Protein Quality and Amino Acid Digestibility
by Carl M. Parsons
Multi-State Poultry Meeting, May 25-27, 1999
Animal protein meals such as feather meal (FM), meat and bone
meal (MBM) and poultry by-product meal (PBPM) are important feedstuffs in poultry
nutrition. These ingredients are high in protein but their protein quality can
vary. For example, Sibbald (1) and Parsons (2) reported that the digestibility
of amino acids, particularly lysine (Lys) and cystine (Cys), in MBM varied greatly
among samples. There are several factors that have been reported or hypothesized
to influence protein quality of animal protein meals. This paper will summarize
some of our recent research to quantitate the effect of factors such as processing
systems, processing temperatures, processing pressure and ash on the protein
quality of animal meals, with the emphasis being on MBM.
ORDER OF AMINO ACID LIMITATION
Before discussing the factors affecting protein quality, I would
like to first discuss which amino acids are most deficient and most important
in animal meals. The order of amino acid limitation or deficiency in MBM and
PBPM determined in my lab are shown in Table 1. These orders of limitation were
assessed using two-week chick growth trials in which either MBM or PBPM provided
the only source of dietary protein. The three most deficient amino acids were
the same for both MBM and PBPM, namely, Cys, Trp and Thr. Cystine and Trp were
much more deficient than the others and Cys was more limiting than Trp in PBPM
but was equally first limiting with Trp in MBM. The more severe deficiency of
Trp in MBM is probably due to the higher ash or bone content in MBM. Bone protein
is very deficient in Trp. When looking beyond the first three limiting amino
acids, the order varied for MBM vs PBPM, particularly with Ile and Met being
more deficient in MBM than PBPM.
Table 1. Order of Amino Acid Limitation
in Meat and Bone Meal and Poultry By-Product Meal for Chicks1
||Meat & bone meal
||Poultry by-product meal
|Phe + Tyr
|1From Wang et al. (3) and Wang and Parsons
FACTORS AFFECTING PROTEIN QUALITY OF ANIMAL MEALS
We have recently confirmed earlier reports that the variation
in amino acid digestibility among samples of MBM is substantial. The results
of an evaluation of 14 random industry samples of MBM are summarized in Table
2. Lysine digestibility varied from 69 to 88% and Cys digestibility varied from
37 to 72% for four selected samples. It is further interesting that the variability
in digestibility was greater for Lys and Cys than for Thr and particularly Met.
Moreover, even though both Met and Cys are sulfur amino acids, the variation
in Cys digestibility is greater than all other amino acids, whereas the variation
in Met digestibility is generally less than all others. Thus, there are considerable
differences in digestibility among specific individual amino acids.
Table 2. Variation in Amino Acid Digestibility Coefficients (%) for Different
Samples of Meat and Bone Meal1
|1Parsons et al. (5).
Effect of raw material source, processing system and
A large study was conducted in cooperation with the Fats and Proteins
Research Foundation in attempt to identify the major commercial factors affecting
protein quality of FM, MBM, and PBPM. Six samples of FM, 32 samples of MBM,
and 12 samples of PBPM were processed in different rendering systems at two
different temperatures (low vs high). The raw material source also varied for
the MBM (beef, pork, mixed species). The latter variable was not found to have
any substantial effect on amino acid digestibility. The type of processing system
and processing temperature significantly affected amino acid digestibility of
FM, MBM, and PBPM. The most striking affects were observed for MBM and a small
portion of the data from that study are summarized in Table 3. The digestibility
of Lys and Cys was considerably higher for MBM produced in processing System
B than in System A. Moreover, System B generally yielded very high Lys digestibility
of 90% or greater. In addition, amino acid digestibility was higher when the
MBM was processed at the lower temperature in both Systems A and B, but the
temperature effect was greater in System A. At least part of the probable explanation
for the lower amino acid digestibilities for System A is that the MBM was processed
at higher temperatures than in System B. Cystine was the amino acid that was
most affected by processing system and temperature, with the high temperature
used in System A yielding very low Cys digestibilities. Finally, it is important
to note that the results for System B show MBM with very high amino acid digestibility
can be produced with good processing procedures.
Table 3. Effect of Processing System and Temperature on Amino Acid Digestibility
of Meat and Bone Meal1
||Digest. Coefficient (%)
||Processing temperature (°C)
|1From Wang and Parsons (6).
Effect of pressure processing
Recently, we have conducted research with MBM to assess the effects
of pressure processing on AA digestibility. The reason for evaluating pressure
processing is due to concerns of bovine spongiform encephalopathy (BSE). The
feeding of BSE-infected MBM to ruminants may cause BSE. It is suspected that
the consumption of meat from BSE-infected cattle may, in turn, cause Creutzfeldt-Jakob
Disease (CJD) in humans. Consequently, extreme restrictions have been placed
on the feeding of MBM in the United Kingdom and the feeding of MBM containing
ruminant tissue to ruminants has been banned in the U.S. BSE and CJD are caused
by heat-stable prion proteins that can be at least partially inactivated by
pressure cooking. The European Union requires that MBM be processed at 3 atmospheres
(30 gauge psi) for 20 minutes at 133° C (271° F) to reduce the risk of BSE and
CJD. Future requirements/regulations for MBM processing are unknown. It is possible
that MBM may have to be pressure processed in the U.S. in the future.
Processing conventionally-rendered MBM at 15, 30, 45 or 60 psi
for 20 min. influenced AA digestibility (Table 4). Pressures of 15 and 30 psi
produced moderate depressions in digestibility of most AA, including Thr, Lys
and Met. The reductions in digestibility of Cys were greater than those for
the other AA. Increasing the pressure to 60 psi produced large decreases in
AA digestibility for all AA, with by far greatest reduction occurring for Cys.
The large reduction in digestibility at 60 psi was due both the destruction
of AA and decreased digestibility of AA that were not destroyed.
Table 4. Effect of Pressure Processing
on Amino Acid Digestibility Coefficients (%) for Meat and Bone Meal1
|Gauge pressure (psi)2
|a-cMeans within a column with no common superscript
differ (P < .05).
|1Shirley and Parsons, unpublished
|2Meat and bone meals processed for 20 min. at the specified
Effect of ash content
Another variable that is alleged to influence protein quality
and amino acid digestibility of animals meals such as MBM and PBPM is ash content.
Meals that contain higher ash are generally considered to be of lower protein
quality and have lower amino acid digestibility. However, when one reviews the
literature, there are limited data to support the effect of ash on protein quality
and little or no data to support an effect of ash on amino acid digestibility.
The results of one of our initial studies to evaluate the effect of ash for
MBM and PBPM are summarized in Table 5. Increased ash content had no negative
effect on amino acid digestibility of MBM or PBPM. In contrast, increased ash
did have a negative effect on protein quality as measured by protein efficiency
ratio (PER) in a 10-day chick growth trial. In the PER trial, 10% CP diets were
fed in which the MBM or PBPM provided the only source of dietary CP. The PER
values were calculated by dividing the weight gain (g) by the protein intake
(g). The reduction in protein quality (PER values) due to increased ash was
not due to reduced amino acid digestibility but was due to poorer total amino
acid balance or profile; that is, the level of sulfur amino acids and Trp per
unit of CP decreased as the ash content increased. The latter effect is probably
due to the increased bone content in the higher ash samples. Bone contains approximately
25 to 30% CP and the protein is of very low quality due to extreme deficiencies
of sulfur amino acids, particularly Cys, and Trp. Thus, our initial results
indicate that the protein quality of MBM and PBPM do, indeed, decrease with
increasing ash content, but the reduction is due to a poorer balance of total
amino acids, not decreased amino acid digestibility.
Table 5. Digestibility of Total Essential
Amino Acids (TEAA) and Protein Efficiency Ratio (PER) of Meat and Bone
Meals (MBM) and Poultry By-Product Meals (PBP) Varying in Ash Content1
||Digest. Coeff. for TEAA (%)
|1From Johnson and Parsons (7) and Johnson et
|2PER = chick weight gain (g) divided by
protein intake (g).
LABORATORY METHODS FOR PREDICTING PROTEIN QUALITY
It continues to be of great importance to nutritionists to be
able to predict protein quality of animal meals with laboratory assays so that
expensive, time consuming animal trials do not have to be conducted. We have
evaluated many different types of assays but have only found a few that are
useful. Both ash and CP level are significantly correlated with PER values (Table
6). The reasons for the ash relationship are discussed in the previous section
and the CP correlation is probably due indirectly to the ash level (i.e., samples
with high CP are low in ash). The pepsin N digestibility assay is also useful
for MBM if the pepsin level is reduced from.2% to .002% or .0002% (Table 6).
We found no benefit to reducing the pepsin level from .002% to .0002% for MBM.
Table 6. Correlation Coefficients Among
Different Assays for 14 Meat and Bone Meals1
|Ash vs protein efficiency ratio
|CP vs protein efficiency ratio
|.2% pepsin vs Lys digestibility
|.002% pepsin vs Lys digestibility
|.0002% pepsin vs Lys digestibility
|1From Parsons et al. (5).
value is significant ( P < .001).
DIETARY FORMULATION WITH ANIMAL MEALS ON AN AVAILABLE
AMINO ACID BASIS
Due to variation in amino acid digestibility values among samples
of animal meals, formulation of poultry feeds containing animal meals on an
available or digestible amino acid basis should be superior to formulation on
a total amino acid concentration basis. The development of the precision-fed
cecectomized rooster assay has resulted in a large increase in digestible amino
acid data and also provided a means of obtaining ingredient digestibility data
within a reasonable amount of time and at a reasonable cost. A number of recent
studies (e.g. Fernandez et al., 9; Rostagno et al., 10) have
shown definite benefits to formulating diets on a digestible amino acid basis
versus a total amino acid basis. The results of a study from my lab for low
and high-quality MBM are shown in Table 7. Inclusion of 10 or 20% of either
low or high-quality MBM into a corn-soybean meal diet on a total amino acid
basis resulted in depressed chick weight gain and/or feed efficiency. When diets
were formulated on a digestible amino acid basis, 10% low-quality or 10 or 20%
high-quality meal had little or no negative effect on performance. These results
clearly illustrate the superiority of formulating diets on a digestible or available
amino acid basis. A negative effect was observed from 20% low-quality MBM even
on an available amino acid basis. Further experiments showed that the latter
negative effect was not associated with amino acids but was due to some other
unknown characteristic of the meal.
Table 7. Dietary Formulation with Low-Quality
(LQ) and High-Quality (HQ) Meat and Bone Meals (MBM) on a Total Versus
an Available Amino Acid (AA) Basis1
||Weight gain (g)
|| Gain:feed ratio
|10% LQ MBM
|20% LQ MBM
|10% HQ MBM
|20% HQ MBM
10% LQ MBM
|20% LQ MBM
|10% HQ MBM
|20% HQ MBM
|1All diets contained 20% CP and were fed to chicks
from 8 to 22 days of age. Lysine and sulfur amino acid digestibility coefficients
(%) for the LQ and HQ MBM were 71 and 62 and 92 and 82, respectively (Wang
and Parsons, 11).
- Sibbald, I.R., 1986. Bulletin 1986-4E, Agriculture Canada.
- Parsons, C.M., 1991. Proc. Tech. Symp., National Renderer's Assoc.
- Wang, Xincheng, Castanon, Fructuoso, and Parsons, Carl M., 1997. Poultry
- Wang, X. and Parsons, C. M., 1998a. British Poultry Science 39:113-116.
- Parsons, C.M., Castanon, F., and Han, Y., 1997. Poultry Sci. 76:361-368.
- Wang, X. and Parsons, C.M., 1998b. Poultry Sci. 77:834-841.
- Johnson, M.L. and Parsons, C.M., 1997. Poultry Sci. 76:1722-1727.
- Johnson, M.L., Parsons, C.M., Fahey, Jr., G.C., Merchen, N.R., and Aldrich,
C.G., 1998. J. Anim. Sci. 76:1112-1122.
- Fernandez, S.R., Zhang, Y., and Parsons, C.M., 1995. Poultry Sci. 74:1168-1179.
- Rostagno, H.S., Pupa, J.M.R., and Pack, M., 1995. J. Appl. Poultry Res.
- Wang, X. and Parsons, C.M., 1998c. Poultry Sci. 77:1010-1015.