Bacteria are ubiquitous, minute unicellular microorganisms and cannot be seen with naked eye. The growth of bacteria depends upon specific kind of food, temperature, pH, moisture and oxygen. Bacteria are much more difficult to kill than moulds and yeast and are the most common causes of food spoilage. They are present in two forms viz. vegetative and spore, which are active and resting form, respectively. The vegetative forms of bacteria are easily destroyed at boiling temperature whereas spores require harsh treatments like application of heat (100°C) for a long time (six hours) or at 121°C under 15 psi pressure for 30 min.
Each kind of bacteria has a definite range of food requirements. Level of moisture in food is important in preventing or allowing the bacterial growth in the food. Bacteria require more moisture than yeasts or moulds. Most bacteria grow best at neutral pH, however, few bacteria also grow in acid or alkaline media. All forms of bacteria are sensitive to acids and can be killed easily in acidic pH at a temperature of boiling water. So foods with high acid content (all fruits, tomatoes, pickles etc.) are processed at 100°C whereas low acid foods such as corn, peas, beans and all vegetables except tomatoes have to be processed at higher temperature (116°C) in a steam pressure to kill bacteria. Moist heat resistant bacteria are present in the soil, hence, preparation and processing of root vegetables require special care. Clostridium botulinum causes spoilage in canned foods.
Bacteria are classified as aerobic, anaerobic and facultative when they require free oxygen for growth, no oxygen and can grow with or without free oxygen, respectively. On the basis of temperature required for growth, bacteria can be grouped into four types viz. mesophiles, obligate thermophiles, facultative thermophiles and psychrophiles requiring temperature below 38°C, 38 to 82°C, below 38 to 82°C and at refrigeration temperatures, respectively.
The most common bacteria causing significant reductions in shelf life of fruits and vegetables is the soft rotting species of the genus Erwinia . Under suitable conditions they produce large quantities of extracellular enzymes which rapidly macerate the tissue which gives unpleasant off-odours.
Some bacteria are also useful particularly in food processing. For example- aerobic bacteria like Acetobacter aceti used in vinegar production and lactic acid bacteria likeLactobacillus plantarum, Pediococcus cerevisiae, Leuconostoc mesenteroides etc. produce lactic acid by fermentation of carbohydrates.