Understanding Foods as Soft Materials
Sixth entry by Kelly Miller, AP225 Fall 2011
Title: Understanding Foods as Soft Materials  Authors: Raffaele Mezzenga, Peter Schurtenberger, Adam Burbide, Martin Michel Journal: Nature Materials, Volume 4, October 2005, pgs. 729-740
Foods are one of the most complex examples of soft condensed matter. Their complexity is a function of many factors: intricate composition, different aggregation states, and many relevant characteristic time and length scales.
Foodstuffs are governed by the rules of soft condensed matter but, all the complicating aspects of real systems also play a large role. Consequently, the study of food as an example of soft matter has deepened our understanding of the field.
This review paper discusses the current understanding of food science, by considering established soft condensed matter methods as well as emerging techniques. The complexity, heterogeneity and multitude of states found within the realm of food as a material provides the field of soft condensed matter physics with a challenge of remarkable importance.
A wide range of systems are described in this paper - a comparison is made between the kinetics of foods structured at small length scales and those with a longer length scale. If correlation length scales are small, very short times are needed to reorganize molecules to attain equilibrium. This is the case with self-assembled liquid crystalline foods (scale is typically a few nanometers), for example. As a result of the short length-scale, these structures are almost always observed at equilibrium. Foam, on the other hand, has length scales on the order of millimeters and therefore, can be stable for long times despite the high internal energy associated with the presence of very large interfaces.
Although this paper discusses many different aspects of soft condensed matter, in the context of food, in order to stay on this week's topic (phases and phase diagrams) I will focus here on the paper's discussion of self-assembled liquid crystalline foods.
Self-Assembled Liquid Crystalline Foods
Liquid crystalline refers to a state of matter that has properties between those of a conventional liquid and those of a solid crystal. Liquid crystalline substances may flow like a liquid, for example, but have molecules oriented like a crystal. Self-assembled liquid crystalline matter describes ordered substances that have formed from a disordered system of pre-existing components due to local interactions among the components themselves.
Unlike foods with larger characteristic length scales, these compounds are self-aggregating molecules governed by intra- and intermolecular forces. Liquid crystalline foods are nanostructured materials whose formation is based on the self-assembly of short surfactants, such as monoglycerides or phospholipids and water. The oil-water interface of salad-dressings, for example, would have self-assembled liquid crystalline structures. In the context of food, these structures can be found in one of the following 3 forms:
1) In the bulk-state or re-dispersed in water to form colloidal dispersions with an internally ordered structure
2) Efficient building blocks for more complex functional foods and can be used as carriers of ingredients spread in either the hydrophobic or hydrophillic phase
3) Nanoreactors - to run and control food-specific chemical reactions within confined geometries and under controlled conditions
(such as the Maillard reaction where amino acids and sugars react to produce aromas and flavors)
Phase Diagrams of Liquid-Crystalline Foods
Liquid-crystalline foods have rich phase diagrams that contain a broad range of structures:
-lamellar phases with amorphous or crystalline lipid domains
-inverted columnar hexagonal cylinders (see inset (a) in Figure 6 above)
-bicontinuous double gyroidal (see inset (b) in Figure 6 above)
-primitive cubic phases (see inset (c) in Figure 6 above)
These mesophases have viscoelastic properties that depend heavily on their specific structure and cover a range of behavior that spans from viscous fluids to rubbery solids.
Thermodynamically, liquid-crystalline food molecules are also complex. Currently, there is no general quantitative theoretical framework available to explain the change of these structures with changes in temperature and composition. What is known is, despite the low relative molecular mass, lipids and water self-assemble into ordered structures due to the large enthalpy of mixing that exists between the two phases formed by:
1) the solution of water and hydrophilic lipid heads
2) the hydrocarbon-based lipid tails
Using the Flory equation, this enthalpy has been estimated to be around <math>\chi </math>=3
For lipids, the threshold in molecular mass of the surfactants for self-assembly is approximately 3-4 segments. For water-lipid systems the order-disorder transition for surfactants is around 12-16 atoms. This explains why in foods, where typical lipid-based surfactants always have a molecular mass beyond this threshold, self-assembly is not uncommon. Self-assembly of liquid-crystalline foods is governed mostly by enthalpy, rather than by competing enthalpy and entropy of mixing. Despite this fact, the phase diagrams in lipid-water systems are still hugely complicated (see figure 7 below).
The concept of critical packing parameter (CPP) has been used to try to classify the various structures of liquid-crystalline foods. This is the ratio between the volume of the hydrophobic lipid tail and the product of the cross-sectional lipid head area and the lipid chain length. From this, different liquid-crystalline phases may be predicted from the curvature of the water-lipid interface. It should be noted however, that the CPP can explain qualitatively a number of features of liquid crystalline foods (for example - if the temperature is raised the number of water molecules hydrating the polar heads of the lipids will decrease, due to breaking of the hydrogen bonds) but, the quantitative details of these phase diagrams still need to be worked out.
A quantitative model, well known in polymer-polymer interfaces, has been proposed in explaining self-assembly for lipids and water as a function of temperature and composition. This model is based on a "mean-field approach" and involves the individual lipid chains "seeing" the field exerted by the other surrounding chains. Despite its success in understanding polymer systems, it has yet (at the time that this review paper was published) to produce quantitative agreement for lipid-water mixtures.
It is amazing the interactions between molecules in a form of matter as prolific as food still have so many aspects that are poorly understood. The fact that there is still a lot of work that needs to be done to understand the phase transitions for liquid-crystalline systems illustrates their complexity and the huge potential for discovery in this field.