2021年2月20日雅思考试机经回忆完整版(4)

2023-06-27 14:49:16 来源：中国教育在线

2021年2月20日雅思考试机经回忆完整版(4) 关于这个问题下面小编就来为各个考生解答下。

2021年2月20日雅思考试机经回忆完整版(4)

Passage 3

Topic

Biology of Bitterness

参考答案:

To many people, grapefruit is palatable only when doused in sugar. Bitterblockers like adenosine monophosphate(单磷酸腺苷) could change that.

There is a reason why grapefruit juice is served in little glasses: mostpeople don't want to drink more than a few ounces at a time. Naringin, anatural chemical compound found in grapefruit, tastes bitter. Some peoplelike that bitterness in small doses and believe it enhances the general flavor,but others would rather avoid it altogether. So juice packagers often selectgrapefruit with low naringin though the compound has antioxidant propertiesthat some nutritionists contend may help prevent cancer and arteriosclerosis.

lt is possible, however, to get the goodness of grapefruit juice

without the bitter taste.l found that out by participating in a test conducted atthe Linguagen Corporation, a biotechnology company in Cranbury,NewJersey. Sets of two miniature white paper cups, labeled 304and 305, wereplaced before five people seated around a conference table.Each of us drankfrom one cup and then the other , cleansing our palates between tastes withwater and a soda cracker. Even the smallest sip of 304 had grapefruit 'sunmistakable bitter bite.But 305 was smoother; there was the sour taste ofcitrus but none of the bitterness of naringin. This juice had been treated withadenosine monophosphate, or AMP, a compound that blocks the bitterness infoods without making them less nutritious.

Taste research is a booming business these days, with scientistsdelving into all five basics-sweet, bitter, sour, salty, and umami, the savorytaste of protein.Bitterness is of special interest to industry because of itsuntapped potential in food.There are thousands of bitter -tasting compoundsin nature.They defend plants by warning animals away and protect animalsby letting them know when a plant may be poisonous.But the system isn'tfoolproof.Grapefruit and cruciferous vegetable like Brussels sprouts and kaleare nutritious despite-and sometimes because of-their bitter-tasting

components.Over time, many people have learned to love them, at least insmall doses."Humans are the only species that enjoys bitter taste," saysCharles Zuker, a neuroscientist at the University of California School ofMedicine at San Diego."Every other species is averse to bitter because itmeans bad news.But we have learned to enjoy it. We drink coffee, which isbitter, and quinine [in tonic water] too. We enjoy having that spice in our lives."Because bitterness can be pleasing in small quantities but repellent whenintense, bitter blockers like AMP could make a whole range of foods, drinks,and medicines more palatable-and therefore more profitable.

People have varying capacities for tasting bitterness, and the

differences appear to be genetic.About 75 percent of people are sensitive tothe taste of the bitter compounds phenylthiocarbamide and

6-n-propylthiouracil.and 25 percent are insensitive.Those who are sensitiveto phenylthiocarbamide seem to be less likely than others to eat cruciferousvegetables, according to Stephen Wooding, a geneticist at the University ofUtah. Some people, known as supertasters, are especially sensitive to6-n-propylthiouraci because they have an unusually high number of taste buds. Supertasters tend to shun all kinds of bitter-tasting things, includingvegetable, coffee, and dark chocolate.Perhaps as a result, they tend to bethin. They're also less fond of alcoholic drinks, which are often slightly bitter.Dewar's scotch, for instance, tastes somewhat sweet to most people." But asupertaster tastes no sweetness at all, only bitterness," says Valerie Duffy, anassociate professor of dietetics at the University of Connecticut at Storrs.

ln one recent study, Duffy found that supertasters consume alcoholicbeverages, on average, only two to three times a week, compared with five orsix times for the average nontasters.Each taste bud, which looks like anonion, consists of 50 to 100 elongated cells running from the top of the bud tothe bottom.At the top is a little clump of receptors that capture the taste

molecules, known as tastants, in food and drink.The receptors function muchlike those for sight and smell.Once a bitter signal has been received, it isrelayed via proteins known as G proteins.The G protein involved in theperception of bitterness,sweetness,and umami was identified in the early1990s by Linguagen's founder, Robert Margolskee, at Mount Sinai School ofMedicine in New York City. Known as gustducin, the protein triggers a

cascade of chemical reactions that lead to changes in ion concentrationswithin the cell.Ultimately, this delivers a signal to the brain that registers asbitter. "The signaling system is like a bucket brigade,"Margolskee says. "Itgoes from the G protein to other proteins."

In 2000 Zuker and others found some 30 different kinds of genes thatcode for bitter-taste receptors."We knew the number would have to be largebecause there is such a large universe of bitter tastants,” Zuker says.Yet nomatter which tastant enters the mouth or which receptor it attaches to, bitter

always tastes the same to us.The only variation derives from its intensity andthe ways in which it can be flavored by the sense of smell."Taste cells are likea light switch,”Zuker says."They are either on or off.”"

Once they figured put the taste mechanism, scientists began to thinkof ways to interfere with it. They tried AMP, an organic compound found inbreast milk and other substances, which is created as cells break down food.Amp has no bitterness of its own, but when put it in foods,Margolskee and hiscolleagues discovered, it attaches to bitter-taste receptors.As effective as it

is，AMP may not be able to dampen every type pf bitter taste, because itprobably doesn't attach to all 30 bitter-taste receptors. So Linguagen hasscaled up the hunt for other bitter blockers with a technology called

high-throughput screening. Researchers start by coaxing cells in culture toactivate bitter-taste receptors.Then candidate substances, culled fromchemical compound libraries, are dropped onto the receptors, and scientistslook for evidence of a reaction.

Tin time, some taste researchers believe, compounds like AMP willhelp make processed foods less unhealthy.Consider, for example, that asingle cup of Campbell's chicken noodle soup contains 850 milligrams ofsodium chloride, or table salt-more than a third of the recommended dailyallowance.The salt masks the bitterness created by the high temperaturesused in the canning process, which cause sugars and amino acids to react.Part of the salt could be replaced by another salt, potassium chloride, whichtends to be scarce in some people's diets.Potassium chloride has a bitteraftertaste, but that could be eliminated with a dose of AMP.Bitter blockerscould also be used in place of cherry or grape flavoring to take the harshness

out of children's cough syrup, and they could dampen the bitterness ofantihistamines, antibiotics, certain HIV drugs, and other medications.

A number of foodmakers have already begun to experiment with AMPin their products, and other bitter blockers are being developed by rival firmssuch as Senomyx in La Jolla,California. In a few years, perhaps, after foodcompanies have taken the bitterness from canned soup and TV dinners, theycan set their sights on something more useful: a bitter blocker in a bottle thatany of us can sprinkle on our brussels sprouts or stir into our grapefruit juice.

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