| In order to ensure food safety and promote the popularization of real-time monitoring of food quality,at the same time,to alleviate the environmental pollution caused by plastic waste and the shortage of petroleum resources,the development of smart packaging films based on biomass resources is urgent.It is of great significance to protecting the "security on the tongue"of the people and building a beautiful China.In this paper,cassia gum(CG)was used to prepare green packaging films,and the effects of three different plasticizers—linseed oil,glycerol,and sorbitol on its film-forming properties were stuied,and the interactions between the components were analyzed.Carboxylated cellulose nanofibers(C-CNCW)were used to improve the films’ mechanical and barrier properties.Natural pigments including purple cabbage extract(PCE)and Rhein(Rhe)were used as pH indicators to prepare pH-respose intelligent films.In order to improve the accuracy of pH-response and to ensure the safety of use,in-situ pH sensitive fibers(IS-pH-SFs)were prepared by anchoring bromothymol blue(BTB)onto cationic pulp fibers(CPFS),and IS-pH-SFs were added into CG matrix to prepare high-precision pH-response films(CG-ISFs).Rheological properties of film-forming solutions and the properties of the films were measured,and the structure of the films was also characterized,to analyze the interaction among each film-forming components.The responsivity of the intelligent films to pH and triethylamine were investigated.The films were used to real-time monitor the freshness of pork and chicken to establish the relationship between the freshness of meat and the chroma of the intelligent films,which provided theoretical and technical basis for broadening the application fields of CG and ensuring food safety and promoting the popularization of food quality monitoring.The concentration of film-forming solution was 0.6 g/100 mL by measuring and analyzing the rheology properties of CG solutions with different concentration.CG alone can’t form a complete film,and is brittle and non-flexible.Flaxseed oil,glycerol,and sorbitol are added to improve its flexibility.It was found that the complete film can be peeled off after adding linseed oil,but because of its hydrophobicity and its poor blending compatibility with CG,the flexibility of the film is poor.Glycerol or sorbitol has good blending compatibility with CG,which can significantly improve the flexibility of the film.Film-forming solutions with different amounts of glycerol and sorbitol still exhibited non-Newtonian fluid characteristics,and the original hydrogen bonding among CG chains was broken and new hydrogen bonds formed with them.FTIR results showed that no chemical interaction between glycerol or sorbitol and CG,only hydrogen bonds were present.SEM results showed that the surface and cross-section of the films were continuous and compact.The elongation at break of CG films increased with the addition of glycerol or sorbitol,and the tensile strength increased with the addition of sorbitol,butcincreased at first and then decreased with the addition of G.The addition of glycerol or sorbitol reduced the barrier property and light transmittance,but slightly improved the thermodynamic stability.A small amount of sorbitol was precipitated after a long period of storage which is due to its large molecular weight and poor blending compatibility with CG.Synthesizing various properties,glycerol was used as the plasticizer and the amount of additive was 45%of the mass of CG.Carboxylated cellulose nanocrystal whisker(C-CNCW)with diameters of 3～10 nm and lengths of 100～500 nm were added into CG matrix to improve the mechanical and barrier properties of the CG-based films.The effects of the amount of C-CNCW on the properties of the film-forming solutions and the films were also investigated.After C-CNCW adding,the film-forming solutions were still non-Newtonian fluids,and new hydrogen bonds were formed among CG and glycerol.With the increase of the amount of C-CNCW,the surfaces and cross-sections of the film became more and more rough,but the film was still continuous and compact.The addition of C-CNCW reduced the transparency of the films,but improved its thermodynamic stability,barrier property and mechanical properties.The performances were best with 4%of C-CNCW addition,WVP、OP and PO were separately decreased from 3.466 g m-1 s-1 Pa-1×10-10、0.2077 cm3 mm m-2 day-1 MPa-1 and 0.550 g mm m-2 day-1 to 2.505 g m-1 s-1 Pa-1×10-10、0.1678 cm3 mm m-2 day-1 MPa-1 and 0.064 g mm m-2 day-1.Tensile strength and heat sealing strength were separately increased from 18.53 MPa、1295.40 N/m to 32.85 MPa、2218.78 N/m.The experiment of packaging corn oil proved that the films containing C-CNCW have a good application prospect,and can be used in the packaging of instant food products,such as condiments,which can achieve tear-free and food safety.Natural pH response factors,purple cabbage extracts(PCE)and Rhein(Rhe)were added to CG matrix to prepare pH-response films.The effects of PCE and Rhe on the rheological properties of the film-forming solutions and the properties of the films were discussed.The addition of PCE and Rhe broken the original hydrogen bonds among the components and formed new hydrogen bonds with each component,but the film-forming solutions were still the non-Newtonian fluids.The transparency,barrier properties and tensile strength of the films decreased with the addition of PCE and Rhe.PCE solution has a good response to pH,and presented different color in different pH buffer solutions.CG-6PCE and CG-3Rhe could response to pH,but the color change was not obvious when pH rangeability was 1.CG-6PCE and CG-3Rhe responsed well to triethylamine,and the larger the relative humidity,the more obvious the color change,the more sensitive the response.CG-6PCE and CG-3Rhe could indicate the freshness of pork and chicken,but there existed a little lag.When the color of the film changes obviously,the meat was spoiled.In order to improve the pH accuracy of the films,BTB was loaded on cellulose fibers(CPFs)to prepare in-situ pH sensitive fibers(IS-pH-SFcs)through chemical bonding.FTIR results showed that the cationic groups and BTB were successfully grafted onto the cellulose skeleton.IS-pH-SFs were sensitive to pH buffer solution from 4 to 8.No BTB released after IS-pH-SFs immersing in water for 24 h and under the action of NaOH and HCl for a short time,which proved that the interaction between BTB and CPFs is strong and can be used safely.The pH-response films(CG-ISFs)with high precision were successfully prepared by adding IS-pH-SFs into CG matrix.The addition of IS-pH-SFs reduced the transparency and barrier properties of the films,and the moderate addition(1%～3%)could enhance its tensile strength.CG-ISFs films responded well to pH and triethylamine,and the higher the humidity,the more significant color change,the higher sensitivity.The release test showed that only a small amount of BTB released under the stimulation and energy supply,which proved the safety of its use.CG-3ISFs film clearly changed from yellow to blue-green when the volatile total nitrogen(TVB-N)of pork and chicken was 12.270 mg/100 g and 13.971 mg/100 g,respectively,and the meat products were near deterioration,indicating that it can accurately monitor and indicate the freshness of them.The blending of IS-pH-SFs with CG improved the tensile strength of the films through the hydrogen bonding among the film-forming components,and endowed the film precise pH-response.The meat quality can be reflected to the merchants and consumers through the color changes visible to the naked eye,which has an important role in promoting the popularization of real-time monitoring of fresh meat products. |
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