Pet food freeze dryer

Pet food freeze dryerThe key to a freeze-drying machine depends on its freeze-drying process. Today, we will learn about some knowledge about the pre freezing stage in a freeze-drying machine. Let's delve deeper into the knowledge of this stage together!

The pre freezing process of pet food freeze-drying machine is to solidify the free water in the solution, giving the dried product the same form as before, preventing irreversible changes such as foaming, concentration, and solute movement during vacuum drying, and minimizing the reduction of substance solubility and changes in life characteristics caused by temperature.

There are two methods for pre freezing solutions: pre freezing inside the freeze-drying box and pre freezing outside the box.

The pre freezing method inside the box is to directly place the product on multiple shelves inside the freeze dryer, and freeze it by the freeze dryer's freezer. For the convenience of entering and exiting the box, a large number of small bottles and ampoules are usually placed in several metal trays and then packed into boxes to improve heat transfer. Some metal plates are made with a removable bottom, and the bottom is removed when entering the box, allowing the small bottle to directly contact the metal plate of the freeze-drying box; For plates that cannot be bottom drawn, it is required that the bottom of the plate be flat to achieve uniformity of the product. The large plasma bottles using the rotary freezing method should be pre frozen and equipped with a metal frame for thermal conductivity before entering the box for freezing.

There are two methods for pre freezing outside the box. Some small freeze dryers do not have a device for pre freezing products and can only use low-temperature refrigerators or alcohol with dry ice for pre freezing. Another type is a specialized rotary freezer, which can freeze large bottles of products into a shell like structure while rotating, and then enter the freeze-drying box.

Pre freezing process of freeze-drying machine:

When the temperature of an aqueous solution drops to a certain point, based on the eutectic concentration of the solution, ice begins to form in the dilute solution. This temperature is called the freezing point. Generally speaking, the freezing point is dominated by concentration and decreases together with concentration. When the temperature of the solution is below the freezing point, a part of the solution will crystallize and precipitate, and the concentration of the remaining solution will increase. In this way, the freezing point will decrease, and then continue to cool. The ice crystals increase with cooling, and the concentration of the remaining solution will increase accordingly. But when the temperature drops to a certain point, the remaining solution freezes completely, and the frozen material is mixed with ice crystals. This temperature is the eutectic point.

After the solution needs to be supercooled to the freezing point and crystal nuclei are formed inside, free water will begin to crystallize in the form of ice, while releasing crystallization heat to raise its temperature to the freezing point. As the crystals grow, the concentration of the solution increases. When the concentration reaches the eutectic concentration and the temperature drops below the eutectic point, the solution will freeze completely.

The number and size of crystal grains in solution crystallization are not only related to the properties of the solution itself, but also to the rate of nucleation and crystal growth. And the two factors of nucleation rate and crystal growth rate vary with temperature and pressure. Therefore, we can control the number and size of crystal grains in the solution by controlling the temperature and pressure. Generally speaking, the faster the cooling rate and the lower the supercooling temperature, the more crystal nuclei are formed, and the crystals are frozen before they can grow. At this time, the more grains are formed and the finer the grains; On the contrary, the fewer the number of grains, the larger the grains.

The shape of crystals is also related to the freezing temperature. When freezing begins near 0 ℃, ice crystals form a hexagonal symmetry and grow forward in six main axis directions. At the same time, several secondary axes also appear, and all ice crystals are connected to form a network structure in the solution. As the degree of undercooling increases, ice crystals will gradually lose their hexagonal symmetry for capacity recognition. In addition, with a high number of nuclei and fast freezing rate, they may form an irregular dendritic shape with any number of axial columns, unlike hexagonal crystals which only have six.

The crystal units formed by the freezing of biological fluids (such as blood plasma, muscle fluid, vitreous humor, etc.) are often similar in type to the ice crystals formed by a single component aqueous solution. The type of crystallization mainly depends on the cooling rate and body fluid concentration. For example, when plasma, muscle slurry, etc. freeze at normal concentrations, they form hexagonal crystal units at higher sub zero temperatures and slow cooling rates. When rapidly cooled to low temperatures, irregular dendritic crystals are formed.

Cell suspensions (such as red blood cells, white blood cells, bacteria, etc. suspended in distilled water, plasma, or other suspension media), when slowly frozen at high to low temperatures, a large amount of ice grows in the suspension, squeezing the cells into a narrow tube between two ice columns. The suspended medium in the tube condenses due to the precipitation of water, and the water inside the cells permeates out of the cells through the cell membrane, causing the concentration of solutes inside the cells. At the same time, the growth of extracellular ice will also force the volume of cellular material to shrink and deform. But at this time, the cells are not frozen. When rapidly freezing at low temperatures, intracellular ice will form inside the cell. The size, shape, and distribution of ice are related to the cooling rate, the presence or absence of protective agents, the properties of protective agents, and the amount of water inside the cell. Generally speaking, the faster the cooling rate and the lower the temperature, the more ice is formed inside the cell. Adding a non permeable protective agent to the suspension can reduce the number of ice formed inside cells during rapid freezing.

The form of solution crystallization has a direct impact on the freeze-drying rate. The gaps left by the sublimation of ice crystals are the escape channels for water vapor during subsequent ice crystal sublimation. The gaps formed by the sublimation of large and continuous hexagonal crystals are large, and the resistance to water vapor escape is small, resulting in a faster drying speed of the product. On the other hand, dendritic and discontinuous spherical ice crystal channels are small or discontinuous, and water vapor can only escape through diffusion or permeation, resulting in a slower drying speed. Therefore, considering only the drying rate, slow freezing is better.

In addition, the freezing rate is also related to the type, capacity, and heat transfer medium of the freezing equipment.

Pre freezing can have a certain destructive effect on cells and life, and its mechanism is very complex. It is generally believed that the mechanical and solute effects caused by water freezing during pre freezing are important factors leading to the inactivation or denaturation of biochemical drugs during freeze-drying. Mechanical effect refers to the increase in volume when water freezes, causing some weak molecular bonds in the active sites of active substances to be broken, resulting in loss of activity; Solute effect refers to the increase in solute concentration caused by water freezing, as well as the change in pH value due to inconsistent solubility changes of various solutes under different temperature conditions, resulting in changes in the environment in which the active substance is located and causing deactivation or denaturation. The following measures can be taken to address this phenomenon:

1. Pre freezing adopts the quick freezing method, which first lowers the shelf temperature to -450 ℃, and then rapidly freezes the product to form fine ice crystals, making it too late to produce mechanical effects.

When selecting buffering agents, it is necessary to choose buffer pairs with comparable solubility