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MICROORG~~ISMS - THEIR RELATION TO

... REFRIGERATION PLANT .OPERATIONS

I. J. Eustace

CSIRO Meat Research Laboratory

. Meat pre$ervation _is princ;ipally. concerned with the' "inhibition o:f microbial spoilage. The value of low-temperature storage has long been recognised and appreciation of this fact led to the storage of meat in caves, .tl;len in cool cellars, in ice-cooled cellars~ and finally in mech8,llically-refrigerated rOOJru:l. ·

. .

Th~:~_bbjectives of microbiological control in meat are two-fold:

1) · To restrict or retard microbial growth so that microorganisms will not cause ch~mical changes which,result·in reJection of the meat by the consumer, .:because qf:-,putrefacti.6n; sour odours. or slime • . . : . . ~ : .. ·, . . . ~·.

.··: :·· ::.

2) To re:d\lce as much as p~~~dble ·the incidence of contamination of meat with food poisoriing bacteria. ·

The pathogenic or :food poisoning :~J;q\lP .Q~_.bact~ria. requires particular mention. Organisms in this group either grow and produce toxins in foods befq_re they are eat_~~ ($.tqphy'tacofJcus~ CZostridiW17} ·or i:!iultiply in the int~stiP.al tract ~rid,c~~~.e~.;~~sease by infection (Salmonella;, Shigella).

. . . . . =·~;. . ,~ ~- . . . .. . :, r . : .· The Australian:·meat i,IJ.Cl,ustry is very conscious of the need to produce

meat which :t,s<a.st•:f'ree· as ·Pif!.cticable of :food poisoni-ng bacteria. Tha obrlous.:)ffr.is:t···atea·· for prEiV~ntive ac-tion is to avoid· contamination,

. . .. . .. _., -- _,.. '·1·.:• ..

part.±.c~ ~~uri rig dressfli~. of the e-a.rcass ~ However, it is important to recognis·e that\ 'tfl.e situation can be aggravated by bacterial growth (i.e. incire~se in nl.mibers) ~f contaminated carcasses are not cooled rapidly •

-·. ~:. ;i : J... .

The types of microorganisms of concern to the ttl.eat ind~try are moulds, ye~~:~s and' t?act~:da. . . .. .t ; • ' ' •.

:· ., .}::-:··: ·:::·:

' · .. , ·· 'Moulds · . .

.: ; Y: MoUlds ~e multicellular, fi.lamentous living organisnis <~hose growth ·on food's is ustiB.,Uy readily recognisable by the whisker;_ like or "fuzzy" appearapee of til~ growth mass •

. · .. :: .. . ·Moulds norihally rep:roQ.upe py spore :formation~· However, they can also

grow by transf~~ ~:f ~·Part of the growth mass •. · • · . . . . . .. . .·.

!lthough the majority of ~oulds grow best at temperatures in the range 20-30 C, many wi~l S:'f()W l}ea.r 0 C. . , .

'Whisker~' a~d ''bi~~k.spot' on meat are due to the growth et moulds. · The temperature B;t whicti.·.meat is held in a freezer is very signi:fftant in relation to moUld growt}l ~· The black-spot ·mould ( CZa.dosponum) can grow at -8°c and the temperatUre should be lower than this i:f meat is to be kept"for a long period.

1 - 2

Yeasts . ' . ~· .

, ~·.·I •

··-······ ..

These are single-celled organisms, rath~r larger than bacteria. Although yeasts are'widely. distributed i'ri-nature, they are not commonly a problem in the meat industry.

,.·.

Bacteria

This group is of most concern. Bacteria are verJ small (0.2-lO.O~m), Single-cell~a organisms. · Thef:· are widely ··distributed in nature, and there are many: $pedes·.· · · '· · . ,. · ·

. ; '. ;: ··~.:. ,· ! .·.: ... :. -~!' · .

. . · Although:·mB.ny ·people· believe· 6thervise ~ most speCies. are harmless and many are, in fact, very benefiCial'·~ · · However;· as a general rul.e, the presence of any bacteria mey be considered detrimental to the quality of meat.··· One· excepti6n. to this ·'is the use. of a. bacte'rio.l· species {Pedioaoaous) to develop the ripened flavour of some continental sausages.

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" Two. relevant :cfonseqtience's of'·th.e very: small size 'of bacteria are · .(T) they· .. can be·readinY .rttans:ferreci from ·h'Ulna.zl hazid.s, .rodents, insects, birds, dust particles, air currents and water sprays, and { 2) their presence is not· obvioUS ·rihtil ~uge numbert;. ar~,-~pre~ent .• ,·.

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Reproduction of bacteria · · ·' > .: '•

. ,-;· ·':.:~\·

· .. · · · The' reprcid:tict'ion of· fbacteH~~. is· by b:i.na.rY fission; that is, one cell· di Vid~s·· to: :give two/ these .:l.wo divide to give four, . an.d. so on. The -time between consecutive divisions is called the generation time. Given favourable conditions, the generat.ion time for .some speGie;; of bacteria is less than' ·20 ininutes. The resUitF is a very :rapid increaf!e. in numbers -a rapid growth' rate/· Lower tempe~atures siov down this rate of division,

. btit even at 2° c some bacteria ( e ~g. 'j:P~'eu4omomz?). can. divide at least once ·every 10 · ho'Urs. There will be more; uthb.n a hundred-fold incr.ease in numbers

·' in .three. deys·; . in eight to nine d.ti~s: one ceil ~auld' increase 'to one million.

The growth patterno'f bacteria.reachiti~.a n~y?meat surface can be divided into four phases (Fig. 1). The lag .phase is a period of adjustment for bacteria in their nev environment~ when any cell damage is repaired and the bacteria adapt to utilise the nutrients now available. During this phase there is no increase in the actual number of bacteria.present. The bacterial cells then begin to divide by bine.ry fissioi>.· This phase is t·he log·~phase or growth ·phase~ At any particule.~ temperature .during the

· log ·phase~· 'the cells ·.continue to ;double· i~ r.t~ber at ,a .defin~ te rate, which also depends on the particular organism and the substrates available 0

Eventually, the nutrient supply becomes limiting ~d .end--:products of '? bacterial· metabolism: 'accuinUlat.~·; The .. growth 'rate. slows and. a .. stage is reached: when there is no riet increase· in the ·number of bacteria. · · This phase is <

called the stationary phase. . ... ~ ·., ····:,· . ,.( ', .. • : ..

However, :for an excellent· substrat~ like meat, 'the number of bacteria present by this stage is very la.rge, a.ro.und .100 milli.on :to 1,000 million

. 2.n.l ~ .... · .. >r: ·\:- ce"lls--per. gram of meat.·_, In the final phase, the death :i>l)ase, there is a net :;· ''.'; ··. ! rdecrease ·in bacterial numbers.' The fin.a;l two phases qf _bact~rial growth ·

::--·:·· :··,:on me-at are associated with meat ¥hich· is by. now loJ:?.s. sppiled.

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'~GROWTH CYCLE OF BACTERIA

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RELATIONSHIP BETWEEN INITIAL CONTAMINATION & RAPIDITY OF SPOILAGE OF t1EAT HELD AT. 5°C fiGe 2 ..

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1 - 5

Factors affecting the numbers of bacteria .. ' '· . : ~ ... ·' . ' .

Ini.tiaZ nwnbers .· ·. ·· ' ~ .

The extent of the initial contamination influences the storage life of the meat. Obviously~ the greater the number of bacteria on the carcass

'Lwhen it enters the chiller,:the lower the· amount- o:f growth that c~ be --.-.. tolerated. Figure 2· .indicates tha~ at a st.orra~ytemperature of 5 C,

··· counts in excess or·· 100 ,:000 per em limit •· the' storage life! (as determined by appearance of visible sliming, for instance) to less than one week. 2 On the other hand•;, · if; the initial count, has; been limited to 100 per em , the storage life will be more than two weeks-.·.

The graph shows another interesting fact in that if the initial number of contaminating bacteria is low, the lag phase will be rather longer than if the initial nuinber is high~·Onee the bacterial population

.,,, . enters :the log phase, however, growth rates are not dependent on initial · contamination. · -

Temperature

Temperature has, without doubt, the greatest effect on the growth rate of. bac:t-eri.a ·on meat~ - '·The lower the • temperature, the· slower the growth ra~es, 'Ley; growth rates are slowed by cooling. In frozen meat held at ... 5 or lower,. bacterial·growth is effectfvely stopped. ·rn addition, the durat-ion of.thelag phase increases at lower-temperatures. Given a par:ticular number.. of bacteria on meat, the temperature of storage Will greatly influence the time to spoilage. Figure 3. shows the times for one Pseudomonas cell to multiply and reach spoilage levels on a moist

·meat ·surface at different temperatures:. Low temperatures favour longer times to spoilage •

. . . ·Different species of bacteria vary in their response to temperature and they may be broadly classified according to their optimum, minimum and maximum temperatures for growth•

TABLE 1:

Group

Thermophiles Mesophiles Psychrophiles Psychrotrophs

Minimum temperature for growth

37-40 8-15

(-5) - (+5) ( -5) - ( +5)

. . . . .·. o··. T~perat~e ( c)

Optimum temperature for growth

45-75 20-45 10-15 10-37

: : ·.1' :·:. ·.. ·. : '.::,

Maximum temperature

·. · .• fi!?.'!:'. growth

60-80 40-50 ~- :20 20-50

. ·.-.r,_,··

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1 - 6

Important point~ to note are: '·'·' ' : ·~: . ~· , ... -...

Growth of mesophiles (coliforms and the pathogenic bacteria such as CLostridium, Satmone L la and Staphy Zoaoaaits 'ate irtcltid~d here) is rapid at th~ body t5mperature of a freshly dr_-~ssed carcass. Growth does not occur below 8 C. · ·

. ; 't :"; .·:· ) :. . · ... i ~; .! :_'

: 2f' . ~~f6~~-:B°C.- 'i.e. in the temperature range applicaole'io·'chilled meat -... :the. psyc:hrophiies and the · psyclirotrdphs (e.g. ·Pseu:d.i:J1h0nas·) ., are the

. importa.rit groups· of bacteria~. These will eventually spoil the meat. . . . . . ... ., .

The optimum growth rates for. the psychrotrophic bacteria occur at a temperature considerably higher than the minimum temperature. figure 4 shows the effect of temperature on the growth of spoilage 'bacteria ori moist meat. · · · '

··' ::,!.)

.. ''. · i'b~s ~·,if bacterial growth is tO b~ avoided, it is important to cool the; c~rcai3s quickly and maintain it at a low temperatur~~ ··The temperature at boning out, packaging, and storage of the cartoned meat mUst be maintained as low as possible.

Moistt.t.r,~ ... ·- .

Like other· ii ving organisms, bacteria need w'ater before they can grow, and, after temperature, availability of moisture is probably the most importent ·requirement for microbial growth on meat. The water must be available and not bound by salt, stigar or· ice. It is the amount of water available rather than the total moistUre content of a food that is the limi tihg ·factor. . .

··A term that is now widely: used' in the context of water relationships of meat and meat spoilage organisms is "water activity", a • This term is a measure of the available water. The a of a solutionwis the ratio of

.. its v'apour pressure 'to -that of pure· water a~ ··the same' temperature; it is inv~:i-sely proport:lonal.to the number of solute molecUles present. The a is equal to percentage relative humidity divided by'J.OO. :Afi:the a decr~ases, more and more species of bacteria are unable to grow. The use of Itrong brines in corned meat, for instance, will effectively reduce the a below the level where problem bacteria can grow. · "' w

···· ·· The importance of surface. drying in ... r.es.tricting. mic.robial growth on meat is universally recognised. From a purely bacteriological point of view it is desirable to get the surfaces of carcasses dry as soon as possible.

Nutrients

f-ieat is an excellent source of nutrients for bacteria.

·-·· ... Microbial g.rowth is strong~ a.f'f.ected.by.the pH of .the .m~O.ium but again there are wide differences between the pH requirements of the various species of bacteria.

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EFFECT OF TEMPERATURE ON·THE TIME FOR BACTERIAL . SPOILAGE OF MOIST f·1EAT (EXPERIMENTALLY HELD AT

99c3% RID

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EFFECT OF TEMPERATURE ON THE GROWTH OF SPOILAGE BACTERIA (EnGo PSUEDOMQNAS) ON MEAT

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Most bacteria can grow within the pH range 4-9 but the maxium growth rate usually occurs around pH_7, i.e. at neutrality. Muscles in the living animal have a pH in the range 6.8-7.2.

-0 r · ?~ • • ~ d · ' b t i th ., -' J;: -·: ~-. 'L> ·-:s'flle;re··lJ~:...an appr~c~~~l~ z:e,, ¥~t!:J.OP,;ln · gro~~-;f,E!.~e., of ac er a as e

. '.[r) ;~~J"'!V :r_:1·rpH ·:f~ll:.tri\1eJ;g-w~ 7~;;particU,lar;J.y:.::l:le~8W d?,lj,.6 •.. Th~ ;:i e~#f}qt .()f P~t, sn the growth · <.i:·u~·!:'.~'vr.: .. ~·o'f·Mi<c:mobacrt;t;;r?:<wn"io~ p~ychrotrqpb~~ ;.'paqtez::j.l.Ull .. cgm,mQnJ;.:t.:fou.n<l'~?m meat, is

shown in Fig. 5. · ......... -........ ·'- ··-~·.- _., '0 ·-""'"'r:'-~J .• '<··~·!!·: . :_!···~:·~:.--c~· ... ~ ":>·~~~ .. ~,:·:J ·.:•.~--:t·:t l.:~ .;.-~ -.:< \:.:::;:-:

~:~"Jl· .. ··C.:> ·::!.--~ ·.· .-:·!::l.:-yr{ .CY .. C".{ -~Jn~-~.;:~ ·· , .. , ~+t:j:.._.:': ::·'Lt ~: _,···.·· ·····; · :::~J·~:.··:rr'.:· --.. -i:.

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· l3ac-t.~~1;!.al species. v,ary! ~ .. t:l7-eif., -~~quir_emen;y;~·;,~a:{p:r pre;f_t=rences for the g~seous compos! tion of t-~---~~r.otm~I:l!S F1Jytp.osp.~:~r-~;t;·, .: -~flc:teria have been classlfied into three, and sometimes four groups by thelr response to oxygen_; _, · .. : , r,, .i:-:L:• t, , . . . _ " , \':.:: .. :!i :: ;;: .1: :!"J:

. ," . . -~ . ' ·: ·. · . .:- ~C{J • rr . ·::j ~·=:.: :_ .; - .: : · ,· •-.~ :~ :1 · · those· ei t~er. preferr:i.ng t.o grow· pi\}le~~ing to grow in an a~mosph~re .. cont~in~p~! .~?~'J~en _(Jf~a·.~ ''

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anaerobic: those requiring an atmosphere devoi<r·or· oxygen for :<, ··:..r.: !•;."·; -:,::: ;:,:. -':!· t~;~:;•-:-S:roj$.4_~J' .Thes,e, a,;r.,e.;~,:~.Pc:/:!'!-ft't!·:~~APi.}:ed by 02 •

;:··"f"rrr .. ,~~~ .:_·;·: ... : ""'r_, .'-.::r·-:r,, c'""": ..... i · r c ~::. - L · -· ·. · .. l-1_....... ~, .. l ·~·. ·.~ : ..... ~ 1 ..:) ..tJ-: 1 ::.~-· :..i~u.:J!·~r.:-(ti·t·::,~i ,"lL.: :r~ -,t ::-.t~'I ... ;#'"i-:-:'1 ~

'.·-·~, <~.i~ :t!.oc•r· ,:faeUlta;t;ive--.- .. -_1 ·:·~:;· thpp~J:Pr;~fe~~R~- an_.L~t~S>~.P~~5~tf+~V'9~;d: ;:Qf: oxygen but ;;_! 2·:·:.· !.:· i ,; · aina~bes: · .. :-::c·j:~~;ffiP:::::s.t'~W.:f,i a;lt~,-1:1., l;'~?.~~d~ r¥.;e ,,,_f~r:~Jt~,..;presence of

·-.r.:.:· ;"-! ;J:·~· :::: t!'j~~f·.·:' ·.::")J ... :.:~i·~p.r·:4 ~--;-~: f-:· .. ~·~-~:::2~8-~P.~· ::>j · J£.::n;:. :::.11::: ···:);)[:~)·: :.-.7 .:tn~::.c·:).t'"7:,.~;_.;;;!.: · ..... ·. ~ ;·~· -~· f1l': ;:··:·:::~ r~~ . ..t;"f:~, ;.·~(.~r::-J·.~.i~=.:""".• .:,JZ"'.f..'2•;,}:·.:·; ~);f b:Jf'!!!·):Jr.~j~ ·· ~; ;_ '7.·i. .J!~)2i"J ::~ ~ :·~.[:-y\:-·: •

. . jr·£: J :::~': micl;[®..eii"'P.~~le~1~:oJ-v~~e1 cJ>:r';~;fn;rr~M;- .. a re.duc¥ <::Ri'Sf~f#l}, r-~~r;)9~~~ratlon but not ·:r · )~:::; · .:_,,: •:, ~,. '.-" · d;; , 1ty;~:es,~_g.p.~; ~'foP< ~sence tf?:,¥r_E;!pf .. ;.,92 .. ~~' ~he partial

. .J :.' . . :f'.·r J::::: presswtet_i:Q.f 2~;E!r1J.-ormal a~~P)~;PQ~.r~. ~~~p~ts them • . · :::>!:.f.::···"t:~:-~:..~!1- ... j~J -';L: .. !J ·. _; ·· :.:J:L.· ·. :.~ :-~···.~ .[J):j:·::.:-: ;-. .

• ·,. ::J , ~,{c:-c,·r.t · ::rMany of ~~':s.pecies c.o~<pnly,,:i~ssociated, ·p..th the _spoilage of fresh .. ;,,J: v:;~':''- me'at; :includi~.rP~e:uc«Jmonas, ~.~;a~robes •. The1 .-t:Zof!,t.1;';{4~UJ:r! .species associate·'

with bone taint is anaerobic and grows deep wi~!l.i~ %'~~m~~t.1~issue.

c'J~f. Li; ·'.":. :· -~al!Q.Q:J1j:c;1i~.?x.;ifte .(C02.) Gonc~~tr~tions higher ~,WID,1~~t of normal .rc.r.~•::.·r:;,;!_:atmosph~l',ti ti.!ll:l~'J);I,.t. many:""])acterl,~~: -,Foncentratiqgs ,:13;;;}-1l~~~7~f?,' 5%· are known

T:'.: ,.,,_, :l"J to inhi-bi t,,,:f!-ome. :;S.P~Cies, includingut'seudomonas.,.-_,an4 t~~~1;irtffiibi tory effect 'J.·:·• ·.· _.;:-~becom~s greater w;l;~h.qo2 concentrat~ons up .to 50% •.. $qni~~p~¢.teria, however,

are resistant to the presence of co2 , e.g. Lactobaci~?:_Y.~vtW~ Mia!'obacte'l'iwn.

If stored in air the storage life of chilled meat, even under ideal circumstances of low initial bacterial contamination, rapid chilling, low holding temperature (near 0°C) and low humidity, will only be approximately three weeks. The normal life under commercial conditions is about 12 days.

The carcass chiller

All freshly-dressed carcasses are contaminated with bacteria. This initial bacterial load has come from hides, viscera, equipment, air, knives, and slaughtermen's hands. Even with reasonable care, the total bacterial2counts on the carcasses 2ntering the chiller may be as high as 10,000/cm for beef and 100,000/cm for mutton and lamb. The actual count will depend on the site/part of the carcass, slaughtering techl1iques, and standard of hygiene observed on the slaughter floor.

; . ' ~ .

. . . . . r . ~ 1. - 1.0

,, .... ·.-~:Beef sides enter the-chiller with surfe.ce:teinperattires of 25-30°C, ·::·.- '!.·· ,, · and,:deep butt temperatures ·or about 38°c. The' deep butt-t"em!>era.tures of

. sheep .. ca.reasses would be ·fiimilar to 'those for"beef~ '"ThEtstir.race temperature

·.: : .. :-:

mey be a little lower, 20-25°C. ·. .._i·· · · ·.T _..

The carcass surfaces are moist, and the meat will. have a_ pH above 6.5. In short, ideal conditions exist for the gr.dWtli''b:t.~ma.ny .. ba.cteria., including Salmone'LZa and other pathogens, and Clostridium, the organism commonly associated with bone taint·~~- Growth rates will be near· maximum unless .~onditions are made less fa?oureble.

It is likely that the majority of the bacterial cells will initially be in the lag phase of growth. Given the conditions cited above, this phase will last no more than 2 · to 3 hours , then the cells will enter the ·log phase. Obv.iously we must aim to prolong the lag phase by reducing the temperature and by reducing the water activity.

. . . :. : ~

'l'o prevent.-.putrefaetion, carcasses should be chilled0as rapidly as

possible. The deep butt temperature should fall. below 1.5 C after 20 hours. · .... Chilling .should continue.until··the centre temperature of"·;the meat is 8°C.

Frequently it is found that: the refrigeration capacitf ot''chillers is insufficient to reduce the meat to the desired temperature when a 24 hour cycle is used. If it is intended to operate carcass chillers on a 24 hr cycle,· adequate refrigeration must be provided and' chillers must not be

·overloaded. Alternatively,- a 48 hour cycle should be used. No further growth of pathogenic organisms will occur once the temperature of 8°C is reached but of course total bacterial counts will continue to increase, though at a much reduced rate, due to the growth· of>p~ychrotrophs , e.g. P_eeudomonae •' .. The. growth of these organisms·~'. howev~r/ vill 'be. very low at or near 0°c·. ;··- . . . ·. : " ,, ; ;</ '. . ·•··· .. ~:.

. vlhen the·· hot. and· moist carcass enters the~ chill~r;~ :Yater wi~l. be removed .trom the surface of the carcass by evi:i.po~t!rtion:: 'R·etrigeration equipment in the .chiller must be capable of remov]j).g silffic:i.ent water

· vapour from the air. to give dry surfaces , ancf thereby' ·reduce the water activity... · ' '' ··' ·

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1- 11

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(J4/SUOU.D..JaUa5) alDJ 4JMOJ9 EFFECT OF PH ON THE GR0\1/TH RATE OF Mio:robaataJ.•ium. AT 25°C.

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F:igure·6 illustrates the pattern of change in water'activity of the surface'-Of' the lean tissue and the subcutaneous adipose tissue of a beef carcass' dUring chilling~ ' 'The water activity of the. surface tissue falls rapidly with the coJ:ilil'l:encement of chilling;. However, tlie rate of water loss soon falls.:below the:~re.te of :rt.igration of 'water to the 'sliz'fadE:i.Jfrom underlying tissue and within an hour the water activity begins to increase again. Within 4 to 5 hours the water activity of a lean meat surface has increased to near the level where growth of bacteria is possible (a :::! 0. 95). Therefore, even under good chilling conditions, there is only a small margin for'errdr. Irieffic:ient chiller operation, e;g. poor air distribution,

. inadequate.·- carcass :spacing, and loading; in hot carcasses alongside cold ca.rcasses~ will quickly lead to conditions favourable for bacterial growth.

: ~; .r ·.·· : .. · Practices such as failure to cool· the chiller adequately prior to

commencement of loading, allowing excessive times for loading, overloading chillers, so that carcasses are touching, ·or operating chillers at

-, : -·- ,-excessively high hilinidi ties and temperatures, delay cooling and/ or drying :r.i ____ , i.:of.;the surface of the meat. Bacte:dafwfll: grow and the storage life will btc:· _,:';-~be- reduced. '

,"u·;;r·:::: ·:j· r······ .<i· ·· Chariges in numbers of bacteria during diillirtg --.:·:.;··f '·;_ 1. _\_; rt.:·· o: ;·~ :: :·' . , · . : - :- ::~ .:

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After chilling

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_. Total .. E. coli ,. '' .;;;.;;...;..;;;;=,_ ______ =-';..,;o;;=-

7,900 ··;'' 5o , ado ~ .~-~

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·5o 7,900

Mutton samples - by excision

The data in _Table 2 illustrate the effect -of suspending mutton carcasses 'i:a};i clo.s,eiy.

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Here there has been no increase in either the total or E.aoti counts of the 'z1on-c~rit~citlhg surfaces during overnight chilling. On the contacting surfaces, the E.coU.count· has increased about ·50 times. Because the growth r.13.;tes. of .E~ qq1~i. and salmonellae on meat are very similarlY influenced by tem:Pei.~~W:',e,,· ·cooling r_egimes which allow growth of E. coU would also allow growth -of Salmone~Za •.

;;cfLrt~P.e living animal, muscles have a pH in the ~ange 6.8-J .2. After sla~q~~'!t> the pH falls as .. gJ,Ycolys~s (conversion of glY,(!pgen in muscles to lactic acid) proceeds, until at 24-~8 hours after death the pH reaches 5. 5-5. 8. This is referred to as the ultimate pH. Most p~·imal cuts have an ultimate pH which is in 'the range 5.5-5.8. As mentioned earlier, bacterial growth slows below pH 6.0. The pH of the meat therefore, is

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1 - 14

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:m,C>~_t.:,f.a:':':our,~bf1e ~Of,b:{l.c~eri&). gr.o~h. in .~.~~ ,~~~~ s;t~-~7,!?, 19;f chilling, at wh~.ch ::tame the car.cass .~s .also warm and mout. In ;t;h~ CS;f?.E¥-.· of "dark­cutting" meat .and some muscl'es, e.g. in shanks and n.~c-~s ;: :·:t~e ultimate pH may be: 6.q-:-6.5 .. Bacterial_gro~h w,i},l be. more rapi<,l, '·part:l,cularly

... the gro~h .of -~orne undesirable 1Jacterial. species. It -is' for this reason tb,at .high:· pH, dark,-cutting me.at is rej~Gt.ed for ageing~ ' . . ...... ·. . . .. . . . . .·· . . . . . ' . . .. ,: ... •; .. ·.·

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. Mou+ds are less of a probiem than. bact:eria, ~ut situations where .moulds grow QP. me1:1.t surfaces. do occur, and profuse . 'whi.skers' of mould are very obvious •. In addition, moulds.can.grow on'ceilings and other areas of chiU,~rs .which are not regularly cleaned. One particularly COI!liilOn area. is ~n ( a.Ild. over) refrig~ration units._. · .

The cond;iti6ns under which ~ost mould gro-rlh occurs on meat are a hllinid. atmosphere and a temperature close to freezing - coriqitions when they can. compete:· adequately with bacteria. Temperature fluctuations in chillers can promote the growth of moulds due to moisture migration and condensation.

In a situation where carcasses are held for some days, e.g. over a long weekend, vi~;i.ble mould growth on tl;te qarcasses can sometimes result, particularl:Y-~if· there' are pockets of' moulds in the chiller to act as sources of contamination e.g. walls, ceilings or refrigeration·units, and if high humidity con:cli ti-oris prevail. .

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Recent New Zealand work has shown that polyurethane-based sealants with or without fungicide were unsuitable for use: in chillers because they support mould growth. Joints between wall panels in areas subject to temperature fluctuations should be sealed· with· silicone-based sealants.

In summary the overall objective of: refrigeration is to achieve carcasses of good microbiological quality. Attainment of this objective will be assisted by close attention to several aspects of carcass chiller operation. These· are: ·" '

1) Ensure that carcasses entering the chiller have minimal bacterial

2)

3) .

contamination.

Keep the chiller clean. The chillerand refrigeration units should be designed to facilitate cleaning and sanitation.-

The carcasses must be loaded into the chiller in batches·to avoid leaving chiller doors open for extended periods. . .

4). Carcasses must be chilled as quickly as possible to reduce deep butt temperatures and thus avoid bone taint, and.· to reduce surface temperature~, to avoid growth of food poisoning bacteria •

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.· · Chilling must be contrbl':l,.ed so . that carcaSf? surfaces • become cold and dry; to minimise the gr_owth of SP()ilage and ·pat~ogenic bacteria .

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Avoid practices which lead to condensation.' of moisture on chiller surfaces~· · •··

·1- 15

Achievement of these objectives requires chilling.space and refrigeration capacity- sufficient to handle the ~a.ximum anticipated -throug~put of the slaughter floor.

1)

· Factors which contribute to poor performance are:

Inadequate cleaning of chillers. Dirty .chillers ;J,e~4.-:G-6 a further load of bacteria and :.moulds; ··on carca$ses. · Dirty. refrigeration units act· as good distributors 'of contamination. · · ,

2) Overloading chillers. If meat surfaces are in contact~ they form warm, moist areas, ideal for bacterial growth.

3) Slow chilling. This results in meat being at elevated temperatures for extended times. If a 24 hour cycle is used, carcasses may not be adequately chilled.

4) High holding temperatures. When hard fat problems are experienced in the boning room, there is a temptation to increase chiller holding temperatures. Bacteriological quality may well suffer.

5) Temperature fluctuations in the chiller. These result in condensation on meat and on walls.

Chilling & freezing of cartoned meat

Bacterial growth does not occur on frozen meat held at recommended temperatures (-18°C or lower). Chilled meat which is vacuum-packaged and held at approximately 0°C has a storage life of at least 10-12 weeks. The normal aerobic spoilage bacteria are effectively inhibited because oxygen is depleted and carbon dioxide, which inhibits aerobes, accumulates.

However, the important fact to recognise is that if beef carcasses are boned out within 24 hours of slaughter, which is the usual situation in Australia, the meat temperatures within a carton will be in the range 10-15°C or even higher. All meat surfaces will remain very moist. The time required to reduce the meat temperature at the centre of the carton to 0°C {or lower) is vitally important. Monitoring of temperatures during

0 freezing has shown that the centre of a carton of meat boned out at 10 C may remain at a temperature suitable for growth of bacteria (i.e. above minus 1.5°C) for as long as 35-40 hours. An initial, high meat temperature within the carton will mean that a longer cooling period is necessary before the temperature drops sufficiently to make the product microbiologically stable. The time between boning and the commencement of chilling or freezing can be considerable in some establishments and must be taken into consideration. If cartons are not correctly spaced in the chiller or freezer, access of air will be impeded. The rate of cooling will be slow.

It is likely, in fact, that a combination of high meat temperatures at boning, excessive delays in transferring cartons to the chiller or freezer, and poor stacking, will lead to conditions which allow for significant growth of food-poisoning and spoilage bacteria.

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1 - 16

Carton chillers and freezers, therefore, have to bring temperetures down from t}le, boned-out t;.emp~rature. of the. meat, not from the nominal t~mperature ~O"f, :the ~arcass chiller~.

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In conclusion, control of bacteria and other microorganisms on meat involves, firstly, avoidance of .contamination of dressed carcasses on the slaughter rio or~ control also involves 'ef.ficient chilling of carcasses to .pr~yent growth of and fu,rther. contamination by, bacteria .. Thirdly it invol ve.s effiCient cni:P.,irig or .. i:t~eZing'' O:f :_boned, cartoned product .

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