Increased Shelf-Life for Refrigerated Fish

Biological spoilage has always been a problem with gathering and transporting food. The gathering of various foods and the actual marketing of the foods may be separated by days or weeks. One of the particularly difficult foods for preservation is fresh fish fillets. Fresh fish is becoming a more popular consumer item. At present, fish are caught in the ocean and iced or immediately frozen for preservation. Upon delivery to retailers, fish shelf-life is extremely short. Not only is there loss due to spoilage but fish lose weight yield due to drip loss. Fresh fish fillets usually maintain a quality shelf-life of only a few days after cut. Methods are needed to extend the shelf-life both for the benefit of the seller and the buyer. Antibiotics such as chlortetracycline have been used to extend the shelf-life of fish fillets but are no longer allowable in the United States. A blend of fumaric acid and benzoate is available but the composition was not particularly effective in a recent study.

The preservation of edible protein as well as yield enhancement is a well studied art which has provided numerous treatment compositions. Each type of protein product, i.e. poultry, fresh meat, eggs, cheese, sausage, shell fish, frozen shrimp and fish, has peculiar biochemical compositions with different types of bacteriological problems, taste problems, and deterioration problems. There are available numerous compositions for increasing preservation and yield which at first blush appear to be similar. However, these compositions are generally useful for only one type of protein. Such compositions are commercially or technically unsuccessful or unacceptable when applied to other types of proteins. The transfer of technology from these areas to fish fillet treatment is seldom possible. For this reason, fish fillets are particularly difficult to preserve and to maintain yield. One of the problems encountered in fish fillet preservation is pH change due to bacterial growth. Since aerobic bacterial growth is prevalent during iced storage, the pH of the fillet tends to rise. This contributes to bacterial growth and flavor problems. A preservative system must counteract both bacterial growth problems and organoleptic problems. A composition for preservation of shell fish is not per se operable with fish fillet. This is particularly true in the case of fresh fish which are not frozen to increase storage stability. Further, food producers, being subject to various labeling and regulatory restrictions as well as customer reaction, desire to avoid any hint of these problems if possible. The more and varied the ingredients in a composition, the greater the risk of encountering a labeling or regulatory problem. A composition with fewer ingredients can provide the food processor with a clearer labeling requirement picture than a composition of many ingredients.

The problem of increasing the shelf-life of fish fillets has been outlined in a patent to Crawford, U.S. Pat. No. 4,431,679, issued Feb. 14, 1984. This patent teaches that fish fillets can be stabilized by a mixture containing sodium tripolyphosphate (STP), sodium hexametaphosphate, and optionally sodium pyrophosphate in combination with from 0.1 to 0.2 part potassium sorbate per part of phosphates (9-16%) of the total composition and citric acid in an amount sufficient to provide a pH within the range of 5.6-6.5. This product is apparently embodied in a commercial product FISH-PLUS.TM. fish preservative. It would be desirable to achieve a formulation in which the level of sorbate can be reduced as this material is presently many times as expensive as the next most expensive ingredient, citric acid. It would also be desirable to achieve a formulation which did not contain citric acid as this is also an expensive chemical which can add an undesirable taste to the fish fillet.

Shimp, U.S. Pat. No. 4,394,396, issued July 17 1983. shows a dip used for retaining the natural appearance and preventing weight loss of thawed or cooked shrimp. The dip is composed of an 80:20 to 60:40 ratio of sodium tripolyphosphate to sodium acid pyrophosphate. The pH of the dip is 6.2-7.1. As stated by Crawford, fish fillet are unique from shell fish, and formulations beneficial to one are not necessarily beneficial to the other.

Kohl, U.S. Pat. No. 3,681,091 shows that 1 percent sodium polyphosphate having an average chain length of 18 was effective in preventing propagation of inoculated S. aureus for 3 days in a liquid test media containing ground fish fillet (Example 17). Kohl also discloses blends of polyphosphates having an average chain length of from about 16 to about 100 and an alkali metal sorbate as providing better taste than a preserving amount of sorbate alone (Example 27 ). Kohl discloses that sodium acid pyrophosphate gave no significant microbial growth inhibitory effect when used alone, and a slight inhibitory effect was shown using a 50:50 blend of sodium acid pyrophosphate and tetrasodium pyrophosphate. Kohl does not show the use of a composition of sodium acid pyrophosphate in combination with sodium hexametaphosphate and a sorbate. The preferred levels of sorbate to polyphosphate of 2:1 to 1:2 in Kohl is an undesirably high level of sorbate. Kohl teaches that the presence of pulpy material may contribute to the breakdown of polyphosphate influence over the performance of the additives.

In accordance with the present invention it has been found that fish fillet can be storage stabilized and yield stabilized without citric acid and the flavor problems attendant with its use by means of a composition containing an alkali metal hexametaphosphate, an alkali metal acid pyrophosphate and a sorbate antimicrobial agent. The detailed amounts and methods of applying the material will become more apparent from the following description.

The compositions of the present invention can stabilize fish fillets at sorbate levels significantly less than required by prior art compositions and without citric acid. The composition of the invention provides effective bacterial, flavor, texture and appearance control at lower cost over that of the compositions of the prior art.

The alkali metal hexametaphosphate usable in the present invention is a medium chain length polyphosphate of the formula: ##STR1## wherein X represents hydrogen or an alkali metal and Y represents an alkali metal, alkali metal being illustrated by potassium and preferably sodium, and N.sub.ave represents an average chain length ranging from about 8 to about 100, and preferably from about 8 to about 37, and more preferably from about 8 to about 5. The alkali metal hexametaphosphate is used in an amount ranging from about 75 percent to about 90 percent based on the total dry weight of the composition. The high SHMP content is important to the more efficient shelf-life extension provided by the invention. The high SHMP content in the compositions of the present invention is considered to achieve an efficient synergism with sorbate that STP-sorbate blends cannot achieve.

The composition of the present invention also contains an alkali metal acid pyrophosphate (an acidic compound as distinguished from the alkaline sodium pyrophosphate also known as tetrasodium pyrophosphate). The alkali metal acid pyrophosphate can be illustrated by sodium acid pyrophosphate and potassium acid pyrophosphate or mixtures thereof. preferably, the sodium acid pyrophosphate is used though the potassium can be used if a low sodium diet is needed.

The sodium acid pyrophosphate is used in an amount sufficient to provide a treatment solution having an acid pH sufficient for effective sorbate use and preferably within the range of from about 5.5 to about 6.5. Preferably the pH is adjusted to a level approximately the natural pH of the treated substrate. The alkali metal acid pyrophosphate is preferably used in an amount ranging from about 9 percent to about 11 percent.

Also included in the composition of the invention is a sorbate food grade antimicrobial agent. Included within this class are sorbic acid and alkali metal sorbates such as sodium and, preferably, potassium sorbates and mixtures thereof.

The sorbate antimicrobial agent is used in an amount sufficient with the sodium hexametaphosphate and sodium acid pyrophosphate to extend the shelf-life of the substrate over a control. The preferred sorbate is preferably used in an amount ranging from about 4 to about 8.5 percent based on the weight of the total composition.

The composition of the present invention can be prepared by dry blending the ingredients or by mixing the ingredients and drying them as desired. This dried composition can be stored until such time as needed for use. Also, the composition can be formed in situ in the treatment tank. The treatment solution can contain any other functional or functionally inert ingredients which have no detrimental effect on the functioning of the treatment solution of the invention. Such materials may include water, fats, oils, color, flavoring and the like. The preferred vehicle is water.

The composition of the invention is used in amounts sufficient to provide a solution having a solids content of the phosphate and sorbate composition ranging from 3 to 20 percent and preferably from about 8 to about 12 percent by weight based on the total weight of the solution, e.g. solvent and the phosphate and sorbate ingredients. The solution is in such concentration that the fish will pick up a sufficient amount of solution to provide the level of shelf-life control desired.

The solution can be applied to the surface of the fish fillet by conventional means including spraying, dipping and flooding or to the interior of the fillet by injection with a solution containing an effective amount of the treatment material composition or a combination of injection and surface treatment. The fillets can be flexed in the solution to improve absorption of the treating solution. The solution is applied to the fillet such that an amount effective to preserve and stabilize the fillet is absorbed. Dipping time is sufficient to cause fish take-up of such an effective amount and can be as short as 10 seconds to hours or longer. The time is reduced by flexing the fillet. Exact times may vary depending on the size of the fillet and species of fish. Concentrations and absorbence rates can be adjusted to meet any governmental requirements or limits.

The invention is illustrated in the Examples which follow. As used in the Examples, the sodium hexametaphosphate had an average chain length of phosphate groups of 12. The best raw appearance rank is 1. Flavor and texture were judged on a hedonic scale of 1 to 9 where 9 is the best unless otherwise stated. Throughout the Examples, the compositions denoted SHMP/SAPP/SORB are the same formulations as in Example I. All percentages are by weight unless otherwise stated.

The following compositions will be used in the Examples unless otherwise indicated. The reference to the % sorbate is intended to mean a formulation as outlined in the Table below:

In these Examples, taste test panel results were conducted on samples which were battered with a commercial mix (MODERN MAID All Purpose Batter) and breaded with a commercial breading mix (REDI-BREADER). The breaded fillets were fried at 176.7.degree. C. (350.degree. F.) for 3.5 minutes on one side and 2.5 minutes on the other. Corresponding portions of each fillet were evaluated for preference by the tasters.

Tasting was done by company personnel and part-time employees who are students in consumer type testing. Only a relatively small number of tasters was involved (4-8). The confidence limits for the data were not expected to be conclusive.

Standard plate counts were determined by an outside laboratory. One hundred gram fillet samples were blended with sterile water and 10 gram portions were diluted with phosphate buffer, pH 7.2. Blender jars were sterilized between samples with alcohol. Microbial incubation was conducted at 32.degree. C. for 48 hours using AOAC techniques. Plate counts were reported as the numerical values or as logarithms in the Tables.

Whole catfish were purchased and filleted locally. The fillets were preweighed. Treating compositions were prepared by dry blending the ingredients as set forth in TABLE II and were dissolved at a 10 percent concentration. Fillets were flexed with 10 percent of treatment solution, based on the weight of the fillets, for 1 minute each, drained and weighed. Flexing was accomplished by forcing each side of the fillets to follow the contours of a bowl containing the treating solution. The fillets were placed in tightly closed polyethylene bags and stored on ice in an insulated box in a 3.3.degree. C. room. The controls were placed in bags after treatment with water only. The samples were evaluated for yield, odor, appearance and flavor as well as bacterial count.

The data in TABLE II shows that after 5 days storage sorbate levels of 4.3 percent or greater provided appreciable microbial inhibition compared to the water control and a treating solution of SHMP/SAPP containing no sorbate. Odor ratings were also improved using sorbate, particularly in the compositions of the invention as compared to the commercial composition. After 13 days, those samples having 11.4 percent and 12.7 percent sorbate showed the most accepted flavor. Flavor results to determine acceptability of the treatment at lower sorbate levels for shorter storage periods of time were not obtained. The microbial data suggest that lower sorbate lewels can be used in the compositions of the invention to significantly inhibit deterioration.

Flounder were treated in this Example using the same agents as in Example 1 . The agents containing different sorbate levels again inhibited microbial growth compared to water control (TABLE III) but not compared to SHMP/SAPP without sorbate. The taste panel results in TABLE IV showed significant improvement over the control over time by the inclusion of even low levels of sorbate with the phosphate. The SHMP/SAPP blend containing 4.3 percent sorbate provided good organoleptic ratings through 9 days storage and ratings which were nearly as high as the commercial product containing 12.7 percent sorbate. This result is surprising since the patent relating to the commercial product (U.S. Pat. No. 4,431,679) specifies that a minimum of 9.1 percent sorbate is required for significant shelf-life extension in a blend containing STP, SHMP, citric acid and sorbate.

Ocean catfish and cod were treated with the agents in TABLE I dissolved at 10 percent concentration. The fillets were flexed in treatment solution as in Example 2. One set of each species was injected using a Townsend.TM. mini-injector.

Fillets were cooked in covered foil pans in a 400.degree. F. oven for 10 minutes. The results of evaluation of the samples are shown in TABLES V and VI:

The microbial growth data of TABLE V showed all treating agents inhibited growth in the two species compared to their respective water controls. The microbial inhibition data were too variable to significantly indicate differences among sorbate levels.

Catfish exhibited flavor ratings for the SHMP/SAPP/SORB samples that were similar to each other at all storage periods. Samples of U.S. Pat. No. 4,431,679 rated higher at Day 2 and lower at Day 9 than those treated using the treatment agents of the invention. Control samples rated low at all storage times.

Cod ratings did not vary much with treatment on Days 2 and 9. However, on Day 6 the 0 percent and 4.3 percent sorbate levels rated lower. The odor ratings for these samples were similar to those obtained by higher sorbate level sample and were higher than controls for all storage periods.

Overall, the taste panel evaluations for the products of the invention in Example 3 show that SHMP/SAPP containing lower sorbate levels provided flavor, texture and odor ratings similar to U.S. Pat. No. 4,431,679 containing 9.9 percent sorbate and ratings that were higher than controls. Microbial inhibition data show the same results. These results show the same patterns as those in the previous runs and confirm that 4.3 percent--8.2 percent sorbate levels are sufficient for providing 3-6 day shelf-life extension in the present invention, whereas U.S. Pat. No. 4,431,679 requires a minimum of 9.1 percent sorbate for significant shelf-life extension. SHMP/SAPP containing 6.3 percent sorbate is recommended as the preferred composition in the present invention. U.S. Pat. No. 4,431,679 shows a preferred composition for FISH-PLUS.TM. which contains 11.5 percent sorbate.

A blend of SHMP/SAPP/SORB containing 83.7 percent SHMP, 10 percent SAPP and 6.3 percent sorbate was dissolved in cold water at a 10 percent concentration. Fifty pound batches of 5-8 ounce catfish fillets were pre-weighed and added to a BAADER Batch Treatment Machine (BTM). Treatment solution was added at 7 percent by weight of the fillets and the batch was agitated in the BTM for 1 minute. Controls were agitated with water. Treated fillets were drained, weighed and packaged in 1 pound styrofoam trays. The trays were packed with ice, held either at 37.degree. F., stored on ice or frozen. They were evaluated periodically for weight changes, microbial counts and organoleptic changes.

For tasting, fillets were battered and breaded, fried in 176.7.degree. C. (350.degree. F.) oil for 6 minutes and rated in blind testing.

Control sample counts showed generally higher microbial counts over time than sample counts stored at 0.degree. C. (32.degree. F.) and 2.8.degree. C. (37.degree. F.).

Flavor results at 0.degree. C. (32.degree. F.) (TABLE IX) inversely parallel the microbial results. Decreases in flavor ratings with storage time generally correspond to microbial increases with storage time. The blend of the invention maintained acceptable flavors for 21days. whereas control samples became unacceptable (ratings below 5) after 12 days Flavor ratings at 2.8.degree. C. (37.degree. F.) showed similar trends and indicated a shelf-life extension for the invention of about 4 days.

Odor scores for raw samples reflect the flavor scores. Panelists generally rated odor "neither like nor dislike" until near the end of the shelf-life period when some unpleasant odors became apparent. In this regard, an odor rating of "3" corresponded to a flavor rating of "5", and indicated the end of shelf-life.

Overall, the results show that catfish are readily treated with compositions of the invention under plant conditions using a commercially available mechanical unit. The yields and shelf-life extension achieved are consistent with previous studies and indicate that the process is practical and economically attractive for commercial use.

Whole catfish were processed as in Example 1. Fillets from each fish were distributed among various treatments. Treating compositions as in TABLE X were dissolved at an 8 percent concentration. Four fillets were flexed with 7 percent by weight based on the weight of the fillets of treatment solution. The controls were placed in bags without treatment. The samples were evaluated for odor, appearance and flavor after 5 and 7 days storage.

Catfish fillets (5 per treatment) were weighed and labeled. The fillets were flexed for 1 minute each with 10 percent by weight of the fillets of treatment solution. The fillets were drained for 30 minutes, reweighed, packaged in tightly sealed polybags and stored over ice in an insulated box in a 3.3.degree. C. room. The amount of solution absorbed was determined based on initial and drained weights; values were reported as percent yield based on the initial weight as 100 percent. Flavor and texture evaluations were conducted after 5 and 8 days. The results are reported in TABLE XII.

SHMP/SAPP/SORB composition provided the best raw appearance. Yields and plate counts are substantially equivalent for the phosphate/sorbate samples and are higher than controls; and 8 day texture/taste panel results are improved for SHMP/SAPP/SORB over FISH-PLUS.TM. fish preservative in the absence of citric acid.

Catfish were treated as in Example 2 except that the treating agents were dissolved to a 10 percent solids solution.

Cod were treated as in Example 7 using the agents set forth in TABLE XIV with the following results:

Weighed freshly cut catfish fillets were flexed with 10 percent weight of fillets treatment solutions having 10 percent concentration of the ingredients listed in TABLE XIV at a commercial plant. The fillets were flexed for 1 minute in a rotating drum containing treatment solution, drained for 5 minutes and reweighed. Fourteen (14) sets of two treated fillets were placed on styrofoam trays and held on ice until sealed with clear wrap. The trays were stored on ice until separated into iced and refrigerated samples the next days.

Four sets, identified herein as Group I, of each treatment were stored refrigerated and tested for odor, appearance and standard plate count changes. Four sets were held at 2.8.degree. C. (37.degree. F.) and four were held on ice, these being identified as Group II. Weight changes, organoleptic and standard plate count evaluations were made. Three sets were frozen to study thaw drip loss and organoleptic changes after extended storage.

Cod were treated in the same manner as Example 8. The following results were obtained.

Flounder fillets were cut from whole fish and treated as in Example 9 at a fish processor plant using the agents listed in TABLE XXII. The samples were tray packed and stored under refrigeration. They were evaluated for odor and appearance. The results are shown in TABLE XXII.

As can be seen from these Examples, the compositions of the present invention use large amounts of sodium hexametaphosphate and small amounts of sorbate while achieving effective fish fillet stabilization. Expensive acids such as citric acid need not be used as required by the prior art. The composition of the invention provides effective bacterial, texture and appearance control at lower cost over that of the compositions of the prior art.