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Most micro-organisms require food, warmth, moisture, oxygen, time and a certain pH level in order to grow and multiply.

Moulds
•Most moulds are saprophytic i.e. they feed on dead organic matter. They grow on many foods especially fruit, bread, jam and cheese.
•Most moulds are mesophiles i.e. they thrive at temperatures between 20 and 45 degrees Celsius. Freezing (-18 degrees Celsius) inactivates mould growth. Temperatures below 15 degrees Celsius including fridge temperatures (1 to 4 degrees Celsius) retard mould growth. Cooking temperatures above 75 degrees Celsius destroy moulds.
•Moulds grow best in humid moist conditions, on foods with water present in liquid form. Frozen foods are unsuitable.
•Moulds are aerobic i.e. they require oxygen. As a result they grow on the surface of solid foods e.g. jam, and throughout open structure foods e.g. bread.
•Moulds favour slightly acidic conditions of pH 4 to pH 6. Extremely acidic or alkaline conditions inhibit mould growth.
•Moulds need time in order to multiply.

Yeast
•Yeast feeds on carbohydrates.
•Yeast thrives at temperatures between 25 to 30 degrees Celsius. Low temperatures inactivate yeast, while high temperatures of about 60 degrees Celsius kill yeast.
•Yeast needs moisture in order to grow.
•Yeast is a facultative organism i.e. it can live with or without oxygen.
•Yeast grows best in an acidic environment.
•Yeast needs time to grow.

Bacteria
•Some bacteria are saprophytic, and some are parasitic. Saprophytic bacteria i.e. those that feed on dead organic matter, include those bacteria which are present in soil and food and cause decomposition. Parasitic bacteria i.e. those that feed on living matter, cause diseases in humans, plants and animals.
•Bacteria have a very wide temperature range. Some are psychrophiles i.e. they thrive at low temperatures of -5 to 20 degrees Celsius, e.g. some clostridia bacteria.
Added 5/10/2013 4:11:49 PM
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