Introduction and Understanding Microbiology
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Microbiology and the problem with food
It is well understood that microorganisms, namely bacteria have been in existence on this planet not long after the time when water first appeared in abundance on the planet, i.e. several thousands of millions of years ago. Why they appeared, well this depends on what you believe: Chance evolution, outer space, or heavenly intervention. What we do know though is that initial conditions on early earth were particularly hostile and this initially restricted the type and distribution of the ancient bacteria.
Environments found today like deep-sea hydrothermal vents and volcanic spring pools may well have been similar to those early conditions and today these geographical features teem with bacterial life experts believe to be similar to those very first organisms. The bacteria are particularly unusual by today's standards in that they extract much of their energy from inorganic chemicals like sulphurous compounds, materials that all other life forms would not consider as being food. However, over the course of time, the state of the planet changed, and with it, bacteria evolved to be able to utilise much more readily available materials based around carbon. Carbon is contained in all life forms and its conversion from one form to another is the basis of the food chain. Plants and photosynthesising bacteria are usually at the base of it, using sunlight to "fix" carbon dioxide into sugars. The organic carbon in these sugars can then be used in the construction of all other materials necessary for life, e.g. fats, proteins, etc.
So where do the bacteria that we perceive as a problem in food production come into this food chain? Ordinarily, bacteria if left to their own devices get on with what they do best; live in the guts of animals, consuming organic material in the way of food eaten by their host animal, or they sit around in the environment, i.e. in water and soils or on plant and animal surfaces, consuming whatever "edible" organic materials come along. Our problem is that food is entirely comprised of organic matter and unlike a piece of meat still on the hoof, it has no immune system to ward off the advances of bacteria. This is where those in the food industry have to take steps towards ensuring food safety and shelf life.
Pathogens and spoilage bacteria
Before considering how to control micro-organisms in food, it is worth clarifying the two categories of organisms to fight against. Many peoples' immediate reaction is to think that the pathogen organisms are the worst and the main priority to remove from food products. No one can argue with this, as we do not want to sell cheese or sandwich filling which is packed with added E. coli O157 or Salmonella. However, more food is condemned each year because of spoilage problems.
Organisms that cause illness in animals and people are referred to as pathogens and anybody who has a basic knowledge of food hygiene should be able to list off several different pathogenic bacteria.
Spoilage bacteria are less tangible and it would seem that preventing their occurrence on food is an almost impossible task. Moreover, the control measures we may take for pathogens do not always work so effectively against the spoilage bacteria. This is essentially because food is not a natural substrate on which pathogens should find themselves. As a rule they much prefer the warm contents of a digestive system, or the sanctuary of skin and hair. Spoilage organisms on the other hand are the bacterial multi-taskers, able to adapt quickly to new environments and opportunities to exploit new nutrient sources.
Growth of bacteria
As with all life, in order to grow, bacteria need nutrients or food. We hopefully appreciate that bacteria are not particularly impressive in size, individual cells being in the region of less than 0.0005 cm in length, thus the amount of food that they require is equally tiny. A solution of sugar in the parts per million concentration range will be adequate to maintain growth for many hours and the more nutrients available, the faster growth occurs.
Bacterial growth does not occur in the same manner as with plants and animals. Growth for bacteria means division, multiplication or replication. By a process referred to by cell biologists as binary fission, one bacterial cell splits down the middle to become two. After a short rest period and with a touch of nutrient absorption, both cells split again giving four from the original one. Theoretically this process could go on indefinitely and bacteria would take over the world within the space of a few days, though this has clearly yet to happen, so why not? In essence it all comes down to limiting factors. Nutrients and growth medium would have to be unlimited, as would water. Moreover, as with all life forms, the spent chemicals of metabolism need to be removed before they become inhibitory and toxic. These limitations fortunately give us a clue as to how we can adjust conditions to dissuade bacteria from taking over the food we produce for human consumption.
Controlling microorganisms in food
So we have established that microorganisms require nutrients which they obtain easily enough from the food we produce for ourselves. Unfortunately the nutrients we need suit bacteria equally so there isn't anything we can leave out of foods to make them unsuitable for bacteria. Fortunately for one sector of the food industry though, some products come already kitted out with their own microbiological protection.
Fruit, vegetables and seeds/nuts if unprocessed will invariably be protected with an outer skin or casing which if undamaged is very effective against the ingress of bacteria and moulds too. Shell eggs also have an effective barrier to the ingress of micro-organisms to assist in their keeping qualities. For the rest of the food industry though, other measures are required.
A key requirement of micro organisms is water and plenty of it, presumably because this reflects their evolutionary origins. Consequently foods suitable for bacterial growth must have reasonable available water content. Available water is not a measure of total water content, but the amount present that is not saturated with salts, sugars and proteins etc dissolved in it. For example, jam appears on the face of it to be a particularly wet food, but actually has a low water activity value because of the high content of dissolved sugar present. Products such as fresh meat have high water activities >0.95 whereas jam may be around 0.65 - 0.70. As a rule no bacteria important in food can grow in products below 0.85. So for certain foods, drying or the addition of solutes like sugar and salt can be used as a processing aid. However, it should be stressed that a lack of water does not necessarily kill bacteria. Of course there are sensitive individuals in the bacterial world that disagree with drying, i.e Campylobacter, but Salmonella has been demonstrated to cause illness after undergoing a drying process; for example the documented case of contaminated spray-dried infant milk powder. Unfortunately, water is not the complete answer since consumers do not want cooked sliced chicken to taste like cardboard between their sandwiches, so what else can we do?
Other than fill the product full of a list of E-number preservatives, which again won't do the whole job, we need to look towards temperature. Heat is useful at both ends of the temperature scale, high temperature can be used at both point of consumption, and in production. At point of use, it is the responsibility of the consumer to apply the correct heat for the desired length of time to ensure suitable cooking, this is typical for fresh meat products, where not only spoilage organisms will be present, but also there exists potential for pathogenic organisms.
In food processing, high temperature can be used to pasteurise or sterilise. In the case of sterilisation, heating is so extreme that all bacteria are destroyed including the super heat resistant spore types. The big problem with sterilisation though is that the re-introduction of bacteria after processing will undo all this good work. In the canning industry, this potential problem is circumvented by placing the food in the final packaging (a tin can) and sealing it before heating. In this way the food is cooked, and sterilised as is the packaging. The downside is that any damage to the can seams may lead to the re-introduction of bacteria.
The dairy and drinks industry also make use of sterilisation through a process of UHT (ultra high temperature) whereby liquids are heated to a minimum of 135°C for a very short time, whilst under pressure to prevent boiling. In the case of UHT, aseptic filling technology is then employed to transfer the sterilised bulk liquid into individual packaging containers. The benefit of sterilised foods is that care need not be taken as to their subsequent storage temperature and the product will have a long-term shelf life.
At the other end of the temperature scale, much use is made of freezing as a means of prolonging food shelf life. Freezing works not by making the environment too cold for bacteria to function, but instead it locks away the one key element vital to growth, this being water. Foods become rock-solid due to the formation of a lattice of ice crystals. Consequently water contained within bacterial cells is also frozen inhibiting any chemical activity. However, once the process of thawing begins, many of the bacteria recover too and will again start multiplying given appropriate temperature conditions. So what are appropriate temperature conditions for bacterial growth in foods? Well, it is a very broad spectrum from a couple of degrees below zero, depending how many dissolved sugars or salts are present in the food, up to 60°C or thereabouts. Within this broad range, it is usual to find that spoilage organisms do best at either end of the scale, specifically with so-called Gram-negative organisms doing best at lower temperatures (and are referred to as psychrotrophs) and Gram-positive spore formers coping at the higher temperatures (thermophiles). The pathogenic organisms prefer an easy time of things and so are best found growing happily away at mid range temperatures (mesophiles). This is not to say that the bacteria always obey these strict margins of growth, instead each group tends to be best adapted at either low, mid or higher temperatures and so become the dominant organisms.
Common sense should tell us therefore that the closer to freezing we hold food, the less active bacteria will be at colonising and spoiling the product. Why this should be so comes down to basic laws of thermodynamics. Bacteria can be considered as chemical factories with their growth and metabolisms requiring thousands of chemical reactions. The lower the temperature, the slower the reaction rate: a rise in temperature of 10°C will double the rate of a chemical reaction. So for all foods that are unsuitable for freezing, drying or sterilising, chilled temperature storage is the only option towards giving at least a few days shelf life, unless other preservatives are to be included.
Again many common foods make use of low pH to reduce the impact of bacteria. Typically the magic number for high acid foods to be free of worries over pathogens is pH 4.5 or below although this is not sufficient to prevent microbiological activity. Lactic acid bacteria - common spoilage organisms found in all food types with sufficient water - are happy operating in the pH 2-3 range. From the fungal order, a significant number of moulds and yeasts are also happy where the vast majority of bacteria cannot cope
As a final point of note where food preservation is concerned, redox potential, i.e. aerobic or anaerobic environmental conditions, is also important. However, changing this with little regard for the microorganisms can sometimes have disastrous consequences. Taking a product traditionally employing over-wrap packaging, and gas-flushing with a modified atmosphere with little oxygen present twinned with longer shelf life could lead to the growth of anaerobes, including strains of Clostridium botulinum. In general, bacteria that are oxygen tolerant (facultative anaerobes) grow best in the presence of oxygen and slowest in anaerobic environments. The bacteria, E. coli and Salmonella fall into this category.