Sorbets are typically made of simple syrup (water and sugar) and suitable flavouring. As with other ice creams, air also needs to be incorporated in the base mixture to avoid rock-solid end results. Sorbets normally do not contain any dairy products, and without the fat these contain, the challenge is usually to keep down the size of the ice crystals. As with ordinary ice cream, large ice crystals will affect the texture negatively, away from the desired “smooth” and closer to the undesired “icy”.
The main keys to success? Enough, but not too much, sugar, and fast freezing!
The basic sugar syrup
Since sorbets can be made on many different things (sweet fruits, more sour fruits, wines of differing sweetness …), some serious makers would insist on matching the proportions (sugar/water-ratio) to the specific fruit (or other flavouring) to be used. A classic proportion is 65 % sugar to 35 % water.
Real serious sorbet-making should, however, preferably use a special instrument to check the sugar content of the syrup. One such instrument is the refractometer, which measures the level of sweetness on a so-called Brix (°Bx) scale. Also used is the density measuring Baumé hydrometer, using the so-called Baumé scale. A sorbet mixture (at least fruit-based ones) should keep 17-18 °Baumé / 30-31 °Brix.
Why? If the sorbet mixture is too sweet, the level of sugar will prevent the sorbet from freezing properly. If the level of sugar is too low, however, the sorbet will freeze too hard, and the flavour is likely to taste “watered down”. Also with regard to the flavour, remember that the un-frozen base should taste somewhat sweeter than you would like, since the cold will dull the sensation of sweetness experienced in the final sorbet.
Some propose a 50% water/50 % sugar-ratio for a versatile base syrup suitable for many occasions. As always, you are invited to experiment around yourself!
If you would like to minimise experimentation but still want to be serious about sugar levels (but not serious, or rich, enough to buy special instruments to check it), I will pass on an old kitchen trick.
The Egg test – estimate sugar levels without using special instruments
This test only requires a fresh egg. On the web, I have seen it well explained by Jenni Field. Basically, you drop the fresh egg (wash it first!) into the mixture. Watch carefully how much of the egg is showing up above the surface once it has resurfaced. If the egg remains submerged, or if less than about 2 1/2 cm is showing above the surface, add more sugar. Likewise, if more than 2 1/2 cm is showing up, add more juice, fruit purée or water to the mixture.
When the “ideal” 2 1/2 cm elevation is achieved, this translates into the likewise ideal 17-18 °Baumé!
(Why use fresh eggs? When eggs become older, they begin to contain more and more air. Eventually, an old egg would begin to float almost regardless of sugar content, which would render the Egg test rather worthless)
How to actually create the simple sugar syrup
There are basically two ways of preparing simple sugar syrup.
Cooking the water and sugar together seems to be the traditional, generally suggested option in most ice cream literature I have come across. Simmering the water and sugar for about five minutes is usually enough to create the viscous blend you want (see below for a chemical explanation of why sugar syrup is something more than just an uncomplicated mixture of water and sugar …).
However, there is also the much speedier way (let us call it the Hurried bartender’s method) which also tend to work quite well, and which does not require any cooling down: Simply mix water and sugar together without any heating. Whisk/stir/shake until all the sugar has dissolved. Voilà – ready to use! Obviously, if you do not find this solution ‘syrupy’ enough, do simmer the mixture!
Extra reading – Sugar syrup chemistry
What happens when sugar mixes with water? Chemically speaking, the sucrose (a disaccharide) splits into the simple sugars glucose and fructose (monosaccharides). At the same time, strong hydrogen bonds are created between the water and the sugar molecules. This is actually very good for sorbets, as the increased amount of sugar molecules not only increases the syrup’s capacity to bind water (obviously super-important for avoiding unwanted iciness and improving texture), lowers the freezing point (increases scoopability) and increases the shelf life. The general sweetness is also increased with about a third (and then in a somewhat “fresher” way, since most of the ‘new’ sweetness comes from the fructose). This process is greatly speeded up by heating, which is probably why boiling sugar syrup has come to be the traditionally preferred method.
A syrup, once made, can be stored in a bottle or closed jar in the refrigerator for about 2-3 weeks. When the syrup starts to become cloudy, it is about to begin to mold and should be discarded.
Base recipe – Sorbet
Prepare a simple sugar syrup with, suggestively, 500 ml (2 cups) sugar and 300 ml (1.3 cup) water [or 500 ml (2 cups) water and 500 ml (2 cups) sugar, depending on which of the two base ratios you would like to follow]. Remember, however, that the important point is not the exact amounts of liquid/sugar above but the proportions used when preparing the sugar syrup (50/50 sugar/water, or 65 sugar/35 water).
If you cook the syrup (about 5 minutes of simmering, or less, should do), let it cool down.
Mix the cooled down syrup with an equal volume of berries- or fruit purée/fruit juice, and add the juice of 1 lemon (in order to avoid an overly sweet sorbet).
Freeze according to the instructions of your ice cream machine. If you do not have an ice cream machine, still-freeze in your freezer (go here for more exact instructions).
Before freezing the mixture, possibly add just a little alcohol (or some inverted sugar, such as corn syrup, agave nectar, glucose syrup or honey), in order to ensure that the sorbet won’t freeze too solid. Just go careful with the alcohol, otherwise the mixture might never freeze.
UPDATE: For general ways to improve sorbets not covered above, check out this general post on various ways to make them last longer in the freezer, and this post devoted to the particular, classic method of adding so-called Italian meringue and taking the sorbets to new heights 😉