Microstructural Analysis of Biomaterials and Food

Our experience of food depends greatly on its microscopic structure. In LINX, DTU develops new 3D-imaging methods for food and other challenging bio-materials.

The core expertise of the Technical University of denmark (DTU), as unfolded in LINX, is imaging – also known as tomography. It uses X-rays (or neutron radiation) to produce 3D-images of materials with micrometer resolution. In this sense, imaging can be regarded as a “super microscope” which sees not only the surface of things but right through them.

Food is an extremely important class of everyday “materials” – usually not thought of as such, simply because we eat them. Nevertheless, the whole sensory impression we get from food depends on its structure at a microscopic level. Crystal formation by water in ice cream makes for a “crunchy” unpleasant eating experience, and meat which has been frozen requires more careful cooking in order to stay tender and not becoming inedibly dry. Why? How? Imaging is, at the outset, an excellent tool for revealing the answers to both questions.

Foodstuffs are challenging materials to scrutinize, however. The difficulties with imaging largely hinge on the facts that (1) food is usually very inhomogeneous on the microscale, with many changing and competing structures, and (2) that most biologic materials are more or less equally transparent to X-rays. In other words, all the main components of food – carbohydrates, protein, fats and fibers – are, in imaging terms, quite similar. None of them truly stand out, and this makes the contrasts between different 3D-regions harder to distinguish. In short, it becomes extremely difficult to produce a sharp image with clear boundaries between different, neighboring “building blocks” throughout the whole sample. Everything smears.

Pattern Recognition

In LINX, DTU takes on this challenge by developing new data-processing methods for imaging. These are based on the mathematics of pattern recognition. While neighboring regions in foodstuffs are hard to distinguish individually, they may nevertheless be brought into focus if they are part of a common, repeated and dominant feature in the food in question: The “true” image can be reinforced (inferred) on the basis of sheer statistics and repetition frequencies. This is for instance the case with chocolate, where sugar particles lie trapped in fat; the statistics of the size and shape of the particles as well as the distance between them can be used for removing the image blur of the border regions. Other examples are plant cells in vegetables or water-ice crystals in ice cream, which both form repeated patterns.

The Gentle Touch

As a parallel, DTU investigates how to minimize the amount of X-ray (or neutron) radiation to which a sample is subjected. Like all organic matter, foodstuffs are damaged by strong radiation. That is to say, a particular sample may change (decompose) continuously during a measurement, which will of course render the results useless. Here, the way forward is to better exploit – or improve – computer algorithms for data treatment in order to minimize the number of “2D” X-ray pictures (as known from the dentist), which are the basis for the construction of the 3D-image.

Among the aims of DTU is to examine water uptake in grain, i.e. studying how drying and re-hydrating affect the structure and thereby subsequent use. The results, and not least the methodology itself, may later be applied to beans and all sorts of seeds. This can also be done as “accelerated live footage” of the sprouting and initial growth of the respective plants which, in turn, can be related to the action of plant enzymes. Understanding such details will not only help food companies develop new products but also to better monitor and test raw ingredients.

Techniques and Methods

A number of properties in functional materials are governed by internal microstructure of the material. The internal microstructure of materials can in many cases be studied with X-ray and neutron imaging techniques. The 3D Imaging center at DTU is focus around the use of X-ray and neutron used for studying microstructures of materials, in most cases hard materials, such as metal, batteries and catalysis. Over the last years the team around the 3D Imaging center at DTU has expanded the collaborations into areas of soft matter such as bio materials and culture heritage.
Within the LINX project there has been a collected effort to try and attract companies within the food and ingredients industry. In order to support this direction, the team at DTU working with the LINX project has strengthened the collaborations within bio materials. A number of these collaborations are described below. A collaboration with Carlsberg on micro malting of barley seeds is described in the outreach work package and is an example of how development projects can be combined with outreach projects with companies.

Imbibition in plant seeds
In this project, we collaborated with the DTU biophysics group and the Carlsberg Laboratory to apply micro-CT to study imbibition in plant seeds. We describe imbibition in real and artificial plant seeds, using a combination of experiments and theory. The data obtained on both natural and artificial seeds collapse onto a single curve that agrees well with our model, suggesting that capillary phenomena contribute to moisture uptake in soy seeds. In both systems, our experiments demonstrate that liquid permeates the substrate at a rate, which decreases gradually over time. Tomographic imaging of soy seeds is used to confirmed this by observation of the permeating liquid using an iodine stain. This result is published in Physical Review E.
Louf, J. F., Zheng, Y., Kumar, A., Bohr, T., Gundlach, C., Harholt, J., Poulsen, H. F., Jensen, K. H. (2018). Imbibition in plant seeds. Physical Review E, 98(4), 1–5. https://doi.org/10.1103/PhysRevE.98.042403

Porous material for water contamination treatment
This is a collaboration with McMaster University (Canada) and University of Copenhagen. We apply micro-CT to study the 3D structure of porous sponges, which can be used for water contamination treatment. Self-assembly of graphene oxide (GO) nanosheets into porous 3D sponges is a promising approach to exploit their capacity to adsorb contaminants while facilitating the recovery of the nanosheets from treated water. Yet, forming mechanically robust sponges with suitable adsorption properties presents a significant challenge. Ultra-strong and highly porous 3D sponges are formed using GO, vitamin C (VC), and cellulose nanocrystals (CNCs) – natural nanorods isolated from wood pulp. CNCs provide a robust scaffold for the partially reduced GO (rGO) nanosheets resulting in an exceptionally stiff nanohybrid. The concentration of VC as a reducing agent plays a critical role in tailoring the pore architecture of the sponges. By using excess amounts of VC, a unique hierarchical pore structure is achieved, where VC grains act as soft templates for forming millimeter-sized pores, the walls of which are also porous and comprised of micron-sized pores. The unique hierarchical pore structure ensures the interconnectivity of pores even at the core of large sponges as evidenced by micro and nano X-ray computed tomography. The unique pore architecture translates into an exceptional specific surface area for adsorption of a wide range of contaminants, such as dyes, heavy metals, pharmaceuticals and cyanotoxin from water. The study is published in Nanoscale.
Zheng, Y., Sørensen, H. O., Bruns, S., Yousefi, N., Hosseinidoust, Z., Tufenkji, N., & Wong, K. K. W. (2018). Hierarchically porous, ultra-strong reduced graphene oxide-cellulose nanocrystal sponges for exceptional adsorption of water contaminants. Nanoscale, 7171–7184. https://doi.org/10.1039/c7nr09037d

Micro-CT study of carbonate rock for CO2 storage
This is in collaboration with Copenhagen University to apply micro-CT technique to study carbonate rock. The quantification of surface area between mineral and reactive fluid is essential in environmental applications of reactive transport modelling. This quantity evolves with microstructures and is difficult to predict because the mechanisms for the generation and destruction of reactive surface remain elusive. The challenge of accounting for the inherent heterogeneities of natural porous media in numerical simulation further complicates the problem. Here we first show a direct observation of reactive surface generation in chalk under circumneutral to alkaline pH using in situ X-ray microtomography. The momentary increase of reactive surface area cannot be explained by a change in fluid accessibility or by surface roughening stemming from mineralogical heterogeneity. We then combine greyscale nanotomography data with numerical simulations to show that similar temporal behavior can be observed over a wide range of pH as porous media dissolve in imposed flow field. We attribute the observation to the coupling between fluid flow and mineral dissolution and argue that the extent of surface generation is strongly correlated with the advective penetration depth of reactants. To conclude, we demonstrate the applicability of using a macroscopic Damköhler number as an indicator for the phenomenon and discuss its environmental significance beyond geologic carbon storage. This is accepted for publication in the Journal of Hydrology.
In this study, we applied micro-CT to study fungal contamination on wood and wallpaper. We also applied the layer detection method, developed by QIM for image segmentation. Approximately 10-30% of North American homes are infested by moisture-induced molds, which can cause adverse health effects, especially for immunocompromised individuals. It was suggested that indoor fungal contamination is associated with sick building syndrome, which often appears as flu-like symptoms, watering eyes and allergic rhinitis. Fungal contamination not only reduced the value of the wallpaper as the cellulose-degrading enzymes can leave a remarkable stain on the material, it could also put the user safety at risk. Recycled pulp is the main source of fungi contamination, due to its less than optimal transportation and store conditions. Compared with virgin fiber pulps, recycled pulp has one thousand times more microorganisms. S. chartraum can survive a wide range of temperature up to 60ºC. As our results suggested the S. chartraum was embedded inside layers of wallpapers, more cautious should be given during the paper recycling process to prevent potential harmful fungi species growth within the paper. The result has been shown on ZEISS XEN workshop. The manuscript is under preparation.

Project Information

Participants: Technical University of Denmark.
Title: Possibilities in X-ray imaging of Bio materials and food industry (DTU GDP).