Agriculture Reference
In-Depth Information
'cook. The system must also ensure that processed and
non-processed cans cannot be mixed.
chamber before sealing. The ends of the can are loosely
attached to permit the escape of air, sealing being
completed when the cans leave the exhauster.
2
Vacuumising
, in which the cold material is filled into
the can, which is then closed in a vacuum-closing
machine, the can being subjected to a high vacuum
during the sealing operation.
Following closure, the cans are usually washed, before
processing, in water at 80-85°C, containing a non-ionic
detergent.
Cooling
Prompt cooling after processing is important, as it checks
the action of heat and prevents undue change in texture
and colour. In addition, cooling reduces the considerable
internal pressure of the cans which builds up during pro-
cessing. The cans may be placed under cold water show-
ers, immersed in a cold water tank or pressure cooled in
the retort. The standard of the cooling water should be
that acceptable for public drinking water supply, that is,
it should be clean and wholesome. Reliance cannot be
placed on chlorination alone, which has little effect on
any organisms if organic matter is present; river water
will require sedimentation and filtration before final
chlorination. The amount of chlorine added to cooling
water should be enough to produce, after 30 minutes
contact time, a free residual chlorine content of 0.5 ppm
or more, and a chlorinated water supply should show no
coliform bacteria in 100 ml water, a standard readily
obtained by effective treatment. In commercial practice,
cans are water cooled to 38°C, and the residual heat dries
the exterior and prevents corrosion.
Processing
With the exception of such foods as sweetened condensed
milk or jam, all canned foods are processed, that is, given
final heating, after hermetic sealing. The term 'process-
ing' is an exact one; it is not sterilisation since certain
canned foods after processing may still contain living
organisms. Although canned foods will keep with cer-
tainty if sterilised, they are then liable to alteration in col-
our and texture. Food to be canned is threatened on the
one hand by bacterial spoilage and on the other by danger
of overheating. The canner therefore chooses a middle
course, the minimum heat employed in processing being
controlled by the nature of the food in the can and the
types and number of bacteria likely to be present.
During processing, heat penetrates to the centre of the
can by conduction and by convection currents. In solid
meat packs, the heat diffuses by
conduction
only, and the
process is therefore slow; the
convection
currents in
loosely packed foodstuffs transfer heat faster. Solids
loosely packed in a liquid will, therefore, heat more rap-
idly than those that are tightly packed. Canned ham,
being the largest and most solid pack of all the canned
foods, requires very careful processing.
In non-acid foods, such as meat, the destruction of
bacterial spores is slow; temperatures of about 115°C are
required for adequate processing within a practical time
limit. In commercial practice, the cans are placed in
metal baskets in closed retorts and processed by pres-
surised steam.
The amount of heat used is based on that required to
destroy the spores of
Clostridium botulinum
, the so-
called botulinum cook. It is quantified as the
D-value
,
which is a measure of the time taken to achieve a 10-fold
reduction in the bacterial numbers at a given tempera-
ture. The accepted standard for a safe heat treatment is
the time/temperature combination which will achieve a
reduction in
Cl. botulinum
by a factor of 10, otherwise
expressed as 10D. Monitoring of this critical control
point must ensure that the time/temperature and
pressure parameters are measured continuously and
automatically to ensure all cans receive the correct
Can washing
Cans that have just been cooled are dirty and greasy on
the outside and are therefore washed in a bath with soap
or saturated with fatty alcohols and rinsed to facilitate
subsequent handling, lacquering and labelling.
Outside lacquering
Commercial lacquer or enamel is a colour varnish con-
taining vegetable or synthetic resin. Lacquer may be
applied to the outside of the tin to prevent external cor-
rosion, particularly when the cans are destined for humid
climates. Although external lacquering is not common
in the canning of vegetables and fruits, it is almost uni-
versal in the salmon canning industry, not only because
the UK market insists on shipments finished in this way
but also because the loss through rusting would other-
wise be enormous.
Container handling
The contents of hot wet cans may be infected if the cans
are subjected to mechanical abuse and exposed to exces-
sive concentrations of micro-organisms around the seam
or seal areas, for example, from operatives' hands. Thus,
manual handling of hot wet cans must be avoided, and it
is wise to discard cans manually handled while, for
example, clearing runway blockages. Surfaces coming
into direct contact with cans must be checked for
efficiency of cleaning (<10 cfu/cm
2
).
