Journal of the Science of Food and Agriculture J Sci Food Agric 79 :786–791 (1999)

Detection of discoloration in thermallyprocessed blue crab meat¹Dina D Requena,1 Scott A Hale,1* David P Green,2 W Fred McClure1 and Brian

E Farkas 31 Dept of Biological and Agricultural Engineering, North Carolina State Univers ity , Raleigh, NC 27695-6725, USA

2 Dept Food Science, Seafood Laboratory ,Morehead City , NC,USA

3 Dept Food Science, North Carolina State Univers ity , Raleigh, NC,USA

Abstract : This study objectively and quantiüably examined the eþ ect of a series of factors on blue

crab meat discoloration. Factors explored include heating process, animal harvest location, and posi-

tion of meat within a container. A Spectrogard colorimeter was used to collect visual reýectance

spectra between 380 and 720 nm. Meat degree of coloration was characterised objectively and rapidly

by using lightness (L), red–green (a) and yellow–blue (b) colour values. Results showed that meat

became darker with increasing heating process; crab harvest location had signiücant eþ ect on the

lightness of the ýesh; and meat that is located in the bottom of a can was darker than that in the top.

This study will serve as a baseline for the development of a coloration quality control system.

1999 Society of Chemical Industry(

Keywords: blue crab meat ; colorimetry ; meat discoloration; reýectance

INTRODUCTION

Colour is one of the most important attributes aþect-ing the appearance and consumer appeal of foods.Food processes that preserve and improve the char-acteristic colour and appearance of products areimportant to food manufacturers. In processing ofblue crab meat, loss of the natural glistening white tooþ-whitish or creamy colour in fresh meat lowersconsumer acceptance and can generate economic lossfor producers.

Discoloration in processed crab meat has beenclassiüed1 according to üve general categories : blue,related to components in crab blood and associatedwith thermal processing ; brown, due to carbo-hydrates associated with the Maillard reaction;black, caused by sulphide reactions of metals duringcanning ; yellow, due to lipid oxidation in frozenstorage; and red, due to muscle pigments found inleg or claw meat packed alongside white body meat.In addition, several chemical and biological factorshave been attributed to development of discolorationin processed crab meat. These include copper,1h4iron,5 moisture content and muscle pH.6 Additionalfactors reported are gender and anatomical struc-ture,5 heat processes and time of year.7

This study investigated meat darkening in com-mercially processed crab meat in an objective, quan-

tiüable manner. The degree of heat treatment,animal harvest location and position of meat within acontainer were considered as possible causativeagents.

MATERIALS AND METHODS

In this study, meat coloration was measured with aSpectrogard colorimeter.8 Two experiments wereperformed. The ürst evaluated the coloration ofcommercially pasteurised meats, which had pre-viously been reported to have a discolorationproblem. Because reports of discoloration appearedto be regional, a second pilot-scale experimentexplored the eþects of animal harvest location andheat processing.

Commercially pasteurised meat coloration

Random samples of Atlantic blue crab meat(Callinectes sapidus) were collected from a NorthCarolina commercial processor experiencing dis-coloration with heat processed (pasteurised) crabmeat. Six containers (108] 177.8mm, 454g) ofbackün crab meat were opened, visually inspectedand photographed. Because the discolorationappeared to be related to the position of the meatwithin the container, the contents of each can were

¹ Paper number 98-04 of the Journal Series of the Department of

Biological and Agricultural Engineering, North Carolina State Uni-

vers ity, Raleigh, NC 27695-7625, USA. The us e of trade names in

this publication does not imply endors ement by the North Carol-

ina Agricultural Res earch Service or criticis m of s imilar ones not

mentioned.

* Corres pondence to : A. Dept of Biological and Agri-Scott Hale,

cultural Engineering, North Carolina State Univers ity, Raleigh, NC

27695-6725, USA

(Received 7 April 1998 ; revis ed vers ion received 15 June 1998;

accepted 16 September 1998)

( 1999 Society of Chemical Industry. J Sci Food Agric 0022–5142/99/$17.50 786

Detecting discoloration in blue crab meat

divided into top and bottom sections, each of thosesections representing one sample. Two sub-samples(c 100–125g) from each sample were then taken ofcolorimetric analysis. Each sub-sample was blendedin a Waring blender (10s, 3] , low speed). Theblended sub-samples were carefully positioned incuvettes with glass windows of 5cm diameter andscanned three times (replicates) in a Spectrogardcolorimeter.8 Each scan produced a visible spectrum(380–720nm) consisting of 34 data points fromwhich colour values such as Lightness (L), red–green(a) and yellow–blue (b) were calculated. A total of 72spectra (36 for the top and 36 for the bottom posi-tions, respectively) and their respective color valueswere obtained. Thus, 216 points were considered forstatistical analysis.

Controlled, pilot-scale pasteurisation

Fresh hand-picked blue crab meat was obtained fromtwo North Carolina commercial processors to studythe eþects of harvest location and heating process ondiscoloration. A bulk sample of meat (4.54kg) camefrom blue crabs that were harvested from CurrituckSound, a brackish water estuary in the northeasterncorner of North Carolina. Another bulk sample ofmeat (4.54kg) came from blue crabs that were har-vested from Palmico River, a salty water tributarycentrally located along the coast.

Each bulk sample was separated into 10 testsamples of 454g. Each test sample was placed into acontainer (108] 177.8mm, 454g) for subsequentcooking. The 10 cans were each equipped with athermocouple that measured the internal center tem-peratures during heating in a water bath. Duringheating, two cans containing meat from each region(4 total) were removed from the cooker as the inter-nal temperature reached 71.1 (F \ 0.09min), 76.7(F \ 0.49min), 82.2 (F \ 3.23min), 87.8(F \ 45.14min) and 87.8¡C (F \ 56.47min), respec-tively. One sample from each can was taken forcolorimetric analysis. Two sub-samples wereobtained from each sample. Each sub-sample wasthen blended in a laboratory blender, then positionedin cuvettes with 5cm diameter glass windowsand scanned three times in the Spectrogard colorim-eter used in the previous experiment. One visiblespectrum of 34 data points with its respective L, a, bcolour values was obtained for each sub-sample. Atotal of 60 spectra were obtained for each harvestregion at all the üve heating processes resulting in atotal of 120 spectra with their respective L, a, bvalues. Thus, 360 points were considered for sta-tistical analysis.

Statistical analysis

A randomised block design with an equal number ofsub-samples (samples) was applied to determine dif-ferences between the eþects of the treatments on thecolour of the meat. SAS general linear modelprocedures9 with partial sum of squares10 were run

for these experimental designs. The L, a, b colourvalues were considered to be responses due to thetreatments. Treatments included position of meatwithin a container, heating process and harvest loca-tion. Position of meat within a container was thetreatment for the experiment on commercially pas-teurised meat. Heating process and harvest locationwere treatments for the controlled, pilot-scale pas-teurization experiment. Experimental, sub-samplingand sampling errors were taken into consideration.For both experiments, the null hypothesis(hypothesis tested) stated that there were no signiü-cant diþerences in the colour values of the meat dueto the treatments.

RESULTS AND DISCUSSION

Colorimetric spectra were represented by a reýec-tance (R) vs wavelength (nm) graph. Figure 1 showsan example of visual spectra of crab meat located intop and bottom positions of a commercially pasteur-ised sample. Figure 2 shows visible spectra of meatssubjected to equivalent heating process (76.7¡C,F \ 0.49min) but taken from diþerent harvest loca-tions. Figure 3 shows visual spectra of meats fromcrabs harvested in the same waterway (PalmicoRiver), which were subjected to all üve diþerentheating processes. Results from Currituck soundwere similar. In general, blue crab meat reýectancepercentages ranged from 20 to 55%, having highervalues with increasing wavelengths. Regions from380 to 500nm showed reýectance values of less than30%, while regions from 500 to 720nm showedincreasing reýectance values greater than 35% forboth experiments. All these spectra show clear dif-ferences between each other. Statistical results tosupport those diþerences are given below.

Commercially pasteurised meat coloration

Table 1 gives the averaged L, a, b colour values foreach can (denoted A to F) and position (top andbottom). Lower values of lightness indicated that themeat was darker. Higher values of a indicated thatthere was more red (]) and less green ([) colour inthe meat source. The b value indicated how muchblue ([) and yellow (]) was reýected by the meatsamples. Thus, it can be observed from the L values,that meat located in the bottom of a can was darkerthat meat located in the top of a can. There was not aclear tendency for the a values. However, for eachcan such value was higher for meat located in thebottom. The b values indicated that meat from thebottom of a can was bluer than that of the top. Thissatisüed the postulation made in Ref 4 that dis-colored meat should show a bluer tendency for a bluetype discoloration.

Table 2 provides the results for the statisticalanalysis for this experiment. The L, a, b values wereconsidered to be responses that vary according to thetreatments. Treatments were the top and bottom

J Sci Food Agric 79 :786–791 (1999) 787

DD Requena et al

Figure 1. Averaged vis ible

s pectra for meat located in

the top and bottom of a

container.

positions and the blocks were the six cans. Thehypothesis formulated (null hypothesis) indicatedthat there were not signiücant diþerences in colourvalues of meat located in the bottom and top of a can.However, at 95% (a \ 0.05) conüdence level there

are signiücant diþerences between the color values ofmeat located in the top and in the bottom of a can.The null hypothesis was rejected for the four sourcesof variation. This is because the probability of asmaller F is less than 0.05 in all the results,(Pr[ F)

Figure 2. Averaged vis ible

s pectra for meat from crabs

harves ted in the Palmico

River and Currituck Sound

which were s ubjected to

equivalent heat treatment

(76.7¡C, F\ 0.49min).

Can TOP BOTTOM

L a b L a b

A 63.5^ 0.090 0.5^ 0.021 11.4^ 0.041 62.1^ 0.212 0.7^ 0.048 11.5^ 0.095B 65.4^ 0.853 [0.2^ 0.050 9.0^ 0.347 63.6^ 0.372 [0.7^ 0.061 8.0^ 0.177C 64.9^ 0.043 0.1^ 0.030 9.9^ 0.039 63.5^ 0.189 [0.3^ 0.085 9.1^ 0.142D 64.9^ 0.043 [0.1^ 0.030 9.8^ 0.056 63.5^ 0.189 [0.4^ 0.037 7.6^ 0.433E 64.9^ 0.043 [0.1^ 0.012 9.3^ 0.065 63.5^ 0.189 [0.3^ 0.140 9.0^ 0.088F 64.9^ 0.043 0.6^ 0.140 10.0^ 0.146 63.5^ 0.189 0.3^ 0.032 9.0^ 0.112

Table 1. AveragedL, a, b values

for meat located in the top and

bottom s ections of a container

788 J Sci Food Agric 79 :786–791 (1999)

Detecting discoloration in blue crab meat

Figure 3. Averaged vis ible

s pectra of meat from crabs

harves ted in the s ame waterway

(Palmico River) and expos ed to

different heat treatments .

except in the case where indicating noPr [ 0.1074,signiücant diþerence in lightness between the cansused for the experiment.

Diþerences of position based on L values were sig-niücant even at 99% (a \ 0.01) conüdence level.These results indicate that meat located in thebottom of a can was darker than meat located in thetop as shown in Table 1 and Fig 1. This is though tobe due to the higher moisture content in the bottompart of the can. The higher the moisture content of asample, the faster its heating rate. The higher themoisture of the sample, the faster it heats up gener-ating the development of faster changes in the meatcolour (darkening). For the a values, there were sig-niücant diþerences between treatments at 95% con-üdence level ; however, no clear tendency wasobtained. The a values were nearly always higher forthe meat located in top of a can. For the b values,there was a signiücant diþerence between treat-ments. This supported the results given in Table 1

indicating that meat located in the bottom of a canwas bluer than that located in the top of a can.

Controlled, pilot-scale pasteurisation

Table 3 provides averaged results of the L, a, bvalues for each harvest region and thermal treatment.There were obvious diþerences for all color valuesbetween the üve heating processes and betweenharvest locations. The L values indicated that meatfrom Palmico River was lighter than that from Cur-rituck sound. Also, the L values decreased withincreased degree of heat processing for both waterlocations, indicating that heating process inýuencedmeat discoloration. There was not a clear tendencyfor the a values, but it appears that they varied dueto increases in heating process. The heating processdid not have an eþect on b values. Statistical resultsdo corroborate these insights.

Statistical results of the diþerence tests for thetreatments are given in Table 4. The L, a, b values

Res pons e Source of variation DF MS F value Pr[F

L Pos ition 1 50.133 12.419 0.0168

Can 5 13.365 3.311 0.1074

Samples 5 4.037 12.429 0.0002

Total 12 0.325 3.394 0.0012

a Pos ition 1 1.163 9.720 0.0263

Can 5 2.233 18.584 0.003

Samples 5 0.120 6.918 0.0029

Total 12 0.017 4.916 0.0001

b Pos ition 1 13.546 7.198 0.0437

Can 5 13.250 7.041 0.0258

Samples 5 1.882 17.413 0.0001

Total 12 0.108 6.426 0.0001

Table 2. General linear model

procedure with partial s um of

s quares for the commercially

pas teuris ed meat s amples

J Sci Food Agric 79 :786–791 (1999) 789

DD Requena et al

Table 3. AveragedL, a, b values for meat from crabs harves ted in Palmico River and Currituck Sound which were s ubjected to different

degrees of heat treatment

Heating proces s Palmico River Currituck Sound

(¡C, min)L a b L a b

71.1, 0.09 65.3^ 0.219 0.3^ .0.054 12.8^ 0.307 60.3^ 0.148 0.1^ 0.029 11.3^ 0.16876.7, 0.49 64.0^ 0.683 0.3^ 0.132 12.0^ 0.395 59.8^ 0.326 0.1^ 0.046 11.3^ 0.09782.2, 3.23 64.3^ 0.228 0.3^ 0.019 11.8^ 0.056 59.14^ 0.286 0.2^ 0.048 11.1^ 0.16487.8, 45.14 62.9^ 0.412 0.3^ 0.055 11.5^ 0.229 58.5^ 0.373 0.1^ 0.019 10.8^ 0.14887.8, 56.47 62.7^ 0.561 0.3^ 0.044 11.6^ 0.322 58.9^ 0.595 0.2^ 0.052 11.1^ 0.190

were considered to be responses that varied due tothe treatments harvest regions (Palmico River andCurrituck sound), heating process, and heatingprocess within a harvest region. The null hypothesisfor all the treatments stated that there were no diþer-ences in meat colour due to the treatments. Asobserved in Table 4, at the 95% conüdence levelthere were signiücant diþerences in the L, a and bvalues obtained for the two harvest locations and sig-niücant diþerences in L values resulting from theheat processes. The diþerences between harvestlocations were signiücant even at higher conüdencelevel (a \ 0.01). These diþerences indicated thatharvest location and heating process were importantcausative agents of meat darkening. Crabs harvestedin Currituck sound produced signiücantly darker(lower L values) meat than those harvested inPalmico River, as Fig 2 shows. Speciüc reasons forthis have not been determined, but salinity of thewater may be one factor inducing the colour varia-tion. Also, meats that were heat processed to 87.8¡Cand F \ 56.47min had the lowest L value; hence,higher thermal treatment led to darkening of themeat.

During heating treatment, several reactions inducechanges in meat texture and color. Meat texture isimproved due to heat treatment. The muscle proteinbecomes ürmer as the collagenic proteins form a gelstructure. Thermally induced color changes thatoccur are due to the Maillard reaction (browndiscoloration), or the reaction of crab blood com-

ponents associated with thermal processing (bluediscoloration).

CONCLUSIONS

The purpose of this study was to objectively andrapidly investigate the eþect of thermal treatmentand harvest location on crab meat coloration. A base-line method for the identiücation of causative agentsof meat darkening (discoloration) was proposed.Meat colour was characterised by L, a, b indexes,which were used as indicators of coloration. Overall,this type of spectrally based measurement was foundto be an appropriate means for determining theeþects of thermal treatment and harvest location onblue crab meat appearance.

Both, the degree of thermal treatment and crabharvest location were important causative agents ofmeat discoloration. Increased heat treatmentadversely aþected meat lightness. Signiücant diþer-ences in coloration were also found to have resultedwhen meats from crabs harvested in diþerent water-ways were compared. Currituck Sound crabs pro-duced darker meats than Palmico River crabs. Theposition of meat within a container was also found tohave a signiücant inýuence on coloration. Meatlocated in the bottom part of a can was darker thanmeat in the top.

This study provided a baseline for the develop-ment of objective coloration quality control strategies

Res pons e Source of variation DF MS F value Pr[F

L Harves t location 1 532.724 283.044 0.0001

Heating proces s 4 14.353 7.434 0.0389

Sample 4 1.921 3.599 0.0172

Total 28 0.560 27.331 0.0001

a Harves t location 1 0.884 31.783 0.0048

Heating proces s 4 0.030 1.047 0.4830

Sample 4 0.028 2.811 0.0442

Total 28 0.011 14.066 0.0001

b Harves t location 1 17.811 23.494 0.0166

Heating proces s 3 3.143 4.035 0.1410

Sample 3 0.776 5.445 0.0056

Total 23 0.150 14.960 0.0001

Table 4. General linear model

procedure with partial s um of

s quares for meat s amples

collected from Currituck Sound

and Palmico River

790 J Sci Food Agric 79 :786–791 (1999)

Detecting discoloration in blue crab meat

for crab meat processing facilities. Coloration valuescan be transformed to tolerance levels for generatinga decision making rule for further qualiücation ofcrab meat. However, further work is needed todevelop a rapid, non-destructive coloration qualitycontrol system which may lead to improvements inthermal processing and other crab meat processoperations with the goal of optimum quality product.

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