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Strawberry’s Properties are More than Skin Deep

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Everyone loves a perfectly ripe strawberry, right? They count towards your five a day and contain important nutrients, vitamins and antioxidants. But what if science could help breeders make them even better?

Balancing sweetness, imparted by sugars such as glucose, fructose and sucrose, with sour characteristics from citric and malic acids is vital for the optimal consumer experience. They also say that we eat with our eyes and as such appearance is an equally important consideration when striving to achieve the best strawberry. Strawberries owe their characteristic red color primarily to derivatives of two compounds, pelargonidin and cyanidin, part of the anthocyanins family of compounds, which are also credited with strawberry’s proposed antioxidant and anti-inflammatory properties. But if we look more closely, there are gaps in our knowledge about the distribution of these important compounds within the fruit.

To address this shortfall, a multicentered team from Teikyo University and Kagoshima University in Japan set out to investigate the levels and distribution of compounds important for the berry’s smell, appearance, flavor and health benefits within the fruit.

The researchers used an innovative method, called imaging mass spectrometry (IMS), which is emerging as a useful tool for investigating not only the content and quantity of compounds within a sample but also their distribution within the test object – in this case strawberry slices.

By combining this technique with the more traditional matrix-assisted laser desorption/ionization mass spectrometry and tandem mass spectrometry (MALDI MS and MALDI MS/MS), which they used to identify the presence and quantities of metabolites in strawberry extract, the scientists were able to obtain a fuller picture of how much of what was where.

As you can imagine, obtaining stable slices of a delicate, watery tissue, like strawberry, able to withstand IMS can be challenging. However, the group found that using liquid nitrogen to flash freeze the slices following emersion in 2% carboxymethylcellulose (CMC) gave good stability. They also found that applying the matrix required for analysis to the strawberry slices using an automatic sprayer gave better results, in terms of the number of metabolites detected and their signal strength, than an airbrush. This has been credited to the more uniform spray delivered by the automatic sprayer and increased temperature at which it delivers the matrix.

The analyses revealed that interestingly, whilst hexose sugars (including glucose and fructose) were distributed throughout the strawberry, sucrose was concentrated near the tip. This accounts for the characteristic sweetness gradient seen from the top to bottom of the Tochiotome strawberry used for the study.

The distribution of the anthocyanin compounds, which range in characteristic color from orange to magenta and violet, was similar to findings in other berries and focused predominantly in the skin, with the corresponding quantities blending to give the distinctive red hew.

Understanding how gene expression patterns and environmental conditions produce these distributions in strawberries could help increase the beneficial properties of the fruit. It is hoped that breeders will be able to use this information to improve berries in the future and the work also demonstrates the power and versatility of IMS.