Encapsulation of β-carotene in alginate-based hydrogel beads: Impact on physicochemical stability and bioaccessibility

Highlights

• β-carotene was incorporated in free nanoemulsion and alginate hydrogel beads.

• Hydrogel beads encapsulation improved β-carotene chemical stability.

• Lipid digestion and β-carotene release was retarded upon encapsulation.

• Hydrogel beads could be designed to adjust the β-carotene digestion fate.

Abstract

Delivery systems are needed to protect carotenoids in foods, but release them at an appropriate location within the gastrointestinal tract (GIT). In this study, β-carotene was incorporated into three different delivery systems: free lipid droplets; filled hydrogel beads formed using 0.5% alginate (“0.5% beads”); and, filled hydrogel beads formed using 1% alginate (“1% beads”). Hydrogel beads were fabricated by injecting an alginate solution into a calcium ion solution using an extrusion device (Encapsulator). Light scattering and confocal microscopy measurements indicated that the 0.5% beads had much smaller diameter (285 μm) than the 1% beads (660 μm). β-carotene encapsulated in free lipid droplets (nanoemulsions) was highly unstable to chemical degradation when stored at elevated temperatures. Conversely, incorporation of the β-carotene-loaded lipid droplets into hydrogel beads greatly improved its chemical stability. Simulated GIT studies indicated that the rate and extent of lipid digestion decreased in the following order: free lipid droplets >0.5% beads >1% beads. The encapsulated β-carotene had a higher bioaccessibility in free lipid droplets than in hydrogel beads, whereas its chemical stability within the GIT was higher in the hydrogel beads, with the 1% beads giving better protection against degradation than the 0.5% beads, which was attributed to differences in hydrogel pore size. Overall, our results provide valuable information for the rational design and development of nutraceutical delivery systems for utilization in functional food products.

Introduction

Carotenoids are natural pigments present in certain fruits and vegetables where they play key roles in photosynthesis and photo–protection reactions (Cazzonelli, 2011, DemmigAdams et al., 1996, Namitha and Negi, 2010). β-carotene, one of the major types of carotenoid, is a natural precursor of vitamin A (Maiani et al., 2009, Rao and Rao, 2007). β-carotene also demonstrates antioxidant and non-antioxidant biological activities, which may account for its ability to reduce the risk of certain chronic diseases including cardiovascular disease, eye disease, and certain cancers (Rao & Rao, 2007). There is therefore interest in fortifying food and beverage products with this potentially beneficial nutraceutical to create products designed to promote human health and wellness (van den Berg et al., 2000). However, the extended hydrocarbon backbone and high degree of unsaturation of β-carotene restrict its incorporation into many foods and beverages because these factors lead to low water-solubility, poor chemical instability and low bioaccessibility (Fernandez-Garcia et al., 2012, Qian et al., 2012b, Yi et al., 2014). Moreover, β-carotene can be chemically transformed within the gastrointestinal tract (GIT), which may alter its potential health benefits (Sy, Dangles, Borel, & Caris-Veyrat, 2015). Consequently, there is a need to develop effective carotenoid delivery systems that can overcome these challenges (Donhowe and Kong, 2014, Lopes et al., 2010).

Colloidal delivery systems are one of the most convenient methods of incorporating lipophilic bioactives (such as carotenoids) into foods (McClements, 2012, Patel and Velikov, 2011, Velikov and Pelan, 2008). These systems consist of small colloidal particles, typically fabricated from lipids, phospholipids, surfactants, and/or biopolymers, which encapsulate the lipophilic bioactives. These colloidal particles are designed to facilitate the incorporation of bioactives into food products, improve their chemical/biochemical stability, and control their GIT fate (McClements, 2014). A typical bioactive-loaded colloidal system can be simply prepared by solubilizing the lipophilic bioactive components within an oil phase and then homogenizing it with an aqueous phase containing an emulsifier to form oil-in-water emulsions or nanoemulsions (Donsi et al., 2011, Marze, 2015, McClements et al., 2007). Previous studies indicate that emulsion-based systems are particularly suitable for encapsulation and delivery of carotenoids such as lutein (Losso, Khachatryan, Ogawa, Godber, & Shih, 2005), lycopene (Boon, McClements, Weiss, & Decker, 2009) and β-carotene (Qian, Decker, Xiao, & McClements, 2012a; Salvia-Trujillo et al., 2013, Silva et al., 2011). In addition, the bioaccessibility and bioavailability of carotenoids may increase considerably after encapsulation in emulsion-based systems (Ribeiro et al., 2006, Salvia-Trujillo et al., 2013).

One limitation of using emulsions or nanoemulsions as delivery systems is they have limited scope for controlling the chemical stability of encapsulated carotenoids. This is because only a thin layer of emulsifier molecules coats the lipid droplets, and so any carotenoids located near the droplet surfaces are prone to chemical degradation promoted by hydrophilic components in the aqueous phase (such as acids, transition metals, or enzymes). In particular, carotenoids have a conjugated polyunsaturated hydrocarbon chain, which makes them highly prone to degradation due to autoxidation promoted by light, heat, singlet oxygen, transition metals, free radicals and highly acidic conditions (Qian et al., 2012a). This problem could be overcome by trapping the carotenoid-loaded lipid droplets inside hydrogel beads (“microgels”). The hydrogel matrix surrounding the carotenoids could then be designed to provide a local environment that protects them from degradation.

Hydrogel beads suitable for utilization in foods are usually constructed from food-grade biopolymers such as proteins and/or polysaccharides using a variety of approaches (Chen et al., 2006, Joye and McClements, 2014, Shewan and Stokes, 2013). The injection-gelation method is one of the simplest approaches for the encapsulation, protection, and delivery of nutraceuticals and other active agents (Shewan & Stokes, 2013). In this method, a biopolymer solution containing the bioactive is injected into another ‘‘hardening’’ solution under conditions that promote biopolymer gelation. This procedure results in the formation of a hydrogel bead with the nutraceuticals trapped inside. The nature of the hydrogel matrix surrounding the bioactive can be designed to improve its physical and chemical stability, as well as to control its GIT fate. Typically, hydrogel beads with a particle diameter greater than about 1 mm are prepared using a syringe with a needle or a pipette (Li, Hu, Du, Xiao, & McClements, 2011). Smaller beads (<1 mm) can be fabricated using an extrusion device with a vibrating nozzle (Encapsulator), which may be an advantage for certain applications. The dimensions of hydrogel beads can be further adjusted by altering several factors including biopolymer and cross-linking ion concentration, instrument parameters, and hardening time (Gombotz and Wee, 2012, Li et al., 2011).

In the present study, microfluidization was used to prepare oil-in-water nanoemulsions containing β-carotene. The β-carotene-loaded lipid droplets were then used as delivery systems themselves, or they were incorporated into hydrogel beads by injecting them and an alginate mixture into a gelling solution (Ca²⁺). Calcium alginate beads formed using this approach are widely used in the pharmaceutical industry to encapsulate and protect bioactive agents (Giri et al., 2012, Lee and Mooney, 2012). Hydrogel beads with different structural, physical, and functional attributes were prepared by using two different alginate concentrations (0.5% and 1%): typically bead hardness increases with increasing alginate, but bead pore size decreases. The chemical stability and simulated GIT fate of β-carotene encapsulated within either nanoemulsions or filled hydrogel beads were compared. The information obtained from this study may facilitate the production of delivery systems that improve the stability and bioavailability of lipophilic bioactives.