Practice Test 30: The Science of Sourdough and Lighthouses After the Keeper

A Sourdough bread is made without commercial yeast. Instead, bakers use a starter, a mixture of flour and water that is left to ferment. Wild yeasts and lactic acid bacteria, which live on grain and in the surrounding air, multiply in the mixture and make it bubble. A well kept starter can be used for years, and some bakers say that theirs has been passed down through generations.

B The two groups of microbes play different roles. The yeasts produce carbon dioxide, which makes the dough rise. The bacteria produce acids, which give the bread its slightly sour taste and also help it to stay fresh for longer. A cooler dough tends to produce more acid, while a warmer one favours the yeasts, so bakers can change the flavour of a loaf by changing the temperature of the kitchen.

C Slow fermentation also changes the dough itself. During the long rise, some of the starch and protein in the flour is broken down. Some researchers suggest that this may make the bread easier to digest for certain people, but the evidence is limited. Sourdough made from wheat is not safe for people with coeliac disease, because the gluten is not removed.

D Keeping a starter alive is a matter of routine. It must be fed regularly with fresh flour and water, and part of it is thrown away each time so that it does not grow too large. A starter that smells strongly of nail polish remover is usually hungry. A large survey of home starters found that the place a starter came from explained very little about the microbes in it, and that the flour and the baker's habits were more important.

A For centuries a lighthouse could not work without a keeper. The keeper trimmed the wicks, polished the lenses, wound the clockwork that turned the light and wrote a log of the weather and passing ships. On remote rocks and islands, keepers and their families lived in great isolation, sometimes waiting weeks for supplies.

B In the twentieth century electric lamps and, later, automatic controls made the keeper unnecessary. Lights could now be switched on by a sensor at dusk and monitored from the shore. In many countries the staff were gradually withdrawn, and a job that had lasted for generations came to an end.

C Satellite navigation has made ships less dependent on lights, yet most authorities have kept many lighthouses in service. They are a backup in case electronic systems fail, and small boats still use them to find harbours and avoid hazards. A lighthouse is also easy to understand and needs no battery or signal on the vessel.

D The buildings themselves have found new uses. Some have become museums, and others are guesthouses or holiday homes. Maintaining a tall tower exposed to salt and storms is expensive, so charities and local communities now run many of them. For visitors they offer a glimpse of a way of life that has almost disappeared.