Stabilization and rheology of concentrated oil-in-water emulsions

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Food and Beverage Chemistry/Biochemistry

Stabilization and rheology of concentrated oil-in-water emulsions using natural emulsifiers: Quillaja saponins and Rhamnolipids Ziqian Li, Lei Dai, Di Wang, Like Mao, and Yanxiang Gao J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b05291 • Publication Date (Web): 29 Mar 2018 Downloaded from http://pubs.acs.org on March 30, 2018

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Journal of Agricultural and Food Chemistry

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Stabilization and rheology of concentrated emulsions using

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natural emulsifiers: Quillaja saponins and Rhamnolipids

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Ziqian Li, Lei Dai, Di Wang, Like Mao*, Yanxiang Gao*

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Beijing Advanced Innovation Center for Food Nutrition and Human Health,

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Beijing Laboratory for Food Quality and Safety, Beijing Key Laboratory of

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Functional Food from Plant Resources, College of Food Science & Nutritional

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Engineering, China Agricultural University, Beijing, 100083, P. R. China

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*Corresponding authors:

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Tel.: + 86-10-62737034

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Fax: + 86-10-62737986

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E-mail: [email protected] (Like Mao), [email protected] (Yanxiang Gao)

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ACS Paragon Plus Environment


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ABSTRACT: Concentrated emulsions are widely used in the cosmetic, personal care, and

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food industries to reduce their costs of storage and transportation and to provide desirable

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characteristics. The current study aimed to produce concentrated emulsions (50 wt% oil) using

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two natural emulsifiers, i.e., quillaja saponins and rhamnolipids. The impact of emulsifier

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concentration on particle size, rheological properties and stability of concentrated emulsions was

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evaluated. The particle size of the emulsions was negatively correlated with concentration of

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either quillaja saponins or rhamnolipids, and rhamnolipids were more effective in producing

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smaller droplets. Both emulsifiers formed stable concentrated emulsions against a series of

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environmental stresses including temperatures (30–90℃), salt concentrations (≤200mM NaCl),

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and pHs (pH 5–8). The rheology tests suggested that concentrated emulsions stabilized by

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quillaja saponins or rhamnolipids presented a shear thinning behavior and had relatively a low

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consistency coefficient.

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KEYWORDS: concentrated emulsions, natural emulsifiers, quillaja saponins, rhamnolipids

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INTRODUCTION

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Emulsions are widely used in the food, personal care, cosmetics, and

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pharmaceutical industries. Conventional diluted emulsions are greatly accepted, but

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in some food applications it is advantageous to apply concentrated emulsions(≥50wt%

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oil ). The droplet concentration can affect the appearance, texture, surface gloss, and

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stability of the final emulsions1. Food manufacturers are often in favor of

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concentrated emulsions rather than diluted emulsions to obtain the desired properties

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of particular products, such as cream, salad dressing, butter. Second, previous studies

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have concluded that oxidation rate of emulsions was reduced with the increase in oil

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content2-3. Third, concentrated emulsions can be used as main agents for storage and

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transportation to reduce costs4-5. In this case, concentrated emulsions are initially

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produced and then diluted when they are applied in the final products. In the field of

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concentrated emulsions, most studies focus on Pickering emulsions. Fan et al.6

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designed medium-chain triacylglycerol (MCT) Pickering emulsions with 50wt%

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MCT

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(octenylsuccinylation treated soluble starch nanoparticle, OSA-SSNP, and insoluble

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starch nanoparticle, ISNP). Zeng et al.7 found that stable concentrated emulsions can

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be prepared with protein/polysaccharide hybrid particles at low concentration (0.5–

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2%). Wang et al.8 proved that zein/chitosan colloid particles were effective in

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stabilizing 50wt% n-tetradecane, which could highly resist coalescence over a

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9-month storage test. However, there was only minor application of Pickering

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emulsions in commercial food products. Therefore, it is desirable to use traditional

prepared

by

different

modified

starch-based

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nanoparticles


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emulsions instead of Pickering emulsions to prepare stable concentrated emulsions

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for food applications.

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Synthetic ingredients can be used to produce concentrated emulsions. For

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example, Tween 80 (used alone or mixed with lecithin) could stabilize emulsions

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with a fish oil concentration of 50%2,9. However, consumers are cautious about the

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synthetic ingredients in foods, and foods with natural ingredients are more

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preferred10-11. Therefore, to meet the need of both manufacturers and consumers, the

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preparation of concentrated emulsions with natural emulsifiers could provide

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solutions for wider applications. There are many types of natural emulsifiers, which

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are allowed to be used in food products, including polysaccharides (e.g. pectin and

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modified starch), proteins (e.g., whey, casein), phospholipids, and small molecular

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weight surfactants12-13 Nevertheless, the utilization of some biopolymers is limited in

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certain food applications. Emulsions stabilized by protein are mostly unstable at high

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ionic strength, high temperatures and pH close to the isoelectric point14. For

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polysaccharide-type emulsifiers, there are usually some difficulties in preparing

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emulsions with very small particle size15-16. Some natural small molecular weight

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surfactants have been shown to effectively form and stabilize emulsions. Yang et.al17

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reported that quillaja saponins were effective emulsifiers to stabilize emulsions

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containing 10% MCT. Quillaja saponins are the extracts isolated from the bark of the

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Quillaja saponaria Molina tree18-20, which are proved to be saponins that contain

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linked triterpenoid and steroid glycosides by glycosylic groups18,21 (Fig. 1). Quillaja

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saponins are surface active because their molecules have both hydrophobic (e.g., 4


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quillaic acid) and hydrophilic regions (e.g., rhamnose, galactose, glucuronic acid)

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19,21

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emulsion and their stability and proved that rhamnolipids could effectively reduce

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the interfacial tension. Rhamnolipids are a type of glycolipids isolated from certain

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microorganisms through fermentation24. Furthermore, rhamnolipids have surface

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activity since there are non-polar fatty acid chains with beta-hydroxyalkanediol

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attached one or two polar rhamnose units in their structures (Fig. 2)25.

. In addition, Helvacı et al.23 explored emulsifying capacity of rhamnolipids in

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In this study, we evaluated the stability of concentrated emulsions prepared with

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different natural emulsifiers (rhamnolipids and quillaja saponins) at different

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concentrations. The impacts of pH (5-8), salt concentration (0-200mM NaCl), and

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temperature (30-90℃) on emulsion stability were explored. Furthermore, emulsions’

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viscosity could increase as the oil concentration increased2, which may influence the

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applications in practice. Therefore, the rheological properties of concentrated

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emulsions were evaluated. The findings of this study are important for the

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application of concentrated emulsions in practice, especially for the production of

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food and beverages.

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MATERIALS AND METHODS

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Materials and Chemicals

Medium chain triglyceride (MCT, Miglyol 812N)

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was purchased from MUSIM MAS (Merch, Indonesia). Quillaja saponins

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(Q-Naturale 200) were purchased from Ingredion Inc (Westchester, IL, USA).

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Rhamnolipids (R90) were purchased from Shaanxi Pioneer Biotech Co (Ltd.

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Shaanxi, China). The other solvents and reagents used were all of analytical grade. 5



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Emulsion preparation Emulsions were prepared by homogenizing the water

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phase and the oil phase at a weight ratio of 1:1 with an emulsifier concentration of

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0.5~3.0wt%. The aqueous phase contained emulsifier (Quillaja saponin /

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Rhamnolipid) and 10mM phosphate buffer solution (pH 7.0). The coarse emulsions

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were made by blending the water phase and oil phase with ULTRA-TURRAX at a

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speed of 10,000rpm for 6 min. Then the final emulsions were formed by preheating

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at 65℃ for 3 min, and then homogenizing the preheated coarse emulsions with a

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microfluidizer (M-110P, Microfluidics, MA) for two passes at 100 MPa.

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Determination of particle size

The particle size was determined using a

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Zeta-sizer Nano-ZS90 (Malvern Instruments, Worcestershire, UK) described by Liu2.

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The emulsion was diluted with phosphate buffer solutions at the ratio of 1:500 before

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analysis to avoid multiple scattering effects.

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The droplet charge (ζ-potential) of the

Determination of droplet charge

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samples was measured using particle electrophoresis(Zeta-sizer Nano-ZS90, Malvern

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Instruments, Worcestershire, UK). Dilute emulsions 500 times using phosphate buffer

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solutions, and then directly inject them into the instrument’s chamber prior to the

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analysis8.

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Measurement of Rheological Properties of Emulsions

Rheological

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measurement was carried out by a dynamic shear rheometer ( TA Instruments,

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Delaware, USA) equipped with the flat plate measurement cell (cone diameter 40 mm)

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at 25℃. 1.5 ml sample was carefully deposited on the plate. The apparent viscosity of

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all the emulsions was measured with varying shear rates (0.1-100 s -1). The shear rate 6


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sweeps were performed on a traditional log scale with 10 points per decade of shear.

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Emulsion stability was measured with an

Measurement of Physical stability

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analytical centrifuge (LUMiSizer, L.U.M.290 GmbH, Germany) based on the method

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of Gao26 with some modifications. It records the changes in the intensity of

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transmitted light (880 nm) when particles move or phases separate during

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centrifugation, and analyzes a pattern of light flux as a function of the radial

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position27-28. Samples were centrifuged at the speed of 4000 rpm for 2h (25 ℃) in this

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study.

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Environmental stability was tested by

Environmental stability testing

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subjecting the emulsions to thermal treatment, pH adjustment or changes in ionic

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strength. Emulsions were then store them in a dark place at ambient temperature for

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24 h.

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Thermal treatment: Emulsions were transferred to glass tubes and then

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incubated in the water bath at various temperatures (30, 40, 50, 60, 70, 80, 90 ℃) for

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half an hour. The processed emulsions were cooled naturally to ambient temperature.

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pH adjustment: Emulsions were adjusted to pH 2、3、4、5、6、7、8、9 using

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0.1 M NaOH or HCl solutions.

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Ionic strength: Emulsions were mixed with buffer solutions (pH7.0 sodium

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phosphate buffer) with different concentrations of NaCl to reach salt concentrations

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of 100、200、300、400、500mM. The final pH of these solutions with adjusted ionic

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strength was 7.0.

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Statistical analysis

All measurements were performed on three prepared 7



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samples, and mean value with standard deviations were reported. Statistical analysis

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was performed with variance (ANOVA), and the difference (p < 0.05) was regarded to

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be significant.

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RESULTS AND DISCUSSION Roles of emulsifier type and concentration in droplet size of emulsions

The

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influence of emulsifier type and concentration on the particle size of concentrated

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emulsions was presented (Fig.3). For both rhamnolipids and quillaja saponins

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stabilized emulsions, when emulsifier concentration was increased from 0.5 wt% to 3

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wt%, particle size was significantly decreased (p

Stabilization and rheology of concentrated oil-in-water emulsions

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