What Einstein Kept Under His Hat: Secrets of Science in the Kitchen (29 page)

(If you ever decide to go into scallop counterfeiting, beware of the tiny, almost microscopic barbs on the skate’s skin. They don’t sting, but they prickle annoyingly. And don’t ask me how I know.)

Known as
raie
in France, the skate is indeed a kind of ray, a term that covers several families of flat, bottom-dwelling fish that, like sharks, have cartilage instead of bones. Rays (family Rajidae) range from the most-often-eaten European skate (
Raja batis
) to stingrays with poisonous tails and giant manta rays that can weigh up to 3,000 pounds. Rays’ bodies are flattened out into ribbed, fanlike “wings” that undulate gracefully for locomotion. They are all edible, but some are not exactly gourmet fare.

Even if cookie-cutter cylinders of skate wing were to be passed off as scallops, they wouldn’t fool anyone who has ever eaten skate, sometimes sold as raja fish. The flavors and meat colors are similar, but the texture is all wrong. The skate’s toothsome, long-stranded texture is more like that of crabmeat than scallop. Be suspicious of any scallop that seems to come apart in strands or layers.

And by the way, those little hollow, rectangular, leathery black “mermaid’s purses” that you see washed up on beaches or tangled in seaweed are the egg cases of skates, originally containing two large eggs and abandoned after the young ’uns have hatched.

My aunt came back from Sicily and brought me some bottarga. I know it’s bottarga because it says so in big letters on the package, but the rest is in Italian. (I didn’t want to sound ungrateful by asking, “What am I supposed to do with this?”) I know it’s some kind of fish eggs, but it’s almost as hard as a rock. What is it, and what can I do with it?

....

I
could tell you it’s rockfish roe, but I won’t.

Bottarga is dried, salted roe from either the Mediterranean tuna (
tonno
in Italian) or the gray mullet (
mugine
).
Bottarga di tonno
(also known as
uovo di tonno,
or tuna eggs) and
bottarga di mugine
are local specialties of Sicily and Sardinia, Italy’s two large Mediterranean islands, and are valued as delicacies in the rest of Italy.

The roe sac is removed as soon as the female fish is caught. It is then washed; salted; pressed, traditionally between wooden planks or marble slabs; and dried, traditionally in the sun, for one or two months. It comes out looking like dark amber wooden boards, firm enough to be grated like Parmesan cheese. The salt helps the drying process by extracting water from the crushed eggs, which glue themselves together because of their albumen and fat.

Tuna bottarga has a bright, sharp salty flavor, whereas the mullet version is somewhat milder. The best thing to do with either is the simplest: make Sardinia’s
spaghetti alla bottarga
. To a plate of cooked spaghetti, add extra-virgin olive oil, chopped garlic, parsley, and red pepper flakes. Toss, and grate some bottarga over the top before serving. Remember that bottarga is a condiment, quite salty and fishy, and a little goes a long way.

I’ve always wondered about seviche, the Latin American seafood dish. Books say the fish is “cooked” just by being marinated in lime juice. Is it really “cooked,” or is it still raw?

....

V
irtually every mention of ceviche (seh-VEE-che; I’ll use the Spanish spelling) by a food writer is accompanied by a gratuitous statement to the effect that lime juice does to protein what heat does to protein, and therefore the fish is essentially “cooked” by the lime juice.

Well, does “cooked” mean cooked, or doesn’t it? And if the quotation marks are necessary, whom, pray tell, is everyone quoting? Apparently it’s a vicious cycle, with everyone quoting everyone else. Let’s just agree that “cooked” means subjected to heat, while “raw” generally means
not
cooked. So take your choice.

But before I serve up your mini-course in protein chemistry, here’s a bit of an appetizer.

Ceviche is made from small pieces of any of several kinds of raw saltwater fish, or from scallops or other shellfish, or squid or octopus, all marinated in lime juice for several hours in the refrigerator, after which some oil, chopped onion and other vegetables, and spices are added and the mélange served cold. If the fish is fresh to begin with—and it absolutely must be—it is safe to marinate it for up to five or six hours; the lime’s acidity (pH around 2.2) is strong enough to retard bacterial growth.

The citric acid in lime juice changes the proteins in fish by a process called
denaturation
. The normally twisted and folded protein molecules are unraveled or unfolded into less convoluted shapes. And the shapes of molecules, especially proteins, are responsible for most of their physical and chemical properties. In other words, they have lost their original natures: they have been denatured.

And yes, the heat of cooking also denatures proteins.

But besides acids and heat, a variety of other kinds of conditions can denature proteins. High concentrations of salts, including table salt—sodium chloride—can do it. Air can do it, as happens in the bubbles formed when cream is whipped. Even alkalis, the opposite of acids, and low temperatures, the opposite of heat, can do it, but less commonly. The analogy with cooking comes only from the fact that heat is the most familiar protein-denaturing agent in the kitchen.

Denaturing or unwinding protein molecules is no great trick, because the bonds that keep them twisted and folded aren’t very strong. Evolution may supply a rationale for that fact: Over the eons, specific proteins have evolved to do specific jobs in specific living organisms, so they have no need to be stable under conditions vastly different from those that prevail in the organisms they serve. Animal muscle is normally only mildly acidic, while body temperatures are relatively low, especially in the case of sea creatures. Thus, meat and fish proteins can be destabilized when subjected to higher acidities and higher temperatures than those in the animal’s muscles. That’s why in making ceviche, fish protein can be denatured by an acid no stronger than lime juice, and even at refrigerator temperatures.

The different denaturing methods complement and enhance one another. For example, the stronger the acid a protein is subjected to, the lower the temperature at which it can be denatured by heat. That’s why meat or fish bathed in a marinade containing lemon or lime juice (citric acid), vinegar (acetic acid), or wine (primarily tartaric and malic acids) will require less cooking time than an unmarinated sample. And if you want to explain that by saying the acid has partially “cooked” the meat, be my guest.

After the protein molecules in a food have been unraveled or unfolded by any of these denaturing environments, they may not stay that way. For one thing, if the conditions should change, they can re-ravel back into their original shapes or something similar. But usually this doesn’t happen, because as they unfold or disrobe, so to speak, the protein molecules expose parts of themselves that previously had been concealed in the folds, and these parts can react with other chemicals in the vicinity that can change their shapes more or less permanently.

Or, the newly denuded sections can bond to one another, making so-called crosslinks that knit the molecules together into tighter structures. That’s why when you either cook a piece of fish or soak it in lime juice to make ceviche, it develops a firmer texture. You’ll notice also that it becomes more opaque, because light rays can’t penetrate the tightly balled-up, crosslinked protein molecules. (The same thing happens to the proteins in an egg white; when cooked it turns from transparent to opaque white.) And under the right conditions, acidified, unfolded protein molecules will stick together and the protein will coagulate, as when cheese curds are formed when lactic acid denatures the casein in milk.

So why are acids so important in cooking? First of all, all our animal and vegetable foods are inherently either slightly acidic or neutral (neither acidic nor alkaline), and food chemistry, including the chemistry of cooking, is therefore very sensitive to even slight changes in acidity. The degree of acidity (expressed as a pH between 0 and 7) is critical to many of the chemical transformations that take place in cooking.

On the other hand, alkalinity (a pH between 7 and 14), the antithesis of acidity, plays virtually no role in cooking. Alkaline chemicals, being mostly unnatural in our foods, have generally deleterious effects on them and are rarely used in cooking. Nature has set the stage for that by making alkaline substances taste disagreeably bitter and soapy. All acids, on the other hand, add sourness—a very important element in our treasury of tastes.

But what about safety? Cooking temperatures will kill all bacteria and most spores, while the acid does so primarily on the surface of the food. Any parasites that may be lurking within the flesh can be killed by freezing or by the heat of cooking, but not by the acid.

Once again, however, if you’re using fresh, inspected fish from a trustworthy supplier, your sushi, sashimi, and ceviche should be quite safe. Just don’t buy your fish from that guy in the 1985 Chevy wagon by the side of the road.

I’ve enjoyed mussels many times in restaurants, but the few times I tried cooking them at home they were gritty and stringy. How should I have cleaned them?

....

Y
ou may have bought “wild” mussels, rather than cultivated or farm-raised mussels. The grit was probably sand, and the “strings” were remnants of their beards, which are routinely removed from the “domesticated” (tame?) ones before they reach the market.

Mussels don’t burrow in the sand as clams do, cement their shells to each other as oysters do, or swim freely as scallops do. They anchor themselves to something stationary by means of a
byssus
, or beard—a clump of tough threads that they manufacture by extruding a liquid protein that hardens in seawater and sticks better than Super Glue to almost anything. In fact, scientists have been trying to reproduce it in the laboratory for possible use in gluing people back together after surgery.

Mussels are raised on underwater hanging hemp ropes in Spain, on bamboo poles in Thailand, on oak boards in France, on “longlines” (Paul Bunyan–sized socks hanging from an underwater clothesline) in Sweden and Canada, and on shallow ocean bottoms in the Netherlands and Maine. In these environments they pick up little if any sand. Their beards are clipped by machine before they’re sent to market, although you may still have to yank a few. Otherwise, cultivated mussels need no cleaning beyond a cold-water rinse. Any whose shells gape open and don’t close when tapped sharply with one of its brethren are dead, and should be discarded.

In the bottom-culture method used in Maine, wild, inch-long baby blue mussels are dredged from selected natural locations and sown by being scattered thinly onto leased beds where, not having to compete for food in the sea-floor jungle, they will grow to two to three inches in 18 to 24 months. In the wild, it might take them 7 to 8 years to reach that size. (If successful at evading ducks, crabs, starfish, and humans, mussels will live for 12 to 13 years, with some growing as old as 50.) Pampered and plump, cultivated mussels in the shell will be at least 25 percent meat by weight, whereas wild ones rarely exceed 15 percent.

Anchored as they are, mussels must depend for food on whatever the ocean currents bring them. Like clams and oysters, they feed by constantly taking in water and filtering out particles of plankton. A typical two-inch-long blue mussel may process 10 to 15 gallons of water per day. Unfortunately, its filtering system can also trap bacteria and other toxic microorganisms. That’s why mussels from polluted waters are so dangerous; bacteria can pile up in a mussel like dirt in a vacuum-cleaner bag. Cultivated mussels are raised in carefully monitored waters.

There are seventeen known species of edible mussels in the world. The one that you’ll see most often is the Atlantic blue mussel (it’s actually blue-black),
Mytilus edulis
, which is raised mostly in the waters off Maine and Prince Edward Island, Canada. A similar species,
Mytilus troesselus
, is grown in the state of Washington. But from May through July, the blue mussels are likely to be devoting all their energies to spawning, which makes them weak and flabby and not good to eat.

Fortunately for mytilophiles, the Mediterranean mussel,
Mytilus galloprovincialis
, which is being cultivated on the West Coast, spawns in January and February, so it’s good eatin’ all summer. It is mild, sweet, plump, and big, ranging up to seven inches long and up to 60 percent meat at its peak.