A novel turmeric drying approach based on supercritical carbon dioxide

Sumanjot Kaur; Arda Tuhanioglu; Ali Ubeyitogullari

doi:10.1038/s41598-025-18958-z

. 2025 Oct 7;15:34994. doi: 10.1038/s41598-025-18958-z

A novel turmeric drying approach based on supercritical carbon dioxide

Sumanjot Kaur ¹, Arda Tuhanioglu ¹, Ali Ubeyitogullari ^1,^2,^3,^✉

PMCID: PMC12504580 PMID: 41057442

Abstract

As a widely used spice, turmeric is rich in curcuminoids, which are known for their anti-inflammatory and antioxidant properties. However, the drying process, especially at high temperatures, often leads to significant curcumin degradation, impacting its bioactivity and quality. This study investigates drying turmeric using an innovative approach based on supercritical carbon dioxide (SC-CO₂), and conventional drying methods (oven drying and freeze-drying) in terms of curcuminoid retention, color, solubility, chemical interactions, crystallinity, and morphology. The results revealed that SC-CO₂ drying preserved up to 82.3 µg curcuminoids /mg dried turmeric, which was significantly higher than the levels retained by oven drying (55.4 µg/mg) and freeze-drying (42.4 µg/mg). Moreover, SC-CO₂ drying exhibited better color retention, with higher L*, a*, and b* values compared to oven-dried samples. Further, SC-CO₂ drying improved curcuminoids’ solubility (0.57 µg curcuminoids/mg turmeric) as compared to oven drying (0.29 µg/mg) and freeze-drying (0.12 µg/mg) at 60 °C, which is critical for enhancing curcumin bioavailability in food and pharmaceutical products. The morphological analysis showed that SC-CO₂ drying maintained the structural integrity of turmeric better than the other methods, making it a promising technique for preventing thermal degradation and oxidation of micronutrients during drying.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-025-18958-z.

Keywords: Turmeric, Curcumin, Curcuminoids, Supercritical carbon dioxide, Drying

Subject terms: Chemical engineering, Chemistry, Materials science

Introduction

Turmeric (Curcuma longa L.), a vibrant golden spice native to Southeast Asia and a member of the ginger family (Zingiberaceae), boasts a rich history dating back to Vedic times^1,2. Renowned for its unique color, aroma, and flavor, turmeric has transcended its culinary role to become a revered medicinal spice. Its bioactive components, particularly curcuminoids, are credited with a number of health benefits, including potent anti-inflammatory and antioxidant properties^1,3,4. However, to harness these therapeutic potentials and ensure long-term storage, fresh turmeric rhizomes necessitate proper drying.

Turmeric typically contains a high moisture content (~ 90%, wet basis), rendering it unsuitable for long-term storage without significant risk of spoilage⁵. Drying plays a crucial role in preserving the quality and bioactive compounds of turmeric. However, the drying method significantly impacts the final product’s physicochemical properties, particularly the retention of curcuminoids, the key bioactive components responsible for turmeric’s therapeutic effects^1,6,7. Traditionally, sun drying has been the most widely used method for drying turmeric due to its simplicity and low cost. However, it offers minimal control over drying conditions (temperature, humidity, air circulation), exposing the product to potential contamination and uncontrolled light exposure. This often leads to significant curcumin degradation and inconsistent product quality. Alternative drying methods involve heated air, and can offer faster drying times. For example, Vallespir, et al.⁸ reported efficient low-temperature ultrasound-assisted convective drying of beetroot when combined with a freezing pre-treatment. Similarly, pulsed-spouted bed microwave freeze-drying reduced drying times and better preserved color and volatile compounds compared to conventional freeze-drying⁹. Despite these advances, microbiological safety remains a significant challenge for the drying industry¹⁰. Also, these methods result in non-homogeneity, overheating, discoloration, and deterioration, negatively impacting the quality of the dried materials^1,11.

Supercritical carbon dioxide (SC-CO₂) drying emerges as a promising alternative due to its unique properties. In this method, CO₂ is pressurized (around 7.4 MPa) and heated (around 31 °C) to a supercritical state, where it acts like both a gas (for deep penetration) and a liquid (for efficient moisture removal). It retains the gas-like characteristic of high diffusivity, allowing for deep penetration into the cellular structure of food particles. Simultaneously, SC-CO₂ possesses a liquid-like density, increasing its solvating power. This enables the removal of moisture in food items^12,13. Furthermore, the low processing temperature minimizes thermal degradation, a major threat to heat-sensitive curcuminoids and the rapid drying times facilitated by SC-CO₂ can significantly reduce/eliminate exposure to detrimental factors like light and oxygen¹⁴. Beyond its effectiveness, CO₂ is a naturally occurring and non-toxic gas, and the closed-loop nature of the system allows for CO₂ recycling and reuse, minimizing environmental impact.

Although SC-CO₂ drying is well-established in the preparation of aerogels¹², its use in food processing is still emerging, like carrots¹⁵, basil¹⁶, coriander¹⁷, apples¹⁴ and tropical fruits such as mangoes and persimmons¹⁸. These studies suggest that SC-CO₂ drying can preserve the structural integrity and bioactive compounds effectively, but the processing parameters impact drying behavior. For example, Brown, et al.¹⁵ identified temperature as a critical factor influencing drying time, yet the interplay between temperature, pressure, and CO₂ density on moisture diffusivity and matrix integrity requires further quantification. Also, Bušić, et al.¹⁶ demonstrated that both pressure and drying duration significantly influence the retention of bioactive compounds and sensory characteristics in dried basil, highlighting the sensitivity of phytochemicals to subtle process variations. Nevertheless, there is no report on drying turmeric using SC-CO₂.

Therefore, the overall goal of this study was to investigate the impact of SC-CO₂ drying on the properties of turmeric. The specific objectives included investigating the influence of different drying methods (SC-CO₂, oven drying, and freeze-drying) on curcuminoids content, morphology, color, solubility, crystallinity, and chemical interactions.

Results and discussion

Turmeric samples were dried using oven, freeze, and SC-CO₂ drying, where oven and freeze drying were employed as the conventional drying approaches¹. While oven drying operates via temperature-induced vapor pressure gradients, it often results in significant degradation of thermolabile compounds, particularly curcuminoids in turmeric⁵. Freeze drying, proceeding through freezing (−40 °C to −50 °C) and sublimation under high vacuum (≤ 100 Pa), better preserves bioactive constituents but can compromise cellular integrity through ice crystal formation and growth¹⁹. SC-CO₂ drying was conducted at 20 MPa And 60 °C based on our preliminary experiment and previous publications, for its superior mass transfer properties¹². At this condition, SC-CO₂ exhibits enhanced diffusivity and low surface tension, which enables efficient moisture removal¹⁵. Additionally, it prevents oxidative deterioration of bioactive compounds since no oxygen is involved during drying.

Moisture content

Table 1 presents the moisture content data of turmeric dried using oven, freeze, and SC-CO₂ drying methods. The statistical analysis showed significant differences among the three drying techniques (p < 0.05). On average, oven-dried turmeric showed the highest moisture content (10.10 ± 0.04%), followed by SC-CO₂ (9.45 ± 0.17%), and freeze-drying showed the lowest moisture content (8.97 ± 0.33%) (p < 0.05). The oven-drying and freeze-drying results were consistent with findings from other studies^1,20,21. For instance, Malik and Kumar⁶ and Prasad, et al.²⁰ reported similar moisture content ranges for oven-dried and solar-biomass dried turmeric (9–11%). A slightly lower moisture content (~ 5%) in solar conduction-dried samples was observed by Borah, et al.²². Furthermore, Llano, et al.¹ revealed ~ 8% moisture content in turmeric dried using a traditional solar, convection oven, and fluidized bed drying. Overall, all three drying techniques were effective in reducing moisture content (< 10%) to levels suitable for preservation, typically below 9–10% for turmeric powder²¹.

Table 1.

The moisture content and water activity of turmeric dried by oven, freeze, and SC-CO₂ drying methods.

Drying method	Moisture content (%, wet basis)	Water activity (a_w)
Oven	10.10 ± 0.04^a	0.54 ± 0.00^c
Freeze	8.97 ± 0.33^b	0.57 ± 0.01^a
SC-CO₂	9.45 ± 0.17^ab	0.55 ± 0.00^b

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*Means in the same column that do not share a common letter are significantly different (p<0.05).

Additionally, the water activity (a_w) values for all dried samples were below 0.6, which is the critical threshold for inhibiting microbial growth and ensuring product safety and shelf stability. Oven-dried turmeric exhibited the lowest a_w (0.54 ± 0.00), followed closely by SC-CO₂-dried (0.55 ± 0.00) and freeze-dried turmeric (0.57 ± 0.01). These results confirm that all drying methods effectively reduced water activity to safe levels, contributing to the microbial stability and prolonged shelf life of the turmeric powders²³.

Color analysis

Table 2 shows significant variations in L* (lightness), a* (redness), and b* (yellowness) values of blanched fresh turmeric and dried turmeric using different methods (oven, freeze, and SC-CO₂). The fresh (blanched) turmeric indicated a moderately light appearance (L* = 58.05 ± 0.22), redness (a* = 20.14 ± 0.18) and strong yellowness (b* = 47.92 ± 2.15), which are characteristic of turmeric’s vibrant golden-orange hue (Fig. 1)²⁴. Freeze-dried turmeric exhibited the highest L* value (76.55 ± 0.04), indicating the lightest color, followed by SC-CO₂ (72.71 ± 0.58) and oven-dried (63.83 ± 0.30) samples. SC-CO₂ dried turmeric showed the highest a* value (20.35 ± 0.95), suggesting more redness, while freeze-dried samples had the lowest (11.86 ± 0.01) (p < 0.05). Similarly, Ray, et al.²⁵ reported a redness value of around 10–15 in the solar, and hot air oven-dried turmeric. The b* values, representing yellowness, were the highest in freeze-dried samples (56.53 ± 0.07), followed by SC-CO₂ (53.72 ± 0.15) and oven-dried (42.64 ± 0.28). Comparing the color changes (ΔE*), freeze drying caused the largest color difference from the fresh sample (ΔE* = 22.02), followed by SC-CO₂ drying (ΔE* = 15.78), while oven drying resulted in the smallest color deviation (ΔE* = 8.23) from the original blanched color^24,25. Ray, et al.²⁵ revealed that high-temperature drying methods like oven drying (60 ℃) resulted in significant color degradation, causing intense Maillard reactions, oxidation, and structural changes in unblanched turmeric samples. In that study, the degradation was primarily attributed to the activity of polyphenol oxidase (PPO) and peroxidase (POD), leading to intense browning. However, in our study, since turmeric was blanched prior to drying, which reduced the enzymatic activity. According to Ray, et al.²⁵ and Aksoy, et al.²⁶ the superior color retention in freeze-dried samples was due to low-temperature drying methods, which probably better preserved heat-sensitive pigments like curcumin in turmeric; however, in our study, the porous structure of freeze-dried turmeric contributed to a higher lightness value, which in turn resulted in an increased ΔE* value. The intermediate color values of SC-CO₂-dried turmeric suggest a trade-off between color preservation and processing efficiency, as noted by Pravallika, et al.²⁷ on SC-CO₂ drying of red bell pepper, apple pieces, and carrot slices.

Table 2.

The color parameters of turmeric dried by oven, freeze, and SC-CO₂ drying methods.

Samples	L*	a*	b*	ΔE*	C	H⁰
Fresh	58.05 ± 0.22^d	20.14 ± 0.18^a	47.92 ± 2.15^c	N/A	51.99 ± 1.91^b	22.83 ± 1.11^a
Oven-dried	63.83 ± 0.30^c	17.64 ± 0.14^b	42.64 ± 0.28^d	8.23 ± 0.27^c	46.14 ± 0.29^c	22.48 ± 0.14^a
Freeze-dried	76.55 ± 0.04^a	11.86 ± 0.01^c	56.53 ± 0.07^a	22.02 ± 0.07^a	57.76 ± 0.07^a	11.85 ± 0.03^b
SC-CO₂-dried	72.71 ± 0.58^b	20.35 ± 0.95^a	53.72 ± 0.15^b	15.78 ± 0.57^b	57.45 ± 0.21^a	20.75 ± 0.94^a

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*Means that do not share a common letter within the same column are significantly different (p<0.05). Fresh: The color of fresh samples after blanching (prior to drying) was reported.

Fig. 1 — Photographs of turmeric powders dried by (a) oven, (b) freeze, and (c) SC-CO₂ drying methods.

Chroma (C) values, which indicate color saturation and vividness, decreased after oven drying (46.14), whereas freeze drying (57.76) and SC-CO₂ drying (57.45) maintained or slightly enhanced the chroma compared to the fresh sample (51.99), indicating superior retention of color intensity by the latter two methods. Further, the hue angle (H°) of fresh turmeric was 22.83°, indicating a reddish-orange tone. After drying, H° decreased slightly to 22.48° (oven-dried), 20.75° (SC-CO₂-dried), and 11.85° (freeze-dried). These shifts suggest a minor yet noticeable color change, especially in the freeze-dried sample, which may be due to altered pigment stability or microstructural differences. The reduction in H° across all drying methods signifies a deepening of color, potentially influenced by non-enzymatic browning or concentration of pigments during moisture loss²⁸.

Curcuminoid content

Table 3 compares the total and individual curcuminoid levels (BDMC, DMC, and CUR) in fresh turmeric versus dried turmeric, with the latter being dried using different methods. The fresh turmeric yielded the highest total curcuminoid content (95.62 ± 6.17 µg/mg on a dry basis). A very close curcumin content of 94 µg/mg in fresh turmeric was reported by Hirun, et al.²³. Furthermore, SC-CO₂ drying closely followed (82.30 ± 1.88 µg/mg) with a nonsignificant difference from the fresh sample (p > 0.05). This demonstrates its efficacy in preserving curcuminoids, potentially attributable to the acidic environment of SC-CO₂, where curcumin is more stable, and the lack of oxygen in the drying environment²⁹. Additionally, in the SC-CO₂ dried sample, BDMC was the only curcumin moiety (26.39 ± 0.59 µg/mg) showing a significantly lower amount compared to fresh turmeric (39.21 ± 2.75 µg/mg) (p < 0.05). Therefore, SC-CO₂ drying demonstrated a superior capacity for preserving curcuminoids with methoxy groups (DMC and CUR) compared to other drying methods. Although BDMC lacks potentially reactive methoxy groups, it may still undergo degradation processes, with pressure and temperature having a significant impact on its recovery³⁰. Furthermore, no significant differences were detected in the levels of the curcuminoids between oven and freeze-drying (p > 0.05). Oven drying (55.35 ± 8.10 µg/mg) likely caused significant thermal degradation, while freeze-drying (42.35 ± 8.70 µg/mg) may have led to structural changes affecting extractability. This aligns with findings by Suresh, et al.³¹ and Bambirra, et al.³²who reported thermal degradation of curcumin during cooking. A previous study indicated that in the process of SC-CO₂ drying, CO₂ penetrates the food and avoids the formation of the vapor-liquid interfaces¹⁴. Initially, it eliminates free water, which is easily extractable, then proceeds to penetrate deeper into the food to remove entrapped water.

Table 3.

The curcuminoid levels in fresh turmeric and oven-, freeze-, and SC-CO₂-dried turmeric samples.

Turmeric samples	Total Curcuminoids (µg/mg sample)	BDMC (µg/mg sample)	DMC (µg/mg sample)	CUR (µg/mg sample)
Fresh	95.62 ± 6.17^a	39.21 ± 2.75^a	20.35 ± 1.18^a	36.03 ± 2.43^a
Oven-dried	55.35 ± 8.10^b	15.39 ± 1.99^c	11.75 ± 1.65^b	28.21 ± 4.46^ab
Freeze-dried	42.35 ± 8.70^b	14.16 ± 2.80^c	8.98 ± 1.83^b	19.21 ± 4.07^b
SC-CO₂-dried	82.30 ± 1.88^a	26.39 ± 0.59^b	20.37 ± 0.95^a	35.53 ± 0.35^a

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BDMC: Bisdemethoxycurcumin, DMC: Demethoxycurcumin, CUR: Curcumin.

*Means that do not share a common letter within the same column are significantly different (p<0.05).

Water solubility of curcuminoids

Figure 2 and Fig. S1 show the water solubility and chromatograms, respectively, of curcuminoids in turmeric dried using oven, freeze, and SC-CO₂ drying. SC-CO₂ drying revealed superior curcuminoids’ solubility (0.25 ± 0.01 µg curcuminoids/mg turmeric at 23 °C, And 0.57 ± 0.01 µg curcuminoids/mg turmeric at 60 °C), followed by oven drying (0.07 ± 0.01 µg curcuminoids/mg turmeric at 23 °C, And 0.29 ± 0.01 µg curcuminoids/mg turmeric at 60 °C), while freeze-drying showed the lowest solubility (0.02 ± 0.01 µg curcuminoids/mg turmeric at 23 °C, And 0.12 ± 0.02 µg curcuminoids/mg turmeric at 60 °C) (p < 0.05). The highest water solubility of curcuminoids in SC-CO₂-dried turmeric might be due to higher curcuminoid retention, an open porous structure that increases water interaction, intermolecular interactions, and reduced crystallinity. In contrast, freeze-dried turmeric exhibited the lowest solubility because of its lower curcuminoids retention. The lower solubility of oven-dried could be attributed to thermal degradation, oxidation, or structural changes in curcumin, as suggested by Guo, et al.³³ and Emelike³⁴.

Fig. 2 — Water solubility (µg curcuminoids/mg sample) of turmeric dried by (a) oven, (b) freeze, and (c) SC-CO₂ drying methods.

All drying methods exhibited significantly enhanced solubilities at elevated temperature (60 °C) compared to room temperature (23 °C) (p < 0.05), with SC-CO₂ drying demonstrating a 2.24-fold increase, oven drying a 4.14-fold increase, and freeze-drying a 6-fold increase at 60 °C. As expected, at higher temperatures, the thermal energy disrupts intramolecular hydrogen bonds and intermolecular forces, exposing polar hydroxyl (− OH) and keto (> C═O) groups, which increase the availability and solubility of curcumin in water²⁹. The higher temperature (60 vs. 23 °C) might alter the intermolecular forces between curcuminoids and other compounds in turmeric, potentially improving its solubility³⁵. These results highlight the complex interaction between drying methods and temperature on the water solubility of curcuminoids in turmeric and emphasize the need to consider both factors in applications where solubility is important, such as food processing or pharmaceutical formulations.

Morphology

Figure 3 presents morphological changes in turmeric dried using oven drying, freeze drying, and SC-CO₂ drying. In the oven-dried turmeric (Fig. 3A, a), strong capillary forces and high surface tension during water removal resulted in a more compact and fused structure with less porosity as the liquid-vapor interface induced significant compressive stresses on cell walls. This aligns with findings by Karam, et al.³⁶ who observed that thermal drying could lead to shrinkage and collapse of cellular structures. The freeze-dried turmeric (Fig. 3B, b) exhibited a more open, porous structure with sheet-like formations, where ice crystal formation and sublimation created a highly macroporous structure by bypassing liquid-phase surface tension effects^27,36,37. Furthermore, the SC-CO₂ dried sample (Fig. 3C, c) showed a preserved microstructure due to near-zero surface tension in the supercritical state, which eliminated capillary stresses during water removal. Similarly, Pravallika, et al.²⁷, Ubeyitogullari and Ciftci³⁸, and Ahmadzadeh and Ubeyitogullari³⁹ found that SC-CO₂ drying can preserve structural integrity better than air drying or freeze-drying. The differences in microstructure could significantly impact properties such as flowability, rehydration capacity, and bioactive compound bioaccessibility. For instance, the more open structure may contribute to better solubility, as noted by Karam, et al.³⁶ and Kaur, et al.¹² while the compact structure might result in slower dissolution^27,36. This open porous structure contributed to the increased water solubility of SC-CO₂-dried turmeric, as reported in Fig. 2. In addition to morphological differences, the drying method can induce structural transformations in curcumin itself. Thermal drying may lead to partial degradation or isomerization of curcumin molecules, potentially reducing their antioxidant activity. Freeze drying, although less harsh, may still promote crystalline formation that can limit curcumin solubility³⁶. SC-CO₂ drying, on the other hand, may help retain curcumin in a more amorphous state with fewer crystalline domains due to its low-temperature and low-stress mechanism. This amorphization can enhance curcumin’s water dispersibility, and overall functional performance in food and nutraceutical applications²⁷.

Crystallinity

XRD patterns of turmeric samples showed peaks at 2θ 15°, 17°, 22° And 25°⁴⁰. Amalraj, et al.⁴¹ revealed the XRD peaks of curcuminoids exist between 2θ 5–30°. Oven-dried turmeric samples showed the highest intensity peaks (Fig. 4), indicating a more crystalline structure with 9.5% crystallinity, and freeze-dried sample revealed a crystallinity of 8.3%. The SC-CO₂ dried samples showed a crystallinity of 7.5%, indicating a dominant amorphous structure¹². The differences in crystallinity can significantly influence the key properties such as solubility, stability, and bioavailability^42,43. For instance, the more amorphous structure of SC-CO₂ dried turmeric might have contributed to its higher solubility, as observed in the solubility data (Fig. 2).

Fig. 4 — XRD patterns of turmeric powders dried by (a) oven, (b) freeze, and (c) SC-CO₂ drying methods.

Chemical interactions

The FTIR spectra of turmeric subjected to different drying methods revealed broadly similar profiles (Fig. 5), indicating the major functional groups in curcuminoids and other polymers. The broad absorption band at 3400–3200 cm⁻¹, attributed to phenolic O-H stretching vibrations, was more intense and broadened in the oven-dried sample^25,41. This broadness may be linked to increased hydrogen bonding due to heat-induced conformational changes that promote intermolecular interactions. This phenomenon is consistent with literature noting broadened O-H bands in formulations with polar-nonpolar layer interfaces due to hydrogen bonding among curcuminoids and surrounding matrices^25,41,44. In contrast, both SC-CO₂ and freeze-dried samples exhibited sharper and slightly blue-shifted O-H bands, indicating less extensive hydrogen bonding and better preservation of native phenolic structures. The strong and well-defined peaks in the aromatic C = C stretching region (1600–1500 cm⁻¹), notably near 1602 cm⁻¹ and 1510 cm⁻¹, were most intense in SC-CO₂ dried turmeric, suggesting minimal oxidation or degradation of the conjugated aromatic systems under non-thermal conditions^25,44. In comparison, the oven-dried sample showed broader and less intense peaks in this region, likely reflecting partial structural degradation due to thermal stress and hydrogen bonding of curcuminoids between the polar-nonpolar layer⁴¹. Additional differentiation was observed in the 1200–1000 cm⁻¹ region, associated with methoxy (C-OCH₃) and ether (C-O-C) stretching vibrations. Peaks around 1281 cm⁻¹ and 1027 cm⁻¹ were clearer and more defined in SC-CO₂ dried turmeric, consistent with improved retention of methoxylated and ether-linked groups. Moreover, characteristic bands around 714 cm⁻¹ and 857 cm⁻¹, which correspond to aromatic ring bending and phenyl ring vibrations in curcuminoids respectively, were better preserved in the SC-CO₂-dried sample^19,25,44. The curcuminoid standard spectrum served as a critical benchmark, displaying sharp, well-resolved peaks across all functional regions. Among the dried samples, SC-CO₂-dried turmeric most closely resembled this standard in both peak position and resolution, indicating superior structural fidelity. In contrast, the oven- and freeze-dried samples showed more pronounced deviations, particularly in the phenolic and aromatic regions, likely due to heat or sublimation-induced molecular perturbations.

Conclusions

This study demonstrates the advantages of SC-CO₂ drying over traditional drying methods, namely oven drying and freeze drying, in preserving the quality and bioactive compounds of turmeric. Among the three drying methods, SC-CO₂ drying showed the highest retention of curcuminoids (82.30 ± 1.88 µg/mg), closely matching the curcuminoid levels of fresh turmeric (95.62 ± 6.17 µg/mg). In comparison, oven drying and freeze drying resulted in much lower curcuminoid retention, at 55.35 ± 8.10 µg/mg and 42.35 ± 8.70 µg/mg, respectively. Additionally, SC-CO₂ dried samples exhibited a lightness (L*) value of 72.71 ± 0.58, intermediate between freeze-dried samples (76.55 ± 0.04) and oven-dried samples (63.83 ± 0.30). The redness (a*) and yellowness (b*) values were highest in SC-CO₂ dried turmeric (20.35 ± 0.95 and 53.72 ± 0.15, respectively), indicating better color retention. Moreover, SC-CO₂ dried turmeric showed the highest curcuminoids solubility (0.57 µg curcuminoids/mg turmeric) at 60 °C, which was significantly higher than both freeze-dried (0.12 µg curcuminoids/mg turmeric) and oven-dried (0.29 µg curcuminoids/mg turmeric) turmeric samples at the same temperature. This might be due to the higher curcumin retention, more porous structure, intermolecular interactions, and reduced crystallinity of SC-CO₂ dried turmeric. Overall, SC-CO₂ drying offers significant improvements in the quality of dried turmeric, including higher curcuminoids retention, better color preservation, enhanced solubility, and favorable structural characteristics. This novel drying method has great potential for the food, nutraceutical, and pharmaceutical industries, particularly in the production of high-quality turmeric powders.

Methods

Materials

Turmeric (Curcuma Aromatica) was purchased from the Fijian Spice Company (NJ, USA). The curcuminoid standard (> 98%) (a mixture of BDMC: Bisdemethoxycurcumin (%2.5 ± 0.2), DMC: Demethoxycurcumin (%18.5 ± 0.1), and CUR: Curcumin (%78.9 ± 0.2)) and ethanol were purchased from Fisher Scientific (PA, USA), and liquid carbon dioxide (> 99.99%) was supplied by Airgas, Inc. (AR, USA).

Sample Preparation

The turmeric rhizomes were sliced into small pieces (1-cm thick) and blanched to eliminate enzyme activity. The blanching was conducted according to Ahmadzadeh, et al.⁴⁵. Briefly, the turmeric samples were steam-blanched in a steamer (Dixie, M-6 Steam Blancher-Cooler) at 90 °C for 3 min. After blanching, the samples were prepared for drying. Fresh turmeric samples were manually minced with a knife for curcuminoids content analysis, and dried samples were ground using a coffee grinder and sieved through a #35 mesh (< 500 μm) for further analysis⁴⁶.

Turmeric drying

Oven drying

The blanched turmeric samples (10 g) (1-cm thick) were spread on a perforated aluminum tray. The drying was conducted at 50 °C to a constant weight¹.

Freeze-drying

The blanched turmeric samples (10 g) were packed into Ziploc bags and frozen for at least 24 h. Then, freeze-drying was conducted at − 45 °C and 7.3 Pa for 48 h using a freeze-dryer (Labconco, MO, USA).

SC-CO₂ drying

The blanched turmeric samples were dried according to Kaur, et al.¹² using a lab-scale SC-CO₂ technology (SFT-120, Supercritical Fluid Technologies Inc., DE, USA). First, the samples were loaded into a thimble and placed in a 100 mL high-pressure vessel with glass wool placed on both ends of the vessel. The vessel and micrometering valve temperatures were set to 60 °C. The micrometering valve was heated to prevent freezing due to the Joule-Thomson effect. After the set temperatures were attained, the system was pressurized to 20 MPa. The SC-CO₂ drying continued for 6 h with a CO₂ flow rate of 3 L/min. Drying conditions for SC-CO₂ were determined through preliminary experiments (pressure: 10–20 MPa, temperature: 40–60 °C, and CO₂ flow rate 1–3 L/min) and literature¹². After the drying process was finished, the system was depressurized. Finally, turmeric samples were collected and stored at room temperature (23 °C) in airtight containers.