Investigation On The Interaction Between Starch And Typical Aromatic Compounds In Cooked Rice During Recrystallization

Convenient rice at room temperature（CRrt）has increasingly attracted consumers’attention because of its convenience,portability,and sanitation.However,flavor deterioration is a major factor constraining the development of the CRrt industry due to the relatively high storage temperature and long shelf life.Such undesirable changes in CRrt during storage are mainly caused by the migratory loss of odor-active compounds,the formation and accumulation of a large amount of deteriorating flavor compounds originated from the lipids’oxidation and hydrolysis.Therefore,it is an important way to regulate flavor deterioration of CRrt by increasing the retention of typical aromatic compounds as well as slowing down their release.Starch is the principal component of rice.During storage of gelatinized starch,both amylose and amylopectin molecules rearrange,i.e.recrystallization,which could complex with various ligands,including some aromatic molecules.However,the relationship between the recrystallization behavior of starch and the aromatic compounds of cooked rice is not clear.Based on this,the complex formation between rice starch and aromatic compounds in cooked rice as well as their interaction during recrystallization nucleation,grain stacking growth and crystal was systemically studied.Moreover,a CRrt product with good flavor quality was produced on the basis of the regulation of starch recrystallization.The interaction between aromatic compounds and rice amylose as well as the controlling release of aroma by amylose was tested.Five aromatic compounds including 2-acetyl-1-pyrroline（2-AP）,hexanal（HEX）,1-octen-3-ol（OCT）,γ-decalactone（DEC）,and 2,3-butanedione（BUT）,were selected as representative of the different types of typical aromas in cooked rice.The formation of the complex of amylose and five aromatic molecules was verified using Fourier transform infrared spectroscopy（FT-IR）,differential scanning calorimetry（DSC）,thermal gravimetric analyzer（TGA）,¹³C cross-polarization/magic angle spinning nuclear magnetic resonance(¹³C CP/MAS NMR),and X-ray diffraction（XRD）.Their complexes showed V_6I-,V_6II-,V_6II-,V_6III-,and B-type crystals,respectively.The molecular dynamics simulation was carried out by Hyperchem software to study the possible forms of different aromatic molecules in the starch chains.The results showed that the aromatic molecules forming V-type complex could enter the spiral cavity of amylose,while the molecules forming B-type complex could locate between the amylose chains.Finally,the retention and controlled release of the five aromatic compounds were analyzed by gas chromatography（GC）.The results showed that the retention rates of aforementioned five aromatic compounds were related to their solubility and hydrophobicity,which were 12.16%,8.72%,24.26%,38.24%,and 8.18%,respectively.In addition,the release rates in amylose matrix were mainly affected by saturated vapor pressure,solubility,boiling point and hydrophobic characteristics of aromatic compounds,which were 2-AP>HEX>OCT>BUT>DEC in order.The nucleation process of recrystallization affects the retention of aromatic compounds.Different recrystallized nuclei of starch and aromatic molecules were prepared by controlling the cooling rate,and their complex behavior was further analyzed by DSC,XRD,attenuated total reflection-Fourier transform infrared（ATR-FTIR）,and GC.The results of DSC showed that the crystal nuclei obtained by high cooling rate had lower thermal stability,looser arrangement of helical clusters,poor compactness and low perfection.The composite analysis by XRD and ATR-FTIR analysis indicated that the relative crystallinity of crystal nuclei decreased at a rapid cooling treatment,as well as long-range and short-range order degree.According to the lattice parameters of crystal nuclei,high cooling rate induced the formation of a loose cluster and a larger cell volume.The GC results showed that the rapid cooling treatment increased the complex content of each aromatic compounds.In conclusion,cooling rate mainly affected the migration of molecular chain and led to different crystal structures.The loose crystal nuclei formed at high cooling rate was conducive to maintaining the aromatic molecules.The difference of crystal nuclei resulted in changes of grain stacking behavior during crystal growth,thus affected the complexation and retention of aromatic compounds.The 2-AP and BUT,which could form V-type and B-type crystals with starch,were selected to further study the growth process of grain packing and the complex behavior of aromatic molecules as affected by the recrystallized nuclei.DSC,XRD,and small angle X-ray scattering（SAXS）were used to analyze the changes of grain stacking,nucleation modes,lattice parameters,and crystal layer structure during the growth process of starch recrystallization.XRD results showed that rapid cooling resulted in the decrease of grain size and the increase of cell volume,which indicated that the imperfect crystal nuclei caused by rapid cooling induced the formation of recrystallized crystals with poor perfection and high heterogeneity.According to SAXS analysis,fractal types of all samples belonged to mass fractal,which was not affected by the cooling rate and aromatic molecules properties.Furthermore,the samples with high cooling rate at the same storage time have thinner crystal layer,a lower density and loose crystal structure.DSC results suggested that the mode of nucleation in starch recrystallization followed an instantaneous mechanism（n<1）at the cooling rate of 0.34°C/min and tended to be continuous（1<n<2）at the cooling rate of 3.04°C/min.In addition,the recrystallization rate decreased from 0.0502 to0.0160 d^-n,indicating the enhanced inhibition effect of BUT molecules on the migration of starch chains,as well as the increased intention of starch on BUT molecules retention.The variation of the V-type crystal during the recrystallization process was minute;thus,in order to enhance the retention of this kind of aromatic compounds forming V-type complexes with starch,it should be controlled in the initial stages of recrystallization.The retention of aromatic molecules can also be affected by its interaction with recrystallized starch.The interaction between BUT and 2-AP with B-and V-type recrystallized starch was systematically investigated using FT-IR,laser confocal micro-Raman（LCM-Raman）,and ¹³C CP/MAS NMR.The results of FT-IR and LCM Raman showed that both BUT and 2-AP molecules could interact with recrystallized starch by hydrogen bond and affect the water molecular environment of starch,but with a different mode of action.¹³C CP/MAS NMR results further indicated that the involvement of aromatic molecules affected the microenvironment of water distribution and the conformation of the starch chains of recrystallized region.The 2-AP molecules may enter the single helix cavity of V-type recrystallized starch,which enhanced the confinement of starch chains.By scanning electrostatic potential of B-and V-type crystal,BUT molecules could interact with starch chains,but the probability of entering the cavities and clusters was relatively small;2-AP molecules could be selectively inserted into the double helix inner wall and the cavities between helical clusters of B-type recrystallized starch.The main form of 2-AP interaction with V-type recrystallized starch is binding with its hydrophobic cavities of single helix.Based on the principle that the retention of aromatic molecules could be improved by regulating the nucleation morphology as well as the growth process of starch recrystallization,a new preparation process of high-quality CRrt had been established.Fast gas chromatography electronic nose（FGC E-nose）,GC-MS,and sensory evaluation methods were used to analyze the effect of cooling rate on the overall flavor of cooked rice.The results showed that the overall flavor of samples treated with rapid cooling was close to fresh rice and obtained optimal sensory evaluation,indicating that rapid cooling was helpful to maintain the flavor of cooked rice.Partial least squares regression（PLSR）further identified that eight flavor compounds（E,E）-2,4-decadienal,2-butyl-2-octenal,1-hexanol,2-propyl-1-pentanol,2-butyl-furan,geranylacetone,2-acetylthiazole,and 2-AP had significantly positive contributions to sensory evaluations.A CRrt product with the texture characteristics similar to the commercial products but good flavor quality was developed by controlling the cooling rate and maintaining the beneficial aromatic compounds.According to the industry standards（SB/T 10652-2012）,the physical and chemical indicators,pollutants and mycotoxins limit,microbial limit,and food additives were tested.The results showed that the prepared CRrt satisfied the production requirements of the industry standards.