The ancient spice turmeric, often referred to as the "Golden Spice of India," originates from the rhizomes of Curcuma longa, a member of the ginger family (Zingiberaceae). Perennial in nature, turmeric is extensively grown in tropical and subtropical regions of the world, including India and China. India is one of the world's top producers and exporters of turmeric. India is the world's top producer, consumer, and exporter of turmeric, with an annual production amount of about 11 lakh metric tonnes. Production of turmeric in India is 80% of the world's turmeric, followed by China (8%), Myanmar (4%), Nigeria (3%), and Bangladesh (3%) [1].

Turmeric (Curcuma longa) rhizome is a common ingredient in Southeast Asian cuisine, valued for its use as a food flavouring, colouring agent, and spice. In addition to enhancing the flavour of food, turmeric, referred to as the "Kitchen Queen," has been used for centuries in traditional medicine as a home remedy for various ailments [2]. In the market, turmeric is typically available as fresh rhizomes or in the more convenient form of dried powder, which is favoured for commercial distribution. The price of turmeric powder varies based on factors such as quality, appearance (colour), moisture content, and levels of curcuminoids and phenolic content [3].

Turmeric is renowned for its health-promoting properties, including antioxidant and anti-carcinogenic effects, which contribute to the prevention of cancer and cardiovascular diseases. Dried rhizomes are the primary form in which turmeric is traded globally, as they can be directly used as a spice or processed to produce turmeric oleoresin and turmeric oil. The bioactive components of turmeric, particularly curcuminoids and volatile oils, have been highlighted in various studies for their health benefits [4,5]. Curcumin, the main curcuminoid, is responsible for turmeric’s bright yellow color, alongside demethoxycurcumin and bisdemethoxycurcumin. Additionally, turmeric’s essential oil, which gives it a spicy and aromatic flavor, contains biologically active constituents such as ar-turmerone, α-turmerone, and β-turmerone.

Drying is one of the oldest and most effective food preservation techniques, crucial for extending the shelf life of perishable items by reducing moisture content, which prevents bacterial growth and spoilage [6]. This method, used worldwide for centuries, is particularly valuable for preserving foods with high moisture levels, like fruits, vegetables, and spices, which are prone to quick deterioration [7]. Drying is another critically important step during the processing of turmeric, with the main aim being reducing moisture from 70–80% at the time of harvest to a safe limit of 10% for grinding or 6% for safe storage [8]. Among the various drying techniques, microwave drying is a common method.

Microwave drying offers the advantage of rapid energy transfer along with efficient mass transfer. This method shortens drying time and operates at lower temperatures, which helps preserve the quality of dried food products, including their colour, texture, and nutritional content, such as proteins, carbohydrates, lipids, antioxidants, and vitamins [9-11]. However, there is limited current research on how microwave drying affects the bioactive components and overall quality of turmeric. Therefore, the objective of this research project is to investigate the impact of microwave power and slice thickness on turmeric qualities, including curcumin content, oleoresin, volatile oil, and antioxidant activity, and to determine the optimal drying conditions for turmeric.

Sample Preparation

The fresh turmeric rhizomes (var. salem) were washed thoroughly in tap water to remove the adhering soil, and undesirable portions were removed manually. These washed and cleaned fresh turmeric rhizomes were used for further study. Finally, the cleaned turmeric rhizomes were sliced to about 2-6 mm in thickness as per experimental design using adjustable vegetable slicers.

Figure 1: Process Flow Chart for Microwave Drying of Turmeric Slices.

Drying Equipment

The microwave dryer available in the Department of Processing and Food Engineering was used for drying turmeric rhizomes. A belt was provided inside the chamber for placing and removing the sample from the continuous microwave dryer. A 250 g sample of turmeric rhizome was taken and spread uniformly over the belt in a single layer. Microwave power and drying time were adjusted to the desired level using the control unit. The sample was weighed at regular intervals by electronic balance. Drying was stopped when the drying mass reached the constant weight and no further drying took place.

Experimental Design

Response Surface Methodology (RSM) was used for designing the experiment trials. A Central Composite Rotatable Design (CCRD) of 2 variables at 5 levels each with 6 center point combinations [12].

Table 1: Treatment Combinations as Per CCRD.

Determination of Biochemical Parameters of Dried Turmeric Rhizomes

Biochemical parameters of dried turmeric rhizomes obtained through different treatments were determined by using standard protocols. All the samples with triplicates were analysed at a time for determination of a particular parameter.

Curcumin

Curcumin content was determined as per the procedure followed by [13].

Curcumin content was estimated by the following formula:

Oleoresin

Oleoresin content was determined as per IS Specification no [14].

Percentage of oleoresin was estimated using the formula:

Volatile Oil

Volatile oil yield was obtained by using the hydro-distillation (using the Clevenger apparatus) method as per FSSAI [15].

The percentage of oil recovered was estimated using the formula:

Antioxidant Activity

Antioxidant activity was measured by the DPPH free radical scavenging method as described by Chandra Shekhar and Anju [16]. From the different concentrations, 200 µg/ml was finalized for determination of antioxidant activity for all samples.

The following formula was used to measure the percent DPPH scavenging effect.

The results obtained and its relevant discussion have been given under the following heads:

Effect of Slice Thickness and Microwave Power on Quality Attributes of Turmeric

Curcumin

The response surface curves showed the effect of slice thickness and microwave power on curcumin, oleoresin, volatile oil, and antioxidant activity (Fig. 2). Curcumin in turmeric slices ranged from 3.49 to 4.32%. The maximum curcumin was observed for the combination of 5.5 mm slice thickness and 300 W microwave power, and minimum curcumin was found for the combination of 4 mm slice thickness and 650 W microwave power. The curcumin was decreased with an increase in microwave power and increased with an increase in slice thickness. This result was in accordance with the results of Kadam et al, who stated that as the thickness of the sample is reduced, the curcumin content goes on decreasing. Nithya et al found similar curcumin content for microwave drying, which was in the range of approximately 2.5 to 4%. Surendhar et al, Gagare et al, and Suresh et al reported that degradation of the curcumin was directly correlated with the drying temperature and time.

Oleoresin

The oleoresin of turmeric slices ranged from 9.54 to 11.27%. The maximum oleoresin was observed for the combinations of 5.5 mm slice thickness and 300 W microwave power, and minimum oleoresin was found for the combination of 4 mm slice thickness and 650 W microwave power. The oleoresin decreased with an increase in microwave power and increased with an increase in slice thickness. Jayashree et al. also found 10.51% oleoresin for 5 mm thick slices of turmeric. However, oleoresins are sensitive to light, heat, and oxygen. The heat treatment of the spice can lead to the degradation of its oleoresin, volatile oil, and pungent principles.

Volatile Oil

Volatile oil of turmeric slices ranged from 4.75 to 5.25%. The maximum volatile oil was observed for the combination of 5.5 mm slice thickness and 300 W microwave power, and minimum volatile oil was found for the combination of 4 mm slice thickness and 650 W microwave power. The volatile oil was increased with a decrease in microwave power and increased with an increase in slice thickness. Increasing the temperature and hence heat resulted in a higher rate of oxidation, resulting in the destruction of the biological structure of glandular trichomes, resulting in the vaporization of aromatic compounds. Low temperature during drying reduced the destructive effect of heat during drying, resulting in a higher essential oil yield.

Antioxidant Activity

The antioxidant activity of turmeric slices ranged from 43.64 to 49.75%. The maximum antioxidant activity was observed for the combination of 4 mm slice thickness and 650 W microwave power, and minimum antioxidant activity was found for the combination of 4 mm slice thickness and 250 W microwave power. The antioxidant activity was decreased with a decrease in microwave power and increased with an increase in slice thickness. Drying and processing methods are reported to have variable effects on the antioxidant activity of plant samples. Impacts include little or no change, significant losses, or enhancement in antioxidant activity. An increase in antioxidant activity from thermal treatment has been reported in red pepper and citrus peels. The increase in antioxidant activity following thermal treatment is attributed to the release of bound phenolic compounds brought about by the breakdown of cellular constituents and the formation of new compounds with enhanced antioxidant potential. Inchuen et al. reported that microwave drying resulted in higher DPPH radical scavenging activity than hot-air drying for curry powder. The increase in microwave power and the increase in DPPH radical potential were observed.

There is higher curcuminoids content and higher antioxidant capacity because these bioactive compounds have antioxidant properties. However, Singh et al. [8] reported other active substances with antioxidant capacity in turmeric, such as ar-turmerone and alpha-turmerone, the main constituents of essential oil and oleoresin. According to Chumroenphat et al, curcumin is degraded and transforms other phenolic compounds, particularly vanillin and ferulic acid, during the drying process. This degradation could be responsible for the final increase in the phenolic content and antioxidant capacity of turmeric. Jose and Joy reported that a decrease in curcumin content reflects the reduction of oleoresin content because curcumin and essential oil constitute more than 80% of the total turmeric oleoresin. A moderate temperature preserves curcuminoids and other compounds [7]. They reported that a longer microwave drying time increased antioxidant activity due to the high content of curcuminoids. It was also reported that microwave-vacuum drying could inhibit enzymatic browning and improve physical appearance as well as maintain bioactive compounds. Furthermore, the optimal condition for providing good quality turmeric was a high microwave power (3500–4000 W) and duration (27–30 min). Optimization for process parameters for curing and microwave drying of turmeric rhizomes was carried out by Gagare et al. Turmeric rhizomes were boiled in a 0.1% sodium carbonate solution for 15–45 min followed by microwave drying at a power of 1000–2000 W. Curcumin content was decreased when microwave power was increased from 1000W to 2000W.

Figure 2: 3D Response Surfaces of the Model Conditions.

Fig. 3 demonstrated a plot of probability and corresponding residuals and a plot of residuals versus the predicted response. Visually, it is revealed that the residuals were distributed closely to the 45-degree line, which suggests that errors follow normal distribution and least-square fit is adequate to produce residuals. The internally studentized residuals vs. run number plots indicated that the distribution of the data was within the 95% confidence interval, as well as all data were distributed in the red line (Fig. 3). These results confirmed the reliability and stability of the computer software based estimation. The data points follow no clear pattern and situate above and below the x-axis equally. These results suggest adequacy of the estimated model and no violation of the independence or constant variance assumption [17].

Figure 3: The Predicted Vs. Actual Value Plots (Left) and Internally Studentized Residual Vs. Run Number Plots (Right) of the Model Condition (A) Curcumin, (B) Oleoresin, (C) Volatile Oil, and (D) Antioxidant Activity.

Statistical Analysis and the Model Fitting

Fitting the data with various models, the ANOVA showed that the curcumin, oleoresin, volatile oil, and antioxidant activity (radical scavenging capacity) were described in quadratic polynomial models. The ANOVA (Table 2) showed that the calculated F-values for curcumin (21.66), oleoresin (41.30), and antioxidant activity (33.52) were significant at p<0.001. At the same time, it possessed a non-significant lack of fit (p>0.05). While the calculated F-value for volatile oil (10.92) was significant at p<0.01. The lack-of-fit p-value was larger than 0.05, meaning it was not significant relative to the pure error. So, non-significant lack of fit was good. Additionally, the low values of pure error showed good reproducibility of the data. The significant F-value and non-significant lack of fit indicate the fitness and reliability of the model for a given response. However, the adequacy of the model needed to be further checked by the coefficient of regression (R²), which is the ratio of the explained variation to the total variation and is a measure of the degree of fit. The closer the value of R² to unity, the better the empirical model fits the actual data [18]. The value of R² greater than 0.8 implies that the model indicates a good fit. The predicted R² is in reasonable agreement with the adjusted R²; i.e., the difference is less than 0.2 for all the models. The high Adeq Precision value (>4) again supported the significance of the model for curcumin, oleoresin, volatile oil, and antioxidant activity. The small value of the coefficient of variation for curcumin (2.01%), oleoresin (1.16%), volatile oil (1.19%), and antioxidant activity (1.13%) explained that the experimental results were precise and reliable. Smaller values of CV give better reproducibility; the CV lower than 10 indicates that we had developed an adequate response model (Table 3). R² and Adj R² also revealed excellent correlations between the independent variables. Table 4 demonstrated the empirical relationship between quality attributes and the test variables in coded units.

Table 2: Analysis of Variance (ANOVA) for Quadratic Model.

Table 3: Fit Statistics for Biochemical Parameters.

Table 4: Regression Equations for Biochemical Attributes.

Where, A and B are the coded factors of slice thickness and microwave power, respectively.

Optimization Process Condition for Sliced Turmeric Rhizomes

The optimum condition for slice thickness and microwave power was determined by the numerical optimization technique, using Design Expert software version 13 (State-Ease Inc., Minneapolis, MN, USA). The limiting conditions applied to the selected constraints during the optimization were given below in Table 5.

Table 5: Constraints, Criteria and Output for Numerical Optimization of Turmeric Slices.

Accordingly, the goals that were fixed for the process variables and response parameters to get the appropriate combination are summarized in Table 5. The equal importance of the value three was given to all the independent variables and response parameters for optimizing the process parameters. Upon the applied constraints, the optimum treatment conditions were obtained as 6 mm slice thickness and 402.952 W microwave power.

RSM was successfully applied to optimize conditions for maximum curcumin, oleoresin, volatile oil, and antioxidant activity (radical scavenging capacity). The curcumin, oleoresin, and volatile oil decreased with an increase in microwave power level. While antioxidant activity increased with an increase in microwave power level. The analysis showed that at the optimized levels of slice thickness and microwave power, it would be possible to increase the curcumin up to 4.273%, oleoresin up to 11.115%, volatile oil up to 5.241%, and antioxidant activity up to 45.897%. Upon the applied constraints, the optimum treatment conditions were obtained as 6 mm slice thickness and 402.952 W microwave power.

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