Source
In general, the internal tissues of healthy animals (meat) and plants (fruits and vegetables) are essentially sterile. However, microorganisms enter foods from both natural and external sources to which they are exposed from the time of production/capture/harvest until the time of consumption. Natural sources for foods of animal origin include skin, hoofs, hair, feathers, gastrointestinal tract, urinogenital tract, milk duct (teat canal). In contrast, natural sources for foods of plant origin include the surfaces of fruits, vegetables, grains and pores of tubers and onions.
In addition, foods contamination can be through air, soil, water, feeds, humans (food handlers), processing equipment, packages and others.
MICROBIAL GROWTH AND NECESSARY FACTORS
The phenomenon of microbial food spoilage occurs due to either microbial growth in a given food or release of microbial enzymes (both extracellular and intracellular) in the food micro-environment. Generally, microbial food spoilage involves several events that usually take place in sequence.
Following the contamination of the food from any of these sources (air, soil, water etc), the food inherent parameters (pH, oxidation-reduction potential, nutrients, antimicrobial substances) as well as the storage conditions must favour the growth of the contaminant and finally, the food must be stored for sufficient length of time to enable the microorganism(s) attain the high population needed to induce undesirable quality changes in the food. Multiplication of microorganism(s) in foods is a very important factor in food spoilage, hence, bacteria are the major spoilage agents because of their short generation time compared with fungi and other microorganisms.
PARAMETERS OF FOODS THAT AFFECT THEIR SPOILAGE
Two categories of parameters critical to microbial food spoilage are:
(1) intrinsic and (2) extrinsic parameters. The intrinsic (inherent) parameters serve as defence mechanisms against the invasion and proliferation of microorganisms in foods. Therefore, the discussion of microbial spoilage of foods will be incomplete
without emphasis on these parameters.
- INTRINSIC PARAMETERS
These inherent parameters are found in animal and plant tissues and they help to prevent or retard the microbial spoilage of foods that are derived from them. The parameters are as follows:
• pH
• Moisture content
• Oxidation- reduction potential (Eh)
• Nutrient content
• Antimicrobial constituents/substances
• Biological barriers/structures
These parameters are discussed briefly as follows:
pH
In general, most bacteria grow best at about pH 6.6 – 7.5 while yeasts and moulds grow at below pH 5.0. It is therefore important to know the minimum pH values for the growth of some food-borne bacteria and fungi.
Minimum pH values for the growth of some foodborne bacteria and fungi.
Microorganism pH
Aeromonas hydrophila 6.0 – 6.5
Bacillus cereus 6.0 – 7.0
Clostridium botulinum 5.0 – 6.0
Clostridium perfringens 6.0 – 6.5
Escherichia coli 0157:H7 5.0 – 6.0
Lactobacillus plantarum 4.5 – 5.5
pH values of some selected foods
Food pH range Food pH range
Beef 5.3 – 6.2 Oysters 5.8 – 6.5
Whereas several factors influence the growth of microorganisms and their ability to cause food spoilage, an obvious relationship/correlation exists between the type of species, the initial population and the type of food. For example, while bacteria are mainly responsible for spoilage of pH neutral foods (meat, fresh milk, poultry), yeasts and moulds are the main spoilage agents of acidic products such as fruits, alcoholic products/soft drinks as well as high sugar products.
The food composition influences the microorganisms and at the same time, the microorganisms influence the food ecosystem. For instance, the initial reaction of most microorganisms in food is acidic because they breakdown carbohydrates to produce organic acids. This alteration of pH by production of acids is often used in
the food fermentation industries. However, growth of moulds in acidic foods such as tomatoes/tomato products results in pH increase leading to proliferation of food borne pathogens and potential health hazards. Thus, lactic acid bacteria (acid producers) tend to lower the pH by production of lactic acid while proteolytic microorganisms such as Pseudomonas spp tend to increase the pH by production of ammonia and other proteolytic compounds.
Microbial spoilage of foods is therefore mainly influenced by the acidic nature of the food and the type of microorganisms present. For example, fruits, fruit juices, beer and wine are spoilt mainly by lactic acid bacteria (acidophiles), moulds and yeasts while most of the other foods will be spoiled by bacteria neutrophiles. It is important to note that microorganisms have special mechanisms to adapt to their environment (including food ecosystems) for survival.
MOISTURE CONTENT/ WATER ACTIVITY
Microorganisms can remain viable in a dried condition but cannot multiply in absence of water or in hure water (i.e no nutrients/solutes). The water in food in both bound and free. Bound water is held by physical forces to macromolecules and is not available to microorganisms for metabolic activity.
SourceWATER ACTIVITY
It is now accepted that the water/ moisture requirements of microorganisms should be described in terms of the water activity (aw) in the environment. This parameter in defined as the ratio want of the water vapour pressure (V.P) of food substracted solution to the vapour pressure of pure water at the same temperature i.e aw= p/po where p is the vapour pressure of the solution and po is the vapour pressure of the solvent normally water. The values of water activity range from 0-1.0. The VP of a liquid depends on the rate of escape of water to the air is measured by the equilibrium relative humidity (ERH).
Consequently, VP and ERH ar related. For example, when pure water is altered by the addition of a solute (e.g sodium chloride), the concentration of water is decreased and the rate of escape from the surface is reduced.
Thus, the water activity of solution is defined in terms of VP and ERH by the formular.
aw = p/po = ERH/100
This formula has been used to determine the water activity of several substances including foods.
Oxidation-reduction potential (Eh) is a measure of the tendency of a reversible system/reaction to give or receive electrons. The usual method of measuring OR/Eh is with a platinum redox electrode attached to a pH/mV (millivolt) meter. The importance of redox potential in foods is that it creates two major Eh requirements for microorganisms where some microorganisms require positive (i.e. oxidized: =+50mv) Eh for growth while others require negative (i.e. reduced: = -40mV) condition. The aerobic microorganisms such as fungi and some bacteria (e.g. Bacillus spp, require positive Eh and Clostridium spp, require negative Eh values, typically – 200mV and below).
However, some microorganisms (e.g. Campylobacter spp and Lactobacillus spp) which reduced Eh conditions and called microaerophiles. In addition, others microorganisms can also grow as aerobic or anaerobic conditions and called facultative anaerobes.
Changes in Eh of food occur as the microorganisms grow in such foods. In general, aerobic microorganisms (especially fungi) cause the spoilage of fruits due to the relatively high Eh and low pH (as earlier indicated). However, the microbial spoilage of meat due to Eh is highly dependent on the type of meat. For example, uncut/solid meat samples (carcasses) have reduced Eh due to less total surface area as compared with comminuted (ground) meat.
Microorganisms as earlier indicated need adequate nutrients for their growth and proliferation. Thus, the nutrient content comprising water, source of energy, source of nitrogen, vitamins and other growth factors and minerals is essential. However, there are variations in microbial requirements for these components. For example, complex carbohydrates such as starches and cellulose as source of energy by simple sugars.
Generally, simple components such as amino acids will be utilized by almost all organisms before any attack is made on the more complex compounds including high molecular weight proteins and polysaccharides. In addition, some microorganisms may require B Interestingly, Gram-negative bacteria and moulds are able to synthesize most or all of their vitamin requirements. Thus, these two groups of microorganisms are capable of spoilage of fruits as result of their lower B vitamin contents, low pH as well as positive Eh all of which contribute to their spoilage by these microorganisms.
SourceANTIMICROBIAL CONSTITUENTS/SUBSTANCES
Several antimicrobial substances are naturally present in some food
and these provide some degree of stability against their spoilage.
Examples are Lactoperoxidase, thiocyanate/ hydrogen peroxide system
Pseudomonas spp, Listeria monocytogenes etc.
2.Extrinsic parameters
These are the parameters involving the strage environment that affect the food itself and the microorganism. They are as follows:
1). Temperature of storage
2). Presence and concentration of gases in the microenvironment
3). Relative humidity of the microenvironment.
These parameters are critical for storage- life and control of microbial growth, thereby influencing the safety of the food. They are discussed briefly as follows:
Temperature of storage: Temperature is one of the most important environmental factors that influences the growth of microorganisms. Generally, microorganisms are classified into three categories based on their temperature of growth.
a)Psychrophiles /psychrotrophs (those that prefer 5oC refrefrigeration temperature and those that grow at between 5oC and 15oC respectively).
b) Mesophiles have 25oC to 40oC as their optimum growth temperature range (most dangerous for pathogenic growth).
c) Thermophiles prefer 45oC to 65oC
Examples of microorganisms in the three categories are shown in
Presence and concentration of gases in the microenvironment
Microorganisms vary significantly in their gaseous concentration requirement. For example, the type of gas in the microenvironment of the food will determine the types of microorganisms that become dominant. While presence of free oxygen in the atmosphere favours the growth of aerobic microorganisms especially fungi, lack of oxygen (e.g carbon dioxide increase) supports facultative anaerobes and they become dominant. The main purpose for gaseous composition manipulation in food microenvironments is to achieve significant commercial benefits. For instance, the use of modified atmosphere storage (MAS) i.e increase in CO2 and reduction in oxygen content is widely employed in international food trade. Especially for shipment of apples, pears and grapes (particularly to many developing countries including ours). The quest to exploit this concept of oxygen manipulation to enhance the shelf-life of fruits has been of research interest to us and others
However it has been shown that increase in CO2 could also result in anaerobic condition that will encourage the growth of food-borne pathogens such as Clostridium botulinum (Jones, 1990).
Relative humidity of the microenvironment
The importance of relative humidity in relation to water activity is the induced spoilage of foods. However, it is important to know that when foods having low values (such as flour) are placed in microenvironment of high relative humidity, the food will pick up (absorb) moisture until equilibrium is established and spoilage especially on the surface will occur. In contrast, foods containing high aw value (e.g.meat) will lose moisture when stored in an environment of low RH thereby, leading to spoilage by moulds, yeasts and some bacteria.
Food Preservation Methods and Strategies
The rationale for food preservation is primarily two-fold: (i) to prevent or minimize the growth of pathogenic and spoilage microorganisms and (ii) to preserve the quality attributes of the food. All foods following harvest, slaughter or manufacture lose quality at some rate which is dependent on the initial microbial profile, food type and composition; formulation (for manufactured/processed foods), storage conditions and so forth.
However, quality loss may be accelerated or minimized at any of the stages and the total perseveration strategy is therefore often multicomponent since one factor/approach is often inadequate. Several food preservation methods exist and they include the following: Drying, smoking, use of low/high temperatures, food preservatives, fermentation, radiation, modified/ controlled atmosphere (MA/CA).These are discussed briefly as follows:
This involves the lowering of the moisture content of food so that microorganisms and their enzymes become inactive. Foods are dried to different levels of moisture and water activity using several drying systems. Some of the foods are referred to as low moisture (LM) (i.e. contain not more than 25% moisture and intermediate moisture (IM) foods that contain between 15% and 50% moisture.
In general, bacteria are the most inhibited by drying since they require higher moisture content than both yeasts and moulds.