Germ-Specific Triacylglycerols as Potential Biomarkers for Authenticating Zhongzi Purple Rice, a Cultivar Recognized for Its Nutritional Value

Abstract

Zhongzi purple rice is recognized as a nutritionally superior whole-grain variety, containing higher levels of protein, iron, dietary fiber, and vitamin B6 compared to conventional rice. While the nutritional profile of Zhongzi purple rice is well-established, the spatial distribution and structural specificity of its lipid components, especially germ-specific triacylglycerols (TAGs), remain poorly characterized. This study employs a multimodal mass spectrometric strategy to investigate the lipidomic uniqueness of the Zhongzi purple rice. The strategy combines matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) for spatial analysis, high-resolution mass spectrometry (HRMS) for primary analysis, and high-resolution tandem mass spectrometry with atmospheric pressure chemical ionization (APCI-HRMS/MS) for secondary analysis. MALDI-MSI identified seven metabolites in Zhongzi purple rice that exhibited germ-specific accumulation. In contrast, these metabolites showed markedly reduced signal intensities and lacked structured spatial distribution in conventional purple, black, and brown rice. Using a combination of lipidomic database interrogation, HRMS molecular formula validation, and APCI-HRMS/MS structural elucidation, we identified these metabolites as unsaturated fatty acid-enriched TAGs. Specifically, they were TAG(16:0/16:0/18:2), TAG(16:0/16:0/18:1), TAG(16:0/18:1/18:3), TAG(16:0/18:1/18:2), TAG(16:0/18:1/18:1), TAG(18:1/18:2/18:2), and TAG(18:1/18:1/18:2). The high unsaturation of these TAGs contributes to improved lipid hydrolysis efficiency, metabolic health benefits, and oxidative stability. These properties highlight Zhongzi purple rice as a potential functional food source. Due to their germ-specific accumulation, these compounds can act as potential biomarkers for authenticating Zhongzi purple rice and detecting adulterants. This provides a practical solution to commercial challenges in the industry. This study pushes the boundaries of cereal lipidomics through the integration of spatially resolved imaging and structural validation. The results shed light on the role of lipids in driving the nutritional and metabolic benefits of whole grains.