Muddling brings out the soft and tender elements of fruit and vegetables by crushing the firmer flesh and revealing the sweet and aromatic inner flavors. Muddling is the hidden secret to blending together flavors without pureeing them, changing their texture. A mint julep is the perfect example of muddling, in this case mint with raw sugar to bring out the flavors instead of pulverizing them.
I like to use wood for my muddling, but high volume bartenders will use a plastic one or one made of stainless steel. Wood is just too difficult to clean on the fly and it cannot be sterilized in the commercial chemicals without deteriorating. You can muddle roasted fruits with syrups and fragile herbs before adding liquor and soda water. That’s what makes for a wonderfully ‘muddled’ cocktail — you’d muddle for only a few presses so you don’t turn the herbs to chlorophyll, making them unpalatable and ruining your cocktail.