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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.
参考译文：
人们之所以偏爱某些食物，主要是因为食物的味道和口感。科学家们对味觉的原理已经有了相当的了解，但却刚刚开始着手破解食物口感的谜题。
如今，美国费城蒙内尔化学感觉中心（Monell Chemical Senses Center）的研究人员已经发现，唾液当中有一种名为淀粉酶（Amylase）的物质，这种酶可以溶解淀粉，可能在我们评价食物口感优劣的过程中发挥着关键的作用。一项新的遗传学研究表明，人体制造淀粉酶的数量因人而异，互相之间的差异大得惊人。口腔里的淀粉酶越多，溶解淀粉食物的速度也就越快。
科学家们认为，这一发现有助于解释，人们为何会对食物产生或滑腻或浓稠、或粘牙或松软的口感，与此同时，不同的口感又会影响我们对食物的偏好。含淀粉的食物为数众多，其中包括布丁和酱料，甚至还包括枫糖。吃到这类食物的时候，有些人可能觉得口感刚刚好，另一些人的感觉则是太过松软，还有些人的感觉又是不够化渣。之所以会有这样的差异，原因就是人体制造淀粉酶的数量各不相同。
淀粉是一种碳水化合物，快速分解淀粉的能力只是决定人们食物偏好的因素之一。味道偏好取决于一个复杂的互动过程，参与互动的各方包括味蕾、口腔和鼻腔里的其他受体，以及所有这些受体发送给大脑的讯息。美国印第安纳州西拉斐特（West Lafayette）普渡大学（Purdue University）的食品及营养学教授马蒂斯（Rick Mattes）说，文化也发挥着一定的作用，因为人们往往会偏爱自己熟悉的食物。除此之外，多次重复有时也会让人们培养起对特定食物的偏好：以牡蛎为例，很多人一开始都不喜欢它那种粘乎乎的口感，吃过几次之后却会喜欢上这种食物。
蒙内尔中心研究员、美国新泽西州新布朗斯维克（New Brunswick）罗格斯大学（Rutgers University）教授布雷斯林（Paul Breslin）说，“我们都有过这样的经历，那就是我们喜欢吃某种东西，人家却觉得这种东西太粘、太滑、太涩或者太软。正是由于这个原因，同一个产品系列当中往往会有几种口感不同的产品”，比如带果粒的橙汁和不带果粒的橙汁。
蒙内尔中心的科学家们说，我们的日常食品当中，60%由淀粉组成或是加有淀粉，有鉴于此，要想搞清人们的口感偏好，关键在于搞清淀粉的消化过程。布雷斯林说，其他研究已经表明，人们偏爱滑腻的口感，也喜欢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，就是因为他对滑腻的口感很有兴趣。此外，淀粉酶的理论也有助于解释人们对冰激凌和酸奶酪等产品的品牌偏好，因为不同品牌的产品加入了不同数量的淀粉。
前些日子，蒙内尔的研究人员让73名成人品尝了一些浓度各不相同的淀粉溶液，又让他们在60秒钟之内作出评价，看看这些没有特殊味道的半透明凝胶状物质粘度如何。这些淀粉可以在几秒钟之内溶解，具体速度则取决于各人制造的淀粉酶数量。
研究人员还从受试者的血样和口腔拭子当中提取了DNA样本，旨在确定负责制造淀粉酶的基因拷贝数和受试者报称的淀粉溶解速度之间的联系。负责制造淀粉酶的基因名为AMY1，研究结果表明，不同的人拥有数量千差万别的AMY1拷贝，与此同时，AMY1拷贝比较多的受试者报称的淀粉溶解速度也比较快。此项研究于上月发表在公共科学图书馆（Public Library of Science）的《公共科学图书馆•综合》（PLoS ONE）杂志上。
目前，蒙内尔中心的研究人员正在进行相关研究，目的是确定进食之后，AMY1拷贝数较高的人是否会出现较高的血糖峰值。此外，研究人员还计划找出较高的淀粉酶产量和食物偏好之间的联系，并且假定淀粉酶产量较高的人将会偏爱淀粉含量较高的食品，因为他们可以让大量葡萄糖更快进入自己的血液。
淀粉酶的作用和淀粉溶解的速度也与糖尿病有关。很有可能，那些能够快速消化淀粉的人更容易出现较高的血糖峰值，这样一来，他们的身体就必须制造出更多的胰岛素。布雷斯林博士的蒙内尔同事、研究论文的第一作者曼德尔（Abigail Mandel）说，身体长期承受这样的重负，这些人就可能会对胰岛素产生抗拒，如果身体制造胰岛素的能力在压力之下陷于崩溃，这些人还可能会罹患糖尿病。
布雷斯林博士说，淀粉酶和唾液中的其他一些，还有助于解释人们的口感偏好为什么会随年龄的变化而变化。举例来说，许多年幼的孩子都不喜欢某些特定的水果，因为他们觉得它们又粘又滑──比如西红柿。不过，人们分泌唾液的速度会随着年龄的增长而衰减。这样一来，他们用嘴巴溶解淀粉的能力就可能会受影响，滑溜的口感也会由此减弱。
