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I made a sauce for lamb from blueberries and , but made too much so put some in the freezer. It wouldn鈥檛 set solid and spent a month being malleable at -7 掳C. It tasted OK when we used it later. Why wouldn鈥檛 it freeze solid?
鈥� Don鈥檛 know the answer, but have you got the recipe?
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Alice Collins, Washington DC, US
鈥� 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 鈥� the highest concentration for which I have data 鈥� doesn鈥檛 start to freeze until the temperature gets down to -4.45 掳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 掳C.
鈥淒issolved solids, especially salts or sugars such as sucrose, lower the freezing point of a solution鈥�
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
鈥� There would have been a lot of alcohol (ethanol) and a lot of sugar (sucrose) in the sauce. Both are very effective at lowering the freezing point. In years past, ethanol was the basis of commercial antifreezes, for example, and I always put some (which in spite of its name is ethanol) in the windscreen washer of my car to prevent it freezing up.
Guy Cox, Electron Microscope Unit, University of Sydney, Australia
鈥� A temperature of -7 掳C is not very cold for a freezer and not adequate for long-term food storage. I would reduce the temperature via the controls if possible, or replace the freezer.
John Kinross, School of Life Sciences, Edinburgh Napier University, UK