Kitchen Mysteries_ Revealing the Science of Cooking Part 5

Salting Why Must Infants Not Be Fed Sausages?

Those nitrates that ecologists condemn for polluting streams and rivers are present in foods preserved with salt. Pota.s.sium nitrate, that is, saltpeter, has been used in this way empirically since the Middle Ages, even since Roman times. In 1891 the biologist H. Polenski demonstrated that bacteria transform saltpeter into nitrite in meat. Then in 1899 came the discovery that the characteristic color of salted products was due to these nitrites and not to the nitrates themselves. In 1901 the biologist John Scott Haldane found that this color resulted from the combination of the chemical group NO with the meat pigments. Finally, in 1929, nitrites were observed to inhibit the development of bacteria. Today, the description of the process is complete: salting, with the use of saltpeter, is an effective conservation method because the nitrate ions of the saltpeter are transformed into nitrite ions, which kill bacteria.

Unfortunately, nitrites are certainly not lacking in toxicity for humans as well. They react with the amino acids that make up proteins and form carcinogenic nitrosamines. Babies, especially, should not absorb nitrites, because these compounds are oxidants. They transform the hemoglobin in the blood into methemoglobin, which no longer transports oxygen. Adults possess an enzyme called methemoglobin reductase that retransforms methemoglobin into hemoglobin, but infants, who do not yet have this protective enzyme, must wait to indulge in sausage, dried meats, and the like.

How Do We Dry Meat Using Salt?

Though nitrited salt is available commercially, we should nevertheless remember that nitrites are not crucial for home salting. A well-implemented brining and drying process will suffice. The salt in a brine acts according to the phenomenon of osmosis already discussed. When a piece of meat is placed in a terrine with a little water and a lot of cooking salt, the water in the animal cells tends to leave the meat until the concentration of salt inside and outside the cells is equal. The salt does not enter the cells, but the water, small molecule that it is, is very mobile.

Thus drained, the meat hardens on the surface, and in this waterless meat bacteria have trouble developing. Why must a little water be added to the terrine? Isn't the cooking salt alone sufficient? With a bit of water, the meat is entirely soaked, so that contact with the salt is improved.40 After undergoing this treatment for some time, the meat is removed from the brine and dried. For a successful drying operation, it is advisable to place the meat in a dry, well-ventilated spot. An uninsulated attic with good ventilation serves well, as does a cool, dry cellar. The meat dries, and, after some time, it can be consumed ... with that great pleasure derived from slowly cured, long-awaited foods.

Microwaves Cooking with Internal Vapor Cooked in a microwave, beef is rejected by taste testers, who find fault with its grayish external color, the uniformity of its internal color, its toughness, its lack of succulence, and its bland taste. And they are right. Microwaves penetrate into the ma.s.s of the foodstuff for several dozen millimeters before being absorbed by the water molecules. These molecules are heated, then vaporized. The temperature never goes above 100C (212F). Now, as we have seen, heating in this way is fatal to meat, which must be heated intensely to achieve the browning produced by Maillard and other similar reactions.

On the other hand, microwaves are good for cooking eggs, for example, in which the proteins coagulation begins at 61C (141F). Placed in a bowl without an ounce of fat, an egg will cook rapidly; its taste is acceptable, and the figure benefits. Scrambled eggs, soft-boiled eggs, omelets, and even souffles can be cooked in this way. Microwaves are useful for fish as well, because they efficiently heat the poaching liquid over which the fish is placed. Similarly, vegetables can be cooked in boiling water, heated by microwaves.

Where Do Microwaves Act?

Let us review the basic principles in order to really understand where microwaves act. Inside a microwave oven is a device called a magnetron that emits electromagnetic waves (that is, vibrations in s.p.a.ce a.n.a.logous to light or to radio waves but with a different wavelength) with a frequency equal to 2400 megahertz. At each point in s.p.a.ce crossed by a microwave beam, the electromagnetic field oscillates 2400 times a second.

Without safeguards, such waves would heat the water in our bodies, and we would boil. Thus the waves are directed by an aluminum tube just inside the oven, and they are sealed within the oven (metallic grating in particular, like the kind used to reinforce microwave oven doors, stops microwaves).

When food is irradiated with microwaves, the waves interact through their electrical field with electrically asymmetric molecules, such as water molecules. The energy given to these molecules is transformed into motion, and the movement of these agitated molecules disturbs the other, unagitated molecules, so that the ma.s.s is put into motion, that is to say, heated. Gradually, the agitated molecules are calmed down by colliding with the surrounding molecules, through their random movement. Since most foods contain large quant.i.ties of water, they are heated because this water becomes agitated, and it is especially the parts of the food containing the most water that are heated the most. Hence the recipe for canard a l'orange canard a l'orange given at the start of this book. given at the start of this book.

A Few Questions and Answers Why have the manufacturers of microwave ovens adjusted the frequency of microwaves so that they are a bit lower than the frequency at which water best absorbs these waves?

Because if we want the inside of foods to be cooked as well as the outside, the microwaves must not be absorbed immediately by the outer layers of the food. If the water on the surface absorbs only some of the microwaves, the rest will permeate the food, where another share will be absorbed.

Why do salted foods heat more quickly than unsalted foods in a microwave oven? Because salt contains ions, and the water molecules that hydrate these ions, by surrounding them, heat more quickly than isolated water molecules.

Why should we not try to heat oil in a microwave oven? Because triglycerides have no chemical groups that can interact efficiently with microwaves. This can easily be demonstrated by putting two gla.s.ses, one filled with water, one filled with oil, in a microwave oven. When the water comes to a boil, the oil will still be cold.

Why does meat cooked by microwaves become grayish-brown? Because the temperature stays below 100C (212F); thus the oxymyoglobin is not denatured and retains its color.

And, to end with something sweet, remember that caramel can be prepared quite easily in a microwave oven. Take a small bowl, place sugar and a bit of water in it, and heat. Caramel rapidly results without any trouble whatsover.

Vegetables COLOR AND FRESHNESS.

A Matter of Water Vegetables, the jewels of the kitchen! Did they not give their names to the great Roman families? Fabius, in honor of faba faba, or feve feve, the broad bean; Lentulus, in honor of the lentil; Piso, in honor of the pea; Cicero, in honor of the chickpea.

Vegetables must be eaten fresh to be good. The soil in which they were cultivated, the climate that brought them to life will sing in one's mouth ... if they are not mangled in the cooking process. Cooking them is a delicate operation. How long must they cook to become sufficiently tender? Must they be tossed into cold or hot water? Must the cooking water be salted? How to retain their bright colors, which seem to be the mark of their freshness?

Before I launch into an examination of this last question, let me recall that a very fresh vegetable is generally tender, and cooking is not of great value to it. On the other hand, for certain older or even dried vegetables, like lentils, rehydration is essential.

In these two cases, the cooking methods are very different, since the object in the first is to retain the emollient moisture of the vegetable and in the second to reintroduce moisture that has been lost.

How Do We Avoid the Discoloration of Green Vegetables When Cooking Them?

The intense green that vegetables acquire after cooking for a few seconds in boiling water results from the release of gases trapped in the s.p.a.ces between the vegetable cells.

Generally, these pockets of air act as magnifying gla.s.ses that highlight the color of the chloroplasts, the green organelles that are responsible for the transformation of carbon dioxide into oxygen.

Vegetables, however, are usually cooked longer than a few seconds, thus destroying the atmosphere that shows these vegetables in their best light. Spinach cooked too long turns brown, sorrel as well; leeks lose their greenness, and so on. How to retain that appetizing color?

The cooks of antiquity were the first to make advances toward explaining this phenomenon. They observed that green vegetables remained very green when saltpeter or ashes were added to cooking water. Why?

When a green vegetable is heated, some of its cells burst, releasing various organic acids. The hydrogen ions of these acids react with chlorophyll molecules (which contribute to the green color of green vegetables) because these molecules contain a large square chemical pattern, the porphyrin group, at the center of which is a magnesium atom. Now, the hydrogen ions have a bad habit of taking the place of the magnesium ion in this porphyrin group and of thus transforming the various kinds of chlorophylls into pheophytins, which absorb different components of light. Instead of retaining all the light rays except those of the color green, pheophytins reflect a mixture of wavelengths that produce the perception of a horrible brown.

But from this a.n.a.lysis emerges a solution: not heating the vegetables for too long, so that the magnesium will remain in its chlorophyll cage.

A few corollaries are equally essential. To retain the color of green vegetables, avoid lidded earthenware pots and opt for steaming, because if they are not immersed in water, the vegetables are not in contact with the hydrogen ions. If vegetables are cooked in water, large quant.i.ties of water should be used. Finally, adding vinegar to the cooking water for green vegetables should be absolutely avoided, as it will enhance the bad effects you wish to avoid. Be aware, too, that many juices from fruits are very acidic (and that the acidity one perceives can be hidden by sugars).

Naturally, inventive cooks have thought of cooking green vegetables in the presence of salts, which provide ions that can occupy the positions hydrogen ions would otherwise take. That is why green vegetables were cooked in copper pans, called "regreening pans," and why, later in history, copper salts were used; with these methods, the green remained intense ... but the vegetables became toxic. Indeed, a law prohibited the practice of adding copper salts in 1902. More recently, processes using zinc ions have been patented.

Adding a base to the cooking water in order to neutralize the acids as they form has also been considered. This solution was already familiar to the Romans. Apicius, famous for his gastronomical extravagances, wrote, "Omne holus smarugdinum fit, si c.u.m nitro coquantur" (All vegetables will be the color of emerald if they are cooked with niter).41 The same effect occurs with ashes, where potash is present. Alas, niter, or saltpeter, and potash ruin the taste. The same effect occurs with ashes, where potash is present. Alas, niter, or saltpeter, and potash ruin the taste.42 How Long Must Vegetables Be Cooked?

Do not hope for a global response to such a question. Fresh asparagus will cook for less time than asparagus kept for a day or two after picking. And regardless of freshness, asparagus will not take as long to cook as potatoes. Still, as is so often the case, an a.n.a.lysis of the problem can guide us in our culinary transformation operations.

The objective is to tenderize the vegetables, the cells of which, unlike animal cells, are each protected by a hard, fibrous wall. Weakened by cooking (the cellulose is not altered chemically, but the pectins and the hemicellulose are), these walls becomes porous, and as their proteins are denatured, they lose their ability to regulate the movement of water from the interior of the cell to the exterior, and vice versa. Water can pa.s.s through the walls, while larger molecules are blocked.

We know that when we put vegetables into unsalted water, they swell because the water enters the vegetable cells as a result of osmosis. On the other hand, if the cooking water has too much salt, the vegetables harden (especially carrots), because the water does not enter the cells to reduce the salt concentration in them-the contrary!

The Mystery of Dried Vegetables The case of dried vegetables (lentils, etc.) is a little different, because the objective there is to reintroduce the water lost in drying them. As I just noted, the cooking water must not be salted. Nevertheless, this precept is not enough, and cooks have perfected a precise methodology for obtaining good results.

The first operation should be a soaking, the aim of which is to soften the external layer of the vegetable and facilitate the subsequent cooking. Often, two hours of soaking is enough to obtain a wrinkled skin. Warm water seems preferable to cold water, because the soaking is thus accelerated. The soaking water is then replaced for cooking.

The cooking water must not be calcareous, cooks say, because if a layer of calcium settles on the skins of the vegetables, it will harden them and prevent them from cooking. Authors like Madame Saint-Ange recommend adding bicarbonate of soda when the water is calcareous. In fact, no layer of calcium forms, but calcium should be avoided nevertheless because it acts as a cement between pectin molecules in the vegetable cell walls, hardening them rather than promoting softening. Madame Saint-Ange was right to recommend bicarbonate of soda. It has two benefits. First, the calcium is precipitated so that it cannot bind the pectins. Second, the water becomes basic, contributing to the pectin separation (we shall witness this effect again later, with regard to jams).

It is also specified that the cooking be gradual. This makes sense in principle, because cooking too rapidly from the outset cooks the exterior part too much, turning it to mush before the center of the vegetable is soft. Likewise, adding cold water if the cooking water boils away should be avoided. The sudden thermal discontinuity can explode the vegetable skins, thus releasing their contents into the cooking water.

Do Carrots Risk Losing Their Color in Cooking?

If the cook is careful, no carrot will ever lose its color. We must understand that the color of vegetables comes from various pigments: chlorophylls (green to blue pigments), carotenoids (yellows, oranges, and reds), and anthocyanins (reds, purples, and blues). If green vegetables are green, it is because they contain chlorophylls. If carrots are orange, it is because they contain, especially, carotenoids.

Now carotenoids, soluble in fat but insoluble in water, are little altered by boiling water. Normally, carrots remain brightly colored (the same is true for tomatoes, though their color is mainly due to lycopene, not carotenoids). In other words, carrots are easy to cook ... so long as a pressure cooker is not used. The pressure that builds in a pressure cooker alters the carotenoid molecules, which then lose their color.

How Do We Cook Potatoes?

Potatoes are made of cells that contain granules of starch. These starch granules become soft, inflated, and jelled when they are immersed in water at temperatures from 58 to 66C (136 to 150F). The perfectly cooked potato is full of these inflated, tender granules, all of which have uniformly reached the temperature of 66C (150F).

Thus sauteed potatoes are better when they have been cooked in water for a few minutes and acquired a jelled outer layer. During cooking, this layer prevents the starch granules from absorbing too much oil, while the external surface can be heated to 160C (320F). The starch contained there deteriorates and reacts, as we saw in the chapter on deep-frying, giving way to a crispy, golden casing.

Can a Dish Containing Vegetables Be Reheated in b.u.t.ter?

Reheating vegetables in b.u.t.ter is often a mistake, because the b.u.t.ter will make the sauce oily, unless, by one means or another, the sauce has been emulsified as a precaution. Furthermore, if a dish contains sauteed vegetables, they will turn brown and dry out when reheated in b.u.t.ter. It is better to use water, in minuscule proportions, to compensate for the loss of the water involved in the initial preparation.

Of course, if a microwave oven is available, the problem of reheating is resolved. What a fine invention!

Why Must Cauliflower Not Be Overcooked?

The various vegetables in the cole family (mustard, brussels sprouts, cauliflower, broccoli, turnips, etc.) contain sulfur compounds, a.n.a.logous to certain aromatic precursors in onions. In these vegetables, however, the sulfur compounds are bound to sugar molecules and odorless as long as they do not come in contact with an enzyme that transforms them into aromatic compounds. This enzyme is inactive in the acidic conditions of normal vegetable tissues. But when the tissues are broken down, the enzymes come into contact with the odorant precursors, unbinding the sugar molecules and releasing the odorant compounds. The chemical weapon, mustard gas, is synthesized from such derivatives (which belong to the family of isothiocyanates).

The vegetables in the cole family were among the first to be a.n.a.lyzed because their strong, persistent odor when cooked suggested that they contained odorant compounds. Thus, beginning in 1928, it was discovered that the extracts of these vegetables, and their derivatives containing cystine (an amino acid), break down into various odorant compounds, especially dihydrogen sulfide, mercaptan, and methyl sulfide. Finally, these compounds react with one another to form trisulfides.

The longer the vegetables in the cole family cook, the greater the number of these molecules and the more the odor increases. Notably, the quant.i.ty of dihydrogen sulfide produced while cooking cauliflower doubles between the fifth and seventh minutes of cooking. The smell soon fills the whole house.

Choose your cooking time according to the degree of tenderness you desire for cauliflower, but do not go too far over that limit!

Why Do Beans Cause Flatulence?

Raffinose, a sugar present, for example, in peas and flat beans, is composed of a chain of three chemical rings, one fructose, one glucose, and one galactose. The table sugar that we eat, composed of glucose and fructose, is broken down by digestive enzymes into its const.i.tuent rings, which are metabolized. On the other hand, we have no enzyme capable of metabolizing galactose. It pa.s.ses intact into the large intestine where it is a.s.similated by the intestinal flora (especially the bacteria Escherichia coli Escherichia coli). The microorganisms of this flora release hydrogen, carbon dioxide, and methane. These are the three gases that inflate the stomach and produce those well-known noisy eruptions.

A good way to eliminate galactose from our vegetables is to let them germinate, because this process creates galactosidase, an enzyme that destroys galactose. They can also be soaked, and the water used for soaking and later that used for cooking discarded.

Sauerkraut and the Miracles of Fermentation We know that sauerkraut is produced by fermenting cabbage in a brine, where the development of certain pathogenic bacteria is blocked while the development of other organisms like Leuconostoc mesenteroides Leuconostoc mesenteroides and and Lactobacillus plantarum Lactobacillus plantarum is encouraged. During this development, the bacteria consume glucose and expel lactic acid, which gives sauerkraut its flavor. is encouraged. During this development, the bacteria consume glucose and expel lactic acid, which gives sauerkraut its flavor.

Lactic acid (C3H6O3) is half a glucose molecule (C6H12O6). It is formed through the anaerobic fermentation (in the absence of oxygen) of sugar and glucose, and it is responsible for muscle aches after sustained exercise when the muscles are deprived of oxygen.

Lactic acid is also found in milk when the milk is colonized by bacteria that make use of its sugar, lactose, and break it down, releasing lactic acid. By increasing the acidity of the milk, lactic acid makes it coagulate. That is how yogurt is produced. Similarly, lactic acid is responsible for the characteristic flavor of pickles and other foods preserved in vinegar.

How to make sauerkraut? It is remarkably simple. Shredded cabbage is placed in salt, and water is added to obtain a salinity of about 2.25 percent. At a temperature of 18 to 21C (64 to 69F), the bacterium Leuconostoc mesenteroides Leuconostoc mesenteroides grows and releases, in particular, lactic acid. Then, when the concentration of lactic acid reaches 1 percent, grows and releases, in particular, lactic acid. Then, when the concentration of lactic acid reaches 1 percent, Leuconostoc mesenteroides Leuconostoc mesenteroides is replaced with is replaced with Lactobacillus plantarum Lactobacillus plantarum. A good level of acidity is attained after about two-and-a-half weeks.

The Ripening of Tomatoes For the end of our journey, let us head toward the sun, with an examination of tomatoes, delicious but ephemeral. Initially green, they progress to a ripe state under the sun's heat, becoming juicy and aromatic ... but not for long, because they soon rot. Half the tomatoes produced, it is estimated, end up spoiling. What a shame!

Could their ripening be controlled, and their rotting avoided? Undoubtedly, because in general with tomatoes, ripening is preceded by increases in the respiration of the vegetable cells and the production of ethylene, a simple organic molecule that acts as a hormone.

T. Oller and his colleagues at the University of Albany have just demonstrated that ethylene is a cause and not an effect of ripening. In other words, one means of slowing down the ripening process in tomatoes consists of putting them in a very well ventilated place, so that they do not remain in contact very long with the ethylene they produce.

Sauces CREAMY, SATINY, FLAVORFUL.

Neither a Juice nor a Puree Before they sang of Trojan heroes or the adventures of Ulysses, the Greek poets invoked the Muses, who were supposed to ensure the truth of their poetic madness. As modern bard of one of the basic components of cooking-the sauces-I invoke Ali-Bab, that early-twentieth-century French engineer who, upon returning from his numerous world travels, offered gourmands the fruits of his long travel experience. His Gastronomie pratique Gastronomie pratique hardly merits its name, but his paragraph on sauces deserves to be quoted: hardly merits its name, but his paragraph on sauces deserves to be quoted: Sauces are liquid food combinations, thickened or unthickened, that serve to accompany certain dishes.Thickened sauces, by far the most important, all consist of a more or less succulent stock, seasoned, and a thickening agent. The number of stocks for sauces is considerable, the number of aromatics very great, and there are many ways to thicken a sauce. Thus, given these circ.u.mstances, it is easy to understand that, with the number of possible combinations being infinite so to speak, here lies a veritable gold mine for the treasure seeker.

What does this quotation teach us? That sauces are thickened to various degrees, but that, generally, a sauce is neither a juice nor a puree. With their highly sophisticated consistency, sometimes syrupy, sometimes creamy, always flavorful, sauces must have a certain quality to accompany fish, meat, vegetables, and desserts.

This precept is implicit in Ali-Bab's description. For sauces, the key words are "consistency" and "flavor." If you examine various recipes for sauces that you have already made, you will see that these two basic elements are present every time: a flavorful liquid and a thickening agent.

If the question of flavor has come up in other chapters, the question of consistency, absolutely crucial, has hardly been mentioned until now. Two recent scientific findings, obtained by researchers in Dijon and Nantes respectively, will convince us of its importance.

First, in Dijon, Patrick Etievant, a physical chemist at INRA, offered tasting panels various strawberry jams in which differing amounts of jelling agents had been added in order to obtain varying degrees of firmness. The same batch of fruit had been used in all of them, and each of the test jams was a.n.a.lyzed for its chemical composition. Verdict: the firmer the jams were, the less flavor they had.

Second, at the INRA station in Nantes, Michel Laroche and Rene Goutefongea studied liver mousses in which, to make them lighter, they replaced part of the fat with a hydrocolloid, that is, essentially, a starch of water and flour. Once again a.s.sisted by taste testers, these two Nantes researchers discovered that the flavor quality of the liver mousses prepared in this way depended on the consistency. The more hydrocolloids the mousses contained, the more they melted in the mouth ... and the better tasting they were.

Thus not only do we expect a particular consistency from a particular dish, but the perception of flavors and scents depends on that consistency.

A Variable Consistency These remarkable findings of modern food science encourage us to a.s.semble the proper gear before taking off to explore the great land of sauces.

The notion of viscosity will be useful to us here. We have seen that a sauce is neither a juice nor a puree. Its consistency, or "viscosity," is somewhere in between. This entry point into the matter allows us to imagine how sauces can be spoiled in the hands of cooks who neglect the important principles. They may be too liquid, too solid, too inconsistent, too full of lumps.

Physics has shown modern gourmands that viscosity is a complex subject and therefore more interesting than they might otherwise have imagined. A few simple culinary experiments will clue us in to this new business.

First, let us dissolve sugar in water. As long as the amount of sugar is small, the solution flows like water, but when the syrup is concentrated, it thickens, sticks to the spoon, and flows with more difficulty. The proper equipment will show us that this viscosity, the inverse of fluidity, remains the same regardless of the speed of the flow: a constant shearing stress applied to a simple solution or a syrup generates a constant speed of flow.

For other fluids, such as mayonnaise, bechamel, and bearnaise sauces, the true sauces in short, this law no longer holds. In some cases, the viscosity diminishes when the speed increases; sometimes, on the contrary, the viscosity increases. Thus a bearnaise sauce that seems very thick, nearly solid when it is sitting in the sauceboat, takes on an angelic fluidity when it pa.s.ses through the mouth, at a speed of some fifty centimeters (about twenty inches) per second. Naturally, the molecular composition of the sauces is responsible for these flow properties.

And at this point let us retreat from our incursions into the territory of pure physics; we have enough gear to set out for the land of sauces.

Is Bearnaise Sauce Warm Mayonnaise?

With regard to mayonnaise, we have previously seen that water, perfectly fluid, and oil, also perfectly fluid, form a viscous mixture, thick enough sometimes to cut with a knife, if they have been combined in an emulsion, that is, into a dispersion of oil droplets stabilized with the help of the surface-active molecules of egg yolk.

The viscosity of emulsions is widely used in cooking. It accounts for the satiny quality of bearnaise sauce, hollandaise sauce, white b.u.t.ter sauce, and even of milk and cream, in which the quant.i.ty of fat dispersed in the water can be as high, respectively, as 4 and 38 percent.

Most of the time, sauce emulsions are of the oil-in-water variety. These are dispersions of droplets of a liquid fatty substance into a continuous phase of water. b.u.t.ter, on the other hand, belongs more to the water-in-oil variety (it is not a true emulsion, however, because part of the fat is solid).

Let us interpret the recipe for hollandaise sauce, of which the famous bearnaise sauce differs only in the seasoning and the quant.i.ty of b.u.t.ter dispersed in the aqueous solution.43 To make a hollandaise sauce, egg yolks are beaten, all by themselves, so as to mix their const.i.tuent parts thoroughly. Then water is added, lemon juice, and salt. The mixture is then heated (in a hot water bath if you are worried that your burner is too hot) and mixed to obtain an initial thickening. At this stage, the egg is forming microscopic aggregates, which add some viscosity to the emulsion. Finally, while whisking, b.u.t.ter is added bit by bit: the whisking separates the fat, which melts into microscopic droplets, and disperses them throughout the mixture, which is, in effect, a water solution. The sauce is removed from the heat as soon as it has thickened and served immediately.

What takes place during these successive operations? First, the surface-active molecules of the egg yolks have been dispersed in the tasty aqueous solution. These molecules are composed of proteins and lecithins.

Then, whisking the sauce while the b.u.t.ter melts separates the fat into droplets, which become coated with the various surface-active molecules already present in the mixture. At the same time, the protein coagulates, forming tiny aggregates that also become dispersed in the aqueous phase. Indeed, both hollandaise and bearnaise are not, strictly speaking, emulsions but rather share the attributes of two physical systems: emulsion and suspension.

Why Does Hollandaise Sauce Thicken?

Why does hollandaise sauce become viscous? Because it is a mixture more complex than pure water, and it flows with difficulty. Remember that it contains microscopic egg protein aggregates and fat droplets, which are bigger than the water molecules and mutually impede one another.

Another effect also takes place. First, the salt adds ions that link to the various electrically charged parts of surface-active molecules. Then, the lemon juice or vinegar causes the protein to become positively charged, which causes forces of electrical repulsion to appear between the egg aggregates and the droplets. All identically charged, the heads of the surface-active molecules repel each other. Their flow is further complicated by this repulsion; the viscosity increases many percentage points. But there is danger: if the temperature is too high, flocculation can occur, and egg protein aggregates can combine into bigger, visible aggregates. Lumps lurk: use your whisk!

Why Is Bearnaise Sauce Opaque?

To make an emulsion-hollandaise, bearnaise, or white b.u.t.ter-we begin with water, which is transparent, and b.u.t.ter, transparent as well when it is melted. Why is the resulting emulsion opaque? Because the light spreading through the sauce is reflected on the surface of the droplets, and it is refracted within the oil. The phenomenon is a.n.a.logous to what we observe when we put broken gla.s.s into a jar: the whole thing looks opaque, even though each individual piece of gla.s.s is transparent. Milk's whiteness and the yellow of a bearnaise sauce or mayonnaise result from this same phenomenon.

Why Do Some Emulsified Sauces Fail?