A piping bag is a kitchen tool used for decorating cakes, pies, cookies, and other pastries, and also for squeezing doughs, batters, creams and puréed ingredients like potatoes onto platters or baking sheets.
A piping bag is basically a bag in the shape of a cone. The bag is filled with the ingredient to be piped and then twisted to force the ingredient toward the pointy end of the cone. The pointy end is open and can be fitted with any number of tips which will produce a different decorative effect.
Piping bags come in all sizes from very large for doing big jobs to extremely tiny ones for doing finely detailed work on cakes and cookies. Pastry cream is often piped through a bag, and choux pastry dough is piped onto a baking sheet for making cream puffs or eclairs. Piping bags are also used for filling pastries, like doughnuts that are filled with cream or jelly.
Piping bags are usually made of canvas, which makes them durable, with a plastic coating on the inside which makes them easy to clean and reuse. But disposable plastic ones are often used, too.
A piping bag is usually fitted with a coupler at the pointy end, which makes it easier to swap out different tips — otherwise, you'd have to reach through the bag of frosting to change the tip.
The easiest way of filling a piping bag is to attach the desired tip, then fit the empty bag point-down into a glass quart jar. Then you can wrap the wide end of the bag over the mouth of the jar like a cuff and then simply spoon your ingredient into the bag.
Also Known As: Pastry bag, decorating bag