Final answer:
Yogurt is a common substance that contains lactic acid due to the lactic acid fermentation process carried out by specific bacteria, which is different from the acetic acid found in vinegar. Olive oil and soap do not contain lactic acid.
Step-by-step explanation:
A common substance that contains lactic acid is yogurt. This is due to the process of lactic acid fermentation, which is carried out by certain bacteria, including those found in yogurt. These bacteria consume sugars and produce lactic acid as a byproduct, causing the milk to thicken and develop the tangy flavor characteristic of yogurt.
Vinegar, on the other hand, contains acetic acid and not lactic acid. Olive oil does not go through a fermentation process and consequently does not contain lactic acid. Soap, consisting of fats or oils and an alkali, also does not contain lactic acid.
The bacteria responsible for this fermentation belong to several gram-positive genera, such as Lactobacillus, Leuconostoc, and Streptococcus, all known as lactic acid bacteria (LAB). These bacteria are also crucial in the production of cheese, where lactic acid plays a similar role in thickening and flavoring the final product.