The Application of Molecular Vibrational Spectroscopy to Determine Selective Bioactive Compounds, Total Antioxidant Activity and The Antimicrobial Effect of Onion (Allium cepa) and Garlic (Allium sativum)
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Analysis of bioactive and antimicrobial components in vegetables and fruits is an important area of food analysis. Fourier transform infrared (FT-IR) spectroscopy and surface enhanced Raman scattering (SERS) spectroscopy are useful tools for analysis of biological materials, and in this study, the type and activity of selected bioactive compounds in onions and garlics were predicted using FT-IR and SERS spectroscopies. Having a new tool for assessing the composition of antioxidant components and total antioxidant activity is an important development to predict the bioactive properties in fruits, vegetables and commercial food products. Research focused on the properties of Allium sp. (red, white and yellow onions, shallot, garlic and elephant garlic) since these have distinct properties of possessing antioxidant and antimicrobial activities. Quercetin concentrations ranged from approximately 400 mg/kg fresh weight to 650 mg/kg fresh weight with red onion having the highest content. Diallyl sulfides provide the greatest contribution to the antimicrobial activity of garlic, contributing roughly 80% of the overall antimicrobial activity against Listeria monocytogenes, Escherichia coli O157:H7 and Campylobacter jejuni. For this current project, FT-IR and SERS spectroscopies were used to: (1) determine quercetin, certain quercetin derivatives and total flavonol contents in onion; (2) develop prediction models for the total antioxidant activity in onion and garlic from spectral features and conventional chemical and chromatographic methods; (3) examine the antimicrobial effect of garlic extracts and garlic-derived organosulfur compounds on specific foodborne pathogens (Listeria monocytogenes, Escherichia coli O157:H7 and Campylobacter jejuni) and compare vibrational spectroscopic results with those from traditional food microbiological methods, morphological assessment, biochemical and immunochemical tests.