IELTS Academic Reading Practice Test 21

Cheese is one of the oldest processed foods and one of the most varied: estimates of the number of distinct types run to well over a thousand. Yet almost all of them are produced by the same basic method. Milk is a mixture of water, fat, a sugar called lactose and proteins, the most important of which for cheese-making are a group known as caseins. The fundamental task of the cheese-maker is to make these proteins clump together, trapping fat and some of the water, and then to separate the solid curd from the liquid that remains, which is known as whey.

Nobody knows exactly when people first made cheese, although a popular legend suggests that it was discovered by accident, when milk stored in a bag made from an animal's stomach turned solid. In 2012 a team of chemists analysed fragments of pottery sieves from sites in the Kuyavia region of Poland, dating from around 7,000 years ago, and found residues of milk fat in them, which they interpreted as the earliest direct evidence of cheese-making. Turning milk into cheese had a clear practical advantage. Fresh milk spoils within hours in a warm climate, whereas cheese can be kept for months. In addition, most of the lactose in milk drains away with the whey, so cheese could be eaten by adults who, like most early farmers, were unable to digest milk sugar.

Modern cheese-making usually begins with pasteurised milk, which has been heated to kill harmful bacteria, although some traditional cheeses are still made from raw milk. The milk is warmed and a starter culture is added. This consists of bacteria that convert lactose into lactic acid, gradually making the milk more acidic, which helps the curd to form and inhibits the growth of unwanted microbes.

Next comes rennet, a substance containing an enzyme called chymosin, which acts on the casein so that the milk sets into a soft gel, typically within an hour. Traditionally, rennet was extracted from the stomach lining of young calves. In 1990, however, chymosin produced by genetically modified microorganisms became the first ingredient made by genetic engineering to be approved for use in food in the United States, and enzymes of this kind are now used to make most of the cheese produced in North America.

The gel is then cut into pieces with blades or wires. This step largely determines the character of the final cheese, because cutting allows whey to escape: the smaller the pieces, the more whey is released and the harder and drier the cheese will be. For soft cheeses such as Camembert, the curd is ladled gently into moulds with little cutting, while for hard cheeses it is cut into pieces the size of rice grains and often heated and stirred to expel still more moisture. In the production of Cheddar, the curd is formed into slabs that are repeatedly stacked and turned, a process known as cheddaring, which squeezes out whey and gives the cheese its dense texture. The curd is then salted, which adds flavour, draws out further moisture and helps to preserve it, before being packed into moulds and pressed.

Few cheeses are eaten immediately. Mild varieties may be ready within a month, whereas Parmigiano Reggiano is aged for at least twelve months and often for two years or more. Most are ripened, or matured, for periods ranging from a few weeks to several years, usually in rooms where temperature and humidity are carefully controlled. During this time, enzymes from the milk, the rennet and the bacteria break down proteins and fats into smaller compounds that create each cheese's distinctive flavour and texture. Around ten litres of milk are needed to make a single kilogram of a hard cheese, and long maturation adds further to the cost, since the cheese occupies storage space and must be turned and inspected regularly.

Microorganisms are responsible for many of the most recognisable features of particular cheeses. Camembert and Brie, for instance, develop a soft white rind formed by a mould that grows on their surface. The blue veins of Roquefort are produced by the mould Penicillium roqueforti; the cheese is pierced with needles so that air can enter and the mould can grow, and by French law it must be matured in the natural limestone caves of Roquefort-sur-Soulzon.

In 1925 Roquefort became the first cheese in France to receive legal protection of its name and place of origin. The holes in Swiss Emmental are created by bacteria that release carbon dioxide during ripening. In 2015, however, Swiss researchers reported that tiny particles of hay in the milk act as starting points around which the gas collects. This explained why Emmental made from milk collected in modern, enclosed milking systems had developed fewer holes in recent years.

Today most cheese is produced on an industrial scale, and the largest factories process millions of litres of milk every day. Small producers have nevertheless survived, and in many countries traditional and farmhouse cheeses have enjoyed a revival, often supported by systems of protected names similar to the one first granted to Roquefort. Whether made in a factory or on a farm, every cheese still depends on the same partnership between milk, microbes and time that farmers discovered thousands of years ago.

Most of the time, the desert locust, Schistocerca gregaria, is an unremarkable grasshopper. Several other locust species, in Africa, Asia, Australia and the Americas, behave in a similar way. The desert locust lives in small numbers across the dry regions of North Africa, the Middle East and south-west Asia, an area of around 16 million square kilometres, and individuals tend to avoid one another. Under certain conditions, however, the same insects undergo a dramatic transformation. They change colour and behaviour, begin to gather in groups and eventually form swarms that can travel across entire continents. According to the Food and Agriculture Organization of the United Nations (FAO), a major plague can affect some 20 per cent of the world's land surface and threaten the livelihoods of a tenth of the world's population.

For a long time, the solitary and swarming forms were thought to be separate species, because they look so different. In 1921 the Russian-born entomologist Boris Uvarov, who spent most of his career in London, showed that they were in fact two phases of a single species, and that one could change into the other. Solitarious locusts are green or brown and well camouflaged, and they fly mainly at night. Gregarious locusts are more active, fly by day and seek out the company of other locusts; their young, which cannot yet fly, are boldly patterned in black and yellow, while mature adults turn bright yellow. The change is not merely cosmetic, since the two phases also differ in body proportions, physiology and even the size of their brains.

The trigger for this transformation is crowding. After good rains, vegetation grows in the desert and locusts breed rapidly. When the plants later dry out, the insects are forced together into the remaining green patches. Experiments have shown that repeated touching of the hind legs, which happens constantly when locusts jostle one another, is enough to start the change. In 2009 researchers at the universities of Oxford and Cambridge reported that this stimulation causes a sharp rise in serotonin, a chemical messenger in the nervous system, and that blocking serotonin prevented locusts from becoming gregarious. Behaviour can switch within a few hours, while the full change in colour and body shape builds up over successive generations.

Once gregarious, locusts actively attract one another. In 2020 a team led by the Chinese scientist Le Kang identified a chemical called 4-vinylanisole, released by gregarious individuals of a related species, the migratory locust, which attracts other locusts of both sexes and all ages. The researchers suggested that substances blocking this signal might one day be used to stop swarms from forming. Before they can fly, young locusts, known as hoppers, march across the ground in bands that may contain millions of individuals. Studies of marching locusts have found that the movement is partly driven by cannibalism, since each insect tries to avoid being bitten by those behind it while advancing on those in front.

A swarm of adult locusts is one of the most destructive phenomena in nature. According to the FAO, a swarm covering a single square kilometre may contain up to 80 million adults and can eat as much food in one day as about 35,000 people. Each locust consumes roughly its own body weight in fresh vegetation daily. Carried by the wind, swarms can travel more than a hundred kilometres in a day, and they have been known to cross the Red Sea and even the Atlantic Ocean: in 1988 swarms from West Africa reached the islands of the Caribbean.

The most recent major upsurge illustrates how quickly a crisis can develop. In 2018 two cyclones brought unusually heavy rain to the remote deserts of the southern Arabian Peninsula, an area rarely visited by survey teams, allowing several generations of locusts to breed undetected. By late 2019 swarms had reached the Horn of Africa, and in early 2020 Kenya experienced its worst locust outbreak in 70 years, while Ethiopia and Somalia faced their worst in 25 years. Swarms also spread east to south-west Asia. A large international control campaign, combined with drier conditions, eventually brought the upsurge to an end.

Because upsurges begin in remote areas, the key to control is early warning. The FAO's Desert Locust Information Service, based in Rome, combines reports from national survey teams with satellite data on rainfall and vegetation to forecast where breeding is likely. Survey officers in affected countries now record their observations on rugged tablet computers that send the data directly to the service. The aim is to treat infestations before swarms form, which is far cheaper and requires much less pesticide than attacking swarms once they are airborne. Most control still relies on chemical insecticides sprayed from vehicles or aircraft, but a biological alternative exists: a product based on the fungus Metarhizium acridum, which infects locusts and grasshoppers but is harmless to most other organisms. It kills more slowly than chemical sprays, so it is best suited to hopper bands in environmentally sensitive areas. History also offers a reminder of how much remains unknown. The Rocky Mountain locust, which formed vast swarms across North America in the nineteenth century, disappeared within a few decades, probably because farming destroyed its breeding grounds in river valleys, and the last known living specimens were collected in 1902.

For much of the twentieth century, most people received their news from a small number of sources: a daily newspaper, a radio bulletin, an evening television programme. Editors decided what mattered, and audiences largely accepted their judgement. That model has been overturned. According to the Reuters Institute's Digital News Report, published in 2025, the proportion of people in the United States who used social media and video platforms for news in a given week reached 54 per cent, overtaking television for the first time. Among younger people in many countries, platforms such as TikTok, YouTube and Instagram have become the main way of encountering news at all. Whether this shift is damaging democracy has become one of the most anxiously debated questions of the age.

The most alarming evidence concerns the spread of falsehoods. In a study published in 2018, Soroush Vosoughi, Deb Roy and Sinan Aral analysed around 126,000 stories shared on Twitter between 2006 and 2017 that had been checked by independent fact-checking organisations. False stories reached more people, and spread more quickly, than true ones; a false story was about 70 per cent more likely to be shared than a true one. Strikingly, the researchers found that automated accounts spread true and false stories at similar rates. The difference was due to human users, who seemed drawn to false stories because they were more novel and provoked stronger emotional reactions, such as surprise and disgust.

A second widely held fear is that social media traps people in 'filter bubbles', a term popularised by the internet activist Eli Pariser in 2011 to describe a situation in which algorithms show users only content that confirms what they already believe. The evidence for this is weaker than is often assumed. Studies of online behaviour have generally found that most people's news diets are more varied than the metaphor suggests, and that the most one-sided consumption is concentrated among a relatively small minority of highly engaged partisans. In a large experiment conducted on Facebook during the 2020 United States election and published in 2023, a team including the political scientist Brendan Nyhan reduced the amount of content that participants saw from like-minded sources. Over the three months of the study, the change did not measurably reduce their political polarisation.

Why, then, do people share false information? One common assumption is that they simply believe whatever suits their political views. Research by the psychologists Gordon Pennycook and David Rand points to a different explanation. In studies published from 2019 onwards, they found that people who were more inclined to think carefully were better at distinguishing true headlines from false ones, regardless of whether the headlines favoured their own political side. Much misinformation, in other words, is spread through inattention rather than conviction. Encouragingly, they also found that simply prompting people to consider whether a headline was accurate improved the quality of the content they subsequently chose to share.

A related approach, known as prebunking, draws on an analogy with vaccination. Rather than correcting false claims after they have spread, it aims to make people resistant to manipulation in advance by exposing them to weakened examples of common techniques, such as emotionally loaded language or false choices between two options. In a study published in 2022, Jon Roozenbeek, Sander van der Linden and colleagues showed short animated videos explaining such techniques to YouTube users in the form of advertisements, and found that viewers became better at recognising the techniques. The effects are modest and fade over time, but because the videos can be distributed cheaply to millions of people, they may still be worthwhile.

The most serious damage that social media has done to news, however, may be economic rather than psychological. As advertising moved online, much of it went to the platforms themselves rather than to the organisations producing the journalism that circulated on them. Newspapers that once depended on classified and display advertising have closed or shrunk, and local reporting has suffered most: in the United States, thousands of local newspapers have disappeared since the early 2000s.

Platforms, moreover, can change the rules at any time. In 2018 Facebook altered its news feed to favour posts from friends and family, reducing the traffic it sent to publishers, and in 2023 its parent company, Meta, blocked news content for users in Canada in response to a law requiring platforms to pay news organisations. Two years earlier, Facebook had briefly blocked news in Australia during a dispute over a similar law.

Where local journalism disappears, the consequences are tangible. Research has linked the closure of local newspapers to less competitive local elections and to higher borrowing costs for local governments, which face less scrutiny once nobody is reporting on them. These effects receive far less attention than viral falsehoods, yet they may matter more. Teaching people to recognise manipulation and encouraging them to pause before sharing are valuable measures, but they do nothing to ensure that someone is attending council meetings and reporting what happens there.

In Britain, the BBC has since 2017 funded local democracy reporters who cover councils for regional news organisations, but such schemes remain small compared with the revenue that has been lost. Social media did not create the appetite for sensational or misleading stories, which is as old as the printing press. What it has done is weaken the institutions that once supplied a reliable alternative, and rebuilding those institutions should be at the centre of any response.