Phenol Red Broth is a liquid medium recommended to differentiate microorganisms based on their ability to ferment specific carbohydrates, which must be added. Enzymatic Digest of Casein provides additional nutrients (nitrogen and carbon) to the microorganisms, while Sodium Chloride serves as source of essential electrolytes and maintains the osmotic equilibrium. The pH indicator Phenol Red shifts from red to yellow upon accumulation of acidic metabolic byproducts, thus revealing carbohydrate fermentation.