The effect of aeration on the performance of docosahexaenoic acid (DHA) production by Schizochytrium sp. was investigated in a 1,500-L bioreactor using fed-batch fermentation. Six parameters, including specific growth rate, specific glucose consumption rate, specific lipid accumulation rate, cell yield coefficient, lipid yield coefficient, and DHA yield coefficient, were used to understand the relationship between aeration and the fermentation characteristics. Based on the information obtained from the parameters, a stepwise aeration control strategy was proposed. The aeration rate was controlled at 0.4 volume of air per volume of liquid per minute (vvm) for the first 24 h, then shifted to 0.6 vvm until 96 h, and then switched back to 0.4 vvm until the end of the fermentation. High cell density (71 g/L), high lipid content (35.75 g/L), and high DHA percentage (48.95%) were achieved by using this strategy, and DHA productivity reached 119 mg/L h, which was 11.21% over the best results obtained by constant aeration rate.
Docosahexaenoic acid (DHA) percentage in total fatty acids (TFAs) is an important index in DHA microbial production. In this study, the change of DHA percentage in response to fermentation stages and the strategies to increase DHA percentage were investigated. Two kinds of conventional nitrogen sources, monosodium glutamate (MSG) and ammonium sulfate (AS), were tested to regulate DHA synthesis. Results showed that MSG addition could accelerate the substrate consumption rate but inhibit lipid accumulation, while AS addition could increase DHA percentage in TFAs effectively but extend fermentation period slightly. Finally, the AS addition strategy was successfully applied in 7,000-L fermentor and DHA percentage in TFAs and DHA yield reached 46.06 % and 18.48 g/L, which was 19.54 and 17.41 % higher than that of no-addition strategy. This would provide guidance for the large-scale production of the other similar polyunsaturated fatty acid, and give insight into the nitrogen metabolism in oil-producing microorganisms.
Different fermentation processes, including batch, fed-batch and repeated fed-batch processes by Schizochytrium sp., were studied and compared for the effective DHA-rich microbial lipids production. The comparison between different fermentation processes showed that fed-batch process was a more efficient cultivation strategy than the batch process. Among the four different feeding strategies, the glucose concentration feed-back feeding strategy had achieved the highest fermentation results of final cell dry weight, total lipids content, DHA content and DHA productivity of 72.37, 48.86, 18.38 g l(-1) and 138.8 mg l(-1) h(-1), respectively. The repeated fed-batch process had the advantages of reducing the time and cost for seed culture and inoculation between each fermentation cycles. The results of fermentation characteristics and lipid characterization of the repeated fed-batch process indicated that this repeated fed-batch process had promising industrialization prospect for the production of DHA-rich microbial lipids.
Aims: To improve the yield and productivity of docosahexaenoic acid (DHA) by Schizochytrium sp. in terms of the analysis of microbial physiology.
Methods and Results: A two‐stage oxygen supply control strategy, aimed at achieving high concentration and high productivity of DHA, was proposed. At the first 40 h, KLa was controlled at 150·1 h−1 to obtain high μ for cell growth, subsequently KLa was controlled at 88·5 h−1 to maintain high qp for high DHA accumulation. Finally, the maximum lipid, DHA content and DHA productivity reached 46·6, 17·7 g l−1 and 111 mg l−1 h−1, which were 43·83%, 63·88% and 32·14% over the best results controlled by constant KLa.
Conclusions: This paper described a two‐stage oxygen supply control strategy based on the kinetic analysis for efficient DHA fermentation by Schizochytrium sp.
Significance and Impact of the study: This study showed the advantage of two‐stage control strategy in terms of microbial physiology. As KLa is a scaling‐up parameter, the idea developed in this paper could be scaled‐up to industrial process and applied to other industrial biotechnological processes to achieve both high product concentration and high productivity.
International audienceHeat treatment is one of the essential operations widely used in most dairy processes, and heat stability is one of the essential properties of milk. Casein micelles are the major component in milk responsible for the heat stability of milk during processing. This study assessed the effects of heat treatment temperature and duration on the average size, turbidity, polydispersity index and heat stability of casein micelles in yak skim milk and distilled water. The results showed that whey protein had an important role in influencing the heat stability of casein micelles. The average size, polydispersity index and turbidity of micelles in skim milk were higher than those of micelles in distilled water in all cases while the heat stability of casein micelles in skim milk was lower than those in distilled water. As a result of the heat treatment, the size of micelles in skim milk increased due to complex of casein/whey protein formed via covalent bonds, whereas it decreased in distilled water attributed to the change of hydrophobicity in micelles. The size distribution of particles broadened with increasing heating temperature, resulting in the increase in turbidity and polydispersity index of casein micelles both in skim milk and distilled water. The micelles in skim milk combined with whey protein during heating. These findings will help processors design appropriate heating conditions for yak milk and yak casein products and help identify new opportunities for product development
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