Application Of Multi-target Hurdle Technology In Controlling Microbial Spoilage During The Storage Of Grass Carp

There is an abandant resources of freshwater fish in china. Grass carp, one of the“four major Chinese carps”is welcomed by consumers for its good edible quality andmoderate sale price. Therefore, output, consumption amount and production value ofgrass carp is always in the leading position of all freshwater fishes. In 2010, output ofgrass carp reached to 422.22 million tons which occupies 20.45% of the total amountof freshwater fish. However, with high water content, neutral pH value, soft anddelicate muscle tissue, active cathepsin, large amount of mucus on its surface, and alarge number of microorganisms in its body ( head ,gill and viscera), freshwater fish iseaaily decayed in the process of storage, transportion, processing and selling. Inaddition, freshwater fish is in a low processing rate (less than 30%) in China.Concequently, with a corruption rate more than 30%, grass carp is still selled alive.This condition leads to a great waste of resources. Thus, it is very significant andurgent to research and develop a new technology of antisepsis and refreshing toimprove quality of fish meat and prolong its shelf life of grass carp.Choosing grass carp as the research object, the present paper analyses thecorruption process of grass carp in low-temperature storage and normal-temperaturestorage. The author also identifies specific spoilage organism(SSO) of fish meat whichis storaged in low temperature. By optimizing and integratedly applicating hurdlefactors such as low-temperature pretreatment, reduction of initial bacterium number,acidification, addtion of biological antistaling agents, the corruption of microbes iseffectively controlled; and shelf life of grass carp is prolonged to 12 days (below thesecond level freshness). Main content of innovative research in this paper goes asfollows:In this paper, all kinds of indexes, including total number of microbial colonies,TBA value, TVB-N value, pH value and protein degradation are studied and analysedwhen the fish is storaged in low-temperature and normal-temperature. It is found thatthe growth of total number of microbial colonies is related to the increase of TBAvalue and TVB-N value and the degradation of protein. Furthermore, total number ofmicrobial colonies can reflect the corruption process of grass carp in the process ofstorage. So microbes are considered to be the major factor which causes the corruptionof grass carp. Total number of microbial colonies in fish meat reaches to 2.46×107CFU/g in one day under the storage temperature of 25℃. What’s more, under thiscondition fish meat corrupts obviously. But, if grass carp is storaged in 4℃for fourdays, total number of microbial colonies only reaches 1.03×106CFU/g, which is abovethe critical freshness standard (≤106CFU/g) of GB 2733-2005. In this condition, corruption becomes slow.The SSO of fish meat of grass carp storaged is separated and identified inlow-temperature. Main SSO are Buttiauxella gaviniae、Pseudomonas fragi、Brochothrix thermosphacta and Psychrobacter immobilis. Only Psychrobacterimmobilis is growing constantly in the refrigeration process. All of the rest of SSOhave a certain adaptive phase before growing slowly.The presnet paper researches control effect of single hurdle factor to microbialcorruption in the process of storaging grass carp; and optimizes corresponding factors:(1) this paper studies cold sterilization pretreatment process of grass carp whichsignificantly reduces the initial bacterial counts. The best cold sterilization process isobtained by response surface methodology: mass concentration of 157 mg/L ClO2,mass concentration of 1.1 g/L of H2O2and soaking time 6.4 min. Under this condition,the rate of bacteria-reducing is 90.51%;（2）Based on the single factor experiments,this study obtaines the optimal condition of acidification by the orthogonal test: thefish meat is soaked in lactic acid / sodium lactate buffer solution (concentration 0.4mol/L and pH 3.0) for 6 min, and then storaged in low-temperature for 6 days. Averagebacteria counts is 5.85×104CFU/g which is much lower than microbiologicalspecifications of critical freshness (<1.0×106); (3) On the basis of single factorexperiments, this paper employs the Box-Behnken experimental design and analyzesexperimental results with response surface methodology to get the optimal conditionof biological preservative treatment of fish: Nisin concentration 0.11%, lysozymeconcentration 0.42% and the.soaking time 5.9min. When fish meat is storaged underthis condition for 6 days，total number of bacteria is 6.485×104CFU/g. Threevalidation tests was conducted under the optimum conditions. Total number of bacteriais 6.61×104CFU/g. Compared with the theoretical prediction value which is6.485×104CFU/g, relative error is very small; (4) This paper studies the effect ofchitosan solution which containes chitosan concentration 1.5% and alcoholconcentration 75%. The result indicates that microbial amount in fish is wellcontrolled by spraying this solution on the surface of fish.In order to get the integrated storage conditon of fish, the hurdle factors includingpretreatment of raw materials, low temperature, cold sterilization, acidification,biological preservation are selected and optimized under the theory of hurdletechnology. The author of this paper sucessfully obtains storage conditon of grass carpwhich integrated multiple barrier effect. It is a good utilization of multi-target hurdle toeffectively control the growth of microbes in fish. And it can also prolongs the shelflife (below the second level freshness) of cold fresh fish to more than 12 days.The result in this paper has the theoretical significance and application value. Itlaids the foundation for the preservation of freshwater fish and cold fresh fishproducts.