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◆Abstract：Agar is a biodegradable polymer material，which is a hydrophilic colloid and has certain application value in many fields.In this paper，the properties of agar and its application in medicine，food and microbiology were reviewed，and made relevant research prospects，in order to provide a theoretical basis for expanding the use of agar and increasing the application value of agar.
◆Key words：agar；property；application；research progress
1 Introduction
Agar，the scientific name agar，also known as agar-agar，frozen powder，large vegetable silk，and it is a water-soluble polysaccharide material extracted from the red algae cell wall.Agar has good thickening，shape retention，and gelation.The gelation of agar is its biggest feature，not only for the preparation of medium，but also in food industry，daily chemical industry and other fields also have a wide range of applications.In this paper，the properties and applications of agar were reviewed，in order to provide a theoretical basis for the research of agar in various fields.
2 The properties of agar
2.1 Gelatinity
Agar can form a gel even at concentrations as low as 0.004%，and the agar gel completely prevents chemical degradation.However，the agar gel is hard，brittle，coarse in structure and poor in transparency，and dehydration occurs after freezing.Therefore，it is often used in combination with other materials to overcome its shortcomings.The combination of agar and carrageenan can improve the softness and elasticity of the product； when combined with dextrin and sucrose，the gel strength can be improved.
2.2 Hysteresis
Because of the unique nature of agar，the temperature of gel formation is lower than the temperature at which the gel melts.The freezing point of 1.5% agar sol is generally between 32 and 43 ℃，and the melting point of gel is generally between 75 and 90 ℃.The melting point temperature is much higher than the freezing point temperature is a unique phenomenon of agar，known as “hysteresis”.
2.3 Solubility
The solubility of agar in water has a certain positive correlation with temperature.It is insoluble in cold water，organic solvents and inorganic solvents，but it can be absorbed 20 times in cold water.It is easily dispersed in hot water to form a neutral sol.The higher the proportion of agarose，the higher the gel strength.
3 The applications of agar
3.1 Medicine
Agar has the advantages of strong water retention performance，lubrication and no irritation，and is very beneficial to intestinal peristalsis，so it can be widely used as a laxative.Agar is also commonly used in pharmaceuticals as a sustained release capsule，a surgical lubricant，and some emulsion type additives.Agar can be used as an emulsifier and stabilizer for the production of ointments in surgical dressings. 3.2 Food industry
Agar is used in juices and beverages as a suspending agent to keep the solids in the beverage evenly suspended.The suspension time is long and the suspending effect is stable.Agar can also be used in cold foods，which can reduce the precipitation of ice crystals and improve the product’s resistance to hot melt，making the product more refreshing，and the tissue is more delicate and smooth.Agar is blended with locust bean gum and gelatin to maintain the texture and aroma of cold foods and to prevent skin crusts and syneresis.
3.3 Microbiology
Agar has the characteristics of being catabolized by most microorganisms，does not contain components that inhibit the growth of microorganisms，has a low freezing point and a high melting point，and can form a strong，elastic，stable and transparent gel.In microbial culture，agar is generally used at a concentration of 1% to 2%.Agar is relatively stable when used in culture because very few organisms are capable of metabolizing agar or agar-decomposing enzymes.
4 Conclusion
Agar has been widely studied and applied in many fields，but some shortcomings of agar itself cannot be ignored.For example，agar has a slower degradation rate and a higher dissolution temperature，and many applications require agar to be easily degraded into agar having a smaller molecular weight and even being soluble at low temperatures.Therefore，overcoming the above shortcomings and how to expand the application of agar will be the direction of future research.
作者簡介
李海燕（1993—），女，汉，山东省泰安市人，研究生在读，青岛大学化学工程专业，食品科学方向。
◆Key words：agar；property；application；research progress
1 Introduction
Agar，the scientific name agar，also known as agar-agar，frozen powder，large vegetable silk，and it is a water-soluble polysaccharide material extracted from the red algae cell wall.Agar has good thickening，shape retention，and gelation.The gelation of agar is its biggest feature，not only for the preparation of medium，but also in food industry，daily chemical industry and other fields also have a wide range of applications.In this paper，the properties and applications of agar were reviewed，in order to provide a theoretical basis for the research of agar in various fields.
2 The properties of agar
2.1 Gelatinity
Agar can form a gel even at concentrations as low as 0.004%，and the agar gel completely prevents chemical degradation.However，the agar gel is hard，brittle，coarse in structure and poor in transparency，and dehydration occurs after freezing.Therefore，it is often used in combination with other materials to overcome its shortcomings.The combination of agar and carrageenan can improve the softness and elasticity of the product； when combined with dextrin and sucrose，the gel strength can be improved.
2.2 Hysteresis
Because of the unique nature of agar，the temperature of gel formation is lower than the temperature at which the gel melts.The freezing point of 1.5% agar sol is generally between 32 and 43 ℃，and the melting point of gel is generally between 75 and 90 ℃.The melting point temperature is much higher than the freezing point temperature is a unique phenomenon of agar，known as “hysteresis”.
2.3 Solubility
The solubility of agar in water has a certain positive correlation with temperature.It is insoluble in cold water，organic solvents and inorganic solvents，but it can be absorbed 20 times in cold water.It is easily dispersed in hot water to form a neutral sol.The higher the proportion of agarose，the higher the gel strength.
3 The applications of agar
3.1 Medicine
Agar has the advantages of strong water retention performance，lubrication and no irritation，and is very beneficial to intestinal peristalsis，so it can be widely used as a laxative.Agar is also commonly used in pharmaceuticals as a sustained release capsule，a surgical lubricant，and some emulsion type additives.Agar can be used as an emulsifier and stabilizer for the production of ointments in surgical dressings. 3.2 Food industry
Agar is used in juices and beverages as a suspending agent to keep the solids in the beverage evenly suspended.The suspension time is long and the suspending effect is stable.Agar can also be used in cold foods，which can reduce the precipitation of ice crystals and improve the product’s resistance to hot melt，making the product more refreshing，and the tissue is more delicate and smooth.Agar is blended with locust bean gum and gelatin to maintain the texture and aroma of cold foods and to prevent skin crusts and syneresis.
3.3 Microbiology
Agar has the characteristics of being catabolized by most microorganisms，does not contain components that inhibit the growth of microorganisms，has a low freezing point and a high melting point，and can form a strong，elastic，stable and transparent gel.In microbial culture，agar is generally used at a concentration of 1% to 2%.Agar is relatively stable when used in culture because very few organisms are capable of metabolizing agar or agar-decomposing enzymes.
4 Conclusion
Agar has been widely studied and applied in many fields，but some shortcomings of agar itself cannot be ignored.For example，agar has a slower degradation rate and a higher dissolution temperature，and many applications require agar to be easily degraded into agar having a smaller molecular weight and even being soluble at low temperatures.Therefore，overcoming the above shortcomings and how to expand the application of agar will be the direction of future research.
作者簡介
李海燕（1993—），女，汉，山东省泰安市人，研究生在读，青岛大学化学工程专业，食品科学方向。