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B C T E R I O L O G IC L C O N S I D E R T I O N S
N C A N N I N G F O O D
D . J . O L I V A N T
B SC
Microbiological Laboratory 1t. I. He hlz Com pany L td
THE problem of obtaining food has always been fol lowed by the problem of a
sat isfactory m ethod for preserving i t for later consum ption. Tradit ional metho ds
of preserv at ion- -dry ing, curing, smoking, pickl ing, use of sugar or sal t, and
various co m bin at io ns- -ha ve a l imited applicat ion. Mo dern method s of can-
ning, freezing, freeze drying and irradiating, have however made possible the
preservation of nearly all foods. Most spoilage of foodstuffs is caused by
various micro-organisms, yeasts, moulds and bacteria. In the canning process,
instead of making it difficult or impossible for the micro-organisms present to
grow, the micro-organisms are destroyed. This is also the case with irradiation.
In canned foods the pH of the produc t is imp ortant , as this affects the type of
micro-organisms that could grow, in an imperfectly sterilized or faulty can. pH
4.5 is generally taken as a dividing line, as below this the heat-resistant spores
produced by certain bacteria, including CI. botulinum (Go rdo n Mu rrel 1967)
will not germ inate or grow. Yeasts, moulds and bacteria al l present prob lem s
where the pH value is below 4.5, because the heat resistance of som e yeasts is as
high as the bacteria tha t can grow in this pH range. They are especial ly imp ortan t
to frui t canners. A bove a pH of 4.5 bacteria a re the ma in spoilage group.
These include the food poisoning organisms, and are therefore most important
to canners of low-acid products.
To prevent mic robial spoilage and possible food poison ing it is first necessary
to destroy~ the bacteria, and then keep them out. M etho ds described to-achieve
this are based on those in use at H. J . Heinz Company Limited.
In canning, the product is sealed into the container and subjected to a heat
treatm ent, using Saturated steam in a pressure vessel s o that a predeterm ined
tem peratur e between 230 and 250F ma y be achieved. This ran ge gives reasona bly
rapid sterilization without overcooking the product.
STERILIZING PROCESSES
When sterilizing at these temperatures, all vegetative cells, yeasts and moulds
are destroyed early on, b ut the spores of BaciUus and Clos tridiumo rganism s can
withstand these tem pera tures for a reasonable t ime. Extensive experiments
have indicated a logari thmic o rder of death for m icro-organisms in response.to
heat . This mean s that the amo un t o f heat that wil l redu ce I00 organisms to 10
will i f applied again reduce I0 to one and so on. There w il l never of course be
fractions of a survivor present , bu t there m ay be one survivor fo r instance, per
billion cans.
The most notorious spore-producing bacterium is
Clostridium botulinum.
T h e
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182 PU BL IC HE AL TH VOL LX XX II NO 4
spores h ave a fairly high heat resistance, surviving boiling for several hours. The
organism produces a toxin which is so lethal that one ounce would ki l l
200,000,000 people.
From studies on this organism by Esty and Meyer (I922) recommendations
were issued that lowered the possibility of a spore surviving the sterilizing
process to once in over a hundred years. This stopped botul ism overnight from
commercial ly canned products in the U.S.A. and spread rapidly to become an
accepted standard. Most products nowadays are given an even higher steril izing
process to destroy more heat resistant org~inisms that can cause spoilage.
These sterilizing processes give a com mer cially sterile pack, a phras e which
admits the impossibil i ty of mak ing every can ever produc ed completely steri le ,
but means that any survival will be of such 10w level, Or unable to grow in the
pack, that virtual sterili ty is obtained. For example, the spores of the therrno-
philic Bacilli, especially
B stearothermophilus
are so heat resistant that perhaps
one survives per 100 cans. However, as these only grow at temperatures above
100F they will only cause spoilage if specially incubated. Normally the spores
remain do rma nt and there i s no evidence whatsoever tha t these do rm ant spores
in any way affect the product . The high heat resistance of the spores of thermo-
philic bacteria makes them a useful tool for checking processes, as tests for their
survival can be made by the incubation of relatively few cans and the results re-
lated to other more heat-sensi t ive organisms.
As the sterilizing process only gives a proportional reduction of the bacteria
present , i t is important that thele should be as low an ini t ial contaminat ion as
possible. This is achieved by examining raw materials o n del ivery to check tha t
they Conform to a strict specification of bacterial load. The storage conditio ns o f
each raw material are also specified to keep the material dry, or frozen, so that
counts do not increase. As prepa ratio n begins howe ver the different mLxes are
often heated, water is added, and conditions favourable to l~acterial growth are
established. Normally the product is fil led at such speed that bacteria cannot
mult iply (under favourable cond it ions the fastest bacterial division takes
twenty minutes), bu t l imits on holding t ime are imposed in the event of a hold-up
or breakdown.
Aft er a ~ti sfa cto ry sterilization process a can is sterile, conta ining no bacteria,
no salmonellae, staphylococci, yeasts, fungi, toxins o r spoilage organisms. There-
fore, any spoilage or food poiso ning Could only come about by entry of organ-
isms through the seams
after
the sterilizing processi Assu ming no tinplate faults
or othe r gross can faults which should be picked but by au toma tic testers and
stat ist ical sampling, this is most l ikely to happen during the water cooling
that is employed at the en d o f the heat ing process.
CAN COOLING
Cans m ust be cooled quickly and sat isfactorily for a num ber of reaso ns. First ly,
air cooling takes too long, and would requir e tooimuch space. Secondly, during
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BACTE RIOLO GICA L CONS IDERA TIONS IN CANN ING FOOD 83
a s low cool the temp era ture of the conten ts of the can would pass gradual ly
throug h the growth te mpe ra ture range of the therm ophi l ic bac ter ia , some of
whose spo res wil l have survived the s ter i l izing process , so tha t they could grow,
causing spoilage. Thirdly, i t is necessary to cool cans quickly to prevent the
conten ts cont inuing to cook, g iv ing darkening and burn t f lavours . Four th ly ,
controlled, quick cooling is necessary for later s tages of the production l ines to
work efficiently.
A typica l mo dern op en top can has an in ter locked so ldered s ide seam. At
e i ther end of th is s ide seam there i s on ly a s ing le th ickness of t inp la te as i t mus t
f i t in to the top and bot t om c losures. The end seam is no t so ldered , bu t i s in ter-
locked by a ro ll ing opera t ion an d conta ins a sea l ing com poun d. Unless there is a
fault in the soldering, the s ide seam is robust and impervious, with the s ingle
th ickness lap as a weak po in t . W ith the ro l led-on ends however , fau l ts a re more
comm on. At the speeds of mode rn l ines i t is imposs ib le fu lly to contro l a l l
the d imens ions of the double seam, only the overa l l p ropor t ions . However ,
whi le the seams are wi th in cer ta in l imi ts the sea l ing com pou nd wi ll take up any
slight discrepancies .
Con e nd ~
~ C a n b o d y
eoling compound
F~G, 1. C ro ss section of a double seam
Can~ are sea led e i ther ho t or under vacuum in order to s top oxidat ive reac-
t ions and because any a i r p resent ins ide a can expands and causes cons iderable
pressure dur ing s teri liz ing . E ve n so , wi th s te ri l iza tion temp era tures of 25 0 F
(15 lb sq. in. of s team) th e internal pressure o f a car t is over 20 Ib sq. in . (Me tal
Box T.C. N o. 1) ye t when cooled i t wi ll hav e a vacu um of perhaps 12 in . Hg.
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184 PU BL IC HE AL TH VOL LXX XII NO 4
A t the end of the ster i l izat ion, therefore, cons iderable changes o f pressure occu r
inside the can. T he me tal wil l be ' con tractin g an d the external pressure wil l
drop, unless air pressu re is applied du ring cooling to replace steam press ure as is
done on many modern s ter i l iz ing systems. These s t ra ins on the seam a t a t ime
when the sea ling comp ound is sof t can resu l t in mom ent ary leakage of the seam.
To d emon st ra te such leaks , cans conta in ing a su i tab le bac ter ia l cu l tu re medium
are retor ted and cooled in water with a very heavy bacter ial load (several mil-
l ion organisms per ml) whi le be ing subjec ted to such mech anica l abuse as m ight
reasonably occur . U nd er these c i rcumstances 7 to 12 per cen t o f cans wi ll b low
(Bashford He rber t 1965). This ha pp en s wi th cans sho ~in g no seam defec ts .
The ra te of spo i lage shows tha t the ho les are exceedingl~ smal l and open only
momen ta r i ly . Wha t happens in an expe r imen t may a l so happen in the canne ry
and th is must be prevented . I t has been shown convincingly tha t sound cans may
leak a t th is per iod of severe s t ra in : The ty pe of o rganism found in spoi l t cans
a lone shows th is . There are two approaches to prevent ing bacter ia be ing taken
in to a ca n f rom cool ing water . On e is to use c lean wa ter and the o ther i s to
=emove the w ater f rom the ou ts ide o f the s te r i lized can as qu ick ly as possible .
Both methods are used together .
COOLING WATER
Cool ing water wi th a count o f less than 100 organisms per ml ( the same as
domest ic d r ink ing water ) i s accepted . To ach ieve cooling water o f th is qual i ty
i t ma y be taken d i rec t f rom th e loca l water b oard , o r be t rea ted a t the cannery .
Cool ing water i s genera l ly re~ ' rcu la ted for cost reasons , bu t e i ther way i t i s
treated with chlorine. Th is is cheap, eff icient and has no harmfuleffects. Chlorina -
t ion i s car r ied ou t au tomat ica l ly by machine , wi th regular checks (a t po in t o f
use) to see tha t i t is functioning correctly . Because recircul ated cooling water is
used - - w h ic h can conta in debr is, c onten ts o f sp li t o r sp i lt cans , g rease e tc.
the f ree or avai lab le ch lorine i s measured . I t i s im por t an t to a l low con tac t t ime
(20 to 40 min utes) af ter chlo rinat ion to give time for the c hlorine to kil l the
bacter ia before the water i s used for cool ing . Suf f ic ien t contac t t ime can be
ar ranged by baf f les in the water tank .
As a rou t ine precaut ion bacter ia l counts a re car r ied ou t on samples of cooling
water to ensure tha t the pur i ty i s main ta ined .
Prope r ch lor ina t ion reduces bac ter ia l contanf ina t ion of cool ing water to a
neglig ib le level. The-M ilne re por t (1 96 4) of the :Aberdeen typhoid ou tbreak ,
whi le lack ing f ina l p roof , concludes t ha t the in fec t ion came f rom a can tha t had
been cooled wi th in fec ted , unchlo r ina ted r iver water an d i t i s poss ib le tha t ha d
proper ly t rea ted water been used the ou tbreak would no t have occur red .
Af ter cans are Cooled to a sa t i s fac tory te mpera ture they pass th rou gh labell ing
and p acking mac hinery . This wi l l una void ably in fl ic t some impacts on the can ,
the two end seams being the most vu lnerab le spots . Whi ls t the exact mec hanism
of en t ry o f bac ter ia in to cans i s n o t loaown, it: i s obvious tha t a f i lm o f water
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BACTERIOL OGICAL CONSIDERA TIONS N CANNIN G FOOD 85
plays an important part . Some bacteria wil l not survive for any period if they
dry up while man y are moti le in liquids. I t is therefore very impo rtant that run-
ways and mach ines are as hygienic as possible. These aims are achieved by atten-
tion to the drying of cans, regular, frequent and specified cleaning down of all
parts of the plant, and careful design of equipment.
~AN DRYING
To stop water being transferred to the runways and machines by the cans, the
cans are dried by various means, depending on the sterilizing system in use.
Cans are cooled to about 100F so that they are above ambient temperature.
Surfactant dips are employed to aid running off and evap orat ion of water and
also have a mild disinfectant action. On continuous cookers air blasts are used
effectively to dry cans.
The importance of drying cans as quickly as possible is shown by cases of
leaker spoilage. In 1957 outbreaks of staphylococcal food poisoning were
experienced which were eventually traced to a carrier who was handling wet
cans (Bashford
et l
1960). This prob lem of undetected carriers am ongst staff is
one that faces al l canners and can only be combated by emphasizing personal
hygiene, and ensuring th at staff imm ediately repo rt a ny cuts or sk in infections.
PLANT CLEANING
Hygienic condit ions are obviously essential to any food production operat ion,
but in a cannery it is especially im por tant to main tain a low bacterial popu lation
on any area that might come into contact with processed cans, especially ones
that are stil l wet.
It has been fo und that in preventing bacterial bui ld-up frequency o f cleaning
is the chief factor, an d therefore w ashing down of equipment takes place at
me albreak s in addit ion to a tho rough clean during the cleaning shift between day
and night production.
Different equipm ent requires different me thods of cleaning, and to ensure
tha t the correct cleaning is carried out, pro ced ure s are specified fo r every piece
of equipment. In som e cases washing and scrubbing by ha nd is necessary, often
preceded by part ial dismantl ing of equipment, while in other cases equipment
is cleaned in place, e .g. by pum ping detergent solut ions throug h pipework, or by
built-in spray systems, and this can be fully automated.
All food machinery should be designed so that i t can be cleaned easi ly, but
also it should be designed to stay clean while in use. It should not splash food,
oi l or water, should have as few places as pos s~ le where these can collect , and any
such traps m ust be readily accessible for cleaning. These considerations app ly to
al l auxil iary equipment--~-pipework, drains, m otors, fans and so on . Furthe r,
al l can hand ling equipm ent m ust give sm ooth passage to the cans , and inflict a
min im um of kn oc ks and bum ps especial ly when the cans are wet .(Doyle 1965).
Other im porta nt hygiene precautions are the cleaning down of walls and over-
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186 PU BL IC HE AL TH VOL. LX XX II NO. 4
head f i tt ings the removal of rubbish dus t d i r t and o i l f rom machines and genera l
good housekeeping and cleanliness . I t is also necessary to exercise control over
birds rode nts and insect pests .
Res ea rch and qua l i ty con t ro l by the canne r s and can maker s have today
advanced canned foods to the s tage where they are as safe as or safer than any
other type .
S U M M R Y
The canning process based on that followed by H. J. Heinz Co. Ltd has been
described throug h foo d preparation filling sterilizing cool ing and drying and
mechanized handling with attention to the risk of bacteriological contamin ation
at each stage. A generalized picture of the steps that are taken by the modern
canner to prevent such contamination is also given.
R E F E R E N C E S
BASHFORD
T. E.,
GILLESPY
T. G.
TOMLINSON
A. J. H. (1960). Staphylococcal food
poisoning associated with proces sed peas. Chipping Campden: The Fruit and Vegetable
Canning and Quick Freezing Research Association.
BASHFORD
T . E .
HERBERT
D, A. (1965). Safeguards against a microbial infection of
canned foods (b). Post-process leakage and its control. R.S.H. Conference on the Safety
of Canned Foods,
DOYLE, E. S. (1965). Jr. Milk Fd. TechnoL, 28, 10, 306--309.
ESTY, J. R. MEYER, K. F. (1922). J. infect. Dis. 31, 650.
GORDON
R. A . ~ U R R E L , W. G . (1967). Fd. Preserv, Q. 27, 1,
REPORT (1964), C~N D. 2542 . The Aberdeen typhoid outbreak 1964. Report of the Inter-
departm ental Com mittee o f Enqu~u'y. Edin burg h: H . M. Stationery Office.
TECHNICAL COMMUNICATION NO. 1. (1941). The pressure cooking and cooling of ~ ans:
Practical Operations. Th e Metal Box Co. Ltd, Research Department.
