Classification of Nano-sized Starch

Nano-sized starch can be roughly divided into two categories based on its preparation methods and properties: SNs and SNCs. Strictly speaking, SNCs are a particular type of SNs. In this chapter, we emphasize the distinction between them in order to better understand nano-sized starch.

Over the last decade, SNCs have attracted growing interest due not only to their nanoscale size but also their renewable and biodegradable nature. SNCs are crystalline platelets resulting from the disruption of the semicrystalline structure of starch granules by hydrolysis of amorphous regions and vary in size from 5 to 7 nm thick, 20−40 nm in length, and 15−30 nm in width. In comparison to native starches, the relative crystallinity (RC) of SNCs is increased by ~10% owing to the preferential hydrolysis of the amorphous regions. Thus, SNCs possess an excellent ability for reinforcing nanocomposites. In general, the yield of SNCs (~15%) is below the theoretical crystallinity of starch (~40%). This is due to the fact that hydrolysis occurs simultaneously in both amorphous and crystalline regions. However, the side chains of amylopectin is arranged in an orderly fashion in crystalline parts, resulting in a much lower rate of hydrolysis than occurs in amorphous regions.

SNCs have attracted much interest from the research community, but studies in this field are now slowly declining. By contrast, SNs are attracting increasing attention. SNs are usually prepared by the regeneration method as spherical or oval structures with a diameter of 10−100 nm. The main differences between SNs and SNCs are the appearance and the preparation protocol. Regarding appearance, SNCs present a platelet shape with an angular (A-type) or rounded (B-type) edge depending on the crystalline type of the native starch. Regarding the preparation protocol, SNCs are usually obtained by hydrolysis (acid or enzymolysis) of the amorphous regions of native starches, and any protocol that accelerates the hydrolysis process can shorten the preparation time. By contrast, SNs are often prepared through the recrystallization or cross-linking of starch molecules. Thus, native starches must be gelatinized then subjected to other pretreatments (such as debranching) to facilitate the formation of SNs. Due to the different preparation mechanisms of nano-sized starch, the yield of SNs is generally much higher than that of SNCs. As a result, more and more researchers are paying attention to SNs.

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