This is different from the case. The casing is a liquid preparation that is added to tobacco to alter its taste or aroma or to improve its ability to retain moisture and hold flavorings. Some may contain simple chemicals, such as citric acid. Flavor casings may include essential oils, which are either alcohol-based or polypropylene glycol (PPG) based. They may also be common food flavorings, such as honey or syrup or brandy, or other commercial alcohol products. Casings that are not intended to alter the flavor or aroma, but rather to increase the hygroscopic (water holding) nature of the tobacco include PPG or glycerin, or both. PPG also acts as a mold inhibitor, while glycerin is mildly bacteriostatic. These latter casings are what allow commercially distributed cigarettes or especially pipe tobacco to retain their soft, look and feel for months or years on the shelf. Essential oils alone tend to rapidly lose their aroma, in the absence of PPG. It is PPG that permits the so-called black Cavendish to remain black in appearance, instead of its natural deep brown. PPG does not impact aroma, but it does affect the taste of tobacco.