91av

ALL over the world, people use acid to preserve fruit and vegetables, creating the sour and delicious foods we call pickles. The microbes that spoil our food have a hard time growing if the pH is lower than 4.5, but we can eat foods with a pH as low as 2 (the lower the pH, the more acidic the substance).

Some pickles are made by salting vegetables or fruit, encouraging the growth of bacteria that produce lactic acid. These include kimchi, which I described in a previous issue (29 February 2020). A quicker and simpler way to make pickles is to add vinegar, which contains ethanoic (aka acetic) acid and typically has a pH of around 2.4. For pickling, you should use vinegar with 5 per cent acidity – this is usually printed on the label.

As well as inhibiting microbes, the acid stabilises plant cell walls, helping vegetables such as cucumbers keep their crunch. Unrefined sea salt contains calcium and magnesium impurities, which also help to reinforce the pectin molecules in cell walls. Table salt is best avoided because it contains anti-caking agents that can turn pickling liquid cloudy.

In addition to vinegar, pickling liquids can include flavourings, such as herbs and spices, and sugar to balance the sour taste. Garlic in pickles can sometimes turn blue or green due to reactions that produce pyrroles. These chemicals can join together to produce colourful polypyrroles, a group that includes the green pigment chlorophyll. Blue or green pickled garlic is safe to eat.

To make a cucumber pickle, slice small cucumbers lengthwise into spears. Cut off and dispose of the flower end (the one with the rough dot), which contains enzymes that accelerate softening. If you are using larger cucumbers, cut them into shorter lengths, or slice thinly into discs instead. Mix the cucumber with salt in a bowl and leave overnight to draw out water.

Heat the vinegar, water, sugar and spices in a lidded saucepan until the sugar has dissolved, then allow to cool. Put the garlic and dill in the bottom of the jars. Drain the liquid from the cucumbers, then pack them in tightly and pour the pickling liquid over them so they are completely covered.

Once packed into jars, pickles can be heat-processed in a pressure-canning machine to kill off any microbes and extend their shelf life further, but this results in a less crunchy and fresh-tasting product. If you aren’t doing this, the pickles should still keep in the fridge for about two months.

Best not to try this at home, but chemistry lecturers sometimes . The high concentration of sodium ions carries the current through the pickle, and the yellow glow is emitted when electrons from excited sodium ions return to a lower energy state. In essence, the pickle functions as an organic light-emitting diode.

For other projects visit newscientist.com/maker.