Foods and Nutrition · Lesson 7

Causes of spoilage, preservation methods, storage and packaging

WAECNECO

What the syllabus asks

WAEC: Food storage and preservation: food spoilage (meaning, causes and agents); food preservation (meaning and importance, general principles, methods); food storage (meaning and importance, perishable and non-perishable foods, storage space and equipment: refrigerator, deep freezer, cupboards, shelves, granaries, barns); Self-employment: packaging (explanation, reasons, qualities and types of…

NECO: Food storage and preservation: food spoilage (meaning, causes and agents); food preservation (meaning and importance, general principles, methods); food storage (meaning and importance, perishable and non-perishable foods, storage space and equipment: refrigerator, deep freezer, cupboards, shelves, granaries, barns); Self-employment: packaging (explanation, reasons, qualities and types of…

What you will learn

Define spoilage and identify its signs, causes and agents.
Explain the importance and principles of preservation.
Describe methods of preservation including traditional Nigerian methods.
Explain storage rules, date marks and equipment.
Explain reasons for packaging and qualities and types of packaging materials.
Calculate microbial growth, brine strength and drying yields.

Key points

Spoilage meaning and signs.
Causes: microbes, enzymes, chemical, physical, pests.
Microbial growth and its conditions.
Importance and principles of preservation.
Heat methods: boiling, pasteurisation, sterilisation, canning, bottling.
Cold methods: refrigeration, freezing, cold chain.
Drying, smoking, salting, sugaring; traditional methods.
Chemical preservatives, fermentation, irradiation, pickling.
Storage rules, date marks.
Storage spaces and equipment.
Packaging: reasons, qualities, materials.

Start here: Food spoilage: meaning and signs

Food spoilage is any change in food that makes it unfit or unpleasant to eat. Spoiled food may change in colour, smell, taste, texture or appearance, and may contain harmful microbes or toxins. Spoilage is different from food poisoning: some spoiled food (sour milk) is not dangerous, while some food that looks and smells normal can carry dangerous bacteria. Both must be prevented.

Signs of spoilage

Off smell: sour (milk, rice), putrid (fish, meat), rancid (oil, groundnuts).
Colour change: green-grey meat, blackened fruit, discoloured bread.
Slime on meat, fish, cooked vegetables and soups.
Mould growth: fuzzy spots on bread, oranges, tomatoes, cooked swallows.
Gas and bubbles: fermenting juices and soups; swollen (blown) cans and packets.
Sour taste in stews kept overnight without reboiling.
Texture change: soft, mushy fruits; hard stale bread; wilted vegetables.
Insects and holes in grains, beans and flour; webbing in flour.
Leaking, dented, rusted or bulging containers.

Perishable and non-perishable foods

Highly perishable: spoil within hours to a few days: fish, meat, poultry, milk, eggs (relatively), cooked foods, leafy vegetables, tomatoes, ripe fruits.
Semi-perishable: keep for weeks with good storage: yam, potatoes, onions, oranges, bread (days), cheese.
Non-perishable: keep for months when dry and protected: grains, dried beans, garri, flour, sugar, salt, dried fish, canned foods, powdered milk.

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Practice questions

1. Water activity below about ____ stops most bacteria.

Show answer

A: 0.9
Moulds can grow to about 0.8.

2. Low-acid canned foods must be processed at about

100 °C
72 °C
60 °C
121 °C

Show answer

D: 121 °C
Destroys Clostridium botulinum spores.

3. Jam keeps because its high sugar

kills all spores
raises pH
draws water out of microbes by osmosis
adds vitamins

Show answer

C: draws water out of microbes by osmosis
About 60–65% sugar.

4. The usual home freezer temperature is about

−50 °C
−18 °C
4 °C
0 °C

Show answer

B: −18 °C
Stops growth, does not kill all microbes.

5. Fish dried from 5 kg (75% water) to 15% moisture weighs about

1.47 kg
1.25 kg
3.75 kg
0.75 kg

Show answer

A: 1.47 kg
1.25 ÷ 0.85.

Key words

Spoilage: Change making food unfit or unpleasant.
Perishable: Spoils quickly.
Spore: Resistant resting form of some bacteria.
Putrefaction: Breakdown of protein by microbes.
Enzyme: Protein catalyst in food.
Pasteurisation: Mild heat treatment.
Sterilisation: Heat killing all microbes.
Canning: Sealing and heat-processing food in cans.
Osmosis: Water movement out of cells by salt or sugar.
Brine: Salt solution.
Freezer burn: Drying of poorly wrapped frozen food.
Cold chain: Unbroken refrigeration from producer to consumer.

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