I cut an underripe avocado in half and put half of it, without the pit but with the peel, in the refrigerator. A day later, I see very small black bumps along the cut surface. What are they? What causes them?
It looks to me (although I'd want to use a microscope to check) like the black dots are xylem. When you cut the fruit, you've severed the xylem and also exposed the flat surface. Three main things have then happened:
- The increased surface area has led to the 'fleshy' part of the fruit contracting as the cells dehydrate.
- The stiffer, lignified xylem tubes have retained their structure and have not contracted with the rest of the fruit, so they've become exposed.
- The exposed xylem have blackened due to oxidation - this eventually happens to all the flesh but, again speculating here, the oil in the fleshy tissue might protect it from oxidation longer than the exposed xylem which don't have the oil coating.