Prevention of fresh meat colour defects - Red Meat Innovation

Meat technology update00/2 – April 2000Reprinted November 2006Prevention of freshmeat colour defectsConsumers usually assess meat qualityin terms of colour, tenderness, juiciness,flavour, fat cover and marbling. However,at retail level, customers are mostinfluenced by meat colour in theirdecision to buy. Colour is perceived to bea valuable guide to the overall quality ofmeat; if a visual appraisal raises anydoubts it is unlikely that purchase of thatparticular item will be considered further.Although there are conflicting scientificstudies about colour relevant to eatingquality, it is the colour of meat that firstinfluences the potential customer andalso affects the judgement of wholesaleand institutional purchasers. It istherefore necessary to have anunderstanding of the variations in meatcolour and how it is affected by variousconditions and handling practices.The basis of fresh chilledmeatcolourair for approximately 30 minutes. Theoxygenated form of the pigment, calledoxymyoglobin, causes the bright redmeat colour expected by the consumer.The red meat layer occurs to a depth ofonly 3-4 mm at 0°C (Figure 1A).After the meat’s prolonged exposure toair, the pigment turns grey or brown justunder the surface. This brown pigment isknown as metmyoglobin (Figure 1B).Systems in the meat and environmentalfactors control the rate at which thepigments change from one state toanother. Generally speaking, pigmentchanges are caused by factors thatreduce the O 2 tension at the meatsurface or cause dissociation of the O 2from oxymyoglobin. With this in mind,conditions of primary importance to meatcolour are temperature, pH and theamount of available oxygen.In the absence of oxygen (O 2 ), freshmeat is purplish-red. This colour iscaused by the presence of a pigmentcalled myoglobin, which, on exposure toair, absorbs oxygen and becomes brightred. Since this occurs progressively,colour appraisal of the meat should becarried out after it has been exposed toFIGURE 1 A simplifiedrepresentation of the formation ofpigments at the surface of meat

attained; therefore, it is incorrectly assessedas dark cutting. Effective electricalstimulation will overcome this effect byaccelerating the rate of pH decline.Normal colour of vacuumpackagedand 100% CO 2flushed chilled meatThe removal of oxygen during vacuumpacking leads to changes in the colour ofmeat. Inside a vacuum package, residualoxygen is consumed and carbon dioxide(CO 2 ) is produced as a result of metabolismby the muscle tissue. The resultantatmosphere contains less than 0.5 per centoxygen, some 20-40 per cent CO 2 withnitrogen making up the remainder.The bright red (oxymyoglobin) colour of freshmeat disappears in the vacuum pack and inthe CO 2 flushed pack as the pigment revertsto its purplish-red form. This is the normaland desirable colour of vacuum-packedmeat. Within a short time of the pack beingopened the surface purple myoglobinchanges to bright red oxymyoglobin and themeat blooms to a bright red colour. Therelationship between pigments is shown inpunctures, or poor air evacuation at the timeof packing and sealing, the meat will turnbrown during storage.The oxygen permeability of the films used forvacuum packing is also very important and isa matter to which the packer should payclose attention. To prevent oxygen fromgaining access to the meat surface, care isneeded to ensure that the appropriatedegree of impermeability is chosen. If thepermeability is a little too high, or if there arepinholes or poor seals, a small amount ofoxygen will gain access to the meat surfaceand its concentration could very easily reachthe critical (low) level at which metmyoglobinformation is most rapid.It is difficult to evacuate all the air from avacuum pack but residual oxygen should beused up by respiration of the meat andmicrobial activity. The speed at which thisresidual oxygen is used up is dependent onthe age of the meat when packed. If themeat is not fresh (i.e. is older than 48 hourspost-slaughter) at the time of packing,insufficient meat respiration to consume theresidual oxygen can cause a problem.In addition, if there is residual oxygen in thepack due to any of these factors, thebrowning problem will be exacerbated byhigh temperatures.Figure 2.FIGURE 2 Some colour relationshipsfound in fresh and vacuum-packagedmeatsBrowningBrowning of meat stored in a vacuumpackage indicates that there is too muchoxygen in the pack. This occurs fastest atmoderate (1%) level. If there are poor seals,Colour problems with beef stored in highconcentrations of carbon dioxide will notoccur provided oxygen is excluded from thepack. If only 0.5-1 per cent of oxygen ispresent, the rate of formation ofmetmyoglobin is higher than in air, andbrowning occurs more rapidly. In addition todiscolouration of the lean surface, problemswith the appearance of fat surfaces mayoccur (brown-grey discolouration). Withlamb, a brownish discolouration of the fellsurfaces may develop after several weeksstorage at 0°C if there is too much oxygenpresent and, if this occurs, the appearance isinferior to that of fresh primals.From a colour point of view the aim ofvacuum or gas packing is to prevent theproduction of undesirable brownmetmyoglobin by reducing the concentration4

of oxygen in the pack to below 0.2%. Toachieve this, the meat should be packed andsealed as quickly as possible after boning orcutting the carcase, provided that thecarcase deep muscle temperature isadequately reduced.This type of browning should not beconfused with cooked meat browning (greybrown),which can be caused by excessiveheat during shrinkage of the packaging film.Retail storage of chilled meat inair or in air-permeable filmBecause the bright red colour is enhancedby high concentrations of oxygen, mostdisplayed meat is either open to air or sealedin an oxygen-permeable wrap. The wrapallows passage of oxygen but preventsundesirable drying of the meat surface.Meat displayed in this fashion usuallybecomes discoloured owing to thedevelopment of brown colour (metmyoglobin)before bacteria have grown sufficiently tocause spoilage. Incorrect storage or handlingat any stage of the chain will reduce retailstorage life.The principal factor affecting the shelf life ofwrapped meat on display is its temperature.This is mainly because the oxidation ofmyoglobin to metmyoglobin is extremelytemperature dependent. However, it is alsopartly because higher temperatures increasebacterial growth on the meat, and the resultis that bacterial metabolites contribute to theincreased oxidation rate.In order to obtain the longest possible shelflife for meat on display, the storagetemperature should be as close to 0°C aspossible. This will keep microbial growth to aminimum and prolong the retention of theattractive red meat colour. As it is impossibleto maintain a steady 0°C in open meatdisplay cabinets, a realistic optimumtemperature range is 0-5°C.Meat should be cooled to below 5°C beforecutting and it should be cut immediately priorto display.Storage after ageingAfter three weeks ageing, aged meatexposed to air discolours in the retailsituation faster than fresh meat because of adiminished ability of any brown colour thathas formed to revert to the desired redcolour. The longer meat is aged, the faster itwill discolour after opening to air because theenzyme system becomes less effective withageing.Table 1 indicates the relative retail displaylife of consumer cuts of beef (stored at about5°C) in conventionally overwrapped andoxygen-enriched modified-atmosphere packs(as a function of the meat storage time in thevacuum pack at 0°C). The longer the periodof storage in the vacuum pack the shorterthe display life of the consumer portions,because of the more rapid conversion ofoxymyoglobin to the brown metmyoglobin.TABLE 1 The relative retail display lifeof consumer cuts of beef as a functionofthe time the meat was stored vacuumpackagedStorage time in thevacuum pack(weeks at 0°C)Retail display life(days)*Overwraped traysBeef and lamb stored in high levels of CO 2appear to retain their retail display life betterthan vacuum-packed lamb. Retail shelf life ofproduct prepared from lamb primals orcarcases stored in a CO 2 atmosphere shouldbe three days after 12 weeks chilled storage.Summary0 2 4 6 83 3 2 2 1The delay of adverse colour changesinvolves efficient refrigeration during chilling,holding, transportation, storage, preparationand display; and proper hygiene, packagingand selection of meat. Attention to the aboveis necessary, but in particular, if fresh meat isto be maintained in good condition withminimum colour deterioration for anadequate commercial display period, the5

temperature of the meat must be keptclose to 0°C.Further ReadingMeat Technology Update Newsletter99/6 ‘ Pre-slaughter aspects of beefeating quality’Meat Technology Update Newsletter99/4 ‘Post-slaughter aspects of beefeating quality’Meat Technology Update No. 97/3(AMT) ‘Lighting in Meat ProcessingAreas’CSIRO Meat Research Newsletter 93/5‘Displaying Meat for Maximum Return’Contact us for additional informationMeat Industry Services is supported by the Australian Meat Processor Corporation (AMPC) and Meat & Livestock Australia (MLA).Brisbane: Sydney: Adelaide:Food Science Australia Bill Spooncer Chris SentancePO Box 3312 PO Box 181 PO Box 178Tingalpa DC QLD 4173 KURMOND NSW 2757 FLAGSTAFF HILL SA 5159Ian Eustace Neil McPhail Alison SmallT +61 7 3214 2117 T +61 7 3214 211 T +61 7 3214 2109 T +61 2 4567 7952 T +61 8 370 7466F +61 7 3214 2103 F +61 7 3214 2103 F +61 7 3214 2103 F +61 2 4567 8952 F +61 8 8370 7566M 0414 336 724 M 0414 336 907 M 0409 819 998 M 0414 648 387 M 0419 944 022