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Spray chill encapsulation of flavors within anhydrous erythritol crystals
LWT - Food Science and Technology 48 (2012) 107e113
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Spray chill encapsulation of flavors within anhydrous erythritol crystals Matthew Sillick *, Christopher M. Gregson Firmenich, Inc., 250 Plainsboro Rd., Plainsboro, NJ 08536, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2011 Received in revised form 13 February 2012 Accepted 20 February 2012
Encapsulate powders are widely used in the food industry to control the release of liquid active ingredients, such as flavors. Spray chilling with a carrier composed of erythritol, an anhydrous sugar alcohol, was investigated as a novel alternative to existing technologies. The carrier entrapped droplets of the liquid actives and solidified via crystallization. The process was performed with active loadings as high as 35 g/100 g. Efficient retention was observed in many cases, but losses increased for more volatile and/or carrier miscible actives. The resulting powders had high bulk density and a free flowing character. The release characteristics were determined by the physical properties of the crystalline carrier. On heating, the actives were afforded significant protection from volatilization until melting of the erythritol at about 120 C. Sorption of moisture was minimal at relative humidity below 92%, at which point the carrier became deliquescent. This delivery system has unique performance characteristics, which may enable improved performance for certain food applications. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Erythritol Microencapsulation Flavor encapsulation Crystalline carrier
1. Introduction Valuable active ingredients, such as flavors, are often encapsulated within a continuous protective matrix in order to facilitate handling, prevent premature evaporation or provide protection against oxidation. Typically, this matrix is a carbohydrate or glycoprotein glass that has been formed either by spray drying or melt extrusion (Gouin, 2004). Glasses have performed successfully in no small part because they are homogenous and free of grain boundaries. This report considers an alternative route to encapsulation based on spray chilling and the crystallization of a nonaqueous carrier. Spray chilling (or spray congealing) is the process of atomizing a molten liquid and cooling the resulting droplets to form prills or powders that are solid at room temperature. Commonly spray chilled carriers for encapsulation include fats, waxes, polyethylene glycols, fatty acids and fatty alcohols (Kjaergaard, 2001). Such lipophilic materials are often miscible and hence poor barriers for many flavor compounds (Benczedi, 2002). The use of a nonmiscible matrix, such as a sugar alcohol, has the potential to overcome this limitation. Early efforts at using a lipophobic crystallizing carrier to entrap flavors were made with sorbitol (Dimick, 1959). Unfortunately, sorbitol melts crystallize slowly (over a time scale of hours) which
* Corresponding author. Tel.: þ1 609 580 4763. E-mail address: matthew.sillick@firmenich.com (M. Sillick). 0023-6438/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2012.02.022
hinders industrialization and makes particle formation by spray processing impractical. Spray chilling of a mannitol carrier, which crystallizes much more readily, has been reported in the pharmaceutical literature (Kanig, 1964; Zajc, Obreza, Bele, & Srcic, 2005). Perhaps in part because of the relatively high melting point of mannitol, adaptation of the process to encapsulate volatile flavors has not been widely described. A similar, but perhaps more suitable carrier might be meso-erythritol (C4H10O4), a linear sugar alcohol with a melting point of 121 C. Molten erythritol also has advantages in that it is a thermally and chemically stable liquid that crystallizes readily when supercooled or supersaturated (Lopes Jesus, Nunes, Ramos Silva, Matos Beja, & Redinha, 2010). This report demonstrates a process of entrapping flavor droplets within erythritol crystals to produce a powder or granular delivery system. Several factors that impact the degree of flavor retention are described. While the thermal and moisture sorption properties of the encapsulates are shown here, demonstration of improved performance within a chewing gum application was reported elsewhere (Gregson & Sillick, 2011). 2. Materials and methods Thermo-mechanical properties of erythritol (ZeroseÒ, Cargill Inc., Minnetonka, MN, USA) were assessed using a rheometer (AR2000) and calorimeter (Q200) both from TA Instruments (New Castle, DE, USA). Viscosity was measured under constant shear flow using the 40 mm 2 cone and plate geometry. Calorimetric phase transitions were observed on heating at 10 C/min in a hermetic
M. Sillick, C.M. Gregson / LWT - Food Science and Technology 48 (2012) 107e113
aluminum pan. The calorimeter had been calibrated for temperature with indium and heat capacity using sapphire discs according to instructions from the manufacturer. Five active ingredients were selected that spanned a range of volatility and hydrophobicity (Table 1): limonene, nicotine (both from TCI America, Portland, OR, USA), methyl salicylate (Alfa Aesar, Ward Hill, MA, USA), cinnamic aldehyde (MP Biomedicals, Solon, OH, USA), and NeobeeÒ M5 (Stepan Company, Northfield, IL, USA). Each was mixed with a surfactant at a 9:1 active:surfactant mass ratio. The surfactant was lecithin (YelkinÒ SS, Archer Daniels Midland Company, Decatur, IL, USA) for most cases. However, because lecithin showed only limited solubility in cinnamic aldehyde, a citric acid ester (GrindstedÒ CITREM, Danisco USA Inc., Fairfield, NJ, USA) was used in this case. These mixtures of active ingredients with surfactant are referred to hereafter as the “oil” phase. Each oil solution was then paired with the molten erythritol carrier at several selected mass ratios between 10 and 40 g/100 g. The two low viscosity fluids (the carrier and the oil) were then placed into a 150 mL pressurizable vessel, which was tempered to 130 C. Mixing was administered for approximately 30 s using a hand-held homogenizer (T25 UltraTurrax, Ika Works, Inc. Wilmington, NC, USA). The mixing element was removed from the vessel and the lid was closed. The melt emulsion was pushed through either a pressure nozzle (for creating a fine spray) or a syringe needle (to prill larger granules) under head nitrogen pressure of 70e350 kPa. The droplets fell approximately 10 cm before landing in a bath of quench fluid. Limonene was select as the quench fluid because it is immiscible with molten erythritol and could be tempered to 0 C to achieve rapid crystallization of the carrier. The hardened particles were collected and laid on an absorbent sheet overnight to allow the quench fluid to evaporate off. The resulting prills and spray powders were examined using an Olympus (Center Valley, PA, USA) BX51 optical microscope to determine encapsulate particle size, entrapped oil droplet size, and to characterize the encapsulate microstructure. Oil droplet size was also confirmed after reconstituting selected samples in water using a Beckman Coulter (Brea, CA, USA) LSÒ13 320 analyzer with the built in Mei model and polarization intensity differential scattering (PIDS) treatment. Moisture sorption was studied using an Surface Measurement Systems (Allentown, PA, USA) Advantage DVS system. Thermal gravimetric analysis was conducted using a TA Instruments Q50 system under flowing nitrogen. Samples were placed within a pan having side walls in order to achieve a roughly consistent surface area when the carrier was in the liquid state. Freely settled powder density was measured by weighing the amount of material necessary to fill a 50 mL graduated cylinder. Tapped bulk density was determined after hand tapping the cylinder 100 times. Retention of oil within encapsulate powders was determined by time-domain low-field nuclear magnetic resonance (TD-LF-NMR) as described by Andrade, Farhat, Aeberhardt, Normand, and Engelsen (2008). This technique measures the fraction of
a complex sample that has sufficient molecular mobility to respond to a change in magnetic field (Hafner, Dardelle, Normand, & Fieber, 2011). A Bruker Optics (Billerica, MA, USA) Minispec 20 MHz spectrometer was used to measure 4 scans of the 90 /180 spin-echo signal with a 3.5 ms delay time, 0.5 ms acquisition window and a 20 s recycle delay. The sample size was approximately 3 g. The spin-echo signal was proportional to the mass of the oil in the preparation and therefore could be calibrated using carefully measured mixtures of each specific active ingredient/surfactant solution and granular erythritol. 3. Results and discussion 3.1. Thermo-mechanical properties of erythritol Viscosity of the anhydrous erythritol melt was found to be 25 mPa s at 130 C and was independent of shear rate (i.e. Newtonian). This low value confirms that it should be possible to atomize molten erythritol by conventional methods such using a pressurized spray nozzle or prilling needle. Viscosity increased modestly on cooling at a constant shear rate of 10 s1 as shown in Fig. 1 and was 725 mPa s at 60 C. The data were fit by a VogeleFulchereTamman trend, which along with its equivalent rearrangement the WLF relation, are commonly used equations for describing the temperature dependence of relaxation within liquids (Angell et al., 1994; Sillick & Gregson, 2009). Below 60 C, the supercooled melt crystallized. Crystallization involves a sudden decrease in free volume. The results show the ensuing departure from the smooth VFT trend. As crystallization progressed, the material took on a white opaque appearance and the apparent viscosity soon exceeded the limit of the instrument. Erythritol was quenched from above its melting point and reheated to give the DSC curve shown in Fig. 2. A glass transition is seen in the range 44 to 40 C, which agrees with previously reported values (Lopes Jesus et al., 2010). Such a low Tg provides confidence that any solid encapsulate particles observed at room temperature must be predominantly crystalline and not glassy. With further reheating, a cold crystallization occurred with onset
1000000 100000 Melt Viscosity (mPa.s)
108
10000 1000 100 10
Table 1 Vapor pressure and logP of active ingredients used in this study.
1
Ingredient
VP at 25 Cb (Pa)
VP at 130 Ca (Pa)
LogPb
Nicotine Methyl salicylate Cinnamic aldehyde NeobeeÒ (octanoic triglyceride) Limonene
4.0 9.3 3.5 8.8*1025 2.1*102
2.0*103 3.9*103 1.8*103 7.2*1014 2.5*104
0.7 2.2 2.1 25 4.5
a Calculated based on VP at 25 C and boiling point using the ClausiuseClapeyron equation. b Taken from an online resource (chemspider.com, 2011).
40
60
80 100 120 Temperature (°C)
140
160
Fig. 1. Melt viscosity of erythritol vs. temperature. Below 60 C the sample crystallized and experimental data (>) departed from the VFT trend line. VFT fit : log h ¼ log ho þ ðB=ðT To ÞÞ log ho ¼ 4:625 B ¼ 640 To ¼ 190:5 K:
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109
Fig. 2. DSC scan of erythritol on reheating at 10 C/min after quenching the melt.
at 3 C and peak in the heat flow signal at 7 C. The quenching temperature for encapsulate processing was chosen to fall in this range. 3.2. Encapsulate particle size and morphology The morphology of the encapsulate particles is decidedly spherical rather than faceted (Figs. 3 and 4). Thus, crystal growth appears to have continued to the boundaries of the preceding melt emulsion droplets without dramatic changes in shape or size. Given the high degree of supercooling and rapid crystallization, crystal growth might be expected to proceed in a normal and continuous (rather than lateral and stepwise) mode (Cahn, Hillig, & Sears, 1964). Similar spherical shapes are reported in other crystalline
Fig. 4. Fine crystalline particles form by spraying (A) pure erythritol and (B) an erythritol/limonene emulsion (90 g/100 g erythritol, 9 g/100 g limonene and 1 g/100 g lecithin) as imaged by slightly uncrossed polarized light. While several gas bubbles are seen within one of the particles of A, particles in B also contain a multitude of fine (w1 mm) oil droplets dispersed throughout the structure.
Fig. 3. Spherically shaped crystalline encapsulate particles formed by prilling. The encapsulate particles were formulated with 90 g/100 g erythritol, 9 g/100 g limonene and 1 g/100 g lecithin.
particles formed by melt spraying, for example artificial snow as well as industrially spray chilled materials (e.g. fertilizer prills, KOH prills and certain metal powders) (Kjaergaard, 2001). The size of the particles was dependent on the atomization technology and process conditions. The particles sprayed through a pressure nozzle tended to have diameters in the range 40e300 mm (based on optical microscopy). The prills, which broke up into droplets after being ejected from a dispensing needle, were larger (500 mme2 mm). A “ballooning” or vacuole formation phenomenon can lead to low bulk densities within spray dried flavor encapsulates (Reineccius, 2001). Bulk densities reported in Table 2 are high and similar to that of as-received erythritol, which suggests that the particles did not contain large internal voids. This theory was largely corroborated by transmitted light microscopy. As the encapsulation within erythritol does not involve volatilization of water, vacuole formation should be largely avoided. However, some porosity was noted in the largest prill particles (generally those
110
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Table 2 Average bulk and tap density of crystalline encapsulate particles for all active ingredients types and loading levels. Bulk density (tapped) (g/mL)
Hausner ratio
Crystalline encapsulates Prills (avg.) Spray powders (avg.)
0.82 0.06 0.78 0.06
1.06 0.01 1.20 0.08
As-received granular erythritol
0.90
1.15
>1.5 mm). These particles tended to have a small hole on their surfaces which lead to a moderately sized internal void. This is likely to be due to the sudden decrease in specific volume upon crystallization. Kjaergaard (2001) has described similar indentations and pores that form because of constrained shrinkage for other types of coarse prills. The Hausner ratio (freely settled density/tapped density) is an empirical metric of the flow characteristics of a powder or granular material (Grey & Beddow, 1969). Powders which have values closer to 1 are often less cohesive and more flowable. The larger prill particles were particularly free flowing and had Hausner ratio values even lower than that of as-received granular erythritol. The finer spray encapsulate powders were less free flowing, yet still had Hausner ratio values less than 1.25. Sparingly flowable powders, by comparison, are typified by values >1.4 (Santomaso, Lazzaro, & Canu, 2003).
3.3. Microstructure and inclusion of oil droplets Many fine oil droplets were observed dispersed throughout the crystalline encapsulate particles. Ensuring that the dispersed oil droplets are suitably small compared to the size of the encapsulate particle is likely to be important in order to minimize active ingredient loss during particle formation. The emulsification protocol was sufficient to produce very fine oil droplets. By microscopy, the mean size appeared to be between 1 and 5 mm. A light scattering measurement of a 20 g/100 g limonene encapsulate powder reconstituted in water showed a d4,3 mean droplet size of 3.05 mm and standard deviation of 1.58 mm. Successful entrapment of the active ingredient within the encapsulate microstructure requires that the oil droplets be accommodated within the crystallizing domain. However, crystallization is often considered to be a purification process involving phase separation and exclusion of impurities. For example, one can imagine active ingredient loss occurring by a process akin to freeze concentration in ice cream; ice crystals form and physically exclude fats, sugars and proteins enriching their concentrations in the interstitial regions. If oil droplets are similarly excluded from a developing erythritol crystal, they too would concentrate and potentially coalesce or even form large channels in the spaces between erythritol crystals. Surprisingly, exclusion can be avoided to a significant extent due to the sufficiently rapid crystallization of erythritol. Evidence for this comes from polarized light microscopy. Fig. 4 shows several small particles formed by spraying pure erythritol and an erythritol/limonene melt emulsion. Fine droplets are seen throughout (B) the encapsulate particles but are not present in (A) the pure erythritol particles. Meso-erythritol forms a tetragonal crystal structure (Bokoe & Powell, 1959) and is therefore birefringent. The particles have concentric and continuous patterns of polarization colors. The order of the colors follow the classical Michel-Levy chart (Pamphlet from Carl Zeiss Light Microscopy, 2005). The color is determined by the phase difference which is the product of
birefringence and particle thickness. Because the particles are spherical and (unlike flat discs) vary in thickness, the colors progress steadily toward the interior. Superimposed on the interference colors is an intensity component. If a crystal is rotated in the field of view, the color intensity varies cyclically from zero at “extinction” when the optic axis of the crystal is parallel to one of the polarizers of the microscope to a maximum when oriented at 45 to the polarizers (Stoiber & Morse, 1994). When rotated on the microscope stage, the entire particle (Fig. 4B) cohesively progresses through maxima and minima in polarization brightness. This implies a continuous orientation of crystallographic axes throughout such particles. Continuous orientation, in turn, indicates that such encapsulate particles are either highly imperfect single crystals or otherwise aligned domains (such as occurs with meso-crystals). If the particles were polycrystalline and randomly oriented, one would expect a more heterogeneous appearance. For example, Fig. 4A shows particles that appear to consist of two unaligned domains. The polarization colors are less bright within the domain closer to extinction. Inclusion is a well known process by which a micron-scale impurity becomes incorporated within a host crystal. In this sense, inclusion differs from other modes of association such as entrapment (in which an active is surrounded by many crystals), molecular inclusion (e.g. cyclodextrin complexation), and solidsolutions (in which guest molecules are accommodated within lattice sites of a host crystal). The presence of droplet or void inclusions within single crystals is common both in geology and various crystallization industries (Rudolph, 2008). For example, Lowenstern (1995) stated that “as many as scores of inclusions” are often found within a single quartz crystal. According to Mullen (1972), inclusions tend to be incorporated under conditions of rapid crystal growth and inhibited diffusion. Meso-crystals are superstructures of parallel or aligned nanoparticles, which by many conventional analyses, including optical microscopy, appear as single crystals (Colfen & Antonietti, 2005). One pertinent aspect of these newly recognized materials is their potential to incorporate significant amounts of impurities such as macromolecules within their superstructure. Incorporation of droplets or larger bodies has not yet been widely described. Both scenarios (inclusion within single crystals and meso-crystalline domains) provide routes for transforming the system from a liquid melt emulsion to a solid crystalline body without the displacement of oil droplets. Large crystals are rare in nature and tend to form over long timescales, growth occurring very slowly. Regarding encapsulation within erythritol, there may be some limit to the size of crystals that is both chemistry and process dependent. While the small sprayed particles (<100 mm), such as those shown in Fig. 4, often appear to consist of a single oriented domain, larger particles were always polycrystalline. The domain “grain size” (which is approximately 200 mm on average) is large compared to the typical size of oil droplets (w3 mm) and thus permits a significant degree of inclusion. Unfortunately, with increasing particle size, the interior becomes increasingly obscured and so determining precisely the degree to which oil exists within or between crystals becomes impossible on intact particles by visual techniques. Crushing the large particles and examining the resulting fragments showed that droplets were also included within crystalline domains inside the particles. Large oriented sections with included oil droplets were observed. 3.4. Moisture sorption Sorption of moisture from humid environments is an example of a property for which there are clear mechanistic differences
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3.5. Thermal stability A sprayed encapsulate powder made with 9 g/100 g loading of limonene was examined by TGA and the result is shown in Fig. 6. The sample was ramped slowly to 100 C, during which time essentially no weight was lost. Any “free” or accessible limonene was apparently removed during the solvent removal step prior to the analysis. The remaining limonene appears to be trapped and resists volatilization. The sample was then warmed quickly to 125 C to melt the erythritol. Significant weight loss ensued. Most of the loss can be attributed to limonene, however in the liquid state erythritol also has a non-negligible vapor pressure. At 100 C (in the super cooled state) pure erythritol in a pan with a surface area of 44 mm2 lost
25 Carbohydrate glass Erythritol encapsulate
20
Temperature
102
Encapsulate Pure Erythritol
100
200 180
140 120 100
96
80 94
60
Temperature (°C)
160
98 Weight (%)
between glassy and crystalline materials. If permitted some simplification, hydrophilic food glasses can be described as absorbing or desorbing water in order to bring their water activity to equilibrium with ambient relative humidity (Bell & Labuza, 2000; Sillick & Gregson, 2010). For a simple non-porous crystal, sorption occurs at the surface and continues when ambient relative humidity exceeds the water activity of the saturated solution (deliquescence). Thus, crystals have differently shaped moisture sorption isotherms from glasses. In general, crystals tend to be more resistant to moisture sorption than their corresponding glasses. For example, amorphous sucrose will absorb sufficient moisture to depress its glass transition temperature to below ambient on exposure to relative humidity above 24% (Roos & Karel, 1993), yet deliquescence of crystalline sucrose occurs at considerably higher relative humidity (w85%) (Yao, Yu, Lee, Yuan, & Schmidt, 2011). Crystalline erythritol is considered to be particularly nonhygroscopic (Embuscado & Patil, 2001). Fig. 5 shows a marked difference in sorption behavior between the crystalline encapsulate particles and an example of a typical glassy carrier (maltodextrin, GlucidexÒ IT19 from Roquette Corporation, Lestrem, France). While the initially glassy carrier continually adjusted its moisture content in response to the changing environment, the crystalline carrier did not. No significant sorption occurred for the erythritol encapsulate until relative humidity exceeded 92%.
111
40
92
20 90 0
200
400
600
0 800
Time (min) Fig. 6. TGA results for a erythritol encapsulate (formulated with 90 g/100 g erythritol, 9 g/100 g limonene and 1 g/100 g lecithin) and as-received erythritol. Volatilization is experienced only after melting the carrier.
weight at a rate of 0.71 mg/min. The encapsulate sample was melted within a similar pan. After an initial very rapid loss on melting, release became zero-order. Between 200 and 400 min the encapsulate sample, which was in the form of a melt emulsion, lost mass at a rate of 4.0 mg/min. The release rate then slowed as it became limited by the amount of limonene remaining in the system. 3.6. Active ingredient loading limit The encapsulation process was conducted with a range of oil loads from 10 to 40 g/100 g. No critical issues were observed related to the creation of encapsulate particles at 10 and 20 g/100 g oil regardless of active ingredient type. Some ingredients could be combined with the erythritol melt at even higher ratios; up to 30 g/ 100 g for methyl salicylate and cinnamic aldehyde and 35 g/100 g for NeobeeÒ. Eventually, each mixture ran into the same limiting phenomenon. When too much flavor oil was added, the mixture formed an erythritol-in-oil emulsion rather than an oil-inerythritol emulsion. This manifested in a notable increase in oil volatilization during the emulsification and spraying stages of the process and the powder after quenching comprised of very fine <1 mm erythritol spheres. Predictably, very little of the active ingredient remained encapsulated within this powder, most of it being lost in the quenching fluid.
Moisture Uptake (wt%)
3.7. Encapsulation efficiency 15
The amount of oil (flavor þ surfactant) retained within the powders was measured by TD-LF-NMR. Encapsulation efficiency (i.e. percentage of oil retained) is plotted in Fig. 7 as a function of loading level. NeobeeÒ has a high logP and as a triglyceride is essentially non-volatile. High encapsulation efficiency (about 90%) was observed at all loading levels. No influence of particle size was observed: both the large prills and the fine sprayed powder retained oil efficiently.
10
5
0
-5 0
600
1200 1800 2400 3000 3600 4200 Time (min)
Fig. 5. Moisture sorption behavior of a erythritol encapsulate (formulated with 90 g/ 100 g erythritol, 9 g/100 g limonene and 1 g/100 g lecithin) and a glassy carbohydrate carrier (maltodextrin).
3.7.1. Effect of volatility Cinnamic aldehyde and limonene were both retained efficiently when added at modest concentrations (w90% efficiency for a 10 g/ 100 g load). However, encapsulation efficiency decreased steadily with increasing oil load (to as low as w63% efficiency for 30 g/100 g loading of cinnamic aldehyde oil). The highly concentrated emulsions produced a noticeable plume of vapor during both homogenization and spraying. This suggests that volatilization of the active
112
M. Sillick, C.M. Gregson / LWT - Food Science and Technology 48 (2012) 107e113 100% Limonene Spray Limonene Prill
Encapsulation Efficiency
90%
Methyl Salicylate Spray Methyl Salicylate Prill Neobee Spray
80%
Neobee Prill Cinnamic Aldehyde Prill
70%
Nicotine Spray
60%
50%
40% 0
10
20 Oil Loading (g/100g)
30
40
Fig. 7. Encapsulation efficiency vs. active ingredient loading for various oil and particle types processed at 130 C.
ingredient was at least partially responsible for the poor encapsulation efficiencies observed. One way of reducing active ingredient volatilization is to reduce the processing temperature. Although erythritol crystallizes rapidly, it can be supercooled. It was possible to allow the emulsion and the entire spray vessel to cool by natural convection to 110 or even 100 C (well below the melting point of 121 C), yet maintain the erythritol in the liquid state. On pressurizing the chamber, all of the melt/emulsion was ejected through the needle to form droplets. “Supercooled” processing was found to provide a very significant improvement in oil retention, as shown in Fig. 8. Presumably, this is primarily due to reduced vapor pressure, which is a strong function of temperature. 3.7.2. Effect of logP A previous section argued that the crystallization of erythritol melts seemed remarkably tolerant of dispersed oil droplets. Molecular impurities may prove more problematic toward active ingredient retention. Low logP ingredients are likely to be soluble to a significant degree in the hot erythritol melt. This may slow crystal growth as impurities limit mass transfer, adsorb onto crystal surfaces, and/or disrupt the crystal lattice (Hartel, 2001). Slow crystallization, in turn, could lead to active ingredient loss if it impacts the degree to which droplets are excluded. Alternatively,
100%
Encapsulation Efficiency
95% 90% 85%
Cinnamic Aldehyde Prill 130 ° C
80%
Cinnamic Aldehyde Prill 100 ° C
75%
Limonene Prill 130 ° C
70% Limonene Prill 110 ° C
65% 60% 0
10
20 Oil Loading (g/100g)
30
40
Fig. 8. Chart illustrating the benefit of supercooled processing for encapsulation of cinnamic aldehyde and limonene.
the loss could occur by diffusion while the carrier is still in the liquid state. The prilled erythritol particles solidified as they sank through the quenching fluid. Solidification was noticeably slower for the particles made using methyl salicylate, cinnamic aldehyde, and (in particular) nicotine. Prills made using high logP oils solidified quickly and were sufficiently solid by the time they reached the bottom of the cylinder that they did not further deform or coalesce. Those made using methyl salicylate and cinnamic aldehyde showed some tendency to stick together at the bottom of the cylinder. Prills made with nicotine (logP ¼ 0.72) solidified very slowly (over several seconds). In order to avoid coalescence, it was found necessary to tilt the graduated cylinder thereby providing the particles extra time to slowly roll to the bottom. The nicotine prills also showed very poor oil retention (w50% was lost).
4. Conclusions Active ingredients were emulsified and sprayed with anhydrous erythritol carriers to yield dense powders. Inclusion of droplets within crystalline domains lead to efficient encapsulation (w90% in many cases), but loss increased for ingredients with high volatility and low logP. This unique approach offers processing advantages such as a very low melt viscosity and no drying requirement. Additionally, crystals can have distinct physical properties, which may provide alternative release mechanisms and performances compared to conventional glassy encapsulation systems.
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M. Sillick, C.M. Gregson / LWT - Food Science and Technology 48 (2012) 107e113 deposits (pp. 71e99). Victoria, British Columbia: Mineralogical Association of Canada. Mullen, J. W. (1972). Crystallization kinetics. In Crystallization (pp. 174e216). Cleveland, OH: CRC Press. Pamphlet from Carl Zeiss Light Microscopy. (2005). Michel Levy Color Chart Polarized Light Conoscopic Determination. Gottengen, Germany. Reineccius, G. A. (2001). The spray drying of food ingredients. In P. Vilstrup (Ed.), Microencapsulation of food ingredients (pp. 151e179). Surrey, England: Leatherhead Publishing. Roos, Y., & Karel, M. (1993). Effects of glass transition on dynamic phenomena in sugar containing food systems. In J. M. V. Blanshard, & P. J. Lillford (Eds.), The glassy state in foods (pp. 207e222). Loughborough: Nottingham University Press. Rudolph, P. (2008). Thermodynamics, origin and control of defects. In H. J. Scheel, & P. Capper (Eds.), Crystal growth technology: From fundamentals and simulation to large-scale production (pp. 73e96). Weinhein: Wiley-VCH Verlag GmbH & Co.
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Santomaso, A., Lazzaro, P., & Canu, P. (2003). Powder flowability and density ratios: the impact of granules packing. Chemical Engineering Science, 58, 2857e2874. Sillick, M., & Gregson, C. M. (2009). Viscous fragility of concentrated maltopolymer/ sucrose mixtures. Carbohydrate Polymers, 78, 879e887. Sillick, M., & Gregson, C. M. (2010). Critical water activity of disaccharide/maltodextrin blends. Carbohydrate Polymers, 79, 1028e1033. Stoiber, R. E., & Morse, S. A. (1994). Crystal identification with the polarizing microscope. New York: Chapman & Hall. Yao, W., Yu, X., Lee, J. W., Yuan, X., & Schmidt, S. J. (2011). Measuring the deliquescence point of crystalline sucrose as a function of temperature using a new automatic isotherm generator. International Journal of Food Properties, 14, 882e893. Zajc, N., Obreza, A., Bele, M., & Srcic, S. (2005). Physical properties and dissolution behaviour of nifedipine/mannitol solid dispersions prepared by hot melt method. International Journal of Pharmaceutics, 291, 51e58.
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LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt
Spray chill encapsulation of flavors within anhydrous erythritol crystals Matthew Sillick *, Christopher M. Gregson Firmenich, Inc., 250 Plainsboro Rd., Plainsboro, NJ 08536, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2011 Received in revised form 13 February 2012 Accepted 20 February 2012
Encapsulate powders are widely used in the food industry to control the release of liquid active ingredients, such as flavors. Spray chilling with a carrier composed of erythritol, an anhydrous sugar alcohol, was investigated as a novel alternative to existing technologies. The carrier entrapped droplets of the liquid actives and solidified via crystallization. The process was performed with active loadings as high as 35 g/100 g. Efficient retention was observed in many cases, but losses increased for more volatile and/or carrier miscible actives. The resulting powders had high bulk density and a free flowing character. The release characteristics were determined by the physical properties of the crystalline carrier. On heating, the actives were afforded significant protection from volatilization until melting of the erythritol at about 120 C. Sorption of moisture was minimal at relative humidity below 92%, at which point the carrier became deliquescent. This delivery system has unique performance characteristics, which may enable improved performance for certain food applications. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Erythritol Microencapsulation Flavor encapsulation Crystalline carrier
1. Introduction Valuable active ingredients, such as flavors, are often encapsulated within a continuous protective matrix in order to facilitate handling, prevent premature evaporation or provide protection against oxidation. Typically, this matrix is a carbohydrate or glycoprotein glass that has been formed either by spray drying or melt extrusion (Gouin, 2004). Glasses have performed successfully in no small part because they are homogenous and free of grain boundaries. This report considers an alternative route to encapsulation based on spray chilling and the crystallization of a nonaqueous carrier. Spray chilling (or spray congealing) is the process of atomizing a molten liquid and cooling the resulting droplets to form prills or powders that are solid at room temperature. Commonly spray chilled carriers for encapsulation include fats, waxes, polyethylene glycols, fatty acids and fatty alcohols (Kjaergaard, 2001). Such lipophilic materials are often miscible and hence poor barriers for many flavor compounds (Benczedi, 2002). The use of a nonmiscible matrix, such as a sugar alcohol, has the potential to overcome this limitation. Early efforts at using a lipophobic crystallizing carrier to entrap flavors were made with sorbitol (Dimick, 1959). Unfortunately, sorbitol melts crystallize slowly (over a time scale of hours) which
* Corresponding author. Tel.: þ1 609 580 4763. E-mail address: matthew.sillick@firmenich.com (M. Sillick). 0023-6438/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2012.02.022
hinders industrialization and makes particle formation by spray processing impractical. Spray chilling of a mannitol carrier, which crystallizes much more readily, has been reported in the pharmaceutical literature (Kanig, 1964; Zajc, Obreza, Bele, & Srcic, 2005). Perhaps in part because of the relatively high melting point of mannitol, adaptation of the process to encapsulate volatile flavors has not been widely described. A similar, but perhaps more suitable carrier might be meso-erythritol (C4H10O4), a linear sugar alcohol with a melting point of 121 C. Molten erythritol also has advantages in that it is a thermally and chemically stable liquid that crystallizes readily when supercooled or supersaturated (Lopes Jesus, Nunes, Ramos Silva, Matos Beja, & Redinha, 2010). This report demonstrates a process of entrapping flavor droplets within erythritol crystals to produce a powder or granular delivery system. Several factors that impact the degree of flavor retention are described. While the thermal and moisture sorption properties of the encapsulates are shown here, demonstration of improved performance within a chewing gum application was reported elsewhere (Gregson & Sillick, 2011). 2. Materials and methods Thermo-mechanical properties of erythritol (ZeroseÒ, Cargill Inc., Minnetonka, MN, USA) were assessed using a rheometer (AR2000) and calorimeter (Q200) both from TA Instruments (New Castle, DE, USA). Viscosity was measured under constant shear flow using the 40 mm 2 cone and plate geometry. Calorimetric phase transitions were observed on heating at 10 C/min in a hermetic
M. Sillick, C.M. Gregson / LWT - Food Science and Technology 48 (2012) 107e113
aluminum pan. The calorimeter had been calibrated for temperature with indium and heat capacity using sapphire discs according to instructions from the manufacturer. Five active ingredients were selected that spanned a range of volatility and hydrophobicity (Table 1): limonene, nicotine (both from TCI America, Portland, OR, USA), methyl salicylate (Alfa Aesar, Ward Hill, MA, USA), cinnamic aldehyde (MP Biomedicals, Solon, OH, USA), and NeobeeÒ M5 (Stepan Company, Northfield, IL, USA). Each was mixed with a surfactant at a 9:1 active:surfactant mass ratio. The surfactant was lecithin (YelkinÒ SS, Archer Daniels Midland Company, Decatur, IL, USA) for most cases. However, because lecithin showed only limited solubility in cinnamic aldehyde, a citric acid ester (GrindstedÒ CITREM, Danisco USA Inc., Fairfield, NJ, USA) was used in this case. These mixtures of active ingredients with surfactant are referred to hereafter as the “oil” phase. Each oil solution was then paired with the molten erythritol carrier at several selected mass ratios between 10 and 40 g/100 g. The two low viscosity fluids (the carrier and the oil) were then placed into a 150 mL pressurizable vessel, which was tempered to 130 C. Mixing was administered for approximately 30 s using a hand-held homogenizer (T25 UltraTurrax, Ika Works, Inc. Wilmington, NC, USA). The mixing element was removed from the vessel and the lid was closed. The melt emulsion was pushed through either a pressure nozzle (for creating a fine spray) or a syringe needle (to prill larger granules) under head nitrogen pressure of 70e350 kPa. The droplets fell approximately 10 cm before landing in a bath of quench fluid. Limonene was select as the quench fluid because it is immiscible with molten erythritol and could be tempered to 0 C to achieve rapid crystallization of the carrier. The hardened particles were collected and laid on an absorbent sheet overnight to allow the quench fluid to evaporate off. The resulting prills and spray powders were examined using an Olympus (Center Valley, PA, USA) BX51 optical microscope to determine encapsulate particle size, entrapped oil droplet size, and to characterize the encapsulate microstructure. Oil droplet size was also confirmed after reconstituting selected samples in water using a Beckman Coulter (Brea, CA, USA) LSÒ13 320 analyzer with the built in Mei model and polarization intensity differential scattering (PIDS) treatment. Moisture sorption was studied using an Surface Measurement Systems (Allentown, PA, USA) Advantage DVS system. Thermal gravimetric analysis was conducted using a TA Instruments Q50 system under flowing nitrogen. Samples were placed within a pan having side walls in order to achieve a roughly consistent surface area when the carrier was in the liquid state. Freely settled powder density was measured by weighing the amount of material necessary to fill a 50 mL graduated cylinder. Tapped bulk density was determined after hand tapping the cylinder 100 times. Retention of oil within encapsulate powders was determined by time-domain low-field nuclear magnetic resonance (TD-LF-NMR) as described by Andrade, Farhat, Aeberhardt, Normand, and Engelsen (2008). This technique measures the fraction of
a complex sample that has sufficient molecular mobility to respond to a change in magnetic field (Hafner, Dardelle, Normand, & Fieber, 2011). A Bruker Optics (Billerica, MA, USA) Minispec 20 MHz spectrometer was used to measure 4 scans of the 90 /180 spin-echo signal with a 3.5 ms delay time, 0.5 ms acquisition window and a 20 s recycle delay. The sample size was approximately 3 g. The spin-echo signal was proportional to the mass of the oil in the preparation and therefore could be calibrated using carefully measured mixtures of each specific active ingredient/surfactant solution and granular erythritol. 3. Results and discussion 3.1. Thermo-mechanical properties of erythritol Viscosity of the anhydrous erythritol melt was found to be 25 mPa s at 130 C and was independent of shear rate (i.e. Newtonian). This low value confirms that it should be possible to atomize molten erythritol by conventional methods such using a pressurized spray nozzle or prilling needle. Viscosity increased modestly on cooling at a constant shear rate of 10 s1 as shown in Fig. 1 and was 725 mPa s at 60 C. The data were fit by a VogeleFulchereTamman trend, which along with its equivalent rearrangement the WLF relation, are commonly used equations for describing the temperature dependence of relaxation within liquids (Angell et al., 1994; Sillick & Gregson, 2009). Below 60 C, the supercooled melt crystallized. Crystallization involves a sudden decrease in free volume. The results show the ensuing departure from the smooth VFT trend. As crystallization progressed, the material took on a white opaque appearance and the apparent viscosity soon exceeded the limit of the instrument. Erythritol was quenched from above its melting point and reheated to give the DSC curve shown in Fig. 2. A glass transition is seen in the range 44 to 40 C, which agrees with previously reported values (Lopes Jesus et al., 2010). Such a low Tg provides confidence that any solid encapsulate particles observed at room temperature must be predominantly crystalline and not glassy. With further reheating, a cold crystallization occurred with onset
1000000 100000 Melt Viscosity (mPa.s)
108
10000 1000 100 10
Table 1 Vapor pressure and logP of active ingredients used in this study.
1
Ingredient
VP at 25 Cb (Pa)
VP at 130 Ca (Pa)
LogPb
Nicotine Methyl salicylate Cinnamic aldehyde NeobeeÒ (octanoic triglyceride) Limonene
4.0 9.3 3.5 8.8*1025 2.1*102
2.0*103 3.9*103 1.8*103 7.2*1014 2.5*104
0.7 2.2 2.1 25 4.5
a Calculated based on VP at 25 C and boiling point using the ClausiuseClapeyron equation. b Taken from an online resource (chemspider.com, 2011).
40
60
80 100 120 Temperature (°C)
140
160
Fig. 1. Melt viscosity of erythritol vs. temperature. Below 60 C the sample crystallized and experimental data (>) departed from the VFT trend line. VFT fit : log h ¼ log ho þ ðB=ðT To ÞÞ log ho ¼ 4:625 B ¼ 640 To ¼ 190:5 K:
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109
Fig. 2. DSC scan of erythritol on reheating at 10 C/min after quenching the melt.
at 3 C and peak in the heat flow signal at 7 C. The quenching temperature for encapsulate processing was chosen to fall in this range. 3.2. Encapsulate particle size and morphology The morphology of the encapsulate particles is decidedly spherical rather than faceted (Figs. 3 and 4). Thus, crystal growth appears to have continued to the boundaries of the preceding melt emulsion droplets without dramatic changes in shape or size. Given the high degree of supercooling and rapid crystallization, crystal growth might be expected to proceed in a normal and continuous (rather than lateral and stepwise) mode (Cahn, Hillig, & Sears, 1964). Similar spherical shapes are reported in other crystalline
Fig. 4. Fine crystalline particles form by spraying (A) pure erythritol and (B) an erythritol/limonene emulsion (90 g/100 g erythritol, 9 g/100 g limonene and 1 g/100 g lecithin) as imaged by slightly uncrossed polarized light. While several gas bubbles are seen within one of the particles of A, particles in B also contain a multitude of fine (w1 mm) oil droplets dispersed throughout the structure.
Fig. 3. Spherically shaped crystalline encapsulate particles formed by prilling. The encapsulate particles were formulated with 90 g/100 g erythritol, 9 g/100 g limonene and 1 g/100 g lecithin.
particles formed by melt spraying, for example artificial snow as well as industrially spray chilled materials (e.g. fertilizer prills, KOH prills and certain metal powders) (Kjaergaard, 2001). The size of the particles was dependent on the atomization technology and process conditions. The particles sprayed through a pressure nozzle tended to have diameters in the range 40e300 mm (based on optical microscopy). The prills, which broke up into droplets after being ejected from a dispensing needle, were larger (500 mme2 mm). A “ballooning” or vacuole formation phenomenon can lead to low bulk densities within spray dried flavor encapsulates (Reineccius, 2001). Bulk densities reported in Table 2 are high and similar to that of as-received erythritol, which suggests that the particles did not contain large internal voids. This theory was largely corroborated by transmitted light microscopy. As the encapsulation within erythritol does not involve volatilization of water, vacuole formation should be largely avoided. However, some porosity was noted in the largest prill particles (generally those
110
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Table 2 Average bulk and tap density of crystalline encapsulate particles for all active ingredients types and loading levels. Bulk density (tapped) (g/mL)
Hausner ratio
Crystalline encapsulates Prills (avg.) Spray powders (avg.)
0.82 0.06 0.78 0.06
1.06 0.01 1.20 0.08
As-received granular erythritol
0.90
1.15
>1.5 mm). These particles tended to have a small hole on their surfaces which lead to a moderately sized internal void. This is likely to be due to the sudden decrease in specific volume upon crystallization. Kjaergaard (2001) has described similar indentations and pores that form because of constrained shrinkage for other types of coarse prills. The Hausner ratio (freely settled density/tapped density) is an empirical metric of the flow characteristics of a powder or granular material (Grey & Beddow, 1969). Powders which have values closer to 1 are often less cohesive and more flowable. The larger prill particles were particularly free flowing and had Hausner ratio values even lower than that of as-received granular erythritol. The finer spray encapsulate powders were less free flowing, yet still had Hausner ratio values less than 1.25. Sparingly flowable powders, by comparison, are typified by values >1.4 (Santomaso, Lazzaro, & Canu, 2003).
3.3. Microstructure and inclusion of oil droplets Many fine oil droplets were observed dispersed throughout the crystalline encapsulate particles. Ensuring that the dispersed oil droplets are suitably small compared to the size of the encapsulate particle is likely to be important in order to minimize active ingredient loss during particle formation. The emulsification protocol was sufficient to produce very fine oil droplets. By microscopy, the mean size appeared to be between 1 and 5 mm. A light scattering measurement of a 20 g/100 g limonene encapsulate powder reconstituted in water showed a d4,3 mean droplet size of 3.05 mm and standard deviation of 1.58 mm. Successful entrapment of the active ingredient within the encapsulate microstructure requires that the oil droplets be accommodated within the crystallizing domain. However, crystallization is often considered to be a purification process involving phase separation and exclusion of impurities. For example, one can imagine active ingredient loss occurring by a process akin to freeze concentration in ice cream; ice crystals form and physically exclude fats, sugars and proteins enriching their concentrations in the interstitial regions. If oil droplets are similarly excluded from a developing erythritol crystal, they too would concentrate and potentially coalesce or even form large channels in the spaces between erythritol crystals. Surprisingly, exclusion can be avoided to a significant extent due to the sufficiently rapid crystallization of erythritol. Evidence for this comes from polarized light microscopy. Fig. 4 shows several small particles formed by spraying pure erythritol and an erythritol/limonene melt emulsion. Fine droplets are seen throughout (B) the encapsulate particles but are not present in (A) the pure erythritol particles. Meso-erythritol forms a tetragonal crystal structure (Bokoe & Powell, 1959) and is therefore birefringent. The particles have concentric and continuous patterns of polarization colors. The order of the colors follow the classical Michel-Levy chart (Pamphlet from Carl Zeiss Light Microscopy, 2005). The color is determined by the phase difference which is the product of
birefringence and particle thickness. Because the particles are spherical and (unlike flat discs) vary in thickness, the colors progress steadily toward the interior. Superimposed on the interference colors is an intensity component. If a crystal is rotated in the field of view, the color intensity varies cyclically from zero at “extinction” when the optic axis of the crystal is parallel to one of the polarizers of the microscope to a maximum when oriented at 45 to the polarizers (Stoiber & Morse, 1994). When rotated on the microscope stage, the entire particle (Fig. 4B) cohesively progresses through maxima and minima in polarization brightness. This implies a continuous orientation of crystallographic axes throughout such particles. Continuous orientation, in turn, indicates that such encapsulate particles are either highly imperfect single crystals or otherwise aligned domains (such as occurs with meso-crystals). If the particles were polycrystalline and randomly oriented, one would expect a more heterogeneous appearance. For example, Fig. 4A shows particles that appear to consist of two unaligned domains. The polarization colors are less bright within the domain closer to extinction. Inclusion is a well known process by which a micron-scale impurity becomes incorporated within a host crystal. In this sense, inclusion differs from other modes of association such as entrapment (in which an active is surrounded by many crystals), molecular inclusion (e.g. cyclodextrin complexation), and solidsolutions (in which guest molecules are accommodated within lattice sites of a host crystal). The presence of droplet or void inclusions within single crystals is common both in geology and various crystallization industries (Rudolph, 2008). For example, Lowenstern (1995) stated that “as many as scores of inclusions” are often found within a single quartz crystal. According to Mullen (1972), inclusions tend to be incorporated under conditions of rapid crystal growth and inhibited diffusion. Meso-crystals are superstructures of parallel or aligned nanoparticles, which by many conventional analyses, including optical microscopy, appear as single crystals (Colfen & Antonietti, 2005). One pertinent aspect of these newly recognized materials is their potential to incorporate significant amounts of impurities such as macromolecules within their superstructure. Incorporation of droplets or larger bodies has not yet been widely described. Both scenarios (inclusion within single crystals and meso-crystalline domains) provide routes for transforming the system from a liquid melt emulsion to a solid crystalline body without the displacement of oil droplets. Large crystals are rare in nature and tend to form over long timescales, growth occurring very slowly. Regarding encapsulation within erythritol, there may be some limit to the size of crystals that is both chemistry and process dependent. While the small sprayed particles (<100 mm), such as those shown in Fig. 4, often appear to consist of a single oriented domain, larger particles were always polycrystalline. The domain “grain size” (which is approximately 200 mm on average) is large compared to the typical size of oil droplets (w3 mm) and thus permits a significant degree of inclusion. Unfortunately, with increasing particle size, the interior becomes increasingly obscured and so determining precisely the degree to which oil exists within or between crystals becomes impossible on intact particles by visual techniques. Crushing the large particles and examining the resulting fragments showed that droplets were also included within crystalline domains inside the particles. Large oriented sections with included oil droplets were observed. 3.4. Moisture sorption Sorption of moisture from humid environments is an example of a property for which there are clear mechanistic differences
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3.5. Thermal stability A sprayed encapsulate powder made with 9 g/100 g loading of limonene was examined by TGA and the result is shown in Fig. 6. The sample was ramped slowly to 100 C, during which time essentially no weight was lost. Any “free” or accessible limonene was apparently removed during the solvent removal step prior to the analysis. The remaining limonene appears to be trapped and resists volatilization. The sample was then warmed quickly to 125 C to melt the erythritol. Significant weight loss ensued. Most of the loss can be attributed to limonene, however in the liquid state erythritol also has a non-negligible vapor pressure. At 100 C (in the super cooled state) pure erythritol in a pan with a surface area of 44 mm2 lost
25 Carbohydrate glass Erythritol encapsulate
20
Temperature
102
Encapsulate Pure Erythritol
100
200 180
140 120 100
96
80 94
60
Temperature (°C)
160
98 Weight (%)
between glassy and crystalline materials. If permitted some simplification, hydrophilic food glasses can be described as absorbing or desorbing water in order to bring their water activity to equilibrium with ambient relative humidity (Bell & Labuza, 2000; Sillick & Gregson, 2010). For a simple non-porous crystal, sorption occurs at the surface and continues when ambient relative humidity exceeds the water activity of the saturated solution (deliquescence). Thus, crystals have differently shaped moisture sorption isotherms from glasses. In general, crystals tend to be more resistant to moisture sorption than their corresponding glasses. For example, amorphous sucrose will absorb sufficient moisture to depress its glass transition temperature to below ambient on exposure to relative humidity above 24% (Roos & Karel, 1993), yet deliquescence of crystalline sucrose occurs at considerably higher relative humidity (w85%) (Yao, Yu, Lee, Yuan, & Schmidt, 2011). Crystalline erythritol is considered to be particularly nonhygroscopic (Embuscado & Patil, 2001). Fig. 5 shows a marked difference in sorption behavior between the crystalline encapsulate particles and an example of a typical glassy carrier (maltodextrin, GlucidexÒ IT19 from Roquette Corporation, Lestrem, France). While the initially glassy carrier continually adjusted its moisture content in response to the changing environment, the crystalline carrier did not. No significant sorption occurred for the erythritol encapsulate until relative humidity exceeded 92%.
111
40
92
20 90 0
200
400
600
0 800
Time (min) Fig. 6. TGA results for a erythritol encapsulate (formulated with 90 g/100 g erythritol, 9 g/100 g limonene and 1 g/100 g lecithin) and as-received erythritol. Volatilization is experienced only after melting the carrier.
weight at a rate of 0.71 mg/min. The encapsulate sample was melted within a similar pan. After an initial very rapid loss on melting, release became zero-order. Between 200 and 400 min the encapsulate sample, which was in the form of a melt emulsion, lost mass at a rate of 4.0 mg/min. The release rate then slowed as it became limited by the amount of limonene remaining in the system. 3.6. Active ingredient loading limit The encapsulation process was conducted with a range of oil loads from 10 to 40 g/100 g. No critical issues were observed related to the creation of encapsulate particles at 10 and 20 g/100 g oil regardless of active ingredient type. Some ingredients could be combined with the erythritol melt at even higher ratios; up to 30 g/ 100 g for methyl salicylate and cinnamic aldehyde and 35 g/100 g for NeobeeÒ. Eventually, each mixture ran into the same limiting phenomenon. When too much flavor oil was added, the mixture formed an erythritol-in-oil emulsion rather than an oil-inerythritol emulsion. This manifested in a notable increase in oil volatilization during the emulsification and spraying stages of the process and the powder after quenching comprised of very fine <1 mm erythritol spheres. Predictably, very little of the active ingredient remained encapsulated within this powder, most of it being lost in the quenching fluid.
Moisture Uptake (wt%)
3.7. Encapsulation efficiency 15
The amount of oil (flavor þ surfactant) retained within the powders was measured by TD-LF-NMR. Encapsulation efficiency (i.e. percentage of oil retained) is plotted in Fig. 7 as a function of loading level. NeobeeÒ has a high logP and as a triglyceride is essentially non-volatile. High encapsulation efficiency (about 90%) was observed at all loading levels. No influence of particle size was observed: both the large prills and the fine sprayed powder retained oil efficiently.
10
5
0
-5 0
600
1200 1800 2400 3000 3600 4200 Time (min)
Fig. 5. Moisture sorption behavior of a erythritol encapsulate (formulated with 90 g/ 100 g erythritol, 9 g/100 g limonene and 1 g/100 g lecithin) and a glassy carbohydrate carrier (maltodextrin).
3.7.1. Effect of volatility Cinnamic aldehyde and limonene were both retained efficiently when added at modest concentrations (w90% efficiency for a 10 g/ 100 g load). However, encapsulation efficiency decreased steadily with increasing oil load (to as low as w63% efficiency for 30 g/100 g loading of cinnamic aldehyde oil). The highly concentrated emulsions produced a noticeable plume of vapor during both homogenization and spraying. This suggests that volatilization of the active
112
M. Sillick, C.M. Gregson / LWT - Food Science and Technology 48 (2012) 107e113 100% Limonene Spray Limonene Prill
Encapsulation Efficiency
90%
Methyl Salicylate Spray Methyl Salicylate Prill Neobee Spray
80%
Neobee Prill Cinnamic Aldehyde Prill
70%
Nicotine Spray
60%
50%
40% 0
10
20 Oil Loading (g/100g)
30
40
Fig. 7. Encapsulation efficiency vs. active ingredient loading for various oil and particle types processed at 130 C.
ingredient was at least partially responsible for the poor encapsulation efficiencies observed. One way of reducing active ingredient volatilization is to reduce the processing temperature. Although erythritol crystallizes rapidly, it can be supercooled. It was possible to allow the emulsion and the entire spray vessel to cool by natural convection to 110 or even 100 C (well below the melting point of 121 C), yet maintain the erythritol in the liquid state. On pressurizing the chamber, all of the melt/emulsion was ejected through the needle to form droplets. “Supercooled” processing was found to provide a very significant improvement in oil retention, as shown in Fig. 8. Presumably, this is primarily due to reduced vapor pressure, which is a strong function of temperature. 3.7.2. Effect of logP A previous section argued that the crystallization of erythritol melts seemed remarkably tolerant of dispersed oil droplets. Molecular impurities may prove more problematic toward active ingredient retention. Low logP ingredients are likely to be soluble to a significant degree in the hot erythritol melt. This may slow crystal growth as impurities limit mass transfer, adsorb onto crystal surfaces, and/or disrupt the crystal lattice (Hartel, 2001). Slow crystallization, in turn, could lead to active ingredient loss if it impacts the degree to which droplets are excluded. Alternatively,
100%
Encapsulation Efficiency
95% 90% 85%
Cinnamic Aldehyde Prill 130 ° C
80%
Cinnamic Aldehyde Prill 100 ° C
75%
Limonene Prill 130 ° C
70% Limonene Prill 110 ° C
65% 60% 0
10
20 Oil Loading (g/100g)
30
40
Fig. 8. Chart illustrating the benefit of supercooled processing for encapsulation of cinnamic aldehyde and limonene.
the loss could occur by diffusion while the carrier is still in the liquid state. The prilled erythritol particles solidified as they sank through the quenching fluid. Solidification was noticeably slower for the particles made using methyl salicylate, cinnamic aldehyde, and (in particular) nicotine. Prills made using high logP oils solidified quickly and were sufficiently solid by the time they reached the bottom of the cylinder that they did not further deform or coalesce. Those made using methyl salicylate and cinnamic aldehyde showed some tendency to stick together at the bottom of the cylinder. Prills made with nicotine (logP ¼ 0.72) solidified very slowly (over several seconds). In order to avoid coalescence, it was found necessary to tilt the graduated cylinder thereby providing the particles extra time to slowly roll to the bottom. The nicotine prills also showed very poor oil retention (w50% was lost).
4. Conclusions Active ingredients were emulsified and sprayed with anhydrous erythritol carriers to yield dense powders. Inclusion of droplets within crystalline domains lead to efficient encapsulation (w90% in many cases), but loss increased for ingredients with high volatility and low logP. This unique approach offers processing advantages such as a very low melt viscosity and no drying requirement. Additionally, crystals can have distinct physical properties, which may provide alternative release mechanisms and performances compared to conventional glassy encapsulation systems.
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