STEAM PASTEURIZATION METHOD FOR FREEZE-DRIED FOOD

Abstract

A method for pasteurizing freeze dried food comprises: 1) Determining the corresponding sterilization temperature according to the food to be sterilized, and determining the target water activity at the sterilization temperature; 2) Cooling the food to a temperature corresponding to the type of the food; 3) Removing the air from the sterilization chamber using a vacuum pump; 4) Introducing water vapor into the sterilization chamber, adjusting the steam flow rate after the food temperature reaches the sterilization temperature, and maintaining the food temperature at the sterilization temperature for a specific duration and ceasing the introduction of water vapor; 5) Capturing steam from the sterilization chamber and reducing the pressure in the chamber; 6) After the pressure in the chamber reaches or approaches the water saturation pressure corresponding to the cooling temperature prior to sterilization, charging clean air into the chamber until the vacuum state is released, and removing the food.

Claims

1. A steam pasteurization method for freeze-dried food, characterized in that the method comprises: 1) Determining the corresponding sterilization temperature according to the type of freeze-dried food to be sterilized, and determining the target water activity at the sterilization temperature; 2) Cooling the food in step 1) to a cooling temperature corresponding to the type of the food; 3) Removing the air from the sterilization chamber where the food is located by using a vacuum pump; 4) Introducing water vapor into the sterilization chamber, adjusting the steam flow rate after the food temperature reaches the sterilization temperature, and maintaining the food temperature at the sterilization temperature for a specific sterilization time and lastly cease the introduction of the water vapor; 5) Capturing steam from the sterilization chamber and reducing the pressure in the sterilization chamber; 6) After the pressure in the sterilization chamber reaches or approaches the water saturation pressure corresponding to the cooling temperature prior to sterilization, clean air is charged into the sterilization chamber until the vacuum state is released, and finally food is removed from the chamber to complete the sterilization process.

2. The steam pasteurization method for freeze-dried food according to claim 1, wherein in the step 2), the cooling temperature is determined as follows:
T.sub.1=T.sub.2(x.sub.2x.sub.1)*(h.sub.1h.sub.2)/c.sub.p Where c.sub.p is the constant pressure specific heat capacity of the food; T.sub.1 is the cooling temperature; T.sub.2 is the sterilization temperature; x.sub.1 is the initial moisture content of the freeze-dried food; x.sub.2 is the moisture content of the food at the sterilization temperature and target water activity which is obtained by using the isothermal moisture absorption curve of the food; h.sub.1 is the enthalpy of saturated water vapor at temperature T.sub.1; h.sub.2 is the enthalpy of saturated water at the sterilization temperature.

3. The steam pasteurization method for freeze-dried food according to claim 1, wherein in the step 1), the sterilization temperature is determined from 60 to 90 C., and the target water activity is from 0.70 to 0.85.

4. The steam pasteurization method for freeze-dried food according to claim 1, wherein in the step 3), air is removed from the sterilization chamber until the absolute pressure in the sterilization chamber is reduced to 100 to 3000 Pa.

5. The steam pasteurization method for freeze-dried food according to claim 1, wherein in the step 4), the water vapor introduced into the sterilization chamber is a negatively pressurized saturated water vapor.

6. The steam pasteurization method for freeze-dried food according to claim 1, wherein in the step 4), when the water vapor introduced into the sterilization chamber is a negatively pressurized saturated water vapor, the pressure of the steam source should be equal to or slightly higher than the water saturation pressure corresponding to the sterilization temperature.

7. The steam pasteurization method for freeze-dried food according to claim 5, wherein in the step 4), the saturation temperature corresponding to the negatively pressurized saturated steam pressure should be higher than the pasteurization temperature by 0 to 10 C.

8. The steam pasteurization method for freeze-dried food according to claim 1, wherein in the step 4), the specific sterilization time is from 10 min to 90 min.

9. The steam pasteurization method for freeze-dried food according to claim 1, wherein the step 5) is specifically carried out by: connecting the sterilization chamber with a cold trap, Condensing or depositing the steam on the surface of the cold trap, lastly capturing the non-condensable gas by a vacuum pump.

10. The steam pasteurization method for freeze-dried food according to claim 8, wherein the surface temperature of the cold trap should be between 40 C. to 10 C.

Description

DETAILED EMBODIMENTS

[0041] Hereinafter, specific embodiments of the present invention will be described in details.

[0042] The sterilization method in the proposed invention comprises: pre-cooling the freeze-dried food to an appropriate temperature, condensing the low-pressure saturated steam on the surface and inside the pores of the food at a lower temperature in the vacuum environment, raising the food temperature such that the moisture content increases and therefore water activity rises rapidly, sterilizing the food for a period of time at the appropriate pasteurization temperature and elevated water activity. After the sterilization is complete, evaporating the condensed water by reducing the pressure (capturing steam), decreasing the temperature and moisture content of the food such that the food is in a suitable state for storage.

[0043] In some embodiments of the invention, the sterilization method comprises the following steps:

[0044] (1) First, selecting a pasteurization temperature suitable for the freeze-dried food and the target water activity at the sterilization temperature, the temperature ranges from 60 C. to 90 C.; and the target water activity ranges from 0.7 to 0.85.

[0045] (2) Cooling the freeze-dried food to be sterilized. The temperature T.sub.1 of the food to be reached before sterilization can be calculated by the following method: according to the law of energy conservation, the heat required for rising the temperature of the material is equal to the heat released by the steam condensation. Without consideration of the heat capacity of the container, the energy balance relationship can be represented as:

c.sub.p*(T.sub.2T.sub.1)=(x.sub.2x.sub.1)*(h.sub.1h.sub.2)(1)

[0046] Where c.sub.p is the constant pressure specific heat capacity of the food;

[0047] T.sub.1 is the temperature to be reached before sterilization of the food;

[0048] T.sub.2 is the sterilization temperature;

[0049] x.sub.1 is the initial moisture content of the freeze-dried food;

[0050] x.sub.2 is the moisture content of the food at the sterilization temperature and target water activity which is obtained by using the isothermal moisture absorption curve of the food;

[0051] h.sub.1 is the enthalpy of saturated water vapor at T.sub.1 temperature;

[0052] h.sub.2 is the enthalpy of saturated water at the sterilization temperature.

[0053] The temperature of the food before the sterilization can be calculated by the following equation:

T.sub.1=T.sub.2(x.sub.2x.sub.1)*(h.sub.1h.sub.2)/c.sub.p(2)

[0054] If T.sub.1 calculated using the above equation (2) is too low, the proposed method will be difficult to execute. As a consequence, the sterilization effect can be achieved by appropriately adjusting the target water activity and extending the sterilization time.

[0055] (3) Removing the air from the sterilization chamber where the food is located. After the absolute pressure in the sterilization chamber is reduced to 100-3000 Pa, stop the air removal.

[0056] (4) Introducing negatively pressurized saturated water vapor into the sterilization chamber. The pressure of the steam source is equal to or slightly higher than the water saturation pressure corresponding to the pasteurization temperature. Adjusting the steam flow rate after the temperature of the food reaches the sterilization temperature, and maintaining the food at the sterilization temperature for a period of 10 minutes to 90 minutes, then cease the introduction of water vapor.

[0057] (5) Connecting the sterilization chamber with a cold trap with a low surface temperature (temperature: from 40 C. to 10 C.), and steam is condensed or deposited on the surface of the cold trap, and then pumping non-condensable gas through a vacuum pump. When the pressure in the sterilization chamber is lower than the water saturation pressure corresponding to the temperature of the food, the condensed water on food surface evaporates and the temperature of the food decreases

[0058] (6) When the pressure in the sterilization chamber reaches or approaches the saturation pressure corresponding to the cooling temperature T.sub.1, vacuum is stopped and clean air is introduced into the sterilization chamber until the vacuum state in the sterilization chamber is released. The food is displaced from the chamber, and finally, the sterilization process is complete.

Example 1

[0059] The sterilization process of the proposed invention is further explained by a preliminary study of the sterilization process of freeze-dried chicken meat with a moisture content of 2.8%.

[0060] (1) Parameter Setting and Calculation

[0061] The pasteurization temperature was set at 75 C., and the target water activity was 0.75. By checking the isothermal moisture absorption curve of the material, the water activity was 0.06 at a temperature of 25 C. and a moisture content of 2.8%. The corresponding moisture content was 8.4% at a temperature of 75 C. and a water activity of 0.75. The constant pressure specific heat capacity of the freeze-dried chicken was 1.7 kJ/(kg C.), and the temperature of the material before steam sterilization was 3 C. as calculated according to the above equation (2).

[0062] (2) Vacuuming

[0063] Several pieces of the freeze-dried chicken were inoculated with Enterococcus faecium (ATCC 8459) at 6.310.sup.8 CFU/g; the freeze-dried chicken was cooled to 1 C. in a cold storage room and then taken out and placed in a sterilization chamber which was vacuumed to 1000 Pa by a rotary vane vacuum pump, and the food temperature was raised to 3 C. during the process.

[0064] (3) Steam Sterilization

[0065] After vacuuming was complete, saturated steam at 78 C. was introduced into the sterilization chamber, the steam was condensed on the surface and inside the pores of the material, and the temperature of the material was raised from 3 C. to 74 C. in 10 minutes. The steam flow rate was adjusted to maintain the temperature of the material at 751 C. for 45 min.

[0066] (4) Steam Capture

[0067] The sterilization chamber was connected with a cold trap which was introduced with refrigerant. The evaporation temperature was maintained at about 20 C. The steam condensed on the surface of the cold trap. When the pressure in the sterilization chamber was reduced to 3000 Pa, non-condensable gas was pumped by a vacuum pump. During the depressurization process, the condensed water on the surface and inside the pores of the material was evaporated, and the water vapor was frosted on the surface of the cold trap, and the temperature of the material was decreased.

[0068] (5) Vacuum Releasing

[0069] When the pressure in the sterilization chamber was reduced to 700 Pa, the temperature of the material was decreased to 20 C. the vacuum in the sterilization chamber was released and the material was taken out.

[0070] (6) Test Results

[0071] After the testing, the moisture content of the material after sterilization was 3.6%, and the number of inoculated Enterococcus faecalis was reduced to 4.010.sup.3 CFU/g, with a reduction of 5.2 log units. By checking the isothermal moisture absorption curve of the freeze-dried chicken at 25 C., the water activity was 0.18 with a moisture content of 3.6%. Although the moisture content and water activity of the materials have increased slightly after sterilization, they have little effect on the quality and shelf life of the product.