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The Science of Vinegar

This pantry staple is one of the most useful ingredients in your kitchen. Here’s why.

by David Joachim, Andrew Schloss

fromFine Cooking
Issue 116

There are dozens of types of vinegar. Anything that contains sugar, from grapes to rice, can be made into this common ingredient. It’s a two-step process: First, yeast feeds on the sugar and ferments it into alcohol; second, harmless bacteria called Acetobacter feed on the alcohol and ferment it into a sour-tasting byproduct, which the French first dubbed vinaigre, or “sour wine.” That sourness, or acidity, is essential to our enjoyment of food—look no further than a salad dressing, sauce, or marinade to taste why. But aside from enhancing flavor, vinegar also has the unique ability to alter the color and texture of foods. Read on to learn how it works its magic.

Why does vinegar taste sour?

Acids are volatile molecules that release positive hydrogen ions. These ions have a pro-found effect on the other molecules in foods (more on that later). They can also affect the cells in your body, but fortunately our taste buds help us regulate the amount of free hydrogen ions we eat by making us perceive acids as sour tasting. A small amount makes food taste bright and fresh, while too much tastes unpleasantly sour.

Most vinegars don’t just taste sour, though. The acid in vinegar is primarily acetic acid, which makes up 4 to 7 percent of table vinegars. Distilled white vinegar, for example, is 5 percent acetic acid and 95 percent water; it has the harsh sourness of acetic acid alone. In other vinegars, that harshness is tamed by other acids. Red wine vinegar tastes of tartaric acid, the structure-building acid in wine, and apple cider vinegar has the green apple flavor of malic acid. Depending on the variety, vinegar can contribute sweet, buttery, malty, and savory flavors, as well as woody aromas from aging in wooden barrels, evidenced in balsamic and sherry vinegars.

How does vinegar affect the color of fruits and vegetables?

The hydrogen ions released from vinegar displace magnesium in the chlorophyll molecules of green vegetables, turning bright green chlorophyll a drab olive color. That’s why flavoring broccoli with vinegar discolors it. To keep green vegetables from discoloring, flavor them with non-acidic ingredients instead, such as citrus zest, herbs, and spices.

Vinegar has the opposite effect on plant pigments known as anthocyanins: It brightens red fruits and vegetables like sour cherries and red cabbage. Anthocyanins appear red under acidic conditions and turn blue under alkaline conditions. When boiled in alkaline cooking water (urban water is often made alkaline to prevent pipe erosion), red cabbage turns turquoise blue. Similarly, sour cherries develop a blue outer ring in muffins because baking powder makes the batter alkaline. The trick to keeping anthocyanin pigments bright red is to maintain acidic conditions. Cooking red cabbage with vinegar keeps it red; cherries stay red in muffins if there’s a little vinegar (or another acid, like buttermilk) in the batter.

Vinegar can also preserve the color of white-fleshed produce like apples and potatoes, which discolor when cut. The cut cells release enzymes that cause phenolic compounds previously held within the cells to bond with oxygen, creating brown-tinted melanin pigments in the flesh. Adding an acid like vinegar slows down the enzyme activity and helps prevent browning.

How does vinegar alter the texture of food?

Vinegar’s free hydrogen ions interact with proteins and starches, altering their molecular structure and changing their texture and consistency. Here are some of the practical ways vinegar can affect texture:

It coagulates protein. Vinegar breaks the chemical bonds that hold protein strings in a twist, causing the proteins to denature or unravel and “tenderize.” This commonly occurs when meat is marinated in a vinegar-based marinade. With continued exposure to acid, the unraveled protein strings eventually bump into each other and form new bonds. These bonds gradually coagulate the proteins, re-forming them into a much more rigid structure. The popular South American raw dish ceviche is based on acid’s ability to denature and coagulate protein without heat: sliced raw fish is “cooked” in acidic ingredients until opaque and firm, but without the flavor changes that would occur if the fish were exposed to high temperatures. (On the other hand, acids can “overcook” meats when they marinate for too long, ultimately making them mushy.)

Similarly, when fresh milk is mixed with vinegar, milk proteins called “casein” coagulate into curds. Buttermilk, yogurt, sour cream, and cheese are all based on this thickening effect. In cheese making, the acid is usually produced through the fermentation of lactobacillus bacteria, but simple cheeses like cottage cheese can be made by adding vinegar to fresh milk.

Protein coagulation also occurs when you poach eggs in water with vinegar. The heat of the poaching liquid alone causes the egg proteins to form bonds that help the egg white set neatly around the yolk, but with the addition of vinegar, these bonds form earlier. Because the proteins coagulate before they are unraveled fully by the heat, they can’t intertwine and bond as tightly with one another. As a result, the cooked eggs are more tender.

It keeps pasta from sticking. When pasta is cooked in alkaline tap water, calcium and magnesium ions in the water can cause the pasta to release more starch, making it sticky on the surface. Adding a tablespoon of vinegar to the pasta water acidifies the water and reduces stickiness. This is helpful when using pasta in a salad, where stickiness is undesirable, and the flavor of the vinegar just blends in with the tartness of the dressing.

It thins overly thickened sauces. When you add a touch of vinegar to a sauce that’s thickened with flour or cornstarch, the acid breaks the starch chains that thicken the sauce into shorter pieces, which thins out the sauce much more than the same amount of water or other nonacidic liquid would.

It makes high-rising egg foams. When you beat egg whites for a meringue or soufflé, tough proteins can coagulate into tiny lumps, making the beaten whites grainy. Adding about 1/4 teaspoon vinegar per egg white slows the formation of sulfur bonds (the tightest and toughest bonds involved in protein coagulation), so that the beaten whites are silky and glossy and rise higher and more evenly when baked. The flavor of such a small amount of vinegar is barely noticeable. (Cream of tartar is an acid and works the same way.)

It maintains the texture of cooked beans. Plant cells contain substances called hemicelluloses, which act as a kind of glue to hold the cells together. Acids make hemicelluloses more stable and less soluble. That’s why baked beans that are cooked in a sweet-and-sour sauce can be simmered for hours and reheated repeatedly without disintegrating.

How Acidic is vinegar?

The acidity of an ingredient is measured by its pH. Pure water is considered neutral and has a pH of 7. Anything with a pH higher than 7 is called alkaline. Anything with a pH lower than 7 is acidic, and the lower the pH number, the stronger the acid. The pH scale is logarithmic, which means that each number in the scale describes acidity that is 10 times stronger than the number preceding it.

 

PH OF COMMON ACIDIC INGREDIENTS
(from strongest to weakest)
 
Ingredient    pH 
Lemon juice 2.1
Vinegar 3
Orange juice 3.2
Apple juice 3.5
Wine 3.3-3.7
Tomato juice 4
Yogurt/buttermilk 4.5
Milk 6.7
Pure water 7

 

Photo: Scott Phillips

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