专业外语-食品化学与食品营养学.doc

Text A Carbohydrates Many carbohydrates（碳水化合物） can be expressed in the form [Cx(H2O)y], but this is not an accurate description of their structure. Carbohydrates are infact polyhydroxy（多羟基） aldehydes（醛） or ketones （酮）and the term also includes any compounds that can be hydrolyzed to form carbohydrates. The simplest carbohydrates are called sugars (or monosaccharides（单糖）) and these may link together to form more complex carbohydrates, starches (or polysaccharides（多糖） ). The diversity of dietary carbohydrates necessitates discussion of several classes of these molecules, ranging from simple sugars to huge, branched polymers. 1.Monosaccharides Monosaccharides or simple sugars are either hexoses (6-carbon) （六碳糖）like glucose（葡萄糖）, galactose（半乳糖） and fructose（果糖）, or pentoses (5-carbon) （五碳糖）like ribose（核糖）. These are the breakdown products of more complex carbohydrates and can be efficiently absorbed across the wall of the digestive tube （消化道）and transported into blood. Simple sugars are monosaccharides. These are characterised by hydroxyl（羟基）, aldehyde and ketone groups（功能团）. Monosaccharides are either aldoses（醛糖） or ketoses（酮糖）. The aldo- and keto- prefix （前缀）identifies the nature of the carbonyl（羰基，羰酰） group, and the –ose suffix（后缀） designates a carbohydrate. In Fischer projections（费歇尔投影式）, if the hydroxyl group at the lowest stereogenic centre（手相中心）, C5 in this case, points to the right, it may be labelled a D-sugar, if it points to the left, it is an L-sugar. The number of carbon atoms is also indicated, so for example, aldohexose is the six-carbon aldehyde sugar, more commonly referred to as glucose. Intramolecular（分子内）, nucleophilic addition （亲核加成）can take place between a hydroxyl and a carbonyl group of the same molecule, resulting in the formation of a cyclic hemiacetal（半缩醛）. Simple monosaccharides consist of aldehyde or ketone groups and both primary and secondary hydroxyl groups, all of which are available for reaction. An alkaline（碱的，强碱的） solution of copper