Classification of foods by shelf life
By how long they keep, foods are perishable, semi-perishable or non-perishable (stable). Moisture is the main reason some spoil in days and others last a year.
What is shelf life?
Shelf life is the period during which a food, kept under the stated storage conditions, stays safe, keeps its expected colour, flavour, texture and nutrients, and matches its label. It depends on the food itself (moisture, water activity, pH), how it is processed and packed, and the storage temperature. 'Use by' and 'best before' dates on labels are based on it.
Three groups of foods
| Group | Keeps for | Examples | How to store |
|---|---|---|---|
| Perishable | A few hours to a few days at room temperature | Milk, curd, paneer, meat, fish, eggs (in hot weather), leafy greens, tomatoes, ripe mango, cooked rice and sambar | Refrigerate or freeze; use quickly |
| Semi-perishable | A few weeks to a few months | Potatoes, onions, garlic, some apples, nuts, atta, rava, cheese | Cool, dry, airy place; airtight containers |
| Non-perishable (stable) | A year or more | Rice, wheat, dry pulses, sugar, salt; canned and dried foods while sealed | Dry, pest-proof, airtight bins; first in, first out |
Why some foods keep longer
- Moisture: high water activity lets microbes grow fast; dry grains and sugar resist them.
- pH: near-neutral foods (meat, milk) favour bacteria; acid foods keep a little longer.
- Living tissue: fruits and vegetables keep breathing and ripening after harvest.
- Natural covers: egg shell, onion skin and grain husk slow spoilage; bruising and cutting speed it up.
- Processing moves a food to a more stable group: milk → milk powder, raw mango → pickle, grapes → raisins, fresh peas → canned peas.
In a canteen or hospital store, the class decides the buying plan: perishables are bought daily or every two days and held in the cold room, semi-perishables weekly, stable foods monthly in bulk. This cuts waste and saves nutrients.
2 marks: “What are perishable foods? Give two examples.” Foods that spoil within hours or a few days at room temperature unless preserved, e.g. milk and fish.
5 marks: “Classify foods based on shelf life with examples.” Three groups with keeping time, four examples each and the storage rule; add how processing moves a food to a more stable group.
Perishable = days, semi-perishable = months, stable = a year: milk, onion, rice.