Sensory, structural breakdown, microstructure, salt release properties, and shelf life of salt-coated air-dried yellow alkaline noodles

Yellow alkaline noodles (YAN) are a staple in many Asian countries and typically include wheat flour, water, salt, and kansui, which impart yellow color and firm, elastic texture. Excessive dietary sodium intake is linked to increased risk of cardiovascular and kidney diseases; many populations exceed WHO recommendations for salt intake, and processed foods such as instant noodles contribute substantially. Reducing salt in noodles must maintain texture and acceptability, yet salt affects processing, sensory quality, and shelf life. Hylon VII (high-amylose corn starch) and Semperfresh (a GRAS sucrose polyester/cellulose/glyceride mixture) are edible coating systems previously used in cereals and produce. The study hypothesizes that applying salt at the noodle surface via HC or SC coatings enhances in-mouth salt release during mastication, preserving perceived saltiness while reducing total sodium. The objective is to evaluate how salt coatings (HC or SC; 0–30% NaCl) influence sodium content, sensory attributes, structural breakdown, microstructure, in vivo salt release (salivary conductivity and pH), and shelf life of air-dried YAN under refrigerated and room-temperature storage.

Salt reduction strategies include partial replacement with alternative salts, designing food structures to optimize sodium release, adding saltiness enhancers, modifying salt physical form, and controlling sodium absorption. Prior work indicated that surface salt can enhance dissolution and perceived saltiness by increasing dissolution rate in saliva, with parallels from potato crisps. Yeoh et al. previously showed that salt-coated YAN (HC and SC coatings with 10–30% NaCl) improved textural, mechanical, and sensory hardness/springiness, with optimal performance at 10% salt. Literature also shows NaCl strengthens gluten, influences starch gelatinization, and affects noodle microstructure and color, while kansui promotes protein cross-linking and firm networks. Shelf life of fresh noodles is typically short due to high moisture; temperature and pH strongly affect microbial growth and product deterioration. Edible coatings can extend shelf life by limiting moisture transfer and microbial proliferation.

Materials: Wheat flour, salt, kansui (9:1 sodium/potassium carbonate), Hylon VII, Semperfresh, and analytical-grade reagents for artificial saliva and analyses. Deionised water used throughout.

Noodle preparation: Base unsalted YAN dough was made from 100 g wheat flour, 50 g water, and 1 g kansui; noodles were formed and divided for coating groups. Commercial YAN (reference) was prepared with 1.75 g salt per 100 g flour equivalent.

Coatings: HC solution prepared as 15% w/v Hylon VII; NaCl added at 0, 10, 20, or 30% w/v; slurry boiled with stirring for 30 min and autoclaved at 110 °C for 30 min; noodles immersed 1 min and air-dried 6 h at 30 °C. SC solution prepared as 5% v/v Semperfresh with NaCl at 0, 10, 20, or 30% w/v; stirred magnetically; noodles immersed 1 min and air-dried 6 h at 30 °C. Sample designations: HC-Na0/10/20/30 and SC-Na0/10/20/30.

Cooking time: Determined per AACC 66-50 (disappearance of white core) with 1:20 noodle:water ratio.

Sodium content: Freeze-dried samples (200–250 mg) digested by microwave, measured by FAAS (589 nm). n=3 per type.

Sensory evaluation: 30 trained panelists (20–40 years) assessed cooked samples (served in soup) on 7-point hedonic scale for taste and saltiness. Randomized presentation; commercial YAN as reference.

Structural breakdown (MEC): Multiple extrusion cell on TA-TX2 texture analyzer, 20 g noodles in 10 mL artificial saliva (NaHCO3, K2HPO4, NaCl, KCl, CaCl2·2H2O, xanthan gum, alpha-amylase; pH 7.0). Test: 20 compression cycles at 10 mm/s. Parameters from exponential decay: Work 1st (total work to break intact noodles), winf (work at infinite cycles), w1 (work associated with degradable structures), n1 (decay rate). n=3.

Digital microscopy and SEM: Cross-sections of raw and cooked noodles (selected 0, 10, 30% salt) imaged at 100× (digital microscope) and SEM (Pt-Pd coated, 100×). Commercial YAN as reference.

In vivo salivary conductivity and pH: 10 healthy subjects (5F/5M, 26–37 y) attended 12 sessions; chewed samples for 0, 5, 10, 15, 20 chews; saliva collected immediately at each interval using syringe/tube method; diluted 1000×; conductivity measured with handheld meter; pH with calibrated pH meter. Blank (no noodles) condition included; commercial YAN as reference.

Shelf life study: Selected 10% salt coatings (HC-Na10, SC-Na10) and unsalted (HC-Na0, SC-Na0) stored in bags at 4 °C (refrigerator) up to 29 days and 25 °C (incubator) up to 4 days; commercial YAN prepared as reference. Microbiological analyses: TPC, coliforms, yeast & mould (Petri film; incubation 37 °C for TPC/coliform 48±2 h; 25 °C for Y&M 5 days), reported as log CFU/g. pH measured with pH meter; color L* measured with spectrophotometer. n=3 per timepoint.

Statistics: Mean ± SD; ANOVA with Tukey’s test; significance at P<0.05 (SPSS v26). Ethics approval obtained; informed consent for human measures.

• Sodium content of cooked noodles increased with coating salt level: HC-Na30 3253 mg/kg and SC-Na30 3006 mg/kg were highest; HC-Na0 450 mg/kg and SC-Na0 403 mg/kg were lowest. All coated noodles contained <50% sodium of commercial YAN (7307 mg/kg, ~1.75% w/w).
• Sensory (7-point hedonic): Taste and saltiness scores increased with salt concentration (P<0.05). HC-Na30 and SC-Na30 had the highest taste and saltiness, approaching commercial YAN; HC-Na0 and SC-Na0 were least liked for taste/saltiness.
• Structural breakdown (MEC): Work 1st ranked HC/SC-Na10 > Na20 > Na30 > Na0. Representative values: Work 1st (J): HC-Na10 2.46±0.13; SC-Na10 2.41±0.06; HC-Na0 1.91±0.04; SC-Na0 1.86±0.08; YAN 0.98±0.02. w1 (J): HC-Na10 1.61±0.07; SC-Na10 1.60±0.03; HC-Na0 1.20±0.05; SC-Na0 1.19±0.04; YAN 1.17±0.04. n1 (J/cycle): highest in Na10 (HC-Na10 4.90±0.05; SC-Na10 4.87±0.02), lowest in Na0. These indicate Na10 noodles were hardest/firmest and required most work to break down, consistent with prior TPA/tensile data.
• Microstructure: Digital microscopy and SEM showed coatings present on surfaces of raw and cooked HC/SC. Noodles with 10% salt displayed denser, more continuous matrices with fewer voids; 30% salt showed ruptured surfaces; 0% salt exhibited larger hollows/voids after cooking, consistent with higher cooking loss and lower mechanical strength previously reported.
• In vivo salivary measures: Conductivity increased with number of chews and salt level: HC-Na30 > SC-Na30 > HC-Na20 > SC-Na20 > HC-Na10 > SC-Na10 > HC-Na0 > SC-Na0. Maximum values at 20 chews (P<0.05), indicating extensive salt release from coatings. Salivary pH also increased slightly with chew count and salt level (range ~6.74–7.21; highest at 20 chews). Commercial YAN yielded higher pH than all coated samples, consistent with higher salt and stimulated salivation.
• Shelf life at 4 °C: Microbial growth slowed vs 25 °C. TPC reached 5 log CFU/g by Day 8 for HC-Na0 and SC-Na0; exceeded 5 log CFU/g by Day 15 for HC-Na10 and SC-Na10. Coliform counts remained <2 log CFU/g through Day 29 for all samples. Yeast & mould exceeded 5 log CFU/g by Day 15 for HC-Na0 and SC-Na0; later for Na10 samples. Approximate shelf life at 4 °C: HC-Na0/SC-Na0 <8 days; HC-Na10/SC-Na10 >8 days.
• Shelf life at 25 °C: TPC and coliforms exceeded limits by Day 1 for all samples. Yeast & mould exceeded 5 log CFU/g by Day 1 for Na0 and by Day 2 for Na10. Shelf life <1 day at 25 °C.
• pH during storage: At 4 °C, pH trends reflected microbial metabolism and potential protein decomposition products; at 25 °C, pH declined continuously over 4 days, indicating rapid deterioration. L* (lightness) decreased over storage at both temperatures, consistent with enzymatic and non-enzymatic browning. Refrigeration mitigated changes in microbiology, pH, and L*.
• Overall, concentrating salt on noodle surfaces enhanced in-mouth salt release and perceived saltiness while total sodium remained substantially lower than commercial YAN. A 10% salt coating optimized structure and shelf life, whereas 30% damaged structure.

The findings support the hypothesis that surface-applied salt via edible coatings enhances the rate of sodium release into saliva during mastication, improving perceived saltiness despite lower total sodium content. Both HC and SC matrices act as carriers for NaCl at the noodle surface, enabling rapid dissolution and ion availability with chewing, as evidenced by increasing salivary conductivity and pH up to 20 chews. Sensory data showed saltiness and taste perception tracked coating salt level; the 30% coatings best matched commercial YAN in saltiness while maintaining about half the sodium content.

Structurally, a 10% coating level reinforced noodle integrity: MEC parameters (higher Work 1st and w1) and microscopy revealed denser, more continuous matrices, suggesting NaCl-mediated gluten strengthening and reduced starch swelling. Conversely, 30% salt induced microstructural rupture and porosity, likely facilitating excessive water penetration and compromising integrity during cooking.

Shelf-life outcomes indicate that modest salt at the surface (10%) can retard microbial growth relative to unsalted coatings under refrigeration, extending usable life beyond 8 days at 4 °C, while room temperature storage remains unsuitable (<1 day). Refrigeration markedly slowed microbial proliferation and physicochemical changes (pH and lightness). Together, these results demonstrate a practical pathway to reduce sodium in YAN without sacrificing sensory quality and with added shelf-life benefits at appropriate storage temperatures.

This study demonstrates, for the first time, the application of salt surface-coatings (HC and SC) to YAN as a means to reduce overall sodium while maintaining saltiness perception and improving structural properties and shelf life. Concentrating salt on the noodle surface accelerates in-mouth release and enhances perceived saltiness. A 10% salt coating (HC-Na10 or SC-Na10) optimizes structural robustness (highest MEC work parameters), provides acceptable sensory attributes, and extends refrigerated shelf life (>8 days), whereas higher levels (30%) compromise microstructure. The approach offers a viable strategy to produce lower-sodium noodles with quality comparable to commercial products.

Future research should: (i) optimize coating composition and thickness for minimal sodium with maximal sensory equivalence; (ii) explore alternative or combined salt replacers and flavor enhancers within coatings; (iii) assess broader sensory attributes and consumer acceptability across demographics; (iv) investigate applicability to other noodle types and starch matrices; and (v) evaluate storage stability over longer periods and supply-chain conditions.

• The in vivo salivary study involved a small, homogeneous sample (n=10, university-based adults), which may limit generalizability.
• Sensory evaluation used trained/experienced panelists (n=30) and assessed only taste and saltiness; broader consumer testing and additional sensory modalities were not conducted.
• Only one noodle formulation base and two coating systems were tested; results may not directly extrapolate to other formulations or processing conditions.
• High salt levels (20–30%) showed structural damage; mechanistic analyses of coating–matrix interactions at high ionic strengths were limited.
• Shelf-life assessment was limited to up to 29 days at 4 °C and 4 days at 25 °C; real-world packaging and distribution conditions were not fully simulated.
• Only NaCl was used; comparisons with salt substitutes or blends in coatings were not included.