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Having decided theFvalue required, it is necessary to ensure that the
F 0 value actually delivered by a particular heating regime achieves this
target value. To do this, the thermal history of the product during
processing is determined using special cans fitted with thermocouples to
monitor the product temperature. These must be situated at the slowest
heating point in the pack where the F 0 value will be at a minimum. The
precise location of the slowest heating point and the rate at which its
temperature increases depend on the physical characteristics of the can
contents. Heat transfer in solid foods such as meats is largely by
conduction which is a slow process and the slowest heating point is the
geometric centre of the can (Figure 4.5). When fluid movement is
possible in the can, heating is more rapid because convection currents
are set up which transfer heat more effectively. In this case the slowest
heating point lies on the can’s central axis but nearer the base.
The slowest heating point is not always easy to predict. It may change
during processing as in products which undergo a sol–gel transition
during heating, producing a broken heating curve which shows a phase

Table 4.4 Typical F 0 values for some canned foods

Food F 0 (min)

Asparagus 2–4

Beans in tomato sauce 4–6

Carrots 3–4

Peas 4–6

Milk pudding 4–10

Meats in gravy 8–10

Potatoes 4–10

Mackerel in brine 3–4

Meat loaf 6

Chocolate pudding 6

Figure 4.5 Conduction and convection heating in cans.KDenotes slowest heating point

74 The Microbiology of Food Preservation