FOOD SPOILAGE: MEANING AND CAUSES

Food is considered spoiled when undesirable change in the colour, flavour, odour or texture has occured. Foreign substances in the food products make food undesirable! The onset of food spoilage is rather indefinite. It is a gradual process occuring as a result of poor sanitation, enzymatic or chemical reactions, improper temperature controls, microbial growth etc. Detecting spoilage relies on being aware of the typical indicators, such as appearance (discolouration or slime), texture, smell or taste.

Shelf life refers to the length of time a food remains palatable- from the time it is harvested or processed until when it is consumed.

Once food is harvested or slaughtered, its plant or animal tissue soon starts to decay.  Microorganisms, such as fungi (molds & yeasts), spoilage bacteria, and their enzymes usually cause the spoilage process. Not all these changes in food are undesirable. Some people like aged beef and cheeses or very ripe fruit. The production of wine and beer involves conversion of sugars to alcohol, while souring of milk is essential in the production of cheese.

However, it is important to remember that some of the conditions that accelerate spoilage, such as inappropriate temperature and moisture control, also encourage the growth of pathogenic microorganisms that cause foodborne illness. Consequently, spoiled food is not just 0an issue of quality, it is also often a question of food safety.

Like us, microbes need food to stay alive. The foodstuffs that keep us healthy also provide the ideal nutrients for the growth of microbes.

Spoiling food

Microbes are all around us – in the air, the soil, water and our bodies. This means they can soon get into food and, if the conditions are right, multiply rapidly. Unfortunately, when certain microbes grow on food, it soon begins to smell nasty, look slimy, change colour, taste awful or even acquire a furry coating. The food ‘goes off’ – it is spoiled. Even though it may not harm us, it is inedible and must be thrown away. There is also a chance that pathogenic microbes are present along with the spoilers.

Around a quarter of the world’s food supply is lost to spoilage by microbes and insects. So what can we do to outwit the microbes and ensure that we consume the food before they do? Over the years man has developed many ways of preserving foods and today’s food technologists have refined the techniques and come up with new ones.

We should remember that spoilage is not necessarily a bad thing. It shows us that a food has not been made or kept in the best conditions, alerting us to the possible presence of pathogenic microbes.  Also decomposition returns the chemicals in food back to the environment, to be used again in the life cycles of Earth.

CAUSES OF THE SPOILAGE OF FOOD

• MICROBIAL
• NON-MICROBIAL

a.            Microbial spoilage

This is the major cause of food spoilage. It occurs as a result of contamination of food by microbes, provision of suitable environment for their growth, and degradation of foodstuffs. There are three types of microorganisms that cause food spoilage -- yeasts, moulds and bacteria. Eating spoiled food caused by bacteria can cause food poisoning.

• 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 temperature and time. If bacterial growth can be controlled, then the major cause of food spoilage would have been prevented.

• 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. They 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 high temperature of 100°C for the appropriate length of time destroys yeasts and moulds.

During the growth of microbes in food, the various organic compounds are decomposed leading to the production of a variety of chemical compounds which may alter the organoleptic properties of food. Sometimes such changes may be desirable or undesirable. This often involves the degradation of proteins, carbohydrates and fats by the microorganisms and their nutrients.

Microorganisms involved in food spoilage

Microorganisms involved in food spoilage(Other than Canned Foods) with some examples of causative organisms
Food	Type of Spoilage	Microorganisms involved
Bread	Mouldy	Rhizopus nigricans Penicillium ,Aspergillus niger
	Ropy	Bacillus subtilis
Maple sap and syrup	Ropy	Enterobacter aerogenes
	Yeasty	Saccharomyces, Zygosaccharomyces
	Pink	Micrococcus roseus
	Mouldy	Aspergillus,Penicillium
Fresh fruits and vegetables	Soft rot	Rhizopus , Erwinia
	Gray mold rot	Botrytis
	Black mold rot	A. niger
Pickles, sauerkraut	Film yeasts, pink yeasts	Rhodotorula
Fresh meat	Putrefaction	Alcaligenes Clostridium Proteus vulgaris Pseudomonas fluorescens
Cured meat	Mouldy	Aspergillus Rhizopus Penicillium
	Souring	Pseudomonas Micrococcus
	Greening, slime	Lactobacillus Leuconostoc
Fish	Discoloration	Pseudomonas
	Putrefaction	Alcaligenes Flavobacterium
Eggs	Green rot	P. fluorescens
	Colorless rots	Pseudomonas Alcaligenes
	Black rots	Proteus
Concentrated orange juice	"Off" flavor	Lactobacillus Leuconostoc, Acetobacter
Poultry	Slime, odor	Pseudomonas , Alcaligenes

 

Common mould on bread

1. Rhizopus stolonifer
2. Penicillin expansum
3. Aspergillus niger
4. Chrysonilia sitophila

Rhizopus stolonifer

Penicillin expansum

Aspergillus niger

NON MICROBIAL SPOILAGE

This comes up as a result of chemical changes within the food itself or reaction between the food and the packaging material.

• Enzymes

Enzymes bring about chemical changes in food. Enzymes can be defined as large protein molecules that act as biological catalysts accelerating the chemical reactions without being consumed to any appreciable extent themselves. The activity of enzymes is specific for a certain set of chemical substrates, and it is dependent on both pH and temperature. All living tissues of palnts and animals maintain a balance of enzymatic activities. The balance is disrupted upon harvest or slaughter. If uncooked foods are not used while fresh, enzymes cause undesirable changes in colour, texture and flavour. Enzymes are destroyed easily by heat processing.

ENZYMES	FOOD PRODUCT	SPOILAGE
Ascorbic acid oxidase	Vegetables	Destruction of Vitamin C
Lipase	Cereals  Milk Edible oils	Discolouration & Hydrolytic rancidity Hydrolytic rancidity
Lipoxygenase	Vegetables	Destruction of Vitamin A
Pectic enzymes	Fruits Citrus juices	Excessive softening Destruction of pectic substances
Peroxidase	Fruits	Softening
Phenolase	Fruits/vegetables	Browning, off-flavours
Protease	Eggs Crabs, Lobster Flour	Reduced shelf life of eggs Overtenderisation Reduction of gluten formation
Thiaminase	Meat, fish	Destruction of thiamine

• Oxidation by air

Atmospheric oxygen can react with some food components which may cause rancidity or color changes.

3. Other factors

• 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.