Starch is one of the most abundant biomaterials in nature. It is found in plant tissues in the form of granules whose shape and size depend on the origin of the starch. Starch consists of two molecules: amylose (also known as the linear polymer) and amylopectin (also known as the branched polymer). Amylose and amylopectin are organised within the granule in a semi-crystalline arrangement, which in turn is made up of double helix bundles formed by adjacent amylopectin chains and amorphous regions rich in amylose [1, 2]. Double helix bundles and amorphous amylose define the lamellar structure of starch, but the orientation of the double helices in a particular spatial arrangement also defines starch polymorphism [2, 3].
Starch is essentially an energy reserve and the predominant food reserve in plants, providing 70–80% of the calories consumed by humans [4]. However, the literature has described various other uses of starch for industrial and technological purposes beyond that of a mere food ingredient: e.g. edible films and coatings [5, 6], bioplastics for packaging [7, 8], scaffolds for wound healing [9–10], specialised hydrogels [11] and more recently in the field of bioprinting [12–14].
However, starch has limitations for use in some industrial applications due to its low solubility in cold water, high viscosity after gelatinisation and tendency to retrograde during storage [15]. Respect to viscosity, the high viscosity of starch during cooking is explained by the complete loss of the semicrystalline arrangement of the granules and the loss of the lamellar structure, which is addressed by the phenomenon of granule swelling, granule disruption and amylose lixiviation that takes place during gelatinisation. On the other hand, viscosity changes also occur during cooling and storage due to the self-association and self-assembly of starch polymers. Thus, the viscosity changes exhibited by starch suspensions during thermal processing and storage, which could imply viscosity increases of several orders of magnitude, are an important drawback of starch for industrial applications. Herein, several strategies have been followed to overcome these drawbacks. For instance, Yu et al [16] tested different doses of electron bean irradiation (EBI) for reducing the viscosity of corn starch, concluding that slurry made from EBI pre-treated cornstarch (45% wt/wt) exhibited a rather low viscosity values and high dextrose equivalent values compared with that of the slurry (45% wt/wt) made from the non-EBI pre-treated cornstarch. Therefore, the use of starch modified by electron beam irradiation treatment could lead to reduce corn syrup processing costs. Likewise, Chen et al [17] modified mung bean starch by using cold plasma (120 W, 5 min) to improve the properties of starch films aimed to produce novel food packing materials. The study reported a decrease in peak viscosity of starch slurry after the cold plasma treatment (tested by Rapid-Visco-Analysis) which correlated with the modification of crystallinity, amylose content and short-range orders. The latter resulted in mung bean starch film casts showing uniform morphology, with higher tensile strength and improved thermal stability compared with the untreated cold plasma starch film. On the other hand, Cui et al [13] has highlighted the relevant role of viscosity in the bioprinting properties of potato starch. In terms of printability an ideal printing material is a viscous paste with a proper response to shear (i.e. shear-thinning with high recovery) or low-viscosity material structured in the printing process. Thus, a printing material based on starch should have a proper viscous to ensure a steady flow rate, but also be self-supportive after deposition [13].
Interestingly, the mechanism underlying the low viscosity of starch is still unclear and there is a general lack of studies in the literature aimed at investigating this behavior. Some studies have suggested that several factors could determine the low viscosity state of a starch, including the cultivar from which the starch was extracted, the size distribution of the starch granules, the swelling power of the granules, the amylose content, the average chain length of amylose and the fine structure of amylopectin [18]. However, studies are not conclusive and some contradictory results have been reported.
Therefore, the use of novel biomaterials, such as low-viscosity starches, in applications that are not yet well explored requires in-depth analysis of physico-chemical characterisation, specifically aimed at understanding how structural changes that occur during processing and storage determine the performance of the material. This is particularly relevant in the case of low-viscosity starches, which are currently available on the market but for which little information on their physicochemical, structural and toxicological properties is available in both technical and safety data sheets.
Hence, the aim of this study was to characterise a low-viscosity potato starch (LVPS) of commercial origin in terms of its pasting properties and retrogradation kinetics, assessed by rheological and mechanical tests, respectively. A normal potato starch (NPS) was used as a control for comparison. Our study was complemented by measurements of thermal properties and cold-water solubility, as well as some structural features, such as polarised microscopy and IR spectroscopy, in both starches.