The lacto-fermentation process (also lactic acid fermentation) is a biological anaerobic degradation process (similar to silage production process in agriculture), but without gas formation. During this process, sugars such as glucose, fructose, and sucrose, are converted into cellular energy and the metabolic by-product lactic acid. Lacto-fermentation is used throughout the world to preserve foods. When the acidity rises due to lactic acid fermenting organisms, many other pathogenic microorganisms are killed. Yogurt and sauerkraut are some of the most famous products produced by lacto-fermentation. Most lactic acid bacteria obtain energy only from the metabolism of sugars and hence are usually restricted to habitats in which sugars are present. The two most important subgroups of the lactic acid bacteria depend on the nature of the products formed from the fermentation of sugars: one group, called homofermentative, produces a single fermentation product, lactic acid, whereas the other group, called heterofermentative, produces other products, mainly ethanol and CO2 as well as lactate. Even though anaerobe, most lactic acid bacteria have the advantage not to be sensitive to O2, allowing them to grow in its presence as well as in its absence (they are aerotolerant anaerobes).