Hierarchical mesoporous silica materials for separation of functional food ingredients — A review

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Abstract

Recently developed mesoporous silica materials are proposed as adsorbents for the separation of food bioactive molecules, due to their narrow pore size distributions and high surface area. These materials can be synthesised with a variety of porous architectures with uniform pore sizes in the mesoporous range, making them attractive candidates for adsorption of biomacromolecules. Research on these materials to date has largely focused on their synthesis, characterisation and applications in catalysis. However, recent developments in the bioadsorption ability and capacity as well as the aqueous stability of mesoporous materials demonstrate their potential as adsorbents for separations in the food industry. This paper reviews the research in this area and identifies the challenges remaining for the application of these materials in food based separations.

Industrial relevance

The increasing demand for health-promoting foods is a key driver for the development of highly selective, cost-effective separation technologies for food bioactive molecules. Separation and purification stages in industrial biotechnology processes can account for up to 70% of the capital and operating costs. A significant portion of the functional food market is devoted to dairy functional foods and ingredients, as milk and whey provide rich sources of bioactive proteins and peptides with a variety of biological and nutritional properties. Hence this paper focuses upon the potential for use of hierarchical mesoporous silica materials for separation of functional food ingredients, taking dairy streams as a representative example.

Introduction

The dairy industry has recognized the value of bioactive components, implementing separation processes, which traditionally employed elevated temperatures, low pH and high shear forces, conditions that can be detrimental to protein functionality (Korhonen, Pihlanto-Leppala, Rantamaki, & Tupasela, 1998). Chromatography has emerged as an important technique for fractionation and purification of proteins from food streams, providing greater selectivity without employing conditions such as high shear or high temperature (Bargeman, 2003, DeSilva et al., 2003, Gerberding and Byers, 1998). Further development and implementation of improved separation technologies would allow food manufacturers to isolate desirable components more efficiently, while maintaining their intrinsic functionality for use as ingredients in functional foods and nutraceuticals.

Adsorbents can provide a non-denaturing and highly selective medium for separation of bioactive molecules. Depending on the target molecule, they can be designed to achieve separations on the basis of size-exclusion, ion-exchange, hydrophobic or affinity interactions (Bargeman, 2003). A new paradigm in adsorbent design was made possible by the development of uniform pore size mesoporous silica materials in the early 1990s (Beck et al., 1992, Yanagisawa et al., 1990). These materials posses highly ordered and tunable structures with pore dimensions in the mesoporous region (2–40 nm). A wide range of porous architectures have been developed, ranging from hexagonally-arrayed linear pores through to interconnected 3-D structures with hierarchical pore dimensions ranging from micro- to mesoporous sizes (Polarz & Smarsly, 2002). Materials can be designed with a range of properties making them ideal potential adsorbents for biomolecule separations. In particular, narrow tunable pore size distributions, along with large surface areas (∼ 1000 m2 g 1) and pore volumes give them potential for high adsorption capacity and selectivity in biochemical separations.

The potential of mesoporous materials is yet to be realised in industrial scale food separations. In order for mesoporous materials to be applied commercially as chromatographic adsorbents in the food industry, factors including their stability under typical food processing conditions, selectivity, capacity, and regeneration efficiency need to be understood and optimised. Suitable chemical surface modifications can be made to these materials to tailor their performance for particular applications. This paper reviews the properties and potential of these materials as well as progress towards their application in food industry separations.

Section snippets

Mesoporous materials

In 1992 scientists at Mobil Research and Development Corporation discovered a new class of mesoporous silica materials, known as the M41S family (Beck et al., 1992). M41S materials, including MCM-41 (“Mobil Composition of Matter 41”) and MCM-48, possessed unique structural and physical characteristics notably uniform pore sizes in excess of 2 nm, surface areas greater than 1000 m2 g 1 and long-range ordering of the packing of pores.

The original synthesis of M41S materials was achieved by the

Biomolecule separations

Although mesoporous silica materials display promising properties for the separation of biological molecules, only limited work in this area has been reported. Han, Stucky, and Butler (1999) investigated the size-exclusion properties of (3-aminopropyl)-triethoxysilane (APTS) functionalized SBA-15 and MCF. Three differently sized proteins, with similar isoelectric points: conalbumin (MW 77 000, pI 6.0), chicken egg ovalbumin (MW 44 000, pI 4.9) and soybean trypsin inhibitor protein (MW14 000 pI

Adsorption capacity and kinetics

The adsorption capacity of an adsorbent is important for separation applications. Adsorption capacity is affected by steric effects and the interaction between the solute and adsorbent surface. Katiyar, Ji, Smirniotis, and Pinto (2005) reported that the loading capacity of myoglobin (pI = 7–7.2) onto SBA-15 was dependant on the solution pH, due to the strong influence of electrostatic interactions. At a pH of 8.5 the adsorption capacity was very low (∼ 25 mg g 1) as both the solute and adsorbent

Stability

In order to apply mesoporous materials to large scale applications in the food industry, stability under operating conditions is crucial. Dairy components are typically separated from aqueous solutions employing chromatography or membrane operations under pressure driven flows. Furthermore, separation media are cleaned and regenerated with alkaline solutions generally containing 0.5–1.0 M sodium hydroxide at 80 °C (Tragardh & Johansson, 1998). Thus chemical, thermal and mechanical stabilities

Challenges remaining

Despite the promising features of mesoporous materials identified above, very few commercial applications exist so far and their potential in bioseparations for the food industry is yet to be realised. A number of research and development challenges need to be addressed before they could be economically implemented in food separations.

Much of the limited adsorption data for biological molecules on mesoporous silica reported to date is based on adsorption from model solutions containing a single

Concluding remarks

Mesoporous silica materials show promise as adsorbents that could be tailored for the efficient separation of functional food ingredients, particularly biological macromolecules. They can be synthesised with high surface areas and pore volumes as well as a variety of porous architectures with uniform pore sizes in the mesoporous range, allowing access for large molecules. Recent developments have demonstrated that a variety of biological macromolecules can be adsorbed in the pores of mesoporous

Acknowledgements

Ryan Brady acknowledges the Australian Government for an Australian Postgraduate Award and CSIRO Food Futures Flagship for a Postgraduate Scholarship. The authors acknowledge the funding support from the University of Melbourne and CSIRO Collaborative Research Support Scheme and access to infrastructure from the Particulate Fluids Processing Centre, a special research centre of the Australian Research Council.

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