Elsevier

Food Chemistry

Volume 275, 1 March 2019, Pages 123-134
Food Chemistry

Encapsulation of stevia rebaudiana Bertoni aqueous crude extracts by ionic gelation – Effects of alginate blends and gelling solutions on the polyphenolic profile

https://doi.org/10.1016/j.foodchem.2018.09.086Get rights and content

Highlights

  • Encapsulation of stevia crude extract within two alginate blends by ionic gelation has been assessed.

  • High encapsulation efficiency of polyphenols was achieved for all stevia-loaded beads.

  • Size of beads decreased as encapsulation efficiency increased for both the blends.

  • Metabolomics profiling identified 479 free and encapsulated polyphenolic compounds.

  • Crosslinking conditions impacted on phenolic profile providing a selective uptake.

Abstract

We formulated and characterised two alginate blends for the encapsulation of stevia extract (SE) via ionic gelation through an extrusion technique. Calcium chloride in SE and calcium chloride solutions were assessed as crosslinkers to overcome phenolic losses by diffusion and increase encapsulation efficiency (EE). Regardless of the blend, all stevia-loaded beads exhibited high EE (62.7–101.0%). The size of the beads decreased as EE increased. Fourier transform infrared analysis showed increased hydrogen bonding between SE and alginates, confirming the successful incorporation of SE within the matrix. Untargeted metabolomics profiling identified 479 free and encapsulated polyphenolic compounds. Flavonoids (catechin and luteolin equivalents) were predominant in SE whereas tyrosols and 5-pentadecylresorcinol equivalents were predominant in all bead formulations. Three-common discriminant compounds were exclusive to each blend and were inversely affected by the crosslinking conditions. Both alginate blends have been shown to be feasible as carrier systems of stevia extracts independent of crosslinking conditions.

Introduction

Encapsulation of polyphenolic-rich crude extracts constitutes a current research need since it can be incorporated in foods to act as a food additive, preventing or delaying food deterioration, as well as functioning as a nutritional supplement with health benefits, giving rise to innovative functional foods (Kurozawa & Hubinger, 2017). Encapsulation technologies have been extensively applied in the pharmaceutical, nutraceutical and food industries as an important tool to enhance the stability of bioactive compounds during processing and storage as well as to improve their bioavailability (McClements, 2015, Nedovic et al., 2011).

Ionic gelation through extrusion dripping represents a simple, efficient and low-cost encapsulation technique that does not require specialised equipment, high temperature, or organic solvents, making it suitable for both hydrophobic or hydrophilic compounds (Đorđević et al., 2015, Nedovic et al., 2011). Because of their ionic gelation properties and biocompatibility, sodium alginates are considered one of the most versatile, natural anionic polymers widely employed as an encapsulation agent of bioactive compounds, including plant extracts polyphenols (López-Córdoba et al., 2014, Lozano-Vazquez et al., 2015, Stojanovic et al., 2012). However, there are limitations commonly associated with hydrophilic bioactive compounds encapsulated with alginates, such as burst release mechanisms and loss by diffusion during gel formation and storage (Arriola et al., 2016, Gombotz and Wee, 2012).

Therefore, a current challenge in the design of carrier systems for water-soluble compounds, such as plant polyphenols, by ionic gelation is to increase encapsulation efficiency and enhance controlled release properties (Kurozawa & Hubinger, 2017). Current approaches include the use of filler materials within the polymer matrix, internal ionic gelation, coating as a barrier as well as the use of natural polymer mixtures (Kurozawa and Hubinger, 2017, Lozano-Vazquez et al., 2015).

Stevia rebaudiana Bertoni (stevia) is a perennial South American herb, also known as ka’a hee or sweet-leaf, originally native to the north-east region of Paraguay (Gaweł-Bȩben et al., 2015, Wölwer-Rieck, 2012). Besides its commercial application due to its foliar steviol glycosides with natural and intense sweetening properties, a growing body of research is pointing to stevia leaf extracts as a multifunctional source of natural bioactive compounds (Ruiz-Ruiz et al., 2015, Shivanna et al., 2013). Different therapeutic properties have been linked to stevia phytoconstituents that comprise vitamins, phytosterols, triterpenes, essential oil components, as well as polyphenols – mostly consisting of flavonoids and phenolic acids – associated with its anti-inflammatory and antioxidant properties (Gaweł-Bȩben et al., 2015, Karaköse et al., 2015, Molina-Calle et al., 2017, Wölwer-Rieck, 2012). Yet, despite the relevance of phenolic compounds, studies involving stevia crude extract characterisation are few and still emerging, especially compared to the extensive work published regarding its glycoside derivatives (Gaweł-Bȩben et al., 2015, Wölwer-Rieck, 2012).

In our previous study, the successful encapsulation of crude stevia extracts with alginates through ionic gelation was described for the first time in scientific literature (Arriola et al., 2016). However, both the alginate employed and the free and encapsulated polyphenolic profile of stevia lacked a thorough characterisation analysis. Furthermore, we observed a significant diffusional migration of phenolic compounds from stevia in and out of beads during both the ionic gelation process and during beads storage.

In this study, we hypothesise that the entrapment efficiency of stevia phenolic constituents by the formulated systems will be enhanced not only by employing alginate blends, but also by employing the extract in the crosslinking solution. Therefore, mixtures of sodium alginates with different molecular characteristics were formulated and characterised as carrier blends. The two blends were formed into plain alginate beads (no stevia in either the bead or the collecting solution) and two types of stevia-loaded beads which were either gelled in calcium chloride solution, or in calcium chloride dissolved in stevia extract. The structural and morphological differences between beads of the same blend and beads across blends were investigated using Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) analyses. Subsequently, secondary metabolites (phenolics and their derivatives) were identified through a comprehensive untargeted metabolomics profiling, an innovative approach in stevia publications.

Section snippets

Chemicals

The Folin–Ciocalteu reagent, gallic acid and calcium chloride dehydrate were purchased from Sigma-Aldrich (Poole, UK). Sodium alginates employed in this study were characterised using 1H nuclear magnetic resonance spectroscopy (ASTM F2259-12) (Chater et al., 2015, Wilcox et al., 2014). Alginates employed in the blend preparations included LFR 5/60, a low viscosity and low molecular weight (MW) (40,000) sodium alginate rich in guluronate Fg 0.64; SF120, a high viscosity and high MW (225,000)

Characterisation of alginate beads

In preliminary tests, all sodium alginates used as ingredients in the blends were assessed for their individual entrapment efficiency of total phenolic content of stevia extracts (data not shown). The alginate LFR5/60 (2% w/v) was selected as the main ingredient of both the blends since it showed the best encapsulation performance (data not shown).

Conclusions

Encapsulation of stevia crude extract within two alginate blends by ionic gelation has been assessed. The impact of carrier formulations and crosslinking conditions on the overall encapsulation performance was investigated aiming to overcome phenolic compound losses by diffusion and, thus, increase encapsulation efficiency. All the formulated stevia-loaded beads reached high encapsulation efficiency values (>60%), independent of the carrier combination or the crosslinking conditions.

Conflict of interest

The authors declare that they have no conflict of interest in this study.

Acknowledgements

This work was supported by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) through Programa de Doutorado Sanduíche no Exterior (PDSE) [99999.006878/2015-06].

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