Thermal Food Processing

Thermal Processing of Ready Meals 371
a botulinum cook, and the standard process is 3 min equivalent at 121.1°C,
referred to as F 0 3. 10–12 All retorting processes target this organism if no other
effective hurdle to its growth is present. However, there is a growing trend to
apply additional hurdles to microbial growth that allow the processor to use a
milder heat treatment, referred to as pasteurization.
Milk is the most widely consumed pasteurized food in the U.K., and the
process was first introduced commercially in the U.K. during the 1930s, when a
treatment of 63°C for 30 min was used. Modern milk pasteurization uses an
equivalent process of 72°C for 15 sec. Pasteurization is nowadays used extensively
in the production of many different types of food, such as fruit products, pickled
vegetables, jams, and ready meals. 13 Food may be pasteurized in a sealed container
(analogous to a canned food) or in a continuous process analogous to an aseptic
filling operation. It is important to note that pasteurized foods are not sterile and
will usually rely on other preservative mechanisms to ensure their extended
stability for the desired length of time.
Whereas thermally processed ready meals for ambient storage have shelf lives
normally measured in years, cook-freeze meals typically have shelf lives measured
in months and cook-chill foods in days.
12.2.3 IN-VESSEL SYSTEMS
The vessel acts in a way similar to that of a heat exchanger in that it raises the
food temperature to that required for pasteurization. Sterilization processes are
not normally applied within vessels because of the need to operate at pressures
above atmospheric, and the benefits of sterilization are lost when the food is
cooled and packaged. A typical vessel size is around 800 to 1000 kg and usually
comprises a hemispherical steam-jacketed base with cylindrical sides (Figure 12.3).
Direct steam injection can be used to effect a more rapid heating rate. A hinged
lid is usually present to reduce heat loss and prevent foreign objects from falling
into the food. With high-viscosity foods it is essential that the food is well mixed;
otherwise, laminar boundary layers develop that reduce the thermal efficiencies
and may assist burn-on to the heated surface. Horizontal agitators with scrapedsurface
blades offer the most effective mixing, although recirculating pumps and
vertical mixing blades are alternatives.
Once pasteurized, the food can be filled either hot or cold into the containers.
A hot-fill process will only require a short hold time at high temperature to ensure
the inside container surfaces are pasteurized. This is usually achieved in a raining
water tunnel pasteurizer, although it is possible to omit this step if:
1. The food’s acidity is high (pH < 3.8)
2. The filling temperature is above 95°C
3. The containers are prewarmed or of low heat capacity
The shelf life of a sauce of low pH will be many months if hot filled, and
determined by its chemistry and not by its microbiology. However, multicomponent