Preparation and Application of Aerogel Colorimetric Labels Based on Aldehydes as Indicators of Walnut Kernel Oxidative Rancidity

Walnut kernels are prone to lipid oxidation and the formation of harmful products such as aldehydes due to their high content of unsaturated fatty acids. In this study, an intelligent tag was developed to monitor walnut kernel oxidative stress in real time based on aldehydes. The tag utilized a high porosity aerogel prepared from carboxymethyl cellulose (CMC) and sodium alginate (SA), and loaded with Congo red and hydroxylamine sulfate as a colorimetric probe. By optimizing the CMC/SA ratio (1:2), an aerogel with excellent hardness (2039.20 g) and cohesiveness (62.85%), porosity of over 94%, and thermal stability was obtained. Its response sensitivity to hexanal and valeraldehyde was noticeable color difference change (ΔE, increased from 39.21 to 49.43). The ΔE was significantly positively correlated with peroxide value (PV, r = 0.84, p < 0.01), acid value (AV, r = 0.98, p < 0.01), and malondialdehyde content (MDA, r = 0.90, p < 0.01). When ΔE ≥ 46, the corresponding PV was about 3.0 mmol/kg, and the label color transitioned from light red to brown, which could accurately indicate the critical point of walnut kernel oxidation and rancidity. Gas chromatography-mass spectrometry (GC-MS) validated the dynamic accumulation pattern of hexanal (0-403.31 µg/kg) as a core biomarker. These results demonstrate the great potential of the aerogel label for semi-quantitative, real-time monitoring of lipid oxidation processes, as a complementary tool to traditional GC-MS analysis. PRACTICAL APPLICATIONS: This aerogel colorimetric label can not only qualitatively indicate the occurrence of oxidation reactions but also quantitatively evaluate the degree of oxidation, providing an efficient solution for oxidation monitoring of nut products.