| | Beer is the product of a yeast fermentation. During the manufacturing process, the population of yeast in the fermentation vessel increases greatly. This yeast can be recycled, usually after an acid wash to control contamination by bacteria, but eventually its ability to ferment drops. The spent yeast is then removed and transported to the MARMITE factory where it is pumped into tanks. The yeast undergoes a process called autolysis, where enzymes present in the cells break down the proteins, nucleic acids, carbohydrates and other components inside the yeast cells. Autolysis is speeded up by heating the yeast to 50°C and adding salt. After about a day most of the proteins are broken down into amino acids. At this stage the liquid cell contents, called the autolysate, are centrifuged and filtered to separate them from the cell walls. The solid residue is used for animal feed. Water is then evaporated from the liquid autolysate in condensers until it reaches the required consistency. The mixture is then blended and flavoured with vegetable extracts and packed into sterile glass jars. Other yeast extracts are produced by boiling spent yeast with hydrochloric acid, which hydrolyses most of the proteins to amino acids. Sodium hydroxide is added to neutralise the acid. The product is concentrated into a paste which tastes salty because sodium chloride (table salt) is produced during the neutralisation of the acid. These yeast hydrolysates are used as additives in food manufacture to give a meaty taste. |
