What Einstein Kept Under His Hat: Secrets of Science in the Kitchen (22 page)
Read What Einstein Kept Under His Hat: Secrets of Science in the Kitchen Online
Authors: Robert L. Wolke
So round two must go to your California friend, who may have been referring to this blackening process which, like many California customs, is not practiced anywhere else in the world. In Greece and Turkey, though, they do use a similar process to make fully ripe blackish olives dead black.
| | OSMOSIS IS A TWO-WAY STREET | |
I tried making strawberry preserves by boiling the berries first, figuring I could add the sugar later. But all I got was mush. What went wrong?
....
O
smosis went wrong. It went in the wrong direction.
Whenever two water solutions containing different amounts of sugar (for example) are on opposite sides of a plant’s cell wall, water molecules will move spontaneously through the cell wall in the direction of the more concentrated (stronger) solution, making it less concentrated—diluting it. That’s osmosis.
When you cooked the berries in plain water without any sugar, water molecules moved into the cells, where some dissolved sugars already existed, until the cells could hold no more water and burst. Ruptured cells, having lost their crisp cellular structure, are mushy cells.
On the other hand, when you cook a fruit in water with lots of sugar—more sugar than exists inside the cells—water molecules move
out of
the cells into the external sugar solution. The cells will shrink like deflated balloons, but they won’t burst and their cell walls will still be more or less intact, retaining their toothsome texture. The berries therefore won’t be softened as much by cooking in sugared water as they would be in plain water.
Sugar also has a strengthening effect on the fruit’s cells even when they’re deflated, because it reacts with the proteins in the cell walls.
In Nature, osmosis moves water from a solution of low concentration (of sugar, salt, etc.) through a cell wall or other kind of membrane, into a solution of higher concentration on the other side, thereby diluting or watering down the more concentrated solution. (See “Osmosis” on the following page.) But food producers often want to make a solution
more
concentrated; that is, to remove water from it—the exact opposite of what osmosis would do.
To accomplish this, they reverse the osmosis process by forcing water out of the concentrated solution, through a membrane, and into a more dilute solution. The process, called reverse osmosis, can require a substantial amount of pressure—as high as 1,000 pounds per square inch—to counteract the natural
osmotic pressure
and reverse the natural direction of water flow.
For example, the watery whey from cheese making was once considered a waste product and, when discarded, an environmental pollutant. But today, through reverse osmosis, the water is removed and the protein is sold to food manufacturers as the “whey powder” or “milk protein concentrate” that you see in the lists of ingredients in processed foods.
Reverse osmosis is also used to purify water. In this case, the pure water “squeezed out” of the impure water is, of course, the desired product.
Sidebar Science:
Osmosis
IN A SOLUTION
of sugar in water, there are both sugar molecules and water molecules. If there aren’t many sugar molecules (that is, if the solution is
dilute
), the water molecules can freely bombard the walls of their container without much interference from the sugar. If those walls happen to be the walls of a plant cell, which are somewhat permeable to water, many of those water molecules will succeed in passing through to the other side. On the other hand, if a sugar solution is strong (
concentrated
), the sugar molecules will interfere severely, and not as many water molecules will succeed in penetrating the cell wall.
So if we have a dilute solution on one side of a cell wall and a concentrated solution on the other, more water molecules will be flowing from the dilute side to the concentrated side than in the opposite direction, as if there were a net amount of pressure (
osmotic pressure
) forcing them in that direction. They will continue flowing that way until the concentrated solution has been watered down (
diluted
) to the same dilution as the dilute solution.
A dilute sugar solution (left side of the water-permeable membrane) and a more concentrated sugar solution (right side of membrane). Because there are relatively more water molecules in the dilute solution (left) than in the concentrated one (right), there is a net tendency (
osmotic pressure
) for water molecules to move though the membrane from the dilute solution into the more concentrated solution (from left to right).*
* For simplicity, I have portrayed the membrane as if it has holes big enough for water molecules to fit through, but not big enough for sugar molecules to pass. In reality, the mechanisms by which animal and plant membranes selectively permit water, but not other kinds of molecules, to pass through are more complicated and in many cases not completely understood.
| | Strawberry Preserves | |
I
n much of the country, locally grown strawberries can be found in farmers’ markets for only a few weeks in late spring. Make the most of this window. Select small, firm but ripe berries in perfect condition. In this method, standing periods alternating with short cooking times yield a preserve with deep red color and fresh flavor.
In making preserves, jams, and jellies, the proportions of the three primary ingredients, fruit pectin, sugar, and acid (lemon juice), are crucial. The gel is formed by the action of the acid on the pectin, so too little pectin or acid will prevent gel formation and you’ll have syrup instead. Too little sugar will make a tough jelly, while too much sugar will make a weak one. Simply put, the ingredients must be measured carefully. That’s why in this recipe the sugar and berries are weighed, rather than measured by bulk.
I usually start this recipe in the late afternoon and finish it up the next morning. Do not double the recipe, because the longer cooking time will break down the berries’ pectin and prevent gelling.
About 2
1
/
4
pounds strawberries
2 pounds (5 cups) sugar
1
/
4
cup freshly squeezed lemon juice
1.
Wash and hull the strawberries. Keep smaller berries whole, but slice larger ones in half.
2.
Weigh out 2 pounds strawberries and place them in a large stainless-steel saucepan or heavy preserving kettle. (I use an enameled cast-iron Le Creuset Dutch oven.) Add the sugar and, using a rubber spatula, gently mix it with the strawberries. Let stand for 4 hours, stirring occasionally.
3.
Place over medium heat, bring to a boil, add the lemon juice, and cook rapidly for 12 minutes. Cover and let stand in a cool place overnight.
4.
In the morning, bring the berry mixture to a boil over high heat, then turn down the heat to low. Remove the berries with a slotted spoon, draining them thoroughly, and spoon them into sterilized half-pint jelly jars, filling them only halfway. Thorough draining is important, because too much liquid in the jars at this stage will thin the jelly.
5.
Bring the syrup remaining in the pan to a boil and cook until thickened or until it registers 224°F (107°C) on a candy thermometer. To test if the syrup is ready, dip a soup spoon into the syrup and hold it horizontally over the pan; the syrup should fall from the spoon’s surface in “sheets.”
6.
Pour the hot syrup over the berries to fill within 1/2 inch of the rim. Wipe the rims clean and seal with rubber-rimmed self-sealing tops and metal ring bands. As each jar is filled and sealed, turn it upside down. Allow it to cool in that position.
MAKES SIX HALF-PINTS
| | BOBBING FOR PUMPKINS | |
I’m planning a party on Halloween at which there will be bobbing for apples. Must I buy a certain kind, or do all apples float?
....
A
lthough most apples will float, they can vary somewhat in their aquatic stabilities. Buy samples of a few varieties several days before the party and test them. Then go back to the store and load up on the best floaters. Asking your guests to bob for apples that sink to the bottom would severely compromise your reputation as a caring host.
Th ere is no easy way to predict whether an object will float or sink in water. You just have to try it. On
The Late Show with David Letterman
, a pair of attractive models drop various objects into a tank of water after Dave and sidekick Paul Shaffer have speculated about whether they will sink or float. Inspired by your question, but unfortunately unable to recruit the models, I decided to play “Will It Float?” on my own.
I went to the supermarket and, to the dismay of the cashier, purchased one each of a variety of fruits and vegetables. (“What’s this?” she often inquired. I answered “Rutabaga” each time, and she was apparently satisfied.) Back home, I filled the kitchen sink with water and, humming my own rendition of Shaffer’s fanfare, dropped them one by one into the water and recorded the results in my laboratory notebook.
Here, then, revealed for the first time in the annals of gastronomic science, are the results of my research. Floaters: apple, banana, lemon, onion, orange, parsnip, Bartlett pear, pomegranate, rutabaga (barely), sweet potato (barely), zucchini. Sinkers: avocado (barely), mango, Bosc pear (barely), potato, cherry tomato.
Almost all of my experimental subjects had difficulty making up their minds as to whether they wanted to sink or float. That’s understandable, because they are all made mostly of water. According to the USDA’s National Nutrient Database for Standard Reference (2003), the edible portions—the flesh—of my subjects range from 73 percent to 95 percent water. They would therefore tend to stay pretty much suspended. In fact, as indicated above, several of them just barely floated or sank.
Note that the USDA’s figures are averages, and my supermarket samples were random individuals. Different varieties and samples of apples and (as I found out) pears may give different results. All in all, though, the odds are good that you’ll find floating apples to bob for.
All of this made me think about the role of density in cooking. Density is a measure of how heavy a substance is for its bulk or volume. It can be expressed as a number of pounds per cubic foot.
Do you remember Archimedes, who jumped out of his bathtub and ran naked and dripping through the streets of Syracuse shouting
“Eureka!”
(which is Greek for “Who stole my towel”)? Well, Archimedes discovered the principle that governs whether an object will sink or float in a fluid.
Once, when he was a little kid in school, Archimedes’ principal . . . no, let’s start over.
Archimedes’ Principle states that “a body immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced.” That statement may be the way we “learned” it in school, but it’s about as illuminating as a firefly wearing an overcoat. How many of us (including our teachers) really understood it? I confess that I never did until I figured it out for myself while fully clothed and dry. Here it is, in a one-paragraph nutshell.
Let’s say we’re bobbing for pumpkins. We’ll completely submerge a 15-inch-diameter pumpkin, which has a volume of one cubic foot, into a big tub of water. One cubic foot of water now has to get out of the way to make room for the pumpkin. That displaced water is necessarily pushed upward—there’s no place else for it to go—so the water level rises. But the water now has a pumpkin-sized hollow in it, and the displaced water wants to flow back down, as is its gravitational habit, and fill it. The only way it can do that is to push the pumpkin back up out of the hollow with whatever force or weight it can muster; for a cubic foot of water, that amount of weight is about 60 pounds. If the one-cubic-foot pumpkin should happen to weigh, say, only 50 pounds, it will be pushed up (buoyed up) by that extra 10 pounds of force from the water. That is, it will float. If that one-cubic-foot pumpkin should happen to weigh 70 pounds, however, it would overcome the 60 pounds of buoyancy and sink.
Conclusion: If an object’s density is less than that of water (which is actually 62.4 pounds per cubic foot), it will float; if its density is greater than that of water, it will sink. (In reality, a 15-inch-diameter pumpkin weighs about 40 pounds and would float.)
What does that mean to us in the kitchen? Here are a couple of examples.
Gnocchi, ravioli, and pierogi will sink at first when you put them into boiling water because they are more dense than the water. But as their starch granules swell in the hot water, their density decreases until they are less dense than water, whereupon they inform you that they are cooked by floating to the surface.
