Enhancing the hydrophobicity of purple sweet potato powder/sodium alginate composite colorimetric labels through beeswax coating coupled with hot pressing.

This study developed a novel technique to prepare hydrophobic colorimetric labels for intelligent packaging. Beeswax was phase-separated in hot ethanol, forming an upper beeswax-ethanol phase (UB) and a lower beeswax sediment phase (LB). A purple sweet potato powder/sodium alginate composite colorimetric label (P label) was first coated with either UB or LB and then hot pressed. Results showed that UB thinly adhered to P label through hydrogen bonds, whereas LB formed a thick deposit on the label with no robust interactions. Subsequent hot pressing enabled UB and LB to tightly bond to the label. Notably, LB coating combined with hot pressing was more effective in enhancing the hydrophobicity of P label, reducing its water vapor permeability by 33.5% and increasing its water contact angle by 123.2%. Moreover, the accuracy of P label in monitoring shrimp freshness was improved by LB coating coupled with hot pressing.