Bio 3 - Microbiology - Food production using yeast

One of the microorganisms that is most useful to people is yeast. The yeasts are single celled organisms. Each yeast cell has a nucleus, cytoplasm and a membrane surrouned by a cell wall. The main way yeast reproduces is by asexual budding - splitting in two to form new yeast cells

When yeast cells have plenty of oxygen they respire aerobically. They break down sugar to provide energy for the cells, producing water and CO2 as waste products.

However yeast can also respire anaerobically. When yeast cells break down sugar in the absence of oxygen, they produce ethanol and CO2.

Ethanol is commonly referred to as Alcohol. The anaerobic respiration of yeast is sometimes called Fermentation

The yeast cells need aerobic respiration because it provides more energy than anaerobic. This allows them to grow and reproduce. However, once there are large numbers of yeast cells, they can survive for a long time in low oxygen conditions. They will break down all available sugar to produce ethanol

We have used yeast in making bread and alcohol as far back as human record go. We know that yeast was used to make bread in Egypt 6000 years ago. Not only that, some ancient wine found in Iran is over 7000 years old.

When fruit fall to the ground and begin to decay, wild yeasts on the skin begin to break down the fruit sugar, producing ethanol and CO2. This can cause some animals who eat the fruit to become drunk. This is most likely how our ancestors discovered alcohol

We now use the same reaction in controlled conditions to make beer and wine. In both cases yeast must be supplied with Carbohydrates to act as an energy source for respiration

Beer making depends on a process called malting:

You soak barley grains in water and keep them warm.

As germination begins, enzymes break down the starch in the barley grains into a sugary solution.

You then extract the sugary solution and use it as an energy source for yeast.

The yeast and sugary solution from malting is then fermented to create alcohol.

You add hops at this stage to give flavour.

Finally you allow the beer to clear and develop its flavour fully before barreling it up to be sold.

Wine making, however, relies on the natural sugar found in fruit like grapes as the energy source for yeast. 

You press the grapes an mix the juice with yeast and water

You then let the yeast respire Anaerobically until most of the sugar has been used up. 

At this stage, you filter the wine to remove the yeast and put it in bottles. It will be stored for some time to mature before being sold.