If the casein in the milk is hydrolyzed into peptides and amino acids, it will lose its opaqueness. Therefore, if the bacteria has the exoenzyme capable of casein hydrolysis, there will be a clear zone around the bacterial growth. If the organism lacks the exoenzyme to break down casein, the Skim Milk agar will remain white and opaque . Fermentation of Carbohydrates Phenol red pH indicator is red at neutral pH and yellow at acid pH . A change in color in the tube from red to yellow indicates that the organism has fermented that particular carbohydrate, producing acid end products.
Gas bubbles in the durham tube indicate gas was also produced from fermentation.
If the phenol red remains red, no acid was produced and the carbohydrate was not fermented . Carbohydrate fermentation producing acid but no gas. Carbohydrate fermentation producing acid and gas. No carbohydrate fermentation. No acid or gas.