How microbes transform sugars into acids and gases
Fermentation is the controlled work of microbes (bacteria, yeasts, or moulds) consuming sugars in the absence of plentiful oxygen and producing acids, gases, or alcohol as byproducts. Lactic acid bacteria drive sauerkraut, yoghurt, and kimchi by producing lactic acid that lowers pH; yeasts drive bread and beer by producing carbon dioxide and ethanol.
The dropping pH and the metabolic byproducts together suppress spoilage organisms, which is why fermentation historically functioned as a preservation method long before refrigeration.
Nutritional and flavour changes during fermentation
Beyond preservation, fermentation can synthesise certain B vitamins and vitamin K2, partially break down antinutrients such as phytates that bind minerals, and predigest some proteins and lactose, which is why some lactose-intolerant people tolerate aged or fermented dairy better. It also generates the complex acids, alcohols, and aroma compounds that give fermented foods their characteristic flavour.
The extent of these changes varies widely by food, culture, and time. This is general educational information, not medical advice; people who are pregnant, immunocompromised, or managing specific conditions should consult a qualified professional about which fermented foods are appropriate.