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The Everyday Chemistry of Cooking
By Simon Quellen Field
Chicago Review Press Incorporated Copyright © 2012 Simon Quellen Field
All rights reserved.
MEASURING AND WEIGHING
In science, and especially in chemistry, careful weighing and measuring are important for reproducible results. If someone cannot reproduce your results, there is little point in doing the experiment.
For people to reproduce a culinary masterpiece, it is important to carefully weigh and measure according to a recipe. But when you're just cooking up some breakfast, it is more important to know why the ingredients are used, and why certain processes are followed. With this knowledge, you create and adjust the food on the fly, substituting some ingredients you have for some you don't, or use up things from the back of the refrigerator before they go bad.
Variations in Recipes
You can get a feel for how important measuring is by comparing recipes. Suppose you look at 10 recipes for homemade cupcakes and compare the ratios of flour and sugar in them:
2.75 1.5 183.33%
2 1.5 133.33%
Standard Deviation 43.47%
The average cupcake has one and a half times as much flour as sugar. But some cupcakes have equal amounts, and some have two and a half times as much flour as sugar. The high standard deviation means that there is a lot of variation among simple cupcake recipes. A good cook can feel free to vary the amount of sugar in the recipe for taste or to compensate for what will accompany the cake, such as icing or bits of fruit in the batter.
Why Sifted Flour?
Some recipes list the ingredients by weight instead of volume. Some cooks swear by weighing everything, to get consistent results. When consistency of results is important, by all means, measure carefully. But when a little variation and creativity are called for, or when you are changing parts of the recipe for whatever reason, judgment and knowledge are more important.
Recipes once called for sifting flour. Flour was something that often had lumps, bits of millstone, or insects in it, so sifting was important. Other reasons have been suggested for sifting, such as aeration, or mixing dry ingredients, but a whisk in a bowl can accomplish both these tasks. The bother of sifting would not be worth it if either of these were the main reason.
So why sift? When ingredients are not weighed, the difference between a cup of flour and a cup of sifted flour can be significant. But a knowledgeable cook can use a bit less flour and avoid the time and mess of sifting.
It is interesting to look at recipes that are very careful to weigh out all of the ingredients yet then call for three eggs, without specifying the weight of the eggs. Eggs vary in weight, but most recipes don't specify the size of the eggs as small, medium, large, extra large, or jumbo. The reason is that it really doesn't matter too much. Whatever the size, the recipe is going to come out just fine. There is a lot of room for variation, and consistent results are usually not as important to the eater as they are to the creator of the recipe, who wants to protect his or her reputation for being reliable.
The best recipes will tell you what to look for in the processing of the food. Instead of giving a precise baking time, a cake will be tested for doneness with a toothpick or the press of a finger. In candy making, the initial amounts of sugar and water are not that important when you are cooking the mixture to a certain temperature or to "hard ball" stage, both of which are measures that tell the cook exactly what the ratios are during cooking.
Density and Good Eggs
In making wine or beer, the density of the mixture, measured by floating a little scale (called a hydrometer) in the water, tells how much sugar, alcohol, and water are in the mix at any given time. A density test can also tell you how fresh your eggs are. Place an egg in water, then dissolve measured amounts of salt into the water until the egg floats. A bad egg will float right away.
You may have noticed at a party that some cans of soda in a tub of ice water float, while others sink. This is caused by density; sodas with sugar in them are at the bottom and the diet sodas are at the top. As an interesting experiment, place a diet soda can in a glass container large enough for it to float, then place a small plastic cup on top. Slowly fill the cup with sugar until the can sinks. You might be amazed at how much sugar it takes to sink the can. There is at least that much sugar in the sodas that sink, but probably more.
Another place where density comes into play in the kitchen is in making hardboiled eggs. The yolk of an egg contains fats and oils and is thus less dense than the white of the egg. This means that if left to itself, the yolk inside will float to the top of the egg and thus be off-center when the egg is cut in half for deviled eggs or sliced into a salad.
To keep the yolk centered, the eggs must be turned frequently while being cooked, keeping the yolk away from the shell. Since the white of the egg cooks on the outside first (where it is closer to the boiling water), the yolk that is turned often will not be able to get past the hardening white and will end up centered.
Some things are easy to measure. Not all cooks have kitchen scales, so many recipes (especially in the United States) call for easy volume measurements. But some things you might care about, such as how many calories are in the food you are making, might at first seem hard to measure at home.
But with a little thought, estimating calories isn't that difficult.
As a general rule, proteins and carbohydrates have about 4 calories per gram, while fat has about 9. You can separate the ingredients by whether they are fats or not, weigh them, and then multiply. Or you can estimate by eye what percentage of the recipe is fats, and pick a number between 4 and 9 that matches the estimate. A little adjustment for water content, and you have a good guess at the number of calories in the food.
A Hostess Twinkie says on the label that it has 4.5 grams of fat (40.5 calories) and 27 grams of carbohydrates (108 calories) for a total of 148.5 calories. One Twinkie weighs 43 grams, and the label says it has 150 calories, so about 3.5 calories per gram.
Take a look at some popular foods:
Beefjerky: 116 calories in 28 grams, or 4 calories per gram
Pork sausage: 95 calories in 28 grams, or 3.4 calories per gram
Air-popped popcorn: 31 calories in 8 grams, or 3.8 calories per gram
Butter: 70 calories in l0 grams, or 7 calories per gram
Bacon: 50 calories in l2 grams, or 4 calories per gram
Buttercream frosting: 100 calories in 26 grams, or 3.8 calories per gram
Enriched flour: 455 calories in 125 grams, or 3.6 calories per gram
Whole wheat bread: 70 calories in 28 grams, or 2.5 calories per gram
A steak: about 2 calories per gram
What you see from the examples above is that until you get to something like pure butter, most processed foods have between 3½ to 4 calories per gram, about the same as pure sugar.
Celery has 0.16 calories per gram, an apple has 0.5 calories per gram, and a carrot has 0.4 calories per gram. These foods are mostly water. So eat fruits and vegetables to fill yourself up if you are watching your calories.
Steaks, chicken, pork chops — even those have fewer calories per gram than popcorn or bread. But within about a factor of two, you can simply weigh the food and figure 1,300 to 1,800 calories per pound. Put your whole meal on a plate and weigh it. If you don't like what the bathroom scale says the next morning, put less on your plate today.
Of course, counting calories to control your weight assumes that your weight is simply a matter of balancing the number of calories you eat with the number of calories you burn. But your body already has mechanisms for doing that balancing. If you starve yourself, your body will stop burning as many calories. If you eat too much, your body will burn more. This is controlled by hormones in your body, the main one being insulin.
Insulin tells the fat cells to take in sugar from the blood. When there is too much sugar in the blood, extra insulin is produced to remove it, and thus extra fat is stored. Foods with high insulin indexes (foods that cause more insulin to be produced than other foods do) can upset the balance that keeps your calorie inputs and outputs matched. This is why low-carbohydrate diets seem to be effective in controlling weight. They prevent excess insulin from being produced and thus prevent extra fat from being stored.
There are many complex interactions in the body that affect the balance that controls fat production. Some are genetic, some are behavioral, some are environmental, and some are caused by infections or disease. Planning effective weight control for an individual will necessarily be an individual exercise, and one diet plan will not work for everyone. But it is important to understand that simply cutting calories or getting more exercise is not the whole story.CHAPTER 2
Foams are fun. Marshmallows, meringues, cakes, whipped cream, cookies, ice cream — all of these are foams.
Foams are formed by several different processes. In many foams, such as whipped cream and beaten egg whites, an interesting thing happens at the interface between water and air. In both of these foams, proteins in the foam are first denatured, which, as the name implies, means that they are changed from their natural state.
Proteins are made up of building blocks called amino acids. Some of these building blocks are attracted to water but avoid oils and fats. Others are attracted to oils and fats but are repelled by water. In the natural state of the protein, the water-loving parts are on the outside of the protein, next to the water, and the water-avoiding parts are tucked inside, away from the water.
Proteins are big molecules, formed of strands and sheets of amino acids, all tangled up into a shape that is important for their natural function. When we beat the cream or the egg whites, the protein unfolds, like a carefully folded origami animal would if you beat it hard with a whisk.
As the protein unfolds, it encounters oils and fats in the cream, as well as air. The water-loving parts of the protein still stay in the water. The water-avoiding parts unfold so they can stick into the fats or into the air, to avoid the water. Eventually, the air bubbles become smaller and smaller as they are beaten, and they become surrounded by a film made of protein, to which some water is still attached. The proteins can now link together to form a tough film that holds the bubbles in shape and prevents them from merging together again.
In whipped egg whites, you get bubbles with a protein film. The water-loving parts stick into the water, and the water-avoiding parts stick into the air bubble.
In whipped cream, you get big bubbles of air surrounded by a film of protein, surrounded by tiny globules of fat stuck to the fat-loving parts of the protein, connected to another film of protein that forms the wall of the next bubble. In between the bubbles of air and the globules of fat, the water-loving parts of the proteins extend into the water.
You can make an egg white foam more stable by increasing the number of places where the proteins bond together. Beating the egg whites in a copper bowl causes the amino acids that have sulfur in them to bond together where the sulfur atoms are. Linking two sulfur atoms in this way forms a disulfide bridge, a very strong chemical bond that helps keep the protein stuck in the new position.
Adding an acid such as lemon juice or cream of tartar can also help form more bonds between the proteins and stabilize the foam, because the acid unravels the protein a bit, allowing the proteins to tangle and bond together.
You probably know that whipped cream forms a foam but whipped milk does not (unless it is heated with steam). The reason lies in the nature of the proteins in milk and cream, and the nature of butterfat. But mostly it lies in the amount of solid fat compared to the amount of water.
Butterfat is a liquid at body temperature — anything above about 90°F (32°C) — but it solidifies when chilled. This is why butter melts in your mouth. To whip cream, you need chilled, solid butterfat. As you beat the cream, it forms bubbles and the proteins denature, with some parts staying in the water and some parts staying in the fat, until you end up with a film of solid fat and protein that traps the air inside, with the water in between the bubbles.
If you beat the cream too much, you can turn the whole thing inside out, with the water trapped inside films of fat and protein, and the air gets out. This is butter. Where cream was tiny bits of fat in liquid water, butter is tiny drops of water in solid fat. One is a liquid and the other is a solid, but both are made of the same stuff.
To keep whipped cream stable, you need to keep the temperature low enough that the fat stays quite solid. You can also stabilize it by adding more protein, such as gelatin or some vegetable gums. Both help to link the proteins together and hold the fat in place.
If cream does not contain at least 30 percent fat, it will be difficult to whip. Most whipping cream is about 36 percent fat. Reduced fat whipping creams need the help of stabilizers. Most common are cellulose-based ingredients called hydrocolloids, or food gums.
As you whip cream, it gradually becomes stiffer. Maximum stiffness happens when the cream just starts to become butter. It will be slightly yellow in appearance, and the volume will have dropped a bit. The stiffness comes from the firm butterfat that has formed larger and larger particles on its way to becoming butter.
If your recipe uses whipped cream as a structural element, such as in cake icing or rosettes on a cream pie, you will want a nice stiff cream. For toppings on strawberry shortcake or other desserts, stopping the whipping when the foam is at peak volume will make it stretch further.
If you want to make a foam out of milk, you must use steam, as in a cappuccino machine. The steam denatures the proteins and links them together and at the same time incorporates air into the foam. When the steam cools, it becomes water again. The foam is full of air, not steam.
Wheat flour contains a protein called gluten, which is formed when enzymes in the flour react with precursor proteins as water is added. Gluten is gluey, and as you mix the batter or knead the dough, the little bits of gluten that form stick together and form rubberlike sheets.
Stirring and folding incorporate air to form little bubbles in the sheets of protein. Yeast or other leavening agents add gas inside the bubbles and make them expand. Heating the dough further changes the protein, denaturing it into a solid.
A Bread Recipe
Basic bread is fairly simple. You need some flour, some water, some yeast, and optionally some sugar or honey, salt, and/or oil, butter, or some other fat.
What do those ingredients do, and how much of each do you need? The flour provides the gluten precursors, starch, flavor, and bulk of the bread. Water is necessary to make the gluten and allow the yeast to multiply and produce carbon dioxide gas. The yeast is there to make the carbon dioxide gas so you get a foam instead of a brick.
All the other ingredients are optional. The salt is not there just as a seasoning; it's there to slow down the yeast. (There really isn't a lot of it in most breads.) If the yeast produces too much gas too fast, faster than the gluten forms, the gas will simply escape as the bubbles pop. But many recipes omit the salt. Some of the gas will escape, but these recipes usually call for the size of the bread to double, which will eventually happen with or without the salt.
Sugar or honey is often added to feed the yeast. But the yeast will find enough food in the flour without it. It will just grow a little more slowly, which (as we saw with adding salt) can be a good thing. But if you are making a lot of bread, and start with a small amount of yeast, you can grow the yeast you need in a little sugar water. The amount of sugar or honey is generally so small that it makes little difference to the taste of the bread.
Excerpted from Culinary Reactions by Simon Quellen Field. Copyright © 2012 Simon Quellen Field. Excerpted by permission of Chicago Review Press Incorporated.
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