Since cassis has a high alcohol content, it will depress the freezing point (apparently at least 7 degrees). In the same way vodka will stay liquid in a freezer, depending on the proof level of the brand used

The sauce would have had a high sugar content, not only from the sugar I assume you would have added to make it, but also from the fruit itself. This would have been concentrated by boiling, as this evaporates the water away and leaves a more concentrated sugar solution behind. The more concentrated the solution is, the lower it's freezing point is; solutions of some chemicals (perchlorates spring to mind) can lower the freezing point of water by tens of degrees celsius. An everyday example of this is the freezing of seawater. Where there's a freshwater lake in close geographical proximity to a sheltered harbour, neither will have much movement most of the time, and when the temperature drops below zero, the lake will start to freeze round the edges long before the water in the harbour. If the temperature were to drop much lover, the effect would become more pronounced, with the lake probably being completely frozen over before the sea started to exhibit ice.

Your very tasty-sounding sauce contains water, polysaccharides, glycoproteins, flavouring compounds, salts, sugars, (including a bit of trehalose, nature's antifreeze!) and neither least nor last, ethanol. At freezer temperatures the fine crystallisation of ice would extract fairly pure water, perhaps together with some organic acids and the like.  

Extraction of that water from the liquid leaves the solution with a higher concentration of small molecules that hinder further freezing, and larger molecules that prevent the formation of large, hard ice crystals. The product is  your slush of ice in strongly flavoured liquid.

The smaller molecules, mainly ethanol, possibly assisted by trehalose and salts, act as antifreezes. Liquid nitrogen freezes ethanol pretty smartly, but other commonly available substances don't, not even dry ice. Accordingly, selective freezing can concentrate alcoholic drinks more conveniently than distillation; the main disadvantage is that desirable flavours such as certain organic acids freeze out, while acrid phenolic compounds pass through in the ethanol. That is fine for your sauce; you get everything back when you re-melt it, but no matter how cold you set it, your freezer gives you an  ethanol slush, which fortunately is more conveniently usable than a solid cassis club.

Personally I dislike mint sauce, but I could go for your blackcurrant recipe!

Cheers,

Jon

Don't know the answer, but have you got the recipe?

Alice Collins, Washington DC, US

The blueberry and cassis sauce probably contained a lot of sugar (natural and added). Dissolved substances lower the freezing point of a liquid.

For instance, a 42 per cent sucrose solution - the highest concentration for which I have data - doesn't start to freeze until the temperature gets down to -4.45 °C. And the zero point of the Fahrenheit temperature scale was, according to some accounts, determined by the temperature at which a mixture of ice and salt froze. This is close to -18 °C.

This lowering of the freezing point aside, there is also the possibility that the sauce was so thick that ice could not form.

As for why your sauce kept so well for a month despite not freezing, the low temperature would have slowed any decomposition, as it would whether a substance freezes or not, while the high sugar content has a preservative effect.

Eric Kvaalen, La Courneuve, France