Distinct Properties and Structures Among B-Crystalline Starch Granules

Starch-related structural basis and enzymatic mechanism of the different appearances of soft rice

To investigate the fine starch structure characteristics and formation mechanism of high-quality appearance soft rice, two high-quality and low-quality soft rice varieties (HA-SR and LA-SR, respectively) were selected. Differences in appearance quality, fine starch structure, and activity of key enzymes involved in starch synthesis during the grain-filling stage were compared. The results showed that compared with LA-SR, HA-SR were less chalky, more transparent, had larger starch grains, a lower content of shorter chains (DP 6-24), a higher content of longer chains (DP ≥ 25), lower relative crystallinity, fewer ordered structures, more amorphous structures and larger thicknesses of semi-crystalline lamellae. In terms of amylase activity during the grain-filling stage, the AGPase and GBSS activities of HA-SR were higher, and the SBE activity of HA-SR was lower compared to LA-SR. In conclusion, higher AGPase activity can produce a higher filling rate resulting in fuller starch grain in soft rice. Fuller starch grains reduce the chalkiness of soft rice. Higher AGPase and GBSS activities and lower SBE activity can result in soft rice with more long-branched and less short-branched amylopectin. Thus, soft rice has lower relative crystallinity and less ordered structure. These structures may facilitate reduce grain chalkiness and improve grain transparency.

Changes in Structural and Thermodynamic Properties of Starch during Potato Tuber Dormancy

The main reserve polysaccharide of plants-starch-is undoubtedly important for humans. One of the main sources of starch is the potato tuber, which is able to preserve starch for a long time during the so-called dormancy period. However, accumulated data show that this dormancy is only relative, which raises the question of the possibility of some kind of starch restructuring during dormancy periods. Here, the effect of long-term periods of tuber rest (at 2-4 °C) on main parameters of starches of potato tubers grown in vivo or in vitro were studied. Along with non-transgenic potatoes, Arabidopsis phytochrome B (AtPHYB) transformants were investigated. Distinct changes in starch micro and macro structures-an increase in proportion of amorphous lamellae and of large-sized and irregular-shaped granules, as well as shifts in thickness of the crystalline lamellae-were detected. The degree of such alterations, more pronounced in AtPHYB-transgenic tubers, increased with the longevity of tuber dormancy. By contrast, the polymorphic crystalline structure (B-type) of starch remained unchanged regardless of dormancy duration. Collectively, our data support the hypothesis that potato starch remains metabolically and structurally labile during the entire tuber life including the dormancy period. The revealed starch remodeling may be considered a process of tuber preadaptation to the upcoming sprouting stage.

Physical and Enzymatic Hydrolysis Modifications of Potato Starch Granules

In this work, a valorization of the starch stemming from downgraded potatoes was approached through the preparation of starch nanoparticles using different physical methods, namely liquid and supercritical carbon dioxide, high energy ball milling (HEBM), and ultrasonication on the one hand and enzymatic hydrolysis on the other hand. Starch nanoparticles are beneficial as a reinforcement in food packaging technology as they enhance the mechanical and water vapor resistance of polymers. Also, starch nanoparticles are appropriate for medical applications as carriers for the delivery of bioactive or therapeutic agents. The obtained materials were characterized using X-ray diffraction as well as scanning and transmission electron microscopies (SEM and TEM), whereas the hydrolysates were analyzed using size exclusion chromatography coupled with pulsed amperometric detection (SEC-PAD). The acquired results revealed that the physical modification methods led to moderate alterations of the potato starch granules’ size and crystallinity. However, enzymatic hydrolysis conducted using Pullulanase enzyme followed by nanoprecipitation of the hydrolysates allowed us to obtain very tiny starch nanoparticles sized between 20 and 50 nm, much smaller than the native starch granules, which have an average size of 10 μm. The effects of enzyme concentration, temperature, and reaction medium pH on the extent of hydrolysis in terms of the polymer carbohydrates’ fractions were investigated. The most promising results were obtained with a Pullulanase enzyme concentration of 160 npun/g of starch, at a temperature of 60 °C in a pH 4 phosphate buffer solution resulting in the production of hydrolysates containing starch polymers with low molecular weights corresponding mainly to P-10, P-5, and fractions with molecular weights lower than P-5 Pullulan standards.

A Review of Starch Biosynthesis in Relation to the Building Block-Backbone Model

Starch is a water-insoluble polymer of glucose synthesized as discrete granules inside the stroma of plastids in plant cells. Starch reserves provide a source of carbohydrate for immediate growth and development, and act as long term carbon stores in endosperms and seed tissues for growth of the next generation, making starch of huge agricultural importance. The starch granule has a highly complex hierarchical structure arising from the combined actions of a large array of enzymes as well as physicochemical self-assembly mechanisms. Understanding the precise nature of granule architecture, and how both biological and abiotic factors determine this structure is of both fundamental and practical importance. This review outlines current knowledge of granule architecture and the starch biosynthesis pathway in relation to the building block-backbone model of starch structure. We highlight the gaps in our knowledge in relation to our understanding of the structure and synthesis of starch, and argue that the building block-backbone model takes accurate account of both structural and biochemical data.