Designing flaxseed oil-in-water emulsion-based stimuli-responsive edible films

Abstract Emulsions are useful for formulating compartmentalized structures with tunable characteristics, making their dried state a potential stimuli-responsive edible substrate for four-dimensional (4D) printing applications. Yet, establishing component, microstructure, and response relationships is crucial for controlled transformations under stimulus exposure, thereby enabling the technology for edible applications. The oil phase incorporation requires particular attention, due to its hydrophobicity and its impact on the response behavior, which can be determined by the swelling rate during water immersion tests. This study focused on flaxseed oil-in-water (O/W) emulsion-based edible films, stabilized by gelatin and lecithin. The dried films based on two oil-to-water (O:W) ratios (5:95 and 10:90 v/v), were submerged in distilled water (pH: 4, 7, and 10). The film-forming emulsions were yield stress fluids exhibiting solid-like properties in the linear viscoelastic region. Network strength and connectivity of emulsions were significantly different based on complex modulus as a function of frequency ( p < 0.05). The films were characterized via thickness (0.19–0.66 mm), weight (0.41–1.18 g), moisture content, opacity, and color. The film matrix properties affected the response behavior of the samples depending on the swelling media. The film survivability depended on the swelling medium pH ( p < 0.05). All samples remained intact overnight at pH 4 and 7. Only sample 5O95W-10 swelled up to 60 min at pH 10, which can be attributed to longer gel setting time and the presence of more protein and emulsifier allowing better network connectivity. Swelling rates of the films were similar at pH 4 and pH 7 ( p > 0.05), and their governing mechanism was less Fickian diffusion. Understanding the effects of the oil phase and the pH of the swelling medium on the response behavior provides insights for tailoring these films as novel substrates for 4D printing and enables controlled transformations.