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A pastry bag (or piping bag in the Commonwealth) is an often cone- or triangular-shaped, hand-held bag made from cloth, paper, or plastic that is used to pipe semi-solid foods by pressing them through a narrow opening at one end, for many purposes including cake decoration. It is filled through a wider opening at the opposite end, rolled or twisted closed, and then squeezed to extrude its contents.
Though a circular nozzle is quite useful for making round shapes and for filling pastries such as profiteroles, many differently shaped nozzles are commonly used to produce star, leaf, and flower-petal shapes.
Aside from icing, pastry bags are commonly used to shape meringue and whipped cream, and to fill doughnuts with jelly or custard. They are used to form cream puffs, éclairs, and ladyfingers. When presentation is especially important, fluted tips can be used to shape savory foods such as filling for deviled eggs, whipped butter, and mashed potatoes (especially for Pommes duchesse).
A high-quality reusable bag is often made from tightly woven nylon, polyester, rubber or waterproofed (plastic-coated) cotton. Medium quality bags are similar, except they are not so tightly woven and may let some contents seep through the weave or the seams. After use, a reusable bag is washed by hand and hung open to dry. A high-quality bag may last for many years.
Pastry bag users who do not have a dishwashing machine may prefer to use disposable bags, and thus avoid hand-washing messy bags. Disposable bags are ready-made in inexpensive plastic. A plastic food storage bag is also commonly used as a pastry bag. For small quantities and fine piping, a pastry bag can be made by rolling cooking parchment or wax paper into a cone, filling it, folding the wide end several times to close it, and then cutting the tip into whatever shape is desired. This is especially useful for small quantities of melted chocolate, since a very small hole can be cut and the bag can be discarded when it cools and becomes clogged.
Tips come in sets of interchangeable pieces; expensive tips can be purchased one at a time. They may be chrome-plated or stainless steel, or plastic. Each tip is cone-shaped, with a base too large to fit through the small opening in the bag; they are to be inserted through the larger opening before food is spooned in. Tips can be used with pairs of adapter rings: an inner ring is dropped inside the bag and pushed part way out the hole, a tip is slipped over the ring, then an outer ring is slipped over the tip and screwed onto the inner ring. This permits the tip to be changed without emptying the bag. Some inexpensive sets are of disposable plastic film with a drop-in ring and screw-on plastic tips (see image above). Many foods (including frosting and pressurized "spray can" whipped cream) can be purchased in disposable packaging designed to serve the function of a pastry bag.