Study On The Key Technologies Of Bacterial Cellulose Fermention In The Citrus Dregs Based Medium

Bacterial cellulose (Bacterial Cellulose, BC), collectively referred to cellulose produced by microorganism, presents in gel-like thick film on the surface of medium, and has some excellent features, such as high mechanical strength, high water-holding capacity, high purity, and high crystallinity, etc. It has been used both as food ingredients and a new type of biological material for food, medicine, paper and other industrial fields.Currently, BC is fermented by Gluacetobacter xylinum in the raw material medium, i.e. coconut juice or pineapple residue, however, its yield and production scale are limited by region and season. Therefore, it is significant to seek the BC high-yield strains, expand the source of raw materials, innovate in the traditional technology and broaden the production scale of BC. Citrus dregs chosen as the main raw material to ferment BC, suitable fermentation strain screened, fermentation medium combined, the culture conditions optimized,and the effect of processing conditions on the BC’s quality were investigated in this paper. The results were summarized as follows:(1) A cellulose-producing strain named CIs51from spoiled Wenzhou mandarin fruit was screened and identified as Gluconacetobacter hansenii by the micrograph morphology observation, physiological and biochemical tests, and16S rDNA sequence analysis, and was named Gluconacetobacter hansenii CIs51.The chemical composition and micro structure of gel film produced by CIs51were determined by using fourier transform infrared (FT-IR) spectrum, X-ray diffractometry (XRD) analysis and scanning electron microscope observation. Results indicated that the main component of the gel membrane is cellulose with its characteristics of the β-configuration glucan, type I crystallization,71%crystallinity index and irregular mesh structure wovened by cellulose.(2) Effects of pretreatment of citrus dregs, yeast fermentation beforehand, main nutrient source, growth factors on the yield of BC produced by Gluconacetobacter hansenii CIs51strain in citrus dregs medium were investigated, and the culture conditions of CIs51including inoculation amount, temperature and culture time were optimized. The suitable pretreatment conditions were shown to be mixing and grinding citrus dregs with distilled water in a ratio of1:8(w/v) and hydrolyzing the pulp with150U/ml pectinase in combination with100U/ml cellulase at45℃for2h. Yeast fermentation beforehand could significantly promote the yield of BC and the two-stage fermentation, a yeast fermentation followed by bacterial fermentation producing cellulose was proposed. The appropriate fermentation conditions for yeast were5%Brix in the medium,1.5%(v/v) inoculum and cultivating at25℃for96h. For the later fermentation of BC, sucrose was chosen as carbon source, and ammonium sulphate as nitrogen source, with the addition amount of30g/L and3g/L respectively. The additions of yeast extract, citric acid and disodium hydrogen phosphateas were determined as5g/L,1g/L and2g/L, respectively, and the inoculation amount of CIs51was10%(v/v). The culture was conducted in stationary at30℃for7d. Under the above conditions, the BC yield of CIs51reached to7.23g/L, which was much higher than those in HS medium and citrus dregs filtrate modified HS (CMHS) medium (4.02g/L and6.57g/L, respectively). The BC film obtained from the optimized cultivation had translucent texture and nice flexibility with thicker fiber bundle and closer network. Therefore, citrus dregs waste could efficiently stimulate the production of BC by strain CIs51, which appeared to have the prospect of industrial application.(3) Effects of processing methods such as pH, the concentration of NaCl, heating and frozen on water content, water holding capacity, firmness and total flavonoids of BC were preliminary studied. The results indicated that the BC produced by CIs51had higher water content (96-99%)and water-holding capacity (2.38-3.79%). The concentration of NaCl had a much obvious impact on the water content, water holding capacity and firmness of BC, while the pH value had more obvious impact on the water holding capacity and firmness of BC. Moreover, temperature had also certain impact on the water holding capacity and hardness of BC. When treated with high temperature, water holding capacity and hardness of BC was reduced, while when frozen, water holding capacity of BC was lower but its hardness increased. A higher level of total flavonoids was observed in frozen BC compared to that in high temperature processed samples.