The depletion of fossil oil reserves and their ever-increasing prices drive the search for alternative cheap and sustainable resources to produce fuels and chemicals that have a minimal impact on the environment, which can produce long-term economic benefits. This shifts the society from fossil fuel-dependence to a more sustainable-based resource economy.

Plant polysaccharides are abundant, and their full utilization potential is vital for a future sustainable bioeconomy because plant polysaccharides are labeled as non-toxic, biodegradable, and biocompatible. Most importantly, they come from renewable resources.

Aerogel is a man-made, ultralight, solid material that is characterized by a highly porous structure and a low density with a very high surface area. Aerogels can be formed by replacing the liquid phase of a gel with air/gas using techniques that maintain the three-dimensional 3D-structure i.e. volume of the gel in a dry state.

In his doctoral dissertation Abdul Ghafar from Faculty of agriculture and forestry at the University of Helsinki obtained bio-based composite aerogels that can be labeled as “green materials,” since the raw materials are isolated from renewable resources i.e. plants. Furthermore, there is no use of hazardous chemicals in the material preparation.

- These bio-based composite aerogels are safe for food and food-related applications because the cross-linking technique does not involve any toxic chemicals during the aerogel processing, says Ghafar.

Polysaccharides have attracted attention over the past few years for bio-based materials development. However, most of the polysaccharides in their native form do not possess the optimal functionality, for example, strong gel forming ability at low concentrations which is required for certain applications like aerogel formation. Therefore, polysaccharides require some modifications to achieve such functionality.

Literature study showed that chemical cross-linking is an easy and quick way to introduce a new functionality to polysaccharides. However, when using modified polysaccharides for material in contact with humans or for human consumption, then chemical cross-linking is considered a safety risk due to the toxicity of the chemical used as a cross-linker. Another drawback of chemical cross-linking is the non-selective catalytic activity of the chemicals, which results in side product formation during modification of polysaccharides. Enzymes were exploited as an alternative way for polysaccharide functionalization in order to avoid the limitations faced by chemical modification

Ghafar suggests galactose oxidase to be used as environmentally friendly modification techniques to develop polysaccharide-based aerogels from two different polysaccharides: guar galactomannan and tamarind seed xyloglucan. He added nanofibrillated cellulose as reinforcing agent to see their effect on the material’s properties. Aerogel is formed using two different methods: freeze drying and supercritical CO 2 drying method. Ghafar studied the effect of these methods and their processing conditions in relation to aerogel’s properties.

Ghafar’s study showed that the freeze drying technique is more efficient in terms of preserving the hydrogel’s original volume when converting to the aerogel, especially as compared to supercritical CO 2 drying, which showed significant volumetric shrinkage during the drying process.

In short, this study showed that the properties of aerogels were dependent on many factors, such as the types of biopolymers i.e. guar galactomannan and tamarind seed xyloglucan used, their interaction with nanofibrillated cellulose, and the morphology of the produced aerogel, including pore sizes and their distribution. This latter characteristic was significantly dependent on the type of drying technique and pre-drying steps, which complicates the prediction of an aerogel’s properties from a hydrogel’s properties.

- To obtain aerogels with desired characteristics we need to select the starting material and drying technique carefully to be able to control the processing parameters, Ghafar sums up.

Potential applications of polysaccharide-based biocomposite aerogels

Primary and secondary packaging materials to avoid mechanical abrasion to fruits and vegetables during transportation. Water absorbent in fresh meat packaging. For absorption and controlled release applications in the food industry, such as for flavor or nutraceutical ingredients or as carrier materials in the pharmaceutical industry.

Dissertation:

M.Sc. Food Technology Abdul Ghafar will defend the doctoral dissertation on 2 March 2018 at 12:00 entitled: "Novel functional materials from upgraded biopolymers: polysaccharide aerogels".

The public examination will take place at the following address: Infocenter Korona, lecture hall 235, Viikinkaari 11, 00790 Helsinki. The opponent is Professor Kristiina Oksman, Luleå University of Technology, Sweden, and the custos is Professor Kati Katina.

The dissertation is also available in electronic form through the Ethesis service.

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