Enzymes & Signs of Food Spoilage

Food Preparation & Nutrition: AQA GCSE, topic 4.1.2

High-Risk Foods

We classify ready-to-eat foods as high-risk foods if they can cultivate dangerous bacteria from being stored incorrectly.

These foods contain lots of protein and water.
Because there's a risk of bacteria multiplying rapidly, these foods cannot be kept safely for long (short shelf life).
We don't classify certain uncooked foods as high-risk foods because we don't consume them uncooked. Eggs are a good example.

Common signs that food is going off include:
Mould growing - on things like bread.
A sour smell - for things like yoghurt.
A slimy texture - for things like chicken.
But sometimes there are no signs that food has been spoiled, so we have to be careful.

Common signs include:
Fresh meat - smelling fresh, feeling firm and appearing bright.
Fresh fish - smelling clean or a little salty, with red-coloured gills, shiny skin and clear eyes.

Enzymes are biological catalysts that increase the rate of chemical reactions. They are proteins that cause fruits to either ripen or brown.

Fruits ripen because of the enzymes they contain.
Ripeness determines a fruit's colour, texture and sweetness.
Enzymes in bananas cause them to soften and sweeten by breaking down the starch they contain.

Slicing fruits and vegetables like pears and potatoes expose their insides to oxygen. The oxygen causes the fruit to brown and the enzymes in the fruit make this process happen faster.
Fruits also start to brown as they start to overripen.

Enzymes function best at an optimal pH. So, introducing an acid can stop enzymes from working properly.
E.g. pouring orange or lemon juice (acidic) over fruits will stop the fruits browning.
Enzymes ripen vegetables, causing them to gradually lose flavour, nutrients, colour and texture. Although freezing vegetables will slow enzyme activity, it won't end enzyme activity. Therefore, we must blanch foods to kill ripening enzymes and make sure vegetables are suitably colourful and nutritious for freezing.