Method and apparatus for treating heat-sensitive products

Abstract

Each unit of a heat-sensitive product is contacted directly or indirectly with a cooling medium so that the unit acquires a condition in which a surface layer thereof has a lower temperature than its inner portions. The product unit is then heated, by electromagnetic energy of at least microwave frequency, from this condition to substantially the desired sterilizing or pasteurizing temperature in a medium having a temperature which will permit such heating. In this way, a substantially uniform heat treatment effect is obtained in the product units.

I claim: 
     
       1. In the processing of a heat-sensitive product by heating it from substantially above 0° C. to a treatment temperature for sterilizing or pasteurizing, the method which comprises subjecting units of said product to the influence of a cooling medium and thereby placing each product unit in a condition wherein a surface layer thereof has a lower temperature than the inner portions thereof, and then subjecting each product unit to a field of electro-magnetic energy of at least microwave frequency to heat the unit from said condition to substantially said treatment temperature and while the unit is in a second medium acting to substantially equalize the heating effect of said field throughout the unit. 
     
     
       2. The method of claim 1, in which said cooling medium surrounds each product unit entirely. 
     
     
       3. The method of claim 1, in which said cooling medium is a liquid. 
     
     
       4. The method of claim 1, in which said second medium has a dielectric constant of at least half of that of the product unit in said field, said second medium surrounding the product unit at least along the boundary lines thereof as seen in a plane perpendicular to the direction of the main application of the field against the product unit. 
     
     
       5. The method of claim 4, in which said electro-magnetic field raises the temperature of said second medium from a starting temperature to substantially said treatment temperature during said electro-magnetic heating of the product unit. 
     
     
       6. The method of claim 4, which comprises also moving the product unit at one speed and said second medium at a different speed through the electro-magnetic field, said relative speeds being controlled to cause said second medium to be heated by the field to substantially said treatment temperature where the product unit leaves the field. 
     
     
       7. The method of claim 1, wherein each product unit is passed through a liquid bath of said cooling medium and then through a second liquid bath of said second medium. 
     
     
       8. The method of claim 1, in which said electro-magnetic heating of each product unit is continued sufficiently to give said surface layer a higher temperature than said inner portions thereof. 
     
     
       9. The method of claim 1, in which said product unit is packaged prior to said processing. .Iadd. 
     
     
       10.  The method of claim 1 in which the units are heated prior to being subjected to the influence of the cooling medium. .Iaddend. .Iadd.11. In the processing of a heat sensitive product by heating it from substantially above 0° C. to a treatment temperature for sterilizing or pasteurizing, the method which comprises subjecting units of said product to a preheating step, subjecting the preheated units to the influence of a cooling medium thus to place each product unit in a condition wherein a surface layer thereof has a lower temperature than the inner portions thereof, then subjecting each unit to a field of electro-magnetic energy of at least microwave frequency, while the unit is in a second medium having a temperature below the temperature of the surface of the unit and a dielectric constant at least half that of the product unit, to heat the unit substantially uniformly to treatment temperature..Iaddend.