Tag Archives: fruit juice

Faking Lateral Flow Tests: the problem with pH

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Fruit juices can be used to generate a fake positive on COVID-19 LFTs

On Thursday last week, I got a message from Prof Mark Lorch — my sometime collaborator on supercharacter-based ramblings.

“Have you seen the reports of kids fooling the Covid lateral flow tests and getting false +ve results by adding orange juice to the devices?” he wondered.

At this point, I had not – but I quickly got up to speed. Mark had previously made an excellent video explaining how lateral flow test (LFT) devices work, so it was just a case of working out, firstly, whether the false positives were reproducible, and secondly, speculating what, exactly, was causing them.

Thus ensued some interesting discussion which ultimately led to a couple of articles from Mark. One at The Conversation and another, slightly more recently, at BBC Future.

I won’t delve into LFT-related science, because Mark has covered it (really, check the video and those articles), but I am going to talk a little bit about pH – the scale chemists use to measure how acidic or alkaline solutions are – because as soon as news of this started to gain traction people, predictably, started trying it out themselves. And that was when things got really interesting.

Image

The buffer included with LFTs is effective at neutralising the pH of solutions, for example, cola

Now, firstly, and importantly: the test kits come with a small vial of buffer solution. Buffers are substances which resist pH changes. As I’ve written before, our bodies naturally contain buffer systems, because keeping the pH of our blood and other body fluids constant is important. In fact, if blood pH varies even a little, you’re in all sorts of serious trouble (which is how we can be certain that so-called “alkaline” diets are a load of hooey). Anyway, the important message is: don’t mix any liquid you’re testing with the contents of that phial, because that will neutralise it.

If you want to try this for yourself, just drop the liquid you want to test directly into the window marked S on the test.

That out of the way, let’s get back to pH. It’s a scale, usually presented as going from 0–14, often associated with particular colours. The 0 end is usually red, the 7 in the middle is usually green, while the 14 end is usually dark blue.

These colours are so pervasive, in fact, that I’ve met more than one person with the idea that acids are red, and alkalis are blue. This isn’t the case, of course. The red/green/blue idea largely comes from universal indicator (UI), which is a mixture of dyes that change colour at different pH values. There’s also a common indicator called litmus (people sometimes mix up UI and litmus, but they’re not the same) which is also red in acid and blue in alkali.

Some species of hydrangea produce pink flowers in alkaline soil, blue in acid soil.

There are actually lots of pH indicators, with a wide variety of colour changes. Phenolphthalein, for example, is bright pink in alkali, and colourless in acids. Bromocresol purple (they have such easy-to-spell names) is yellow in acids, and violet-purple in alkalis.

Many plants contain natural indicators. Just to really mix things up, some species of hydrangea produce flowers that are blue-purple when they’re grown in acidic soil, and pink-red in alkaline conditions.

Bottom line? Despite the ubiquitous diagrams, pH has nothing to do with colour. What it is to do with is concentration. Specifically, the concentration of hydrogen ions (H+) in the solution. The more H+ ions there are, the more acidic the solution is, and the lower the pH. The fewer there are, the less acidic (and the more alkaline, and higher pH) it is.

In fact, pH is a log scale. When the concentration changes by a factor of 10, the pH changes by one point on the scale.

This means that if you take an acid with pH of 2, and you dilute it 1 part to 10, its pH changes to 3 (i.e. gets one point more alkaline, closer to neutral). Likewise, if you dilute an alkali with a pH of 10 by 1:10, its pH will shift to 9 (again, closer to neutral).

And what this means is that the pH of substances is not a fixed property.

Louder for anyone not paying attention at the back: the pH of substances is not a fixed property!

Yes, we’ve all seen diagrams that show, for example, the pH of lemon juice as 2. This is broadly true for most lemons, give or take, but if you dilute the lemon juice, the pH rises.

Apple juice dropped directly into the test window gives an immediate “positive” result.

I am by no means an expert in commercial, bottled lemon juice, but I reckon a lot of them have water added – which may well explain why @chrismiller_uk was able to get a positive result, while @BrexitClock, using a French bottle of lemon juice, couldn’t.

Mark and I concluded that the pH you need to aim for is probably around 3–4. Go too low, and you don’t get a positive (and you might wipe out the control line, too). Likewise, too high also won’t work.

Myself, I tried apple juice. I couldn’t find the indicator colour key for my indicator paper (I really must clear out the drawers one of these days) but it’s broadly the same as Mark’s cola photo, up above. In other words, the apple juice is about pH 3. And it gives a beautiful positive result, immediately.

One more interesting observation: Mark recorded some time-lapse video comparing orange juice to (sugar-free) cola. It shows the cola test line developing a lot more slowly. We’re not entirely sure why, but it may be pH again: orange juice almost certainly has a lower pH than cola.

For any parents reading this thinking we’re being terribly irresponsible, fear not: as Prof Lorch has made clear in his articles, you can identify a fake by waiting a few minutes and then dropping some of the buffer solution provided in the test window. As I said above, this will neutralise the pH, and the positive test line will disappear. Extra buffer won’t change a genuinely-positive test, because the antibodies bind very tightly (more technical info here). To quote Mark: “you’d need a swimming pool’s worth of buffer to have any chance of washing [the antibodies] off.”

Alternatively, you can just watch your teenager as they do their tests, and make sure they’re not getting up to anything nefarious…

Have you tried to trick an LFT? If you have, share your results! Look us up on Twitter: @chronicleflask and @Mark_Lorch or add a comment below. We’d love to see your photos!

Do you want something non-sciency to distract you from, well, everything? Why not take a look at my fiction blog: the fiction phial? You can also find me doing various flavours of editor-type-stuff at the horror podcast, PseudoPod.org – so head over there, too!

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No, ketchup does not cause cancer

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ketchup and coke

Do these things really cause breast cancer? (Spoiler: no)

Less than two days into the new year, and I’d already found what might well be one of the silliest health headlines of the year. What is it I hear you ask? Well, it was in a national newspaper on New Years Day, and it was this:

“Sugar found in ketchup and Coke linked to breast cancer”

This, to borrow a favourite line from an online greetings card company, had me rolling my eyes so hard I could practically see my brain. Why? Because even without reading any further, I knew immediately that it was the equivalent of saying, “too much of thing found in most stuff might cause cancer!”

But let’s not be one of the 70% of users that only read the headline, let’s dig a little further. The newspaper article, which in fairness isn’t too bad – it’s just a bit of a silly headline, alludes to work carried out the University of Texas’ MD Anderson Cancer Centre. If you click on the link I’ve added back there, you’ll see that MD Anderson’s headline was:

“Sugar in Western diets increases risk for breast cancer tumors and metastasis”

Note, they just say ‘sugar’, not sugar in two apparently randomly-selected foodstuffs. The researchers divided mice into four groups, fed some a diet high in sucrose (more commonly called table sugar – in other words, the stuff in the sugar bowl) and compared them to others fed a low-sugar, ‘starch-controlled’ diet. They found that the high-sugar diet lead to increased tumour growth, particularly in mammary glands.

I’ve covered forms of sugar before but still, here’s a quick reminder before we go any further: this is a molecule of sucrose:

Saccharose2

Sucrose

Sucrose is made of two ‘bits’ joined together: one unit of fructose and one unit of glucose.

157px-Alpha-D-Glucopyranose

Glucose

These two molecules are what chemists call isomers. They contain the same number and type of atoms, just joined up differently. They’re both sugars in and of themselves. Glucose is used directly by cells in your body for energy. Fructose, on the other hand, is trickier. It has a lower glycemic index than glucose, in other words, it doesn’t raise your blood sugar as rapidly as glucose, but this doesn’t mean it’s healthier. It’s metabolised almost exclusively in the liver and, long story short, invariably ends up being converted into, and stored as, fat.

179px-Beta-D-Fructofuranose

Fructose

Fruit is high in fructose, and fructose tastes very sweet to us (sweeter than either glucose or sucrose). This is nature’s way of telling us, and other animals that might eat the fruit, that it’s high in nutrients. From the plant’s point of view, it’s an incentive to eat the fruit and, ahem, spread the seeds around.

Humans have, of course messed around with this perfectly sensible survival mechanism by stuffing all kinds of easily-available and not particularly nutrient-rich foods with fructose, and herein lies the problem. Co-author of the paper that started all this, Lorenzo Cohen, Ph.D., professor of Palliative, Rehabilitation, and Integrative Medicine, said “we determined that it was specifically fructose, in table sugar and high-fructose corn syrup […] which was responsible for facilitating lung metastasis and 12-HETE production in breast tumors.” Notice that he mentions fructose in table sugar; this is because, once you eat sucrose, it breaks down into units of glucose and fructose.

The article goes on to suggest that sugar-sweetened beverages are a significant problem, so was the newspaper wrong to pick on Coke? It’s a popular drink after all, and a standard can of Coca-Cola contains approximately 35 grams of sugar (which might come from either sucrose or high fructose corn syrup mainly depending on where you buy it). The guidance for adults is no more than 30 grams of sugar per day, so a single can of regular Coca-Cola would take you over that limit, and it’s very easy to drink two or even three cans without giving it a second thought.

sugar

Soft drinks and fruit juice both contain a lot of sugar

However, the same goes for pretty much any non-diet soft drink.  Pepsi, for example, has a similar amount. Lemonade can be even more sugary, with some drinks hitting 40 grams per 330 ml can. Ginger beer might well be the worst; there are 53 grams per 330 ml in Old Jamaica Ginger beer for example. Fruit juice is no better, with many juices containing 35 g of sugar per 330 ml, although at least fruit juice might contain some other nutrients such as vitamin C.

So really, I’d say it’s a bit unfair to single out Coke in a headline like this.

What about the ketchup (note they didn’t pick a specific brand here, just generic ‘ketchup’)?

Well, ketchup IS high in sugar. It contains about 24 grams of sugar per 100 grams. But hang on, 100 grams of ketchup is quite a lot. A more realistic serving size of a tablespoon is only about 15 grams, which works out at about 3.5 grams of sugar. Still quite a lot, but probably a drop in the ocean compared to all the sugar in cake, bread, drinks, fruit juice, breakfast cereals and the tubs of Roses and Quality Street you scoffed over Christmas. Unless you make a habit of drinking ketchup by the bottle (apparently some people do) this is frankly a ridiculous foodstuff to pick on.

I imagine that someone did a quick search for ‘foods that contain fructose’ and picked Coke because, well, everyone knows that Coke’s bad, right? So that sounds credible. And ketchup because we all sort of suspect it’s probably not that healthy, but it hasn’t been the subject of a health scare recently so that makes it stand out. Great clickbait, bad science.

mouse

Mice are not people

Plus, let’s be absolutely clear, the study was in mice. Mice are not people. While a study that shows an effect in mice is an interesting start, and may well be good reason to conduct more studies, quite possibly in humans, it’s not proof that this mechanism exists in humans. Humans have, after all, evolved to eat a very different diet to mice. There might well be a link, but this doesn’t prove it, and even if a link does exist we certainly can’t say anything about the significance or size of it from this research.

I’m not a dietician, but I’m going to go out on a (fairly sturdy) limb here and say that cutting back on sugar will not do you any harm and is likely to be a jolly good thing. Let’s also be clear that sugar in fruit juice, agave, honey etc is still sugar and is no healthier than table sugar. Eating too much of the sweet stuff is almost definitely bad for your waistline and, as we all learned as children, bad for your teeth too – something which is often overlooked but really shouldn’t be, poor dental health having been linked to other serious health problems including diabetes and heart disease.

ketchup on bread

Maybe cut back on the fried ketchup sandwiches

But, and here’s my big problem with the newspaper’s headline, none of this means that Coke and ketchup directly cause breast cancer which is how, I fear, some people will interpret it. Cut out sugary fizzy drinks by all means, and perhaps ditch the ketchup sandwiches (especially fried ones), but please don’t worry that the occasional dollop of red sauce is going to kill you. I’m pretty certain it won’t.

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A small edit was made on 6th January to clarify that pure fructose isn’t used as an ingredient in Coke, but rather high fructose corn syrup.