Basics of experiments
A food experiment is a careful test. Careful means safe, clean and exact.
- Safety: wear goggles, gloves and a coat. Tie back long hair. Never taste anything in a lab.
- Clean tools: wash and dry glass before use. A dirty flask can spoil the answer.
- Weighing: a balance gives mass in grams (g). Press tare (zero) with the empty dish on it.
- Sample: a small part that stands for the whole food. Mix well before you take it.
- Records: write the date, sample name, every reading and the unit. Repeat the test at least twice.
Chemical experiments: acid by titration
Titration finds how much acid or alkali is in a food, for example in vinegar, juice or curd.
- Put a measured amount of food liquid in a flask and add 2 drops of indicator.
- Fill the burette with alkali of known strength (for example 0.1 M).
- Add the alkali slowly while you swirl the flask.
- Stop at the end point: the first lasting pink colour.
- Read how many mL you used. That volume tells you the amount of acid.
Other chemical tests: pH with a meter, water content by drying in an oven, and salt or sugar by simple tests.
Microbiology experiments: plate count
Microbes are too small to see, but they grow into colonies you can count. One colony grows from one cell or a small clump, so we say CFU (colony forming unit).
- Mix 10 g of food in 90 mL of sterile water. This is 10-1.
- Move 1 mL into 9 mL of water. This is 10-2. Repeat for 10-3, 10-4.
- Put 1 mL of each on a plate with agar (a jelly food for microbes). Warm it for 1 to 2 days.
- Count the plate that has 25 to 250 colonies. Fewer is not exact. More is too crowded.
CFU per gram = colonies counted × dilution factor. Work in clean, aseptic style: flame the loop, keep lids on, and kill used plates in a pressure cooker or autoclave before you throw them away.
Key formulas and definitions
- Dilution factor = 10<sup>n</sup> for a 10<sup>-n</sup> tube
- CFU/g = colonies ÷ (volume plated in mL × dilution)
- Moles of alkali = molarity × volume (L)
- Good plate range: 25 to 250 colonies
Worked examples
1. You must weigh 10 g of sample in a dish that weighs 25 g. What should the balance show if you do not tare?
Without tare, dish + sample = 25 + 10 = 35 g. With tare you would see 10 g.
2. In a titration 8.0 mL of 0.1 M alkali reached the end point. How many moles of alkali were used?
Volume = 8.0 mL = 0.008 L. Moles = 0.1 × 0.008 = 0.0008 mol. One mole of alkali balances one mole of a one-acid food, so the acid is about 0.0008 mol.
3. A 10<sup>-2</sup> plate (1 mL plated) shows 180 colonies. Find CFU per gram.
CFU/g = 180 × 100 = 18,000 = 1.8 × 10<sup>4</sup> CFU/g.
4. Why is the 10<sup>-3</sup> plate with 18 colonies not used?
It has fewer than 25 colonies. A tiny count is not exact, so pick the 10<sup>-2</sup> plate with 180.
5. You mix 10 g of food with 90 mL water, then 1 mL of that with 9 mL water. What is the dilution now?
First tube 10<sup>-1</sup>, second tube 10<sup>-2</sup>. So the food is diluted 100 times.
Common mistakes
- Not zeroing the balance, so the dish weight gets added to the sample.
- Going past the pink end point. One extra drop too many gives a wrong answer.
- Counting the plate with too many colonies. Choose 25 to 250.
- Forgetting to write the unit or the dilution beside a reading.