Factors affecting microbes in foods
The microbial flora of food consists of microorganisms associated with the raw material, processing and those surviving pasteurization and preservation treatments.
There are about 25 genera of bacteria in food causing either spoilage and or user in desirable effects.
Examples: Clostridium, Bacilli, Serratia, Staphylococcus, Pediococuus, Acetobacter, Acinetobacter, Alcaligens, Bacteroids, Citrobacter, Corynebacterium, Erwinia, Escherichia, Flavobacterium, Lactobacillus, Micrococcus, Pedicoccus, Streptococcus, Streptomyces, Shigella, Enterobacter, Leuconostoc, Pseudomonas, Proteus, etc.
There are also 18 known fungi known in food:
Fusarium, Geotricum, Alternaria, Cladosporium, Rhizopus, Sporotricum, Penicillium, Mucor, Aspergillus, etc.
Only 12 genera of yeast that are known to be involved in food amongst are:
Saccharomyces, Torulopsis, Mycoderma, Rhodotorula, Trichosporium, Candida, Berthanomyces, etc
Microorganisms in food products are basically affected by two distinct factors:
1. Extrinsic factors and
2. Intrinsic factors.
This are factors in the environment external to the food, which affect both the microorganisms and the food itself during processing and storage. Extrinsic factors include Temperature, humidity, and oxygen.
Different microorganisms grow over a wide range of temperatures.
There basically three temperature types: Mesophiles, Psychrophiles, Thermophiles
Psychrophiles are microorganisms that grow at low temperatures. (0-20°C), 15°C optimum. Example; Bacillus, Pschrophilus etc. Psychrophiles are the major course of refrigerated food spoilage.
Mesophiles grow best at room temperature. They have optima around 20- 45°C and often have a temperature minimum of 15 to 20oC and a maximum of about 45°C. Most human pathogens fall under this group because of the normal 370C body temperature.
Thermophiles grow at high temperatures (between 55°C and 85°C), optima between 55°C and 66°C. Hyperthermophile are those organisms that usually have optima between 85°C and about 113°C. Temperature is of paramount importance in food safety because if the growth temperature ranges for dangerous microorganisms are known, it helps in employing appropriate production temperature and time for foods that require heating. It also aids in selecting the proper temperature for food storage to make them less able to grow and reproduce.
An important factor for the growth of microorganisms at the food surfaces is the humidity of the storage environment. Dry conditions are devoid of water for microbial activities referred to as water activity and thus better for food storage than moist conditions. Foods stored in a dry atmosphere, therefore, have a longer shelf life than foods stored in a humid environment.
Some microorganisms require oxygen in order to grow and multiply. Such organisms are called aerobic microorganisms. An example is Escherichia coli; On the other hand, there are some microorganisms that grow without oxygen, called anaerobic microorganisms. For example Clostridium botulinum, the bacterium causes botulism in very low oxygen environments as is in canned foods. Obligate aerobe: are those that completely depends on atmospheric oxygen for growth e.g. protists and fungi. Organisms can be classified based on oxygen requirements as follows;
Facultative anaerobe: are those that do not require oxygen for growth but grows better in its presence e.g. Escherichia, Enterococcus.
Aerotolerant anaerobe: grow equally well in the presence of oxygen e.g. Streptococcus pyogenes.
Obligate anaerobe: does not tolerate oxygen and dies in its presence e.g. Clostridium, Bacteroides
Obligate aerobe: grow only in the presence of oxygen.
Microaerophile: requires oxygen level between 2 10% for growth and is damage by atmospheric oxygen levels (20%) e.g. Campylobacter, Spirillum volutans.
These are factors that exist as part of the food product itself. For example, fish have certain characteristics that may promote the growth of certain microorganisms. Below are the common intrinsic factors that affect the growth and multiplication of microorganisms in foods;
pH is the measure of acidity or alkalinity of an environment on a scale that has neutral at pH7. Environments that are acidic have pH values below 7; those that are alkaline have pH values above 7. Most microorganisms grow best at close to the neutral pH value (pH 6.6 to 7.5). Only a few microorganisms grow in very acid conditions below a pH of 4.0. Bacteria grow at a fairly specific pH for each species, but fungi grow over a wider range of pH values. Microbes can be classified based hydrogen potential as follows;
Acidophiles: have growth optimum between pH 0 and 5.5 e.g. Sulfolobus, Picrophilus
Neutrophiles: have growth optimum between pH 5.5 and 8.0 e.g. Escherichia, Euglena
Alkaliphiles (alkalophiles): have growth optimum between pH 8.0 and 11.5 e.g. Bacillus acidophilus
Extreme alkaliphiles: have growth optima at pH 10 or higher
Moisture content (water activity, aw)
Microorganisms are living cells, thus require a moist environment to grow in. The water requirements of microorganisms for growth are described in terms of water activity (aW). Also referred to as a measure of how much water is available or present. The water activity of pure water is aW = 1.00. Fungi can grow even at a lower aw = 0.7 Most foodborne pathogenic bacteria require aW to be greater than 0.9 for growth and multiplication.
Microorganisms require a range of nutrient-rich foods in nitrogen, vitamins, and minerals to aid proper growth and multiplication. Therefore vast species of microorganisms are feasible on such food, unlike food that may be deficient. Consequently, the spoilage rate is faster for the former than the later.