What Einstein Told His Cook (33 page)

You may have noticed that knives kept on a magnetic rack do become magnetized. (Try picking up paper clips with them.) And according to Professor Bob O. Handley of MIT’s Materials Science and Engineering Department, a magnetized piece of steel will be somewhat stiffer than when it is not magnetized. It can probably then be sharpened to a keener edge and may stay sharp longer in use.

But don’t count on it. Knife blades are made of several different steel alloys, and some of them may not retain their magnetism very long. The stiffening effect isn’t likely to be very large in any case.

On the other hand, the careless use of a magnetic rack can indeed damage your knives if you whack or drag the cutting edges against the magnet bar when removing them or putting them back. That may be how the story got started that magnetic racks can dull the edges.

If you are worried about nicking your knife blades through hasty grabbing from a magnetic rack, you might prefer to keep them in a countertop wooden rack. Some people think that’s really the best way. But who, besides Martha Stewart and the recipients of wedding gifts, owns a set of perfectly graduated knives, all tucked away in their custom-fitted, wooden slots? The downsides are that the slots are hard to clean and it’s not easy to tell from the protruding handles which knife you’re grabbing. With a magnetic rack on the wall, you can always select the right one for the job.

As every culinary textbook warns, a sharp knife is a safe knife; it won’t slip off the food onto a finger. There are many good electric and manual knife sharpeners on the market, so the time-honored and time-consuming method of honing them on a stone is no longer necessary.

But a word of warning: Those brute-force sharpeners containing two interleaved sets of disks through which you draw the knife blade scrape off wholesale slivers of metal that will stick to the blade if it’s magnetized. (These sharpeners are not recommended unless you like knives that get progressively skinnier.) Metal slivers are not nice to eat, however, so after you use such a sharpener wipe the knife carefully with a wet paper towel. That’s a good idea if you keep your knives on a magnetic rack no matter what kind of sharpener you use, because abraded metal particles can be invisibly small.

GIVING A BRUSH THE BRUSH-OFF

I can’t seem to keep my pastry brushes clean or undamaged. I must have bought ten new ones in the past year. Any suggestions?

Y
es. Wash them properly and don’t use them for unintended purposes.

Two pastry brushes (top) and a basting brush (bottom).

After being used to brush on an egg wash or melted butter, a pastry brush becomes gummy and rancid unless you wash it thoroughly before putting it away. Wet it with hot water and work up a lather by swishing it around on a cake of soap, as if lathering up a shaving brush. Then work the lather well into the bristles against the palm of your hand. Or else plunge it up and down in a container of hot water and dishwashing liquid. In either case, rinse it well with hot water and air-dry it thoroughly before putting it away in the drawer.

Regarding the damage: Don’t confuse a pastry brush with a basting brush, as several articles in popular food magazines have done. They’re two separate tools, designed to do different jobs.

Pastry brushes aren’t made to withstand heat; their soft, natural boar bristles can melt if used to apply oil or sauce to hot foods in the oven or on the grill. The longer-handled basting brushes, on the other hand, with their stiffer, synthetic bristles, can take the heat without melting.

Just as a pastry brush shouldn’t be used for basting, a basting brush is too stiff for use on delicate pastry.

JIFFY LUBE

To cut down on my use of fat, I put some oil in a spray bottle, but all it did was shoot out a heavy, calorie-laden stream. Is there a better way to make my own “cooking spray”?

Y
es, there is a better way.

Ordinary plastic spray bottles are made to spray watery liquids, not oily ones. Water is thinner (less viscous) than oil and breaks up easily into a mist, but the paltry pressure from a trigger pump isn’t enough to break oil down into microscopic droplets, the way a pressurized aerosol can can.

Cookware stores and catalogs sell olive oil sprayers that are great for oiling frying pans and grill pans, “greasing” baking pans, making garlic bread, spraying salad greens, and many other uses. You put the oil in them and pressurize them by pumping the cap. The oil then sprays out in a fine mist at the touch of a button, just as if from an aerosol can.

A JUICY STORY

I often make lemon curd as a filling for small tarts, and of course I always use freshly squeezed lemon juice. But it seems that I waste a lot of juice by not getting it all out. Is there some way to get the maximum amount of juice out of a lemon or lime?

Y
ou’ll read in some food books and magazines that you should roll it firmly on the counter. Others recommend microwaving it for a minute or so. These actions sound perfectly reasonable, but I have always wondered whether they really work.

I had a chance to find out when my friend Jack, who loves to find bargains, discovered that a local supermarket was overstocked with limes and selling them at twenty for a dollar. With visions of endless margaritas dancing in his head, he bought forty for himself and called me to spread the news.

What an opportunity! Here was my chance to do the experiment I’d always wanted to do. But from my long experience as an academic scientist I knew that a proposal to the National Science Foundation would be unlikely to earn the necessary funding. So I dipped into my own reserves, procured four dollars’ worth of limes without any competitive bidding or even so much as a purchase order, and delivered them personally via Toyota to my lab—uh, kitchen. They were large, green, good-looking Persian limes, the most common type in American supermarkets.

I wanted to find out whether heating a lime (or lemon; the principles should be the same) in a microwave oven or rolling it on the counter before squeezing will really produce more juice. I had always been suspicious of these recommendations which, like many tenets of kitchen lore, have never (to my knowledge) been scientifically investigated. I wanted to test them with all the rigor of a controlled scientific experiment. I did that, and the results may surprise you.

Here, in the lab-notebook style that they taught me to use in high school science, is what I did.

E
XPERIMENT NO
. 1

Procedure:

I divided 40 limes into four groups. (The math was easy.) One group, I microwaved for 30 seconds in an 800-watt oven; the second, I rolled firmly on the counter beneath the palm of my hand; the third, I both rolled and microwaved; to the fourth I did nothing, as a control. I weighed each lime, gave it its treatment, if any, cut it in half, extracted the juice with an electric juicer, and measured the amount of juice obtained. I then compared the yields in milliliters of juice per gram of fruit. I’ll spare you the details of the weight, volume, and temperature measurements and the statistical analysis of the data.

Results and Discussion:

There was no detectable difference among the four groups of limes. Neither microwaving, nor rolling, nor rolling and microwaving produced any increase in the yield of juice.

Why should it, really? A fruit contains a certain amount of juice, depending on its variety, its growth conditions, and its post-harvest handling. Why on earth should anyone expect warming it or man-handling it to change that amount of juice? That’s the part of citrus folklore that never made sense to me, and I have now proved it wrong.

But of course an electric juicer extracts virtually all of the juice that the lime contains. Maybe microwaving and rolling make it easier to get the juice out. When squeezing by hand, you may therefore get more juice for the same amount of squeezing pressure.

E
XPERIMENT NO
. 2

Procedure:

I divided another two dollars’ worth of limes into four groups as before, but this time I squeezed them as hard as I could by hand. Naturally, I got less juice: on average, less than two-thirds of the machine yield. A much stronger man could undoubtedly get more. But I flatter myself that my right-hand strength probably exceeds that of the typical female cook.

Results and Discussion:

Hand-squeezing the limes as they came from the store yielded an average of 61 percent of their total juice. Microwaving yielded 65 percent, while rolling yielded 66 percent. All three of these results are the same, within experimental error. My skepticism was again justified; neither rolling nor microwaving prior to hand-squeezing significantly increases the amount of juice obtained.

But here’s the big surprise: Rolling followed by microwaving made the limes so easy to squeeze that they yielded 77 percent of their total juice, some 26 percent more than untreated limes. They practically gushed juice, and I had to cut them over the juice collector to avoid losing any.

Here’s what I concluded must be going on: Rolling breaks open some of the vacuoles—those little pillowcases full of juice in the fruit’s cells. But the juice still can’t flow out very easily because its surface tension (the “surface glue” that makes drops of liquid want to remain spherical) and its viscosity (its non-flowability) are both pretty high. But when the liquid is subsequently heated, its surface tension and viscosity drop substantially and the juice can flow out more easily, much more easily than I would have expected without looking up the actual viscosities. At the average before-and after-microwaving temperatures, it turns out that water (close enough to lime juice) flows four times as easily when it’s hot. So the rolling breaks open the floodgates and the heating allows the flood to flow more easily.

Summary

If you are using an electric or mechanical juicer, rolling and/or microwaving will accomplish nothing. The same goes for those ribbed wooden or plastic reamers and the old-fashioned ribbed-glass juicers, because they also release virtually all of the juice that the fruit contains.

But if you are hand-squeezing limes and have a microwave oven, roll them on the counter and then microwave them. Rolling alone makes them softer and they appear to be juicier, but it hardly affects the yield. Microwaving alone accomplishes little more than to make the juice uncomfortably hot: 170 to 190ºF in my experiments.

Although I didn’t test them, I would expect that the same techniques to produce similar results with lemons. I’ve told Jack to keep an eye out.

Finally, what is the maximum amount of juice you can expect to get out of a lime? Limes are particularly fickle fruits, and recipes should therefore specify a number of ounces, rather than “juice of half a lime.” The average electric-juicer yield of all my Persian limes was exactly two ounces, whereas rolling, zapping, and hand-squeezing yielded an average of 1.5 ounces. The champion in my sample contained 2.5 ounces, while two very healthy-looking specimens yielded only three-tenths of an ounce each.

As a result of my experiments, I now have enough lime juice for 130 margaritas. Just give me a little time. (If you’d like to join me, see the recipe on page 247.)