Abstract: 【Objective】Lactic acid fermentation process by Lactobacillus rhamnosus using sweet potato residue was studied in this paper in order to provide new technical strategy for utilization of sweet potato dregs.【Method】Firstly, the major elements of sweet potato starch residue were measured by elemental analyzer, meanwhile, there were four fermentation groups carried out with four different time points during the logarithmic growth phase of L. rhamnosus which was identified through OD value of bacteria, the optimal seed age was confirmed taking the fermentation efficiency of lactic acid production using liquid glucose culture as index of investigation. Then on this basis, the sweet potato dregs were used by L. rhamnosus through solid state fermentation (SSF) in the following research: firstly, the effects of inoculation amount,temperature, CaCO₃ quantity, nitrogen sources on lactic acid production were investigated through single factor experiments. In addition, the effects of nitrogen sources on viable bacteria counts in fermenting mash after fermentation were studied also. Then the optimal combination was established by further orthogonal design.【Result】Element analysis results showed that sweet potato starch residue was full of C and H, with weight percentage content of 40.34% and 6.16%, respectively. The content of N was only 0.32%. Although C was quite rich in sweet potato starch residue, N was deficient comparatively, it was necessary to add N source to promote growth and metabolism of L. rhamnosus furthermore. The growth curve of L. rhamnosus identified by ODvalue showed that the OD value of L. rhamnosus increased rapidly after inoculated for 2 h, which means that the logarithmic growth phase began. The OD value wasn’t change approximately after 8 h, which means that the stationary growth stage began. It was confirmed that the logarithmic growth phase of L. rhamnosus was 2 h to 8 h. The fermentation efficiency was the highest, reached 92.39%, when the seed age got to 4 h, and the content of residual glucose was the lowest, reached 0.59 g·L^-1, it was confirmed that the optimal seed age of lactic acid fermentation was 4 h. The results of batch single factor experiment through SSF showed successively: The highest fermentation efficiency of 83.87% reached at 10%. The highest fermentation efficiency of 85.55% reached at 37℃, when CaCO₃ quantity ranged from 1% to 7%, the highest fermentation efficiency of 90.24% reached at 5%; the fermentation efficiency of urea group reached 91.01% among 4 kinds of inorganic nitrogen source; when urea quantity ranged from 0.4% to 1.6%, the highest fermentation efficiency of 94.13% reached at 0.8%, the viable bacteria counts were 4.32×10⁸ cfu/g. Based on the results of single factor experiments above, the optimal CaCO₃ quantity of 5% was suitable for neutralizing the lactic acid produced by initial sugar in fermentation system of 10%, so it was unnecessary to change the optimal CaCO₃ quantity, and it wasn’t investigated in the next orthogonal design. Besides, adding cellulase as investigation factor, designing the orthogonal experiment with four factors and three levels. Then, the fermentation optimum conditions were obtained, which were: the inoculation size 10%, urea amount 0.8%, cellulose amount 0.4%, temperature 35℃, CaCO₃ quantity 5%, fermentation efficiency and viable bacteria counts, respectively, reached (96.55±0.866)% and 3.04×10⁸ cfu/g under these conditions. 【Conclusion】A low-cost, simple-process, high-efficiency process condition was established which was not only suitable for lactate industry but also beneficial to sweet potato starch processing farmers.

Key words: Lactobacillus rhamnosus, sweet potato residue, fermentation, lactic acid