1. Workshop: Got Protein?
By Ricardo Chiesa
The purpose of this workshop was to test the protein content of different types of
milk, using the Bradford Protein Assay. This method uses Coomassie blue dye, which
binds to the side chains of specific amino acids. In this experiment, the protein that was
quantified was casein, the most abundant protein in milk. This blue staining can bind to 13
different amino acids of the 224 amino acids casein has. First of all, I diluted two samples
containing different types of milk with phosphate buffered saline (PBS 1x), and then it was
diluted again with the protein dye reagent. Afterwards, 7 sample standards with different
known concentrations of casein were prepared, adding the protein dye reagent to each of
the standards. The purpose of this step was to visually compare the test samples (milks)
with the known concentration samples, and to decide what the unknown concentrations of
the milks were. Later, I proceeded with the analysis of all the samples using a
spectrophotometer to measure the absorbance, which was going to be proportional to the
protein concentration. Once this information was obtained, I prepared a standard curve with
the values and, finally, the exact concentration of protein in the test samples was calculated
in mg/mL.