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3.1: Carbohydrate Fermentation

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    During fermentation most bacteria convert carbohydrates into organic acids, with or without the production of gas. One can test for this by adding a pH indicator and an inverted tube (a Durham tube) to the culture medium. We will use phenol red as the pH indicator. If acid is produced the phenol red will turn yellow (pH below 6.8). Any gas produced will form a bubble in the inverted tube.


    1. Obtain a tube of phenol red glucose, lactose, sucrose broth, each with an inverted Durham tube.
    2. Inoculate each with your assigned organism by swishing it in the broth.
    3. Incubate for at least 24 hours and record the results.


    pH/Gas Production Color Result
    Alkaline or no change Red/fuchsia Negative
    Acidic without gas Yellow Positive for fermentation
    Acidic with gas Yellow with bubble Positive for fermentation with gas production


    Left: Negative Result
    Right: Positive Result with Bubble

    This page titled 3.1: Carbohydrate Fermentation is shared under a CC BY license and was authored, remixed, and/or curated by Molly Smith and Sara Selby (GALILEO Open Learning Materials) .

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