Drying Behavior and Effect of Drying Temperatures on Cyanide, Bioactive Compounds, and Quality of Dried Cassava Leaves

Abstract

In this study, the drying behavior and quality of the dried leaves of cassava (Manihot esculenta Crantz) of the ‘Rayong 5’ cultivar from Thailand were investigated. An increase in the drying temperature resulted in an increased drying rate and a reduction in drying time. The Page model provided the best fit for describing the drying characteristics of cassava leaves, with the entire drying process occurring in the falling rate period. The results showed that cyanide content was sensitive to high temperatures, with drying at 80 °C being the most effective method for toxin elimination. Prolonged drying periods lead to the degradation of vitamin C. Drying cassava leaves at 50–80 °C did not significantly affect β–carotene levels. However, lutein, chlorophyll–a, and chlorophyll–b were reduced after drying. The drying processes did not change the crude proteins content but increased the levels of histidine, alanine, and aspartic acid. In this study, high-temperature, short-time drying was identified as the optimal condition for detoxification, maintaining nutrients, and preserving the color of dried cassava leaves.

1. Introduction

Cassava (Manihot esculenta Crantz) is one of the most important staple crops in tropical and subtropical regions. Thailand is the third largest cassava producer, with a production of 30.6 million tonnes over 1.5 million ha [1]. Cassava is a vital source of carbohydrates from its roots, while its leaves are often abandoned in the field. Cassava leaves are rich in nutrients (proteins, vitamins) and bioactive compounds (carotenoids, chlorophylls), making them valuable supplementary sources of nutrition for humans [2,3]. However, their high cyanide content 1315.97 ± 141.64 mg/kg (dry basis, d.b.) limits direct consumption. In some African and Asian countries, cassava leaves are consumed as vegetables after appropriate processing [4,5]. Fresh cassava leaves contain 63.41–73.14% (wet basis, w.b.) moisture [3], which gives them a short shelf life. Drying is commonly used to stabilize perishable produce by reducing the water content to a safe level, thereby preventing microbial spoilage.

The drying characteristics and modeling of the drying process are crucial for determining the optimal drying conditions and predicting the drying time needed to achieve the target moisture content of dried products. The drying behavior of agricultural products could be described using various models; however, thin-layer models are widely used to describe the drying behavior of different types of leaves [6]. Several mathematical models are suitable for drying leaves, such as the Lewis model for black tea [7], the Page model for bay leaves and thymus leaves [8,9], the Henderson and Pabis model for mint leaves [10], and the Verma model for parsley leaves [11]. For cassava, the thin-layer drying model has been studied for cassava root pulp [12]. However, the drying of cassava leaves has not been examined, indicating a need for further investigation to determine a suitable mechanism and drying behavior for cassava leaves.

The heat during the drying process can affect the quality of dried leaves. Bioactive compounds content, including luteoxanthin, lutein, zeaxanthin, β–carotene, tocopherol, vitamin C, chlorophyll–a, chlorophyll–b, and antioxidant activity, decreased in Moringa oleifera leaves after hot-air drying [13]. Different drying temperatures can significantly impact the final quality of dried leaves. In dried sea buckthorn leaves, carotenoids, chlorophylls, and total phenolics decrease as drying temperatures increase [14]. Similarly, vitamin C in dried stevia leaves decreased with increasing drying temperature, while total phenolics and total flavonoids increased after drying at 30 °C, 40 °C, and 50 °C but decreased at high temperatures (60 °C, 70 °C, and 80 °C) [15]. The cyanide content in cassava leaves is retained at 64.9% after oven drying [16]. The optimum temperature for linamarase activity is 55 °C [17].

This study explored efficient drying temperatures for maximizing the quality of dried cassava leaves. The drying process and product quality were optimized by analyzing drying behavior using an appropriate kinetic model at various temperatures. The effects of hot-air drying at temperatures ranging from 40 to 80 °C on leaf quality were assessed. The findings will help identify the optimal drying conditions for maintaining nutrients and bioactive compounds and eliminating cyanide in cassava leaves.

2. Materials and Methods

2.1. Materials

Cassava leaves (Manihot esculenta Crantz) from the ‘Rayong 5’ cultivar, grown for 6 months in Lao Khwan District, Kanchanaburi Province, Thailand (latitude 14°25′39.1″ N, longitude 99°43′19.8″ E), were used in this study. Leaves at seventh to ninth positions from the shoot apex, along with petiole, were harvested by hand in the morning (6:00–8:00 a.m.) These leaves were selected for their high nutrients, bioactive compounds, and total cyanide content [3].

The fresh leaves were kept in a polystyrene box with ice after harvest and during transportation to the laboratory at Silpakorn University in Nakhon Pathom province, which took about 2 h. The samples were stored in a cold room at 10 ± 2 °C until use.

2.2. Drying Characteristic

The drying characteristics of cassava leaves were studied using a high-precision hot-air laboratory dryer at the Institute of Agricultural Engineering, Hohenheim University [18]. Approximately 1 cm was removed from both tips and stalks of the leaves, which were then chopped into cross-sectional lengths of 2–3 cm. For each batch, the chopped samples were unfolded and evenly spread onto a stainless-steel mesh tray, maintaining a load of 1 kg per 1 m², with a sample thickness of approximately 4 cm. During the experiments, the leaves were dried without turning. Drying temperatures were set at 40, 50, 60, 70, and 80 °C, with a constant air velocity of 0.5 m/s using over- and underflow mode. The specific humidity of the drying air was maintained at 25 g/kg dry to simulate typical conditions in Thailand. The drying experiments were conducted in duplicate. During the drying process, the samples were automatically weighed every 30 min, and the mass was recorded to monitor water loss until a constant mass was achieved. The recorded sample mass was used to calculate the moisture content at specific time intervals using Equation (1). The dried samples were immediately packed and sealed in aluminum foil bags and left overnight at room temperature to achieve homogeneous moisture content. The moisture content of samples was analyzed as described in Section 2.4. The moisture ratio (MR) during drying was calculated using Equation (2), while the drying rate (DR) at each drying temperature was determined using Equation (3). Six mathematical-thin layer models (Lewis, Page, Henderson and Pabis, Two–term exponential, Verma, and Diffusion approximation) were applied to the experimental data to identify the most suitable model describing the drying behavior of cassava leaves. The coefficient of determination (R²), mean absolute percentage error (MAPE), and root mean square error (RMSE) for the drying models were calculated using Equations (4), (5), and (6), respectively. The most suitable drying model was selected based on the lowest MAPE and RMSE values, along with the highest R² value.

$${\mathrm{MC}}_{\mathrm{t}}=100 \times \left(1-\left({\displaystyle \frac{{\mathrm{W}}_{\mathrm{eq}}}{{\mathrm{W}}_{\mathrm{t}}}}\times {\displaystyle \frac{{100-\mathrm{MC}}_{\mathrm{eq}}}{100}}\right)\right)$$

(1)

$$\mathrm{MR}={\displaystyle \frac{{\mathrm{MC}}_{\mathrm{t}}{-\mathrm{MC}}_{\mathrm{eq}}}{{\mathrm{MC}}_0{-\mathrm{MC}}_{\mathrm{eq}}}}$$

(2)

$$\mathrm{DR}={\displaystyle \frac{{\mathrm{MC}}_{\mathrm{t}+\mathrm{dt}}{-\mathrm{MC}}_{\mathrm{t}}}{\mathrm{dt}}}$$

(3)

$${\mathrm{R}}^2=1-{\displaystyle \frac{{{\displaystyle \sum }}_{\mathrm{t}=1}^{\mathrm{n}}{\left({\mathrm{MR}}_{\exp }{-\mathrm{MR}}_{\mathrm{pre}}\right)}^2}{{{\displaystyle \sum }}_{\mathrm{t}=1}^{\mathrm{n}}{\left({\mathrm{MR}}_{\exp }-\stackrel{-}{{\mathrm{MR}}_{\mathrm{pre}}}\right)}^2}}$$

(4)

$$\mathrm{MAPE}={\displaystyle \frac{100}{\mathrm{n}}}{{\displaystyle \sum }}_{\mathrm{t}=1}^{\mathrm{n}}\left|{\displaystyle \frac{{\mathrm{MR}}_{\exp }{-\mathrm{MR}}_{\mathrm{pre}}}{{\mathrm{MR}}_{\exp }}}\right|$$

(5)

$$\mathrm{RMSE}=\sqrt{{\displaystyle \frac{1}{\mathrm{n}}}{{\displaystyle \sum }}_{\mathrm{t}=1}^{\mathrm{n}}{\left({\mathrm{MR}}_{\exp }{-\mathrm{MR}}_{\mathrm{pre}}\right)}^2}$$

(6)

where t is drying time, MC₀ is initial moisture content, MC_t is moisture content at time t min, MC_t+dt is moisture content at time t + dt min, MC_eq is moisture content at equilibrium, W_t is sample weight at time t min, W_eq is sample weight at equilibrium, ME_exp is experimentally observed moisture ratio, and MR_pre is predicted moisture ratio.

2.3. Drying for Quality Analysis

The effects of the drying temperature on the dried-leaf quality were carried out using a hot-air convective cabinet dryer (12 kW/380V, Kluay Nam Thai, Bangkok, Thailand) at the Department of Food Technology, Silpakorn University. The leaves were washed and air-dried to remove excess water using forced air from an electric fan for 10 min. The leaves were then cross-sectionally chopped into 2–3 cm lengths. In each run, 250 g of fresh samples were evenly spread on a 50 × 50 cm² stainless steel mesh tray. The leaves were dried at 40, 50, 60, 70, and 80 °C. The hot air was circulated by a fan at approximately 0.5 m/s over the sample trays. The samples were dried without turning until the midrib became brittle, achieving moisture content and water activity (a_w) levels below 10% w.b. and 0.60, respectively. The cabinet dryer was preheated for 1 h at each temperature before loading the sample trays. Triplicate runs were conducted for each drying temperature.

The dried samples were ground using a blender (HR2118, Philips, Eindhoven, The Netherlands) and sieved through a 500 µm laboratory test sieve (D-42759, Retsch, Haan, Germany) before being packed in aluminum foil bags. The samples were stored at –20 °C until chemical analysis.

2.4. Quality Analysis

For quality measurement, all analyses were conducted in triplicate, except for amino acids, which were analyzed once for each drying run. The CIE color values, including lightness (L*), green–red color (a*), and blue–yellow color (b*), of the dried sample were measured using a color meter (ColorFlex EZ, HunterLab, Reston, VA, USA). Hue angle (h°) and chroma (C*) were then calculated. Water activity (a_w) was measured using a water activity meter (AQUALAB 4TE Meter, Pullman, WA, USA). The moisture content was determined by drying in a hot-air oven (FD53, Binder, Tuttlingen, Germany) at 105 °C for 24 h [19]. The total cyanide was measured using picrate paper kits [20], and crude proteins were analyzed using the Kjeldahl method [19]. Amino acid profiles were analyzed following the European Commission regulation VO (EC) Nr. 152/2009 III F for all amino acids, except tryptophan, which was analyzed using the VO (EC) Nr. 152/2009 III G method [21]. The analysis was performed using liquid ion exchange chromatography systems (Biochrom30, Biochrom, Cambridge, UK) at the Core Facility Hohenheim (CFH), Stuttgart, Germany.

Bioactive compounds, β–Carotene, lutein, chlorophyll–a, and chlorophyll–b, were analyzed using high-performance liquid chromatography (HPLC) according to the method of Lee et al. [22], with some modifications, as described in a previous study [3]. Standard reagents β-carotene, lutein, chlorophyll–a, and chlorophyll–b were obtained from Sigma-Aldrich and prepared in acetone at concentrations 0 to 100 mg/kg. The leaf samples (1 g fresh or 0.5 g dried) were extracted with 30 mL of acetone at 35–40 °C for 90 min in an ultrasonic bath (T780/H, Transsonic, Elma, Germany). The mixture was filtered through a 0.45 µm PTFE syringe filter before HPLC analysis. The HPLC system (Shimadzu, Kyoto, Japan) included an LC-20 AT pump, DGU-20A degassing unit, SIL-20A HT autosampler, SPD-M20A UV detector, CBM-20A controller, and CTO-20A column oven. Separation was performed using a C30 column (Stability 100, 5 µm, 250 mm × 4.6 mm I.D.) with a guard column (ST15.30.S2546, Dr. Maisch, Ammerbuch, Germany) at 30 °C. The mobile phase consisted of 75% methanol (solvent A) and 100% ethyl acetate (solvent B). The gradient flow (1.0 mL/min) was set as follows: 0–15 min, 30–90% B; 15–20 min, 90–30% B; followed by a constant 30% B until the end of the run at 25 min. A photodiode array detector at 450 nm measured the peak areas for quantification. Vitamin C was determined using HPLC (Shimadzu, Kyoto, Japan) according to the method of Valente et al. [23]. Each sample of cassava leaves (1 g fresh or 0.5 g dried) was placed in a small mortar with 5 mL of extraction solution containing 10% (w/v) perchloric acid and 1% (w/v) meta–phosphoric acid. The sample was crushed for 2 min; then, another 15 mL of the extraction solution was added, and it was left for 10 min. The sample was centrifuged (Sorvall RC6 superspeed centrifuge, Kendro, Hanau, Germany) for 10 min at 4 °C and 15,000 rpm. The solutions were then filtered through a 0.45 µm nylon membrane. The components were separated using a Luna C18(2) 100A column (5 µm, 250 mm × 4.6 mm I.D.) (Phenomenex, Torrance, CA, USA). The mobile phase consisted of 20 mM ammonium dihydrogen phosphate mixed with 0.015% (w/v) meta–phosphoric acid in HPLC water, adjusted to pH 3.5 with 85% orthophosphoric acid. The mobile phase was set at a flow rate of 0.6 mL/min. Ascorbic acid quantification was performed at 254 nm. The standard curve was prepared from L–ascorbic acid (Chem Supply, South Australia, Australia) diluted with the mobile phase.

2.5. Statistical Analysis

Mathematical models for leaf drying were fitted by non-linear regression method using the Solver program in Microsoft Excel (Microsoft Excel 2016, Microsoft Corporation, Redmond, WA, USA). Analysis of variance (ANOVA) and Duncan’s multiple range test were used to compare the means of chemical values in fresh and dried cassava leaves at different drying temperatures. Statistical analysis was conducted using SAS software (version 9.2, SAS Institute Inc., Cary, NC, USA)

3. Results and Discussion

3.1. Drying Characteristic of Cassava Leaves

The initial moisture content of fresh cassava leaves was 69.50 ± 0.79% w.b. After drying, the moisture content decreased to a constant value (MC_eq) of 8.26 ± 0.04, 5.53 ± 0.33, 4.46 ± 0.13, 3.50 ± 0.11, and 3.01 ± 0.02% w.b. at drying temperatures of 40, 50, 60, 70, and 80 °C, respectively (Figure 1A). The drying times required to reach MC_eq were 20, 7, 6, 3, and 2.5 h for these respective temperatures. The Page model is consistent with previous studies indicating that higher drying temperatures result in shorter drying times due to increased drying rates [24,25]. However, higher drying temperatures can also lead to the degradation of nutrients and bioactive compounds. Figure 1C illustrates the variation in drying rate with the moisture ratio during cassava leaf drying at various temperatures. Initially, the drying rate was notably high and gradually decreased as the moisture ratio declined. The drying process occurred during the falling rate period at all temperatures, with no constant rate period detected due to the insignificant amount of unbound moisture in the leaves [6].

Six widely used thin-layer drying kinetic models were applied to fit the drying curve of cassava leaves, and the statistical results are presented in

Table 1. Drying models fitted at different temperatures and statistical results.

Models Equation *	Temperature (°C)	Parameters			R2	MAPE	RMSE
		k	a	n, g, b
Lewis MR = exp(−kt)	40	0.2242			0.9895	2.6157	0.0045
	50	0.4826			0.9860	5.0923	0.0096
	60	0.7593			0.9873	8.8639	0.0099
	70	1.1796			0.9879	9.9045	0.0145
	80	1.8493			0.9939	15.2210	0.0115
Page MR = exp(−ktn)	40	0.1570		1.2065	0.9982	0.6074	0.0019
	50	0.3843		1.2419	0.9973	1.6251	0.0042
	60	0.6566		1.2913	0.9988	1.6186	0.0031
	70	1.1590		1.3277	0.9999	0.9707	0.0014
	80	2.0584		1.3374	0.9998	3.6879	0.0018
Henderson and Pabis MR = a·exp(−kt)	40	0.2352	1.0533		0.9920	2.1846	0.0040
	50	0.5020	1.0440		0.9882	4.5775	0.0088
	60	0.7868	1.0423		0.9891	7.8898	0.0091
	70	1.2063	1.0278		0.9889	9.4084	0.0139
	80	1.8624	1.0103		0.9941	14.9701	0.0114
Two-term exponential MR = a·exp(−kt) + (1 − a)exp(−kat)	40	0.2994	1.7341		0.9980	0.8556	0.0020
	50	0.6574	1.7682		0.9969	2.2231	0.0045
	60	1.0661	1.8330		0.9982	2.1726	0.0037
	70	1.7191	1.9075		0.9996	2.2986	0.0027
	80	2.7258	1.9866		0.9997	3.3528	0.0024
Verma MR = a·exp(−kt) + (1 − a)exp(−gt)	40	0.1435	5.9405	0.1316	0.9967	1.9392	0.0026
	50	0.2653	7.9531	0.2437	0.9976	1.7214	0.0039
	60	0.4698	10.2358	0.4469	0.9945	7.7550	0.0065
	70	0.6640	9.6509	0.6225	0.9965	2.2694	0.0078
	80	3.3682	23.6361	3.4944	0.9998	3.5129	0.0021
Diffusion approximation MR = a·exp(−kt) + (1 − a)exp(−kbt)	40	0.3481	5.1768	1.1411	0.9984	0.6725	0.0018
	50	0.2731	4.7597	0.8614	0.9976	1.7214	0.0039
	60	0.4800	5.6582	0.9101	0.9945	7.7364	0.0065
	70	0.6812	5.5214	0.8887	0.9965	2.2555	0.0078
	80	0.3481	5.1768	1.1411	0.9997	3.3528	0.0024

MR: moisture ratio; t: time (min); k, a, n, g, b: model parameters. *: [6].

. The Page model was identified as the most suitable for describing the drying behavior of cassava leaves at the temperatures between 40 and 80 °C, as it yielded the highest R² (0.9973–0.9999) and lowest MAPE (0.6074–3.6879%) and RMSE (0.0014–0.0042). This model is often used to describe the drying of various leaves, such as thymus, pandanus, olive, kaffir lime, mint, CTC tea, moringa, coriander, parsley, rosemary, nettle, and bay [6]. The drying curves fitted with the Page model are shown in Figure 1A. The moisture ratio continuously decreased during drying. The k value in the model equations, which represents the drying rate, increased with higher temperatures. Figure 1B shows the Arrhenius plot between ln k and the inverse temperature (1/T) according to Equation (7):

$$\ln \mathrm{k}=\ln \mathrm{A}-{\displaystyle \frac{\mathrm{E}}{\mathrm{RT}}}$$

(7)

where A is pre-exponential factor, E is activation energy (kJ·mol⁻¹), R is gas constant (8.314 J·mol⁻¹K⁻¹), and T is the drying temperature (K). The result shows that the Arrhenius equation well described the relationship between k and the drying temperature. The activation energy was 57.6 kJ·mol⁻¹_, which is close to that for rue leaves (60.58 kJ·mol⁻¹) [26], but much higher than those for bushbuck leaves (26.05 kJ·mol⁻¹) [27] and broccoli leaves (27.06 kJ·mol⁻¹) [28].

3.2. Effect of Drying on Cassava Leaves Quality

3.2.1. Quality of Fresh Cassava Leaves

The chemical composition of fresh cassava leaves varied depending on the environmental conditions, such as rainfall and temperature, on different harvest dates. On average, fresh samples contained 72.34 ± 0.89% w.b. of moisture content, 35.02 ± 1.78 g/100 g d.b. of crude proteins, 971.25 ± 197.45 mg/100 g d.b. of vitamin C, 136.84 ± 5.68 mg/100 g d.b. of β–carotene, 81.62 ± 3.70 mg/100 g d.b. of lutein, 542.90 ± 32.62 mg/100 g d.b. of chlorophyll–a, 253.74 ± 13.59 mg/100 g d.b. of chlorophyll–b, and 1315.97 ± 141.64 mg/kg d.b. of total cyanide. The average amount of 18 amino acids in fresh cassava leaves is shown in Figure 2, indicating that the major amino acids were glutamine (3.87 ± 0.27 g/100 g d.b.), aspartic acid (3.50 ± 0.16 g/100 g d.b.), and leucine (3.17 ± 0.21 g/100 g d.b.), with lower amounts of cysteine, methionine, tryptophan, and histidine.

3.2.2. Effect of Drying Temperatures on Cassava Leaves Quality

Table 2. Drying time, moisture content, water activity (a_w), and retention (%) of compositions in dried cassava leaves at different drying temperatures.

Drying Temperatures	Time (h:min)	Moisture (% w.b.)	aw	Retention (%)
				Crude Proteins	Vitamin C	β-Carotene	Lutein	Chlorophyll–a	Chlorophyll–b	Total Cyanide
Fresh	-	72.34 ± 0.89 a	0.99 ± 0.00 a	100.00 a	100.00 ab	100.00 ab	100.00 a	100.00 a	100.00 a	100.00 a
40 °C	14:00 ± 0:00	8.93 ± 0.17 b	0.56 ± 0.02 b	103.56 ± 2.83 a	29.25 ± 10.27 e	91.55 ± 5.83 c	87.84 ± 4.35 d	66.77 ± 1.48 bc	68.70 ± 2.27 b	82.46 ± 9.02 b
50 °C	5:34 ± 0:07	8.09 ± 1.34 bc	0.50 ± 0.09 bc	100.77 ± 2.45 a	57.50 ± 10.97 d	95.95 ± 0.80 bc	89.22 ± 2.03 cd	71.40 ± 5.63 bc	72.40 ± 6.19 b	79.16 ± 2.56 b
60 °C	2:52 ± 0:28	6.64 ± 1.24 c	0.41 ± 0.09 c	99.52 ± 2.38 a	75.68 ± 2.25 c	94.80 ± 1.31 bc	86.67 ± 2.36 d	66.03 ± 5.07 c	66.94 ± 5.06 b	66.95 ± 2.01 c
70 °C	1:33 ± 0:03	7.29 ± 1.00 bc	0.46 ± 0.07 bc	102.07 ± 1.80 a	88.15 ± 12.01 bc	99.77 ± 1.68 ab	92.93 ± 0.36 bc	72.85 ± 1.32 b	73.88 ± 2.37 b	54.36 ± 10.60 d
80 °C	1:01 ± 0:01	6.41 ± 0.70 c	0.41 ± 0.05 c	100.54 ± 4.97 a	106.60 ± 5.12 a	102.87 ± 4.00 a	94.73 ± 2.67 b	71.89 ± 3.49 bc	72.15 ± 4.75 b	37.40 ± 3.06 e

Different superscripts in the same column indicate significant differences in means (Duncan’s test, n = 3; p < 0.05).

presents the drying time, moisture content, water activity, and the percentage of remaining components of cassava leaves dried at different temperatures using a cabinet dryer. Drying cassava leaves at 40 °C required 14 h to achieve a a_w less than 0.6, whereas drying at 80 °C reduced the drying time by more than 90%.

The final moisture content of the dried leaves ranged from 6.41 to 8.93% w.b., with a_w between 0.41 and 0.56. Leaves dried at higher temperatures had lower final moisture and a_w compared to those dried at lower temperatures. This was because the drying process was stopped when the midribs of the leaves became dry and brittle, which may be attributed to the over-drying of the lamina leaf area.

Cyanide is the natural toxin which limited the use of cassava leaves for human consumption. The total cyanide content in dried cassava leaves decreased as the drying temperature increased. Although higher temperatures can reduce the cyanide content, they can also lead to the degradation of other nutrients and bioactive compounds. Leaves dried at 40 °C retained 82.46% of cyanide (1084.55 ± 165.55 mg/kg), while at 80 °C, only 37.40% (490.86 ± 52.36 mg/kg) persisted. These results are consistent with previous studies showing lower cyanide content at higher temperatures and longer drying times [29,30]. Temperature was identified as the primary factor influencing the cyanide content. This study demonstrated that drying alone might not sufficiently reduce cyanide content to the recommended safe level of 10 mg/kg for human consumption, as suggested by FAO/WHO [31]. Therefore, additional cooking or processing of dried cassava leaves before consumption is necessary. Moreover, a long drying process at low temperatures may not effectively prevent mold formation and could result in significant nutrients and bioactive compounds loss.

At drying temperatures of 70 and 80 °C, the vitamin C content remained relatively high, retaining 88.15% and 106.60% of the levels found in fresh leaves, respectively. In contrast, drying at lower temperatures, which required a longer duration, significantly reduced vitamin C content. At 40 °C, only 29.25% of the vitamin C content remained. These findings highlight that the drying time significantly impacts the degradation of vitamin C in cassava leaves. Similar results have been observed in previous studies, showing higher losses of vitamin C occur when drying cabbage outer leaves at lower temperatures and for longer durations [32]. Roshanak et al. [33] reported that tea leaves dried at 80 °C retained more vitamin C than those dried at 60 °C or sun-dried. Dried dill greens at 50 °C retained 81.37% of the vitamin C content compared to fresh samples [34].

The remaining β–carotene in cassava leaves dried at 50, 60, 70, and 80 °C was not significantly different from that in fresh leaves, retaining 95.95%, 94.80%, 99.77%, and 102.87%, respectively. However, at 40 °C, the β–carotene content was significantly lower, at 91.55%. The higher drying temperatures, which reduced drying time, resulted in less β–carotene degradation, as observed in dried apricots [35]. Similarly, Muratore et al. found that drying tomatoes at 80 °C retained more β–carotene compared to drying at 40 °C or 60 °C [36].

The lutein content significantly decreased after drying with a cabinet dryer at all temperatures, with retention levels ranging from 86.67% to 94.73%. Similarly, drying curry leaves in a hot-air oven at 60 °C retained 51.61% of lutein [37], while drying Moringa oleifera leaves at 50 °C for 12 h retained 47.7% of trans-lutein [13]. In guabiju and red guava, lutein content was reduced to 25.42% and 33.85%, respectively, after being hot-air dried at 70 °C [38]. Moreover, lutein content in marigold flowers decreased to approximately half of the fresh samples concentration when dried at 60 °C for 4 h [39]. Lutein concentration can change due to isomerization, epimerization, oxidation, or other degradation processes during heat treatment [40].

Chlorophyll–a and chlorophyll–b were significantly reduced after drying with a cabinet dryer, with contents decreasing to 66.03–72.85% and 66.94–73.88%, respectively. A similar result was found in a previous study by Naidu et al. [34], who found that hot-air drying dill greens at 50 °C for 6.5 h reduced chlorophyll–a and chlorophyll–b to 60.29% and 80.95% of their values in fresh samples. These reductions in chlorophylls in dried cassava leaves were due to the degradation caused by heat and chlorophyllase enzyme activity from the leaf tissues [22].

Crude proteins content of cassava leaves after drying remained comparable to that of fresh samples. This finding aligns with a previous study, which reported that the crude proteins content remained at 99.63% in cassava leaves dried in a solar tunnel dryer [41].

Most of the amino acids remained stable after drying, though some showed significant changes (

Table 3. Changes in amino acids content (%) in dried cassava leaves at different drying treatments compared to that in fresh leaves.

Amino Acid	Fresh	40 °C	50 °C	60 °C	70 °C	80 °C
Essential amino acid
Histidine	100.00 b	112.32 ± 5.07 a	111.46 ± 2.80 a	108.09 ± 2.84 a	111.92 ± 1.60 a	111.85 ± 4.15 a
Isoleucine	100.00 a	105.56 ± 5.18 a	105.08 ± 1.87 a	101.49 ± 4.30 a	101.72 ± 3.84 a	100.52 ± 4.98 a
Leucine	100.00 a	105.63 ± 3.34 a	105.43 ± 1.91 a	101.69 ± 3.08 a	102.45 ± 3.73 a	101.62 ± 5.26 a
Lysine	100.00 a	102.11 ± 2.35 a	102.75 ± 1.50 a	99.76 ± 3.80 a	100.33 ± 2.81 a	100.06 ± 5.73 a
Methionine	100.00 a	107.37 ± 3.68 a	105.71 ± 4.82 a	103.75 ± 1.80 a	103.98 ± 1.47 a	103.40 ± 7.01 a
Phenylalanine	100.00 a	105.87 ± 3.57 a	105.18 ± 1.92 a	101.44 ± 3.49 a	101.71 ± 4.01 a	101.30 ± 5.39 a
Threonine	100.00 b	108.64 ± 3.18 a	107.20 ± 2.46 a	103.32 ± 2.73 ab	104.09 ± 3.13 ab	102.89 ± 5.24 ab
Tryptophan	100.00 a	109.58 ± 7.49 a	107.11 ± 7.77 a	104.96 ± 10.51 a	105.67 ± 11.02 a	105.39 ± 10.04 a
Valine	100.00 a	106.12 ± 5.75 a	105.48 ± 2.64 a	101.30 ± 3.22 a	101.26 ± 3.20 a	99.89 ± 4.96 a
Non-essential amino acid
Alanine	100.00 b	110.14 ± 3.73 a	113.61 ± 2.11 a	114.16 ± 2.67 a	115.03 ± 2.32 a	110.77 ± 5.34 a
Arginine	100.00 b	104.36 ± 3.46 b	106.41 ± 1.39 a	102.48 ± 3.24 b	103.95 ± 3.09 b	102.46 ± 5.18 b
Aspartic	100.00 abc	105.32 ± 3.11 a	100.39 ± 1.49 ab	94.62 ± 3.44 bcd	94.00 ± 3.34 cd	93.57 ± 5.40 d
Cysteine	100.00 b	137.85 ± 8.11 a	137.52 ± 7.28 a	128.50 ± 13.71 a	129.37 ± 8.68 a	135.44 ± 14.02 a
Glutamine	100.00 bc	106.56 ± 4.06 a	104.73 ± 2.16 ab	99.01 ± 3.70 bc	97.00 ± 2.11 c	95.93 ± 4.70 c
Glycine	100.00 a	105.62 ± 3.75 a	104.60 ± 2.40 a	100.18 ± 2.58 a	101.14 ± 3.08 a	100.58 ± 4.40 a
Proline	100.00 a	108.52 ± 1.01 a	106.31 ± 1.52 a	102.65 ± 4.32 a	101.63 ± 8.04 a	101.64 ± 7.98 a
Serine	100.00 b	106.60 ± 2.19 a	103.77 ± 2.59 ab	100.13 ± 1.27 b	101.56 ± 3.72 ab	101.87 ± 4.06 ab
Tyrosine	100.00 a	105.27 ± 4.77 a	105.48 ± 4.17 a	100.32 ± 2.74 a	100.65 ± 2.93 a	99.48 ± 3.86 a

Different superscripts in the same row indicate significant differences in means (Duncan’s test, n = 3; p < 0.05).

). Histidine, alanine, and cysteine content in dried cassava leaves significantly increased at all drying temperatures (40–80 °C). These results align with a previous study, which found that the amino acid profile (g amino acid/kg total proteins) of cooked cassava leaves was not significantly changed from raw cassava leaves, except for a significant increase in alanine after thermal cooking [4]. Cooking cassava leaves in boiling water reduced total proteins and amino acid content by around 58% due to diffusion and thermal degradation [4]. Weiss et al. [42] reported that the eight amino acids (glycine, cysteine, aspartic acid, asparagine, glutamic acid, glutamine, arginine, and histidine) decomposed when heated between 185 °C and 280 °C. However, drying cassava leaves at the highest temperature (80 °C) specifically degraded aspartic acid content.

Table 4. Appearances of dried and ground cassava leaves from different drying treatments, along with their CIE color values.

Drying Methods	Dried Leaves	Color Values of Ground Leaves
		Ground Leaves	L*	a*	b*	Hue Angle	Chroma
40 °C			41.65 ± 0.87 a	−10.71 ± 0.19 a	27.99 ± 1.43 a	110.96 ± 0.62 a	29.97 ± 1.41 a
50 °C			41.50 ± 0.20 a	−10.75 ± 0.16 a	27.82 ± 1.33 a	111.14 ± 0.74 a	29.82 ± 1.28 a
60 °C			41.28 ± 0.35 a	−10.69 ± 0.23 a	27.73 ± 1.69 a	111.11 ± 0.77 a	29.72 ± 1.66 a
70 °C			40.79 ± 0.10 ab	−10.94 ± 0.19 a	28.54 ± 0.60 a	110.98 ± 0.69 a	30.57 ± 0.51 a
80 °C			40.20 ± 0.54 b	−11.32 ± 0.17 b	28.52 ± 1.02 a	111.65 ± 0.59 a	30.68 ± 0.99 a

Different superscripts in the same column indicate significant differences in means (Duncan’s test, n = 3; p < 0.05).

shows the appearance and color values of dried cassava leaves. Leaves dried at all temperatures exhibited similar appearances. The drying temperature did not significantly affect the color of dried leaves in terms of hue and chroma value (p > 0.05). However, the color values revealed that the −a* value, which indicates the greenness of cassava leaves, was more negative for leaves dried at 80 °C compared to other temperatures. Additionally, the L* value of cassava leaves dried at 80 °C was significantly lower than that of leaves dried at lower temperatures, which took longer. The slightly greener color of dried leaves obtained at higher drying temperatures was due to the inactivation of polyphenol oxidase enzyme, leading to a reduction in enzymatic browning [43].

4. Conclusions

In this work, higher drying temperatures resulted in shorter drying times in a hot-air drying of cassava leaves. The Page model provides the best fit for describing the drying behavior. Although higher temperatures can lead to the degradation of nutrients and bioactive compounds, temperature was the major factor affecting the cyanide content, whereas vitamin C was more sensitive to the drying duration. Leaf pigments, including carotenoids and chlorophylls, degraded more at lower temperatures and longer drying times than at higher temperatures with shorter durations. The drying process had minimal impact on the proteins content and amino acid profiles. In this study, the drying time and temperature did not significantly affect the product appearance. However, higher temperatures could lead to a change in the color of the dried leaves. In conclusion, to preserve cassava leaves effectively for further use, this study recommends drying at a high temperature (80 °C). This method is most effective in reducing cyanide; preserving nutrients, bioactive compounds, and color; and minimizing the processing time.