FAO Corporate Document Repository FISH CANNING PROCESSING Source Click Here 1. CANNING PRINCIPLES 1.1 Introduction to Canning The purpose of thermal processing during manufacture of canned fishery products is the destruction of bacteria by application of moist heat. Only having satisfied the safety requirementsofprotectingconsumerhealth,andthecommercialrequirementsof preventingnon-pathogenicspoilage,doesthecannersetaboutchoosingathermal process schedule that will optimise the sensory quality of the finished product. Of the bacteria contaminating fishery products, some (the pathogenic bacteria) cause food poisoning while others only spoil the food. Of particular concern to fish canners is the possibility of there being contamination by Clostridium botulinum which, if present, can form heat resistant spores capable of withstanding a mild thermal process. As this microorganismcangrowatthepHoffishfleshitisimportantthattheprocessor ensure that all his cans have received a process that is sufficiently severe to kill spores andvegetativeformsofthebacterium.SurvivalofClostridiumbotulinum,afterthe thermal process, is an extreme health risk as low-acid canned foods (pH >4.5) support growth of the organism, and under certain conditions will also favour formation of the neurotoxin responsible for outbreaks of botulism. Sterilization is a heat treatment given foods capable of supporting the growth of heat resistantsporeformingbacteria.Sterilizationprocessesdestroyallpathogenic contaminants and all other micro organisms capable of growing under normal storage conditions; survivors of the process will be extremely heat resistant spores which pose nohealthriskandonlygrowatelevatedtemperatures(=40C).Ratherthanmake cannedfoodsabsolutelysterile,cannersaimfor"commercialsterility"whichmeans that the contents are safe (as all pathogenic microorganisms have been destroyed) and shelf-stableatnormalstoragetemperatures.Werethethermalprocessdesignedto makeallcansabsolutelysterile,therewouldbeunnecessarylossofsensoryand nutritional quality without there being any increase in the safety of the product. The higher the temperature of sterilization the greater is the rate of thermal destruction, which is why canners process their canned fish in steam under pressure rather than in water at atmospheric pressure. The rate of thermal destruction is also affected by the nature of the product (liquids heat faster than solids) and the container size (large cans offishpackedinbrinetakelongertoreachlethaltemperatures,thandosmallcans containing the same product). The total sterilization effect of a thermal process can be expressedasthesumofallthesterilizationeffectsachievedbyallthetime-temperaturecombinationsthroughouttheentirethermalprocess.Byconvention, sterilizing effect is expressed in standard units of minutes at 121.1 C, so that. an entire processing cycle is expressed as being equivalent, to holding the product at 121.l C for a given time. The unit of sterilization is the Fo unit, where an Fo value of one minute isequivalenttoholdingtheproductat121.1Cforoneminuteandthencoolingit instantly:Fish Canning Process1FAO Corporate Document Repository 1.2 Selection of Thermal Processing ConditionsThepurposeofsterilizingcansoffisheryproductsistoridthecontainerandthe contents of all pathogenic micro-organisms and to prevent. spoilage by non-pathogenic contaminantsundernormalstorageconditions.Selectionofprocessingconditions necessary to fulfill these criteria is based upon experimental studies in which the rate of heat penetration to the slowest heating point (SHP) of the container is measured during simulatedretortingcycles.Thedatafromthesetrials(orfromsuitablereference sources)are.usedbyfishcanningtechnologiststodeterminetheprocessing temperatures and times necessary to render the canned product commercially sterile. Manufacturers of canned fish (and all low-acid canned foods) can specify their thermal processesintermsoftargetFovalues,where theFovalueisameasureofthermal processingseverity.HavingselectedanappropriateFovalue(whichmaybefarin excess of that required to reduce to an acceptably low level, the probability of survival of Clostridium botulinum spores -as may be the case when the process is designed to bring about bone softening) the canner then adopts a time and a temperature for the thermal process which will ensure its delivery at the SHP of the container. 1.3 Heat Resistance of Bacterial SporesThe heat resistance of bacterial spores is specified by the time required to kill 90 per centofthepopulationatconstanttemperature;thisenablesacomparisonofheat resistanceofsporesofmanydifferentbacteria.Formostsporesofimportancein cannedfoodspoilagetheirheatresistanceismeasuredat121.1C(250F),a common retorting temperature, and is expressed as the D value. A typical plot of the number of survivors against heating time is shown in Figure 1. It can be seen that the time to reduce the population from 1 000 000 to 100 000 is the same as that required to reduce it from 100 to 10. That is, the D value is constant for specific bacterial spores when they are subjected to heat at constant temperature. In Table 1 are summarised the D values of bacterial spores important in canned foods. Destruction of all spores of Clostridium botulinum is the minimum safety requirement , whenthermallyprocessinglow-acidcannedfoods.Cannersaimtoreducethe probability of one spore surviving the thermal process to such a low level that, for all practical purposes, the contents of the container pose no health risk due to survival of Clostridiumbotulinum(spores).Experiencehasshownthataprocessequivalentin sterilising effect to twelve decimal reductions of the population of Clostridium botulinum issufficienttoprotectconsumersafety.Suchaprocessisreferredtoasa"12D" process and it is equivalent to holding the contents of the container at 121.1 C for 2.8 min (12 D=12 x 0.23 =2.8 min). A process as severe as this will satisfy requirements (underconditionsofgoodmanufacturingpractice);however,itwillbeinsufficientto reduce to a commercially acceptable level, the probability of survival for the extremely heat resistant spores (with D values of 2.0 to 5.0 min) of non-pathogenic bacteria. This iswhycannedfishmanufacturersselectathermalprocesswhichgoesbeyondthe safety requirements of destruction of Clostridium botulinum. Fish Canning Process2FAO Corporate Document Repository Figure 1 Survivor curve for bacterial destruction at constant temperature Although the probability of survival for spores of non-pathogenic heat resistant bacteria may be several thousand times that for Clostridium botulinum spores, their presence is of no great concern to canners for two reasons:i.should they lead to spoilage, there is no associated health risk, andii.theyonlygrowattemperaturesabove40C(i.e.,theyarethermophilic)and their optimum growth temperature is around 55 C, which is above that in most warehouses ) and retail outlets .Table 1 Decimal reduction times (D-values) ofbacteriaimportantinlowacid canned foods. Organism D value (min. at 121.1 C) B. stearothermophilus4.0 - 5.0 C. thermosaccharlyticum3.0 - 4.0 D. nigrificans *2.0 - 3.0 C. botulinum (A & B)0.1 - 0.23 C. sporogenes (P.A. 3679)0.1 - 1.5 B. coagulans0.01 - 0.07 * Formerly C. nigrificans 1.4 Lethality of Heat During Heating and Cooling Although by convention the sterilising effect of a process is expressed in standard units ofminutesat121.1C(thesymbolusedisFo).theproductinsideacandoesnot instantaneouslyreachprocessingtemperatureandinsomecasesofconduction heating,thetemperatureatthethermalcentreofthecanneverreachesthatofthe heating medium (which need not be at 121.1 C) .This paradox is resolved by making Fish Canning Process3FAO Corporate Document Repository use of a relationship which shows that the rate of change in the thermal destruction of bacteria (i.e. the rate of change in their D values) is logarithmic around temperatures commonly used in heat sterilisation. This means that the lethal rate of destruction at any temperature can be related to that at a reference temperature. This relationship is graphically represented . in Figure 2 which shows a thermal death time curve passing through 1 min at 121.1 C. This "phantom" curve shows that relative to the lethal rate of unity at 121.1C the lethal rates at 91.1, 101.1, 111.1, 131.1, 141.1 and 151.1 C are 0.001, 0.01, 0.1, 10, 100 and 1 000, respectively. Thesterilisingeffectofathermalprocess(theprocessFovalue)canthereforebe computed by integrating the combined lethal effect of exposure at all time/temperature combinations throughout the process. This means that a process that delivers an Fo value of 2.8 min (the so called 12D process for Clostridium botulinum) is equivalent in . sterilising effect to heating the contents of the can to 121.1 C instantly, holding it at that temperature for 2.8 min, and then cooling it instantly. Similarly, a process for solid style canned tuna packed in 84 x 46.5 mm cans may have a target Fo value of 10 min, which can be achieved by processing for 74 min at 116 C or 50 min at 121.1 C. With each process, however, the sterilising effect is the same as, and equivalent to, holding the can of tuna at 121.1 C for 10 min under conditions of instantaneous heating and cooling. 1.5 Calculating Fo Values To be sure of commercial sterility the Fo value at the SHP, the thermal centre of the container,mustbesufficienttokillallClostridiumbotulinumandreducesurvival probabilities for other more heat resistant bacteria to an acceptable level. It is assumed that bacterial spores will randomly contaminate the fish and that therefore they may be located at the SHP. Although a pessimistic approach, this caters for the ``worst case`` scenario on which product safety must be based. The measure Fo value heat penetration studies are conducted for representative packs of the canned fish filled to the maximum fill weight likely to be encountered. These cans are then fitted with thermocouple probes which must be located so as to measure the temperature at the SHP. (As can-to-can variation in the rate of heat penetration can be significant, it is recommended that at least twelve replicates are tested before data from theslowestheatingofallthetestcansareusedtocomputetheFovalueforthe process).The thermocouples are connected to digital or graphical recorders, some of which indicate the product temperature during the thermal process, while others can be purchased which automatically compute Fo value. Where automatic computation is not possible, the temperature-time data can be used in a number of ways to calculate Fo value.Fish Canning Process4FAO Corporate Document Repository Figure 2.Thermal death time curve passing through 1 min at 121.1 C. 1.5.1 The improved general method A plot of temperature versus time is made on specially constructed lethal rate paper which has on its left-hand vertical axis product temperature (on a log scale) while on theotherverticalaxisisdrawnlethalrate(onalinearscale).Thusforeach temperaturecanbeshownthecorrespondinglethalrate.Timeisplottedalongthe horizontal axis, using a convenient scale. The area under the graph which represents the product of exposure time at all lethal rates throughout the process, is then divided by the area equivalentto that of an Fo valueof unity. This yields the total sterilising effect,ortheFovalue,fortheprocess.InFigure3isshownahypotheticalheat penetration curve for a semi-solid product processed for 40 min at 120 C.Fish Canning Process5FAO Corporate Document Repository Figure 3. Heat penetration lethal rate curve The temperature profile shown is that of the slowest heating point. By counting squares or using a planimeter the area under the graph is found to be 71 cm, while the area correspondingtooneunitoflethality(Fo=1)is4cm.Thereforethetotalprocess lethality can be calculated, =71/4 =17.5 min This means the total sterilising effect of the process is equivalent to 17.5 minutes at 121.1 C, assuming instantaneous heating and cooling. We have now expressed the severity of sterilisation, as experienced at the slowest heating point of the can.Intheworkedexample,theretortwasnotoperatingatthereferencetemperature (121.1 C) nor did the product reach retort temperature. It is important not to confuse thespecificationfortheprocess(40min/120C)withFofortheprocess.Aprocess Fish Canning Process6FAO Corporate Document Repository specification alone indicates little about the total process lethality. It would be possible to have a process specification of 60 min at 121.1 C and Fo values of, say, 6.2 min and 11.5 min for 450-g and 225-g cans respectively, the different process severity in thiscasereflectingcansize.Similarmodeofheating(convection/conduction),pack weight and fill temperature can all affect the Fo value even though retorting conditions may be constant. TO SUMMARIZEi.The improved general method takes account of the entire heating and cooling effectsincludinganychangesinheatpenetrationratescausedbyproduct gelation or liquefaction.The thermal characteristics of the product need not be known. While this makes Fo calculation simple it limits the versatility of the technique .Theoretically the thermal centre of a conduction heating can is at the geometric centre, while that for a convection heating product is slightly below this on the vertical axis. It is important to locate thermocouple tips at the thermal centre for solidpacksheatingbyconductionbutlesssoforconvectionheating,asthe rapidheattransfercausedbyconvectioncurrentspreventsanysignificant heating lags.1.5.2 The trapezoidal integration method. Amathematicalmethodinwhichthetime-temperaturedataareusedtomeasure changes I in lethality during heating and cooling. By using standard time intervals the lethal value: is computed in stages and the cumulative L value for the process is found without the need for graphical representation of the heating and cooling curves. The Fo value for the process is calculated by summing all the L values and multiplying this value by the standard time interval between readings. Thetrapezoidalmethodalsoallowssimplecalculationofthecontributiontototal process lethality of the heating and cooling portions of the process. In Table 2 are shown L values and in Table 3 is shown a worked example in which temperature was recorded at 5 minute intervals for a process of 60 minutes at 121.1 C. TocalculateFofortheprocess:SummingtheLvaluesgives2.925whichwhen multiplied by 5 (the time interval between readings) gives an Fo value of 14.6 min. To calculate Fo for the heating phase: The sum of L values at time 25 and 60 min (0 and 0.776) is divided by 2 and this value (0.388) is added to the sum of L values from time 30 to 55 min. This gives 1.730 which when multiplied by 5 yields on Fo of 8.6 min for the process lethality at the stage when the steam was turned off. The Improved General Method which relies on a temperature-time plot. for the entire process is the most accurate of all methods for calculating Fo value and for this reason is frequently quoted as the "reference method". Like the Trapezoidal Method there are no assumptions made regarding product heating and cooling characteristics, however the benefits of accuracy have to be balanced against the lack of versatility. Data from Fish Canning Process7FAO Corporate Document Repository one set of trials cannot easily be used to calculate Fo values when product temperature and/or retort temperature are (is) altered. This means that once process conditions are alterednewtemperature-timedatamustbecollectedunderthenewexperimental conditions. Table 2. Values of L for temperature ranging from 90 C to 130.9 C in 0.1 C intervals C0.00.10.20.30.40.50.60.70.80.9 900.0010.0010.0010.0010.0010.0010.0010.0010.0010.001 910.0010.0010.0010.0010.0010.0010.0010.0010.0010.001 920.0010.0010.0010.0010.0010.0010.0010.0010.0010.002 930.0020.0020.0020.0020.0020.0020.0020.0020.0020.002 940.0020.0020.0020.0020.0020.0020.0020.0020.0020.002 950.0020.0030.0030.0030.0030.0030.0030.0030.0030.003 960.0030.0030.0030.0030.0030.0030.0040.0040.0040.004 970.0040.0040.0040.0040.0040.0040.0040.0050.0050.005 980.0050.0050.0050.0050.0050.0050.0060.0060.0060.006 990.0060.0060.0060.0070.0070.0070.0070.0070.0070.008 1000.0080.0080.0080.0080.0090.0090.0090.0090.0090.010 1010.0100.0100.0100.0100.0110.0110.0110.0110.0120.012 1020.0120.0130.0130.0130.0130.0140.0140.0140.0150.015 1030.0150.0160.0160.0170.0170.0170.0180.0180.0190.019 1040.0190.0200.0200.0210.0210.0220.0220.0230.0230.024 1050.0250.0250.0260.0260.0270.0280.0280.0290.0300.030 1060.0310.0320.0320.0330.0340.0350.0350.0360.0370.038 1070.0390.0400.0410.0420.0430.0440.0450.0460.0470.048 1080.0490.0500.0510.0520.0540.0550.0560.0580.0590.060 1090.0620.0630.0650.0660.0680.0690.0710.0720.0740.076 1100.0780.0790.0810.0830.0850.0870.0890.0910.0930.095 1110.0980.1000.1020.1050.1070.1100.1120.1150.1170.120 1120.1230.1260.1290.1320.1350.1380.1410.1450.1480.151 1130.1550.1580.1620.1660.1700.1740.1780.1820.1860.191 1140.1950.2000.2040.2090.2140.2190.2240.2290.2340.240 1150.2450.2510.2570.2630.2690.2750.2820.2880.2950.302 1160.3090.3160.3240.3310.3390.3470.3550.3630.3720.380 1170.3890.3980.4070.4170.4270.4370.4470.4570.4680.479 1180.4900.5010.5130.5250.5370.5500.5620.5750.5890.603 1190.6170.6310.6460.6610.6760.6920.7080.7240.7410.759 1200.7760.7940.8130.8320.8510.8710.8910.9120.9330.955 1210.9771.0001.0231.0471.0721.0961.1221.1481.1751.202 1221.2301.2591.2881.3181.3491.3801.4131.4451.4791.514 1231.5491.5851.6221.6601.6981.7381.7781.8201.8621.905 1241.9501.9952.0422.0892.1382.1882.2392.2912.3442.399 1252.4552.5122.5702.6302.6922.7542.8182.8842.9513.020 Fish Canning Process8FAO Corporate Document Repository C0.00.10.20.30.40.50.60.70.80.9 1263.0903.1623.2363.3113.3883.4673.5483.6313.7153.802 1273.8903.9814.0744.1694.2664.3654.4674.5714.6774.786 1284.8985.0125.1295.2485.3705.4955.6234.7545.8886.026 1296.1666.3106.4576.6076.7616.9187.0797.2447.4137.586 1307.7627.9438.1288.3188.5118.7108.9139.1209.3339.550 Note: z =10 C T =product temperature Table 3. Trapezoidalmethodforintegrationoflethal rate data to calculate Fo value Time(min) Temperature (C) LL/t Fo(min) 0240 524.50 10340 15540 2072.50 25870 30980.005351050.02540110.50.08745114.50.219501170.389551190.617601200.776 1.730 8.6 * STEAM OFF 651200.776701060.031758802.925 14.6 Fish Canning Process9