Thursday, November 22, 2012

What makes Brussels sprouts bitter?

I originally looked this up in the hopes of giving people advice on how to make their Thanksgiving Brussels sprouts more palatable.  I hadn't really expected what I found.


Brussels sprouts (Creative Commons,
courtesy of Wikipedia/Eric Hunt)
The simple answer is that it's from sulfur compounds common to the broccoli family (Brassicaceae).  The highest-profile of these is sulforaphane, one of several similar chemicals that brassicas use to repel chewing insects and animals, because they taste bad.  (Go figure).  But why are sprouts sometimes bitter and sometimes not, and how can you cook them so they aren't bitter?

Plants respond to stress in various ways.  When they can't get enough water, or the weather is too hot, or they're getting chewed on by bugs or diseases, they produce chemicals to make themselves hardier and less tasty.  Brussels sprouts like lots of water and cool weather, to the point that a few light frosts make them sweeter.  (They build up sugars in the leaves to act as antifreeze when the temperature dips below freezing.) So stressed sprouts are more likely to be bitter, as are ones harvested when they're too mature.  Letting them dry out or sit too long on the produce counter doesn't help either.  If you want the best sprouts, get ones from a major producer or a local farm which are small, rock-hard, and bright green.  The sprouts-on-a-stalk aren't bad either, as they tend to be younger and the stalk keeps them from drying out quickly.  Keep them cold until you're ready to use them.

As for how to cook them, that's more complicated.  Sulforaphone is a potent anti-cancer agent, which goes beyond the usual free-radical scavenging job of antioxidants and actually has the potential to turn on tumor-suppressor genes in cells which are turning cancerous.1 So really, that bitter taste is good for you, in a much more direct way than just "eating your vegetables".  What's more, it's made by breaking apart a bigger molecule with an enzyme called myrosinase, and that doesn't happen until the plant is injured (such as by chewing).  Myrosinase, like many enzymes, is a pretty fragile protein that is damaged by heating, so the more you cook the sprouts, the less there is.  If you cook them too much, it doesn't matter how much of sulforaphone's parent chemical is in the sprouts, it will never be activated.  (Though you can introduce more myrosinase to cooked Brussels sprouts with the addition of horseradish, mustard, or something like broccoli sprouts.2)

So if you want that anticancer activity, cook them lightly, either by steaming, microwaving on low power, or frying them gently in a pan with a little oil; boiling pulls a bunch of the good compounds out into the water, and heats the enzymes too much. In addition, chewing thoroughly activates myrosinase to make sulforaphone. The catch is that you have to put up with some bitterness, which some people can taste more than others (though we haven't found a clear genetic link yet3). On the other hand, if you have really fresh, young sprouts, they have more than enough sweetness to take off that bitter edge. Kids probably still won't like them, but if you can handle a little hops in your beer or a nice dark chocolate, brussels sprouts shouldn't be any more of a stranger to your diet.

Saturday, November 17, 2012

Why do plants in shallow soil end up so short?

This was a real question I fielded last night.  My brother had observed that in areas where the soil is thin -- say, up in the mountains -- the plants there are shorter than their deep-rooted counterparts, sometimes dramatically so.  He said that it seemed to affect all sorts of plants, too, like there was some sort of "as below, so above" rule.  He wasn't sure why.


Photo courtesy of the blogging nurseryman, via Flickr.
He's quite correct; dwarfing is common in plants for various reasons, and one of the places where you really notice it is in areas with shallow topsoil.  Donner Pass in California is a good example: you can look out your car window and see old weathered pines that are only four feet tall.

Just having no room to send roots down doesn't dwarf a plant, though.  In marshy areas, plants can get by with a cupful of soil and send out abundant top growth.  Palms grow tall in lots of places, but their root balls are naturally small (look at pictures of uprooted palms in hurricane areas).  So why the difference?

It's a question of resources.  Plants all need two things: water and nutrients.  In fact, water is the carrier for those nutrients, which get dissolved and taken up into the rest of the plant.  If the soil is shallow, there is no deep reserve of water that the plant can rely on between rains; the soil starts to dry out on the surface, and it's stuck.  As the soil gets drier, it gets harder and harder to pull water out of it, and what the plant can get goes only so far.  Literally: like sucking on a straw stuck into a corked bottle, the plant can get water only so many inches above the ground before it just won't go any further.  Any growth above that wouldn't survive in dry spells.  Some plants can "suck" harder than others, or get by with less water and nutrients overall, which is why some of them end up a bit taller.

Thin soil also tends to be poor in nutrients, as there aren't many things to refresh it (like river silt) and rain easily leaches out what's already there.  Without the basic building blocks like nitrogen and magnesium, plants have to live within their means, and build small.  Add wind whipping around mountain peaks or across deserts, and there are more advantages to hunkering down.  Blowing wind evaporates water from leaves that may be hard to come by, and staying low and dense can help deflect it.

All of this together means that plants which normally grow tall and full, like pine trees, turn into wizened dwarfs where the soil is only a few inches deep.  It's not universal, either among regions or among species, but it's enough of a trend that it's hard not to notice.

Oh, and in case you were wondering, the little pots bonsai trees are kept in aren't what keep the trees short.  Proper bonsai care does involve pruning roots periodically to help them fit in the pot -- but the pruning and repotting leads to a very dense, healthy root system buried in extremely rich soil.  They are kept well-watered, and without diligent pinching up top, most of them would double in size in short order, even with such tiny pots.  Bonsai trees are like miniature athletes in the plant world: fit, vigorous, and pampered.

Welcome to Street-Clothes Science

This blog came out of the realization that people -- average, ordinary people -- don't know much about the Mysterious and Complicated Realm of Science.  It's not that science is always complex, or full of jargon, or about things that don't matter in everyday life.  It's that it's so rarely explained in a way that everyone can understand.  Basic science is something as easy as looking out the window and paying attention to what you see.  It affects every part of modern life, and we need to understand it to make good choices about our food, our lifestyles, and who and what we vote for.  This blog is an attempt to bring ordinary people and science closer together.

This is not a place for politics, or closed minds, or emotional arguments.  It is here to lay out the facts as we know them, with minimal bias.  It cannot address issues of personal health or safety, only the technical background that helps you evaluate those things for yourself, or ask educated questions of professionals.  The best opinions are based on a thorough understanding of the real world, and this blog is here to help you get that.

To that end, let's take off the lab coats, put away the degrees, and talk like regular people would over lunch.

About Alison
I'm a plant person, first and foremost.  I have a Bachelor's degree in plant biology and a Master's degree in plant pathology, both from the University of California at Davis; I'm working as a professional gardener and consultant right now.  That wasn't my first career choice, however, and the string of colleges I attended gave me a wide array of academic disciplines and viewpoints.  (My first major was in quantum physics.)  I picked up chemistry, biochemistry, microbiology, physiology, ecology, entomology, physics, mathematics, and a dab of geology, along with things like women's studies, Russian folklore, and the history of Cuba.  That's just on the academic end; I've dabbled in electronics, computers, carpentry, cooking, bicycle repair, machine shop, writing fiction, graphic design, and various crafts.

What this sort of background has given me is a sense that lots of things interconnect.  The more of those things you grasp, the easier it is to understand why something works the way it does.  Complex systems aren't that complex if you break them down enough, and show how the pieces are like the pieces of other things we know.  I've been told I'm good at that.

A memory I think about when explaining things happened when I was a freshman physics major.  I was sitting in my first physics course, in my first term, when the professor wrote a formula on the board.  I had just started calculus, so when he started with a swooping curve, I was a bit lost.  He paused, as if he could pick my confusion out of the whole lecture hall, and said, "I realize not everyone here has had calculus yet; this is an integral.  Think of it as a big S, which stands for sum -- it means you take all these little bits and add them together to get the total."  And with that, he covered two weeks of calculus lecture in less than a minute.  When the much more technical explanation came along in calculus the next semester, I clung to that one-sentence description as a roadmap to what was really going on.  That taught me the power of simple, plain-English explanations.  It's the sort of thing I strive to do.

Onward
I welcome your questions, as it's easiest to explain when answering a specific question. In the meantime, I'll try to touch on a few topics that have been in the news, or which have come up in conversation.  If something doesn't make sense, please tell me -- I want these posts to be easy to understand.

Thanks for reading!