www.elsevier.nlrlocateraqua-online
Assessment of depuration system and duration on
gut evacuation rate and mortality of red swamp
crawfish
W. Ray McClain
)Louisiana State UniÕersity Agricultural Center, Louisiana Agricultural Experiment Station, Rice Research
Station, P.O. Box 1429, Crowley, LA 70527-1429, USA
Accepted 19 November 1999
Abstract
Two depuration, or purging, systems were examined for their effect on survival and evacuation
Ž .
rate of intestinal contents of the hindgut of crawfish Procambarus clarkii . In the immersion system, crawfish were held in baskets suspended in the water column and subjected to profuse aeration and low water exchange. Crawfish in the water spray system were maintained in shallow
Ž .
pools 1.3-cm deep water and in a constant mist environment. Loading rate for each system was 14.5 kg crawfishrm2 surface area. Similar mortality was observed from the two systems and
averaged 9% and 12% for 24 and 48 h of purging, respectively. Cannibalism occurred in both purge systems; however, the mean net hindgut evacuation rate for crawfish was similar for the two systems and averaged 57%, 63%, 78%, and 81% for 12-, 24-, 36-, and 48-h purge durations, respectively. Evacuation efficiency after 12 and 24 h was positively correlated with water temperature. Of the total amount of intestinal contents excreted during 48 h of purging, 70%, on average, was excreted within the first 12 h. Mortality increases as purge duration increases; therefore, the shortest and most practical duration that results in a product acceptable by the
Ž .
consumer will be the most cost effective. A 12-h or overnight duration in well-managed systems may be sufficient for most markets in Louisiana. q2000 Elsevier Science B.V. All rights reserved.
Keywords: Crawfish; Crayfish; Purging; Depuration; Mortality; Evacuation
)Tel.:q1-318-788-7531; fax:q1-318-788-7553.
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E-mail address: rmcclain@agctr.lsu.edu W.R. McClain .
0044-8486r00r$ - see front matterq2000 Elsevier Science B.V. All rights reserved.
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( ) W.R. McClainrAquaculture 186 2000 267–278
268
1. Introduction
Ž . Ž .
Louisiana USA has a thriving crawfish Procambarus clarkii and P. zonangulus production industry. During the 1997–1998 season, farm production from 44,500 ha totaled 16.4 million kg, which was worth US$22.5 million. The natural fishery of wild
Ž
crawfish accounted for an additional US$19 million Louisiana Cooperative Extension .
Service, 1998 . Farm-raised crawfish are also produced in other southern states of the US, albeit in much smaller quantities. The preferred way to prepare crawfish for the consumer is to boil the whole animal in seasoned water. The consumer then extracts the abdominal meat by hand, which is also the preferred method for eating crawfish in Europe — a major export market for Louisiana’s largest, most valuable crawfish. A portion of the annual harvest is also commercially processed for the abdominal meat ŽHuner et al., 1994 ..
To render a more appealing product for the whole-crawfish market, crawfish are sometimes placed in depuration or ‘‘purging’’ systems for 1 or 2 days. In these systems, crawfish are confined in water or very humid environments and food is withheld. This process cleans the exoskeleton of mud and debris, and eliminates or greatly reduces
Ž .
digesta in the hindgut intestine , which many consumers find unappealing. When the
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exoskeleton is removed ‘‘peeled’’ from the abdomen, the hindgut, which lies dorsally along the abdomen, is exposed. The full, dark colored intestine of an unpurged crawfish conspicuously contrasts with the light colored meat. The exposed contents of the full intestine are particularly offensive if the intestinal wall is ruptured during peeling, contaminating the meat with fecal material. Purging mitigates this unappealing aspect of crawfish consumption. The intestine of a fully purged crawfish is smaller, translucent, and much less conspicuous. Purging also serves to remove ingesta from the foregut and clean the branchial chamber of grit and mud, which contaminate the cooking water when multiple batches of crawfish are boiled.
In the southern USA, many laypeople improperly define purging as placing crawfish in a tub of water with 400–500 g of sodium chloride for a few minutes. This does little to remove the contents of the intestine and is simply little more than a cursory wash. The salt has little effect. Purging, to the extent that intestinal contents are significantly reduced, requires additional time.
Some people, especially Louisianaians, who have been eating crawfish for many years, are accustomed to non-purged crawfish and do not find them objectionable; nonetheless, many would probably prefer a purged product if the higher costs were reasonable. Although the current market for purged crawfish is relatively small due to
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lack of public awareness and price differences 15–25% higher , purging has contributed Ž
to repeat sales and loyalty to certain producers or distributors Haby and Younger, 1989; .
Avery and Lorio, 1996 . Promotions of purged crawfish in non-traditional locales have helped to foster new markets. At one time, purging was also thought to increase shelf
Ž
life of crawfish held in a cooler until cooking Haby and Younger, 1989; Huner and
. Ž .
Barr, 1991 , but this claim has been refuted McClain, 1994 . However, purging may benefit other aspects of crawfish processing, such as lower bacterial counts in peeled
Ž .
Ž .
Huner et al. 1994 review some of the commercial depuration systems that can be
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classified as either immersion types crawfish are totally immersed in water or spray
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types non-inundated crawfish exposed to water spray or mist . Few studies have Ž
evaluated the purging process Lawson and Drapcho, 1989; Lawson et al., 1990; .
McClain, 1994; McClain et al., 1994 , and the scope and findings of those studies were limited. Many aspects of crawfish depuration remain undocumented. This study was conducted to evaluate several aspects of the purging process. The main objectives were to examine the efficiency at which crawfish evacuate the contents of the hindgut under ‘‘typical’’ purging conditions and to compare mortality and the efficiency of gut evacuation in two types of depuration systems at a common loading rate. Evacuation efficiency was examined at 12-h intervals, in lieu of 24-h intervals commonly used in the industry, to obtain more accurate assessments of gut evacuation rate.
2. Materials and methods
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Red swamp crawfish P. clarkii , the most abundant commercial species, were used for this study and were produced at the Louisiana Agricultural Experiment Station’s Rice Research Station, Crowley, LA. During 1997 and 1998, crawfish were harvested Ž0800–0900 h from commercial-type rice-field ponds using baited wire mesh traps.. They were immediately placed into hard plastic crates and put into one of two
Ž depuration systems within 2 h after capture. Crawfish harvested on four dates April to
.
June in 1997 were subjected to an immersion system of depuration, and those harvested
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on seven dates February to May in 1998 were depurated in immersion or spray type systems. The 11 test periods encompassed a wide array of environmental and biological conditions typical of the crawfish harvest season in Louisiana.
In the immersion system crawfish were fully immersed in water under conditions of profuse aeration and a low rate of water exchange. The spray system consisted of a
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non-immersion spray with high rate of water exchange. The immersion system Fig. 1 was a smaller version of a commercial type, developed by the author in 1981 and subsequently adopted by many that commercially purge crawfish. It consists of an
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oblong, convex-bottom tank 2570 l with suspended baskets that are placed crosswise inside the tank and hold crawfish in the water column. The tank is equipped with a solid partition in the center to provide two similar, but separate, units supporting six baskets each. The basket frames are semi-circular in shape and made from aluminum tubing
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covered with perforated aluminum sheets 4.8-mm mesh . Crawfish cling to all surfaces
Ž 2.
of the baskets and occupy the entire submerged surface area 1.2 m within a three-dimensional space. A loading rate of 14.5 kg crawfishrm2 surface area was chosen to obtain maximum use of available surfaces and was within the 10–24 kgrm2
Ž
range of densities commonly used in commercial purging systems Lawson and Drap-.
cho, 1989 . Profuse supplemental aeration was supplied via a rotary vane compressor and diffused through two 15.2-cm ‘‘airstones’’ placed directly beneath each basket. Flow-through water was added to each half of the tank at a rate of 0.6 lrmin. The tank was cleaned and filled with clean water prior to each test date.
Ž
The spray system consisted of twenty-two, 38-l plastic containers bottom surface
2.
( ) W.R. McClainrAquaculture 186 2000 267–278
270
Fig. 1. This immersion system for purging crawfish utilizes low water exchange and profuse aeration and is designed with hinged baskets for efficient delivery of the washed and purged product.
cm of standing water and a spray nozzle 51 cm above the bottom. Crawfish were partially immersed with only the lower one-half of the animal under water. A fine mist
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of water 0.3 lrmin was continuously directed into the container, resulting in high
Ž 2 .
humidity and a steady exchange of water. The crawfish loading rate 14.5 kgrm area was the same as that of the immersion system; however, crawfish were confined to a
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single plane bottom in this system.
Both systems used a common source of well water filtered through a commercial water softening system. The quality of water from this source was previously determined to be suitable for purging crawfish; influent and effluent characteristics of the water are
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presented in McClain 1994 and McClain et al. 1994 . Water temperature during this
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study was monitored in the ponds at harvest data loggers and in the depuration systems
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during purging minimumrmaximum thermometers .
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Randomly selected crawfish were allocated at the prescribed loading rates to two
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baskets one in each tank half of the immersion system for each test period in 1997. In
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For assessment of hindgut content weight, five mature and five immature crawfish were collected prior to depuration and then from designated replicates at each 12-h sample period. Equal numbers of males and females were collected when possible.
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Crawfish were immediately and individually frozen y188C and at a latter date thawed, weighed, and dissected. The intestine, or hindgut, was separated at its junction to the midgut and left attached to the anus. The telson, with intact anus and attached hindgut, was removed from the abdomen and centered over a pre-weighed aluminum pan. Contents of the hindgut were collected in the pan by flushing the gut with 3 ml of distilled water dispensed from a small syringe through the anus. Gut contents from each of the five mature and five immature crawfish were pooled, and dry weight was recorded after drying overnight at 758C. Dry gut content weight was computed as a percent of whole wet crawfish weight, wet abdominal muscle weight, and dry abdominal muscle weight. Gut evacuation rate was determined as the mean percentage loss for each 12-h duration, and net incremental loss was defined as the percent of the mean total loss Žafter 48 h that occurred at each 12-h increment. Mortality that occurred during purging. was determined by closely examining every animal after removal from designated containers in each purge system. Sexual maturity of dead and surviving crawfish was
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also determined as described by McClain 1994 .
The experimental approach consisted of a randomized incomplete block design with test date as the blocking factor. Data were analyzed with the General Linear Model
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procedure in a factorial arrangement of treatments system and time interval using the Ž
Micro SAS Statistical Software System Statistical Analysis Systems version 6.10, SAS .
Institute, Cary, NC . Main effects were tested for interactions and analyzed
indepen-Ž .
dently if no significant interaction PF0.05 occurred. For the independent analyses, if significant differences were present, Duncan’s multiple range test was used to separate treatment means at PF0.05. The relationship between water temperature and gut
Ž .
evacuation rate or crawfish mortality was determined by simple linear correlation analysis.
3. Results
Ž .
About 950 kg of crawfish )35,000 individuals were examined for purge-related mortality over the 2-year period of investigation. The percentage of crawfish dying during purging for each year, and the proportion of immature crawfish in the catch and those that died during purging are depicted in Table 1. The maturity status of a substantial portion of dead crawfish could not be identified in 1998 as a result of cannibalism. Minimal unidentifiable remains were found in 1997. The data indicate that mortality increased with an increase in duration of purging, with a greater incidence of death among immature crawfish. Mortality was highly variable with respect to test
Ž .
period Table 2 , and no trend relative to harvest period or temperature was observed. Mortality after 24 and 48 h of purging averaged 9% and 12%, respectively. Although results were highly variable, no significant differences due to purge system type were
Ž
observed for mortality or composition regarding identifiable or unidentifiable maturity
. Ž .
( ) W.R. McClainrAquaculture 186 2000 267–278
272 Table 1
Ž .
Mean percent maturity and mortality for crawfish ns35,264 subjected to purging
Hour Percent Percent Percent Percent
immature of catch mortality immature of total dead unidentifiable of total dead
1997
About 1500 crawfish were dissected for determination of hindgut content. The
Ž .
average amount of hindgut contents dry weight basis voided during purging, based on
Ž .
percentage of whole crawfish weight, generally declined with time Table 2 . The maximum net evacuation efficiency was -90%, and crawfish averaged 57, 63, 78, and 81% evacuation of the hindgut after 12, 24, 36, and 48 h of purging, respectively. Of the 11 evaluation period averages, only one mean net evacuation peak occurred within the
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first 12 h of purging 26 May 1998 ; no peaks occurred at 24 h, four occurred at 36 h, and six occurred after 48 h of purging. Mean percent evacuation for each 12-h increment was generally positively correlated with water temperature at harvest and during purging Žminimum, maximum, and median for separate correlations; temperature at harvest had.
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the higher correlations, which were only significant at the 12-h rs0.705; Ps0.016
Table 2
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Water temperature 8C at harvest and during purging, mean "SD percent net evacuation of hindgut contents, and percent mortality for each test period
Date Temperature Percent gut evacuation Percent mortality
Table 3
Response values for crawfish purged in either spray and submerged systems for 12-, 24-, 36-, or 48-h durations during 1998. Means followed by superscripts were significantly different, and other mean
compar-Ž .
isons within category by duration were not significantly different P)0.05
12 h 24 h 36 h 48 h
and 24-h rs0.870; Ps0.0005 durations. The correlation coefficients r for the
36-Ž . Ž .
and 48-h periods were 0.282 Ps0.401 and y0.433 Ps0.1835 , respectively.
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Net gut evacuation rate was slightly affected by purge system type Table 3 . Only at
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the 36-h period was there a significant difference Ps0.03 , with crawfish in the immersion system having the higher rate. The most notable difference between the two systems is that the mean peak in net gut evacuation occurred at 36 h in the immersion system and at 48 h in the spray system.
Because there was a general lack of differences due to purge system type and with no significant interaction between system and purge duration, data were pooled with regard
Ž .
to computations and analyses of hindgut content as a percent of tissue weights Table 4 . Overall, the purging process resulted in a significant reduction in the percentage of contents of the hindgut, although the reductions at each 12-h increment were not always
Ž .
significant P)0.05 . There were small differences and no obvious trend due to maturity. Approximately 70% of the net total excrement loss occurred during the first 12 h, about 77% occurred within 24 h of purging, and 97% occurred within 36 h of purging ŽTable 4 ..
4. Discussion
It is widely accepted that purging, especially the immersion process, is effective in
Ž . Ž
cleaning the exterior and branchial chamber of crawfish Haby and Younger, 1989; .
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Table 4
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Mean hindgut content weight dry as a percent of wet whole crawfish weight, wet abdominal muscle weight, and dry abdominal muscle weight before and after
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purging. Net increment loss for each relative tissue comparison was calculated as a percent of the net total content loss that occurred at each 12-h increment of purge
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duration. Means followed by the same superscript within a column were not significantly different P)0.05
Hour Based on wet crawfish Based on wet abdominal muscle Based on dry abdominal muscle
Percent dry Net increment loss Percent dry Net increment loss Percent dry Net increment loss gut content as percent of total loss gut content as percent of total loss gut content as percent of total loss
A A A
0 0.174 – 1.19 – 5.96 –
B B B
12 0.075 70.2 0.47 73.5 2.37 73.4
B B B
24 0.065 77.3 0.43 78.0 2.18 77.3
C C C
36 0.038 96.5 0.23 97.9 1.16 98.1
a C C C
48 0.033 100 0.21 100 1.07 100
a Ž . Ž .
Few differences between the two purge system types were observed relative to gut evacuation rates and mortality at the loading rates tested. The spray system used in this study maintained a shallow layer of water in which the crawfish resided. Personal experience and anecdotal observation in the use of spray systems that did not allow for standing water suggest that mortality of crawfish after 24 h may be higher than that in
Ž .
well-maintained immersion systems. Lawson and Drapcho 1989 reported that crawfish
Ž .
purged equally as well in spray and immersion flow-through systems, although they did not state whether standing water was present in the spray system. They further reported a possible trend of lower mortality in the spray system, but there were no significant differences.
Mortality during purging is a major factor that leads to higher prices for the product andror directly impacts producer profits. In this study, survival was significantly affected by purge duration, regardless of system type. Mortality after 48 h of purging
Ž .
averaged 63% higher than that after 24 h, in 1998 Table 1 . Although mortality was not measured at the 12- and 36-h interval, it is suspected that mortality increases as purge duration increases for any given set of conditions. In other reports, purge-related
Ž . Ž
mortalities after 24 h albeit highly variable averaged -10% in one study McClain et
. Ž . Ž .
al., 1994 and 11.6% in another study McClain, 1994 . Lawson and Baskin 1985 reported a mean death loss of 24% for crawfish purged for 5 days at a comparable loading rate. Crawfish maintained in refrigerated storage for the same 5 days in that
Ž .
study averaged only 3% mortality. Lawson et al. 1990 reported a significant increase in mortality as the loading rate increased from 4.9 to 34.1 kgrm2 in a water spray
system.
Ž
Results of this study corroborated earlier findings McClain, 1994; McClain et al., .
1994 that showed much higher mortality of immature crawfish. Immature crawfish are likely to be more susceptible to purge-related mortality due to higher proportions of pre-and post-molt individuals, thinner exoskeletons, pre-and generally smaller sizes. Lower mortality after 48 h of purging during 1997 may be the result of a lower proportion of immature crawfish composing the catch.
Acute outbreaks of vibriosis may also be the cause for some observances of high
Ž .
mortality among crawfish during purging Thune et al., 1991 , especially with the red swamp crawfish. P. clarkii appears to be more sensitive to the Vibrio organism than P.
Ž . Ž
zonangulus Huner, 1995 . Many other factors, both before e.g., stress in traps,
. Ž
handling practices, and length of time out of the water and during purging e.g., loading .
rates, size variation, and water quality , are likely to influence survival and should be investigated further.
The depuration process was not completely effective in voiding all contents from the hindgut of crawfish in this study. The gut appeared to be completely voided in many individuals, while variable amounts remained in a few individuals, even after 48 h.
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( ) W.R. McClainrAquaculture 186 2000 267–278
276
higher incidence of cannibalism was apparent after 36 and 48 h, as evidenced by a larger
Ž .
percentage of unidentifiable remains Table 1 . Cannibalism was also a probable cause
Ž .
for peaks of evacuation prior to the 48-h interval for certain dates Table 2 .
Ž .
Based on a moisture content of 31% unpublished data, 200 crawfish , the wet gut content of unpurged crawfish represents less than 2% of the wet abdominal muscle weight. However, the dark, full intestine is conspicuous and, although it is normally
Ž .
Fig. 2. Exposed intestine hindgut of cooked whole crawfish and abdominal muscle from crawfish that were
Ž . Ž .
discarded, it can be unappealing to many consumers. Whereas, on average, some gut content remains after purging, the exposed abdomens of non-purged crawfish are much
Ž .
less appealing than those of purged specimens Fig. 2 .
Ž .
Most )70% of the net total gut contents voided were evacuated during the first 12 h of purging, especially at the higher temperatures, when most of the annual harvest occurs. In addition, the lowest mortality presumably occurred at 12 h, representing the lowest economic loss for a producer who employs purging. Only another 7% of the gut content were evacuated after an additional 12 h of purging, when mortalities are likely to increase substantially. Most of the benefits of cleaning the exterior of crawfish should also be realized within the first 12 h of purging. Therefore, assuming consumer acceptance, a 12-h purging duration is recommended, followed by storage in refrigerated coolers if necessary. Mortality is reduced when crawfish are properly held in refrigerated
Ž .
storage rather than in purging systems Lawson and Baskin, 1985; McClain, 1994 . In summary, the immersion and spray depuration processes were effective in signifi-cantly reducing the content of the hindgut, and evacuation rate and mortalities for both systems were comparable. Because purge-related mortalities increase with time, purge duration should be held to a minimum. Purging to increase product acceptance does exact a cost by at least as much as the percentage of mortalities. Additionally, one should consider the added cost associated with capital expenditures and operating expenses when marketing or purchasing purged crawfish. Cost and energy factors were not considered in this research, but findings that the different purge systems produced similar outcomes under the conditions investigated, should lend support to those who will decide which system is the most efficient in terms of capital expenditure, operating cost, energy requirement, space, and other efficacy issues. More effort needs to be
Ž .
directed toward determining the minimal purge duration i.e., level of gut evacuation that is acceptable to the consumer and other optimum operating conditions, such as rates of loading, flow, and aeration.
Acknowledgements
Appreciation is extended to John Sonnier for his invaluable assistance with data collection and processing. Thanks are also extended to Drs. Robert Romaire and Jay Huner for their critical reviews and constructive comments. This work was supported by the Louisiana Agricultural Experiment Station and approved for publication by the Director, Louisiana Agricultural Experiment Station, as manuscript number 99-86-0189.
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