Encapsulation of flaxseed oil with plant-based protein concentrates: a potential functional food application

Abstract

The overall aim of this research was to develop a protein plant-based encapsulated oil with desirable functional properties to protect and deliver the core material within gastrointestinal environments. In the first part of the study (Chapter 3), the most suitable methods for extraction of a flaxseed protein concentrate (FPC) from cold-pressed flaxseed meal (FM) with a high content of protein were screened. The effect of processing conditions on the extraction efficiency and protein yield of all extracted fractions of flaxseed meal was investigated. The flaxseed protein concentrates produced by alkali isoelectric precipitation (A-FPC), enzymatic (E-FPC), and enzyme-solvent assisted (ES-FPC) were characterised in terms of proximate composition, protein purity, protein structure, thermal and physicochemical properties and compared with pea protein concentrate (PPC). The characterisation of extracts was affected by the methods of extraction. The highest protein yield with the lowest purity was recorded for A-FPC, while ES-FPC showed the lowest yield of protein extraction with the highest purity. All types of protein concentrates were soluble in aqueous system in the following order E-FPC = A-FPC > PPC > ES-FPC. The water holding capacity (WHC) showed a similar result to the solubility. ES-FPC showed the lowest water (WHC) and oil (OHC) holding capacity. All the protein concentrates were well distributed in an aqueous system and produced droplets ≤ 1 μm at different residence times of sonication, with higher emulsion stability for A-FP E-FPC. The FTIR analysis, showed similar spectra with differences in band sizes. A-FPC and E-FPC showed higher thermal stability. In Chapter 4, the SDS-PAGE result showed higher molecular weight heterogeneity in PPC, in contrast to the all types of FPCs which showed a limited range of MW bands between ~10-~50 kDa with the highest proportion of MW at 25–30 kDa and 35-40 kDa. All types of protein concentrates showed their maximum solubility at alkali pH with the minimum solubility at the isoelectric point (IP) of FPCs (~pH 4.0) and PPC (~pH 5.0). All emulsions were similar in polydispersity with mono-modal droplet distribution and size of ≤ 0.43 μm that carried a net negative charge at neutral conditions (pH 7.0). A-FPC showed significantly higher average surface hydrophobicity (H0: 66.14) than that of ES-FPC (52.63), and E-FPC (43.27) and was comparable to PPC (68.47). A-FPC displayed significantly (p < 0.05) the highest emulsion capacity (EC: 87.91%), emulsifying activity index (EAI: 87.18 m2/g) and emulsifying stability index (ESI: 12.51 min) compared to the other protein concentrates. All the emulsions showed shear thinning behaviour with the highest viscosity for those prepared with A-FPC. Based on the suitable functional properties of E-FPC and proper viscosity, this type of protein extract was selected for emulsion formulation and spray drying in the following chapters. In Chapter 5, E-FPC-stabilised emulsions were formulated based on the different ratio of protein concentrate to the oil content. The physicochemical properties (e.g., oil droplet size, stability, etc.) of emulsions were significantly affected by the different ratio of emulsion components. The ratio of coating: oil 2:1 was assessed as the best emulsion formulation and considered for the next chapter of study. In Chapter 6, emulsions using either flaxseed protein concentrate (FPC) or pea protein concentrate (PPC) as a coating material were produced before feeding to the spray dryer. The physicochemical properties of FPC and PPC emulsions as well as the encapsulated oils were affected by the type of coating material. In contrast to the larger oil droplet size of emulsions produced with FPC (697 nm) than PPC (274 nm) the FPC emulsions showed higher viscosity and stability, and they showed a significantly lower increase in polydispersity index (PDI) of reconstituted emulsion after drying. The moisture content and water activity were determined to be in the range of 1.15-2.24 % and 0.09-0.2 for all spray dried samples respectively. The FPC capsules showed significantly lower surface oil and higher encapsulation efficiency. And also these type of capsules showed higher capability of dispersion of particles in water by showing the higher water solubility index (WSI: 88.20%) and higher water absorption index (WAI: 4.64 g.g-1) than those produced with PPC. Both types of encapsulated powders showed poor flowing properties and lower density and lighter colour was found for PPC capsules. The larger particle size with less pores was observed for FPC capsules. The oxidative stability evaluation of encapsulated oils (Chapter 7) showed both types of coating materials could delay the oxidation of flaxseed oil compared to that of crude oil over 4 weeks of storage and higher oxidative stability was observed for FPC capsules. Moreover, the unsaturated fatty acids reduction was significantly (p < 0.05) lower in encapsulated oils than that of crude flaxseed oil (control). The capsules were able to deliver 52%-77% of the encapsulated oil with lower percentage of oil released by FPC capsules during an in vitro gastrointestinal digestion. This study showed the flaxseed protein concentrate can be considered as an alternative emulsifier for plant-based encapsulation systems to protect and deliver core materials.