- Email: [email protected]
Honorary note
Advances in Colloid and Interface Science 150 (2009) 2–4
Contents lists available at ScienceDirect
Advances in Colloid and Interface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c i s
Honorary note Ton Van Vliet ⁎ TI Food and Nutrition, c/o Wageningen University, P.O. Box 8129, 6700 EV Wageningen, Netherlands
behaviour of the emulsion droplets during processing and afterwards, one must be able to characterize them thoroughly. It is not enough to determine an average particle diameter, but it is essential to determine the whole droplet size distribution and changes therein on processing of the milk; moreover, the composition of the surfactant layer has to be known. To this end, he developed and refined suitable methods. The work on the characterization of emulsions brought him in contact with colloid (and interface) science. It formed in fact the basis of his main scientific interest during the rest of his career, that is, to combine fundamental concepts developed in colloid science with technological problems in food manufacture and of food properties, with an emphasis on dairy products. He turned out to be excellent in making this connection, but not only in doing that. If it appeared that concepts developed in colloid science were not worked out sufficiently to hold for the complicated food or model systems studied, he was often able to further develop these concepts. In that way he contributed also to the development of colloid science. Thereby he was successful in avoiding the two kinds of failure that he distinguished for studies that involve combining technology and basic science [1]: Pieter Walstra is by training a food technologist whose main research was based on his conviction that to really understand what happens during the manufacture of many food products and to explain several characteristics of these products, the application of colloid and interface science is essential. He was born in 1931. After finishing secondary school he started a study in Dairy Technology at the Wageningen Agricultural University in 1948, where he obtained the M.Sc. degree in 1955. After military service he started in 1957 with a PhD study on “Gravimetric methods for the determination of the fat content of milk and milk products”. He obtained the PhD degree in 1961. From 1957 onwards he was a lecturer and later professor at the department of Dairy Science and Technology, which soon merged with the fairly new departments of Food Science and of Food Engineering. In the mean time, he had started research on the effect of homogenization on the properties of milk, an important topic in dairy technology. This process causes disruption of the fat droplets present in the native milk, which results in a strongly increased stability of the milk against creaming and coalescence of the fat droplets. Very soon he realized that for obtaining a profound understanding of the
⁎ Tel.: +31 317 480136. E-mail address: [email protected]. 0001-8686/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cis.2008.08.007
1. Failures due to insufficient understanding of the underlying theories, typically made by classical product technologists. 2. Failures due to application of a (single) theory or a (single) analytical method, without proper knowledge of the system studied, typically made by specialized scientists. Another characteristic of Pieter Walstra is his broad scientific interest. Probably, he is best known by colloid scientists for his work on emulsions, but for dairy science another subject was at least equally important. This concerned the concept he postulated that the casein micelles in milk are stabilized by a hairy layer providing steric repulsion. This had large consequences for the thinking about the formation of milk gels by acidification or by addition of a proteolytic enzyme, the basis of yogurt and cheese manufacture, respectively. Also his work on so-called fractal aggregation of the destabilized casein particles in milk has a much broader application than food science. Finally he contributed significantly to the application of concepts developed within the field of fracture mechanics to the yielding and fracture behaviour of soft solids; crystallization of (milk) fat; and untangling the reactions occurring during heat coagulation of milk. As mentioned above, Pieter Walstra's involvement in colloid science started with his work on emulsions. An excellent example of the overview he had of this field is the chapter on emulsions in volume V, Soft Colloids, of Fundamentals of Interface and Colloid Science [2]. In his emulsion work three main aspects can be distinguished:
T.V. Vliet / Advances in Colloid and Interface Science 150 (2009) 2–4
emulsion formation (e.g. [3]), emulsion stability during storage [4] and partial coalescence. Emulsion formation focused on the mechanisms determining final droplet size after homogenization of an emulsion. Based on the Kolmogorov theory of turbulent flow, he derived and confirmed experimentally that the average droplet size must be proportional to the homogenization pressure p to the power −0.6, which was confirmed by experimental results. In subsequent studies on droplet-break up and stabilization he distinguished different regimes according to (i) flow type: turbulent or laminar; (ii) nature of the forces acting on the droplets: inertial (pressure fluctuations) or frictional (viscous stress); and (iii) bounded or unbounded flow. Moreover, equations were deduced giving the time scale of various processes, such as drop deformation or break-up, drop encounters, adsorption of surfactants, etc. Another break-through was the development of a method to determine recoalescence of newly formed drops during the emulsification process. In that way it could be shown that prevention of recoalescence is generally not due to colloidal repulsion between drops, but to interfacial tension gradients on the drops, which strongly retard outflow of the continuous phase from the film between two drops. This is governed by the magnitude of a dimensionless Marangoni number. Madr =
d lnγ d lnC
ð1Þ
where Madr is the Marangoni number of the newly formed droplet surface, γ the surface tension and Γ the surface excess. It was found that the main reasons why small-molecular surfactants are more efficient than polymers in giving an emulsions with small oil droplets is their much higher Marangoni number in combination with a smaller interfacial tension. With respect to emulsion stability, various instability mechanism were distinguished and it was established how they depend on each other. Work focused on sedimentation and coalescence. Regarding coalescence various regimes were distinguished and factors were defined that governs the boundaries between stable and unstable, especially a Weber number Wedr = external stress × stress concentration factor = Laplace pressure
ð2Þ
was found to be often the most important variable governing coalescence rate. In foods the oil in O/W emulsions is nearly always a mixture of triacylglycerols, implying that at low temperatures part of the oil is crystallized. Some crystals may protrude from the droplet surface in the continuous phase and rupture the film between approaching films. This leads to oil–oil contact between two droplets but generally not to coalescence: the crystals in the drops tend to form a continuous network in a droplet that prevents this. The result is that aggregates of partly crystalline droplets are formed. The phenomenon was called partial coalescence and it is an essential process during the churning of cream to form butter and in the making of whipped cream. Partial coalescence rate is enormously increased by agitation. Next to his work on emulsions, around 1980 a second research line was established, focused on the stability of the casein micelles in milk. This concerns proteinaceous particles containing four different species of casein molecules, calcium phosphate and some other salts and much water. Their average radius is somewhat below 0.1 μm. They do not aggregate in milk, but can be made to aggregate by acidification to a pH below 5 (yoghurt preparation) or by the action of rennet, a proteolytic enzyme (cheese making). Initially, most researchers tried to explain the stability based on DLVO type interactions. In 1979 Walstra postulated [5] that the micelles have a hairy outer layer consisting of the C-terminal part of κ-casein that would provide steric (besides electrostatic) repulsion, and that upon action of rennet enzymes the protruding ‘hairs’ would be cut off, thereby making the
3
micelles prone to aggregation. This was shown to be indeed the case by application of dynamic and static light scattering [6]. When studying the clotting of milk it was postulated that unhindered perikinetic aggregation of particles would always lead to gel formation. When the first articles appeared about fractal aggregation this fell in place: cluster–cluster aggregation produces fractal clusters that decrease in density when they grow. Finally, the volume fraction of particles in the clusters will equal the volume fraction of the particles in the system and the clusters then form a space-filling network, hence, a particle gel [7]. The formation and properties of ‘fractal’ gels was intensively studied. It includes e.g. factors affecting the dimensionality and lower cut-off length of the fractal region, the relation between particle concentration and rheological properties (modulus, yield stress and strain) and gel permeability and effects due to slow reorganisation of the gel structure. Several simple scaling laws were derived and checked [8]. Not all particle gels were found to have a fractal structure. For instance on cooling a liquid fat, nucleation and crystal growth generally proceed for a considerable time. Already at a low volume fraction of crystallized fat a fractal network is formed, but with time more crystals will be formed and deposited in the holes of the primary network, which may cause loss of all fractal characteristics [9]. In practice, problems related to aggregation of colloidal particles are due to the occurrence of visible changes in the system, be it the emergence of visible flocs, the formation of a gel, or the separation of a dispersion into layers. Halving times calculated by using Smoluchowski equations often do not give adequate prediction regarding the rate of these processes. By taking also fractal aggregation into account, it turned out that the relation between the halving time and the ‘visible aggregation time’ can vary enormous, by several orders of magnitude, strongly depending on conditions. A range of equations were derived describing these relations and compared with literature [10]. If bonds between aggregating particles are (still) not irreversible, (slow) rearrangements may occur in the aggregates formed and in the final gel network. This was shown to be a very important aspect in the syneresis process, i.e. expelling liquid from the gel. This process forms an essential part of cheese making and has been studied both experimentally and by modelling [11,12]. Pieter's interest in the mechanical properties of cheese brought him in contact with fracture mechanics. He realised that if one want to raise the level of the research in food science on fracture of foods during its preparation and consumption one should use concept developed in fracture mechanics. This approach turned out to be very successful. An overview of many aspects of this work is presented in a review paper published in the Faraday Discussions [13]. Last but not least Pieter Walstra was an excellent teacher. His lectures for students were always well organized and clearly presented and, as a result of that, well attended. He always stressed the importance of understanding the topic rather than just knowing the facts. Both in university lectures and in post-doc courses he was able to explain fundamental concepts from colloid science in such a way that they could be understood and applied by food scientists. During his career he supervised 32 Ph D students. By his co-workers he was known for his careful writing of his own publication and editing those of others. He has written many reviews, four text books and a monograph, for the most part together with co-workers of his own group or with colleagues from other groups. Well known is his book on Physical Chemistry of Foods. Some of these books he wrote after his official retirement in 1996. As mentioned above it is Pieter Walstra's conviction that for a good understanding of the manufacturing of food products and of their characteristics colloid and interface science is essential. His drive to advocate this was also clear from his firm support of the still ongoing series of conferences on Food Colloids. He largely contributed to their success by lecturing at many of these conferences, organising one of
4
T.V. Vliet / Advances in Colloid and Interface Science 150 (2009) 2–4
them in 1992 and participating in setting up the scientific programme as a member of the international scientific committee. During the conference in 2002 he gave a clear overview of the importance of these conferences as mean to stimulate the application of basic physical and physico-chemical sciences in the field of food science and technology [1]. Pieter is one of the scientists who greatly contributed to this development. References [1] Walstra P. Studying food colloids; past present and future. In: Dickinson E, Van Vliet T, editors. Food Colloids: Biopolymers and materials, Royal Society of Chemistry; 2003. p. 391–9. [2] Walstra P. Emulsions. In: Lyklema J, editor. Fundamentals of Interface and Colloid Science. Soft ColloidsElsevier Academic Press; 2005. Chapter 8. [3] Walstra P, Smulders PEA. Emulsion formation. In: Binks BP, editor. Modern Aspect of Emulsion Science. Royal Society of Chemistry; 1998. p. 56. [4] Walstra P. Emulsion stability. In: Becher P, editor. Encyclopedia of Emulsion Technology, vol. 4. Dekker; 1996. p. 1.
[5] Walstra P. The voluminosity of bovine casein micelles and some of its implications. J Dairy Res 1979;49:317. [6] Walstra P, Bloomfield VA, Wei GJ, Jenness R. Effect of chymosin action on the hydrodynamic diameter of casein micelles. Bioch Biophys Acta 1981;669:258. [7] Bremer LGB, Van Vliet T, Walstra P. Theoretical and experimental study of the fractal nature of the structure of casein gels. J Chem Soc, Faraday Trans I 1989;85: 3359. [8] Bremer LGB, Bijsterbosch BH, Schrijvers R, Van Vliet T, Walstra P. On the fractal nature of the structure of acid casein gels. Colloids Surf 1990;51:159. [9] Walstra P, Kloek W, Van Vliet T. Fat crystal networks. In: Garti N, Sato K, editors. Crystallization Processes in fats and Lipids Systems. Dekker; 2001. p. 289–328. [10] Bremer LGB, Walstra P, Van Vliet T. Estimations of the aggregation time of various colloidal systems. Colloids Surf A 1995;99:121. [11] Van Dijk HJM, Walstra P, Schenk J. Theoretical and experimental study of onedimensional syneresis of a protein gel. Chem Eng J 1984;28:B43. [12] Van Vliet T, Walstra P. Water in casein gels: how to get it out or keep it in. J Food Eng 1993;22:75. [13] Van Vliet T, Walstra P. Large deformation and fracture behaviour of gels. Faraday Discuss 1995;101:359.
Contents lists available at ScienceDirect
Advances in Colloid and Interface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c i s
Honorary note Ton Van Vliet ⁎ TI Food and Nutrition, c/o Wageningen University, P.O. Box 8129, 6700 EV Wageningen, Netherlands
behaviour of the emulsion droplets during processing and afterwards, one must be able to characterize them thoroughly. It is not enough to determine an average particle diameter, but it is essential to determine the whole droplet size distribution and changes therein on processing of the milk; moreover, the composition of the surfactant layer has to be known. To this end, he developed and refined suitable methods. The work on the characterization of emulsions brought him in contact with colloid (and interface) science. It formed in fact the basis of his main scientific interest during the rest of his career, that is, to combine fundamental concepts developed in colloid science with technological problems in food manufacture and of food properties, with an emphasis on dairy products. He turned out to be excellent in making this connection, but not only in doing that. If it appeared that concepts developed in colloid science were not worked out sufficiently to hold for the complicated food or model systems studied, he was often able to further develop these concepts. In that way he contributed also to the development of colloid science. Thereby he was successful in avoiding the two kinds of failure that he distinguished for studies that involve combining technology and basic science [1]: Pieter Walstra is by training a food technologist whose main research was based on his conviction that to really understand what happens during the manufacture of many food products and to explain several characteristics of these products, the application of colloid and interface science is essential. He was born in 1931. After finishing secondary school he started a study in Dairy Technology at the Wageningen Agricultural University in 1948, where he obtained the M.Sc. degree in 1955. After military service he started in 1957 with a PhD study on “Gravimetric methods for the determination of the fat content of milk and milk products”. He obtained the PhD degree in 1961. From 1957 onwards he was a lecturer and later professor at the department of Dairy Science and Technology, which soon merged with the fairly new departments of Food Science and of Food Engineering. In the mean time, he had started research on the effect of homogenization on the properties of milk, an important topic in dairy technology. This process causes disruption of the fat droplets present in the native milk, which results in a strongly increased stability of the milk against creaming and coalescence of the fat droplets. Very soon he realized that for obtaining a profound understanding of the
⁎ Tel.: +31 317 480136. E-mail address: [email protected]. 0001-8686/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cis.2008.08.007
1. Failures due to insufficient understanding of the underlying theories, typically made by classical product technologists. 2. Failures due to application of a (single) theory or a (single) analytical method, without proper knowledge of the system studied, typically made by specialized scientists. Another characteristic of Pieter Walstra is his broad scientific interest. Probably, he is best known by colloid scientists for his work on emulsions, but for dairy science another subject was at least equally important. This concerned the concept he postulated that the casein micelles in milk are stabilized by a hairy layer providing steric repulsion. This had large consequences for the thinking about the formation of milk gels by acidification or by addition of a proteolytic enzyme, the basis of yogurt and cheese manufacture, respectively. Also his work on so-called fractal aggregation of the destabilized casein particles in milk has a much broader application than food science. Finally he contributed significantly to the application of concepts developed within the field of fracture mechanics to the yielding and fracture behaviour of soft solids; crystallization of (milk) fat; and untangling the reactions occurring during heat coagulation of milk. As mentioned above, Pieter Walstra's involvement in colloid science started with his work on emulsions. An excellent example of the overview he had of this field is the chapter on emulsions in volume V, Soft Colloids, of Fundamentals of Interface and Colloid Science [2]. In his emulsion work three main aspects can be distinguished:
T.V. Vliet / Advances in Colloid and Interface Science 150 (2009) 2–4
emulsion formation (e.g. [3]), emulsion stability during storage [4] and partial coalescence. Emulsion formation focused on the mechanisms determining final droplet size after homogenization of an emulsion. Based on the Kolmogorov theory of turbulent flow, he derived and confirmed experimentally that the average droplet size must be proportional to the homogenization pressure p to the power −0.6, which was confirmed by experimental results. In subsequent studies on droplet-break up and stabilization he distinguished different regimes according to (i) flow type: turbulent or laminar; (ii) nature of the forces acting on the droplets: inertial (pressure fluctuations) or frictional (viscous stress); and (iii) bounded or unbounded flow. Moreover, equations were deduced giving the time scale of various processes, such as drop deformation or break-up, drop encounters, adsorption of surfactants, etc. Another break-through was the development of a method to determine recoalescence of newly formed drops during the emulsification process. In that way it could be shown that prevention of recoalescence is generally not due to colloidal repulsion between drops, but to interfacial tension gradients on the drops, which strongly retard outflow of the continuous phase from the film between two drops. This is governed by the magnitude of a dimensionless Marangoni number. Madr =
d lnγ d lnC
ð1Þ
where Madr is the Marangoni number of the newly formed droplet surface, γ the surface tension and Γ the surface excess. It was found that the main reasons why small-molecular surfactants are more efficient than polymers in giving an emulsions with small oil droplets is their much higher Marangoni number in combination with a smaller interfacial tension. With respect to emulsion stability, various instability mechanism were distinguished and it was established how they depend on each other. Work focused on sedimentation and coalescence. Regarding coalescence various regimes were distinguished and factors were defined that governs the boundaries between stable and unstable, especially a Weber number Wedr = external stress × stress concentration factor = Laplace pressure
ð2Þ
was found to be often the most important variable governing coalescence rate. In foods the oil in O/W emulsions is nearly always a mixture of triacylglycerols, implying that at low temperatures part of the oil is crystallized. Some crystals may protrude from the droplet surface in the continuous phase and rupture the film between approaching films. This leads to oil–oil contact between two droplets but generally not to coalescence: the crystals in the drops tend to form a continuous network in a droplet that prevents this. The result is that aggregates of partly crystalline droplets are formed. The phenomenon was called partial coalescence and it is an essential process during the churning of cream to form butter and in the making of whipped cream. Partial coalescence rate is enormously increased by agitation. Next to his work on emulsions, around 1980 a second research line was established, focused on the stability of the casein micelles in milk. This concerns proteinaceous particles containing four different species of casein molecules, calcium phosphate and some other salts and much water. Their average radius is somewhat below 0.1 μm. They do not aggregate in milk, but can be made to aggregate by acidification to a pH below 5 (yoghurt preparation) or by the action of rennet, a proteolytic enzyme (cheese making). Initially, most researchers tried to explain the stability based on DLVO type interactions. In 1979 Walstra postulated [5] that the micelles have a hairy outer layer consisting of the C-terminal part of κ-casein that would provide steric (besides electrostatic) repulsion, and that upon action of rennet enzymes the protruding ‘hairs’ would be cut off, thereby making the
3
micelles prone to aggregation. This was shown to be indeed the case by application of dynamic and static light scattering [6]. When studying the clotting of milk it was postulated that unhindered perikinetic aggregation of particles would always lead to gel formation. When the first articles appeared about fractal aggregation this fell in place: cluster–cluster aggregation produces fractal clusters that decrease in density when they grow. Finally, the volume fraction of particles in the clusters will equal the volume fraction of the particles in the system and the clusters then form a space-filling network, hence, a particle gel [7]. The formation and properties of ‘fractal’ gels was intensively studied. It includes e.g. factors affecting the dimensionality and lower cut-off length of the fractal region, the relation between particle concentration and rheological properties (modulus, yield stress and strain) and gel permeability and effects due to slow reorganisation of the gel structure. Several simple scaling laws were derived and checked [8]. Not all particle gels were found to have a fractal structure. For instance on cooling a liquid fat, nucleation and crystal growth generally proceed for a considerable time. Already at a low volume fraction of crystallized fat a fractal network is formed, but with time more crystals will be formed and deposited in the holes of the primary network, which may cause loss of all fractal characteristics [9]. In practice, problems related to aggregation of colloidal particles are due to the occurrence of visible changes in the system, be it the emergence of visible flocs, the formation of a gel, or the separation of a dispersion into layers. Halving times calculated by using Smoluchowski equations often do not give adequate prediction regarding the rate of these processes. By taking also fractal aggregation into account, it turned out that the relation between the halving time and the ‘visible aggregation time’ can vary enormous, by several orders of magnitude, strongly depending on conditions. A range of equations were derived describing these relations and compared with literature [10]. If bonds between aggregating particles are (still) not irreversible, (slow) rearrangements may occur in the aggregates formed and in the final gel network. This was shown to be a very important aspect in the syneresis process, i.e. expelling liquid from the gel. This process forms an essential part of cheese making and has been studied both experimentally and by modelling [11,12]. Pieter's interest in the mechanical properties of cheese brought him in contact with fracture mechanics. He realised that if one want to raise the level of the research in food science on fracture of foods during its preparation and consumption one should use concept developed in fracture mechanics. This approach turned out to be very successful. An overview of many aspects of this work is presented in a review paper published in the Faraday Discussions [13]. Last but not least Pieter Walstra was an excellent teacher. His lectures for students were always well organized and clearly presented and, as a result of that, well attended. He always stressed the importance of understanding the topic rather than just knowing the facts. Both in university lectures and in post-doc courses he was able to explain fundamental concepts from colloid science in such a way that they could be understood and applied by food scientists. During his career he supervised 32 Ph D students. By his co-workers he was known for his careful writing of his own publication and editing those of others. He has written many reviews, four text books and a monograph, for the most part together with co-workers of his own group or with colleagues from other groups. Well known is his book on Physical Chemistry of Foods. Some of these books he wrote after his official retirement in 1996. As mentioned above it is Pieter Walstra's conviction that for a good understanding of the manufacturing of food products and of their characteristics colloid and interface science is essential. His drive to advocate this was also clear from his firm support of the still ongoing series of conferences on Food Colloids. He largely contributed to their success by lecturing at many of these conferences, organising one of
4
T.V. Vliet / Advances in Colloid and Interface Science 150 (2009) 2–4
them in 1992 and participating in setting up the scientific programme as a member of the international scientific committee. During the conference in 2002 he gave a clear overview of the importance of these conferences as mean to stimulate the application of basic physical and physico-chemical sciences in the field of food science and technology [1]. Pieter is one of the scientists who greatly contributed to this development. References [1] Walstra P. Studying food colloids; past present and future. In: Dickinson E, Van Vliet T, editors. Food Colloids: Biopolymers and materials, Royal Society of Chemistry; 2003. p. 391–9. [2] Walstra P. Emulsions. In: Lyklema J, editor. Fundamentals of Interface and Colloid Science. Soft ColloidsElsevier Academic Press; 2005. Chapter 8. [3] Walstra P, Smulders PEA. Emulsion formation. In: Binks BP, editor. Modern Aspect of Emulsion Science. Royal Society of Chemistry; 1998. p. 56. [4] Walstra P. Emulsion stability. In: Becher P, editor. Encyclopedia of Emulsion Technology, vol. 4. Dekker; 1996. p. 1.
[5] Walstra P. The voluminosity of bovine casein micelles and some of its implications. J Dairy Res 1979;49:317. [6] Walstra P, Bloomfield VA, Wei GJ, Jenness R. Effect of chymosin action on the hydrodynamic diameter of casein micelles. Bioch Biophys Acta 1981;669:258. [7] Bremer LGB, Van Vliet T, Walstra P. Theoretical and experimental study of the fractal nature of the structure of casein gels. J Chem Soc, Faraday Trans I 1989;85: 3359. [8] Bremer LGB, Bijsterbosch BH, Schrijvers R, Van Vliet T, Walstra P. On the fractal nature of the structure of acid casein gels. Colloids Surf 1990;51:159. [9] Walstra P, Kloek W, Van Vliet T. Fat crystal networks. In: Garti N, Sato K, editors. Crystallization Processes in fats and Lipids Systems. Dekker; 2001. p. 289–328. [10] Bremer LGB, Walstra P, Van Vliet T. Estimations of the aggregation time of various colloidal systems. Colloids Surf A 1995;99:121. [11] Van Dijk HJM, Walstra P, Schenk J. Theoretical and experimental study of onedimensional syneresis of a protein gel. Chem Eng J 1984;28:B43. [12] Van Vliet T, Walstra P. Water in casein gels: how to get it out or keep it in. J Food Eng 1993;22:75. [13] Van Vliet T, Walstra P. Large deformation and fracture behaviour of gels. Faraday Discuss 1995;101:359.