Powder sterilization technology using superheated steam

There are two sterilizing systems by heating; dry heating and wet one. The effect of sterilization is quite different between them.For example, when sterilizing Clostridium Sporogens by heating, the D-value (time to kill 90% of bacteria) at 120oC is 115 to 195 minutes by dry heating, and only 0.18 to 1.4 minutes by wet heating. This shows that the wet heating by steam is far more advantageous than the dry heating by high temperature air or indirect heating.

However, after sterilization by wet heating, the materials must be dried or crushed due to increased moisture content or formation of lumps and the system may be stopped with sticky or hygroscopic materials. Since “Superheated Steam” is not only excellent in sterilization effect and easy to use but has the properties both of dry and wet heating, it is best-suited to sterilize the powdery or granular foods.

If the saturated steam at a certain pressure is further heated, it is called superheated steam. Fig.1 shows the state diagram of water. Here, the lower part from saturated steam curve shows a hot water zone and the upper part a superheated steam zone. For example, as the water is heated at the pressure of 0.2 MPaG, it becomes saturated steam at 133oC and when it is further heated, it becomes superheated steam. If it is heated up to 180oC, it is called a superheated steam of 0.2 MPaG and 180oC. The difference of 47oC from saturated temperature of 133oC is called “Degree of Superheat.”

Superheated steam set at proper degree of superheat has advantages both of dry heat and wet one, to sterilize the materials stably, not wetting them too much.

Fig.2 shows the comparison between destruction ratio of thiamin by heating and thermal death toll of index bacteria spores.
For example, point A (116oC, 1,300 sec.) and point B (137oC, 10 sec.) are on the same F0=6.0 min. line and have the same effect in sterilization. Here, the destruction ratio of thiamin must be highlighted. It is 20% at point A and only 1% at point B. It means that under the same sterilization effect, the material deteriorates less by high-temperature short-time heating (point B) than low-temperature long-time one. (point A)

In our system, almost all the materials can be sterilized for 4 seconds approximately.

Fig.3 is an example of sterilization that the material of 20oC is supplied into pneumatic pipes and sterilized by superheated steam of 0.2 MPaG at 153oC. (degree of superheat of 30oC) Time from feed to discharge is shown on the horizontal line, and temperatures for superheated steam and material and moisture content of material are shown on the vertical line.

Here, the temperature of superheated steam goes down by depriving the heat soon after the material is supplied. Then, it return to the original 153oC by receiving the heat from the outside of the pipe. Receiving the wet heat from superheated steam (0.2 MPaG), the material temperature instantly goes up to the saturated temperature of superheated steam, 133oC. At the same time, the moisture content of material also goes up. However, the material starts drying by apparent heat from superheated steam and the moisture content becomes close to the original one. In this process, the material temperature is kept at 133oC for approximately 4 seconds. Finally, the product is discharged to atmosphere.
Please note that the material temperature never goes up above the saturated temperature equivalent to superheated steam pressure (in this case, 133oC) though it is exposed to the apparent temperature of superheated steam.  It is because given heat is used to evaporate the water of material and from wet heat. While the evaporation continues, the material temperature is kept at the saturated temperature. As soon as the material is discharged to atmosphere, the material temperature drops closely to 100oC, boiling temperature of atmospheric pressure.

This means that the ultimate sterilizing temperature is not the superheated steam temperature, but the saturated temperature of superheated steam pressure. To increase the sterilizing temperature, the pressure must be increased. Under atmosphere, the material temperature cannot be over 100oC while the material is wet. Maximum steam pressure is 0.3 MPaG for model KPU and 0.55 MPaG for model SIRV

Please see the flow sheet on page 3 of this catalog. High temperature and pressure superheated steam runs at high speeds (20 to 30 m/s) in the pneumatic piping to circulate in the heating / sterilization line with steam circulation blower. The powdery or granular material is continuously supplied through constant feeder into pneumatic pipe with high pressure feed rotary valve. It goes in the piping with superheated steam to be sterilized instantly. The material is recovered with high pressure cyclone and discharged from the compressed line through high pressure discharge rotary valve. It takes approximately 4 seconds for these processes.

Since the material from high pressure discharge rotary valve is entrained with a small amount of steam, it should be separated in the steam separation cyclone. The material is transferred in the cooling line to be recovered with product recovery cyclone. All the piping and cyclones after high pressure discharge rotary valve are sanitary design to prevent secondary contamination. The cooling air comes from aseptic filter (HEPA) and it is compressed.