Plant sanitation - Gracey's Meat Hygiene - page 89

If one or more factors are limited by the cleaning condi-
tions, for example, mild chemicals must be used to avoid
corrosion of a surface and/or contact time is very lim-
ited, then one or more of the other factors must be raised
to compensate, for example, water pressure and/or
temperature.

should be avoided where the nature of the production or
cleaning process poses a corrosion risk.
Terrazzo and concrete may both become porous and
cracked if mistreated and are liable to damage by acids.
Paints and other similar coatings can vary enormously
in their resistance to attack by chemicals and hot or pres-
surised water and, once flaking, present a risk of foreign
body contamination to the food. Chemical-resistant res-
ins are available, but it is important to match the coating
to the production and cleaning environment expected.
Plastics and rubbers also vary greatly. At their worst,
they can swell on contact with some detergents, which
may affect smooth running of machinery if present in
gaskets, bearings, etc., or become embrittled by heat,
light or chlorine. Some are surprisingly absorbent of
soils, especially colourings, mineral oils, smoke, etc., and
may even play the generous host to moulds.
The main point to look for in choosing surface mate-
rial type is compatibility with the production environ-
ment, both physical and chemical, and with the cleaning
regime. Compatibility with each other is also very
important, particularly when two different metals such
as mild and stainless steel are in contact in a moist envi-
ronment. Galvanic corrosion invariably takes place, caus-
ing the 'lower' metal to corrode rapidly. This will also
occur in welds which are poorly executed or where the
choice of welding rod is incorrect.

Chemical and physical reactions of cleaning
Detergency involves many different reactions, physical
and chemical, which depend on the nature of the soils to
be removed and the nature of the detergent employed to
remove them.

Physical reactions
The primary physical reactions are the following:

Wetting Wetting is defined as the displacement of
one fluid from a solid surface by another. The displaced
fluid may be air or some liquid or semi-liquid such as
grease. The fluid displacing it is, for the purpose of our
discussions, water or a detergent solution. Water alone is
not sufficiently wet to displace many types of soils or
even to displace air from water-repellent or 'hydropho-
bic' surfaces, for example, water droplets on a Teflon fry-
ing pan. In these cases, the water curls up under its own
surface tension into droplets. Lack of wetting will pre-
vent cleaning taking place.
To achieve wetting of such surfaces, chemical agents
that have particular surface properties are employed:
'surfactants' or 'wetting agents. These are organic mole-
cules, which are different at each end. One end is essen-
tially hydrocarbon in nature and closely resembles
grease, oil or fat, that is, 'hydrophobic. The other end is
either ionised to give a positive or negative charge or
consists of oxygen-containing groups. In either case, this
end strongly attracts water, that is, it is 'hydrophilic. The
result is a dual-nature molecule, which concentrates
itself at the interface between the water and the surface
and allows wetting to take place as shown in Figure 5.1.
The nature of the surfactant - whether it foams or de-
foams, how it wets different surfaces and emulsifies dif-
ferent fats or how biodegradable it is - depends upon its
exact design. There are many hundreds commercially
available, which may be used in detergents and disinfect-
ants, either alone or in combination.

Energies of cleaning
A principle of prime importance is that every cleaning
process, of whatever kind, always involves a combination
of four factors:

1 Thermal energy , in the form of hot water or steam. As
a rough guide, an increase in temperature by 10°C in a
detergent solution doubles the rate of the chemical
reactions involved in cleaning.
2 Mechanical energy , in the form of brushes, water jets,
turbulent flow in pipes or even the micro-agitation
produced by the bursting of foam bubbles. In clean-
ing-in-place (CIP) of pipe systems, a flow rate of about
2 m/s is needed to ensure turbulence and avoid lami-
nar flow, discussed in more detail later.
3 Chemical energy , which depends on the nature and
concentration of the detergent used.
4 Time , which varies from hours in the case of soak
cleaning to seconds in the case of tray or crate cleaning
in industrial washing machines.
It is essential to understand the interrelation between
these factors. Failure to do so will often lead to very poor
cleaning results. While it is impossible, because of the
complexity of the cleaning reactions, to be mathematical
about it, there must be a balance between the four factors.

Penetration Wetting is the first essential step in the
removal of the soil. As wetting agents allow the detergent
to displace air from surfaces, detergents are able to pen-
etrate deep into porous dry deposits much faster than
water alone. In doing so, the other active components of
the detergent are enabled to react with soil components
deep in the deposit at a much earlier stage.