Heating Methods
Proper
heating and cooling of precooked food products are critical to the prevention
of food-borne illnesses. The objective is to provide you with knowledge about
the types of pathogenic organisms which may be present on these foods, the
proper interventions to control them as a public health hazard, and with
procedures for assuring proper cooking and cooling of them where you will recognize
inadequate processes associated with the cooking and cooling while discussing the
hazards associated with foods and the cooking and cooling processes.
Cooking
of the food products changes its color, texture, arrests enzymatic reactions
and generally makes food more palatable while improving the taste and aroma.
However, from a food safety point of view, it’s most important objective is to
kill or inactivate spoilage of food and pathogenic organisms.
There
are various types of cooking methods and controls available to assure the
elimination of pathogens or sometimes heat degradable chemical compounds such
as enzymes or hormones. But it is important to know how we transfer the heat to
the products and its effectiveness in the cooking processes.
Conduction
The first method of heat transfer is
conduction. Conduction (or heat conduction) is the transfer of heat energy
by microscopic diffusion and collisions of particles or
quasi-particles within a body due to a temperature gradient. The
microscopically diffusing and colliding objects include molecules, electrons,
atoms, and phonons. They transfer microscopically disorganized kinetic and
potential energy, which are jointly known as internal energy. Conduction can
only take place within an object or material, or between two objects that are
in direct or indirect contact with each other. Conduction takes place in all
forms of ponderable matter, such as solids, liquids, gases and plasmas. Whether
by conduction or by thermal radiation, heat spontaneously flows from a body at
a higher temperature to a body at a lower temperature. In the absence of
external drivers, temperature differences decay over time, and the bodies
approach thermal equilibrium.
Heating by conduction is a slow
process in which heat is applied to the food container, and the heat is passed
on to the food. In conduction heating, heat is transferred through the food
being cooked one particle at a time (from one molecule to the next). This type
of heating is typical for solid foods such as a turkey or a roast being cooked
in an oven. To evaluate the adequacy of cooking, you must know where the
coldest point is in the food. The coldest point in conduction heating is
usually either the geometric center, or farthest point from the heat source.
Convection
A faster method of heating is
convection where heat penetration is augmented by movement in the food. Convection is
the concerted, collective movement of groups or aggregates of molecules within fluids (e.g., liquids,
gases) and rheids, either through advection or through diffusion or as a
combination of both of them. Convection of mass cannot take place in solids, since
neither bulk current flows nor significant diffusion can take place in solids.
Diffusion of heat can take place in solids, but that is called heat
conduction. Convective heat transfer is one of the major modes of heat
transfer, and convection is also a major mode of mass transferring fluids.
Convective heat and mass transfer take place both by diffusion – the
random Brownian motion of individual particles in the fluid – and by advection,
in which matter or heat is transported by the larger-scale motion of currents
in the fluid.
Convection heating can only occur in
foods that can move within the cooking vessel. This movement is referred to as convection
currents, and uneven heating within the food brings them about. For example, in
a pot of stew heat moves through the food container walls and heats the
material nearest to the wall of the pot. As this part of the food becomes warmer
it tends to rise, and the cooler material at the center of the container sinks.
These convection currents speed the heating process, and make it more uniform. The
coldest spot in convection heating is no longer the geometric center but is
nearer the bottom center of the container where the currents diverge.
Forced Convection
Forced convection is a mechanism, or type of transport in
which fluid motion is generated by an external source (like a pump, fan,
suction device, etc.). It should be considered as one of the main methods of
useful heat transfer as significant amounts of heat energy can be transported
very efficiently. For even faster heating and more uniformity, forced
convection is used. This is convection heating that is facilitated by stirring
or agitation. Stirring moves the food around in the heating container and by
doing so speeds the heating process.
There are a variety of methods for
stirring foods at the food processor level, but at the retail level the cook
simply stirring the pot usually accomplishes it. The location of the cold spot
in forced convection heating depends on the type of stirring involved, however
if very active stirring is involved, cold spots are virtually eliminated.
Forced convection can also be observed in forced air ovens where forced air
circulation facilitates faster heat transfer on the surface of the product.
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