Preparation of indicator cards for freshness detection of rice with four different processing degrees during storage based on olfactory visualization technology

Rice deteriorates during storage by producing volatile amines and aldehydes. Based on olfactory visualization technology, this study prepared a chitosan-carboxymethylcellulose-Congo red (CS/CMC-CR) indicator card, which can specifically detect changes in the main deteriorative flavor substances in rice, thereby realizing the visual determination of rice freshness. This indicator card also possesses the characteristics of environmental friendliness, cost-effectiveness, and convenient operation. Experiments show that hexanal is the main characteristic flavor compound for rice deterioration during storage, and Congo red exhibits the highest sensitivity to hexanal—with a significant color change observed when the hexanal concentration reaches 2.24 mg/kg. The interference of other volatile flavor compounds on the color of the Congo red indicator card was eliminated. Furthermore, the CS/CMC-CR composite indicator card has a denser structure and better crystallinity than single materials, with improved mechanical properties (maximum tensile strength of 37.10 MPa) and barrier properties, achieving nearly 100% accuracy in rice freshness detection.