Combined 6-benzylaminopurine and H2O2 stimulate the astaxanthin biosynthesis in Xanthophyllomyces dendrorhous.

Astaxanthin is one of the most attractive carotenoids due to its high antioxidant activity and beneficial biological properties, while Xanthophyllomyces dendrorhous is one of its main microbial sources. Since astaxanthin is synthesized as a response to oxidative stress, several oxidative agents have been evaluated to increase X. dendrorhous astaxanthin yields. However, the extent of the stimulation is determined by the cellular damage caused by the applied oxidative agent. Phytohormones have also been reported as stimulants of astaxanthin biosynthesis acting directly on its metabolic pathway and indirectly promoting cellular resistance to reactive oxygen species. We reasoned that both oxidative agents and phytohormones lead to increased astaxanthin synthesis, but the latter could mitigate the drawbacks of the former. Thus, here, the stimulation on astaxanthin biosynthesis, as well as the cellular and transcriptional responses of wild type X. dendrorhous to phytohormones (6-benzylaminopurine, 6-BAP; abscisic acid, ABA; and indole-3-acetic acid, IAA), and oxidative agents (glutamate, menadione, H2O2, and/or Fe2+) were evaluated as a single or combined treatments. ABA and 6-BAP were the best individual stimulants leading to 2.24- and 2.60-fold astaxanthin biosynthesis increase, respectively. Nevertheless, the effect of combined 6-BAP and H2O2 led to a 3.69-fold astaxanthin synthesis increase (0.127 ± 0.018 mg astaxanthin/g biomass). Moreover, cell viability (> 82.75%) and mitochondrial activity (> 82.2%) remained almost intact in the combined treatment (6-BAP + H2O2) compared to control (< 52.17% cell viability; < 85.3% mitochondrial activity). On the other hand, mRNA levels of hmgR, idi, crtYB, crtR, and crtS, genes of the astaxanthin biosynthetic pathway, increased transiently along X. dendrorhous fermentation due to stimulations assayed in this study. KEY POINTS: • Combined 6-BAP and H2O2 is the best treatment to increase astaxanthin yields in X. dendrorhous. • 6-BAP preserves cell integrity under oxidative H2O2 stress conditions. • 6-BAP and H2O2 increase transcriptional responses of hmgR, idi, and crt family genes transiently.

Exploring volatile compounds and microbial dynamics: Kluyveromyces marxianus and Hanseniaspora opuntiae reduce Forastero cocoa fermentation time.

Traditional cocoa bean fermentation is a spontaneous process and can result in heterogeneous sensory quality. For this reason, yeast-integrated starter cultures may be an option for creating consistent organoleptic profiles. This study proposes the mixture of Hanseniaspora opuntiae and Kluyveromyces marxianus (from non-cocoa fermentation) as starter culture candidates. The microorganisms and volatile compounds were analyzed during the cocoa fermentation process, and the most abundant were correlated with predominant microorganisms. Results showed that Kluyveromyces marxianus, isolated from mezcal fermentation, was identified as the dominant yeast by high-throughput DNA sequencing. A total of 63 volatile compounds identified by HS-SPME-GC-MS were correlated with the more abundant bacteria and yeast using Principal Component Analysis and Agglomerative Hierarchical Clustering. This study demonstrates that yeasts from other fermentative processes can be used as starter cultures in cocoa fermentation and lead to the formation of more aromatic esters, decrease the acetic acid content.