A big corny-worny thing

Many years ago I found myself in Sergi Farm, the last working farm in the formerly agricultural Boston suburb of Belmont, the town where I grew up. I happened to have a rare opportunity that most people probably will never get a chance to have -- the chance to try a fresh ear of sweet corn right off the stalk, out in the field. There's really nothing quite like it -- it's not soggy from boiling, it's just sweet and crisp and rather creamy. If you ever do get the chance, don't say no.

Sweet corn is a rite of summer for most Americans -- unlike the starchy dent corn that makes up the backbone of American agriculture or the dense, hard kernels of popcorn that are a staple of entertainment venues everywhere, sweet corn at its best is essentially candy thinly disguised as a vegetable. And although it can be had year round, it's only at its best either frozen (if you've got a Trader Joe's nearby, their supersweet corn from Oregon is beyond awesome) or dead fresh, picked in the summer and rushed to the market no later than the next day. Sweet corn is an institution in American cookery, from the very earliest pre-Columbian days when some Indian farmer discovered an odd mutant ear with sweet, moist kernels in the cornfield. Which is what makes the odd genetics of sweet corn very interesting.

The story of sweet corn breeding is the story of several rather strange mutations. The most significant is in a gene known conventionally as su (for "sugary"); the standard su gene controls the process of polymerizing sugar to starch, and the mutant su gene that makes sweet corn possible changes the enzyme that su coded for to make it less efficient, giving the harvester about half a day's window (and sometimes a lot less) to either chill or cook the corn before it begins to turn starchy. This is where traditional sweet corn varieties hit the wall -- while they're fine for immediate consumption or processing, they don't really keep very well.

Although heirloom vegetables are very common these days, there's very little heirloom sweet corn out there because of the limitations of su corn. The seeds exist -- Seeds of Change, among other seed producers dedicated to organic and heirloom vegetables, will sell them to you -- but the old cliche that you walk out to the cornfield but run back was created precisely for these old varieties. Since more stable sugar meant more sales for the farmer and less waste for the greengrocer, the late 19th and early 20th centuries saw the development of what would later be called se corn, named for a gene known as sugary enhanced that actually increased the proportion of sugar to starch at harvest, sometimes fourfold. While the se mutants did little to slow down the enzyme action of the mutant su gene, the sheer sugar overload led to corn that could store at room temperature for up to a week without going starchy. se varieties are fairly widely available for home gardening purposes and probably make up the bulk of what you'll find in more mainstream seed catalogs like Burpee and Thompson and Morgan; they don't have the same sort of cachet as the old heirloom varieties, but they do a pretty good job of balancing keeping and flavor. But sometimes... sometimes that just isn't enough. Enter sh2.

The shrunken-2 gene produces some odd results -- it practically shuts down the starch-conversion enzymes, as well as causing the kernels to shrivel up like raisins as they dry (if you've seen wrinkled sweet corn seeds in garden shops, that's what's going on there -- normal dent corn simply contracts on the top, creating the characteristic "dent" that it's named for). The modern supersweet corns, first developed in the 1950s but popularized in the 1980s, contain honking huge doses of sugar even compared with the se varieties, and their enzyme activity slows almost to a standstill for a period of weeks. Though initially unsuccessful because of the expense of the seeds (most of which are hybrids rather than open-pollinated), when sh2 varieties took hold, they became ragingly popular. It would seem, then, that corn producers have found a best-of-both worlds variety -- like I said at the very beginning of the article, candy thinly disguised as a vegetable. What could possibly go wrong?

Well, the first is an accident of genetics. sh2 is a recessive gene, and corn is wind-pollinated, with the pollen being released in rather vast quantity. As a result, in order to produce properly sh2 varieties have to be quarantined from other varieties in order to allow the gene to be expressed; the supersweets must either be grown chronologically out of phase with other corn crops or physically isolated. The former requires a fairly long growing season, which is not available to all the corn-growing areas that might care to grow supersweet corn. The latter is more complicated -- a farmer must have either very large fields (as well as, presumably, something to plant between the different corn crops) or a way to build a physical barrier (sunflowers, which are usually taller than corn plants, can sometimes suffice). The plants are also less hardy overall, being more sensitive to cold. But there's even another solution. I subscribe to the annual catalog from Vesey's Seeds, a seed company based in Prince Edward Island that specializes in short-season seeds (a dire necessity in Canadian agriculture). Vesey's did not until very recently carry sh2 corn, but they are a huge supplier of an entirely different class that they refer to as sy or "synergistic". sy corn is an interesting genetic form in and of itself; I don't think the term refers to a specific gene as such, but it essentially refers to the fact that sy varieties combine sh2 and se kernels on the same cob. They're hardier plants as well, taking better to short seasons and cold soils, and they aren't hamstrung by the quarantine requirement. While not by nature quite as sweet as straight sh2 corn, sy corn is a perfect example of creating a workaround in adverse conditions by taking advantage of the oddities of mutation.

So that's the buck-fifty tour of sweet corn genetics; there's a lot more, especially as the corn genome was completely sequenced earlier this year, a huge step forward in keeping on top of one of the world's most important agricultural crops. (I'm sure some of the Tangled Bank readers are looking at me like "did this silly little food writer wannabe just try to explain corn 101 to a bunch of biology PhDs?") But just for fun, think about this: the supersweet thing doesn't just apply to corn. If you've ever wondered why you can't buy fresh shell peas much outside farmstands, peas have many of the same starchification problems as corn. I leave you with Jung Seed's Eclipse pea, the first commercially available high-sugar pea.

Further reading

Parsons, Russ, How To Pick A Peach, Houghton-Mifflin, 2007, ISBN 9780547053806. Parsons, a food writer for the LA Times, takes his second hack at food science in a book on making the most of your produce dollar. Much of the most up-to-date information here came from his book.
Rupp, Rebecca, Blue Corn and Square Tomatoes. A fun little trip through the garden that goes into dramatic detail about the origins of our produce, including bitter Himalayan cucumbers and the weird Mesoamerican grass known as teosinte that was ancestral to modern corn. Published in the mid-80s, it's somewhat out of date but still very good for basic information.