Anaerobic Respiration in Micro-organisms
In the previous articles i talked about Aerobic and Anaerobic respiration, no let's talk about Anaerobic respiration in micro-organisms :)
Micro-organisms derive their energy by a form of anaerobic respiration called fermentation. Through anaerobic respiration, yeat forms alcohol and carborn dioxide as by-products, whilst certain bacteria form a number of acids (citric and oxalic) as by-products. These substancess have great commercial value. In industries, micro-organisms are used to produce these substances in large quantities. The field using these biological process for such production is called biotechnology.
Yeast is a fungus that has important uses in winemaking and bread making. The fermentation process occurring in yeast cells is used in the production of alcoholic beverages and the products of fermentation depends on the source of sugar used in anaerobic reespiration as shown below.
In the making of red wine , selected red grapes are crushed in the machine. The resulting solids and juices are then replaced into a fermentation vat. The rich flavour and the red colour of the wine comes from components of the grape skin and seeds. Fermentation is then allowed to proceed at a controlled temperature. In the making of white wine, the solids that form after crushing are omitted.
In addition to alcohol, carbon dioxide is also produced during fermentation by yeast. Yeast is used in the making of bread. When bread, flour is mixed with water to make a dough. Flour contains starch and some of this breaks down to the sugar maltose when the flour is moistened. Yeast is added to the dough and breaks down the sugar as it respires. Initially, while there is still air in the dough, the yeast respireea aerobically. When is used up, the yeast respires anaerobically. The carbon dioxide is produced and bubbles of this gas get trapped in the dough, making it rise. The alcohol formed during the process eveporates from the dough during baking. The yeast is killed when the bread is cooked.
Some bacteria, called lactic acid bacteria, produce lactic acid through anaerobic respiration. The lactic acid is responsible for making milk or cream turning sour and reducing its pH. The low pH stops many other undesirable bacteria whereas most lactic acid bacteria continue growing. Human have made use of thse bacteria in cheese and yogurt production.
The following steps are involved in the making of cheese :
- Lactic acid bacteria added to the milk forming lactic acid in the milk.
- The enzyme rennin, which is a protease, added to the milk. Rennin coagulates the milk protein. At one time the rennin was obtained from the stomach of calves. Now, however, several spieces of fungus secrete rennin in large quantities, some naturally and some through genetic engineering.
- The milk curdle into solid curd, which is then cut into slabs. The remaining liquid (whey) left behinds after the curdling process is drained away.
- The curd is then compressed into blocks that are then placed on shelves and left to ripen/age by the action of further micro-organisms. The longer the ripening period , the more acidic or sharper the taste.
The texture or flavour of the cheese or yogurt dends on the strains of bacteria used in their production.