Food safety has been a concern for a long time. The process of food deterioration is associated with both the loss of nutrients and the production of harmful substances. Therefore, many foods are packaged to slow down deterioration and ensure freshness [1]. Currently, most food packages are labelled with “Best Before” and/or “Use By” expiration dates [2]. However, consumers expect to know the freshness of the food they buy in real time, and traditional food packaging cannot meet this demand. As a result, intelligent packaging that can monitor food in real time while ensuring the quality of packaged food is being researched [3].

Many intelligent packaging materials visually communicate information about the freshness of food to consumers through a colour change of smart label. Because food spoilage releases acids or alkalis that alter the pH of the surrounding environment, many smart labels rely on pH-sensitive pigments. For example, protein-rich food releases ammonia and volatile amines when rotting [4,5], which will increase the pH of the surrounding environment. Fruits, vegetables, mushrooms and milk release acidic substances when they spoil [6,7], which decrease the pH of the surrounding environment. Among pH response indicators, synthetic dyes are more difficult to apply because they are often toxic and polluting, which results in strict dosage control when they are used. And among the natural dyes, anthocyanins have the widest pH indication range, thus, their utilization on smart labels is increasing [1,8,9]. These molecules are not only safe and widely available, but most also have health care benefits [3,10]. This is perfectly in line with consumer demands for food packaging, and they are excellent substitutes for synthetic dyes. Many studies have used natural anthocyanins as pH-responsive factors for smart packaging, such as blueberry anthocyanins [11], red cabbage anthocyanidins [12], Lycium ruthenicum Murr anthocyanidins [13], black rice bran anthocyanins [14] and black plum peel anthocyanidins [15]. Some of them are derived from fruits, vegetables, food crops, medicinal herbs and feedstuffs. Therefore, those may compete with people's daily needs, so that they are not conducive to the expansion of production. In addition, the composition and source of anthocyanins can have significant effects on the structures and properties of films [16]. Hence, studies on anthocyanins from different sources are necessary. Padus virginiana ‘Canada Red’ is an ornamental plant with purple-black fruits when ripe that are rich in anthocyanins, but too astringent to be eaten and have not yet been applied to smart film.

The matrix complexed with anthocyanins has a huge impact on the performance and promotion of smart packaging. Previously, plastic products have been widely favoured for their excellent performance and low price, making them the food packaging of choice. However, the harmful effects of microplastics on human body are increasing obviously,. Moreover, due to the aggravation of white pollution and the rise in environmental awareness, alternatives to plastics are being sought [17,18]. Plant polysaccharides have been heavily researched due to their renewable, widely sourced, degradable, and environmentally friendly characteristics. κ-Carrageenan, an anionic polysaccharide extracted from algal plants, consists of galactose and 3,6-anhydrogalactose linked by glycosidic bonds and contains sulphonic acid groups, making it a superior material for preparation of biodegradable films [19]. However, pure κ-carrageenan film has poor properties, which can not meet the needs of packaging. Hence, it is usually combined with other substances in the application process. Among them, nanocellulose, starch nanocrystals and ZnO nanoparticles are commonly used as film enhancers, and these nanomaterials are uniformly dispersed in the films, which can greatly improve the mechanical properties and barrier properties of the films. But they tend to cost more [20]. Lignin is also an useful raw material for film preparation because it has numerous rigid benzene ring structures that can enhance the mechanical properties of films, but it also has unique structures such as carbonyl groups and phenolic hydroxyl groups that can improve UV shielding of films [21]. Furthermore, lignin also enhances the hydrophobicity of films, which helps to prevent smart labels from being damaged by excessive water absorption during use. Lignin is mainly found in lignified plants, among which the lignin content of rice straw is 15–25 %. While rice straw is a rich biomass resource, it is also a type of agricultural and forestry residue that is difficult to treat. In the past, rice straw was mostly incinerated, but in response to global heating, many countries and regions have begun to strictly control this process [22]. Therefore, using lignin from rice straw stalks in film preparation not only improves the performances of the films, but also alleviates the problem of straw incineration. In our previous study [23], straw lignin enhanced the mechanical properties, hydrophobicity, and haze of κ-carrageenan film, as well as gave the film the UV shielding and antioxidant properties.

In this study, pH-responsive intelligent films were prepared using κ-carrageenan as matrix, sorbitol as plasticiser, lignin extracted from straw as enhancer, and anthocyanin-rich P. virginiana peels extract as an intelligent response factor. Additionally, the effects of P. virginiana peels extract addition on the structures and properties of the films and the ability of intelligent films to monitor the freshness of chicken breast meat were studied.