microbiological characteristics of pacific shrimp (pandalus jordani)l
TRANSCRIPT
APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Apr. 1977, p. 853-859Copyright © 1977 American Society for Microbiology
Vol. 33, No. 4Printed in U.S.A.
Microbiological Characteristics of Pacific Shrimp (Pandalusjordani)l
J. S. LEE* AND D. K. PFEIFERDepartment of Food Science and Technology, Oregon State University, Corvallis, Oregon 97331
Received for publication 10 November 1976
Microorganisms associated with Pacific shrimp (Pandalus jordani) were iso-lated and identified. Those on the iced raw shrimp, which yielded an averagecount of 1.6 x 106, were predominantly Moraxella, Pseudomonas, Acinetobac-ter, Arthrobacter, and Flavobacterium-Cytophaga spp. The blanching and peel-ing reduced the microbial level to 3.3 x 104 and also selectively eliminatedMoraxella spp. The microbial flora changed after each processing sequence, andthe heat sensitivity and growth characteristics of the representative microbialgroups suggested that the presence of Arthrobacter and Acinetobacter spp. inpeeled shrimp may indicate inadequate cleaning of raw shrimp or a shorterblanching time. The presence of Moraxella and Flavobacterium-Cytophaga spp.would indicate the degree of secondary contamination, and the presence ofPseudomonas spp. would indicate the shelf-age of the processed shrimp.
Since we made a microbiological survey ofPacific shrimp (Pandalus jordani) processingin Oregon in the late 1960s (6), two importantchanges have taken place. The shrimp landingsnow have quadrupled to over 20 million poundsper annum, and nearly all landed shrimp arepeeled by the Laithram model PCA machine(Laithram Co., New Orleans, La.).Although the shrimp are still harvested from
the same bed, at 60 to 90 fathoms, 12 to 15nautical miles off shore, the trip now takesfrom 2 to 5 days instead of the 1-day trip thatwas common in the 1960s. The shrimp are icedon board and, with a few exceptions, the entirecatch of a trip is held in a common hold. Theage of the shrimp upon landing, therefore, mayrange from less than 1 day to 4 days out ofwater.Upon landing, the shrimp may be held on ice
for an additional day or immediately washedfree from ice and processed. The raw shrimp aretransported to a holding tank and are steam-blanched at 100°C (212°F) for 2 to 3 min. Peelingis accomplished by reciprocating rubber rollers,the meat and shell are separated by densitydifferential under running water, and thepeeled shrimp are hand-sorted, brined, packedin 5-lb (no. 10; about 2,267.5 g) cans, and frozen.
In this study, we examined the raw shrimp toassess the effect of the changes in harvestingpractice on the microbial quality. The peeledshrimp were examined to determine the impact
I Technical paper no. 4403 of the Oregon AgriculturalExperiment Station.
of the mechanical peeling, and the individualprocessing method was studied to evaluate theoverall microbial quality of the shrimp as it iscurrently being processed.Attempts were also made to determine the
cause of the microbial quality difference byidentifying the microbial population to the ge-nus level and examining the low-temperaturegrowth and heat inactivation characteristics ofthe major microbial groups.
MATERIALS AND METHODSShrimp samples. Raw shrimp samples were ob-
tained at the receiving dock, and the peeled shrimpwere obtained from the packaging tables of commer-cial processing plants, between June and Septemberof 1974 from Astoria and Coos Bay, Ore. The proc-essing samples were obtained in July 1975 fromAstoria. Samples were placed in sterile jars, trans-ported to the laboratory in an insulated container,and examined within 20 min of sampling.
Plating procedure. A 50-g portion of the samplewas blended in 450 ml of Butterfield's phosphatebuffer (11) for 2 min at 2,000 rpm in an Osterizer.The blended sample was then diluted in buffer, andthe appropriate dilutions were spread-plated in du-plicate on tryptone-peptone-extract (TPE) agar (6).
Identification procedure. After 48 h of incubationat 25°C, colonies on the plates were counted, and allthe colonies from a plate that contained approxi-mately 100 isolated colonies were transferred to amaster plate. The procedure for the identification ofmicrobial isolates to genus level was the previouslydescribed replica-plating method (7).
Screening for growth and heat sensitivity. Colo-nies that grew after 48 h at 25°C on TPE agar wereremoved aseptically and suspended in TPE broth.
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854 LEE AND PFEIFER
The cell density was adjusted to 0.1 at 420 nm on theoptical density scale of a Spectronic-20 spectropho-tometer (Bausch & Lomb, Inc.), using 11.7-mm in-ner diameter matched test tubes. Then one drop(0.04 ml) of the cell suspension from a standard 1.0-ml serological pipette (Corning no. 7086) was deliv-ered to a 16-mm culture tube containing 10 ml ofTPE broth.The inoculated tubes were incubated at test tem-
peratures of 10, 25, and 30°C for growth and at 50,55, and 60°C for heat sensitivity testing. Duplicatetubes were used for each treatment.
Heat sensitivity. A 500-ml round-bottom flaskcontaining 100 ml of TPE broth was placed on aheating mantle (catalogue no. 0-406, Glas-col, TerreHaute, Ind.) and heated to constant temperature of52 + 1°C with a magnetic stirrer. At zero time, aknown concentration of cells was introduced into theflask, and the thermal inactivation characteristicswere determined by periodically withdrawing andspread-plating the suspension on TPE agar.Growth characteristics. A 50-ml amount of TPE
broth in a 300-ml side-arm flask (stock no. 2573-14133, Bellco Glass, Inc., Vineland, N.J.) was inocu-lated with a standardized concentration of cells andincubated with shaking at 10, 20, and 30°C in aPsycrotherm incubator (New Brunswick ScientificCo., New Brunswick, N.J.). The growth was deter-mined by optical density at 400 Am with a Spec-tronic-20 spectrophotometer.
NaCl. The NaCl content of the brine was deter-mined with Quantab chlorine titrator 1176 (AmesCo., Elkhart, Ind.).
RESULTS AND DISCUSSIONRaw shrimp. Table 1 lists the generic compo-
sition of the bacterial flora in six raw shrimpsamples obtained at various docksides of theOregon coast during the 1974 and 1975 season.The samples were arranged from I to VI in the
order of increasing microbial count. The countranged from 1.6 x 105 for sample I to 3.2 x 106for sample VI. The arithmetic mean was 1.6 x106.Baer et al. (1) reported a geometric mean of
2.2 x 105 from 1,462 raw shrimp samples exam-ined in a nationwide survey. The two countsare, however, not comparable because the incu-bation temperature of 35°C employed by thesurvey was shown to yield a count that is onelog smaller than that at 25°C (6), and also thelatter samples were from the retail level.A study similar to ours was done by Vander-
zant et al. (12) with the Gulf of Mexico shrimps(Penaeus aztecus and P. setiferus). The micro-bial load of shrimp at landing was determinedat 28°C incubation. Their counts ranged from8.1 x 103 to 1.3 x 106. They also noted thevariations in the length of trip, which rangedfrom 1 to 8 days at sea, irregularities in icingand handling practices on board, and the condi-tions of the bins.
Table 1 shows the microbial load and theflora of raw shrimp obtained from two ports inOregon. The similarity of the microbial loadshown by various samples of Pacific shrimp (P.jordani) in this study may occur because therelatively recent origin of this industry in Ore-gon has resulted in maintenance of a reasona-bly uniform harvesting method.The microbial flora of the raw shrimp were
also very similar. Moraxella spp. were the pre-dominant microbial group in all samples, andPseudomonas spp., especially types III and IV,Acinetobacter spp., Arthrobacter spp., and Fla-vobacterium-Cytophaga spp. were also preva-lent (Table 1).
TABLE 1. Percent distribution of microorganisms in Pacific shrimp (P. jordani) as landed at dockside
Raw shrimp sampleaMicroorganism
I II III IV V VIPseudomonasType I..................................... 1 6 0 1 7 2Type II .................................... 1 1 1 0 5 2Types III and IV ............. .............. 8 15 17 13 22 13
Moraxella ................................... 38 32 56 60 30 34Acinetobacter ................................ 24 15 5 13 4 16Flavobacterium-Cytophaga .................... 16 16 8 7 7 12"Coliforms".................................... 0 0 0 0 1 1Bacillus ..................................... 0 0 1 0 0 1Lactobacillus ................................. 0 0 0 1 1 1Arthrobacter ................................. 12 16 10 4 21 14Micrococcus ................................. 0 0 0 0 1 0Yeasts ..................................... 1 0 2 0 0 2Unidentified ................................. 0 0 0 2 1 2
a The numbers of colonies identified for samples I to VI were 160, 157, 95, 118, 229, and 131, respectively.The microbial counts per gram for samples I to VI were 1.6 x 105, 3.1 x 105, 9.1 x 105, 2.4 x 106, 2.4 x 106, and3.2 x 106, respectively.
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The data could not be compared directly withour earlier results (6), because the microbialgrouping scheme has since been changed, but itappears that the change ofthe microbial flora ofraw shrimp, if any, has not resulted from thechange from 1-day trips of the 1960s to thepresent 2- to 4-day trips.Vanderzant et al. (12) showed that the pre-
dominant microbial groups of the Gulf coastshrimp were Pseudomonas spp. and "coryne-form" (Arthrobacter) spp. They further notedthat one of the characteristics of the high-countsamples was the predominance ofthe Pseudom-onas spp. We had one sample (sample V) inwhich Pseudomonas was the predominant or-
ganism.Another interesting aspect of the microbial
flora of Pacific shrimp (P. jordani) was its simi-larity to that of Dungeness crab (Cancer mag-
ister; 8). Such similarity might have been dueto the relatedness of the respective environ-ments from which the two seafood species are
harvested. This supports the contention thatthe environment is the major factor that dic-tates the composition of microbial flora in sea-
foods (10).Peeled shrimp. The microbial composition of
six peeled shrimp samples is presented in Table2. The samples were obtained independently ofthe raw shrimp samples I to VI. Therefore, thesample numbers in Tables 1 and 2 do not corre-
spond to each other.The most noticeable difference from the raw
shrimp was the greater variation in the micro-bial count and the microbial flora composition
MICROBIAL QUALITY OF SHRIMP 855
in the peeled shrimp. The count ranged from6.2 x 102 for sample I to 1.3 x 106 for sample VI.The arithmetic mean of the counts, 3.3 x 104,was higher than the 6.6 x 103 and 1.2 x 104 thatwe observed from the hand-picked shrimp (5). Ifmicrobial level is to be used as a sole criterion ofshrimp quality, the shift from hand peeling tothat of a machine did not seem to have resultedin an appreciable improvement.Two ofthe peeled shrimp samples (samples II
and VI) showed Pseudomonas III and IV spp.
predominating, whereas Acinetobacter spp.,
Flavobacterium-Cytophaga spp., and Arthro-bacter spp. variously predominated in differentsamples. It is noteworthy that Moraxella spp.,which were predominant in raw shrimp, wereall but eliminated from the peeled shrimp. Wehave previously noted the selective eliminationof Moraxella spp. from smoked salmon (7), butthe opposite was observed with the cookedDungeness crab (8).Processing evaluation. We observed during
our subsequent plant survey that, although thepeelers employed by all the processors were theidentical Laithram model PCA, no two plantsused the same processing schedule.Raw shrimp were transported to the peeler in
some plants via a pump, in others a bin con-taining approximately 1,000 lb (453 kg) ofshrimp plus ice was dumped into the holdingtank of the machine, and still others washedthe shrimp free from ice and the washed shrimpwere held in the holding tank. The temperatureof the water in the holding tank, therefore,ranged from that ofmelting ice to 26.7°C (80°F),
TABLE 2. Percent distribution of microorganisms in shrimp, blanched and peeled by machine
Blanched and peeled shrimp sampleaMicroorganism
Ib II III IV V VI
PseudomonasType I.................................... 0 0 8 0 0 0Type II ........... ......................... 2 0 7 4 3 0Types III and IV ............ ............... 7 76 2 56 4 1
Moraxella ........... ........................ 2 0 2 0 2 0Acinetobacter ................................ 28 16 55 16 35 13Flavobacterium-Cytophaga .................... 37 3 19 16 25 26"Coliforms".................................... 0 0 0 1 0 0Bacillus .................................... 2 0 1 0 2 1Lactobacillus ................................ 2 0 0 0 * 2 1Arthrobacter ................................. 15 5 0 7 23 57Staphylococcus ............................... 4 0 0 0 0 0Micrococcus ................................. 0 0 5 0 3 0Yeasts ............ .......................... 0 0 0 0 0 1Unidentified ................................. 2 0 2 0 0 0
a The numbers of colonies identified for samples I to VI were 68, 37, 136, 71, 93, and 133, respectively. Themicrobial counts per gram for samples I to VI were 6.2 x 102, 1.1 x 104, 1.1 X 104, 1.8 x 104, 2.3 x 104, and 1.3x 106, respectively.
b The sample numbers do not correspond to those of Table 1.
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856 LEE AND PFEIFER
and the shrimp could be held in the tank from 0to 60 min, depending on the location of theshrimp in the tank in relation to the peeler.The peeled shrimp were also transported
from the peeler to hand-sorting tables via a ductin a continual stream. Little microbial consid-eration was apparently given to the design ofthese ducts and chutes. In some cases shrimpwere collected from up to four peelers in a com-mon duct, and apparently the resulting bottle-neck served as a flow regulator. Some proces-sors, no doubt in the spirit of good manufactur-ing practice, isolated the sorting area awayfrom the peeler and then had to resort to theelaborate duct work to transport the peeledshrimp over the fin-fish filleting tables. Otherscollected the peeled shrimp in a bin and trans-ported them as a batch to the sorting tables.One interesting arrangement we observed wasa sorting area above the peeler. Although thetwo areas were physically separated by thefloor and walls, the steam from the peeler wasevident in the sorting room.Another variable introduced is the handling
of the peeled shrimp. After hand sorting, theshrimp were washed and brined. The strengthand the freshness of the brine varied widely.Some processors chilled the brine but most didnot.
After several attempts to quantitatively dif-ferentiate each processing variable, in terms ofits influence on the microbial composition ofshrimp, we abandoned this approach. We werenot able to control the processing schedule. Thevolume of shrimp processed changed from dayto day, which greatly influenced the rate ofprocessing. Nevertheless, we have consistentlyisolated Arthrobacter spp. in greater proportionfrom plants that employed minimal washing ofraw shrimp. According to Vanderzant et al. (13)Arthrobacter spp. were the predominant micro-organisms of pond-reared shrimp, and the pondwater frequently yielded over 90% Arthrobacterspp. Apparently the Arthrobacter spp. of Pa-cific shrimp (P. jordani) were of similar mudorigin.We also isolated higher proportions of Pseu-
domonas spp. from shrimp hung up on thepeeler or the transporting duct. Some of suchshrimp samples, however, also yielded micro-bial flora predominated by Arthrobacter spp. orFlavobacterium-Cytophaga spp.
Because of the difficulty in examining allpossible combinations of variables, we concen-trated our efforts instead on two plants, one (A)which handled the shrimp with high counts andanother (B) which usually had low counts. Themicrobial population of the shrimp as it passed
APPL. ENVIRON. MICROBIOL.
through the processing line in a given day wasthen thoroughly investigated. The two plantsselected were both small one-peeler operations.At 7:00 a.m., both plants started processing theshrimp which was landed the night before andheld on ice overnight. Samples for microbialanalyses were taken just before the morningcoffee break of 9:00 to 9:30 a.m. The shrimpbeing processed were from a 3-day trip andcould have been out of the water for 4 days bythe time of processing. Plant A washed the iceoff the shrimp before transporting them to themachine in a 700-lb (318-kg) tote bin. Thewashed shrimp were emptied into the holdingtank and slowly fed to the peeler. The tempera-ture gradient of the tank water was 100C (50°F)to 25.6°C (78°F). Plant B emptied the ice andshrimp directly into the holding tank; thus, thetemperature gradient in the tank was not uni-form. Nevertheless, the water temperaturenear the peeler had warmed to 24.4°C (76°F).The steam pressure of the peeler was set at 22lb/in2 and the blanching time was 2 min 47 s atplant A. Plant B had the steam pressure set at23 lb/in2 and used an exposure time of 2 min 32s. The steam pressure was fixed by the servicerepresentative of the peeler manufacturer, andthe processor could only vary the exposure timeto obtain desired yield. The temperature of thesteam at the point of contact with the shrimp,although it was difficult to measure accurately,was approximately 820C (180°F). The peeledshrimp from plant A were transported via ashort duct to a sorting room, which was walledoff from the rest of the processing plant. Therethe shrimp were hand-sorted, brined in chilledrecirculating brine of 5.4% NaCl at 1.1°C(340F), and packaged to be frozen. Plant B, onthe other hand, collected 50 lb (22.7 kg) ofshrimp in a stainless-steel container. The con-tainer when filled was transported on a cart tothe processing area, which was also walled offfrom the rest of the processing plant. Thepeeled shrimp were then hand-sorted, washedin 10.4% NaCl brine, and packaged. The brinein plant B was not chilled and its temperaturewas 14.4°C (580F).Table 3 compares the microbial counts of
shrimp, water, and brine, obtained from thetwo plants. The microbial loads of the rawshrimp of both plants were comparable. Thefinished products, however, showed more thana log difference in the microbial counts. Whenthe two columns in Table 3 are compared, itbecomes apparent that the major difference be-tween plants A and B had emerged after thepeeled shrimp had been brined. The microbialload of plant A shrimp after brining was re-
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duced by a log, whereas that of plant B in-creased slightly.Comparison of microbial flora. Tables 4 and
5 list the percent generic distribution of micro-bial flora of the shrimp, holding-tank water,and brine obtained from plants A and B. Themajor difference from the data in Tables 1 and 2was the greater diversity of microorganismsfound in off-the-processing line samples. Farberand Lerke (2) have noted a positive correlationbetween the diversity ofthe microbial flora andthe freshness of seafoods. The observed diver-sity of microbial flora was apparently due to thelack of opportunity for any microbial groups toestablish predominance. Again, the Moraxellaspp. seemed to have been selectively eliminatedfrom the peeled shrimp (Tables 2, 4, and 5).
It is also interesting to note that the shrimp
TABLE 3. Microbial counts ofshrimp and processingenvironment samples obtained from two plants
Microbial count/g or mlaSample
Plant A Plant B
Iced whole shrimp frombox .................. 9.0 X 105 2.1 x 106
Shrimp from holding tankon a machine ......... 3.5 x 105 1.6 x 106
Holding-tank water ..... 2.0 x 105 1.4 x 105Shrimp blanched and
peeled ................ 3.4 x 103 1.1 X 104Shrimp after hand sorting 8.5 x 103 2.0 x 104Shrimp after brine wash. 6.2 x 102 2.3 x 104Brine water . . 1.1 X 103 4.5 x 104Shrimp from packaging
table ................. 2.0 x 103 6.8 x 104
a Counts were made from a single determinationof the composite samples collected at each step.
MICROBIAL QUALITY OF SHRIMP 857
that passed through the chilled brine (1.1°C,34°F) brine of plant A yielded a greater propor-
tion of Pseudomonas spp. (Table 4). Althoughthe chilled brine seemed to have successfullykept the microbial load of the shrimp down, thelow temperature apparently had also favoredthe selection of this recognized spoilage bacte-rium (10).The Hugh-Leifson glucose-fermentative,
gram-negative rods identified as "coliforms"were tested for their reactions to indole, methylred, Voges-Proskauer, and citrate. Noneshowed the typical reactions of Escherichiacoli type I or type II (++-- or -+--).
Characterization of isolates. From the fore-going, it became apparent that the ability ofmicroorganisms to withstand heat inactivationand their ability to grow rapidly, at ambient torefrigeration temperatures, were the two majorfactors that contributed to their selection dur-ing shrimp processing. Data indicated thatMoraxella spp. would be very heat sensitiveand Pseudomonas spp. would grow most rap-
idly at the refrigeration temperatures (Tables2, 4, and 5).Ten cultures each of Pseudomonas, Mor-
axella, Acinetobacter, Flavobacterium-Cyto-phaga, and Arthrobacter spp., isolated fromthree raw shrimp samples, were selected atrandom and screened for their relative heatsensitivity and growth potential. The cultureswere inoculated into a series of tubes with opti-cally standardized inocula. The inoculatedtubes were then heated at 50, 55, and 60°C in a
water bath. Sets of tubes were removed periodi-cally at 5-min intervals and incubated at 25°C.Heat sensitivities of the cultures were deter-mined by turbidity end points, and the cultures
TABLE 4. Percent distribution of microorganisms isolated from shrimp, water, and brine ofprocessing plantA
Iced Raw Tank Blanched Sorted Brined Brine FinishedMicroorganism" shrimp shrimp water shrimp shrimp shrimp (1.10C) shrimp
PseudomonasType I................... 5 6 1 0 0 0 4 4Type II .................. 3 6 6 6 5 0 3 4Types III and IV ......... 9 18 14 6 10 30 36 36
Moraxella ................. 27 20 8 0 3 3 3 3Acinetobacter .............. 11 16 8 48 38 20 16 13Flavobacterium-Cytophaga.. 24 13 22 18 13 17 10 21"Coliform"................... 0 0 0 0 3 3 0 0Bacillus ................... 3 6 11 0 3 6 3 1Lactobacillus ..........0.... 0 2 0 0 2 3 4Arthrobacter ............... 17 14 22 21 22 14 12 6
Micrococcus ...........0....O 1 0 3 7 5 6Yeasts .................... 0 0 2 0 0 0 1 0Unidentified ............... 2 0 2 0 0 0 5 0
a The numbers of colonies identified for each sample from iced shrimp to finished product, from left to
right, were 66, 128, 99, 33, 63, 71, 77, and 67, respectively. The counts are given in Table 3.
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TABLE 5. Percent distribution of microorganisms isolated from shrimp, water, and brine ofprocessingplant B
Iced Raw Tank Blanched Sorted Brined Brine Finishedshrimp shrimp water shrimp shrimp shrimp (14.4°C) shrimp
PseudomonasType I................... 6 3 3 1 3 1 4 2Type II .................. 3 3 4 3 3 1 0 0Types III and IV ......... 9 11 10 9 13 9 10 17
Moraxella ................. 21 16 15 3 6 2 3 5Acinetobacter .............. 26 19 13 27 23 20 22 29Flavobacterium-Cytophaga . 3 11 20 9 15 25 17 19"Coliform".................... 3 3 4 0 4 2 1 2Bacillus ................... 4 7 3 0 2 5 5 2Lactobacillus .............. 0 0 2 0 3 5 6 2Arthrobacter ............... 12 17 22 40 19 22 13 14Micrococcus ............... 4 3 3 9 5 5 11 5Staphylococcus ............. 0 2 0 0 4 1 3 2Yeasts .................... 3 3 2 0 0 1 4 0Unidentified ............... 3 2 0 0 0 1 1 0
a The numbers of colonivs identified for each sample from iced shrimp to finished product, from left toright, were 98, 113, 107, 70, 96, 89, 108, and 42, respectively. The counts are given in Table 3.
were then arranged in the order of heat resist-ance. After this preliminary screening, cul-tures with the median heat resistance fromeach genus were chosen, and the heat-inactiva-tion kinetics at 52°C were determined by theviable count of the survivors. The 10 culturesfrom each genus screened showed either identi-cal end points or 10 min variation.
Table 6 lists the time in minutes required toinactivate 90% ofthe test cultures. Vibrio para-haemolyticus (FDA 8700) from a 1973 Marylandfood-poisoning outbreak (3) was included as acontrol, and the Staphylococcus and Micrococ-cus data from our earlier work (7) were alsoincluded for comparison. The data show thatAcinetobacter and Arthrobacter spp. were themost heat resistant, and Flavobacterium-Cyto-phaga and Moraxella spp., the least.
The heat-sensitivity data would explain thecause for the selective elimination ofMoraxellaspp. in blanched shrimp. They also explain thepredominance ofArthrobacter and Acinetobac-ter spp. in peeled shrimp. But they fail to ac-count for the presence of heat-sensitive Flavo-bacterium-Cytophaga spp. in the same shrimp.Growth potential. The same 10 cultures of
each genus were also screened for their growthat 10, 25, and 300C. The inoculated tubes wereincubated at the above temperatures, and thegrowth was followed by a spectrophotometer at400 ,um. The cultures with the median valueswere again selected by comparing the slope ofthe optical-density plots. Some of the culturesused for heat-inactivation study, therefore,were not the same as those selected for thegrowth experiment.The representative cultures from each genus
were then inoculated into side-arm flasks and
the optical density was measured. Generationtime in hours was determined from the expo-nential phase of the curve of a semilogarithmicplot.The generation times of six cultures are pre-
sented in Table 7. As expected, Pseudomonasspp. showed the shortest generation time, 3.9 hat 10°C. Moraxella spp., with a generation timeof 4.2 h, however, was equally capable of rapid
TABLE 6. Heat inactivation kinetics of shrimpisolates
Microorganism D52 (min)aAcinetobacter.8.0Arthrobacter.6.3Micrococcus. 6.3Staphylococcus. 1.5Pseudomonas.1.2Moraxella.0.7Vibrio parahaemolyticus (FDA 8700).. 0.7Flavobacterium-Cytophaga 0.4
a Average of at least two determinations.bData from previous work (7).
TABLE 7. Generation time ofshrimp isolates at threetemperaturesa
Generation time (h) atMicroorganism
1o0c 25°C 30°CPseudomonas ...... 3.9 + 0.30 1.2 + 0.10 1.0 ± 0.17Moraxella ......... 4.2 + 0.17 1.3 + 0.10 1.2 + 0.10Arthrobacter ....... 8.0 - 0.17 1.4 ± 0.21 0.9 ± 0.20Flavobacterium-Cy-
tophaga ......... 8.0 + 0.20 1.7 ± 0.10 1.6 ± 0.17Acinetobacter ...... 8.1 ± 0.36 1.5 + 0.17 1.2 ± 0.26Vibrio parahaemo-
lyticus .......... 13.5 ± 0.75 0.8 ± 0.17 0.1 ± 0.02
a Three determinations.
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growth at 100C. The growth rates ofArthrobac-ter, Flavobacterium-Cytophaga, and Acineto-bacter spp. at 100C were twice as slow as thoseof Pseudomonas and Moraxella spp.With the exception of mesophilic V. parahae-
molyticus, the growth rates of shrimp isolates,irrespective of their generic identity, were verysimilar at 25 and 300C. The data, therefore,suggest that the microbial population in shrimpthat are kept above refrigeration temperatureswould have remained diverse. The difference inthe microbial flora of shrimp that were brine-chilled (Table 4) and unchilled (Table 5) mighthave been due to the stronger selective pressureexerted by the lower temperature.The obligate psychrophiles are known to
grow poorly at room temperature and fail tocompete successfully with the facultative psy-chrophiles (9). According to Harder and Veld-camp (4), it is a necessary trade-off required forthe advantage of low-temperature growth. Ap-parently, Pseudomonas and Moraxella sp. didnot have to sacrifice their 30°C growth poten-tials for their rapid 100C growth advantage.
ACKNOWLEDGMENTSThis research was sponsored by the Oregon State Uni-
versity Sea Grant College Program, which is supported byNational Oceanic and Atmospheric Administration Officeof Sea Grant, Department of Commerce, under grant 04-6-158-44004.
LITERATURE CITED
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MICROBIAL QUALITY OF SHRIMP 859
Microbiological quality of frozen breaded fish andshellfish products. Appl. Environ. Microbiol. 31:337-341.
2. Farber, L., and P. Lerke. 1961. Studies on the evalua-tion of freshness and on the estimation of the storagelife ofraw fishery products. Food Technol. 15:191-196.
3. Fishbein, M., and B. Wentz. 1973. Vibrioparahaemoly-ticus methodology for isolation from seafoods andepidemic specimens. J. Milk Food Technol. 36:118-123.
4. Harder, W., and H. Veldkamp. 1971. Competition ofmarine psychrophilic bacteria at low temperatures.Antonie van Leeuwenhoek J. Microbiol. Serol. 37:51-63.
5. Harrison, J. M., and J. S. Lee. 1968. Microbiologicalevaluation of Pacific shrimp processing. Appl. Micro-biol. 18:188-192.
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