Many recipes call for “reduction.” Reducing a liquid by boiling it down until it is half or third or a quarter of the original volume intensifies the flavor. A concentrated liquid adds more flavor than a thin liquid. Great sauces are made from reductions because the flavor base packs a wallop. Reductions are critically important to pan sauces, bearnaise, and beurre blanc sauces.

How far should a liquid be reduced? Look for surface tension. When water boils, the bubbles disappear immediately. When syrup boils, the bubbles linger with more surface tension. A liquid is reduced enough when the bubbles look like syrup.

What liquids should be used? Traditionally, wine and stocks are boiled down to make flavorful sauces. Reduced veal stock is known as demi-glace, which is liquid gold in making French recipes. Although red or white wines will make delicious reductions, I often substitute port wine because the flavor is so much richer.

Since it is autumn and I’m obsessed with apples, I’ve been boiling down apple cider to make wonderful salad dressings (just add apple cider vinegar and olive oil), a sauce for poached trout (just whisk cold butter into the reduction), and my current obsession of baked apple sabayon.

Apple Sabayon

for the reduced cider:

• 2 cups apple cider

In a saucepan, bring the cider to a boil and cook until reduced to 1/4 cup. Set aside.

for the sabayon:

• 1/4 cup reduced cider
• 4 egg yolks
• 1/4 cup sugar
• 1 tablespoon rum or calvados (apple brandy)

In a stainless steel bowl, whisk together the cider, yolks, sugar, and rum with a dash of salt. Set over a pan of simmering water and whisk constantly until thick and fluffy like hollandaise. Serve over baked apples. To bake apples, remove the core with an apple core and trim the bottom so that the apple stands up. Dot the apples with butter and brown or maple sugar. Cover. Bake at 350 degrees for about 45 minutes, or until softened.

The next sauce-making class at Kitchen Conservatory is Sauce Sense on November 3.