Benders'dictionary of nutrition and food technology

xerosis Abnormal dryness of conjunctiva, skin or mucousmembranes.xerostomia Dry mouth, a common side effect of a variety ofdrugs.See also ptyalism.X-ray diffraction Technique for determination of crystal structures(e.g. of proteins) by analysis of the diffraction pattern of abeam of X-rays shone through the crystal.xylanase Mixture of enzymes of fungal or bacterial origin thathydrolyse xylans: β-1,4-endoxylanase (EC 3.2.1.8) and β-dxylosidase(EC 3.2.1.32). Sometimes added to poultry andpig feed to increase the digestibility of cereal non-starchpolysaccharides.xylans Polysaccharides of xylose, not digested, part of nonstarchpolysaccharide;a major component of hemicellulose.xylitol A five-carbon sugar alcohol found in raspberries, endive,lettuce; 80–100% of the sweetness of sucrose; used in sugar-freehard sweets and gelatine gums. Apart from being of low cariogenicity,xylitol is said to have an effect in suppressing the growthof some of the bacteria associated with dental caries (see toothfriendlysweets).xyloascorbic acid Term used for ascorbic acid (vitamin c) todistinguish from isoascorbic acid (see erythorbic acid), whichis araboascorbic acid and has only slight vitamin C activity.xyloglucan One of the hemicelluloses in plant cell walls, linkingcellulose fibres. A component of non-starch polysaccharide.xylose Pentose (five-carbon) sugar found in plant tissues mainlyas polysaccharides (xylans); 40% as sweet as sucrose. Alsoknown as wood sugar. Mainly excreted unmetabolised, and usedto test carbohydrate absorption.xylulose Pentose (five-carbon) sugar occurring as a metabolicintermediate in the pentose phosphate pathway.Y517YAC Yeast artificial chromosome, a specialised cloning vectorthat can carry large DNA inserts.Yakult TM yogurt containing live Lactobacillus spp. which are probiotics.Originally Japanese name for yoghurt fermented with L.casei.yam Tubers of perennial climbing plants of a number of speciesof Dioscorea, D. rotundala white yam, and D. cayenensis, yellowor Guinea yam, water, trifoliate or Chinese yam. A major foodin parts of Africa and also the Far East. In USA sweet potatoes(see potato, sweet) are sometimes called yam.