MALCOLM B. HALE and MELVIN E. WATERS

Frozen Storage Stability ofWhole and Headless FreshwaterPrawns, Macrobrachium rosenbergii

Introduction

The culture of freshwater prawns,Macrobrachium rosenbergit: in temper­ate zones is potentially profitable(Roberts and Bauer, 1978; Liao andSmith, 1980). The prawns must be sea­sonally batch harvested, however, beforewater temperatures become lethal(Brody et aI., 1980); thus, most of theprawns must be frozen for later market­ing. Commercial operations in moretropical areas, such as Hawaii, selective­ly harvest larger prawns from year­round culture ponds for marketing as afresh product, primarily to the restau­rant trade. Recently, rapidly increasingproduction has stimulated interest in themarketing of frozen products (Reddy etal.,1981).

Little information is available on the

Malcolm B. Hale and Melvin E. Waters are withthe Charleston Laboratory, Southeast FisheriesCenter. National Marine Fisheries Service.NOAA, Charleston, SC 29412.

ABSTRACT- The freshwater prawn,Macrobrachium rosenbergii, is being increas­ingly cultured in tropical regions and hasaquacultural potential in temperate zones.Very little information on frozen storage sta­bility has been published, however. In thisstudy, frozen glazed prawns were storedwhole (both raw and precooked) and de­headed (raw)for 9 months at -200e. Chem­ical, physical, microbiological, and sensoryevaluations were made at intervals during

/8

frozen storage stability of freshwaterprawns. Miyajima and Cobb (1977) re­ported their preliminary observationson prawns frozen and stored both wholeand headed and either glazed or vacu­um packaged. They measured nitroge­nous compounds (ammonia, trimethyl­amine, and total volatile nitrogen) andbacterial counts. A significant decreasein organoleptic quality of heads-onprawns was noted after more than 6months of storage, but this was not indi­cated by their chemical or microbiolog­ical test results. Nip and Moy (1979)evaluated prawns that had been frozenby several different methods and storedfor 1 month at -150e. Textural changeswere noted but there were no significantlosses in sensory quality due to frozenstorage.

This study was conducted in coopera­tion with personnel of the South Caro­lina Marine Resources Research Insti­tute (MRRI), who have been developingculturing methods for prawns for severalyears. Our objectives were to evaluatethe frozen storage stability of headed

storage. All samples had a firmer textureafter 3 months offrozen storage but the sam­ples stored and cooked whole had a consis­tently softer texture than those stored as tails.This was due more to the prawns beingboiled whole than to changes during frozenstorage. An increase in total volatile nitrogenbetween 6and 9months was not reflected inother measurements, but there was a generaldecline in acceptability over the course ofthe study.

prawns and whole prawns which couldbe marketed as a gourmet restaurantproduct, as in Hawaii.

Materials and Methods

The Macrobrachium prawns used inthis study were provided by MRRI per­sonnel from the October 1979 harvest ofexperimental culture ponds. The freshlyharvested prawns were immersed in icewater and transported to the MarineResources Center where they weregraded. Large prawns weighing about50 g (9/pound) were selected for storagewhole and medium prawns weighingabout 25 g (18/pound) were selected forstorage as tails.

Three experimental packs were pre­pared: 1) Raw tails, 2) raw whole prawns,and 3) prawns that were boiled, frozen,and stored whole (precooked). Theprawns were layered on trays and rapid­ly frozen in a Hobart l blast freezer to aninternal temperature of -34°e. Thefrozen prawns were glazed by beingdipped twice into ice water before theywere packaged in 5- pound waxed paper­board cartons and stored at - 20°e. Areference standard sample was pre­pared, consisting of raw frozen tails (18count). The tails were glazed, packed inwaxed cartons, wrapped with PVC film,sealed in freezer bags, and stored at-40°e. Sufficient samples were pre­pared for evaluation at zero time and 1,3, 6, and 9 months. The evaluationsincluded organoleptic ratings of appear­ance, flavor, texture, and overall accept­ability plus mechanical shear values,total volatile nitrogen (TVN), pH, andmicrobiological analyses.

Sensory Evaluation

Sensory evaluations were made by atrained taste panel consisting of 121abo­ratory personnel. All tests were madewith the cold, boiled product. Experi­mentally stored tails, and the referencestandard, were thawed, peeled, and de­veined, cooked in boiling 2 percent salt­water for 2 minutes and rinsed with coldwater. The whole, raw stored samples

'Mention of trade names or commercial firmsdoes not imply endorsement by the NationalMarine Fisheries Service, NOAA.

Marine Fisheries Review

800

96 7

X Toils (row)

• Whole (row)

... Whole (precooked)

31'-----'---~-~--'-~--'-~~~

o

9

8

.~ 6~~ 5cEJ: 4

6 789

X Toils (row)

• Whole (row)

... Whole (precooked)

3 4

5

3

9

8

(;>£ 4

.~ 6

.~

Months at -200 (

Figure I. - Average flavor intensity offrozen prawns as perceived by tastepanel (scale of I to 9. standard refer­ence = 5.0\.

Months at -200 (

Figure 2.-Texture (lirmness) of fro­zen stored prawns relative to standardreference sample at 5.0.

Months frozen storage

Figure 3. - Average mechanical shearvalues for experimental and standardreference samples.

were cooked in boiling 3 percent salt­water for 3 minutes, rinsed, deheaded,peeled, and deveined. The third set ofsamples, precooked whole as aboveprior to frozen storage, were thawed,headed, peeled, and deveined. All sam­ples were refrigerated for at least 2hours between cooking and sensoryevaluation.

The taste panelists rated the samplesfor appearance, flavor, and texture rela­tive to the reference standard sample. A9-point rating scale was used in whichthe fifth, or center point, was labeled asequal to the standard. The overall ac­ceptability was rated on a 9-point he­donic scale without reference to thestandard. Ratings ranged from "like ex­tremely" at 9 through "neutral" at 5 to"dislike extremely" at 1.

Physical Analyses

The texture of the prawns was evalu­ated mechanically by use of a Kramershear press. Peak heights of force curveswere measured for triplicate 100 g sam­ples of cooked, chilled tail meat.

Chemical Analyses

Total volatile nitrogen (TVN) analy­ses were conducted using the modifiedConway microdiffusion technique de­scribed by Obrink (1955) and furthermodified by Cobb et al. (1973). The pHwas measured by direct contact of a

combination pH electrode with thethawed, macerated flesh.

Microbial Analyses

The total aerobic plate count (TAPC),most probable number (MPN) of coli­form bacteria and Eschen'chia coli, andthe presence or absence of salmonellawere determined according to FDA'sBacteriological Analytical Manual forFoods (AOAC, 1976). Standard platecount agar was used as the plating me­dium for the TAPe. Plates were incu­bated at two different temperatures:35°C for 48 hours and 22°C for 5 days.Suspect salmonella cultures were sub­jected to serological testing for identifi­cation. The TAPC is reported as pergram of sample, coliforms and E. coli asMPN per 100 g of sample, and salmo­nella as presence or absence of the or­ganism. Reported results are the aver­age of three replicates.

Results and Discussion

Flavor and Appearance

Sensory evaluations of flavor and ap­pearance showed no definite trends overthe 9-month period of frozen storage.The taste panel evaluations of flavor in­tensity are shown in Figure 1. The testscale ranged from 1 (much milder) to 9(much stronger) with 5 being equal inflavor to the reference standard sample

stored at -40°e. Moderate increases inflavor intensity over the first 3 months ofstorage leveled out over the remainderof the study.

Texture

The taste panel evaluations of textureare shown in Figure 2. The raw samplesincreased in firmness (relative to thestandard) during the first 3 months offrozen storage. After 9 months thewhole prawns were slightly softer andthe tails slightly firmer than initial values(relative to the standard). The softertexture of the whole raw and precookedsamples was due primarily to the factthat they were cooked whole, in contactwith the enzymes of the hepatopan­creas, while the raw tails and standardreference samples were boiled as peeledtails. This is indicated by the texturedifference (Fig. 2) between the tails andwhole prawns at the beginning of thestorage study. The shear values for repli­cate 100 g samples, as measured withthe mechanical shear press, are shownin Figure 3. The pattern is similar to thatperceived by the taste panel but shows amore pronounced increase in firmnessfor the raw experimental samples at 3months. The standard reference sampleshowed a moderate but steady increasein shear value. The precooked samplehad a very slight increase in firmnessover the 9-month frozen storage period.

December 1981,4]112) /9

Month

o 0.0022

0.0394 0.0001

Table 3.-P values 01 the WelchAN OVA tests to test equality 01means among product lorms lorTVN and shear value at each stor­age time. Equa lity 01 means is re­jected when P<0.05.

0.004

0.0075

00004

Shear value

P value

TVN

0.1432

00914

0.8948

Month

9

3

6

Item 0 3 6 9

Whole vs. Flavorprecooked Texture

Acceptability

Whole vs. Flavortail Texture

Acceptability

Precooked Flavorvs. tail Texture

Acceptability

Table 1.-Total volatile nitrogen(TVN, mg N/100g), totalaerobic plate count (TAPe), and pH of trozen Macro-brachium prawns.

Storage Sample' TAPC/g------

period code TVN @22°C @35°C pH

0 T-R '8.16 78Xl0' 64Xl0' 6.30

1 month T-R-20 1287 55Xl0' 55Xl0' 6.60T-R-40 14.40 670W-R-20 14.29 658W-C-20 1017 660

3 months T-R-20 13.04 660T-R-40 12.79 660W-R-20 12.60 6.50W-C-20 11.78 690

6 months T-R-20 10.03 17X10' 18Xl0' 6.43T-R-40 1080 6.45W-R-20 1282 6.45W-C-20 1185 665

9 months T-R-20 26.22 47X10' 35Xl0' 6.28T-R-40 2572 6.30W-R-20 20.09W-C-20 17.72 655

'T-R-20 ~ Tails. stored at 20°C; T-R-40 Tails. 40°C;W-R-20 ~ Whole. raw. 20°C; W-C-20 Whole. pre-cooked. -20°C.'the TVN and TAPe values are averages of three repli-cate analyses each.

Table 2.-Results 01 nonparametric Wilcoxon signed­rank tests on each pair 01 products at each storage lime.• indicates rejection 01 equality 01 median at the sig­nificance level a ~ 0.05.

9

Months at -20°C

Figure 4. - Acceptability of frozenstored prawns as judged by taste panel(9 point hedonic scale).

scoring was limited to nine discrete val­ues. The Friedman two-way analysis ofvariance (ANOYA) tests indicated thatat a = 0.05 there were significantdifferences between sample medians fortexture but not for flavor or acceptabil­ity. The nonparametric Wilcoxonsigned-rank test was applied to pairs ofproducts at each storage time with theresults as shown in Table 2. The wholeand precooked whole samples were sig­nificantly softer than the tails.

The results of the Welch ANOYAtests for equality of product means forTVN and mechanical shear press valuesare listed in Table 3. The differences inshear press values are highly significant.

Conclusions

The Macrobrachium prawns thatwere frozen and stored whole, eitherraw or precooked, had a significantlysofter texture than the samples stored asraw tails. The softness was due primarilyto the samples being cooked whole, incontact with digestive enzymes, ratherthan to changes occurring during stor­age. This was indicated by the differ­ence between firmness of tails and wholeprawns at zero time, which persistedduring frozen storage.

Raw samples stored at - 20°C devel-

This stability may be attributed to a par­tial destruction of enzymes by cooking.

The increased firmness of the Macro­brachium with storage was not expe~ted.Webb et al. (1975), however, reportedthat brown shrimp, Penaeus aztecus,showed increased mechanical shearvalues after storage for 60 days at- 20°e. Hose and Lightner (1980) re­ported that the tail muscle of cultured Pstylirostns produced small amounts (upto 4.8 ppm) of formaldehyde duringpostmortem decomposition. Trimethyl­amine oxide (TMAO) is the precursor offormaldehyde (which causes toughen­ing of the flesh) and dimethylamine(DMA) in gadoid fish species. No TMAwas detected in our analyses of fresh orstored Macrobrachium, however.

Chemical and Microbiological Tests

The results of analyses for TYN,TAPC, and pH are listed in Table 1. TheTVN values remained relatively low forall samples through the first 6 months ofstorage. The higher values for the final9- month samples have not been ex­plained and were not reflected in thesensory evaluations. The bacterial platecounts decreased steadily over the fro­zen storage period. Although not shown,coliform counts dropped from 60/100 gto 9/100 g MPN and neither E. coli norsalmonella was detected. The pH valuesremained relatively stable throughoutthe storage period.

Acceptability

The average taste panel judgments onthe overall acceptabilities of the exper­imental products are shown in Figure 4.There was a general downward trend inacceptability and deviations could berelated to texture. The whole raw sam­ple tended to be too soft after cookingwhole but the increased firmness at 3months resulted in an increase in ac­ceptability, which was followed by a de­cline. Conversely, the tails were disa­greeably tough at 3 months and thedecreased firmness at 6 months corre­sponded with a slightly higher accept­ability.

Statistical Analysis

The taste panel results required anal­ysis by nonparametric statistics because

20 Marine Fisheries Review

oped peak firmness after 3 months.There was little change in the texture ofthe precooked prawns but they werevery soft from the beginning and were ofborderline acceptability. The accept­ability of the raw samples declined dur­ing storage and if a taste panel rating of5 is taken as the borderline of accept­ability, the storage life is estimated to beabout 7 months for whole prawns andabout 10 months for tails. The frozenstorage life for whole prawns could begreater if other methods for cooking areused.

Acknowledgments

We thank Ted Smith and other MRRIpersonnel for supplying and grading thefresh Macrobrachium prawns and Ly-

December 1981,43(12)

sander Ng for statistical analysis of theexperimental data.

Literature Cited

AOAC. 1976. Bacteriological analytical manualfor foods. Assoc. Off. Analytical Chem..Wash., D.C.

Brody, T., D. Cohen, A. Barnes, and A. Spector.1980. Yield characteristics of the prawn Mac­robrachium rosenbergii in temperate zoneaquaculture. Aquaculture 21 :375-385.

Cobb, B. E, I. Alaniz, and C. A. Thompson, Jr.1973. Biochemical and microbial studies onshrimp: Volatile nitrogen and amino nitrogenanalysis. J. Food Sci. 38:431-436.

Hose, J. E., and D. V. Lightner. 1980. Absenceof formaldehyde residues in penaeid shrimpexposed to formalin. Aquaculture 21 :197-201.

Liao, D. S., and T. I. J. Smith. 1980. The mar­keting opportunity for freshwater shrimp inSouth Carolina: A preliminary survey. InProc. 5th Annual Trop. Subtrop. Fish. Tech­no!. Conf., p. 67-69. Texas A&M Univ. Pub!.TAMU-SG-81-101.

Miyajima. L. S., and B. E Cobb. 1977. Prelimi­nary observations on the frozen storage stabil­ity of the freshwater prawn, Macrobrachiumrosenbergii. Proc. 2nd Annual Trop. Subtrop.Fish. Techno!. Conf.. p. 253- 261. Texas A&MUniv. Pub!. TAMU-SG-78-101.

Nip, W. K.,andJ. H. Moy.1979. Effectoffreez­ing methods on the quality of the prawn, Mac­robrachium rosenbergii. Proc. World Mari­cult. Soc. 10:761-768.

Obrink, K. J. 1955. A modified Conway unit formicrodiffusion analysis. Biochem. J. 59:134­136.

Reddy. S. K., W. K. Nip, and C. S. Tang. 1981.Changes in fatty acids and sensory quality offreshwater prawn (Macrobrachium rosen­bergii) stored under frozen conditions. J. FoodSci. 46:353-356.

Roberts, K. J., and L. L. Bauer. 1978. Costs andreturns for Macrobrachiumgrow-out in SouthCarolina, U.S.A. Aquaculture 15:383-390.

Webb, N. B., A. J. Howell, B. C. Barbour, R. J.Monroe, and D. D. Hamann. 1975. Effect ofadditives, processing techniques and frozenstorage on the texture of shrimp. J. Food Sci.40:322-326.

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