|[Image: Photo of purging crawfish]|
W. Ray McClain
The preferred way to cook crawfish is to boil the whole animal in seasoned water and then serve it so the consumer extracts the abdominal muscle, or tail meat, by hand. When the exoskeleton is peeled from the abdomen, the intestine is often exposed and can be unappealing. The full, dark-colored intestine of unpurged crawfish contrasts conspicuously with the light-colored meat. It can be particularly offensive if the intestinal wall is ruptured during peeling, contaminating the meat with gritty fecal material.
For a more attractive product, crawfish are sometimes placed in depuration or “purging” systems for one or two days. Crawfish are confined in water or high humidity environments and food is withheld. This process cleans the exoskeleton of mud and debris and eliminates or greatly reduces digesta in the intestine. The intestine of a fully purged crawfish is smaller, translucent and much less conspicuous. Because purging removes ingesta from the stomach and cleans the gill chamber of grit and mud, the water used to boil crawfish does not become contaminated.
Placing crawfish in salted water for several minutes before cooking is sometimes improperly referred to as purging, but this process does little to remove contents of the intestine and is little more than a cursory wash. The salt has little effect.
Some people who have eaten crawfish for many years are accustomed to non-purged crawfish and do not find them objectionable. Nonetheless, most would prefer a purged product if the additional costs were minor. Purging has contributed to repeat sales and loyalty to certain suppliers and has helped foster new markets in non-traditional locales.
Because costs of purged crawfish are 15 percent to 25 percent higher, the current market for purged crawfish is relatively small. The higher cost of purged crawfish comes from the capital expenditures and operating expenses needed for purging, the extra labor required and crawfish death associated with the purging process. Research has been conducted at the LSU Agricultural Center’s Aquaculture and Rice Research stations to better characterize the purging process and to develop protocols for reducing cost.
Two basic types of purging systems have been used in the industry. Immersion, which is the most common, involves totally immersing the crawfish. The spray system involves exposing the crawfish to water spray or mist. Both types were evaluated for their effect on crawfish survival and evacuation rate of intestinal contents. The findings showed that the systems were similar.
Crawfish mortality during purging averaged 8 percent after 24 hours and 12 percent for 48 hours and was similar to previous findings. Of the total amount of intestinal contents excreted during 48 hours of purging, 70 percent, on average, was excreted within the first 12 hours. Because purgerelated mortality increases with time, the shortest acceptable purge duration will be the most cost effective. A 12-hour (or overnight) purge is sufficient for cleaning the exterior of crawfish and for the evacuation of most of the intestinal contents. A 12-hour duration will incur the least mortality and use less energy. Therefore, approximately 12 hours is likely to become the recommended purge duration.
Research is evaluating ways to increase the efficacy of purging. Preliminary findings suggest that segregating crawfish by size and decreasing the loading rates during purging lessens mortality.
W. Ray McClain, Professor, Rice Research Station, Crowley, La