问题 问答题 论述题

试述非酶褐变对食品质量的影响。

答案

参考答案:

(1)非酶褐变对食品色泽的影响

非酶褐变反应中产生二大类对食品色泽有影响的成分,其一是一类分子量低于1000水可溶的小分子有色成分;其二是一类分子量达到100000水不可溶的大分子高聚物质。

(2)非酶褐变对食品风味的影响

在高温条件下,糖类脱水后,碳链裂解、异构及氧化还原可产生一些化学物质,如乙酰丙酸、甲酸、丙酮醇、3-羟基丁酮、二乙酰、乳酸、丙酮酸和醋酸;非酶褐变反应过程中产生的二羰基化合物,可促进很多成分的变化,如氨基酸在二羰基化合物作用下脱氨脱羧,产生大量的醛类。非酶褐变反应可产生需要或不需要的风味,例如麦芽酚和异麦芽酚使焙烤的面包产生香味,2-H-4-羟基-5-甲基-呋喃-3-酮有烤肉的焦香味,可作为风味增强剂;非酶褐变反应产生的吡嗪类等是食品高火味及焦糊味的主要成分。

(3)非酶褐变产物的抗氧化作用

食品褐变反应生成醛、酮等还原性物质,它们对食品氧化有一定抗氧化能力,尤其是防止食品中油脂的氧化较为显著。它的抗氧化性能主要由于美拉德反应的终产物-类黑精具有很强的消除活性氧的能力,且中间体-还原酮化合物通过供氢原子而终止自由基的链反应和络合金属离子和还原过氧化物的特性。

(4)非酶褐变降低了食品的营养性

氨基酸的损失:当一种氨基酸或一部分蛋白质参与美拉德反应时,会造成氨基酸的损失,其中以含有游离ε-氨基的赖氨酸最为敏感。糖及Vc等损失:可溶性糖及Vc在非酶褐变反应过程中将大量损失,由此,人体对氮源和碳源的利用率及Vc的利用率也随之降低。蛋白质营养性降低:蛋白质上氨基如果参与了非酶褐变反应,其溶解度也会降低。矿质元素的生物有效性也有下降。

(5)非酶褐变产生有害成分

食物中氨基酸和蛋白质生成了能引起突变和致畸的杂环胺物质。美拉德反应产生的典型产物D-糖胺可以损伤DNA;美拉德反应对胶原蛋白的结构有负面的作用,将影响到人体的老化和糖尿病的形成。

阅读理解

Dolphins are our friends. They can use tools to solve problems although they don't have hands.   For example, scientists have ever found that a dolphin can make a shy eel(鳗鱼) come out of its hole with a dead fish.

Dolphins use different sounds to exchange information with each other. Scientists in Hawaii have developed a sign language to give instructions to dolphins, and the results are amazing. They find that dolphins not only understand the meaning of individual (个别的) words, but also understand the importance of word order in a sentence.

Most animals enjoy playing — but dolphins seem to like making their games as challenging as possible. On a beautiful day in 1997, scientists at a beach watched a little dog going into the sea and swimming towards the dolphins. To their surprise, the dolphins came up to the dog and then started throwing it into the air. The dog seemed to enjoy the "game" and continued playing with the dolphins for more than an hour.

Dolphins live in a very different world from ours, but they are very good at our "IQ tests” . For example, they will jump out of the water when they hear a whistle (口哨) noise. They do this because they will get a fish as a prize.

There is still much to learn about these cute problem-solvers, but it is certain that watching a dolphin show is fun.

根据短文内容,选择最佳答案。(5分)

小题1: What can a dolphin use to make an eel come out of its hole?

A.Its mouth.

B.Its head.

C.A dead fish.

D.A little dog.小题2:How do dolphins exchange information with each other?

A.By using tails.

B.By using tools.

C.By using words.

D.By using sounds.小题3:Where did the dolphins throw the dog when playing?

A.Onto its back.

B.Into the air.

C.Onto the beach.

D.Into the water.小题4:Why do the dolphins jump out of the water when they hear a whistle noise?

A.Because they want to get a fish as a prize.

B.Because they enjoy playing on the water.

C.Because they want to play with other dolphins.

D.Because the dolphin keepers want them to come out.小题5: Which of the following is TRUE according to the passage?

A.Dolphins understand many languages.

B.Dolphins are a kind of clever animals.

C.Scientists have known dolphins well enough.

D.Dolphins seem to make their games relaxing.

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