Extraction, characterization and usage of pectin from sugar beet pulp waste

Abstract

The worldwide accumulation of substantial agro-industrial waste causes serious economic and ecological problems. Agro-industrial wastes are the byproducts produced during the processing of crops in the food industry. Converting by-products from the processing of plant foods into valuable functional products helps mitigate the waste problem in the food industry. Sugar beet pulp is the residual by-product after sugar is extracted from sugar beets. It is an underutilized waste currently used for animal feed or as feedstock in biorefinery facilities. Sugar beet pectin is a promising variant of pectin derived from sugar beet pulp. In 2021, Turkey imported approximately 676,000 kilograms of pectin. Given its extensive applications in the Turkish food industry, this import volume is expected to increase in the coming years. Turkey is the world's fifth-largest sugar beet producer, but despite the great potential, currently there is no facility producing pectin. Pectin is a heteropolysaccharide made up of α-D-galacturonic acid units connected at the 1,4 position, with additional side chains of neutral sugars including 1,5-linked αL-arabinose and 1,4-linked β-D-galactose. Pectin is found in the middle lamella of plant cell walls providing structural support and protection with cellulose and hemicellulose. Pectin is classified into two types based on its degree of methylation (DM): Low methoxy pectin (LMP) with a DE of less than 50%, and high methoxy pectin (HMP) with a DE greater than 50%. Pectin is widely utilized in the food industry, mainly serving as a gelling and thickening agent in jams, marmalades, and sugar confectionery, and a stabilizer in milk and fruit products. In this study, pectin was extracted from sugar beet pulp waste using conventional extraction (CE), ultrasound-assisted extraction (UAE), and microwave-assisted extraction (MAE) methods. The objective was to determine the effect of extraction methods on its physicochemical properties and performance in fermented milk beverage. An aqueous solution of homogenized sugar beet pulp waste acidified with citric acid was prepared for extraction. CE involved heating the solution to 85°C in a water bath. For UAE, an ultrasonic processor with maximum power of 400 W and a frequency of 24 kHz was used. A sonication amplitude of 50% was applied to the solution for 15 minutes to heat to the same temperature as in CE. MAE was performed using a microwave unit with a maximum power of 1400 W and a frequency of 2450 MHz was used. The sample solution was subjected to a power of 420 W for 15 min to reach the same temperature as others. Heated solutions were mixed for 60 min and homogenized. After cooling and removing sedimented particles, pectin was separated from the remaining solution by using ethanol. Obtained precipitate was then purified by dialysis to remove small molecules and then dried. The extraction yield, degree of esterification (DE) and galacturonic acid content (GalA) of pectin extracts were determined. Pectin extracts were also analyzed using FTIR spectroscopy for structural elucidation in the wavenumber range of 650-4000 cm-1 . Dried pectin was used in a fermented milk beverage and rheological properties and serum separation level of the beverages were measured. Pectins were also used in an emulsion model for determining their effect on emulsion stability. The properties of the extracted pectin samples were compared with those of commercial citrus pectin. Pectin yields ranged from 5.3% to 10.3%, with the highest yield of 10.3% achieved by UAE. The CE took 35 minutes to reach a comparable yield, while MAE and UAE required only 15 minutes. Notably, the protein content in the pectin sample from UAE was 8.6%, significantly higher than those from CE at 6.6% and MAE at 6.2%. It is important to emphasis that lower protein content is indicative of higher sample purity. Pectin samples extracted via UAE, MAE, and CE showed similar degrees of esterification, 75.8%, 72.7%, and 72.3%, respectively. All extraction methods produced DE values above 50%, qualifying that the pectin was high methoxyl pectin (HMP). GalA content in pectin samples ranged from 56.8% to 61.3% with no significant difference among the value in terms of extraction method. FTIR spectra of sugar beet pectin samples showed peaks at similar wave numbers. All pectin samples exhibited absorbance peaks at 1643.57 cm⁻¹, 1732.69 cm⁻¹, and 1729.18 cm⁻¹, which are attributed to the C=O stretching of methyl-esterified carboxyl groups and carboxylate ions, respectively. FTIR analysis revealed that the different extraction methods had minimal impact on the chemical structure of pectin. In fermented milk beverages prepared with pectin from UAE, MAE, and CE, as well as commercial pectin, significant increases in apparent viscosity, flow behavior index and consistency coefficient values were observed compared to those of the control sample without any additive. All fermented milk beverages showed pseudoplastic behavior. The serum separation levels in the pectin containing beverage samples were lower than that of the control sample (13.6%) after 7 days of storage. The sample with commercial citrus pectin had a lower serum separation level than those of the other samples. In addition, pectins obtained with UAE and MAE provided a higher stability in emulsion compared to pectin from CE and commercial citrus pectin. These results showed that the sugar beet pectin had a potential for use as a food ingredient. In this study, UAE, MAE, and CE generally yielded sugar beet pectin with similar properties and performance in fermented milk beverage and emulsion. However, UAE and MAE can be preferred to reduce extraction time significantly compared to CE. UAE can also be beneficial to increase protein content in the pectin in some cases. Pectin extraction from sugar beet pulp can be a way to upcycle this valuable waste in a sustainable food production system.