The science journal Angewandte Chemie (“Applied Chemistry”) is one of my favorite chemistry magazines. The news is timely and the articles are well-written. Angewandte is one of the top European journals, drawing some of the most prestigious authors and scientists to contribute. Occasionally, an article comes along which is a wonderful discovery but which makes me wonder why it took until the 21st century for scientists to stumble across it. Such was the case with a recent article from some Austrian chemists which details a curious phenomenon in the skins of some yellow fruits.
The scientists were analyzing the skins of fruits to determine how the chemical composition of the skins changed during the ripening and storage process. Trying to monitor a crop so that the fruit is picked at precisely the best moment, and monitoring the health of the fruit from that point onwards so that overripe fruit is not sold to customers are both important tasks for a grocer. Theoretically, if food scientists developed an accurate chemical analysis for the compounds present in fruit skin when the fruit was at its peak ripeness, they would be able to time harvests more accurately and would spend less money disposing of spoiled produce. With that in mind, the scientists from Austria took fruit skins and ground them into a pulp, extracting the pulp and analyzing it using a high-pressure liquid chromatograph (HPLC). I’ve written about HPLCs before here on Associated Content, and how they function; they enable chemists to separate complex mixtures of molecules and to analyze each individual chemical component by itself.
When the results came back, the chemists noticed that the molecular components present at the peak ripeness of the yellow fruit gave a huge response in the ultraviolet detector of the HPLC instrument. This means that whatever chemicals were present were great chromophores, or absorbers for light energy. The researchers theorized that this meant the compounds were somehow related to chlorophyll, which is one of the best chromophores present in unripened (green) fruit. After all, the whole purpose of chlorophyll is to absorb light. Careful analysis of the precise components showed the molecules present in the ripe fruit to be catabolites of chlorophyll – remnants of chlorophyll that had begun to break down as a result of the ripening process. Even more importantly, these catabolites were fluorescent. They glowed a light blue color upon irradiation with ultraviolet light.
Fluorescence is a phenomenon that occurs when a molecule absorbs a particle of light (as in the case of chlorophyll), but has no real need for the extra energy it takes in (as in a “ruined” piece of chlorophyll – a catabolite). To get rid of the extra energy, the molecule reemits light. The light it reemits is not the same color, or wavelength, as the light which it absorbs; the color is shifted depending on the chemical structure of the molecule. In the case of these fruit skin chemicals, ultraviolet light was absorbed and then reemited as blue light.
The benefits of this research become apparent when you consider that this blue color reaches its maximum intensity when the fruit is at its peak ripeness. Monitoring intensity of light is an easy task; scientists have all manner of rugged, hand-held instruments that can accomplish this task. This technology is therefore immediately applicable to the problem of monitoring fruit during storage. Simply by shining a black light on the fruit and analyzing the amount of blue light which is given off by the surface of the fruit, scientists can determine if the fruit still needs a few days of sitting around to reach optimum ripeness, or if waiting any longer to sell the fruit will hurt the quality. Fruit that is spoiled will not emit the blue light at all, and neither will fruit that is “green” – green, ironically, because that is the color of chlorophyll which hasn’t begun to break down as yet.
This work is a great example of how some of chemistrys most complex principles – fluorescence, biochemistry, light harvesting – can be transformed into a simple, hand-held package that anyone can use. It takes a little bit of innovation at the front end of things, but after that, the “fruits” of the discovery can be enjoyed by all.
The source of this article can be found at: http://www3.interscience.wiley.com/journal/121448271/abstract