C: Food Chemistry JFS C: Food Chemistry Carbon Monoxide Treatments to Impart and Retain Muscle Color in Tilapia Fillets D. MANTILLA, H.G. KRISTINSSON, M.O. BALABAN, W.S. OTWELL, F.A. CHAPMAN, AND S. RAGHAVAN ABSTRACT: Carbon monoxide (CO) has been used for improving the color of muscle foods. In the current study, we compared the postmortem treatment of tilapia fillets with 100% CO and euthanasia of live tilapia with CO for their ability to stabilize the color of white and red muscle of tilapia fillets. Both postmortem CO treatment and CO euthanasia were effective in increasing the redness (a* value) and lightness (L* value) of tilapia white and red mus- cle. Fillets obtained from CO-euthanized tilapia showed significantly higher a* and L* values during 1 mo of frozen storage at –20 ◦ C and subsequent thawing and storage at 4 ◦ C for 18 d. The amount of CO present in the red and white muscles decreased during the 18 d of storage at 4 ◦ C. There was no significant difference in the pH, drip, or thaw loss of CO-treated tilapia fillets compared to the untreated fillets. Keywords: carbon monoxide, color, pH, storage, tilapia Introduction Q uality of aquatic foods can be influenced by a variety of pre- and postmortem conditions. Among them, the type of han- dling before slaughter (Lowe and others 1993) and the method of slaughter could play important roles (Erikson 2001; Robb 2001; Morzel and van de Vis 2003). Several methods of slaughter, such as electrical stunning (Robb and others 2002), immersion in liquid ice (Huidobro and others 2001), and euthanizing using carbon diox- ide (CO 2 ) (Kestin and others 1995), are currently being used in the fish industry. Some fish industries utilize a combination of tech- niques such as CO 2 and live chilling (Erikson and others 2006) to minimize stress and prolong the onset of rigor. An animal is con- sidered to be in a state of stress when it is required to make abnor- mal or extreme adjustments to its physiology or behavior to cope with adverse aspects of its environment and handling (Fraser and others 1975). When fish are under stress, oxygen is not available in sufficient amounts and the anaerobic pathway becomes pre- dominant, resulting in reduced glycogen content. This depletion of glycogen could result in a rapid decrease in pH and faster on- set of rigor. Hence, proper handling practices of live fish could pro- long the onset of rigor. The advantage of having a longer onset of rigor is that processing of the raw material can start immediately after slaughter, as otherwise the process cannot start until rigor has been resolved (Skjervold and others 2001; Lauritzsen and oth- ers 2004; Kristoffersen and others 2006a). Moreover, increased han- dling stress could adversely affect the texture (Sigholt and others 1997), leading to softening and gaping of the muscle tissues (Robb and others 2000; Kristoffersen and others 2006b) and an overall loss in the quality of fish fillets. Generally, CO 2 is used for anesthetizing fish (Erikson and others 1997) and reducing stress before slaughter. In the current study, we studied the effect of carbon monoxide (CO) dissolved in water as an alternative way of anesthetizing tilapia (Oreochromis aureus). Several researchers have already shown the MS 20070899 Submitted 12/4/2007, Accepted 2/20/2008. Authors Mantilla, Kristinsson, Balaban, Otwell, and Raghavan are with Aquatic Food Prod- ucts Program, Dept. of Food Science and Human Nutrition and author Chapman is with Dept. of Fisheries and Aquatic Sciences, Univ. of Florida, Gainesville, FL 32611, U.S.A. Direct inquiries to author Kristinsson (E-mail: hordur@ufl.edu). ability of CO to improve the color and quality of fish (Brown and others 1980; Chow and others 1998; Hsieh and others 1998). CO sta- bilizes the red color of muscle foods by binding with Fe +2 of muscle heme, forming a cherry red carboxymyoglobin/hemoglobin. Since CO has a higher binding affinity to heme than oxygen, treatment of muscle foods with CO could result in a prolonged and enhanced red color of meat. Numerous researchers have studied the use of CO as a constituent gas in modified atmospheric packaging of fish and meat products (Brown and others 1980; Jayasingh and others 2001). However, few researchers have studied the effect of CO as an anesthetic on the quality of fish. The objective in this study was to investigate the color retention and quality of fillets obtained from CO euthanized tilapia and compare them with postmortem CO- treated tilapia fillets and tilapia fillets exposed to no gas treatment. Changes in muscle parameters such as color, color stability, CO uptake and stability, muscle pH, and water holding capacity were investigated. Materials and Methods Materials Live tilapia (Oreochromis niloticus) were obtained from Evans Farms in Pierson, Fla., U.S.A. They were transferred live to the labo- ratory and were held for < 5 h in oxygenated holding tanks prior to euthanasia by exposure to CO. The chemicals used for analy- ses were of American Chemical Society (ACS) grade and were pur- chased from Fisher Scientific (Santa Clara, Calif., U.S.A.). Treatment of tilapia using saturated solution of CO The holding tank was a closed system tank constructed with transparent Plexiglas (36 × 16 × 12 inches). The tilapia were eutha- nized with CO saturated water prepared by injecting 100% CO (sol- ubility of CO at 20 ◦ C was 2.32 mL/100 mL water) into the circula- tory system of the holding tank to saturate the tank water (Mantilla 2005). All experiments were performed at ambient room tempera- ture (21 ◦ C). The fish were maintained in the euthanizing tank until they were confirmed dead by visual inspection. On average, 31 min were needed for the completion of the euthanasia process. During every trial, 13 fish were euthanized. To flush the remaining CO out of the tank, air was flushed in and the CO was converted to CO 2 C390 JOURNAL OF FOOD SCIENCE—Vol. 73, Nr. 5, 2008 C  2008 Institute of Food Technologists doi: 10.1111/j.1750-3841.2008.00757.x Further reproduction without permission is prohibited