Transglycosylation of Steviol Glycosides and Rebaudioside A: Synthesis Optimization, Structural Analysis and Sensory Profiles

Prebiotic potential of new sweeteners based on the simultaneous biosynthesis of galactooligosaccharides and enzymatically modified steviol glycosides

Prebiotics are known for their health-promoting functions associated with the modulation of the colonic microbiota and the products of fermentation. Recently, single-pot syntheses of galactooligosaccharides in combination with steviol glycosides (mSG-GOS) have been developed. This work was conducted to evaluate their prebiotic effect by using faecal inoculum from healthy human donors during in vitro batch fermentations. Additionally, their relative sweetness was evaluated to determine their suitability as food ingredients. The results showed a significant growth (p < 0.05) of bacteria, including the genera Bifidobacterium, Bacteroides and Clostridium, and a corresponding increase in short-chain fatty acids (SCFA) in comparison to either positive and negative controls. The sweetness equivalence to 1 % w:v of SG-GOS was 0.8 % w:v when compared to sucrose. Considering the bacteria and organic acids analyses and their sweetness values of these new biosynthesized compounds, SG-GOS could act as a prebiotic sweetener with potential health benefits warranting further evaluation through human studies.

Genomic analysis of Paenibacillus macerans strain I6, which can effectively saccharify oil palm empty fruit bunches under nutrient-free conditions

The improper disposal of palm oil industrial waste has led to serious environmental pollution. In this study, we isolated Paenibacillus macerans strain I6, which can degrade oil palm empty fruit bunches generated by the palm oil industry in nutrient-free water, from bovine manure biocompost and sequenced its genome on PacBio RSII and Illumina NovaSeq 6000 platforms. We obtained 7.11 Mbp of genomic sequences with 52.9% GC content from strain I6. Strain I6 was phylogenetically closely related to P. macerans strains DSM24746 and DSM24 and was positioned close to the head of the branch containing strains I6, DSM24746, and DSM24 in the phylogenetic tree. We used the RAST (rapid annotation using subsystem technology) server to annotate the strain I6 genome and discovered genes related to biological saccharification; 496 genes were related to carbohydrate metabolism and 306 genes were related to amino acids and derivatives. Among them were carbohydrate-active enzymes (CAZymes), including 212 glycoside hydrolases. Up to 23.6% of the oil palm empty fruit bunches was degraded by strain I6 under anaerobic and nutrient-free conditions. Evaluation of the enzymatic activity of extracellular fractions of strain I6 showed that amylase and xylanase activity was highest when xylan was the carbon source. The high enzyme activity and the diversity in the associated genes may contribute to the efficient degradation of oil palm empty fruit bunches by strain I6. Our results indicate the potential utility of P. macerans strain I6 for lignocellulosic biomass degradation.

Efficient Bioconversion of Stevioside and Rebaudioside A to Glucosylated Steviol Glycosides Using an Alkalihalobacillus oshimesis-Derived Cyclodextrin Glucanotransferase

The enzymatic transglycosylation of steviol glycosides can improve the edulcorant quality of steviol glycosides. Cyclodextrin glucanotransferase (CGTase) is one of the most popular glucanotransferases applied in this reaction. Herein, the CGTase-producing strain Alkalihalobacillus oshimensis CGMCC 23164 was isolated from Stevia planting soil. Using mass spectrometry-based secretome profiling, a high-efficiency CGTase that converted steviol glycosides to glucosylated steviol glycosides was identified and termed CGTase-13. CGTase-13 demonstrated optimal transglycosylation activity with 10 g/L steviol glycoside and 50 g/L soluble starch as substrates at <40 °C. Under the above conditions, the conversion rate of stevioside and rebaudioside A, two main components of steviol glycosides, reached 86.1% and 90.8%, respectively. To the best of our knowledge, this is the highest conversion rate reported to date. Compared with Toruzyme^® 3.0 L, the commonly used commercial enzyme blends, glucosylated steviol glycosides produced using CGTase-13 exhibited weaker astringency and unpleasant taste, faster sweetness onset, and stronger sweetness intensity. Thus, CGTase provides a novel option for producing high-quality glucosylated steviol glycoside products and has great potential for industrial applications.

Transglycosylation of Stevioside by a Commercial β-Glucanase with Fungal Extracted β-Glucans as Donors

Abstract

Alternative sweeteners, such as steviol glucosides from the plant Stevia rebaudiana Bertoni, are becoming increasingly popular for the design of next-generation foodstuffs. However, the bitter aftertaste of native steviol glucosides is one of the main reasons behind consumer reluctance towards stevia-containing products. Biocatalysis could be a sustainable solution to this problem, through addition of glucosyl moieties to the molecule. Glycoside hydrolases are enzymes performing transglycosylation reactions, and they can be exploited for such modifications. In the present work, the commercial β-glucanase Finizym 250L® was employed for the transglycosylation of stevioside. After optimization of several reaction parameters, the maximal reaction yield obtained was 19%, with barley β-glucan as the glycosyl donor. With the aim to develop a sustainable process, β-glucan extracts from different fungal sources were prepared. Pulsed Electric Field pretreatment of mycelial biomass resulted in extracts with higher β-glucan content. The extracts were tested as alternative glucosyl donors, reaching up to 15.5% conversion yield, from Pleurotus-extracted β-glucan. Overall, in the present work a novel enzymatic process for the modification of stevioside is proposed, with concomitant valorization of β-glucans extracted from fungal biomass, potentially generated as a byproduct from other applications, in concert with the principles of circular economy.

Graphical Abstract

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2019-2020

This review is the tenth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2020. Also included are papers that describe methods appropriate to analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. The review is basically divided into three sections: (1) general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, quantification and the use of arrays. (2) Applications to various structural types such as oligo- and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals, and (3) other areas such as medicine, industrial processes and glycan synthesis where MALDI is extensively used. Much of the material relating to applications is presented in tabular form. The reported work shows increasing use of incorporation of new techniques such as ion mobility and the enormous impact that MALDI imaging is having. MALDI, although invented nearly 40 years ago is still an ideal technique for carbohydrate analysis and advancements in the technique and range of applications show little sign of diminishing.

Metabolic engineering for the synthesis of steviol glycosides: current status and future prospects

With the pursuit of natural non-calorie sweeteners, steviol glycosides (SGs) have become one of the most popular natural sweeteners in the market. The SGs in Stevia are a mixture of SGs synthesized from steviol (a terpenoid). SGs are diterpenoids. Different SGs depend on the number and position of sugar groups on the core steviol backbone. This diversity comes from the processing of glycoside steviol by various glycosyltransferases. Due to the differences in glycosylation, each SG has unique sensory properties. At present, it is more complicated to extract high-quality SGs from plants, so the excavation of the metabolic pathways of engineered microorganisms to synthesize SGs has been extensively studied. Specifically, the expression of different glycosyltransferases in microbes is key to the synthesis of various SGs by engineered microorganisms. To trigger more researches on the functional characterization of the enzymes encoded by these genes, this review describes the latest research progresses of the related enzymes involved in SG biosynthesis and metabolic engineering.Key points• Outlines the research progress of key enzymes in the biosynthetic pathway of SGs• Factors affecting the catalytic capacity of stevia glucosyltransferase• Provide guidance for the efficient synthesis of SGs in microbial cell factories.

High-Yield Synthesis of Transglycosylated Mogrosides Improves the Flavor Profile of Monk Fruit Extract Sweeteners

Luo Han Guo fruit extract (Siraitia grosvenorii), mainly composed of mogroside V (50%), could be considered a suitable alternative to free sugars; however, its commercial applications are limited by its unpleasant off-notes. In the present work, a central composite design method was employed to optimize the transglycosylation of a mogroside extract using cyclodextrin glucosyltransferases (CGTases) from three different bacteriological sources (Paenibacillus macerans, Geobacillus sp., and Thermoanaerobacter sp.) considering various experimental parameters such as maltodextrin and mogroside concentration, temperature, time of reaction, enzymatic activity, and pH. Product structures were determined by liquid chromatography coupled to a diode-array detector (LC-DAD), liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS), and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). Sensory analysis of glucosylated mogrosides showed an improvement in flavor attributes relevant to licorice flavor and aftereffect. Consequently, an optimum methodology was developed to produce new modified mogrosides more suitable when formulating food products as free sugar substitutes.