Products that need freeze-drying are generally pre-prepared into an aqueous solution or suspension, so their freezing point is different from that of water. Water freezes at 0 ℃, while seawater freezes at a temperature lower than 0 ℃ because seawater is also an aqueous solution of various substances. The experiment indicates that the freezing point of the solution will be lower than that of the solvent.
In addition, the freezing process of the solution is also different from that of pure liquid. When pure liquid such as water freezes at 0 ℃, the temperature of the water does not decrease until all water freezes, which indicates that pure liquid has a fixed freezing point. The solution is different. It does not completely condense into a solid at a fixed temperature, but at a certain temperature, the crystals begin to precipitate. With the decrease in temperature, the number of crystals continues to increase. Finally, the solution does not condense completely. In this way, the solution does not condense at a fixed temperature. Instead, it condenses in a certain temperature range, and the temperature at which crystals begin to precipitate when cooled is called the freezing point of the solution. The temperature at which all the solution condenses is called the freezing point of the solution. Because the freezing point is the starting point of melting (i.e. melting point). For the solution, it is the point where the solute and solvent melt together. So it's also called co-melting point. It can be seen that the freezing point and co-melting point of the solution are different. The co-melting point is the temperature at which the solution really solidifies.
Obviously, the concept of CO melting point is important for freeze-drying, because freeze-dried products may contain substances such as salts, sugars, gelatin, proteins, blood cells, tissues, viruses, bacteria, etc. Therefore, it is a complex liquid, and its freezing process must also be a complex process. Similar to the solution, it also has a temperature at which all of it condenses into a solid. Co melting point. Because freeze-drying is carried out under a vacuum. Only after the products are completely frozen can they be sublimated under a vacuum. Otherwise, when some liquid exists, it will not only evaporate rapidly under vacuum, resulting in the concentration of liquid and the reduction of the volume of freeze-dried products; Moreover, the gas dissolved in water will quickly emerge under vacuum, resulting in the appearance of liquid boiling, which makes the freeze-dried products bubble and even comes out of the bottle. This is what we don't want. Therefore, at the beginning of sublimation, the freeze-dried products must be cooled to the temperature below the co melting point, so that the freeze-dried products can be completely frozen.
In the freezing process, it is impossible to determine whether the product is completely frozen into a solid from the observation of the appearance; It is impossible to determine the internal structural state of the product by measuring the temperature. With the change of product structure, the change of electrical properties is very useful, especially when the freezing is resistivity measurement, which can let us know whether the freezing is in progress or has been completed. After all freezing, the resistivity will be very large, so the solution is ionic conductive. Freezing is that ions will be fixed and cannot move, so the resistivity increases significantly. When there is a small amount of liquid, the resistivity will decrease significantly. Therefore, measuring the resistivity of the product will be able to determine the co-melting point of the product.
The normal co-melting point measurement method is to immerse a pair of platinum electrodes into the liquid product, insert a thermometer into the product, cool them to a low temperature below - 40 ℃, and then slowly heat up the frozen product. Wheatstone bridge is used to measure its resistance. When the resistance suddenly decreases, the temperature at this time is the co-melting point of the product. The instrument will be powered by the DC bridge because the whole process of electrolysis will be recorded.
It can also be measured by simple methods. Insert two copper wires of appropriate thickness and mutual insulation into the container containing the product as electrodes. Insert a thermometer near the copper electrode, the insertion depth is similar to that of the electrode, put them together near the observation window hole in the freeze-drying box, fix them with appropriate methods, and then pre-freeze them together with other products. At this time, we use a multimeter to continuously measure the resistance value in the cooling process, and determine the common melting point according to the change of resistance value.
Connect the electrode lead with the multimeter through a switch, regardless of the positive and negative poles. If there is no wire leading out connector in the freeze-drying box, two thin wires can be led out from the crack of the box door, and some vacuum sealing wax can be coated near the wire, so as not to affect the vacuum degree.
After the thermometer drops to 0 ℃, start measuring and making records. Put the change-over switch of the multimeter at the highest gear for measuring resistance（ × 1Kor × 10K）。 Since the direct current is used in the multimeter, in order to prevent electrolysis, turn off the switch immediately after each measurement, and record the temperature and resistance values measured one by one. In the beginning, the resistance value is very small, and then gradually increases. At a certain temperature, the resistance increases suddenly and is almost infinite. At this time, the temperature value is the eutectic point value.
The co-melting point measured by this method has a certain error because there is some electrolysis at the copper electrode. The multimeter has no bridge sensitivity for high resistance; In addition, the change of resistance in the freezing process and melting process is not exactly the same, but the measured value still has a practical reference value.
The value of CO melting point ranges from 0 ℃ to - 40 ℃, which is related to the variety of products, the type, and the concentration of protective agents. The list of common melting points of some substances is for reference because the actual freeze-dried products have other components. So it's different.
When the product is freeze-dried, it needs to be put into a suitable container, and then pre-frozen before sublimation drying. The pre-freezing process is not only to protect the main properties of substances; Moreover, it is necessary to obtain that the frozen product has a reasonable structure, which is conducive to the sublimation of water; There should also be an appropriate loading capacity for future application.
There are usually two ways to repack products: bulk and bottled. Metal plates, lunch boxes or glassware can be used in bulk; Bottles are made of glass bottles and ampoules. Glass bottles have plasma bottles. Vaccine bottles and penicillin vials, ampoules also include flat bottom ampoules, long ampoules, and round ampoules; These need to be determined according to the future use of the product, and the bottle needs to be equipped with a suitable rubber stopper.
Table 22 co melting point of some substances (℃)
Material co melting point
0.85% sodium chloride solution-22
10% sucrose solution-26
40% sucrose solution-33
10% glucose solution-27
2% gelatin, 10% glucose solution-32
2% gelatin, 10% sucrose solution-19
10% sucrose solution, 10% glucose solution, 0.85% sodium chloride solution - 36
Skimmed milk-26
Horse serum-35
All containers shall be cleaned and sterilized before sub packaging.
The products that need to be lyophilized need to be prepared into a certain concentration of liquid. In order to ensure a certain shape after drying, the best material content is between 10 ~ 15%.
There is a certain ratio of surface area to thickness when the product is sub-packed into the container. The surface area should be larger and the thickness should be smaller. Large surface area is conducive to sublimation, and large product thickness is unfavorable to sublimation. Generally, the sub packaging thickness shall not be greater than 10mm. Some products require large bottles. When a large number of products are freeze-dried, they can be frozen into shell shape by rotary freezing method, or the inclined container can be frozen into the inclined plane, so as to increase the surface area and reduce the thickness.
