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Studies on pentosans in Indian wheat varieties varying in chapati making quality

Mr., Revanappa (2009) Studies on pentosans in Indian wheat varieties varying in chapati making quality. PhD thesis, University of Mysore.

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Abstract

Cereals are staple foods for human nutrition and their use into a wide range of products is of great economic importance. Wheat is one of the major cereals across the world and is used mainly for the preparation of bread. Wheat in India is consumed mainly in the form of unleavened flat bread known as chapati. Chapaties constitute an important source of dietary proteins, calories, some of the vitamins and minerals for a large section of Indian population. Arabinoxylans are the pentose containing carbohydrate polymers and hence are often referred to as pentosans. Arabinoxylans (pentosans) were first identified in wheat flour among cereals and they have been of interest to cereal chemists and technologists because of their technological importance. Pentosans (arabinoxylans) have been shown to significantly affect cereal-based processes such as milling, brewing and bread making. Furthermore, they offer nutritional benefits of soluble and insoluble dietary fiber, and because of the presence of phenolic moieties in their molecular structures, they also offer antioxidant properties. Arabinoxylans constitute a significant portion of human dietary fiber intake because they occur in wide variety of cereals and are consumed in large quantities. Pentosans are the major non-starch polysaccharides in wheat and consist of xylan backbone with branches of arabinose residues in various linkages. The pentosans are known to play an important role in water balance of dough, rheological properties of dough, retrogradation of starch and bread making quality. These arabinoxylans compete with other constituents of dough ii for water added to the flour. The ability of arabinoxylans to imbibe and hold water was found to increase with the cross linking density of the gel network. The chemical composition and structures of arabinoxylans vary significantly in different cereals and among wheat varieties and has been a subject of great academic interest. Variation in the amount, chemical composition and structures of pentosans brings about differences in their physico-chemical, solubility and functional properties. Pentosans in different varieties of wheat are known to vary and have relation to bread / chapati making quality. To establish molecular basis of arabinoxylan functionality, it is necessary to gain more insight into the structural diversity of arabinoxylans from wheat varieties differing in chapati making quality. There have been few studies to relate the wheat chapati making quality with respect to structure-function relationship of pentosans. Hence, the present study is aimed to undertake a detailed investigation on the carbohydrate profile and structure–function relationships of pentosans in Indian wheat varieties varying in their chapati making quality. The thesis is presented in 5 chapters 1. Introduction, 2. Materials and Methods, 3. Results and Discussion, 4. Summary and Conclusion, 5. Bibliography 1. First chapter consists of introduction on wheat and its chemical composition. Structural features of cereal pentosans reported in literature, physicochemical properties and nutritional significance of pentosans are presented. These aspects are cited with appropriate references. The first chapter ends with scope and objectives of the present investigation. 2. Second chapter deals with various materials used and methodologies employed during the course of the investigation. It deals with various analytical methods, isolation procedures and combination of methods employed in structural analysis of pentosans. iii 3. Third chapter deals with the results obtained and are discussed with contemporary results. This is divided into four sub-sections. Each sub-section has a brief Introduction, followed by Results and Discussion. 3.1. Carbohydrate composition of pentosans isolated from wheat varieties having differences in chapati making quality Different varieties of wheat were screened for chapati making characteristics, of which DWR-162 and GW-322 showed good and MACS-2496 and HD-2189 revealed poor chapati making quality. They were chosen to study the carbohydrate profile and elucidate structural features of arabinoxylans. Various polysaccharide fractions such as water-soluble, barium hydroxidesoluble and alkali-soluble polysaccharides (viz., hemicelluloses A and B) were isolated from these wheat varieties and their carbohydrate compositions are studied. Water-soluble fractions showed glucose as the major sugar along with small amounts of arabinose and xylose. Barium hydroxide-soluble polysaccharides were rich in arabinose and xylose with small amounts of glucose. Hemicellulose A fractions contained mainly xylose. Arabinose, xylose and glucose were the major sugars observed in hemicellulose B fractions. Alkali-insoluble residue was basically cellulosic in nature and was strongly associated with arabinoxylans. Differences were observed in arabinose/xylose (A/X) ratios in different fractions. Arabinose to xylose (A/X) ratio was more in varieties having good chapati making quality compared to poor chapati making varieties. iv 3.2. Phenolic acid profile and antioxidant properties of wheat varieties Both free and bound phenolic acids contained ferulic and p-coumaric acids. Free phenolic acids in addition contained protocatechuic, vanillic and syringic acids. Ferulic acid was the predominant phenolic acid present in bound form. The contribution of bound phenolic acids to the total phenolic acid contents and antioxidant activities was higher than that of free phenolic acid in all the wheat varieties. Bound phenolic extracts of MACS-2496 showed highest antioxidant activity followed by HD-2189. In free phenolic acids, HD-2189 followed by MACS-2496 revealed maximum antioxidant activity. Dietary fibre contents and their chemical composition differed in these varieties. Dietary fibre analysis revealed that whole-wheat flours from MACS- 2496 and HD-2189 had higher amounts of soluble and insoluble dietary fibres. Arabinose, xylose and glucose were the major sugars present in both soluble and insoluble dietary fibres. 3.3. Structural studies on purified pentosans (arabinoxylans) from wheat varieties differing in chapati making quality Pentosans (arabinoxylans) purified from barium hydroxide-soluble (BE) polysaccharides and hemicelluloses B (Hem B) were rich in arabinose and xylose with glucose as a minor contaminant. Hemicellulose A (Hem A) was rich in xylose along with small amounts of arabinose and glucose. The purification of pentosans was done by glucoamylase digestion and alcohol precipitation. Methylation analysis of pentosans from BE of wheat varieties indicated a xylan backbone in β-(1-4) linkages to which arabinose residues are substituted at O-3 or at both O-2 and O-3. The wheat varieties, DWR-162 and GW-322 known for good chapati making quality had branched arabinoxylans to a greater degree compared to MACS-2496 and HD-2189 having poor chapati making quality. Arabinoxylans from DWR-162 and GW-322 revealed high amounts of disubstituted xylose residues. Terminal arabinosyl residues were present in furanose form, whereas the xyloses were present in pyranose form. v 13C and 1H NMR results substantiated the differences observed by methylation data. Signals due to anomeric carbon atoms of xylose residues were observed in the region between 101.01- 103.2 ppm and were characteristic features of β-linked xylopyranose residues. The signals due to anomeric carbon atoms of α-linked arabinofuranose were observed at 108.42- 109.23 ppm. 1H NMR analysis of arabinoxylans from wheat varieties indicated signals around 5.40, 5.30, and 5.20 corresponding to anomeric protons of α-Larabinofuranose substituted at O-3 (monosubstituted) and at both O-3 and O-2 (disubstituted) of xylose residues, respectively. Fourier transform infrared spectroscopy (FT-IR) of arabinoxylans showed the characteristic bands which could be assigned to the associated water, C-O, C-C and C-OH stretching and bending modes. Optical rotation measurements indicated that the xylan backbone of arabinoxylans was mainly made up of β- linkages. Periodate oxidation and Smith degradation analysis results indicated higher degree of substitution in varieties such as DWR-162 and GW-322. These results were further substantiated by the release of formic acid and products obtained as glycerol, arabinose and xylose, in Smith degradation samples. Hemicellulose A mainly consisted of xylose along with small amounts of arabinose residues. High negative optical rotation value indicates preponderance of β-linkages in the xylan backbone. Pentosans from hemicellulose B of wheat varieties showed the basic structure of xylan backbone in β-(1,4) linkages to which arabinose residues are attached at O-3 and /or O-2 and O-3 positions by α-linkages. Methylation analysis revealed that arabinoxylans (AX) from DWR-162 and GW-322 are more branched. Proton NMR analysis of hemicellulose B fractions from wheat varieties revealed signals for anomeric protons of terminal α-D-arabinofuranosyl residues around 5.2-5.4 ppm and of β-D-xylopyranosyl residues around 4.4-4.7 ppm. FT-IR spectra showed the bending and stretching vibration characteristics of arabinoxylans. Ferulic acid contents were more in good chapati making varieties compared to poor varieties. vi 3.4. Changes in textural quality and arabinoxylan characteristics of wholewheat dough by the incorporation of peroxidase Effect of peroxidase on textural parameters such as hardness, cohesiveness and adhesiveness and physico-chemical characteristics of wheat dough pentosans (arabinoxylans) were studied in the DWR-162 wheat variety. Significant increase in dough hardness was observed with the incorporation of peroxidase in the dough and it was maximum at 1mg peroxidase /100 g flour, with a hardness value of about 7.16 N. Adhesiveness decreased in dough with the incorporation of peroxidase. Pentosans isolated from dough incorporated with peroxidase (1 mg) had higher molecular weight compared to control. Peroxidase (1 mg) treated doughs also yielded pentosans with higher viscosities compared to control. Pentosans from peroxidase treated dough had higher arabinose to xylose ratio, ferulic acid and protein contents compared to native dough extracts. Thus, addition of peroxidase to dough may catalyze cross linking of arabinoxylans as well as formation of protein-arabinoxylan complexes that could be responsible for the alteration of physico-chemical properties of dough. Fourth Chapter deals with Summary and Conclusions of the present study described in third chapter. Fifth Chapter: The thesis concludes with a Bibliographic citation of all the chapters.

Item Type: Thesis (PhD)
Uncontrolled Keywords: Cereals, pentosans, Arabinoxylans, non-starch polysaccharides, chapati
Subjects: 500 Natural Sciences and Mathematics > 04 Chemistry and Allied Sciences > 13 Carbohydrate Chemistry
600 Technology > 08 Food technology > 21 Cereals > 04 Wheat
Divisions: Dept. of Biochemistry
Depositing User: Food Sci. & Technol. Information Services
Date Deposited: 01 Mar 2011 09:56
Last Modified: 28 Dec 2011 10:21
URI: http://ir.cftri.com/id/eprint/9953

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