Analysis of the physiochemical properties of rice induced by postharvest yellowing during storage

Insight into the relationship between cell wall, intracellular starch structure and physicochemical properties of cassava cells from cassava varieties

Cassava cell flour can expand the food industrial availability of cassava resources. In this study, cassava cells were isolated from eight cassava varieties to analyze the composition, structure, and physicochemical properties. The smaller particle size in CS4 led to the lowest swelling power and viscosity, which further reduced the modulus (G', G") and shear stress of the cassava cell gel. The higher starch content in CS3 and CS8 (88.86 %, 87.99 %) increased the swelling and solubility of the cells, resulting in high viscosity and gel strength. Thicker cell walls presented a higher content of cell wall polysaccharides, hindering the interaction of water, heat, and digestive enzymes with intracellular starch, leading to higher gelatinization temperatures and lower digestion rates. In addition, the B1 chain promoted the formation of the starch crystalline region and increased the gelatinization temperature and enthalpy of cell flour. Based on cluster analysis and correlation analysis, the differences in the functional properties of cassava cell flour were related to cell components, morphology, and intracellular starch structure. The study provided a theoretical basis for the application of cassava cells in the food industry.

Rheological properties of indica rice determined by starch structure related enzymatic activities during after-ripening

The processing quality of indica rice must undergo ripening after harvest to achieve stability and improvement. However, the mechanism underlying this process remains incompletely elucidated. Starch, the predominant component in indica rice, plays a crucial role in determining its properties. This study focused on analyzing the rheological properties and starch fine structure, as well as the related biosynthetic enzymes of indica rice during the after-ripening process. The results showed that after-ripened rice exhibited increased elastic modulus (G') and viscous modulus (G″), accompanied by a decrease in the loss tangent (Tan δ), indicating an enhancement in viscoelasticity and the gel network structure. Moreover, the proportions of amylopectin super long chains (DP 37-60) decreased, while those of medium chains (DP 13-24 and DP 25-36) or short chains (DP 6-12) of amylopectin increased. Additionally, the activities of starch branching enzyme (SBE) and starch debranching enzyme (DBE) declined over the after-ripening period. Pearson correlation analysis revealed that the rheological properties of after-ripened rice were correlated with the chain length distribution (CLD) of starch, which, in turn, was associated with its related endogenous enzymes. These findings provied new insights into understanding the quality changes of after-ripened indica rice.

Flavonoids‑targeted metabolomic analysis following rice yellowing

Flavonoids are the main metabolites responsible for yellowing of rice. However, the accumulation pattern of flavonoids and the metabolic basis of flavonoid biosynthesis during rice yellowing remain unclear. Thus, flavonoid-targeted metabolomics was used to investigate the composition and concentration of flavonoids in rice during yellowing. The results indicated the differential flavonoids at Month 3 and Month 5 of storage were more in composition and concentration with higher antioxidant capacity. Accumulated flavonoids were mainly flavones, flavonols, isoflavones, and anthocyanidins, of which rutin, farrerol, naringenin, cyanidin 3-rutinoside, and diosmetin were the indicators of rice yellowing. Metabolic association among flavonoids demonstrated the formation of yellow pigments was jointly induced by flavones, flavonols, isoflavones, and anthocyanidins metabolism. Examination of flavonoid metabolism presented in this study enhanced current understanding of the relationship between flavonoid metabolites and development of rice yellowing. It also offers a theoretical basis for targeted prediction of rice yellowing in the future.

Physicochemical and volatile characteristics present in different grain layers of various rice cultivars

Five rice cultivars were applied for investigating effect of milling degree on rice physicochemical properties. The first layer had the lowest peak viscosity, followed by the second and third layers, indicating the effect of non-starchy components on starch gelatinization behaviors. Consistently, more content of non-starch components in the first layer led to an enhanced gelatinization temperature. Rheological study demonstrated the G' and G" were successively increased as the layer moved inward, indicating a stronger gel network due to the increased amylose content and crystallinity in the corresponding layer. This is the first study to reveal the second layer has the highest digestibility, suggesting both non-starch components and starch structure control starch digestion. Furthermore, analysis of volatile compounds found alcohols and ketones concentrated in the first layer, whilst compounds including (E,E)-2,4-decadienal, 3-octanone and 3-nonen-2-one only existed in the second layer, serving as an indicator for managing the rice quality during milling.

A review on rice yellowing: Physicochemical properties, affecting factors, and mechanism

Yellowing is a critical issue that reduces quality and commodity value of rice. This article presents an overview on rice yellowing and the mechanism of rice yellowing was addressed as the emphasis. The change of physicochemical and nutritive properties in yellowed rice depends on the exposure temperature and time, as well as rice cultivar. The temperature and moisture on rice yellowing were dominant. There is no consensus on the relationship between microorganisms and rice yellowing. The occurrence of yellowing is mainly associated with heat stress induced by heaping heat or respiration of grain, and the yellowing is the collective result of primary and secondary metabolism. The upregulation of flavonoids is the direct cause of rice yellowing, which can be used as metabolic markers of rice yellowing. The Maillard reaction also contributes to yellowing during storage. Aeration and cooling are recommended to lessen the occurring of rice yellowing during commercial storage.

Differences in starch multi-layer structure, pasting, and rice eating quality between fresh rice and 7 years stored rice

With the continuous improvement of rice production capacity and the accumulation of reserves year by year, rice sometimes has to be stored for a long time. However, long-term storage of rice has poor sensory properties, which may be related to the structural changes of starch. Different from the previous studies on short-term storage of rice (often 3–12 months), the focus of this study was to understand the differences in starch multi-layer structure, pasting, and rice eating quality between 7 years stored rice and fresh rice. Our research indicated that 7 years stored rice showed higher hardness and lower stickiness compared to fresh rice, which ultimately led to poorer eating quality. These bad changes were related to differences in starch multi-layer structure. The 7 years stored rice had lower amylose content, a lower thickness of crystalline lamellae and short-range ordered structure of starch, and more large starch granules. In particular, the volume mean diameter of 7 years starch was more than 4 times that of fresh starch. 7 years stored rice had more large granular starch and unstable crystal structure, which led to the increase of pasting temperature and the decrease of gelatinization enthalpy during starch gelatinization, and ultimately reduced the eating quality of the rice.

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7 years stored rice had higher hardness and poorer eating quality.

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Volume mean diameter of 7 years stored starch was 4 times larger than fresh starch.

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7 years stored rice had lower short-range order structure of starch.

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The pasting temperature of 7 years stored starch was higher than fresh starch.

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Higher pasting temperature and lower gelatinization enthalpy reduced the eating quality.

Research advance in gas detection of volatile organic compounds released in rice quality deterioration process

Rice quality deterioration will cause grievous waste of stored grain and various food safety problems. Gas detection of volatile organic compounds (VOCs) produced by deterioration is a nondestructive detection method to judge rice quality and alleviate rice spoilage. This review discussed the research advance of VOCs detection in terms of nondestructive detection methods of rice quality deterioration, applications of VOCs in grain detection, inspection of characteristic gas produced during rice spoilage, rice deterioration prevention and control, and detection of VOCs released by rice mildew and insect attack. According to the main causes of rice quality deterioration and major sources of VOCs with off-odor generated during rice storage, deterioration can be divided into mold and insect infection. The results of literature manifested that researches mainly focused on the infection of Aspergillus in the mildew process and the attack of certain pests in recent years, thus the research scope was limited. In this paper, the gas detection methods combined with the chemometrics to qualitatively analyze the VOCs, as well as the correlation with the number of colonies and insects were further studied based on the common dominant strains during rice mildew, that is, Aspergillus and Penicillium fungi, and the common pests during storage, that is, Sitophilus oryzae and Rhyzopertha dominica. Furthermore, this paper pointed out that the quantitative determination of characteristic VOCs, the numeration relationship between VOCs and the degree of mildew and insect infestation, the further expansion of detection range, and the application of degraded rice should be the spotlight of future research.

Association of enriched metabolites profile with the corresponding volatile characteristics induced by rice yellowing process

Change in metabolites and volatiles during yellowing process in six rice cultivars was analyzed. Based on the yellowness, the study indicated Japonica was more prone to yellowing than Indica rice. Metabonomics analysis showed most differential metabolites were up-regulated, in which pathways of flavone and flavonol biosynthesis were significantly enriched following the yellowing process. Meanwhile, 54 differential metabolites were overlapped in six comparative groups, which is characterized by commonly-shared metabolic regulation pathway in each rice. Phenylalanine content was increased, followed by the enhanced phenylpropanoids formation, showing transformation between primary and secondary metabolites during yellowing process. Furthermore, 43 volatile compounds were identified, and the yellowed rice had more volatiles, including ketones, alcohols, esters and hydrocarbons, suggesting a positive correlation with the yellowing. Compounds 6-methyl-5-hepten-2-one and 6,10,14-trimethyl-2-pentadecanone were increased steadily during yellowing process, which may be applied for monitoring rice yellowing progress. This investigation provides further insight for revealing rice yellowing mechanism.

Influence of Electron Beam Irradiation on the Moisture and Properties of Freshly Harvested and Sun-Dried Rice

Moisture content is an important factor that affects rice storage. Rice with high moisture (HM) content has superior taste but is difficult to store. In this study, low-dose electron beam irradiation (EBI) was used to study water distribution in newly harvested HM (15.03%) rice and dried rice (11.97%) via low-field nuclear magnetic resonance (LF-NMR). The gelatinization, texture and rheological properties of rice and the thermal and digestion properties of rice starch were determined. Results showed that low-dose EBI could change water distribution in rice mainly by affecting free water under low-moisture (LM) conditions and free water and bound water under HM conditions. HM rice showed smooth changes in gelatinization and rheological properties and softened textural properties. The swelling power and solubility index indicated that irradiation promoted the depolymerization of starch chains. Overall, low-dose EBI had little effect on the properties of rice. HM rice showed superior quality and taste, whereas LM rice exhibited superior nutritional quality. This work attempted to optimize the outcome of the EBI treatment of rice for storage purposes by analyzing its effects. It demonstrated that low-dose EBI was more effective and environmentally friendly than other techniques.