Practice using food microbes
Many foods are made by microbes. In a school practical we choose the microbe, give it food and warmth, and watch it work.
- Curd: lactic acid bacteria eat milk sugar and make lactic acid. The acid makes milk proteins stick together, so milk sets.
- Bread and idli batter: yeast and bacteria eat sugar and make carbon dioxide gas. The gas makes tiny bubbles, so the dough rises.
- Pickles and kimchi: salt keeps bad microbes away while acid-making bacteria grow.
How to run a curd practical
- Wash your hands and use a clean, boiled-and-cooled jar.
- Warm the milk to a little above body temperature (about 40 °C).
- Stir in one spoon of curd as starter.
- Cover and keep it warm for 6 to 8 hours.
- Record the time it set, the taste and the smell. Try a second jar kept in the fridge and compare.
Why does the practical work? Microbes need four things
Microbes grow when they have food, water, warmth and time. In curd, milk gives food and water, the warm room gives warmth, and the starter gives the microbes. Change one thing, such as keeping the jar cold, and the change shows in how fast curd sets.
Always control one thing at a time. This is called a fair test.
Microbiological testing of food
A food can look fine and still carry too many harmful microbes. So factories and labs test food on a nutrient plate.
Plate count method (serial dilution)
- Dilute: mix 1 mL of the food with 9 mL of sterile water. This is a 1 in 10 dilution. Repeat from tube to tube: 1:100, 1:1000 and so on.
- Spread: put 1 mL (or 0.1 mL) of one dilution on a plate of agar jelly (nutrient medium).
- Incubate: keep the plate warm for one to two days. Each living microbe grows into one visible colony.
- Count: count colonies on a plate that has between 30 and 300. Too few is not reliable, too many cannot be counted.
- Multiply: CFU per mL = colonies × dilution ÷ volume plated.
Safe working and what the result means
- Use clean tables, flame or alcohol, gloves and masks. Never taste anything made in a lab.
- Close used plates and give them to the teacher to sterilise before throwing away.
- Test results are compared with a limit set for that food. Raw milk, ready-to-eat salad and packaged water all have different limits.
- We also look for indicator microbes, such as coliform bacteria, which tell us that the food may have been touched by dirt or waste.
Key formulas and definitions
- CFU per mL = (number of colonies × dilution factor) ÷ volume plated (mL)
- Dilution factor of 1:10 = 10; two steps of 1:10 = 100
- Good plate: 30 to 300 colonies
- Key terms: starter, colony, serial dilution, incubation, agar, CFU, indicator microbe
Worked examples
1. A plate with 1 mL of a 1:1000 dilution grows 45 colonies. How many CFU per mL were in the original food?
CFU per mL = 45 × 1000 ÷ 1 = 45,000 CFU/mL.
2. Plating 0.1 mL of a 1:100 dilution gives 60 colonies. Find CFU per mL.
CFU/mL = 60 × 100 ÷ 0.1 = 60,000 CFU/mL.
3. Three plates from dilutions 1:10, 1:100 and 1:1000 (1 mL each) show 500, 52 and 6 colonies. Which plate do you use and what is the answer?
Use the plate with 30 to 300 colonies: 52. CFU/mL = 52 × 100 = 5,200 CFU/mL.
Common mistakes
- Counting a plate with 800 colonies. It is too crowded; use a higher dilution instead.
- Forgetting to multiply by the dilution factor, so the answer is far too small.
- Opening plates in a dirty room, so outside microbes spoil the result.
- Thinking all microbes in curd are harmful. Most are helpful lactic acid bacteria.