Optimization of the process for carotenoid production from bean curd residue fermented by Rhodotorula

Abstract: Bean curd residue as livestock feed contains crude protein, crude fat and other nutrients. However, more than 50% contents in bean curd residue are the indigestible dry matters (mainly cellulose and hemicellulose), while the absorption of nutrients can also be interfered by the trypsin inhibitor and other anti-nutritional factors. Consequently, the specific pretreatment of bean curd residue can increase the digestibility, absorption rate, utilization of feed nutrients, and the production performance of livestock and poultry. In order to improve the feeding value of bean curd residue, this study aims to explore an optimization technology for carotenoid production from the fermented bean curd residue by Rhodotorula. Firstly, the yield of reducing sugar was taken as the index in the acidolysis solution of bean curd residue, while a single factor test was carried out on four factors: hydrochloric acid concentration, solid-liquid ratio, acidolysis temperature and acidolysis time. In L9 (34) orthogonal test, the optimal combination of parameters for the yield of reducing sugar from bean curd residue by acidolysis was as following: hydrochloric acid concentration was 1.0 mol/L, solid-liquid ratio (g/mL) was 1:10, acidolysis temperature was 100 ℃, and acidolysis time was 3.0 h. At this condition, the yield of reducing sugar reached 29.06%±0.07% in the optimal acidolysis solution of bean curd residue. Rhodotorula was selected to ferment the acidolysis solution of bean curd residue rich in reducing sugar, and thereby produce functional nutrient carotenoids. The yield of carotenoid was taken as the indictor, and the single factor test was carried out on 7 factors, such as fermentation temperature and time, which can affect the fermentation of acidolysis solution of bean curd residue by Rhodotorula. In the single factor test, Plackett-Burman test regression was used to screen out the main influencing factors. The results showed that the main factors influencing the yield of carotenoid were the pH value of the acidolysis solution of bean curd residue, inoculum amount of Rhodotorula, fermentation temperature and fermentation time. Three levels of Box-Behnken response surface test were designed to optimize the four main factors as independent variables, and the yield of carotenoid as the response value. The optimal combination of technological parameters was obtained: the pH value of the fermented acidolysis solution of bean curd residue was 6.0, liquid volume was 80 mL/ (500 mL), inoculation age was 48 h, inoculation amount was 11% (the concentration of seed liquid was 8.5×109 CFU/mL), rotation speed was 60 r/min, fermentation temperature was 31℃, fermentation time was 128 h. In this case, the yield of carotenoids reached (2.65±0.02) mg/L, which was 67.7% higher than that by non-optimized fermentation techniques. In Rhodotorula, there were only simple nutrient requirements, including short fermentation cycle, non-toxic metabolites, easy to synthesize plentiful carotenoid in a short period of time. Carotenoid can serve as antioxidant to enhance immunity, thereby to prevent cardiovascular and cerebrovascular diseases, and cancer. Therefore, the deep-processing is expected that the bean curd residue can be translated into carotenoid functional feed with high value-added and nutrition. The optimized technique for carotenoid production from the fermented bean curd residue by Rhodotorula can provide a promising prospect and remarkable potential market for livestock feed.