Apple cider vinegar is acidic. The main ingredient in apple cider vinegar is acetic acid; therefore, the acid content is quite high.
Apple cider vinegar is made by squeezing apples, which draws out their liquid. Yeast and bacteria are added, which begins the fermentation process. During this process, the sugars in the apple juice are converted to alcohol. After a time, a second fermentation process is started and the alcohol turns into vinegar in a process that involves acetic acid bacteria. When the vinegar is finished, it has a sour taste due to the acetic and malic acids that developed during the fermentation process.