Four teacher packs have been pre

Yeasts are important in the food industry for two processes: baking bread and making alcohol. Cider, wine and beer production are examples of centuries-old biotechnology using yeast.

Yeast, a microscopic fungus, is present naturally in orchards, in the soil and sometimes even in the air.

When yeast respires without oxygen it is called fermentation.

Yeast uses sugars (in the case of cider the sugars in the apple juice) as its energy source and produces alcohol, carbon dioxide and some energy. Over 8000 strains of yeast have been classified but only a few are used in cider-making.

Dissolve the sugar into enough warm water (not hot water or the yeast culture will be killed) to fill the plastic bottle. Fermentation must be carried out in the absence of air to make alcohol. Add the yeast powder and mix. Fit a deflated balloon over the neck of the bottle and let the bottle stand in a warm place not below room temperature, but not too hot either. The ideal temperature for yeast to be active is between 20^o and 30 ^o_.Within a day the balloon should start to fill with CO₂which is a waste product of the yeast.

Paul Mahoney writes about alternative ways of extracting the CO₂ on the website

Questions:

What is yeast?
How is yeast used in the food industry?
Which gas is given off during the fermentation process?
What energy source does yeast use?
What is the word equation for yeast fermentation?

To make cider you need cider apples. These varieties of apple are small, hard and not very good to eat, as they are fibrous and quite bitter. They are high in tannin and low in acidity. Over 350 different varieties of cider apples have been cultivated. Once the apples are harvested, they are washed and pressed. This juice is then fermented. At the end of this section you will see instructions for making apple cider. Cider is an alcoholic drink, which usually contains between 3.5% and 7.5% alcohol.

Even though people have been producing primitive alcoholic drinks for thousands of years, it was not until the 1857 that Louis Pasteur proved the presence of yeast in the fermentation process. Originally, cider was made by utilising the naturally present yeasts in the apples and in the barrels, but factory cider producers now use culture fermentations to ensure consistent product quality.

The traditional method involves running the apple juice into a wooden barrel with the bung removed. No yeast is added as the natural yeast on apple skins already present is relied on. The fermentation starts in 1-2 days and continues for several weeks. When fermentation is complete, the bung is replaced and the cider is matured for 5-9 months. Some people make home-made cider using this method. The bung is removed to give the carbon dioxide a chance to escape.

Factory cider fermentation: The juice is treated with sulphur dioxide to inhibit natural yeasts and then cultured yeasts are added. This gives the cider maker more control over the fermentation process. The cider ferments in either large wooden vats (the cider company Westons, for example, use old oak vats, some of which are over 200 years old. Squeak, the oldest vat, holds 42,107 gallons of cider) or in stainless steel tanks (Bulmer’s famous Strongbow vat is the largest vessel for storage of alcoholic liquid in the world, holding 1.6 million gallons.)

Yeasts require an abundance of oxygen for growth. The fermentation process actually takes a day or two to start, because the yeast feeds off the oxygen present in the juice until this is gone. Sometimes food spoils because of unwanted fermentation. [If you are unlucky, for example, you could buy a pot of yoghurt that has gone off because of fermentation. You can usually notice that there might be a spoilage problem because the lid is raised due to the carbon dioxide which is pressing on it from inside the pot. When you open the lid, it goes “pop” as the gas is released.]

When the alcohol concentration reaches about 10% the alcohol damages the yeast and fermentation stops. You can study the growth pattern of yeast cultures.

Mix the yeast with the sugar solution and then take a drop of this mixture and place it under the microscope. Count the number of cells. Do this every half hour and keep track of your numbers on a graph. At some point the yeast population will stop growing. Why is this?

Questions:

Why is cider an alcoholic drink?
Who originally proved the presence of yeast in the fermentation process?
Why do yeast populations stop growing at some point?
What is the main difference in the fermentation process between the traditional method of making cider and the factory method?

Use cider apples which are in good condition (avoid rotten ones) and leave for a couple of weeks to soften the skin. Buy or borrow a fruit press (you might also be able to purchase newly pressed apple juice from a cider maker or grower) and press your apples. The juice should be put in a special plastic container or wooden cask. (The apple skins can be composted)

For a higher alcohol content you will need larger quantities of juice, as the fermentation process will go on for longer. Place the cask in a cool place with the bung end on top. Some cider makers add special yeasts, others rely on the yeasts present naturally.

After about 48 hours the fermentation process will begin. Keep the bung open during fermentation, so that the froth created by the carbon dioxide can escape. When the fermentation process is nearly finished, remove some cider, add sugar (2 – 4 lb per gallon depending on how sweet you want it) and slowly reintroduce it to the cider in the cask. You will need to dissolve the sugar over heat – only reintroduce the sweetened cider when it has cooled. If you can’t fit all the sweetened liquid into the cask at once, add it gradually. Fermentation will now continue for about 2 weeks. You can tell it has finished when bubbles stop forming on the surface.

Make sure the container or cask is completely full before sealing it or the cider can turn to vinegar. Let the cider mature, the longer the better. Some cider makers leave it for 8 months. While the cider matures it reduces. Keep the cask topped up with the cider you siphoned off earlier to make room for the sweetened juice.

In an interview for the apples and pears past oral history project, Geoff Morris mentions a man has set up courses for cider making and says this about the process:

“Peter Mitchell has had a big effect, he has had money to produce courses, people have gone on his course and learnt how to do cider making. It’s not just a matter of, in simple terms, you press your apples, you get the juice into a container and you let it ferment. But the production of cider is the controlled fermentation. If you let things go naturally what you will end up with is vinegar. So cider production is a matter of knowing enough biochemistry, microbiology to be able to produce a good product. You must stop the fermentation at a certain point. You can filter it which will take out the yeasts and bacteria. You can discourage wild yeasts with the use of sulphur dioxide, you can discourage certain bacteria by the use of sulphur dioxide. I think that Peter Mitchell has shown people how you can control fermentation to get the cider that they want to produce, not the cider that they get.”

For more interviews with people involving the cider making process, consult the articles listed on the Apples and Pears Past Home Page.