Smart packaging materials that can indicate the freshness of food align with the preferences of contemporary consumers. To promote environmental sustainability, there is growing emphasis on researching and developing smart films made from natural polymers instead of synthetic ones [1]. Natural polymers like proteins and polysaccharides have various functional properties, including excellent biocompatibility and edibility, making them ideal choices for creating smart packaging films [2,3].

Pea protein isolate (PPI) is a promising protein source for film production due to its availability, cost-effectiveness, and high lysine content. Unlike soy protein isolate, PPI does not cause allergies [4,5]. However, PPI films have limited barriers and mechanical properties that restricts their use in food packaging. To overcome these limitations, various approaches have been employed such as physical treatment, crosslinking, and plasticization. For instance, high-intensity ultrasound treatment effectively improved the tensile strength and hydrophobicity of PPI films [3]. Recently, Cheng et al. used the high-pressure homogenization method to modify the structure of PPI to improve the mechanical properties of PPI films [6]. On the other hand, Perez-Puyana et al. prepared PPI-based films with enhanced mechanical properties using heat treatment and UV radiation as physical crosslinking methods [7]. Additionally, incorporating formaldehyde or monoglycerides significantly improved the mechanical properties while glycerol enhanced the hydrophobicity of PPI films [8,9]. Despite these advancements in enhancing PPI film properties, there are drawbacks: specialized equipment is required for physical methods which limits their universal applicability; formaldehyde is toxic and cannot be used in edible films. Therefore, further advancements are necessary to enhance the properties of PPI films.

Polysaccharides can combine with proteins to form composites through hydrophobic and electrostatic interactions [10], enhancing the properties of protein films [11,12]. Cellulose nanofibers (CNFs), derived from cellulose, an abundant and easily modified polysaccharide, have a nanoscale size, excellent biodegradability, and mechanical strength. They are widely used in the construction of environmentally friendly and sustainable food packaging [13]. Recent studies have shown that CNFs could increase film thickness and improve tensile strength in protein-based films [14,15]. However, CNFs have drawbacks such as water swelling and insufficient interaction with proteins, which limit their use as packaging material. In a recent study by Larraza et al. [16], it was demonstrated that carboxyl groups on the surface of carboxylated cellulose nanofibers (CCNFs) facilitated their interaction with proteins, thereby enhancing the mechanical properties and thermal stability of protein films. However, non-covalent interactions like hydrogen bonds and electrostatic interactions between CCNFs and proteins remain weak and have limited enhancement effect on protein films, requiring further improvement. Studies have shown that covalent bonds formation through Schiff base reactions was a feasible approach to enhance protein films' properties [17]. For example, dialdehyde glucomannan formed by periodate oxidation could enhance the properties of gelatin films via Schiff base reaction [18]. Therefore, oxidizing CCNFs with periodate to form DCCNFs is expected to enhance PPI films' properties through Schiff base reaction.

It is worth noting that in addition to favorable physicochemical properties, smart films also need convey the freshness of food through color changes. This primarily relies on how the color-responsive components in the film react to food spoilage products. Hence, choosing suitable color-responsive components is vital for smart films. Anthocyanins, such as black carrot anthocyanin and roselle anthocyanin, are polyphenolic flavonoids that can change colors under different pH conditions [19]. They have been successfully incorporated into smart packaging films to monitor the freshness of fish, shrimp, and pork due to their ability to change color within a wider pH range [20,21]. Bilberry (Vaccinium myrtillus L.) is a popular berry among consumers because it is rich in anthocyanins and other active substances like quercetin, flavanols, and catechins [22,23]. Undoubtedly, bilberry extracts (BE) can serve as a color-responsive component in smart packaging films to indicate food spoilage through color changes induced by pH.

To the best of our knowledge, there is a lack of research on using dialdehyde polysaccharides as cross-linking agents to improve the physicochemical properties of protein-based smart films with natural anthocyanin extracts. Therefore, this study aims to develop pH-responsive smart films with enhanced physicochemical properties by introducing DCCNFs as a cross-linking agent and BE as an indicator agent into the PPI matrix through the casting method. The effects of DCCNFs and BE on the chemical structures, morphology, thermal stability, physical properties, and pH responsiveness of the film were thoroughly investigated. Subsequently, the pH-responsive smart film was utilized to monitor pork spoilage during storage. This study provides a smart biopolymer-based packaging material that has good mechanical properties and pH responsiveness.