Fermentation is a process by which food is metabolized by microorganisms (bacteria or yeast) to form new byproducts. The process may be catabolic or anabolic where compounds are broken down and new compounds are made. Generally fermentation refers to the the breakdown of carbohydrates although other food components such as proteins and lipids are also metabolized during the process. To differentiate fermentation of carbohydrates from fermentation of proteins and lipids, the fermentation of proteins is referred to as proteolysis, and the breakdown of lipids is called lipolysis. Proteolysis (also called putrefaction) produce rotten off odors while lipolysis produce rancid off odours. The products of fermentation on the other hand are generally, alcohol, acetic acid, carbon dioxide and flavor compounds that can be utilized in preservation and improving sensory acceptability of foods. Common processes utilizing fermentation include the making of yogurt, cheese, sauerkraut, bread, wine, and vinegar. Key factors affecting fermentation rates include starter culture concentration, temperature, substrate concentration, acidity/pH, salt concentration, oxygen, and fermentation time.
How Some Foods are Fermented
Yogurt: Yogurt is made using live bacteria; generally Lactobacillus bulgaricus, Streptococcus thermophilus and Lactobacillus acidophilus. They work together to convert the lactose sugar in milk to produce lactic acid and characteristic yogurt flavors. This lactic acid in turn causes the proteins in the milk to coagulate to form a thick gel. The process begins with heating of the milk to temperatures ranging between 180oF-195oF. The purpose of heating is to break down whey proteins in the milk, which will later help in gel formation. Heating also evaporates some of the moisture, causing the yogurt to have a thicker consistency. After heating, the milk is cooled to the fermentation temperature required to support bacteria growth (113oF-115oF), and the bacteria is added. During fermentation, lactic acid is produced, which changes the structure of another type of protein in the milk called casein by removing its negative outer charge, making it neutral. This change in the charge of casein causes it to clump together along with the whey proteins to form a thick gel. Fermentation usually last between 7-12 hours.
Cheese (Cheddar): Cheese is made by adding starter culture bacteria from the genera lactococcus, lactobacillus, and streptococcus. Activity of these bacteria produces lactic acid which is responsible for curdling milk by neutralizing the negative charge on kappa-casein of casein micelles in milk. Kappa-casein has a negative charge, causing casein to repel and making them soluble in water. However, with the removal of the negative charge, casein no longer repels, and hence they clump together to form a curd. Rennet may also be added to increase curdling. Rennet is a combination of enzymes harvested from the stomachs of calves. The key enzyme is rennin (also called chymosin) which is responsible for chopping off kappa-casein from casein micelles, thereby contributing to curdling. Cheese made with rennin is much harder than cheese that is solidified with acid alone. Other enzymes in rennet such as lipases (enzymes that break down lipids) and other proteases, contribute to flavor development during cheese aging. Most of the rennet used today is produced from fungus that have been genetically modified to produce rennin. This is done by extracting DNA from the stomach of a calf placing it in a fungus DNA. The DNA is like a recipe of instructions that can be transferred from one organism to the next. Once the fungus has the transcript of the recipe to make rennin, it can do so on its own.
Steps in cheddar cheese making include:
- Adding bacteria culture
- Adding rennet (mentioned as coagulant)
- Draining whey
- Cutting into blocks
During aging, flavors develop due to enzymatic processes that metabolize proteins, lipids and remaining lactose. This may take months or years depending on the level of flavor complexity required.
Sauerkraut: Cabbage is chopped and enclosed in a container under anaerobic conditions with up to 2% salt (NaCl). The salt retards pathogens, however good bacteria are able to tolerate this salt concentration. These good bacteria are naturally present on the surface of the cabbage. They include Leuconostoc mesenteroides, Lactobacillus cucumeris, and Lactobacillus pentoaceticus. They are not totally removed when the cabbages are washed. However use of chlorine water to wash the cabbage or blanching is not done prior to processing since these steps would deactivate a significant number of the microorganisms. Leuconostoc mesenteroides is first activated during the fermentation to produce acetic acid (major product), lactic acid, alcohol and carbon dioxide. At about 1% acidity, the L. mesenteroides is inactivated. However L. cucumeris is more tolerant and can survive up to approximately 1.5% acidity. At this point, they are deactivated. However L. pentoaceticus is still able to continue metabolizing acetic acid until approximately 2.5% acid. Complete fermentation can take up to 20 days.
Bread: Yeast (Saccharomyces cerevisiae) metabolize starch and reducing sugars (e.g. fructose, sucrose, and maltose) to produce carbon dioxide, ethanol, and bread flavors. The carbon dioxide is responsible for aeration of the dough and development of volume. The yeast is deactivated during baking.
Wine: Yeast (Saccharomyces cerevisiae) is added to fruit juice. The yeast metabolize carbohydrates under anaerobic conditions to produce ethanol and flavors. fermentation continues until the concentration of ethanol is too high for further yeast activity. This is usually around 12-15%.
Vinegar: Ethanol, produced from wine fermentation is oxidized (i.e. broken down in the presence of oxygen) to acetic acid (vinegar). Bacteria used for acetic acid production is from the genus acetobacter. Acetobacter starter culture is available commercially or you can make your own starter culture by adding unpasteurized vinegar to wine. Unpasteurized vinegar contains live acetobacter that are reactivated in alcohol (the substrate). The new acetobacter produced will be formed as a scum on the surface which can be removed and used as a “mother of vinegar” to produce more vinegar.