nutritional properties of milk powders - US Dairy Export Council

7 NUTRITIONAL PROPERTIES OF MILK POWDERS
Protein Efficiency Ratio
A protein of high quality is one that supplies
all essential amino acids in quantities
adequate to meet the individuals need
for growth.
Federal regulations have stipulated that the
protein efficiency ratio (PER) using a rat
growth assay, utilizing casein as a reference
protein (AOAC 1975), be used as a means of
evaluating protein quality (Food and Drug
Administration, 1986). A number of other
countries have similar regulations. The
standard protein casein has a PER of 2.7
(FAO, USDA, National Dairy Council).
Any protein with a PER greater than 2.7 is
regarded as an excellent quality protein
(see table on page 42 for a comparison of
PER values).
The PER method has been criticized for
its lack of precision, reproducibility and
appropriateness for measuring protein
quality (McLaughlan et al, 1980, Bender,
1982, Sarwar et al 1989). In 1993, the
FDA replaced the PER with the Protein
Digestibility-Corrected Amino Acid Scoring,
or PDCAAS, method (Hopkins,1982, Fomon,
1993). It combines a measure of protein
digestibility with an amino acid score based
on a comparison with a natural or hypothetic
reference protein (Young and Pellett, 1991). If
the PDCAAS score if greater than or equal to
1.00, the protein is a good source of essential
amino acids.
Several amino acid scoring systems
have been developed to compare the
concentration of a limiting essential
amino acid in a protein of interest, to the
concentration of those amino acids in a
reference protein (Fomon 1993). There are
differences in protein digestibility based
on the protein’s configuration, amino acid
bonding, presence of interfering components
of the diet (e.g. fiber, tannins, phytate),
presence of antiphysiologic factors or adverse
effects of processing (Fomon 1993). It should
be noted that protein quality may be changed
without modifying a specific amino acid.
This may occur as a result of processing for
example. In processing soy protein, moist
heat inactivates the trypsin inhibitors that
otherwise interfere with digestion, thus
protein quality is improved without changing
the protein score. Conversely the processing
may negatively affect the availability of
lysine as a result of Maillard reactions
(Fomon 1993).
Net Protein Utilization
Net Protein Utilization (NPU) is another
biological measure of protein quality that is
often used, which includes an evaluation of
protein digestibility as well as the content
of essential amino acids.
Protein Digestibility Corrected
Amino Acid Score (PDCAAS)
Although PDCAAS scoring is considered
an appropriate test of protein quality, it too
has its limitations (Darragh, 1999). There is
considerable debate regarding the adequacy
of the reference proteins, the truncation of
the PDCAAS, the accuracy of scoring amino
acid availability and the impact of the
anti-nutritional factors (e.g. heat treatment)
on the PDCAAS score.
Calculating PDCAAS requires an approximate
nitrogen composition, the essential amino
acid profile and the true digestibility score.
Whey proteins have a higher score than
virtually all other protein sources.
PDCAAs of Key Protein Sources
Ingredient
PDCAAS
Milk* 1.23
Whey protein* 1.15
Egg white 1.00
Soybean protein .94
Whole wheat-pea flour (**) .82
Chick peas (garbanzos) .69
Kidney beans .68
Peas .67
Sausage, pork .63
Pinto beans .61
Rolled oats .57
Black beans .53
Lentils .52
Peanut meal .52
Whole wheat .40
Wheat protein (gluten) .25
Source: FAO, except for * European Dairy
Association/ U.S. National Dairy Council (from INRA,
France. Unpublished data).
**An example of mutual supplementation. Combining
whole wheat and pea flours yields a protein with a
higher PDCAAS than that of either product alone.
Health Benefits of Milk Proteins—
An Overview
Individual milk proteins have been shown
to exhibit a wide range of beneficial
functions including enhancing calcium
absorption and immune function. The
antimicrobial mechanisms of whey
proteins such as immunoglobulins,
lactoferrin, lactoperoxidase, lysozyme
and glycomacropeptide are now
well documented (see table on page 44).
Immunoglobulins from milk and whey
have a prophylactic and therapeutic effect
(clinically established in human adults)
against specific micro-organisms causing
diarrhea, gastritis and dysenteria. In addition
to containing minerals that enhance bone
growth, whey protein was recently reported
to contain an active fraction that stimulated
the proliferation and differentiation of
cultured osteoblasts (bone-forming cells)
(reviewed by Miller 2000).
Mature cow's milk contains an average 32g
of protein per liter, mostly casein (26g/l) and
whey (6g/l) (Whitney et al. 1976; Jenness,
1979; Davies, Law, 1980; Stewart et al., 1987;
Leonil et al., 2001). The caseins have been
divided into four subclasses, -s 1 casein,
-s 2 casein, -casein, -casein s 1 . Bovine
whey protein is constituted by numerous
soluble proteins including -lactoglobulin
(3g/l), -lactalbumin (1g/l), Bovine serum
albumin (0.4g/l), lactoferrin (0.1g/l)
immunoglobulins (