PER is a method to metabolize or determine the quality of protein in foods. Quality is measured by the amount of usable protein and the growth resulting from it through an animal assay. Formerly, this method was used as the standard method for all protein quality analysis. However, there is some question as to whether or not it is a valid measurement. This is because PER does not account for the differences in amino acid requirements between humans and rats (Seligson and Mackey, 1984), nor does PER account for the protein needed for cell maintenance. Therefore, PER results often overestimate the requirements for some amino acids and underestimate others. Specifically, PER tends to underestimate the protein quality of lysine-deficient proteins such as wheat gluten (Hackler, 1977).
However, since the PER is an in vivo test, protein digestibility and amino acid bioavail-ability are encompassed to some extent within the assay. Despite these advantages, it is difficult to determine the individual contribution of digestibility and bioavailability of individual amino acids, or of individual proteins in a complex mixture, on overall protein quality. There are indications that the assay can be shortened from 4 weeks to 2 weeks with little loss in accuracy (Hackler, 1977).
A minimum of 10 rats per assay group (i.e., 10 control group, 10 test group): male weanlings (Sprague-Dawley), 50 to 70 g from the same colony, 21 to 28 days old
PER control group diet (see formulas provided in Critical Parameters; casein is assigned a PER of 2.5; results for the test protein or proteins are normalized against this value in an attempt to reduce interlaboratory variation) PER test group diet (see formulas provided in Critical Parameters) Commercial laboratory rat chow Diet to be tested
Individual stainless steel screen-bottom cages with removable food cups Animal room maintained at 18° to 26°C, 40% to 70% relative humidity, and 12 hr light/dark cycle
BASIC PROTOCOL 1
1. House animals in individual stainless steel screen-bottom cages in a room maintained at 18° to 26°C with a 12 hr light/dark cycle and 40% to 70% relative humidity. Provide food and water ad libitum.
2. For a 2-day acclimation period, feed the rats a commercial laboratory rat chow diet.
3. Record weight of each animal before test trial begins.
Initial weight of rats should be between 50 and 70 g. Distribute rats between diet treatment groups so that the average weight and range of weights will be similar for each of the test groups.
4. Feed rats the specified diet for a 28-day trial period.
Rats are fed the test diet ad libitum. Rats on the reference protein (control) diet are also fed ad libitum.
5. Two times a week, remove and weigh the food cups. Fill with fresh diet and reweigh prior to placement back into the cage. Change water in water bottle.
6. Record body weight of each rat every 7 days, and at the end of the trial period.
7. Calculate the PER based on weight gained by test group in grams divided by the total amount of protein consumed (unit B1.2). Total protein is calculated based upon measurement of total nitrogen in the diet.
The final PER value is an average of the weight gain and protein intake of the entire test group at day 28.
PER = weight gain of test group (g) total protein consumed (g)
8. Calculate the relative PER (RPER).
RPER provides a value for the test protein relative to casein when casein is run as part of the same experiment.
PER of casein
ALTERNATE PROTOCOL 1
Analyses of Protein Quality
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