VOL-9 Test 1 パッセージ 3 — IELTS リーディング練習

A There is a reason why grapefruit juice is served in little glasses. Most people don't want to drink more than a few ounces at a time. This is because a natural chemical compound found in grapefruit, naringin, has a bitter taste. Some people like that bitterness in small doses and believe it enhances the general flavour, but others would rather avoid it altogether. So juice packagers often select grapefruit with low naringin content, even though the compound has antioxidant properties that some nutritionists contend may help prevent many serious diseases.

A There is a reason why grapefruit juice is served in little glasses. Most people don't want to drink more than a few ounces at a time. This is because a natural chemical compound found in grapefruit, naringin, has a bitter taste. Some people like that bitterness in small doses and believe it enhances the general flavour, but others would rather avoid it altogether. So juice packagers often select grapefruit with low naringin content, even though the compound has antioxidant properties that some nutritionists contend may help prevent many serious diseases.

B It is possible, however, to get the goodness of grapefruit without the bitter taste. In a test conducted by a biotechnology company, sets of two miniature white paper cups, labelled 304 and 305, were placed before five people seated around a conference table. Each person drank from one cup and then the other, cleansing their palates between tastes with water and a cracker. Even the smallest sip of 304 had grapefruit's unmistakeable bite. But 305 was smoother; there was the sour taste of citrus but none of the bitterness of naringin. This juice had been treated with AMP, a compound that blocks the bitterness in foods without making them less nutritious.

C Taste research is a booming business these days, with scientists delving into all five basics: sweet, bitter, sour, salty, and umami (the savory taste of protein). However, bitterness is of special interest to the food industry because it is present in so many different types of food. There are thousands of bitter-tasting compounds in nature. They defend plants by warning animals away and protect animals by letting them know when a plant may be poisonous. But the system isn't foolproof. Grapefruit and bitter green vegetables like Brussels sprouts and kale are nutritious despite - and sometimes because of - their bitter-tasting components.

D 'Humans are the only species that enjoys a bitter taste,' says Charles Zuker, a neuroscientist at the University of California School of Medicine at San Diego. 'Every other species is averse to bitter because it means bad news. But we have learned to enjoy it. We drink coffee, which is bitter, and quinine (in tonic water) too. We enjoy having that spice in our lives.' Because bitterness can be pleasant in small quantities but repellent when intense, bitter blockers like AMP could make a whole range of foods, drinks and medicines more palatable.

E People have varying capacities for tasting bitterness and the differences appear to be genetic. About 75% of people are sensitive to the taste of some well-identified bitter compounds. Those who are sensitive seem to be less likely than others to eat bitter vegetables, according to Stephen Wooding, a geneticist at the University of Utah. Some people who have an unusually high number of taste buds, and are known as 'supertasters', are especially sensitive to these compounds. Supertasters tend to shun all kinds of bitter-tasting things, including vegetables, coffee, and dark chocolate.

F The tongue is covered with taste buds. Under the microscope each one looks like an onion, consisting of 50 to 100 elongated cells running from the top of the taste buds to the bottom. At the top is a group of receptors that captures the taste molecules, known as tastants, in food and drink. The receptors function much like those for sight and smell. Once a bitter signal has been received, it is relayed via proteins known as G proteins. The G protein involved in the perception of bitterness, sweetness and umami was identified in the early 1990s by Robert Margolskee, at Mount Sinai School of Medicine in New York. The protein triggers a cascade of chemical reactions that lead to ion concentrations within the cell. Ultimately, this delivers a signal to the brain that registers as bitter.