• Dr Michael Mosley and James Wong explore why strawberries are so acidic yet taste so sweet. (Getty Images)Source: Getty Images
How do strawberries fool us into thinking they're sweet when they're actually pretty acidic? We find out why in Secrets of Your Food.
Yasmin Noone

23 May 2018 - 7:51 AM  UPDATED 26 Sep 2018 - 7:21 AM

Strawberries are one of the most luscious and versatile fruits, distinctively loved around the world for their sweet flavour.

Ironically, the strawberry is also regarded as a health food that can be consumed in large proportions compared to other sweet-tasting fruits because of its low sugar content.

According to myfitnesspal, one cup of raw strawberry halves only contains 49 calories and seven grams of sugar.

Science journalist, Dr Michael Mosley tests the actual sweetness of strawberries the new SBS three-part series Michael Mosley’s Secrets of Your Food by conducting an experiment to compare the sugar content of blueberries to strawberries in episode two.

“That means [strawberry juice] contains nearly half as much sugar per gram as there is in blueberries: I’m genuinely surprised by that."

Dr Mosley squeezes a few drops of juice out of each handful of ripe fruit into a petri dish. He then uses a refractometer to test the sugar level of fresh strawberry and blueberry juice.

“What happens when light passes through any liquid is that it gets bent,” Dr Mosley says in episode two (scroll on down to watch the entire episode online via SBS On Demand). “The more sugar there is in that liquid, the more [light] will get bent.”

He first tests blueberry juice, which yields a sugar score of 13. Strawberry juice is then tested, producing a score of eight.

“That means [strawberry juice] contains nearly half as much sugar per gram as there is in blueberries: I’m genuinely surprised by that. So why is it that a strawberry tastes so sweet when it contains that much sugar?”

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The answer is complicated but fascinating. Botanist and show co-host, James Wong, explains that strawberries actually have a very high acid content.

Wong measures the pH of fresh strawberry juice to test its acidity. With seven being perfectly neutral and one being very acidic, strawberries sit at 3.5. Strawberries reap the same acidic measure as grapefruit, despite tasting a lot sweeter. To put this figure in perspective, wine also sits at a 3.5 pH level, vinegar yields a pH of 2.9 and black coffee has a pH value of five.  

“Strawberries have a cunning ability to hide their acidity,” says Wong.

He adds that strawberries also don’t start out as sweet red-coloured fruits.

“Strawberry plants have specifically evolved this sweet succulent fruit to encourage animals to eat them,” explains Wong. “That’s because when these seeds pass through the digestive tracts of an animal, they are deposited – with a bit of fertiliser – far and wide, helping the strawberries’ empire grow.”

“Strawberries have a cunning ability to hide their acidity."

But, he says, this only works when the seed is fully mature and ready to sprout. Up until this point, strawberries are green and full of acid that makes them taste sour.

The sour flavour of an unripe strawberry is a deliberate animal deterrent. “The brain [of the animal which is trying to eat the strawberry before maturation] interprets this taste as unpleasant and a sign that the food could be spoilt or unfit to eat,” says Wong. “It’s a biological reaction that plants use to their own ends: a taste strong and repellent enough to put most animals off.

“But just at the right moment when the seeds have matured, the strawberry needs to mask this acidity to make it more palatable. All the acid is still there [in the strawberry] but the fruit becomes flooded with sugar, produced when hormones from the seeds announce they are ready to be eaten. The sugars react with other plant molecules and make attractive red pigments [in the strawberries] that say ‘eat me’.”

It's estimated that the sugar content of strawberries increase from five percent in unripe green fruit up to nine per cent when fully ripe, according to an article in The Conversation based on studies from University of Birmingham. The article also explains that the acidity decreases during ripening making ripe strawberries taste a lot sweeter. 

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The deceiving scent of a strawberry

There’s also another key reason why strawberries taste so sweet but are actually very acidic and low in sugar.

“Part of the secret to its success is its smell,” says Dr Mosley. “Surprisingly strawberries have altered the way they taste using the power of smell.”

Dr Mosley explains that strawberries contain a host of molecules that give the fruit their characteristic scent but also boost our taste sensation around sweetness.

Around 20-to-30 of the 350-plus molecules which are present in strawberry vapour are important to the fruit's sweet flavour. These molecules blend together to produce an aroma that deceives our brain into thinking we are getting a lot more sugar than we actually are.

“Now despite the fact that I now know that an awful lot of the sweetness I think I’m experiencing in my mouth is actually coming from stuff that’s going into my nose, it hasn’t diminished my pleasure at all,” says Dr Mosley. “I still find strawberries deliciously sweet.

“It’s a clever trick. By boosting how sweet the fruit needs to be the plant needs to give away less sugar while still encouraging us to spread its seeds.”

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