Characterisation of plant oil bodies and their application as delivery systems of bioactive compounds : a thesis presented in partial fulfilment of the requirements for the degree of Master of Food Technology at Massey University, Manawatū, New Zealand
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Date
2021
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Massey University
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Abstract
Oil bodies are small, spherical organelles that store triacylglycerols (TAG) in plants. They are derived as natural oil-in-water emulsions and their unique interface composed of a monolayer of phospholipids embedded with proteins provides great stability against physical and chemical stresses. This study aimed to characterise oil bodies extracted from hemp and mustard seeds and determine their potential as delivery systems to encapsulate β-carotene, a model of hydrophobic bioactive compound. Hemp and mustard oil body fractions obtained after aqueous extraction were characterised in terms of particle size, surface charge, microstructure, and behaviour at different pH (2-10) conditions and ionic strengths (0-1,000 mM NaCl). β-carotene was then encapsulated into oil body systems at a concentration of 400 μg/g oil using intact and disrupted oil bodies. The colloidal and β-carotene stability of these oil body systems were analysed for 14 days at different storage temperatures (4°C, 22°C, and 50°C) and light conditions (with and without light at ambient temperature) by determining their particle size, surface charge, colour, and β-carotene content. Mustard oil bodies were entrapped in flocs of extraneous proteins even when extracted at alkaline pH (9), making it difficult to achieve their successful extraction. Hemp oil bodies, however, were isolated with minimal flocculation. They exhibited high electrostatic stability at neutral pH, aggregated at pH values close to the isoelectric point of the oleosins (pH 4 and 5), and had reduced ζ-potential with the addition of salt (NaCl ≥ 62.5 mM). Hemp oil bodies were used to encapsulate β-carotene in delivery systems. In intact oil bodies, the use of solvents (ethanol, hexane, and diethyl ether) did not enhance the partitioning of β-carotene to the TAG core of oil bodies (low encapsulation efficiency of <40%). In contrast, heating (50°C, 10 min) and mild sonication were able to directly incorporate crystalline β-carotene into the hemp oil body fraction. When oil bodies were homogenised, their membrane material fragments were able to stabilise the interface of newly formed oil droplets, but flocculation was observed. Heat-induced destabilisation of oil bodies (stirred for 1 h at 70°C) led to the extraction of oil body membrane materials (OBMM), which were used as emulsifiers of β-carotene-loaded hemp oil emulsions. During storage for 14 days, these β-carotene-loaded oil body systems remained stable at 4°C and 22°C, but storage at 50°C caused a significant reduction (p<0.05) in their colloidal and chemical stability. However, their stability was unaffected by the presence of light during storage at room temperature. Compared to the WPI-stabilised emulsion, the OBMM-stabilised emulsion exhibited a comparable colloidal stability while the homogenised and non-homogenised oil bodies had a similar retention of β-carotene. Overall, oil bodies can be utilised as encapsulation systems for bioactive compounds. These systems exhibit comparable stability to protein-stabilised emulsions during storage. However, a balance between their colloidal and chemical stability must be achieved to enhance their functionality for commercial application. Further characterisation of the composition of their membrane materials is recommended to fully elucidate the mechanisms by which they can stabilise emulsions and protect the bioactive compound against degradation.
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