One-day aged casein was prepared by adding 5 g of casein (Sigma, St Louis, MO, C7078) to 400mL of 0.31% (w/v) NaOH solution (Sigma, St Louis, MO, 415413) and stirring for 30 minutes. After the casein was fully dissolved, 1.2 g of boric acid (B0394) and 2.2 g of sodium tetraborate decahydrate (Sigma, St Louis, MO, S9640) were added to the stirred solution. The pH was adjusted to 8.5 with 1.0M HCl. Ultrapure water was added to bring the volume to 500 mL. The solution was filtered using a Nalgene 0.2 µm filter unit and placed in a 40°C oven for 24 hours. The solution was aliquoted and frozen at -20°C until use.