A Novel Indicator Based on Polyacrylamide Hydrogel and Bromocresol Green for Monitoring the Total Volatile Basic Nitrogen of Fish

An intelligent indicator was developed by immobilizing bromocresol green (BCG) within the polyacrylamide (PAAm) hydrogel matrix to monitor the total volatile basic nitrogen (TVB-N) content of fish. The FTIR analysis indicated that BCG was effectively incorporated into the PAAm through the formation of intermolecular hydrogen bonds. A thermogravimetric analysis (TGA) showed that the PAAm/BCG indicator had a mere 0.0074% acrylamide monomer residue, meanwhile, the addition of BCG improved the thermal stability of the indicator. In vapor tests with various concentrations of trimethylamine, the indicator performed similarly at both 4 °C and 25 °C. The total color difference values (ΔE) exhibited a significant linear response to TVB-N levels ranging from 4.29 to 30.80 mg/100 g at 4 °C (R2 = 0.98). Therefore, the PAAm/BCG indicator demonstrated stable and sensitive color changes based on pH variations and could be employed in smart packaging for real-time assessment of fish freshness.

Metal-Organic Framework Flowers as a Naked-Eye Colorimetric Indicator of Trace Water

Convenient and sensitive trace water indication is of great significance in various industrial processes. Here, a flower-like metal-organic framework Cu-FMM is assembled from ultrathin nanosheets that change its coordination structure reversibly with the capture and loss of water molecules, enabling sensitive trace water naked-eye colorimetric indication ability. A recognizable black/yellow color change can be observed when the dried Cu-FMM is exposed to the atmosphere or solvent with trace water as low as RH 3% and a water content of 0.25‰ (v/v) and further enables potential trace water imaging applications. The excellent accessibility of the multi-scale pore structure of Cu-FMM contributes to a fast response time of 3.8 s with good reversibility (>100 cycles), outperforming traditional coordination polymer humidity sensors. The present study provides new inspirations for the design of sensitive and applicable naked-eye water indicator materials that are applicable to in situ and continuous monitoring in industrial processes.

New perspectives on electrospun nanofiber applications in smart and active food packaging materials

Packaging plays a critical role in determining the quality, safety, and shelf-life of many food products. There have been several innovations in the development of more effective food packaging materials recently. Polymer nanofibers are finding increasing attention as additives in packaging materials because of their ability to control their pore size, surface energy, barrier properties, antimicrobial activity, and mechanical strength. Electrospinning is a widely used processing method for fabricating nanofibers from food grade polymers. This review describes recent advances in the development of electrospun nanofibers for application in active and smart packaging materials. Moreover, it highlights the impact of these nanofibers on the physicochemical properties of packaging materials, as well as the application of nanofiber-loaded packaging materials to foods, such as dairy, meat, fruit, and vegetable products.

A review on colorimetric indicators for monitoring product freshness in intelligent food packaging: Indicator dyes, preparation methods, and applications

Intelligent food packaging system exhibits enhanced communication function by providing dynamic product information to various stakeholders (e.g., consumers, retailers, distributors) in the supply chain. One example of intelligent packaging involves the use of colorimetric indicators, which when subjected to external stimuli (e.g., moisture, gas/vapor, electromagnetic radiation, temperature), display discernable color changes that can be correlated with real-time changes in product quality. This type of interactive packaging system allows continuous monitoring of product freshness during transportation, distribution, storage, and marketing phases. This review summarizes the colorimetric indicator technologies for intelligent packaging systems, emphasizing on the types of indicator dyes, preparation methods, applications in different food products, and future considerations. Both food and nonfood indicator materials integrated into various carriers (e.g., paper-based substrates, polymer films, electrospun fibers, and nanoparticles) with material properties optimized for specific applications are discussed, targeting perishable products, such as fresh meat and fishery products. Colorimetric indicators can supplement the traditional "Best Before" date label by providing real-time product quality information to the consumers and retailers, thereby not only ensuring product safety, but also promising in reducing food waste. Successful scale-up of these intelligent packaging technologies to the industrial level must consider issues related to regulatory approval, consumer acceptance, cost-effectiveness, and product compatibility.

Multicolor PEGDA/LCNF Hydrogel in the Presence of Red Cabbage Anthocyanin Extract

Colorimetric indicator gels were developed by incorporating anthocyanin (AC) obtained from red cabbage into poly (ethylene glycol) diacrylate (PEGDA)-based hydrogel containing lignocellulose nanofiber (LCNF). The PEGDA-based hydrogel was prepared by mixing all of the mentioned components at the specific composition, and the hydrogels were cured under UV light (245 nm) for 1 min. The pH-response, UV absorption, swelling ratio, and mechanical properties of PEGDA/LCNF were determined. It was further found that PEGDA and LCNF mount play an important role in adjusting the mechanical properties of PEGDA/LCNF. In general, the presence of LCNF improved the mechanical properties and swelling ratio of PEGDA. The incorporation of red cabbage anthocyanin into the PEGDA/LCNF film showed multicolor response when specific pH buffers were introduced. Based on the multicolor response of PEGDA/LCNF/CA, this gel film indicator can be developed as a food freshness indicator that focuses on the detection of ammonia and amine compound.

Application of colorimetric indicators to predict the fermentation stage of kimchi

Cabbage kimchi, a popular side dish in Korean cuisine, produces several fermentation by-products (FBPs). Kimchi is praised for its flavor, taste, and texture when suitably fermented at 0.7% to 0.9% total acidity, or a pH of approximately 4.1. Beyond this acidity level, the quality of the product decreases, negatively impacting consumers' purchase preferences. Therefore, the current study seeks to develop an optimally fermented (OptF) kimchi indicator that can be inserted into product packaging to evaluate its utility at 4 and 10 °C. A gradual change in the total color difference (ΔE) was observed during the kimchi fermentation stage, and the highest ΔE values were observed at 4 (34.87) and 10 °C (37.99), after 9 weeks. Moreover, the color-change response function value F(Xc) was more linear at 4 and 10 °C (0.981 and 0.984, respectively) compared to the ΔE over time, during kimchi fermentation. Coefficients of determination for F(Xc)-carbon dioxide (0.983), F(Xc)-pH (0.979), and F(Xc)-titratable acidity (0.974) were sufficient to meet the optimal polynomial regression model, while that for F(Xc)-lactic acid bacteria (0.881) was not. Standardized residuals of predicted data indicated that 95% of the residuals were in the range of -2.0 to 2.0. The regression analysis further suggested that the OptF kimchi indicator could be used as a kimchi fermentation indicator. PRACTICAL APPLICATION: Cabbage kimchi, a traditional Korean fermented food, produces several fermentation by-products. After the optimal fermenting stage, the sensory evaluation of cabbage kimchi and consumers' purchase preference decreases. This study describes an optimally fermented kimchi indicator and its utility at 4 and 10 °C. Our results demonstrate the ability of this indicator to predict the freshness and fermentation stage of kimchi without the need for sensory evaluation. This method could help increase the purchase preference for commercial kimchi.

An inkjet-printed sulfonephthalein dye indicator array for volatile amine detection

Colorimetric indicators are versatile for applications such as intelligent food packaging, for reflecting the actual quality and/or monitoring distribution history of food products. In this study, a colorimetric indicator array composed of sulfonephthalein dyes was successfully developed by piezoelectric inkjet printing, for the detection of volatile amines-the primary spoilage gases for fish and seafood products. The printing inks were formulated in water/ethanol/1-butanol mixture. By refilling the printer's cartridges with our formulated inks and controlling the red, green, and blue color parameters, a 7 × 9 indicator array was printed onto inkjet transparency films. The color response of the indicator array was tested with different volatile amines at various concentrations. The array indicator was capable of discriminating six different volatile amines: ammonia, trimethylamine, dimethylamine, triethylamine, piperidine, and hydrazine. The printability of the inks was investigated by characterizing their density, surface tension, and dynamic viscosity, dictating that all formulated inks were printable fluid. The microstructural morphologies of the printed dyes on transparency films were evaluated using scanning electron microscopy. Interactions of the dye with the volatiles were studied by Fourier transform infrared spectroscopy. In summary, the piezoelectric inkjet printing method presented in this study offers a convenient, efficient, and flexible means to fabricate colorimetric indicators for detecting food spoilage volatiles. These indicators are promising as sensing components in intelligent packaging systems, to reveal the freshness of fish products by correlating with quality parameters such as total volatile basic nitrogen, microbial growth, and sensory attributes. Further studies on the feasibility of using the array indicators in real food packaging systems, development of strategy to mitigate the potential migration of the indicator dyes, and designing array patterns optimal for machine/human interpretation, are important to commercialize the technology. PRACTICAL APPLICATION: Piezoelectric inkjet printing offers a convenient way to fabricate sensing materials and aligns with industrial packaging operations. The use of indicators on food package helps consumers more accurately perceive real-time food quality information.

Bacterial Bioprinting on a Flexible Substrate for Fabrication of a Colorimetric Temperature Indicator by Using a Commercial Inkjet Printer

Background:

Bacterial sensors are recommended for medical sciences, pharmaceutical industries, food industries, and environmental monitoring due to low cost, high sensitivity, and appropriate response time. There are some advantages of using bacterial spores instead of bacteria in vegetative forms as spores remain alive without any nutrient for a long time and change to vegetative form when a suitable environment is provided for them.

Methods:

For biosensor fabrication, it is important to define how the bacterial spores are delivered to the substrate media. The main purpose of this paper is an investigation of transferring bacterial spores on a flexible substrate media using a commercial inkjet printer (HP Deskjet 1510). It should be noted that in the previous researches, the special printers were used to transfer bacteria on rigid films.

Results:

These printed bacterial spores are used as a colorimetric temperature indicator. The custom-made bio-inks are prepared by bacterial spores along with a gelling agent and pH indicator.

Conclusions:

Finally, transformation of bacterial spores into vegetative bacteria is occurred by changing of temperature. A color change in the bio-prints is demonstrated because the bacterial transformation and growth change the environmental pH to an acidic level.