Fabrication and characterization of highly re-dispersible dry emulsions
Graphical abstract
Introduction
Emulsions have been widely used for different objectives in the food, nutrition, and pharmacy industries (McClements, 2015). One of their major applications is as encapsulants and delivery carriers for functional ingredients, due to their ease of preparation, maintenance of the physical and chemical stability of encapsulated compounds, potential controlled release and target delivery, and low cost. Emulsion-based carriers can be employed to functionally deliver a variety of lipophilic nutrients, such as carotenoids (Mao, Wang, Liu, & Gao, 2017; Wei, Sun, Dai, Zhan, & Gao, 2018), polyphenols (Lu, Kelly, and Miao, 2016), vitamins (Parthasarathi, Muthukumar, & Anandharamakrishnan, 2016), ω-3 fatty acids (Karthik & Anandharamakrishnan, 2016), and probiotics (Gbassi & Vandamme, 2012). Incorporation of these health-beneficial nutrients into structured emulsions can not only increase their stability and shelf-life, but also can significantly improve their oral bioavailability and thus their health benefits.
However, liquid emulsions are dynamically unstable systems, and their stability decreases with storage time, leading to shortened shelf-life and thus limited application in food industry. In addition, transportation, storage and packaging of liquid emulsions can incur high cost. Hence, strategies must be applied to increase the long-term stability (shelf-life) of liquid emulsions and decrease their transportation/and storage cost at the same time. Several approaches have been developed to improve the long-term stability of liquid emulsions. Among these, microencapsulation technology is always considered to be an ideal way of achieving this (Rosenberg, Talmon, & Kopelman, 1988).
Microencapsulation is a packaging technology by which liquid droplets or solid particles are packed into continuous shells. The shells (or 'walls') are designed to protect the encapsulated material (‘core’) from factors that may cause its deterioration. In the food industry, the technology has been mainly used for the encapsulation of volatiles and environment-sensitive materials. Spray-drying is one of the mostly used microencapsulation technique for food preservation, which is also a good way of extending the shelf-life of liquid emulsions through drying them into powders (Vega, 2006; Gharsallaoui et al., 2010).
Spray-drying process can potentially promote the instability of emulsions by altering their interfacial properties (Gharsallaoui et al., 2010). It is therefore important to properly formulate emulsions those are stable to drying, and/or suitable for conversing into powders. In addition, formulation of liquid emulsions can significantly influence their drying process, the properties of obtained emulsion powders (Jafari, 2017), and the properties of re-constituted powdered emulsions in water. In addition, properties of emulsion droplets is closely related to their digestion, release of ingredients from droplets and subsequent absorption of these ingredients in the gastrointestinal tract (GIT) (Lu et al., 2017a,b; Lu, Zheng, & Miao, 2018; McClements & Li, 2010). Hence, maintaining the uniformity of emulsion droplet structure before and after drying becomes a critical issue in the drying of emulsions. If the powdered emulsions show good re-dispersibility in water and re-constituted emulsions still have intact droplet structure and good stability, such a drying process (including the formulation of liquid emulsions) is always preferred by researchers and manufacturers.
Many strategies have been developed to obtain optimized formulations of liquid emulsions suitable for spray-drying, such as multilayer emulsions (Wei et al., 2018a, 2018b), addition of soluble ingredients as ‘wall’ materials into the water phase of emulsions before drying process, or combined use of both. Commonly used ‘wall’ materials include maltodextrin, gum arabic, dairy proteins, lactose, and cellulose (Aghbashlo, Mobli, Madadlou, & Rafiee, 2012; Calvo, Hernández, Lozano, & González-Gómez, 2010; Jayasundera, Adhikari, Aldred, & Ghandi, 2009). However, a high levels of wall ingredients were always used in drying process, which not only can decrease the content of bioactive ingredients encapsulated in the emulsions but also can significantly increase the cost of the production of powdered emulsions. For example, maltodextrin (MD), mostly used ‘wall’ materials in drying liquid emulsions, was added to the water phase of emulsion in a level of 8%–30% (w/w in liquid emulsion) with the objective of obtaining stable and highly re-dispersible emulsion powders (Gharsallaoui et al., 2010; Jang, Kim, Oh, & Lee, 2014). Therefore, new wall materials, which can produce stable emulsion powders at a low level of addition, are required.
Our previous studies showed that konjac glucomannan (KGM) in the water phase of emulsions can form an intermolecular entanglement, which can significantly enhance the stability of whey protein-stabilized emulsion droplets and thus can potentially act as the protective skeleton and ‘wall’ material in spray-, or freeze-drying of emulsions (Lu et al., 2018). Meanwhile, emulsions containing KGM in the water phase demonstrated sustained release of entrapped nutrients. In addition, emulsion-based carriers with monoglyceride (MG) in the oil phase (Lu et al., 2017b) can significantly improve the bioavailability of encapsulated bioactive nutrients. However, whether these previously-formulated emulsion-based functional delivery systems can be dried into stable powders is still not clear. Meanwhile, little is known about the influence of KGM and MG on the properties of obtained dried emulsions and the properties of their reconstitutions in water.
This study was therefore conducted to prepare dry emulsions with KGM and MG structured liquid emulsions. The effects of KGM and MG on the properties of dry emulsions by both spray-drying and freeze-drying was also studied. β-carotene was incorporated into the oil phase of liquid emulsions as an indicator to provide potential useful information of using structured powdered-emulsion as functional delivery systems for functional lipophilic ingredients.
Section snippets
Materials
All-trans-β-carotene (>93%, UV) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Whey protein isolate (70% β-lactoglobulin and 18% α-lactalbumin) was purchased from Davisco Food International (Le Sueur, MN, USA). Sunflower oil (Solesta, >98% fat) was purchased from a local supermarket (ALDI, Fermoy, Co. Cork, Ireland). Monoglyceride (glycerol monostearate, Danisco, Denmark) was purchased from Cloverhill Food Ingredients Ltd (Cork, Ireland). Konjac glucomannan (KGM) powder was obtained
Scanning electronic microscopy (SEM) observation
Five liquid emulsions were formulated to investigate the possibility of obtaining spray-dried emulsion powders by using low level of ‘wall’ materials. As is shown in Fig. 1, all liquid emulsions were successfully spray-dried into dry powders, which usually showed approximately spherical particles with a concavo-convex surface (Fig. 1f–j). The liquid oil droplets with irregular shapes (Fig. 1h, red arrow) were embedded within the ‘wall’ materials (MD or KGM) and located on the surface of the
Conclusions
Emulsion powders were obtained through spray-, or freeze-drying of KGM or MG structured O/W emulsions. The introduction of KGM and MG significantly reduced the level of wall material (MD). All emulsion powders showed rapid re-hydration in water. Spray-drying process increased the mean droplet size of KGM, and MD structured emulsions, while the opposite result was observed for freeze-drying process. KGM significantly decreased the initial viscosity (p < 0.05) but increased the creaming stability
Funding
This work was sponsored by Teagasc-The Irish Agriculture and Food Development Authority (RMIS6821), the Shanghai Pujiang Program (19PJ1406500) and the Start-up Program of Shanghai Jiao Tong University (19X100040028).
Notes
The authors declare no conflict of interest.
CRediT authorship contribution statement
Wei Lu: Conceptualization, Methodology, Writing - original draft, Data curation. Valentyn Maidannyk: Writing - review & editing. Alan L. Kelly: Writing - review & editing. Song Miao: Supervision, Conceptualization, Writing - review & editing, Funding acquisition, Project administration, Investigation.
Acknowledgement
The support of Teagasc, University College Cork (UCC), and Shanghai Jiao Tong University (SJTU) on the instruments, and/or facilitates relevant to the experiments and manuscript writing was highly acknowledged.
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