Microbes in household products

What are microbes in household products?

In plain words: many foods in an Indian kitchen are made with the help of microbes. They turn milk into curd, make batter rise, and give cheeses their holes and flavours.

Bacteria and many fungi can be grown on nutritive media to form colonies that can be seen with the naked eye; such cultures are useful in studying micro-organisms (NCERT Chapter 8, Introduction, page 149). NCERT asks which kingdoms contain micro-organisms, and which are only microscopic (page 149). Of the five kingdoms, micro-organisms occur mainly in Monera, Protista and Fungi; Kingdom Plantae also has unicellular algae such as Chlorella (NCERT Class 11 Biology, Chapter 3, page 26). Monera (bacteria) and Protista, the single-celled eukaryotes, are the microscopic ones; the striking exception is a slime mould’s plasmodium, which can spread over several feet. Fungi range from unicellular yeasts to mushrooms (NCERT Class 11 Biology, Chapter 2, pages 12–16; reasoned from NCERT).

Curd: lactic acid bacteria

• Micro-organisms such as Lactobacillus and others, commonly called lactic acid bacteria (LAB), grow in milk and convert it to curd (page 151).
• During growth, the LAB produce acids that coagulate and partially digest the milk proteins (page 151). Making lactic acid without oxygen is the lactic acid fermentation of Class 11.
• A small amount of curd added to fresh milk as inoculum or starter contains millions of LAB, which multiply at suitable temperatures and convert the milk to curd (page 151).
• Curd also has better nutritional quality: the change increases vitamin B₁₂ (page 151).
• In our stomach too, the LAB play a very beneficial role in checking disease-causing microbes (page 151).

Fermented dough: idli, dosa and bread

• The dough used for foods such as dosa and idli is fermented by bacteria. Its puffed-up appearance is due to the production of CO₂ gas (page 151).
• The dough used for making bread is fermented using baker’s yeast (Saccharomyces cerevisiae) (page 151).

NCERT asks two questions here (page 151):

• Which metabolic pathway forms the CO₂? Fermentation, the breakdown of sugar without oxygen (NCERT Class 11 Biology, Section 12.3). In bread dough it is the yeast’s alcoholic fermentation, in which pyruvic acid gives ethanol and CO₂. Beyond the NCERT text: in idli batter the gas comes from lactic acid bacteria such as Leuconostoc mesenteroides, which are heterofermentative. Unlike the lactic acid fermentation of Class 11, which releases no CO₂, theirs gives both lactic acid and CO₂, so the batter rises and turns sour (primary idli-fermentation studies, including Mukherjee et al., 1965 and Mandhania et al., 2019).
• Where do the bacteria come from? NCERT leaves this as a question (page 151). Our reasoning: usually no starter is added to idli or dosa batter, so the bacteria must come from the soaked grains themselves and from the surroundings: utensils, hands and air.

Toddy and other fermented foods

• Toddy, a traditional drink of some parts of southern India, is made by fermenting sap from palms (page 151).
• Microbes are also used to ferment fish, soyabean and bamboo-shoots to make foods (page 151).

Cheese

• Cheese is one of the oldest food items in which microbes were used. Its varieties are known by their characteristic texture, flavour and taste, the specificity coming from the microbes used (page 151).
• The large holes in ‘Swiss cheese’ are due to the production of a large amount of CO₂ by a bacterium, which NCERT names Propionibacterium sharmanii (page 151). Beyond the NCERT text: this bacterium breaks down the lactate in the young cheese into propionate, acetate and CO₂, and bubbles of the gas trapped in the cheese become its holes (Wikipedia, “Propionibacterium freudenreichii”, facts in our words).
• ‘Roquefort cheese’ is ripened by growing a specific fungus on it, which gives it a particular flavour (page 151). Beyond the NCERT text: the fungus is the mould Penicillium roqueforti (Wikipedia, “Roquefort”).

NCERT slip (page 151): the bacterium of Swiss cheese is spelt shermanii, not “sharmanii”: its full name today is Propionibacterium freudenreichii subsp. shermanii, and the NCBI Taxonomy database (US National Library of Medicine, public domain) lists that name and finds no “sharmanii”. In an exam, Propionibacterium shermanii is the correct spelling; an option printed as NCERT spells it means the same bacterium.

Solved examples

Common mistakes to avoid

Quick revision summary

• Microbes are everywhere; not all are harmful, and we use them or their products every day.
• Curd: Lactobacillus and other LAB grow in milk; their acids coagulate and partly digest milk proteins; a little curd is the starter; curd has more vitamin B₁₂; LAB in the stomach check disease-causing microbes.
• Dough: idli and dosa dough fermented by bacteria, bread dough by baker’s yeast; CO₂ makes it puff up.
• Toddy: fermented palm sap (southern India). Fish, soyabean and bamboo-shoots are also fermented into foods.
• Cheese: Swiss cheese holes from CO₂ made by a bacterium (Propionibacterium shermanii; NCERT prints “sharmanii”); Roquefort ripened by a fungus.

Questions students ask

Are lactic acid bacteria the same as Lactobacillus?

Lactic acid bacteria (LAB) are a larger group than Lactobacillus: NCERT itself writes “Lactobacillus and others commonly called lactic acid bacteria (LAB)” (page 151). Beyond the NCERT text, the US National Library of Medicine’s MeSH thesaurus (public domain) files “lactic acid bacteria” under the order Lactobacillales, gram-positive bacteria “that have the ability to ferment sugars to lactic acid”, and that order also takes in Lactococcus, Streptococcus, Leuconostoc and Pediococcus. MeSH now files the species once called Lactobacillus plantarum under a new genus, as Lactiplantibacillus plantarum, so Lactobacillus is one genus among many lactic acid bacteria.

What is the difference between curd and yoghurt?

Beyond the NCERT text, which does not mention yoghurt, curd and yoghurt are both milk fermented by lactic acid bacteria, but India’s food standards treat them as separate products: FSSAI regulation 2.1.13 (fermented milk products) asks only for lactic acid bacteria in dahi, the Indian curd, while yoghurt needs Streptococcus thermophilus growing with Lactobacillus delbrueckii subsp. bulgaricus or, in alternate-culture yoghurt, with Lactobacillus species (facts in our words). The standard also gives the two separate names on the label. So, by our reading, the difference lies in the bacteria: yoghurt’s are named, while curd may be set by a wider range of lactic acid bacteria.

Is curd a probiotic?

Beyond the NCERT text, which does not use the word, curd is not automatically a probiotic just because it contains live lactic acid bacteria: the US National Center for Complementary and Integrative Health (NCCIH, public domain) defines probiotics as “live microorganisms that are intended to have health benefits when consumed or applied to the body” and names yoghurt and other fermented foods as places where they occur. The same source says that for most uses researchers still cannot tell the helpful probiotics from the unhelpful ones, so, by our reading, the name ‘probiotic’ belongs to particular tested strains rather than to curd in general, whose own benefits NCERT gives on page 151.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. What do lactic acid bacteria produce in milk, and what does it do to the milk proteins?

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   Acids, which coagulate and partially digest the milk proteins (NCERT page 151).

2. Q2. Which microbe ferments the dough used for bread?

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   Baker’s yeast, Saccharomyces cerevisiae (NCERT page 151).

3. Q3. What is toddy, and how is it made?

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   A traditional drink of some parts of southern India, made by fermenting the sap of palms (NCERT page 151).