Also known as superfine sugar, Bar Sugar, Berry Sugar, Castor Sugar, Extra Fine Sugar, Fruit Sugar, Instant Dissolving Sugar, Ultrafine Sugar (Canadian Sugar Institute)
Caster (Castor) sugar is white refined Cane or Beet sugar with superfine grains. Its crystal size is the finest of all types of granulated sugar. Caster sugar is valued for its quick dissolving properties.
It’s ideal for meringues but can also be used in baked goods, desserts, cocktails and drinks. It also adds volume when it is whisked with butter resulting in a light textured cake.
Castor sugar is excellent for sprinkling over fruit or cereals, or in creamed mixtures.
It’s used commercially in powdered preparations and dissolves easily in cold beverages.
Unrefined golden caster (castor) sugar is also available. It’s made from unrefined cane sugar and is popular for it’s pale golden colour and subtle buttery taste. It has the natural coating of cane molasses.
How to store caster sugar
Store in an airtight container in a cool dry cupboard or pantry.
Point to Note
Frankly speaking, I used Castor sugar when I was a student in Europe and the normal white granulated sugar sold here in Canada has got very fine grains and would still be a good substitute for castor sugar, in my opinion. If you disagree, please let me know. Below is caster on the left and granulated table sugar on the right.
myfavouritepastime.com
My Favourite Pastime 
Castor sugar is great for pavlovas. I’ve never spelt it caster, but I understand that form is acceptable in many other countries.
It’s like Pitta and Pita or Dal and Dhal. It’s also good for meringues!
Liz