensively investigated for individual proteins
and mixes, particularly with a view to precisely predicting the
effects. However from the point of view of the feedstuff evaluation
and formulation, the use of ileal digestible amino acids represents a
real progress compared to using the total amino acid content and is
a concept which should be more widely adopted by feed formulators.
By accounting for the different losses in the process of digestion/absorption,
for example of lysine in cereals, a more accurate estimate of
the requirement for supplementation can be made.
In ruminant nutrition, the reason behind the attempts to protect dietary
protein is to avoid the degradation of high quality proteins in the
raw materials by ruminal degradation. It is possible to protect proteins
using several procedures such as heat treatment, chemical treatment
or modification, and inhibition of proteolytic activity and identification
of naturally protected protein. The use of these techniques in comparison
to the usual sources of dietary proteins improves the supply
of amino acids, resulting in a better performance by the animal and
less N-emissions into the environment.
Frequently the terms digestibility and availability are interchanged.
However, this can result in incorrect estimates of requirements since
even digested and absorbed amino acids are not always completely
available for protein synthesis. These can be estimated for limiting
amino acids from N-balance trials or measurements of weight gain.
Such measurements take into account the availability of amino acids
at the metabolic level, a factor that is not considered by measuring lit digestibility
alone. Only at the metabolic level can the term availability
be truly justified for an amino acid.
As shown in Table 4 the digestibility of amino acids can be estimated
for various raw materials. In ruminants digestible amino acids are estimated
as shown in Table 5. Apart from diet formulation based on
the total amino acid content, research is in progress to determine requirements
of key digestible amino acids. Work is also in progress to
estimate the availability of amino acids at the physiological level but
further research is required.
The availability of amino acids in cereals and protein-rich ingredients
that have not been damaged by heat can be estimated by digestibility
alone. For protein sources which have undergone thermal treatment
in vitro availability tests are available.
The value of using standardized ileal digestible amino acids was
demonstrated in trials with pigs. Although all diets were formulated
with the same amount of digestible lysine, the efficiency of growth decreased
with increasing intensity of the heat treatment. It has proven
difficult to accurately estimate the degradation rate or catabolism of
the essential amino acids in metabolic processes. In the future it will
be even more important to be able to accurately measure amino acid
catabolism in order to precisely estimate the requirement of utilisable
amino acids, particularly for the limiting amino acids.
Future research must be aimed at developing adequate systems for the
evaluation of amino acid availability, which are able to provide reliable
predictions of availability under the conditions of modern feed processing.
The limitations of the present system that considers only digestion
and absorption will not be sufficient to satisfy future requirements.