Research On Real-time Monitoring Of The Freshness Of Typical Cold Meat Based On Colorimetric Smart Indicator Tags

The non-freezing treatment of cold fresh meats（0-4℃）during stransportation protects the original taste and nutrition of the meats to the greatest extent,while it also provides a suitable circumstance for microorganisms to decompose the meat nutrients and release metabolites.As a result,the freshness of meats decreases rapidly,leading to a short shelf life.Although conventional physical and chemical methods can accurately determine the meat freshness,they are generally time-consuming and cannot determine the real-time meat freshness.Meanwhile,the rapid and non-destructive detection methods usually require specialized instruments and complex data processing.Intelligent colorimetric labels could indicate food quality through color changes,with the advantages of intuitiveness and simplicity,and have been successfully used for real-time monitoring of meat freshness as well.Volatile biogenic amines and sulfur-containing compounds are mainly produced during the decomposition of proteins and amino acids of meats,and therefore they are generally regarded as characteristic compounds for evaluating the freshness of meats.Currently,natural pigments based bogenic amines-sensitive labels generally meet the shortages of insufficient light stability and leachingof pigments,and colorimetric sulfide-sensitive labels generally meet the shortages of material security and a single color intensity change.Hence,in this study,in terms of the bogenic amines-sensitive labels,we managed to respectively develop anti-ultrovilet（UV）light and leaching-resistent labels;In terms of the bogenic amines-sensitive labels,we managed to respectively develop green-synthesized and multicolor responsive labels.These labels were successfully used to monitor the freshness of typical livestock meat,poultry,marine fish and freshwater fish.The main research contents are as follows:（1）Development of an anthocyanin-based label with anti-ultraviolet ability and its response to biogenic amines of pork.Anthocyanins（AN）are easy to degrade under UV light,leading to the color fading of labels.In this study,we proposed to use rutile titanium dioxide（TiO₂）nanoparticles to barrier UV light and thus to protect anthocyanins.A bilayer film was fabricated with sol-gel method.The upper and lower layers were gellan gum-TiO₂（GG-TiO₂）composite film and agar-anthocyanins（GG-AN）composite film.The illumination stability of the bilayer film and its response to biogenic amines from typical livestock（pork）were investigated.The results indicated that the illumination stability increased with the increase of the TiO₂ concentration.The relative color changes decreased from 14%of AG-AN/GG film to 8.4%of AG-AN/GG-2%TiO₂ film,under ultraviolet light illumination for 35 h.The linear detection range of the film to trimethylamine was 0-330μM,and the limit of detection was 18μM.The film could react with volatile biogenic amines generated from pork during storage,and thus showed a red-to-green color change.Hence,the developed label had good anti-UV light ability and can be used to monitor meat freshness.（2）Development of a leaching-resistant label based on liquid embedding techonoly and its response to biogenic amines of salmon.AN in hydrophilic labels are easy to leach in high-humidity packaging environment,leading to decrease of anthocyanins content in the labels.In this study,we proposed to microencapsulate the anthocyanins through water in oil（W/O）emulsion.The W/O emulsion was fabricated with the high-pressure homogenization method and then was coated on the hydrophobic polyvinylidene fluoride film to form a compostite film.The leaching issue of anthocyanins in water and the response of the composite film to biogenic amines from typical marine fish（salmon）were investigated.The results indicated that due to the microencapsulation of anthocyanins,the leaching rate of anthocyanins decreased from99.7%to 15.37%,whne the composite films were immersed in water for 72 h.The linear detection range of the film to ammonia is 7.4-333μM,and the limit of detection is 3.56μM.The film could react with volatile biogenic amines generated from salmon during storage,and thus showed a red-to-green color change.Hence,the developed label had good anthocyanins leaching-resistant ability and can be used to monitor meat freshness.（3）Development of a leaching-resistant label based on solid embedding techonoly and its response to biogenic amines of silver carp.Curcumin in hydrophilic labels are easy to leach in high-humidity packaging environment,leading to decrease of curcumin content in the labels.In this study,we proposed to embed the curcumin into plastic polymer.The LDPE-curcumin composite film was developed through melting extrusion method.The leaching issue of curcumin in pH buffer solutions and the response of the composite film to biogenic amines from typical freshwater fish（silver carp）were investigated.The results indicated that the leaching rate of curcumin in the film immsered in pH 12 buffer solution for 5 d was merely 0.33%.The linear detection range of the film to ammonia was 0-240μM,and the limit of detection was 0.18μM.The film could react with volatile biogenic amines generated from silver carp during storage,and thus showed a yellow-to-brown color change.Hence,the developed label had good curcumin leaching-resistant ability and can be used to monitor meat freshness.（4）Development of a label with green synthesized silver nanoparticles（AgNPs）and its response to hydrogen sulfide of chicken breast.Some hydrogen sulfide-sensitive materials are developed with cumbersome synthesis process and toxic reagents.In this study,we proposed to synthesis the hydrogen sulfide sensor with biopolymers.AgNPs was synthysized by using the edible exopolysaccharide gellan gum（GG）and silver nitrate through a one-step heating method.The foundamental characteristics of AgNPs and its response to hydrogen sulfide from typical poultry meat（chicken breast）were investigated.The results indicated that the AgNPs with negative charged surface have high stability during storage,and at alkaline and high ion strength conditions.When reacting with hydrogen sulfide,a silver sulfide（Ag₂S）layer was formed on the surface of AgNPs to form Ag@Ag₂S core-shell nanoparticles,which changed the color of the AgNPs solution from yellow to colorless.The linear detection range of AgNPs to hydrogen sulfide was 5-85μM,and the limit of detection was 0.81μM.Then,agar was used to fix AgNPs to prepare a portable hydrogel sensor.The hydrogel could selectively react with hydrogen sulfide generated from chicken breat during storage,and thus showed a yellow-to-colorless color change.Hence,the developed label was safe and easy to frabricate,and can be used to monitor meat freshness.（5）Development of a label with dual emmision silicon quantum dots-silver nanoclusters（SiQDs-AgNCs）hybrid and its response to hydrogen sulfide and methanthiol of beef.The color changes of some sulfide sensors merely have a single color change,namely the change of color intensity,which is not conducive to naked eye observation.In this study,we proposed to develop a hydrogen sulfide and methanthiol sensor with dual colors.SiQDs-AgNCs hybrid with dual emission lights was developed based on the fluorescence resonance energy transfer effect,namely the emission light of the SiQDs,as an energy donor,could excite the energy acceptor AgNCs to generate emission light.The fluorescent characteristics of SiQDs-AgNCs and its response to methanthiol from typical livestock meat（beef）were investigated.The results indicated that SiQDs-AgNCs had dual emission peaks of 468 nm and 610 nm,under the excitation light of 400 nm.The orange-red fluorescence of AgNCs was quenched and the blue-green fluorescence of SiQDs was enhanced,when the hybrid reacted with hydrogen sulfide and methanthiol.The linear detection ranges of SiQDs-AgNCs to hydrogen sulfide and methanthiol were 1.125-17μM and 1.125-38.25μM,respectively,and the limits of detection were 0.82μM and 0.89μM,respectively.The SiQDs-AgNCs was embedded in agar gel and then coated on the polyvinylidene fluoride film to prepare a fluorescent film.SiQDs-AgNCs could selectively react with hydrogen sulfide and methanthiol generated from beef during storage,and thus showed a red-to-blue color change.Hence,the developed label had a dual color signal that was easy to recognize by naked eye,and can be used to monitor meat freshness.The intelligent colorimetric labels developed in this study could indicate the freshness of typical meats.Hence,this study could provide new ways for the preparation and application of intelligent meat freshness labels.