Technique: Zipper Bag Freezing
If you don't have an ice cream maker, this is an easy and economical method. You will need 2 different zipper bags, one large, e.g. 2 Gallon size, and one smaller, such as a quart bag. Place about two ice cube trays worth of ice in the larger bag along with 1/2 a cup of salt. Fill the smaller bag with your ice cream, sorbet, or granita mixture and seal. Next, place the smaller bag inside the bigger bag with the ice and salt, sealing up the whole thing. Now shake and massage the whole thing for approximagtely 10-15 minutes until the texture of the liquid in the inside bag gets thick and reaches your desired texture. If it doesn't freeze comepletey, or you want to use later, simply remove the small bag, rinse in cold water, and place in the freezer. The reason this works it that the salt reduces the freezing temperature of water in the ice, turning it into a liquid below 32 degrees Fahrenheit (zero Celsius). This much colder liquid is more efficient at absorbing the heat form the smaller bag and freezing it.