Germinated rice: an overview of gaba, phenolic components and antioxidant activity

This article provides an overview of various studies reporting the correlation and mechanism of increased GABA, phenolic components, and antioxidant reactions in sprouted rice. Sprouted rice is viewed as a functional food because it contains nutrients such as GABA, antioxidants, and other metabolites that are excellent for health. The germination process significantly increases the availability of nutrients such as GABA by 15.4-fold and antioxidant activity by 0.43%. Germination not only adds to the nutrients already present but also introduces new components such as inositol, g-oryzanol, ferulic acid, phytic acid, zinc, tocotrienols, potassium, and prolyl endopeptidase inhibitors. Thus, sprouted rice products can be used as a means of dietary improvement and also as health-promoting foods.


Introduction
Rice (Oryza sativa L.) is the most popular grain and is used by an increasing number of people as a staple food for half of the world's population.Recently, rice germination has become a new research object.The treatment done to obtain germinated rice is to soak grain in water until sprouting.Several studies have shown that the sprouting process changes the chemical components of rice and improves its functional and nutritional properties [1].Sprouted rice releases minerals that lower the concentration of phytic acid, making it more easily absorbed by the body [2].The increase in nutrition of sprouted rice is also due to the increase in amino acids, as the stored proteins are broken down by the action of amino acids [3], and an increase in amino acid bioavailability [4] as well as the reduction of some antinutritional compounds [5].As a functional food or nutraceutical food, consuming sprouted rice can promote human healing, which is very widely used as a leading research subject in the field of food science.Several studies have shown that sprouted rice contains good minerals, vitamins, and bioactive components, especially g-oryzanol, tocopherols, and tocotrienols, GABA, and non-protein amino acids that have important biological activities.It is known that sprouted rice has many pharmacological properties, namely as an antihypertensive and antihypertensive agent, and has the ability to reduce the consequences of several dangerous diseases such as cancer, diabetes, cardiovascular disease, and Alzheimer's disease [6].
The resulting changes in the nutritional, physical properties and chemical components of rice are caused by the breakdown of large molecules such as starch, fiber, and protein by hydrolytic enzymes activated during germination [4].Germination also showed an increase in bioactive compound components, including antioxidants [7].Increased antioxidant activity is characterized by the collection of phenolic compounds and flavonoids, namely flavonols, chalcones flavnes, anthocyanins, flavanoids, tannins, ascorbic acid and tocopherols at various grades of rice during germination [8] Sostin rice, sprouted rice contains almost four times more fiber, Vitamin E, niacin and lysine and ten times higher GABA content [9].GABA acts as a neurotransmitter in the brain that controls high blood pressure and inhibits cancer cell growth [10].Understanding the germination process has great significance for explaining the transformation mechanism of nutrients, chemical composition and functional properties of rice.Therefore, this paper reviews the role and mechanism of rice germination in the addition of GABA, phenolic components phenolic components and antioxidant reactions.

Germinated rice definition
Sprouted rice is seen as a functional product due to its nutritional content such as GABA, antioxidants and other metabolites that are very good for health.The germination process leads to improved nutrition by removing phytic acid, breaking down proteins, vitamins, and enzymes allowing these essential nutrients to be more easily absorbed during the digestive process [10].It has been reported that sprouted rice can improve brain function and lower blood fat.The physical and chemical changes that occur in germinated rice are caused by acylated steryl glucoside (ASG) which functions to increase enzyme content, normalize malfunctioning enzymes, prevent diabetes and help control blood sugar levels [11].Rice germination also affects the structure and flavor of rice.Compared to unsprouted rice, its softer texture is a result of phytic acid and minerals during sprout growth [12].Sprouted rice takes less time to cook as the water that enters the grains during germination causes the rice to bulk up [13].
Germination not only improves existing nutrients but also presents new components.Significantly improved nutrients are GABA, vitamin E, lysin, niacin, magnesium, vitamin B1, vitamin B6 and also dietary fiber [14].Inositol, ferulic acid, phytic acid, tocotrienols, potassium, zinc, g-oryzanol, and prolylendopeptidase inhibitors are other nutrients in germinated rice that also increases [15].Inositol, ferulic acid, phytic acid, tocotrienols, potassium, zinc, g-oryzanol, and proly-lendopeptidase inhibitors are other nutrients in germinated rice that also increases [16].They also reported specifically related gaba content of rice germinating that there was a tenfold increase when compared to regular milled rice and twice as high as non-sprout brown rice.Furthermore, they found that there was a total increase in protein while the calorie and sugar content decreased in rice germinated [17].Several other studies have reported that there is a significant effect between the amount of protein and glutamic acid, as well as the content of glutamic acid and GABA at the time of germination [18].Soaking time affects the amount of GABA, where the longer the soaking will increase the GABA content [19].After 24 hours of germination, the relevant nutrient content increased more than that of ungerminated rice, namely fructose 3.4-fold, reducing sugar 2.75-fold, and GABA 7.97-fold [20].Consuming sprouted rice continuously can prevent headaches, inhibit colon cancer, relieve constipation, prevent diabetes and heart disease [11].During the menopause period GABA intake can suppress blood pressure and autonomic disorders as well as improve sleep [21 Nisar et al. reported that consumption of sprouted rice effectively controlled blood sugar levels in patients with high blood sugar levels without increasing insulin action [22].In addition to containing other beneficial components, these foods also contain abundant neurotransmitters as well as dietary fiber that stimulates intestinal peristalsis [12].It is also reported to contain considerable phytic acid, which has strong anti-cancer effects and inhibits prolyl endopeptidase involved in peptide metabolism [23].Germinated rice increases protease activity 1.5-fold.They also hypothesized that the decrease in protein and hypersensitivity levels was most likely due to the abundant protease activity after germination [24].Current research is increasingly proving that the nutritional and medicinal value of rice is enhanced through the sprouting process.In addition to the increase as mentioned, there is also a decrease in indigestible components, namely protease inhibitors and lectins [26 there is also a collection of polyphenols and vitamin C [8], they also reported that in germinated rice contained many bioactive compounds such as antioxidants, antidiabetics and anticancers.The method of making germinated rice through a fairly simple stage [10], which is soaked in water at 35-40 ° C for 12 hours and during the soaking period, the soaking water is replaced every 3-4 hours with the aim of preventing fermentation processes that can cause unwanted odors and to maintain water conditions.The process will produce sprouts with a length of 0.5-1 mm, at which stage the nutrients in rice are maximum.Then the drying process is carried out using an oven with a temperature of 45℃ for 4 hours [25].

GABA content
The higher bioactive content in sprouted rice compared to regular rice causes sprouted rice products to play an important role in the world of food and health [6].Of these bioactive compounds, the most dominant is the research topic GABA which functions as an important inhibitor in the brain [27].GABA also acts as a cancer cell growth inhibitor, can control blood pressure and cholesterol, and plays an active role in pain and anxiety disorders [28].GABA is produced through plant metabolic processes on the GABA-shunt pathway, where GABA derived from L-glutamate is converted by glutamate decarboxylase (GAD); however, it can also be converted to succinic semi-aldehyde (SSA) by GABA transaminase (GABA-T) [29].GABA synthesis is directly proportional to the increase in pyridoxal phosphate (PLP) coenzyme, which controls blood fat and sugar [24].The rise of research related to GABA content in rice, it is generally reported that GABA increases during germination through protein metabolism and other components in seeds [30].Several experiments conducted proved that high GABA content is effective for lowering blood pressure, blood sugar recovery symptoms of alcohol addiction [31], being the main neurotransmitter [10], and help improve memory and learning skills tried in mice [32].
The germination process activates many enzymes, most notably the enzyme glutamate decarboxylase (GAD) which converts L-glutamic acid into GABA [23].Gaba content in rice germinates differently per cultivar.Non-germinated rice GABA content in several cultivars namely 0.19, 0.24, 0.34, 0.39 and 0.51 mg/g respectively for kultivar PL2, CN1, SP1, PT1 and KDML 105 [33].After being condemned there was an increase of about 6.58, 6.70, 5.11, 4.80, and 4.08 times respectively, so that the final gaba content of rice germinated sequentially on all cultivars was 1.25, 1.59, 1.74, 2.01 and 2.45 mg/g.Some rice cultivars that increase GABA content is most likely due to metabolic changes in the seed, the presence of damaged components after hydration that produce dissolved nitrogen compounds such as GABA.Their results support previous research that studied GABA accumulation in Korean and Japanese brown rice, GABA concentration increased higher than 13, which means a 3.41-fold increase from ungerminated brown rice [29].While other reasons related to the increase in GABA in germinated rice are related to the breakdown of other compounds and protein synthesis as well as several other components [34], however, it can also be caused by proteins that break down into amides as a source of nitrogen for embryonic growth during germination [35].In addition to gad enzymes, another enzyme involved is gluttamine-oxyglutarate-aminotransferase [22], which accelerates the work of gad enzymes causing glutamic acid to form and transform into GABA higher during germination [28].On the other hand, some evidence suggests that glutamate synthase (GOGAT) and glutamine synthase (GS) function in the accumulation of glutamic acid in the course of germination [12].
Research on GABA enhancement in germinated rice continues to be updated, with many factors beginning to be linked and controlled for optimal GABA enhancement and microbial reduction [3].They managed to increase the GABA content to 25.5mg/100g by spraying ethanol gas for 21 hours.Another problem that arises is in the drying treatment given to germinated rice.After germination, the moisture content of rice will increase until it reaches 40% wet weight, then the drying process is carried out until the moisture content drops to 10-13% wb [12].Drying using high temperatures will certainly give an adverse effect to GABA content that has properties susceptible to heat.Bread roasting using rice germinated at 200°C for 20 minutes, eliminating GABA content entirely [25].In addition, the use of heating to remove germs in germs germs also proves to reduce GABA content [31], likewise with the treatment of pratanak in rice [36].Jongyingcharoen et al (2016) do not provide drying treatment of germinated rice, but perform a brewing and incubation process that has been shown to increase GABA content by 15 times that of native rice (1.31 mg/100 g DM) to 17.74 mg/100 g FW, which turns out to be slightly higher than the GABA content of KDML 105 GBR cultivar rice [35] [37].High GABA content (19.71 mg / 100 g DM) in rice germinated cultivar KDML 105 is obtained by brewing at 35 °C for 4 hours and then incupulated at the same temperature for 20 hours [35].
In his latest study Ding et al [38] stated that the need for regular germination treatment, especially using CO2 for more than 6 hours, can produce high GABA content of sostin rice.While in brown rice germination for 72 hours, GABA increased 15.4 times (2.91 mg/100 g).Furthermore, [38] utilize sonication technology that provides ultrasonic waves to rice after being condemned.They found that there was an increase of 60-69% times higher than rice germinating without sonication treatment.The result is a new method to optimize the accumulation of bioactive contents during germination, not only GABA but also trehalose, succinic acid, arabinose, fructose and other carbohydrate components.The mechanism of increasing GABA by ultrasonic wave administration is described through the GABA-shunt pathway according to several previous studies [39], where H+ atoms also play an important role during GABA metabolism [40].Glutamate is the precursor of GABA, which plays a role in stimulating the activity of GAD, is the main enzyme for the formation of GABA, then through alanine aminotransferase, glutamate produces pyruvate, then alanine and α-ketoglutaric acid [41].In this study, the accumulation of GABA increases was as direct as previously reported [42 By applying ultrasonic waves (25 kHz, 16 W/L) for 5 minutes after 66 hours of germination, the GABA content of brown rice became 75.82 mg/100 g or increased by 69.2% compared with rice germinated without ultrasonic waves [38].

Antioxidant and phenolic components
In addition to changes in biochemical characteristics and responsiveness, germination also exhibits a collection of bioactive compounds, some of which such as antioxidant phenolic components [7].After germination, the color of the rice will change.The color change in rice germination results from the addition of phenolic and flavonoid groups, including ascorbic acid, chalcones, tannins, flavones, flavandiols, anthocyanins, and tocopherols, which automatically increase antioxidant activity [8].Some bioactive groups that are also contained in sprouted rice are α-tocopherol, thiamine, niacin, and pyridoxine which are much higher than non-germinated rice [43].The accumulated increase in bioactive compounds during germination is influenced by several factors namely cultivars, pH, immersion aeration, temperature and immersion time [44].Total phenolics increase significantly due to germination, but different content in each type of rice that is condemned [45].[46] The increase in these phenolic compounds is based on enzyme activation during germination through the phenylpropanoid pathway and the hydrolysis of polysaccharides bound to the cell wall to the cell wall [47].It can also be caused by the activation of saccharolytic enzymes that hydrolyze starch under aerobic conditions so that many phenolic compounds are released [48].This explains the accumulation of free phenolic acids as well as phenolic hydrolysis, and the decrease of hydroxycyclic sucrose esters in germinated rice [49].While research conducted on the phenolic content of several rice cultivars, also experienced a higher increase from the initial content which ranged from 939.2 to 1,567.4 µg/g.Furthermore, after germination the phenolic content increases which is in line with the increase in phenol, flavonol and tannin content during germination.This is in line with the increase in the content of phenols, flavonols and tannins during germination [50].Another class of phenol, tocopherol (vitamin E) is an antioxidant that breaks down in fats [51].Increase in tocopherols twice as high in rice germinated [52].The increase is likely due to the activity of some enzymes that lower the macro content and produce other compounds that are important for growing buds, which refer to the cleavage, solubilization of bound components and the collection of several bioactive compounds [53].A-tocopherol and g-oryzanol also increased during germination.This compound is a beneficial and important antioxidant that is generally contained in rice bran, which serves to protect rice from oxidation [54].During germination, these compounds increase and migrate due to enzyme activity from the bran into the rice grains, thus being reported as the cause of the increase in antioxidants [55].G-oryzanol is a triterpene-like compound produced through the mevalonate pathway and dependent on ferulic acid bound by ester bonds, while a-tocopherol is synthesized through the shikimate and nonmevalonate pathways [56].
Elevated phenolic compounds generally result from new syntheses and de-novo transformations [57].Glucose as the main trigger for the synthesis of phenolic compounds, and several important molecular formation routes, shikimate, phenyl-propanoid and tannin pathways, glycolysis, acetate/malonate, which participate in the conversion of phenolic content [58].It can also be said that the amount of free phenolics has increased due to the destruction of the walls in the germination process [59].Increased insoluble form during germination contributes to an increase in insoluble phenolic hydrolysis [22].One of the factors for the increase in phenolic content of brown rice after germination is caused by an increase in the amount of hydrolytic enzymes due to cell wall damage during germination [21].This rice accumulates phenolic compounds with higher potency than non-germinated rice, making it more suitable for human consumption [60].This is also consistent with previous studies that germinated brown rice has higher flavonoid content [42].The increase in phenolic content is inversely proportional to phytic acid, where during germination the phytic acid content decreases.Phytic acid in brown rice decreased by 0.43% after germination [7].The correct explanation for this is due to increased phytase activity.Increased phytase activity leads to the synthesis of inositol and phosphoric acid, which are active in the germination process [47].The increase in phenolic and bioactive compounds, and the decrease in phytic acid content actively contributed to the increase in antioxidant activity [61].In previous findings, it was reported that extracts bound to germinated brown rice had higher antioxidant capacity compared to the original seeds [42].A recent study reported that the antioxidant activity of ungerminated rice ranged from 34.6 to 43.9%, which then increased to 60.9 to 77.5% after germination [7].This explains that the antioxidant capacity of bound extracts correlates with phenolic bounds, further phenolic-bound changes can directly alter antioxidant capacity [62].

Conclusion
From the above literature review, it is clear that sprouted rice products can be used for food renewal and also as health-promoting foods.The reason behind such a statement is that there are many findings relating to its high GABA and antioxidant content and its biofunctional properties for maintaining health.