Mould has always been a menace on my blocks of Gouda and Edam cheese, which I store under a cheese cover. Recently my wife told me to put a lump of sugar under the cover with the cheese, and I have not seen mould since. The sugar gets moist and slowly dissolves, but nothing else seems to happen to it. My wife learned this from her mother, and so it must be an old and possibly widespread remedy. Why and how does it work?
This habit is also common in northern Germany, where the explanation given is quite simple. The sugar lump takes up moisture from the air trapped under the cheese cover, slowly dissolving as it does so. The relatively dry air reduces the suitability of the environment for cheese moulds.
David Fleet, Suederstapel, Germany
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The sugar cube absorbs water, lowering the relative humidity so that mould can no longer grow on the surface of the cheese. Salt would work just as well, as would saturated solutions of sugar or salt 鈥� saturated solutions are those that still contain some undissolved solute.
This forms the basis of humidity control in museum display cabinets. If the humidity is too high, undesirable moulds grow, but if it is too low, wood and leather might crack. Saturated solutions of different salts can peg the relative humidity anywhere from 10 per cent to 90 per cent. For example, a saturated solution of lithium chloride will maintain a relative humidity of 11 per cent, while a saturated solution of common salt keeps the relative humidity at around 70 per cent.
鈥淭he sugar cube absorbs water, lowering the relative humidity so mould can no longer grow鈥�
John Hobson, Devizes, Wiltshire, UK
The sugar will draw liquid from the air by its intrinsic hygroscopicity 鈥� its tendency to absorb moisture. This is the reason sugar cakes in the damp, and in the process it will suppress the growth of mould or bacteria.
The mechanism is related to the one that protects honey from microbial growth. Honey is so effective at this that it was once spread on wounds to prevent infection.
Honey suppresses mould and bacterial growth thanks to its high concentration of sugar. By drawing water away from its surroundings, the sugar dessicates any fungal and bacterial cells and spores in the honey. Spores or cells must feed to reproduce and so they absorb food in contact with their cell membranes or which their excreted enzymes have released. Sugar in the honey will draw water out of the cell, which will either kill it or encourage it to live on in the spore phase and eschew reproduction until it encounters a more benevolent environment. This is the spore鈥檚 job, and so it sits and waits and ceases to be active in the honey.
Bill Jackson, Toronto, Ontario, Canada