Is Plant-Based Milk as Protein-Packed as Dairy Milk?

This project follows the Scientific Method. Review the steps before you begin.

Experiment Background

Protein test strips allow for semi-quantitative protein measurements. This means that the colors correspond to an approximate but not exact protein concentration, which can be inferred using the table below (Table 1). Alternatively, you can use the protein quantification scale in Figure 2 or on the test strip bottle to determine the amount of protein from your protein test strip color and the corresponding protein concentration in grams per liter (g/L). 

You may notice a big gap between some of these colors and numbers. Like many chemical reactions to measure protein levels, this test is better at detecting smaller amounts of protein. Therefore, samples with lower protein concentrations will be easier to compare. How will this inform what samples you test and your interpretation of the results?

Image Credit: Laura Ohl, Ph.D. / Science Buddies

Refer to the instructions below on how to interpret these results.

To effectively compare and contrast the amount of protein in different plant-based milks, we must look at the amount of protein in extreme cases on the protein quantification scale. Water has no protein, so it is a good example of what a sample without protein looks like as our negative control. If it contains no protein, it should be yellow on the protein test strip. On the other hand, dairy milk has a lot of protein, so it is a good example of what a sample with high protein content looks like as our positive control. It should show a turquoise-blue color on the protein test strip. Using these controls, we can more easily compare the protein levels in plant-based milk sources to see if they have low or high protein content. This method helps us understand the protein levels through comparison and helps us see if the test strips work well. 

Image Credit: Laura Ohl, Ph.D. / Science Buddies

Our water negative control should remain yellow in color, while our dairy milk should turn dark blue in color indicating a lot of protein. Refer to the experimental protocol below for more details.

Before Starting the Experiment

1. Create a data table like the one below (see Table 2).
2. Write down which plant-based milk source you think will have the most protein in order from 1-5, from the least(1) to the most(5) in your data table. What informed your ranking?
3. Mix the milk in the container before pouring it into a cup to get a similar representative sample for each test.
4. Pour each solution into its own individually labeled cup.
   1. Note: Do not use this same cup for experimentation since it is now cross-contaminated with your spit.
5. Look at the appearance of the solution (water or milk) and write down what you see in the data table below. Does this change your prediction? If so, update your table with your new predictions.
6. Before starting the experiment, taste each solution and rank them based on your preference from worst(1) to best(5). Which sample tastes the best to you? Do you think this will correspond with the amount of protein or not? If so, update your table with your new predictions.
   1. WARNING: Do not taste anything you are allergic to or intolerant to. Safety comes first!

Experiment Steps

1. Mix the milk in the container before pouring for a good representative sample.
2. Pour a similar amount of each solution (water or milk) into a new cup.
   1. Note: We recommend just enough to dip a protein test strip into, about 1 inch or 2.4 cm deep. 
   2. REMINDER: Do not use the cups from before the experiment, as they are contaminated with your spit.
3. Put on gloves to protect the protein test strips from touching your hands.
   1. CAUTION: Your hand has lots of proteins all over it because our skin is made of cells containing proteins! Wear gloves to avoid contaminating your test strips and ensure an accurate result for more reliable comparisons.
4. With gloved hands, open the plastic bottle containing the protein test strips.
5. Grab the top of the test strip out of the protein test strip bottle. Avoid touching the yellow part of the test strip to anything. This yellow part of the test strip contains our special chemical dye, which is where our chemical reaction occurs.
   1. Note: See Figure 2 for a visual reference of where to hold the test strips to prevent cross-contamination. 
6. Dip the pale yellow piece of protein test strip into the milk for 2 seconds before removing the paper from the solution.
7. Wait for 1 minute to allow the chemical reaction to occur.
8. Watch the color change and visually compare it to the protein quantification scale and your controls (See Figures 2 and 3 for a visual reference).
9. Record your results in your data table.
   1. Write down the observed color. 
   2. Write down the approximate quantity by comparing the protein quantification scale and controls.
10. Rank the milk sources based on their quantified protein content in grams per liter (g/L) by ranking them in order on a scale of 1-5, from the least(1) to the most(5).
11. Repeat your experiment with multiple trials to make sure your results are reproducible.
12. Answer the questions in the conclusion section to better understand the results of your experiment.

Conclusions

1. Is there a relationship between milk's appearance and its protein content? What about its taste and protein content?
2. Graph the amount of protein in each sample in a bar graph. Is it easier to visualize which samples have the most protein using this method instead of a ranking system?
3. How did your predicted protein quantity compare to your measured protein quantity? What surprised you the most?
4. Were there any samples that were hard to compare? Why do you think that is?
5. Which plant-based milk had the most protein? Which one had the least?  
6. What would you do differently in your next experiment? What other solutions or tests could you do next? Check out the variations below!