Barbara Mitcalfe, of Kelburn, asks :-
What are the occasional, tiny, hard bits present in tamarillos eaten either raw or cooked? They do not appear to be structural, i.e. they seem to be unattached to any particular part at all, except the inside of the fleshy wall of the fruit.
Robin MacDiarmid, a plant virologist with Plant and Food Research, responded.
I had to wait for the tamarillo season to start to obtain samples. I extracted some of these bodies and we stained them with phloroglucinol to detect any lignin present.
They are full of lignin! They are therefore most likely to be composed of sclerid cells (schlerenchyma) that are most closely related to fibre cells except they are short and irregularly shaped. The function of them in this case is unknown. Often sclerids can be present for structural support (e.g. in water lilies) and sometimes for protection (e.g. macadamia nuts) but sometimes I think they are just there to taunt scientists and school students alike. Another taunting example is in pears, sclerids cause that rough texture in the flesh. Why? We don't yet know.
I attach an image of cells after the body had been stained and then sectioned. The stain has only penetrated the outside few cells (red) while the inner cells appear cream.