Promotion of monacolin K production in Monascus extractive fermentation: the variation in fungal morphology and in the expression levels of biosynthetic gene clusters.

BACKGROUND: Monacolin K, an important secondary metabolite of Monascus, possesses a cholesterol-lowering effect and is widely used in the manufacture of antihypertensive drugs. In the present study, we constructed an extractive fermentation system by adding non-ionic surfactant and acquired a high monacolin K yield. The mechanism was determined by examining both cell morphology and the transcription levels of the related mokA-I genes in the monacolin K biosynthetic gene cluster. RESULTS: The monacolin K yield was effectively increased to 539.59 mg L-1 during extraction, which was an increase of 386.16% compared to that in the control group fermentation. The non-ionic surfactant showed good biocompatibility with Monascus. Electron scanning microscopy revealed alterations in the morphology of Monascus. The loosened mycelial structure and increased number of cell surface wrinkles were found to be related to the increased cell-membrane permeability and extracellular accumulation of monacolin K. Gene expression levels were measured via a quantitative reverse transciptase-polymerase chain reaction. By contrast, in the control group, mokA, mokB, mokC, mokD and mokF showed higher-level and longer-term expression in the extractive fermentation group, whereas mokE and mokG did not present a similar trend. The expression levels of mokH and mokI, encoding a transcription factor and efflux pump, respectively, were also higher than the control levels. CONCLUSION: The addition of a non-ionic surfactant to Monascus fermentation effectively increases the yield of monacolin K by transforming the fungus morphology and promoting the expression of monacolin K biosynthesis genes. © 2021 Society of Chemical Industry.

Effect of nonionic surfactant Brij 35 on morphology, cloud point, and pigment stability in Monascus extractive fermentation.

BACKGROUND: Nonionic surfactant Brij 35 in submerged fermentation of Monascus can significantly increase Monascus pigment yield. Here, the effects of nonionic surfactant Brij 35 on Monascus pigment secretion in extractive fermentation are discussed in terms of cell morphology, cloud point change, and pigment stability. RESULTS: At Brij 35 concentrations up to 32 g L-1 , the higher concentrations led to the loosening of the network structure on the surface of the fungal wall, enhanced cell wall permeability, and increased abundance of lipid droplets. Alternatively, when the concentration of Brij 35 exceeded 32 g L-1 , a large amount of substances accumulated on the surface of the fungal wall, permeability reduced, and the degree of oil droplet dispersion in cells decreased. Further, during extractive fermentation, Brij 35 induced formation of a grid structure on the fungal wall surface beginning on day 2, increased the number of intracellular lipid droplets, and promoted intracellular pigment secretion into the extracellular environment. When the cloud point temperature in the fermentation system approached that of fermentation, the nonionic surfactant exhibited stronger Monascus pigment extraction capacity, thereby enhancing pigment yield. Hence, Brij 35 can improve pigment stability and effectively reduce damage caused by natural factors, such as light and temperature. CONCLUSION: Brij 35 promotes the secretion of pigment by changing the fungal wall structure and cloud point, as well as by improving pigment stability. © 2020 Society of Chemical Industry.

Volatile compounds analysis and biodegradation strategy of beany flavor in pea protein.

Pea protein has attracted the attention of consumers owing to its non-GMO, hypoallergenic, and high nutritional value, however, the beany flavor limits its applicability. Hence, GC-MS and GC-IMS were applied to assess the influence of Saccharomyces cerevisiae and Lactobacillus plantarum on the beany flavor of pea protein. Specifically, 51 and 65 volatiles were detected in pea proteins via GC-MS and GC-IMS, respectively. Odor activity value results showed that the components contributing to the beany flavor mainly included: 2-pentylfuran, (E, E)-2,4-decadienal, hexanal, nonanal, (E, E)-2,4-nonadienal, octanal, (E)-2-nonenal and (E)-2-octenal. S. cerevisiae and L. plantarum fermentation could remove 79.65% and 78.94% of the major beany flavor aldehydes from pea protein, respectively. Principal component analysis and hierarchical cluster analysis indicated that the unfermented and fermented pea protein samples can be distinguished by volatile compounds. These results provide a reference for the removal of beany off-flavors from pea proteins by fermentation.