Effects of wall materials on the physicochemical properties of spray-dried bitter gourd (*Momordica charantia* L.) powders

Bitter gourd (Momordica charantia L.) is traditionally used in Chinese medicine for conditions such as heat stroke, fever, dysentery, gastrointestinal issues, and inflammation. Modern studies indicate hypoglycemic, anti-inflammatory, antioxidant, anticancer, antimalarial, anti-HIV, and immunoregulatory activities, attributable to its nutrients and bioactive compounds (saponins, polysaccharides, polyphenols, flavonoids, essential oils). However, the fruit’s strong bitterness—linked to triterpenoid saponins such as momordicine I and momordicosides—limits consumer acceptance and overall use, and fresh fruit has a short shelf life. To enhance usability and health benefits, various bitter gourd products have been developed, yet bitterness remains inadequately masked. Spray drying is widely used in the food industry for drying and encapsulation. Common wall materials include starches, lipids, and proteins; prior studies have shown gum arabic (GA), soybean protein isolate (SPI), maltodextrin (MD), sodium alginate, whey protein, and a lecithin-calcium caseinate mixture (LCC) can influence bitterness masking and physicochemical properties. Research gaps include limited systematic comparisons of different wall materials for bitterness masking and retention of functional components in bitter gourd. The purpose of this study was to identify optimal wall materials for spray-dried bitter gourd powders (SD-BGP) by comparing their effects on bitterness, physicochemical attributes, and retention of bioactives, laying groundwork for bitterness encapsulation technology that reduces bitterness and improves active substance retention.

Prior work demonstrates encapsulation and drying aids can modulate quality and bioactive retention. Whey protein isolate and maltodextrin co-encapsulation protected active compounds during storage in paprika and cinnamon oleoresins (Ferraz et al.). MD as wall material reduced hygroscopicity and maintained acceptable color with potent antioxidant activity in spray-dried amla (Mishra et al.). Whey protein concentrate as wall material reduced bitter taste and hygroscopicity of whey protein hydrolysate (Ma et al.). GA attenuated bitterness of hydrolyzed casein in spray-dried products (Subtil et al.). MD and GA encapsulated aqueous bitter melon extract, protecting bioactives (Tan et al.). Despite these, comprehensive comparisons across multiple wall materials for bitter gourd powders—especially regarding bitterness-masking efficacy and bioactive retention—are scarce, and encapsulation wall material choices for bitter gourd have been limited.

Materials: Fresh bitter gourd variety 'Lvbaoshi' sourced in Guangzhou, China. Food-grade wall materials: soybean protein isolate (SPI), gum arabic (GA), maltodextrin (MD), resistant starch (RS), and a lecithin-calcium caseinate mixture (LCC). LCC was prepared by mixing soybean lecithin and calcium caseinate at 1.5:1.0 and homogenizing. Preparation of feed: Fruits washed, deseeded, cut and crushed to slurry (90 s). Each wall material was mixed with bitter gourd slurry at a 1:1 ratio on a solids basis. Total solids adjusted to 9%. The feed was beaten at 14,000 rpm for 15 min on ice, then homogenized at 30 MPa for 5 min. Spray drying: Conducted on LPG-5 spray dryer at inlet air temperature 130 °C, atomization speed 100 rpm, feed rate 2 rpm. Powders collected, sealed, stored at −20 °C. Physicochemical analyses: - Moisture content (drying oven at 105 °C to constant weight; triplicates). - Water activity (LabSwift-aw at 25 °C, 30 min; triplicates). - Water-soluble index (WSI): 2.5 g sample dispersed in 30 mL water, vortexed, centrifuged (5000 rpm, 10 min), supernatant dried to constant weight for WSI calculation. - Instant solubility/dispersion time: 50 mg dissolved in 1 mL deionized water; time to complete reconstitution measured (triplicate). - Color: UltraScan VIS colorimeter for L*, a*, b*, chroma, hue angle; images captured. - Morphology: SEM (Hitachi SU-70), 200×, gold sputter-coating, 2 kV. - Particle size distribution: MS 3000 laser analyzer (dry), refractive index 1.52, absorptivity 0.1; volume-weighted D[4,3] and span. - Hygroscopicity: 5 g powder at 25 °C and 75% RH (NaCl saturated solution) over 0–120 h with periodic weighing; moisture uptake curves. Bitterness evaluation: - Taste dilution analysis (TDA): SD-BGP (encapsulated) at 50 mg/mL; OD-BGP (oven-dried) and NE SD-BGP (nonencapsulated) at 25 mg/mL; serial 1:1 dilutions with triangular tests; TD value recorded. - Electronic tongue (Astree): Extracts prepared in water; PCA performed; taste intensities (bitterness, sourness, sweetness, umami, saltiness) measured. Bioactive retention and functional assays: - Total phenols: Folin–Ciocalteu; retention ratio calculated relative to pre-drying content. - Total flavonoids: NaNO3/AlCl3/NaOH colorimetric method; retention ratio calculated. - Total saponins: Vanillin–perchloric acid colorimetry; retention ratio calculated. - Vitamin C: AOAC 984.26 fluorometric method (o-phenylenediamine); retention ratio calculated. - Antioxidant activity: ORAC assay (Trolox equivalents, µmol TE/g DW). - α-Glucosidase inhibitory activity: PNPG assay; IC50 values determined; acarbose as positive control. Statistics: All measurements in triplicate; results as mean ± SD. One-way ANOVA with Duncan’s test (SPSS 18.0); significance at p<0.05.

- Moisture and water activity (Table 1): NE SD-BGP had highest moisture (9.67%) and aw (0.32). All wall materials significantly reduced both. Moisture content descending after encapsulation: RS (7.13%), LCC (5.83%), SPI (5.07%), MD (4.10%), GA (4.01%). Water activity descending: LCC (0.27), SPI/RS (0.25), GA (0.22), MD (0.21). - Solubility and dispersion (Table 2): NE WSI 37.6%. Wall materials increased WSI to 40.5–84.8%. GA (80.5%) and MD (84.8%) highest WSI but had poorer dispersibility: dispersion times GA 256 s and MD 55.43 s vs SPI 4.89 s, LCC 5.82 s, RS 1.20 s, NE 2.53 s. - Color (Table 3): Wall materials increased lightness (L*), yellowness (b*), and chroma, and reduced redness (a*). Protein-based materials, especially SPI, best preserved greenish hue; RS powders were more yellowish. - Morphology (SEM): NE showed globular particles with aggregation. Encapsulation reduced aggregation and produced intact microcapsules. MD and RS yielded predominantly globular particles (RS slightly aggregated). GA particles were umbilicated/globular. SPI and LCC yielded umbilicated, irregular, wrinkled surfaces with larger particle sizes—typical of protein-based walls. - Particle size distribution (Fig. 3a, Table 4): D[4,3] ranged 50.43–102.86 µm; span 1.06–2.27. SPI had largest D[4,3] (102.86 µm); RS had largest span (2.27). GA and MD produced smaller, more uniform particles (spans ~1.06–1.07). Larger, less uniform particles (SPI/LCC/RS) associated with better dispersibility. - Hygroscopicity (Fig. 3b): LCC showed greatest reduction in hygroscopicity, followed by SPI; MD and RS also reduced hygroscopicity vs NE. GA decreased hygroscopicity only initially (first 24 h) but ultimately led to higher hygroscopicity than NE. - Bitterness (TDA, Fig. 4): OD-BGP most bitter; NE SD-BGP strongly bitter. Protein walls reduced TD markedly: LCC TD=2 (8× lower than NE), SPI TD=4 (4× lower). RS TD=8; GA and MD showed no significant TD decrease vs NE. - Electronic tongue (PCA and intensities, Fig. 5, Table 6): PC1+PC2=96.24%; PDI=98% indicated clear discrimination. Bitterness intensity order (low→high): LCC < SPI < RS < GA < MD < NE < OD. Bitterness-masking ratios vs OD and NE: LCC 70.53% and 61.64%; SPI 56.84% and 43.84%. GA showed moderate masking; MD lowest among walls. - Retention ratios of bioactives (Fig. 7): • Total phenols: Increased after spray drying. NE increased most (346.00%). Among walls, LCC highest (249.76%), GA≈MD intermediate; RS lower; SPI lowest among walls. • Total flavonoids: Decreased overall. NE 38.99%. Encapsulation improved retention: SPI 75.53% (highest), GA 71.37%, LCC 53.11%, RS 49.01%, MD lowest and not significantly different from NE. • Total saponins: Decreased overall. NE 52.21%. Encapsulation improved retention: LCC 74.00% (highest), MD 70.38%, SPI 65.83%, RS 61.13%, GA 57.68%. • Vitamin C: NE 1.28% (nearly lost). Encapsulation protected vitamin C: LCC 51.69% (highest), SPI 29.17%, GA 26.85%, RS 24.48%, MD 20.97%. - Antioxidant capacity (ORAC, Fig. 8): Fresh 4348.78 µmol TE/g DW; NE increased to 11746.15. SPI highest 22209.76; LCC next highest; GA and MD similar to fresh; RS lowest 2075.19. Order (low→high): RS < Fresh < GA ≈ MD < NE < LCC < SPI. - α-Glucosidase inhibition (Fig. 9, Table 7): IC50 (mg/mL): acarbose 2.14; fresh 3.08; NE 27.56 (lowest activity). Among walls: SPI 5.20 (highest activity among SD-BGPs), LCC 8.73, MD 17.43, GA 17.46, RS 17.82. Inhibition correlated with saponin retention.

Encapsulation with selected wall materials reduced moisture and water activity below thresholds (moisture <6%, aw <0.3 for most walls), helping to inhibit biochemical and microbial reactions and potentially extending shelf life. Film formation during spray drying (rapid migration of wall materials to the droplet surface) promotes water removal. Particle morphology and size influenced reconstitution: GA and MD produced smaller, more uniform particles with lower moisture, enhancing solubility (higher WSI) but impairing dispersibility due to increased interparticle contact; SPI/LCC/RS formed larger, wrinkled particles, improving dispersibility. Protein-based walls, especially SPI and LCC, better preserved green color via rapid surface film formation and protection of pigments. Hygroscopicity was lowest with LCC and SPI; GA’s highly hydrophilic, ramified polysaccharide structure led to long-term higher hygroscopicity relative to NE. Bitterness reduction was strongest with protein-based walls (LCC > SPI), likely due to elastic protein films, wrinkled/umbilicated particle surfaces, larger particle sizes, and interactions that slow release of hydrophobic bitter saponins. RS also reduced bitterness via lower solubility and slower release. High solubility of GA/MD likely promoted bitter compound release, explaining weaker masking. Electronic tongue data aligned with sensory TDA, indicating robust taste differentiation among samples. Encapsulation improved retention of key bioactives (flavonoids, saponins, vitamin C) relative to NE powders; LCC best protected saponins and vitamin C, while SPI best preserved total flavonoids. Total phenols increased after spray drying, especially in NE, possibly due to release of bound phenolics; encapsulation moderated this increase by entrapping phenolics. Enhanced retention with protein-based walls supported higher ORAC indices (SPI, LCC), with potential contributions from intrinsic antioxidant properties of proteins or associated components. α-Glucosidase inhibitory activity of SD-BGPs tracked with saponin retention, with SPI and LCC maintaining the highest activities among encapsulated samples. Differences from prior studies (e.g., GA vs MD effects) may stem from differences in core material (slurry vs extract) and process specifics.

All five wall materials (SPI, GA, MD, RS, LCC) improved key quality attributes of spray-dried bitter gourd powders by reducing moisture, water activity, browning, agglomeration, and perceived bitterness, while enhancing solubility to varying degrees. Protein-based walls were most effective overall: SPI best preserved color, maximized antioxidant capacity and α-glucosidase inhibitory activity, and provided the highest retention of total flavonoids; LCC most effectively reduced hygroscopicity and bitterness, and provided the highest retention of total saponins and vitamin C. GA and MD enhanced solubility but were less effective at bitterness masking; GA showed long-term higher hygroscopicity. RS improved dispersibility and reduced bitterness moderately. Findings guide selection of wall materials for specific product goals and may be extended to other fruit/vegetable powders with similar characteristics. Future work could optimize wall material combinations and spray-drying conditions to further improve bitterness masking and bioactive retention, and evaluate storage stability and sensory acceptance in consumer products.

The study evaluated five wall materials at a fixed wall-to-core solids ratio (1:1) and a single spray-drying condition set (e.g., inlet 130 °C), which may limit generalizability across different formulations or process parameters. Comparisons with prior literature may be affected by differences in core material (whole-fruit slurry here vs extracts elsewhere). Long-term storage stability, release kinetics in complex food matrices, and comprehensive sensory consumer testing were not reported.