In order to test for these fermentation products, you inoculate and incubate tubes of media containing a single carbohydrate (such as lactose or maltose), a pH indicator (such as phenol red) and a durham tube (a small inverted tube to detect gas production). If the particular carbohydrate is fermented by the bacterium, acid end products will be produced which lowers the pH, causing the pH indicator to change color (phenol red turns yellow) .If gas is produced along with the acid, it collects in the durham tube as a gas bubble.In order to test for these fermentation products, you inoculate and incubate tubes of media containing a single carbohydrate (such as lactose or maltose), a pH indicator (such as phenol red) and a durham tube (a small inverted tube to detect gas production). If the particular carbohydrate is fermented by the bacterium, acid end products will be produced which lowers the pH, causing the pH indicator to change color (phenol red turns yellow) .If gas is produced along with the acid, it collects in the durham tube as a gas bubble