Food Freshness Indicator Packaging with Natural Coloring

The quality characteristics of food are mainly reflected in its texture, flavor, color, appearance and nutritional value, and these characteristics can change during transportation and storage. Ensuring food safety prevents foodborne illness and maximizes the use of food by reducing spoilage and losses due to microorganisms. Packaging is an important process for preserving and ensuring the quality of food for export, storage and final consumption. Traditional food packaging generally has four basic functions: protection, communication, convenience and containment [1], but is limited to protecting food from external influences that may accelerate food spoilage, such as gases, moisture, light, temperature and microorganisms. In the past few years, food packaging technology has made progress to overcome the challenges posed by the globalization of the market, increased environmental regulatory requirements, and emerging concerns about consumer preferences and food safety [2]. Routine physical and chemical testing and microbiological analysis cannot provide timely, intuitive feedback to the average consumer. Therefore, smart packaging that can provide information, enhance safety, and improve the quality of packaged food during storage, transportation, and distribution has been developed.

Food quality needs to be monitored from two aspects: one is to protect consumers from food-borne diseases, and the other is to improve the productivity of the food industry and reduce losses caused by food spoilage [3]. Smart packaging is used as a communication tool to monitor changes in internal and external environmental conditions and their impact on packaged foods, so as to better communicate between stakeholders in the supply chain, including producers, retailers and consumers [4]. Information about the source, authenticity, ingredients, expiration date of consumption and consumption of food can be given to consumers in an intelligent way, and the product status of the food supply chain can be tracked. It has functions such as anti-theft and anti-counterfeiting, such as radio frequency identification (RFID) and electronic article surveillance (EAS). This not only meets the actual needs of consumers, but also ensures the effective circulation of food in the market.

Smart packaging can be classified into three categories: indicators, data carriers and sensors [5]. Among these, pH indicators have been used to monitor and indicate the freshness of stored foods due to their efficiency and simplicity, as colorimetric indicators will display a specific color change when reacting with food metabolites in the form of a gas sensor. These smart packaging systems are usually attached, embedded or printed on the food packaging material in the form of a label to monitor the quality of the food product [6]. In this way, consumers can distinguish fresh food from inferior food based on visual color differences without having to open the packaging. Common chemical indicators are synthetic dyes such as bromothymol blue [7], bromocresol green, bromocresol purple [8], methyl red [9], cresol red and chlorophenol [10], which are considered potentially harmful to human health and therefore unsuitable for use in food packaging. In recent years, scientists have also been looking for natural colorants that can replace synthetic dyes. There has been increasing research and application of natural coloring such as anthocyanins, curcumin, alizarin, madder and betaine [11].

This article reviews the research progress of different types of natural coloring in food freshness indicator packaging, lists their application in different food types, and analyzes and prospects the current research.

1 Natural coloring-based freshness indicators

1.1 Anthocyanins

The preferred Natural Coloring needs to be able to respond to changes in the food itself or its surroundings (such as temperature, pH,

microbial growth, volatile substances, etc.) in the packaging and provide users with easy-to-see and accurate information. In recent years, anthocyanin-rich extracts from plants have been widely used [12]. This is because they can change color according to pH changes over a wide pH range. Anthocyanins are of great importance for the monitoring of food quality, shelf life and ultimately as color indicators in food packaging.

Considering their stability, the most commonly used plants for anthocyanin extraction are red cabbage and purple sweet potato, which account for more than 35% of all studies, as they have high levels of anthocyanins and peonins in their mono- and diacetylated forms. Compared to non-acylated anthocyanins, they have better thermal and light stability and exhibit a wider pH-dependent chromatography [13]. Jiang et al. [14] prepared a packaging film for monitoring changes in fish freshness by mixing a mixture of carboxymethyl cellulose and starch with anthocyanins extracted from purple sweet potatoes. Zhang et al. [15] developed an intelligent indicator film based on a biodegradable membrane and roselle anthocyanins. The results showed that the indicator film can be used to intelligently monitor the freshness of pork. In addition to traditional sources of anthocyanins, the application of anthocyanins from different sources in food freshness indicator packaging is shown in Table 1.

As can be seen from Table 1, anthocyanins have a wide range of applications as indicators in films for indicating food freshness, and there are abundant sources from which to obtain them, as well as a wide range of substrate choices. In addition to pH sensitivity, the activity and food packaging application performance of indicator films containing anthocyanins were tested, and indicator films rich in anthocyanins also have excellent antioxidant and antibacterial properties [19,24-26]. However, Zhai et al. [33] prepared a smart film using agar, gelatin and carrot anthocyanins and tested the color stability of the anthocyanin-based indicator film at different temperatures (4, 25 and 37 °C). They observed that the color of the indicator film changed over time, with the red color fading over time and the discoloration becoming more pronounced with increasing storage temperature. This indicates that the discoloration of anthocyanins at high temperatures is mainly due to the oxidation of anthocyanins and their relatively low thermal stability. Therefore, in the application of freshness indicator packaging, maintaining the stability of anthocyanins at different temperatures and preventing the deterioration of the pigment under environmental conditions such as light, heat and oxygen is the first problem that needs to be solved.

1.2 Curcumin

Curcumin (CR) is a polyphenolic compound extracted from the herb turmeric and the turmeric rhizome, and has anticancer, anti-inflammatory, antioxidant and antibacterial activities [36]. Curcumin-containing films are highly sensitive pH-responsive detection materials, and their color changes are visible to the naked eye [37]. It has attracted interest in the preparation of composite films for packaging because it provides additional functional properties such as enhanced mechanical strength, UV protection, antioxidant properties and antimicrobial activity. The color change of curcumin under different pH conditions is the result of changes in its main structure. The molecular conformation of curcumin depends on the pH, polarity and temperature of the solution. At neutral and alkaline pH, the α and β-unsaturated β-diketone parts of the curcumin structure act as hydrogen donors, leading to hydrolysis and degradation of curcumin. The curcumin structure exists as a ketene-enol tautomer depending on the nature of the solvent. At acidic or neutral pH, it exists as a diketone and is yellow in color. At an alkaline pH, it loses the proton on the phenolic group and decomposes rapidly, mainly in the form of enol, and the color changes to red [38, 39].

Ma et al. [40] successfully prepared an indicator film based on a mixed matrix of tara gum and polyvinyl alcohol (PVA), to which curcumin was added. The film showed obvious color changes in an NH3 environment with different relative humidities and in experiments applied to detect the freshness of shrimp, indicating that the film can be used as a sensor in the food industry. Liu et al. [41] prepared an indicator film based on k-carrageenan (Car) and curcumin and applied it to monitor the freshness of shrimp and pork. In addition to improving thermal stability, an appropriate amount (no more than 3%) of CR can significantly improve the film's resistance to oxidation, moisture and tensile strength. In addition, the film has good control over the release of CR and has a clear release mechanism, which is conducive to sustainable sensing of pH changes. The film exhibits strong red shifts (i.e., yellow-red) under alkaline conditions, showing good discoloration properties.

It can be seen that curcumin can not only be used as an indicator in food freshness indicator packaging, but that the addition of an appropriate amount of curcumin can also improve the relevant properties of the indicator film. However, due to the limited range of curcumin discoloration, in recent years, some studies have used curcumin and other pigments to make mixed indicators for use in food freshness indicator packaging.

Chen et al. [42] prepared pH-sensitive films containing a natural mixed dye of curcumin (CR) and anthocyanin (ATH) as a packaging indicator label for non-destructive real-time detection of fish freshness. The PVA/glycerin film with a ratio of 2:8 (v/v) of CR and ATH can display three different colors, representing the freshness, medium freshness, and spoilage of the packaged fish. In addition, stability experiments have shown that the mixed dye indicator enhances the stability of the anthocyanin monomer dye. Zheng Hui et al. [44] prepared an intelligent indicator label based on a mixed pigment of red cabbage anthocyanins and curcumin to compensate for the limitation of the small indication range of the single curcumin pigment. There is a significant color difference between fresh, sub-fresh and rotten pork, and it has a good ability to distinguish the freshness of pork.

Compared with a single indicator, a mixed indicator can not only expand the range of color changes [43], but also more easily distinguish the freshness of food, and can also improve the instability of some single Natural Coloring. Therefore, the application of mixed indicators is also a future development trend.

1.3 Alizarin

Alizarin is a new intelligent option for monitoring pH changes, with improved physical, chemical and functional properties of biopolymer-based films [45]. Alizarin (1,2-dihydroxy-anthraquinone, C14H8O4) is a Natural Coloring extracted from the roots of the madder plant, and has been used industrially since ancient times as a red fabric dye and as a staining reagent in biological research [46]. Alizarin has also attracted considerable attention as an acid-base indicator. As the pH changes from acidic to alkaline, protons in the molecular structure of alizarin are transferred, and the two hydroxyl groups bind to the adjacent carbonyl oxygen atom to form an intramolecular hydrogen bond, changing the color from yellow to purple [45]. Since microbial decay can release volatile alkaline compounds that cause an increase in pH, the pH-responsive discoloration of alizarin can be used as a natural coloring to indicate the freshness of food [47].

Aghaei et al. [48] developed a cellulose acetate nanofiber film with added alizarin as a real-time indicator of fish spoilage. Rainbow trout fillets were stored at refrigerator temperature (4 °C). With an increase in the total volatile basic nitrogen (TVB-N) and an increase in the pH of the product, the color of this packaging sensor can be visually observed at 6 and 12 d. The addition of alizarin as a dye produced the expected visual color change.

In addition, Ezati et al. [45, 49-50] have conducted a large number of studies using natural coloring made from alizarin. A pH-responsive functional film was prepared using chitosan as the base material. As the pH of the packaged fish changes, the color of the composite coating changes from khaki to light brown, indicating that the fish is beginning to spoil. In subsequent research, Ezati et al. [49] used a pH-sensitive indicator film prepared with alizarin added to a starch-cellulose paper matrix to detect the freshness of rainbow trout at 4 °C. The added alizarin had good compatibility with other ingredients, and the color stability was higher after being stored at 4 ℃ for two months, which could better identify the beginning of the spoilage of refrigerated fillets. Ezati et al. [50] also applied an alizarin-dyed indicator film to beef. The alizarin-based cellulose-chitosan pH sensitive indicator film can also track the freshness of ground beef during storage by perceivable color changes. The above studies have shown that alizarin can be used as a good indicator and added to indicator packaging. The resulting indicator film shows good UV barrier properties, and its elongation at break, surface hydrophobicity, thermal stability and antioxidant activity are all improved.

1.4 Shikonin

Another Natural Coloring is the naphthoquinone pigment, also known as alizarin, which is usually extracted from the dried roots of the arnica plant. Arnebia euchroma is a plant native to Xinjiang that is widely used in traditional Chinese medicine. The main chemical component of arnica root extract is the enantiomeric naphthoquinone, alizarin or alizarin. Natural naphthoquinones are lipophilic red pigments. In addition to helping wound healing, it also has antibacterial, anti-inflammatory and anticancer effects [51, 52]. Naphthoquinone pigments are sensitive acid-base indicators and are relatively stable compared to anthocyanins, and can be used as a new type of Natural Coloring dye [53]. The main structure of a naphthoquinone compound consists of two parts: a naphthazoline group and a chiral six-carbon side chain. The naphthyridin part of shikonin is chemically active and sensitive to light, heat, acids, alkalis, etc. [54]. Therefore, there has been little research on naphthyridin compounds as indicator membrane dyes.

Huang et al. [55] studied the development and testing of a new indicator membrane for monitoring fish freshness using Natural Coloring (arnebia euchroma root, AEREs) extracted from arnebia euchroma root and agar as raw materials. After the addition of AEREs, the water solubility, swelling ratio, and elongation at break of the colorimetric film were all reduced to a certain extent. The prepared indicator film was used to monitor the freshness of Wuchang sea bream under refrigerated (4 ℃) and room temperature (25 ℃) conditions. The results showed that the indicator film with a lower AEREs content had a more obvious color change during the fish spoilage process. Poor mechanical properties may be related to the choice of substrate. In order to improve the mechanical strength of the indicator film, Dong et al. [56] studied the preparation of a new colorimetric sensing film using biodegradable cellulose and naphthoquinone dye. The film has high tensile strength (227 MPa) and hydrophobic properties (water contact angle 112.2°). The film changes significantly from rose red to purple to blue-violet. The color change can be correlated with the TVB-N and (total viable count, TVC) content of shrimp and pork measured by standard laboratory methods, and can be used for visual monitoring of the freshness of shrimp and pork. In addition, Ezati et al. [57] prepared an indicator by adsorbing natural shikonin on cellulose paper, which can be used in smart food packaging for fish and pork to monitor the quality of packaged food in real time.

The addition of shikonin not only improves the antioxidant activity, thermal stability and water resistance of the indicator film, but also makes the indicator film more sensitive to color changes. Compared with other natural colorings, the indicator film containing shikonin still shows high stability after long-term storage, with stable and sensitive color changes.

2 Application research of natural coloring in various food packaging

2.1 Fresh meat

Fresh meat consists of very complex compounds, mainly water, protein and fat, and is very susceptible to physiological changes. Due to the perishable nature of fresh meat, microbial activity can easily increase, and therefore the quality of the meat deteriorates very quickly [58]. Therefore, the meat industry needs powerful and simple tools to determine the quality of packaged meat, predict shelf life and the degree of deterioration.

As decay progresses, some compounds are produced through microbial metabolism, such as biogenic amines, ammonia, hydrogen sulfide, indole and organic acids [59], which cause strong odors and discoloration. These substances can also trigger the discoloration of freshness indicator films. Freshness indicators for raw meat are mainly based on a quality-indicating relationship established with metabolites related to meat type, spoilage flora and storage process. Most natural colorings can be used as freshness indicators in fresh meat products because at least one indicator compound is produced or present during spoilage. Table 2 lists the current research status of freshness indicators in fresh meat products in recent years.

As can be seen from Table 2, freshness indicator films based on natural coloring have been partially applied in pork, chicken and beef. Natural Coloring is also mainly based on anthocyanins extracted from various plants. Among them, there have been more studies on the freshness monitoring of pork. For different types of raw meat, freshness indicator films based on Natural Coloring can all change color with the pH value of the sample during storage, which is closely related to the deterioration of the food itself and has a good indication effect. However, due to the vigorous growth and reproduction of microorganisms in raw meat at room temperature, the indicator film changes rapidly. For meat products that have undergone refrigeration and frozen preservation treatments, further research is needed to determine how sensitive Natural Coloring is at low temperatures, whether the color change of the indicator film is obvious, and whether it can effectively and accurately determine the freshness of meat products.

2.2 Raw aquatic products

Aquatic products are rich in nutrients such as protein, highly unsaturated fatty acids, vitamins and trace elements[67]. However, due to enzymatic reactions and microbial contamination, aquatic products are highly susceptible to microbial spoilage[68]. With the continuous growth of China's fishery production, coupled with the characteristics of diverse aquatic products, production affected by seasonal temperatures, and perishable raw materials and products[69], it is essential to assess the freshness of aquatic products in the supply chain.

Under the action of enzymes and microorganisms, seafood is prone to the decomposition of substances such as proteins and amino acids, accompanied by a series of biochemical reactions that lead to spoilage [70]. They are mainly decomposed through three mechanisms: enzymatic decomposition, microbial spoilage and lipid oxidation. As with meat products, when seafood products undergo microbial degradation, microbial metabolites such as volatile basic nitrogen, trimethylamine, sulfides, esters, aldehydes and ketones are produced, which are characteristic of the rancid smell of fish and seafood [71]. The main products of rancidity are ammonia and amines [72]. These compounds accumulate in the headspace of the packaging, causing a change in the pH of the indicator film due to its alkalinity [73]. As a result, the natural indicator shifts towards a more alkaline color, which conveys information about the quality of the fresh seafood.

Due to the wide variety of seafood, the application of freshness indicator films based on natural coloring is currently focused on shrimp and fish. However, if it is to be applied in practice, further research is needed on the relationship between the freshness of various seafood and the discoloration of the indicator film, in order to make a more accurate and comprehensive assessment, ensure the safety of seafood, and reduce waste.

2.3 Milk

Milk is also an essential food in everyday life. In addition to water, milk contains some carbohydrates, proteins, fats, as well as other vitamins and minerals. Freshness indicators are essential for determining the shelf life and quality of the product during distribution and consumption. Most milk and dairy products are distributed, handled and stored at refrigerated temperatures. Therefore, while storing milk at low temperatures, it is also necessary to have sensitive indicators to indicate the freshness of the milk. This is the most challenging task for indicator packaging based on natural pigments. In the past decade, there has been a lot of research on milk freshness indicator packaging based on Natural Coloring, as shown in Table 4.

In recent years, freshness indicator films based on Natural Coloring have been more widely used in milk, mainly using anthocyanins extracted from plants as dyes. Most studies have shown that when milk is stored at room temperature for 48 hours, the acidity increases during the process of spoilage, and the pH value decreases from 6.5-6.8 to below 5. The color of the indicator film also changes, eventually becoming purplish red, indicating that the milk has gone bad. Using this characteristic, freshness indicator films for food have certain application value in detecting the freshness of milk.

2.4 Fruit

At present, food freshness indicator packaging based on natural coloring is mostly used in meat, aquatic products and milk. It is less commonly used in other types of food, and there has been little research on fruit freshness indicator labels. The color change of this type of indicator film is mainly due to the respiration of fruit after picking [78]. As the quality of fresh fruit decreases, respiration produces a large amount of acidic gas CO2, which causes the indicator film to change color as the pH changes. Wang Guilian et al. [79] designed an intelligent indicator label for fruit freshness using a carrot pigment solution as a pH indicator, and applied it to strawberries. The results showed that the indicator label can indicate different color changes in strawberries at different stages, so the maturity and freshness of strawberries can be judged by the color change of the label.

Feng Gang et al. [80] used anthocyanins extracted from blueberry peel to replace methyl red and bromothymol blue, mixed with methyl cellulose to prepare an indicator film for application in blueberry packaging. By comparing the color difference of the indicator label, the effects of three factors on the color development of the indicator label were investigated: peel concentration, initial pH of the filtrate, and dipping time of the filter paper, and the optimal extraction process was selected. The final freshness indicator label for blueberries performed well during application. Since the internal environment of most fruits does not change significantly over a short period of time, and natural coloring is susceptible to environmental (humidity, temperature, and CO2) influences, synthetic dyes are more suitable for freshness indicator films for fruits because they are characterized by obvious color changes, rapid response, and strong stability.

3 Conclusion and outlook

 Natural Coloring is attracting a lot of attention in smart packaging technology because of its intelligent color-changing properties in response to changes in environmental pH. Freshness indicator packaging films can help visually identify the status of food products in real time, which is very helpful for assessing the safety and quality of packaged food on site. The effectiveness of Natural Coloring as a pH indicator shows its high application potential in food packaging. However, there is still a large gap between China and foreign countries in terms of research and application. In terms of technical issues, the ambient temperature, humidity, tightness of the packaging and the use of desiccants all have an impact on the accuracy of the information provided by natural dyes. There are also often safety issues to be resolved with regard to the indicator solution and the substrate itself. Therefore, it is possible to exploit the technical advantages of a wide range of Natural Coloring and new materials (such as nanomaterials, natural materials or food-grade materials, etc.). While improving the accuracy of the indicator, it is also important to ensure the safety of the food itself and to improve the environmental impact of natural dyes.

In addition to the relevant technical factors, consumers' perception of imperfectly labeled packaging is one of the main problems hindering its penetration in the food market. Consumers are worried that innovative packaging may affect the quality of food. Relevant industry standards need to be developed, for example, to ensure that food packaging companies are clear about how to safely and appropriately use active and intelligent material items; to ensure that the food contact materials used in labeled packaging are safe and that their ingredients are not transferred to food; and to ensure that food manufacturers are standardized and reasonable in the use of labeled packaging. Finally, only by ensuring the reliability of the information provided by the packaging can we avoid misleading consumers and provide them with a better consumer experience.

With the continuous advancement of technology, different scientific fields are converging, and freshness indicator packaging technology is constantly being developed and improved. Although there are still many obstacles to overcome in terms of applicability and safety, through the application of various technologies and the continuous innovation and development of various research, smart freshness indicator packaging for food will gradually enter the public eye.

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