Revealing the structure of high-water content biopolymer networks: Diminishing freezing artefacts in cryo-SEM images

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Journal Article
Food Hydrocolloids, 2017, 73 pp. 203 - 212
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© 2017 Elsevier Ltd High-water content biopolymer gels are widely used in diverse areas of research and in many commercial processes, including a significant proportion of importance to the food and pharmaceutical industries. The internal structures of these hydrogels are complex and span many length scales, the specifics of which are important in determining functionality. Elucidating these structures is critically important in extending and tailoring their use. Cryo-scanning electron microscopy (cryo-SEM) is a popular method for structural elucidation. A critical step in this imaging technique is cryo-fixation. However, consideration of the literature reveals that images of inefficiently cryo-fixed networks remain widely reported as being representative of the native structure of gels despite being significantly damaged by ice formation. Here, we show that the application of slow freezing methods, like the very commonly used slushy nitrogen method, indeed yield erroneous structures and produce singularly common structures for both protein and polysaccharide hydrogels. On the contrary, we demonstrate, by rapidly freezing thin samples of low concentration protein amyloid fibril hydrogels using a propane jet freezer, that cryo-SEM imaging of biopolymer hydrogels in their native state is feasible. In contrast, while the use of cryo-protectants is successful in reducing ice crystallisation, it can affect the bonding within hydrogels and hence, modify the structure under examination.
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