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Tuesday, 9 July 2019

When is a Strawberry No Longer a Strawberry?

You take that first bite, tearing through the cells of the red berry and allowing its fragrance to erupt. Juices ooze, coating your mouth. The sweetness, modulated with a slight bitter hint, hits your tongue and lights up your senses, and the distinctive flavour of the strawberry overflows.

Why do we love strawberries so much?


Why do we love strawberries so much? There’s something alluring about them, something intense and complex in the flavour we experience that is not matched by any other berry – its chemistry.

When we recognise a flavour – the combined experience of taste, mouth feel and (crucially) smell – we are actually recognising the aromatic chemistry of a food, the volatile molecules given off when we bring it close to our nose or after we bite into it. When we understand the molecules that make up the distinct chemical profile of a flavour, we can replicate it by creating those chemicals in a lab and stirring them together. This helps us make artificial banana flavour, artificial vanilla flavour, and artificial fresh grass smell. But strawberries are awkward. Strawberry flavour is made up of a chemical medley of many, many different molecules (more than 350), variation exists strawberry species to strawberry species, and we just don’t know how important each chemical is.

This is in direct contrast to the raspberry, a humble berry that we recognise as 4-(4-hydroxyphenyl)butan-2-one, or “raspberry ketone”. Whilst cranberries and blackberries also contain this odour, they mix it with other chemicals, whilst in the raspberry it is exclusively responsible for the raspberry profile we instantly know.

Raspberry ketone, the molecule behind the smell we know so well. © Things We Don't Know.


The proof is in the pudding…



Strawberry tart: may also be made with raspberries. © Things We Don't Know.

Although our shop-bought strawberries are grown, rather than made in a lab, they never seem to taste as good as the screwed up, ugly-looking wild ones. This is because their chemical profile is much less complex: bigness, prettiness, shelf-life and disease-resistance were selected over flavour, resulting in rampant genetic loss of flavour molecules.

Not that domestic strawberries were selectively bred, as such: they were the result of accidental cross-pollination when hardy Virginian strawberries met large Chilean strawberries in France. The resultant “garden strawberry” rapidly became popular, and was thereafter deliberately bred, surpassing the popularity of the wild musk and wood strawberries, even though they didn’t smell as good.

Now scientists think that understanding wild strawberries better – and identifying what makes them so mouthwatering – could provide us the key for selectively breeding greater tastiness into our domestic varieties. The diversity of flavours in the wild strawberry probably evolved as the strawberry spread to different environments, where its survival depended on appealing to different animals that would feed on its fruit and disperse its seeds.

Researchers estimate that 20 to 30 of the 350 volatile strawberry molecules matter when it comes to human appreciation of the flavour. The others may be important for attracting other strawberry-loving animals (which means, theoretically, we might be able to breed a strawberry that drives us wild, but that birds turn their beaks up at), or they may help preserve the strawberry, or ripen it.

The Molecular Strawberry


To investigate these molecules, the odour molecules in wild strawberries like the musk and wood strawberry were isolated, identified and tested[1]. Although it was confirmed that no one molecule coded for strawberry recognition, six key ones were pinpointed[2]: cis-hexanal, which alone smells like green grass or unripe strawberries; ethyl butanoate, a “pineapple ester”; methyl butanoate, which also smells like pineapples, and is used to make pineapple and orange flavoured liqueurs; methyl isobutyrate, a fruity, rum-like compound; furaneol, another pineapplely molecule, known as strawberry furanone and used for most artificial strawberry scents; and finally diacetyl, a molecule that is produced naturally in wines during a secondary fermentation processing known as malolactic fermentation. During malolactic fermentation, malic acid is turned to lactic acid, producing small amounts of diacetyl as a byproduct. However, our human noses are so incredibly sensitive to diacetyl, we detect it even in only trace amounts, as a buttery flavour in champagnes or strawberries.

Six key molecules identified in the odour of wild strawberries. Alone, most of them smell like pineapple. © Things We Don't Know.

Other strawberry molecules include methyl anthranilate, a fluorescent molecule with musky berry smell, and methyl cinnamate, a strawberry-cinnammon-like chemical.

These top flavour molecules were identified painstakingly by researchers, who isolated 12, and made up various combinations of 11 (along with pectin, sugar and acids, in roughly the quantities found in fresh strawberry juice), testing them on volunteers to see if thought they were smelling the real thing. The six core molecules always had to be present in roughly the right proportions, otherwise the people decided they were actually smelling peaches or pineapple[2].

So How does Pineapple + Pineapple = Strawberry?


None of these molecules smell like strawberry alone: strawberry flavour can only be made by combining them. This is known as synergistic: our brain codes the mixture of molecules not as a mixture, but as a single, new odour, much the way that mixing paint together produces a new colour. For example, a binary mixture of just ethyl butanoate and diacetyl makes a butterscotch smell. Our brain interprets “new” smells from combinations in a very similar way to how it registers one smell overpowering another.

Even if scientists measure the chemical composition of a strawberry and combine all the chemicals in these amounts, the smell may not be quite like the real strawberry. This is because the flesh of the strawberry affects the volatility of the compounds, and so how quickly and strongly they reach our noses. To mix “strawberry flavour” in different foods – bread, for example – we would need different amounts of our strawberry molecules. This is why scientists are so keen to simplify a strawberry down to as few molecules as possible.

Musk strawberry, hanging out. Dendrofil via Wikipedia Commons.

What is a Strawberry?


The unpindownable chemical nature of the strawberry suggests that its output – smell – must also be difficult to pin down. Obviously the participants in the study discussed above had to do just that – distinguish the “strawberry threshold”, the chemical line beyond which what follows is no longer a strawberry – but how can we be sure this is not subjective? Perhaps it depends on how many strawberries you ate as a child, whether you have ever eaten wild ones, or whether you like pineapple. The borderline between a “bad strawberry” and “less strawberry” may not be a solid one, not least because of the variation between strawberries, mostly because of their harvest and growing conditions.

The smell of a strawberry is stronger when the hormone auxin reaches its peak concentration because, at this point, the cell walls of the strawberry break down, releasing the aroma. Auxin also controls the sugar and acid contents, decreasing the acid to make it less tart and increasing the sugar up to as much as 9% of the strawberry, so the ripe strawberry is irresistibly sweet.

At least, for the run-of-the-mill garden strawberry. But then there are the odd ones. Like bubbleberries. A novel item on the Waitrose shelves, “bubbleberries” are marketed by Waitrose as tasting like bubblegum and looking like miniature strawberries. So does that mean they don’t taste like strawberries? – because that’s exactly what they are: musk strawberries. Although they contain odour molecules like terpenoids that don’t occur in the garden strawberry, they also contain more of the molecules that do: if you like, they are more strawberry.

To find out more about the chemistry of flavour, explore the topic or see our other blog post.

The sweet truth about strawberries. This graphic is reprinted with permission from Chemical & Engineering News. To see more of C&EN's Periodic Graphics series, visit http://cenm.ag/periodicgraphics.

For a deep dive into the language of smell, read more on our blog.
 
References
why don't all references have links?

[1] Ulrich, Detlef, and Klaus Olbricht. Diversity of volatile patterns in sixteen Fragaria vesca L. accessions in comparison to cultivars of Fragaria× ananassa. Journal of Applied Botany and Food Quality 86.1 (2013).

[2] Schieberle, P., and T. Hofmann. Evaluation of the character impact odorants in fresh strawberry juice by quantitative measurements and sensory studies on model mixtures. Journal of Agricultural and Food Chemistry 45.1 (1997): 227-232.

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