Fermentation

Fermentation is a metabolic process that converts sugar to acids, gases or alcohol. It occurs in yeast and bacteria, and also in oxygen-starved muscle cells, as in the case of lactic acid fermentation. Fermentation is also used more broadly to refer to the bulk growth of microorganisms on a growth medium, often with the goal of producing a specific chemical product. French microbiologist Louis Pasteur is often remembered for his insights into fermentation and its microbial causes. The science of fermentation is known as zymology.

Fermentation takes place in the lack of oxygen (when the electron transport chain is unusable) and becomes the cell’s primary means of ATP (energy) production. It turns NADH and pyruvate produced in the glycolysis step into NAD⁺ and various small molecules depending on the type of fermentation (see examples below). In the presence of O₂, NADH and pyruvate are used to generate ATP in respiration. This is called oxidative phosphorylation, and it generates much more ATP than glycolysis alone. For that reason, cells generally benefit from avoiding fermentation when oxygen is available, the exception being obligate anaerobes which cannot tolerate oxygen.

Fermentation in food processing

Fermentation in food processing is the conversion of carbohydrates to alcohols and carbon dioxide or organic acids using yeasts, bacteria, or a combination thereof, under anaerobic conditions. Fermentation usually implies that the action of microorganisms is desirable. The science of fermentation is also known as zymology or zymurgy.

The term "fermentation" is sometimes used to specifically refer to the chemical conversion of sugars into ethanol, a process which is used to produce alcoholic beverages such as wine, beer, and cider. Fermentation is also employed in the leavening of bread (CO₂ produced by yeast activity); in preservation techniques to produce lactic acid in sour foods such as sauerkraut, dry sausages, kimchi, and yogurt; and in pickling of foods with vinegar (acetic acid).

Natural fermentation precedes human history. Since ancient times, however, humans have been controlling the fermentation process. The earliest evidence of an alcoholic beverage, made from fruit, rice, and honey, dates from 7000–6600 BC, in the Neolithic Chinese village of Jiahu, and winemaking dates from 6000 BC, in Georgia, in the Caucasus area. Seven-thousand-year-old jars containing the remains of wine, now on display at the University of Pennsylvania, were excavated in the Zagros Mountains in Iran. There is strong evidence that people were fermenting beverages in Babylon c. 3000 BC,ancient Egypt c. 3150 BC, pre-Hispanic Mexico c. 2000 BC, and Sudan c. 1500 BC.

Tea processing

Tea processing is the method in which the leaves from the tea plant Camellia sinensis are transformed into the dried leaves for brewing tea.

The categories of tea are distinguished by the processing they undergo. In its most general form, tea processing involves different manners and degree of oxidation of the leaves, stopping the oxidation, forming the tea and drying it.

The innate flavour of the dried tea leaves is determined by the type of cultivar of the tea bush, the quality of the plucked tea leaves, and the manner and quality of the production processing they undergo. After processing, a tea may be blended with other teas or mixed with flavourants to alter the flavour of the final tea.

History

The history of the tea processing corresponds intimately with the role that tea played in Chinese society and the preferred methods of its consumption in Ancient Chinese society.

Green

The ancient Chinese society first encountered the tea plant in what is now southern China and processed it as another medicinal herb for use in Chinese herbology. The processing technique used to process fresh tea leaves was to immediately steam the fresh tea leaves and dry them for preservation, which is likely the most ancient Chinese form of tea leaf processing. This processing method was perfected near the end of the Han Dynasty (206 BCE-220 CE) and produced a dried tea that would be classified today as "green tea" and quite similar to modern Japanese sencha. For consumption, dried tea leaves were either decocted with water around with other herbs, or ground into a powder to be taken straight or in a liquid.