Course 26.1Lesson 2 of 3

Fermented drinks and the acids of industry

Yeast turns the sugar of juices and malted cereals into ethanol. Wine and beer are made without distillation, and whisky, brandy and rum are made by distilling the fermented broth. Other microbes make acids for industry: citric, acetic, butyric and lactic acid.

about 25 min1 simulation1 practice question

Lesson 2 of 3Show all lessons

Try the code and the simulations beside the lesson.

Sugar, yeast and a bubbling jar

When fruit juice is left in a warm place, it starts to bubble. Yeast cells from the air and the skin of the fruit are at work. They ferment the sugar. Fermentation is the breakdown of sugar by microbes without oxygen. The products are ethanol and carbon dioxide.

Industry uses the same idea in a controlled way.

Fermented drinks

People have used yeast for a very long time to make drinks. The yeast is Saccharomyces cerevisiae, which is also called brewer's yeast. It is the same species as the baker's yeast of bread. It ferments malted cereals and fruit juices and makes ethanol.

The kind of drink depends on the raw material and on the processing, with or without distillation.

Wine and beer are made without distillation.
Whisky, brandy and rum are made by distilling the fermented broth.

A flow chart. On the left, three boxes one below the other with arrows pointing down. The top box is the raw material, malted cereals or fruit juice. The middle box is the yeast Saccharomyces cerevisiae. The bottom box is the fermented broth of ethanol and carbon dioxide. From the broth, a long arrow marked no distillation leads to a box on the right that says Wine and Beer. A second arrow leads up and to the right into a box called Distillation, and from it an arrow leads to a box that says Whisky, Brandy and Rum.

Distillation means heating the broth and cooling the vapour back to a liquid. Ethanol boils at about 78 °C and water at 100 °C. So the vapour is richer in ethanol than the broth was, and the liquid that is collected is stronger.

More about the drinks. NCERT does not name the raw materials. Books give them as grape juice for wine, malted barley for beer, malted grain for whisky, wine or fruit juice for brandy, and the juice or molasses of sugar cane for rum.

Chemicals made by microbes

Industry uses microbes to make organic acids and other chemicals. Learn the table, because it is asked again and again.

Product	Microbe	Group
Citric acid	Aspergillus niger	fungus
Acetic acid	Acetobacter aceti	bacterium
Butyric acid	Clostridium butylicum	bacterium
Lactic acid	Lactobacillus	bacterium
Ethanol	Saccharomyces cerevisiae	yeast

Acetic acid is the acid of vinegar. Citric acid is used in foods and drinks. Lactic acid is also the acid of curd. Of the four acids, only citric acid comes from a fungus. The other three come from bacteria.

Two tables. The first, Chemicals made by microbes, has the columns Product, Microbe and Group. Citric acid: Aspergillus niger, a fungus. Acetic acid: Acetobacter aceti, a bacterium. Butyric acid: Clostridium butylicum, a bacterium. Lactic acid: Lactobacillus, a bacterium. Ethanol: Saccharomyces cerevisiae, a yeast. The second, Enzymes and bioactive molecules, has the columns Product, Source and Use. Lipases, from microbes, remove oily stains in detergents. Pectinases and proteases, from microbes, clear the haze of bottled juices. Streptokinase, from Streptococcus, is a clot buster for a heart attack or a blocked vessel. Cyclosporin A, from Trichoderma polysporum, is an immunosuppressant after an organ transplant. Statins, from Monascus purpureus, lower the cholesterol of the blood.

You should find that a drink made from more sugar is stronger, and that a lower yield makes it weaker. A fermented drink without distillation stays weak, because the ethanol slows down and kills the yeast cells.

Try it in Python

The program works out the ethanol that the yeast makes, the strength of the drink and the spirit that distillation could give.

Practise with new numbers

Common mistakes

Saying that wine and beer are distilled. They are not. Whisky, brandy and rum are.
Saying that citric acid comes from a bacterium. It comes from the fungus Aspergillus niger.
Mixing the acid and the microbe. Acetic acid goes with Acetobacter aceti. Butyric acid goes with Clostridium butylicum.
Thinking that brewer's yeast is a different species from baker's yeast. Both are Saccharomyces cerevisiae.
Forgetting the density. Mass of ethanol divided by its density gives its volume.

Remember these

Fermentation: sugar to ethanol and carbon dioxide by yeast, without oxygen.
Saccharomyces cerevisiae is brewer's yeast and baker's yeast. Wine and beer: no distillation. Whisky, brandy and rum: distillation.
Citric acid: Aspergillus niger. Acetic acid: Acetobacter aceti. Butyric acid: Clostridium butylicum. Lactic acid: Lactobacillus. Ethanol: Saccharomyces cerevisiae.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

Which yeast is used to make wine, beer and bread?

Which drinks are made without distillation, and which with it?

Name the microbe that makes citric acid, and say what kind of microbe it is.

Which microbe makes acetic acid, and which makes butyric acid?

Why is the liquid collected by distillation stronger than the broth?

References

NCERT (2023). Biology, Textbook for Class XII, chapter 8, Microbes in Human Welfare.
Walker G. M. and Stewart G. G. (2016). Saccharomyces cerevisiae in the production of fermented beverages. Beverages, 2(4), 30. doi:10.3390/beverages2040030.
Madigan M. T. et al. (2019). Brock Biology of Microorganisms, 15th edition, Pearson, the chapter on industrial microbiology.