Heat Sterilization (Technology)
Flushing the retort with incoming steam to remove air from the retort
Reason for removing air from retort:
Air forms a thin boundary layer around containers, which reduces heat transfer from steam to containers
It may be difficult to heat solid and highly viscous foods. Cooking for longer time to attain desirable temperature at thermal center may cause overcooking.
Therefore, to increase heat transfer
Containers are agitated
Thinner containers are used
Heating by Hot Water
Used for mainly glass containers, flexible pouches and polymer trays
Glass containers are thicker and have lower thermal conductivity
Hot water provides lower thermal shock to containers than steam
Flexible pouches and polymer trays are thinner. Therefore, they can be heated more rapidly
Heating by flames
Used mainly for small cans
Types of Containers used in the retorts:
Glass jars or bottles
Removal of air from the headspace of containers
Reasons for Exhausting
To prevent air expansion with heat. Helps to remove distortion of containers
Removal of oxygen prevents internal corrosion in containers
Removal of oxygen prevents oxidative changes in foods
Principle of Exhausting
Gases occupy greater volume at a higher temperature
Methods Used for Exhausting Containers
Hot filling the food
Cold filling the food and then heating the container and contents with partially sealed lid
Applying vacuum to remove air
Flushing the cans with steam
mixed or intermediate.
the majority of food products (liquid food
products with solid particles)
a mixture of conduction and convection.
The heating and cooling characteristics are
less predictable and must be measured very
Three steps in retorts
Horizontal or vertical
End over End Agitation of containers
Advantages and Disadvantages of Continuous retorts
1. Gradual changes in pressure and temperature.
2. Less strain on cans
1. High capital investment
2. Breakdown will stop the process line
Variables Monitored in Retorts
Temperature of the raw material
Temperature of steam
Temperature of cooling water
Heating and cooling rates
Example: For a certain food F10120 = 10 min is needed for commercial sterility. Two alternative procedures:
1.Heat food instantaneously to 1200C, hold at this temperature for 10 min and cool instantaneously. F=10(T-q)/z tf= 10(120-120)/10 x 10 = 10min.
2. Heat food inst. To 1400C, hold at this T for 0.1min and cool inst. F= 10(140-120)/10 x 0.1 = 10min.
Suppose this food contains a valuable enzyme with a z-value of 50C0which requires 4 min at 1200C for inactivation. At 1400C time required for inactivation will be:
t = 4 x 10(120-140)/50 = 1.6 min.
Processing time needed Time needed for enzyme inactiv.
Procedure 1: 10 min 4 min
Procedure 2: 0.1 min 1.6 min
Ultra High Temperature/Aseptic Processing
Sterilized food packed in sterile containers under aseptic conditions.
1.Product with higher organoleptic and nutritional quality,
2.Possibility to use large containers to pack the food,
3.Extended possibilities for using packaging materials of many package sizes, shapes and materials,
4.Handling of containers during subsequent sterilization is avoided, recontamination risk during cooling is minimized.
Processing at a very high temperature for a very short time
UHT is used for liquid foods
Manufacturers of UHT Heat Exchangers
Tubular APV, Nuova Frau
Plate Stork, Alfa Laval
Direct Heating Alfa-Laval, APV, Stork
Electric resistance APV, Elecster
Examples of liquid foods processed using UHT
Fruit Juices and concentrates
Ice Cream Mix
Soups and rice desserts
Principle of UHT
For a given increase in temperature, the rate of destruction of micro-organisms and many enzymes increases faster than the rate of destruction of nutrients and sensory components.
A disadvantage of UHT
In milk some enzymes such as proteases and lipases are more heat resistant
They can cause flavor changes during prolonged storage
Process conditions are independent of container size
Enzymes and probiotics with low heat sensitivity can be added to the Food After Sterilization
Retorting (Canning) Vs. UHT (Aseptic Processing