The metabolic process by which the skeletal muscles of humans, bacteria, and yeast produce energy (ATP) in the absence of oxygen is called fermentation.
Fermentation is divided into ethanol fermentation and lactic acid fermentation.
In ethanol fermentation (yeast and some bacteria) the end product of glycolysis is converted into ethanol and carbon dioxide, commonly used for making food products such as beer, wine, and bread.
In lactic acid fermentation the end product of glycolysis is converted to lactic acid
The first fermented food was yogurt.
Yogurt was fermented by humans without the use of any processing in industries.
In North America, the milk from various sources like camels, sheep, goats, and cattle was fermented using microflora and ideal climatic conditions, giving rise to the first-ever documented fermented food item, yogurt.
Yogurt is made with lactic acid fermentation in which a small amount of starter or yogurt contains Lactobacillus bulgaricus and Streptococcus thermophilus bacteria ferments the sugar present in milk (lactose) to form lactic acid.
The increase in lactic acid production decreases the pH of milk, thereby forming a soft gel-like liquid called yogurt.