Rapid and High-Yield Recovery of Sodium Alginate from Undaria pinnatifida via Microwave-Assisted Extraction

Abstract

Alginate, a promising biopolymer in the food, biomedical, pharmaceutical, and electronic materials industries, is characterized by its biodegradability, biocompatibility, low toxicity, and gel-forming properties. It is most abundantly found in brown algae. However, conventional dilute acid and alkali extraction methods face limitations in commercialization due to their long processing time, low throughput, and high solvent requirements. In this study, a microwave-assisted extraction (MAE) process for sodium alginate was designed to improve extraction efficiency. The solid/liquid ratio, extraction temperature, and extraction solvent concentration were major variables affecting sodium alginate extraction from Undaria pinnatifida (sea mustard). They were then statistically optimized using response surface methodology. Under optimal conditions (13.27 g/L, 91.86 °C, 2.51% (w/v), and 15 min), the yield was 38.41%, which was 93.43% of the theoretical content of sodium alginate in Undaria pinnatifida. Our work has confirmed the productivity and industrial feasibility of the efficient extraction of sodium alginate from marine biomass, and we hope that it will serve as an encouraging case for the application of biopolymers as one of the desirable options for alternative petrochemicals to construct a sustainable society.

1. Introduction

Since the Industrial Revolution, it is a well-known fact that greenhouse gas emissions have contributed to global warming and adversely affected air quality, climate, and the environment, severely deteriorating human health [1,2]. In response, various policies have been established worldwide, notably the adoption of the Paris Agreement in the 2015 United Nations Framework Convention on Climate Change (UNFCCC) meeting. Additionally, the Intergovernmental Panel on Climate Change (IPCC) emphasizes the need for fossil fuel reduction, gradual phase-out, expansion of renewable energy, and improvement in energy efficiency to achieve carbon neutrality [1]. In this context, exploration and transition from fossil fuel to carbon-neutral and renewable resources have become essential. Biomass holds tremendous potential in meeting these societal demands, with the development of biorefineries significantly contributing to solving sustainability and environmental issues.

One promising resource in biorefineries is alga, which requires no farmland or fertilizer and has the characteristics of rapid reproduction and high yield [3,4]. Particularly, brown algae, due to their high consumption in Asia, account for 47.3% of the world’s cultivation at 16.4 million tons [5]. With this increasing production, waste generated during processing poses a problem, affecting local environments through landfills and incineration [6]. Since 2013, ocean dumping has been prohibited under the London Convention, leading to a significant increase in waste disposal costs. Therefore, exploring the utilization of such waste is necessary. Undaria pinnatifida, Saccharina japonica, and Pyropia yezoensis account for almost 97% of Korea’s total algae production [7]. Undaria pinnatifida (sea mustard) contains various physiologically active compounds, including monosaccharides, polysaccharides (alginate, saccharin, mannans), polyphenols, polyunsaturated fatty acids, peptides, phytosterols, and vitamins, making it a useful resource [8].

Among the components of Undaria pinnatifida, mucilaginous polysaccharide alginate is highly valued due to its biocompatibility, biodegradability, antibacterial properties, and ease of gel formation [9]. It plays a crucial role in various fields, such as food industry thickeners, wound-healing agents, and drug delivery systems [10]. Alginate, abundantly found in brown algae, is globally estimated to have a production of at least 30,000 tons and is projected to reach a market value of 1.07 billion dollars by 2028 [9]. However, the biorefining process of alginate typically requires a low yield, high energy input, and long processing time [11]. To overcome these limitations, microwave-assisted extraction (MAE) has been proposed. MAE has been widely employed for the extraction of active compounds from natural products and is known as an efficient extraction method [9]. This technology demonstrates compatibility with the biorefinery concept by offering rapid hydrolysis and low energy consumption compared to traditional methods. Although MAE has been proven to be effective for various extractions, research on its suitability for rapid alkali extraction of alginate from brown algae has not been vigorously pursued [12,13,14].

The goal of this study was to optimize the extraction process of sodium alginate from Undaria pinnatifida using MAE. Using response surface methodology, we derived the optimal conditions based on key parameters such as solid/liquid ratio, extraction temperature, and extraction solvent concentration. This approach aims not only to improve the yield of extracted alginic acid sodium, but also to apply this process to biorefinery systems, promoting sustainability and economic viability. Previous studies on microwave-assisted sodium alginate extraction from Undaria pinnatifida lacked the capability to explore optimized conditions for multiple variables and precise control of these variables through sensors. However, our research has overcome these limitations by optimizing a process influenced by multiple factors, thereby enabling the determination of maximum yield. Additionally, our microwave system allows for the precise control of variables, enhancing the accuracy of our research and providing insight into energy consumption, which is beneficial for industrial applications. This research is expected to make a significant contribution to biorefining and sustainable material science by providing a new, efficient, and scalable method for extracting sodium alginate.

2. Materials and Methods

2.1. Materials

Undaria pinnatifida was purchased from Green Natural (Seoul, Republic of Korea). Sodium alginate to be used as a standard substance was obtained from Sigma-Aldrich (St. Louis, MI, USA). Sulfuric acid (95% H₂SO₄) and calcium carbonate (CaCO₃) to be used for analyzing monosaccharides and sodium alginate compositions in Undaria pinnatifida were purchased from Daejung (Busan, Republic of Korea). Sodium carbonate (Na₂CO₃) and hydrochloric acid (0.1 M HCl) to be used for the sodium alginate extraction of Undaria pinnatifida and pH neutralization were acquired from Daejung and Junsei (Kyoto, Japan), respectively. Phosphoric acid (H₃PO₄) and sodium hydroxide (NaOH) for HPLC analysis were obtained from Sigma-Aldrich (USA) HPLC. Water was purchased from Honeywell (Charlotte, NC, USA).

2.2. Analysis Method of Sugars and Sodium Alginate in Undaria pinnatifida

Monosaccharide compositions of Undaria pinnatifida were analyzed according to the standard procedure of the National Renewable Energy Laboratory (NREL). First, 0.025 g of the sample was hydrolyzed with 250 µL of 72% H₂SO₄ at 30 °C for 1 h, and then further hydrolyzed with 7 mL of deionized water at 121 °C for 1 h. The hydrolyzed sample was quantified using HPLC. The sodium alginate content was analyzed following the procedure of a previous study [15]. Briefly, 0.1 g of the sample was reacted with 10 mL of 0.4 M H₂SO₄ at room temperature overnight, and then filtered to obtain residues. These residues were reacted with 20 mL of 3% Na₂CO₃ at 50 °C overnight and diluted with 80 mL of DW. Then, 1 mL of the diluted sample was reacted with 6 mL of 95% H₂SO₄ at room temperature for 20 min. Half (3.5 mL) was used as a blank. To the other half (3.5 mL), 0.3 mL of carbazole–ethanol solution was added and reacted at room temperature for 45 min. The reacted sample was quantified using a UV-Vis spectrum.

2.3. Effects of Variables on Sodium Alginate Yield Using Microwave-Assisted Extraction

The solid/liquid ratio, extraction temperature, and extraction solvent concentration were identified as variables significantly affecting sodium alginate extraction from Undaria pinnatifida. Ranges for solid/liquid ratio, extraction temperature, and extraction solvent concentration were set to be 20~100 g/L, 50~90 °C, and 1~10% (w/v), respectively. The impacts of these variables were investigated. The comprehensive process of sodium alginate extraction from Undaria pinnatifida is depicted in Figure 1. Sodium alginate was extracted using a microwave system (Multiwave 5000, Anton Paar, Austria). The extraction temperature was meticulously regulated using onboard sensors coupled with the device’s integrated reaction control system. The system was calibrated to attain the desired temperature within a span of 5 min. The extraction time and solvent amount were set to be 5 min and 20 mL, respectively. After extraction, the samples underwent centrifugation (centrifuge 5804, Eppendorf, Germany) at 8000 g for 10 min to separate the supernatant. This supernatant was neutralized with 0.1 M HCl and diluted 100-fold with deionized water. The diluted sample’s yield was then determined using high-performance liquid chromatography (HPLC). The sodium alginate yield was calculated using the following equation:

$$\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}\left(\mathrm{\%}\right)=\frac{\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{o}\mathrm{f}\mathrm{s}\mathrm{o}\mathrm{d}\mathrm{i}\mathrm{u}\mathrm{m}\mathrm{a}\mathrm{l}\mathrm{g}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{e}(\mathrm{g}/\mathrm{m}\mathrm{L})}{\mathrm{S}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d}/\mathrm{L}\mathrm{i}\mathrm{q}\mathrm{u}\mathrm{i}\mathrm{d}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}(\mathrm{g}/\mathrm{m}\mathrm{L})}\times 100$$

(1)

After extraction, the separated supernatant was mixed with ethanol in a 1:3 v/v ratio to precipitate sodium alginate. The resulting sodium alginate solid was then washed with ethanol, methanol, and acetone, and dried at 70 °C. The final sample was analyzed using Fourier transform infrared spectroscopy (FT-IR).

2.4. Experimental Design and Statistical Optimization

To determine the optimal extraction conditions for sodium alginate from Undaria pinnatifida, the D-optimal design of response surface methodology (RSM) was used. Independent variables affecting sodium alginate extraction, such as solid/liquid ratio (x₁), extraction temperature (x₂), and extraction solvent concentration (x₃), were selected (

Table 1. Extraction factors and their levels in response surface methodology.

Factor	Units	Symbol	Low Level	High Level
Solid/liquid ratio (x1)	g/L	x1	10	30
Extraction temperature (x2)	°C	x2	50	100
Extraction solvent concentration (x3)	% (w/v)	x3	1	5

), with the response being the yield (%) of sodium alginate. The relationship between variables affecting the response was estimated using the following quadratic equation:

$$\mathrm{Y}={\mathsf{\beta }}_0+\sum {\mathsf{\beta }}_{\mathrm{i}}{\mathrm{x}}_{\mathrm{i}}+\sum {\mathsf{\beta }}_{\mathrm{i}\mathrm{j}}{\mathrm{x}}_{\mathrm{i}}{\mathrm{x}}_{\mathrm{j}}+\sum {\mathsf{\beta }}_{\mathrm{i}\mathrm{i}}{\mathrm{x}}_{\mathrm{i}}^2$$

(2)

where Y is the dependent variable (sodium alginate yield), x_i and x_j are independent variables, β₀ is the offset term, β_i is the first-order model coefficient, β_ii is the second-order model coefficient for variable i, and β_ij is the linear model coefficient for the interaction between variables i and j [16].

2.5. Analytical Methods

2.5.1. HPLC

Extracted samples were diluted and quantified using high-performance liquid chromatography (HPLC) according to a previous method, with slight modifications [17]. Quantification was performed using a Shimadzu HPLC system (SPD-20A, Shimadzu, Japan) equipped with a UV detector and a C18 stationary phase column (5 μm, 4.6 × 150 mm). The mobile phase was a buffer solution (0.5 mL of phosphoric acid in 1 L of deionized water). Its pH was adjusted to 7.0 using 1 M NaOH. The flow rate was set to be 0.6 mL/min. The temperature was set at 25 °C. An injection volume of 20 μL was used. Sodium alginate was analyzed at a wavelength of 200 nm.

2.5.2. FT-IR

Functional groups of precipitated and dried sodium alginate from the extract were identified using Fourier transform infrared spectroscopy (FT-IR; FT/IR-1600, Jasco, Japan) [18]. Measurements were recorded in the range of 4000~500 cm⁻¹, with a resolution of 4 cm⁻¹. The IR laser wavenumber, scanning speed, and aperture were set to be 15802.00 cm⁻¹, 2 mm/s, and 7.1 mm, respectively.

3. Results and Discussion

3.1. Analysis of Components in Undaria pinnatifida

To evaluate the potential of Undaria pinnatifida as a source of sodium alginate, the carbohydrate compositions of Undaria pinnatifida were investigated (

Table 2. Contents of monosaccharides and polysaccharides in Undaria pinnatifida.

Algae	Monosaccharides/Polysaccharide Contents (%)
	Glucose	XMG	Arabinose	Sodium Alginate
Undaria pinnatifida	4.9	3.0	-	41.11

-: not detected, XMG: xylose/mannose/galactose.

). Quantitative analysis of monosaccharides showed the presence of 4.9% glucose and a combined 3.0% of xylose/mannose/galactose. However, arabinose was not detected. The content of sodium alginate was quantified at 41.11%, which is consistent with previous reports showing that the sodium alginate content in Undaria pinnatifida was 35~40 wt% [19,20,21]. Research has also shown that spore leaves, which are often discarded for edible use, contain the highest amount of sodium alginate [22]. This analysis confirms the high potential of Undaria pinnatifida as a biomass resource given its high sodium alginate content and the utilization of otherwise discarded parts.

3.2. Investigation of Variables Affecting Sodium Alginate Microwave-Assisted Extraction

Previous studies have suggested that solvent type, solvent concentration, solid-to-liquid ratio, temperature, and time can affect sodium alginate extraction from brown algae [9,23,24,25]. Solvent concentration, solid-to-liquid ratio, and temperature were found to be significant variables. Their effects on sodium alginate extraction from Undaria pinnatifida were studied. Alkaline solvents are reported to be favorable for extracting sodium alginate from algae, as alginic acid, mainly present as calcium salts in brown algae, undergoes ion exchange to form soluble sodium alginate [26]. Acid pretreatment has been reported to enhance extraction efficiency, although the difference is minimal and non-essential [26]. An alkaline single-step process also simplifies the procedure, offering environmental and economic benefits.

Preliminary investigations were carried out to set significant ranges for RSM. The variables were the extraction temperature (50, 70, 90 °C), extraction solvent concentration (1, 5, 10% (w/v)), and solid/liquid ratio (20, 60, 100 g/L). The results of the preliminary experiments are presented in

Table 3. Preliminary experiments on three significant variables.

Sodium Alginate Yield (%)
	Solid/liquid ratio	20	60		100
Extraction temperature
50		33.57	22.03		13.82
Extraction solvent concentration: 1%
	Extraction Temperature	50	70		90
Extraction solvent concentration
1		33.57	36.48		37.40
Solid/liquid ratio: 20 g/L
	Extraction solvent Concentration	1	5		10
Solid/liquid ratio
20		33.57	25.14	12.49
Extraction temperature: 50 °C

. The solid/liquid ratio showed a significant decrease in yield as it increased from 20 g/L to 100 g/L. The decrease in solid/liquid ratio increased the amount of alkaline solution used for a certain quantity of algae, allowing more Na+ ions to be exchanged with Ca2+ ions and increasing the amount of soluble sodium alginate formed. However, very low S/L ratios could complicate industrial applications. Hence, a range of 10~30 g/L was set as the optimum. The extraction temperature showed increased yields as the temperature increased from 50 °C to 90 °C. This was attributed to enhanced penetration of the alkaline solution into cell walls, increasing the solubility and dissociation rate of sodium alginate [17]. Therefore, the extraction temperature range for optimization was set to be 50 °C to 100 °C. The extraction solvent concentration showed a decrease in yield with increasing concentration. This was because sodium alginate decomposed into monocarboxylic and dicarboxylic acids at high alkali concentrations, particularly at pH levels above 10 [27].

For sodium alginate extraction using alkali, preliminary experiments defined the following ranges for significant variables: 10~30 g/L for solid/liquid ratio, 50~100 °C for extraction temperature, and 1~5% for extraction solvent concentration. These ranges were consistent with previous research [24].

3.3. Optimization of Alkali Extraction Conditions for Undaria pinnatifida Using RSM

To optimize the alkali extraction conditions for sodium alginate, D-optimal designs and response surface methodology (RSM) were employed. RSM is a statistical and mathematical tool for the reliable data analysis of interactions between multiple variables and their effects on the response. D-optimal designs can efficiently reduce the number of required test runs compared to other RSMs [28]. A total of 20 experiments were designed to study the effects of three independent variables (x₁= solid/liquid ratio, x₂ = extraction temperature, x₃ = extraction solvent concentration) on the sodium alginate yield (Y) (

Table 4. Results of extraction yield of sodium alginate with the response surface design.

Run	Extraction Parameters			Yield (%)
	x1: Solid/Liquid Ratio (g/L)	x2: Extraction Temperature (°C)	x3: Extraction Solvent Concentration (% (w/v))
1	18	50	3	29.91
2	10	100	1	36.02
3	30	50	5	21.12
4	30	80	3	34.73
5	30	50	1	27.35
6	18	50	3	29.73
7	30	100	5	36.26
8	28	75	5	34.58
9	10	100	4	37.66
10	23	100	1	34.99
11	30	50	1	27.23
12	18	80	3	35.54
13	10	50	5	24.80
14	10	67	1	36.03
15	30	100	5	34.93
16	23	69	1	33.22
17	10	50	5	24.26
18	18	80	5	34.82
19	28	55	3	27.28
20	10	67	1	36.01
Extraction time: 5 min

).

Regression analysis based on experimental data (Table 4) yielded the following predictive quadratic polynomial model:

$$\mathrm{Y}=35.7-1.45{\mathrm{x}}_1+4.50{\mathrm{x}}_2-1.12{\mathrm{x}}_3+0.54{\mathrm{x}}_1{\mathrm{x}}_2+0.51{\mathrm{x}}_1{\mathrm{x}}_3+2.10{\mathrm{x}}_2{\mathrm{x}}_3-0.67{\mathrm{x}}_1^2-2.86{\mathrm{x}}_2^2-1.27{\mathrm{x}}_3^2$$

(3)

Here, factors were investigated in relation to their impact on sodium alginate yield (Y).

Table 5. Analysis of variance (ANOVA) for the response surface model.

Source	Sum of Squares	Degree of Freedom	Mean Square	F Value	p-Value Prob > F
Model	448.90	9	49.88	54.29	<0.0001
x1: S/L	20.62	1	20.62	22.45	0.0008
x2: Temperature	202.60	1	202.60	220.51	<0.0001
x3: Concentration	11.82	1	11.82	12.87	0.0050
x1x2	2.02	1	2.02	2.20	0.1690
x1x3	2.03	1	2.03	2.21	0.1677
x2x3	30.41	1	30.41	33.10	0.0002
x12	1.22	1	1.22	1.32	0.2766
x22	27.06	1	27.06	29.45	0.0003
x32	5.14	1	5.14	5.59	0.0396
Residual	9.19	10	0.92
Lack of Fit	8.13	5	1.63	7.68	0.0215
Pure Error	1.06	5	0.21
Cor Total	458.09	19

presents ANOVA results for the second-order response surface model. The model and its terms were significant, as indicated by a high F value of 54.29 and a low p-value of < 0.0001. Independent variables x₁, x₂, and x₃ were significant (p < 0.05) model terms for the yield of sodium alginate. The interaction term x₂x₃ (p = 0.0002) was significant, unlike x₁x₂ (p = 0.1690) and x₁x₃ (p = 0.1677). The coefficient of determination (R²) was very high at 0.9799, well above the significant value of 0.9 [29]. The coefficient of variation (CV) was low at 3.0121%, far below the 10% threshold [29], indicating the model’s accuracy and reliability. The adequate precision was high, at 23.4970, significantly above the desirable value of 4 [30], showing that the model could be effectively used to navigate the design space.

Interactions between independent variables and their effects are visualized in 3D response surface plots, which are shown in Figure 2. Figure 2a displays the interaction between S/L and temperature, showing an increase in sodium alginate yield with decreasing S/L and increasing temperature. The yield was particularly high (above 35%) at temperatures above 90 °C. Figure 2b illustrates the interaction between S/L and concentration, where a decrease in both S/L and concentration increased the yield of sodium alginate, showing less variation in the yield due to these variables in the specified range. Figure 2c shows the interaction between temperature and concentration. The increasing temperature enhanced the sodium alginate yield. The yield also increased with decreasing concentration as the temperature increased.

The optimal conditions for the highest yield of sodium alginate using a microwave system were found to be a solid/liquid ratio of 13.27 g/L, an extraction temperature of 91.86 °C, and an extraction solvent concentration of 2.51% (w/v). Under these optimal conditions, the model predicted a sodium alginate yield of 37.79%, which was validated by an experimental yield of 36.21% (

Table 6. Predicted and experimental values under optimal conditions.

Factor		Actual Level
Solid/liquid ratio (x1)		13.27 g/L
Extraction temperature (x2)		91.86 °C
Extraction solvent concentration (x3)		2.51% (w/v)
Response	Predicted		Experimental
	37.79%		36.21%

).

3.4. Effect of Extraction Time on Yield

The optimal extraction time was investigated under the optimal conditions confirmed using RSM (Figure 3). The extraction times were set as 5, 15, 30, 60, and 120 min, with the highest yield of 38.41% observed at 15 min. It was also found that, under the optimized high-temperature conditions, a prolonged extraction time caused more sodium alginate degradation, resulting in decreased yield. The decrease in yield over time might have been due to decomposition of sodium alginate into new dicarboxylic saccharinate compounds under high temperature and alkaline conditions [31]. This aligns with previous studies using microwave and ultrasound hybrid systems, suggesting a negative impact of increased extraction time on sodium alginate yield. Therefore, the extraction time is a crucial factor affecting the yield of sodium alginate, with the optimal time being 15 min. This finding serves as an important indicator for optimizing the sodium alginate extraction process.

3.5. FT-IR

The FT-IR spectra in Figure 4 show peaks representing functional groups present in the sodium alginate structure. A notable peak at 1025 cm⁻¹ represented C-C stretching vibrations of the pyranose ring. The peak at 1100 cm⁻¹ was attributed to C-O stretching vibrations of the C-O-C glycosidic bond. Previous papers have identified G-blocks at the 1025 cm⁻¹ band and M-blocks at the 1100 cm⁻¹ band [32]. The peaks at 1595 cm⁻¹ and 1406 cm⁻¹ were attributed to asymmetric stretching vibrations of carboxylate groups (COO^-) and weak carboxylate symmetric vibrations of mannuronate and guluronate residues, respectively [17]. The extracted sodium alginate showed similar functional groups to commercial sodium alginate, suggesting a similar structure composed of guluronic and mannuronic acid units.

3.6. Evaluation of the Sodium Alginate Extraction Process

The mass balance for the sodium alginate extraction process according to the extraction system is presented in Figure 5. The MAE sodium alginate extraction process was optimized at an S/L ratio of 13.27 g/L, an extraction temperature of 91.86 °C, a solvent concentration of 2.51%, and an extraction time of 15 min. Under these conditions, approximately 384.1 g of sodium alginate was obtained from 1000 g of Undaria pinnatifida, equating to 93.43% of the theoretical sodium alginate content of 411.1 g. The maximum yield achievable through hydrothermal extraction was 286.1 g, even after 30 min, whereas our process achieved a 1.34 times higher yield in just 15 min. This aligns with previous research suggesting that microwaves could accelerate the extraction process and increase the efficiency through electromagnetic effects on the cell structure [19]. However, the potential of microwaves to disrupt polymer chains necessitates consideration when setting optimal extraction conditions [21]. From an energy perspective, while hydrothermal extraction consumes approximately 1500 W of energy over a duration of 4 h, our process utilizes a maximum of 900 W of energy for just 20 min. This substantial reduction in both energy and time consumption makes our method both environmentally friendly and economically efficient.

Table 7. Comparison of sodium alginate yield with previous literature.

Algae	Solvent and the Others	Extraction Condition	Extraction Time	Yield	Reference
Sargassum	Pretreatment, 3.75% alkali, 12.63 mL/g, 80 °C	Hydrothermal extraction	6 h	20.76%	[17]
Sargassum muticum	Pretreatment, 3% alkali, 86 °C, Precipitated with 93% EtOH	Hydrothermal extraction	3 h	13.57%	[24]
Padina pavonica	Pretreatment, 4% alkali, 50 mL/g, 50 °C	Hydrothermal extraction	5 h	36.5%	[34]
Ecklonia radiata	Pretreatment, 0.2 M alkali, 45 °C	Hydrothermal extraction	2 h	45%	[23]
Macrocystis pyrifera	Pretreatment, 1 N alkali, 25 mL/g, 60 °C	Hydrothermal extraction	2 h	34%	[35]
Sargassum cristaefolium	Pretreatment, pH 10.3 alkali, 31.1 g/L	Extrusion system (2.95 rpm)	6.8 min	34.96%	[25]
Sargassum angustifolium	Pretreatment, 5% alcalase, pH 8, 50 °C, 3% alkali, pH 11, 65 °C	Enzyme-assisted extraction	24 h (alcalase)/3 h (alkali)	3.50%	[36]
Sargassum muticum	20 mL/g, 25 °C	Ultrasound-assisted extraction (150 W)	30 min	15%	[37]
Nizimuddinia zanardini	Pretreatment, 29 mL/g, 67 °C	Microwave-assisted extraction (400 W)	19 min	31.39%	[38]
Undaria pinnatifida	2.51% alkali, 13.27 g/L, 92 °C	Microwave-assisted extraction	15 min	38.41%	This study

presents studies of sodium alginate extracted from algae through various methods and conditions. Our process was observed to provide a shorter extraction time and higher yield compared to other methods. The process of extracting from Sargassum cristaefolium using an extrusion system achieved a similar yield in a shorter timeframe than our method [25]. However, our process holds an advantage in terms of overall processing time, as it does not include a preprocessing stage. Compared to traditional hydrothermal extraction, which typically features high extraction temperatures, long extraction times, and no safety hazards, microwave systems offer the advantages of high extraction speed, low energy consumption, and minimal waste production [33]. Thus, in the current context of increasing interest in sustainable development, microwave systems can be a viable eco-friendly technology in extraction processes. In this study, a process was designed to recover a high yield of 38.41% sodium alginate in just 15 min, and was compared to other processes.

4. Conclusions

This study proposed a new biorefinery approach for utilizing Undaria pinnatifida as an effective source for sodium alginate extraction and identified the optimal extraction conditions through statistical analysis. The optimal conditions established for sodium alginate extraction from Undaria pinnatifida were a solid/liquid ratio of 13.27 g/L, an extraction temperature of 91.86 °C, and an extraction solvent concentration of 2.51% (w/v). Employing these parameters, microwave-assisted extraction achieved a maximum yield of 38.61% sodium alginate in just 15 min, significantly outperforming traditional hydrothermal extraction by 1.34 times. Currently, commercial microwave systems provide high productivity, with a processing capacity of 2500 kg/h and an output of 200 kW. This research demonstrates the feasibility of applying this method on a large commercial scale, allowing for optimized productivity and economic efficiency. This outcome lays a vital foundation for future studies in terms of assessing the potential application of sodium alginate in diverse sectors, including the food industry, medical field, biotechnology, and 3D printing, thereby highlighting its broad industrial potential.