Due to the possible relationship between the consumption of synthetic dyes in food and their adverse behavioral and neurological effects in some children, there is a growing interest for using natural colorants to replace these artificial dyes. Anthocyanins (ACNs), for instance, are a group of natural pigments carried in many fruits and vegetables (blueberry, purple sweet potato, red cabbage and many others). Anthocyanins provide red or purple to blue color at different pH levels. In addition to their visual appearance, ACNs were proposed to have multiple health benefits including anti-inflammatory and anti-carcinogenic activity, preventing cardiovascular disease, and facilitating weight management. However, ACNs have been reported to be sensitive to their ambient environments, including pH, light, oxygen and heat.
One way to protect ACNs against these environmental factors is encapsulation. Encapsulation is a delivery system for sensitive bioactive compounds (e.g. ACNs) which is usually applied to improve the stability, to maximize the retention, and to control the release at the target locations of the encapsulated bioactive agents. Encapsulation in biopolymer based hydrogels provides excellent opportunities for the food industry because these carbohydrates are from natural resources and are approved by U.S. Food and Drug Administration for use in food materials.
The major objective of this dissertation was to develop a pH responsive hydrogel system using food-grade polymers to encapsulate ACNs, and to achieve further release of the bioactive compounds at simulated target conditions.
To start with, a biopolymer mixture composed of alginate (Al) and pectin (P) that can form hydrogel were used to generate particles using extrusion method in low pH buffer. We also produced novel disc shaped particles which can potentially enhance the particle adhesion in intestines, our ultimate target location. As the pH increases, Al-P hydrogels go through a gel-sol transition and the dissolution kinetics of the hydrogel dominates the bioactive compound release. According to our study, the volume change of spherical and disc shaped particles showed that the hydrogel particles would be stable in low pH beverages (pH below 3.0) during storage. At pH 5.0 and 7.0, hydrogel particle dissolution due to increased electronic repulsive forces, measured as the particle size change followed a zero-order kinetic model. The 2.8% TGC 43-57 wt% Al-P disc particles had the fastest and the 2.2% TGC 82-18 wt% Al-P spherical particles had the slowest volume dissolution rate at pH 7.0 and 37°C. Activation energies of hydrogel particles were significantly affected by pH, particle shape and Al to P ratio. Such a smart biopolymer system which responds to pH provides an opportunity to use food as a vehicle for targeted delivery of bioactive compounds.
Then, encapsulation of purple corn (PC) and blueberry (BB) extracts in alginate-pectin hydrogel particles was achieved to protect the bioactive compound—anthocyanins (ACNs) from degradation. The alginate-pectin hydrogel particles containing PC or BB extracts were produced by extrusion method in pH 1.2 buffer. Factors including initial ACNs concentration in particle, shaple of particles (disc and sphere), alginate-pectin ratio, total gum concentration (TGC), ACN source, and curing bath conditions were chosen to evaluate their influences on encapsulation efficiency (EE) of ACNs using mass balance. In general, the initial ACN concentration and particle shape showed no influence on EE, while the alginate-pectin ratio, TGC, ACN source and the pH of the curing bath showed major impact on EE.
Loss of ACNs during short-time storage after encapsulated in Al-P hydrogel particles was also investigated in pH 3.0 buffer. The effect of alginate to pectin ratio, TGC, particle shape, particle weight to solution volume (wt/vol), ACN source and storage temperature on the EE at equilibrium during storage was evaluated. The higher alginate content particles showed higher EE at equilibrium during storage using PC or BB extracts, compared with low alginate content particles. High TGC improved the percent EE at equilibrium during storage significantly for both ACN sources, and higher amount of ACN was retained in spherical shaped particles than disc shaped particles.
Also, ACN stability in particles or in solution was evaluated during storage under light or dark condition at 20°C. The degradation of ACN in solution showed first-order kinetics while the degradation of ACN in particle followed zero-order kinetics. The stability of ACN encapsulated in alginate-pectin particles was significantly improved over ACN in solution during storage in both light and dark conditions.
Lastly, the diffusion study of ACNs from the Al-P gel was evaluated. Using a well-mixed and temperature-controlled system, the diffusion behavior of PC and BB ACNs from Al-P hydrogel particles were investigated. The diffusion coefficients for ACN were determined based on the mathematical approach using Fick’s second law. Various experimental conditions were evaluated to assess the behavior of the diffusivity of different initial ACN concentration in particles (20.75, 87.78, 114.96 and 173.23 µg/ml), alginate to pectin ratios (82-18 and 43-57 Al-P), total gum concentrations (2.2% and 2.8%), particle shapes (disc and spherical shaped), ACN sources (PC and BB ACN) and temperatures (4, 24 and 37°C). Results showed the diffusion coefficient of ACNs was significantly affected by the particle shape and the operating temperature, while independent from the initial ACN concentration in particle, alginate to pectin ratio, total gum concentration and ACN source. In all scenarios, the diffusion coefficient calculated was noticeably smaller than the diffusion coefficient of ACN in pure water calculated by using Wilke & Chang’s equation (1955). Partition coefficient was evaluated for all the studied conditions, and showed strong dependent all the evaluated parameters except the initial ACN concentration in particles. These results can be used to design the hydrogel particles to achieve different equilibrium state during diffusion, and the significant higher activation energy using alginate-pectin hydrogel particles suggested the potential of using this matrix for processes those undergoes strong temperature fluctuation.