Chapter 6 Purification, Rheological Characterization, and Visualization of Viscous, Neutral, Hetero-exopolysaccharide Produced by Lactic Acid Bacteria S. Ikeda, D. Kondoh, N. P. D. Aryantini, T. Urashima, and K. Fukuda Abstract Viscous exopolysaccharide (EPS)-producing lactic acid bacteria (LAB) have received increasing interest in the dairy industry because of their capability to improve the texture and mouthfeel of fermented dairy products. To date, enormous efforts have been made to reveal the relationship between texture and EPS production in fermented milk products such as yogurt. However, the structure-rheology relationship of EPSs themselves is not yet well understood due to their low yields in general and their wide variety of chemical structures. In this chapter, we describe common techniques for the purification, visualization, and rheological analysis of viscous EPSs produced by LAB. Key words Atomic force microscopy, Dynamic viscoelasticity, Light microscopy, Transmission elec- tron microscopy, Negative staining 1 Introduction Some lactic acid bacteria (LAB) produce exopolysaccharides (EPSs), which can be divided into two different types: homo-EPS consisting of a single sugar residue such as dextran, a polymer of glucose, and hetero-EPS, which is a polymer of repeating units of various sugar residues [1]. In general, the former has a relatively simple structure and high production yields compared to the latter, owing to the attributes of their biosynthetic pathways. Several homo-EPSs have already been commercialized, but progress is hindered to some extent for hetero-EPS produced by LAB, mainly due to their low production yields (ranging from a few to hundreds of milligrams), with a few exceptions [1]. Despite the technical setbacks, studying the relationship between chemical structures and physicochemical properties of hetero-EPSs produced by LAB is important for their industrial application. Furthermore, such knowledge would be useful in interpreting the reported health Makoto Kanauchi (ed.), Lactic Acid Bacteria: Methods and Protocols, Methods in Molecular Biology, vol. 1887, https://doi.org/10.1007/978-1-4939-8907-2_6, © Springer Science+Business Media, LLC, part of Springer Nature 2019 55