First, drink some black coffee. Next, rub your tongue against the roof of your mouth. It should feel a little rough, like very fine sandpaper: the tiny bumps on your tongue, called papillae, are raised just enough to create friction against your palate. If you now add cream to your coffee and try again, the sensation should be much smoother — almost velvety. A layer of fat and mucous is now coating your tongue, providing lubrication and preventing friction. What you have just done was, until very recently, the most accurate method for evaluating the oral perception of fat — the precise degree of tongue-coating creaminess in milk, mayonnaise, or chocolate pudding.
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