Dehydration, Microbes, and Spoilage
Topic 2. Food Spoilage
1. What is food spoilage?
Food spoilage means the original nutritional value, texture, flavour of the food are damaged, the food become harmful to people and unsuitable to eat.
2. Causes of the spoilage of food
The picture of spoiled food
There are three types of microorganisms that cause food spoilage -- yeasts, moulds and bacteria.
- Yeasts growth causes fermentation which is the result of yeast metabolism. There are two types of yeasts true yeast and false yeast. True yeast metabolizes sugar producing alcohol and carbon dioxide gas. This is known as fermentation. False yeast grows as a dry film on a food surface, such as on pickle brine. False yeast occurs in foods that have a high sugar or high acid environment.
- Moulds grow in filaments forming a tough mass which is visible as `mould growth'. Moulds form spores which, when dry, float through the air to find suitable conditions where they can start the growth cycle again.
- Mould can cause illness, especially if the person is allergic to molds. Usually though, the main symptoms from eating mouldy food will be nausea or vomiting from the bad taste and smell of the mouldy food.
- Both yeasts and moulds can thrive in high acid foods like fruit, tomatoes, jams, jellies and pickles. Both are easily destroyed by heat. Processing high acid foods at a temperature of 100°C (212°F) in a boiling water canner for the appropriate length of time destroys yeasts and moulds.
|Storage rot in grapes caused by Botrytis cinerea.|
|Storage rot in strawberry caused by Botrytis cinerea.|
|Blue mould rot in tomato caused by Penicilliumi spp. (also by Fusarium spp.)|
|Black mummy rot of grapes caused by Guignardia bidwellii|
|Watery soft rot in apple caused by Sclerotinia sclerotiorum.|
|Blue mould on oranges caused by Penicillium digitatum.|
- Bacteria are round, rod or spiral shaped microorganisms. Bacteria may grow under a wide variety of conditions. There are many types of bacteria that cause spoilage. They can be divided into: spore-forming and nonspore-forming. Bacteria generally prefer low acid foods like vegetables and meat. In order to destroy bacteria spores in a relatively short period of time, low acid foods must be processed for the appropriate length of time at 116°C (240°F) in a pressure canner. (Temperatures higher than 100°C [212°F] can be obtained only by pressure canning.)
- Eatting spoiled food caused by bacteria can cause food poisoning.
|Soft rot in tomato caused by Erwinia carotovora.|
Enzymes are proteins found in all plants and animals. If uncooked foods are not used while fresh, enzymes cause undesirable changes in colour, texture and flavour. Enzymes are destroyed easily by heat processing.
- Oxidation by air
Atmospheric oxygen can react with some food components which may cause rancidity or color changes.
- Infestations (invasions) by insects and rodents, which account for huge losses in food stocks.
- Low temperature injury - the internal structures of the food are damaged by very low temperature.
|Low Temperature Injury|
|Internal mahogany browning of potato caused by low temperature injury.|
|Chilling injury in cucumber caused by low temperature. Note the watery surface.|
Topic 1. Microbes and Food Spoilage
Topic 2. Food Spoilage
Topic 3. Ecosystem, Food Chain and Food Web
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