The Invisible Living World: Beyond Our Naked Eye · Lesson 6 of 9
Microbes in Food and Fermentation
“Compare two food experiments to understand yeast, curd bacteria and conditions for growth.”
• Explain how yeast changes dough and how a comparison tests its effect. • Explain how Lactobacillus changes milk to curd and why warmth matters. • Distinguish the gas produced by yeast from the acid that sours curd. • Relate microbial growth to food spoilage and preservation.
Microbes can change food
A fruit left out may develop a powdery or cotton-like microbial growth. Microorganisms are common in air, soil and on surfaces, so food can come into contact with them. Some changes spoil food; others are deliberately used to prepare it. Pickles and murabbas keep better when enough salt or sugar makes conditions unsuitable for many microbes. “Preserved” does not mean that no microbes exist anywhere around the food.
Why does yeast dough rise?
The chapter compares two bowls of flour and a pinch of sugar, kneaded with warm water. Only bowl A receives yeast; bowl B is the comparison. Both are covered with a damp cloth and kept warm for four to five hours. Dough with yeast may rise, become softer and fluffier, and smell different. The comparison helps connect that change to yeast rather than to flour, water or warmth alone.
Yeast is a unicellular fungus. It uses food and releases energy for growth; as it acts on sugars, carbon dioxide forms bubbles trapped in the dough. Those bubbles make it expand and become airy. A little alcohol can also form, contributing to a changed smell. This useful microbial process helps make breads and cakes. Some bacteria also help ferment batters for idli and dosa and dough for bhatura.
A food-changing process carried out by microorganisms; the products depend on the organism and conditions.
Problem
A rises but B changes little. What does the comparison support?
- 1.The bowls were treated alike apart from added yeast.
- 2.Yeast is a likely cause of the extra rising; carbon dioxide bubbles explain the fluffy texture.
How does milk become curd?
The second comparison adds the same spoonful of curd starter to two bowls of milk. Bowl A has lukewarm milk and is kept warm; bowl B has cold milk and is kept cool. Predict and then record the appearance and colour in the chapter’s observation table. After several hours or overnight, A typically sets as curd and tastes a little sour. B may not set in that time, though it might become somewhat sour.
Curd contains bacteria, including Lactobacillus. They use lactose, the sugar in milk, and make lactic acid, which gives the sour taste. Warm conditions favour their growth and activity. This differs from the yeast-dough example: the highlighted product in curd is an acid, while carbon dioxide bubbles are central to rising dough.
Problem
Both bowls receive starter. Why does the warm bowl set before the cold bowl?
- 1.The starter introduces curd-forming bacteria to both bowls.
- 2.Temperature is the main planned difference; bacteria work more effectively in suitable warmth, so the warm milk sets sooner.
Problem
Why might curd left warm become more sour by the next day?
- 1.Curd bacteria may continue using milk sugar if conditions allow.
- 2.More lactic acid can accumulate, increasing sourness; compare storage conditions before judging the amount.
| Food change | Microbe | Main observation explained |
|---|---|---|
| Dough rises | Yeast, a fungus | Carbon dioxide bubbles give a fluffy texture |
| Milk sets and sours | Lactobacillus and other curd bacteria | Lactic acid forms from milk sugar |
| Batter ferments | Useful bacteria including Lactobacillus | Batter changes during preparation |
Fermentation does not always have one identical result. Gas explains rising dough; lactic acid explains sour curd.
Check your understanding
In each comparison, identify what was kept similar, what changed, what you observed and what that observation allows you to conclude.
Quiz
What is the planned difference between dough bowls A and B?
What makes yeast dough fluffy?
Which substance is mainly responsible for curd’s sour taste?
Why is the cold milk bowl useful in the curd activity?
Why can enough salt or sugar help preserve certain foods?
Practice Problems
- Explain the roles of yeast, sugar and warm conditions in the dough comparison.
- What would be missing from the evidence if both bowls had yeast?
- Describe the predicted results for warm and cool milk with curd starter and explain why they may differ.
- Compare the main products highlighted for yeast dough and curd.
- A bowl of curd becomes more sour after a day. Give two plausible factors to examine.
- Why does observing mould on fruit not mean that all microbes are harmful?
Key Takeaways
• Microbes can spoil food or be used deliberately to prepare it. • Yeast releases carbon dioxide that makes dough airy. • Curd bacteria convert milk sugar and form sour lactic acid. • Warmth affects microbial activity; controlled comparisons help reveal its effect. • Enough salt or sugar can discourage the growth of many food-spoiling microbes.