General mechanisms of weak acid-tolerance and current strategies for the development of tolerant yeasts.

Yeast cells are often subjected to various types of weak acid stress in the process of industrial production, food processing, and preservation, resulting in growth inhibition and reduced fermentation performance. Under acidic conditions, weak acids enter the near-neutral yeast cytoplasm and dissociate into protons and anions, leading to cytoplasmic acidification and cell damage. Although some yeast strains have developed the ability to survive weak acids, the complexity and diversity of stresses during industrial production still require the application of appropriate strategies for phenotypes improvement. In this review, we summarized current knowledge concerning weak acid stress response and resistance, which may suggest important targets for further construction of more robust strains. We also highlight current feasible strategies for improving the weak acid resistance of yeasts, such as adaptive laboratory evolution, transcription factors engineering, and cell membrane/wall engineering. Moreover, the challenges and perspectives associated with improving the competitiveness of industrial strains are also discussed. This review provides effective strategies for improving the industrial phenotypes of yeast from multiple dimensions in future studies.

Alkali extraction and in vitro antioxidant activity of Monascus mycelium polysaccharides.

In the present work, alkali extraction technology was used to optimize the extraction of Monascus mycelium polysaccharides for the first time. The extracting parameters of alkali extracted Monascus mycelium polysaccharides were optimized by Box-Behnken design (BBD). The optimum conditions were extraction temperature 49 °C, alkali concentration 7%, solvent/material ratio 23:1 (ml/g) and extraction time 2.3 h with an enhanced yield of 10.1%, compared with the yield 4.76% of hot water extraction, indicating that alkali extraction is a more efficient way. In order to discuss the biological activity of alkali extracted polysaccharides, we compared the in vitro antioxidant activity of alkali extracted polysaccharides (AMP) with hot water extracted polysaccharides (HMP). The result showed that AMP have the similar capability of scavenging both superoxide radical and 2, 2-diphenyl-1-picrylhydrazyl (DPPH) radical of HMP in vitro. Therefore, alkali extraction technology is not only a high-efficiency way to extract AMP, but also can retain the natural antioxidant activities of AMP, which can be used in pharmaceutical and food industries.

Advances in the Application of the Non-Conventional Yeast Pichia kudriavzevii in Food and Biotechnology Industries.

Pichia kudriavzevii is an emerging non-conventional yeast which has attracted increased attention for its application in food and biotechnology areas. It is widespread in various habitats and often occurs in the spontaneous fermentation process of traditional fermented foods and beverages. The contributions of P. kudriavzevii in degrading organic acid, releasing various hydrolase and flavor compounds, and displaying probiotic properties make it a promising starter culture in the food and feed industry. Moreover, its inherent characteristics, including high tolerance to extreme pH, high temperature, hyperosmotic stress and fermentation inhibitors, allow it the potential to address technical challenges in industrial applications. With the development of advanced genetic engineering tools and system biology techniques, P. kudriavzevii is becoming one of the most promising non-conventional yeasts. This paper systematically reviews the recent progress in the application of P. kudriavzevii to food fermentation, the feed industry, chemical biosynthesis, biocontrol and environmental engineering. In addition, safety issues and current challenges to its use are discussed.

Sequential Production of á´ -xylonate and Ethanol from Non-Detoxified Corncob at Low-pH by Pichia kudriavzevii via a Two-Stage Fermentation Strategy.

Improving the comprehensive utilization of sugars in lignocellulosic biomass is a major challenge for enhancing the economic viability of lignocellulose biorefinement. A robust yeast Pichia kudriavzevii N-X showed excellent performance in ethanol production under high temperature and low pH conditions and was engineered for ᴅ-xylonate production without xylitol generation. The recombinant strain P. kudriavzevii N-X/S1 was employed for sequential production of ᴅ-xylonate and ethanol from ᴅ-xylose, feeding on ᴅ-glucose without pH control in a two-stage strategy of aerobic and shifting micro-aerobic fermentation. Acid-pretreated corncob without detoxification and filtration was used for ᴅ-xylonate production, then simultaneous saccharification and ethanol fermentation was performed with cellulase added at pH 4.0 and at 40 °C. By this strategy, 33.5 g/L ᴅ-xylonate and 20.8 g/L ethanol were produced at yields of 1.10 g/g ᴅ-xylose and 84.3% of theoretical value, respectively. We propose a promising approach for the sequential production of ᴅ-xylonate and ethanol from non-detoxified corncob using a single microorganism.

Transcriptional profiling reveals molecular basis and the role of arginine in response to low-pH stress in Pichia kudriavzevii.

The non-conventional yeast Pichia kudriavzevii is considered to be a promising biotechnological host for the production of organic acids under low-pH conditions. However, little is known about the low-pH stress response in P. kudriavzevii, which significantly restricts its future development. In this study, P. kudriavzevii N-X showed great tolerance to low-pH stress, but the cell aggregation upon extremely acidic conditions might be unfavorable for low-pH fermentation. We therefore conducted RNA-Seq to compare global gene expression of P. kudriavzevii N-X in response to different pH stresses. Totally 434 genes were identified to be differentially expressed genes (DEGs), and annotation and enrichment analysis suggested that multiple genes associated with regulation of membrane lipid composition, filamentous growth and arginine metabolism were differentially expressed. The increased specific activity of arginase and intracellular ammonia concentration of P. kudriavzevii cultured at pH 2.0 further implied potential roles of arginine in response to extreme low-pH conditions. Extracellular supplementation of 5 mM arginine resulted in increased pHi and cell growth at pH 2.0, meanwhile the cell aggregation was partially suppressed. Additionally, overexpression of ARG J involving in arginine synthesis can also enhance the cell growth and reduce the aggregation effect. These results suggested that increasing arginine flux might be an alternative approach in the developing of P. kudriavzevii as a platform host for production of organic acids under low-pH conditions.