Betaine, a simple natural zwitterion, is currently attracting widespread attention. Although historically labeled as an osmoregulator in agriculture and a methyl donor in animal nutrition, the molecule is now being repositioned at the forefront of green chemistry and materials science due to its unique physicochemical structure. This review critically explores the expanding horizon of betaine applications, bridging the gap between its established biological functions and its emerging roles in recently reported technologies, such as deep eutectic solvents (DESs), cocrystal engineering, and sustainable polymer synthesis. Beyond summarizing its versatile functionality across biomedicine, food science, and industrial formulations, we provide a comprehensive bibliometric analysis to map the evolution of research trends, identifying a clear focus toward industrial ecology and advanced materials. By synthesizing current advancements and discussing potential future directions, this work highlights betaine not merely as a supplement, but as a versatile molecular component with potential applications in sustainable materials and chemical engineering processes.

Deep-fat frying is widely used, but high temperatures and complex food matrices promote oil deterioration and harmful substance formation, posing risks to food safety and oil quality. This study evaluated five vegetable oils—sunflower oil (SFO), canola oil (CNO), palm oil (PO), cottonseed oil (CSO), and soybean oil (SBO)—during deep-fat frying of crispy meat to elucidate oil deterioration and contaminant formation patterns. After 32 h of frying, total polar compounds (TPCs) of PO and CNO were 29.8% and 32.6%, significantly lower than the other oils. Similar trends were observed for total oxidation value (TOTOX), carbonyl value (CV), and polar polymers, suggesting higher oxidative stability of PO and CNO, as confirmed by principal component analysis (PCA). Initial monochloro-1,2-propanediol esters (MCPDEs) and glycidyl ester (GE) in PO were relatively high (e.g., 3-MCPDE: 3630 μg/kg) but decreased over time during frying, whereas levels in SFO, CSO, and SBO remained low. Pearson’s correlation analysis indicated diacylglycerols (DAG) and monoacylglycerols (MAG) were positively correlated with MCPDEs and GE (p < 0.05). L* and b* values were positively correlated with polar polymers and contaminants, indicating that color parameters may serve as rapid, non-invasive auxiliary indicators of oil quality but should be combined with other indices for accurate evaluation.