Fluorescence from aging bananas
In October of 2009, Rich Blatchly and I offered a workshop for 24 middle and high school teachers from western Massachusetts as part of the STEM program at UMass. The workshop was entitled "Illuminating Life" and sought to use types of bio and chemluminescence to teach aspects of photophysical processes. After an 1.5 hour lecture about fluorescence and phosphorescence, we used a recent paper by Columbia University's Nick Turro as a "prompt" and gave the teachers the challenge to come up some experiments that they might be able to bring back to the classroom. The paper from PNAS is entitled "Fluorescent chlorophyl catabolites in bananas light up blue halos of cell death."
We provided the teachers with a copy of the paper from PNAS and a challenge assignment to explore this phenomenon and conceive a lab that they could bring back to their own schools and children. We also provided two dozen bananas of various "ripenesses", about a dozen apples, household solvents (rubbing alcohol, witch hazel, ammonia, bleach, vinegar, and Iron-Out -a chelating agent to get rid of hard water precipitates), cutting and grinding kitchen tools, canning jars and measuring cups so that they could pour out the solvents. Appropriate warnings were issued about not mixing solventsa. The teachers were also provided them with hand held battery operated UV lights that can be purchased for about $10.00 and are used by stores to check counterfeit money. I also had some stronger lab UV lights and gave them the cautions about using them. They were instructed how to use the lamps safely.
First, the class spent about 15 minutes observing the bananas under UV irradiation (as well as skin spots, old scars, and diamond rings) noting that bruised bananas did not fluoresce the way that the skin around the naturally formed brown spots fluoresce. Our apples had a very weak surface fluorescence which appeared to wash away (pesticide trace?) Then most of the teachers took up the implicit invitation to try to isolate the chlorophyl derived pigment (CDP). Some went right for the rubbing alcohol, given their having been prompted by the methanol isolation method in the Turro paper. The groups decided that each group would do one alcohol extraction and one other extraction. The blue fluorescence from the extracted CDP was very exciting to see in the isopropanol. Most surprising for them was the ammonia extraction, which shifted the fluorescence to the green. Most of the other solvents did not seem to extract or solubilize the CDP in our hands. We got the highest yield by dissecting out the spots and surrounding skin and then placing the spots in a small blender with solvent. Sadly, nobody did anything with the Iron-out because I wondered if it might be able to remove the Mg+2 from the chlorophyll and make it flouresce (Rich and I will do this when we have time).
That was fun enough, but we also spent some time wondering how we could take these observations and turn them into a lab appropriate for middle and high school kids. The teachers thought that their classes might be able to test the "ripening" gas produced by other fruit (such as apples) when placing objects in a bag with bananas which started out all an equal shade of yellow. Same size bags could hold one, two, or three apples and each have one banana. After a given amount of time (say one day), the bags could be opened and the bananas tested for ripeness. The appearance of the blue fluorescence would be used as an indicator of the amount of ripening experienced by the banana, and might be related to how many apples had shared the bag with it. I'm not sure that would work, but I thought you might enjoy their creativity.