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Design of liquid emulsions to structure spray dried particles
Journal of Food Engineering xxx (2015) xxx–xxx
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Design of liquid emulsions to structure spray dried particles Maria del Rayo Hernandez Sanchez ⇑, Marie-Elisabeth Cuvelier, Christelle Turchiuli AgroParisTech, UMR 1145 Ingénierie Procédés Aliments, 1 Avenue des Olympiades, F-91300 Massy, France INRA, UMR1145 Ingénierie Procédés Aliments, F-91300 Massy, France CNAM, UMR1145 Ingénierie Procédés Aliments, F-91300 Massy, France
a r t i c l e
i n f o
Article history: Received 19 November 2014 Received in revised form 17 February 2015 Accepted 13 July 2015 Available online xxxx Keywords: Emulsification Spray drying Encapsulation Emulsion
a b s t r a c t The formulation and structure of initial liquid emulsion have an impact on the efficiency of encapsulation (encapsulated oil quantity) in spray dried particles (dried emulsions) and must be adapted to spray drying. Two different protocols were tested varying the concentration of emulsifier (TweenÒ 20), conditions of homogenization and concentration of wall material (Maltodextrin and Agave Inulin). The concentration of emulsifier and wall material should allow avoiding micelle formation or coalescence. Results show that it was possible to produce stable emulsions with an oil droplet size around 2 lm containing 40% w/w of total dry matter (g/100 g emulsion) using maltodextrin as wall material and 3.5% TweenÒ 20 (3.5 g/100 g oil) or using Agave Inulin in emulsions with a total dry mater content of 50% w/w (50 g/100 g emulsion) with a minimal concentration of emulsifier (1.17 g/100 g oil). The spray drying of these emulsions produced fine powders (<22 lm) with a monodispersed size distribution and good oil encapsulation efficiency (>88%). Ó 2015 Published by Elsevier Ltd.
1. Introduction Lipophilic active compounds such as polyunsaturated fatty acids (PUFAs), vitamins, antioxidants have shown to have positive effects on human health. In addition, antioxidants are useful to protect PUFAs from oxidation and preserve the organoleptic properties of foods. Indeed, PUFAs are mainly composed of large unsaturated hydrocarbon chains, they suffer oxidation when exposed to light, metals, heat and oxygen. The oxidation is the main cause of the degradation and deterioration of their properties, of the appearance of undesirable compounds and of the loss of their nutritional value (Kolakowska and Sikorski, 2011; Choe and Min, 2006; Frankel, 1998; McClements, 1999). In order to preserve and to prolong their functional properties, lipophilic compounds can be dispersed in oil-in-water emulsions and later encapsulated by spray drying. The first step of lipophilic compounds encapsulation by spray drying consists therefore in the preparation of an initial oil-in-water liquid emulsion with formulation, structure and properties suitable for further spray drying. Emulsion properties must also provide efficient encapsulation of the active components and in some cases make possible the reconstitution of the initial liquid emulsion by dissolution of the powder. Spray drying is a unit
operation in which a liquid, suspension or emulsion is pumped, atomized and dried in contact with a hot air current to produce a powder. It is a technique widely used for the encapsulation of active compounds into a solid matrix to provide them protection against degradation and volatile losses. The protection is mainly given by the wall material, which forms a physical barrier that encloses droplets or particles of the active compound encapsulated. The nature and concentration of wall material used, the percentage of oil, the viscosity and oil droplet size of emulsion as well as the drying conditions (e.g. liquid feed rate, drying air temperatures, air flow rate, etc.) will have an impact on the properties of the end product and thus, on the encapsulation efficiency (% oil encapsulated) (Gharsallaoui et al., 2007; Krishnaiah et al., 2012; Turchiuli et al., 2013, 2014; Janiszewska et al., 2014). Carneiro et al. (2012) stated that viscosity and oil droplet size of the emulsion positively affected the encapsulation efficiency and the oil retention during the encapsulation of coffee oil. However, a negative effect was found when the oil concentration and inlet temperature of drying air increased. Therefore, the homogenization conditions, the concentration and nature of wall material and emulsifier and percentage of oil have to be carefully studied to obtain initial emulsion that must:
⇑ Corresponding author at: AgroParisTech, 1 Avenue des Olympiades, 91300 Massy, France. E-mail addresses: [email protected] (M.R. Hernandez Sanchez), [email protected] (M.-E. Cuvelier), christelle. [email protected] (C. Turchiuli). http://dx.doi.org/10.1016/j.jfoodeng.2015.07.036 0260-8774/Ó 2015 Published by Elsevier Ltd.
Please cite this article in press as: Hernandez Sanchez, M.d.R., et al. Design of liquid emulsions to structure spray dried particles. Journal of Food Engineering (2015), http://dx.doi.org/10.1016/j.jfoodeng.2015.07.036
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M.R. Hernandez Sanchez et al. / Journal of Food Engineering xxx (2015) xxx–xxx
1. Have an oil droplet size distribution in relation to the size of the dry particles obtained from the sprayed droplets. For a given formulation, the emulsion size distribution depends on the equipment and energy input during homogenization. Knowing that powder particles formed by spray drying have diameters between about 20 and 50 lm at pilot scale and between about 50 and 200 lm at industrial scale, the emulsion size should not be larger than about 2 and 10 lm respectively (Gharsallaoui et al., 2007; Turchiuli et al., 2013, 2014). 2. Be physically stable until and during spray drying, under mechanical and thermal stress, in order to ensure correct oil distribution within the matrix of encapsulation (Janiszewska et al., 2014). 3. Have a formulation allowing encapsulation by spray drying. That means a viscosity compatible with pumping, pulverization and drying; and the presence of wall materials soluble in water, allowing forming solutions with a reasonable viscosity at high solids concentration (e.g. up to 40–60 g/100 g liquid feed), with a good drying ability (not too sticky), that does not react with the encapsulated component during processing and upon storage, providing protection against environmental factors (e.g. oxygen, heat, light, humidity) and leading to powders with good end-use properties (Desai and Park, 2005; Gharsallaoui et al., 2007). Wall materials are generally polymers that will increase the viscosity of the aqueous phase and limit both diffusion of active molecules and mobility of oil droplets in the liquid emulsion. Among the polymers used, maltodextrins are products of partial hydrolysis of starch that are widely used in encapsulation. They can be classified depending on their degree of hydrolysis, which is expressed as dextrose equivalents (DE). Due to their high solubility in water, low permeability to oxygen and low cost, maltodextrins are adequate for spray drying. However due to their lack of emulsifying properties, they need to be associated with surface active molecules in order to increase the physical stability and oil retention. Agave inulin, a fructo-oligosaccharide can be also used as wall material due to its nutritive and beneficial effects (Gharsallaoui et al., 2007; Jafari et al., 2008; Madene et al., 2006; Turchiuli et al., 2013, 2014). Emulsifiers (e.g. sodium caseinate, whey protein and gelatine) or surfactants (e.g. phospholipids, free fatty acids, monoacylglycerols and synthetic surfactants) that reduce the interfacial tension in emulsions when adsorbed at the water/oil interface are used to stabilize liquid emulsions (Genot et al., 2003; McClements, 2010; Vaclavik and Christian, 2008; Appelqvist et al., 2007). But, wall materials and surfactants can also have a negative impact in terms of physical stability of emulsions when added in certain concentrations. When high molecular polysaccharides or surfactants are added in excess, they can remain free in the aqueous phase stimulating the depletion mechanism, with, as a consequence, the flocculation and coalescence of oil droplets. During depletion, a modification of the rheological properties of the aqueous phase around the oil droplets takes place, promoting a decrease of the number of surfactant molecules adsorbed at the oil–water interface and then causing a reinforcement of the attraction forces between oil droplets. When the attraction forces become greater than the repulsive forces, the mechanism of flocculation occurs (Klinkesorn et al., 2004; Udomrati et al., 2013) causing an increase of the oil droplet size that becomes too large compared to the dry particle diameter. Furthermore, an excess of emulsifier would stimulate the formation of micelles; these micelles can interact with active components (e.g. phenolic antioxidants) and associate them, modifying their partitioning between the different phases of the emulsion and therefore their activity (Coupland and McClements, 1996).
The aim of this work was to propose protocols and formulations to design stable oil-in-water liquid emulsions with an oil droplet size around 2 lm, to get good dispersion and protection of the oil phase within dry particles of about 20 lm obtained by spray drying at pilot scale. Formulation was tested in terms of TweenÒ 20 concentration (1.17–20 g Tween 20 Ò/100 g oil) and nature and concentration of wall material (maltodextrin (MD) and agave inulin (I)) in order to find a compromise to avoid both depletion phenomena and excess of emulsifier. For the preparation of the liquid emulsions, two protocols were tested with respectively one or two steps. 2. Material and methods 2.1. Materials Commercial sunflower oil containing 11% w/w saturated, 29% w/w mono-unsaturated and 60% w/w poly-unsaturated fatty acids and 0.05% w/w a-tocopherol was used as model oil for lipophilic compounds encapsulation. The emulsifier TweenÒ 20, was purchased from Sigma–Aldrich (FR). Maltodextrin (Glucidex DE 12) was purchased from Roquette (FR) and agave inulin (Oligofructine) was purchased from Nutriagaves (MX). 2.2. Preparation of emulsions 2.2.1. Protocol 1 – Direct emulsification Protocol 1 (Fig. 1a) consists in a direct emulsification of all the ingredients. First, an aqueous phase was prepared by slow dissolution of wall material (MD) in ultrapure water under mechanical stirring with a 3-bladed propeller stirrer (Eurostar, IKA, FR) at room temperature. Then, 3.5 or 20 g/100 g oil TweenÒ 20 were incorporated and sunflower oil (4 g/100 g emulsion) was added. Emulsion (E1) was obtained by homogenization for 20 min with a rotation speed of 3500 rpm using a rotor–stator homogenizer (AXR, Silverson Machines Ltd, FR) (Turchiuli et al., 2014). 2.2.2. Protocol 2 – Double emulsification + dilution Protocol 2 (Fig. 1b) consists in first, a 2-step emulsification without wall material to reach the required emulsion size (e.g. about 2 lm) and second, the addition of an aqueous phase containing the wall material (dilution). The preparation of the pre-emulsion (Pre-E2) was adapted from Berton et al. (2012) and consisted in adding oil and TweenÒ (1.17–3.50 g/100 g oil) in ultrapure water to produce an emulsion with 30% w/w of oil. The ingredients were pre-homogenized at 16,000 rpm for 3 min with a rotor–stator homogenizer (Ultra-Turrax T25 basic, IKA, FR). The second homogenization was carried out with a two-stages high-pressure valve homogenizer (1001 L PANDA, Soavi NIRO, FR) using different homogenization pressures (35, 50 and 300 bars). An aqueous phase containing the wall material (MD or I) was prepared separately under mechanical stirring with a 3-bladed propeller stirrer (Eurostar IKA, FR) at room temperature. Finally, the newly formed aqueous phase was added to the Pre-E2 under gentle stirring in order to produce the final emulsion (E2) with 4 g oil/100 g emulsion and different dry matter content (10–60 g/100 g emulsion). The composition of the different emulsions prepared with both protocols is shown in Table 1. 2.3. Characterization of liquid emulsions 2.3.1. Size distribution of oil droplets The size and size distribution of oil droplets in initial emulsions and reconstituted emulsions were determined by laser diffraction
Please cite this article in press as: Hernandez Sanchez, M.d.R., et al. Design of liquid emulsions to structure spray dried particles. Journal of Food Engineering (2015), http://dx.doi.org/10.1016/j.jfoodeng.2015.07.036
M.R. Hernandez Sanchez et al. / Journal of Food Engineering xxx (2015) xxx–xxx
(a)
(b)
Protocol 1 Direct emulsification (Turchiuli et al., 2014)
MD + Tween20 + Water
+
3
Protocol 2 2-steps emulsification (Adapted from Berton et al., 2012) Sunflower oil + Tween20 +Water
Sunflower oil
Homogenization by rotor-stator device (AXR, Silverson Machines Ltd, Fr)
Homogenization by :Rotor-stator device (Ultra-Turrax T25 basic, IKA, Fr) and High-pressure device (SoaviNIRO 1001 L PANDA, Fr)
Pre-Emulsion (Pre-E2)
Emulsion (E1)
+ MD or I + Water
Emulsion (E2) Fig. 1. Protocols for preparation of emulsions: (a) protocol 1 – direct emulsification and (b) protocol 2-double emulsification + dilution.
Table 1 Composition of the emulsions prepared with Protocol 1 (Emulsions E1 n° 1 and n° 2) and Protocol 2 (Emulsions E2 n° 3 to n° 14) using maltodextrin DE12 (MD) and inulin (I) as wall material. Dry matter (g/100 g emulsion) = oil + TweenÒ 20 + wall material. Emulsion
Dry matter
Oil
MD
I
Water
TweenÒ 20
g/100 g emulsion E1 E2
Protocol 1 n° 1 n° 2 Protocol 2 n° 3 n° 4 n° 5 n° 6 n° 7 n° 8 n° 9 n° 10 n° 11 n° 12 n° 13 n° 14
2.5. Characterization of spray dried emulsions Spray dried powders produced were stored in hermetic jars at 20 °C. They were unfrozen at ambient temperature (at least 15 min) prior analysis.
g/100 g oil
40
4
35.2 36
60
0.8 0.14
20 3.5
10 20 25 30 40 40 40 40 45 50 55 60
4
6 16 21 26 36 36 36
90 80 75 70 60 60 60 60 55 50 45 40
0.05
1.17
0.14 0.20 0.05
3.5 4.85 1.17
36 41 46 51 56
TweenÒ 20 (g/100 g oil) = Indicates the quantity of TweenÒ 20 regarding the oil content.
(Mastersizer 2000, Malvern, FR) in wet mode (Hydro 2000) using 1.33 as the refraction index value for the dispersing water and 1.475 for the oil droplets (measured at 20 °C).
2.3.2. Apparent viscosity The apparent viscosity g (Pa s) of the different emulsions prepared and their rheological behaviour were measured at 20 °C using a rotational rheometer with coaxial cylinders (Rheomat R180, Lamy, FR) with velocity gradients c (s1) up to 1500 s1.
2.5.1. Size distribution of dried particles Particle size distribution of dried emulsions was measured by laser diffraction (Mastersizer 2000, Malvern, GB) using an automated dry powder dispersion (Scirocco 2000). Compressed air pressure was fixed to 4 bars and vibration rate to 50% using the fine sieve, to ensure the dispersion of cohesive dried particles. 2.5.2. Surface oil content (non encapsulated oil) The surface oil fraction in spray dried particles (non encapsulated oil) was recovered by a solvent extraction according to the GEA Niro Method A 10 (GEA, 2005). 10 g of spray dried emulsion were weighed into a 250 mL Erlenmeyer Flask and 50 mL of petroleum ether were added. The flask was closed and agitated during 15 min in a shaking device. The agitation speed was controlled in order to move the sample but not to splash it up on the sides of the upper half of the flask. After the agitation was stopped, 25 mL of extract solution were taken and placed into pre-weighed aluminium dish. Samples were first let for solvent evaporation in the fume hood, and after total evaporation of petroleum ether, dried in an oven for one hour at 105 °C. Finally, the surface oil content (g surface oil/100 g powder) was expressed as follows (Eq. (1)) (GEA, 2005):
a 50 100 % Surfaceoil ¼ v qa b
ð1Þ
where v, volume of extract taken (mL); a, weight of residue after oven drying (g); b, weight of powder (spray dried emulsion) used (g) and q, oil density (g mL1) (e.g. 0.94 g mL1in this study). 3. Results and discussion
2.4. Preparation of spray dried emulsions 3.1. Protocol 1 Liquid emulsions were spray dried in a pilot scale spray dryer (Niro Minor, Niro, DA). It is a one step co-current spray dryer with an evaporative capacity comprised between 1 and 4 kg h1. For spraying, a rotary wheel was used with a rotation speed of 25,000 rpm (5.5 bar compressed air). The emulsions flow rate was 25 g min1. Drying air was taken from the ambient by a fan (43 Hz) with a flow rate of 110 kg h1. The inlet air temperature was fixed at 200 °C and the outlet temperature varied from 94 to 110 °C.
Protocol 1 was used in previous studies on the encapsulation of lipophilic compounds (Fuchs et al., 2006; Tonon et al., 2011; Frascareli et al., 2012; Turchiuli et al., 2013, 2014). It requires only one step of homogenization and allowed obtaining stable emulsions, suitable for the encapsulation of lipophilic compounds (fish oil, sunflower oil). In these studies, emulsifying properties were brought by acacia gum which was also used as wall material and therefore added in large quantity (up to 30 g/100 g emulsion) for
Please cite this article in press as: Hernandez Sanchez, M.d.R., et al. Design of liquid emulsions to structure spray dried particles. Journal of Food Engineering (2015), http://dx.doi.org/10.1016/j.jfoodeng.2015.07.036
M.R. Hernandez Sanchez et al. / Journal of Food Engineering xxx (2015) xxx–xxx
Volume (%)
4
14
3.5 g Tween/100 g emulsion
12 10
20 g Tween/100 g emulsion
8 6 4 2 0 0,01
0,1
1
10
100
1000
10000
Oil droplet size (μm) Fig. 2. Oil droplet size distribution of emulsions n° 1 and 2 with 40 g of total dry matter/100 g emulsion obtained with protocol 1 using maltodextrin DE12 (MD) as wall material and 3.5 and 20 g of TweenÒ 20/100 g oil respectively.
the encapsulation of flaxseed oil, coffee oil and olive oil (Tonon et al., 2011; Frascareli et al., 2012; Turchiuli et al., 2013, 2014). In the present study, TweenÒ 20 was chosen for its emulsifying properties to stabilize oil droplets, but did not act as a wall material. It was therefore expected to lower its concentration in order to avoid micelles formation due to the presence of free molecules not adsorbed at the oil/water interface. Emulsions n° 1 and 2 were therefore prepared using protocol 1 with a quantity of TweenÒ 20 respectively of 20 g/100 g oil, in agreement with the previous studies, and of 3.5 g/100 g oil, to reduce the risk of micelles formation (Table 1). The use of excess of emulsifier (20 g/100 g oil) allowed producing an emulsion E1 n° 1 with a monomodal size distribution, and a median oil droplet size suitable for oil encapsulation by spray drying (d50 = 1.7 lm) with 80% of the oil droplets with diameters between 0.9 and 3 lm (Fig. 2). But, when decreasing the emulsifier content, a bimodal oil droplet size distribution was obtained, with 80% of the oil droplets with a diameter between 5.2 and 17 lm and only 10% with a diameter below 5.2 lm (d50 = 9.8 lm for 3.5 g TweenÒ 20/100 g oil) (Fig. 2). In this last case, the increase of the homogenization time or intensity did not make it possible to decrease the emulsion size. This may be due to the depletion phenomena occurring when using maltodextrin with low emulsifier content and when all the ingredients of the emulsions are subjected to a direct emulsification (Klinkesorn et al., 2004; Udomrati et al., 2013). Small oil droplets in the searched size range are formed during homogenization, but they flocculate and coalesce rapidly due to large attraction forces. 3.2. Protocol 2 The first part of protocol 2 was adapted from a study of Berton et al. (2012) who produced oil-in-water emulsions with a median oil droplet size of about 2 lm using TweenÒ 20 as emulsifier in very low concentration (<2 g/100 g oil). In that study, the oil concentration in the emulsion was larger than the one required for the present study (e.g. 30 g oil/100 g emulsion compared to 4 g oil/100 g emulsion) and no wall material was added in the aqueous phase. It was therefore proposed to first produce concentrated emulsions with the required oil droplet size using a similar protocol and formulation and then to dilute these pre-emulsions with the wall material solution to obtain a final emulsion with the required oil content (4 g/100 g emulsion). 3.2.1. Influence of homogenization pressure on size distribution of preemulsions E2 (Pre-E2) Pre-emulsions containing oil (30 g/100 g emulsion), TweenÒ 20 (1.17 g/100 g oil) and water were pre-homogenized using rotor– stator homogenizer and then submitted to high pressure homogenization. Different homogenization pressures were tested in order to check the influence on the pre-emulsion size distribution. The
use of low homogenization pressure (35 and 50 bars) resulted in emulsions with a large fraction of oil droplets above 10 lm resulting in median diameters d50 of 4.8 and 5 lm respectively with a span value of 2 in both cases (Fig. 3) therefore too large for an efficient encapsulation in this study. However, as the pressure was increased, the oil droplet size was reduced. The homogenization at higher pressure (300 bars) caused a reduction of 50% of the droplet size (d50 = 2.2 lm, span = 2.2) compared to the homogenization at lower pressures. Janiszewska et al. (2014) homogenized lemon aroma emulsions (2–10 g/100 g emulsion) using a protocol very similar to the one described in the present paper. They used a mixture of maltodextrin DE 10 and acacia gum (7:1). The homogenization was carried out at 400 and 600 bars. Results showed that the emulsion size (d50 = 0.20–0.35 lm) was not affected by the homogenization pressure, but by the increase of aroma oil fraction. For the purpose of the present research, the homogenization pressure for the production of pre-emulsions Pre-E2 was therefore set at 300 bars. 3.2.2. Influence of wall material concentration and nature on viscosity and size distribution of Emulsions E2 As stated above, it was possible to produce a pre-emulsion pre-E2 with the desired oil droplet size for the purposes of this research. However, this emulsion needs the addition of wall material for further encapsulation by spray drying. An aqueous phase was therefore added in the pre-emulsion pre-E2 varying either the concentration or nature of wall material, with the aim of producing an emulsion (E2) maintaining the oil droplet size of 2 lm. Emulsions E2 n° 3–7 (Table 1), prepared from the same pre-emulsion pre-E2 (d50 = 2.2 lm – 1.17 g TweenÒ 20/oil), were obtained adding aqueous phases containing maltodextrin DE12 with different concentrations. They showed an oil droplet size around 2 lm when the MD content was increased up to 26 g/100 g emulsion (Fig. 4). However, emulsions suffered a dramatic change of oil droplet size when the maltodextrin content was higher, resulting in an oil droplet size of 7.8 lm when the MD content was 36 g/100 g emulsion (emulsion n° 7 – total dry matter content 40 g/100 g emulsion). This size increase is the result of coalescence/flocculation of oil droplets in the presence of certain concentrations of wall material. This instability phenomenon is produced under particular circumstances such as the non-absorption of polymers (wall materials) in the aqueous phase of emulsion. The critical flocculation concentration (CFC) provides the minimum polymer concentration required for promoting flocculation, therefore, it is a useful tool in terms of quality control of emulsions in presence of polysaccharides (Klinkesorn et al., 2004; Udomrati et al., 2013). In the case of maltodextrins, it was demonstrated that a reduction of the dextrose equivalent value (DE) decreased the CFC value, which was attributed to the increase of the molecular weight and viscosity of maltodextrin solutions (Klinkesorn et al., 2004). The CFC of MD DE12 in emulsions with 35 bars 50 bars 300 bars
Fig. 3. Oil droplet size distribution of pre-emulsions E2 (Pre-E2) composed by 30 g oil/100 g emulsion and TweenÒ 20 (1.17 g/100 g oil) homogenized at different pressures.
Please cite this article in press as: Hernandez Sanchez, M.d.R., et al. Design of liquid emulsions to structure spray dried particles. Journal of Food Engineering (2015), http://dx.doi.org/10.1016/j.jfoodeng.2015.07.036
M.R. Hernandez Sanchez et al. / Journal of Food Engineering xxx (2015) xxx–xxx
10 MD
8
d50 (μm)
I
6 4
CFC CFC
2 0 0
10
20
30
40
50
60
Wall Material (g/100 g emulsion) Fig. 4. Estimation of critical flocculation concentration (CFC) for emulsions E2 (1.17 g TweenÒ 20/100 g oil) with maltodextrin (MD) or inulin (I) as wall material from the evolution of emulsion median diameter (d50) as a function of wall material concentration (g/100 g emulsion).
I - 51 g/100 g
η (Pa.s)
0,08 0,06
MD – 36 g/100 g I – 46 g/100 g I – 41 g/100 g I – 36 g/100 g
0,04 0,02 0 200
700
1200
γ (s )
5
In order to produce emulsions E2 adequate for spray drying, it is necessary to increase the dry matter content in the emulsion keeping constant the oil droplet size. This can be achieved either by increasing the emulsifier content or using another wall material. Emulsions E2 n° 8 and 9 were therefore prepared with a MD DE12 content of 36 g/100 g emulsion increasing the TweenÒ 20 content to 3.5 and 4.85 g/100 g oil respectively (total dry matter content 40 g/100 g emulsion). This allowed obtaining emulsions E2 with the required oil droplet size (1.5 lm in both cases) and viscosity (about 0.04 Pa s) (Figs. 5 and 6). When using inulin as wall material instead of maltodextrin (emulsions E2 n° 10–14) with a minimal content of TweenÒ 20 (1.17 g/100 g oil), the oil droplet size distribution remained unchanged (d50 1.7–1.8 lm) when the concentration of I was increased up to 46 g/100 g emulsion (Fig. 4), with the CFC in E2 containing I about 50 g/100 g emulsion. Regarding viscosity (Fig. 5), emulsions E2 containing either 50 g/100 g total dry matter with inulin as wall material and 1.17 g TweenÒ 20/100 g oil (n° 12) or 40 g/100 g total dry matter with maltodextrin DE12 as wall material and 3.5 or 4.85 g TweenÒ 20/100 g oil (n° 8 and 9) showed similar apparent viscosities (around 0.04 Pa s), which is suitable for spray drying (Turchiuli et al., 2014).
-1
3.3. Spray dried emulsions
Fig. 5. Apparent viscosity of emulsions E2 with different concentrations of maltodextrin DE12 (MD) and inulin (I): 36 g/100 g emulsion of MD, 36, 41, 46, and 51 g/100 g emulsion of I, respectively in emulsions n° 8, 10, 11, 12 and 13.
(a) Maltodextrin
10
1.17 g Tween/ 100 g emulsion 3.50 g Tween/ 100 g emulsion 4.85 g Tween/ 100 g emulsion
Volume (%)
8 6 4 2 0 0,01
0,1
1
10
100
1000
10000
Oil droplet size (μm)
Volume (%)
10
(b) Inulin
8
1.17 g Tween/ 100 g emulsion
6 4 2 0 0,01
0,1
1
10
100
1000
10000
Oil droplet size (μm) Fig. 6. Oil droplet size distribution in emulsions E2 with (a) 40 g/100 g emulsion total dry matter using maltodextrin DE12 (MD) for different concentration of TweenÒ 20 (g/100 g oil) and (b) with 50 g/100 g emulsion total dry matter using inulin (I) with a minimal concentration of TweenÒ 20 (1.17 g/100 g oil).
1.17 g TweenÒ 20/100 g oil is about 26 g/100 g emulsion, which means that MD has to be added at lower concentration to avoid depletion phenomena. However, for the purposes of the encapsulation by spray drying, higher concentration of dry matter (about 40 g/100 g emulsion when using MD as wall material) and viscosity of the emulsion are necessary (Turchiuli et al., 2013, 2014).
Emulsion E1 n° 1 prepared using protocol 1 with maltodextrin as wall material and an excess of TweenÒ 20; and emulsions E2 n° 8 and 12 prepared using protocol 2 with respectively maltodextrin and inulin as wall material were spray dried in similar conditions. For emulsions n° 1 and n° 8, containing maltodextrin, the total dry matter content was 40 g/100 g emulsion whilst for emulsion n° 12, containing inulin, it was 50 g/100 g emulsion. In emulsions n° 1 and n° 8, oil (4 g oil/100 g emulsion) represented 10 g/100 g of dry matter and 8 g/100 g of dry matter for emulsion n° 12. For all the three, the apparent viscosity was about 0.04 Pa s and the oil droplet median diameter was about 2 lm (e.g. 1.7, 1.5 and 2.1 lm for emulsions n° 1, 8 and 12 respectively). The physical stability of the three emulsions was checked along the process (from production step to end of spray drying). They were found to be stable for at least 2 h provided they were not agitated. Three powders were obtained and characterized for the particle size distribution and non-encapsulated oil content (Table 2, Fig. 7). As expected, the median particle size of the three powders was about 20 lm, probably due to the fact that, despite the dry matter content was different (composition and nature), they had similar apparent viscosity and were spray dried in similar conditions. This is in agreement with the results of Turchiuli et al. (2014). They obtained powders with median diameters between 24 and 26 lm spray drying, in similar conditions and equipment, olive oil emulsions (10 g oil/100 g dry matter) with maltodextrin, acacia gum and inulin as wall materials used alone or in mixtures with different ratios. The use of acacia gum increased the emulsion viscosity (e.g. up to about 0.2 Pa s). This may explain the bigger size of the powders they obtained. Assuming no oil loss during spray drying (Cuvelier et al., 2014), the theoretical total oil content of the powders obtained was 10 g/100 g powder for emulsions n° 1 (powder P1-MD) and n° 8 (powder P2-MD) and 8 g/100 g powder for emulsion n° 12 (powder P2-I). In any case, the encapsulation efficiency was good with less than 12 g/100 g of the total oil non encapsulated corresponding to less than 1.2 g of oil at the particle surface in 100 g of powder. However, the non encapsulated oil content was different for the three powders. The smaller one was obtained for powder P1-MD corresponding to an excess of TweenÒ20 and the higher one was obtained for powder P2-MD for which depletion phenomena was
Please cite this article in press as: Hernandez Sanchez, M.d.R., et al. Design of liquid emulsions to structure spray dried particles. Journal of Food Engineering (2015), http://dx.doi.org/10.1016/j.jfoodeng.2015.07.036
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M.R. Hernandez Sanchez et al. / Journal of Food Engineering xxx (2015) xxx–xxx
Table 2 Particle size (d10, d50 and d90) and percentage of surface oil (non encapsulated oil) of dry emulsions. Initial emulsion
Dry emulsion
Particle size (lm) d10
d50
d90
n° 1 (protocol 1) n° 12 (protocol 2) n° 8 (protocol 2)
P1-MD P2-I P2-MD
11.3 6.3 8.4
21 18 19
43.3 61 40.6
12
Volume (%)
0.74 ± 0.03 0.80 ± 0.01 1.12 ± 0.03
partitioning between the different phases of the dry emulsion and therefore their protection.
P1-MD P2-I P2-MD
10
% Surface oil (g oil/100 g powder)
8
Acknowledgment
6
The authors would like to thank the Mexican National Council of Science and Technology (Consejo Nacional de Ciencia y Tecnología, CONACYT) for financial resources provided to Maria del Rayo Hernandez Sanchez.
4 2 0 0,01
0,1
1
10
100
1000
10000
Parcle size (μm) Fig. 7. Particle size distribution of dry emulsions produced with initial liquid emulsion obtained using protocol 1 (P1-MD) and protocol 2 (P2-I and P2-MD). (MD: maltodextrin DE12; I: inulin).
the more susceptible to occur. These values of surface oil content are consistent with those obtained by Turchiuli et al. (2014) and Aghbashlo et al. (2012). They are quite lower, but spray drying conditions and emulsion composition were different.
4. Conclusion Two protocols were tested for the production of oil-in-water emulsions with oil droplet size distribution, formulation, viscosity and physical stability suitable for further oil encapsulation by spray drying. The first protocol, with a direct homogenization of all the components, required an excess of emulsifier due to depletion phenomena occurring when adding a polymeric wall material. The second protocol proposed allowed overcoming this phenomena. It consisted in the production of a pre-emulsion without wall material, for which it was possible to obtain the required oil droplet size (e.g. about 2 lm) followed by the addition of a concentrated aqueous solution of the wall material in a second step to reach the composition suitable for spray drying. With this second protocol, it was possible producing stable emulsions with the desired oil droplet size using a lower quantity of emulsifier. The use of agave inulin instead of maltodextrin DE 12 as wall material was also found to decrease the quantity of emulsifier (TweenÒ 20) necessary to produce stable emulsions due to a higher CFC value. Emulsions containing either maltodextrin or inulin as wall material, containing different concentration of emulsifier and produced using the two protocols were spray dried. For all the powders obtained, the encapsulation efficiency was good with up to 92% of the oil encapsulated in the solid matrix. The proposed protocols could be applied to formulate emulsions containing other wall materials (maltodextrin with DE, products with health benefits) and emulsifiers. Encapsulation efficiency in terms of active molecules stability during storage has now to be evaluated in order to check the influence of the nature of the wall material and of the presence of an excess of emulsifier. Actually, the formation of micelles between free molecules of emulsifier and active molecules (e.g. antioxidants) may modify their
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Design of liquid emulsions to structure spray dried particles Maria del Rayo Hernandez Sanchez ⇑, Marie-Elisabeth Cuvelier, Christelle Turchiuli AgroParisTech, UMR 1145 Ingénierie Procédés Aliments, 1 Avenue des Olympiades, F-91300 Massy, France INRA, UMR1145 Ingénierie Procédés Aliments, F-91300 Massy, France CNAM, UMR1145 Ingénierie Procédés Aliments, F-91300 Massy, France
a r t i c l e
i n f o
Article history: Received 19 November 2014 Received in revised form 17 February 2015 Accepted 13 July 2015 Available online xxxx Keywords: Emulsification Spray drying Encapsulation Emulsion
a b s t r a c t The formulation and structure of initial liquid emulsion have an impact on the efficiency of encapsulation (encapsulated oil quantity) in spray dried particles (dried emulsions) and must be adapted to spray drying. Two different protocols were tested varying the concentration of emulsifier (TweenÒ 20), conditions of homogenization and concentration of wall material (Maltodextrin and Agave Inulin). The concentration of emulsifier and wall material should allow avoiding micelle formation or coalescence. Results show that it was possible to produce stable emulsions with an oil droplet size around 2 lm containing 40% w/w of total dry matter (g/100 g emulsion) using maltodextrin as wall material and 3.5% TweenÒ 20 (3.5 g/100 g oil) or using Agave Inulin in emulsions with a total dry mater content of 50% w/w (50 g/100 g emulsion) with a minimal concentration of emulsifier (1.17 g/100 g oil). The spray drying of these emulsions produced fine powders (<22 lm) with a monodispersed size distribution and good oil encapsulation efficiency (>88%). Ó 2015 Published by Elsevier Ltd.
1. Introduction Lipophilic active compounds such as polyunsaturated fatty acids (PUFAs), vitamins, antioxidants have shown to have positive effects on human health. In addition, antioxidants are useful to protect PUFAs from oxidation and preserve the organoleptic properties of foods. Indeed, PUFAs are mainly composed of large unsaturated hydrocarbon chains, they suffer oxidation when exposed to light, metals, heat and oxygen. The oxidation is the main cause of the degradation and deterioration of their properties, of the appearance of undesirable compounds and of the loss of their nutritional value (Kolakowska and Sikorski, 2011; Choe and Min, 2006; Frankel, 1998; McClements, 1999). In order to preserve and to prolong their functional properties, lipophilic compounds can be dispersed in oil-in-water emulsions and later encapsulated by spray drying. The first step of lipophilic compounds encapsulation by spray drying consists therefore in the preparation of an initial oil-in-water liquid emulsion with formulation, structure and properties suitable for further spray drying. Emulsion properties must also provide efficient encapsulation of the active components and in some cases make possible the reconstitution of the initial liquid emulsion by dissolution of the powder. Spray drying is a unit
operation in which a liquid, suspension or emulsion is pumped, atomized and dried in contact with a hot air current to produce a powder. It is a technique widely used for the encapsulation of active compounds into a solid matrix to provide them protection against degradation and volatile losses. The protection is mainly given by the wall material, which forms a physical barrier that encloses droplets or particles of the active compound encapsulated. The nature and concentration of wall material used, the percentage of oil, the viscosity and oil droplet size of emulsion as well as the drying conditions (e.g. liquid feed rate, drying air temperatures, air flow rate, etc.) will have an impact on the properties of the end product and thus, on the encapsulation efficiency (% oil encapsulated) (Gharsallaoui et al., 2007; Krishnaiah et al., 2012; Turchiuli et al., 2013, 2014; Janiszewska et al., 2014). Carneiro et al. (2012) stated that viscosity and oil droplet size of the emulsion positively affected the encapsulation efficiency and the oil retention during the encapsulation of coffee oil. However, a negative effect was found when the oil concentration and inlet temperature of drying air increased. Therefore, the homogenization conditions, the concentration and nature of wall material and emulsifier and percentage of oil have to be carefully studied to obtain initial emulsion that must:
⇑ Corresponding author at: AgroParisTech, 1 Avenue des Olympiades, 91300 Massy, France. E-mail addresses: [email protected] (M.R. Hernandez Sanchez), [email protected] (M.-E. Cuvelier), christelle. [email protected] (C. Turchiuli). http://dx.doi.org/10.1016/j.jfoodeng.2015.07.036 0260-8774/Ó 2015 Published by Elsevier Ltd.
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1. Have an oil droplet size distribution in relation to the size of the dry particles obtained from the sprayed droplets. For a given formulation, the emulsion size distribution depends on the equipment and energy input during homogenization. Knowing that powder particles formed by spray drying have diameters between about 20 and 50 lm at pilot scale and between about 50 and 200 lm at industrial scale, the emulsion size should not be larger than about 2 and 10 lm respectively (Gharsallaoui et al., 2007; Turchiuli et al., 2013, 2014). 2. Be physically stable until and during spray drying, under mechanical and thermal stress, in order to ensure correct oil distribution within the matrix of encapsulation (Janiszewska et al., 2014). 3. Have a formulation allowing encapsulation by spray drying. That means a viscosity compatible with pumping, pulverization and drying; and the presence of wall materials soluble in water, allowing forming solutions with a reasonable viscosity at high solids concentration (e.g. up to 40–60 g/100 g liquid feed), with a good drying ability (not too sticky), that does not react with the encapsulated component during processing and upon storage, providing protection against environmental factors (e.g. oxygen, heat, light, humidity) and leading to powders with good end-use properties (Desai and Park, 2005; Gharsallaoui et al., 2007). Wall materials are generally polymers that will increase the viscosity of the aqueous phase and limit both diffusion of active molecules and mobility of oil droplets in the liquid emulsion. Among the polymers used, maltodextrins are products of partial hydrolysis of starch that are widely used in encapsulation. They can be classified depending on their degree of hydrolysis, which is expressed as dextrose equivalents (DE). Due to their high solubility in water, low permeability to oxygen and low cost, maltodextrins are adequate for spray drying. However due to their lack of emulsifying properties, they need to be associated with surface active molecules in order to increase the physical stability and oil retention. Agave inulin, a fructo-oligosaccharide can be also used as wall material due to its nutritive and beneficial effects (Gharsallaoui et al., 2007; Jafari et al., 2008; Madene et al., 2006; Turchiuli et al., 2013, 2014). Emulsifiers (e.g. sodium caseinate, whey protein and gelatine) or surfactants (e.g. phospholipids, free fatty acids, monoacylglycerols and synthetic surfactants) that reduce the interfacial tension in emulsions when adsorbed at the water/oil interface are used to stabilize liquid emulsions (Genot et al., 2003; McClements, 2010; Vaclavik and Christian, 2008; Appelqvist et al., 2007). But, wall materials and surfactants can also have a negative impact in terms of physical stability of emulsions when added in certain concentrations. When high molecular polysaccharides or surfactants are added in excess, they can remain free in the aqueous phase stimulating the depletion mechanism, with, as a consequence, the flocculation and coalescence of oil droplets. During depletion, a modification of the rheological properties of the aqueous phase around the oil droplets takes place, promoting a decrease of the number of surfactant molecules adsorbed at the oil–water interface and then causing a reinforcement of the attraction forces between oil droplets. When the attraction forces become greater than the repulsive forces, the mechanism of flocculation occurs (Klinkesorn et al., 2004; Udomrati et al., 2013) causing an increase of the oil droplet size that becomes too large compared to the dry particle diameter. Furthermore, an excess of emulsifier would stimulate the formation of micelles; these micelles can interact with active components (e.g. phenolic antioxidants) and associate them, modifying their partitioning between the different phases of the emulsion and therefore their activity (Coupland and McClements, 1996).
The aim of this work was to propose protocols and formulations to design stable oil-in-water liquid emulsions with an oil droplet size around 2 lm, to get good dispersion and protection of the oil phase within dry particles of about 20 lm obtained by spray drying at pilot scale. Formulation was tested in terms of TweenÒ 20 concentration (1.17–20 g Tween 20 Ò/100 g oil) and nature and concentration of wall material (maltodextrin (MD) and agave inulin (I)) in order to find a compromise to avoid both depletion phenomena and excess of emulsifier. For the preparation of the liquid emulsions, two protocols were tested with respectively one or two steps. 2. Material and methods 2.1. Materials Commercial sunflower oil containing 11% w/w saturated, 29% w/w mono-unsaturated and 60% w/w poly-unsaturated fatty acids and 0.05% w/w a-tocopherol was used as model oil for lipophilic compounds encapsulation. The emulsifier TweenÒ 20, was purchased from Sigma–Aldrich (FR). Maltodextrin (Glucidex DE 12) was purchased from Roquette (FR) and agave inulin (Oligofructine) was purchased from Nutriagaves (MX). 2.2. Preparation of emulsions 2.2.1. Protocol 1 – Direct emulsification Protocol 1 (Fig. 1a) consists in a direct emulsification of all the ingredients. First, an aqueous phase was prepared by slow dissolution of wall material (MD) in ultrapure water under mechanical stirring with a 3-bladed propeller stirrer (Eurostar, IKA, FR) at room temperature. Then, 3.5 or 20 g/100 g oil TweenÒ 20 were incorporated and sunflower oil (4 g/100 g emulsion) was added. Emulsion (E1) was obtained by homogenization for 20 min with a rotation speed of 3500 rpm using a rotor–stator homogenizer (AXR, Silverson Machines Ltd, FR) (Turchiuli et al., 2014). 2.2.2. Protocol 2 – Double emulsification + dilution Protocol 2 (Fig. 1b) consists in first, a 2-step emulsification without wall material to reach the required emulsion size (e.g. about 2 lm) and second, the addition of an aqueous phase containing the wall material (dilution). The preparation of the pre-emulsion (Pre-E2) was adapted from Berton et al. (2012) and consisted in adding oil and TweenÒ (1.17–3.50 g/100 g oil) in ultrapure water to produce an emulsion with 30% w/w of oil. The ingredients were pre-homogenized at 16,000 rpm for 3 min with a rotor–stator homogenizer (Ultra-Turrax T25 basic, IKA, FR). The second homogenization was carried out with a two-stages high-pressure valve homogenizer (1001 L PANDA, Soavi NIRO, FR) using different homogenization pressures (35, 50 and 300 bars). An aqueous phase containing the wall material (MD or I) was prepared separately under mechanical stirring with a 3-bladed propeller stirrer (Eurostar IKA, FR) at room temperature. Finally, the newly formed aqueous phase was added to the Pre-E2 under gentle stirring in order to produce the final emulsion (E2) with 4 g oil/100 g emulsion and different dry matter content (10–60 g/100 g emulsion). The composition of the different emulsions prepared with both protocols is shown in Table 1. 2.3. Characterization of liquid emulsions 2.3.1. Size distribution of oil droplets The size and size distribution of oil droplets in initial emulsions and reconstituted emulsions were determined by laser diffraction
Please cite this article in press as: Hernandez Sanchez, M.d.R., et al. Design of liquid emulsions to structure spray dried particles. Journal of Food Engineering (2015), http://dx.doi.org/10.1016/j.jfoodeng.2015.07.036
M.R. Hernandez Sanchez et al. / Journal of Food Engineering xxx (2015) xxx–xxx
(a)
(b)
Protocol 1 Direct emulsification (Turchiuli et al., 2014)
MD + Tween20 + Water
+
3
Protocol 2 2-steps emulsification (Adapted from Berton et al., 2012) Sunflower oil + Tween20 +Water
Sunflower oil
Homogenization by rotor-stator device (AXR, Silverson Machines Ltd, Fr)
Homogenization by :Rotor-stator device (Ultra-Turrax T25 basic, IKA, Fr) and High-pressure device (SoaviNIRO 1001 L PANDA, Fr)
Pre-Emulsion (Pre-E2)
Emulsion (E1)
+ MD or I + Water
Emulsion (E2) Fig. 1. Protocols for preparation of emulsions: (a) protocol 1 – direct emulsification and (b) protocol 2-double emulsification + dilution.
Table 1 Composition of the emulsions prepared with Protocol 1 (Emulsions E1 n° 1 and n° 2) and Protocol 2 (Emulsions E2 n° 3 to n° 14) using maltodextrin DE12 (MD) and inulin (I) as wall material. Dry matter (g/100 g emulsion) = oil + TweenÒ 20 + wall material. Emulsion
Dry matter
Oil
MD
I
Water
TweenÒ 20
g/100 g emulsion E1 E2
Protocol 1 n° 1 n° 2 Protocol 2 n° 3 n° 4 n° 5 n° 6 n° 7 n° 8 n° 9 n° 10 n° 11 n° 12 n° 13 n° 14
2.5. Characterization of spray dried emulsions Spray dried powders produced were stored in hermetic jars at 20 °C. They were unfrozen at ambient temperature (at least 15 min) prior analysis.
g/100 g oil
40
4
35.2 36
60
0.8 0.14
20 3.5
10 20 25 30 40 40 40 40 45 50 55 60
4
6 16 21 26 36 36 36
90 80 75 70 60 60 60 60 55 50 45 40
0.05
1.17
0.14 0.20 0.05
3.5 4.85 1.17
36 41 46 51 56
TweenÒ 20 (g/100 g oil) = Indicates the quantity of TweenÒ 20 regarding the oil content.
(Mastersizer 2000, Malvern, FR) in wet mode (Hydro 2000) using 1.33 as the refraction index value for the dispersing water and 1.475 for the oil droplets (measured at 20 °C).
2.3.2. Apparent viscosity The apparent viscosity g (Pa s) of the different emulsions prepared and their rheological behaviour were measured at 20 °C using a rotational rheometer with coaxial cylinders (Rheomat R180, Lamy, FR) with velocity gradients c (s1) up to 1500 s1.
2.5.1. Size distribution of dried particles Particle size distribution of dried emulsions was measured by laser diffraction (Mastersizer 2000, Malvern, GB) using an automated dry powder dispersion (Scirocco 2000). Compressed air pressure was fixed to 4 bars and vibration rate to 50% using the fine sieve, to ensure the dispersion of cohesive dried particles. 2.5.2. Surface oil content (non encapsulated oil) The surface oil fraction in spray dried particles (non encapsulated oil) was recovered by a solvent extraction according to the GEA Niro Method A 10 (GEA, 2005). 10 g of spray dried emulsion were weighed into a 250 mL Erlenmeyer Flask and 50 mL of petroleum ether were added. The flask was closed and agitated during 15 min in a shaking device. The agitation speed was controlled in order to move the sample but not to splash it up on the sides of the upper half of the flask. After the agitation was stopped, 25 mL of extract solution were taken and placed into pre-weighed aluminium dish. Samples were first let for solvent evaporation in the fume hood, and after total evaporation of petroleum ether, dried in an oven for one hour at 105 °C. Finally, the surface oil content (g surface oil/100 g powder) was expressed as follows (Eq. (1)) (GEA, 2005):
a 50 100 % Surfaceoil ¼ v qa b
ð1Þ
where v, volume of extract taken (mL); a, weight of residue after oven drying (g); b, weight of powder (spray dried emulsion) used (g) and q, oil density (g mL1) (e.g. 0.94 g mL1in this study). 3. Results and discussion
2.4. Preparation of spray dried emulsions 3.1. Protocol 1 Liquid emulsions were spray dried in a pilot scale spray dryer (Niro Minor, Niro, DA). It is a one step co-current spray dryer with an evaporative capacity comprised between 1 and 4 kg h1. For spraying, a rotary wheel was used with a rotation speed of 25,000 rpm (5.5 bar compressed air). The emulsions flow rate was 25 g min1. Drying air was taken from the ambient by a fan (43 Hz) with a flow rate of 110 kg h1. The inlet air temperature was fixed at 200 °C and the outlet temperature varied from 94 to 110 °C.
Protocol 1 was used in previous studies on the encapsulation of lipophilic compounds (Fuchs et al., 2006; Tonon et al., 2011; Frascareli et al., 2012; Turchiuli et al., 2013, 2014). It requires only one step of homogenization and allowed obtaining stable emulsions, suitable for the encapsulation of lipophilic compounds (fish oil, sunflower oil). In these studies, emulsifying properties were brought by acacia gum which was also used as wall material and therefore added in large quantity (up to 30 g/100 g emulsion) for
Please cite this article in press as: Hernandez Sanchez, M.d.R., et al. Design of liquid emulsions to structure spray dried particles. Journal of Food Engineering (2015), http://dx.doi.org/10.1016/j.jfoodeng.2015.07.036
M.R. Hernandez Sanchez et al. / Journal of Food Engineering xxx (2015) xxx–xxx
Volume (%)
4
14
3.5 g Tween/100 g emulsion
12 10
20 g Tween/100 g emulsion
8 6 4 2 0 0,01
0,1
1
10
100
1000
10000
Oil droplet size (μm) Fig. 2. Oil droplet size distribution of emulsions n° 1 and 2 with 40 g of total dry matter/100 g emulsion obtained with protocol 1 using maltodextrin DE12 (MD) as wall material and 3.5 and 20 g of TweenÒ 20/100 g oil respectively.
the encapsulation of flaxseed oil, coffee oil and olive oil (Tonon et al., 2011; Frascareli et al., 2012; Turchiuli et al., 2013, 2014). In the present study, TweenÒ 20 was chosen for its emulsifying properties to stabilize oil droplets, but did not act as a wall material. It was therefore expected to lower its concentration in order to avoid micelles formation due to the presence of free molecules not adsorbed at the oil/water interface. Emulsions n° 1 and 2 were therefore prepared using protocol 1 with a quantity of TweenÒ 20 respectively of 20 g/100 g oil, in agreement with the previous studies, and of 3.5 g/100 g oil, to reduce the risk of micelles formation (Table 1). The use of excess of emulsifier (20 g/100 g oil) allowed producing an emulsion E1 n° 1 with a monomodal size distribution, and a median oil droplet size suitable for oil encapsulation by spray drying (d50 = 1.7 lm) with 80% of the oil droplets with diameters between 0.9 and 3 lm (Fig. 2). But, when decreasing the emulsifier content, a bimodal oil droplet size distribution was obtained, with 80% of the oil droplets with a diameter between 5.2 and 17 lm and only 10% with a diameter below 5.2 lm (d50 = 9.8 lm for 3.5 g TweenÒ 20/100 g oil) (Fig. 2). In this last case, the increase of the homogenization time or intensity did not make it possible to decrease the emulsion size. This may be due to the depletion phenomena occurring when using maltodextrin with low emulsifier content and when all the ingredients of the emulsions are subjected to a direct emulsification (Klinkesorn et al., 2004; Udomrati et al., 2013). Small oil droplets in the searched size range are formed during homogenization, but they flocculate and coalesce rapidly due to large attraction forces. 3.2. Protocol 2 The first part of protocol 2 was adapted from a study of Berton et al. (2012) who produced oil-in-water emulsions with a median oil droplet size of about 2 lm using TweenÒ 20 as emulsifier in very low concentration (<2 g/100 g oil). In that study, the oil concentration in the emulsion was larger than the one required for the present study (e.g. 30 g oil/100 g emulsion compared to 4 g oil/100 g emulsion) and no wall material was added in the aqueous phase. It was therefore proposed to first produce concentrated emulsions with the required oil droplet size using a similar protocol and formulation and then to dilute these pre-emulsions with the wall material solution to obtain a final emulsion with the required oil content (4 g/100 g emulsion). 3.2.1. Influence of homogenization pressure on size distribution of preemulsions E2 (Pre-E2) Pre-emulsions containing oil (30 g/100 g emulsion), TweenÒ 20 (1.17 g/100 g oil) and water were pre-homogenized using rotor– stator homogenizer and then submitted to high pressure homogenization. Different homogenization pressures were tested in order to check the influence on the pre-emulsion size distribution. The
use of low homogenization pressure (35 and 50 bars) resulted in emulsions with a large fraction of oil droplets above 10 lm resulting in median diameters d50 of 4.8 and 5 lm respectively with a span value of 2 in both cases (Fig. 3) therefore too large for an efficient encapsulation in this study. However, as the pressure was increased, the oil droplet size was reduced. The homogenization at higher pressure (300 bars) caused a reduction of 50% of the droplet size (d50 = 2.2 lm, span = 2.2) compared to the homogenization at lower pressures. Janiszewska et al. (2014) homogenized lemon aroma emulsions (2–10 g/100 g emulsion) using a protocol very similar to the one described in the present paper. They used a mixture of maltodextrin DE 10 and acacia gum (7:1). The homogenization was carried out at 400 and 600 bars. Results showed that the emulsion size (d50 = 0.20–0.35 lm) was not affected by the homogenization pressure, but by the increase of aroma oil fraction. For the purpose of the present research, the homogenization pressure for the production of pre-emulsions Pre-E2 was therefore set at 300 bars. 3.2.2. Influence of wall material concentration and nature on viscosity and size distribution of Emulsions E2 As stated above, it was possible to produce a pre-emulsion pre-E2 with the desired oil droplet size for the purposes of this research. However, this emulsion needs the addition of wall material for further encapsulation by spray drying. An aqueous phase was therefore added in the pre-emulsion pre-E2 varying either the concentration or nature of wall material, with the aim of producing an emulsion (E2) maintaining the oil droplet size of 2 lm. Emulsions E2 n° 3–7 (Table 1), prepared from the same pre-emulsion pre-E2 (d50 = 2.2 lm – 1.17 g TweenÒ 20/oil), were obtained adding aqueous phases containing maltodextrin DE12 with different concentrations. They showed an oil droplet size around 2 lm when the MD content was increased up to 26 g/100 g emulsion (Fig. 4). However, emulsions suffered a dramatic change of oil droplet size when the maltodextrin content was higher, resulting in an oil droplet size of 7.8 lm when the MD content was 36 g/100 g emulsion (emulsion n° 7 – total dry matter content 40 g/100 g emulsion). This size increase is the result of coalescence/flocculation of oil droplets in the presence of certain concentrations of wall material. This instability phenomenon is produced under particular circumstances such as the non-absorption of polymers (wall materials) in the aqueous phase of emulsion. The critical flocculation concentration (CFC) provides the minimum polymer concentration required for promoting flocculation, therefore, it is a useful tool in terms of quality control of emulsions in presence of polysaccharides (Klinkesorn et al., 2004; Udomrati et al., 2013). In the case of maltodextrins, it was demonstrated that a reduction of the dextrose equivalent value (DE) decreased the CFC value, which was attributed to the increase of the molecular weight and viscosity of maltodextrin solutions (Klinkesorn et al., 2004). The CFC of MD DE12 in emulsions with 35 bars 50 bars 300 bars
Fig. 3. Oil droplet size distribution of pre-emulsions E2 (Pre-E2) composed by 30 g oil/100 g emulsion and TweenÒ 20 (1.17 g/100 g oil) homogenized at different pressures.
Please cite this article in press as: Hernandez Sanchez, M.d.R., et al. Design of liquid emulsions to structure spray dried particles. Journal of Food Engineering (2015), http://dx.doi.org/10.1016/j.jfoodeng.2015.07.036
M.R. Hernandez Sanchez et al. / Journal of Food Engineering xxx (2015) xxx–xxx
10 MD
8
d50 (μm)
I
6 4
CFC CFC
2 0 0
10
20
30
40
50
60
Wall Material (g/100 g emulsion) Fig. 4. Estimation of critical flocculation concentration (CFC) for emulsions E2 (1.17 g TweenÒ 20/100 g oil) with maltodextrin (MD) or inulin (I) as wall material from the evolution of emulsion median diameter (d50) as a function of wall material concentration (g/100 g emulsion).
I - 51 g/100 g
η (Pa.s)
0,08 0,06
MD – 36 g/100 g I – 46 g/100 g I – 41 g/100 g I – 36 g/100 g
0,04 0,02 0 200
700
1200
γ (s )
5
In order to produce emulsions E2 adequate for spray drying, it is necessary to increase the dry matter content in the emulsion keeping constant the oil droplet size. This can be achieved either by increasing the emulsifier content or using another wall material. Emulsions E2 n° 8 and 9 were therefore prepared with a MD DE12 content of 36 g/100 g emulsion increasing the TweenÒ 20 content to 3.5 and 4.85 g/100 g oil respectively (total dry matter content 40 g/100 g emulsion). This allowed obtaining emulsions E2 with the required oil droplet size (1.5 lm in both cases) and viscosity (about 0.04 Pa s) (Figs. 5 and 6). When using inulin as wall material instead of maltodextrin (emulsions E2 n° 10–14) with a minimal content of TweenÒ 20 (1.17 g/100 g oil), the oil droplet size distribution remained unchanged (d50 1.7–1.8 lm) when the concentration of I was increased up to 46 g/100 g emulsion (Fig. 4), with the CFC in E2 containing I about 50 g/100 g emulsion. Regarding viscosity (Fig. 5), emulsions E2 containing either 50 g/100 g total dry matter with inulin as wall material and 1.17 g TweenÒ 20/100 g oil (n° 12) or 40 g/100 g total dry matter with maltodextrin DE12 as wall material and 3.5 or 4.85 g TweenÒ 20/100 g oil (n° 8 and 9) showed similar apparent viscosities (around 0.04 Pa s), which is suitable for spray drying (Turchiuli et al., 2014).
-1
3.3. Spray dried emulsions
Fig. 5. Apparent viscosity of emulsions E2 with different concentrations of maltodextrin DE12 (MD) and inulin (I): 36 g/100 g emulsion of MD, 36, 41, 46, and 51 g/100 g emulsion of I, respectively in emulsions n° 8, 10, 11, 12 and 13.
(a) Maltodextrin
10
1.17 g Tween/ 100 g emulsion 3.50 g Tween/ 100 g emulsion 4.85 g Tween/ 100 g emulsion
Volume (%)
8 6 4 2 0 0,01
0,1
1
10
100
1000
10000
Oil droplet size (μm)
Volume (%)
10
(b) Inulin
8
1.17 g Tween/ 100 g emulsion
6 4 2 0 0,01
0,1
1
10
100
1000
10000
Oil droplet size (μm) Fig. 6. Oil droplet size distribution in emulsions E2 with (a) 40 g/100 g emulsion total dry matter using maltodextrin DE12 (MD) for different concentration of TweenÒ 20 (g/100 g oil) and (b) with 50 g/100 g emulsion total dry matter using inulin (I) with a minimal concentration of TweenÒ 20 (1.17 g/100 g oil).
1.17 g TweenÒ 20/100 g oil is about 26 g/100 g emulsion, which means that MD has to be added at lower concentration to avoid depletion phenomena. However, for the purposes of the encapsulation by spray drying, higher concentration of dry matter (about 40 g/100 g emulsion when using MD as wall material) and viscosity of the emulsion are necessary (Turchiuli et al., 2013, 2014).
Emulsion E1 n° 1 prepared using protocol 1 with maltodextrin as wall material and an excess of TweenÒ 20; and emulsions E2 n° 8 and 12 prepared using protocol 2 with respectively maltodextrin and inulin as wall material were spray dried in similar conditions. For emulsions n° 1 and n° 8, containing maltodextrin, the total dry matter content was 40 g/100 g emulsion whilst for emulsion n° 12, containing inulin, it was 50 g/100 g emulsion. In emulsions n° 1 and n° 8, oil (4 g oil/100 g emulsion) represented 10 g/100 g of dry matter and 8 g/100 g of dry matter for emulsion n° 12. For all the three, the apparent viscosity was about 0.04 Pa s and the oil droplet median diameter was about 2 lm (e.g. 1.7, 1.5 and 2.1 lm for emulsions n° 1, 8 and 12 respectively). The physical stability of the three emulsions was checked along the process (from production step to end of spray drying). They were found to be stable for at least 2 h provided they were not agitated. Three powders were obtained and characterized for the particle size distribution and non-encapsulated oil content (Table 2, Fig. 7). As expected, the median particle size of the three powders was about 20 lm, probably due to the fact that, despite the dry matter content was different (composition and nature), they had similar apparent viscosity and were spray dried in similar conditions. This is in agreement with the results of Turchiuli et al. (2014). They obtained powders with median diameters between 24 and 26 lm spray drying, in similar conditions and equipment, olive oil emulsions (10 g oil/100 g dry matter) with maltodextrin, acacia gum and inulin as wall materials used alone or in mixtures with different ratios. The use of acacia gum increased the emulsion viscosity (e.g. up to about 0.2 Pa s). This may explain the bigger size of the powders they obtained. Assuming no oil loss during spray drying (Cuvelier et al., 2014), the theoretical total oil content of the powders obtained was 10 g/100 g powder for emulsions n° 1 (powder P1-MD) and n° 8 (powder P2-MD) and 8 g/100 g powder for emulsion n° 12 (powder P2-I). In any case, the encapsulation efficiency was good with less than 12 g/100 g of the total oil non encapsulated corresponding to less than 1.2 g of oil at the particle surface in 100 g of powder. However, the non encapsulated oil content was different for the three powders. The smaller one was obtained for powder P1-MD corresponding to an excess of TweenÒ20 and the higher one was obtained for powder P2-MD for which depletion phenomena was
Please cite this article in press as: Hernandez Sanchez, M.d.R., et al. Design of liquid emulsions to structure spray dried particles. Journal of Food Engineering (2015), http://dx.doi.org/10.1016/j.jfoodeng.2015.07.036
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M.R. Hernandez Sanchez et al. / Journal of Food Engineering xxx (2015) xxx–xxx
Table 2 Particle size (d10, d50 and d90) and percentage of surface oil (non encapsulated oil) of dry emulsions. Initial emulsion
Dry emulsion
Particle size (lm) d10
d50
d90
n° 1 (protocol 1) n° 12 (protocol 2) n° 8 (protocol 2)
P1-MD P2-I P2-MD
11.3 6.3 8.4
21 18 19
43.3 61 40.6
12
Volume (%)
0.74 ± 0.03 0.80 ± 0.01 1.12 ± 0.03
partitioning between the different phases of the dry emulsion and therefore their protection.
P1-MD P2-I P2-MD
10
% Surface oil (g oil/100 g powder)
8
Acknowledgment
6
The authors would like to thank the Mexican National Council of Science and Technology (Consejo Nacional de Ciencia y Tecnología, CONACYT) for financial resources provided to Maria del Rayo Hernandez Sanchez.
4 2 0 0,01
0,1
1
10
100
1000
10000
Parcle size (μm) Fig. 7. Particle size distribution of dry emulsions produced with initial liquid emulsion obtained using protocol 1 (P1-MD) and protocol 2 (P2-I and P2-MD). (MD: maltodextrin DE12; I: inulin).
the more susceptible to occur. These values of surface oil content are consistent with those obtained by Turchiuli et al. (2014) and Aghbashlo et al. (2012). They are quite lower, but spray drying conditions and emulsion composition were different.
4. Conclusion Two protocols were tested for the production of oil-in-water emulsions with oil droplet size distribution, formulation, viscosity and physical stability suitable for further oil encapsulation by spray drying. The first protocol, with a direct homogenization of all the components, required an excess of emulsifier due to depletion phenomena occurring when adding a polymeric wall material. The second protocol proposed allowed overcoming this phenomena. It consisted in the production of a pre-emulsion without wall material, for which it was possible to obtain the required oil droplet size (e.g. about 2 lm) followed by the addition of a concentrated aqueous solution of the wall material in a second step to reach the composition suitable for spray drying. With this second protocol, it was possible producing stable emulsions with the desired oil droplet size using a lower quantity of emulsifier. The use of agave inulin instead of maltodextrin DE 12 as wall material was also found to decrease the quantity of emulsifier (TweenÒ 20) necessary to produce stable emulsions due to a higher CFC value. Emulsions containing either maltodextrin or inulin as wall material, containing different concentration of emulsifier and produced using the two protocols were spray dried. For all the powders obtained, the encapsulation efficiency was good with up to 92% of the oil encapsulated in the solid matrix. The proposed protocols could be applied to formulate emulsions containing other wall materials (maltodextrin with DE, products with health benefits) and emulsifiers. Encapsulation efficiency in terms of active molecules stability during storage has now to be evaluated in order to check the influence of the nature of the wall material and of the presence of an excess of emulsifier. Actually, the formation of micelles between free molecules of emulsifier and active molecules (e.g. antioxidants) may modify their
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Please cite this article in press as: Hernandez Sanchez, M.d.R., et al. Design of liquid emulsions to structure spray dried particles. Journal of Food Engineering (2015), http://dx.doi.org/10.1016/j.jfoodeng.2015.07.036