An innovative hydrogel-based colorimetric freshness indicator array for smart packaging of bananas, apples, and pears

Real-time monitoring of volatile organic compounds (VOCs) released from perishable fruits is essential for predicting their freshness. Herein, we designed a non-destructive and portable hydrogel-based colorimetric freshness indicator array for on-site monitoring of acetone, acetaldehyde, and propionaldehyde markers in yellow fruits such as bananas, apples, and pears. These VOCs serve as critical indicators of fruit spoilage. The colorimetric sensor array is constructed from a calcium alginate pad (CAP) modified with a hydrogel blend of xanthan (XG) and tragacanth gum (TG), incorporating zinc oxide nanoparticles (ZnO-NPs) as a stabilizing filler. Additionally, the array utilizes glycerol as a crosslinker, along with bromocresol purple (BCP), methyl orange (MO), BCP/thymol blue (ThB), and bromocresol green (BCG) as color-sensitive materials. This array is integrated with smartphone technology and color analysis software, allowing for real-time detection of color changes. The results demonstrate a strong correlation between the concentrations of acetaldehyde, propionaldehyde and acetone, and the color intensity, with measurable ranges of 0.117–3.92 ppm, 0.153–5.202 ppm and 0.088–5.032 ppm and the respective limits of detection were determined to be 0.11, 0.14 and 0.07 ppm, respectively. Furthermore, the sensors exhibited no adverse effects on water vapor permeability or the VOC materials. They showed a desirable recovery, enabling effective monitoring of quality changes in fruits during storage at both room temperature and in refrigeration. This research provides valuable insights for the design and development of solid-state strip sensors and simple packaging indicators, facilitating the assessment of fruit freshness through smartphone-based online signaling and data analysis.