A student uses a quantitative Biuret assay and a colorimeter to investigate the protein concentration of five different plant-based milk alternatives (labeled P to T).
Before measuring the absorbance of each sample, the student calibrates the colorimeter to zero absorbance using a 'blank' cuvette containing only distilled water and Biuret reagent. Explain why this calibration step improves the accuracy of the protein concentration measurement.
Table 1 shows the protein concentration stated on the manufacturer labels in grams per 100 mL for each milk sample. Table 2 shows the measured absorbance at 540 nm (in arbitrary units, AU) across three experimental trials.
Table 1: Stated protein concentration
| Milk Alternative | Protein concentration (g per 100 mL) |
|---|---|
| P | 0.4 |
| Q | 1.1 |
| R | 2.4 |
| S | 3.3 |
| T | 4.5 |
Table 2: Absorbance at 540 nm (AU)
| Milk Alternative | Trial 1 | Trial 2 | Trial 3 | Calculated Mean |
|---|---|---|---|---|
| P | 0.08 | 0.09 | 0.07 | 0.08 |
| Q | 0.21 | 0.23 | 0.22 | 0.22 |
| R | 0.48 | 0.89 | 0.46 | 0.47 |
| S | 0.65 | 0.67 | 0.66 | 0.66 |
| T | 0.88 | 0.91 | 0.91 | 0.90 |
Identify the anomalous reading in Table 2, stating the milk alternative and trial number. Explain how this anomaly should be treated when calculating the mean absorbance.
Use the data to explain whether the results support the conclusion that milk alternatives with higher manufacturer-stated protein concentrations have a higher actual protein concentration.
Name two other classes of large biological macromolecules, besides proteins, that are commonly found in plant-based milks and serve as energy sources.
