Low Sweetness Research Articles

Galacto-oligosaccharides as functional foods and their properties

Galacto-oligosaccharides (GOS) are classified as prebiotics due to their indigestible nature. GOS are regarded as resistant to salivary digestion and are not utilized by the mouth. Thus, they can be utilized as substitutes for low-cariogenic sugar. The structure of GOS mixtures is determined by the differentiation between the transferase reaction and hydrolysis. Transgalactosylation can be achieved intramolecularly when the glycosidic bond in lactose cleaves and reassembles itself on another glucose molecule. Microbes can be used as a source for the β-galactosidase enzyme or as agents to produce GOS units. Commercial β-galactosidase enzymes likewise do have a great potential for their use in GOS synthesis. These transgalactosyl reactions could find useful applications in dairy. Since 70% of the world's population cannot tolerate lactose, lactose utilization could only be enhanced through hydrolysis into its components, the monomer sugars of D-glucose and D-galactose. GOS can be produced industrially using whey or lactose as the substrate. Trisaccharide (β1-4 or β1-6 galactosyllactose) are generally the major products of this process. In comparison with lactose and other saccharide molecules, GOS has been shown to have low carcinogenicity, low calorific value, and low sweetness. This review focuses on GOS production, and the physicochemical characteristics, physiological effects, and applications of these prebiotics are summarized.

Effects of a fruit-ice combination flavor on appeal and sensory experience of vaping and moderation by preexisting e-cigarette flavor preference.

E-cigarettes with ice flavors, which are products with a cooling agent added to a characterizing flavor (e.g., grape-ice), are widely sold. Whether ice flavors appeal to only those who already use them or a to wider population is not widely understood. This secondary analysis of a clinical laboratory experiment tested effects of experimental exposure to e-cigarettes with fruit-ice versus other flavors on the appeal and sensory attributes of vaping and whether fruit-ice effects are moderated by preexisting e-cigarette flavor preference. In a controlled double-blind within-subject randomized online experiment, adults who currently use e-cigarettes, N = 85, M (SD) = 35.5 (13.2) years, 50% female, 54.3% White, self-administered e-liquids varying in flavors (fruit-ice [grape + menthol], tobacco-only, dessert-only [caramel], and fruit-only [strawberry]). Participants rated each product's appeal (liking, disliking, willingness to use again) and sensory attributes (sweetness, smoothness, harshness, bitterness, and coolness). In the overall sample, fruit-ice flavor produced (a) higher appeal, sweetness, smoothness, and coolness and lower bitterness and harshness compared to tobacco and dessert-only flavors and (b) higher coolness and lower sweetness than fruit-only flavors. The appeal-enhancing effect of fruit-ice (vs. tobacco and dessert-only flavors) was stronger among those with preexisting preferences for menthol/mint, fruit, and ice flavors, but not for those who typically used tobacco-only flavors. Our findings suggest that restrictions on fruit-ice-flavored e-cigarettes may reduce the appeal of vaping, particularly among vapers with preferences for products with cooling, fruit, and/or sweet sensory attributes. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Effects of postharvest nitric oxide and 24-Epibrassinolide treatments on cultivated blueberry (Vaccinium corymbosum L.): From flavor and antioxidant properties to edibility evaluation

Blueberry is a tender and perishable berry. This study identified the effects of nitric oxide (NO) and 2,4-Epibrassinolide (EBR) on the flavor, antioxidant properties, and edible value of blueberry. The results indicated that 1.0 mmol L−1 sodium nitroprusside (SNP, a NO donor) and 0.8 mg L−1 EBR groups showed high sourness and low sweetness in taste. SNP preserved the content of volatile flavor compounds on 8 d, while EBR preserved that on 16 d. The principal component analysis (PCA) plot from the flavor well differentiated between the treatment and control blueberry. Moreover, the biplot shows the correlation between volatile flavor compounds and taste perception. From the perspective of antioxidant properties, SNP and EBR inhibited the accumulation of H2O2, which showed a negative correlation with total phenolic (TPC), total flavonoid content (TFC), and ascorbic acid (AsA) content. SNP and EBR increased enzyme activities, such as catalase (CAT), peroxidase (POD), superoxide dismutase (SOD), ascorbic acid peroxidase (APX), and glutathione reductase (GR), antioxidant activity, including 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging and ferric ion-reducing antioxidant power (FRAP), and the gene expression levels of VcSOD, VcAPX, and VcGR. The random forest model showed that AsA content, weight loss rate, decay incidence, TFC, TPC, Methyl-2-methylbutyrate, POD activity, and DPPH radical scavenging activity played critical roles in the edible value. Collectively, NO and EBR preserved the flavor, increased antioxidant capacity, and maintained postharvest quality.

Cryoprotective effect of epigallocatechin gallate replacing sucrose on Hypophythalmichthys molitrix surimi during frozen storage.

The present study aimed to investigate the cryoprotective effect of epigallocatechin gallate (EGCG) replacing sucrose on surimi during frozen storage. Substitution or partial substitution of 0.1% EGCG for sucrose (1.5%) was added to surimi, and the surimi samples without and with commercial cryoprotectants (4% sucrose and 4% sorbitol) were used as the control group. The results obtained suggest that, with the increase in frozen storage time, the structural performance of surimi protein gradually weakened (e.g. the decrease in the surface hydrophobicity, the increase in the total sulfhydryl and solubility, and the protein myosin heavy chain bands became shallow) and surimi gel quality gradually deteriorated (e.g. the decrease in water-holding capacity, gel strength and all texture profile attributes). However, compared with the other three group surimi samples during the frozen period, the surimi proteins with partial replacement of sucrose by EGCG had a higher total sulfhydryl group content and solubility of proteins, as well as lower surface hydrophobicity of protein, suggesting that the addition of EGCG as a partial substitute for sucrose can enhance the antifreeze ability of surimi. Meanwhile, the surimi gel with the partial replacement of sucrose by EGCG had a higher water retention capacity, gel strength and texture attributes (e.g. hardness, springiness, cohesiveness, chewiness, and resilience), indicating that the addition of EGCG as a partial substitute for sucrose can inhibit the deterioration of surimi gel quality. Overall, EGCG partially replacing sucrose can play an alternative cryoprotectant with a lower sweetness to prevent the quality of surimi from deteriorating. © 2024 Society of Chemical Industry.

Cryoprotective effects and mechanisms of soybean oligosaccharides on the grass carp (Ctenopharyngodon idellus) surimi during frozen storage.

Conventional cryoprotectant mixtures (sucrose and sorbitol) impart excessive sweetness and calories to surimi. Therefore, there is a need to explore alternative cryoprotectants with low sweetness and low-calorie content. The cryoprotective effects and possible mechanisms of soybean oligosaccharides (SBOS) on the frozen stability of grass carp (Ctenopharyngodon idellus) surimi were investigated during 120 days of frozen storage in a comparison with commercial cryoprotectants (4% sucrose and 4% sorbitol, w/w). SBOS at 6-8% (w/w) and commercial cryoprotectants could restrain water mobility and reduce thawing loss of frozen surimi by increasing non-freezable water content. SBOS could maintain the structural stability of proteins by preventing sulfhydryl groups from being rapidly oxidized to disulfide bonds, retarding the reduction of the solubility, Ca2+-ATPase activity and α-helix content of myofibrillar proteins (MP), as well as hindering the increasing surface hydrophobicity of MP of surimi during 120 days of frozen storage. The introduction of SBOS increased the gel strength and water-holding capacity of frozen-stored surimi. Compared with commercial cryoprotectants, 8% SBOS was more effective in stabilizing protein structure, whereas it was slightly less effective with respect to ice-forming inhibition. The results obtained in the present study suggest that 8% SBOS could be potentially developed as a new cryoprotectant for surimi as a result of its ice-forming inhibition abilities and protein structure stability. © 2024 Society of Chemical Industry.

Sweet and sour sips: No effect of repeated exposure to sweet or sour-tasting sugary drinks on children's sweetness preference and liking

Health agencies advocate reducing children's sweetness exposure to lower sweetness preference or liking to ultimately lower sugar intake. However, the relationship between sweetness exposure, preference, and liking remains unclear. This work investigated the influence of exposure to a sucrose-containing sweet or sour-tasting drink on sweetness preference and liking for sweet and sour products in 4-7-year-old children (n = 65). The children were randomized into three groups with one daily exposure to either the sweet drink, sour drink, or water (control group) for 14 days. Sweetness preference was assessed at baseline (t1), day 15 (t2), and two months after the intervention (t3), using a forced-choice, paired comparison test with five beverages varying in sweetness intensity. Hedonic liking for the intervention drinks, a sweet and sour yogurt, and a sweet and sour candy was evaluated using a 5-point pictorial scale. Linear mixed models revealed a significant increase in sweetness preference from t1 to t3 (F(2) = 7.46, p < 0.001). However, ANCOVA analysis indicated that this effect was not caused by the intervention. Based on linear mixed models, we observed that children's hedonic liking for sweet and sour products remained stable from t1 to t3 and was not influenced by the intervention. These findings suggest that 14 exposures to a sucrose-containing sweet or sour-tasting drink did not affect sweetness preference or liking in 4-7-year-old children.

Sensory analysis of sweetness in viscous solutions and gels with agar

AbstractWe prepared 81 samples of sweetened viscous solutions and gels with agar concentrations ranging from 0.025% to 0.60% and compared their sweetness intensities to reference solutions without agar. Sweetened viscous solutions containing agar showed significantly higher sweetness intensity in six solutions containing D‐glucose, three solutions containing sucrose, and four solutions containing D‐sorbitol compared to the reference solutions. Notably, some sweetened viscous solutions with extremely low viscosity showed higher sweetness intensity than the reference solution, which is a novel finding. Sweet samples containing agar showed significantly lower sweetness intensity than the reference solution in 15 samples containing D‐glucose, 17 samples containing sucrose, and 15 samples containing D‐sorbitol, supporting previous theories. We performed a simple regression analysis on the sweetness intensity of viscous solutions and gels containing agar and their physical properties. Our results showed a strong correlation between sweetness intensity and physical properties. These findings suggest that as sweetness intensity decreases, physical properties tend to increase.Practical ApplicationsTo prevent noncommunicable diseases such as diabetes, it is recommended to practice carbohydrate restriction and reduce the consumption of sweet foods. In this study, upon being offered extremely low‐viscosity sweetened solutions, we anticipate perceiving a slightly stronger sweetness and an increased sense of satisfaction in contrast to tasting sweetened aqueous solutions without viscosity. These findings hold the potential to provide pioneering insights into the interplay between taste and texture for entities such as food manufacturers. They could facilitate applications designed to elevate the quality of health‐conscious food production.

Sensitive to music? Examining the crossmodal effect of audition on sweet taste sensitivity

Previous research has shown that music can influence taste perception. While most studies to date have focused on taste intensity ratings, less is known about the influence of musical stimuli on other parameters of taste function. In this within-subjects experiment (N = 73), we tested the effects of three sound conditions (High Sweetness soundtrack – HS; Low Sweetness soundtrack – LS; and Silence – S) on sweet taste sensitivity, namely, detection and recognition. Each participant tasted nine samples of sucrose solutions (from 0 g/L to 20 g/L) under each of the three sound conditions in counterbalanced order. We assessed the lower concentrations at which participants were able to detect (detection threshold) and correctly identify (recognition threshold) a taste sensation. Additionally, the intensity and hedonic ratings of samples above the recognition threshold (7.20 g/L) were analyzed. Affective variations (valence and arousal) in response to the sound conditions were also assessed. Although music did not lead to significant differences in mean detection and recognition thresholds, a larger proportion of sweet taste recognitions was observed at a near-threshold level (2.59 g/L) in the HS condition. The intensity and hedonic ratings of supra-threshold conditions were unaffected by the music condition. Significant differences in self-reported mood in response to the sound conditions were also observed. The present study suggests that the influence of music on the sweet taste perception of basic solutions may depend on the parameter under consideration.

Chitosan-based fruit coating as postharvest treatments on two pineapples (Ananas comosus L. Merr) clones

Production of fresh pineapple fruit is dominated by the MD2 clone, while the GP3 clone is used as canned fruit. To be promoted as fresh fruit, the GP3 has some defects, namely a less sweet taste and sensitivity to internal browning (IB) disorders. The clones need fruit coatings, and the two might get responded differently. This study aimed to study the effects of chitosan-based fruit coating mixed with palm stearin and Aloe vera gel on the fruit qualities of the two clones, as well as to seek the potential of the GP3 clone as a fresh fruit. The study used 2 x 4 factorials. The first factor was two clones (GP3 and MD2). The second factor was fruit coatings (control, chitosan, palm stearin + chitosan, and Aloe vera gel + chitosan). The fruits were stored at 7 °C. The results showed that the GP3 started to show the incidence of IB disorders on the 21st day, 2 weeks earlier than the MD2. The MD2 showed a °Brix of 16.82% and vitamin C of 391.45 mg/L, which were significantly higher than the GP3. Meanwhile, the GP3 showed 0.60% acidity, which was significantly higher than the MD2. These indicated that the GP3 should not be used for fresh fruit consumption, mainly because of its sensitivity to IB and lower sweetness compared to the MD2 clone. The chitosan treatment, which was added with Aloe vera or palm stearin, showed a higher vitamin C value than the other treatments.

Insights into Chemical Structure-Based Modeling for New Sweetener Discovery.

The search for novel, natural, high-sweetness, low-calorie sweeteners remains open and challenging. In the present study, the structure-based machine learning modeling and sweetness recognition mechanism were investigated to assist this process. It was found that whether or not a compound was sweet was closely related to molecular connectivity and composition (the number of hydrogen bond acceptors and donors), tpsaEfficiency, structural complexity, and shape (nAtomP and Fsp3). While the relative sweetness of sweet compounds was more determined by the molecular properties (tpsaEfficiency and Log P), structural complexity and composition (nAtomP and ATSm 1). The built machine learning models exhibited very good performance for classifying the sweet/non-sweet compounds and predicting the relative sweetness of the compounds. Moreover, a specific binding pocket was found for sweet compounds, and the sweet compounds mainly interacted with the VFT domain of the T1R2-T1R3 through hydrogen bonds. In addition, the results indicated that among the sweet compounds, those that were sweeter bound to the VFT domain stronger than those that had low sweetness. This study provides very useful information for developing new sweeteners.

Characterization of key sweet taste compounds in Camellia nanchuanica black tea

Camellia nanchuanica gained attention for having original tea tree characteristics and producing black tea with appealing sweetness. To better study the sweet taste of Camellia nanchuanica black tea, 30 kinds of Camellia nanchuanica black tea (NCBT) samples were classified into three types (high, middle and low sweetness) according to the differences in sweetness. A total of 844 metabolites were detected in NCBT; among them, there were 155 differential metabolites. Four differential metabolites (phloridzin, dulcitol, l-alanine, and sucrose) positively correlated with sweetness and showed higher VIP (variable importance for the projection) values (>1), which made main contributions to the sweetness of NCBT. Phloridzin and l-alanine might be the key sweet compounds of NCBT due to higher DoT (dose-over-threshold) values. Addition experiments further verified that phloridzin was the key sweet taste compound because phloridzin significantly increased the sweet taste of low sweetness samples. Notably, although NCBT24, 26, and 28 belonged to high sweetness samples, phloridzin content was lower. Based on the interactions of binary mixture, hyper-addition effects of phloridzin, dulcitol, l-alanine, and sucrose existed in NCBT24, 26 and 28.

Sensory Profile on Robusta Coffee by Rate-All-That-Apply (RATA)

Coffee flavor strongly influences consumers preference. Geographical location is one factor influencing the flavor of Robusta coffee. Descriptive sensory using RATA was used to differentiate the sensory profiles of several Robusta coffees. Sensomic approach by Principal Component Analysis (PCA) success- fully differentiated Robusta coffee from Jember Regency, East Java, Indonesia. It showed that Robusta from different growing locations has specific sensory characteristics. Robusta coffee from Gunung Malang, Tanggul, and Pakis was characterized by sweet and acid aromas, while coffee from Kemiri was characterized by sweet and sour taste, brown color, bitter aroma, and rough mouthfeel. Meanwhile Sidomulyo I and Sidomulyo II coffees, which were grown near each other and at similar altitudes, have similar characteristics, although the process was different. Both coffees have bitter aftertastes and bodies, burnt aromas, astringent flavors, and high levels of bitterness. In contrast, Robusta Rowosari, Tugusari, and Badean were characterized by low sweetness, sourness, bitter aroma, and rough mouthfeel.

Contextual processing and its alterations in patients with addictive disorders

Contextual processing is implicated in the pathophysiology of addictive disorders, but the nature of putative deficiencies remains unclear. We assessed some aspects of contextual processing across multimodal experimental procedures with detoxified subjects who were dependent on opioids (n = 18), alcohol- (n = 20), both opioids and alcohol (n = 22) and healthy controls (n = 24) using a) facial- and b) emotionally laden images; c) gambling task and d) sucrose solutions. Healthy subjects displayed consistent response pattern throughout all categories of the presented stimuli. As a group, dependent subjects rated (i.e., valuated) attractive and average faces respectively more and less attractive in comparison to controls. Dependent subjects' motivational effort, measured in the units of computer keypress to determine the attractive faces' viewing time, accorded the valuational context but was diminished relatively to the average faces’ valuation. Dependent subjects’ motivational effort for pleasant and aversive images respectively mirrored the attractive and average faces; their neutral images’ motivational effort was incongruent with the valuational context framed by the intermixed images. Also, dependent subjects’ emotional responses to counterfactual comparisons of gambling outcomes were unmatched by the riskiness context. Moreover, dependent subjects failed to show greater liking of sweet solutions that normally accompanies low sweetness perceptual context indicative of higher sucrose concentration needed for maximal hedonic experience. Consistent differences among the dependent groups (opioid vs. alcohol vs. comorbid) on the above procedures were not observed. The present findings suggest that opioid and/or alcohol dependence may be associated with amplified hedonic and motivational valuation of pleasant stimuli and with a disrupted link between behavioral/emotional responsivity and contextual variations. Further research is warranted to unravel the distinctive features of contextual processing in opioid- vis-à-vis alcohol addiction and how these features may interrelate in comorbid conditions.

Suppression of sweetness: evidence for central mechanism for suppression of sweetness from sucrose by citric acid.

The underlying mechanisms of taste interactions in humans are not well understood, and three mechanisms have been proposed, namely a chemical interaction, a peripheral physiological, and a central mechanism. In the present study, it was investigated which of these mechanisms causes the suppression of sweetness by citric acid. This was investigated using a split-tongue gustometer that can stimulate the two sides of the tongue with different stimuli simultaneously, enabling a comparison of sucrose and citric acid presented either separately on each side of the tongue simultaneously or in a mixture on one side. Two studies were conducted using low (Study 1; n = 50) and high (Study 2: n = 59) concentrations of sucrose (2.5% (w/w) and 10% (w/w), respectively), and citric acid (0.14% (w/w) and 0.18% (w/w), respectively). In neither of the studies was there a significant difference in sweetness intensity ratings between the two conditions where sucrose and citric acid were presented either separately or in a mixture form. However, both showed significantly lower sweetness ratings than without citric acid indicating suppression of the sweetness of sucrose from citric acid. This provides strong evidence for a central mechanism for the suppression of the sweetness of sucrose by citric acid. This mechanism seems to be equal in high and low concentrations of both sucrose and citric acid.

Just noticeable difference in sweetness perception of cola: Small changes in sugar are noticeable.

These data could help researchers and industry product developers know how much sugar can be reduced from a cola-flavored beverage before the change is apparent to consumers.

Sweet music influences sensory and hedonic perception of food products with varying sugar levels

Reducing sugar intake is an important nutritional goal in most developed countries. Despite the health consequences of excessive sugar consumption (e.g., non-communicable diseases), individuals are often reluctant to shift dietary habits due to the high hedonic appeal of sugar-rich products. Manipulating the intrinsic sensory attributes of foods (such as color or aroma) has been put forward as a promising framework for enhancing taste perception and increasing acceptance of low-sugar products. So far, it is less known whether extrinsic sensory cues may have similar effects.In this within-subjects experiment (N = 106, 64 % women), we tested how auditory stimuli (i.e., music) may impact the perception and acceptance of products varying in sweetness levels. Participants tasted samples of two product categories (vegetables and cookies), with higher (carrots and cookies) and lower sweetness levels (cucumbers and 0 % sugar cookies), while listening to previously tested soundtracks that were strongly (vs weakly) associated with sweetness.Results showed that the high “sweetness” soundtrack increased the sweetness ratings of all products compared to the low “sweetness” soundtrack. Participants also reported higher preference and more favorable intentions of future consumption when the high “sweetness” soundtrack was played. Overall, these findings suggest that extrinsic sensory cues, namely music, may aid in reducing sugar intake by increasing the acceptance of products with lower sugar content.

Assessment of gelatin hydrolysates from threadfin bream (Nemipterus hexodon) skin as a cryoprotectant for denaturation prevention of threadfin bream natural actomyosin subjected to different freeze-thaw cycles

Gelatin hydrolysates prepared from threadfin bream skin (GH) using lizardfish stomach pepsin (GHLP) with different degrees of hydrolysis (DHs: 10–40%) were studied as a potential alternative to commercial cryoprotectant (sucrose/sorbitol blend; SuSo, 1:1) used in frozen surimi. The characteristics of threadfin bream natural actomyosin (NAM) containing 8% GHLP with different DHs or SuSo were compared to control NAM before and after freeze-thaw treatment. After 3 and 6 freeze-thaw cycles, all present cryoprotectants retarded the protein changes. Among all samples, the highest Ca2+-ATPase activity, total sulfhydryl and solubility with the coincidental lowest disulfide bond content were observed in NAM added with GHLP having 20% DH after repeated freeze-thawing. Also, GHLP could prevent lipid oxidation in NAM as shown by the lower TBARS value. When cryoprotective properties of GHLP with 20% DH and GH prepared by papain (GHP) (DHs: 10–40%) were compared, the cryoprotectant properties of GHLP were similar to that of GHP. However, GHP with 10% DH exhibited a higher protective effect on protein denaturation. Hence, both GHLP and GHP with suitable DH can be a new generation cryoprotectant with the lower sweetness in surimi products.

Study Protocol: Sensory Effects of Replacing Sugar Sweetened Sodas With Unsweetened, Flavored Sparkling Water

Abstract Objectives Excess sugar consumption contributes to numerous chronic diseases. Prior work has typically emphasized replacing sugar-sweetened beverages, a key source of excess dietary sugar, with low-calorie sweetened beverages. Our protocol aims to instead replace sugar sodas with unsweetened, flavored sparkling water products, which are available in the same containers and at comparable prices to sugary sodas. Methods Participants who consume at least 2 sugar-sweetened sodas per day are being recruited for this project. Flavored, unsweetened sparkling waters will be provided to participants for 2 weeks, with sensory evaluations at baseline, 1 week, and 2 weeks. At the end of the 2-week intervention, we will provide participants with information on the costs and how to find the flavored, unsweetened sparkling water products. Two weeks post intervention, participants will be asked whether they continued to consume the sparkling waters or if they reverted back to sugar-sweetened soda consumption. We will use linear mixed models with subject as a repeated measure to determine the effect of replacement of sugar-sweetened sodas beverages with sparkling waters on: liking of sweetness in soda-like beverages, “just about right” concentration of sugar/intensity of sweetness in soda-like beverages, and post-intervention beverage selection. Results As this is a protocol abstract, results have yet to be obtained. However, we hypothesize that participants who comply with substituting sparkling waters for sweetened sodas will exhibit sensory shifts to prefer lower concentrations of sugar and liking for lower sweetness intensity in beverages. We expect these participants will also report continuing to consume the sparkling water products at 2 weeks post intervention. Conclusions Data from this study will inform future work on whether commercially available water products could shift sensory preferences for sweetness, which could aid in interventions to curb added sugar intake. Funding Sources This study is supported by a Pilot and Feasibility Award within the Indiana CTSI/CDMD, an initiative funded by the NIH/NIDDK.

Identification of Key Gene Networks Controlling Soluble Sugar and Organic Acid Metabolism During Oriental Melon Fruit Development by Integrated Analysis of Metabolic and Transcriptomic Analyses.

Oriental melon (Cucumis melo var. acidulus) is one of the most economically important fruit crops worldwide. To elucidate the molecular basis related to soluble sugar and organic acid metabolism in the fruits of two oriental melon cultivars with different sweetness, we performed integrated metabolomic and transcriptomic analyses of the fruits of ‘Tianbao’ (A) with high sweetness and ‘Xiaocuigua’ (B) with low sweetness at different ripening stages. The high accumulation of sucrose, D-glucose, D-(+)-raffinose, and the relatively lower citric acid and malic acid might contribute to the sweet taste of A. By screening the differentially expressed genes (DEGs) and correlation analysis of the DEGs and differentially accumulated metabolites, we deduced that the B cultivar might promote the conversion of glucose and fructose into intermediate compounds for downstream processes such as glycolysis. The tricarboxylic acid (TCA) cycle might also be enhanced compared to A, thus resulting in the differential accumulation of soluble sugars and organic acids, ultimately causing the taste difference between the two oriental melon cultivars. Our finding provides important information for further exploring the metabolic mechanisms of soluble sugars and organic acids in oriental melon.

Efficient D-allulose synthesis under acidic conditions by auto-inducing expression of the tandem D-allulose 3-epimerase genes in Bacillus subtilis

BackgroundD-allulose, a hexulose monosaccharide with low calorie content and high sweetness, is commonly used as a functional sugar in food and nutrition. However, enzyme preparation of D-allulose from D-frutose was severely hindered by the non-enzymatic browning under alkaline and high-temperature, and the unnecessary by-products further increased the difficulties in separation and extraction for industrial applications. Here, to address the above issue during the production process, a tandem D-allulose 3-epimerase (DPEases) isomerase synergistic expression strategy and an auto-inducible promoter engineering were levered in Bacillus subtilis 168 (Bs168) for efficient synthesis of D-allulose under the acidic conditions without browning.ResultsFirst, based on the dicistron expression system, two DPEases with complementary functional characteristics from Dorea sp. CAG:317 (DSdpe) and Clostridium cellulolyticum H10 (RCdpe) were expressed in tandem under the promoter HpaII in one cell. A better potential strain Bs168/pMA5-DSdpe-RCdpe increases enzyme activity to 18.9 U/mL at acidic conditions (pH 6.5), much higher than 17.2 and 16.7 U/mL of Bs168/pMA5-DSdpe and Bs168/pMA5-RCdpe, respectively. Subsequently, six recombinant strains based on four constitutive promoters were constructed in variable expression cassettes for improving the expression level of protein. Among those engineered strains, Bs168/pMA5-PspoVG-DSdpe-PsrfA-RCdpe exhibited the highest enzyme activity with 480.1 U/mL on fed-batch fermentation process in a 5 L fermenter at pH 6.5, about 2.1-times higher than the 228.5 U/mL of flask fermentation. Finally, the maximum yield of D-allulose reached as high as 163.5 g/L at the fructose concentration (50% w/v) by whole-cell biocatalyst.ConclusionIn this work, the engineered recombinant strain Bs168/pMA5-PspoVG-DSdpe-PsrfA-RCdpe was demonstrated as an effective microbial cell factory for the high-efficient synthesis of D-allulose without browning under acidic conditions. Based on the perspectives from this research, this strategy presented here also made it possible to meet the requirements of the industrial hyper-production of other rare sugars under more acidic conditions in theory.