Thermal Food Processing

Thermal Processing of Ready Meals 365
Both retail and catering markets are important for ready meals and have seen
tremendous growth recently as new processes such as sous vide increase in
popularity. Sous vide is a technology that originated in France as a mild thermal
process (e.g., 70°C for 40 min) for the manufacture of high-quality foods sold
to the catering sectors. 3 More recent sous vide processes have targeted the psychrotrophic
strains of Clostridium botulinum (e.g., 90°C for 10 min) that have
required the process severity to be increased. Section 12.3 discusses microbiological
targets and how thermal processes are validated.
Packaging formats vary significantly for ready meals and are linked with the
methods of production, storage, and consumer preference. Paperboard, plastic,
and aluminum trays are typical for chilled and frozen ready meals, where the
thermal processes are most likely to be applied to the food components before
they are packaged. These packaging formats often have multiple compartments
in which the components, such as meat, vegetables, and rice, are placed separately,
and each given a different thermal process. The processes given tend to be referred
to as cooking steps, but there is a microbiological kill associated with these steps,
and so it is correct to refer to this as thermal processing. The microorganism
targets are discussed later in Section 12.3.
Production methods for single-component or multicomponent ready meals
are varied, but can be categorized into those where the thermal processing step
occurs prior to packaging (e.g., heat exchangers) or after packaging (e.g., retorts).
The former method has the most variety in terms of the production methods,
where factories making short-shelf-life meals can be divided into high and low
care (or risk) areas. Methods of ready-meal manufacture are discussed in more
detail in Section 12.2.
12.2 METHODS OF MANUFACTURE
In order to understand how thermal processing can be used to manufacture a
complex food such as a ready meal, it is important to know how microorganisms
behave. In the hazard analysis and critical control point (HACCP) context, food
safety with respect to microorganisms is controlled by quantifying their introduction
to the food, growth within that food, and survival through the production
stages. 4 A factory HACCP plan will consider introduction, growth, and survival
at all stages of manufacture.
There are categories of temperature sensitivity of microorganisms that help
to define and position the thermal processes that are applied to ready meals:
• Psychrotrophic (cold tolerant), which can reproduce in chilled storage
conditions, sometimes as low as 4°C. Having evolved to survive in
extremes of cold, these are the easiest to destroy by heat.
• Psychrophilic (cold loving), which have an optimum growth temperature
of 20°C.
• Mesophilic (medium range), which have an optimum growth temperature
between 20 and 44°C. These are of greatest concern with ready meals.