Pectin isolation and characterization from six okra genotypes

doi:10.1016/j.foodhyd.2017.06.014

Food Hydrocolloids

Volume 72, November 2017, Pages 323-330

https://doi.org/10.1016/j.foodhyd.2017.06.014 Get rights and content

Highlights

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Okra pectin has always low degree of methylation and high degree of acetylation.
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“Hairy” and “smooth” pectins can be isolated depending on the genotype.
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Okra pectin is of high weight-average molar mass (>700 g mol⁻¹).

Abstract

Pectin was isolated by aqueous extraction at pH 6.0 from the pods of six different okra genotypes (Abelmoschus esculentus L.). Genetic diversity was determined using fragment length analysis (FLA) of ten simple sequence repeat (SSR) markers. Physical and chemical evaluation of pectin was performed by means of FT-IR and NMR spectroscopy, sugar composition analysis (GC-MS), size exclusion chromatography coupled to multi-angle laser light scattering (SEC-MALLS), dilute solution viscometry and steady shear rheology assisted by principal component analysis (PCA). Each of the SSR markers detected on average 4.1 alleles and revealed unique genotypes for each sample. Extraction yield was between 11 and 14% resulting in pectin with galacturonic acid content between 43 and 63%, low degree of methyl-esterification (17–25%) and high degree of acetylation (20–40%). All samples were of high weight-average molar mass (M_w) (700–1700 × 10³ g mol⁻¹) and sugar composition analysis revealed the structural diversity of samples with HG/RG-I ratios ranging between 1.3 and 3.1. The present work shows that individual okra genotypes provide pectin with different structural properties that could potentially provide a new source of functional pectin for the food or pharmaceutical industries.

Graphical abstract

Introduction

Okra (Abelmoschus esculentus L.) is cultivated throughout the tropical, sub-tropical and temperate regions of the world including the shores of the Mediterranean Sea owing to its high economic and nutritional value. Pectin has been identified as the responsible hydrocolloid for the viscous texture of okra extracts and is of major technological interest for food and pharmaceutical applications (Alba et al., 2015, Georgiadis et al., 2011, Ghori et al., 2014, Ghori et al., 2017). The functional properties of pectins are typically influenced by their chemical and macromolecular properties such as molecular weight, charge and charge-density, polymer conformation, and flexibility of polymer chains (Kontogiorgos et al., 2012, Ndjouenkeu et al., 1996). Depending on the plant source, genotype, stage of ripening and extraction method, pectins can exhibit heterogeneity in macromolecular characteristics, which subsequently affect their functional properties (Alba & Kontogiorgos, 2017).

Okra pectins have been previously isolated by following various extraction strategies using aqueous buffers (Alamri et al., 2012, Alba et al., 2015, Archana et al., 2013, Georgiadis et al., 2011, Samavati, 2013, Sengkhamparn et al., 2009, Woolfe et al., 1977, Zheng et al., 2014). Isolated okra pectins are rich in rhamnogalacturonan-I (RG-I) segments with varying composition of side chains and molecular weights ranging from 10 – 767 × 10³ g mol⁻¹. Although the effect of extraction conditions on structural and macromolecular characteristics of okra polysaccharides is well investigated and understood, the impact of different okra genotypes on those features has not yet been evaluated. The variability of chemical structures of cell wall polysaccharides (e.g., pectin and hemicellulose) is related to both genetic and developmental factors and has been the subject of several studies in dicotyledonous plants focusing on investigation of the compositional changes of cell-wall polysaccharide structural domains (Gálvez-López et al., 2012, Lahaye et al., 2013, Lahaye et al., 2014, Lahaye et al., 2012).

The understanding of the impact of each structural parameter (e.g., neutral sugar composition, degrees of methylation and acetylation) of cell-wall polysaccharides may serve as the basis for plant design with remodelled functionality. Isolation of pectin from different okra genotypes may also result in structural dissimilarities and consequently may impact functional properties of okra pectin isolates. The aim of the present work, therefore, was to investigate the physicochemical properties of extracted pectins of six different okra genotypes in order to evaluate their potential as novel functional ingredients for the food and pharmaceutical industries.

Section snippets

Cultivation of okra genotypes and pectin isolation

Okra genotypes (Asha, Agbagoma, Asontem, Balabi, Sengavi and Penkrumah) were cultivated in Ghana (Akrofu, Volta Region) from October 2015 to January 2016 and all agricultural practices including thinning, weed control and watering were carried out under controlled environmental conditions. The soil at the experimental site was sandy-loam with the rainfall pattern remaining very low (<20 mm) whereas temperature ranged between 22 and 31 °C. A standardized crop descriptor for okra (Resources, 1991

Genetic diversity of okra samples

SSR (microsatellite) markers were used to determine the genetic diversity and relationships between the six samples of okra. The SSR profile of a diploid or polyploid individual may not always express the genotype, as the observed banding pattern cannot make the distinction between homology of fragments of the same size and the possibility of loss of PCR products (Kosman & Leonard, 2005). Therefore, SSRs were considered as dominant markers, with no assumptions made on the genetic nature of the

Conclusion

In the present work, the structural features of pectin extracted from six okra genotypes were investigated. Regardless of the genotype, extracted okra pectin had low degree of methylation (i.e., LM-pectin) and high degree of acetylation. Despite these common structural similarities, sugar molar ratios and variability in the HG and RG-I domains revealed a range of backbone structures. Genotypes Asontem, Agbagoma and Sengavi show extensive branching compared to the other three genotypes. In Asha

Acknowledgements

The authors are grateful for the financial support from the Robert S. McNamara Fellowships Program (RSM) Award and KNUST Research Fund (KReF).

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Pectin isolation and characterization from six okra genotypes

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Cultivation of okra genotypes and pectin isolation

Genetic diversity of okra samples

Conclusion

Acknowledgements

Food Hydrocoilloids

Colloids and Surfaces B: Biointerfaces

Carbohydrate Polymers

Journal of Chromatography A

Analytical Biochemistry

Carbohydrate Research

International Journal of Biological Macromolecules

Carbohydrate Polymers

Analytical Biochemistry

Food Hydrocolloids

Food Hydrocolloids

International Journal of Biological Macromolecules

Carbohydrate Research

International Journal of Biological Macromolecules

Food Hydrocolloids

Postharvest Biology and Technology

LWT - Food Science and Technology

Analytica Chimica Acta

Postharvest Biology and Technology

Carbohydrate Polymers

Food Chemistry

Carbohydrate Polymers

Food Hydrocolloids

Food Hydrocolloids

Carbohydrate Polymers