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

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
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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.
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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.
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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.
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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.
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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.