Anaerobic Respiration & Fermentation (Microbiology) What are the bubbles in fermented medium made of? Describe the process that they come from. Solution Answer: The bubbles in the fermented medium is made of carbon dioxide and hydrogen. Microbes ferment glucose/dextrose to produce gases such as carbon dioxide and hydrogen. The medium most commonly used is phenol red glucose broth and is composed of 0.5-1.0% glucose. As the broth gets fermented, acidic end products are produced. This reduces the pH of the media which can be detected as the phenol red changes its colour from red to yellow. Inverted tubes called Durham tubes can be attched to the tubes of the broth which trap any carbon dioxide produced as a result of fermentation..