Study of total fructan and fructooligosaccharide content in different onion tissues Laura Jaime, 1 Francisco Martı ´nez, 1 Maria A Martı ´n-Cabrejas, 1 Esperanza Molla ´, 1 Francisco J Lo ´pez-Andre ´u, 1 Keith W Waldron 2 and Rosa M Esteban 1 * 1 Departamento de Quı´mica Agrı´cola, Facultad de Ciencias, UniversidadAuto ´ noma de Madrid (UAM), E-28049 Madrid, Spain 2 Institute of Food Research, Norwich Research Park, Norwich NR4 7UA, UK Abstract: The objective of this work was to determine the fructan and fructooligosaccharide (FOS) content of different onion tissues in order to evaluate the potential use of onion by-products from the food industry as a source of FOS and fructans. Assays with two methods were carried out to optimise the extraction procedure. The main FOS, namely kestose (GF 2 ), nystose (GF 3 ) and fructofurano- sylnystose (GF 4 ), were measured directly using standard sugars. The method for total fructans was based on enzymatic treatment (Novozym 230) of ethanolic/aqueous extract followed by determination of released fructose and glucose by HPLC. Data showed a clear predominance of GF 2 in every onion tissue and no occurrence of highly polymerised fructans. The tissues richest in fructans were the ¯eshy layers, so that the outer two ¯eshy layers turn out to be the best onion by-product as a possible fructan source. # 2000 Society of Chemical Industry Keywords: onion (Allium cepa L); by-products; fructans; fructooligosaccharides INTRODUCTION Fructans comprise the main reserve polysaccharide in several plants, including onion, asparagus root, bana- na, wheat, barley, Jerusalem artichoke, etc. 1 Fructans consist of a series of homologous oligo- and polysac- charides of fructose which can be considered as derivatives of sucrose. 2 However, several structurally different oligosaccharides have been referred to as fructooligosaccharides (FOS). It is generally accepted that FOS is a common name only for fructose oligomers that are mainly composed of kestose (GF 2 ), nystose (GF 3 ) and fructofuranosylnystose (GF 4 ), in which fructosyl units (F) are bound by b linkage at the position of sucrose (GF) respectively. 3 Fructans and FOS can be obtained from natural resources such as chicory inulin by partial hydrolysis with endoglycosidases or from sucrose through the transfructosylating action of b-fructofuranosidases and b-D-fructosyltransferases from micro-organisms and plants such as Aspergillus sp, Aureobosidium sp, Arthrobacter sp, Fusarium sp, etc. 1,4 The recent use of FOS as a food ingredient has triggered much research on their possible health effects; 5 in this sense, FOS are included in the prebiotics classi®cation. 6 FOS resist hydrolysis by digestive enzymes and enter the caecum and colon intact. No change in blood glucose is noted after oral ingestion, and they are considered safe for diabetics. Moreover, FOS have a low sweetness intensity, since they are only about one- third as sweet as sucrose. This property is quite useful in various kinds of foods where the use of sucrose is restricted by its high sweetness. When FOS reach the large intestine, they are highly fermented, mainly by Bi®dobacteria, into short-chain carboxylic acids (acet- ate, propionate, butyrate), which are responsible for a pH decrease in the intestine. This property, together with their effect on gastric emptying and intestinal motility, allows them to be classi®ed as soluble dietary ®bres. 7,8 FOS offer several important physiological properties, such as decreasing the levels of serum cholesterol, phospholipids and triglycerides, and they are also non-cariogenic substances. 3 Because of the great interest of consumers in diet foods, several companies are trying to obtain permis- sion to use FOS as food ingredients in the USA and Europe, where they are not yet being marketed. The approval of FOS in Japan prompted the establishment of an acceptable daily intake 3 of about 0.8 g kg 1 body weight day 1 . The occurrence of fructans in some Allium species has been known since 1894. 9 The content, distribu- tion and structure of fructooligomers in onion bulbs were investigated extensively by Bacon 10 and Darby- shire and Henry. 11,12 At present, new techniques such as HPLC with refractive index or pulse amperometric detection 13,14 and mass spectrometry 15 are being used to quantify fructans and FOS. Approximately 450 000 t of onion waste is produced (Received 20 December 1999; accepted 20 September 2000) * Correspondence to: Rosa M Esteban, Departamento de Quı ´mica Agrı ´cola, Facultad de Ciencias, Universidad Auto ´noma de Madrid (UAM), E-28049 Madrid, Spain Contract/grant sponsor: European Union; contract/grant number: FAIR-CT96-1184 # 2000 Society of Chemical Industry. J Sci Food Agric 0022±5142/2001/$30.00 177 Journal of the Science of Food and Agriculture J Sci Food Agric 81:177±182 (online: 2000)