The Characterization of Sugar Beet Pectin

Abstract

The need to increase the use of low valued co-products derived from the processing of sugar beets has prompted the investigation of the structure of the pectin extracted from sugar beet pulp. The characterization of sugar beet pectin is essential as it has the potential to be used in the production of industrial products, e.g., as an emulsifying agent in food systems. This added use of sugar beet pectin should be of help to sugar beet growers and processors by increasing the demand and value of their by-product without increasing the cost of sugar to the consumer. Here we discuss the characterization of sugar beet pectin utilizing the EcoSEC GPC System with an internal dual-flow differential refractive index detector and UV detector coupled to multi-angle light scattering, quasi-elastic light scattering, and differential viscometry. Implementing this multi-detector SEC technique allowed for the determination of the molar mass averages, in a calibrant-independent fashion, as well as several sizing parameters.

Introduction

An estimated 2 million tons of dry sugar beet pulp is generated annually by U.S. industries as a result of the extraction of sugar from sugar beets1. Currently sugar beet pulp is mainly dried and sold as low-value animal feed at little profit because of the costly energy required to dry it for storage and shipment. Sugar beet pulp, especially the high molar mass pectin portion, has the potential to be an enormous untapped source of a valuable polysaccharide for U.S. industry. The need to increase the utilization of low-value co-products derived from the processing of sugar beets has prompted the investigation of the structure of the pectin extracted from sugar beet pulp. Sugar beet pulp on a dry weight basis is composed of about 67% plant cell wall polysaccharides, 19% of which are pectin, 21% pectin-associated arabinan, and 24% cellulose¹, all of which can potentially add value to the pulp if isolated and characterized. The pectin in sugar beets has different chemical features than that from other sources of pectin, i.e. citrus, as the former tends to have a higher degree of acetylation, a higher natural sugar content, and contains feruloyl groups. Furthermore, unlike citrus pectin which is currently used as a gelling and thickening agent, sugar beet pectin (SBP) has very poor gelling properties, thus the isolation and characterization of it could result in new applications, especially in the production of industrial products. Thus far, SBP has demonstrated the potential to be used as an emulsifying agent in food systems, as it has been found to reduce the interfacial tension between oil and water phases1. The capability of SBP to reduce surface tension has been attributed to the presence of acetyl groups and hydrophobic proteins in SBP preparations².In order to better understand the ability of SBP to act as an emulsifier, we have investigated the structure of SBP using size exclusion chromatography coupled to a multiplicity of physical detection methods, namely multi-angle light scattering (MALS),  quasi-elastic light scattering (QELS), multi-wavelength UV, differential refractometry (RI), and differential viscometry (VISC), and corroborated these results with results from atomic force microscopy¹.

Experimental Conditions

Sugar beet pectin was prepared using microwave-assisted flash-extraction as described in reference 1. Size exclusion chromatography analysis of the sugar beet pectin was performed on a system consisting of an EcoSEC GPC System (Tosoh Bioscience) with an internal dual-flow differential refractive index detector and UV detector connected in series to a HELEOS II MALS photometer (Wyatt Technology Corp.), a QELS photometer (Wyatt), and a ViscoStar differential viscometer (Wyatt). The UV absorbance was monitored at wavelenghts of 310, 278, and 250 nm. The solvent and mobile phases were water with 0.05 M NaNO₃ and 0.01% NaN₃, at a flow rate of 0.7 mL/min. 200 μL injections of 1 mg/mL solutions were injected onto a column bank consisting of three TSKgel GMPWXL columns (30 cm x 7.8 mm) with a particle size of 13 μm obtained from Tosoh Bioscience. These mixed-bed columns have a separation range, based on polyethylene oxides, of 1000 to 8 x 10⁶ g/mol. Detectors, pump oven, and column oven were maintained at 35°C. Data acquisition and processing were performed using Wyatt’s ASTRA 5.3.4.16 software. The HELEOS II detector was normalized in-house using a pullulan standard, with a molar mass of 47,300 g/mol, while calculation of interdetector delays and interdetector band broadening correction were performed using BSA. Calibration of the MALS unit was performed using toluene. The ∂n/∂c of sugar beet pectin was determined previously to be 0.130 mL/g.

Results and Discussions

As described above, the EcoSEC GPC System equipped with TSKgel SEC columns was coupled to a train of MALS, QELS, UV, RI, and VISC detectors to determine the molar mass and size of sugar beet pectin. The results of the experiments are given in Table 1.

Table 1. Physical Properties of Sugar Beet Pectin

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