Monday, January 11, 2016

Trimming Root Vegetables--Do We Really Have To?

We made a big pan of roasted root veggies last night.  Potatoes, carrots, and onions.  Yum.  In the middle of judiciously cutting off all the green parts of the skins and all the eyes out of the potatoes, we started to wonder how much of this tedious prep work was really necessary.  Everyone knows that green potatoes will murder you in your sleep if you eat them, right?  And green-shouldered carrots are probably just as bad?  Green-shouldered onions will probably make you cry while they do it.

But what if these silent killers were just getting a bad rap?  As it turns out, some are, some, maybe not.  Read on--the Homestead Laboratory investigates!

solanine and chalconine
For potatoes, the green color comes from chlorophyll, but these guys (the glycoalkaloids solanine and chalconine) are the toxic part.  Plants in the nightshade family use them as a defense mechanism, and they ramp up production in response to stress or light.  Also in response to light, they ramp up chlorophyll production, which is why the green color is associated with the toxicity.  Strictly speaking, however, the green color is not required for high glycoalkaloid content. The glycoalkaloids are also bitter, though, so we can still tell when there is a lot of them without having to have any analytical equipment fancier than a tongue.

Green potatoes on cutting board
Next, is there enough of the glycoalkaloids to actually do any damage?  The dose commonly cited to cause toxic effects for solanine is 2 mg/kg body weight, or 140 mg for a 70 kg person.  The half life in the body for humans is 1-2 months, which works out to a total steady-state body burden of 50 mg if the intake of solanine is 1 mg/day.  How many potatoes would you have to eat to take in 1 mg/day of solanine?  Normal solanine contents of potato tubers are about 7.5 mg/100 g fresh weight (varying widely across samples).  The green parts can have more than 200 mg solanine/100 g fresh weight.  For reference, the four potatoes in the picture cumulatively weigh 211 g, and the stripes on the cutting board are 1" wide.  So, it wouldn't take much if you ate taters every day.    Looks like we'll definitely continue to trim the green parts, and probably space out our potato eating a little more, too. 

On the other hand, most researchers seem to consider the eyes as part of the tuber, so if they aren't sprouting, it seems there's no need for us to worry about additional solanine coming from the untrimmed eyes.  Looks like we can save ourselves a lot of time on eye-trimming.  However, if the eyes are sprouting at all, the solanine content in the tuber can go up, down, or stay the same, depending on the variety.  One thing is clear, though--the sprouts have the most solanine of all, so we'll be staying away from them!  In that case, we'll definitely trim more liberally, and probably make a bit of effort to ease off on our potato intake for a while once that dish is gone.

Also, some solace for french fry and potato chip lovers--the frying process doubles as a high-temperature extraction (reducing the content of the solanine in the fries) because the glycoalkaloids are somewhat oil-soluble.  Probably still would be good to not eat the green ones.

Carrot with greenshoulder
Carrots get some green on their shoulders from the sunlight, too, but aren't in the nightshade family.  So, while the green color still comes from chlorophyll, the glycoalkaloids don't come along with it.  The green color does still bring some bitterness, but the molecules that cause it have not been identified (or at least, they hadn't as of 2007, and we couldn't find any more recent papers).  It's known that some types of molecules, including terpenoids, can result in a bitter flavor, even in non-greened carrots , but apparently in the green parts, these are not the terpenoids we're looking for.  One would think they'd also have looked for glycoalkaloids.  But the whole carrot plant, including the top, is edible, so the green parts of the carrot roots might be unpalatable, but not likely dangerous.  Similarly for onions, the greening that happens on the shoulders of the bulb when exposed to sunlight is due to chlorophyll, but not likely dangerous.  It might even be beneficial due to concomitant flavonoid formation!  Maybe the carrots are also making flavonoids.

Of course, if the green parts of the carrots and onions end up being too bitter for us, chicken taste buds might handle them better.  Guten apetit!

What do you do with the green parts of your root veggies?


  1. That's a big carrot! Thanks for breaking it all down for us! I always wondered about the green parts.

    1. We were wondering, too. And of course, I wanted to throw out as little as possible, even if it was going to the chickens... :-)

  2. except potatoes, we eat the green parts.

    1. Good to hear! Hopefully we'll both be eating green-shouldered vegetables for many years to come!