Reader Question: Steroidal Glycosides in Solanum Maternum Fruit

The tamarillo fruit is the fruit of a subtropical Solanum shrub

A reader contacted me via email recently posing a very intriguing question regarding the levels of steroidal glycoalkaloids present in a close relative of the tamarillo (tree tomato). I've reproduced an excerpt of his correspondence below and I ask any readers with practical experience in this area or with academic interest in characterizing the steroidal natural products in this wild fruit to get in touch with me (brian.heasley2009@gmail.com) so that I can relay the information to the relevant parties.

Hello Brian,

Thanks for your blog information in http://modernsteroid.blogspot.co.nz/2014/06/tomato-steroidal-glycoalkaloids.html

For context, I grow a wild South American Solanum species, Solanum maternum, which I doubt has ever been investigated for the glycoalkaloids present (it was discovered relatively recently).

It is the (extremely) close relative of the 'Tamarillo' ('tree tomato'), Solanum betaceum (synonym Cyphomandra betacea), a fruit in limited international commerce, and also grown here in New Zealand.

While in potatoes the 'normal' level of glycoalkaloids of 20-100 mg/kg is said to be 'not of concern' - albeit a 'safe' level has not been established - introducing genes from wild species of potato may inadvertently introduce undesirable higher levels of glycoalkaloids. In addition, new glycoalkaloids, 'untested' in human diet, may be introduced.

Solanum maternum has total resistance to the fungus disease powdery mildew; S. betaceum, the fruit of commerce, is susceptible. S. maternum is of interest for breeding for disease resistance - just as are wild potato species for the domesticate.

Fruit of S. maternum have a mix of acidity, unusual floral flavor notes - and a savage bitterness and throat-gripping 'acridity' that render it inedible. The Tamarillo has significant bitterness in the fruit wall, but not in the pulp cavity, and it is the pulp only that is eaten. Nevertheless, past consumer testing has shown some people report a 'catching' in the throat when they eat Tamarillo.

I haven't been able to find any internet information on the glycoalkaloid load in tamarillos, but assume, based on their historical wide acceptance, that it is relatively low. I assume that S. maternum has a 'high' glycoalkaloid load. There may be other chemicals present in the maternum fruit that adds to the general 'acridity' or 'heat'. But it has never been investigated...Checking the internet (I have no background in chemistry), I see there are some simple reagents which give a color precipitate in the presence of glycoalkaloids. An industrial chemist might make one or more for me, but these do not seem to be able to give a quantitative indication of levels, which is really what I am after. This is a faint hope - but I have to ask - do you know of any newly invented 'field kit' for a rough quantitative indication of glycoalkaloid levels? Alternatively, do you think there would be sufficient academic interest in characterizing the steroidal complexes in this wild fruit (ideally, from my point of view, in comparison with the domesticate, and also in comparison with fruit from a cross of the two species)?