pH-sensitive smart indicators based on cellulose and different natural pigments for tracing kimchi ripening stages

Five natural pigments including water-soluble [butterfly pea (BP), red cabbage (RC), and aronia (AR)] and alcohol-soluble [shikonin (SK) and alizarin (ALZ)] were extracted, characterized, and loaded onto cellulose for preparing pH-sensitive indicators. The indicators were tested their color response efficiency, gas sensitivity, response to lactic acid, color release, and antioxidant activity. Cellulose-water soluble indicators showed more obvious color changes than alcohol-soluble indicators in lactic acid solution and pH solutions (1-13). All cellulose-pigment indicators exhibited prominent sensitivity to ammonia compared to acidic vapor. Antioxidant activity and release behavior of the indicators were influenced by pigment type and simulants. Kimchi packaging test was carried out using original and alkalized indicators. The alkalized indicators were more effective in showing visible color changes during kimchi storage than the original indicators, and cellulose-ALZ displayed the most distinct color change from violet (fresh kimchi, pH 5.6, acidity 0.45 %) to gray (optimum fermented kimchi, pH 4.7, acidity 0.72 %), and to yellow (over fermented kimchi, pH 3.8, acidity 1.38 %) which followed by BP, AR, RC, SK respectively. The findings of the study suggest that the alkalization method could be used to show noticeable color changes in a narrow pH range for application with acidic foods.

 

Comments:

The mentioned study focused on the extraction, characterization, and application of five natural pigments as pH-sensitive indicators. The pigments included butterfly pea (BP), red cabbage (RC), aronia (AR), shikonin (SK), and alizarin (ALZ). It appears that the pigments were extracted and loaded onto cellulose for the preparation of the indicators.

The study assessed various properties and functionalities of the cellulose-pigment indicators. Here are the key findings:

1. Color response efficiency: The cellulose-water soluble indicators (BP, RC, and AR) exhibited more noticeable color changes compared to the alcohol-soluble indicators (SK and ALZ) in lactic acid solutions and solutions with varying pH levels (ranging from 1 to 13).

2. Gas sensitivity: All cellulose-pigment indicators showed prominent sensitivity to ammonia when compared to acidic vapor. This suggests that the indicators are more responsive to basic (alkaline) environments.

3. Response to lactic acid: The cellulose-pigment indicators demonstrated color changes in the presence of lactic acid. Further details about the specific responses were not provided.

4. Color release: It is unclear what exactly was meant by "color release" in this context. Without more specific information, it's difficult to interpret this aspect of the study.

5. Antioxidant activity: The indicators' antioxidant activity was evaluated, although details regarding the results and the specific effects of different pigments and simulants were not provided.

The study also conducted a kimchi packaging test using the original and alkalized indicators. The alkalized indicators proved more effective in displaying visible color changes during kimchi storage compared to the original indicators. Among the pigments tested, cellulose-ALZ showed the most distinct color change from violet (indicating fresh kimchi with a pH of 5.6 and acidity of 0.45%) to gray (indicating optimum fermented kimchi with a pH of 4.7 and acidity of 0.72%), and finally to yellow (indicating over-fermented kimchi with a pH of 3.8 and acidity of 1.38%). The pigments BP, AR, RC, and SK followed ALZ in terms of the intensity of color changes observed.

In summary, the study suggests that alkalization can be used to achieve noticeable color changes within a narrow pH range, particularly for applications involving acidic foods like kimchi. The cellulose-pigment indicators, loaded with the mentioned natural pigments, demonstrated pH sensitivity, gas sensitivity, response to lactic acid, and potential for use in monitoring the fermentation process of kimchi.

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