Abstract Thailand’s transition toward a super-aged society is increasing the demand for ready-to-eat (RTE) foods that meet the nutritional and texture needs of elderly consumers. This study developed a quinoa-fortified kale soup and evaluated the effects of formulation and retort sterilization on its physical, chemical, microbial, and sensory properties. Three kale-to-quinoa ratios (35:5, 30:10, and 25:15) were analyzed for protein content, chlorophyll retention, viscosity, CIE Lab* color, and total soluble solids. The 30:10 ratio was selected for sterilization at 121°C for 10, 15, and 20 min. All treatments achieved commercial sterility (F₀ = 4.43–15.22 min; <10 CFU/g). Sterilization increased lightness, yellowness, viscosity, and protein content, whereas chlorophyll decreased with increasing heating time. Sensory results indicated that the 10-min process best preserved flavor, color, and overall acceptability.

Overall, quinoa–kale soup sterilized at 121°C for 10 min yields a microbiologically safe, nutrient-dense, and easy-to-consume product suitable for elderly individuals with chewing or swallowing difficulties. The findings provide a practical framework for developing plant-based, elderly-friendly RTE foods with improved nutritional quality and shelf stability.

Keywords: Kale, Quinoa, Retort Pouch, Soup, Sterilization

Funding: The authors are grateful for the research funding provided by the Rajamangala University of Technology Thanyaburi (RMUTT), Pathum Thani, Thailand.

Citation:  Langkapin, J., Oupathumpanont, O., and Parnsakhorn, S. 2026. Development of a quinoa-fortified kale soup and optimized retort sterilization process for elderly nutrition. Natural and Life Sciences Communications. 25(2): e2026040.

Graphical Abstract:

INTRODUCTION

Thailand is undergoing a rapid demographic transition, with its elderly population projected to constitute approximately 28-30% of the total population by 2030-2033, marking the country’s entry into a super-aged society (Nation Thailand, 2023; World Health Organization, 2023; National Statistical Office of Thailand, 2025). This shift increases the urgency for targeted nutritional strategies to support the health, dietary needs, and quality of life in older adults. While exercise interventions such as chair stands and flexibility training have been shown to improve physical fitness in elderly populations (Fauro et al., 2022; Tessier et al., 2025), nutritional approaches to addressing age-associated declines in chewing, swallowing, and nutrient absorption remain underexplored.

Convenience foods, including ready-to-eat (RTE) and ready-to-heat (RTH) meals, are commonly consumed by the elderly but frequently exhibit high sodium levels, insufficient protein content, and unsuitable textures for those with masticatory or swallowing impairments (Jang and Lee, 2017). For example, retort-processed, branched-chain amino acid (BCAA) fortified meat-based mousses have demonstrated improvements in texture, color, and short-term stability (Lee and Shin, 2023); however, plant-based alternatives, which could offer additional nutritional and sensory benefits, are less developed within this sector.

Kale (Brassica oleracea L. var. sabellica) and quinoa (Chenopodium quinoa Willd.) are promising nutrient-dense candidates for elderly nutrition. Kale provides antioxidant phenolic compounds such as quercetin and kaempferol with potential cardiovascular benefits (Korus et al., 2007; Kim et al., 2008; Kasprzak et al., 2018). Quinoa supplies high-quality complete proteins and micronutrients and contains functional starches that improve viscosity and mouthfeel (Contreras-Jiménez et al., 2019; Alandia et al., 2020; Agarwal et al., 2023). Thermal processing can degrade some bioactive compounds in quinoa but can be optimized to maintain its nutritional quality (Sharma et al., 2022).

Retort sterilization at around 121°C is the standard for producing shelf-stable soups with ensured microbial safety (Catauro, 2012; Abhishek et al., 2014). However, such processing can negatively impact chlorophyll content and sensory properties in plant-based soups (Shaha et al., 2017; Benjakul et al., 2018; Ebrahimi et al., 2023). Most research to date focuses on meat- or single-ingredient-based soups, with limited studies on protein-enriched plant-based soups tailored for elderly consumers.

This study addresses these gaps by developing and optimizing a quinoa-fortified kale soup designed for Thailand’s elderly population. It assesses the effects of retort sterilization on physicochemical properties including color, chlorophyll retention, viscosity, protein content, microbial safety, and sensory acceptance. The findings aim to inform the production of nutritionally balanced, texture-appropriate, and microbiologically safe RTE soups meeting the unique dietary and sensory requirements of an aging society.

MATERIALS AND METHODS

Preparation of quinoa-fortified kale soup

Fresh kale was trimmed and blanched in a temperature-controlled water bath (LABTECH LWB-122D, South Korea) at 95–98 °C for 3 min, then immediately cooled in ice water, drained, and reserved. Quinoa was dry-roasted over low heat for 5–7 min, cooked in a 1:6 quinoa-to-water ratio by boiling and simmering for 10–15 min until tender, then cooled.

Formulations were developed through preliminary trials that screened kale–quinoa ratios and seasoning levels using processing stability, viscosity, chlorophyll retention, and sensory acceptance as selection criteria. The standardized salt (3%) and cream (24%) levels were retained because they provide controlled sodium per serving and a smooth, energy-dense texture suitable for older adults with chewing, swallowing, and age-related taste-perception decline (Strom et al., 2013; Aguilera and Park, 2016). The three ratios in Table 1 represent the most feasible range. Ingredients were weighed according to Table 1, mixed for 10 min, homogenized at 30,000 rpm for 15 min, and analyzed in triplicate for total dissolved solids, viscosity, chlorophyll content, color, and protein concentration.

Table 1. Kale-to-quinoa ratios used in the three soup formulations.

Sample preparation for retort pouch sterilization

Following preliminary evaluation, the quinoa-fortified kale soup formulation with optimal physical and nutritional quality was selected for retort sterilization. Portions (220 g) were packed in stand-up retort pouches (10 × 16 × 4 cm; Pathum Flex Packaging Co., Ltd., Thailand), degassed, and heat-sealed. For F₀ determination, a Type T thermocouple was inserted at one-third of the pouch height (coldest point), and the pouch was resealed.

Thermocouple-equipped pouches were sterilized in a high-pressure water spray retort (National Direct Network Co., Ltd., Thailand) at 121°C for 10, 15, and 20 min, with product and retort temperatures recorded at 1-min intervals using a data logger (National Instruments Model NI 9211, Hungary). Cooling was achieved by water spray under controlled pressure until ≤40°C. F₀ values and lethal rates were calculated using Patashnik’s (Shaha et al., 2017) method, Equations (1) and (2).

Where F₀ represents the cumulative thermal lethality expressed as the equivalent sterilization effect at 121°C; L is the microbial inactivation rate per unit time; T is the temperature at the product’s coldest point (°C); T_ref is the reference temperature (°C); z denotes the thermal resistance of the target microorganism (°C); and Δt is the time interval during heating.

Physicochemical properties

Microbial colony count

Colony counts were performed following Maturin and Peeler’s method as outlined in FDA BAM Online (Lee and Shin, 2023).

Chlorophyll Analysis

Chlorophyll a, chlorophyll b, and total chlorophyll were quantified via a modified Barros et al. (2011) method. Approximately 200 mg of fresh or dried sample was extracted with 10 mL acetone–hexane (4:6 v/v), stirred for 5 min, and filtered through No. 4 filter paper. The extract volume was adjusted to 10 mL. Absorbance at 645 nm and 663 nm was measured using a spectrophotometer (Model 6505 UV/VIS, Jenway Ltd., UK), and chlorophyll concentrations were calculated using Equations (3), (4), and (5). Where A₆₆₃ and A₆₄₅ are absorbance values at 663 nm and 645 nm, respectively; V is the volume of the extract (mL); and W is the sample weight (g).

Protein analysis

Protein content was determined using an in-house protocol adapted from AOAC (2019) and AOAC 991.20 (2000). The nutritional analysis was limited to protein enhancement and chlorophyll retention in line with the study objectives; other nutritional parameters (e.g., vitamins, minerals, dietary fiber, and lipids) were not included.

Total soluble solids

Total soluble solids were measured using a HITECH RHB Series refractometer and expressed as °Brix, indicating approximate sugar content.

Color measurement

Color parameters (L*, a*, b*) were recorded using a JC801 colorimeter (Tokyo, Japan). L* indicates lightness (0 = black, 100 = white), a* denotes red (+) to green (−), and b* denotes yellow (+) to blue (−). The device was calibrated with a standard white plate (L* = 98.11, a* = −0.11, b* = −0.08) before measurement.

Viscosity

Viscosity was measured using a Brookfield Viscometer DV II (USA) with spindle No. 3 at 10 rpm.

Sensory evaluation

The sensory evaluation was conducted with 30 untrained panelists aged 40–65 years, representing pre-elderly and elderly consumers relevant to the target population. Panelists were habitual consumers of vegetable-based soups and reported no sensory, olfactory, or swallowing impairments that could influence evaluation. Participants evaluated the quinoa-fortified kale soup samples, including the unsterilized K:Q (30:10) formulation and the samples sterilized at 121°C for 10, 15, and 20 min. A 9-point hedonic scale (1 = extremely dislike; 9 = extremely like) was used to assess appearance, color, flavor, texture, taste, and overall acceptability, following Mahattanatawee et al. (2012). All samples were served at 45–50 °C and presented in randomized order, with drinking water provided for palate cleansing between samples to minimize bias.

Statistical analysis

Sterilized soup samples were prepared in three independent batches and analyzed for physicochemical properties in triplicate (n = 3) to ensure reproducibility. After preparation, samples were cooled to room temperature (25 ± 2 °C) and stored at 4 ± 1 °C in airtight containers. Analyses were conducted after 7 days of storage. Data are presented as mean ± standard deviation. Statistical analysis was performed using one-way analysis of variance (ANOVA), followed by Duncan’s multiple range test at a 95% confidence level (P ≤ 0.05) to evaluate significant differences among treatments. All analyses were conducted using IBM SPSS Statistics version 27.

RESULTS

The soup, composed primarily of kale and quinoa, was formulated in three ratios: K:Q (35:5), (30:10), and (25:15) (Table 1). Each formulation was evaluated for color parameters (L*, a*, b*), chlorophyll content, total soluble solids, viscosity, and protein content, with results summarized in Table 2 and illustrated in Figures 1–2. One formulation was selected for subsequent heat sterilization testing.

Effect of quinoa fortification on color, chlorophyll content, and physicochemical properties of quinoa-fortified kale soup

Color value

The color parameters L*, a*, and b* of quinoa-fortified kale soup differed significantly (P ≤ 0.05) among formulations (K:Q 35:5, 30:10, and 25:15) (Figure 1). The L* value (lightness) was highest in K:Q 25:15, significantly exceeding those in K:Q 35:5 and K:Q 30:10. All a* values were negative, indicating dominant green hues. K:Q 35:5 and K:Q 30:10 showed significantly stronger greenness than K:Q 25:15. The b* (yellowness) value was significantly lowest in the K:Q 25:15 sample. Increasing the quinoa ratio from 35:5 to 25:15 significantly increased L* and b* and decreased a* (P ≤ 0.05), indicating dilution of green pigments due to reduced contributions of kale chlorophylls and carotenoids.

Figure 1. Color changes (L*, a*, b*) across three formulations of quinoa-fortified kale soup. ab different letters in each test condition were significantly different (P ≤ 0.05) when compared by DMRT (mean ± SD).

Chlorophyll content

In kale soup fortified with quinoa at ratios of 35:5, 30:10, and 25:15 (K:Q), chlorophyll a, chlorophyll b, and total chlorophyll significantly decreased (P ≤ 0.05) with increasing quinoa content (Figure 2). The highest chlorophyll total occurred in the 35:5 formulation, while the lowest was observed in 25:15. This decline confirms quinoa’s dilution of chlorophyll pigments, with 35:5 retaining the greatest phytochemical potential and 30:10 showing intermediate pigment preservation.

Figure 2. Changes in chlorophyll a, chlorophyll b, and total chlorophyll across three quinoa-fortified kale soup formulations. ab different letters in each test condition were significantly different (P ≤ 0.05) when compared by DMRT (mean ± SD).

Physicochemical properties

Table 2 summarizes total soluble solids (TSS), viscosity, and protein content in kale soup fortified with quinoa at ratios of 35:5, 30:10, and 25:15 (K:Q). TSS (°Brix) was highest in the 35:5 (4.93) and 30:10 (4.87) formulations, with no significant difference between them (P > 0.05), but both were significantly greater than the 25:15 formulation (4.37). Viscosity (cP) increased significantly from 839.16 cP in 35:5 to 1,053.67 cP in 30:10 and 1,073.00 cP in 25:15. Protein content rose significantly from 1.14 g/100 g in 35:5 to 1.70 g/100 g in 30:10 and 2.73 g/100 g in 25:15. Increasing quinoa content decreased TSS while significantly enhancing viscosity and protein (P ≤ 0.05), due to quinoa’s lower soluble solids and higher starch–protein contribution. These changes increase nutritional density and produce a thicker consistency suitable for elderly-friendly formulations.

Table 2. Total soluble solids, viscosity, and protein content across three quinoa-fortified kale soup formulations.

Note: The mean ± standard deviation of three replicated experiments. ab different letters in each column were significantly different (P ≤ 0.05) when compared by DMRT (mean ± SD).

Heat treatment effects on temperature profiles and F₀ Values of quinoa-fortified kale soup

The K:Q 30:10 formulation was selected for further testing due to its balanced retention of green color and chlorophyll, total soluble solids comparable to the 35:5 ratio, increased viscosity without further thickening at higher quinoa levels, and significantly enhanced protein content, optimizing color, texture, and nutrition. This study investigates the effects of sterilization heating at 121°C for 10, 15, and 20 min on the physical and chemical properties of quinoa-fortified kale soup packaged in retort pouches. Results are detailed in Figures 3–8 and Table 3.

Figures 3–5 show that quinoa-fortified kale soup required sterilization at 121°C for 10, 15, and 20 min, corresponding to total process times of approximately 80, 90, and 100 min, respectively, with a 5–6 °C temperature difference between the product and the sterilizer. The F₀ values, calculated using the Patashnik method (Shaha et al., 2017), were 4.43, 11.73, and 15.22 min. All values exceeded the Ministry of Public Health requirement of ≥ 3 min for low-acid foods in hermetically sealed containers (Ministry of Public Health, 2013), confirming the adequacy of the applied sterilization conditions.

Figure 3. Heat transfer and F₀ values for quinoa-fortified kale soup in retort pouches sterilized at 121°C for 10 min.

Figure 4. Heat transfer and F₀ values for quinoa-fortified kale soup in retort pouches sterilized at 121°C for 15 min.

Figure 5. Heat transfer and F₀ values for quinoa-fortified kale soup in retort pouches sterilized at 121°C for 20 min.

Effects of retort sterilization on color, chlorophyll, physicochemical properties, and microbial safety of quinoa-fortified kale soup 

Color value

Figure 6 shows the L*, a*, and b* color values of quinoa-fortified kale soup in retort pouches before sterilization (K:Q 30:10) and after sterilization at 121°C for 10 (S(121-10)), 15 (S(121-15)), and 20 min (S(121-20)), with all parameters significantly differing among treatments (P ≤ 0.05). The unsterilized sample had the lowest L* value, indicating a darker color, a strongly negative a* value reflecting intense greenness from kale chlorophyll, and a low b* value, indicating minimal yellowness. Sterilization for 10 min significantly increased L*, suggesting partial chlorophyll degradation and lightening of color, while a* shifted toward less negative values, indicating decreased greenness. The b* value increased notably, reflecting enhanced yellowness from heat-stable carotenoids. At 15 min, L* remained elevated and was significantly higher than in S(121-10), a* continued moving toward zero, indicating further chlorophyll-to-pheophytin conversion under acidic, high-temperature conditions, and b* increased slightly, maintaining the yellow hue trend. After 20 min, L* peaked, a* reached its least negative value, and b* was highest, indicating maximal lightness, minimal greenness, and pronounced yellowness consistent with advanced chlorophyll degradation and carotenoid retention.

Figure 6. L*, a*, and b* color values of quinoa-fortified kale soup in retort pouches before and after sterilization at 121°C for 10, 15, and 20 min.
ab different letters in each test condition were significantly different (P ≤ 0.05) when compared by DMRT (mean ± SD).

Chlorophyll content

Figure 7 showed that the sample before sterilization with a kale-to-quinoa ratio of 30:10 had the highest levels of chlorophyll a and b, particularly chlorophyll b, indicating intact pigments and effective retention of kale’s green color (Korus et al., 2007; Ayaz et al., 2008). Sterilization at 121°C for 10 min caused a significant decrease (P ≤ 0.05) in chlorophyll a and b. Extended sterilization to 15 min further reduced chlorophyll content significantly. After 20 min, chlorophyll levels reached their lowest, reflecting near-complete pigment breakdown and substantial green color loss. Given chlorophyll’s antioxidant capacity and potential anti-inflammatory benefits, degradation contributes not only to color loss but also to reduced functional nutritional value. However, a 10-min process retains more chlorophyll and thus higher antioxidant potential compared with longer heating durations.

Figure 7. Chlorophyll values of quinoa-fortified kale soup in retort pouches before and after sterilization at 121°C for 10, 15, and 20 min. ab different letters in each test condition were significantly different (P ≤ 0.05) when compared by DMRT (mean ± SD).

Physicochemical and microbiological properties

Table 3 presents total soluble solids (°Brix), viscosity, protein content, and colony-forming units (CFU/g) of quinoa-fortified kale soup in retort pouches before and after sterilization at 121°C for 10-, 15-, and 20-min. Significant differences (P ≤ 0.05) were observed among treatments. Total soluble solids increased significantly from 4.86 °Brix in the unsterilized sample (K:Q 30:10) to approximately 6.00 °Brix in all sterilized samples, with no significant differences among sterilization durations (P > 0.05). Viscosity rose markedly after sterilization, from 1,053.67 cP in the unsterilized sample to 9,506.67 cP after 10 min, then decreased slightly to 8,476.65 cP and 8,573.33 cP at 15 and 20 min, respectively, with all differences significant (P ≤ 0.05). Protein content increased progressively with sterilization time, from 1.70 g/100 g pre-sterilization to 1.87, 1.96, and 2.07 g/100 g at 10, 15, and 20 min, respectively, with all pairwise differences significant (P ≤ 0.05). Microbial counts decreased significantly from 2×10² CFU/g in the unsterilized sample to below detectable limits (<10 CFU/g) after all sterilization treatments (P ≤ 0.05), meeting the Ministry of Public Health’s standard (Announcement No. 355, 2013) of less than 1×10⁴ CFU/g for sealed food products.

The absence of detectable microbial growth after retort processing confirms commercial sterility and indicates that the product is microbiologically safe immediately after sterilization under sealed-package conditions. These findings demonstrate process adequacy but do not provide evidence of time-dependent shelf-life stability, as no extended storage or challenge testing was performed. Retort sterilization also increased TSS, viscosity, and protein content. Viscosity was highest at 10 min and declined with longer heating, consistent with structural disruption of the starch–protein matrix. The increase in protein content likely reflects concentration effects and greater extractability associated with heat-induced denaturation.

Table 3. Total soluble solids, viscosity, protein content, and colony-forming units of quinoa-fortified kale soup in retort pouches before and after sterilization at 121°C for 10, 15, and 20 min.

Note: * The mean ± standard deviation of three replicated experiments. ab different letters in each column were significantly different (P ≤ 0.05) when compared by DMRT(mean ± SD).

Sensory evaluation

The sensory evaluation of quinoa-fortified kale soup showed significant differences (P ≤ 0.05) across sterilization times. For appearance, the unsterilized sample K:Q (30:10) scored highest at 7.5, slightly exceeding the 7.5 of the 10-min sterilized sample (S(121-10)), while scores declined further for S(121-15) and S(121-20) to 6.8 and 6.3, respectively. Flavor scores peaked at 8.0 for S(121-10), whereas K:Q (30:10) and S(121-15) scored 6.9 and 6.5, respectively, and S(121-20) decreased significantly to 5.7. Color followed a similar trend, with K:Q (30:10) scoring 7 and decreasing to 6.8, 5.6, and 4.7 for S(121-10), S(121-15), and S(121-20), respectively. Texture scores also declined after sterilization, with K:Q (30:10) scoring 7.5, and samples sterilized for 10, 15, and 20 min scoring 6.4, 6.5, and 6.4, respectively. Taste results were consistent, with K:Q (30:10) scoring 8.0, S(121-10) 7.0, S(121-15) 6.1, and S(121-20) 5.1. Overall acceptability mirrored these trends, with K:Q (30:10) and S(121-10) achieving similarly high scores of 7.0 and 7.2, while S(121-15) and S(121-20) scored 5.8 and 5.2. The 10-min sterilized sample showed the highest sensory liking, while scores declined at 15-20 min due to pigment loss, flavor volatilization, and textural breakdown, indicating that 121°C for 10 min provides the optimal balance of safety, quality, and acceptability.

Figure 8. Sensory scores of quinoa-fortified kale soup in retort pouches before and after sterilization at 121°C for 10, 15, and 20 min. ab different letters in each test condition were significantly different (P ≤ 0.05) when compared by DMRT (mean ± SD).

DISCUSSION

The higher L* value in K:Q 25:15 reflects quinoa’s light beige color diluting kale’s dark green chlorophyll pigments. This aligns with Elango et al. (2023), who reported that lighter grains increase lightness in green leafy vegetable products. The reduction in green intensity (a*) with increasing quinoa content suggests that quinoa reduces green intensity through pigment dilution, as it lacks chlorophyll but contains carotenoids, consistent with Heaton and Marangoni (1996), who reported that increased cereal flour reduces green intensity by decreasing chlorophyll content in vegetable-based products. The increased b* value (yellowness) in K:Q 25:15 is likely due to carotenoids such as lutein and zeaxanthin, and Maillard reaction products formed during thermal processing. Yang et al. (2024) similarly observed increased yellowness in thermally processed quinoa products attributed to pigment release and non-enzymatic browning. At the molecular level, quinoa’s low chlorophyll content and its flavonoid and phenolic compounds interact with kale pigments during heating, leading to partial chlorophyll degradation into pheophytins, which decreases a* and increases b*. Heat-induced carotenoid isomerization further stabilizes yellow coloration (Sharma et al., 2022). Overall, increasing quinoa substitution enhances lightness and yellowness while reducing green intensity, reflecting the pigment composition of quinoa and kale and their pigment–protein–phenolic interactions during thermal processing.

The observed decline in chlorophyll content reflects quinoa’s minimal chlorophyll compared to kale, which dilutes the overall pigment concentration. Chlorophyll a and b are the primary green pigments in leafy vegetables, with chlorophyll b predominant in kale (Korus and Kmiecik, 2007; Liu et al., 2021). Thermal processing likely accelerated chlorophyll degradation to pheophytins, compounds with diminished green coloration, further reducing the soup’s color intensity (Sharma et al., 2022). Because chlorophylls and their derivatives contribute antioxidant activity and may support oxidative stability during storage, reduced chlorophyll retention at higher quinoa levels also implies a decline in potential functional nutritional value. This suggests that the kale-quinoa ratio influences not only color but also bioactive pigment retention relevant to elderly-targeted functional foods. Therefore, the quinoa-to-kale ratio critically influences chlorophyll content and, consequently, the soup’s visual quality and potential consumer acceptance.

The decline in TSS with increased quinoa content likely results from quinoa’s lower soluble sugar and carbohydrate levels compared to kale, leading to dilution of soluble solids (Angeli et al., 2020). The increase in viscosity reflects quinoa’s higher protein and starch content, which enhances thickening through water binding and gel formation. The increase in quinoa content not only elevated protein and viscosity as expected but also provided meaningful nutritional and functional benefits for elderly consumers. The rise in protein from 1.14 to 2.73 g/100 g increases the per-serving protein contribution, supporting dietary strategies to mitigate age-related muscle loss. The higher viscosity reflects starch–protein network formation, which enhances bolus cohesion and may facilitate safer swallowing in individuals with chewing or dysphagia risk. However, the accompanying reduction in chlorophyll and phytopigments indicates a trade-off between protein enrichment and bioactive retention, suggesting that the 30:10 ratio represents a balanced compromise between nutritional improvement, functional texture, and phytochemical preservation.

The lack of significant difference in viscosity between 30:10 and 25:15 suggests that viscosity plateaus beyond a certain quinoa proportion (Segev et al., 2025). The observed increase in protein content aligns with quinoa’s superior protein content relative to kale, confirming that quinoa fortification effectively enhances the nutritional quality of the soup (Arguello-Hernández et al., 2024). Additionally, quinoa contains all essential amino acids, making this protein enhancement particularly meaningful for older adults who require higher-quality protein to mitigate age-related sarcopenia. Overall, increasing quinoa ratios in kale soup decreases total soluble solids due to dilution but increases viscosity and protein content due to quinoa’s macromolecular composition, thereby impacting the soup’s texture and nutritional quality key factors for product development and consumer acceptance.

The F₀ values reflect the cumulative lethal effect at a reference temperature of 121°C over the heating, holding, and cooling phases, ensuring commercial sterility. Comparable studies reported similar ranges: Shaha et al. (2017) obtained F₀ values of 7–11 min at 121°C. Catauro (2012) achieved an F₀ of 6.0 min for fermented pork soup; Bindu et al. (2007) processed black clam with an F₀ of 9 min and ~44 min total process time; and Majumdar et al. (2015) achieved F₀ values of 5–8 min for fish curry at 116°C with a 55.61 min total process time. These findings confirm that the sterilization conditions applied in the present study are sufficient to ensure product safety and meet commercial sterility standards. Importantly, achieving microbial safety at the shortest viable process time (10 min) helps preserve pigments, proteins, and functional compounds, illustrating the relevance of thermal optimization for nutrient retention in elderly-oriented formulations.

These results align with known thermal pigment transformations in green leafy vegetables, where chlorophyll degrades to pheophytin under heat and acidic conditions, shifting color from green to olive-brown, while heat-stable carotenoids increase yellowness (Korus et al., 2007; Elango et al., 2023). The retort pouch sterilization method has been shown to affect physicochemical properties, including color changes due to thermal processing (Shaha et al., 2017; Benjakul et al., 2018). The observed progressive decrease in greenness and increase in lightness and yellowness reflect expected chlorophyll degradation and carotenoid retention in kale and quinoa products subjected to retort sterilization (Kim et al., 2008; Agarwal et al., 2023). These pigment changes also correspond to shifts in antioxidant potential, as chlorophyll degradation often reduces antioxidant activity while carotenoids remain comparatively stable. This indicates that processing time influences not only appearance but also functional nutritional quality. Overall, sterilization at 121°C progressively reduced greenness while increasing lightness and yellowness, supporting documented thermal pigment transformation mechanisms in green leafy vegetables and protein-enriched foods.

The observed decrease in chlorophyll a and b resulted from heat-induced degradation, converting chlorophyll into pheophytin, which is paler and less stable (Martins et al., 2023). High heat and prolonged exposure during retort sterilization disrupt chlorophyll’s molecular structure, a process accelerated by the acidic sterilization environment (Heaton and Marangoni, 1996). Extended exposure to high temperature and acidity faded the soup’s green color (Benjakul et al., 2018). Chlorophyll a and b are primary pigments in green leafy vegetables responsible for light absorption during photosynthesis (Elango et al., 2023). The decline in chlorophyll corresponds with a color shift from green to yellow, as carotenoids—more heat-stable pigments remain largely intact (Sharma et al., 2022). Because carotenoids (e.g., lutein and zeaxanthin) play roles in visual health and cognitive support—areas of particular concern in aging populations—their relative stability during sterilization provides added functional value to the product despite overall pigment loss. In conclusion, sterilization at 121°C significantly diminishes chlorophyll content in quinoa-fortified kale soup over time, aligning with observed color changes and altered physical properties post-thermal processing in retort pouches (Abhishek et al., 2014; Benjakul et al., 2018).

The increase in total soluble solids after sterilization likely reflects concentration effects due to water evaporation and chemical changes during thermal processing (Shaha et al., 2017; Benjakul et al., 2018). The observed changes in viscosity, including the peak at 10 min followed by slight decreases at longer times, can be attributed to protein and starch denaturation, molecular aggregation, and moisture loss during heating (Benjakul et al., 2018). These rheological changes directly influence swallowability and perceived thickness, traits essential for developing elderly-appropriate soups that avoid aspiration risks while maintaining palatability. The progressive rise in protein content is likely caused by concentration effects from water loss and structural modifications that enhance protein solubility (Shaha et al., 2017).

The marked reduction in microbial counts confirms the effectiveness of retort sterilization in achieving microbial inactivation and meeting food safety requirements. Although the results verify post-process commercial sterility, microbial and physicochemical changes during storage were not evaluated; therefore, no shelf-life claims are made. Future studies should include accelerated and real-time storage testing to establish practical shelf life and stability under ambient and chilled conditions. Overall, the findings indicate that thermal processing ensures microbiological safety while simultaneously altering the physicochemical properties of the protein-enriched kale soup, with implications for texture and nutritional quality.

Sterilization time had a pronounced effect on sensory quality, with acceptance declining sharply when heating exceeded 10 min. The 10-min treatment yielded the highest scores for appearance, flavor, color, and texture, whereas the 15- and 20-min treatments showed significantly lower ratings across all attributes. Reduced acceptance at longer sterilization times is primarily due to heat-induced degradation of pigments, nutrients, and volatile compounds. Prolonged heating accelerates chlorophyll loss, causing dull color, while also diminishing flavor through volatilization and Maillard reaction–derived cooked notes. Extended heating further weakens texture by disrupting cell structure and starch integrity, resulting in a softer, less cohesive mouthfeel. These combined effects explain the substantial decline in sensory acceptance at 15 and 20 min.

Sterilization at 121°C for 10 min best preserves the sensory quality of quinoa–kale soup, while longer heating causes pigment loss, protein denaturation, and Maillard reactions that reduce color, flavor, and texture. Shorter processing times also help retain heat-sensitive nutrients and bioactive compounds, making the 10-min treatment most suitable for elderly consumers. Overall, the findings highlight the need to balance microbial safety with sensory and nutritional quality during thermal processing.

CONCLUSION

Thailand’s rapid shift toward a super-aged society creates demand for ready-to-heat (RTH) foods that meet elderly nutritional and textural needs, yet current commercial soups are often high in sodium and low in essential nutrients. This study developed quinoa-fortified kale soup (35:5, 30:10, 25:15), selecting the 30:10 ratio for retort sterilization at 121°C for 10–20 min. All processes exceeded low-acid food safety standards (F₀ = 4.43–15.22 min), increased total soluble solids, viscosity, and protein, and eliminated detectable microbes. However, extended heating degraded chlorophyll, increased lightness/yellowness, and reduced sensory acceptance; 10 min produced the highest flavor and overall liking. Results demonstrate that retort processing can yield microbiologically safe, nutrient-rich quinoa–kale soups tailored for elderly consumers, with 10 min at 121°C optimizing both quality and safety. Future research should incorporate a broader nutritional profile and explore non-thermal or mild-heat technologies to reduce nutrient and color loss, extend shelf life, and refine textures for elderly consumers with varying chewing and swallowing abilities.

ACKNOWLEDGEMENTS

Financial support was provided by Rajamangala University of Technology  Thanyaburi (RMUTT), Faculty of Engineering, Rajamangala University of Technology Thanyaburi, Thailand.

AUTHOR CONTRIBUTIONS

Jaturong Langkapin: Conceptualization (Lead), Methodology (Lead), Data Curation (Equal), Formal Analysis (Equal), Writing  Original Draft (Equal); Orawan Oupathumpanont: Validation (Supporting), Investigation (Supporting), Resources (Supporting), Writing Review & Editing (Supporting); Sunan Parnsakhorn: Supervision (Lead), Project Administration (Lead), Funding Acquisition (Lead), Writing Review & Editing (Lead).

CONFLICT OF INTEREST

The authors declare that this original manuscript is not under consideration elsewhere, has been checked for plagiarism, and involves no conflicts of interest. As the corresponding author, I confirm that all authors have read, approved, and agreed to its submission.

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