<h1 class="pgc-h-arrow-right" data-track="1" > species and distribution of seaweed</h1>
Seaweed is a type of plant that grows in the ocean, relying on the energy required for photosynthesis to synthesize itself to synthesize organic matter, we usually say that seaweed mainly refers to brown algae, red algae, green algae three gates of marine algae general term.
Brown algae include macrophyllum, sargasson and spp. In the Pacific Ocean and the South Pole, macroalgae and cysticum are distributed, and kelp is abundant in the Pacific coast and the British Isles, and is common in the Gulf Stream and sargasso sea, and the genus is widely distributed in the intertidal zone of the British Isles. In China, the phylloscopus brown algae is distributed along all coasts, but the distribution of genera is gradually decreasing from north to south.
Red algae include palmar red algae, nori, stone cauliflower, carragee and so on. Palm rotundifolia is mainly distributed on both sides of the North Atlantic, Carragee is mainly distributed in the Rocky Coast of the Atlantic Ocean, and Nori is mainly distributed in the British Isles, Japan, South Korea and the coast of China. The genus Lithocarpus is a worldwide red algae that is widely distributed. In China, the number of phylum erythroidus is distributed in various coastal areas.
Green algae are mainly distributed in fresh water, but also in the shade and dampness of seawater, accounting for only 10%. The main species of green algae in seawater are lithopods, filamentos, tube algae and tube algae. The green algae in the ocean are mainly distributed along the ocean coast and attach to the rocks in shallow stalls. The temperature of the sea water determines the distribution of green algae in the ocean. Green algae in freshwater are widely distributed, not limited by water temperature, and are distributed all over the world. The distribution of the number of phylum green algae genera in China shows a trend of increasing from north to south.
With the increase in people's demand for seaweed, the seaweed produced in nature can not meet the demand for industrial production, and now in China and Southeast Asia, it mainly relies on artificial breeding to meet the needs of industry. There are a large number of seaweed breeding bases along the coast of China. The extraction of sodium alginate is mainly based on brown algae as raw material.
<h1 class="pgc-h-arrow-right" data-track="7">2</h1>
Sodium alginate is a polysaccharide biopolymer extracted and refined from kelp, giant algae and other brown algae, which is a white or light yellow powder. Sodium alginate is a good food additive. In the United States, it is known as a "wonderful food additive"; in Japan, it is known as a "longevity food".
<h1 class="pgc-h-arrow-right" data-track="9" > third, physicochemical properties</h1>
1. Composition
Sodium alginate (C6H7O6Na)n is mainly composed of sodium salts of alginate, consisting of β-D-mannuronic acid (M unit) and α-L-gulocuronic acid (G unit) relying on β-1,4-glycoside bonds and consisting of different proportions of GM, MM and GG fragments. 2、
2. Molecular weight
The molecular weight of commercial sodium alginate is usually like polysaccharides and is relatively dispersed. Therefore, the molecular weight of one sodium alginate usually represents the average of all molecules in the group. The most common ways to express molecular weight are numerical mean molecular weight (Mn) and heavy average molecular weight (Mw).
In multidispersioned molecular groups, Mw > Mn. The coefficient of Mw/Mn is the dispersibility index, and the index of sodium alginate products is in the classic range of 1.5 to 2.5. The most common method of determining molecular weight is calculated based on intrinsic viscosity and light scattering measurements.
3. Molecular formula
The molecular formula is (C6H7NaO6)x, and the molecular structure is as follows:
4. pH value
Sodium alginate is slightly soluble in water and insoluble in most organic solvents. It is soluble in alkaline solutions, making the solution viscous. Sodium alginate powder becomes wet in contact with water, and the hydration of the particles makes its surface sticky. The particles then quickly bond together to form clumps, which are slowly completely hydrated and dissolved. If the water contains other compounds that compete with alginate to hydrate, sodium alginate is more difficult to dissolve in water. Sugars, starches or proteins in water can reduce the hydration rate of sodium alginate, and it is necessary to prolong the mixing time. Salts of monovalent cations , such as NaCl , have a similar effect at concentrations above 0.5 percent. The pH of sodium alginate in 1% distilled aqueous solution is about 7.2.
5. Stability
Sodium alginate is hygroscopic, and the amount of moisture contained in equilibrium depends on the relative humidity. Dried sodium alginate is fairly stable to store at temperatures of 25 °C and below in a well-sealed container. Sodium alginate solution is stable at pH 5 to 9. The degree of polymerization (DP) and molecular weight are directly related to the viscosity of the sodium alginate solution, and the decrease in viscosity during storage can be used to estimate the degree of depolymerization of sodium alginate. High polymerization sodium alginate is less stable than low polymerization sodium alginate. Sodium alginate has been reported to be hydrolyzed by protons, depending on time, pH and temperature. Propylene glycol alginate solution is stable at room temperature and pH 3 to 4; when pH is less than 2 or greater than 6, the viscosity will decrease quickly even at room temperature.
<h1 class="pgc-h-arrow-right" data-track="22" > third, the main features</h1>
(1) Characteristics of sodium alginate
Sodium alginate is a byproduct of iodine and mannitol extracted from brown algae kelp or sargasso, and its molecules are formed by β-D-mannuronic acid (β-D-mannuronic, M) and α-L-gulocuronic acid (α-L-guluronic, G) by pressing the (1→4) bond. The aqueous solution of sodium alginate has a high viscosity and has been used as a thickener, stabilizer, emulsifier and so on for food. Sodium alginate is a non-toxic food that has been included in the United States Pharmacopeia as early as 1938. Sodium alginate contains a large amount of -COO-, which can exhibit polyanionic behavior in aqueous solutions, has a certain adhesion, and can be used as a drug carrier for the treatment of mucosal tissue. Under acidic conditions, -COO- is transformed into -COOH, the degree of ionization is reduced, the hydrophilicity of sodium alginate is reduced, the molecular chain contracts, and when the pH value increases, the -COOH group continues to dissociate, the hydrophilicity of sodium alginate increases, and the molecular chain stretches. Therefore, sodium alginate has a pronounced pH sensitivity. Sodium alginate can quickly form gels under extremely mild conditions, when there are Ca2+, Sr2+ and other cations present, Na+ on the G unit and divalent cations undergo ion exchange reactions, G units accumulate to form a crosslinked network structure, thereby forming a hydrogel. Sodium alginate forms gels under mild conditions, which avoids inactivation of active substances such as sensitive drugs, proteins, cells and enzymes. Due to these excellent properties, sodium alginate has been widely used in the food industry and medicine.
(2) Advantages of sodium alginate
Sodium alginate, as a thickener for beverages and dairy products, has unique advantages in thickening: sodium alginate has good fluidity, making the added drink taste smooth; and can prevent the viscosity decline during product disinfection. When using sodium alginate as a thickener, products with large molecular weight should be used as much as possible, and Ca should be added in an appropriate amount. It can greatly improve the viscosity of sodium alginate.
Sodium alginate is a high-grade stabilizer for cold drinks such as ice cream, which can make cold foods such as ice cream produce a smooth appearance and smooth taste. Because calcium alginate can form a stable thermal irreversible gel, it will not become rough (ice crystal growth) during transportation and storage, and will not deform the ice cream caused by temperature fluctuations; at the same time, this ice cream has no peculiar smell when eating, which not only improves the expansion rate but also improves the melting point, so that the quality and efficiency of the product are significantly improved. The product has a smooth, delicate and good taste. The amount of addition is low, generally 1-3%, and the amount of foreign addition is 5-10%.
Sodium alginate as a stabilizer for dairy products and beverages, stable frozen milk has a good taste, no stickiness and stiffness, when stirred, there is a stickiness, and there is a sense of hysteresis.
<h1 class="pgc-h-arrow-right" data-track="32" >, the source of sodium alginate</h1>
Sodium alginate is mainly derived from the phylum of brown algae, such as kelp, macroalgae, sargasso, etc., mainly in the cell matrix and cell wall of seaweed cells[8], and gives cells certain mechanical properties. Myklestad studies found alginate in cells with a dominant alginate
If it is in the form of calcium alginate, it is also partially present in the form of magnesium alginate, potassium alginate and sodium alginate. Among the different types of seaweed, seaweed has been found
The molecular weight of the acid varies, and the content is also different, the amount of alginic acid in the algae cells
It is about 20% of the dry weight, and its content changes with the season.
<h1 class="pgc-h-arrow-right" data-track="37" >4. Extraction of sodium alginate</h1>
(1) Extraction principle
In the late 19th century, British chemists studied extracts in brown seaweed and found that the extracts had the ability to concentrate solutions, form gels and form films. The purpose of alginic acid extraction is to convert insoluble calcium and magnesium salts into soluble sodium alginate (NaAlg). If the raw material is digested with sodium carbonate, it is used to convert calcium alginate into sodium alginate by ion exchange. The reaction formula is:
Ca(Alg) 2 + 2Na + →2NaAlg + Ca2 +
If the raw material is pretreated with acid and digested with sodium carbonate, the reaction formula is:
Ca( Alg) 2 + 2H + →2HAlg + Ca2 + ,HAlg + Na + →NaAlg +H +
In China, the main raw material for extracting sodium alginate is kelp.
(2) Extraction method
Since the commercialization of alginate, many people have studied its extraction process
There are many process methods, but most of them are similar, and the process is shown in Figure 1, which mainly includes three major steps: pretreatment, digestion and purification. The study of the sodium alginate extraction process is centered on how to increase the yield of alginate, reduce its degradation and improve its appearance during pre-treatment, digestion and purification steps.
Pretreatment of raw materials
Pretreatment plays an important role in the later extraction of the product. In earlier studies, most scholars believed that pretreatment of sodium alginate with dilute acids helped to improve the extraction rate; And in the early industrial production patent of sodium alginate, it is also described that pretreatment with acid is a necessary step for alkaline extraction of sodium alginate. However, the general pretreatment needs to be soaked, the time is longer, if the acid treatment, it will accelerate the rupture of the alginic acid molecular chain, affecting the viscosity, and at the same time, in the subsequent alkali extraction process, there will also be a phenomenon of darker color, affecting the quality of the product. Le Gloahec and Herter et al. proposed the need for increased destaining, and they treated the clarified sodium alginate extract with gelatinous aluminum hydroxide; After the acid treatment, the formalin solution was soaked, and it was found that chlorophyll and other colored substances were fixed in the epidermal cells of the seaweed, and in the process of alkali extraction, the color deepening phenomenon was significantly reduced, and formaldehyde had a destructive effect on the fibrous tissue of the plant cell wall, which was conducive to the replacement and dissolution of alginate in the subsequent digestion process. Haug A believes that the darkening of the color may be caused by phenolic compounds, if sodium hypochlorite is used for decolorization, it will have a great impact on the viscosity of the product, and long-term bleaching and decolorization will degrade the alginate molecular chain, resulting in a decrease in the viscosity of the product. Since then, researchers have generally used hydrochloric acid with formalin solution for pretreatment. Gustavo et al. conducted a detailed study of hydrochloric acid pretreatment. Their pretreatment method is to soak the raw materials overnight with a certain concentration of formalin solution, then soak overnight with a certain concentration of hydrochloric acid, and finally digest and extract with sodium carbonate.
The results show that the yield of sodium alginate increases with the increase of the concentration of the acid used, while the viscosity decreases with the increase of acid concentration. Since then, similar methods have been commonly used in pretreatment studies. He Junhong, He Zhonglin and others believe that formalin may have residues in the process of sodium alginate extraction, and is a carcinogen, but glutaraldehyde is a disinfectant that replaces formalin in hospitals for disinfection, and glutaraldehyde may destroy the cell wall of kelp, promote alginate dissolution and can achieve the purpose of fixing pigments, so they try to use glutaraldehyde with a mass concentration of 1% instead of formaldehyde to pretreat kelp. The results show that the use of glutaraldehyde is larger than the production of formaldehyde under the same conditions, but the viscosity of the resulting product is low. Cheyenne Ethan et al. of Qingdao University used acid in combination with sodium trimethionate for pretreatment and invented a method for preparing sodium alginate with low viscosity. The pretreatment method of Song Fangfang et al. is to first soak the raw materials in water until they are fully swollen and then crushed, and then use malic acid, ethanol, and table salt to stir freeze-thaw, and then carry out subsequent digestion and extraction.
When Rostami et al. investigated the relationship between the molecular weight of sodium alginate and the biological characteristics, in order to obtain sodium alginate of different molecular weights, they used four different pretreatment methods of distilled water, hydrochloric acid, proteolytic enzyme and cellulase to treat the raw materials. The results showed that the yield of water treatment and proteolytic enzyme treatment was the lowest, and the yield after hydrochloric acid treatment was significantly higher than that after water and proteolyase treatment, and the yield after cellulase treatment was the highest among all treatment methods. Nowadays, researchers are more likely to pretreat raw materials with enzymes [27 - 28], and the advantages are
By reducing the use of acid can avoid the degradation of alginic acid, while the enzyme also has a specificity, can destroy the cell wall, so that alginate can be more dissolved and replaced, but with enzyme pretreatment can not fix the pigment, so it is necessary to increase the decolorization of the ring, which will promote the degradation of alginate. In addition to the above pretreatment methods, more people use ethanol in the laboratory
Perform a soaking process. The above method can be obtained by pre-treatment of sodium alginate in the laboratory extraction
Very good results, but in the actual industrial production of sodium alginate, considering the cost of reasons, more or use acid and formaldehyde treatment.
Digestion of raw materials
After the raw material is pretreated, the alginic acid needs to be extracted from the cell wall of the seaweed, that is, the alginate is converted into soluble sodium alginate, thereby removing the alginic acid from the seaweed, which is digestion. Digestion is the main factor affecting the extraction rate, and its common method is alkali extraction. In the early days, some people used sodium hydroxide to digest, because sodium hydroxide polluted the environment and was highly corrosive, and later they were digested with sodium carbonate solution. The main influencing factors of digestion are sodium carbonate concentration, digestion temperature, digestion time and digestion pH.
Zvered et al. digest at 27°C with a 1% to 2% sodium carbonate solution
1 ～ 2 h。 Secconi proposes a pH of 5 to 7. Digestion between 5 ammonia, potassium and sodium salts for 5 to 24 h. Arvizu-Higuera et al. [31] through research
It is found that digesting pH greater than 10 works better. He Junhong, He Zhonglin and others used 1% sodium carbonate solution and added a small amount of sodium hydroxide to study the effect of pH on digestion reaction, and the results showed that with the increase of pH, the yield of sodium alginate increased significantly, from 33. 91% increased to 93. 87% believe that controlling high pH during digestion reactions has a positive effect on the extraction rate of the product. Pan Maodong et al. conducted a more detailed study on the extraction process of Hainan sargassonia from sodium alginate, and conducted a one-factor analysis of the digestion conditions affecting the extraction rate, and the results showed that when the concentration of sodium carbonate was 2%, its extraction rate reached the highest, which was 36. 8% 。 At a digestion temperature of 50°C, the extraction rate is highest at 40. 7%, digestion time of about 3h, the highest extraction rate, 35. 1% 。 The resulting sodium alginate has a viscosity of 185 mPa·s. Tian Hongyun[20] The effect of ultrasonic-microwave collaborative digestion on yield was analyzed in detail by univariate and orthogonal tests, and the viscosity of sodium alginate obtained after optimizing the process conditions reached 3670mPa·s and the extraction rate was 88. 6% 。 Song Yanxian et al. used cellulase-assisted sodium carbonate solution to extract sodium alginate, and orthogonal tests were used to determine the optimal extraction rate 49. 8% 。 Youssouf et al. treated the digestion of the raw materials with sonication by soaking the vacuum-dried raw materials overnight at room temperature with 80% ethanol, then filtering them with a microporous nylon membrane with a pore size of 10 μm, washing with distilled water, adding 2% sodium hydroxide solution, and then treating them with ultrasonic waves with a power of 150W and a frequency of 25kHz. They end up with a 55% extraction rate, which can shorten the extraction time. Digestion is an important part of alginate extraction, and the quality of digestion effect has a great impact on yield. From the green preparation, not only is the alginic acid effectively extracted from the cell wall of the seaweed, and the degradation of the molecular chain is reduced during digestion, but also the use of environmentally friendly extraction methods is the most concerned issue in the digestion process.
Purification of alginate solution
After the raw materials are digested, an impure sodium alginate solution mixture is obtained and needs to be purified. The general steps of purification are: Digested sodium alginate solution → filtered → hydrochloric acid (acid coagulation) or calcium chloride → extraction filtration → decalcification with hydrochloric acid solution or decalcification with sodium chloride solution ion exchange→ dissolving with sodium carbonate → filtering → ethanol precipitation → drying → finished product. In the early days, acid was generally coagulated, because of its low cost and application, but it took a long time, and it was easy to degrade the molecular chain of alginic acid during the extraction process, resulting in a low viscosity of the product. In order to improve the extraction efficiency and product quality of alginic acid, calcium coagulation is gradually used, because it can reduce the production time, on the one hand, the degradation time of the alginate molecular chain is reduced, on the other hand, the impact of acid on the molecular chain is reduced, thereby improving the viscosity of the product and is widely used, but the cost of calcium chloride is higher than that of hydrochloric acid. Wang Xiaohua et al. compared the purification process of acid coagulation and calcium coagulation, and the results showed that the sedimentation rate of acid coagulation was slower, the particle size of the obtained precipitate was smaller, which affected subsequent filtration, and the extraction rate of the product was also low; The sedimentation rate of calcium chloride coagulation is faster, the particle size of the obtained precipitate is larger, which is conducive to subsequent filtration, and the calcium ion replacement is removed by ion exchange, and the yield obtained is higher. After systematic research on the tracoagulation-ion alteration method, the best process conditions were obtained, the purification time was greatly shortened, and the viscosity of the product could reach 2840mPa·s. The purification method of Leinfelder et al. [34] is to use a hydrochloride of alginate extracted with EDTA with 37. 5% ethanol is precipitated with stirring with air, and the precipitate is 0. 5 mol/L potassium chloride solution is dissolved, repeatedly precipitated 2 times and then dissolved, the dissolved solution is dialyzed with a dialysis bag, followed by the third precipitation, and finally dried to obtain a high-purity alginate product. Their aim is to prepare alginates compatible with cells at the molecular level, a method that is expensive and suitable for laboratory preparation.
Gomez et al. studied the effects of three different purification methods on product yield and rheological properties, the first method being: precipitate the digested solution with ethanol→ extract → sodium alginate products with ethanol sox. The second is: hydrochloric acid precipitate after the digestion of the solution → the precipitate is dissolved with sodium carbonate solution→ the solution is precipitated with ethanol→ extracted with ethanol sox→ to obtain sodium alginate product; The third is: the solution after digestion with calcium chloride chelate→ precipitated with hydrochloric acid → chelate extracted with→ water sox
The solution was dissolved with sodium carbonate solution→ the solution was precipitated with ethanol→ and the sodium alginate product was obtained by extracting → with ethanol sox. It is obtained by comparing three purification methods in calcium ions
Precipitating under it, followed by proton exchange in an acidic medium, will result in the lowest molecular mass of sodium alginate and poor mechanical properties. The first method is precipitated directly with ethanol, with few purification steps, and the product obtained without acid treatment exhibits the best yield and rheological properties. In the laboratory, in order to simplify the purification step, the alginate can be precipitated directly from the digested alginate solution with ethanol, repeatedly precipitated, and then dried to obtain the product. Purification is an important part of the extraction of sodium alginate, which affects the efficiency of extraction and the purity of the product during the extraction process.
bibliography
[1] Quan Weiyan, Yang Ziming, Li Sidong, et al. Research Progress on The Extraction of Sodium Alginate[J]. Shandong Chemical Industry,2018,47( 19): 56 - 59.