Heat treatment of heat-sensitive products

Abstract

Separate units of a heat-sensitive product are each pre-treated by heating at least the main part of the unit to a pre-heating temperature of at least 50° C., a cooling medium being brought into direct or indirect contact with the unit so that its surface layer is prevented from acquiring a substantially higher preheating temperature than its inner parts; and each product unit is then heated to substantially sterilizing or pasteurizing temperature by electro-magnetic energy of at least microwave frequency.

I claim: 
     
       1. In the processing of a heat-sensitive product by subjecting it to a sterilizing or pasteurizing treatment temperature, the method which comprises pre-treating units of the product by heating at least the main part of each said unit to a pre-heating temperature of at least 50° C. .Iadd.interrupting said pretreatment heating.Iaddend., and subjecting each unit to the influence of a cooling medium to prevent the surface layer of said unit from acquiring a substantially higher pre-heating temperature than the inner parts of said unit, and then heating each product unit to substantially said treatment temperature by subjecting the unit to electromagnetic energy of at least microwave frequency. 
     
     
       2. The method of claim 1, in which said pre-heating temperature is between 60° and 80° C. 
     
     
       3. The method of claim 1, in which said pre-heating is effected by bringing first a heating medium and then a cooling medium into contact with the surface of the product unit. 
     
     
       4. The method of claim 3, in which the temperature of said cooling medium is higher than the temperature of the product unit before said heating medium was brought into contact with the product unit. 
     
     
       5. The method of claim 1, in which said pre-heating of the product unit is effected by an electro-magnetic field. 
     
     
       6. The method of claim 1, wherein portions of the product unit are heated to a temperature above said pre-heating temperature for a period of time harmless to said portions, the product unit then being kept in contact with a medium having a temperature no higher than said pre-heating temperature. 
     
     
       7. The method of claim 1, wherein said pre-treatment gives a surface layer of the product unit a lower temperature than its inner portions when said heating by electro-magnetic energy is initiated. 
     
     
       8. The method of claim 1, wherein the product unit is packaged before said heating thereof to substantially said treatment temperature. 
     
     
       9. The method of claim 1, in which the product unit is surrounded with an aqueous liquid while effecting said heating thereof to substantially said treatment temperature. 
     
     
       10. The method of claim 9, wherein the temperature of said aqueous liquid is controlled to give it substantially said treatment temperature when said electro-magnetic heating is completed. .Iadd. 11. In the processing of a heat sensitive product by subjecting it to a sterilizing or pasteurizing treatment temperature, the method which comprises pretreating units of the product by heating at least the main part of each said unit to a preheating temperature of at least 50° C., interrupting said pretreatment heating, subjecting each preheated unit to a medium having a temperature less than the temperature of the surface of said unit to prevent the surface layer of said unit from acquiring a substantially higher preheating temperature than the inner parts of said unit and then heating each product unit to substantially said treatment temperature by subjecting the unit to electro-magnetic energy of at least microwave frequency, while said unit is surrounded with an aqueous liquid having a temperature less than the surface temperature of the units..Iaddend.