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Product engineering of dispersed systems
Trends in Food Science & Technology 14 (2003) 9–16
Review
Product engineering of dispersed systems Helmar Schubert*, Karin Ax and Olaf Behrend University of Karlsruhe, Institute of Food Process Engineering, D-76128 Karlsruhe, Germany (Tel.: +49-721-608-2497; fax: +49-721-694-320; e-mail: [email protected])
Product engineering is the design of products with desired properties using the methods of process engineering. To reduce the number of process parameters, necessary to obtain the desired properties, a characteristic feature of the respective product is helpful. For a dispersed system this characteristic feature is its complex structure on a microscale. This micro-structure is the link between the process and the properties of the final product. The principles of product engineering of dispersed systems are explained and demonstrated by means of different examples: Though there are limits to this concept, certain properties of dispersed systems can be obtained by determining and applying property and process functions. # 2003 Elsevier Science Ltd. All rights reserved.
Introduction Dispersed systems, e.g. emulsions, foams, suspensions or powders, are commonly found in many branches of industries, ranging from chemical products to cosmetics, pharmaceuticals, food and innovative materials like metal and ceramic foams. Dispersions, in general, are systems with a characteristic structure on a micro-scale, which determines many of their physical properties and thus the quality and functionality of the product; their macroscopic appearance is often homogeneous, though. Stability, rheology, sensory and optical properties or * Corresponding author.
mass transfer kinetics are only some of the properties, which are largely influenced by the micro-structure of a dispersed system. The correlation between a specific property and the micro-structure is called property function (Rumpf, 1967) (Fig. 1). The use of property functions only makes sense, if the main properties are clearly determined by the micro-structure. For dispersed systems this condition is usually fulfilled (Borho, Polke, Wintermantel, Schubert, & Sommer, 1991). Often, the micro-structure can be sufficiently described by a small number of characteristic parameters (e.g. mean values). In this case, properties are independent of the process conditions used to obtain these structural parameters. The structure is determined by the choice of ingredients as well as the preparation process. Changes in the recipe are often limited by functional demands of the final product or they are subject to legal restrictions. Knowledge about the, often cruical, influence of process parameters on structural characteristics is, therefore, indispensible. The desired micro-structure can be obtained by the choice of suitable process conditions, the relation between both is described by the process function (Borho et al., 1991; Polke, 1995). Thus, product engineering of dispersed systems means the design of products with desired properties, taking into account the methods of process or chemical engineering. The micro-structure of the system may be used as linking factor between the process and product properties.
Examples of product engineering with practical relevance Liquid dispersions: emulsions Emulsions are dispersions of two or more insoluble liquids, e.g. water and oil, with the dispersed phase distributed in form of small droplets in the continuous phase. From a thermodynamic point of view such systems are unstable. As a consequence, energy input is required in the dispersion step, e.g. mechanical energy for droplet disruption. Afterwards, the dispersed state obtained has to be stabilized over the desired period of storage and usage of the respective product. Both dispersion and subsequent stabilization determine mean droplet size and droplet size distribution which are important characteristics of the emulsion’s micro-structure. Dispersion is most commonly obtained by means of different mechanical emulsification devices, the most important of which are shown in Fig. 2 (Schubert & Ax,
0924-2244/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S 0 92 4 - 2 24 4 ( 0 2 ) 0 0 24 5 - 5
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Fig. 1. Product engineering of dispersed systems. Explanation see text.
Fig. 2. Processes and technical equipment for emulsification. For further information see Schubert and Behrend (2000).
2001). A general distinction has to be made between systems with droplet disruption starting from a coarse premix (rotor-stator, high-pressure and ultrasound systems) and direct dispersion, i.e. droplet formation by means of micro-porous structures. In the former type of apparatus, mechanical energy is dissipated in the liquid in turbulent or laminar flow or by cavitation. The resulting inertial or viscous forces acting on dispersed droplets cause their deformation and break-up. An alternative process is the direct dispersion of the internal phase by droplet formation at microporpous membranes (Mine, Shimizu, & Nakashima, 1996; Schro¨der, 1999). The liquid to be dispersed passes the membrane and forms droplets at its surface, which are detached into the flowing continuous phase.
A process similar to membrane emulsification is the so-called microchannel emulsification (Kawakatsu, Kikuchi, & Nakajima, 1997). Instead of membranes specifically manufactured elements with channels of well defined geometry in the range of several micrometers are applied. This method allows production of practically monodispersed emulsions (Tong, Nakajima, Nabetani & Kikuchi, 2000). This is probably due to a droplet detachment mechanism governed by interfacial instabilities. In emulsification devices usually employed, droplet disruption of a coarse premix is induced by various mechanisms. Shear forces in laminar flow and shear or inertial forces in turbulent flow are predominant in case of rotor-stator systems (Karbstein, 1994; Stang, 1998). In high-pressure systems the efficiency of droplet disruption
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largely depends on the geometry of the homogenizing valve: Depending on the hydrodynamic conditions elongational forces in laminar flow and/or turbulence may cause droplet deformation and disruption. Furthermore, liquid cavitation may provide conditions, which contribute to the disruption. In ultrasound fields cavitation is the main effect, which leads to droplet break-up (Behrend & Schubert, 2001). Depending on the respective predominant effect, droplet disruption is influenced by several physical parameters, like viscosity of the dispersed and continuous phase, interfacial tension or density of the liquids (Walstra, 1993; Walstra & Smulders, 1997). Besides, power density (power input per unit volume) and residence time of the emulsion in the dispersing zone of a continuously operated apparatus are key process parameters which influence the emulsification result (Koglin, Pawlowsky, & Schno¨ring, 1981). Based on the results of Koglin et al. (1981) and in good agreement with results for solid particle comminution (Weit & Schwedes, 1986) and cell disruption (Bunge & Schwedes, 1993) the concept of energy density was proved suitable for droplet disruption in continuous mechanical emulsification (Karbstein, 1994; Schubert & Karbstein, 1994): According to experimental results the mean droplet diameter of an emulsion can be described as function of both power density and residence time by a power law (eqn 1). The characteristic exponents are often similar. Thus, the product of both parameters, i.e. the energy density (energy input per unit volume), is the determining process parameter with respect to droplet disruption. b2 1 x3;2 / Pb ðPV tr Þb ¼ Eb V V tr
ð1Þ
where x3;2 volume-surface mean droplet size (Sauter diameter), ð1 x3;2 ¼ xq2 ðxÞ dx 0
q2 ðxÞ surface frequency distribution, PV power density, tr residence time, EV energy density, b1 ; b2 ; b dimensionless characteristic exponents. Eqn 1 is a practical approximation for rotor-stator systems (Karbstein, 1994) and ultrasound homogenizers (Behrend, 2002). For high-pressure homogenizers (homogenizing pressure pH=EV) and droplet disruption in laminar shear or elongational flow eqn 1 can be derived from theoretical considerations (Schubert & Ax, 2001; Walstra & Smulders, 1997). The value of the characteristic exponent contains information about the droplet disruption mechanism. Typically, values are found between b=0.35 (turbulent inertial forces) and b=1 (laminar shear or elongational flow). Membrane emulsification is an exception, where often b > 1 (Schro¨der, 1999).
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Eqn 1 relates an important structural characteristic of the emulsion, volume-surface mean droplet size, to a process parameter, energy density, and is, therefore, an important example for an explicit process function in terms of the concept given in Fig. 1. Besides the possibility to estimate the droplet size to be expected in a certain emulsification process, energy density is a useful parameter for scaling-up purposes in practice and to compare droplet disruption efficiencies of different apparatus. Fig. 3 gives typical emulsification results for different mechanical apparatus in terms of the Sauter diameter of the dispersed phase as function of energy density. The expected linear relation between both on a log-log scale is approximately found for all systems. Nevertheless, at constant energy density there are significant differences between the apparatus. The lowest efficiency of disruption for this emulsion system is obtained with the colloid-mill as typical rotorstator system. Better results are found for the highpressure systems, though the different valve geometries give results, varying with a factor of 10 or more in droplet size. The ultrasound-system provides disruption with an efficiency close to that of the better high-pressure systems. Due to the specific mechanism of step-wise droplet disruption in the cavitating sound field typical droplet size distributions for systems of the latter type are often broad and bimodal (Behrend & Schubert, 2001). In terms of energetic efficiency the membrane emulsification gives the best results for low dispersed phase contents and fine-dispersed emulsions (Schro¨der, 1999). This is due to the fact, that droplets do not have to be disrupted, but are formed by direct dispersion at the pores of the membrane. Energy is needed only to overcome capillary effects in the membrane structure and to detach the droplets from the pores by the current of the continuous phase. Droplet size is mainly determined by the pore size of the membrane and the (dynamic) interfacial tension of the system (Schro¨der, Behrend, & Schubert, 1998; Schro¨der & Schubert, 1999). As shown in Fig. 3, the efficiency of membrane emulsification is largely determined by the dispersed phase (volume) fraction ’. For all other mechanical emulsification systems discussed here, the emulsification result is independent of ’, as long as droplet coalescence during and shortly after the disruption step is avoided due to the choice of an suitable emulsifier and adequate process conditions. Fig. 3 is a summary and comparison of some important process functions for the respective apparatus and may serve as part of the basis for product engineering of emulsions in practice. As an example, emulsion formulations are often used as drug delivery or nutritive systems in parenteral applications. To avoid the danger of particle induced embolisms, emulsion droplet size distribution must not exeed a certain maximum droplet diameter. An indispensible quality aspect or property of
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Fig. 3. Comparison of emulsification results for different apparatus.
such a product is, therefore, its safe applicability. The respective property function be expressed by the simple condition (eqn 2) xmax < 2 mm
ð2Þ
A combination of the empirical relation between maximum and mean droplet size found by Armbruster (1990) (eqn 3) xmax / x3;2
ð3Þ
and the concept of energy density allows to choose a suitable process (Fig. 3) and process conditions (eqn 1). Another important application of oil-in-water emulsions is the use as carrier systems for lipophilic substances such as carotenoids. Carotenoids are natural pigments occuring in many fruit and vegetables. Epidemiological studies indicate that various positive health effects are correlated with a high intake of carotenoid rich food (Bendich & Olson, 1989; Mayne, 1996). As carotenoids are insoluble in water and have only a very limited solubility in vegetable oil, they are often only poorly absorbed in the intestine. The fraction of carotenoids taken up from food strongly depends on their physical form. Carotenoids dissolved in vegetable oil are considered as ideal for absorption, whereas only a small amount of carotenoids is taken up from raw vegetables (Parker, 1997; West & Castenmiller, 1998; Williams, Boileau, & Erdman, 1998). In the following, the fraction of carotenoids absorbed from food will be called bioavailability. Carotenoids can be formulated in dispersed systems in order to obtain a continuous aqueous phase and to
enhance bioavailability. In an aqueous suspension of carotenoid crystals, bioavailability depends on particle size. Decreasing particle size results in a better uptake of carotenoids (Horn, 1989; Kolk, 1974). This correlation can be considered as property function. With regard to good bioavailability, oil-in-water emulsions with carotenoids dissolved in the dispersed oil phase are promising formulations. These emulsions are prepared by dissolving carotenoid crystals in vegetable oil at high temperatures and subsequently dispersing the hot lipophilic phase in the aqueous phase (Ax, Schubert, Briviba, Rechkemmer, & Tevini, 2001). Oil-in-water emulsions could combine the advantages of the good bioavailability of an oily carotenoid solution and applicability as water-dispersible system. Moreover, in the dispersed oil droplets crystallization of carotenoids is inhibited and thus they can be kept in supersaturated solution (Bunnell, Driscoll, & Bauernfeind, 1958; Schweikert & Kolter, 1997). It can be assumed that the maximum supersaturation increases with decreasing droplet size, because nucleation is less likely to occur. In order to prevent crystallization of carotenoids in the oil droplets, the property function is the correlation that links the propability of carotenoid crystals with droplet size at a certain concentration. The process function, that is necessary to prepare a carotenoid emulsion of that droplet size, is given by the concept of energy density (eqn 1). The principle of product engineering can, therefore, be applied for preparing an emulsion with carotenoids in supersaturated solution in the oil droplets. The concept of energy density and the refering eqn 1 describes only droplet disruption in an emulsification process. The dispersed state of the final product is also determined by the extend of droplet coalescence superimposing
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the disruption in the apparatus (Armbruster, 1990; Karbstein, 1994). The extent of coalescence depends on the hydrodynamic conditions in the apparatus, but largely on the stabilizing properties of the surface active ingredients used, i.e. the emulsifier. A method allowing to determine the stabilizing properties of emulsifiers qualitatively under conditions close to practice was developed by Danner, Heubel, Schubert, Polke, and Stang (2000). In this so-called colouring method two emulsion premixes are produced under similar conditions, with the dispersed phase coloured with blue or yellow dye. A mix of equal parts of both systems is processed in a discontinuous emulsification apparatus. Samples of the product are taken at different emulsification times and examined by means of a microscope and a image analysis software. As a result of coalescence the amount of mixed-coloured droplets (green) rises as function of time. The kinetics of this colour shift allow to assess the stabilizing properties of different emulsifiers. Figure 4 shows the experimental results for two typical food emulsifiers. Egg yolk, which contains proteins and phospholipids as surface active components, is typically used for mayonnaise-type products and gives good long-term stability (during storage), but poor shortterm stability (Karbstein, 1994). Tween 80 (E 433) is a low molecular weight surfactant with good stabilizing properties in the short-term range (in the process). In case of the egg yolk-stabilized emulsion the area amount of mixed-coloured (green) droplets increases significantly faster than for the system containing Tween 80. This is in good agreement with results of continuous mechanical emulsification, which indicate a significant influence of coalescence for the referring system with egg yolk (Karbstein, 1994; Stang, 1998).
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Though the concept of energy density holds for the droplet disruption independently of the surfactant applied, its practical value as process function also depends on the efficiency of droplet stabilization due to the adequate choice of emulsifying agents. Recently, the colouring method has been improved by using a theory which allows to calculate coalescence probability of droplets (see Fig. 4) on the basis of experimental data (Danner, 2001). Thus, the method is suitable to determine the short-term stabilizing properties of emulsifiers quantitatively under conditions close to practice.
Powdered products Powdered products are another group of dispersed systems with large practical relevance. Most important quality characteristics of powders are handling aspects like flow behaviour or dust prevention, and, as many powders are intermediate products, instant properties, i.e. wetting behaviour, dispersability or solubility. These properties are, again, largely determined by the microstructure of the powdered product. Fine powders usually have poor wetting properties, which can be significantly improved by the formation of agglomerates. Main structural characteristics are primary particle size, agglomerate size, porosity of agglomerates and the porosity of the powder bulk as well as the binding structures between the particles (liquid or solid bridges, van-der-Waals forces). The nonstationary wetting kinetics of a liquid for a certain nonswelling and unsoluble powder bulk is characterized by the specific wetting time, which is a function of physical and structural parameters of the powder (eqn 4) (Schubert, 1993):
Fig. 4. Stabilizing properties of different emulsifiers; results obtained by the colouring method (Danner et al., 2000).
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twet ¼ 15
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1" 1 l H 2 " x l coseff
ð4Þ
with twet wetting time, " porosity of the powder bulk, x particle (agglomerate) size, l liquid viscosity, l surface tension of the liquid, eff effective contact angle, H wetted powder height. Eqn 4 describes an explicit property function for instant products. Decreasing wetting times are obtained, if agglomerate size and bulk porosity, which must not exceed a critical value (Schubert, 1978), are increased. Generally, an agglomerate size in the range of xaggl > 250 ::: 300 mm
ð5Þ
gives satisfying instant properties for many practically relevant food products (Schubert, 1993; Wollny, 2002). For agglomerates of this size, the condition "bulk < "bulk;crit
ð6Þ
is usually fulfilled, as attractive van-der-Waals forces are too small compared to the agglomerate weight, to form a loose bulk structure. Subsequent to the wetting of the powder bulk wetting of the single agglomerates, desagglomeration and dispersion of the primary particles is required. Here, the structural characteristics of the agglomerates are the dominating factors. Optimal porosity and pore structure are required for a fast wetting without gas trapping in the interior of the agglomerates (Schubert, 1978, 1993). Solid bridges of an easily soluble material (e.g. salt, sugar) as bindig structures between primary particles provide good dispersability. If the powder material tends to swell or is soluble in the dispersing liquid, this has to be taken into account for the description of property functions. A swelling process as well as partial dissolution alters the bulk structure of the powder and may cause interruption of the wetting process at a critical powder height in the former case (Wollny, 2002). Jet agglomeration is an example for an appropriate method to design agglomerates of a specific structure (Hogekamp, 1999; Wollny & Schubert, 2000). Loose pre-agglomerates of the initial powder are formed in the dosing device, e.g. a vibrating channel, and enter the apparatus in free fall. In the wetting zone the particles mix with steam, which partly condenses on their surface and forms liquid bridges between the primary particles. Soluble components of the powder dissolve in the liquid and form solid bridges, as soon as the water is removed in the subsequent drying zone. In this process, the micro-structure of the product is determined mainly in the pre-agglomeration step; the subsequent treatment gives the desired agglomerate strength (Wollny, 2002; Wollny & Schubert, 2000).
Typical results for characteristic wetting times of different fractions of an agglomerated cocoa beverage powder are shown in Fig. 5 (Kyaw Hla & Hogekamp, 1999). The given sieve mesh size represents the maximum agglomerate size of the respective fraction. The fine fractions below 0.2 mm show long wetting times of several minutes, which are inacceptable for practical applications. But then, there is a sharp decrease in wetting time for powder fractions with a maximum particle size larger than 0.28 mm. Typical values are of the order of several seconds, with a slight decrease for increasing agglomerate size. This result is in good agreement with eqn 5 and several other experimental results which were reported and explained earlier (Schubert, 1993). Alternative agglomeration methods are required, if the primary particle diameter is on the nanometer-scale. Dry agglomeration gives products with loose structure and large porosity, often above the critical value, due to the dominating effect of van-der-Waals forces. In wet agglomeration capillary forces during the drying process lead to compact structures, with poor dispersability. A promising method for the agglomeration of such systems is freeze spray drying of nano-particle suspensions, colloidal (e.g. protein) or molecular (e.g. sugar) solutions (Maa, Nguyen, Sweeney, Shire, & Hsu, 1999; Moritz, 1995; Moritz & Reetz, 1993; Niediek, 1982; Reetz & Moritz, 1992). Porosity of the final product is determined by the concentration of suspended or dissolved solids; agglomerate size is adjusted by the droplet size in the spraying process. Further specific properties may be attributed to the internal structure of the particles: amorphous sugar produced from sugar solutions in this process is advantageous compared to the crystalline form due to improved aroma retention in the glassy state of sugar (Niediek, 1982). There are certain practical limits to the applicability of the product engineering concept discussed here (Fig. 1), though. This is the case, if the relevant characteristics of the micro-structure of a system are very complex, e.g. if property and process functions have to be determined taking into account structural parameters on a molecular scale. An important example is the formulation of starch based instant powders by extrusion processes. Key quality aspects of these powders are specific weight, instant properties or viscosity and mouth feeling of the reconstituted starch gel (Schuchmann & Danner, 2000). On the one hand, these properties are closely related to the molecular structure of the starch, which, on the other hand, depends on the complex combination of a large number of process parameters. The determination of the respective property and process functions would require extensive investigations and is often limited as necessary analytical methods are not available. For such systems, it is more efficient from a practical point of view to correlate properties with process parameters
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Fig. 5. Wetting time of agglomerated cocoa beverage powders data from Kyaw Hla & Hogekamp, 1999.
immediately and to dispense with a detailed characterization of the micro-structure and the underlying physical effects (Schuchmann & Danner, 2000).
Summary The task of product engineering is to produce products of a certain quality, i.e. with specific properties. The tools to attain this target are the methods of process engineering. All properties are the result of certain physical, chemical or biological effects in the product, which are determined by the choice of ingredients and processing conditions. The understanding of these basic mechanisms is the key to a purposeful design of product properties. In case of dispersed systems, like emulsions, foams, suspensions or powders, which are of increasing practical relevance in many branches of industries, the linking factor between product properties and processing conditions is the system’s micro-structure. Many important properties of dispersed systems are largely determined by structural parameters (e.g. mean particle size, volume ratio of the phases and others like particle shape, particle size distribution etc.). It is often possible to describe this correlation in the form of explicit property functions. On the other hand, the choice of process conditions (e.g. apparatus design, energy input, additives) decides about these structural characteristics of the dispersion, the dependencies described by process functions. If it is possible to attain these property and process functions for a certain product, the amount of experimental work necessary can be significantly reduced. Variations of certain parameters are then based on a deeper understanding of the mechanisms rather than on empirical correlations and phenomenological descriptions.
Emulsions as liquid dispersions and powdered products are important examples for dispersions with great practical relevance. Driven by this importance and as the result of extensive research on such dispersed systems there are examples for explicit property and process functions for both groups of dispersions. The concept of energy density is an example for a process function with wide applicability in emulsification processes. It allows estimation of the mean emulsion droplet size to be expected under given process conditions. As key structural parameter mean droplet size determines properties like physical stability, rheology or bioavailability of functional ingredients. Wetting time and dispersability are important quality parameters for instant powders. These are closely linked to the particle or agglomerate size, the powder bulk porosity, agglomerate porosity and binding mechanisms, respectively. This powder micro-structure is influenced by the choice of agglomeration method and process parameters, like dosage, external stress during agglomeration, additives, humidity and others. Due to the complexity of the underlying effects often only simplified descriptions of the reality are attainable. Nevertheless, the respective correlations are good approximations with a definite practical value. There are limits to the applicability of this concept of property and process functions. If the decisive mechanisms are too complex or adequate methods for the characterization of the relevant intermediate structure (e.g. in case of certain molecular effects) are not available, the concept of property and process function is not useful. An example are extrusion processes, where, according to the present state of the art, direct correlations between the process and properties are easier to handle.
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Polke, R. (1995). Qualita¨tssicherung und Qualita¨tsmanagement Beispiele aus der Feststoffverfahrenstechnik. Lecture notes, Universita¨t Karlsruhe (TH) Reetz, T., & Moritz, T. (1992). Sinterfa¨higes keramisches Spru¨hgranulat und Verfahren zu dessen Herstellung. Patentoffenlegung, DE 4118752A1 Rumpf, H. (1967). U¨ber die Eigenschaften von Nutzsta¨uben. Staub, Reinhaltung der Luft, 27, 3–13. Schro¨der, V. (1999). Herstellen von O¨l-in-Wasser-Emulsionen mit mikroporo¨sen Membranen. Thesis, Universita¨t Karlsruhe (TH) Schro¨der, V., Behrend, O., & Schubert, H. (1998). Effect of dynamic interfacial tension on the emulsification process using microporous, ceramic membranes. Journal of Colloid and Interface Science, 202, 334–340. Schro¨der, V., & Schubert, H. (1999). Influence of emulsifier and pore size on membrane emulsification. In E. Dickinson (Ed.), Food emulsions and foams (pp. 70–80). Cambridge: The Royal Society of Chemistry. Schubert, H. (1978). Optimierung der Gro¨ße und Porosita¨t von Instantagglomeraten in bezug auf eine schnelle Durchfeuchtung. Verfahrenstechnik, 12, 296–301. Schubert, H. (1993). Instantization of powdered food products. Int. Chem. Eng., 33, 28–45. Schubert, H., & Ax, K. (2001). Verbesserung der gesundheitlichen Qualita¨t von Lebensmitteln durch Erho¨hung und Modifikation des Carotinoidgehaltes — von der Gemeinschaftsforschung zum Leitprojekt. In Innovationsfeld Lebensmittel (Vol. 59, pp. 106–141). Bonn: Diskussionstagung, Forschungskreis der Erna¨hrungsindustrie e.V. (FEI) Schubert, H., & Behrend, O. (2000). Engineering aspects of noval food emulsifying processes. Proc. 8th Int. Congr. Eng. Food (ICEF 8) (Vol. I, pp. 623–627), Puebla, Mexico, 9–13 April 2000 Schubert, H., & Karbstein, H. (1994). Mechanical emulsification. In T. Yano, R. Matsuno, & K. Nakamura (Eds.), Developments in food engineering. Proc. 6th Int. Congr. Eng Food (ICEF 6) (Vol. 2), Chiba, Japan, 23 May 1993 (pp. 9–14). London: Blackie Academic & Professional. Schuchmann, H., & Danner, T. (2000). Product engineering using the example of extruded instant powders. Chemical Engineering & Technology, 23, 303–308. Schweikert, L., & Kolter, K. (1997). Stabile, zur parenteralen Verabreichung geeignete Carotinoid-Emulsionen. EP 0800824 A Stang, M. (1998). Zerkleinern und Stabilisieren von Tropfen beim mechanischen Emulgieren. Thesis, Universita¨t Karlsruhe (TH) Tong, J., Nakajima, M., Nabetani, H., & Kikuchi, Y. (2000). Behaviour of monodispersed oil-in-water microsphere formation using microchannel emulsification technique. In I. Endo, T. Nagamune, S. Katoh, & T. Yonemoto (Eds.), Bioseparation engineering (pp. 155–160). Amsterdam: Elsevier Science. Walstra, P. (1993). Priciples of emulsion formation. Chemical Engineering Science, 48, 333–349. Walstra, P., & Smulders, I. (1997). Making emulsions and foams An overwiew. In E. Dickinson, & B. Bergenstahl (Eds.), Food colloids (pp. 367–381). Cambridge: The Royal Society of Chemistry. Weit, H., & Schwedes, J. (1986). Maßstabsvergro¨ßerung von Ru¨hrwerkskugelmu¨hlen. Chem. Ing. Techn., 58, 818–819. West, C. E., & Castenmiller, J. (1998). Quantification of the ‘‘SLAMENGHI’’ factors for carotenoid bioavailability and bioconversion. Int. J. Vit. Nutr., 68, 371–377. Williams, A. W., Boileau, T. W., & Erdman, J. W. (1998). Factors influencing the uptake and absorption of carotenoids. Proceedings of the Society for Experimental Biology and Mecicine, 218, 106–108. Wollny, M. (2002). U¨ber das Gestalten der Eigenschaften von Instantprodukten durch das Verfahren der Strahlagglomeration. Thesis, Universita¨t Karlsruhe (TH) Wollny, M., & Schubert, H. (2000). Product engineering of instant products by jet agglomeration. In Proc. 8th Int. Congr. Eng. Food (ICEF 8) (Vol. I), Puebla, Mexico, 9–13 April 2000
Review
Product engineering of dispersed systems Helmar Schubert*, Karin Ax and Olaf Behrend University of Karlsruhe, Institute of Food Process Engineering, D-76128 Karlsruhe, Germany (Tel.: +49-721-608-2497; fax: +49-721-694-320; e-mail: [email protected])
Product engineering is the design of products with desired properties using the methods of process engineering. To reduce the number of process parameters, necessary to obtain the desired properties, a characteristic feature of the respective product is helpful. For a dispersed system this characteristic feature is its complex structure on a microscale. This micro-structure is the link between the process and the properties of the final product. The principles of product engineering of dispersed systems are explained and demonstrated by means of different examples: Though there are limits to this concept, certain properties of dispersed systems can be obtained by determining and applying property and process functions. # 2003 Elsevier Science Ltd. All rights reserved.
Introduction Dispersed systems, e.g. emulsions, foams, suspensions or powders, are commonly found in many branches of industries, ranging from chemical products to cosmetics, pharmaceuticals, food and innovative materials like metal and ceramic foams. Dispersions, in general, are systems with a characteristic structure on a micro-scale, which determines many of their physical properties and thus the quality and functionality of the product; their macroscopic appearance is often homogeneous, though. Stability, rheology, sensory and optical properties or * Corresponding author.
mass transfer kinetics are only some of the properties, which are largely influenced by the micro-structure of a dispersed system. The correlation between a specific property and the micro-structure is called property function (Rumpf, 1967) (Fig. 1). The use of property functions only makes sense, if the main properties are clearly determined by the micro-structure. For dispersed systems this condition is usually fulfilled (Borho, Polke, Wintermantel, Schubert, & Sommer, 1991). Often, the micro-structure can be sufficiently described by a small number of characteristic parameters (e.g. mean values). In this case, properties are independent of the process conditions used to obtain these structural parameters. The structure is determined by the choice of ingredients as well as the preparation process. Changes in the recipe are often limited by functional demands of the final product or they are subject to legal restrictions. Knowledge about the, often cruical, influence of process parameters on structural characteristics is, therefore, indispensible. The desired micro-structure can be obtained by the choice of suitable process conditions, the relation between both is described by the process function (Borho et al., 1991; Polke, 1995). Thus, product engineering of dispersed systems means the design of products with desired properties, taking into account the methods of process or chemical engineering. The micro-structure of the system may be used as linking factor between the process and product properties.
Examples of product engineering with practical relevance Liquid dispersions: emulsions Emulsions are dispersions of two or more insoluble liquids, e.g. water and oil, with the dispersed phase distributed in form of small droplets in the continuous phase. From a thermodynamic point of view such systems are unstable. As a consequence, energy input is required in the dispersion step, e.g. mechanical energy for droplet disruption. Afterwards, the dispersed state obtained has to be stabilized over the desired period of storage and usage of the respective product. Both dispersion and subsequent stabilization determine mean droplet size and droplet size distribution which are important characteristics of the emulsion’s micro-structure. Dispersion is most commonly obtained by means of different mechanical emulsification devices, the most important of which are shown in Fig. 2 (Schubert & Ax,
0924-2244/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S 0 92 4 - 2 24 4 ( 0 2 ) 0 0 24 5 - 5
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Fig. 1. Product engineering of dispersed systems. Explanation see text.
Fig. 2. Processes and technical equipment for emulsification. For further information see Schubert and Behrend (2000).
2001). A general distinction has to be made between systems with droplet disruption starting from a coarse premix (rotor-stator, high-pressure and ultrasound systems) and direct dispersion, i.e. droplet formation by means of micro-porous structures. In the former type of apparatus, mechanical energy is dissipated in the liquid in turbulent or laminar flow or by cavitation. The resulting inertial or viscous forces acting on dispersed droplets cause their deformation and break-up. An alternative process is the direct dispersion of the internal phase by droplet formation at microporpous membranes (Mine, Shimizu, & Nakashima, 1996; Schro¨der, 1999). The liquid to be dispersed passes the membrane and forms droplets at its surface, which are detached into the flowing continuous phase.
A process similar to membrane emulsification is the so-called microchannel emulsification (Kawakatsu, Kikuchi, & Nakajima, 1997). Instead of membranes specifically manufactured elements with channels of well defined geometry in the range of several micrometers are applied. This method allows production of practically monodispersed emulsions (Tong, Nakajima, Nabetani & Kikuchi, 2000). This is probably due to a droplet detachment mechanism governed by interfacial instabilities. In emulsification devices usually employed, droplet disruption of a coarse premix is induced by various mechanisms. Shear forces in laminar flow and shear or inertial forces in turbulent flow are predominant in case of rotor-stator systems (Karbstein, 1994; Stang, 1998). In high-pressure systems the efficiency of droplet disruption
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largely depends on the geometry of the homogenizing valve: Depending on the hydrodynamic conditions elongational forces in laminar flow and/or turbulence may cause droplet deformation and disruption. Furthermore, liquid cavitation may provide conditions, which contribute to the disruption. In ultrasound fields cavitation is the main effect, which leads to droplet break-up (Behrend & Schubert, 2001). Depending on the respective predominant effect, droplet disruption is influenced by several physical parameters, like viscosity of the dispersed and continuous phase, interfacial tension or density of the liquids (Walstra, 1993; Walstra & Smulders, 1997). Besides, power density (power input per unit volume) and residence time of the emulsion in the dispersing zone of a continuously operated apparatus are key process parameters which influence the emulsification result (Koglin, Pawlowsky, & Schno¨ring, 1981). Based on the results of Koglin et al. (1981) and in good agreement with results for solid particle comminution (Weit & Schwedes, 1986) and cell disruption (Bunge & Schwedes, 1993) the concept of energy density was proved suitable for droplet disruption in continuous mechanical emulsification (Karbstein, 1994; Schubert & Karbstein, 1994): According to experimental results the mean droplet diameter of an emulsion can be described as function of both power density and residence time by a power law (eqn 1). The characteristic exponents are often similar. Thus, the product of both parameters, i.e. the energy density (energy input per unit volume), is the determining process parameter with respect to droplet disruption. b2 1 x3;2 / Pb ðPV tr Þb ¼ Eb V V tr
ð1Þ
where x3;2 volume-surface mean droplet size (Sauter diameter), ð1 x3;2 ¼ xq2 ðxÞ dx 0
q2 ðxÞ surface frequency distribution, PV power density, tr residence time, EV energy density, b1 ; b2 ; b dimensionless characteristic exponents. Eqn 1 is a practical approximation for rotor-stator systems (Karbstein, 1994) and ultrasound homogenizers (Behrend, 2002). For high-pressure homogenizers (homogenizing pressure pH=EV) and droplet disruption in laminar shear or elongational flow eqn 1 can be derived from theoretical considerations (Schubert & Ax, 2001; Walstra & Smulders, 1997). The value of the characteristic exponent contains information about the droplet disruption mechanism. Typically, values are found between b=0.35 (turbulent inertial forces) and b=1 (laminar shear or elongational flow). Membrane emulsification is an exception, where often b > 1 (Schro¨der, 1999).
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Eqn 1 relates an important structural characteristic of the emulsion, volume-surface mean droplet size, to a process parameter, energy density, and is, therefore, an important example for an explicit process function in terms of the concept given in Fig. 1. Besides the possibility to estimate the droplet size to be expected in a certain emulsification process, energy density is a useful parameter for scaling-up purposes in practice and to compare droplet disruption efficiencies of different apparatus. Fig. 3 gives typical emulsification results for different mechanical apparatus in terms of the Sauter diameter of the dispersed phase as function of energy density. The expected linear relation between both on a log-log scale is approximately found for all systems. Nevertheless, at constant energy density there are significant differences between the apparatus. The lowest efficiency of disruption for this emulsion system is obtained with the colloid-mill as typical rotorstator system. Better results are found for the highpressure systems, though the different valve geometries give results, varying with a factor of 10 or more in droplet size. The ultrasound-system provides disruption with an efficiency close to that of the better high-pressure systems. Due to the specific mechanism of step-wise droplet disruption in the cavitating sound field typical droplet size distributions for systems of the latter type are often broad and bimodal (Behrend & Schubert, 2001). In terms of energetic efficiency the membrane emulsification gives the best results for low dispersed phase contents and fine-dispersed emulsions (Schro¨der, 1999). This is due to the fact, that droplets do not have to be disrupted, but are formed by direct dispersion at the pores of the membrane. Energy is needed only to overcome capillary effects in the membrane structure and to detach the droplets from the pores by the current of the continuous phase. Droplet size is mainly determined by the pore size of the membrane and the (dynamic) interfacial tension of the system (Schro¨der, Behrend, & Schubert, 1998; Schro¨der & Schubert, 1999). As shown in Fig. 3, the efficiency of membrane emulsification is largely determined by the dispersed phase (volume) fraction ’. For all other mechanical emulsification systems discussed here, the emulsification result is independent of ’, as long as droplet coalescence during and shortly after the disruption step is avoided due to the choice of an suitable emulsifier and adequate process conditions. Fig. 3 is a summary and comparison of some important process functions for the respective apparatus and may serve as part of the basis for product engineering of emulsions in practice. As an example, emulsion formulations are often used as drug delivery or nutritive systems in parenteral applications. To avoid the danger of particle induced embolisms, emulsion droplet size distribution must not exeed a certain maximum droplet diameter. An indispensible quality aspect or property of
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Fig. 3. Comparison of emulsification results for different apparatus.
such a product is, therefore, its safe applicability. The respective property function be expressed by the simple condition (eqn 2) xmax < 2 mm
ð2Þ
A combination of the empirical relation between maximum and mean droplet size found by Armbruster (1990) (eqn 3) xmax / x3;2
ð3Þ
and the concept of energy density allows to choose a suitable process (Fig. 3) and process conditions (eqn 1). Another important application of oil-in-water emulsions is the use as carrier systems for lipophilic substances such as carotenoids. Carotenoids are natural pigments occuring in many fruit and vegetables. Epidemiological studies indicate that various positive health effects are correlated with a high intake of carotenoid rich food (Bendich & Olson, 1989; Mayne, 1996). As carotenoids are insoluble in water and have only a very limited solubility in vegetable oil, they are often only poorly absorbed in the intestine. The fraction of carotenoids taken up from food strongly depends on their physical form. Carotenoids dissolved in vegetable oil are considered as ideal for absorption, whereas only a small amount of carotenoids is taken up from raw vegetables (Parker, 1997; West & Castenmiller, 1998; Williams, Boileau, & Erdman, 1998). In the following, the fraction of carotenoids absorbed from food will be called bioavailability. Carotenoids can be formulated in dispersed systems in order to obtain a continuous aqueous phase and to
enhance bioavailability. In an aqueous suspension of carotenoid crystals, bioavailability depends on particle size. Decreasing particle size results in a better uptake of carotenoids (Horn, 1989; Kolk, 1974). This correlation can be considered as property function. With regard to good bioavailability, oil-in-water emulsions with carotenoids dissolved in the dispersed oil phase are promising formulations. These emulsions are prepared by dissolving carotenoid crystals in vegetable oil at high temperatures and subsequently dispersing the hot lipophilic phase in the aqueous phase (Ax, Schubert, Briviba, Rechkemmer, & Tevini, 2001). Oil-in-water emulsions could combine the advantages of the good bioavailability of an oily carotenoid solution and applicability as water-dispersible system. Moreover, in the dispersed oil droplets crystallization of carotenoids is inhibited and thus they can be kept in supersaturated solution (Bunnell, Driscoll, & Bauernfeind, 1958; Schweikert & Kolter, 1997). It can be assumed that the maximum supersaturation increases with decreasing droplet size, because nucleation is less likely to occur. In order to prevent crystallization of carotenoids in the oil droplets, the property function is the correlation that links the propability of carotenoid crystals with droplet size at a certain concentration. The process function, that is necessary to prepare a carotenoid emulsion of that droplet size, is given by the concept of energy density (eqn 1). The principle of product engineering can, therefore, be applied for preparing an emulsion with carotenoids in supersaturated solution in the oil droplets. The concept of energy density and the refering eqn 1 describes only droplet disruption in an emulsification process. The dispersed state of the final product is also determined by the extend of droplet coalescence superimposing
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the disruption in the apparatus (Armbruster, 1990; Karbstein, 1994). The extent of coalescence depends on the hydrodynamic conditions in the apparatus, but largely on the stabilizing properties of the surface active ingredients used, i.e. the emulsifier. A method allowing to determine the stabilizing properties of emulsifiers qualitatively under conditions close to practice was developed by Danner, Heubel, Schubert, Polke, and Stang (2000). In this so-called colouring method two emulsion premixes are produced under similar conditions, with the dispersed phase coloured with blue or yellow dye. A mix of equal parts of both systems is processed in a discontinuous emulsification apparatus. Samples of the product are taken at different emulsification times and examined by means of a microscope and a image analysis software. As a result of coalescence the amount of mixed-coloured droplets (green) rises as function of time. The kinetics of this colour shift allow to assess the stabilizing properties of different emulsifiers. Figure 4 shows the experimental results for two typical food emulsifiers. Egg yolk, which contains proteins and phospholipids as surface active components, is typically used for mayonnaise-type products and gives good long-term stability (during storage), but poor shortterm stability (Karbstein, 1994). Tween 80 (E 433) is a low molecular weight surfactant with good stabilizing properties in the short-term range (in the process). In case of the egg yolk-stabilized emulsion the area amount of mixed-coloured (green) droplets increases significantly faster than for the system containing Tween 80. This is in good agreement with results of continuous mechanical emulsification, which indicate a significant influence of coalescence for the referring system with egg yolk (Karbstein, 1994; Stang, 1998).
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Though the concept of energy density holds for the droplet disruption independently of the surfactant applied, its practical value as process function also depends on the efficiency of droplet stabilization due to the adequate choice of emulsifying agents. Recently, the colouring method has been improved by using a theory which allows to calculate coalescence probability of droplets (see Fig. 4) on the basis of experimental data (Danner, 2001). Thus, the method is suitable to determine the short-term stabilizing properties of emulsifiers quantitatively under conditions close to practice.
Powdered products Powdered products are another group of dispersed systems with large practical relevance. Most important quality characteristics of powders are handling aspects like flow behaviour or dust prevention, and, as many powders are intermediate products, instant properties, i.e. wetting behaviour, dispersability or solubility. These properties are, again, largely determined by the microstructure of the powdered product. Fine powders usually have poor wetting properties, which can be significantly improved by the formation of agglomerates. Main structural characteristics are primary particle size, agglomerate size, porosity of agglomerates and the porosity of the powder bulk as well as the binding structures between the particles (liquid or solid bridges, van-der-Waals forces). The nonstationary wetting kinetics of a liquid for a certain nonswelling and unsoluble powder bulk is characterized by the specific wetting time, which is a function of physical and structural parameters of the powder (eqn 4) (Schubert, 1993):
Fig. 4. Stabilizing properties of different emulsifiers; results obtained by the colouring method (Danner et al., 2000).
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twet ¼ 15
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1" 1 l H 2 " x l coseff
ð4Þ
with twet wetting time, " porosity of the powder bulk, x particle (agglomerate) size, l liquid viscosity, l surface tension of the liquid, eff effective contact angle, H wetted powder height. Eqn 4 describes an explicit property function for instant products. Decreasing wetting times are obtained, if agglomerate size and bulk porosity, which must not exceed a critical value (Schubert, 1978), are increased. Generally, an agglomerate size in the range of xaggl > 250 ::: 300 mm
ð5Þ
gives satisfying instant properties for many practically relevant food products (Schubert, 1993; Wollny, 2002). For agglomerates of this size, the condition "bulk < "bulk;crit
ð6Þ
is usually fulfilled, as attractive van-der-Waals forces are too small compared to the agglomerate weight, to form a loose bulk structure. Subsequent to the wetting of the powder bulk wetting of the single agglomerates, desagglomeration and dispersion of the primary particles is required. Here, the structural characteristics of the agglomerates are the dominating factors. Optimal porosity and pore structure are required for a fast wetting without gas trapping in the interior of the agglomerates (Schubert, 1978, 1993). Solid bridges of an easily soluble material (e.g. salt, sugar) as bindig structures between primary particles provide good dispersability. If the powder material tends to swell or is soluble in the dispersing liquid, this has to be taken into account for the description of property functions. A swelling process as well as partial dissolution alters the bulk structure of the powder and may cause interruption of the wetting process at a critical powder height in the former case (Wollny, 2002). Jet agglomeration is an example for an appropriate method to design agglomerates of a specific structure (Hogekamp, 1999; Wollny & Schubert, 2000). Loose pre-agglomerates of the initial powder are formed in the dosing device, e.g. a vibrating channel, and enter the apparatus in free fall. In the wetting zone the particles mix with steam, which partly condenses on their surface and forms liquid bridges between the primary particles. Soluble components of the powder dissolve in the liquid and form solid bridges, as soon as the water is removed in the subsequent drying zone. In this process, the micro-structure of the product is determined mainly in the pre-agglomeration step; the subsequent treatment gives the desired agglomerate strength (Wollny, 2002; Wollny & Schubert, 2000).
Typical results for characteristic wetting times of different fractions of an agglomerated cocoa beverage powder are shown in Fig. 5 (Kyaw Hla & Hogekamp, 1999). The given sieve mesh size represents the maximum agglomerate size of the respective fraction. The fine fractions below 0.2 mm show long wetting times of several minutes, which are inacceptable for practical applications. But then, there is a sharp decrease in wetting time for powder fractions with a maximum particle size larger than 0.28 mm. Typical values are of the order of several seconds, with a slight decrease for increasing agglomerate size. This result is in good agreement with eqn 5 and several other experimental results which were reported and explained earlier (Schubert, 1993). Alternative agglomeration methods are required, if the primary particle diameter is on the nanometer-scale. Dry agglomeration gives products with loose structure and large porosity, often above the critical value, due to the dominating effect of van-der-Waals forces. In wet agglomeration capillary forces during the drying process lead to compact structures, with poor dispersability. A promising method for the agglomeration of such systems is freeze spray drying of nano-particle suspensions, colloidal (e.g. protein) or molecular (e.g. sugar) solutions (Maa, Nguyen, Sweeney, Shire, & Hsu, 1999; Moritz, 1995; Moritz & Reetz, 1993; Niediek, 1982; Reetz & Moritz, 1992). Porosity of the final product is determined by the concentration of suspended or dissolved solids; agglomerate size is adjusted by the droplet size in the spraying process. Further specific properties may be attributed to the internal structure of the particles: amorphous sugar produced from sugar solutions in this process is advantageous compared to the crystalline form due to improved aroma retention in the glassy state of sugar (Niediek, 1982). There are certain practical limits to the applicability of the product engineering concept discussed here (Fig. 1), though. This is the case, if the relevant characteristics of the micro-structure of a system are very complex, e.g. if property and process functions have to be determined taking into account structural parameters on a molecular scale. An important example is the formulation of starch based instant powders by extrusion processes. Key quality aspects of these powders are specific weight, instant properties or viscosity and mouth feeling of the reconstituted starch gel (Schuchmann & Danner, 2000). On the one hand, these properties are closely related to the molecular structure of the starch, which, on the other hand, depends on the complex combination of a large number of process parameters. The determination of the respective property and process functions would require extensive investigations and is often limited as necessary analytical methods are not available. For such systems, it is more efficient from a practical point of view to correlate properties with process parameters
H. Schubert et al. / Trends in Food Science & Technology 14 (2003) 9–16
15
Fig. 5. Wetting time of agglomerated cocoa beverage powders data from Kyaw Hla & Hogekamp, 1999.
immediately and to dispense with a detailed characterization of the micro-structure and the underlying physical effects (Schuchmann & Danner, 2000).
Summary The task of product engineering is to produce products of a certain quality, i.e. with specific properties. The tools to attain this target are the methods of process engineering. All properties are the result of certain physical, chemical or biological effects in the product, which are determined by the choice of ingredients and processing conditions. The understanding of these basic mechanisms is the key to a purposeful design of product properties. In case of dispersed systems, like emulsions, foams, suspensions or powders, which are of increasing practical relevance in many branches of industries, the linking factor between product properties and processing conditions is the system’s micro-structure. Many important properties of dispersed systems are largely determined by structural parameters (e.g. mean particle size, volume ratio of the phases and others like particle shape, particle size distribution etc.). It is often possible to describe this correlation in the form of explicit property functions. On the other hand, the choice of process conditions (e.g. apparatus design, energy input, additives) decides about these structural characteristics of the dispersion, the dependencies described by process functions. If it is possible to attain these property and process functions for a certain product, the amount of experimental work necessary can be significantly reduced. Variations of certain parameters are then based on a deeper understanding of the mechanisms rather than on empirical correlations and phenomenological descriptions.
Emulsions as liquid dispersions and powdered products are important examples for dispersions with great practical relevance. Driven by this importance and as the result of extensive research on such dispersed systems there are examples for explicit property and process functions for both groups of dispersions. The concept of energy density is an example for a process function with wide applicability in emulsification processes. It allows estimation of the mean emulsion droplet size to be expected under given process conditions. As key structural parameter mean droplet size determines properties like physical stability, rheology or bioavailability of functional ingredients. Wetting time and dispersability are important quality parameters for instant powders. These are closely linked to the particle or agglomerate size, the powder bulk porosity, agglomerate porosity and binding mechanisms, respectively. This powder micro-structure is influenced by the choice of agglomeration method and process parameters, like dosage, external stress during agglomeration, additives, humidity and others. Due to the complexity of the underlying effects often only simplified descriptions of the reality are attainable. Nevertheless, the respective correlations are good approximations with a definite practical value. There are limits to the applicability of this concept of property and process functions. If the decisive mechanisms are too complex or adequate methods for the characterization of the relevant intermediate structure (e.g. in case of certain molecular effects) are not available, the concept of property and process function is not useful. An example are extrusion processes, where, according to the present state of the art, direct correlations between the process and properties are easier to handle.
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