Use of pulsed-light to treat raw channel catfish fillets.

J.E. Figueroa-Garcia (2)

J.L. Silva (1)

T. Kim (2)

J. Boeger (3)

R. Cover (3)

Fish shelf-life is determined mainly by microbiological spoilage. Shelf-life of raw, refrigerated channel catfish (Ictalurus punctatus) fillets ranges from 4--14 days. A new processing technology uses very brief intense pulses of broad-spectrum white light to reduce microbial loads of products. Catfish fillets were subjected to 0.25 and 0.50 [J/cm.sup.2] and 2--4 flashes, for a total of 0.5, 1, and 2.0 [J/cm.sup.2] Psychrotrophic plate counts were reduced (p [less than or equal to]0.05) by 1.5 log CFU/g after treatment. Increased Hunter 'L' and 'b' values were observed after 4 and 2 days, respectively, on treated fillets. Thiobarbituric acid oxidation values (TBARs) increased for all treated fish after 2 days storage. Gapping was not affected by treatment and storage time. Decreased water holding capacity and moisture were observed for all catfish fillets during storage time.

Annual (live weight) catfish sales have grown from 87 million kg to more than 270 million kg in 2000. The U.S. per capita consumption has more than doubled to 0.51 kg in 2000 (USDA, 2001) in the past 10 years. Most of the catfish produced is in the form of fillets (Silva and Dean, 2001). Fresh shelf-life is limited by initial microbial load, temperature, handling, and other factors. Due to increased distribution channels and further distances to markets, it is necessary to search for methods and procedures to increase product's shelf-life. Conventional methods of extending shelf-life of channel catfish such as ice and refrigeration may not be sufficient to reach further markets from the center of the source, the Mississippi Delta. Investigations on the use of modified atmospheres (Ashie et al., 1996; Silva and White, 1994), sanitizers and antimicrobials (Handumrongkul and Silva, 1994; Efiuvwevwere and Ajiboye, 1996), and super chilling(Ashie et al., 1996) have shown the possibility of extending shelf-life by 2X to 5X. However, some of these technologies pose safety concerns, use unapproved additives, or may not be economically feasible. Non-thermal processes (Barbosa-Canovas et al., 1998) are being explored to treat products for increased shelf-life, increased keeping quality, and safety. Amongst these is pulsed light. The Pure Bright(R) process (Barbosa-Canovas et al., 1998) utilizes flashes of intense broad-spectrum pulse light to sterilize or decrease microbial load in pharmaceuticals, medical devices, packaging, and water. This process utilizes flashes of very short duration (<1 ins) of light contained in the visible, UV, and JR spectrum.

The system delivers a spectrum of light much more intense than sunlight or UV alone on the earth's surface. Since it is short in duration, the product suffers little, if any, temperature increases, thus maintaining its "freshness."

The system offers other advantages such as being able to act on the surface of prepackaged products and avoiding post-process contamination. The process has proven to be effective against spore and non-spore formers and achieving many log reductions in "transparent" materials (Barbosa-Canovas et al., 1998; Stier, 2000). The objectives of this study were to evaluate initial and periodic quality and microbiological counts of pulsed-energy light-treated catfish fillets stored at 2[degrees]C.

MATERIALS AND METHODS

Preparation of Catfish fillets--Catfish fillets (fresh) were secured from a catfish processing plant (Macon, MS) during the summer of 1999. The fillets were placed on ice, transported, and packaged individually in sterile 532 mL Whirl-Pak sterile sampling bags (Fisher Scientific, Pittsburgh, PA). The fillets were placed in ice chests with ice and sent overnight to Pure Pulse (San Diego, CA) for treatment. Upon receipt the next day, fillets were exposed to 0.25 and 0.5 J/[cm.sup.2] per flash using 2 and 4 flashes at each energy level. The product was then placed in ice chests and shipped on ice overnight to MSU. Upon receipt (2nd day of life of product), fillets were analyzed. Fillets were held at 2[degrees]C and analyzed after 2, 4, 6, 9, and 13 days of processing.

Psychrotropic plate counts--A 25 g sample of aseptically cut catfish fillet was blended with 225 mL of 0.1 % wt/vol. sterilized Bacto [R] -peptone solution (DIFCO, Detroit, MI) during 1 min on Stomacher bags (model 400 closure bags 6041/CLR, Seward Limited, London, U.K.). A Stomacher 400 (Tekmar [R], Cincinnati, OH) was used for the preparation of the samples.

Serial dilutions were made by placing 1 mL of the homogenate into dilution tubes with 0.1% peptone solution. Appropriate dilutions were plated (duplicates) onto Petrifilm [TM] Aerobic Count (APC) plates (The 3M Corp., Minneapolis, MN) in duplicate. The plates were incubated at 20[degrees]C for 72-96 h, and all colonies were enumerated (APHA. 1992). These were done throughout the life of the fish fillets.

Color measurement--A Hunter Labscan 6000/45[degrees] spectrocolorimeter (Hunter Associates Laboratory, Fairfax, VA) was used to evaluate the color. Three measures were made on the anterior, middle, and posterior part of the skin side of a catfish fillet for the color reading. The fillet was placed on a 13 mm port and the data gathered with a Universal Software Version 3.1. Hunter color was expressed as 'L' (lightness) where 100 represent lightness and 0 darkness, 'a' (redness +, greenness -), and 'b' (yellowness +, blueness -). Hue was calculated as [tan.sup.-1] (b/a).

Determination of thiobarbituric acid reactive substances (TBARs)--TBARs were determined using a distillation method as described by Tarladgis and Watts (1960). Ten grams of each sample of fillet were placed in a 800 mL Kjeldahl flask containing 97.5 mL distilled water and 2.5 mL 4 N HCl. One or two drops of antifoam (Fisher Scientific) and two to three glass beads were added. The flasks were then connected to a Kjeldahl distillation apparatus and 50 mL of distillate were collected. Five milliliters of distillate was transferred to a test tube containing 4 mL TBA reagent. The solutions were mixed and the test tube was stoppered before being placed in a boiling water bath for 30 mm. After cooling, a Coleman 101 Spectrophotometer (Coleman Instruments, Maywood, IL) was used to measure the absorbance of the mixture at a wavelength of 538 nm, after collection with a reagent blank. The absorbance was multiplied by a constant of 7.8 to express TBARs as mg malonaldehyde per 1000 g of fillet (Guillen-Sans and Guzman-Ch ozas, 1998).

Gapping--Fish fillets are often produced in which the muscle flakes, called myotomes, separate, creating the effect called "gapping." Gapping appears to be the result of the breakdown of connective tissue protein, stroma and related components which hold the fish muscle together. The gapping score of the fillets was recorded according to the method of Love et al. (1972). using a 5-point scoring system: 0 (no gaping or longitudinal splitting) to 5 (dropping to pieces).

Water holding capacity--Ground meat from fillet samples (10 g) was placed in 50 mL screw-capped Fisherbrand [R] polypropylene centrifuge bottles (Fisher Scientific), tightly sealed, and weighed. Samples were heated for 30 mm in a 70[degrees]C water bath. The cooked samples were allowed to cool at ambient temperature and reweighed to measure any evaporative loss. Bottles were then centrifuged at 1300 g (CU-500 centrifuge, International Equipment Co., Needham Heights, MA) for 15 mm. The supernatant was withdrawn using a pasteur pipette and weighed. Moisture content of the raw sample and the supernatant was determined by drying 2 to 3 g of sample in a 121[degrees]C oven (Fisher Isotemp [R] Oven 300 Series Model 318F. Fischer Scientific Co. Houston, TX) for two hours, and weight and moisture content were calculated after drying (Trout, 1988). The WHC was calculated by the following formula:

WHC = % Shrinkage = (Wt before centrifugation - Wt after centrifugation/Wt before centrifugation) x 100

Moisture content--Two grams of fish were shredded by hand, placed on a pre-weighed, pre-dried aluminum dish and placed on an oven at 100[degrees]C (Fisher Scientific Isotemp [R] oven, Model 318F) and let dry for 16 h. The percent moisture was calculated using the following equation:

% moisture = (IW - FW)/IW x 100

Where IW = Initial weight (g) and FW = Final weight (g).

Experimental analysis and statistical design--Data were arranged in a 5 (treatments) x 6 (storage time dates) factorial with 3 replications arranged in a completely randomized design. Analysis of data was performed using PROC GLM (SAS, 1996). Whenever significant (p [less than or equal to] 0.05) mean differences were separated using Fisher's Protected LSD (Steel et al., 1996).

RESULTS AND DISCUSSION

Treated samples had a longer lag phase, being 1.5-2.0 log CFU/g less than untreated samples by the ninth day (Fig 1). Huang and Toledo (1982) reported 2.6 log reduction in PPC of Spanish mackerel skin by subjecting to UV light (120-180 mW/c[m.sup.2]) at 253.7 nm. However, their maximum reduction in croaker and mullet were of 1.06 and 1.13 log CFU/c[m.sup.2]. Dunn et al. (1988) reported a 3 log CFU/g reduction in PPC and coliforms on fish subjected to high-pressure wash prior to pulsed-light. MacGregor et al. (1998) reported that pulsed light emissions could significantly reduce populations of Esherichia coli O157:H7 and Listeria monocytogenes on exposed surfaces with exposure times that are 4-6 orders of magnitude lower than those required using continuous UV light sources. The PPC were similar for all samples by the thirteenth day of storage. Since pulsed-light is a surface treatment, the process is difficult in fish fillets due to their irregular surface and the ability of bacteria to adhere and "hide" on t he muscular structure (Kim et al., 2000).

Hunter 'L' (brightness) values increased after 2-4 days of treatment, decreased by the sixth day, and remained steady thereafter for all treatments (Fig. 2). Fish treated for a total of 2.0 J/c[m.sup.2] had higher (P [less than or equal to] 0.05) 'L' values than untreated samples (Fig. 2). Hunter 'a' values fluctuated, but were similar (P [less than or equal to] 0.05) to initial values, regardless of treatment (Fig. 3). Hunter 'b' (yellowness) values increased slightly (P [less than or equal to] 0.05) initially but remained steady thereafter regardless of treatment (Fig. 4). These color data show that only whiteness was affected positively by the high dose treatment, and that changes in hue (pigmentation changes, oxidation) were not affected by treatment or storage (Fig. 5). Initial moisture, water-holding capacity (WHC), and gapping scores were not affected by treatments (Data not shown). This shows that pulsed-light flashes of up to 0.50 J/c[m.sup.2] and up to four flashes over ([less than or equal to]2.0 J /c[m.sup.2]) did not change the characteristics of "freshness" of catfish fillets. However, moisture decreased with storage and WHC increased until 6 days and decreased thereafter (Table 1). Moisture migration may be remediated with better packaging while reduced WHC is the result of protein denaturation and enzymatic breakdown (Ashie et al., 1996).

Table 1

Moisture, water holding capacity, and gapping score of catfish
fillets stored at 2[degrees]C.

Storage time   Moisture  Gapping  Water holding
(days)          (%)      score    capacity (%)

0              81.6 a    0.0 a       10.8 d
2              76.6 bc   0.0 a       14.3 c
4              77.5 b    0.0 a       18.3 b
6              73.6 c    2.3 b       22.7 a
9              79.4b a   2.3 b       20.5 ab
13             77.0b c   2.5 b       19.2 b

LSD(0.05)       3.5      0.25         3.3

abc--Means within a column not followed by same letter differ (p [less
than or equal to 0.05)

Oxidative rancidity (TBARs) increased after two days for all treated fish as compared to the untreated samples (p [less than or equal to] 0.05), but were not different thereafter (Fig. 6). The TBARs levels did not reach anywhere near levels of rancidity for seafood (Sinnhuber and Yu, 1958).

Further research is needed to ascertain if higher doses or number of flashes is effective in enhancing shelf-life of catfish fillets. Nonetheless, this process proved effective in maintaining "freshness" of catfish fillets. Furthermore, since this can be used in prepackaged product, this process may be effective in killing pathogens in individually packaged fillets, especially those that are precooked.

CONCLUSIONS

This study indicated that pulsed light was effective in reducing microbial flora on catfish fillets and did not affect gapping score, initial moisture, and WHC in fillets. Slight changes of TBARs, Hunter 'L,' 'a,' and 'b' values were observed for treated fillets during the initial 2-3 days.

Therefore, pulsed light may have potential as a catfish fillet sanitizer. However, pulsed light may not be enough for eliminating bacteria attached or hidden on the fish surface. For further research, pulsed light may be used in combination with other alternative processing technologies, including various powerful oxidizing agents such as ozone, hydrogen peroxide, and peroxiacetic acid.

ACKNOWLEDGMENTS

Approved for publication as Journal Article No. J-10005 of the Mississippi Agricultural and Forestry Experiment Station, Mississippi State University. Research funded in part by the Mississippi Agricultural and Forestry Experiment Station Project and by USDA-ARS project No. 320329-081100. The authors acknolowledge the assistance of Maxwell Technologies (San Diego, CA).

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(1, 2.) Department of Food Science and Technology, Mississippi State University, Mississippi State, MS 39762,

(3.) Maxwell Technologies, San Diego, CA

(1.) Author for correspondence. [e-mail]

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