Unit 2.14, exercise 3, page 62.0:00
Why does food go bad?0:23
Food and Acidity the PH scale, which ranges from 1 to 14, measures whether something is an acid or an alkali.0:27
A solution with a pH value under 7 is an acid, whereas a solution with a PH value of more than 7 is an alkali.0:36
Anything with a ph value of 7 is considered neutral.0:45
For example, citrus fruits, which have a pH value of about 2, are acidic while pure water is neutral.0:49
The acidity of food is important because it affects the taste and the rate at which microorganisms grow in the food.0:58
Food and Microorganisms Molds, yeasts and bacteria are three types of microorganisms that cause food to spoil.1:07
Molds are microorganisms made of spores that move through the air and and land on food.1:18
We can avoid eating mouldy food because mold grows in colonies so we can see it.1:24
Yeasts are single celled organisms that can be found on the skin of fruit which make fruit go bad.1:30
Bacteria are single celled microorganisms which multiply by splitting in two.1:37
They are the most dangerous microorganisms because they are not visible to the naked eye.1:43
E. Coli and Salmonella are two deadly bacteria that can hide in food.1:49
They need warmth, moisture and time to grow on food.1:55
So we can prevent bacterial food poisoning by keeping dishes, surfaces and utensils clean, cool and dry.1:58
Oxidization Exposure to oxygen in the air also affects food.2:08
For example, an apple will quickly turn brown once it has been cut or peeled.2:15
This is because of a reaction between the natural proteins called enzymes in the fruit and the oxygen in the air.2:20
We can reduce this effect and stop foods such as fruit and vegetables from turning brown by adding a natural acid, for example lemon juice, putting them in boiling water for 10 seconds to destroy the enzymes, or by preserving2:27
them in a sugary solution such as syrup.2:40
Temperature we can help food to stay fresh longer by either heating or cooling it.2:45
UHT products are termed long life because they are ultra heat treated.2:52
This means they are heated to a temperature of about 149 degrees centigrade for six to nine seconds and then quickly cooled.2:58
This destroys the bacteria.3:07
Keeping food in the fridge at 0 degrees centigrade to 3 degrees centigrade will slow the growth of bacteria and keep food fresh for longer.3:10
Freezing food at minus 18 degrees centigrade or lower keeps bacteria dormant.3:18
We should always make sure that we heat frozen foods to a temperature of 72 degrees centigrade or more to cook the food properly and kill any bacteria present this way we can avoid getting food poisoning.3:24
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