A special case of flavor extraction is the making of flavor extracts themselves: preparations that serve as instant sources of flavor for other dishes. The most common materials used for extraction are oils, vinegars, sugar syrups (especially for flowers), and alcohols (for example, a neutral vodka for flavoring with citrus peel). The herb and/or spice is usually bruised to damage the cellular structure and make it easier for the liquid to penetrate and aromas to escape. Oils, vinegars, and syrups are often heated before the herb or spice is added to kill bacteria and facilitate their initial penetration into the tissue, then are allowed to cool to avoid changing the flavor. Delicate flowers may require less than an hour to flavor a syrup, while leaves and seeds are usually infused in the extracting liquids for weeks at a cool room temperature. When the extract has reached its desired strength, the liquid is strained off and then stored in a cool, dark place.