1 Introduction

Alfalfa is harvested multiple times annually, boasting a high yield and quality of roughage. Its robust adaptability to diverse climates and soil conditions contributes to its longevity. Alfalfa enhances soil structure and can be cultivated in both agricultural fields and pasture meadows, characterized by relatively low establishment costs. Because of its superior characteristics, alfalfa is defined as the “queen of fodders” [1, 2]. It offers an inexpensive and rich source of crude protein for livestock and has a high digestibility [3]. Alfalfa contains vitamins, minerals, carotene, and other pigments [4, 5]. There are various uses in livestock operations, such as fodder, roughage, hay, and pellets [6]. Because of these superior properties, it is primarily cultivated worldwide.

Alfalfa herbage should be dried rapidly before feeding it to animals [7]. Elevated moisture content can compromise herbage quality by promoting mold growth [8]. Natural sun drying is commonly employed for alfalfa due to its energy efficiency and minimal environmental impact [9]. However, open-sun drying is disadvantaged by exposure to rain, prolonged drying times, and nutrient loss. Mechanical drying methods are utilized to preserve nutrients and maintain freshness [7]. The drying process in alfalfa aids in nutrient preservation during transportation and storage [10] and mitigates losses in yield and quality [11]. Freeze-drying is effective in preserving unstable components found in fresh feedstuffs [12]. However, it requires expensive equipment and is impractical for large-scale agricultural commodities [13]. Microwave drying shows promise due to its ability to produce high-quality end products, uniform energy distribution, and high thermal conductivity. This method also minimizes thermal degradation and offers energy savings [14]. Microwaves ensure rapid and uniform heat distribution throughout the product, partially preserving enzyme activity [15, 16], making it successful for drying various plant materials [17]. Hybrid drying combines the advantages of air-convective and microwave drying, enhancing overall drying efficiency [18]. This innovative approach consumes less energy and yields high-quality products [19]. Optimal outcomes depend on precise control of temperature and microwave power settings [20].

Edible fibers, along with a variety of beneficial minerals and vitamins, are present in high-quality alfalfa hay. For dairy cattle, alfalfa serves as a palatable, high-energy, and protein-rich feed option. Due to nitrogen-fixing bacteria in its roots, alfalfa not only enhances soil nitrogen content but also ensures the production of high-protein feed in nitrogen-deficient soils [21]. Alfalfa ranks among the plants with the highest forage value and is commonly fed to animals as dried grass fodder. Moreover, alfalfa is currently one of the most extensively harvested feed crops globally, prized for its high nutritional content when selected for animal consumption [22]. Drying can easily alter alfalfa roughage's nutritional composition and appearance. During the process of drying samples, various physicochemical changes are encountered, and such changes alter the quality traits and nutritional composition of the end products [23]. Significant degradations are encountered in quality traits when the products are exposed to high temperatures for extended durations [24]. Recent research has focused on the chemical composition (protein, ash, oil, ADF, NDF, ADL, fiber), mineral content, energy, and digestible nutrients of animal feeds. Because of the increasing consumer interest in fatty acids, especially in the composition of total polyunsaturated fatty acids of dairy and meat products obtained from ruminant animals, research has also been conducted on the fatty acid profile of forage crops [13]. Alfalfa hay contains 23–26% crude protein, 18–23% acid detergent fiber, 27–32% neutral detergent fiber, 11–12% crude ash, 2010–260 relative feed values, 13–23 g/100 g linoleic acid, 41–59 g/100 g alpha-linolenic acid, 21–35% saturated fatty acid, 2–3% monounsaturated fatty acid and 61–76% polyunsaturated fatty acid [13, 25, 26]. Additionally, alfalfa is rich in phytochemicals such as phenols, flavonoids, vitamins (A and C), phytoestrogens, amino acids, and minerals [27, 28].

Previous studies mainly focused on air-convective drying in moringa [29], savory leaves [30], corn-soy [31], sorghum [32], fodder [33], forage grass [34], microwave drying in alfalfa [35, 36], freeze drying in chestnut [37], barley grass [38, 39], and hybrid drying in rice [40] drying and nutritional traits of forage crops. However, no study in which detailed drying profile, energy aspects, and color characteristics of alfalfa has been reported. Therefore, various drying techniques were applied in the present work. The main scope of this study is to investigate the effects of drying techniques on alfalfa. For this purpose, the chemical composition, fatty acids, carotene, mineral, and color properties of dried products were determined. In addition, moisture content, moisture ratio, drying rate, and energy aspects were revealed. Drying methods were also compared, and the systems yielding the best outcomes for all traits were identified.

2 Material and method

2.1 Fresh product

The “Magnum” alfalfa commercial cultivar was used in the present experiments. The research was conducted for growing seasons in Kayseri, Turkey (39°48’N, 38°73’E). Manual sowing was practiced on the 1st of May 2019 to have 2.5 kg of seeds per decade. With the sowing, 30 kg/ha of nitrogen (N) and 100 kg/ha of phosphorus (P2O5) were applied to experimental plots. The plants were grown with relevant institutional, national, and international guidelines and legislation. Alfalfa samples were harvested at 10% flowering period of the 1st cut. Plants were harvested from about 10–12 cm in height with a mowing machine. Before drying, harvested plants were thoroughly mixed to ensure homogeneity, and their physical cleanliness was checked. Harvested samples were kept at + 4 °C and 90% until drying. The stages of the alfalfas processing are presented in Fig. 1.

2.2 Drying process

Open-sun drying, shade drying, greenhouse drying, microwave (at 100W, 200W, 300W) drying, air-convective (100 °C, 80 °C and 60 °C) drying, hybrid (air-convective + microwave: 200W + 60 °C, 200W + 80 °C) drying and freeze drying (-55 °C) were used as drying treatments, and drying times are 45, 112, 91, 400, 105, 65, 660, 1080, 2640, 120, 90, and 4320 min, respectively. Air-convective, microwave, and hybrid drying were performed at an airflow rate of 0.5 m s−1. A hybrid oven (Arçelik KMF 833 I, Turkiye) offering combined air-convective and microwave drying was used for the hybrid drying of alfalfa samples (Fig. 2). In the hybrid dryer, microwave, and air-convective characteristics could simultaneously be used. Ambient temperature could be arranged from 40 °C to 280 °C. Air-convective drying processes were run at 0.5 m s−1 air flow rate and 70 °C temperature. The oven has a fan for air circulation, perforated polyamide platforms and trays to hold the samples. Open-sun drying was performed under direct sunlight on 50 × 50 cm drying papers at a mean temperature of 25.8 °C and mean relative humidity of 48.75%. Images of the drying greenhouse are presented in Fig. 3. Samples were dried in the greenhouse at 28.6–44.1 °C (mean temperature: 35.02 °C) and 34.20% relative humidity. The average solar radiation energy value of the region for the drying period is 62.2 kWh m−2 month−1. Greenhouse drying was performed in an 8 mm polycarbonate-covered (8 mm, 80–85% light transmittance) greenhouse (6 × 12 m = 72 m2) with a circulation and 4 ventilation fans and 1 circulation fan. Greenhouse sidewalls are 4 m high and ridge height is 5 m. Climate parameters (temperature, relative humidity, wind speed, and direction) could be arranged with the aid of a control panel equipped with sensors. The lyophilizator (Christ ALPHA 2–4 LSCplus, Osterode am Harz, Germany) as a freeze dryer is operated at -55 °C condenser temperature and 10–3 mbar pressure. Condenser volume and vacuum pump capacity are 5.8 lt and 5.4 m3, respectively. Alfalfas were preserved before the drying process in the freezer (VWR Symphony Model 414005–087, United States) at ultra-low temperature (-80 ºC) for 24 h.

Fig. 1
figure 1

Fresh alfalfa (a), harvest (b), weight measurements (c), drying and milling (d), weighing for biochemical analysis (e), labeling and packing (f, g), and burning for ash content (h)

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Fig. 2
figure 2

Schematic representation of microwave-convective hybrid dryer

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Fig. 3
figure 3

Images of drying greenhouse: (a) External view of the greenhouse, (b) Heater, (c), Ventilation and circulation fans, (d) Solar panels, (e) Tables where drying takes place, (f) Controller

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2.3 Drying kinetics, drying rate, and modeling of thin-layer drying

Drying-induced weight changes were converted into moisture contents and moisture ratios. The moisture content at a time t was calculated using the equation [41].

$$MR=\frac{{M}_{t}-{M}_{e}}{{M}_{0}-{M}_{e}}$$
(1)

where, Mt: Moisture at time t (d.b.), kg kg−1, M0: Initial moisture (d.b.), kg kg−1, Me: Equilibrium moisture (d.b.), kg kg−1.

The drying rate, which indicates the amount of moisture released from the product per unit of time, was calculated using the following equation [42].

$$DR=\frac{{M}_{t+dt}-{M}_{t}}{dt}$$
(2)

where DR is the drying rate (g water g dry matter-1 min), Mt + dt is the change in moisture at t dt (g water. g dry matter -1), and dt is the drying time (minute).

This study considered ten different mathematical drying models (Table 1). SigmaPlot software was used to model drying curves. Model performance was assessed through coefficient of determination (R2) and standard error of estimate (SEE). The model with the highest R2 and lowest SEE was identified as the best-fitting model for the relevant drying process [42].

$$SEE=\left[\sum_{i=1}^N\frac{\left({MR}_{exp,i}-{MR}_{pre,i}\right)^2}{N-2}\right]^{0.5}$$
(3)

where, MRexp,I: Experimental moisture ratio obtained from drying experiments, MRpre,I: Predicted moisture ratio, N: Number of experimental data, z: Number of parameters used in each model.

Table 1 Thin-layer mathematical models
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2.4 Color measurements

Color measurements were performed on fresh and dried alfalfa samples with a colorimeter (CR-400 Konica Minolta, Tokyo, Japan). CIELab color space was used for L*, a*, and b* values (the superscript “0” indicates the value of fresh samples). Color change (ΔE) and chroma (C) values were calculated using the following equations [50, 51].

$$C=\sqrt{{(a*)}^{2}+{(b*)}^{2}}$$
(4)
$$\Delta E=\sqrt{{(L*-L{*}_{0})}^{2}+{(a*-a{*}_{0})}^{2}+{(b*-b{*}_{0})}^{2}}$$
(5)

2.5 Thermal and energy efficiency analysis

The energy consumption of the drying process was measured with a digital watt-meter. Specific energy consumption (SEC), specific moisture absorption rate (SMER), energy efficiency (ηen), and latent, specific heat capacity and thermal efficiency values were calculated by following equations [51,52,53,54].

$$SEC=\frac{{E}_{c}}{{m}_{w}}$$
(6)
$$SMER=\frac{{m}_{w}}{{E}_{c}}$$
(7)
$${\eta }_{en}=\frac{{m}_{w}{\lambda }_{wp}}{{E}_{c}}$$
(8)
$$\frac{{\lambda }_{wp}}{{\lambda }_{w}}=1+23\text{exp}(-0.4X)$$
(9)
$${\eta }_{th}=\frac{PA\text{}{\lambda }_{w}({M}_{o}-{M}_{e})}{Ft(100-{M}_{e})}$$
(10)

where mw: Mass of evaporated water (kg), X: Sample moisture (kg water kg dry matter−1), λw: Latent heat of water (J kg−1).

$$\begin{array}{c}{\lambda }_{w}=2.503\times {10}^{6}-2.386\times {10}^{3}(T-273.16)\\ 273.16\le T({}^{o}K)\le 338.72\end{array}$$
(11)
$$\lambda_w=\left(7.33\times10^{12}-{1.60\times10^7T}^2\right)^{0.5}.338.72\leq T\left(^{o} K\right)\leq533.16$$
(12)

where λwp: Product's latent heat (J kg-1), P: Product's amount in the system (kg m−2), A: Tray area (m2), mw: Product's amount of water evaporated (kg), F: Dryer's heating capacity (kW), t: Drying time (min).

2.6 Effective moisture diffusivity

Fick's second law of diffusion determined the effective moisture diffusion (Deff) value. The general solution of the formula is presented below, assuming that the moisture transfer is only by diffusion and shrinkage, constant diffusion coefficients and temperature are negligible [55]:

$$MR=\frac{{M}_{t}-{M}_{{}_{e}}}{{M}_{o}-{M}_{e}}=\frac{8}{{\pi }^{2}}\sum_{n=1}^{\infty }\frac{1}{{(2n+1)}^{2}}\text{exp}\left(-\frac{{(2n+1)}^{2}{\pi }^{2}{D}_{eff}t}{4{L}^{2}}\right)$$
(13)

where Deff is the effective moisture diffusivity (m2 sec−1), L is the half thickness of the product (m), and t is the drying duration.

The first term of the above equation is used in the solution for longer drying durations. The slope of drying time vs. ln (MR) plot yields k0 in the equation below.

$$\text{ln}\left(MR\right)=\text{ln}\left(\frac{8}{{\pi }^{2}}\right)-\left(\frac{{\pi }^{2}{D}_{eff}t}{4{L}^{2}}\right)$$
(14)
$${k}_{0}=\frac{{D}_{eff}{\pi }^{2}}{4{L}^{2}}$$
(15)

2.7 Forage chemical composition

Dried alfalfa samples were ground and passed through a 1 mm sieve (IKA MF 10.2, Staufen, Germany) to prepare them for chemical analysis. For crude ash contents, samples were ashed in a muffle furnace at 550 °C for 8 h. The ether extraction method and a Soxhlet collector [56] were utilized for crude oil contents. The Kjeldahl method obtained the nitrogen (N) content of samples. The equation of Nx6.25 was used to determine crude protein contents [57]. An ANKOM 200 Fiber Analyzer device (ANKOM Technology Corp. Fairport, NY, USA) [58] was used for neutral detergent fiber (NDF), acid detergent fiber (ADF), acid detergent lignin (ADL), and crude fiber analyses. For lycopene and β-carotene analysis, 1 g of sample was added to 50 mL 96% ethanol and vortexed. The samples were centrifuged at 15.000 rpm for 5 min and the supernatant was taken. Sample readings were made on a UV/visible spectrophotometer at wavelengths of 663, 645, 505, and 453 nm [59].

2.8 Forage mineral contents

Alfalfa samples were acid digested in nitric perchloric acid [44], and an ICP-OES device (Agilent 5800) was used to determine sample P, K, Mg, Ca, Na, S, Mn, Fe, Cu, B, and Zn, contents [60].

2.9 Forage fatty acid analyses

A GC system (Schimadzu, GC 2010 plus) with a flame ionization detector (Schimadzu, Kyoto, Japan), a capillary column (i.d. 0.25 mm) was operated at an injection volume of 0.6-µL and a split ratio of 1:50 for fatty acid analyses of the samples [61]. Resultant peaks were identified by comparing retention times with authentic standards (Supelco #37, Supelco Inc., Bellefonte, PA, USA, L8404 and O5632, Sigma).

2.10 Uncertainty analysis

Uncertainty analysis is a delicate approach to error assessment, crucial for verifying the accuracy of measured data in experimental studies. Errors occurring during experiments are significant factors influencing precision. In this research, the total measurement error of a parameter was computed using Eq. (16), which accounts for fixed, random, and procedural errors [62].

$${W}_{{}_{R}}={\left[{\left(\frac{\partial R}{\partial {x}_{1}}{w}_{1}\right)}^{2}+{\left(\frac{\partial R}{\partial {x}_{2}}{w}_{2}\right)}^{2}+{\left(\frac{\partial R}{\partial {x}_{3}}{w}_{3}\right)}^{2}+.........+{\left(\frac{\partial R}{\partial {x}_{n}}{w}_{n}\right)}^{2}\right]}^{1/2}$$
(16)

where, R is the magnitude to be measured; x1, x2, x3 and …xn are n number of independent variables influencing this magnitude; w1, w2, w3, …wn error rates for each independent variable and WR is total uncertainty of magnitude R.

2.11 Statistical analysis

SAS software [63] was used for variance analysis of resultant data. Differences in treatment means were compared by the Least Significance Test (LSD).

3 Results

3.1 Thin-layer mathematical drying models

Typical characteristic drying curves of 12 different drying processes are given in Fig. 4. Drying conditions close to each other in terms of drying duration were presented together. Moisture ratio curves are primarily designated by microwave power, drying air temperature, and moisture. The slope of the moisture ratio curve increases with increasing microwave power, air temperature, and decreasing moisture. The decrease in the vertical direction was greater, especially at 200 W and 300 W drying powers. The moisture content of alfalfa tended to decline steadily with drying time until it approached equilibrium moisture. In the first drying stage, a sharp decrease in humidity can be observed up to the first 4–8 h with hot air, the first 10–20 min with microwave drying, and 30–40 min with hybrid drying. At this stage, moisture loss was assumed to be from the stomata. Cut and sun-exposed feed promoted initial drying. Then, only lower reductions were detected. Further decreases are almost unnoticeable at higher drying durations of the second stage.

Fig. 4
figure 4

Drying curves of alfalfa hay under different drying processes: Open-sun and greenhouse drying (a), shade and freeze drying (b), air-convective drying (c), microwave drying (d), hybrid drying (e)

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Experimental data on the drying kinetics of alfalfa were modeled using ten empirical equations frequently used in thin-layer drying (Table 2). The highest R2 values (0.9956, 0.9990, and 0.9969, respectively) were obtained from the Jena&Das model in open-sun, shade, and freeze-drying. In greenhouse drying, the highest R2 values were obtained from Verma (0.9981) and Logistic (0.9980) models. At 100, 200, and 300W microwave powers, the highest correlation coefficients (0.9989, 0.9964, and 0.9963, respectively) and SEE values (0.0116, 0.0227, and 0.0250, respectively) were obtained again from Jena&Das model. In air-convective drying, different models were prominent for every condition. Page model (R2: 0.9978) yielded the best outcomes at low temperature (60 °C), Jena&Das (R2: 0.9976) at medium temperature (80 °C), and Jena&Das (R2: 0.9885) at high temperature (100 °C). The Jena&Das model again yielded the best outcomes in hybrid drying at 200W + 60 °C (R2:09969; SEE:0.0261) and 200W + 80 °C (R2:09969;0.0438). Considering the R2 and SEE values, the Jena&Das model yielded better outcomes under entire drying conditions, except for greenhouse and 60 °C air-convective drying, which still yielded good outcomes.

Table 2 Estimated values of different thin-layer models for drying alfalfa samples
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3.2 Color attributes

Color characteristics of alfalfa dried under different drying conditions are provided in Table 3. L*, a*, b*, and C values for fresh products were measured with the values of 17.98, -6.17, 30.99, and 31.65, respectively. The highest L* value (47.41) was determined at 200W microwave drying and the lowest (35.86) in shade drying. This study measured all a* values as (-). They were indicating a dominant green color. The highest a* values were determined at 200W + 80 °C (-1.82) and 200W + 60 °C (-1.91) in hybrid drying. The greatest b* (64.90) and C (65.38) values were measured at 60 °C air-convective drying. The highest color change (ΔE) (43.03) was found in 60 °C air-convective and the lowest (20.12) in shade drying.

Table 3 Color attributes of alfalfa samples
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3.3 Energy aspects

The shortest drying durations were obtained from 300W (65 min) and 200W + 80 °C (90 min) drying conditions, while the longest was from shade drying (14,400 min). The highest energy consumption (0.86 kWh) was found in 300W drying, the lowest in freeze drying (0.06 kWh), the greatest total energy consumption (285.12 kWh) was seen in air-convective drying at 80 °C and the lowest in 200W + 80 °C hybrid drying (54.00 kWh). It was determined that the hybrid system (200W + 80 °C) provided 18.94% energy saving compared to the air-convective system (80 °C) and shortened the drying duration by 8.33 times. In this sense, the highest SEC value (1.67 kWh kg−1) was calculated for 200W + 80 °C hybrid drying and the lowest (0.26 kWh kg−1) for air-convective drying at 60 °C. The greatest SMER value (1.74 kg kWh−1) was seen in 100W drying, and the lowest (0.43 kg kWh-1) in 300W drying. The greatest energy efficiency (15.38%) was obtained from 200W + 80 °C hybrid and the lowest (0.85%) from freeze drying. The greatest thermal efficiency (11.50%) was seen in 300W drying and the lowest (0.11%) in freeze drying. The greatest Deff (5.68 × 10 − 9 m2s−1) was seen in 300W drying and the lowest (1.19 × 10 − 10 m2s−1) in 60 °C air-convective drying (Table 4).

Table 4 Energy aspects of drying conditions
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3.4 Chemical composition and minerals

Chemical analysis results are provided in Table 5. Drying treatments significantly affected the chemical composition of alfalfa samples (p ≤ 0.01). The lowest crude protein ratio (16.32%) was obtained from 100 ◦C drying and the greatest from 60◦C (22.10%) and open-sun (22.01%) drying methods. For crude fiber contents, the lowest value (19.88%) was seen in 200 W drying and the highest (23.18%) in 100 ◦C drying. Freeze drying and shade drying were also placed into the highest group. Crude ash contents ranged between 8.22—11.35%, with the lowest value from the shade and the greatest from freeze drying. For NDF contents, the lowest value (35.14%) was seen in freeze and the highest (47.29%) in shade drying. The NDF value of shade-drying was also placed into the highest group. For ADF ratios, the lowest group included open-sun and 200 W drying, and the highest group included 100 °C and shade drying. The lowest ADL content (3.20%) was seen in freeze and the highest in 200 W + 80 °C (4.14%) and 300 W (4.11%) drying. Crude oil contents ranged between 1.27—2.81%, with the lowest value from shade drying and the highest from 200 W + 80 °C drying. The highest beta carotene content (0.46 mg/kg) was seen in freeze and the lowest (0.21 mg/kg) in 100 °C drying. For lycopene contents, the greatest values were seen in the freeze, 60 °C, and 200W drying operations, and the lowest in 200W + 60 °C drying.

Table 5 The influence of different drying methods on the chemical composition of alfalfa hay
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Mineral composition of alfalfa samples are provided in Table 6. The highest Mg (1458.63 ppm), Ca (14,242.64 ppm), K (8493.69 ppm), and S (1443.49 ppm) contents were seen in freeze drying. The lowest Mg (846.51 ppm), Ca (7423.90 ppm), and S (627.11 ppm) contents were seen in greenhouse drying. Na contents varied between 216.86 ppm (200 W + 60 °C) and 494.91 ppm (60 °C). The lowest P content (1091.96 ppm) was obtained from greenhouse drying, and the highest value (1599.59 ppm) was obtained from 100 W microwave drying. The lowest values of all microelements were seen in the greenhouse drying method. The highest Cu content (8 ppm) was obtained from 200 W drying. In freeze drying, the highest Fe and Zn contents (135.45 and 33.68 ppm) were seen in 100 °C drying and Mn, B, and Ni contents (25.41, 60.80, and 1.11 ppm, respectively).

Table 6 The influence of different drying methods on the mineral composition of alfalfa hay
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Drying treatments significantly affected fatty acid compositions (Table 7). The highest 16:00, 18:00, and saturated fatty acids were obtained from freeze drying, the highest 14:00 and 16:00 contents were seen in open-sun drying, the highest 18:01 from 200W + 80 °C hybrid drying, and the highest 18:02 content from shade drying. The highest UFA and PUFA values were found in 60, 80, and 100 °C drying operations. The lowest 14:00 and 18:3 contents were seen in shade drying, the lowest 16:00 and 18:01 contents in 100 °C drying, and the lowest 18:02, saturated fatty acids and unsaturated fatty acids were obtained from freeze drying.

Table 7 The influence of different drying methods on the fatty acid composition of alfalfa hay
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3.5 Uncertainty analysis of measurements

Uncertainty of measured parameters and total uncertainties in L*, a*, b*, C, ΔE, DT, Ec, Total Ec, SEC, SMER, ηen, ηth, Deff, CP, CF, CA, NDF, ADF, ADL, CO, β-C, Lyc, Mg, Ca, K, Na, P, S, Cu, Fe, Mn, Zn, B, Ni, 14:00, 16:00, 18:00, 18:01, 18:02, 18:3, SFA, UFA, and PUFA calculations are provided in Table 8. The uncertainty values determined for the drying analysis of alfalfa were significantly lower than the acceptable threshold of 5%

Table 8 Uncertainties of the experimental measurements
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4 Discussion

In this section, roughage quality, drying, and energy aspects of alfalfa samples were discussed in detail, and the most suitable drying methods were reported based on the examined properties. Present findings generally comply with the previous literature. However, it should be considered that genetic structure, sowing time, climate, and environmental factors may have significant effects on alfalfa yield and forage quality [25]. Drying procedures that rapidly remove moisture from the product and inhibit enzymatic activity are pretty beneficial for the preservation of the nutritional properties of the end products [15]. Slower drying durations facilitate the loss of volatile organic compounds and increase protein degradation rates [64]. High drying temperatures reduce crude protein contents and increase ammonia evaporation [64]. In particular, temperatures of 100 °C and above increase the losses of non-protein nitrogenous volatile substances and ammonia [65]. Therefore, drying temperature and durations must be managed precisely [66]. In the present study, open-sun and 60 °C air-convective drying yielded the highest protein ratios—however, the greatest protein content of alfalfa from 70 °C drying [67]. The greatest crude ash content was obtained from the drying method. Generally, increasing drying temperatures or microwave powers reduced crude ash contents. Drying at high temperatures or power might have facilitated the leaching of soluble inorganic compounds, thus decreasing crude ash contents [9, 68]. It was indicated in a previous study that mineral compositions varied significantly with drying methods, durations, and temperatures [69, 70] reported increasing and decreasing mineral contents with increasing temperatures [29]. Arslan and Özcan [30] reported that mineral concentrations changed with drying methods and temperatures. The highest Mg, Ca, K, Na, S, Mn, B, and N values were obtained from freeze drying, the highest Na content from 60 °C, Cu from 200 W, P from 100 W, Fe, and Zn from 100 °C drying. The mineral element's response to drying methods, temperature, and power varied considerably.

van den Berg et al. [71] hypothesized that the inactivation of enzymes and degradation of nutrients may increase bioactive compounds, while Kim et al. [72] reported a loss of antioxidants. It was reported that heat treatments could result in significant loss of epoxy-carotenoids and carotenes [27]. In this study, decreasing β-carotene contents were seen with increasing temperatures. Pinar et al. [9] reported reduced antioxidant compounds with drying processes. Maurya et al. [73] indicated that microwave vacuum drying provided faster heating rates and shortened drying time, reducing carotene losses. This study showed the highest lycopene and β-carotene in microwave and freeze-drying.

Goossen et al. [13] indicated 18:02 and 18:3 as the dominant fatty acids of alfalfa. Drying methods and temperatures may significantly influence the oil ratio and fatty acid composition [9, 74], indicating that dried samples had greater oil content than fresh ones. It was noted that hot-air drying facilitated the enzymatic degradation of PUFA [13]. The highest UFA and PUFA values were determined from the hot-air drying method. Low oil content at other drying temperatures may have resulted from heat-induced enzymatic hydrolysis and lipid oxidation [75, 76], which revealed higher oil content for open-sun drying than for oven and microwave drying. However, such a case varies considerably with the cultivars and species. In the present study, sun drying was prominent for 14:00 and 16:00 fatty acids, while shade drying was prominent for 18:02 fatty acids.

With prolonged drying durations, soluble carbohydrates decrease, and the fiber ratio increases [77,78,79]. Slow-drying methods are expected to increase NDF and ADF ratios [79]. In shade drying, respiration continued for a long time and thus had the highest fiber contents. High temperatures facilitate the formation of Maillard Products. The number of these complexes and NDF and ADF ratios increased with increasing air temperatures [15, 80, 81], indicating that oven drying-induced complexes did not dissolve in detergent extractions, and then ADF and NDF ratios increased. Parissi et al. [82] showed that the NDF and ADL content of roughages increased with the formation of insoluble condensed tannin-protein polymers. In this study, shade-drying yielded the highest NDF and ADL ratios, but 100 °C air-convective drying was also placed into the highest group for ADL and 200 W + 80 °C hybrid-drying for NDF. Delgado et al. [37] reported the lowest NDF ratio of chestnuts for freeze drying.

Mathematical modeling has been successfully applied to define the most appropriate drying processes [42]. The highest modeling results in all methods were obtained from shade and greenhouse drying. Heating is modeled as a typical sigmoidal drying behavior with periods of constant and decreasing velocity. Drying rates increased at high temperatures and microwave powers. The drying behavior turned into a decreasing trend when moisture reached the region of 0.3 (Fig. 2). Farhang et al. [35] used 900, 720, 540, 360, and 180 W microwave powers for alfalfa and indicated similar results for Henderson & Pabis and Wang & Singh. Time-dependent drying curves had similar slopes at similar microwave powers. Darvishi [36] conducted a study for thin-layer alfalfa drying at various microwave powers (900, 720, 540, 360, and 180 W) and obtained results similar to the present Page model. [34] used air-convective (40, 50, 60, and 70 °C) and microwave (180, 360, 540, and 720W) drying for forage grass and presented similar R2 results with the present Page and Henderson & Pabis.

Present findings revealed that shade drying better preserved greenness, probably due to drying-induced degradation and browning of chlorophyll. Higher temperatures, microwave powers, and longer drying durations facilitate chlorophyll decomposition and browning. Shade drying is beneficial in preserving the color of alfalfa during drying since enzyme activity is inactivated and enzymatic reactions are inhibited [83].

Cao et al. [39] freeze-dried barley grass with and without ultrasonic pretreatment and reported that decreasing ultrasound pretreatment powers decreased greenness (a ∗) and lightness (L ∗) values, while the yellowness (b ∗) values remained at the same levels. Such a case was attributed to the difference between light absorption and reflection due to damage to the microstructure of barley grass. Since barley grass was tolerant, ultrasonic powers had little effect on yellowness (b ∗). Cao et al. [38] reported that the products dried at high microwave powers had higher L* (65.01) values. Contrary to current findings, Liu et al. [83] reported the lowest a* value (-8.82) for vacuum freeze-drying, followed respectively by hot-air drying (-7.38) and far-infrared drying (-6.56). The lowest color change (ΔE) was also reported for vacuum freeze-drying.

Present findings revealed that increasing temperatures and microwave powers reduced drying durations but increased energy consumption. High power and temperatures increased the desorption temperature to speed up the drying process, so these methods' energy consumption was relatively reduced. Although energy consumption was higher in microwave drying than in air-convective drying, energy efficiency was higher. For greater appliance efficiency, higher product loads should be used [9, 42, 84], which demonstrated that the hybrid dryer (microwave + air-convective) offered less drying durations and energy consumption and yielded better quality traits for alfalfa. Cao et al. [38] reported drying durations as 600, 720, and 840 min for freeze drying (FD) and 360, 420, and 480 min for microwave-assisted freeze drying (MFD), respectively. It was emphasized that MFD increased the drying rate and reduced energy consumption by about 48% compared to FD. Kliuchnikov [33] used a new drying technique that periodically changes between 30 °C and 60 °C and reported the energy consumptions of wheat, rye, and clover as 5.73, 6.45, and 7.74 MJ kg−1, respectively. Ihediwa [34] reported effective moisture diffusivity values as between 4.44 × 10–9 and 1.26 × 10–8 m2 s−1 for air-convective drying at 40, 50, 60 and 70 °C temperatures and as 5.22 × 10–10 and 4.94 × 10–9 m2 s−1 for microwave-drying at 180, 360, 540 and 720W powers. The SEC values ranged between 12.2—29.1 kWh kg water−1 and SMER values between 0.28—0.94 kg water kWh−1. Motevali et al. [85] reported SEC values for mushrooms dried convectively at different temperatures (40—90 ºC) between 47.88—93.45 kWh kg−1. Liu et al. [86] indicated specific moisture absorption ratios for garlic as 1.924—2.232 kg kWh−1. Beigi [54] reported nth values as between 6.57—8.06%.

Cao et al. [39] freeze-dried barley grass and reported total energy consumption as 6.275 MJ g−1 for untreated samples and 5.175 MJ g−1 for ultrasonic (UT) pre-treated samples. de Brito et al. [32] dried sorghum samples at 50, 60, and 70 °C temperatures and product load densities of 2, 3, and 4 kg and indicated SEC values as between 6837.19—14,514.01 kJ kg−1. Nanvakenari et al. [40] reported drying durations between 90 – 2400 min and energy consumption values between 0.079—0.804 kWh.

Granella et al. [87] dried maize and soybean samples at 30, 40, and 50 °C convective drying temperatures and different ozonation durations and reported Deff values as between 0.57 × 10–9 and 6.89 × 10–9 m2 s−1 for maize and between 8.99 × 10–9 and 33.49 × 10–9 m2 s−1 for soybean. Soares et al. [88] dried Persian and arrowleaf clover samples at 40, 45, and 50 °C convective dryers and reported Deff values as between 3.61 × 10–11—6.81 × 10–11 m2 s−1 for Persian clover and between 6.76 × 10–11—1.15 × 10–10 m2 s−1 for arrowleaf clover. Darvishi et al. [89] rapidly reduced moisture and increased Deff values with increasing thermal energy levels.

Farhang et al. [35] applied various microwave powers (900, 720, 540, 360, and 180 W) to dry alfalfa samples and reported practical moisture diffusivity values as between 2.01 × 10–6 m2 s−1 and 4.34 × 10–6 m2 s−1, which were more significant than the present findings. This may be due to different microwave powers, dried product density, and cultivars. Darvishi [36] microwave powers and reported practical moisture diffusion values as between 3.10 × 10–9—8.21 × 10–9 m2 s−1 and energy consumptions between 3.87—9.98 MJ kg water−1.

5 Conclusion

Alfalfa samples were exposed to 12 different drying treatments. Present findings revealed that drying techniques, air temperature, and power significantly affected the product's quality traits.

  • Open sun-drying and 60 °C air-convective drying were revealed effective for increasing the protein ratio.

  • Shade and hybrid drying methods were determined effective in enhancing the fiber ratio.

  • Freeze-drying yielded successful results for several mineral properties.

  • Models such as Jena&Das, Logistic, Alibaş, and Page were found suitable for analyzing the experimental data.

  • Optimal color properties were achieved with 60 °C air-convective drying.

  • Air-convective and freeze-drying systems had the highest total energy consumption (Ec).

  • Microwave and hybrid systems were identified as the most energy-efficient methods, respectively.

The present results indicate that hybrid drying, which integrates varying drying temperatures and microwave powers, has the potential to substantially reduce drying times and energy consumption while markedly enhancing the overall energy efficiency of the drying process. Further research could explore optimizing specific combinations of drying parameters to maximize efficiency gains and minimize environmental impact in agricultural drying practices. In addition, researchers could perform to improve the drying characteristics of different forage crops with infrared and solar hybrid dryers. Continuing research endeavors focused on preserving nutritional properties could potentially enhance milk and meat yields by optimizing the quality of animal feed consumption.