The areas covered in this article are:
Microbiology is a complicated subject and it can be overwhelming sometimes, so we’re going to try and cover just the parts you need to know, as simply as possible.
We’re going to look at what microbiology is, what the four types are, which microbes are good, which are bad and which are just plain ugly!
Microbiology – what is it?
Microbiology is the science of microscopic organisms. So, organisms that you can only see under a microscope. These organisms are sometimes called microbes. In the food and drink industry we’re concerned with four types of microbe:
Bacteria (or bacterium when you’re taking about just one) is a single cell microbe.
A virus needs another living cell to multiply. It finds a host, such as a bacteria cell and gets the bacteria to create more viruses for it, by attaching itself to it and injecting its DNA code into it.
A yeast is another single cell organism, but one from the fungus (as in mushrooms) family.
A mould is also from the fungus family but it’s multicellular, meaning that it’s made up of more than one cell. These cells are formed in what are called hyphae, which are a bit like fingers – where the cells link together to form a strand.
The good, the bad and the ugly…
Not all microbes are bad, so you don’t have to worry about them all.
The ‘bad’ microbes that make us ill, are called pathogens (also known as pathogenic). Interesting but useless fact for you; the word ‘pathogen’ comes from the Greek words, ‘path’ meaning suffering and ‘genes’ meaning producer of. So, pathogen is the ‘producer of suffering.’ Pathogenic bacteria has the ability to make us sick, so it’s these bacteria that cause food poisoning.
There are good microbes too, which you need to survive. For example, you have good bacteria and yeast in your gut which aid your digestion – these are the live microbes that they put into probiotic drinks.
Then there are what we call the ugly. These are the microbes that aren’t pathogenic, and therefore, don’t make you ill. But, you still don’t want them in the products that you produce, as they will cause spoilage problems. Spoilage – simply put, is where the product starts to decay or rot. This then causes quality problems; resulting in off-flavours, odours or a change in the way in which the product looks, making it look ‘off.’
So, to summarise – the good you don’t need to worry about.
The bad (pathogenic) you need to eliminate or reduce to acceptable levels, as these will make your customers sick. You do this using hazard analysis principles to control product safety.
And, the ugly (spoilage) you need to reduce wherever possible, as they will cause complaints. You do this using process control and GMP.
Bacteria and how they multiply
Bacteria, when you’re talking about just one – is called a bacterium. A bacterium is made up of just one single cell.
On the outside of the cell are fimbriae. The bacteria use these to attach themselves to other bacteria or surfaces.
As you can see in this example, this bacterium also has three long strands called flagella. The bacteria use these flagella to give them motion, to help them ‘swim along.’
The bacterium is contained by a cell wall. The cell wall is protected by a capsule layer and inside the cell wall there is a flexible lining called the plasma membrane.
The inside of the cell is made up of cytoplasm, ribosomes and DNA. These elements make up the ‘machine’ of the bacterium cell – allowing it to produce energy and also to multiply.
Here’s an example of what the single cell bacterium looks like:
The different bacteria shapes
There are many shapes of bacteria, as you can see from this picture.
Multiplication of bacteria
Bacteria are asexual – this means that they aren’t like us, as they don’t need a partner to multiply. A bacterium can become two bacteria all by itself. Then those two bacteria can each multiply again on their own and so, they become four bacteria.
The process that the bacteria use to multiply, is called binary fission. Binary fission literally means, splitting in half. Let’s walk through the process, step-by-step.
Binary fission – Step 1
The bacterium starts as just one cell.
Binary fission – Step 2
In order to become two cells, the bacterium starts to replicate all of the internal parts of the cell – the mechanics of the cells that we talked about earlier, the cytoplasm, ribosomes and DNA. As it does this, the size of the cell gets bigger so it also makes an extra cell wall, capsule layer and more of the plasma membrane. You can see in this picture, as the cell gets bigger it starts to separate and it starts to look more like two cells stuck together.
Binary fission – Step 3
With all the internal parts of the cell complete, the internal parts of the cells divide completely. The cell now just has to complete the production of the outer elements.
Binary fission – Step 4
And when this is complete, the bacteria divide completely. One has now become two.
So how long does this take?
The speed at which a bacterium can divide, is a really key point for you when you’re trying to manage food safety. There are many factors which affect the speed at which a bacterium can divide. And it’s these factors, that you use to your advantage to stop them from dividing and growing to unsafe numbers.
In the right conditions, a pathogenic (remember pathogens are the bacteria that cause food poisoning) bacteria can divide every ten to twenty minutes. This means if you start off with one bacterium, even with a multiplication time of twenty minutes, after four hours you would have over 8,000 bacteria.
Bacteria need the right environment for binary fission to happen. Each species of bacteria like different environments but as a general rule they need time to multiply, with the right:
- Oxygen levels
- pH levels
The temperature has to be right for the specific type of bacteria, but most like temperatures within the ‘danger zone’.
The danger zone is between 6°C and 62°C.
5°C or less, growth is slowed right down. 63°C or more, growth also slows and bacteria start to die off.
Gram-positive or Gram-negative?
You may have heard people talking about whether bacteria are Gram-positive or Gram-negative. You shouldn’t worry about this too much, as it’s above and beyond what you need to know to manage food safety effectively.
But just in case it’s of interest to you, here are the basics.
The staining technique will stain Gram-positive bacteria violet. Gram-negative bacteria don’t pick up the stain, hence why they’re called ‘negative’.
There are lots of reasons why the Gram-positive bacteria stain violet and Gram-negative bacteria don’t, but you don’t need to know that level of detail. The most important thing that you need to remember is that you can get both Gram-negative pathogenic bacteria (e.g. Salmonella) and Gram-positive pathogenic bacteria (e.g. Listeria).
Viruses and how they reproduce
Previously, we’ve looked at how bacteria are asexual and how they multiply through the process of binary fission. In comparison, viruses can’t multiply by themselves at all, they have to get another living cell to do it for them. This is because a virus is a parasite. A parasite is an organism that lives in or on another organism, which is called its host. The host provides the parasite with the things it needs to survive.
A virus uses another living cell to multiply for it. The process it uses to do this is quite amazing. But, before we go into how viruses reproduce, let’s look at their structure.
As you can see, there are quite a lot of component parts to the virus, but the part that you really need to be aware of is the DNA in the centre, which is being protected by the envelope. Then there are glycoproteins on the outer surface – these are also known as ‘keys.’ The DNA and the keys are really important when it comes to the virus multiplying, so we’ll come onto that soon.
Just like bacteria, viruses come in different shapes.
The one in the diagram above is called an enveloped or spherical virus, because it’s contained within an envelope layer.
There are three other shapes of virus. The polyhedral and the helical are contained within an outer layer of what’s called capsomeres. These are balls of protein that link together to make up the surface of the virus.
The fourth virus shape is the complex bacteriophage. This looks very different to the others and it also doesn’t have the keys on the outside. Its DNA is held in the head of the bacteriophage (the crystal shape at the top). At the base of the bacteriophage, it has legs which it uses to land on its host and then pins underneath, which it uses to inject into the host.
So, as we’ve said a virus can’t reproduce itself. It has to have a host to reproduce for it. But how does it do this? This is a complex process, so we’re going to really simplify it.
All viruses contain DNA inside. The DNA is their instructions on how to make a virus. It’s like a recipe, and if you follow it, you can create a virus.
Because the virus can’t multiply on its own – it needs a living cell to do this for it. The virus gets the living cell to use the DNA instructions to make lots of viruses for it. But to do this, the virus has to find a host and take over it. This is where the keys come in.
Living cells have a security feature on the outside of them, to stop unwanted visitors being able to get inside. To keep them safe, they have locks on the outside of the cell and you need the right key to fit the lock, in order to get inside.
The viruses that have the keys look for a host where their key fits the lock of the living cell. Once they find one that fits, they use the key to allow themselves access into the living cell. The living cell thinks that everything is ok, because after all, the virus has entered with a key, so it must have passed security OK. Once the virus has access inside, it provides the living cell with its own DNA.
Inside the living cell there’s a nucleus which contains a copying machine. Normally this machine would be copying its own living cell’s DNA, by feeding the DNA string into one end and out of the other comes two copies. But now the virus DNA is inside, the machine starts to copy the virus DNA, making more of it.
The living cell then uses this DNA to build more viruses. This continues until the living cell is so full of new viruses that it’s created, that eventually it explodes, releasing the viruses and killing the host.
The size of it
Because viruses have to find and enter a host, they are much smaller than living cells or bacteria. An E.coli bacterium for example is about 3000 x 1000nm in size. Whereas a bacteriophage is about 225nm in height.
Also, like other viruses, Norovirus can live on surfaces and in the environment for a long time before they’re picked up by food or our hands.
The result of infection from Norovirus is sickness and diarrhoea.
Today, Coronavirus is clearly the virus that’s the main concern. The results of infection are clearly very different to Norovirus, as Coronavirus syptoms are not sickness and diarrhoea – this attacks breathing functions.
So, how do we protect ourselves?
Viruses are very susceptible to heat, so you can kill them by thoroughly cooking your food.
Preventing food from becoming contaminated is obviously the best solution. The most common way of product getting contaminated is when an infected person doesn’t wash their hands after going to the toilet. However, the most effective way of the virus travelling is from aerosolised vomit, where an aerosol particle can travel up to nine feet! This highlights how important it is to make sure that staff are trained to report illness immediately. And, when someone does become sick, the bodily fluids procedure is implemented effectively, to clean up affected areas and prevent cross-contamination.
Yeast and its beneficial applications
You’ll probably notice that yeast is quite similar to a bacteria in the way it looks and how it multiples. There are good types of yeast that we can use to our advantage. There are also ugly types of yeast that spoil our food through decomposition. Yeasts don’t tend to be an issue when it comes to food poisoning, so we’ll just focus on just the good and the ugly.
The yeast cell
Yeast is a microscopic fungi, but the yeast cell is about three or four times larger than a bacteria cell. It has a cell wall and membrane like a bacteria cell. Inside the cell is a nucleus, which is the ‘machine’ of the cell.
Multiplication of yeast cells
A yeast cell is asexual like bacteria cells, so it can multiple all on its own – but the process that yeast uses is a little different to bacteria.
The bacterium cell splits in two, through the process of binary fission. A yeast cell grows a little bud on the outer surface of the cell, which eventually break off as its own cells. And more than one bud can be growing at any one time. This process is called ‘budding’. You can see in the picture the yeast cells budding.
Do yeast cells breathe?
Well, they don’t breathe like we do, but they do take in gas and convert it as part of their digestion process. Yeast cells can breathe either with or without oxygen:
- When yeast cells breathe using oxygen, this is called aerobic
- When yeast cells breathe without oxygen, this is called anaerobic
You may be wondering why is this important? Well, during aerobic respiration a yeast cell will convert sugar to produce carbon dioxide and water. But, when they’re in an environment where there’s no oxygen, the anaerobic respiration of the yeast cell means that it converts sugar to produce ethanol (or alcohol).
In the food and drink industry this process of converting sugar to either carbon dioxide or ethanol, is called fermentation.
You can see the gas structure developing in the dough in this image.
Baker’s yeast is used to create fermentation of the dough during bread making. The yeast converts the sugar, aerobically (using oxygen) in the dough to create carbon dioxide.
If the gluten proteins in the dough have been sufficiently developed, through mixing, they will produce a network of fine strands. As the yeast produces carbon dioxide, the gas bubbles produced get held in this fine matrix and this causes the bread to rise.
Beer, wines and spirits fermentation
Yeast specifically designed for beer, wines and spirit production is used to create the alcohol in the product.
The yeast in the liquor is held under anaerobic (without oxygen) conditions, so that it converts the sugars into ethanol. The sugar comes from the fruit or grain that’s being used to ferment the product. If a spirit is being made, then this liquor is then distilled down further to increase the alcohol content.
You either love it or hate it!
I’m sure you know exactly what product I’m talking about! Yeast is also used to produce yeast extract, which we use as a paste on our toast. We also add it to other products too. For example, did you know yeast extract is added to dry roasted peanuts?
We can’t talk about yeast, without talking about the ugly side to it. Spoilage is basically the decomposition of food. Yeast has a part to play in spoilage and because yeasts can be airborne, the yeast cells can settle on open food and start the spoilage process. There’s a whole load of different species of yeast that cause spoilage such as Candida, Cryptococcus and Saccharomyces.
Mould and how it spreads
Yeast is a single cell fungus; whereas mould is multicellular, meaning that they’re made up of more than one cell.
You can see from the picture that the cells are joined together to make what’s called hyphae. There are two sorts of hyphae; Septate and Coenocytic. Septate have a septum that separates the nucleus, and Coenocytic hyphae have none. The diagram shows what both of these types of mould look like. Each of these strands of hyphae make up the mould spore. It’s these hyphae that make the mould spore look ‘hairy.’
Mould spores are much bigger than bacteria. They’re still very small, roughly a few microns in size, which means that they’re not visible to the naked eye. When they grow however, they become large enough to be visible – as you can see here on this petri dish.
Mould spores need to have a source of food to sit on, as they can’t generate their own. This is why you typically see mould growing on the surface of food. The mould will spread by increasing the length and connections of the hyphae. The mould also releases news spores into the air, which allows it to become airborne and settle on other surfaces and food. In the petri dish picture above you can just make out in the new spores which will be released into the atmosphere.
Moulds tend to like moist and warm conditions, the optimum temperature being about 30°C. They’re not tolerant to hot conditions, so cooking and baking kill off the mould spores.
This means that foods such as bread, which are susceptible to mould growth are contaminated after baking. Similarly, cheese is susceptible to mould growth and contamination tends to happen during the packing of the cheese or from inadequate sealing of the pack.
The key elements of the micro curve
As you can see from the diagram, there are four main parts to the curve:
- Lag phase
- Log phase
- Stationary phase
- Death phase
The lag phase
In the lag phase the bacteria are getting used to, or ‘settling in’ to their surroundings. Therefore, while they’re doing this they’re not multiplying.
The log phase
As long as they have the right conditions to do so, at this stage the bacteria are multiplying.
The stationary phase
At this stage, the bacteria are dying at the same rate that they are multiplying, meaning that the overall numbers of bacteria stay the same. Some of the bacteria die; because as the bacteria multiply, they generate waste materials, which are toxic to them and so, this kills off some of the bacteria.
The death phase
Here the bacteria have got to the point where they can’t multiply anymore. This may be because they’ve used up all the environmental resources available to them, such as space or food. The conditions have also become unfavourable for growth, because of the amount of waste that they’ve created.
You can use your understanding of the micro curve to, to help ensure that you don’t provide the bacteria with the right conditions, at the right time (as in, for the specific phase) to enable them to grow and cause food poisoning.
Extending the lag phase
You need to make sure that you don’t give bacteria conditions which they’re happy in. If the conditions that the bacteria are given, are not to their liking, this will mean that it will take them longer to get used to their environment, which means that the lag phase will last longer. This is where the intrinsic parameters of the food are really important.
What are the intrinsics?
Intrinsics are the inherent parameters of the food, that restrict growth, such as pH or Aw. If you remember when in the bacteria section of this article, we talked about the environmental factors that bacteria need to grow and multiply. The intrinsics of the food, are these environmental factors.
If the food’s intrinsics provide an environment that the bacteria don’t like, they’ll not ‘settle in’. Meaning that the lag phase will increase in time.
Slowing down the log phase
The log phase is where the bacteria multiply. Here you have to make sure that you don’t give them the conditions that they like to grow and multiply. The intrinsic parameters will help here, but you can also apply what are called preservation hurdles.
What are preservation hurdles?
Preservation hurdles are where you give the bacteria a number of ‘hurdles’ that they need to overcome, in order to grow. Think of them as jumping hurdles on a running track – the bacteria has to jump over each hurdle to get to the end of the race (and multiply). If you put a number of hurdles, one after the other, the bacteria has to jump over each one to get to the end. Each extra hurdle that you apply, makes it more difficult for the bacteria to get over.
Preservation hurdles are adding a combination of things to the food that the bacteria don’t like. You may already have pH or Aw, these are both hurdles. But you can add other hurdles such as temperature (chilled, frozen or hot holding), chemicals such as nitrates or nitrites (for smoked foods) or drying for example.
Using the stationary phase to our advantage
You can also help to stop the bad bacteria (pathogenic) from growing, by helping the good bacteria to grow. If the food contains good bacteria, the good bacteria will grow and take up all of the environmental conditions that the bad bacteria need, until eventually there’s no resource left for the good bacteria either.
This is essentially what’s happening in yoghurt or cheese making. You add lots and lots of good bacteria to the milk and give them the conditions to grow. If there’s any bad bacteria in the milk, they don’t have the opportunity to grow, as they’re overtaken by the good bacteria. This means, that those bad bacteria that do manage to grow, are probably equalled by the number of bad bacteria that are dying, due to the lack of resources.
Using the death phase
This is the phase that’s most obvious to controlling bacteria, as here you can apply heat treatment for example, and kill off the bacteria.
You can however, kill off bacteria in other ways. If you think about it, heat treatment is an extreme condition that bacteria don’t like – temperature. There’s lots of other conditions that bacteria don’t like either so if you apply them in their extreme, they can kill. Take pH for example – if you add pH conditions to bacteria in its extreme, it becomes acid e.g. caustic acid, or alkaline e.g. bleach, which you know when you apply it to bacteria as a cleaning chemical it will kill. You obviously don’t want to add caustic acid or bleach to food as that would be unsafe in itself, but moving the pH scale of the food into conditions that specific types of bacteria don’t like will kill it off.
You can use Aw or dehydration in the same way. Bacteria need moisture to grow, so by dehydrating food this gives an extreme environment that the bacteria can’t survive in. A food with low Aw provides the environment that bacteria can’t survive in as there’s little or no moisture available to the bacteria.
Aw – available water
Let’s take this opportunity to explain Aw. Think of a glass of pure water. In a glass of pure water, all of the water is available, this means it has an Aw of 1.0. If you add something that’s soluble to the water such as sugar or salt, the sugar or salt molecules becomes chemically bound to the water molecules. This means that the water molecules that are ‘bound’ to the sugar or salt molecules are not ‘available’ anymore – they can’t be used. The more sugar or salt you add to the water, the less amount of water in it is available. This means that the Aw reduces to 0.9, then 0.8 and so on. The lower the Aw – the less available the water is.
Food poisoning or foodborne disease?
So, what’s the difference between food poisoning and foodborne disease, and what do we mean by each? This is an interesting question because it seems to depend on who you ask!
Many of the typical food safety books and accredited food safety courses, will say that there’s a difference between food poisoning and foodborne disease. They define them as follows…
Is where you become sick, generally fairly quickly after eating contaminated food, typically within a week. The contamination can be from pathogenic bacteria, mycotoxins or from chemicals.
The pathogenic bacteria that cause food poisoning, can make you sick:
- Due to the quantity (high numbers) of bacteria you eat, or
- due to the fact that they grow to inside you to levels that make you sick, and/ or
- due to the fact that the bacteria have created toxin, and it’s the toxin that makes you sick.
Examples of these would be E.coli, Salmonella, Staph aureus, Bacillus cereus and Clostridium botulinum.
Foodborne disease on the other hand, is different from food poisoning, because you don’t need to eat large quantities of the microorganisms to become sick, there’s sufficient in the food to create illness. This can be from pathogenic bacteria, or from viruses.
Foodborne disease also takes a lot longer to come on, some taking up to a month. Examples of these would be Campylobacter, Listeria monocytogenes and Norovirus.
The problem with this distinction
To us, these definitions don’t give a clear distinction between food poisoning and foodborne disease.
- If foodborne disease is where the food – as it is, will make you sick if you eat it. Why don’t mycotoxins fit into this category? Mycotoxins also produce long-term illnesses (or diseases) so this doesn’t make sense.
- If the onset of foodborne disease is typically longer than food poisoning, then why is E.coli food poisoning (onset is between 10 and 72 hours) and Norovirus is foodborne disease (onset 24 to 48 hours)?
By trying to categorise the illnesses this way it causes confusion, as the distinction between them isn’t clear.
What the FSA say…
The FSA seem to use the terms food poisoning and foodborne disease, but they don’t define exactly what they mean by either term. However, when you read information published on their website about reported food poisoning figures, it seems like they define them as follows:
- Foodborne disease – is the ‘thing’ in the food that makes you ill, such as the pathogen, virus or toxin.
- Food poisoning – is the effect that the foodborne disease has on you, i.e., the illness.
This can be seen from the following statement, taken from their website:
“The data from this study, coupled with data from official statistics, refines our previous estimates of the real burden of foodborne disease and so will help focus efforts to reduce levels of food poisoning in the UK.”
What the FDA say
The FDA use the terms foodborne disease and foodborne illness, in the same way the FSA use them:
- Foodborne disease – is the ‘thing’ in the food that makes you ill, such as the pathogen, virus or toxin.
- Foodborne illness – is the effect that the foodborne disease has on you, i.e., the illness.
Why does it all matter?
So, does it matter what you call it? Probably not. You just need to be aware of the different definitions. If we had to pick one to use, we’d probably go with FDA’s – foodborne disease and foodborne illness, as that makes the most sense.
What does matter; is that you know what effects the different pathogens, viruses and toxins have on the body when they’re consumed. This is important, in order for you to understand how to prevent illness in the first place and also; in the event that you do get an illness complaint, you know what questions to ask to try to determine what pathogen may have caused it. Knowing which pathogen may have caused it will aid you to carry out a thorough investigation.
Bacteriophage in cheese
by Paul Thomas
Paul Thomas is a diary specialist. He teaches cheese making at the School of Artisan Food and helps cheese makers with a whole host of production problems.
Bacteriophage is a virus that attacks the starter culture bacteria in the fermentation of cheese. It results in what’s known as slow or dead vats, where the TA (titratable acidity) can’t be achieved, which means the cheese has to be put to waste. Bacteriophage is the result generally of poor hygiene controls and in the piece below, Paul explains how it doesn’t just cause quality issues, but it puts the safety of the cheese at risk too.
Despite the anxiety which it inspires among some cheesemakers, bacteriophage is sometimes neglected when food safety concerns are considered. When I recently proposed to speak about it at a microbiology training event, several people questioned whether it was perhaps not more of a quality issue.
Whether it is made from raw or pasteurised milk, the safety of cheese relies on hurdles which limit the growth of harmful bacteria. Lactic acid bacteria metabolise lactose, the milk sugar, into lactic acid. This removes a food source for potential pathogens and reduces the pH, making the resulting food less hospitable to these bugs. These starter bacteria also provide positive microbial competition while the cheesemaking process removes some of the moisture, in the form of whey. Finally, salt may be added, which binds some of the water, making it inaccessible to bacteria in the resulting cheese.
For many cheese types, the pH will rise during ripening – less in hard cheeses and more in mould ripened and washed rind cheeses. Despite the removal of one of the main hurdles, for many cheeses, the safety of the product is not compromised to the degree that we might have expected. This is largely due to competition posed by the cheese microflora as it reaches a stationary state of growth. Unfortunately, while this hurdle is arguably one of the most significant barriers to pathogen growth, it is one of the most difficult to quantify.
The only means that a food manufacturer has of measuring the successful growth of a protective microflora is to measure pH development during the make. Surely then an impaired acidification could indicate a greater susceptibility to the growth of pathogenic bacteria?
It is fairly typical to see elevated levels of process hygiene indicators (E. coli or Coagulase Positive Staphylococci) in dairy products which are affected by bacteriophage, either directly because of the loss of starter activity or indirectly because bacteriophage can sometimes be associated with a wider breakdown in hygienic standards. The intention of regulation (EC) 2073/2005 is that such process hygiene indicators should be used as an early warning, alerting the manufacturer to the presence of a developing issue before a major food safety hazard such as Listeria monocytogenes, Salmonella or Staphylococcal enterotoxin is identified.
While the presence of bacteriophage issue can be identified quickly by a laboratory, it’s erratic nature can sometimes leave a cheesemaker slow to realise that it may be present. It may not affect every batch consistently and it can be hard to spot in the case of the long, slow acidification used in the production of many soft cheeses. Its presence may also be disguised by some early acidity development, sometimes attributed to the presence of coliform bacteria.
Where elevated hygiene indicators are coupled with partial or complete starter failure, the presence of bacteriophage should be considered and investigated with some urgency. Where it is identified, the increased sampling for microbiological hazards may be appropriate and where there is reason to question the safety of the batch, the sample number specified in EC 2073/2005 should be respected as a minimum though it is important to note that even with five samples, microbiological testing is unlikely to detect a pathogen with a low prevalence. Many phage problems can be remedied by changing the starter cultures used, but It is also worth scrutinising the hygiene of the production process objectively to prevent a recurrence.
Frequently asked questions
What are pathogenic bacteria?
They’re the bacteria that cause food poisoning.
What are the four stages of the micro curve?
Lag, log, stationery and death – You can use the micro curve to prevent pathogenic growth in food.
What are food intrinsics?
Intrinsics are the inherent parameters of the food, that restrict growth, such as pH or Aw. Bacteria need certain environmental factors to grow and multiply. The intrinsics of the food, are these environmental factors.
What are preservation hurdles?
Preservation hurdles are where you give the bacteria a number of ‘hurdles’ that they need to overcome, in order to grow. Think of them as jumping hurdles on a running track – the bacteria has to jump over each hurdle to get to the end of the race (where it can multiply). If you put a number of hurdles, one after the other, the bacteria has to jump over each one to get to the end. Each extra hurdle that you apply, makes it more difficult for the bacteria to get to the end of the race.
What is Aw (available water)?
In a glass of pure water, all of the water is ‘available’ and it has an Aw of 1.0. If you add something that’s soluble to the water, such as sugar or salt, the sugar or salt molecules become chemically bound to the water molecules. When this happens the water molecules that are ‘bound’ are not ‘available’ anymore and can’t be used. The more sugar or salt you add to the water, the more bound the water becomes, which reduces the Aw to 0.9, then 0.8 and so on. The lower the Aw, the less available the water is. Pathogenic bacteria need water to be able to grow, so you can apply a low Aw to stop them growing.
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