Extending food shelf life

How can plasma help to keep fruit and veg good for longer?
21 January 2020

Interview with 

James Walsh, University of Liverpool




Once food has been grown or raised, it needs to be distributed to customers. And food is routinely shipped or flown all around the world, so storing food in a way to minimise spoilage during the trip is a key area ripe for research. One idea being developed involves bathing food items with chemicals that can boost the defences of fruits and vegetables, and kill the microbes that make things go bad so they last longer. Chris Smith spoke to James Walsh from the University of Liverpool...

James - Well, there's several non-thermal processing technologies currently in use and currently being investigated as well. An example would be ultraviolet light, so we expose products to ultraviolet light and that can kill microbes on the surface. And another quite topical area is chemical methods, so chlorine washing or applying ozone to decontaminate those products.

Chris - And are there no negative effects on the products by doing that? I would imagine that bathing something in ultraviolet light, which is quite high energy, quite ionizing, might be bad. Does it not affect the flavour?

James - Well, there's actually very strict guidelines on that so if there's any change to the products, so nutritional changes or changes to coluor or flavour or anything like that. Then that novel processing technology is subjected to some extremely sort of strict guidelines and there has to be several years of very expensive testing to prove it's actually safe to use before any consumer would ever eat a product treated by that method.

Chris - And with these sorts of methods, how much shelf life can you add?

James - Well, it depends a lot on the method and it depends a lot on the product as well. The particular method I am interested in, which is using plasma to treat products, that can, from the literature, it appears to give between one and four days additional shelf life depending on the type of product you're actually treating.

Chris - Now when you say plasma James, most people when you say plasma, we'll think of the stuff that washes around in your bloodstream and carries blood cells along. This is a totally different type of plasma, isn't it?

James - Absolutely. So this is a plasma, as in the fourth state of matter. So if you imagine you took an ice cube and you warmed it up, you'd get a liquid, you warm that liquid, you get steam. If you warm that steam, what comes next? Well that's actually a plasma. And in that plasma you've actually got a soup of ions, which have got a positive charge, electrons which you've got a negative charge and lots of reactive chemical species. And that's what really does the business in plasma technology.

Chris - How would it be used then? Would you generate the plasma and then spray it on to the food stuff as it sort of goes along on a conveyor belt?

James - That's essentially the idea. Although the plasma is only there whilst you use electricity to generate the plasma. The plasma is only there while you're applying the electricity. So it's really generated when and where you like it. So as you can imagine, you create a plasma or over a conveyor belt, the products pass underneath and they get exposed to all these chemical species and some UV light as well, and that's what actually damages the spoilage organisms on the surface.

Chris - So it zaps the bugs, things like fungi and moulds that would want to degrade the food, but it doesn't degrade the food item itself?

James - Well it's a very fine line where we're sort of treading there. So it's, there's a vast amount of research on finding the correct dose of plasma species to be able to get rid of those organisms that we don't want on the surface, but not have any sort of underpinning impact on the food matrix, that's really the key to this technology.

Chris - I spoke to Kirsty Bayliss who's a researcher at Murdoch University in Western Australia and she's doing something similar. I spoke to her about a year ago and she said that the effect it has on the foods that they've tested, she said to me quite literally, she can take an avocado and have it last in her fridge for three weeks, not three days. And she wasn't exaggerating. She was saying though that the extension in shelf life she's getting is way beyond what can be accounted for just on the basis of suppressing or removing the microbes, because after all they will just come back again later from the environment. So she was thinking there must be something else going on and perhaps that the exposure to these plasmas changes almost like the immune system of the fruits and vegetables. It makes them more resilient and resistant so they can fend off assault from the microbial flora and fauna better.

James - It's definitely a possibility, although I'm not so familiar with that area of research. An alternative sort of theory for that could be that if there's water on the product, you actually activate that water, you give it some chemical reactivity and that lasts much longer than natural gas plasma you're applying. So it has a longterm effect for sure that can give shelf life extensions for longer than a few days.

Chris - When you do the numbers, do the maths, as they like to say, on this, does it translate into a viable prospect? As in there's gotta be infrastructure put in place to do this. You've got to have the machinery, you've got to have plant that's capable of actually processing things in this way. When you add up the cost of all of that, does it translate into a benefit?

James - The figures are still being run, I'd say. But actually plasma technology is a very low cost because it doesn't use any consumables. You don't have to make anything in a factory and transport it to where you are processing your products. You actually create it in situ using only air and electricity. And also the amounts of electricity, the power is, is very low. It's sort of hundreds of watts per square metre of plasma generation. So that's, that's really low. It's less than boiling a kettle for example.

Chris - So when you make your cost benefit analyses for this sort of work, what's your bottom line? How much do you think that the industry could save if you were to implement this technology?

James - Well the types of products we're interested in, commodity products, say tomatoes and salad leaves and things like that. There's a tiny margin of profit made by those food producers on those. So we really, we have to get those costs down for the plasma treatment to be sub pennies per product as it were, to make it viable.

Chris - But the return for them? How much are they actually going to potentially save and make if they can extend the shelf life?

James - There's an enormous amount for both producers and consumers to save. If you can extend shelf life just by a single day. So producers, obviously they can employ people to not work weekends for example, so they can have that extra day storage. Or it was passed onto the supermarkets, they can last longer on the shelves. And of course the biggest loss of food in the UK is in households, 70% of it. So that extra one day shelf life may translate into maybe an £800 a year saving for an average family. So it's quite enormous.


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