Most bacteria grow best at neutral or weakly alkaline pH usually between 6.8 and 7.5.
Some bacteria can grow within a narrow pH range of 4.5 and 9.0, e.g. salmonella
Other microorganisms especially yeasts and molds and some bacteria grow within a wide pH range, e.g. molds grow between 1.5 to 11.0, while yeasts grow between 1.5 and 8.5.
Acid foods :
are those that have a natural pH of 4.6 or less
are low acid foods to which acids are added
Low acid foods:
are those with a pH greater than 4.6 and awof 0.85 or more
growth, toxin production, spore germination, and heat resistance of microorganisms
Water activity is defined as the vapour pressure of a food substance to that of water at the same temperature.
Availability of water for biological functions and relates to water present in a food in “free” form is measured by water activity (aw)
As aw↓, heat resistance↑.
Pure water has aw of 1.0
Microorganisms will not grow in foods with aw≤ 0.60.
Bacteria require aw of ≥ 0.90
Fungi require aw of ≥0.80
Aw can be reduced by :
Removing water (partial) –Drying
Addition of solutes –Preservation
Redox Potential & O2
ability to generate energy by depend on the redox potential of foods.
Microorganisms have been grouped as aerobes, facultative anerobes, anaerobes or microaerophiles
Anaerobicor facultatively anaerobic sporeformers are most likely to grow in canned foods .
Microaerophilic bacteria are most likely to grow in vacuum packed foods since they have low oxygen tension.
Aerobic bacteria are likely to grow on the surface of raw meat.
Aerobic molds will grow in insufficiently dried or salted products
proteins, carbohydrates, lipids, water, energy, nitrogen, sulphur, phosphorus, vitamins, and minerals for growth.
Influences the type of microorganisms that will grow & the products that they will produce during growth.
When a microbial cell is growing in a food, the nutrients supplied by the food include:
carbohydrates, proteins, lipids, minerals and vitamins.
All foods contain these 5 major nutrients but nutrients vary greatly with the type of food.
Normally found in food vary greatly in nutrient requirement with bacteria requiring the most followed by yeast and molds.
Various foods have inherent antimicrobial substances that prevent (inhibit) microbial attack.
Such inhibitors are like lactinin and anti-coliform factors in milk and lysozyme in eggs.
Types of antimicrobial substances:
compounds stop division of microorganism
compounds killing microorganisms.
Types of antimicrobial effect:
Compounds damaging structure of cell or its function
(cell wall, cytoplasmic membrane, ribosomes)
Compounds affecting microbial enzymes
(oxidative agents, chelating agents, heavy metals, antimetabolites)
Compounds reacting with DNA
( chemical mutagens –alkylating or deaminatingagents, cytostatic)
biological structures that prevent microbial entry.
For example, meat has fascia, skin and other membranes that prevent microbial entry.
Eggs have shell and inner membranes that prevent yolk and egg white from infection.
refer to the food composition and characteristics that influence microbial growth in the food.