Asparagus’ Breaking Point

IT’S spring at last: prime time for a vegetable that does not go gently into our food chain, that keeps binding up its harvest wounds even as it’s shipped and stored, growing and bending upward to find the sun. A vegetable whose texture horticultural engineers test by tapping it with a tiny hammer and listening to it vibrate. Asparagus is the hardest-living stalk in the produce business, a challenge to farmers and grocers and cooks alike. The stalks quickly toughen from the bottom up, and it’s no snap to tell the tough portion from the tender.

Asparagus spears are the day-old shoots of a perennial in the lily family. In spring, the persistent root mass unlocks its stores of last summer’s sugars, and fuels the growth of shoots above ground. The shoots normally grow into stalks several feet high, stiffen themselves against the elements, and unfurl feathery branches to photosynthesize and bear small flowers and fruits.

I’ve never succeeded in growing asparagus, but those who have know that once the plant breaks the soil’s surface, it can shoot up 6 to 10 inches in a day. A single day’s growth is what’s usually harvested, still mainly by hand.

About half the world’s asparagus is harvested white, the spears protected by soil or row covers from sunlight that triggers chlorophyll production. It is somewhat bitter, sweeter and more delicately flavored, and more expensive.

Green asparagus spears, cut down just hours after they’ve hit daylight and turned color, are the most lively of all our vegetables, furiously turning the sugars absorbed from their roots into energy and new tissues.

The harvest doesn’t stop them. Even cut off from their roots, the asparagus spears keep growing at the tip. If they’re stored lying down, the tips rise away from the pull of gravity, and can bend 60 degrees or more from the stalk before they run out of energy.

This negative geotropism doesn’t harm the eating quality of asparagus, just its straightness. But the spears also lose much of their sugars, using them to toughen their wounded ends with an increasingly fibrous sheath, which lies just under the green skin and quickly becomes too stringy to chew.

Most of this loss of sweetness and toughening happens in the first day after harvest. Farmers can minimize it by chilling new-cut asparagus right away. But a delay of just four hours between harvest and chilling causes the spears to toughen significantly. So does allowing the chilled spears to warm up to 60 degrees or more in a grocery display or at the farmers’ market.

Hence the cook’s challenge. Some portion of each spear’s butt end is inedible. White ends are unfailingly tough, but green ones can be almost as bad. Peeling deeply will remove the fibrous sheath (and is essential for white asparagus), but it’s a lot of work and the spears end up oddly two-toned and two-textured. So how do you know where to trim a spear so that it won’t be unpleasantly stringy at one end?

The standard advice, going back to Fannie Farmer and beyond, is to “let the asparagus tell you” by bending the stalk until it snaps, which, according to this doctrine, is at the point where acceptable texture meets tough. This practice apparently derives from a centuries-old harvest method. Michigan asparagus is largely snap-harvested, and is said to have less white and fibrous tissue than spears cut from the roots.

I’ve been a spear-snapper too, but I’m regularly annoyed by fibrous spears among the tender ones, and I have wondered just how reliable snapping is. Over the course of a few weeks, I snapped a total of 130 spears, then steamed them and bit into the wide end. About a third were unpleasantly stringy. Some bunches from a farmers’ market on a warm day had more stringy ends than tender ones.

I got much more reliably tender results simply by cutting the spears evenly to between 6 and 7 inches from the tip. But this can leave almost half of the stalk behind. So I tried slicing all but the very bottoms into millimeter-thin rounds. Fibers cut that short are barely noticeable. The rounds are ringed in green and crunchy when raw. I munch on them while cooking and scatter the rest around the cooked spears for contrast. They also cook in seconds in a hot soup or stir-fry.

One other tip: thick, short spears give the greatest proportion of tender tissue to stringy. And apparently we barely know what thick is. According to the food scholar Alan Davidson, some 1930s French varieties produced bland spears 2 inches in diameter and more than a pound in weight. Bland or not, those sound like heirlooms worth unearthing.

May 6, 2009

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