Q: What thickener is best for sauces and gravies, and when should I add it? I always use flour, but it tends to clump. A: Most sauces and...

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Q: What thickener is best for sauces and gravies, and when should I add it? I always use flour, but it tends to clump.

A: Most sauces and gravies are thickened with some kind of starch. The most common are flour and cornstarch, though potato starch, arrowroot and tapioca flour also work well. Every cook has a favorite thickening ingredient and method, and each has its merits — the key is not so much choosing the right thickener as adding it properly.

If you attempt to thicken a pan sauce or gravy by simply stirring flour into the simmering liquid, you will inevitably end up with lumps. This is because the starch around each lump of flour expands rapidly when it comes into contact with hot liquid, forming a sort of waterproof gel that prevents the granules from separating properly. The same is true for any other starch.

To prevent this, you need to separate the granules before adding them to the sauce so that they can slowly disperse and expand to create the desired thickening effect.

You can accomplish this in several ways. The first is to use what’s called a roux. Made from a mixture of fat — either pan drippings or butter — and flour, a roux is slowly cooked on its own before it is added to the sauce. The fat helps the starch to expand and separate, and it lubricates it so it can be smoothly incorporated into the liquid. A roux should be cooked, then cooled slightly, then whisked into the sauce when you’re ready to thicken it. The precooking also eliminates the unpleasant raw-flour taste that sometimes occurs if a sauce isn’t simmered long enough.

Another method is to use what is called beurre manié or kneaded butter. This is essentially the same as a roux, only the flour is worked into the butter by hand or with a fork, then formed into small balls and added, uncooked, to a sauce.

This works as a last-minute thickener, but it should be used sparingly — too much may leave a floury taste behind.

Perhaps the easiest and quickest thickening method is to use a slurry, which is cornstarch stirred into a small amount of cold water or stock, then whisked into a simmering sauce. It thickens almost immediately, and creates a slightly glossy appearance. The same method can be used for nearly any starch, but it’s important not to overestimate the amount of starch you need, or to overheat the sauce — too much heat can cause the starch to break down and the sauce to thin.

Every recipe has different requirements, but as a rule, if you want a medium-thick sauce or gravy, you should add about 2 tablespoons of flour per cup of liquid. If using potato starch, arrowroot or tapioca, you’ll need about half as much, and if you’re using cornstarch, slightly less than that — about 2 to 2 ½ teaspoons per cup.

Q: How do you clean the inside of a washing machine?

A: Many appliances need regular maintenance: The inside of an oven, for example, needs to have burns and spills cleaned out, just as a coffee maker needs to be flushed with vinegar occasionally. It doesn’t always occur to us, however, that our washing machine requires similar upkeep.

Technically, dirt and lint should vanish with the rinse water at the end of a wash cycle. But there’s often a small amount of residue left behind.

This buildup might appear as spots on your just-washed laundry (though stains are typically not permanent — relaundering in a cleaned machine should eliminate them), or it may just seem like your clothes aren’t getting as clean as they should.

In any case, such residue eventually inhibits your washing machine from operating at peak performance.

Most accumulated dirt on the inside of the washer can simply be wiped away with a clean, damp cloth. Running a short, hot wash cycle with detergent, then rinsing with plain water in the empty machine, should take care of any remnants. Performing this chore periodically should keep your machine working well.

If your washing machine is exceptionally dirty or requires sanitizing, make a disinfecting solution of chlorine bleach and detergent — use 3/4 cup bleach and 1 tablespoon powdered laundry detergent for every gallon of warm water — and let it sit in the machine for a few minutes. Then drain and rinse a few times with plain water to eliminate any traces of dirt or bleach.

Questions may be sent to mslletters@marthastewart.com or Ask Martha, care of Letters Department, Martha Stewart Living, 11 W. 42nd St., New York, N.Y. 10036. Sorry, no personal replies.