Influence of Fermentation on Functional Properties and Bioactivities of Different Cowpea Leaf Smoothies during In Vitro Digestion.

This study investigated the effects of Lactiplantibacillus plantarum 75 (LAB 75) fermentation at 37 °C for 48 h on the pH, total soluble solids (TSS), colour, total titratable acidity (TTA), carotenoids, and bioactivities of cowpea leaf smoothies from three cultivars (VOP 1, VOP 3, and VOP 4). Fermentation reduced the pH from 6.57 to 5.05 after 48 h. The TTA increased with the fermentation period, whilst the TSS reduced. Fermentation of the smoothies resulted in the least colour changes (∆E) in VOP 1 after 48 h. Fermentation of cowpea smoothies (VOP 1, VOP 3, and VOP 4) improved the antioxidant capacity (FRAP, DPPH, and ABTS), which was attributed to the increase in total phenolic compounds and carotenoid constituents in all of the fermented cowpea smoothies. VOP 1 was further selected for analysis due to its high phenolic content and antioxidant activity. The VOP 1 smoothie fermented for 24 h showed the lowest reduction in TPC (11%) and had the highest antioxidant (FRAP, DPPH, and ABTS) activity. Ltp. plantarum 75 was viable and survived the harsh conditions of the gastrointestinal tract, and, hence, could be used as a probiotic. VOP 1 intestinal digesta showed significantly higher glucose uptake relative to the undigested and the gastric digesta, while the gastric phase had higher levels of α-amylase and α-glucosidase compared to the undigested samples.

Changes in antinutrients, phenolics, antioxidant activities and in vitro α-glucosidase inhibitory activity in pumpkin leaves (Cucurbita moschata) during different domestic cooking methods.

Pumpkin leaves (Cucurbita moschata) were subjected to different household cooking methods (boiling, microwaving, steaming, and stir-frying) to evaluate their effect on antinutrients, phenolic compounds, antioxidant properties (ABTS, and DPPH) and in vitro α-glucosidase activity. All cooking methods studied significantly reduced the antinutrients and antioxidant activities, whilst phenolic compounds p-coumaric and ferulic acids significantly increased. The cooking methods reduced the oxalates by more than 50%, tannins by 47% and phytates by 79.22%. Steaming and boiling resulted in highest concentrations of p-coumaric (195.40 mg kg-1) and ferulic acids (103.90 mg kg-1) compared to other methods. Overall, boiled leaves retained the highest total phenolic compounds, whilst steamed leaves retained the highest antioxidant capacity. Raw pumpkin leaf extracts showed higher in vitro α-glucosidase inhibitory effects than the cooked leaves. Thus, cooking affected the inhibitory effect of in vitro α-glucosidase activity. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10068-021-00916-w.

Influence of Different Types of Drying Methods on Color Properties, Phenolic Metabolites and Bioactivities of Pumpkin Leaves of var. Butternut squash (Cucurbita moschata Duchesne ex Poir).

Leaves of pumpkin species var. Butternut squash (Cucurbita moschata Duchesne ex Poir) is a popularly consumed leafy vegetable in the Southern African region. Traditional vegetables are commonly sun-dried as a method of postharvest preservation during the off-season. However, different drying methods affect the superior quality, functional properties, and bioactivities of the final product. Therefore, in this study, var. Butternut squash (C. moschata) underwent different drying methods, such as freeze-, oven, sun-, solar, and microwave drying to evaluate the color properties, pigments, phenolic metabolites, in vitro antioxidants, and antidiabetic activities. Results indicate that freeze-drying retained the total chlorophyll content with green color by reducing the color difference (ΔE), improved the concentration of different phenolic metabolites and the content of ascorbic acid, and enhanced the FRAP, ABTS activities and the inhibitory effects of α-glucosidase, and α-amylase. Freeze-dried leaves contained the highest concentrations of quercetin 3-glucoside 7-rhamnoside (rutin), quercetin 3-galactoside, isorhamnetin-3-galactoside-6″-rhamnoside, isorhamnetin-3-O-rutinoside compared with the leaves that underwent four other drying treatments and raw leaves. The OPLS-DA and the UPLC-QTOF/MS and chemometric approach showed that the peak at m/z 609, 1441 (quercetin 3-galactoside 7-rhamnoside) separated the freeze-dried leaves of var. Butternut squash (C. moschata) from the other four drying treatments. Therefore, freeze-drying is highly recommended to obtain good quality leaf powders that are rich in functional compounds and bioactive properties for use as functional ingredients.

Changes in Phenolic Metabolites and Biological Activities of Pumpkin Leaves (Cucurbita moschata Duchesne ex Poir.) During Blanching.

Pumpkin leaves (Cucurbita moschata Duchesne ex Poir.) are popularly consumed in Sub-Saharan Africa and Asia. Blanching the leaves before drying is a method of preservation during off-season. In this study, different blanching treatments and media are used to test the changes in non-targeted phenolic compounds, antioxidant capacity (FRAP and ABTS activity), in vitro α-glucosidase activity and cell cytotoxicity of pumpkin leaves. Steam blanching in plain water led to the highest retention of total phenolic content and reduced the loss of quercetin 3-glucoside 7-rhamnoside (Rutin), kaempferol 7-neohesperidoside, isoorientin 2″-O-rhamnoside, isorhamnetin-3-O-rutinoside, quercetin 3-galactoside, coumaroyl glucaric acid, isorhamnetin-3-galactoside-6″-rhamnoside, 2-caffeoylisocitric acid, quercetin 3-galactoside 7-rhamnoside by (3.04%), (7.37%), (10.65%), (10.97%), (14.88%), (16.1%), (16.73%), (18.88%), and (23.15%), respectively, and coumaroyl isocitrate increased by 14.92%. Candidate markers, 2-O-caffeoylglucaric acid, 2-(E)-O-feruloyl-D-galactaric acid, quercetin 3-galactoside 7-rhamnoside (rutin) and unidentified compounds ([(M-H) 677.28 and at RT 21.78] were responsible for the separation of the steam blanched samples in plain water from the other blanching treatments. Steam blanching in plain water increased the antioxidant capacity (FRAP and ABTS activity). There were no cytotoxic effect or inhibitory effect of α-glucosidase activity detected in the raw or blanched pumpkin leaves. Thus, this study recommends steam blanching in plain water for African cuisine, and confirms it is safe to consume pumpkin leaves frequently.

Comparison of Phenolic Compounds, Carotenoids, Amino Acid Composition, In Vitro Antioxidant and Anti-Diabetic Activities in the Leaves of Seven Cowpea (Vigna unguiculata) Cultivars.

Cowpea is a well-known nutrition rich African leafy vegetable that has potential to sustain food and nutrition insecurity in sub-Saharan Africa. Consumption of cowpea legumes is associated with reduced risk of type 2 diabetes mellitus. Therefore, the present study was designed to evaluate the (i) variation in phenolic metabolites in seven cowpea cultivars (VOP1, VOP2, VOP3, VOP4, VOP5, VOP7, and VOP8 using UHPLC coupled with high resolution Q-TOF-MS technique, (ii) in vitro antioxidant activity using ferric reducing/antioxidant capacity (FRAP) assay (iii) in vitro anti-diabetic effects and (iv) composition of carotenoids and amino acids of theses cowpea cultivars. The results of this study demonstrated that gentisic acid 5-O-glucoside, quercetin 3-(2G-xylosylrutinoside) and Quercetin 3-glucosyl-(1->2)-galactoside were highest in VOP1 VOP4 and VOP5, respectively. High inhibition (>50%) of α-glucosidase and α-amylase activities was shown by the leaf extracts (50 and 25 mg/mL) of VOP1 and VOP4. Cowpea cultivars VOP1 and VOP4 demonstrated the highest gene expression levels of regulation of glucose transporter GLUT4 in C2C12 skeletal muscle cells, similar to insulin. A positive correlation exited between the phenolic components and the inhibitory effect of antidiabetic enzymes and FRAP activity. Cytotoxic effect was not detected in vitro in any cowpea cultivar. Lutein (124.6 mg/100 g) and all-trans-beta-carotene (92.6 mg/100 g) levels were highest in VOP2 and VOP1, respectively. Cowpea cultivars VOP3 and VOP4 showed potential to fulfil the daily requirements of essential amino acids. Thus, based on this information, cowpea (leaves) genotypes/cultivars can be selected and propagated for the further development of supplementary foods or functional food ingredients.

Electrospun Carbon Nanofibers as an Electrochemical Immunosensing Platform for Vibrio cholerae Toxin: Aging Effect of the Redox Probe.

An electrochemical immunosensor for Vibrio cholerae toxin (VCT) has been developed using electrospun carbon nanofibers (CNFs) as the electrode platform. To fabricate the immunosensor, the anti-cholera toxin antibody (Ab) was covalently immobilized on the electrode platforms using the carbodiimide chemistry for the amide bond formation. Every step of the formation of the immunosensor and the subsequent binding of the VCT subunit antigen (Ag) was electrochemically interrogated. The immunosensor gave excellent reproducibility and sensitivities: limits of detection (ca. 1.2 × 10-13 g mL-1), limits of quantification (ca. 1.3 × 10-13 g mL-1), and a wide linear range for the anti-cholera detection of 8 orders of magnitude (10-13 to 10-5 g mL-1). One of the key findings was the enhanced sensitivity of the VCT detection using aged rather than the freshly prepared redox probe, described here as Redox Probe Aging-Induced Sensitivity Enhancement ("Redox-PrAISE"). The Redox-PrAISE was found more useful in the real application of these immunosensors, showing comparable or even better sensitivity for eight real cholera-infested water samples than the conventional clinical culture method. This immunosensor shows promise for the potential development of point-of-care diagnosis of VCT. Importantly, this study highlights the importance of considering the nature of the redox probe on the electrochemical sensing conditions when designing impedimetric immunosensors.

Impact of Household Cooking Techniques on African Nightshade and Chinese Cabbage on Phenolic Compounds, Antinutrients, in vitro Antioxidant, and ß-Glucosidase Activity.

Different household cooking techniques (boiling, steaming, stir frying, and microwave) were tested on the changes of targeted phenolic compounds, antioxidant property (ferric reducing-antioxidant power (FRAP) activity), α-glucosidase activity, antinutritive compounds, and sensory properties in commonly consumed traditional leafy vegetables in Southern Africa, the non-heading Chinese cabbage (Brassica rapa L. subsp. chinensis) and African nightshade (Solanum retroflexum Dun). Stir frying increased kaempferol-3-O-hydroxyferuloyl-trihexoside, kaempferol-dihexoside, sinapoyl malate, rutin, and isorhamnetin-O-dihexoside in Chinese cabbage leaves, followed by steaming. Similarly, stir frying increased kaempferol-3-O-rutinoside, chlorogenic acid, caffeoylmalic acid, and quercetin-3-O-xylosyl-rutinoside in nightshade, followed by steaming. Biomarkers, sinapoyl malate (Chinese cabbage) and caffeoylmalic acid (nightshade), separated the stir frying from the other cooking techniques. Steaming and stir-frying techniques significantly increased the FRAP activity; whereas boiling and microwaving reduced the tannin, oxalate, and phytate contents in both leafy vegetables and steroidal saponins in nightshade. Stir-fried nightshade leaf extract showed the most effective inhibition against α-glucosidase activity, with an IC50 of 26.4 µg ml-1, which was higher than acarbose, a synthetic compound (positive control; IC50 69.83 µg ml-1). Sensory panelists preferred the stir-fried Chinese cabbage and nightshade leaves, followed by steamed, microwaved, and boiled vegetables.