Consumers now demand safe food with long shelf life, prompting researchers and the food industry to explore new technologies for food packaging (Biji et al., 2015). Smart film is a type of packaging that can provide timely information to consumers about the freshness and spoilage of food. In addition, it represents the observations of packaged products' quality and the conditions inside the packaging, including temperature, pH, oxygen, and microbial metabolites (Oliveira Filho et al., 2021).
Smart packaging developed recently is pH indicator films that provide qualitative information through visual color changes. Although most smart films are produced from non-renewable sources of synthetic petrochemical products, currently there is much interest in biodegradable alternatives (Apriliyanti et al., 2018, Hu et al., 2020, Jamróz et al., 2019, Kanatt, 2020, Naghdi et al., 2021, Qin et al., 2020, Yao et al., 2020, Yao et al., 2021). Biopolymers such as polysaccharides and proteins from different sources have been explored to make biodegradable smart films due to their abundance, non-toxicity and biocompatibility (Alizadeh-Sani et al., 2020 ).
Traditionally, compounds such as bromocresol purple, cresol red, bromocresol blue and bromophenol blue have been used as color indicators in smart packaging (Poyatos-Racionero et al., 2018). However, the use of these indicators for food is limited due to their potential toxicity (making food unsafe and posing a health risk), as well as their inedible nature (resulting in changing the organoleptic characteristics of foods) (Alizadeh-Sani et al., 2020, Liu et al., 2019). Therefore, pH-sensitive, edible, and non-toxic indicators, preferably extracted from natural sources, have been the goal of recent studies to develop intelligent food packaging (Moradi et al., 2019).
Natural plant pigments, such as curcumin, anthocyanin, chlorophyll, and betalain, can change color in different pH ranges due to their structural instability; therefore, they can be used as natural, non-toxic pH indicators with potential applications in smart packaging (Priyadarshi et al., 2021). Betalains generally show a stable structure at pH 3–7, while they exhibit structural and color changes from pink/red/purple to yellow/orange under alkaline conditions (pH > 7) (Qin et al., 2020), as shown in Figure 1. Depending on their stability as a function of pH, the color of betanin is generally stable in the pH range of 3–7, with optimum pH between 4 and 5. In this pH range, betanin is bright greenish-red and turns blue-violet as the pH increases. When the pH reaches an alkaline level, betanin is degraded by hydrolysis, resulting in a golden brown color (Delgado-Vargas, Jiménez, & Paredes-López, 2000).
Figure 1. Application of smart film containing betalain extract in food quality observation.
Betalain extracts were obtained from different sources, mainly peel of red dragon fruit (Hylocereus polyrhizus), bougainvillea (Bougainvillea glabra), red flesh of red dragon fruit (H. polyrhizus), beetroot (Beta vulgaris), Amaranth leaves and powder (Amaranthus tricolor L.), and cactus fruit extract (Opuntia ficus-indica), were used as natural indicators in developing smart films. These smart films can be synthesized from various biopolymers (such as glucomannan–polyvinyl alcohol, quaternary ammonium chitosan/fish gelatin, potato starch, Furcellaran, starch/polyvinyl alcohol, polyvinyl alcohol, gelatin, and tara gum/polyvinyl alcohol) for quality monitoring of fresh products (mainly shrimp, chicken and fish) (Aprilidiyanti et al., 2018, Hu et al., 2020, Jamróz et al., 2019, Kanatt, 2020, Naghdi et al., 2021, Qin et al., 2020, Yao et al., 2020, Yao et al., 2021). The smart films with pH control are made of betalain-rich biopolymers and are manufactured using a molding technique. After drying, the smart film can be either in direct contact with food or combined with packaging material to interact with compounds (mainly total volatile base nitrogen (TVBN)) released in the voids of the packaging during food spoilage. The interaction of the smart film with food or with the voids at a particular pH changes the color of the films, visually indicating whether the product is fresh or spoiled (Figure 1). For protein-rich foods such as meat and seafood, which are easily damaged by microorganisms and chemicals, the spoilage process is accompanied by the release of various volatile nitrogenous compounds such as ammonia and biological amines, forming unpleasant tastes (Comi, 2017). The released compounds are closely related to changes in the pH of the food and storage environment and, therefore, can be monitored and detected by consumers by using a pH-sensitive indicator film (Rodrigues et al., 2021).
Typically, shrimp spoilage forms basic compounds such as ammonia, dimethylamine, and trimethylamine. Smart films based on biopolymers supplemented with betalain extracts will detect the presence of these compounds and indicate the quality of shrimp during storage. Tan et al. (2020) developed a starch/polyvinyl alcohol film combined with betalain-rich dragon fruit peel extract to monitor shrimp freshness. The film changed from pink to yellow clearly when reacting with volatile nitrogen compounds (TVBN 19.38 mg/100 g) accumulated during shrimp decomposition. Hu et al. (2020) also monitored the freshness of shrimp stored at 20°C for 48 h using a film based on a combination of quaternary ammonium chitosan/fish gelatin and amaranth betalain extract. A marked color change from pink to yellow was observed in this study when the accumulation of TVBN reached the maximum limit (followed by Chinese standards for seafood (20 mg/100 g)). This film produced using 5 and 10% amaranth extracts, shows high sensitivity when it detects food spoilage with a color change in 24 hours - this is the time it takes for the product to reach the maximum acceptable TVBN limit.
Yao et al. (2021) developed a smart film formulated from starch, polyvinyl alcohol, betalain-rich red fleshy dragon fruit pulp, prickly pear, red beet, globo flower, and red amaranth leaf extract for monitoring shrimp quality. The film color gradually changed from purple/red to yellow with a cumulative increase in TVBN, reaching 21.88 mg/100 g in 24 h. The study showed that the red flesh dragon fruit extract was the most suitable for monitoring shrimp freshness, with the highest color variation (ΔE > 5).
The smart film developed with betalain extract also monitors the spoilage of other protein-rich products such as fish and chicken. Kanatt (2020) has produced a smart film based on polyvinyl alcohol/gelatin combined with betalain-rich Amaranth extract to monitor the freshness of fish and chicken. The film's color changed from red to yellow in both products, correlating with an increase in pH, accumulation of TVBN, and bacterial growth. However, the color change was more evident in fish than in chickens.
The films based on the mixture of glucomannan/polyvinyl alcohol and red dragon fruit peel extract showed effective monitoring of fish fillet spoilage. The film's color changed markedly from pink to yellow after eight days of storage at 4°C. This change coincided with a cumulative increase in TVBN from 2.4 mg/100 g to 39.74 mg/100 g and was detected when the cumulative level was higher than 20 mg/100 g (Aprilidiyanti et al., 2018).
The smart starch film combined with betacyanin extract from bougainvillea was developed by Naghdi et al. (2021) to detect contamination of the entire Caspian Sea. The study observed a visual color change from pink to yellow according to an increase in cumulative TVBN from 7.01 mg/100 g to about 30 mg/100 g and an increase in the number of microorganisms from 2.41 log CFU/g to 7.59 log CFU/g. The color change begins on the eighth day of storage, and the film is almost entirely yellow by the twelfth day as the fish gradually changes microbiologically and chemically.
The efficiency of indicators is also highly dependent on their chemical composition and chemical interactions with the film-forming polymer. Jamróz et al. (2019) reported that furcellaran film mixed with extracts of beetroot, elderberry, blueberry, green tea, or yerba mate were not suitable as pH indicators for monitoring the spoilage of Atlantic mackerel fillets as stored at 2 °C, because the color change of the smart film is not efficient.
Thus, efficient smart films developed using susceptible pH-sensitive indicators and substrates for TVBN detection must also be applied under suitable storage temperature conditions. However, film sensitivity also needs further investigation into film effects on other damaged metabolites, such as biogenic amines (histamine, caverine, and putrescine), not only ammonia. Moreover, the temperature investigated must also simulate the actual storage conditions of foods. Nevertheless, films based on biopolymers and betalains have shown promising results for application to innovative packaging materials.
References:
Josemar Gonçalves de Oliveira Filho, Mirella Romanelli Vicente Bertolo, Murilo Álison Vigilato Rodrigues, Guilherme da Cruz Silva, Giovana Maria Navarro de Mendonça, Stanislau Bogusz Junior, Marcos David Ferreira, Mariana Buranelo Egea, Recent advances in the development of smart, active, and bioactive biodegradable biopolymer-based films containing betalains, Food Chemistry, Volume 390, 2022, 133149.
Uyen P.N. Tran, PhD (Faculty of Engineering and Technology, Van Hien University)