Microbial spoilage is caused by microorganisms like fungi (moulds, yeasts) and bacteria. They spoil food by growing in it and producing substances that change the colour, texture and odour of the food. Eventually the food will be unfit for human consumption.
When food is covered with a furry growth and becomes soft and smells bad, the spoilage is caused by the growth of moulds and yeasts. Microbial spoilage by moulds and yeasts includes souring of milk, growth of mould on bread and rotting of fruit and vegetables. These organisms are rarely harmful to humans, but bacterial contamination is often more dangerous because the food does not always look bad, even if it is severely infected. When microorganisms get access to food, they utilise the nutrients found in it and their numbers rapidly increase. They change the food's flavour and synthesise new compounds that can be harmful to humans. Food spoilage directly affects the colour, taste, odour and consistency or texture of food, and it may become dangerous to eat. The presence of a bad odour or smell coming from food is an indication that it may be unsafe. But remember that not all unsafe food smells bad.
What is the difference between food contamination and food spoilage?
Food contamination is when food is contaminated with microorganisms or substances and eating it could result in foodborne disease. Food spoilage is any undesired change in the natural colour, taste or texture of food items that makes it unfit for consumption because it has lost its quality and nutritional value.
The term contact spoilage is used when microbial spoilage is the result of direct contact or touching between the food and any contaminated or unclean surface such as shelves, food preparation boards or unwashed hands. It also includes food-to-food contact, for example between cooked meat and raw meat or between rotting fruit and sound fruit.