Retort Control in the Food Industry using the C360
AG/RCP-007_2
7
Preheat Period
During the first cycle, the vertical retort is filled with water prior to loading the jars. On
subsequent cycles, the water used to cool down the last batch remains in the retort.
The temperature of this water is too low for the next batch of hot-product filled jars –
glass cannot withstand a thermal shock of more than 28
°
C (50
°
F).
The water must therefore be heated with steam to a predetermined temperature before
the jars are immersed. This operation is called preheat. The preheat water temperature
must not be more than 8
°
C (15
°
F) above product temperature to avoid raising the
internal jar pressure which would cause the jar lids to open.
Preheating in a horizontal retort is not possible because the jars must be loaded and
the door closed before water can be added. The water used to fill the retort must be
preheated externally.
Raise to Cook Temperature Period
The period in which the water is heated from initial temperature to cook temperature is
referred to as the 'raise to cook' period and should be kept to a minimum to prevent
overcooking.
The Cook Period
Both temperature and time are critical for the same reasons as outlined in steam cook.
The Pressure Cool Period
The internal jar pressure must be lowered sufficiently to prevent it from forcing the jar
lids open after cooking. To do this, retort pressure is maintained above jar internal
pressure as cooling water is circulated through the retort to cool the product and
reduce the pressure in the jar. Temperature reduction is done quickly to stop the
cooking of the product.
Pressure Relief Period
After the jars have been cooled sufficiently, the retort pressure may be reduced to
atmospheric (blown down) without danger to the containers.
Atmospheric Cooling Period
After blowdown, and with the retort cover or door open, cooling water can be circulated
through the retort in order to continue the product cooling process. Alternatively, the
jars can be removed and submerged in a cooling canal before labeling.
