Thanksgiving food science: Five holiday flubs explained
By Robert L. Wolke,
We’re advised each Thanksgiving not to whip mashed potatoes with a mixer because they’ll get gummy. What’s happening there?
Potatoes are about 79 percent water and 15 percent to 20 percent starch. Success or failure in mashing is primarily a matter of how you treat that starch.
Inside each of the potatoes’ cells are hundreds of granules that under a low-powered microscope look like plump little pillows. Inside those granules is a clear, thick paste of starch that the plant manufactured during its photosynthesis days as nourishment for its future generations.
Break open too many of those granules, letting too much of the starch paste leak out, and you’ll end up with pasty mashed potatoes. So unless you want to use the result for affixing wallpaper, don’t use a food processor or a blender. Their high-powered blades can reduce the potatoes to a puree, which is great for juicy, non-starchy fruits and vegetables. But by the time a potato is squished to that degree, most of its starch granules have been torn open, spilling their gluey contents.
Mixers can do both mixing and beating/aerating. However, beating potatoes in a mixer in an attempt to make them fluffy is almost as bad as using a blender. It’s okay to use a mixer on very low speed to distribute additives such as butter and milk. But beating them too vigorously will break down their starch granules into glue just as a blender does.
The best tools for mashing starchy potatoes such as russets or Yukon Golds (the waxy reds are not preferable for mashing) are — you guessed it — ricers and mashers. Ricers look like overgrown garlic presses. Cooked potatoes are extruded through a plate with holes approximately the size of grains of rice. This does little damage to the starch granules, thus keeping the potatoes from turning gummy.
Lacking a ricer, I prefer the kind of potato masher that has a flat plate perforated with square or rectangular holes through which the potatoes are extruded, just as in a ricer. But use a straight up-and-down motion; sidewise swipes can squish granules open, resulting in glue.
The frozen bird
OMG! I forgot to defrost the turkey. It’s still in the freezer and my Thanksgiving guests will be arriving in six hours. I called my mother and all she did was LOL. Then she said, “Don’t worry. Just roast it without thawing it. I’ve been doing that for years.”
People do this every year, and reputedly a tender, juicy bird is ready in about five hours. So if you have to deal with a frozen turkey on Thanksgiving morning, all is not lost. Put on an apron and get cracking. But whatever you do, don’t try any quick-thaw schemes such as microwaving it or soaking it in hot water. You would be asking for food poisoning, because the turkey’s surface would warm first and remain at bacteria-friendly temperatures for hours.
An obvious concern is that the skin will burn while the rest of the bird remains underdone. And wouldn’t the bag of giblets packed inside the cavity leak bacteria-laden juices that won’t reach a microbe-lethal temperature?
Fear not. The USDA, the FDA, the Mayo Clinic, Iowa State University Extension and that supreme court of all things turkey — the Butterball Turkey Talk-Line — have all assured the public that a frozen turkey is even safer than a fresh one because it won’t drip contaminated juices all over your refrigerator and kitchen counter during or after defrosting.
If you want to try it, excellent directions can be found online.
How does it work? One might think that the first couple of hours in the oven defrost the turkey and the subsequent hours cook it. But that’s not the way it works. Instead, the heat slowly works its way in from all sides, leaving a trail of successive defrosting and roasting.
But why doesn’t the skin burn to a crisp after five hours in an oven at the recommended temperature of 325 degrees? Because the interior of the turkey is at such a low temperature, any heat absorbed by the skin from the oven’s hot air is conducted into the body at an especially rapid rate, so the skin stays relatively cool.
That’s a consequence of how heat moves through solids from Point A to an adjacent, cooler Point B. The bigger the difference in temperature between A and B, the faster the heat moves between them. It’s like putting a cup of hot coffee on a frozen steak: The heat of the coffee will warm the steak, but the steak also cools the coffee faster than if the cup was just sitting on the table.
In the turkey, this process repeats itself layer by layer as the heat penetrates into the turkey, thereby preventing any part from retaining its heat long enough to overcook.
But don’t try to speed things up with a higher oven temperature. That can indeed burn the skin and leave the cavity full of “snowball stuffing.”
Why does pan gravy so often turn out to be either greasy or lumpy?
Making good pan gravy from a meat or poultry roast is easy in theory — and just as easy to botch. Although the ingredients are few, they must be combined in the right proportions by volume: one part fat, one part flour (or thickener) and eight or more parts broth. Often, that will be two tablespoons each of fat and flour to each cup of broth. The fat contributes a smooth, unctuous mouth feel plus all those savory, fat-soluble flavors, while the broth adds water-soluble flavors and determines the ultimate amount of finished product. More broth yields more but thinner gravy.
The secrets of good gravy involve physics and chemistry. The physical challenge is getting all the drippings out of the pan and separating them into a fatty part and a watery part. The chemistry is that flour plus water makes a gluey paste, so they must be kept apart until ready for the thickening step. That trick is accomplished by coating the flour particles with a barrier of fat so the water can’t get to them.
First, the physics: The fat and juices have to be separated from each other, because the fat will need to be measured separately. Many recipes instruct us to skim off the fat in the pan with a spoon or ladle. Good luck with that. Spoons and ladles are usually deeper than the layer of fat in the pan, so it’s difficult to maneuver them in such a way that only the fat flows into them. You can’t get all the fat out, and that’s the primary cause of greasy gravy.
Instead, tilt the pan (after removing the turkey, vegetables and other solids) and use a flexible spatula to scrape all the liquids into a gravy separator. The liquids will settle into two distinct layers: fat on top and watery juices on bottom. The bottom layer can then be poured into the broth to enhance its flavor.
With the roasting pan set over a range burner or two, measure the amount of fat in the separator, pour it back into the pan and whisk in an equal measure of flour to form a roux. That coats the flour particles with water-impenetrable fat so they can’t stick together to make lumps. Cook it for a couple of minutes until it begins to take on color; this will mellow the flour’s raw taste and make the gravy beautifully brown. When you then whisk in the broth, the fat-coated flour particles will become homogeneously dispersed throughout the liquid.
The final step is to cook the whole mixture at a low temperature, which strips the fat coating from the starch grains. They then take on water, swell and rupture, releasing their glutinous content and evenly thickening the liquid. So no lumps. The fat is still there, but it is spread out in such tiny globules that they can’t coalesce into puddles. So no grease.
What is the chemical makeup of chicken skin? How bad for you is it, nutritionally, if the fat beneath it has been rendered during cooking?
In recent years, chicken skin has been scorned as a nutritional bete noire, along with salt, sugar, saturated fats and virtually anything that has been refined by human intervention. The abominable boneless, skinless (and may I add tasteless) chicken breast has become ubiquitous.
I can understand ditching the bones, but I and many other people insist that the skin is the best thing about a roasted chicken or turkey, in terms of flavor and texture.
Raw chicken skin consists of about 13 percent protein, 32 percent fat and 55 percent water. Turkey skin is 13 percent protein, 39 percent fat and 48 percent water. Those components are arranged, as in many other animals (including us mammals), in three layers of cells: the outer epidermis, the middle dermis and the inner hypodermis. The proteins, mainly collagen, elastin and keratin, are distributed throughout the layers in different amounts: collagen and elastin mostly in the dermis and the fat (adipose) cells mainly in the hypodermis. Elastin makes the skin somewhat rubbery. It is actually stretched over the bird’s body under tension, like a wet suit on a scuba diver.
The fat, of course, is the source of some people’s nutritional concern. According to the USDA, raw chicken fat contains 9 percent saturated, 14 percent monounsaturated and 7 percent polyunsaturated fatty acids. (Turkey: 11 percent saturated, 15 percent monounsaturated and 12 percent polyunsaturated.) This works out to about 3 grams of saturated skin fat per pound of whole chicken. An ounce of the skin of turkey has 3 grams of saturated fat. During roasting, much of the fat is rendered and drips to the bottom of the pan, so by the time it gets to the table, there’s even less fat in the skin.
Is this a cause for alarm, a reason to throw away our poultry skins? Not when they are considered a delicacy in many less-paranoid parts of the world. Are Jews of Eastern European heritage going to give up their gribenes or grieven — crisp chicken-skin cracklings, a kosher version of pork chicharrones? Or the Japanese their torikawa — crisp grilled chicken skin? Must I stop stripping the skin from the breast of a rotisserie chicken and eating it first?
Never. Not over a lousy 3 grams of fat.
What makes cranberries gel?
One word: pectin.
Pectin is a complex carbohydrate (a polysaccharide) found mainly in the cell walls of fruits and certain other parts of plants. Apples, citrus rinds and cranberries are particularly high in pectin.
When an acid is present, pectin reacts with sugar to form a gel, a homogeneous dispersion of water molecules throughout a molecular network within the solid carbohydrate. The resulting substance has jellylike properties. It behaves somewhat like a liquid, yet holds its shape when cooled, like a solid.
As in most chemical reactions, pectin and sugar must be present in a certain proportion for the conversion to occur properly. The right proportions, by weight, are one part sugar and one part water to two parts cranberries. For example, 6 ounces (about 3 / 4 cup) each of sugar and water to a 12-ounce bag of cranberries. The berries themselves contain the necessary acid. Insufficient sugar, in particular, can lead to a sauce that won’t gel, and you may wind up with cranberry soup.
Too much water — more than is necessary to dissolve the sugar and hydrate the pectin — also will keep the sauce from setting. An easy way to fix soupy sauce is to cook it longer to evaporate excess water. Just remember that it won’t “set up” until it has cooled so the melted gel solidifies. (Strictly speaking, the words “melt” and “solid” don’t apply to gels.) But be careful: If you cook the sauce too long, you’ll wind up with red rubber.
Hint: The directions on a bag of fresh cranberries can’t be beat.
Wolke is a professor emeritus of chemistry at the University of Pittsburgh and a former Food section columnist.