Microbes in household food

A spoonful that changes a bowl of milk

Think of how curd is made at home. You warm some milk, add a spoonful of curd and leave the bowl in a warm place. By the morning the whole bowl has set as curd.

Nothing was added except a few living things that are too small to see. They are microbes: bacteria, fungi, protists and viruses. Many microbes cause disease. Many more are friends of people. This chapter is about the friends.

Curd and the lactic acid bacteria

The spoonful of curd is the starter, also called the inoculum. It holds millions of lactic acid bacteria, written LAB. One example is Lactobacillus. The bacteria divide and divide in the warm milk, and they turn it into curd.

• The sugar of milk is lactose. The bacteria ferment it and make lactic acid.
• The acid makes the main milk protein clump together and partly digests it. The milk sets.
• Curd has more vitamin B12 than milk, so the curd is more nutritious.
• In our stomach, LAB help to keep down disease causing microbes.

Milk has a pH of about 6.7. The main milk protein, casein, clumps when the pH falls to about 4.6. This is why the milk sets only when enough acid has formed.

Dough, idli and dosa

The batter that is used for idli and dosa is also fermented by bacteria. The puffed up look of the batter comes from carbon dioxide gas.

Bread dough is fermented by baker's yeast, Saccharomyces cerevisiae. Yeast is a single celled fungus. It uses the sugar in the dough and makes ethanol and carbon dioxide.

The gas is trapped in the dough as bubbles, and the dough rises. In the oven the ethanol evaporates, and the bubbles set into the soft inside of the loaf.

Toddy and other fermented foods

Toddy is a traditional drink of some parts of southern India. It is made by fermenting the sap of palms. The sweet sap is fermented by yeasts and bacteria that are already in the air and the pot. NCERT does not name the microbes of toddy.

People also ferment fish, soyabean and bamboo shoots into foods with the help of microbes.

Cheese

Cheese is one of the oldest foods made with microbes. The milk is first set, the solid part is pressed, and then microbes ripen it. Different cheeses get their own texture and flavour from the microbes that are used.

• Swiss cheese has large holes. A bacterium called Propionibacterium sharmanii makes a large amount of carbon dioxide in the cheese, and the gas leaves holes.
• Roquefort cheese is ripened by growing a specific fungus on it. The fungus gives the cheese its flavour.

A note on the name. NCERT writes sharmanii. Many other books write shermanii, and a modern book may call the bacterium a subspecies of Propionibacterium freudenreichii. Remember the genus Propionibacterium and the carbon dioxide.

You should find that more starter makes the milk set sooner, and that a slower division makes it set later. In a real kitchen, the temperature and the kind of milk change the time too.

Try it in Python

The program prints the count of bacteria and the pH of the milk hour by hour, and says when the milk has set.

Practise with new numbers

Common mistakes

• Saying that yeast makes curd. Curd is made by lactic acid bacteria.
• Saying that the dough rises because of oxygen. The gas is carbon dioxide.
• Calling yeast a bacterium. Yeast is a fungus.
• Saying that a fungus makes the holes of Swiss cheese. A bacterium does, with carbon dioxide.
• Mixing up the two cheeses. Swiss cheese has holes from a bacterium. Roquefort cheese is ripened by a fungus.

Remember these

• Curd: lactic acid bacteria, for example Lactobacillus. The starter is the inoculum. Lactose becomes lactic acid. The vitamin B12 increases.
• Dough: baker's yeast, Saccharomyces cerevisiae, makes carbon dioxide. Idli and dosa batter: bacteria make the gas.
• Toddy: fermented sap of palms.
• Swiss cheese: Propionibacterium sharmanii, carbon dioxide, large holes. Roquefort cheese: a specific fungus, for flavour.

Check yourself

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

1. What is the starter that is added to warm milk to make curd, and what does it hold?

2. Which sugar do the lactic acid bacteria use, and which acid do they make?

3. What makes bread dough rise?

4. Which microbe gives Swiss cheese its holes, and which gas does it make?

5. Which cheese is ripened by a fungus?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 8, Microbes in Human Welfare.
• Walstra P., Wouters J. T. M. and Geurts T. J. (2006). Dairy Science and Technology, 2nd edition, CRC Press, the chapters on the acidification of milk and on cheese.
• McGee H. (2004). On Food and Cooking, Scribner, the chapters on milk and on bread.