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Why people prefer certain foods over others depends largely on a combination of taste and texture. While taste sensations are fairly well understood, scientists are just beginning to unravel the mystery of food texture.
Now, researchers at the Monell Chemical Senses Center in Philadelphia have found that an enzyme in saliva called amylase, which breaks down starch into liquid, could play a key role in determining the appeal of various textures of food. A new genetic study shows that people produce strikingly different amounts of amylase, and that the more of the enzyme people have in their mouth the faster they can liquefy starchy foods.
Scientists think this finding could help explain why people experience foods as creamy or slimy, sticky or watery, and that this perception could affect our preference for foods. For the numerous foods that contain starch, including pudding, sauces and even maple syrup, what can feel just right to some people is experienced as too runny or not melting enough for others because they produce different amounts of the enzyme.
The ability to quickly break down starch, which is a type of carbohydrate, is only one part of the puzzle that determines what people like to eat. Taste preferences are driven by a complicated interaction between taste buds and other receptors in the mouth and nose, and the messages they send to the brain. Culture plays a role, as people tend to like foods that are familiar, says Rick Mattes, a foods and nutrition professor at Purdue University in West Lafayette, Ind. And repetition sometimes can win out: Many people initially don't like oysters because of their slimy texture, for instance, but can come to enjoy them after several tries.
'We all have had the experience of liking a food that someone else complains is too tacky, or slippery, or gritty, or pulpy,' says Paul Breslin, a researcher at the Monell center and a professor at Rutgers University in New Brunswick, N.J. 'This is why a given line of product often comes in different textural forms,' such as orange juice with and without pulp, he says.
Starch comprises or is added to about 60% of the foods people typically eat, so determining how it is digested is key to understanding food-texture preferences, Monell center scientists say. Other research has shown that people have a preference for creamy sensations as well as for foods that start off solid and melt in the mouth such as ice cream and chocolate, says Dr. Breslin, who began the current research because of his interest in creaminess. Amylase also could help explain individual preferences for different brands of ice cream or yogurt, for instance, because they contain different amounts of added starch.
In their recent work, Monell researchers had 73 adults swirl around in their mouths solutions made up of different concentrations of starch -- blobs of translucent gelatinous substances with no particular taste -- and rate their runniness over the course of 60 seconds. Depending on the amount of amylase individuals produced, the starch could be reduced to liquid within seconds.
The researchers also took DNA samples of the participants from a blood sample or cheek swab and studied the link between the numbers of copies of a gene that turns on the production of amylase and how quickly the participant reported the sample turned runny. The findings showed that the number of copies of the gene, called AMY1, varied widely between individuals. People with higher numbers of gene copies reported that the starch turned to liquid more quickly. The study was published last month in PLoS ONE, a journal of the Public Library of Science.
The Monell researchers are now investigating whether people with more AMY1 copies see larger spikes in blood glucose after eating. They also plan to study the link between greater amylase production and food preferences, hypothesizing that people who make more of the enzyme will prefer starchy products because they get a faster blast of glucose into their bloodstream.
The role of amylase and the rate of starch breakdown also has implications for diabetes. People who digest starch quickly could be more likely to have larger spikes in blood-sugar levels and thus a need for the body to generate more insulin. This continued demand on the body might lead these people to become insulin resistant or even diabetic if the body's ability to produce insulin breaks down, says Abigail Mandel, Dr. Breslin's colleague at Monell and first author on the study.
Amylase and other enzymes in saliva could also help explain food-texture preferences that are known to vary with age, Dr. Breslin says. For instance, many young children dislike certain fruits because of a perceived sliminess -- think of the inside of a tomato. But people's saliva-flow rate tends to slow with age, which might affect their ability to break down starch in the mouth and reduce sensations of sliminess.
參考譯文：
人們之所以偏愛某些食物，主要是因為食物的味道和口感。科學家們對味覺的原理已經有了相當的了解，但卻剛剛開始著手破解食物口感的謎題。
如今，美國費城蒙內爾化學感覺中心（Monell Chemical Senses Center）的研究人員已經發現，唾液當中有一種名為淀粉酶（Amylase）的物質，這種酶可以溶解淀粉，可能在我們評價食物口感優劣的過程中發揮著關鍵的作用。一項新的遺傳學研究表明，人體制造淀粉酶的數量因人而異，互相之間的差異大得驚人。口腔里的淀粉酶越多，溶解淀粉食物的速度也就越快。
科學家們認為，這一發現有助于解釋，人們為何會對食物產生或滑膩或濃稠、或粘牙或松軟的口感，與此同時，不同的口感又會影響我們對食物的偏好。含淀粉的食物為數眾多，其中包括布丁和醬料，甚至還包括楓糖。吃到這類食物的時候，有些人可能覺得口感剛剛好，另一些人的感覺則是太過松軟，還有些人的感覺又是不夠化渣。之所以會有這樣的差異，原因就是人體制造淀粉酶的數量各不相同。
淀粉是一種碳水化合物，快速分解淀粉的能力只是決定人們食物偏好的因素之一。味道偏好取決于一個復雜的互動過程，參與互動的各方包括味蕾、口腔和鼻腔里的其他受體，以及所有這些受體發送給大腦的訊息。美國印第安納州西拉斐特（West Lafayette）普渡大學（Purdue University）的食品及營養學教授馬蒂斯（Rick Mattes）說，文化也發揮著一定的作用，因為人們往往會偏愛自己熟悉的食物。除此之外，多次重復有時也會讓人們培養起對特定食物的偏好：以牡蠣為例，很多人一開始都不喜歡它那種粘乎乎的口感，吃過幾次之后卻會喜歡上這種食物。
蒙內爾中心研究員、美國新澤西州新布朗斯維克（New Brunswick）羅格斯大學（Rutgers University）教授布雷斯林（Paul Breslin）說，“我們都有過這樣的經歷，那就是我們喜歡吃某種東西，人家卻覺得這種東西太粘、太滑、太澀或者太軟。正是由于這個原因，同一個產品系列當中往往會有幾種口感不同的產品”，比如帶果粒的橙汁和不帶果粒的橙汁。
蒙內爾中心的科學家們說，我們的日常食品當中，60%由淀粉組成或是加有淀粉，有鑒于此，要想搞清人們的口感偏好，關鍵在于搞清淀粉的消化過程。布雷斯林說，其他研究已經表明，人們偏愛滑膩的口感，也喜歡那些入口即溶的食物，比如冰激凌和巧克力。布雷斯林之所以投入目前的研究，就是因為他對滑膩的口感很有興趣。此外，淀粉酶的理論也有助于解釋人們對冰激凌和酸奶酪等產品的品牌偏好，因為不同品牌的產品加入了不同數量的淀粉。
前些日子，蒙內爾的研究人員讓73名成人品嘗了一些濃度各不相同的淀粉溶液，又讓他們在60秒鐘之內作出評價，看看這些沒有特殊味道的半透明凝膠狀物質粘度如何。這些淀粉可以在幾秒鐘之內溶解，具體速度則取決于各人制造的淀粉酶數量。
研究人員還從受試者的血樣和口腔拭子當中提取了DNA樣本，旨在確定負責制造淀粉酶的基因拷貝數和受試者報稱的淀粉溶解速度之間的聯系。負責制造淀粉酶的基因名為AMY1，研究結果表明，不同的人擁有數量千差萬別的AMY1拷貝，與此同時，AMY1拷貝比較多的受試者報稱的淀粉溶解速度也比較快。此項研究于上月發表在公共科學圖書館（Public Library of Science）的《公共科學圖書館•綜合》（PLoS ONE）雜志上。
目前，蒙內爾中心的研究人員正在進行相關研究，目的是確定進食之后，AMY1拷貝數較高的人是否會出現較高的血糖峰值。此外，研究人員還計劃找出較高的淀粉酶產量和食物偏好之間的聯系，并且假定淀粉酶產量較高的人將會偏愛淀粉含量較高的食品，因為他們可以讓大量葡萄糖更快進入自己的血液。
淀粉酶的作用和淀粉溶解的速度也與糖尿病有關。很有可能，那些能夠快速消化淀粉的人更容易出現較高的血糖峰值，這樣一來，他們的身體就必須制造出更多的胰島素。布雷斯林博士的蒙內爾同事、研究論文的第一作者曼德爾（Abigail Mandel）說，身體長期承受這樣的重負，這些人就可能會對胰島素產生抗拒，如果身體制造胰島素的能力在壓力之下陷于崩潰，這些人還可能會罹患糖尿病。
布雷斯林博士說，淀粉酶和唾液中的其他一些，還有助于解釋人們的口感偏好為什么會隨年齡的變化而變化。舉例來說，許多年幼的孩子都不喜歡某些特定的水果，因為他們覺得它們又粘又滑──比如西紅柿。不過，人們分泌唾液的速度會隨著年齡的增長而衰減。這樣一來，他們用嘴巴溶解淀粉的能力就可能會受影響，滑溜的口感也會由此減弱。
