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How to estimate the Phe content of a food from its protein content: Part 3

May 10, 2013

This blog entry is the third in a series of four posts that introduce this simple web app which estimates the phenylalanine (Phe) content of a food from its protein content.

https://engineering.purdue.edu/brl/PKU/method_0.html

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A little word about precision and round-up.

In the  previous post, I explained how to use the protein content listed on the Nutrition Fact Label of a food to obtain an upper bound and a lower bound on the Phe content of the food. The explanation assumed that the protein content was rounded up to the nearest gram, and so the true protein content could have been up to 0.5g more/less than the stated protein content.

However, the protein content is not always rounded up to the nearest gram. Sometimes, in particular on the label of some food produced in Asia, the protein content is rounded to the nearest 0.1g. For example, a package of ramen noodles might state that it contains 1.4g of protein per serving. In that case, you can simply enter 1.4 into our app, and it will automatically take into account the higher precision of the protein content. More specifically, it will multiply 1.35 by 20 to find the minimum Phe, and it will multiply 1.45 by 64.5 to find the maximum Phe.

As you might have learned in high school chemistry, the last digit of a rounded up number indicates the precision of the measurement.  So while a rounded up protein content of  1g would indicate a true protein content between 0.5 and 1.5, a rounded up protein content of 1.0 would indicate a true protein content between 0.95 and 1.05.

Try it! Enter “1.0” for the protein content in our app and check the result. You will get a different maximum and minimum Phe content then if you had entered “1”.

More in Part 4.

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