The process of extracting longan polysaccharides from longan.

1. Introduction

Longan, also known as Dimocarpus longan Lour., is a popular fruit in many Asian countries. It is not only delicious but also rich in various bioactive components, among which longan polysaccharides are of particular interest. Longan polysaccharides have been found to possess a variety of biological activities such as antioxidant, immunomodulatory, and anti - diabetic properties. Therefore, the extraction of longan polysaccharides has become an important research area in recent years. This article will comprehensively introduce the process of extracting longan polysaccharides from longan.

2. Pretreatment of Longan Fruits

2.1 Selection of Longan Fruits

The first step in the extraction process is the careful selection of longan fruits. Fresh longan fruits should be chosen as they contain higher amounts of polysaccharides compared to stale or damaged ones. When selecting, fruits with intact skins, no signs of decay, and a plump appearance are preferred. This is because any damage or decay may lead to the degradation of polysaccharides or the contamination of other substances, which will affect the quality and yield of the extracted polysaccharides.

2.2 Cleaning of Longan Fruits

After selection, the longan fruits need to be thoroughly cleaned. This is to remove any dirt, dust, pesticides, or other contaminants on the surface of the fruits. Cleaning can be carried out by rinsing the fruits with clean water several times. In some cases, a mild detergent solution can be used for more effective cleaning, but it must be ensured that the detergent is completely removed during subsequent rinsing to avoid any interference with the extraction process.

2.3 Preparation of Longan Pulp

Once the fruits are clean, the next step is to prepare the pulp. The longan fruits are peeled to remove the outer shell, and then the seeds are removed. The remaining pulp can be further processed depending on the extraction method to be used. For example, it can be cut into small pieces or mashed to increase the surface area for better extraction efficiency.

3. Extraction Techniques

3.1 Hot - Water Extraction

Hot - water extraction is one of the most common methods for extracting longan polysaccharides.

1. Firstly, the prepared longan pulp is placed in a suitable container.
2. Then, a certain amount of water is added to the pulp. The ratio of longan pulp to water is an important factor that affects the extraction efficiency. Generally, a ratio of 1:10 to 1:20 (weight/volume) is often used.
3. Next, the mixture is heated to a proper temperature. The temperature usually ranges from 60°C to 100°C. Proper temperature control is crucial during this process. If the temperature is too low, the extraction efficiency will be low as the polysaccharides may not be fully dissolved in the water. On the other hand, if the temperature is too high, it may cause the degradation of polysaccharides, reducing their biological activities.
4. After heating for a certain period of time (usually 1 - 3 hours), the mixture is cooled down to room temperature.
5. Finally, the extract is obtained by filtering the cooled mixture through a filter paper or a filter membrane to remove the insoluble residues.

3.2 Enzyme - Assisted Extraction

Enzyme - assisted extraction is another effective method.

1. To start with, the longan pulp is mixed with a buffer solution to adjust the pH value. The pH value is usually adjusted to the optimal range for the enzyme to work, which is different for different enzymes.
2. Then, a specific enzyme is added to the mixture. Commonly used enzymes for longan polysaccharide extraction include cellulase, pectinase, etc. The amount of enzyme added also needs to be carefully controlled, usually ranging from 0.1% to 1% of the weight of the longan pulp.
3. After adding the enzyme, the mixture is incubated at a certain temperature for a specific period of time. For example, the incubation temperature may be around 40°C - 60°C, and the time may be 1 - 2 hours. During this process, relevant factors such as agitation speed need to be regulated to ensure the enzyme can fully react with the substrate.
4. Finally, the reaction is stopped by heating the mixture to inactivate the enzyme. Then, the extract is obtained by filtering as in the hot - water extraction method.

3.3 Ultrasonic Extraction

Ultrasonic extraction has been increasingly used in recent years due to its high efficiency.

1. First, the longan pulp is placed in an ultrasonic extraction device.
2. Then, a suitable solvent (usually water) is added to the pulp. Similar to hot - water extraction, the ratio of longan pulp to solvent is also important.
3. Next, the ultrasonic device is set to operate at a certain frequency and power. The frequency usually ranges from 20 kHz to 100 kHz, and the power may be adjusted according to the amount of longan pulp. Ultrasonic extraction helps to break the cell walls of longan cells more effectively, which improves the extraction efficiency of polysaccharides.
4. After ultrasonic treatment for a certain period of time (usually 15 - 60 minutes), the extract is obtained by filtering the mixture.

4. Separation and Purification

4.1 Alcohol Precipitation

After obtaining the extract, the next step is separation and purification. Alcohol precipitation is a commonly used method.

1. Firstly, a certain amount of alcohol (usually ethanol) is added to the extract. The final concentration of alcohol in the mixture is usually adjusted to about 70% - 80% (volume/volume). This is because polysaccharides are generally less soluble in high - concentration alcohol, so they will precipitate out.
2. Then, the mixture is left to stand at a low temperature (usually 4°C) for a certain period of time (usually 12 - 24 hours). During this time, the longan polysaccharides will gradually precipitate at the bottom of the container.
3. Finally, the supernatant is removed by decantation or centrifugation, and the precipitated polysaccharides are collected.

4.2 Column Chromatography

Column chromatography is often used for further purification of longan polysaccharides.

1. First, a suitable chromatographic column is selected, such as a DEAE - Sepharose column or a Sephadex G - series column.
2. Then, the collected polysaccharide precipitate is dissolved in a small amount of buffer solution and loaded onto the top of the column.
3. Next, different eluents are used to elute the polysaccharides. For example, a gradient of NaCl solutions with different concentrations can be used. As the eluent passes through the column, the polysaccharides with different properties will be separated and eluted at different times.
4. Finally, the fractions containing the purified longan polysaccharides are collected and combined.

5. Drying and Storage

5.1 Drying of Longan Polysaccharides

After separation and purification, the longan polysaccharides need to be dried. There are several drying methods available, such as freeze - drying, vacuum drying, and air - drying.

• Freeze - drying is a preferred method as it can preserve the structure and biological activities of polysaccharides well. In freeze - drying, the polysaccharide solution is first frozen at a very low temperature (usually - 80°C or lower), and then the water is removed by sublimation under vacuum conditions.
• Vacuum drying is also a commonly used method. In this method, the polysaccharides are dried under reduced pressure at a relatively low temperature (usually 40°C - 60°C). This method can also effectively remove water while minimizing the damage to the polysaccharides.
• Air - drying is the simplest method, but it may take a longer time and may be more likely to cause some changes in the properties of polysaccharides due to the relatively long exposure to air.

5.2 Storage of Longan Polysaccharides

Once the longan polysaccharides are dried, they need to be stored in a suitable environment.

• They should be stored in a dry and cool place to prevent moisture absorption and degradation. A desiccator or a sealed container can be used to store the polysaccharides.
• It is also important to protect the polysaccharides from light, especially direct sunlight, as light may cause photochemical reactions and affect the stability of the polysaccharides.
• Moreover, the storage temperature should be controlled within a certain range. Generally, a temperature of - 20°C to 4°C is considered suitable for long - term storage.

6. Conclusion

The extraction of longan polysaccharides from longan involves multiple steps, including pretreatment of fruits, extraction using different techniques, separation and purification, and finally drying and storage. Each step is crucial and affects the quality and yield of the final product. With the continuous development of extraction and purification technologies, it is expected that more efficient and high - quality longan polysaccharide extraction methods will be developed in the future, which will further promote the research and application of longan polysaccharides in the fields of food, medicine, and cosmetics.

FAQ:

What are the main steps in the pretreatment of longan fruits for longan polysaccharide extraction?

The main steps in the pretreatment of longan fruits for longan polysaccharide extraction are choosing fresh longan fruits, cleaning them thoroughly, and preparing the pulp for the subsequent extraction process.

How does hot - water extraction of longan polysaccharides work?

Hot - water extraction of longan polysaccharides works by heating the longan pulp in water at a proper temperature. This temperature needs to be carefully controlled to ensure effective extraction of polysaccharides from the longan without causing degradation or other unwanted chemical reactions.

What are the key factors in enzyme - assisted extraction of longan polysaccharides?

In enzyme - assisted extraction of longan polysaccharides, the key factors include the addition of specific enzymes that are suitable for breaking down the cell walls of longan cells to release polysaccharides. Also, relevant factors such as pH, temperature, and enzyme concentration need to be regulated precisely to optimize the extraction process.

How does ultrasonic extraction contribute to the extraction of longan polysaccharides?

Ultrasonic extraction contributes to the extraction of longan polysaccharides by creating cavitation effects. These cavitation effects can disrupt the cell walls of longan cells more effectively, increasing the release of polysaccharides and thus improving the extraction efficiency.

What methods are used for separation and purification of longan polysaccharides after extraction?

After extraction, alcohol precipitation and column chromatography are used for separation and purification of longan polysaccharides. Alcohol precipitation helps to precipitate the polysaccharides out of the solution, and column chromatography can further purify the polysaccharides by separating them from other impurities based on different chemical properties.

Related literature

• Extraction and Bioactivity of Longan Polysaccharides: A Review"
• "Optimization of Longan Polysaccharide Extraction Process and Its Antioxidant Activity"

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