Equipment: Electron Microscopy
Light microscope resolution is limited to around 0.25 microns by the wavelength of light. An accelerated electron beam has a much smaller wavelength; consequently, much higher resolution is possible. The electron beam is focused by electromagnetic “lenses” and images are formed either by scanning the surface of a bulk sample such as in scanning electron microscopy (SEM); or by looking through a very thin sample as in transmitted electron microscopy (TEM). Traditionally, electron microscopy of food samples required extensive sample preparation involving chemical fixation, dehydration and possibly resin embedding. However, recent advances in the technology, in particular field emission electron optics, low-vacuum technology and cryo-preparation mean that it is now possible to visualize “difficult” food samples close to their natural state at a resolution of a few nanometers and with little or no sample preparation. These include liquids, frozen products and samples with high moisture, fat or sugar contents such as yoghurt (Fig. 6), dairy spreads and ice cream. The newly commissioned SEM at Moorepark uses the latest field emission technology and is fitted with secondary, backscattered and transmitted electron detectors and most importantly the cryo-stage. An additional variable pressure mode allows examination of partially hydrated samples. The transmitted detector permits high-resolution imaging of samples that have been prepared for TEM such as ultra-thin resin sections or negatively stained food macromolecules.
Figure 6. Cryo-scanning electron microscope image of yoghurt showing rod and cocci-shaped
starter culture bacteria (yellow) embedded in a matrix of milk protein particles.