Effect of fermentation on concentration of functional compounds in malted brown rice flour

Introduction: Nowadays, by increasing the awareness of people on the importance of healthy diet and its effects on prevention of many diseases, researchers have been focused on the formulation and production of functional foods for their positive effects on improving the health. Regarding to the role of  bread in supplying a major part of the daily energy of community members, it is possible to fortify this product during production process with many functional compounds. Brown rice has high fiber content and is known as a rich source of vitamins and antioxidants due to the presence of bran layer on the seed. Many studies have shown that germination can improve the concentration of functional compounds such as gamma- aminobutyric acid, phenolic compounds and antioxidants in brown rice, so it can be used for production of functional foods or fortification . One of  the important processes during the bread making is fermentation. Regarding to  the probable effects of fermentationon on the concentration of functional compounds in sourdough, in the present project the effect of fermenting the malted or germinated brown rice flour by  two lactic acid bactria, Lactobacillus sakei and Lactobacillus sanfranciscensis, seperately and in  the co-culture form besides the baker^,s yeast was investigated and the potential of resulted sour doughs for using in the formulation of functional fermented products such as bread was determined.
Materials and methods: The brown rice sample was steeped for 24 hours at 30 ᵒC, then germinated for 48 hours at the same temperature of steeping. The germinated sample was dried to approximately 10% moisture content then ground and sieved to 40 mesh flour.The resulted flour samples were fermented with each of Lactobacillus sakei and Lactobacillus sanfranciscensis, seperately and in  the co-culture form (once with  the optical density of 2.9 or 10 ⁸ CFU/ml and another time with  the optical density of 1.45 or 0.5 ×10⁸ CFU/ml) in the presence of baker^,s yeast till reaching to pH=4.9, then changes in the concentrations of the functional compounds such as free and bound phenolic compounds and their related DPPH free radical scavenging capacity, gamma-aminobutyric acid and free amino acids contents of sourdough samples was studied. The gamma-amino butyric acid and amino acid profile were measured by HPLC method, free and bound phenolic compounds were determined by using of folin ciocalteu reagent and DPPH radical scavenging capacities were measured by using of spectrophotometric method in all of the samples. A dough sample which was fermented only by Baker^,s yeast (Saccharomyces cerevisiae) was used as control. The effect of each independent variable on the concentration of functional compounds was determined by using of one way ANOVA, by SPSS software (version 20 ). All data were first checked for their normal distribution and also the homogeneity of variances. Duncan^, s multiple range test was used for comparing the means at α=0.05 significant level. All tests were performed in triplicate other than amino acid profile determination.
Results and discussion: According to the obtained results, fermenting with L.sakei and L.sanfranciscensis separately or in the co-culture form, caused significant increase and decrease of free and bound phenolic compounds respectively compared to those of malted brown rice flour (p≤0.05). The highest free phenolics content was observed at sourdoughs containing the mixture of lactic acid bacteria with OD of 2.9 and 1.45. Free phenolic compounds of sourdough sample which wasfermented by L.sakei, showed the highest antioxidant capacity, whilst the highest free DPPH radical scavenging activity of bound phenolic compounds was obsereved in sourdough that was fermented with the mixture of L.sanfranciscensis and L.sakei with optical density of 1.45. In general, the results indicated on the higher antioxidant activity of free and bound phenolic compounds of L. sakei fermented sample and sourdough containing the mixture of L. sakei and L. sanfranciscensis with optical density of 1.45 respectively. The type of microbial flora for fermenting the dough had significant effect on GABA synthesis. The control sample and the sour dough fermented by L. sakei had the lowest and the highest GABA contents respectively. It seems that the difference in the GABA contents of the samples with different starters is related to the presence and amount of glutamate decarboxylase (GAD) in samples. The mentioned enzyme converts the glutamate to GABA. Also, fermenting of sourdough with L.sakei, resulted in the most free amino acids contents. The highest essential amino acid contents was observed at sour dough fermented by the mixture of L.sakei and L. sanfranciscensis with OD of 2.9.
Conclusion: Considering the obtained results, L.sakei bacteria can be suggested to sourdough industry as a suitable lactic acid bacteria for fermentation of malted brown rice flour with the aim of improving its functional compounds specially gamma-amino butyric acid. Gamma-amino butyric acid widely known as GABA, is a 4-carbon nonproteinogenic amino acid that is ubiquitous in microorganisms, plants, animals, and humans. It has been reported to possess numerous physiological functions in different organisms. It serves as a major inhibitory neurotransmitter in the central nervous system in humans and animals by mediating inhibitory synaptic currents between the pre- and postsynaptic membrane. It provides beneficial effects for human health by regulating blood pressure and the heart rate, helping with recovery from chronic alcohol-related symptoms, alleviating pain and anxiety, controlling stress and inhibiting cancer cell proliferation. According to  the scientific articles, L.sakei is a microorganism with short lag phase and high growth rate among other Lactobacilli microorganisms. This bacteria is able to inactivate pathogens and spoilage microorganisms through acid and bacteriocin production and there is no report on its toxic or pathogenic effects, so it is mostly used for fermentation of sausages as starter culture. In the present study, based on the obtained results, the type of the fermenting microorganism and its concentration in the dough affected the functional properties of the resulted sourdough, also using of  L.sakei for fermenting the malted brown rice flour with the aim of producing a functional sour dough, resulted in improving of  the  gamma-amino butyric  acid concentration and DPPH radical scavenging capacity, so producing of a functional bread rich in  gamma-amino butyric acid, free amino acids and free phenolic compounds with antioxidant activity would be possible.