Keeping food from spoiling
Look at twelve kitchen foods, from sun-dried fish to mango achar, find out how long each keeps and what stops the microbes, and sort them by the way they are preserved: drying, cold, heat or pickling.
Core concept
Food spoils when microbes grow in it; microbes need water, warmth and time, so food keeps longer when we take the water out, keep it cold, heat it to kill the germs, or pickle it in acid or oil
Topic: Chemistry in the kitchen
Food goes bad because tiny living things, bacteria and moulds, grow in it. Like all living things, microbes need water, warmth and food, and given time they multiply fast. People have kept food for thousands of years by taking away what microbes need. Drying takes the water out: sun-dried fish and chillies keep for months, and lots of salt or sugar does the same job by drawing the water out of the microbes. Cold slows microbes down (a fridge) or stops them (a freezer). Heat kills them: boiling milk, or heating food and sealing it in a tin so no new germs get in. Pickling puts food in acid: A substance that tastes sour, like lemon juice or vinegar. It turns blue litmus red. All key words (vinegar or lemon) or under oil with salt and spices, where microbes cannot grow.
What you will learn
- Explain why food spoils.
- Name four ways to preserve food and how each stops microbes.
- Sort everyday foods by the way they are preserved.
Before you start
- Know that microbes are living things too small to see.
Materials
- Cards or pictures of the twelve foods
Equipment
- None
Procedure
- Pick a food card and press “What stops the microbes?” and “How long does it keep?”.
- Do this for at least six foods.
- Sort every food by the way it is preserved.