Ice cream - the chemistry
The best ice cream has a wonderful flavour, texture and body. It should melt in the mouth to a creamy liquid at a temperature which feels neither too cold nor too warm.
It is essential to get the balance of ingredients right to make good ice cream and the relative quantities of fat, water and sugar need to be carefully adjusted.
Ice cream freezes at different temperatures depending on the amount of sugar. The greater the amount of sugar in the mixture, the lower the freezing temperature; this affects the size and structure of the ice crystals, and therefore the hardness of the finished product.
The fat (from milk, cream or egg yolks) forms an emulsion and this helps keep ice crystals small by physically getting in the way as the crystals try to clump together and grow. It is the fat element which produces a rich ice cream and also adds to the impression of smoothness on the tongue. (Many commercial ice cream manufacturers add other, cheaper fats to their ice creams, along with emulsifiers to keep the fat in suspension and to try to maintain a good texture.)
Equally important to good ice cream is the amount of agitation (churning) given during the freezing process, and the rate of freezing. A smooth ice cream is formed when the mixture is churned throughout the freezing process because the ice crystals are prevented from aggregating into larger lumps. Churning introduces air into the mixture as it freezes - cheap commercial ice creams often contain as much as 50% air, such that 'frozen foam' would be a more accurate description of this inferior product.
Rapid freezing encourages the formation of many very small ice crystals, rather than fewer large ones, thus producing a smooth-textured ice cream or sorbet. Rough textured granitas are left to freeze more slowly and the ice crystals are only broken up during a later stage of freezing.
The addition of flavouring ingredients, noteably alcohol, chocolate and/or fruit purées will also alter the freezing properties. Alcohol, in particular, significantly lowers the freezing point - if too much is added it is possible that the product will never freeze. So beware - in the case of alcohol in ice cream, it is definitely not a case of 'the more the merrier' ;)