There are several possibilities why honey may become sour during storage and use:

1. Yeast or bacterial infection: Honey contains microorganisms, especially yeasts and bacteria. If honey is exposed to air or the container is unclean, bacteria or yeast may cause honey yeast to ferment and produce acid.

2. Other microorganisms: In addition to yeast and bacteria, other microorganisms such as mold may also cause acidification of honey. Mold growing in honey produces acidic metabolites.

3. Oxidation: Natural enzymes in honey may cause oxidation reactions, leading to acidification of the honey. Oxidation is often associated with prolonged exposure to oxygen when honey is exposed to air, or is heated and processed.

In addition to the above reasons, the quality of honey is also related to the types of plants and nectar the bees feed. The nectar of some plants that produce honey may itself contain some acidic substances, which may cause the honey to become sour during the process of bees collecting and making honey.

Therefore, in order to prevent honey from becoming sour, the storage environment of honey should be kept clean, avoid exposure to air, and be properly sealed and stored. Also, choose fresh, high-quality honey and use it under the right conditions.