The enzyme caseinase is secreted out of the cells into the surrounding media, catalyzing the breakdown of milk protein (casein) into small peptides and individual amino acids which are then taken up b...The enzyme caseinase is secreted out of the cells into the surrounding media, catalyzing the breakdown of milk protein (casein) into small peptides and individual amino acids which are then taken up by the organism for energy use or as building material. The hydrolysis reaction causes the milk agar, normally the opacity of real milk, to clear around the growth area as the casein protein is converted into soluble and transparent end products.