Food Microbiology


Food microbiology is the study of the microorganisms which inhabit food. These microorganisms can have beneficial or detrimental effects on food. The beneficial effects of microorganisms have been utilised in the food industry where they help provide taste and texture to food, including the use of microorganisms to produce bread, cheese, vinegar, beer and wine. A great example is the specially cultured moulds that are used in the production of specialty cheese – forming the coloured veins in blue and gorgonzola cheese, and the edible rinds on brie and camembert cheese.

On a negative side microorganisms can cause food borne illnesses and food spoilage. The main types of microorganisms present in food are bacteria and fungi. Moulds are the most common microorganisms which cause food spoilage. Moulds are microscopic fungi which consist of filaments of cells which join together forming a network within foods. Moulds develop most rapidly in damp humid conditions.  Moulds are generally not harmful to the consumer although a small portion of moulds are capable of producing toxins which are hazardous to health. Moulds can, however, affect the sight and texture of foods causing a fluffy growth on food which is typically coloured white, grey, yellow or blue depending on the type of mould.

Yeasts are another type of fungi which cause foods to spoil. This time food spoilage is caused by the ability of yeast to act on foods such as fruit juices and syrups and cause fermentation. Foods that are most effected by yeast fermentation include fruit juices, honey, jams and jellies.  Bacteria also cause food to spoil e.g. lactobacillus and lactococcus bacteria cause milk to sour, while some bacteria are also pathogenic causing disease

The growth and development of any microbes on or in food depends on a range of factors including water, pH, oxygen and temperature as well the physical structure of the food itself.




Food Spoilage by Enzymes
Enzymes are catalysts in living cells which are capable of speeding up or slowing down a chemical reaction without being altered by the reaction itself. Enzymes are a natural constituent of food and they are also released by bacteria which invade food to allow them to obtain nutrients from the food. Enzymes themselves catalyse a wide variety of functions in plants and animals. In relation to foods, enzymes can help to control ripening and changes in the flavour, texture, nutritional content and colour of different foods.

The action of enzymes has had benefits to food manufacture. Examples of enzymes used in the food industry include proteases which are used to help tenderise meat, lipase which helps provide flavour to foods such as chocolate and cheese, and amylase which converts starches to sugars in industries such as brewing and baking.  

Unfortunately enzymes also have a negative side as they start to degrade food causing spoilage e.g. enzyme reactions cause the blackening of foods such as apples and bananas. Enzymes specifically linked to food spoilage include lipase, which causes cereals to discolour and promotes hydrolytic rancidity in milk and oils; ascorbic acid oxidises which causes the destruction of vitamin C in vegetables; pectic enzyme which promotes the softening and browning of fruits; and protease which leads to a reduction of gluten formation in flour.

Measures to reduce food spoilage catalysed by enzymes
1.    Store foods at cold temperatures - enzymes become less active and even inactive at low temperatures.  Refrigerating food slows down the action of enzymes (as well as slowing down the multiplication of microbes) while freezing slows the action of enzymes still further, or even retards enzyme activity. In contrast, higher temperatures promote the action of enzymes as well as enabling the multiplication of microbes - this is why foods spoil more rapidly at higher temperatures.
2.    Use enzyme inhibitors which interact with enzymes to prevent them working in a normal manner e.g. sulphur dioxide is used to inhibit the enzymatic browning of fruits and vegetables
3.    Blanch fruits and vegetables  before freezing to inactivate enzymes
4.    Change pH by the addition of an acid such as citric acid/ascorbic acid/lemon juice, this is beneficial as enzyme activity is dependent on pH and lowering pH to below 4 inactivates enzymes.

Food-borne Disease
In addition to causing the loss of food through food spoilage, microbes are also responsible for a variety of food borne diseases. Before experimenting with food preservation it is essential to gain a basic understanding of food borne disease in order to prevent it from occurring.

Food-borne diseases fall into two main categories i.e. food borne infections and food intoxications (food poisoning).

Food-borne Infections
Food borne infections can result from consuming foods contaminated with pathogenic (disease causing) microorganisms, most commonly bacteria, followed by viruses then parasites. These pathogens invade and multiply in the small intestine and cause a range of symptoms including diarrhoea, vomiting, abdominal cramps, headache and fever. The most common food borne infections are caused by salmonella, E-coli and campylobacter. Salmonella is a bacterial infection passed to humans from domestic and wild animals including pigs, cattle or eating under cooked poultry and poultry products.  E-coli is associated with eating under cooked contaminated ground beef or drinking unpasteurised milk or contaminated water supplies and campylobacter is a bacterium which causes acute diarrhoea as a result of eating contaminated foods

Sources of food borne infections include the people handling food (due to inadequate hand washing and personal hygiene) and the equipment used in food preservation and packaging, which may be contaminated during food processing.  Other sources include storing foods for too long or at the wrong temperature allowing for bacterial regrowth, or the contamination of foods prior to collection, for example animal products may be contaminated with an infection such as salmonella, shellfish may be contaminated in their environment e.g. with faecal material.

One vital point to consider is that not all infections are caused by animal products such as meat/poultry and eggs, as in fact many outbreaks are also attributed to fruits and vegetables.  Fruit and vegetables may be contaminated during production/growing, harvesting and picking. Sources of contamination include contaminated water used for irrigation and contamination from manure. Further sources of contamination exist when it comes to processing fruit and vegetables e.g. by washing, preparing and packing and distributing  produce with contaminated water supplies, using contaminated equipment or passing on food borne infections from the food handlers.