Make Ice Cream in a Plastic Bag

Yes, it sounds dangerous and the potential for messes seems highly likely, but you’ll be surprised at much you’ll enjoy it when you make ice cream on your boat miles from any shop.
Getting Started
As a general guide, the following quantities will be enough to make around one scoop of the frozen creamy stuff, so adjust your ingredients depending on how much you’re craving.

• 1/2 cup milk or 1/4 cup of milk and 1/4 cup of fresh pouring cream
  1/2 teaspoon vanilla
  1 tablespoon sugar
  4 cups crushed ice
  6 tablespoons salt
  2 0ne litre size Zip-loc bags
  1 four litre size Zip-loc freezer bag
  a hand towel or gloves to keep fingers from freezing as well!

Take a small sized sealable (Ziploc type) bag and add 1/2 cup of milk, (or 50/50 milk and cream), and 1 tbsp of sugar. For added flavour, use 1/2 tsp of vanilla. Seal it up, getting rid of as much air as you can. Now double bag it inside a similar-sized bag.

Throw 4 cups of crushed or cubed ice with 6 tbsp of rock salt in a large freezer bag. You can use any kind of salt, as you won’t actually be eating it but it does work better with larger salt. The salt is there to lower the freezing point of water, if you want to get technical. Put in your little double bag, and seal the large bag around it, getting rid of as much air as possible.

Wrap the bag in the towel or put your gloves on, and shake and massage the bag, making sure the ice surrounds the cream mixture. Five to eight minutes is adequate time for the mixture to freeze into ice cream.
Make Your Ice Cream Extra Yummy
Now that you know the basic steps, get creative and up the yum factor by using flavoured milk, adding some fruit puree towards the end of the process, or even a spoon or two of ready-made fudge sauce. You can also try substituting cream in place of milk for a bit more calorie dense, but super yummy treat
Tips
Freezer bags work best because they are thicker and less likely to develop small holes, allowing the bags to leak. You can get away with using regular Zip-loc bags for the smaller one litre sizes, because you are double-bagging. Especially if you plan to do this indoors, we strongly recommend using large size freezer bags. We also find that a small splash of water with the salt and ice expedites the process, no to much just 2-3 tablespoons.

• If you are making a larger quantity expect that the process will take a bit longer.
• Want it in a cone? Simply cut the corner of the ziplock bag and pipe it into the cone.

What does the salt do?
Just like salt used on icy roads in the winter, salt mixed with ice in this case also causes the ice to melt. When salt comes into contact with ice, the freezing point of the ice is lowered. Water will normally freeze at 0 degrees C. A 10% salt solution freezes at -6-7 degrees C, and a 20% solution freezes at -16 degrees C. By lowering the temperature at which ice is frozen, we are able to create an environment in which the milk mixture can freeze at a temperature below 0 degrees C into ice cream.