PROCESS FOR PREPARING PORTIONS OF A FROZEN FOODSTUFF

Abstract: A process for preparing portions of a frozen foodstuff, comprising the steps of: i) partially freezing a foodstuff to achieve a mouldable consistency in a screw thread scraped surface heat exchanger; ii) dividing the foodstuff into portions; and iii) freezing the portions of foodstuff. The foodstuff may be a vegetable, or a sauce containing vegetable/meat/fish/poultry/pasta pieces.

Description: PROCESS FOR PREPARING PORTIONS OF A FROZEN FOODSTUFF FIELD OF THE INVENTION The present invention relates to a process for preparing portions of a frozen foodstuff. BACKGROUND ART Foodstuffs such as spinach are commonly sold in the form of large frozen blocks which are difficult for consumers to use if they only require a portion of the product . An existing method of forming portioned products involves using pressure to compress the foodstuff into channels; the foodstuff is frozen, thereby forming large frozen slabs, and cut, using a rotating saw, into frozen blocks weighinq 50 grams each, for example. The rotating saw creates gaps between the blocks by removing, and thereby wasting, part of the frozen foodstuff. Cream spinach, which is normally a blend of chopped or sieved spinach and cream, is also sold in the form of frozen blocks . These are formed by pouring the blend into the moulds of a Pello freezer (Registered Trade Mark, available from Frigoscandia) , which freezes the blend to form portions havinq a shape dictated by the mould. French patent no. 74 04593 describes a process for preparing cubes of frozen foodstuff, such as spinach; a mould comprising a plurality of cells in a honeycomb style is used to form the cubes. A bulky de-moulding machine is used to remove the cubes from the moulds . British patent no. 1016574 describes a process for the treatment of vegetables such as spinach, or fruit pulp, which includes the steps of pumping the chopped vegetable or the fruit pulp continuously through a cooled scraped surface heat exchanger which freezes sufficient water in the material to render it extrudable and thereafter subjecting the material to continuous extrusion. Foodstuffs such as spinach, cream spinach, cabbage etc comprise leaves or parts of leaves of the original vegetable. The prior art processes typically cause considerable damage to the leaves, resulting in a frozen product which, when thawed, has lost a lot of the leaf structure. It is an object of the present invention to provide a process for preparing portions of a frozen foodstuff, such as spinach, in which damage to particulates can be minimised. The inventors have discovered that a screw thread scraped surface heat exchanger can be advantageously used to partially freeze food stuff to achieve a mouldable consistency leading to surprisingly little damage to the particulates included in the foodstuff . SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a process for preparing portions of a frozen foodstuff, the process comprising the steps of: (i) partially freezing a foodstuff to achieve a mouldable consistency using a screw thread scraped surface heat exchanger; (ii) dividing the foodstuff into portions; and (iii)freezinq the portions of foodstuff. Preferably, the process is continuous. A gas such as air may be introduced prior to or during step (i) to aerate the foodstuff and thereby modify its texture. The foodstuff is one which can be partially frozen to achieve a mouldable consistency. When the foodstuff enters the scraped surface heat exchanger, it is in a flowable state, for example a liquid, puree, paste or mixture of liquid and particulates . It may comprise particulates . Preferably, it is spinach or pieces of meat, poultry, fish, vegetable, fruit and/or pasta in a sauce or paste-like sauce concentrate. For example, the foodstuff maybe leaf spinach (chopped or whole) , cream spinach, pasta in a cheese or tomato sauce or red cabbage and apple in a sauce . The present invention may be used to process foodstuffs comprising particulates having a maximum dimension of from 3 to 30mm. Hence the integrity of particulates such as pieces of herbs, vegetables, meat etc, including the structure of whole spinach leaves, may be substantially preserved. The particulates may be of any shape depending upon their source. For example, the particulates may comprise sections of leaf or leaves. In this case, the dimension mentioned above is a maximum dimension. In some cases, particulates such as leaves which are flat may become rolled during extrusion procedure. In this case, the maximum dimension of the rolled-up particulate is preferably in the range 3-30 mm. When required, the foodstuff can be mixed in between the partial freezing step (i) and the division step (ii) . The screw thread scraped surface heat exchanger may be a single or twin screw scraped surface heat exchanger. These types of heat exchanger are found to provide mild treatment of the foodstuff, resulting in little damage to any particulates present. Moreover, the heat transfer of these exchangers is efficient. In order to reduce damage to particulates in the foodstuff, it is beneficial to ensure that the residence time of the foodstuff in the scraped surface heat exchanger is controlled. Further, efficient heat transfer is required to ensure rapid freezing of the foodstuff and low shear must be applied to the foodstuff . Single and double thread scraped surface heat exchangers are found to provide all of these characteristics. The person skiled in the art will be able to control the residence time of food stuff in the screw scraped surface heat exchanger with reference to the screw geometry (in particular, number of thread starts, pitch of screw, depth of flights, space between flights and the walls of the heat exchanger) and rate of rotation of the screw. A twin screw scraped surface heat exchanger can provide a better control of feed rate of foodstuff and it can be used to apply more pressure to the foodstuff. However, a single screw scraped surface heat exchanger is preferred for simplicity. To achieve partial freezing such that the foodstuff has a mouldable consistency, the temperature of the foodstuff is lowered to below its freezing point. However, the temperature reached is a poor indicator of the required consistency as, ideally, the foodstuff need only be taken a part of the way through its latent heat zone . Since the consistency of the partially frozen foodstuff relates to the number of ice crystals present, the correct consistency for mouldability may be measured using a cone penetrometer . Cone penetrometers measure the distance that a cone, which is falling under gravity, penetrates a foodstuff within a controlled period of time . As measured by a cone penetrometer, the foodstuff preferably has a consistency after step (i) that allows a cone to penetrate it to a distance of from 5 to 34 mm; the most preferred penetration distances within this range will depend on the foodstuff being processed. The correct consistency for mouldability may also be measured usinq a strucktograph. Strucktographs measure the resistance force of a foodstuff by pushing the foodstuff against a spring-mounted cone. As measured by a strucktograph, the foodstuff preferably has a resistance force of from 0.1 to 4.5 N after step (i) ; more preferably, it has a resistance force of from 0.3 to 2.5 N after step (i) . The most preferred resistance forces within this range will depend on the foodstuff being processed. Details of the methods used to measure these penetration distances and resistance forces are qiven below. The optimum conditions for partially freezing a foodstuff to achieve a mouldable consistency are dependent on the foodstuff in question. The outlet of the screw thread scraped surface heat exchanger is preferably configured to allow onward feeding of the partially frozen foodstuff with minimum damage to particulates . The person skilled in the art will be able to provide a suitably designed heat exchanger outlet . A pressure gradient may be applied to the foodstuff by pressurising means upstream of the scraped surface heat exchanger. Preferably, however, the pressure provided by such pressurising means (the "over pressure") is relatively low, preferably not exceeding 1 x 10 Pa. Preferably, the motion of the screw of the screw scraped surface heat exchanger is sufficient to feed the foodstuff through the screw thread scraped heat exchanger without an additional pressure gradient. Preferably, the total pressure at the exit of the heat exchanger is preferably less than 4 x 10 5 Pa, more preferably less than 1 x 10 5 Pa. Preferably, the pressure drop between the inlet and the outlet of the screw thread heat exchanger is relatively low, being preferably about 1 x 10 Pa. Means may be provided for pressurising the partially frozen foodstuff downstream of the heat exchanger, but this is not preferred. The foodstuff fed to the screw thread scraped heat exchanger may comprise separate streams of different foodstuffs which are mixed in the extruder. Separate streams of different foodstuff may be dosed into the foodstuff at points along the screw thread scraped heat exchanger or at the outlet thereof . Delicate components such as paprika layers can be mixed in a nozzle. The foodstuff may be divided into portions in step (ii) by means of a cryogenic device; by passing it through an extrusion nozzle having a cutting device; or by placing it in the moulds of standard moulding equipment. In a preferred embodiment, the foodstuff is partially frozen in a screw thread scraped surface heat exchanger fitted with an extrusion nozzle and cutting device at its outlet . The portioned foodstuff is frozen either before or after placing the portions in the packs. For example, the foodstuff is portioned, frozen and then flow wrapped. A standard spiral or belt freezer, an air blast freezer or a plate freezer may be used to freeze the portions of foodstuff. A suitable cryogenic device for portioning the partially frozen foodstuff is a set of cryogenic blades . Cryogenic blades comprise a series of hollow blades having liquid nitrogen passing therethrough to lower their temperatures to less than or equal to -80°C, preferably to about -90°C. At these temperatures, the foodstuff does not adhere to the blades. Hence, the dividing process is hygienic and involves no wastage. When a cryogenic device is used to portion a foodstuff , the foodstuff is divided as rapidly as possibly (eg for 1 second when cryogenic blades are used to portion spinach) to prevent the foodstuff from being frozen by the blades , as this would have the undesirable result of expanding the foodstuff. The time for which a blade is in contact with a foodstuff may be controlled such that a frozen crust is formed on the foodstuff, without the foodstuff expanding sufficiently to adhere to the blade. The foodstuff may also be divided by metal or non-metal blades which have been chilled to a temperature in the range of from 0 to +5°C. At these temperatures, excessive bacterial qrowth is prevented. Division by cryogenic or chilled blades may be used to form portions of foodstuff that are not discrete but that are partially separated by gaps and are interconnected at their bases . The consumer subsequently separates the portions by snapping them apart along the lines created by the gaps . This interconnection spaces the portions during packing, storaqe and distribution. Whilst not being completely frozen, the foodstuff has a sufficiently-viscous mouldable consistency that it does not flow into the gaps formed by the blades . DETAILED DESCRIPTION OF THE INVENTION Examples of the products and processes of the invention will now be described to illustrate, but not to limit, the invention, with reference to the accompanying figures in which: figure 1 is a graph showing consistency measurements for sieved spinach; figure 2 is a graph showing consistency measurements for a sauce for pasta; and figure 3 is a graph showing consistency measurements for red cabbage in sauce . Cone Penetrometer To measure penetration distances using a cone penetrometer, a part-frozen, mouldable foodstuff is filled into a container and the surface levelled with a knife. A cone is adjusted such that its tip is as close to the surface of the foodstuff as possible. The cone is allowed to fall under gravity for 5 seconds . After that period the cone is automatically held in place and the penetration distance is measured. In the following examples, the cone used had an angle of 40° and had, together with the attached support shaft, a combined weight of 160g. The results are the average of a number of measurements . Where a food stuff comprises particulates, penetration results will be different depending upon whether the cone of the penetrometer is passing through the partially frozen continuous liquid/ice phase or if it meets a particle. For this reason, it is preferred that a plurality of measurements are taken. Measurements where the cone encountered a particulate will be apparent from an exceptionally low penetration (for example less than half of the average of all the measurements) . Such anomolous results can be rejected. The remainder of the results can then be used to calculate a corrected average. S t ruckt ocf r aph A suitable strucktograph is available from Brabaender; it comprises a table and a cone. To measure resistance forces using a strucktograph, a part- frozen, mouldable foodstuff is filled into a cylindrical container and the surface levelled with a knife . The filled container is placed on the table of the strucktograph. This table is moved with constant speed against the cone. The cone is connected to a force spring, which registers the resistance force of the foodstuff. In the following examples, the cone was Brabaender model no 449648 (cone shape 90°) , the cylindrical container had a height of 20mm and a diameter of 53mm, the speed of the table was 75mm/min, and the table was moved against the cone for 10 seconds. NB For different samples of a partially frozen foodstuff, the penetration distance of the cone and the resistance force of the foodstuff may vary, depending on the composition of the sample (eg. the volume and hardness of any particulates present; the dry matter to liquid ratio) . Examples 1 to 3 These are simulation experiments designed to find the penetration distances for certain foodstuffs that correlate to a desired mouldable consistency. Example 1 Portions of frozen sieved spinach were mixed with varying amounts of unfrozen spinach to achieve five samples having a liquid content of approximately 40, 50, 60, 75 and 80% respectively. The consistency of these samples was measured using a cone penetrometer. Figure 1 shows the graphic results of the consistency measurements for sieved spinach. Results It can be seen that a penetration distance ranging from 10 to 25 mm is most preferred, whilst a penetration distance ranging from 10 to 30 mm is adequate. In this example, the temperature of the sieved spinach is lowered to a range of from -1.4 to -2.1°C, preferably to a range of from -1.6 to -2.1°C, to achieve the desired mouldable consistency via partial freezing. Example 2 Portions of a frozen pasta sauce comprising minced beef, onion and tomato paste were mixed with varying amounts of unfrozen pasta sauce to achieve five samples having a liquid content of approximately 45, 50, 60, 70 and 75% respectively. The consistency of these samples was measured using a cone penetrometer. Figure 2 shows the graphic results of the consistency measurements. Results It can be seen that a penetration distance ranging from 10 to 20mm is most preferred, whilst a penetration distance ranging from 10 to 25mm is adequate. In this example, the temperature of the pasta sauce is lowered to a range of from -6.6 to -4.4°C, preferably to a range of from -6.6 to -5.2°C, to achieve the desired mouldable consistency via partial freezing. These temperatures are lower than those for the sieved spinach of example 1, since the tomatoes in the sauce contain sugar; this lowers the freezing point of the product . Example 3 Portions of frozen red cabbage and apple in sauce were mixed with varying amounts of unfrozen red cabbage and apple to achieve five samples having a liquid content of approximately 65, 70, 80, 90 and 100% respectively. The consistency of these samples was measured using a cone penetrometer. Figure 3 shows the graphic results of the consistency measurements . Results It can be seen that a penetration distance ranging from 10 to 17mm is most preferred, whilst a penetration distance ranging from 10 to 25mm is adequate. In this example, the temperature of the foodstuff is lowered to a range of from -2.0 to +3.0°C, preferably to a range of from -2.0 to 0°C, temperature of the foodstuff is lowered to a range of from -2.0 to +3.0°C, preferably to a range of from -2.0 to 0°C, to achieve the desired mouldable consistency via partial freezing. These temperatures are higher than those for the sieved spinach of example 1, since the foodstuff contains lard in its sauce; the lard increases the consistency of the product . Examples 4 to 7 In the following Examples 4-7, a single screw scraped surface heat exchanger was used. The screw diameter was 203 mm. The screw was 750 mm in length. The threads had a channel depth of 15mm. There were two screw starts and the pitch was a constant 115 mm. Example 4 Chopped spinach with cream was pumped through a screw thread scraped surface heat exchanger, which was cooled by ammonia. The cream spinach was cooled to different temperatures within the heat exchanger; the spinach achieved a mouldable consistency, but did not completely solidify. At each temperature, measured quantities of the partially frozen spinach were extruded through a multiple nozzle and cut into portions. The penetration distance of the spinach was measured at each temperature . The spinach was then frozen using a standard belt freezer. Results Temperature (°C) Penetration Distance (mm) -1.6 22 -1.7 17 -2.2 13 -2.5 17 -2.7 14 Example 5 Chopped spinach with cream was pumped through the screw thread scraped surface heat exchanger, which was cooled by ammonia. The cream spinach was cooled to -1.5 °C within the heat exchanger; the spinach achieved a mouldable consistency, but did not completely solidify. Measured quantities of the partially frozen spinach were extruded through a multiple nozzle and cut into portions. The resistance force for the spinach was measured using a strucktograph. The spinach was then frozen using a standard belt freezer. Results The cream spinach had a resistance force of 1.8 N. Example 6 The method of example 5 was repeated, except that cream leek, cream endives and cream savoy cabbage were each used The method of example 5 was repeated, except that cream leek, cream endives and cream savoy cabbage were each used instead of cream spinach. Results The cream leek had a resistance force of 2.3 N: the cream endives had a resistance force of 1.8 N: the cream savoy cabbage had a resistance force of 1.7 N. Example 7 Chopped meat (particle size < 6mm) was mixed with binding agents, chopped onion and spices, at a temperature about +10°C. The mixture was pumped through the screw thread scraped heat exchanger which was cooled by ammonia. The mixture was cooled to a temperature in the range -1.8°C to -2.5°C in the heat exchanger. The mixture achieved a moldable consistency but did not solidify. The product was then portioned into burgers which were then frozen using a standard belt freezer. Inspection of the product showed that the structures of chopped meat particles were substantially intact.

Claims: CLAIMS 1. A process for preparing portions of a frozen foodstuff, the process comprising the steps of: (i) partially freezing a foodstuff to achieve a mouldable consistency using a screw thread scraped surface heat exchanger; (ii) dividing the foodstuff into portions; and (iii) freezing the portions of foodstuff. 2. A process as claimed in claim 1 that is continuous . 3. A process as claimed in claim 1 or 2 , wherein the screw thread scraped surface heat exchanger has an outlet which is fitted with an extrusion nozzle and a cutting device for dividing the foodstuff into portions . 4. A process as claimed in any preceding claim, wherein the foodstuff has a consistency after step (i) that results in a cone of a cone penetrometer penetrating it to a distance of from 5 to 34 mm in 5 sees. 5. A process as claimed in claim 4 , wherein the cone penetrates the foodstuff to a distance of from 10 to 25mm. 6. A process as claimed in any preceding claim, wherein the foodstuff has a resistance force, as measured by a strucktograph, of from 0.1 to 4.5 N after step (i) . 7. A process as claimed in claim 6, wherein the foodstuff has a resistance force of from 0.3 to 2.5 N. 8. A process as claimed in any preceding claim, wherein the foodstuff is aerated either before or during step (i) . 9. A process as claimed in any preceding claim, wherein the foodstuff is divided into portions by means of a cryogenic device. 10. A process as claimed in any preceding claim, wherein the foodstuff comprises particulates . 11. A frozen portion of a foodstuff prepared in accordance with the process claimed in any preceding claim.

Inventors: AHRENS HANS-WERNER, BINLEY GARY NORMAN, BUSCHMANN URBAN VALENTIN, GRABER HANS JUERGEN, KIRK ELLIOTT, MCBRIDE MALCOLM JOHN, SHARP DAVID GEORGE, AHRENS, HANS-WERNER, BINLEY, GARY, NORMAN, BUSCHMANN, URBAN, VALENTIN, GRABER, HANS, JUERGEN, KIRK, ELLIOTT, MCBRIDE, MALCOLM, JOHN, SHARP, DAVID, GEORGE
Applicants: UNILEVER NV, UNILEVER PLC, UNILEVER N.V.
Application year: 1998
Main classification: A23B4
IPC codes: A23B4, A23B7, A23G9, A23L3