Science notes, unit 8

Food preservation technology

Textbook Science Year 6 (DLP, Semakan 2017), pages 127–146

What this chapter covers

You can explain what food spoilage is and what causes it (8.1.1)
You can name the characteristics of spoilt food (8.1.2, 8.1.3)
You can explain the meaning and purpose of food preservation (8.2.1)
You can describe the methods of food preservation and link each to the growth factors of microorganisms (8.2.2, 8.2.3, 8.2.4)
You can explain how food is preserved by combining methods (8.2.5)
You can explain the importance of food preservation technology (8.2.6)
You can carry out a food preservation project (8.2.7)

Have you ever opened a packet of nasi lemak that was left in a hot car for hours? It smells bad and you cannot eat it. In this unit you will find out why food spoils, how people stop it from spoiling, and why this technology matters.

8.1Food spoilage

Food spoilage happens when microorganisms such as bacteria and fungi decompose food from complex to simpler forms. Decomposition is the breaking down of organic matter into simpler forms by microorganisms.

Factors that make food spoil

Microorganisms grow well when they have these four things:

water (and humidity in the air)
air
a suitable temperature (not too cold, not too hot)
suitable acidity

Characteristics of spoilt food

We can use our senses of sight, smell, taste and touch to detect spoilt food: a change in texture or colour, a bad smell, a sour or unpleasant taste, and a sticky or slimy feel.

Food	What happens when it is spoilt
Milk	Bubbly and lumpy; smells bad; tastes sour because bacteria produce acid.
Bread and rice	Covered with black spots called moulds; rice becomes slimy and smelly.
Fruits and vegetables	Mouldy; texture, taste and colour change.
Beef and chicken	Look blackish, smell bad and feel slimy.
Fish, squids and prawns	Texture changes, smell bad, feel sticky and mushy.

A slice of bread covered with grey-green mould — Mouldy bread: fungi grow where there is water and humidity.

Figure: Science Year 6 textbook, p.128.

Do not taste food on purpose to check whether it is spoilt. Look, smell and touch only, and throw it away if you are not sure.

The rice in Alia's lunch box is slimy and smells bad. Which factor in the box helped the microorganisms to grow?

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The water content in the rice and the warm temperature (also humidity in the air).

8.2Food preservation

Food preservation is the process of slowing down food from getting spoilt. Its purpose is to prevent or slow down the growth of microorganisms.

Each method of preservation takes away, or changes, one of the growth factors of microorganisms.

Methods of food preservation

Method	How it works	Examples
Drying	Heat removes the water content, so microorganisms cannot survive.	Dried fish, dried chillies
Boiling	High temperature kills most microorganisms.	Durian jam, chilli paste
Cooling	About 4 °C slows down growth.	Vegetables, fruits, dairy products
Freezing	0 °C and below stops growth.	Beef, fish, seafood
Vacuum packing	Air is removed, so microorganisms cannot grow.	Beef, vegetables, fruits
Canning	Food is cooked, sealed airtight in a can and heated; no air and high temperature.	Sardines, pineapple, vegetables
Bottling	Food is boiled, cooled and sealed in airtight bottles; high temperature and no air.	Sauces, beans, seafood
Pasteurising	Heated at 63 °C for 30 minutes or 72 °C for 15 seconds, then cooled quickly to 4 °C. UHT: 135 °C for 2 seconds.	Milk, juice
Pickling	Soaked in vinegar or a concentrated sugar or salt solution; unsuitable acidity.	Mangoes, fish, chillies
Salting	A large quantity of salt removes moisture.	Fish, eggs
Smoking	Slow drying with smoke removes water.	Beef, duck, chicken, fish, bananas
Waxing	A wax coat stops air and water from entering.	Vegetables, fruits

Dried fish and dried chillies — Drying removes water.

A refrigerator holding vegetables, fruit and milk — Cooling at about 4 °C.

Raw beef, fish and seafood — Freezing at 0 °C and below.

Figures: Science Year 6 textbook, p.132.

Remember: Cooling slows growth down; freezing stops it. Pasteurising is heating and then cooling.

Do not write "kills bacteria" for every method. Drying, salting and smoking take away water; vacuum packing removes air; pickling changes the acidity.

Example: link method to factor

Salted fish keeps well because salt removes water from the fish. Without water, microorganisms cannot grow.

Name the preservation method for (a) sardines in a tin, (b) fresh milk heated at 72 °C for 15 seconds, (c) pickled mango.

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(a) Canning (b) Pasteurising (c) Pickling.

One food, many methods

The same food can be preserved in more than one way. Chillies can be dried in the sun (water removed), pickled in vinegar, salt and sugar (acidity), frozen (low temperature) or boiled into a paste (high temperature). In the boiled chilli paste, cooking oil forms a layer on top that keeps air out.

In the drying project the chillies lose mass because water leaves them, so the final mass is less than the initial mass.

Bar chart: mass of chillies 150 g on day 1, 90 g day 3, 55 g day 5, 30 g day 7 — Sample results (made up for this site): 150 g of chillies lose 120 g of water in 7 days and become 30 g.

Combining methods

Some foods are preserved by combining methods so they last longer. A cut fish is smeared with a large quantity of salt and then dried in the sun. Salting removes moisture and drying removes more water. Pickled fish uses salt and cooking oil: salt removes water and the oil keeps air out.

Why is a large quantity of salt used to preserve fish?

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Salt removes the water from the fish. Without water, microorganisms cannot grow.

8.3Importance of food preservation technology

Importance	Example
Food has a longer shelf life	Cencaluk and crackers last until next year.
Food wastage is avoided	Leftover rendang is dried to make beef floss.
Food is easier to store	Canned and bottled food needs little space.
Out-of-season food is available all year	Durian, rambutan and mangosteen jam.
Food is easier to export	Coconut milk products sent by ship to other countries.
Instant food can be made	Instant porridge and ready-to-eat rice, useful in drought or disaster.

Did you know? Preserved food has an expiry date, such as "use before 14 April 2021". Eat it before that date.

Give two reasons why we preserve durians in the fruit season.

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So we can enjoy the fruit all year (out-of-season supply) and so the extra fruit is not wasted.