Final answer:
Curdling of milk, digesting proteins in the stomach, fermenting dough, and spoiling of food involve microbial fermentation, which uses microorganisms to convert sugars into acids or gases. This process is central to food production and preservation across various cultures.
Step-by-step explanation:
The commonality between the changes in curdling of milk, digesting proteins in the stomach, fermenting dough, and the spoiling of food is that they all involve microbial fermentation. This biological process utilizes bacteria, yeast, or mold to convert sugars into alcohol, gases, or organic acids. In the case of curdling of milk, this involves the conversion of lactose into lactic acid by microbes, which leads to the precipitation of milk protein-casein and forms a solid mass, commonly found in products such as cheese and yogurt. Digesting proteins in the stomach also involves fermentation; here, the stomach acts as a fermentor, where enzymes and hydrochloric acid break down the food into chyme. As for fermenting dough, yeast consumes sugars and releases carbon dioxide, causing the dough to rise. Spoiling of food is also a fermentation process, where the growth of unwanted microorganisms leads to alterations in flavor, texture, and safety of the food.
The production of foods like yogurt, pico de gallo, sauerkraut, and many others involves harnessing the acidic by-products of fermentation, which provide not only flavor but also protective benefits, such as lowering pH to inhibit the growth of pathogenic bacteria, as outlined in one of the reference options provided: Lowering the pH to kill pathogenic bacteria. Moreover, these acidic conditions, like those favoring lactic acid bacteria, make certain foods stable and safe for consumption over longer periods than their non-fermented counterparts, thus playing a critical role in food preservation.