1-12. THERMAL PROCESSING
Food is prepared for canning and placed in containers. High-acid foods are
processed with moderate heat treatment, since the acid itself inhibits bacterial growth.
Low-acid foods require a much higher temperature. Times and temperatures are
usually given in the contractual documents. A vacuum is created in the container by
one of several methods.
a. Vacuum Closing. In this method, machines are used to close cans of cold-
pack products in a vacuum chamber (example, canned ham).
b. Preheating. The product is heated to 165oF (74oC) or above and held at the
temperature during packing. The heat expands the can's contents. After the contents
are retorted and cooled, they contract and produce a vacuum.
c. Exhaust Box. This method, sometimes called exhausting, uses a tunnel in
which filled cans are heated, causing the contents to expand. After they are retorted
and cooled, they contract and produce a vacuum.
d. Steam Jet (Steam Vac). Live steam at 250oF (122oC) is injected into the
headspace of the filled can until the end is double-seamed into place. The steam
condenses as it cools, creating a vacuum.
Immediately after the air has been exhausted, the can is sealed by machine.
The filled can is then placed in the retort for thermal processing. A retort is any closed
vessel or other equipment used for thermal processing at high temperatures (greater
than 212oF). This processing has two purposes: to produce a commercially sterile
product and to cook the material so that it can be consumed with a minimum of further
preparation. An example of a retort is a pressure cooker.
a. Vertical Retort. The vertical retort consists of a large pressurized steam
cooker that either sits on the floor (for a single batch processing) or extends beneath the
floor (for processing several batches at a time). It is loaded and unloaded from the top.
b. Horizontal Retort. Essentially, this is a vertical retort that has been
positioned on its side and modified to admit steam along the length of the top side. It is
loaded from the end.
c. Continuous Retort. This is the most modern unit and is now used in many
commercial plants. Its three sections are the preheater, the cooker, and the cooler.
Each unit contains a revolving reel and a spiral track. The cans are continuously fed
into individual compartments of the retort and rotated by movement of the reel while, at
the same time, the track guides them continuously from inlet to outlet.