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Liquid absorption capacity of carriers in the food technology
Powder Technology 134 (2003) 201 – 209 www.elsevier.com/locate/powtec
Liquid absorption capacity of carriers in the food technology Heidi Lankes a,*, Karl Sommer b, Bernd Weinreich c a
b
Kraft Foods Manufacturing GmbH and Co. KG, Lo¨rrach 79539, Germany Lehrstuhl fu¨r Maschinen- und Apparatekunde, Technische Universita¨t Munich, Freising 85350, Germany c Adalbert-Raps-Zentrum fu¨r Arznei- und Gewu¨rzpflanzenforschung, Freising 85350, Germany Received 20 August 2001; received in revised form 25 June 2002; accepted 10 June 2003
Abstract If a liquid should be sprayed on a powder that acts as a carrier, the Concentrated Powder Form (CPF) technology can be used. In the food industry, there is a large number of products available in powder form, which can be used as carriers for liquid components. But there is a big difference between them in view of the maximum liquid absorption capacity. To find out which attributes affect the liquid absorption capacity of these carriers, several natural substances and model particles with defined uniform diameter and spherical shape were loaded with increasing amounts of different liquids. The aim of this study is to find out more about the casual relationship between the characteristics of powders and liquids, as well as the loading capacity. Therefore, a correlation between the tapped density and the maximum loading capacity is discussed. To determine the maximum loading capacity, a simple method using a filter was developed. D 2003 Published by Elsevier B.V. Keywords: Liquid addition; Carrier; Pulverization; Agglomeration; Spraying; Supercritical fluid
1. Introduction If frozen pizzas are produced in an industrial way, the same amount of spice is spread on each pizza. Hence, the spice must be standardized. One example is paprika pepper, which is traditionally dried and ground. But the sensorial, physical and chemical characteristics of dried spices are influenced by environment, climate, soil conditions, time of harvesting and post-harvest handling [1]. Nowadays, a highly concentrated extract showing constant attributes can be produced, for example, with supercritical CO2 extraction. Extracts are produced from fresh or coarsely ground spices, and they are standardized in colour, aroma and, sometimes, for their antioxidant activity. In addition, they are more concentrated than dried or fresh spices and can be used in lower concentrations [1]. The disadvantage of these extracts is their high viscosity. Therefore, they must be pulverised again. The newly developed paprika powder has constant values of colour, taste and flavour. This is one example of many others in the food technology, where a
* Corresponding author. Tel.: +49-7621-4147314; fax: +49-76214148314. E-mail address: [email protected] (H. Lankes). 0032-5910/$ - see front matter D 2003 Published by Elsevier B.V. doi:10.1016/S0032-5910(03)00124-4
liquid is formulated in a powder matrix in which the powder acts as a carrier.
2. Producing a powder from a liquid with the CPF technology A possibility to turn a liquid into a powder is to spray the liquid on a carrier with the Concentrated Powder Form (CPF) technology [2]. In this technology, the liquid and a supercritical gas, mostly carbon dioxide, are mixed within a static mixer. The formed solution is then expanded rapidly and during the expansion, the liquid dissipates in very fine droplets, which are mixed with the powder, which acts as a carrier. In Fig. 1, the principle of the CPF technology is shown. The CPF technology is already implemented in the spice industry and in the production of animal feed. It allows high loading concentrations up to 90 mass% of liquid, whereas the product remains a free-flowing, homogenous powder. Free flowing means in this context that the ffc value, as defined by Jenike [3], is no case less than 4. This flowability of the product depends to a great extent on the used carrier. In many applications, the loading should be as high as possible, because the carrier is only an auxiliary material for
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Fig. 2. Different binding mechanisms between liquid and solid.
Fig. 1. Principle of the CPF technology. (1) Carrier; (2) liquid; (3) supercritical CO2; (4) static mixer (temperature and pressure above the critical point of CO2); (5) nozzle; (6) cyclone; (7) product.
maximum loading capacity of different carriers in powder form.
3. Maximum loading capacity the liquid component. The maximum loading capacity depends mainly on the attributes of the carrier, which is discussed below. Another challenge can be to spray very small amounts of liquid as uniform as possible onto large quantities of powder. Even though the CPF technology is not applied in many industries until now, it is a promising technology for further applications. Imaginable are, for example, applications in the aroma and flavour industry in combination with controlled release or in the cosmetic industry for atomising of emulsions. Although there are a lot of similarities between the discussed technology in this article and the wet agglomeration process [4 –8], it is important to distinguish between them. The aim of agglomeration is to add single particles together to form an agglomerate (granule). Iveson et al. [5] distinguish between three key areas: The wetting and nucleation, the consolidation and growth as well as the breakage and attrition. The wet granulation process is performed by spraying a liquid binder onto the particles. The first step, the wetting and nucleation takes in the same way place in the CPF technology. But further on, there is a difference: In the wet agglomeration process, the liquid binds the particles together by capillary and viscous forces until more permant bonds are formed by drying or sintering. The last two operations do not take place in the CPF technology because the aim of this application is not the production of stable agglomerates but to store the liquid in the bulk. Thus, only the first step, the wetting and nucleation is directly comparable with the CPF technology. The aim of this article is therefore to discuss the
If a liquid is sprayed on a solid, the liquid in the liquid – solid system can be immobilized (absorbed or retained) in different ways, which are presented in Fig. 2. The liquid can be present in the following forms: 1. 2. 3. 4.
adsorption layers on the surface of particles, capillary liquid inside the porous system of a particle, liquid bridges or liquid that fills zones between particles.
Between these mechanisms, more distinctions can be made: For example, the capillary liquid can be free and mobile or strongly bonded. Fig. 3 illustrates the classification, which can be made with regard to the amount of liquid. Thereby, it is a precondition that the fluid, which is combined with the bulk solid, is a wetting fluid. With a small amount of liquid, there
Fig. 3. Liquid distribution in a bulk solid [8].
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Fig. 6. Influence of the mass on the results of the filter method: edible oil on cellulose.
Fig. 4. Wetting and nucleation: (a) initial nuclei formation; (b) liquid migration [5].
To obtain these products, a wetting and nucleation process takes place, similar to the one described by Iveson et al. [5] and shown in Fig. 4. In this article, the droplet size relative to the particle size is probably in most cases not as large as in the picture: Due to the expansion of the mixture of liquid and supercritical CO2, the liquid dissipates in very fine droplets with diameters around 100 Am. The mean particle size of the used carriers varied between 10 and 1000 Am, except for Aerosil R972, which has single particles with a mean diameter of 16 nm and which are much smaller. 3.1. Filter method to test the maximum loading capacity
are single liquid bridges between the particles (Fig. 3a). Increasing the liquid, a state will be achieved, where liquid bridges and liquid-filled areas coexist (Fig. 3b). A further increase of the amount of liquid causes the filling of all areas with liquid (Fig. 3c) and afterwards a drop is formed (Fig. 3d) [9]. In practice, the intention is to achieve state c: As much liquid as possible should be absorbed from the powder without getting free liquid in form of droplets. The powder should stay homogeneous and without free and mobile liquid in the bulk.
Fig. 5. Developed test method to determine the maximum loading capacity.
A simple test method (Fig. 5), which uses a filter, was developed to detect the maximum absorption capacity [10]. For the test, 2 – 5 g of product is placed on a filter (Schleicher & Schuell, 589, Schwarzband). Thereby, the mass of product depends on the bulk density because the predetermined area of the filter should be covered with wet powder. Within one test series, the mass of the product on the filter stays constant. Then the sample is compressed with 500 g for 1 min and, afterwards, the increase in weight of the filter is measured. The setting compression with 500 g for 1 min is selected because a great number of measurements show very reproducible results. However, other mass and time combinations were tested, and in all test series, the weight of the filter increases with increasing liquid content of the product in the way the two examples with cellulose and edible oil show in Fig. 6 and Fig. 7: At the beginning, the amount of liquid on the filter only increases in small steps and when the
Fig. 7. Influence of the time on the results of the filter method: edible oil on cellulose.
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Table 1 Different carriers used in the CPF technology Carrier
Aero Myl 33
Paselli MD6 Aerosil R972
Supplier
Su¨dsta¨rke, Rettenmaier Schrobenhausen, and So¨hne, Germany Rosenberg, Germany Category Potato starch Sugar Mean diameter 188 F 25 Ama 60 F 2 Ama Bulk density 123 F 3 411 F 11 [kg/m3] Tapped density 177 F 8 553 F 21 [kg/m3]
Degussa, Frankfurt, Germany
Sipernat 50S Degussa, Frankfurt, Germany
Silicic acid Silicic acid 16 nmb 6 F 1 Ama 59 F 3 103 F 8 74 F 1
Fig. 9. Edible oil absorption capacity of Sipernat 50S.
123 F 16
a
The particle size distributions were determined by using a laser diffraction spectrometer ‘‘HELOS’’ (Sympatec). b Supplier’s specification.
Table 2 Different liquids used in the CPF technology
Fig. 10. Edible oil absorption capacity of Paselli MD6.
Liquid
Water
Triacetin
Edible oil
Supplier
Tap water, Freising, Germany 1.002 F 0.007
Raps and Co., Kulmbach, Germany 1.155 F 0.020
Raps and Co., Kulmbach, Germany 0.911 F 0.013
68.3 F 2.9
36.15 F 0.36
33.20 F 1.18
1.11 F 0.25
21.7 F 1.84
69.27 F 0.76
Density q at 20 jC [kg/dm3] Surface tension c at 20 jC [N/m] Viscosity g at 20 jC [mPa s]
maximum liquid holding capacity of the powder is exceeded, a significant increase can be measured. The significant increase at the end of the measurements is even more distinct with higher weight or longer time. But 500 g and 1 min were sufficient in all examples, and the risk that the filter is saturated with liquid is minor in this case. All measurements were performed three times, and the results represented in the graphs are medians with their confidence intervals (95% probability). It is important to mention that this method is only a qualitative method and not a quantitative one: As described in a former publication [10], it must be taken into account that it is not possible to compare the absolute quantities of liquid on the filter directly between products with different carriers. The competition between the filter and the porous system of the bulk affects the absolute quantities and
Fig. 11. Maximum liquid absorption versus tapped density.
different carriers show a different pore structure. But the run of the curve indicates at which point the limit of the liquid loading capacity is exceeded. With this simple test, an instrument was created to determine the maximum liquid loading capacity of carriers. Another method to demonstrate the so-called overwetting or overmassing is given by Kristensen [4]. The degree of liquid saturation of agglomerates during granulation of calcium hydrogen phosphate (x50 about 10 Am) with a aqueous solution in a high shear mixer is described. In the article, the mean granule size is plotted versus the moisture Table 3 Maximum loading capacity versus tapped density: recent results
Fig. 8. Edible oil absorption capacity of Aero Myl 33.
Name
Category
Tapped density [kg/m3]
Maximum loading capacity [%]
C-Pur Vitacel P290 C-Pulp Tex Aero Myl 33 Sipernat 50S Cabosil
Sugar Cellulose Maize starch Potato starch Silicic acid Silicic acid
616 449 315 177 123 51
25 35 43 58 70 80
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Table 5 Sipernat 2200
a
Fig. 12. Influence parameters on the bulk and the tapped density.
content, and it is shown that there is a very narrow margin between the liquid content required and the contents giving rise to rapid, uncontrolled growth.
4. Applying different powders and liquids In the food technology, many different powders that can be used as carriers are available, for example, starch, silicic acid, cellulose and sugar. The amount of liquid that can be sprayed on these powders without forming wet agglomerates varies in a wide range. The aim of this research project is to find out a correlation between the characteristic attributes of these carriers and the maximum liquid absorption capacity of the particles. Therefore, carriers have to be selected, characterised and then loaded with increasing amounts of liquid. The liquids, which are pulverised in the food industry, range, for example, from highly viscous spice extracts to fluids as aqueous solutions. Thus, different powders and different liquids are used as model substances in this project. The carriers are specified in Table 1 and the liquids in Table 2. 4.1. Variation of powders During the first test series, three different powders, Aero Myl 33, Sipernat 50S and Paselli MD6, were loaded with
Supplier’s specification. The particle size distributions were determined by using a laser diffraction spectrometer ‘‘HELOS’’ (Sympatec). b
increasing amounts of edible oil. The products were investigated visually, and, afterwards, the filter method was applied. The results are shown below. In the graphs, the amount of edible oil, which migrates on the filter during the test, is plotted versus the loading in mass%. As Fig. 8 shows, almost no edible oil went on the filter at loadings up to 40 mass%. The amount of edible oil, which is released from the product between 50 and 60 mass% rises slowly. At a loading of 65 mass%, the quantity is obviously higher than before and also the confidence interval is bigger. This is due to two reasons: Firstly, the amount of liquid on the powder is high, so more liquid migrates to the filter. Secondly, the loading of the product starts to become inhomogeneous, so that samples with different amounts of liquid occur and the liquid mass at the filter varies in a wider range than before. This result fits very well to practical experiences in the past: If Aero Myl 33 is loaded with a higher concentration than 60 mass%, free liquid and wet agglomerates occur in the loaded product. The silicic acid Sipernat 50S tolerates the highest loading amount (Fig. 9). Even with a loading of 70 mass%, there is no significant increase of released liquid. A further experiment was performed, loading the silic acid with 80 mass% liquid concentration. The result was no powder anymore, but a wet paste.
Table 4 Cellulose particles
a
The particle size distributions were determined by using a laser diffraction spectrometer ‘‘HELOS’’ (Sympatec).
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Fig. 13. Water absorption capacity of Sipernat 2200.
In Fig. 12, the influence parameters on the tapped density qTa and the bulk density qB are shown. The main influence parameters are: The pore volume of open and closed pores of the single particles ePa and agglomerates eAg, the void volume between the particles and/or agglomerates in the bulk eB and the solid density qS. Because most of the substances used in this project have a solid density qS between 1.5 and 2.5 kg/m3 and the void volume is always in the same order of magnitude as shown below, the most important influence parameter on the maximum loading capacity must be the porosity of the particles ePa and the agglomerates eAg. This correlation can be shown with the following equation: qTa ¼ qS ð1 eB Þð1 eAg Þð1 ePa Þ
Fig. 14. Triacetin absorption capacity of Paselli MD6.
The last carrier tested was Paselli MD6, which can be only loaded up to a concentration of 20 mass% (Fig. 10). In a former work, a correlation between the tapped density and the maximum loading capacity was found [11]. In all recent experiments with new carriers, the same trend could be observed. The lower the tapped density is, the higher the loading capacity. In Fig. 11, some results of both projects are shown: Each dot represents a carrier, and all recent results are written in Table 3; all the others are discussed in detail in the work of Gru¨ner [11]. As shown in Fig. 11, there is a strong correlation between the tapped density and the maximum absorption capacity of the carriers. Therefore, it is of special interest on which properties the tapped density depends on. The tapped density qTa is measured with a Pharma Test Tap Testing Instrument PT-TD (Pharmatest, Hainburg, Germany). About 200 cm3 of the powder are weighed into the graduated glass cylinder and compressed by means of strokes with a height of 3 F 0.1 mm. After 1250 strokes, the volume is read off, and the test serial is repeated until the difference between two volumes is no more than 2%.
As long as qS and eB are constant, the main influence parameters on the tapped density are the porosity of the single particles and the agglomerates. These voids are filled by liquid, which explains the strong correlation between the taped density and the maximum loading capacity. To demonstrate the correlation between the porosity of the particles and agglomerates, the tapped density and the maximum absorption capacity, some theoretical considerations and a test with two different model particles were performed. If monodisperse, spherical particles are used, the porosity of the bulk is about eB = 0.4 at the end of the measurement of the tapped density, as various tests have shown. This is reasonable because the value of 0.4 is between the porosity of a cubically (eB = 0.477) and the densest (eB = 0.259) packed bulk of monodisperse spheres. This value is also given as point of reference for estimations in the literature [12,13]. Two materials, one cellulose and one silicic acid, were used as carriers, because they consist of almost monodisperse, spherical particles. The cellulose particles have no particle porosity at all, the silicic acid particles in contrast are highly porous. At the beginning, all attributes of the cellulose particles, called Cellets (Syntapharm, Mu¨hlheim-Ruhr, Germany), were measured and the particles were loaded with edible oil using the CPF technology. All characteristics are listed in Table 4. Although the tapped and the bulk density should be independent from the particle size, the values of the meas-
Fig. 15. Triacetin on Paselli MD6: (a) 40 mass%; (b) 45 mass%; and (c) 50 mass%.
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Fig. 18. Solid particles around a stable drop. Fig. 16. Edible oil absorption capacity of Aerosil R972.
urements show a variation of about 5%. However, this can be explained, because the particles are not exactly monodisperse and spherical, so that minor variations of the bulk porosity occur. With a mean tapped density of 880 kg/m3, the porosity eB of the bulk is 0.0392. These particles were loaded with edible oil with the CPF technology and the result is that even a loading of 5 mass% is too high and the products were not free-flowing powders anymore, because there was a great amount of free liquid in the bulk. Afterwards the silicic acid, Sipernat 2200 (Degussa), was characterised (see Table 5) and loaded with edible oil. It is assumed that the bulk porosity is again by 0.39 after measuring the tapped density qTa. Then the particle porosity ePa can be calculated and found to be 0.68. These highly porous particles were loaded with water using the CPF technology and the maximum loading capacity was about 70 mass%, as Fig. 13 shows. This example obviously shows that the particle porosity affects the maximum liquid capacity of a powder to a great extent. Sipernat 2200 (high porosity) can be loaded up to 70 mass% with water, whereas Cellets (without any pores) cannot be loaded with 5 mass%. Again, the filter method appears to be very useful to determine the maximum absorption capacity. Especially the graph in Fig. 13 shows an ideal run of the curve. 4.2. Variation of liquids Another influence parameter on the maximum loading capacity is the used liquid. On Paselli MD6, one more liquid, Triacetin, was sprayed in addition to edible oil. On
the silicic acid, Aerosil R972 edible oil, Triacetin and water were loaded. If Fig. 14 is compared with Fig. 10, it can be seen that more Triacetin than edible oil can be sprayed on Paselli MD6. Again, different scales are applied because the amounts of edible oil, which migrate to the filter differ from the amounts of Triacetin. The products with 40 and 45 mass% Triacetin are still homogenous and free flowing, whereas only 20 mass% edible oil can be loaded on Paselli MD6. The product with a loading of 50 mass% Triacetin is no longer a free-flowing powder: It becomes a bulk of wet agglomerates. Fig. 15 obviously shows the difference between 45 mass%, which is the limit of the loading capacity and 50 mass%, where the limit is exceeded. Considering the different densities of the liquids, qTriacetin = 1.2 kg/m3 and qEdible oil = 0.9 kg/m3, the difference is not as big as at first view. Still, there is a small influence that is not yet investigated completely. As these example shows, there is only little influence of the liquid if
the liquid is wetting (solid – liquid contact angle < p/2), a conventional morphology is produced and the liquid is dissipated into small droplets. Here, one big advantage of the CPF technology in comparison to other spray technologies becomes obvious: Even with high viscous extracts, very fine droplets can be produced, because the extract is mixed with supercritical CO2 and, thereby, a mixture of extract and CO2 with a low viscosity is formed. This is discussed in detail in the work of Lankes et al [10]. On the other hand, the liquid has an important influence, if it is, for example, non-wetting (solid liquid contact angle >p/2) and the droplets are bigger than the solid particles or if
Fig. 17. Drop of water on Aerosil R972 (a) and Aerosil R972 loaded with 95 mass% of water (b).
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Table 6 Dry content of three samples Sample number
Sample 1
Sample 2
Sample 3
Mean value
Water content in %
95.13
95.12
94.96
95.07 F 0.24
there is a crosslinking as the next two examples will demonstrate. One test series was carried out with Aerosil R972. This silicic acid was firstly loaded with edible oil. The results of the filter method are plotted in Fig. 16: The maximum loading capacity is about 70 mass% and a loading of 80 mass% is too high. This silicic acid consists of particles with a hydrophobic surface and a drop of water does not penetrate in the bulk (solid – liquid contact angle >p/2) (Fig. 17a). Nevertheless, with the CPF technology, a homogenous and free-flowing powder was produced, which is composed of 5 mass% Aerosil R972 and 95 mass% water (Fig. 17b). It is assumed that this product contains small drops of water, which are covered with Aerosil R972 particles, as schematically represented in Fig. 18. According to Palzer [14], this occurs if a powder is mixed with a non-wettable, solidifying or viscous liquid in the form of droplets, which are much bigger than the solid particles. In the case of Aerosil R 972 and water, the liquid is non-wettable and the size of the silica particles is very small (Table 2), so the droplets are bigger than the particles. This phenomenon is also described in an article about melt agglomeration by Schæfer and Mathiesen [15], who proposed two different nucleation mechanisms, depending on the relative size of the droplets to the particles. If the drop is large compared to the particle, as it is in the example, immersion of the smaller particles into the liquid droplet will occur. The assumption that the product consists of water droplets covered by small particles is confirmed by the high water activity. The aW value of this product is 0.9. The aW value describes the available water for microorganisms and refers to the vapour phase that is in equilibrium with a solid or a solution [16]. Furthermore, the homogeneity of the product is remarkable. From an amount of about 1 kg, three samples of about 5 g were taken and dried at 105 jC for 12 h. In Table 6, the results are shown.
Fig. 19. Aerosil R972 with 95 mass% water: drying with air at 25 jC and 0% relative humidity.
Fig. 20. Edible oil absorption capacity of Aerosil R972.
Additionally, the product was dried in a Dynamic Vapour Sorption (DVS) System (Surface Measurements Systems, London, UK) with air (25 jC and 0% relative humidity). The graph in Fig. 19 shows the decreasing mass plotted versus time. The mass at the beginning was 17.05 g, and after 50 s, only 0.80 g was left. This verified the loading of 95 mass% water once again. The shape of the curve shows that the water is removed rapidly and completely from the silicic acid. This observation confirms the assumption that this product consists of water droplets surrounded with solid particles. During these measurements, pictures of the loaded powder were made in the DVS System. The diameter of the droplets is about 100 Am. Even though the structure of this product differs to the conventional morphology, the filter method works, as Fig. 20 shows.
5. Conclusion If particles are loaded with liquids in the CPF technology, products with a high amount of liquid can be produced. This CPF technology is already implemented in the spice industry and the production of animal feed and is a promising technology for further applications. The developed filter method can be applied to determine the maximum loading capacity in addition to the optical control. The results of the experiments with various products demonstrate that the test is a useful instrument to determine the maximum absorption capacity. It is important to mention that this method is only a qualitative method and not a quantitative one. The maximum loading capacity depends mainly on the characteristics of the particles if a wetting liquid (solid – liquid contact angle < p/2) is used. The correlation between the taped density and the maximum loading capacity of the powders can be attributed to the particle and the agglomerate porosity, and it is only valid as long as the solid density is in the same order of magnitude. Therefore, the particle and the agglomerate porosity are considered as the most important influence parameter, because these voids are filled with liquid during the loading process. The influence of liquid is quite small in comparison to the influence of the particles discussed previously.
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Acknowledgements The authors are grateful for the financial support to this work by the Adalbert-Raps-Stiftung, Kulmbach, Germany.
[9]
[10]
References [1] S.R. Uhl, Handbook of Spices, Seasonings, & Flavorings, CRC Press LLC Headquarters, Boca Raton, FL 33431, USA, 2000. [2] B. Weinreich, et al., Method for producing a powder product from a liquid substance or mixture of substances. European Patent EP 1021241 A, 2000. [3] A.W. Jenike, Storage and flow of solids, Bulletin 123 of the Utah Engineering Experiment Station, University of Utah, Salt Lake City, UT, 1964, revised 1980. [4] H.G. Kristensen, Particle agglomeration in high shear mixers, Powder Technol. 88 (1996) 197 – 202. [5] S.M. Iveson, J.D. Lister, K. Hapgood, B.J. Ennis, Nucleation, growth and breakage phenomena in agitated wet granulation processes: a review, Powder Technol. 117 (2001) 3 – 39. [6] A. Goldzal, J. Bousquet, Wet agglomeration of powders: from physics toward process optimization, Powder Technol. 117 (2001) 221 – 231. [7] P.C. Knight, A. Johnson, H.G. Kristensen, T. Schæfer, J.P.K.: an investigation of the effects on agglomeration of changing the speed of a mechanical mixer, Powder Technol. 110 (2000) 204 – 209. [8] X. Pepin, S.J.R. Simons, S. Blanchon, D. Rosseti, G. Couarraze,
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Hardness of moist agglomerates in relation to interparticle friction, granule liquid content and nature, Powder Technol. 117 (2001) 123 – 138. H. Schubert, Kapillardruck und Zugfestigkeit von feuchten Haufwerken aus ko¨rnigen Stoffen (Capillary pressure and tensile strength of moist bulk solids from granular substances), Chem.-Ing.-Tech. 45 (6) (1973) 396 – 401. H. Lankes, K. Sommer, B. Weinreich, Spru¨hbeladung bis ans Maximum—Wie viel Flu¨ssigkeit ko¨nnen Tra¨gerstoffe aufnehmen? (Loading up to the maximum—how much liquid can carriers absorb?), Lebensmitteltechnik 6 (2001) 50 – 52. S. Gru¨ner, Entwicklung eines Hochdruckspru¨hverfahrens zur Herstellung hochkonzentrierter flu¨ssigkeitsbeladener Pulver (Development of a high pressure spraying technology for producing high concentrated liquid loaded powders), Thesis, University of Erlangen-Nuremberg, 1999. M. Stieß, Mechanische Verfahrenstechnik 1 (Mechanical Engineering 1), Springer-Verlag, Berlin, Heidelberg, 1992. T. Neeße, M. Fahland, Model calculation for porosity, pore size distribution and flow resistance in filter cakes, Aufbereit.-Tech. 35 (3) (1994) 117 – 124. S. Palzer, Anreichern und Benetzen von pulverfo¨rmigen Lebensmitteln mit Flu¨ssigkeiten in diskontinuierlichen Mischaggregaten (Enrichment and wetting of food in form of a powder with liquids in discontinuous mixers), Thesis, Technische Universita¨t Munich, Germany, 2000. T. Schæfer, C. Mathiesen, Melt pelletization in a high shear mixer: IX. Effects of binder particle size, Int. J. Pharm. 139 (1996) 139 – 148. H.G. Schlegel, Allgemeine Mikrobiologie (General Microbiology), Georg Thieme Verlag, Stuttgart, New York, 1992.
Liquid absorption capacity of carriers in the food technology Heidi Lankes a,*, Karl Sommer b, Bernd Weinreich c a
b
Kraft Foods Manufacturing GmbH and Co. KG, Lo¨rrach 79539, Germany Lehrstuhl fu¨r Maschinen- und Apparatekunde, Technische Universita¨t Munich, Freising 85350, Germany c Adalbert-Raps-Zentrum fu¨r Arznei- und Gewu¨rzpflanzenforschung, Freising 85350, Germany Received 20 August 2001; received in revised form 25 June 2002; accepted 10 June 2003
Abstract If a liquid should be sprayed on a powder that acts as a carrier, the Concentrated Powder Form (CPF) technology can be used. In the food industry, there is a large number of products available in powder form, which can be used as carriers for liquid components. But there is a big difference between them in view of the maximum liquid absorption capacity. To find out which attributes affect the liquid absorption capacity of these carriers, several natural substances and model particles with defined uniform diameter and spherical shape were loaded with increasing amounts of different liquids. The aim of this study is to find out more about the casual relationship between the characteristics of powders and liquids, as well as the loading capacity. Therefore, a correlation between the tapped density and the maximum loading capacity is discussed. To determine the maximum loading capacity, a simple method using a filter was developed. D 2003 Published by Elsevier B.V. Keywords: Liquid addition; Carrier; Pulverization; Agglomeration; Spraying; Supercritical fluid
1. Introduction If frozen pizzas are produced in an industrial way, the same amount of spice is spread on each pizza. Hence, the spice must be standardized. One example is paprika pepper, which is traditionally dried and ground. But the sensorial, physical and chemical characteristics of dried spices are influenced by environment, climate, soil conditions, time of harvesting and post-harvest handling [1]. Nowadays, a highly concentrated extract showing constant attributes can be produced, for example, with supercritical CO2 extraction. Extracts are produced from fresh or coarsely ground spices, and they are standardized in colour, aroma and, sometimes, for their antioxidant activity. In addition, they are more concentrated than dried or fresh spices and can be used in lower concentrations [1]. The disadvantage of these extracts is their high viscosity. Therefore, they must be pulverised again. The newly developed paprika powder has constant values of colour, taste and flavour. This is one example of many others in the food technology, where a
* Corresponding author. Tel.: +49-7621-4147314; fax: +49-76214148314. E-mail address: [email protected] (H. Lankes). 0032-5910/$ - see front matter D 2003 Published by Elsevier B.V. doi:10.1016/S0032-5910(03)00124-4
liquid is formulated in a powder matrix in which the powder acts as a carrier.
2. Producing a powder from a liquid with the CPF technology A possibility to turn a liquid into a powder is to spray the liquid on a carrier with the Concentrated Powder Form (CPF) technology [2]. In this technology, the liquid and a supercritical gas, mostly carbon dioxide, are mixed within a static mixer. The formed solution is then expanded rapidly and during the expansion, the liquid dissipates in very fine droplets, which are mixed with the powder, which acts as a carrier. In Fig. 1, the principle of the CPF technology is shown. The CPF technology is already implemented in the spice industry and in the production of animal feed. It allows high loading concentrations up to 90 mass% of liquid, whereas the product remains a free-flowing, homogenous powder. Free flowing means in this context that the ffc value, as defined by Jenike [3], is no case less than 4. This flowability of the product depends to a great extent on the used carrier. In many applications, the loading should be as high as possible, because the carrier is only an auxiliary material for
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Fig. 2. Different binding mechanisms between liquid and solid.
Fig. 1. Principle of the CPF technology. (1) Carrier; (2) liquid; (3) supercritical CO2; (4) static mixer (temperature and pressure above the critical point of CO2); (5) nozzle; (6) cyclone; (7) product.
maximum loading capacity of different carriers in powder form.
3. Maximum loading capacity the liquid component. The maximum loading capacity depends mainly on the attributes of the carrier, which is discussed below. Another challenge can be to spray very small amounts of liquid as uniform as possible onto large quantities of powder. Even though the CPF technology is not applied in many industries until now, it is a promising technology for further applications. Imaginable are, for example, applications in the aroma and flavour industry in combination with controlled release or in the cosmetic industry for atomising of emulsions. Although there are a lot of similarities between the discussed technology in this article and the wet agglomeration process [4 –8], it is important to distinguish between them. The aim of agglomeration is to add single particles together to form an agglomerate (granule). Iveson et al. [5] distinguish between three key areas: The wetting and nucleation, the consolidation and growth as well as the breakage and attrition. The wet granulation process is performed by spraying a liquid binder onto the particles. The first step, the wetting and nucleation takes in the same way place in the CPF technology. But further on, there is a difference: In the wet agglomeration process, the liquid binds the particles together by capillary and viscous forces until more permant bonds are formed by drying or sintering. The last two operations do not take place in the CPF technology because the aim of this application is not the production of stable agglomerates but to store the liquid in the bulk. Thus, only the first step, the wetting and nucleation is directly comparable with the CPF technology. The aim of this article is therefore to discuss the
If a liquid is sprayed on a solid, the liquid in the liquid – solid system can be immobilized (absorbed or retained) in different ways, which are presented in Fig. 2. The liquid can be present in the following forms: 1. 2. 3. 4.
adsorption layers on the surface of particles, capillary liquid inside the porous system of a particle, liquid bridges or liquid that fills zones between particles.
Between these mechanisms, more distinctions can be made: For example, the capillary liquid can be free and mobile or strongly bonded. Fig. 3 illustrates the classification, which can be made with regard to the amount of liquid. Thereby, it is a precondition that the fluid, which is combined with the bulk solid, is a wetting fluid. With a small amount of liquid, there
Fig. 3. Liquid distribution in a bulk solid [8].
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Fig. 6. Influence of the mass on the results of the filter method: edible oil on cellulose.
Fig. 4. Wetting and nucleation: (a) initial nuclei formation; (b) liquid migration [5].
To obtain these products, a wetting and nucleation process takes place, similar to the one described by Iveson et al. [5] and shown in Fig. 4. In this article, the droplet size relative to the particle size is probably in most cases not as large as in the picture: Due to the expansion of the mixture of liquid and supercritical CO2, the liquid dissipates in very fine droplets with diameters around 100 Am. The mean particle size of the used carriers varied between 10 and 1000 Am, except for Aerosil R972, which has single particles with a mean diameter of 16 nm and which are much smaller. 3.1. Filter method to test the maximum loading capacity
are single liquid bridges between the particles (Fig. 3a). Increasing the liquid, a state will be achieved, where liquid bridges and liquid-filled areas coexist (Fig. 3b). A further increase of the amount of liquid causes the filling of all areas with liquid (Fig. 3c) and afterwards a drop is formed (Fig. 3d) [9]. In practice, the intention is to achieve state c: As much liquid as possible should be absorbed from the powder without getting free liquid in form of droplets. The powder should stay homogeneous and without free and mobile liquid in the bulk.
Fig. 5. Developed test method to determine the maximum loading capacity.
A simple test method (Fig. 5), which uses a filter, was developed to detect the maximum absorption capacity [10]. For the test, 2 – 5 g of product is placed on a filter (Schleicher & Schuell, 589, Schwarzband). Thereby, the mass of product depends on the bulk density because the predetermined area of the filter should be covered with wet powder. Within one test series, the mass of the product on the filter stays constant. Then the sample is compressed with 500 g for 1 min and, afterwards, the increase in weight of the filter is measured. The setting compression with 500 g for 1 min is selected because a great number of measurements show very reproducible results. However, other mass and time combinations were tested, and in all test series, the weight of the filter increases with increasing liquid content of the product in the way the two examples with cellulose and edible oil show in Fig. 6 and Fig. 7: At the beginning, the amount of liquid on the filter only increases in small steps and when the
Fig. 7. Influence of the time on the results of the filter method: edible oil on cellulose.
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Table 1 Different carriers used in the CPF technology Carrier
Aero Myl 33
Paselli MD6 Aerosil R972
Supplier
Su¨dsta¨rke, Rettenmaier Schrobenhausen, and So¨hne, Germany Rosenberg, Germany Category Potato starch Sugar Mean diameter 188 F 25 Ama 60 F 2 Ama Bulk density 123 F 3 411 F 11 [kg/m3] Tapped density 177 F 8 553 F 21 [kg/m3]
Degussa, Frankfurt, Germany
Sipernat 50S Degussa, Frankfurt, Germany
Silicic acid Silicic acid 16 nmb 6 F 1 Ama 59 F 3 103 F 8 74 F 1
Fig. 9. Edible oil absorption capacity of Sipernat 50S.
123 F 16
a
The particle size distributions were determined by using a laser diffraction spectrometer ‘‘HELOS’’ (Sympatec). b Supplier’s specification.
Table 2 Different liquids used in the CPF technology
Fig. 10. Edible oil absorption capacity of Paselli MD6.
Liquid
Water
Triacetin
Edible oil
Supplier
Tap water, Freising, Germany 1.002 F 0.007
Raps and Co., Kulmbach, Germany 1.155 F 0.020
Raps and Co., Kulmbach, Germany 0.911 F 0.013
68.3 F 2.9
36.15 F 0.36
33.20 F 1.18
1.11 F 0.25
21.7 F 1.84
69.27 F 0.76
Density q at 20 jC [kg/dm3] Surface tension c at 20 jC [N/m] Viscosity g at 20 jC [mPa s]
maximum liquid holding capacity of the powder is exceeded, a significant increase can be measured. The significant increase at the end of the measurements is even more distinct with higher weight or longer time. But 500 g and 1 min were sufficient in all examples, and the risk that the filter is saturated with liquid is minor in this case. All measurements were performed three times, and the results represented in the graphs are medians with their confidence intervals (95% probability). It is important to mention that this method is only a qualitative method and not a quantitative one: As described in a former publication [10], it must be taken into account that it is not possible to compare the absolute quantities of liquid on the filter directly between products with different carriers. The competition between the filter and the porous system of the bulk affects the absolute quantities and
Fig. 11. Maximum liquid absorption versus tapped density.
different carriers show a different pore structure. But the run of the curve indicates at which point the limit of the liquid loading capacity is exceeded. With this simple test, an instrument was created to determine the maximum liquid loading capacity of carriers. Another method to demonstrate the so-called overwetting or overmassing is given by Kristensen [4]. The degree of liquid saturation of agglomerates during granulation of calcium hydrogen phosphate (x50 about 10 Am) with a aqueous solution in a high shear mixer is described. In the article, the mean granule size is plotted versus the moisture Table 3 Maximum loading capacity versus tapped density: recent results
Fig. 8. Edible oil absorption capacity of Aero Myl 33.
Name
Category
Tapped density [kg/m3]
Maximum loading capacity [%]
C-Pur Vitacel P290 C-Pulp Tex Aero Myl 33 Sipernat 50S Cabosil
Sugar Cellulose Maize starch Potato starch Silicic acid Silicic acid
616 449 315 177 123 51
25 35 43 58 70 80
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Table 5 Sipernat 2200
a
Fig. 12. Influence parameters on the bulk and the tapped density.
content, and it is shown that there is a very narrow margin between the liquid content required and the contents giving rise to rapid, uncontrolled growth.
4. Applying different powders and liquids In the food technology, many different powders that can be used as carriers are available, for example, starch, silicic acid, cellulose and sugar. The amount of liquid that can be sprayed on these powders without forming wet agglomerates varies in a wide range. The aim of this research project is to find out a correlation between the characteristic attributes of these carriers and the maximum liquid absorption capacity of the particles. Therefore, carriers have to be selected, characterised and then loaded with increasing amounts of liquid. The liquids, which are pulverised in the food industry, range, for example, from highly viscous spice extracts to fluids as aqueous solutions. Thus, different powders and different liquids are used as model substances in this project. The carriers are specified in Table 1 and the liquids in Table 2. 4.1. Variation of powders During the first test series, three different powders, Aero Myl 33, Sipernat 50S and Paselli MD6, were loaded with
Supplier’s specification. The particle size distributions were determined by using a laser diffraction spectrometer ‘‘HELOS’’ (Sympatec). b
increasing amounts of edible oil. The products were investigated visually, and, afterwards, the filter method was applied. The results are shown below. In the graphs, the amount of edible oil, which migrates on the filter during the test, is plotted versus the loading in mass%. As Fig. 8 shows, almost no edible oil went on the filter at loadings up to 40 mass%. The amount of edible oil, which is released from the product between 50 and 60 mass% rises slowly. At a loading of 65 mass%, the quantity is obviously higher than before and also the confidence interval is bigger. This is due to two reasons: Firstly, the amount of liquid on the powder is high, so more liquid migrates to the filter. Secondly, the loading of the product starts to become inhomogeneous, so that samples with different amounts of liquid occur and the liquid mass at the filter varies in a wider range than before. This result fits very well to practical experiences in the past: If Aero Myl 33 is loaded with a higher concentration than 60 mass%, free liquid and wet agglomerates occur in the loaded product. The silicic acid Sipernat 50S tolerates the highest loading amount (Fig. 9). Even with a loading of 70 mass%, there is no significant increase of released liquid. A further experiment was performed, loading the silic acid with 80 mass% liquid concentration. The result was no powder anymore, but a wet paste.
Table 4 Cellulose particles
a
The particle size distributions were determined by using a laser diffraction spectrometer ‘‘HELOS’’ (Sympatec).
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Fig. 13. Water absorption capacity of Sipernat 2200.
In Fig. 12, the influence parameters on the tapped density qTa and the bulk density qB are shown. The main influence parameters are: The pore volume of open and closed pores of the single particles ePa and agglomerates eAg, the void volume between the particles and/or agglomerates in the bulk eB and the solid density qS. Because most of the substances used in this project have a solid density qS between 1.5 and 2.5 kg/m3 and the void volume is always in the same order of magnitude as shown below, the most important influence parameter on the maximum loading capacity must be the porosity of the particles ePa and the agglomerates eAg. This correlation can be shown with the following equation: qTa ¼ qS ð1 eB Þð1 eAg Þð1 ePa Þ
Fig. 14. Triacetin absorption capacity of Paselli MD6.
The last carrier tested was Paselli MD6, which can be only loaded up to a concentration of 20 mass% (Fig. 10). In a former work, a correlation between the tapped density and the maximum loading capacity was found [11]. In all recent experiments with new carriers, the same trend could be observed. The lower the tapped density is, the higher the loading capacity. In Fig. 11, some results of both projects are shown: Each dot represents a carrier, and all recent results are written in Table 3; all the others are discussed in detail in the work of Gru¨ner [11]. As shown in Fig. 11, there is a strong correlation between the tapped density and the maximum absorption capacity of the carriers. Therefore, it is of special interest on which properties the tapped density depends on. The tapped density qTa is measured with a Pharma Test Tap Testing Instrument PT-TD (Pharmatest, Hainburg, Germany). About 200 cm3 of the powder are weighed into the graduated glass cylinder and compressed by means of strokes with a height of 3 F 0.1 mm. After 1250 strokes, the volume is read off, and the test serial is repeated until the difference between two volumes is no more than 2%.
As long as qS and eB are constant, the main influence parameters on the tapped density are the porosity of the single particles and the agglomerates. These voids are filled by liquid, which explains the strong correlation between the taped density and the maximum loading capacity. To demonstrate the correlation between the porosity of the particles and agglomerates, the tapped density and the maximum absorption capacity, some theoretical considerations and a test with two different model particles were performed. If monodisperse, spherical particles are used, the porosity of the bulk is about eB = 0.4 at the end of the measurement of the tapped density, as various tests have shown. This is reasonable because the value of 0.4 is between the porosity of a cubically (eB = 0.477) and the densest (eB = 0.259) packed bulk of monodisperse spheres. This value is also given as point of reference for estimations in the literature [12,13]. Two materials, one cellulose and one silicic acid, were used as carriers, because they consist of almost monodisperse, spherical particles. The cellulose particles have no particle porosity at all, the silicic acid particles in contrast are highly porous. At the beginning, all attributes of the cellulose particles, called Cellets (Syntapharm, Mu¨hlheim-Ruhr, Germany), were measured and the particles were loaded with edible oil using the CPF technology. All characteristics are listed in Table 4. Although the tapped and the bulk density should be independent from the particle size, the values of the meas-
Fig. 15. Triacetin on Paselli MD6: (a) 40 mass%; (b) 45 mass%; and (c) 50 mass%.
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Fig. 18. Solid particles around a stable drop. Fig. 16. Edible oil absorption capacity of Aerosil R972.
urements show a variation of about 5%. However, this can be explained, because the particles are not exactly monodisperse and spherical, so that minor variations of the bulk porosity occur. With a mean tapped density of 880 kg/m3, the porosity eB of the bulk is 0.0392. These particles were loaded with edible oil with the CPF technology and the result is that even a loading of 5 mass% is too high and the products were not free-flowing powders anymore, because there was a great amount of free liquid in the bulk. Afterwards the silicic acid, Sipernat 2200 (Degussa), was characterised (see Table 5) and loaded with edible oil. It is assumed that the bulk porosity is again by 0.39 after measuring the tapped density qTa. Then the particle porosity ePa can be calculated and found to be 0.68. These highly porous particles were loaded with water using the CPF technology and the maximum loading capacity was about 70 mass%, as Fig. 13 shows. This example obviously shows that the particle porosity affects the maximum liquid capacity of a powder to a great extent. Sipernat 2200 (high porosity) can be loaded up to 70 mass% with water, whereas Cellets (without any pores) cannot be loaded with 5 mass%. Again, the filter method appears to be very useful to determine the maximum absorption capacity. Especially the graph in Fig. 13 shows an ideal run of the curve. 4.2. Variation of liquids Another influence parameter on the maximum loading capacity is the used liquid. On Paselli MD6, one more liquid, Triacetin, was sprayed in addition to edible oil. On
the silicic acid, Aerosil R972 edible oil, Triacetin and water were loaded. If Fig. 14 is compared with Fig. 10, it can be seen that more Triacetin than edible oil can be sprayed on Paselli MD6. Again, different scales are applied because the amounts of edible oil, which migrate to the filter differ from the amounts of Triacetin. The products with 40 and 45 mass% Triacetin are still homogenous and free flowing, whereas only 20 mass% edible oil can be loaded on Paselli MD6. The product with a loading of 50 mass% Triacetin is no longer a free-flowing powder: It becomes a bulk of wet agglomerates. Fig. 15 obviously shows the difference between 45 mass%, which is the limit of the loading capacity and 50 mass%, where the limit is exceeded. Considering the different densities of the liquids, qTriacetin = 1.2 kg/m3 and qEdible oil = 0.9 kg/m3, the difference is not as big as at first view. Still, there is a small influence that is not yet investigated completely. As these example shows, there is only little influence of the liquid if
the liquid is wetting (solid – liquid contact angle < p/2), a conventional morphology is produced and the liquid is dissipated into small droplets. Here, one big advantage of the CPF technology in comparison to other spray technologies becomes obvious: Even with high viscous extracts, very fine droplets can be produced, because the extract is mixed with supercritical CO2 and, thereby, a mixture of extract and CO2 with a low viscosity is formed. This is discussed in detail in the work of Lankes et al [10]. On the other hand, the liquid has an important influence, if it is, for example, non-wetting (solid liquid contact angle >p/2) and the droplets are bigger than the solid particles or if
Fig. 17. Drop of water on Aerosil R972 (a) and Aerosil R972 loaded with 95 mass% of water (b).
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Table 6 Dry content of three samples Sample number
Sample 1
Sample 2
Sample 3
Mean value
Water content in %
95.13
95.12
94.96
95.07 F 0.24
there is a crosslinking as the next two examples will demonstrate. One test series was carried out with Aerosil R972. This silicic acid was firstly loaded with edible oil. The results of the filter method are plotted in Fig. 16: The maximum loading capacity is about 70 mass% and a loading of 80 mass% is too high. This silicic acid consists of particles with a hydrophobic surface and a drop of water does not penetrate in the bulk (solid – liquid contact angle >p/2) (Fig. 17a). Nevertheless, with the CPF technology, a homogenous and free-flowing powder was produced, which is composed of 5 mass% Aerosil R972 and 95 mass% water (Fig. 17b). It is assumed that this product contains small drops of water, which are covered with Aerosil R972 particles, as schematically represented in Fig. 18. According to Palzer [14], this occurs if a powder is mixed with a non-wettable, solidifying or viscous liquid in the form of droplets, which are much bigger than the solid particles. In the case of Aerosil R 972 and water, the liquid is non-wettable and the size of the silica particles is very small (Table 2), so the droplets are bigger than the particles. This phenomenon is also described in an article about melt agglomeration by Schæfer and Mathiesen [15], who proposed two different nucleation mechanisms, depending on the relative size of the droplets to the particles. If the drop is large compared to the particle, as it is in the example, immersion of the smaller particles into the liquid droplet will occur. The assumption that the product consists of water droplets covered by small particles is confirmed by the high water activity. The aW value of this product is 0.9. The aW value describes the available water for microorganisms and refers to the vapour phase that is in equilibrium with a solid or a solution [16]. Furthermore, the homogeneity of the product is remarkable. From an amount of about 1 kg, three samples of about 5 g were taken and dried at 105 jC for 12 h. In Table 6, the results are shown.
Fig. 19. Aerosil R972 with 95 mass% water: drying with air at 25 jC and 0% relative humidity.
Fig. 20. Edible oil absorption capacity of Aerosil R972.
Additionally, the product was dried in a Dynamic Vapour Sorption (DVS) System (Surface Measurements Systems, London, UK) with air (25 jC and 0% relative humidity). The graph in Fig. 19 shows the decreasing mass plotted versus time. The mass at the beginning was 17.05 g, and after 50 s, only 0.80 g was left. This verified the loading of 95 mass% water once again. The shape of the curve shows that the water is removed rapidly and completely from the silicic acid. This observation confirms the assumption that this product consists of water droplets surrounded with solid particles. During these measurements, pictures of the loaded powder were made in the DVS System. The diameter of the droplets is about 100 Am. Even though the structure of this product differs to the conventional morphology, the filter method works, as Fig. 20 shows.
5. Conclusion If particles are loaded with liquids in the CPF technology, products with a high amount of liquid can be produced. This CPF technology is already implemented in the spice industry and the production of animal feed and is a promising technology for further applications. The developed filter method can be applied to determine the maximum loading capacity in addition to the optical control. The results of the experiments with various products demonstrate that the test is a useful instrument to determine the maximum absorption capacity. It is important to mention that this method is only a qualitative method and not a quantitative one. The maximum loading capacity depends mainly on the characteristics of the particles if a wetting liquid (solid – liquid contact angle < p/2) is used. The correlation between the taped density and the maximum loading capacity of the powders can be attributed to the particle and the agglomerate porosity, and it is only valid as long as the solid density is in the same order of magnitude. Therefore, the particle and the agglomerate porosity are considered as the most important influence parameter, because these voids are filled with liquid during the loading process. The influence of liquid is quite small in comparison to the influence of the particles discussed previously.
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Acknowledgements The authors are grateful for the financial support to this work by the Adalbert-Raps-Stiftung, Kulmbach, Germany.
[9]
[10]
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