Cassia spectabilis DC seed galactomannan: Structural, crystallographical and rheological studies

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Abstract

The seeds of Cassia spectabilis DC (family: Leguminoseae), an Indian fast growing spreading tree, contain about 40% of endosperm and possess the characteristics of becoming a potential source of commercial gum. The purified galactomannan shows Mw 1.1×106, intrinsic viscosity [η] 615 mL/g with k′=1.706×10−1, and a mannose to galactose ratio of 2.65. The hydrolysis of the fully methylated polysaccharide reveals clearly the expected structure of legume galactomannans. The orthorhombic lattice constants of the hydrated gums are as follows: a=9.12 Å, b=25.63 Å and c=10.28 Å. The results of X-ray fiber studies show that the b dimension of the unit cell is very sensitive to relative humidity (RH), galactose substitution and orientation of the films. The probable space group symmetry of the unit cell is P21212. Rheological studies of the galactomannan have shown that the transition from semi-dilute to dilute regime occurs at a critical concentration Cc*=2.75. The slope of the log–log plot of specific viscosity versus C at zero shear rate is 5.87 in the more concentrated regime. The viscoelastic and critical shear rate behavior indicate the characteristics of a coil polymer. The large dependence of the viscosity on the coil overlap parameter is probably due to polymer–polymer interactions and peculiarity of the galactose distribution along the chain. Above 20 g/L concentration, the rheological behavior of the gum is like the one of a weak-gel.

Introduction

Seed galactomannans 1, 2, commonly known as seed gums, have widespread industrial applications [3]in food, paper, textile, petroleum, pharmaceuticals, cosmetics, etc. They are mostly found in the endosperm of leguminous seeds as cell wall storage component and energy reserve. Galactomannans have the fundamental structure consisting of a main chain of β-(1→4)-d-mannopyranose units substituted by single α-d-galactopyranose units at O-6, although there are few deviations from this basic structure. They differ from each other in mannose:galactose ratio and fine structure regarding distribution of single galactose branches on the main chain, thereby causing variations in solubility, rheology and other properties.

Cassia spectabilis DC, Local name: Archibald's Cassia and Calceolaria Cassia, is a fast growing spreading tree, up to 20 m in height, native from tropical America and introduced in some parts of India [4]. The plant is reported to be a source of senna and may prove to be a cheaper substitute for the true senna. According to Hussain and Kapoor [5], it is enumerated as Senna spectabilis (DC) Irwin and Barneby sp. C. spectabilis DC. The seeds are dicotyledonous, brown, more or less quadrate and medium sized (wt. of 100 seeds; 2.84 g) having about 40% endosperm. In view of this high content of endosperm gum, and to test its characteristics of becoming a potential source of commercial gum, C. spectabilis seed galactomannan has been studied in terms of structural, crystallographical and rheological aspects.

Section snippets

Experimental

The crude gum, isolated through a dry/wet milling process, was purified by dispersing the gum in water and precipitation with EtOH. The resulting polysaccharide was complexed with Ba(OH)2 followed by decomplexing with an excess of 2 N AcOH, regeneration of the polymer with EtOH and continuous stirring with aq EtOH followed by centrifugation. It was successively washed with 70, 80, 90 and 95% EtOH. The sample was finally purified by dialysis and filtration through various Millipore membranes.

The

Isolation and purification

The endosperm of the seeds contains the water-soluble galactomannan, while the rest of the seed meal mainly contains pentoses. The analysis of seed components is presented in Table 1. The endosperm was separated from the seeds by dry and wet milling processes using various mixers, sieves and grinders. The yield of the crude gum was 31–34% in the dry process compared to 22–25% in the wet process. This is due to the fact that during dry milling of seeds, small portions of seed coat and hull

Conclusion

The galactomannan obtained from the seeds of C. spectabilis is characterized by a low galactose content (Man/Gal=2.65). Its Man:Gal ratio lies between those of commercial Guar (1.63) and locust bean (3.12) gums. Methylation and 13C NMR studies reveal that the gum has the basic structure of legume galactomannans. X-ray structural studies show that the b dimension of the unit cell is sensitive to relative humidity (RH), galactose substitution and orientation of the films. The probable space group

Acknowledgements

V.P. Kapoor expresses his gratitude to the French Ministry of Foreign Affairs for the grant of a high level fellowship for one year and to Dr. P.V. Sane, Director, NBRI, Lucknow, India for his keen interest in this work.

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