Sensory science and Covid

What is parosmia, and how does it impact eating?
04 August 2020

Interview with 

Barry Smith, SAS; Jane Parker, Reading University; Tristan Welch, Parker's Tavern


A man's nose.


With the cake half way cooked on the Naked Scientists' BBQ, Chris Smith spoke to sensory scientist Barry Smith and flavour chemist Jane Parker about the sensory experience of food, and how Covid-related changes in smell and taste can impact how Covid sufferers experience eating...

Chris - We are making a cake on the barbecue. Tristan, do you want to give us an update? How are we doing? It's been on what, 10 minutes?

Tristan - Yeah, and you can see the cake mix is bubbling up. It's rising. They look like mini souffles almost, but the top of the paper is starting to toast a little bit with some of the mix that's stuck on the side. That's a great indicator that -

Chris - We're not going to eat the paper, are we?

Tristan - No, but it's just an indication that things are cooking. Well, I dunno, maybe. I mean, sausage-flavoured upside down cake. You never know!

Chris - No, seriously, this is amazing. They've risen right out of the top of the tins and they're beginning to look really amazing, as are my sausages. And it smells good!

Tristan - Yeah, it smells great doesn't it? I've got to say that they are bubbling away beautifully. Just gonna take a few more minutes!

Chris - He's a confident man isn't he! With us also is Barry Smith who's a senses scientist and is joining us to tell us about some of the ways in which my senses are being stimulated today. Now Barry, people characteristically say that things do taste better when cooked and consumed out of doors. Is there any science behind that?

Barry - I think there might be. I like the idea that our senses are being bombarded while we're waiting for the food to cook. We're seeing the colour of the flame. If you had charcoal Chris you'd have a lovely grey-blue flame there. You're smelling the barbecue. You're hearing the sizzling and all of these things are giving you expectations about what you're going to taste. And they're probably giving you expectations that are going to bias you and make it even more pleasurable to taste the food. People are also tempted, I think, when they're eating barbecue food to eat with our fingers. And of course that's a very natural thing to do. People say we eat first with our eyes, but we eat second with our fingers. Picking something up you will gauge from your fingers, which are very sensitive, what's the texture of this? Is it sticky? Is it sharp? Is it crunchy? It's soft, squidgy? And that gives you a good expectation about the texture as you put it in your mouth. So I think all of those things are being primed especially well. If you're in a kitchen, you might smell the cooking smells, but things might be in the oven unseen. You don't see the smoke and you don't want that indoors. So I think this is making a big difference.

Chris - In terms of flavours and things, Barry - you were talking about the sort of tactile nature of food and that being important, that sensation is obviously being integrated with all of the other assaults on your senses when it comes with eating posh-nosh and fine food. If you make the food spicy, that's not actually a taste or a flavour is it, the spiciness? It's just a burning sensation. But does that add a dimension as well? Is that an additional stimulation that makes the food have an extra something?

Barry - Yes, it does. This is one of the hidden flavour senses and it's employing the trigeminal system. So as you know, Chris, the trigeminal nerve is the fifth cranial nerve. It comes from behind the ear. It goes to the eyes, the nose and the mouth. And it's the one that rings bells when you have too much wasabi and you feel it at the bridge of your nose, you feel that ouch sensation. And also if you have too much irritation of the trigeminal nerve, the eyes will water because they think they're under attack when you're having too much spice. That trigeminal nerve makes peppermint tastes cool in the mouth, mustard taste hot, and therefore even spices like vanilla - which seem very gentle, it's a spice - these are just giving you that tingle. Ginger, obviously garlic, all those other things, black pepper. And these are wonderful flavours to add. So you need touch, you need taste, you need smell, and trigeminal stimulation to get the flavours we love.

Chris - I've got to ask you Barry because of course, in recent months, we've seen the case definition for a person with coronavirus change. It went from just have you got a cough and a fever, to have you got a cough, a fever, have you lost your sense of smell and taste. So what's the implication for people who have had coronavirus and have seen their sense of smell and taste change in response to this?

Barry - Well this is very well known to Jane and to me since we were part of a team with colleagues from ear nose and throat surgery campaigning to get the government to recognise that loss of smell and taste were symptoms. So very often people will say, I've lost my ability to taste food. Now, when that happens, I'm not sure it is taste. I think it's mainly smell. And I usually ask them to put a bit salt on their tongue, a bit of sugar, or lemon juice, and I say can you taste that? And if people say they can taste that, but that's virtually all they can taste, then you know what they've lost is their sense of smell. And it's really at that point that they recognised how much smell was contributing to the pleasure of eating. The fruity notes, the meaty notes, that taste of an onion, none of that comes from the tongue - that's all combining what the tongue provides with aromas in the nose. So a lot of people started to say their food was dull, their coffee tasted of nothing. And of course, if you think of coffee without the wonderful smell of freshly brewed coffee, it's just hot water with a bitter flavour. So a lot of people noticed first in their food that they were lacking their sense of smell because of one of the symptoms of COVID-19. And probably because the nose is a major infection site for the virus.

Chris - This is dubbed parosmia, isn't it, Jane? What are you actually doing to try and understand more about what flavours people can and can't perceive? And is it true also that people are saying that when their sense of smell and taste does begin to return post-COVID that actually it doesn't come back all at once and everything begins to taste normal all at once?

Jane - Well, what Barry was describing was anosmia, and that's when you completely lose your sense of smell. And many people with COVID will get that back after several weeks. But some of the more unfortunate people, it will linger for a bit longer. And one of the things that happens in the recovery process is the parosmia which is when everyday smells can be distorted and are repulsive. And frankly, people in severe cases, they can't be in the same room as people cooking, people drinking coffee, cooking for the family becomes a problem. Eating becomes a problem. It can have an impact on mental health. So we've been carrying out a study, which started before COVID, because this also happens with other viruses - so it's known before this parosmia phase. So we've been looking at what foods in particular trigger this parosmia, because it's only certain foods. And then what chemicals, what aroma compounds in particular. And I have to say that a barbecue must be a parosmics nightmare because the things that we're finding that are triggering parosmia - coffee is one of them and coffee is the most common, but after that it's meat and chicken and beef. And it's the cooking process, which is often described as the worst. It's onions, it's peppers, it's cucumber. So even the salad, it's not just the cooking process. You know, it's a series of really potent aroma compounds, which are the triggers.

Chris - What could a person who's suffering with this expect then? And what can they eat?

Jane - Although we've got a group of things that parosmics tend to hate, the things that they like is a lot more variable. But it's generally bland things, it's pasta, there's a lot of fruit and veg that is okay for many parosmics, but it's not without exception. You know, watermelon is horrible for some people and that's really quite bland.

Chris - If you're someone who's unlucky enough to find themselves in that position, what's going to happen to you? Is this going to be the rest of your life or will it go away?

Jane - Well, the good news is that for many, many people, it will improve over time. One of the things that is recommended by ENT specialists is to carry out what's known as smell training, where you sit in a quiet room twice a day, and you've got a range of essential oils and you may smell nothing to begin with, but just that act of smelling the essential oil and thinking about it and going through that process helps you mentally. And it also helps we believe to stimulate regrowth of the neurons, which have been destroyed as a result of COVID.

Chris - Which sounds promising. So do you have any feeling yet for what proportion of people are going to improve? What proportion will probably stay the same and what proportion may even get worse?

Jane - I think it's unlikely that anybody will get worse. We just don't know because we haven't had enough people going through the parosmia stage - there's a three month delay. So we're only just now getting the influx of parosmia people. So we don't know enough about how many people will recover. I know that from other non-COVID parosmics there are some who don't recover, you know, even eight, 10 years down the line, there will be one or two. So we can't promise that everybody will recover, but most people do, but it does take years. You know, you'll make a slow change from if foods are really repulsive and horrible things will start to get slightly better. They'll never return exactly to normal, but you'll learn to live with them. You'll learn to recognise coffee as what it is now rather than hate it, but it does get there.


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