The application and mechanism of action of Michelia alba oil vapour in GABA enhancement and microbial growth control of germinated brown rice

Abstract

Germination of brown rice as a functional ingredient in various food products has been found to increase the γ-aminobutyric acid (GABA). Due to its high nutritional impact, germinated brown rice (GBR) grains are easily contaminated with bacteria, yeast/mould. Bacillus cereus is a foodborne pathogen and spoilage bacteria commonly found in GBR. This study was undertaken to use Michelia alba oil vapour (600, 1200 and 2400 μl L −1 ) to increase GABA during germination of brown rice grains over 48 h. The shelf life of the treated GBR was then determined during storage at 30 ± 2 °C for 28 days by finding physiochemical and physical changes in the treated rice and undertaking sensory evaluation. A mode of action was studied by using a scanning electron microscope (SEM) and gas chromatography-mass spectrometry (GC-MS). Results showed that the treated brown rice grains at 1200 μl L −1 before the germination process showed the highest GABA content in the shortest time at 18 h, reducing cooking time and having a softer texture compared to the control sample. The treated rice's aerobic plate count, yeast/mould count and B. cereus levels were better able to meet the microbiological criteria for rice and rice products (<5 log 10 cfu g −1 ) after 14 days of storage, when compared to the control, where the microbial growth exceeded the criteria within 7 days. Morphology of B. cereus was found to be destroyed due to the leaking of the cell wall after exposing the rice grain to M. alba . Trans -caryophyllene and β-elemene were the two main compounds of M. alba found in the treated rice, which are the key to destroying the cell wall and cell membrane of the bacteria. In addition, the GBR was found to be lighter in colour when exposed to a higher concentration of M. alba. These findings will help to improve GABA content during soaking in a short time period and make it safe to consume, thus developing the GBR industry. • M. alba oil enhanced γ-aminobutyric acid in brown rice grain during germination. • M. alba oil vapour inhibited bacteria, yeast/mould and B. cereus ~ two-fold. • M. alba oil offered higher L* value content in germinated brown rice (than control). • M. alba oil could reduce the hardness and the cooking time of GBR. • Caryophyllene and β-elemene were destroyed the structure of B. cereus .