Structure and mechanical properties of fat crystal networks (original) (raw)
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Structure and functionality of edible fats
Soft Matter, 2012
Fat-structured food materials are an important component of our diet. The role that fat plays in material functionality, flavor perception, texture and health characteristics is due in large part to its physical properties. An understanding of these physical properties is relevant from scientific, technological and medical perspectives. The physical properties of fat materials, are, in turn, governed by a complex confluence of the various structural levels in a fat material beginning with triglyceride molecules. The formation of nanoscale structural elements by these molecules is the first step in the formation of a fat material as we know it. This review shows how these microstructural elements can be imaged and characterized. It is also shown that the formation of these nanocrystals is affected by the attendant crystallization parameters. Through simulation and a discussion of van der Waals forces, it is shown that these nanoscale elements assemble into colloidal aggregates with fractal character. The influence of microstructure on the mechanical properties of a fat material is explained using a variety of mechanical models. Lastly, this review examines methods by which the properties and characteristics of the various structural levels can be engineered. Shear has been shown to affect the polymorphism and phase transition kinetics of triglyceride crystals. As well, shear has been shown to modify the aggregation of nanocrystals, with consequences for the porosity and diffusivity of oil through the fat crystal network.
Crystal Growth & Design, 2011
The study of microstructure in polycrystalline materials such as edible fats has become increasingly important since the macroscopic properties of such materials depend on the structure of their crystal networks. 1À7 Edible fats are a class of plastic materials composed of a continuous network of crystalline fat suspended in an oil phase. It is widely held that the balance between van der Waals attractions and Brownian motion causes the suspended crystals to aggregate and form a three-dimensional (3-D) network via diffusion-limited clusterÀcluster aggregation. 10,11 The properties of this 3-D network depend not only on the amount and distribution of the network mass but also on the properties of the individual particles. These include the size, shape, and arrangement of the crystals. A significant amount of research has shown that the habit of fat crystals is greatly affected by heat, mass, and momentum transfer conditions established during the crystallization process. 6,12À14 Therefore, by modifying processing conditions (i.e., crystallization temperature, cooling rate, and agitation rate), the crystal habit and the subsequent properties of the crystal network can be tailored. It has been shown that the crystallization temperature affects the packing of the triacylglycerol (TAG) molecules and thus influences the microstructure of the crystallized fats. 6,15,16 Marangoni and McGauley (2003) reported that while cocoa butter crystallized at different temperatures showed similar polymorphism, the resultant material exhibited dissimilar microstructure. 6 Several research groups have demonstrated the effects of cooling rate on the nucleation, polymorphism, and aggregation of small crystalline particles in fat materials. 5,17À20 They observed that the slow cooling of milk fat results in a small number of large crystals, whereas rapid cooling produces numerous small crystallites with a fairly uniform size distribution.
Scaling Behavior of the Elastic Modulus in Colloidal Networks of Fat Crystals
Journal of Physical Chemistry B, 2004
The scaling relationship between the shear elastic modulus and the solid fat content (SFC) was determined for anhydrous milk fat (AMF), palm oil (PO), and cocoa butter (CB). The fats were diluted with canola oil to achieve specific SFCs and crystallized at 5°C for 24 h. SFC decreased linearly by increasing the canola oil mass fraction as determined by pulsed NMR. Log-log plots of the shear storage modulus (G′) versus the solids' volume fraction (Φ ) SFC/100) of the diluted fats were used to determine the fractal dimension (D) of the networks. Three different linear regions were identified for the range of dilutions studied. The scaling relationship of the stress at the limit of linearity (σ o ) to Φ indicated that the fats were in the weak-link rheological regime in all three regions. These results suggested that three different types of weak-link rheological regimes could be present in the same material depending on the SFC. Polarized light microscopy showed that varying the solid fat content (SFC) modified the microstructure of these fats. In general, at low SFCs, large crystal clusters were observed, while at high SFCs, only a fine crystal mass was detected. Crystallite morphology and size distribution was also affected by dilution. The onset of crystallization temperature (T c ) and the peak melting temperature decreased with decreasing SFC in the three fats; however, plots of T c versus SFC demonstrated the existence of distinct linear regions that were similar to those identified in the rheological data. Moreover, Hildebrand plots also demonstrated the existence of distinct linear regions, of characteristic solution behavior, which agreed closely with crystallization and rheological results. We propose that, upon dilution, changes in crystal phase behavior lead to changes in the crystallization kinetics of the fats. This in turn translated into alterations in the microstructure of the fat, which ultimately affected its mechanical properties.
Microscopic and rheological studies of fat crystal networks
Journal of Crystal Growth, 1999
This talk discusses the quantification of microstructure in fat crystal networks by using the relationship of the elastic moduli (G) to the solid fat content (SFC) via the fractal dimension (D) of the network. Results from application of a scaling theory developed for colloidal gels [W.H. Shih et al., Phys. Rev. A 42 (1990) 4772] to chemically and enzymatically interesterified and non-interesterified butterfat/canola oil mixtures and to cocoa butter and Salatrim2+ are presented and discussed. In situ images from confocal laser microscopy and polarized light microscopy of the crystal network of butterfat and butterfat-canola oil blends yielded fractal dimensions of D"1.88 and D"2.02$2% respectively, which are in good agreement. The use of rheological measurements to calculate a fractal dimension of the crystal network of butterfat-canola oil blends yielded D"1.99, in good agreement with D calculated from the confocal laser microscopy and the polarized light microscopy images. 0022-0248/99/$ -see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 -0 2 4 8 ( 9 8 ) 0 1 0 1 6 -1
Nanoscale structure intercrystalline interactions in fat crystal networks
Current Opinion in Colloid & Interface Science, 2011
The functional attributes of fat-structured food products such as butter, margarine, chocolate, and ice cream are strongly influenced by the structure and physical properties of an underlying fat crystal network present in the material. Fat crystal networks are arranged in a hierarchical manner with characteristic and quantifiable nano and mesoscale structures. Recent studies carried out by our group have demonstrated that the formation of such a fat crystal network starts with the association of nanoplatelets at the lowest constitutional level. These nanoplatelets interact and aggregate via van der Waals's forces into larger fractal structures, which eventually form a 3-dimensional network responsible for the solid-like characteristics of the material. The purpose of this review is to summarize recent efforts in the characterization and quantification of these recently discovered crystalline nanoplatelets and to discuss the role of van der Waals interactions between them. In addition a brief discussion of previous fractal model will be presented. The new experimental findings on the nanostructural level will then be used to validate our fractal structural-mechanical model of fats . These new insights will contribute to our knowledge of the nature of fat crystal network in plastic fats at different length scales and the relationship of these structural characteristics to the function and properties of fats.
Current Opinion in Colloid & Interface Science, 2014
Triacylglycerols (TAGs) are the majority molecules present in edible fats and oils. Many of the functional characteristics of fat products depend on the colloidal fat crystal network present. Identifying the hierarchies of these colloidal networks and how they spontaneously self-assemble is important to understand their functionality and the oil binding capacity, and new insights into the nano-to meso-scale structure in these colloidal fat networks have been reported in recent years. Ultra small angle X-ray scattering (USAXS) is a technique new to the study of edible oil structures and, when combined with modelling and computer simulation, has enabled significant advances to be made in understanding the nano-to micro-scale crystalline structures of edible oils. In the four years since crystalline nanoplatelets (CNPs) were characterized, models have been made of these highly anisotropic nanoscale structures in which they were treated as the primary unit. In those models, CNPs were represented as close-packed rigid layers of spheres, so chosen because the van der Waals sphere-sphere interaction is known. The intent of the models was to predict the hierarchy of colloidal fat networks that would self-assemble from the components in edible oils. Initially, CNP aggregation was modeled under the assumption that all CNPs are present before aggregation begins and that their solubility in the liquid oil is very low. The models successfully predicted the fractal dimensions subsequently measured using USAXS. This brief review reports on some of the latest models and simulations together with the results of USAXS experiments carried out on binary lipid systems, such as SSS in OOO, as well as certain complex systems that contain many different TAG molecules. The excellent agreement between the two approaches has established that USAXS is a powerful tool in the elucidation of the nano-to meso-length scales in fats and oils. 1-INTRODUCTION Edible fats are a class of colloidal gels in which an oleogel crystal network is formed as the triacylglyceride (TAG) molecules crystallize from the melt. Crystalline nano-platelets (CNPs) [1-6] have been found to be the basic components of the complex macro-colloidal structure of these edible fats. These CNPs aggregate to form
Thermodynamic and kinetic aspects of fat crystallization
Advances in Colloid and Interface Science, 2006
Naturally occurring fats are multi-component mixtures of triacylglycerols (TAGs), which are triesters of fatty acids with glycerol, and of which there are many chemically distinct compounds. Due to the importance of fats to the food and consumer products industries, fat crystallization has been studied for many years and many intricate features of TAG interactions, complicated by polymorphism, have been identified. The melting and crystallization properties of triacylglycerols are very sensitive to even small differences in fatty acid composition and position within the TAG molecule which cause steric hindrance. Differences of fatty acid chain length within a TAG lead to packing imperfections, and differences in chain lengths between different TAG molecules lead to a loss of intersolubility in the solid phase. The degree of saturation is hugely important as the presence of a double bond in a fatty acid chain causes rigid kinks in the fatty acid chains that produce huge disruption to packing structures with the result that TAGs containing double bonds have much lower melting points than completely saturated TAGs. All of these effects are more pronounced in the most stable polymorphic forms, which require the most efficient molecular packing. The crystallization of fats is complicated not just by polymorphism, but also because it usually occurs from a multi-component melt rather than from a solvent which is more common in other industrial crystallizations. This renders the conventional treatment of crystallization as a result of supersaturation somewhat meaningless. Most studies in the literature consequently quantify crystallization driving forces using the concept of supercooling below a distinct melting point. However whilst this is theoretically valid for a single component system, it can only at best represent a rough approximation for natural fat systems, which display a range of melting points. This paper reviews the latest attempts to describe the sometimes complex phase equilibria of fats using fundamental relationships for chemical potential that have so far been applied to individual species in melts of unary, binary and ternary systems. These can then be used to provide a framework for quantifying the true crystallization driving forces of individual components within a multi-component melt. These are directly related to nucleation and growth rates, and are also important in the prediction of polymorphic occurrence, crystal morphology and surface roughness. The methods currently used to evaluate induction time, nucleation rate and overall crystallization rate data are also briefly described. However, mechanistic explanations for much of the observed crystallization behaviour of TAG mixtures remain unresolved.
Relating structure of fat crystal networks to mechanical properties
Food Research International, 1999
This paper reviews the identification of the various levels of structure present in fat crystal networks, and the development of analytical techniques to quantify these levels. The relationship of the various levels of structure to macroscopic physical indicators of the mechanical strength of the network is discussed. The analysis of the microstructural level of the network via fractal geometrical methods
Structural and Mechanical Properties of Fats Quantified by Ultrasonics
Journal of The American Oil Chemists Society, 2007
Since the velocity of an ultrasonic wave through a material depends on its density, bulk modulus (K), and shear modulus (G), a new approach to determine the shear elastic modulus and the mass fractal dimension (D) in a fat crystal network was developed. An ultrasonic chirp wave containing a range of frequencies and amplitudes, was used to estimate the structural and mechanical properties of palm oil based fats, crystallized under shear at three different temperatures (20, 25, and 30 °C). Considering the fat crystal network as a two-phase system (i.e. liquid and solid fat) the velocity of sound in both phases was obtained separately, assuming that the speed of sound in the oil phase was inversely dependent on the temperature. A constant shear modulus for the solid fraction was obtained experimentally by rheology, which was independent of the sample’s nature. These parameters were used for the determination of sample compressibility and its corresponding shear modulus by ultrasonic velocimetry. In addition fractal dimensions (D) were determined by using the relationship of the shear elastic modulus (G) to the mass fraction of the solid fat (φ) in a weak-link regime. The obtained results are comparable and consistent with previously reported fractal dimension values. This method allows online determination of the shear modulus of fats and could be potentially applied for quality control purposes in manufacturing.
Quantification of the physical structure of fats in 20 minutes: Implications for formulation
Lipid Technology, 2014
One challenge facing the fat industry involves finding healthy fat-replacers that do not compromise the functionality of the product made with them. For the past three years crystalline nanoplatelets (CNPs) have been reported as the smallest crystal unit in different edible fat systems. This paper summarizes the latest understanding in the area of CNP aggregation and the structures that emerge from their aggregation when using the techniques of ultra small angle X-ray scattering and modelling with computer simulation. An understanding of how these CNPs aggregate should allow the engineering of new healthy fat-replacers.