Yeast and molds & some bacteria grow within a wide pH range, ( Molds grow between 1.5 - 11.0)
Some bacteria can grow within a narrow pH range of 4.5 & 9.0 (Salmonella)
Most bacteria grow best at neutral or weakly alkaline pH usually between 6.8 & 7.5
Such inhibitors are like lactinin & anti-coliform factors in milk & lysozyme in eggs.
TYPES OF ANTIMICROBIAL SUBSTANCES:
Compounds killing microbes
Compounds stop division of microbes
Antimicrobial substances in food inhibit microbial growth.
The nutrients composition of a food influences the type of microbes that will grow & the product that they will produce during growth.
when a microbial cell is growing in a food, the nutrients supplied by the food include carbohydrates, protein, lipid, minerals & vitamins
Microbes need proteins, carbohydrates, lipids, water, energy, nitrogen, sulphur, phosphorus, vitamins & minerals for growth
The growth of microbes & their ability to generate energy by the specific metabolic reactions depend on the redox potential of foods.
Anaerobic or facultative anaerobic sporeformers are most likely to grow in canned foods
The growth in presence & absence of free oxygen, microbes have been grouped as aerobes, facultative anaerobes, anaerobes or microaerophiles,
Microaerophilic bacteria are most likely to grow in vacuum packed foods since they have low oxygen tension while aerobic bacteria are likely to grow on the surface of raw meat.
Aerobic molds will grow in insufficiently dried or salted products.
As aw decreases, the heat resistance increases.
Pure water has aw of 1.0
The water activity affects the growth, toxin production, spore germination & heat resistance of microbes
Microbes will not grow in the foods with aw less than 0.60
Most fungi require aw of more than 0.80
Water activity can be reduced by removing water and by addition of solutes
Most bacteria require aw of more than 0.90
For example: Meat has fascia, skin & other membranes that prevent microbial entry
Some food have biological structures that prevent the microbial entry
Eggs have shell & inner membranes that prevent yolk & egg white from infection
RELATIVE HUMIDITY OF ENVIRONMENT
Measures of the water activity of the gas phase
Relative humidity and water are interrelated
When food commodities have water activity are stored in high relative humidity, water transfers from gas phase into food.
PRESENCE & CONCENTRATION OF GASES IN THE ENVIRONMENT
Carbon dioxide regulates cell growth of some bacteria
Oxygen influences the redox potential of microbial associations.
Carbon dioxide is used in packaging of some food items in order to to control the growth of microbes.
TEMPERATURE OF STORAGE
Every 10 degree Celsius rise doubles the catalytic rate of enzyme & every 10 degree Celsius reduces it to half.
Optimum temperature of growth
These organisms grows at 25 to 40 degree celsius
None of mesophilic bacteria are able to grow below 5 degree Celsius or above 45 degree Celsius
These grow at 45 degree celsius
Often their optimum growth temperature is between 50 & 70 degree celsius
These grow best at 20 to -10 degree celsius in unfrozen media
Causes food spoilage at low temperatures
Foods are exposed to different temperature from time of production until the time of consumption