'Food Goes Nano' — New Horizons for Food Structure Research

By Martin E. Leser, Martin Michel, and Heribert J. Watzke

NESTLÉ RESEARCH CENTER LAUSANNE, NESTEC LTD, VERS-CHEZ-LES-BLANC, CH-1000 LAUSANNE 26, SWITZERLAND

1 Introduction

Food is such a familiar material to all of us that it is easy to overlook the science input required at all stages of its 'life cycle', from raw materials, production, storage and distribution, through to consumption and to the final body effects. The relevant properties of foods, such as texture, taste, colour, viscosity, stability, mouth-feel and nutritional functionality, are not simply the result of the presence of all the ingredients mixed together during processing, but are the outcome of ingredients interacting to form a certain three-dimensional structure. The dominant underlying phenomena can be quite complicated and their understanding requires the use of a multi-disciplinary approach including physical, physicochemical, chemical, biological, biochemical, and nutritional competencies.

Analyzing the knowledge gained in the study of Food Science and Engineering so far, we realize that most of our understanding is on the chemistry and functionality of single food molecules. This involves flavour chemistry, oxidation phenomena, nutritional functionality, etc., on the one hand, and the bulk behaviour of materials and systems, such as the rheological or textual properties, on the other hand. The investigation of structure formation in food materials has been ignored for a long time, since food engineers know little about the underlying science linking food structure to product properties. We still know almost nothing about the intermediate length scale, the nano- or mesoscale, which is the natural threshold where all living systems and man-made systems work. This is the scale where the first level of organization of molecules is established. Here, the basic properties and functions of food material structures and systems are defined, and, even more importantly, these can be changed as a function of the organization of matter via 'weak' molecular interactions. We believe that the relevant scale of focus in future food structure research will shift from the macro- and micro-level to the nano-level. 'Nanoscience' looks at the world as it works on the nanometre scale. It involves research on structures having at least one dimension in the range from about one up to several hundred nanometres. The age of 'nanotechnology', which means finding applications out of nanoscience concepts, has not really started yet.

2 Food is Made from Biomaterials

When speaking about nanoscience in relation to food, it is clear that the two terms are already intrinsically connected to each other in the sense that food is made of biomaterials, the result of Nature's own nanotechnology. Amongst Nature's nanotechnology products are milk proteins, e.g., the caseins or lactoglobulins, which are synthesized by epithelial cells in the mammary gland from amino acids extracted from the blood. A protein is a construction from thousands of atoms arranged in a precise molecular structure that spans some tens of nanometres. Concerning protein structure formation, there is evidence that the formation of casein micelles takes place in the secretory vesicles in the mammary gland, and that this is orchestrated with the transport of ions, the phosphorylation and glycosylation of the caseins, and lactose synthesis. Moreover, the intravesicular ionic environment and the casein concentration seem to change continuously during the 20 minutes required for casein micelle assembly.

Milk proteins and phospholipids have been used for many decades as emulsifiers and texturizing agents in food production. On the molecular level, these ingredients and their chemical properties are very often intrinsically the same in both their biological and food environment. By using specific chemical and physical modifications we can also add new functionalities to these molecules. There are, however, fundamental differences between the self-assembly occurring in living systems and that occurring in man-made materials. The specificity of the molecules in biological systems, such as organelles, cells or total organisms, is determined by a so-called 'coded self-assembly process'. The instructions for the system design are built into its components based on the instructions of the genome. The whole system is kept thermodynamically in balance by energy-dissipating molecules such as ATP. In contrast to living organisms, the self-assembly of food ingredients such as proteins and low-molecular-weight surfactants is purely thermodynamically driven.

So far, the potential of self-assembly has not been intentionally used in the manufacturing of food materials. We are convinced that a better understanding of the supramolecular structuring principles observed in nature will uncover new phenomena and lead to new manufacturing processes for high-added-value food products and ingredients. A prerequisite for this approach is a proper understanding of the scientific principles of molecular interactions, leading to supra-molecular structures, and their influence on the generation and stabilization of higher hierarchical structures, e.g., oil droplets, air cells, etc., present in most food materials.

3 Hierarchical Structures in Food

Food structures are formed on a wide range of length scales, from millimetres down to nanometres. Prominent structural entities on the microscopic length scale (called also 'microstructure') are oil or water droplets in emulsions, such as in milk or margarine, gas bubbles in foams, such as in mousses or ice-cream, and fat crystals, such as in chocolate. Examples of structural entities on the nano-length scale ('nanostructure') are association colloids (micelles, liposomes, liquid crystalline phases, etc.) made of surfactant molecules like phospholipids or monoglycerides, physical entanglement structures in gel networks, and the tertiary structure of biopolymers, such as proteins or polysaccharides. Figure 1 shows some examples of surfactant association colloids, which are formed when the surfactant is dissolved in water. Surfactant bilayers, for instance, are straight-forward nanoscopic supramolecular structures. It is important to note that nano-structures and microstructures are not only different with respect to their size but also with respect to their thermodynamic properties: while microstructures are thermodynamically unstable, and after a certain time will be destroyed (they have to be kinetically stabilized in order to retard their destruction and give the product the desired shelf-life), nanostructures are thermodynamicaly stable, unless the surfactant molecules react with the environment or become degraded.

Once the individual structural entities of the first hierarchical level are formed, they are free to interact with each other forming larger functional units on a higher hierarchical level. Due to the progress made in colloidal science on model food systems, we understand some of the most essential principles that govern the formation of higher levels of hierarchical structures in dispersed systems like emulsions or particle gels. Prominent examples are the aggregation phenomena observed in oil-in-water emulsions or in particle gels. However, the main focus of these studies, so far, is on visualization of the formed structures and on investigation of the specific interaction forces exerted between single microstructures. One of the greatest challenges that food colloid science faces today is to understand the physical principles governing interactions among individual components or particles, not only of the same order of size, but also of very different sizes. It becomes more and more evident that the essential properties of food colloids critically depend upon phenomena or processes taking place on very different scales of length and time. The development of a 'multi-scale food colloid science', as illustrated in Figure 2, seems therefore to be the next step ahead in food structure research. In this effort, computer simulations will be even more important than they are already today.

Another piece of evidence which significantly influences hierarchical structure formation in food materials is related to the fact that commercial food ingredients are multi-component mixtures and are never pure in terms of their molecular composition. Mainly, structure formation on the nanoscale is influenced by molecular heterogeneity. In order to apply nanoscience approaches successfully in food structure research, it will be important to understand the influence of 'impurities', present in commercial ingredient formulations, on the formation of association colloids, physical entanglement structures, or the established tertiary structure of proteins.

4 Structure Formation in Food: 'Bottom-up' versus 'Top-down' Structuring

The difference in the thermodynamic properties of nano- and microstructures is the basis for their different formation strategies. While microstructures are created as a result of energy input (e.g., homogenization of oil-water mixtures), nanostructures are formed spontaneously after mixing the appropriate ingredients in the appropriate concentrations and under the appropriate physicochemical conditions. The former structuring approach is called 'top-down' structuring, referring to the fact that large (infinite) structures are broken down into smaller ones mostly on the micrometre length scale (this is also referred to as a 'miniaturization process'). Note that there is a minimum structural size that can be attained using 'top-down' structuring concepts due to a limitation on the energy density achievable in food unit operations. The spontaneous formation of self-assembly nanostructures or higher hierarchy structures is called 'bottom-up' structuring; this is the strategy adopted in biological systems for the hierarchical structuring of molecules. The 'bottom-up' strategy of non-food materials fabrication is a new trend in the nanotechnology of soft materials. 'Bottom-up' structuring is also called 'molecular nanotechnology' because of the molecule-by-molecule or component-by-component formation of hierarchical structures. It is considered as 'advanced nanotechnology', the nanotechnology for tomorrow, whereas 'top-down' structuring is the more 'primitive' technology mainly applied in today's industrial production.

Over the past decade, using the self-assembling nature of artificially designed molecules, chemists have succeeded in constructing many kinds of nanometrescale molecular assemblies, e.g., molecular recognition-directed molecular assemblies, surfactant bilayer membranes, Langmuir-Blodgett films, self-assembled monolayers, or, alternatively, deposited polyelectrolyte multilayers. Nanometre-scale molecular self-assembly is governed by the delicate balance of different non-covalent forces exhibited between molecules, such as electrostatic versus van der Waals forces. Nanometre-scale molecular self-assembly is the first step in the 'biomimetic' approach of the 'bottom-up' strategy for materials fabrication. Note that 'self-assembly' stands here for the autonomous organization of components into patterns or structures without human intervention.

The second step of the 'bottom-up' strategy is to organize the nanometer-scale molecular assemblies into larger systems, which are on the mesoscopic scale from 10 nm to the sub-micrometre range; this is the nanotechnology of 'soft materials'. This represents an unexplored field, especially in food science and engineering.