FACTORS AFFECTING MICROBIAL GROWTH AND SURVIVAL IN FOODS
Microbial growth is an autocatalytic process. No growth will occur without the presence of at least one variable cell and the rate of growth will increase with the amount of viable biome present. Bacteria reproduce by an asexual process called binary fission. In this series of events, the chromosome duplicates and the cytoplasm splits into two equal halves, each with one chromosome. The meso some may be involve as it grows out from the lateral surface of the cell membrane and separates the chromosomes. There are no mitotic structures involved as in eukaryotic cells.
Once the division has been completed, the organism begins to mature and produce those biological chemicals and structures that make them unique. The time interval until the completion of the next division is known as the generate time/ doubling time. It has been found to vary considerably for different organisms. For example, Staphylococcus aureus has a generate time of 30mins, Mycobacterium tuberculosis has a generation time of 18 hours; and Treponema palladium, it is a long 33hours. One of the most remarkable generation times is the 20min for Escherichia coli when it grows at its most rapid pace. If one were to begin with a single rod at 8:00am this morning, one would have two rods by 8:20, four by 8:40 and eight by 9.am. Sixty – four rods would have been produced by 10.am and just over one billion by 6pm.
BACTERIAL GROWTH CURVE
The original members of the population may consist of the few hundred that entered the human respiratory tract or were transferred to a new tube of growth medium in the laboratory.
A simple analysis of this curve can distinguish four major phases. In the first, the lag-phase encompasses the first few hours in which the bacterial acclimate themselves to the environment. There is no apparent growth while the inoculums adjusts to the new environment, synthesizes the enzymes required for its exploitation and repairs any lesions resulting from earlier injury, e.g. freezing, drying, and heating.
The population then enters an active stage of growth known as the logarithmic phase often shortened to log phase. Cell rapidly increase in mass, and then reproduce by fission. The pop is at its biochemical optimum and research experts are often performed at this time.
Finally, changes in the medium as a result of exponential growth bring this phase to an end as key nutrients become depleted or inhibitory metabolites accumulate and the culture moves into the stationery phase. In the culture tube nutrient may have become scarce, waste product may have accumulated or the medium may have become dehydrated.
If these factors continue, they will overwhelm the population and the decline phase will ensue. A bacterial capsule may protect the cell from the environments and allow it to survive longer. Flagella may permit the organism move away from the colony and locate nutrients. If the organism is a species of Bacillus or Clostridium, the vegetative cells will probably have become spore and the stationary phase may be extended for months or years. For many species of bacteria, through the history of the population soon comes to an end with the death of all cells.
CHEMICAL FACTORS THAT AFFECT MICROBIAL GROWTH (INTRINSIC FACTORS)
1. NUTRIENT CONTENT
Microrgs can use foods as a source of nutrients and energy. In order to grow and function normally, the microrgs of importance in foods require the following (a) water (b) source of energy (c) source of nitrogen (d) vit &related growth factors (e) minerals.
Molds have the lowest req followed by yeasts, gram-ve bacteria, and gram +ve bacteria. As sources of energy foodborne microrgs may utilize sugars, alcohols and amino acids some few microrgs are able to utilize complex carbohydrates such as starches and cellulose as sources of energy by first degrading these compounds to simple sugars. Fats are used also by microrgs as sugars of energy, but these compounds are attacked by a relatively small number of microbes in foods the gram- negative bacteria and molds are able to synthesize most or all of their requirements consequently, these two groups of organisms may be found growing on foods low in B vits. Fruits tend to be lower in B vits than meats and this fact, along with the usual low ph and positive Eh in fruits, help to explain the usual spoilage of these products by molds rather than bacteria
2. ANTIMICROBIAL CONSTITUENTS
The stability of some foods against attack by microorganisms is due to the presence of certain naturally occurring substances that have been shown to have antimicrobial activity among these are eugenol in cloves, allicin in garlic, cinnamic aldehyde and eugenol in cinnamon. Cow milk contains several antimicrobial substances, including lactoferrin, conglutinin and the lactoperoxidase system. Casein, as well as some free fatty acids that occur in milk, have been shown to be antimicrobial. Eggs contain lysozyme, as does milk, and this enzyme along with conalbumin, provides fresh eggs with a fairly efficient antimicrobial system. The hydroxyl cinnamic acid derivatives found in fruits, vegetables, tea, spices, molasses and other plant sources all show antibacterial and some antifungal activity.
3. PH
It has been well established that most microrgs grow best at Ph values around 7.0 (6.6-7.5), while few grow below 4.0. Bacteria tend to be more fastidious in their relationship to Ph than molds and yeasts with pathogenic bacteria being the most fastidious. It is a common observation that fruits generally undergo mold and yeast spoilage and this is due to the capacity of these organisms to grow at Ph values < 3.5, which is considerably below the minima for most food spoilage and all food poisoning bacteria. Most meats and sea foods have final ultimate Ph of about 5.6 and above. This makes these products susceptible to bacteria as well as to mold & yeast spoilage. Most vegetables have higher Ph values than fruits and consequently vegs are subjected to bacterial than fungal spoilage. Some foods are characterized by inherent activity others owe their acidity or PH to the actions of certain microrgs. The latter type is referred to as biological activity and is displayed by products such as fermented milks regardless of the source of acidity the effect on keeping quality appears to be the same.
Some foods are able to resists changes in Ph than others. Those that tend to resist changes in PH are said to be buffered. Meats are more highly buffered than vegs contributing to the buffering capacity of meats are their various proteins. Vegs are generally low in proteins and consequently lack the buffering capacity to resist changes in their PH by the growth of microrgs.
4. MOISTURE CONTENT
One of the oldest methods of preserving foods is drying or desiccation, and precisely how this method came to be used is not known. The preservation of foods by drying is a direct consequence of removal or binding of moisture, without which microg do not grow. Water reqs of microrgs is defined in terms of the water activity (aw) in the environment. This is defined by the ratio of the water vapor pressure of pure water at the same temp.
Aw¬¬¬¬=p/p0 where p = vapor pressure of solution and p0 = vapor of solvent (usually H20).
In general bacteria require higher values of aw for growth than fungi, with gram-negative bacteria having higher reqs than gram+ves. Most spoilage bacteria do not grow below aw 0.91, while spoilage molds can grow as low as 0.80. Just as yeast and molds grow over a wider ph range than bacteria, the same is true for aw.
Certain relationship have been shown to exist among aw, temperature and nutrition first at any temp the ability of microrgs to grow is reduced as the aw is lowered second, the range of aw over which growth occurs is greatest at the optimum temp for growth, and third, the presence of nutrients increases the range of aw over which the orgs can survive.
5. OXIDATION –REDUCTION POTENTIAL
The O/R potential of a substrate may be defined generally as the ease with which the substrate loses or gains electrons.
When an electricity or compound loses or gains elections, the substrate is said to be oxidized, while a substrate that gains electrons becomes reduced. Oxidation may also be achieved by the addition of oxygen.
A substance that readily give up electrons is a good reducing agent, and one that readily takes up electrons is a good oxidizing agent aerobic microgs req positive eh values (oxidized) for growth while anaerobes req negative eh value (reduced)
The Rh potential of a food is determined by:
1) The characteristic Rh potential of the original food.
2) The poising capacity, ie the resistance to change in potential of the food.
3) The oxygen tension of the atm about the food
4) The access that the atm has to the food. Some aerobic bacteria actually grow better under slightly reduced conditions, and these organisms are often referred to as microaerophiles eg. Lactobacilli and Streptococci.
Some bacteria have the capacity to grow under either aerobic or anaerobic condition
- Facultative anaerobes. Most molds and yeasts encountered in and on foods are aerobic, although a few tend to be facultative anaerobes.
Microorganisms affect the eh of their environments during growth just as they do PH. Aerobes can lower the Eh of their environment while anaerobes cannot. As aerobes grow, O2 in the medium is deplated, resulting in the lowering of eh.
BIOLOGICAL STRUCTURES (PHYSICAL)
The natural covering of some foods provides excellent protection against the entry and subsequent damage by spoilage organisms. Structured as the testa of seeds, the outer covering of fruits, the shell of nuts, the hide of animals, and the shell of eggs. The outer shell and membranes of eggs, if intact, prevent the entry of nearly all microorganisms when stored under the proper conditions of humidity an temp. Fruits and vegetables with damaged covering undergo spoilage much faster than those not damage. The skin covering of fish and meat such as beef & pork prevents the contamination & spoilage of these foods, partly because it tends to dry out faster than freshly cut surfaces.
EXTRINSIC PARAMETERS (PHYSICAL)
The extrinsic parameters of foods are those properties of the storage environment that affect both the foods and their microorganisms. Those of importance to the welfare of food borne organisms are
1. Temp of storage
2. Relative humidity of the environment and
3. Presence and conc of gases in the env.
TEMPERATURE OF STORAGE
It is customary to place micro organisms into three groups based on their temp reqs for growth. Those organisms that grow well at air below 7oC and have their optimum between 20o and 30oC are referred to as psychrotrophs. Those that grow well between 20oC and 45oC with optima between 30o & 40oC are referred to as mesophiles, while those that grow well at and above 45oC with optima between 55oC and 65oC are referred to as thermophiles.
The psychrotrophs found most commonly on foods are those that belong to the general Pseudomonas and Enterococcus. These organisms grow well at refrigerator temps and cause spoilage of meats, fish poultry, eggs and other foods normally held at this temp. Some organisms can grow over a range from 0º and 30oC or above eg Enterococcus fecalis. Most thermophilic bacteria of impt in foods belong to the genera Bacillus and Clostridium i.e canning industry. Many molds are able to grow at refrigerator tempt, notably some stains of Aspergillus, Cladosporium, which may be found growing on eggs, sides of beef and fruits. Yeasts grow over the psychotrophic and mesophilic temp ranges but generally not within the thermophilic range.
The quality of the food product must also be taken into account in selecting storage tempt.Temp of storage is the most input parameter that affects the spoilage of highly perishable foods.
RELATIVE HUMIDITY OF ENVIRONMENT
The R.H of the storage environment is impt both from the stand point of aw within foods and the growth of microorganisms at the surfaces. When the aw of a food is set at 0.60, it is impt that this food be stored under conditions of R.H. that do not allow the food to pick up moisture from the air and thereby increase its own surface and surface aw to a point where microbial growth can occur. When foods with low aw values are placed in environs of high R.H, the food with a high aw lose moisture when placed in an environment of low R.H There is a relationship between R.H and temp that should be borne in mind in selecting proper storage envs for the storage of foods. Foods that undergo surface spoilage from molds, yeasts and certain bacteria should be stored under conditions of low R.H. In selecting the proper environmental conditions of R.H. Consideration must be given to both the possibility of surface growth and the desirable quality to be maintained in the foods in question. By altering the gaseous atm, it is possible to retard surface spoilage without lowering RH.
Presence and concentration of Gases in the Environment
The storage of food in atmosphere containing used amount of co2 up to about 10% is referred to as “controlled atmosphere” or modified atmosphere (MA) storage. The conc of CO2 generally does not exceed 10% and it applied either from mechanical source or by the use of dry ice (solid CO2). Co2 has been shown to retard fungal rotting of fruits caused by (Apples & pears) a large variety of fungi (mechanism of action not known)
It has also been known for many years that ozone added to food storage env has a preservative effect on certain foods. (Parts per million ppm) effective against spoilage organisms. Since it is a strong oxidizing agent, it should not be used on high-lipid content foods because it would cause an increase in rancidity. Both CO2 and O3 are effective in retarding the surface spoilage of beef under long-term storage.
Cardinal temps for microbial growth
Group Minimum Temp (oc) optimum Max
Thermopiles 40 – 45 55 – 75 60 – 90
Mesophiles 5- 15 30 – 40 40 – 47
Psychrophiles - 5 - + 5 12 – 15 15 – 20
Facultative psycrophiles) -5-+5 25 – 30 30 – 35
BIOTIC FACTORS INFLUENCING MICROBIAL GROWTH IN FOODS
The Biotic Factors which influence microbial growth in foods are Growth rate, Metabolism and Antagonism.
Growth
In a food environment, an organism grows in a characteristic manner and at a certain characteristic rate. The length of the lag phase, the generation time, and total cell yield are all determined by genetic factors. Accumulation of metabolic products may limit the growth of particular species. If the limiting metabolic product can be used as a substrate by other species, these new other species may now take over (partly or wholly), creating what is known as an association or a succession. Due to the complex of continuing interactions between environmental factors and microorganisms, a food at any one point in time has a characteristic flora, known as its association. The microbial profile which changes continuously resulting in one association succeeds another in what is called succession. In the microbial deterioration and spoilage of foods, this phenomenon has been observed repeatedly. As long as metabolically active organisms remain, they continue to interact, so that dominance in the flora occurs as a dynamic process. Based on their growth-enhancing or inhibiting nature, these interactions are either antagonistic or synergistic.
Antagonism
Another biotic factor influencing the growth of microorganisms in foods is the competition for nutrients and other factors which promote growth. In the same way that humans compete for food when kept together within the same environment, microorganisms also compete.
In food systems, antagonistic processes usually include competition for nutrients, competition for attachment/adhesion sites (space), unfavorable alterations of the environment, and a combination of these factors. An example of this antagonistic factor is raw ground beef. Even though Staphylococcus aureus is often found in low numbers in minced meat, staphylococcal enterotoxin is not usually produced. The reason is that the Pseudomonas-Acinetobacter-Moraxella association that is always present in ground beef grows at a higher rate, outgrowing the staphylococci. Organisms of high metabolic activity may consume required nutrients, selectively reducing these substances, and inhibiting the growth of other organisms. Depletion of oxygen or accumulation of carbon dioxide favors facultative obligate anaerobes which occur in vacuum-packaged fresh meats held under refrigeration. Staphylococci are particularly sensitive to nutrient depletion. Coliforms and Pseudomonas spp. may utilize amino acids necessary for staphylococcal growth and make them unavailable. Other genera of Micrococcaceae can utilize nutrients more rapidly than staphylococci. Streptococci inhibit Staphylococci by exhausting the supply of nicotinamide or niacin and biotin. Staphylococcus aureus is a poor competitor in both fresh and frozen foods. At temperatures that favor staphylococcal growth, the normal food flora offers protection against staphylococcal growth through antagonism, competition for nutrients, and modification of the environment to conditions which are less favorable to S. aureus. Competition between microorganisms may be affected by changes in the composition of the food, and changes in intrinsic or extrinsic factors.
Metabolism
Metabolic products from one organism can be absorbed and utilized by other organisms. Changes in pH may promote the growth of certain microorganisms whilst discouraging others. A very common example is in natural wild fermentations especially used in most West African countries to preserve foods, in which acid production during the process establishes the dominance of acid tolerant organisms such as the lactic acid bacteria. Changes in Aw in the food can influence symbiosis. At warm temperatures, Clostridium perfringens if present can lower the redox potential in the tissues of freshly slaughtered animals so that even more obligate anaerobic organisms can grow. There are some associations where maximum growth and normal metabolic activity are not developed unless both organisms are present. These associations are known as Symbiotic associations.
In conclusion, the INTRINSIC FACTORS of the food which affect microbial growth are factors which are related to the food itself. These are pH, nutrients, Aw, Redox potential, and presence of antimicrobial agents.
The EXTRINSIC FACTORS are factors which are related to the environment in which the food is stored. These are the temperature of storage, the composition of gases and the relative humidity.