Microbiological shelf-life: how to set a safe shelf-life

Linda Everis, Principal research officer

In this whiteboard presentation, Linda Everis talks us through how to set a shelf-life and some ways of measuring microbiological shelf-life. Linda covers:

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Linda Everis

About Linda Everis

Linda Everis joined Campden BRI in 1995 as a Senior Technician in the Microbiological Analytical Services group having graduated from the University of Wales Aberystwyth with a BSc in Biology.

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I'm here today to talk to you about how you would go about setting a safe shelf-life. But before we start on that we need to decide what shelf life is. So, when talking about shelf-life we mean the time after production that a product remains acceptable for consumption. So, that can be sensory properties have to remain as they would be and chemical properties and also microbiological characteristics, so by that we mean that any of the spoilage organisms present need to be there and not giving you a deterioration in select ability but also the product needs to remain safe that's obviously key.

Why is it really important to make sure that you've got the correct shelf-life? Well need to have the correct shelf life because if you don't the product may not be economically viable you may end up with lots of food waste and also you could end up with lots of consumer complaints so therefore potential loss of reputation.

So, if we're talking about how you actually set a shelf-life there's two approaches that can be taken and one of which is where you look at the quality aspects so you're looking at the sensory properties chemical properties and, as I mentioned, in the microbiological properties so what we'd be looking at is in terms of micro we will just be looking at how the spoilage organisms, ones that would naturally be present in most batches and how they grow and affect the products quality.

But key is the safe shelf-life so what you will have assessed during your HACCP is are there any pathogens that could be present that might give you issues with safety and food poisoning. So, if there are there are two approaches that you can take and the first of which is predictive modelling. So, this is a really really quick and easy technique that can be used to assess if pathogens will grow in your product. So, you already know from your HACCP whether the product type is associated with particular organism types, you'll know if the pH, water activity, temperature will allow these organisms to grow. So, you will know which organisms are going to be an issue. So you can take those organisms and you can use a really easy to use computer modelling system where you generate growth curves like these so what you can do you can you can think about what organisms are important and then produce growth curves so that you're able to see whether the organisms will grow under the temperature pH, salt, water activity conditions of your product. So, if we take this graph here as an example, we've got the time here in days and the log count, so what we're looking at how many organisms grow in a particular time. We've got botulinum here as an example and Listeria so your shelf life might be here so you can say perhaps botulinum wouldn't grow but Listeria would grow within your shelf life so therefore Listeria might be the organism that limits your life.

What you can also do is you might have a similar product but you might have changed pH, water activity (slightly), storage temperature might have changed maybe lower maybe higher you might have changed salt so if you've changed any characteristics of the product you can then have a look and see how that would affect growth so what we've got here is a graph that shows you again time and the numbers and we've got say for example three different salt levels but equally we could have three temperatures or three pH’s so we’ve got three, two and one percent salt, so no growth at three percent but if you drop this to two you do get growth much faster growth than one percent. So, you can see if that's going to affect growth. So, when we're looking at modelling, we are sometimes looking to say for some organisms like Salmonella or botulinum whether there's been any growth at all, ideally those organisms won't be present but if they were are we going to see any signs of growth. For organisms like Listeria, for example, there's some legislation that says you cannot have more than 100 CFU per gram in a ready-to-eat product, so you can use the model to see whether the Listeria level will be more than 100 at the end of your life.

You can also, for bacillus and staph aureus, need to have grown to a lot higher level to be able to produce the toxin and it's the toxin that's an issue so you instead of looking for a level of just a hundred you'd be looking to see when a level of say maybe 104 105 has been reached, so something along these lines.

If you've done the predictive modelling and it's shown that growth hasn't occurred but you want to back that up just to be sure, or if the modelling has shown that there are potential issues with growth and there might be growth over shelf life but you think that perhaps the model might be a bit failsafe as they can be sometimes but you want to prove why the growth will occur, we could look at taking that one stage further and you can do inoculated challenge test studies so this is where you actually deliberately inoculate the food with your test organism. So, if we take Listeria for an example, we'd grow up the bacteria usually overnight, it involves inoculating small amounts into the food, and then we test that on day zero so that we know what level is present, then we would inoculate enough samples and store them and incubate them at the relevant test conditions. So, it always says make sure that you use conditions that realistic so maybe five, to start off with, representing manufacture and storage and they may be slightly higher when you've got your consumer storage at home. We then obviously test maybe weekly if the products got a month shelf life maybe longer if it's got a lot longer shelf life. Well look at and count the results, so we'll numerate their colonies and we'll see whether there has been growth. We then would interpret it. So, has there been growth of say Salmonella or botulinum? Is there potential for a level of more than 100 of Listeria to be present? Is there a level of bacillus and staph aureus present that might give you an issue with toxin?

So, in summary, if you want to set a safe shelf-life there are a few ways you can go about it. You can do the really easy quick to use predictive modelling in the first instance to actually hone in and find out where your risks are, what organisms are going to be an issue where you need to focus, and then you can move on and do the challenge test studies which will then improve whether growth is going to occur or not.

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