The Future of Food Science? by Chris Parmenter
When it comes to microscopy in food science, there are two common options. The first is the original, light microscopy, which is used today in almost every lab and has been joined by laser confocal microscopy. Both are diffraction limited, however, the use of fluorescent dyes, markers and proteins add a colourful display to otherwise monochrome images and can aid with the differentiation of the many components of food systems, i.e. fats, proteins, carbohydrates, etc.
A confocal laser scanning micrograph of bread dough; starch shown in green, protein in red. (Image courtesy of Dr Mark Auty, Teagasc Food Research Centre, Ashtown, Dublin, Eire http://www.teagasc.ie/ )
The second option is electron microscopy. As an electron microscopist I appreciate the resolving power of the electron, and in particular the depth of field that scanning electron microscopy offers. Despite this, the main issue is that vacuum (needed for electrons to flow) and water or oil don’t mix and tend to be sucked out of the sample over time. To overcome this it is necessary to resort to cryogenic preservation or chemical fixation, both of which can be complicated and result in the rearrangement, shrinkage or damage to the structures within a sample. Another issue is that samples are often prepared for EM as thin sections (by ultramicrotomy) or fractured to reveal a random cross-section through the sample to give a surface image. Focused ion beam (FIB) SEM instruments can address this, by allowing in-situ sectioning and imaging, however, this approach can’t really be considered non-destructive.
I recently co-organised the RMS ‘Focus on Food’ meeting (http://www.rms.org.uk/events/pastevents/index/focusfood) and alongside the presentations on optical and electron microscopes that I expected to see, there were two that presented data from X-ray micro computed tomography (CT) by Pieter Verboven (KU Leuven, Belgium) (http://www.kuleuven.be/wieiswie/en/person/u0018628) and Gerard van Dalen (Unilever, The Netherlands) (http://www.microscopy-analysis.com/magazine-issue/volume-26-issue-2-march-2012). The technique is one that approaches the resolution of SEM, is non-destructive and generates 3D information. X-rays form the images instead of electrons or light and the sample rotates during imaging, allowing a 360 degree view of the sample. Since the X-rays pass through the samples, interacting based on the internal structure, the computed image is representative of different materials, voids, liquids, etc.
X-ray tomography of pores inside Braeburn apple tissue. Image used with permission from division BIOSYST-MeBioS of KU Leuven (www.mebios.be or www.insidefood.eu from http://www.sciencedirect.com/science/article/pii/S0925521412001901 by Els Herremans et al. Journal of Post Harvest Biology and Technology, Vol 75, Jan 2013, pp 114-124 available via http://dx.doi.org/10.1016/j.postharvbio.2012.08.008, - Reproduced with permission from authors.)
Unlike electrons, X-rays can pass through air, which means no cryogenic or fixation steps - a significant advantage for food samples. Within the last few years these instruments have become increasingly affordable, due in part to advances in multi-core computer processing, thus cutting the time needed to compute the multiple projections of the object of interest into a 3D tomogram. Instruments are available from a number of manufacturers including Bruker (http://www.bruker.com/products/preclinical-imaging/x-ray-micro-ct.html), GE (http://www.ge-mcs.com/en/phoenix-xray.html) and XRadia (http://www.xradia.com/?page_id=282) who were recently acquired by Carl Zeiss. One down side is the resolution, which is typically limited to around 500 nm, (less if you have a synchrotron handy or pay for the top of the line instruments), however, I’d suggest this is more than made up for by the 3D nature of the data.
In summary, understanding food samples over a range of length scales is crucial and X-ray tomography offers a way to information in 3 dimensions. While X-ray CT will not replace EM and light instruments, I think it is set to be a weapon of choice for food scientists who seek an 3D overview of samples and will regularly be used alongside the established arsenal of light and electron microscopies.
What’s your experience? If you have used X-ray techniques or run an X-ray instrument, do you think it is set to be the new solution for food science? Are you a food scientist using these techniques? Let us know what you’ve used it for.
Posted by Chris Parmenter 26 July 2013