Microencapsulation of Astaxanthin with Different Blends of Protein and Soluble Corn Fibre Using Spray Drying

Abstract

Astaxanthin is an active antioxidant and has beneficial effect on human health. However, its hydrophobic nature and susceptibility to light, heat and oxygen limit its application in most food systems. This study aimed to encapsulate astaxanthin using milk proteins and carbohydrate, in order to improve its stability and application in food systems. Whey protein isolate (WPI) and sodium caseinate (SC) are well known encapsulants and possess antioxidant properties. Soluble corn fibre 70 (SCF70) with DE 20 is believed to exhibit antioxidant activity and to improve encapsulating capacity of protein-based wall systems. They were therefore selected as the wall materials for encapsulation of astaxanthin. The wall solution was prepared from dissolving the emulsifier and carbohydrate containing a total of 20-24 wt% solids in water. The astaxanthin emulsions were prepared by two-stage homogeniser at 80 + 800 bars after passing through the homogeniser 4 times. The emulsion were then converted into powders with 0.33 wt% astaxanthin by spray drying at 160°C inlet air temperature and 70°C outlet air temperature. The properties of the emulsions were evaluated including droplet size, size distribution, zeta potential, and viscosity. The powder produced from spray drying were characterised by chemical and physical tests including water activity, microencapsulation efficiency, surface properties and oxidative stability. The interaction between wall materials was studied using Fourier Transform Infrared Spectroscopy (FTIR) while the bioaccessibility was investigated in an in vitro digestion study. Results show that droplet size of the astaxanthin emulsions was below 200 nm and size distribution appeared to be narrowly distributed. Therefore, particle size would have little effect on the physicochemical properties of spray dried powders. The low viscosity of the parent emulsions probably exhibited little effect on the spray drying process. It was found that the reconstituted emulsion and parent emulsion both had droplet size below 200 nm. This indicates that the astaxanthin emulsions could be stable during spray drying. Scanning electron microscopy reveals that formation of surface dents on some samples caused by particle shrinkage during early drying process. Particle with WPI based wall systems had smoother outer surface than those formulated with SC based wall matrices, suggesting that WPI could be a suitable encapsulating agent in combination with soluble corn fibre 70. Microencapsulation efficiency of the microencapsulated astaxanthin was above 88%, indicating the wall matrices were effective in preventing penetration of the organic solvent into the microcapsule. Storage test was conducted at 45°C and 33% relative humidity, under air and nitrogen environment. The oxidative stability of the astaxanthin microcapsules was determined by measuring peroxide value and p-anisidine value. Results show that surface oil might not be related to the oxidative stability of the microcapsules and other factors might adversely affect the oxidative stability. Changes in the physical state of the amorphous powders due to the difference in water activity between the powders and storage environment might influence the oxidative stability and the astaxanthin content in the microcapsules. Results indicate that wall composition may have little effect on the oxidative stability of the microencapsulated astaxanthin. Astaxanthin content in microcapsules with high oil content decreased slightly faster than in those containing less oil content. Oxidative stability of the microcapsules could be related to the antioxidant activity of raw materials. The FTIR results indicated the possibility of Maillard reaction products formation, which may also influence the oxidative stability of the microcapsules. The in vitro digestion results suggested that the digestivity of the WPI based wall systems might be better than that of the SC based wall systems, as the bioaccessibility of the microcapsule were higher. The presence of dietary fibre (i.e., SCF70) and wall thickness might affect the in vitro digestivity of the microcapsules. Overall, the best formulation showing the best bioaccessibility (71.67%) is the WPI/SCF 70 ratio of 1/0.5 and the wall/core ratio of 2. In summary, this research has shown that microencapsulation of astaxanthin by spray drying technique is capable of producing a more stable microcapsule that has potential application in food system.