| Summary: | 碩士 === 國立海洋大學 === 食品科學系 === 91 === The effects of shrimp chitosan and various chitosan hydrolysate products on the spore germination and cell proliferation of Bacillus cereus were investigated, and the application of chitosan on the preservation of cooked rice was further studied. Four strains of Bacillus cereus including the standard strain CCRC 10603, and three isolates (No.1, No. 2, and No. 3) from lunch box were tested. After 24 h incubation in nutrient broth (NB) containing 150 ppm chitosan with 95% deacetylation degree (DD95), the survivals for the strains of CCRC and No. 2 were reduced by 5.67 and 6.32 log CFU/mL, respectively, compared to the control; while no survival was detected for the strains of No. 1 and No. 3. Cellulase in the technical grade was used to degrade DD95. The hydrolysate from 10 h hydrolysis was demonstrated to have the strongest antibacterial activity. The low-molecular-weight chitosan (LMWC) and chitooligosaccharides were separated from this hydrolysate. The bacteriocidal activity against B. cereus for these products ranked in the decreasing order was DD95, followed by LMWC, then by chitooligosaccharides. Chitooligosaccharides at 150 ppm had little activity against B. cereus.
The surface charges of B. cereus cell increased from the mid-exponential phase (5h) to the early stationary phase (9 h) but at the late-stationary phase (24 h) the charges decreased. After reaction with DD95 in NB for 30 min, the surface of the B. cereus cells with 9 h-age was coated with DD95 and this coating intensity increased with the increasing of time, as demonstrated by the scanning electronic microscopic observation.
The D90 and D100 of these strains changed from 30.77 ~ 46.51 and 1.95 ~ 2.56 min in the control, respectively, to 4.68 ~ 7.91 and 0.72 ~ 0.80 min, respectively, in the presence of 80 ppm of DD95. The D90 and D100 of these strains in the presence of 80 ppm LMWC were 7.47 ~ 10.17 and 0.89 ~ 0.93 min, respectively.
At 37oC and 18oC the addition of 2000 ppm DD95 could effectively reduce the spore germination in cooked rice; while the chitosan had little activity against the spore of B. cereus at 5oC. The spores gradually died-off in cooked rice for both control and experimental groups.
Keyword: Bacillus cereus; Chitosan; Low-molecular-weight chiotsan (LMWC); Antibacterial activity; Cooked rice.
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