You’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.
EXPERIMENT NO. 1
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.
EXPERIMENT NO. 2
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.
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.
We can only assume that our lime-juicing technique works just as well on lemons, since we haven’t received any word from Jack on lemon bargains. It’s well worth the modest effort of squeezing them to make this delicious spread for toast or biscuits.
It also makes a great tart or cake filling and is wonderful in a jelly roll. It will keep for months in the refrigerator.