| Gamma-aminobutyric acid(GABA)is an inportant inhibitory neurotransmitter in the human body,which could activate nerve cells in the brain,delay memory decline and dementia,stabilize blood pressure,relieve anxiety,and other physiological functions.China's Ministry of Health recommends the daily intake of GABA is 500 mg.However,the GABA content in most existing foods is generally too low to meet the elderly daily need.Rice and rice products are essential staple foods for the Asian population The content of free amino acid,vitamins,and GABA significantly increase after germinating,while anti-nutritional factors decrease significantly.In addition,flavor compounds also accumulate during germination,which contributes to an inprovement of palatability.Some literature reported that hypoxia stress or ultrasonic treatment can significantly increase the content of GABA,polyphenols,and other metabolites during millet,soybean,and peanut germination.However,the effects of hypoxia stress time interval,stress durations and high-intensity ultrasonic field excitation on the GABA content,the glutamate decarboxylase(GAD)activity of brown rice during germination,and the mechanisms have not been reported.This study aimed to sift the special rice varieties of germinating de-hulled rice with high GABA content and GAD activity.Then,the change of GABA accumulation in germinating de-hulled rice was determined at low temperature,low oxygen storage environment,CO2 or N2 hypoxia treatment and high-intensity ultrasonic excitation.The specific research contents and results were as follows:1.Screening of rice varieties with high GABA content and GAD activityHusked rice yield,episperm color,GABA content and GAD activity before germination and after 72 h-germination were measured in 3 batches of 114 pieces of hybrid rice combinations during 2010-2011.The results show that,for different rice hybrid combination,there was a significant difference in GABA content and GAD activity before germination and after 72 h-germination The sandy beige(red),brown and purple-black rice varieties we have shown higher GABA content and GAD activity than those of conventional varieties after 72 h-germinatioa GABA content and GAD activity after 72 h-germination were defined as targets,a red rice variety with higher GABA was selected from 114 pieces of hybrid rice combinations,and its GABA content(measured by HPLC)after 72 h germination was 111.0 mg/100g,respectively,which was 6.89 times of that before germination(16.1 mg/100g).2.Effects of storage temperature and gas environment on the content of GABA in brown rice before and after germinationIn this study,brown rice samples were stored at different temperature(4?,10?,-18?,room temperature)in vacuum and different gas environment(CO2,N2,air,lvacuum)at 10?.The GABA content,GAD activity and protease activity were determined during storage for 270 d.The germination rate,cell membrane system integrity,free fatty acid value,malonyl dialdehyde(MDA)content,morphology(SEM)of the brown rice were also analyzed after storage for 360 d.There were significant changes in GABA content and GAD activity of the germinating brown rice after storage in low temperature and low oxygen environment for 72 h.Low temperature and low oxygen storage can delay the GABA accumulation ability of germinating brown rice,decrease GAD enzyme activity and increase fatty acid value.The structure of rice germ and the integrity of cell membrane considerably maintained,which contributed to reducing lipid peroxidation and the inhibition MDA increasing effectively.Brown rice that stored at low temperature(10?),CO2 or N2 hypoxic conditions for 90 days,followed by 72 h germination,the GABA content in brown rice was higher than those at the room temperature and air storage,33.77%and 22.77%,respectively,and the activity of GAD enzyme was higher than the control,35.11%and 22.47%,respectively.3.Effects of hypoxia stress on the content of GABA in germinating de-hulled riceThe effects on GABA content and GAD activity in germinating brown rice of hypoxia stress induced by gas(CO2 and N2)at different stages(processing time,the length of time)of germinating were studied.Gas type,stress treatment time(germination time)and stress treatment duration length have shown significant effects on GABA content and GAD activity in germinating brown rice.Hypoxia treatment contributed to decreasing the GABA content in germinating brown rice during the early stage of germination(CO2:0-24 h;N2:0-48 h)and significantly increased the GABA content during later germination(CO2:24-72 h;N2:48-72 h).Additionally,the degree of GABA accumulation in germinating de-hulled rice of hypoxia stress induced by CO2 was significantly better than that of induced by N2.Filled with CO2 for hypoxia stress treatment and sustained 6 h at 66-72 h germinative period,the GABA content in germinating de-hulled rice was significantly higher than that in other treated periods,and the GABA content was 91.06%higher than that without hypoxia stress treatment.4.Effects of ultrasonic treatment on GABA accumulation and nutritional quality of germinating de-hulled riceThe effects of high-intensity ultrasound(25 kHz)treatment and treatment period on the quality of germinating de-hulled rice and the accumulation amount of GABA and other nutrients were investigated.The results showed that the GABA,Vitamin B2,starch content,glucose content,the pasting viscosity and falling number of rice flour was significantly affected by high field strength ultrasonic treatment during the germination of de-hulled rice.The high-intensity ultrasonic field can penetrate the pericarp of de-hulled rice,and give rise to the formation of cracks and voids in the interior.It has a considerable ejffect on the molecular morphology of proteins,starch grains,and the biofilm structure.Ultrasonic treatment can significantly increase the content of GABA and VB2 in germinating de-hulled rice,facilitate starch hydrolysis and dramatically decrease in falling number and RVA viscosity of germinating brown rice flour.The content of GABA and VB2 in germinating brown rice significantly increased after ultrasonic field treatment for 5 min after soaking or at the 66 h germination period,then with further 72 h germination treatment5.The metabolic mechanism of GABA in the enriched germinating de-hulled rice under hypoxia stress and ultrasonic field stimulationThe metabolites(amino acids,sugars,other organic acids and phosphoric acid)in germinating de-hulled rice during hypoxia stress or high-intensity ultrasound stimulation were determined by GC/MS plant metabolomics analysis platform.Combined with multivariate statistical analysis method,different metabolites were analyzed.The metabolic mechanisms of GABA in germinating rice during hypoxia stress and ultrasonic field excitation were elucidated.Under hypoxia stress,a large number of organic acids accumulated in germinating de-hulled rice and then caused tissue acidification.During this process,GAD activity was improved and GABA-T activity was limited,thus promoted GABA pathway of glutamate decarboxylation and inhibited GABA transamination reaction,which resulted in increased GABA accumulation After high-intensity ultrasonic treatment,the activity of endogenous enzymes,such as amylase,protease,GAD,etc.,were significantly increased,the metabolic capability significantly improved.There was an increase in GABA accumulation-related metabolites such as glutamic acid,alanine,and succinic acid.The improvement of polyamine material degradation and inhibition of glutamate transamination reaction further contributed to the accumulation of GABA. |
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