Structural and molecular basis of starch viscosity in hexaploid wheat

Effects of nitrogen fertilizer application on the physicochemical properties of foxtail millet (Setaria italica L.) starch

The use of nitrogen fertilizer is a crucial agronomic practice to increase crop output and quality. This study investigated the impact of five nitrogen application levels (0, 60, 135, 210, and 285 kg N/hm2) on the physicochemical properties of foxtail millet (FM) starch. Optimal nitrogen application (210 kg N/hm2) significantly increased L*, a*, and b* values, water and oil absorption capacity, water solubility, and swelling power of starch. The number of small starch granules increased as the nitrogen application rate increased, but the granule morphology and typical A-type pattern did not change among the treatments. Nitrogen application increased the relative crystallinity and ordered structure, resulting in a higher gelatinization enthalpy. Compared to the control group (7.02 J/g), the enthalpy increased by 21.94 %, 66.38 %, 73.50 %, and 103.28 % under the nitrogen application rates, respectively. Moreover, nitrogen application greatly increased the percentage of A and B3 chains while it lowered the apparent amylose content, peak viscosity, and final viscosity. The effects of 210 and 285 kg N/hm2 treatments on the water solubility and swelling power, water and oil absorption, and light transmission of starch were greater compared to the 60 and 135 kg N/hm2 treatments. These results indicate that nitrogen fertilization significantly affects the physicochemical properties of FM starch.

A review of the effects of fermentation on the structure, properties, and application of cereal starch in foods

Fermentation is one of the oldest food processing techniques known to humans and cereal fermentation is still widely used to create many types of foods and beverages. Starch is a major component of cereals and the changes in its structure and function during fermentation are of great importance for scientific research and industrial applications. This review summarizes the preparation of fermented cereals and the effects of fermentation on the structure, properties, and application of cereal starch in foods. The most important factors influencing cereal fermentation are pretreatment, starter culture, and fermentation conditions. Fermentation preferentially hydrolyzes the amorphous regions of starch and fermented starches have a coarser appearance and a smaller molecular weight. In addition, fermentation increases the starch gelatinization temperature and enthalpy and reduces the setback viscosity. This means that fermentation leads to a more stable and retrogradation-resistant structure, which could expand its application in products prone to staling during storage. Furthermore, fermented cereals have potential health benefits. This review may have important implications for the modulation of the quality and nutritional value of starch-based foods through fermentation.

Starch granules and their size distribution in wheat: Biosynthesis, physicochemical properties and their effect on flour-based food systems

Starch is a vital component of wheat grain and flour, characterized by two distinct granule types: A-type starch (AS) with granules larger than 10 µm in diameter, and B-type starch (BS) with granules measuring no more than 10 µm in diameter. This review comprehensively evaluates the isolation, purification, and biosynthesis processes of these types of granules. In addition, a comparative analysis of the structure and properties of AS and BS is presented, encompassing chemical composition, molecular, crystalline and morphological structures, gelatinization, pasting and digestive properties. The variation in size distribution of granules leads to differences in physicochemical properties of starch, influencing the formation of polymeric proteins, secondary and micro-structures of gluten, chemical and physical interactions between gluten and starch, and water absorption and water status in dough system. Thus, starch size distribution affects the quality of dough and final products. In this review, we summarize the up-to-date knowledge of AS and BS, and propose the possible strategies to enhance wheat yield and quality through coordinated breeding efforts. This review serves as a valuable reference for future advancements in wheat breeding.

Overexpression of a wheat α-amylase type 2 impact on starch metabolism and abscisic acid sensitivity during grain germination

Despite being of vital importance for seed establishment and grain quality, starch degradation remains poorly understood in organs such as cereal or legume seeds. In cereals, starch degradation requires the synergetic action of different isoforms of α-amylases. Ubiquitous overexpression of TaAmy2 resulted in a 2.0-437.6-fold increase of total α-amylase activity in developing leaf and harvested grains. These increases led to dramatic alterations of starch visco-properties and augmentation of soluble carbohydrate levels (mainly sucrose and α-gluco-oligosaccharide) in grain. Interestingly, the overexpression of TaAMY2 led to an absence of dormancy in ripened grain due to abscisic acid (ABA) insensitivity. Using an allosteric α-amylase inhibitor (acarbose), we demonstrated that ABA insensitivity was due to the increased soluble carbohydrate generated by the α-amylase excess. Independent from the TaAMY2 overexpression, inhibition of α-amylase during germination led to the accumulation of soluble α-gluco-oligosaccharides without affecting the first stage of germination. These findings support the hypotheses that (i) endosperm sugar may overcome ABA signalling and promote sprouting, and (ii) α-amylase may not be required for the initial stage of grain germination, an observation that questions the function of the amylolytic enzyme in the starch degradation process during germination.

Late-Maturity Alpha-Amylase in Wheat (Triticum aestivum) and Its Impact on Fresh White Sauce Qualities

When wheat experiences a cold-temperature ‘shock’ during the late stage of grain filling, it triggers the abnormal synthesis of late-maturity α-amylase (LMA). This increases the enzyme content in affected grain, which can lead to a drastic reduction in falling number (FN). By commercial standards, a low FN is taken as an indication of inferior quality, deemed unsuitable for end-product usage. Hence, LMA-affected grains are either rejected or downgraded to feed grade at the grain receiving point. However, previous studies have found no substantial correlation between low FN-LMA and bread quality. The present study extends previous investigations to semi-solid food, evaluating the physical quality of fresh white sauce processed from LMA-affected flour. Results show that high-LMA flours had low FNs and exhibited poor pasting characteristics. However, gelation occurred in the presence of other components during fresh white sauce processing. This demonstrates that LMA-affected flours may have new applications in low-viscosity products.

Parameterizing starch chain-length distributions for structure-property relations

Understanding starch structure-property relationship is important for the development of new generation of starch-based foods with desirable functions. Recent developments of methodologies on the characterisation of starch molecular structures, especially how to parameterize the starch chain-length distribution (CLD) by few biologically meaningful parameters have brought new insights to explain starch physicochemical properties from molecular levels. Especially, it has shown that gelatinization temperatures are largely controlled by amylopectin short chains, while the retrogradation rate of starch molecules is controlled by amylose content, amylose short to medium chains, amylopectin external and internal chain length. Starch pasting and digestion properties are also controlled to a significant extent by its CLD. With extensive discussion of correlative and casual relations between starch CLD with its physicochemical properties, this review aims to establish a holistic starch structure-property relationship. It enables food producers to develop functional foods based on a precise understanding of starch structure-property relations.

Engineering high α-amylase levels in wheat grain lowers Falling Number but improves baking properties

Late maturity α-amylase (LMA) and preharvest sprouting (PHS) are genetic defects in wheat. They are both characterized by the expression of specific isoforms of α-amylase in particular genotypes in the grain prior to harvest. The enhanced expression of α-amylase in both LMA and PHS results in a reduction in Falling Number (FN), a test of gel viscosity, and subsequent downgrading of the grain, along with a reduced price for growers. The FN test is unable to distinguish between LMA and PHS; thus, both defects are treated similarly when grain is traded. However, in PHS-affected grains, proteases and other degradative process are activated, and this has been shown to have a negative impact on end product quality. No studies have been conducted to determine whether LMA is detrimental to end product quality. This work demonstrated that wheat in which an isoform α-amylase (TaAmy3) was overexpressed in the endosperm of developing grain to levels of up to 100-fold higher than the wild-type resulted in low FN similar to those seen in LMA- or PHS-affected grains. This increase had no detrimental effect on starch structure, flour composition and enhanced baking quality, in small-scale 10-g baking tests. In these small-scale tests, overexpression of TaAmy3 led to increased loaf volume and Maillard-related browning to levels higher than those in control flours when baking improver was added. These findings raise questions as to the validity of the assumption that (i) LMA is detrimental to end product quality and (ii) a low FN is always indicative of a reduction in quality. This work suggests the need for a better understanding of the impact of elevated expression of specific α-amylase on end product quality.

Suppression of glucan, water dikinase in the endosperm alters wheat grain properties, germination and coleoptile growth

Starch phosphate ester content is known to alter the physicochemical properties of starch, including its susceptibility to degradation. Previous work producing wheat (Triticum aestivum) with down-regulated glucan, water dikinase, the primary gene responsible for addition of phosphate groups to starch, in a grain-specific manner found unexpected phenotypic alteration in grain and growth. Here, we report on further characterization of these lines focussing on mature grain and early growth. We find that coleoptile length has been increased in these transgenic lines independently of grain size increases. No changes in starch degradation rates during germination could be identified, or any major alteration in soluble sugar levels that may explain the coleoptile growth modification. We identify some alteration in hormones in the tissues in question. Mature grain size is examined, as is Hardness Index and starch conformation. We find no evidence that the increased growth of coleoptiles in these lines is connected to starch conformation or degradation or soluble sugar content and suggest these findings provide a novel means of increasing coleoptile growth and early seedling establishment in cereal crop species.

Down-regulation of Glucan, Water-Dikinase activity in wheat endosperm increases vegetative biomass and yield

A novel mechanism for increasing vegetative biomass and grain yield has been identified in wheat (Triticum aestivum). RNAi-mediated down-regulation of Glucan, Water-Dikinase (GWD), the primary enzyme required for starch phosphorylation, under the control of an endosperm-specific promoter, resulted in a decrease in starch phosphate content and an increase in grain size. Unexpectedly, consistent increases in vegetative biomass and grain yield were observed in subsequent generations. In lines where GWD expression was decreased, germination rate was slightly reduced. However, significant increases in vegetative growth from the two leaf stage were observed. In glasshouse pot trials, down-regulation of GWD led to a 29% increase in grain yield while in glasshouse tub trials simulating field row spacing and canopy development, GWD down-regulation resulted in a grain yield increase of 26%. The enhanced yield resulted from a combination of increases in seed weight, tiller number, spikelets per head and seed number per spike. In field trials, all vegetative phenotypes were reproduced with the exception of increased tiller number. The expression of the transgene and suppression of endogenous GWD RNA levels were demonstrated to be grain specific. In addition to the direct effects of GWD down-regulation, an increased level of α-amylase activity was present in the aleurone layer during grain maturation. These findings provide a potentially important novel mechanism to increase biomass and grain yield in crop improvement programmes.

Diversity in amylopectin structure, thermal and pasting properties of starches from wheat varieties/lines

Structural, thermal and pasting diversity of starches from Indian and exotic lines of wheat was studied. Majority of the starches showed amylose content ranging between 22% and 28%. Endotherm temperatures (T(o), T(p) and T(c)) of the starches showed a range between 56-57, 60 -61 and 65.5-66.5 degrees C, respectively. Exotherms with T(p) between 87.0 and 88.2 degrees C were observed during cooling of heated starches, indicating the presence of amylose-lipid complexes. Exotherm temperatures were negatively correlated to swelling power. Amylopectin unit chains with different degree of polymerization (DP) were observed to be associated with pasting temperature, setback and thermal (endothermic T(o), T(p), and T(c)) parameters. Amylopectin unit chains of DP 13-24 showed positive relationship with endothermic T(o), T(p) and T(c). Pasting temperature showed positive correlation with short chains (DP 6-12) while negative correlation with medium chain (DP 13-24) amylopectins. Setback was positively correlated to DP 16-18 and negatively to DSC amylose-lipid parameters.