A method for processing prawns

Abstract: Fresh whole prawns are dipped in a solution of sodium metabisulphite and then sorted and graded. The prawns are then arranged in an open-topped container in a generally linear pattern with their shells uppermost. The graded prawns are blast frozen and a first glazing liquid is applied to achieve a weight ratio of glazing liquid to prawns of approximately 1:8. After freezing for a period of at least 8 hours, the frozen prawns are glazed a second time by spraying a second glazing liquid onto the surface of the frozen prawns. The glazing liquids in one or both glazing steps may contain a polyphosphate based glazing compound.

Description: The invention relates to a method for processing prawns and in particular for processing fresh whole prawns. FR-A-2 286 606 discloses a process for conservation of crustaces comprising contacting the crustaces to metabisulfite and producing a single glazing layer. US-A- 2 723 645 discloses a process for glazing pieces of meat with two glazing layers. According to the invention, there is provided a method for processing prawns comprising the steps of:- dipping fresh whole prawns in a preservative liquid; sorting and grading the dipped prawns; arranging prawns of a particular grade in an open-topped container; blast freezing the graded whole prawns in the open-topped container; first glazing the blast frozen prawns by spraying a first glazing liquid at a temperature of from -3°C to +3°C onto the surface of the prawns in the open-topped container; freezing the part-glazed prawns; second glazing the frozen part-glazed prawns by spraying a second glazing liquid at a temperature of from -3°C to +3°C onto the surface of the frozen part-glazed prawns in the open-topped container; and packaging the container of frozen glazed prawns. In a particularly preferred embodiment of the invention, the glazing liquid in the first glazing step is delivered onto the prawns to achieve a weight ratio of from 0.5:8 to 1.5:8 of first glazing liquid to whole prawns. Most preferably, the first glazing liquid is delivered onto the prawns to achieve a weight ratio of approximately 1:8. In a preferred arrangement, the second glazing liquid in the second glazing step is delivered onto the part glazed prawns to achieve a weight ratio of from 0.5:8 to 1.5:8 of second glazing liquid to whole prawns. Most preferably, the second glazing liquid is delivered onto the part glazed prawns to achieve a weight ratio of approximately 1:8. In a particularly preferred embodiment of the invention, at least one of the glazing liquids includes a polyphosphate-based glazing compound. In this case, preferably the glazing compound is present in the glazing liquid in an amount of from 0.1% to 0.3% by weight, most preferably approximately 0.2% by weight. In a preferred embodiment of the invention, both glazing liquids include a polyphosphate-based glazing compound. In a preferred embodiment of the invention, the part glazed prawns are frozen at a temperature of approximately -20° for a period of at least 8 hours prior to second glazing. Preferably, the prawns are dipped in the preservative for a period of from 7 to 10 minutes. In a further embodiment of the invention, the preservative is sodium metabisulphite. In this case, the preservative is present in a dipping liquid in an amount of from 2% to 3% by weight of preservative per volume of dipping liquid, most preferably approximately 2.5% w/v. In one embodiment of the invention, the part glazed prawns are blast frozen in a tunnel freezer at a temperature of no more than -65°C for a period of at least 10 minutes. In a particularly preferred embodiment of the invention, the prawns are arranged in the open topped container in a generally linear pattern. In this case, most preferably the container includes a plurality of generally parallel rows of linearly arranged whole prawns. Typically, the frozen glazed prawns are packaged by covering the open-top of the container with a transparent packaging film or a cardboard lid. The invention also provides prawns whenever processed by the method of the invention. The invention will be more clearly understood from the following description thereof given by way of example only. In the method for processing prawns according to the invention, fresh whole prawns are first transferred from fish boxes into dipping baskets which are then dipped in a preservative liquid. The preservative liquid is typically formed by dissolving sodium metabisulphite in water to achieve a concentration of typically 2.5% weight by volume of sodium metabisulphite to water. The prawns are dipped in the preservative liquid thus formed for a period of typically from 7 to 10 minutes to achieve a level of SO 4 of approximately 800 parts per million. The prawns are removed from the dipping baskets and then sorted and graded into a number of different grades such as the following:- Grade Number of Prawns per Kg 1 4-7 2 8-12 3 13-16 4 17-20 5 21-30 6 31-40 7 41-50 8 51-60 9 61-80 After grading, the prawns of a particular grade are arranged in an open-topped container in a generally linear pattern. The prawns are not curled up into a spiral-type shape but are rather laid end-to-end in substantially generally parallel rows of linearly arranged whole prawns which are arranged with their shells uppermost. The graded whole prawns are then blast frozen by passing the open-topped containers containing the prawns through a tunnel freezer at a temperature of -65°C, for a period of at least 10 minutes, preferably 12 minutes. After blast freezing, the blast frozen prawns are glazed with a first glazing liquid in a first glazing step. The first glazing is achieved by passing the open-top containers containing the blast frozen prawns under two banks each containing double spray nozzles which spray a first glazing liquid onto the surface of the blast frozen prawns in the open-topped container. The glazing liquid is at a temperature of from -3° to +3°C, preferably approximately 1°C and is sprayed at a rate to achieve a weight ratio of first glazing liquid to whole prawns of from 0.5:8 to 1.5:8, most preferably approximately 1:8. The part glazed prawns from the first glazing step are then frozen in a freezer at a temperature of not less than -20°C for a period for at least 8 hours, most preferably from 12 to 24 hours. After freezing, the frozen part glazed prawns are removed from the freezer and are glazed again in a second glazing step by spraying a second glazing liquid onto the surface of the frozen part-glazed prawns in the open-topped container again from a double bank of twin head spray nozzles. The second glazing liquid is also sprayed at a temperature of from -3°C to +3°C, most preferably approximately +1°C. The second glazing liquid is delivered onto the frozen part glazed prawns to achieve a weight ratio of from 0.5:8 to 1.5:8 of second glazing liquid to whole prawns and most preferably at a rate to achieve a weight ratio of approximately 1:8. To achieve a particularly transparent glaze the glazing liquid used in one or preferably both glazing steps is substantially demineralised. For example, the glazing liquid is preferably filtered to remove minerals from the liquid. After glazing, the container of frozen glazed prawns is packaged, typically by covering the open-topped container with a food grade transparent packaging film or a cardboard lid. Preferably, at least one and ideally both of the glazing liquids contain a glazing compound, especially sodium potassium polyphosphates in an amount of 0.1% to 0.3%, and most preferably approximately 0.2% by weight of the glazing liquid. One such glazing compound is available under the Trade Name Ifisol. Preferably, the glazing compound is introduced into a tank containing the glazing liquid and the sprayed glazing liquid which does not impinge on the open-topped container containing the blast frozen prawns is recycled. We have found that by glazing the prawns in two separate stages with an intermediate freezing stage, a highly glazed finished packaged product is achieved. This not only adds to the appearance of the finished product but also, particularly in association with the glazing compound, results in reduced cook-out losses with a more stable product. The prawns processed in this way have a reduced potential to desiccate.

Claims: A method for processing prawns comprising the steps of:- dipping fresh whole prawns in a preservative liquid; sorting and grading the dipped prawns; arranging prawns of a particular grade in an open-topped container; blast freezing the graded whole prawns in the open-topped container; first glazing the blast frozen prawns by spraying a first glazing liquid at a temperature of from -3°C to +3°C onto the surface of the prawns in the open-topped container; freezing the part-glazed prawns; second glazing the frozen part-glazed prawns by spraying a second glazing liquid at a temperature of from -3°C to +3°C onto the surface of the frozen part-glazed prawns in the open-topped container; and packaging the container of frozen glazed prawns. A method as claimed in claim 1 wherein the glazing liquid in the first glazing step is delivered onto the prawns to achieve a weight ratio of from 0.5:8 to 1.5:8 of first glazing liquid to whole prawns. A method as claimed in claim 2 wherein the first glazing liquid is delivered onto the prawns to achieve a weight ratio of approximately 1:8. A method as claimed in any of claims 1 to 3 wherein the glazing liquid in the second glazing step is delivered onto the part glazed prawns to achieve a weight ratio of from 0.5:8 to 1.5:8 of second glazing liquid to whole prawns. A method as claimed in claim 4 wherein the second glazing liquid is delivered onto the part glazed prawns to achieve a weight ratio of approximately 1:8. A method as claimed in any preceding claim wherein at least one of the glazing liquids includes a polyphosphate-based glazing compound. A method as claimed in claim 6 wherein the glazing compound is present in the glazing liquid in an amount of from 0.1% to 0.3% by weight. A method as claimed in claim 7 wherein the glazing compound is present in an amount of approximately 0.2% by weight. A method as claimed in any of claims 6 to 8 wherein both glazing liquids include a polyphosphate-based glazing compound. A method as claimed in any preceding claim wherein the part glazed prawns are frozen at a temperature of approximately -20° for a period of at least 8 hours prior to second glazing. A method as claimed in any preceding claim wherein the prawns are dipped in the preservative for a period of from 7 to 10 minutes. A method as claimed in any preceding claim wherein the preservative is sodium metabisulphite. A method as claimed in claim 12 wherein the preservative is present in a dipping liquid in an amount of from 2% to 3% by weight of preservative per volume of dipping liquid. A method as claimed in claim 13 wherein the preservative is present in an amount of approximately 2.5% w/v. A method as claimed in any preceding claim wherein the part glazed prawns are blast frozen in a tunnel freezer at a temperature of no more than -60°C for a period of at least 10 minutes. A method as claimed in any preceding claim wherein the prawns are arranged in the open topped container in a generally linear pattern. A method as claimed in claim 16 wherein the container includes a plurality of generally parallel rows of linearly arranged whole prawns. A method as claimed in any preceding claim wherein the frozen glazed prawns are packaged by covering the open-top of the container with a transparent packaging film.

Inventors: SCALLAN ALEC, SCALLAN, ALEC
Applicants: HELIX PARK FARM LTD, HELIX PARK FARM LIMITED
Application year: 1996
Main classification: A23B4
IPC codes: A23B4