Preparation Of Paper Indicator Card Of Fish Freshness Based On Gas Sensors

With the development of our country's economy,the living standards of residents have rapidly increased,and our requirements for the freshness of food raw materials are becoming higher and higher.As the main source of protein,the freshness of fish is an important issue of concern to us.The freshness of fish is usually identified by sensory methods,chemical methods,etc.,which are inaccurate or take a long time.The electronic nose can be used to detect the volatile gas components of fish and quickly obtain the freshness level,but its equipment is expensive and the detection process is complicated.Using some gas-sensitive materials can prepare an colorimetric array sensor.Then we can quickly extract characteristic information to form an image by using the sensor.After analyzing and processing the image,the qualitative analysis of the gas composition of the fish is performed to determine its freshness.The sensor can be prepared as a card and placed in the product packaging box as a real-time freshness indicator.However,the gas-sensitive materials and substrates commonly used in gas-sensitive array sensors are expensive,and the gas-sensitive material solvent is toxic,which is not suitable for real-time detection of food.This study attempts to use chromatographic filter paper which is cheap and environmentally friendly as the base material,optimize and modify the gas-sensitive materials suitable for the filter paper,and prepare the freshness indicator card to provide real-time freshness analysis basis for the preservation and transportation of fish.(1)Comparison of properties of four different substrates.By comparing the size uniformity and color stability of spotting on four kinds of substrates(PVDF membrane,chromatography filter paper,nylon membrane,nitrocellulose membrane),and combining with the cost,the chromatography paper was used as the substrate to prepare the gas sensor by phase transformation method.Then we used the paper-based sensor to monitor the freshness of the bass,and compared with the PVDF membrane.(2)Analysis of freshness evaluation indexes and research on changes of volatile gas components during storage of bass.Fresh bass was collected and stored at 4?.The volatile base nitrogen content,pH value and total number of colonies of three samples were measured every 24 h for a total of 10 d.Based on the above date and the relevant national standards,we analyzed the change law of freshness grades during storage.Based on this,GC-MS was used to study the volatile gas components of different freshness of bass.The results showed that the end point of first grade freshness appeared on the 6th day and putrefaction occurred on the 9th day,and Hexanal,Trimethylamine and Methanethiol were identified as the characteristic volatile gases.(3)Sensitization of gas sensitive material.The color sensitivity of the indicator was improved according to the high concentration of trimethylamine,hexanal and methanethiol at different storage stages.Nine indicators sensitive to trimethylamine were screened out from sisiteen pH indicators,and the organic acid base was used to approach the indicator color mutation point to improve the sensitivity.Aiming at hexanal,which was the characteristic gas in the early storage period of perch,2,4-dinitrophenylhydrazone(DNPH)was selected as aldehyde gas specific derivative,PEG was used as stabilizer.We improved the color change significantly,to increase its sensitivity by the rapid derivatization of gas.Then 5,5'-dithio-bis(2-nitrobenzoic acid)(DTNB)was selected as a sulfur gas specific derivative,and PEG was used as a stabilizer to rapidly derivate sulfur gas under alkaline condition.The color change of pH indicators is accelerated,and the sensitivity is improved.(4)Preparation of indicator card which can identity the freshness of fish.The waterproof property and stability of aldehyde sensitive indicators were improved by sol-gel method.The paper-based array sensor was prepared with modified materials and chromatography filter paper,and was used to detect the freshness of bass during cold storage.The results show that the sensor can successfully identify the first degree of freshness,the second degree of freshness and spoilage of perch.