| Tofu is an excellent representative of traditional Chinese food with high nutritional value and soft and smooth taste.It is deeply loved by Chinese people.But tofu’s high water content is prone to deterioration and corruption,which leads to its short shelf life,which severely restricted the large-scale development of the intensive tofu industry in our country.In order to solve the problem of short shelf life of beancurd,this paper tested the microbial status of each link in the production process of boxed beancurd,analyzed the source of microorganisms causing product spoilage,isolated and identified the spoilage bacteria,and used secondary sterilization technology to reduce the impact of spores on the shelf life of products.The results are as follows:(1)Isolation and identification of boxed tofu spoilage bacteria.Taking packaged lactone bean curd and packaged firm bean curd produced by a famous soybean product company in Zhengzhou as the research object,the storage life of two kinds of bean curd under different storage temperatures(4 ℃,25 ℃,37 ℃)was determined by sensory evaluation and colony count.The main spoilage microorganisms in fresh tofu samples and tofu samples at different storage temperatures were isolated and identified by traditional separation and purification techniques and 16 S r RNA molecular identification techniques.The results showed that the shelf life of boxed tofu was very short at room temperature,no more than 2 days.The lower the storage temperature is,the longer the storage life is.Under the same storage temperature,the storage life of lactone tofu was longer than that of firm tofu.The shelf life of the two kinds of boxed tofu is more than 30 d at 4 ℃,the shelf life of boxed firm tofu and lactone tofu is about 36 h and 48 h at 25 ℃,and the shelf life of boxed firm tofu and lactone tofu is only about 12 h and 30 h at 37 ℃.52 strains of putrid bacteria were isolated and identified from fresh boxed tofu and stale boxed tofu stored at different temperatures,mainly Bacillus sp(43 strains in total): Among them,Bacillus cereus(11 strains),Bacillus sp(9 strains),Bacillus subtilis(7 strains),Bacillus licheniformis(3 strains),Rummeliibacillus pycnus(3 strains),Brevibacillus agri(2 strains),Bacillus tequi Lensis(2 strains),Lysinibacillus sp,Baci LLus thuringiensis,Baci LLus jeotga Li,Baci LLus siamensis,Brevibaci LLus sp,Baci LLus pumi Lus 1 strain each;There were 9 strains of other putrefying bacteria,including 2 strains of Chryseobacterium sp,2 strains of Enterococcus faecium and 2 strains of Enterococcus durans,1 strain of Pantoea agg Lomerans,Staphy Lococcus sp and 1 strain of Microbacillus lactobacillus.Storage temperature had a greater effect on the diversity of rotting bacteria in boxed lactone tofu,but had a lesser effect on boxed firm tofu,and there were more rotting bacteria species in boxed lactone tofu at 4 ℃ and 25 ℃.(2)Study on spoilage characters of main spoilage bacteria.The putrefaction characteristics of several main putrefaction bacteria were studied by adding them to the tofu in a box after high temperature sterilization and storing at 25 ℃.The results showed that there were obvious differences between the boxed firm tofu and lactone tofu,the former mainly for sticky and water loss,the latter mainly for irritating odor and bag expansion.The putrid traits of different putrid bacteria are also different.Bacillus cereus can cause firm tofu to brown,Bacillus subtilis and Bacillus licheniformis cause firm tofu to bloat,Pantoea agg Lomerans form a layer of membrane on the surface of lactone tofu,and Bacillus microflora can produce mucus in lactone tofu.(3)Microbial source analysis of boxed tofu.Through the detection of microbial changes in each link of the production of boxed tofu,the law of microbial growth and pollution sources were determined.The results showed that the microbial quantity of lactone tofu increased rapidly in the process of soaking beans,grinding and hot pulp cooling,but decreased rapidly in the process of boiling and heating molding.The microbial quantity of firm tofu increased rapidly in the steps of soaking beans,grinding,pressing and cutting,but decreased rapidly in the steps of boiling,crouching and sterilization.In the two kinds of tofu production process,the total number of colonies decreased significantly and the total number of spores decreased slightly after heat treatment,and there was no significant difference between the total number of colonies and the total number of spores.The changes of the total number of colonies and spores in the two kinds of tofu production process showed that the microorganisms in the industrial production of boxed lactone tofu mainly came from the soybean raw material carrying and equipment pipeline pollution.The microorganisms in the vacuum packaged firm tofu produced in the laboratory were mainly from the soybean raw materials carried and the environment,equipment and personnel pollution.The residual spores in boxed tofu products are the main microorganisms that cause tofu spoilage,which mainly come from soybean raw materials and equipment pollution.(4)Study on germination characteristics of spores.Bacillus subtilis spores,Bacillus cereus spores and Bacillus licheniformis spores,which appear more frequently in boxed tofu,were selected as objects to study their germination characteristics and germination promotion methods,were studied to provide theoretical basis for the application of secondary sterilization technology.The results showed that the temperature suitable for germination of Bacillus subtilis spores and Bacillus cereus spores was about 40 ℃,and the temperature suitable for germination of Bacillus licheniformis spores was about 40～50 ℃.When germination temperature was 40 ℃,the maximum OD value of Bacillus subtilis spores,Bacillus cereus spores and Bacillus licheniformis spores was reached at about 10 h,6 h and 6 h,respectively.In the germination treatment at 40 ℃,the germination rates of Bacillus subtilis spores,Bacillus cereus spores and Bacillus licheniformis spores reached the highest at 8 h,2 h and 6 h,respectively,and the germination rates were 99.61%,91.69% and 54.30%,respectively.(5)Study on application of secondary sterilization technique in boxed tofu.The vacuum package of old tofu made in laboratory was used as the research object.First,the old tofu was sterilized for the first time to kill the nutritious cells.Then,under suitable conditions,the residual spores of old tofu germinated quickly and transformed into vegetative cells.Finally,the second sterilization was carried out to kill the vegetative cells after the germination of the spores.The results showed that the storage life of old tofu could be extended effectively by using the conditions of sterilization temperature 95 ℃,time 30 min,germination temperature 40 ℃,time 6 h.The storage life of old tofu at 25 ℃ could reach 8～10 d,which was 6 d longer than that of single sterilization treatment. |
PDF Full Text Request