Postharvest Variations of Rice Physicochemical Traits and Volatile Compounds under Granary Conditions During Long-Time Storage

Rice quality is influenced by many factors,viz.,temperature,humidity,atmosphere and storage time.To uncover quality variations of rice after postharvest,we estimated the physicochemical properties and volatile compounds of anindicarice Zhongzheyou 8 under granary conditions (20 °C-25 °C in summer and 10 °C-15 °C in other seasons) for three years.During three-year storage,head rice rate and brown rice rate decreased from 67.02% to 61.00%and from 75.62% to 69.74%,respectively,while fissure rate increased from 4.67% to 12.33%,and the yellow rice rate grew from 0.21% to 1.58%.Increased water absorption,hardness,gumminess and chewiness,and decreased pH exhibited the degradation of rice quality in cooking and texture properties after three-year storage.Rice yellowing happened in the first year,and became stronger as time elapsed.Meanwhile,the expansion rate also gradually increased during storage.By the gas chromatography-ion mobility spectrometry (GC-IMS),aldehydes and ketones of main volatile organic compounds(VOCs) significantly increased after three-year storage,which indicated rice oxidation level aggravated.The results revealed that rice quality decline was inevitable during storage even under an appropriate condition.To get the good rice quality,rice should be kept at low or quasi low temperature,and the best storage time was within one year.

Because of seasonal production of rice,most harvest rice is obliged to store at least one year for later processing and consumption.The rice storage characteristics are mainly affected by temperature,humidity,atmosphere and duration(Ding et al,2018),while rice aging spontaneously happens during storage.The rice aging will change the chemical and physical properties of rice,while the yellowing will also happen in this process.The traditional storage quality indicators mainly include germination rate,fat acidity and enzyme activity(polyphenol oxidase and peroxidase),and these indicators can reflect the rice characteristics (Shu et al,2021).For estimating the commercial value,rice processing and cooking qualities should be also studied,which include water absorption,extensibility,texture properties,thermodynamic properties,and volatile compounds (Li et al,2021;Liu et al,2021).The aroma profiles are major VOCs,which not only contribute to rice flavors,but also indicate rice quality variations,such as rice aging and yellowing (Xu et al,2021).As an emerging technology,based on its fast analytical capability and high detection accuracy,GC-IMS is often used to analyze volatile components of agri-food products and evaluate rice quality,which has better precision analysis than gas chromatography-mass spectrometer(GC-MS) with more information of VOCs (Gu et al,2021).Many studies have already focused on these physicochemical variations of rice under different storage conditions (Zhou et al,2010;Azizi et al,2019;Wu et al,2019;Yi et al,2020),and most of them were conducted under laboratory conditions during a short period,which can find the transformations of some rice typical metabolic compound during this process,but cannot represent actual variations in grain bins (Zhou et al,2016;Yuan et al,2019).Hence,in this study,we analyzed VOC profiles and physicochemical properties of rice in different storage years,to discover the quality-change patterns of rice during long-term storage as well as give optimum storage suggestion to maintain the rice quality.

For processing quality,head rice rate and brown rice rate directly affect the rice processing properties (Tong et al,2019).As shown in Fig.1-A,all indices decreased with the extension of storage period.After three-year storage,head rice rate and brown rice rate decreased from 67.02% to 61.00% and from 75.62% to 69.74%,respectively.Conversely,fissure rate increased from 4.67% to 12.33%,so did the yellow rice rate,which grew from 0.21% to 1.58% during this process.Water absorption can influence the sensory evaluation (Diao et al,2019).As shown in Fig.1-B,the water absorption rate and rice expansion rate significantly promoted after two-year storage,whereas this trend suspended in the 3rd year.Furthermore,the pH value and soluble solid content started to reduce in the 2nd year.For pH value,it was more sensitive to storage time,which had significant decrease in following each year.Texture property is another rice taste value,and can represent the metabolism happened in rice during storage.We found that rice gradually became hardened during storage,with hardness value significantly higher in the 2nd and 3rd year than the fresh one(0) and the 1st year (Fig.1-C).The adhesiveness and cohesiveness were not stable.Springiness,which reflects deformational character of rice related taste value,was higher in the 2nd and 3rd years.The elasticity determines rice chewiness.Meanwhile,gumminess may also reveal rice sticky condition.Springiness and gumminess increased by 25.1% and 67.1% after three-year storage compared with the fresh one,respectively.The chewiness had the same variation trend.Rice color,an important sensory property,severely influences consumers’ acceptance.Maillard and caramelization reaction and rice acidification can mainly attribute to rice color changes(Diao et al,2019;Xu et al,2019),along with other factors,viz.,microorganism and other reactions like enzymatic browning(Auksornsri and Songsermpong,2017).L*,a* and b* values represent degree of lightness to darkness,the degree of redness to greenness,and the extent of yellowness to blueness,respectively.According to rice color testing results,rice was darkening during storage as L* values decreased and b* values increased with no significant changes of a* values (Fig.1-D),which are consistent with Ziegler et al (2017) except for the increased b* values in the study.Peak viscosity reflects the expansion property of starch granules,and its descent suggests the decreasing of swelling property (Ma et al,2021).As storage at the 3rd year,the peak viscosity values decreased from 2 964.33 to 2 353.00 cP (Fig.1-E).The breakdown values (subtraction of peak and trough viscosity),which display the thermal stability starch (Zhu et al,2017),decreased from 674.00 (0 year) to 414.67 cP (3rd year) as storage extending,and the decrease trend of the setback values (subtraction of final and trough viscosity) was from 1 288.67 (0 year) to 1 157.33 cp(3rd year),which suggested the rice structure was rearranged when amylose declined.For vitamin E content,although there was no significant reduction between the fresh one and the 1st year,the obvious changes happened in the 2nd and the 3rd years,with the contents decreased to 7.05 and 3.72 mg/kg (Fig.1-F).The significant loss of vitamin E in rice indicates the decreased nutrition and antioxidant ability during rice storage.The composite scores can reflect rice quality,and the minimum score means the worst quality.The scores got less as time went on,and those at the 3rd storage year reached the lowest value(Fig.1-G).The VOCs can also reflect rice yellowing progress(Liu et al,2021),and their concentration varied with the extension of storage.Hence,VOCs can reflect the deteriorated degree of rice.The diversity and content of VOCs were up to the highest after three years (Fig.1-H).These major substances are aldehydes,ketones with minor several acids,alcohols and furans.The ascendant aldehydes during storage are mainly derived from oleate hydroperoxide decomposition,while hexanal is from linoleate decomposition (Yuan et al,2019).Most ketones directly come from fatty acid oxidation.Besides,heterocyclic,which produced from Maillard reaction,is an important substance for rice aging (Zhang et al,2019).Other VOCs,2-heptanone and 6-methyl-5-hepten-2-one,are the key aromatics in rice yellowing (Liu et al,2021).2-pentylfuran was also detected in this study.The increasing of its concentration showed that rice had significantly aged in the storage.Meanwhile,1-octen-3-ol was found in storage during different kinds of rice yellowing (Zhang et al,2020).Thus,aldehydes,ketones,alcohols and heterocyclic compounds of VOCs can reflect rice oxidation and yellowing degree,and aldehydes are one of the main compounds leading to unpleasant olfactory sensation.Therefore,the increasing concentration of aldehydes indicates the elevation of oxidation and rancidity in rice,besides hexanal,decanal and (E)-2-nonenal can represent oxidation of rice as well.The results of fingerprint analysis showed that hexanal,nonanal,octanal,decanal and heptanal had the highest concentration in the 3rd storage year.Ketones often appears in rice deterioration process,of which hexanone,2-heptanone,6-methyl-5-hepten-2-one,acetone,butanedione,pentone and butanone were ascent in this study.Two heterocyclics of 2-pentylfuran and 1-octen-3-ol were also found among these VOCs (Fig.1-H).Additionally,principal component analysis of rice suggested that the samples from the 1st and the 2nd storage years shared similar VOC profile,but were significant different from the 3rd storage year (Fig.1-I).It suggested the stable quality of rice during the first two years,especially the 1st year,had higher quality than those during more one-year storage.

ACKNOWLEDGEMENTS

This study was supported by Zhejiang Lingyan Research Plan Program (Grant No.2022C020202),Hangzhou Initiative Design for Agriculture and Social Development Research Program(Grant No.20190101A07),and Zhejiang A&F University Scientific Research Development Fund Project (Grant No.2019FR014) in China.

SUPPLEMENTAL DATA

The following material is available in the online version of this article at http://www.sciencedirect.com/journal/rice-science;http://ricescience.org.File S1.Methods.