The Application of Natural Colorants in Food Freshness Indicating Films
2025-12-01
1.Main Natural Colorants and Their Characteristics
Anthocyanins are a type of water-soluble pigment widely present in plants. They can exhibit significant color changes within a wide pH range, from red under acidic conditions to purple under neutral conditions, and blue under alkaline conditions. This unique color-changing property makes them an ideal material for food freshness indicators.
Studies have shown that red cabbage and purple sweet potatoes are the best raw materials for extracting anthocyanins, as they have high contents of monomethylated and dimethylated anthocyanins, with better thermal stability and light stability, and exhibit a wider pH-dependent chromatographic range. In reported studies, the proportion of these two plant raw materials exceeds 35%.
Practical application research shows that the packaging film prepared by mixing purple sweet potato anthocyanins with carboxymethyl cellulose and starch can effectively monitor the freshness changes of fish. Similarly, intelligent indicator films based on biodegradable membranes and rosehip anthocyanins have also been successfully applied to the monitoring of pork freshness. However, the stability of anthocyanins at high temperatures remains a problem that needs to be solved. Research has found that as the storage temperature increases, the color change of the anthocyanin indicator film becomes more obvious, mainly due to the oxidation reaction of anthocyanins and the lower thermal stability.
02. Curcumin
Curcumin is a polyphenolic compound extracted from the rhizome of turmeric. It not only possesses various biological activities but also serves as a highly sensitive acid-base indicator. The color change of curcumin is due to the interconversion of its molecular structure: it exists in the form of diketone in an acidic or neutral pH environment and appears yellow, while it mainly presents red in an alkaline pH environment.
Studies have confirmed that the indicator film prepared by adding curcumin to talaloy and polyvinyl alcohol as the base material shows significant color changes in different relative humidity ammonia environments and shrimp freshness detection. The indicator film based on k-carboxymethylcellulose and curcumin not only improves thermal stability but also significantly enhances the membrane's antioxidant, moisture resistance and tensile strength.
To overcome the limitation of the limited color change range of curcumin, researchers developed a mixed indicator system. The pH sensing film prepared by mixing curcumin with anthocyanin in a 2:8 ratio can present three different colors, representing the freshness degree of fish, medium freshness and spoilage. This mixed dye not only expands the color change range but also enhances the stability of a single dye.
03. Cinnamoyaniline
Cinnamoyaniline is a natural pigment extracted from the roots of the cinchona plant. As a new type of pH indicator, it shows promising application prospects in intelligent packaging. As the pH changes from acidic to alkaline, the cinnamoyaniline molecules undergo proton transfer and form intramolecular hydrogen bonds, resulting in a color change from yellow to purple.
Studies have shown that the acetate cellulose nanofiber film with cinnamoyaniline added can be used as a real-time indicator for fish spoilage. Under storage conditions at 4°C, as the total amount of volatile basic nitrogen increases and the product pH rises, this packaging sensor can observe obvious visual color changes on the 6th and 12th days. The pH-responsive functional membrane prepared with chitosan as the substrate changes its color from khaki to light brown as the pH of the packaged fish changes, effectively indicating the spoilage process of the fish.
04. Rubiadin
Rubiadin is a naphthalene quinone pigment extracted from the dried roots of Rubia cordifolia. It has better stability compared to anthocyanins. The naphthalene ring part in its molecular structure is chemically active and is sensitive to light, heat, acid, and alkali. Studies have shown that the natural pigment extracted from the roots of Rubia cordifolia, when combined with agar to form an indicator membrane, changes from the initial pink color to blue-purple color when monitoring the freshness of fish. The color change is very obvious.
To enhance the mechanical strength of the indicator membrane, the colorimetric sensing films prepared using biodegradable cellulose and rubiadin dyes exhibit high tensile strength and hydrophobic properties. The color change is closely related to the volatile basic nitrogen content of shrimp and pork.
2.Preparation method of freshness indicator film
01. Electrospinning technology
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