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How do you calculate protein score?

Published in Protein Analysis 3 mins read

Calculating a "protein score" can refer to several different methods of assessing protein quality. The specific method used determines the calculation. Here are two common interpretations and their corresponding calculations:

1. Percentage of Protein in a Mixture

This method determines the proportion of protein present in a given mixture by weight.

Calculation:

  1. Determine the weight of protein (in grams) in the mixture.
  2. Determine the total weight of the mixture (in grams).
  3. Divide the weight of the protein by the weight of the mixture.
  4. Multiply the result by 100 to express it as a percentage.

Formula:

Percentage Protein = (Weight of Protein (g) / Weight of Mixture (g)) * 100

Example:

If a 100g sample of a protein bar contains 20g of protein, then:

Percentage Protein = (20g / 100g) * 100 = 20%

2. Net Protein Value (NPV)

This method considers both the protein content and the amino acid profile (specifically the limiting amino acid). The limiting amino acid is the essential amino acid present in the lowest amount relative to the body's needs.

Calculation:

  1. Determine the Amino Acid Score: Identify the limiting amino acid in the protein source and calculate its score. The amino acid score is the ratio of the amount of the essential amino acid in the protein being evaluated to the amount of that amino acid in a reference protein (e.g., egg protein). This is often expressed as a percentage. For example, if the protein has 50% of the recommended amount of lysine compared to the reference protein, its lysine score would be 50.
  2. Determine the Percentage Protein: Calculate the percentage of protein in the food, as described in section 1 above.
  3. Calculate NPV: Multiply the lowest amino acid score (expressed as a decimal) by the percentage protein.

Formula:

Net Protein Value (NPV) = (Lowest Amino Acid Score / 100) * Percentage Protein

Example:

Suppose a food contains 25% protein and its limiting amino acid (lysine) has a score of 60. Then,

NPV = (60/100) * 25 = 15

This means that 15% of the food's weight contributes to utilizable protein, considering its amino acid profile.

In summary, calculating the protein score depends on the specific measurement you are trying to obtain. Whether it's the straightforward percentage of protein in a sample or a more complex metric like the Net Protein Value that incorporates amino acid profiles, understanding the underlying principles is key.

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