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BACKGROUND: The effects of alkaline stress on -PGA biosynthesis and the mechanism involved have not been investigated in glutamate-free medium. RESULTS:Effects of alkaline pH stress on the yield and molecular weight of -PGA were investigated in a 50 L bioreactor, and transcriptional levels of relevant genes were measured to explain the mechanism. After alkaline stress treatment, the maximum -PGA yield reached 36.26 g L −1 , increased by 79% compared with the control. The transcription levels of -PGA synthetase genes pgsB and pgsC, as well as their closely related regulation factors degU and swrA, were improved 18.9-fold, 31.2-fold, 3.0-fold and 6.3-fold, respectively. Moreover, transcriptional analysis showed that alkaline stress enhanced the biosynthesis of -PGA precursor (glutamic acid) and reduced the formation of byproducts (acetoin and 2,3-butanediol), which contributed to the improved production of -PGA. In addition, alkaline stress reduced the -PGA molecular weight, which might be due to the enhanced expression of -PGA hydrolase PgdS. CONCLUSION: The alkaline stress-mediated strategy was developed to regulate the yield and molecular weight of -PGA in the glutamate-free medium, and the mechanism involved was explained through transcriptional analysis.

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Basil (Ocimum L.) is widely used as a flavor ingredient, however research on basil flavor is limited. In the current study, nine basil species were selected, including Ocimum basilicum L.var. pilosum (Willd.) Benth., Ocimum sanctum, Ocimum basilicum cinnamon, Ocimum gratissimum var. suave, Ocimum tashiroi, Ocimum basilicum, Ocimum americanum, Ocimum basilicum ct linalool, and Ocimum basilicum var. basilicum, and their fragrance and flavor characteristics were assessed by sensory evaluation. The results indicated that Ocimum basilicum var. basilicum and Ocimum gratissimum var. suave have a strong clove smell and exhibited a piquant taste. Metabolomics and volatilomics analyses measured 100 nonvolatile metabolites and 134 volatiles. Differential analysis showed that eugenol, γ-terpinene, germacrene D and malic acid were among the most varied metabolites in basil species. Combined with sensory evaluation results, correlation analysis revealed that β-pinene and γ-cadinene contributed to the piquant smell, while eugenol and germacrene D contributed to the clove smell, and malic acid and L-(−)-arabitol contributed to the sweet flavor in basil. This study provided comprehensive flavor chemistry profiles of basil species and could be used as a guide for basil flavor improvement. The better understanding of objective sensory attributes and chemical composition of fresh basil could introduce the improved cultivars with preponderant traits, which is also in accordance with the various demands of breeders and growers, food producers, and consumers.

Zingiberaceae plants are widely used in the food and pharmaceutical industries; however, research on the chemical composition and interspecific differences in the metabolome and volatilome of Zingiberaceae plants is still limited. In this study, seven species of Zingiberaceae plants were selected, including Curcuma longa L., Zingiber officinale Rosc., Alpinia officinarum Hance, Alpinia tonkinensis Gagnep, Amomum tsaoko Crevost et Lemarie, Alpinia hainanensis K. Schum. and Amomum villosum Lour. Myristica fragrans Houtt. was also selected due to its flavor being similar to that of the Zingiberaceae plant. The metabolome and volatilome of selected plants were profiled by widely targeted approaches; 542 volatiles and 738 non-volatile metabolites were detected, and β-myrcene, α-phellandrene and α-cadinene were detected in all the selected plants, while chamigren, thymol, perilla, acetocinnamone and cis-α-bisabolene were exclusively detected in certain Zingiberaceae plants. Differential analysis showed that some terpenoids, such as cadalene, cadalene-1,3,5-triene, cadalene-1,3,8-triene and (E)-β-farnesene, and some lipids, including palmitic acid, linoleic acid and oleic acid were amongst the most varied compounds in Zingiberaceae plants. In conclusion, this study provided comprehensive metabolome and volatilome profiles for Zingiberaceae plants and revealed the metabolic differences between these plants. The results of this study could be used as a guide for the nutrition and flavor improvement of Zingiberaceae plants.

In the present study, poly-γ-glutamic acid (γ-PGA) was applied to functionalize the silica-coated magnetic nanoparticles for solid-phase extraction of Cu(ii) and Pb(ii).