Method of vacuum drying of cooked food

Abstract

A method of vacuum drying of cooked food, wherein cooked food is cooled in a cooling device to a temperature from +5°C to negative temperature, at which temperature all the components of the cooked food have already been completely frozen and have transitioned to the solid phase, whereupon the cooked food is subjected to a maximally 5 mbar pressure in the vacuum drying chamber and subsequently heated to a temperature from 0°C to 60°C, wherein the heating is carried out to such a temperature within that temperature range at which the pressure in the vacuum drying chamber is not undesirably increased and the temperature within that temperature range is gradually increased depending on the remaining water content in the cooked food until the cooked food is dried.

Description

[0001] Method of vacuum drying of cooked food

[0002] Technical field

[0003] The invention relates to a method of vacuum drying of cooked food.

[0004] Background of the prior art

[0005] In the prior art, among the present methods of vacuum drying of foods, there is known, for example, a process referred to as lyophilization or freeze drying. Lyophilization is a process that allows many foodstuffs such as fruits, vegetables, cheeses, and herbs to be dried.

[0006] The freeze-drying process, lyophilization, uses two factors, extremely low temperature, and pressure. The foodstuff to be freeze-dried is first frozen, and all the water in the foodstuff is therefore transitioned into a solid state. The water is then evaporated when the pressure is decreased. One of the greatest advantages of this method is therefore that there is no direct transition of water from the liquid state to the gaseous state during the process, as in conventional drying. As a result, the dried material is not damaged.

[0007] Lyophilization usually has three phases. First, the foodstuff containing water is frozen and then dried in two drying phases.

[0008] In freezing, the foodstuff containing water is frozen below the temperature at which it cannot exist in a liquid state. This ensures that melting instead of sublimation does not occur. Microscopic damage to the foodstuff can occur when the water freezes. Therefore, this phase must be carried out very carefully. Usually, the foodstuff is frozen to a temperature between -20 °C and -50 °C.

[0009] During primary drying, the air pressure in the freezing device is decreased to several hundred Pa. Then so much heat is supplied (mainly by conduction and radiation, heat convection is practically not applied) that the frozen water can start to sublimate. This removes about 95 % of the water from the foodstuff. Sublimation should not be carried out too quickly to avoid damaging the dried food. Drying usually takes several hours to several days. The resulting water vapour desublimates on condensers that have a temperature below -50 °C. This prevents water vapour from entering the extractor pumps, which would impair their efficiency. Determining the end of the primary drying process is essential for understanding the entire drying process of the foodstuff. For biotechnology lyophilizers, there are several advanced methods of determining the point at which primary drying is completed, and commonly used methods include comparing the temperature of the shelves with the temperature of the dried foodstuff or comparing the values between a Pirani vacuum meter and a capacitance manometer.

[0010] The process then proceeds to the secondary drying phase, which removes the remaining unfrozen water molecules that hold onto the surface of the solids due to adsorption. The temperature in the drying chamber is raised to a maximum of 60°C, which breaks the bonds between the water and the solid material. The pressure can be further decreased to fractions of Pa depending on the needs of the material and its state of drying. After the secondary drying phase, 1-4 % water remains in the material.

[0011] The applicant for this application had a need to offer lightweight dried cooked dishes which would have the same quality of the cooked dish or the input foodstuffs for the preparation of the cooked dish and whose price, depending on the investment and operating costs of their production, would be competitive on the market. It was also a requirement that the dried cooked dishes offered should have all the qualities and characteristics of lyophilized dishes, e.g., rapid rehydration and preparation without cooking.

[0012] The applicant first tried drying the cooked food by the usual lyophilization process, which proceeded as follows. The cooked food was deep frozen to a temperature typically between -20 and -50°C to prevent the cooked food from "melting" or even "boiling" during the pressure decreasing phase and the first sublimation phase, whereupon it was dried in two drying phases in a vacuum drying chamber in which the pressure was decreased typically to a value of up to 1 mbar to create a vacuum. Once the vacuum in the chamber was achieved, the cooked food was heated during the first drying phase and the frozen water sublimated into water vapour, which was drawn off the cooked food and collected on evaporators, where it again transitioned to frozen water. In this way, about 90 % of the water was removed from the cooked food during the longest drying phase. The temperature of the cooked food was maintained below the freezing point at all times. During the secondary drying phase, to remove the remaining water from the cooked food, the temperature of the cooked food was raised above the freezing point, thus breaking the bond between the cooked food and the water.

[0013] Based on the results of practical tests of vacuum drying of cooked food according to the usual lyophilization, it was found that this method produces dried food with the desired properties and qualities. However, after evaluating the investment and operating costs, the applicant concluded that the price of the dried foods produced in this way is high and uncompetitive, in particular because of the high operating costs of deep-freezing caused by the high energy consumption during deep-freezing and the high investment costs due to the need to acquire a drying chamber with complicated technology, in particular a extractor pump, to achieve a pressure decrease in the chamber of up to 1 mbar. The long time required for deep-freezing was also an obvious disadvantage of this method.

[0014] Because of these disadvantages of vacuum drying by conventional lyophilization, the applicant has initiated trials, testing and evaluation of various changes to the lyophilization process which have resulted in a functional technological process for drying cooked food which achieves the same quality of dried cooked food as using conventional lyophilization, but with a saving of about 30 % of the energy consumed in the drying process.

[0015] Accordingly, the object of the invention was to provide a modified lyophilization method of vacuum drying which achieves dried cooked food of the same quality as the conventional lyophilization method of vacuum drying, but with significantly lower energy consumption and significantly shorter freezing time in the freezing step of the vacuum drying process.

[0016] Summary of the subject matter of the invention

[0017] The objective of the invention has been achieved by a novel method of vacuum drying cooked food, wherein significantly lower energy consumption and shorter freezing times are achieved in the freezing step due to the absence of deep freezing of the cooked food.

[0018] The subject matter of the present invention is a method of vacuum drying of cooked food, the principle of which is that the cooked food is cooled in a cooling device to a temperature from +5°C to negative temperature, at which temperature all the components of the cooked food have already been completely frozen and have transitioned to the solid phase, whereupon the cooked food is subjected to a maximally 5 mbar pressure in the vacuum drying chamber and subsequently heated to a temperature from 0°C to 60°C, wherein the heating is carried out to such a temperature within that temperature range at which the pressure in the vacuum drying chamber is not undesirably increased and the temperature within that temperature range is gradually increased depending on the remaining water content in the cooked food until the cooked food is dried.

[0019] Food is understood in the context of the present invention to mean any food or any combination of foods that are suitable for human consumption.

[0020] From these dishes are excluded within the scope of the invention those dishes which are not suitable for the method according to the invention, i.e., which do not achieve the desired effect of the method according to the invention. Since there are a countless number of different dishes, it is not possible, in such a large variety of all possible dishes, to define precisely the group of dishes which are not suitable for the method according to the invention. However, it may be noted that, as with the known lyophilization method, in particular, dishes which are made from ingredients having a fat and sugar content of more than 50 % by weight are not suitable for the method according to the invention.

[0021] It is apparent from the foregoing that foods suitable for the method according to the invention can be determined primarily on the basis of whether or not they achieve the desired effects of the method according to the invention when specifically used in the method according to the invention. For example, based on tests, the inventors have found that tomatoes as such are not suitable for the preparation of food in the method according to the invention. However, if tomatoes are part of a particular food and are present in it to a certain percent proportion, then processing such food in the method according to the invention will achieve the desired effects. For example, a tomato sauce which contains a higher proportion of tomatoes is not very suitable for the method according to the invention, while a Bolognese sauce which contains a lower proportion of tomatoes is suitable for the method according to the invention. It is also apparent that it is not possible to determine precisely the proportion of tomatoes at which a dish is suitable for the method according to the invention, since the final effect is influenced by other ingredients in addition to tomatoes which combine with the tomatoes in the cooked dish.

[0022] A cooked food is understood in the context of the invention to mean a food of which all of the ingredients from which the food is prepared have undergone a cooking process, i.e., have been exposed to a temperature of at least 100°C for a suitable period of time at atmospheric pressure.

[0023] The cooling device in the context of the invention means any type of cooling device capable of cooling cooked food within a defined temperature range. The cooling device may be part of a vacuum drying chamber, but it is usual that it forms a separate unit.

[0024] A component of cooked food is understood to be a part of the cooked food which is defined in such a way that the temperature at which that part is completely frozen can be determined for that part and is different from the temperatures of complete freezing of the other parts so defined. A cooked food component so defined may be, for example, water, salt, protein, sugar and starch, or meat, potato, carrot, and tofu.

[0025] The state at which all the components of the cooked food have already frozen completely and transitioned to the solid phase is understood to be the state at which all the components of the cooked food are already in the solid phase, i.e. none of the components of the cooked food is already in the liquid phase.

[0026] The temperature in the defined range to which cooked food in the refrigeration system cools down is understood to be the temperature of the cooked food as a whole. Assuming that the components of the cooked food are defined by their different temperatures of complete freezing, as mentioned above, and these temperatures of complete freezing are expressed by the temperatures of melting of those components (e.g. the melting temperature of water is 0°C, potatoes -23°C and chicken -20°C), then the overall temperature to which the cooked food as a whole is cooled is determined on the basis of the melting temperature of the component of the cooked food which has the lowest melting temperature of all the components of the cooked food (from the examples of melting temperatures given, such a temperature would be the melting temperature of potatoes). For a cooked dish containing, for example, ten components, the temperature to which the cooked dish cools as a whole is determined by the melting temperature of the component of the cooked dish which has the lowest melting temperature of all ten component.

[0027] The cooked food is cooled to a temperature within a defined range under normal atmospheric pressure, with the cooling time determined by the hygiene standard.

[0028] The advantage of this step of the method according to the invention, i.e., cooling of the cooked food in a cooling device to a temperature from +5°C to a negative temperature, at which temperature all the components of the cooked food have already been completely frozen and transitioned to the solid phase, as compared to drying methods of the prior art, e.g. conventional lyophilization method of vacuum drying, consists in the fact that the cooked food does not have to be cooled below the temperature of freezing of all the components of the cooked food, i.e. to a temperature of up to -20°C to -50°C as in the case of conventional lyophilization method of the state of the art. Practical tests of the method according to the invention have shown that, in this method, to achieve the desired effects, the cooked food only needs to be cooled to a temperature in the range of -5°C to 0°C, taking into account a temperature range of up to 5°C by which the overall temperature of the cooked food may be increased during subsequent handling of the cooled cooked food, in order to reliably maintain the temperature to which the cooked food as a whole needs to be cooled.

[0029] In the drying methods of the prior art, e.g., conventional lyophilization method, foods identical to those dried by the method according to the invention are deep frozen to temperatures as low as about -30°C, i.e., to temperatures significantly below the freezing temperature of all components of the cooked food.

[0030] The advantage of this step of the method according to the invention, i.e., that the cooked food does not need to be deep-frozen but only cooled or shallow-frozen, leads to lower technical requirements for cooling equipment, shorter cooling times and lower operating costs for cooling device compared to the drying methods of the prior art.

[0031] A vacuum drying chamber is understood in the context of the invention to mean any type of conventional lyophilizing chamber for drying foods, which allows the creation of a vacuum while heating the cooked food to a desired temperature within a defined range of 0 to 60°C. In the method according to the invention, no specially designed device is required and therefore conventional lyophilization device will be sufficient. The method according to the invention is achieved by a combination of a different treatment of the cooked food entering the vacuum drying chamber and a software control of the drying cycle.

[0032] Decreasing the pressure in the vacuum drying chamber leads to a lower boiling point of the components of the cooked food. In the method according to the invention, the cooked food has an inlet temperature at which, during the process of decreasing the pressure in the vacuum drying chamber, some of the components of the cooked food transition to a liquid state or remain in this liquid state and, in an extreme case, some of the components may even be boiled for a short time as a result of the lowering of the boiling point. However, since the vacuum, i.e. deep vacuum, significantly reduces the temperature (the vacuum temperature is about -270°C), it also ensures that all the component of the cooked food are in a frozen state, i.e. in the solid phase, at the end of this step, which ensures the desired state of the cooked food for the next drying step, which of step is the gradual heating.

[0033] The advantage of the method according to the invention is that, in the case of cooked food, the fact that some components are transitioned to a liquid state or are subjected to boiling for a short period of time during the pressure lowering in the vacuum drying chamber is not undesirable and does not lead to any deterioration in the grade and quality of the resulting dried food.

[0034] In contrast, the methods of the state of the art avoid this phenomenon by freezing the food to temperatures below -30°C, ensuring that all food components remain completely frozen when the pressure is lowered.

[0035] The subsequent heating of the cooked food in the vacuum drying chamber is carried out in the same way as in vacuum drying methods of the state of the art, e.g. a conventional lyophilization method. The reason for this step is to ensure faster sublimation of the frozen water in the cooked food components to water vapour and to trap the water vapour on the evaporator of the vacuum drying chamber. The essence of sublimation of frozen water to water vapour is the direct transition of water from the solid state to the gaseous state without an intermediate liquid state, which is achieved by reducing the pressure below the triple point of the water phase diagram. Heating of the cooked food is carried out to a temperature within the defined temperature range at which there is no undesirable increase in pressure in the vacuum drying chamber. The reason for this measure is that the amount of vapour removed from the cooked food should be in balance with the ability of the evaporator to trap the vapour and thus remove it from the chamber space. This is because if the temperature to which the cooked food is heated is too high (or rises at a faster rate), more vapour will be formed than the evaporator can capture, thereby increasing the pressure in the chambre. This would result in the so-called "collapse of the lyophilizate", i.e., thawing of the cooked food, which is undesirable at this phase of drying, as it leads to a deterioration in the quality of the resulting dried cooked food. This step is common to conventional lyophilization method, but in the method according to the invention it has been shown by tests that a decrease in pressure to 5 mbar in the method according to the invention, instead of to a lower pressure of up to 1 mbar in conventional lyophilization method of the state of the art, in the vacuum drying chamber, is sufficient to achieve the desired effects of the present invention method.

[0036] The temperature from the defined range is gradually increased depending on the remaining water content in the cooked food until the cooked food is dried. The heat to raise the temperature of the cooked food is supplied to the cooked food in various ways, e.g. directly by heating elements arranged in the heating shelves on which the trays containing the cooked food are placed, or indirectly by distributing the warm air, optimally as evenly as possible throughout the chamber.

[0037] During the drying step of the method according to the invention, the degree of final drying, i.e. the volume of water remaining in the cooked food, is determined from the rate at which the cooked food removes heat from the heating shelf, i.e. from the power that the heating shelf must exert to maintain the selected temperature). When the heating shelf is set at the highest desired temperature and is exerting about 2-4 % of its rated power to maintain that temperature, the cooked food is considered to be completely dry. This fact is experimentally verified by measuring the water activity of the cooked food. There are other methods of determining the degree of final drying which can be used in the method according to the invention, for example, comparing the temperature measured on the surface of the cooked food with the temperature measured inside the cooked food. Other methods obvious to one skilled in the art may also be used. Preferably, in the vacuum drying method according to the invention, the cooked food is divided into selected sub-units of the cooked food and these selected sub-units are dried separately in the vacuum drying chamber, i.e., they are not dried in the chamber together with the other selected sub-units of the cooked food.

[0038] Selected sub-units of a cooked dish are understood to be sub-units, such as meat, side dish (i.e., potatoes, rice, etc.), sauce, which are usually defined sub-units of a cooked dish in the field of gastronomy, and which are apparent to a person skilled in the art of gastronomy.

[0039] The selected sub-units of the cooked dish, such as meat, sauce, and side dish, contain different components of the cooked dish or the same components of the cooked dish but in different proportions.

[0040] These selected sub-units contain e.g., different proportions of frozen water. These different proportions of frozen water, in combination with the other components in the sub-units, cause these sub-units to require different drying times in the vacuum drying chamber. It follows that if said sub-units of cooked food were dried together in the method according to the invention in the vacuum drying chamber, one sub-unit of cooked food, e.g. with a lower proportion of frozen water, would dry to the desired water proportion in a shorter time than another sub-unit, e.g. with a higher proportion of frozen water.

[0041] It is therefore preferred, from the point of view of the overall practical efficiency of drying the cooked food, to dry the selected sub-units of the cooked food separately in the vacuum drying chamber.

[0042] Examples of embodiments of the invention

[0043] For a better understanding of the invention, the invention will now be described by means of example embodiments. These example embodiments of the invention are to be understood as examples which serve only to further explain the invention and which in no way limit the scope of protection of the invention as defined by the appended patent claims. The first example of the invention is the processing of a cooked dish sold under the name "Chicken Tikka Masala with Basmati Rice" in the method according to the invention.

[0044] The starting product for said example of a method according to the invention is a cooked dish comprising three selected sub-units, namely meat, sauce, and a side dish. All of these sub-units of the cooked food are separately processed in the following manner prior to processing in the method according to the present example of the invention:

[0045] - The meat comprising the chicken breast meat in this example embodiment was marinated in a mixture of spices and yoghurt, baked as a whole in a convection oven with this mixture and then cut into pieces of about 20 x 20 mm;

[0046] - the sauce composed in this example of a sauce comprising cream, chopped tomatoes, onions, garlic, and a mixture of spices was cooked in a large volume kettle;

[0047] - the rice, which in this example is basmati rice, has been cooked in a convection oven so that it is in an 'al dente' state, i.e., it is not completely in a soft state, but is no longer in a hard state.

[0048] The sub-units of the cooked food thus processed were then placed individually on stainless steel trays suitable for vacuum drying. Each sub-unit of cooked food was spread out on the tray in a layer with a predetermined height. Specific layer heights were experimentally determined for each of the three selected sub-units. The height of the layer of the cooked food sub-unit on the tray, and therefore the volume of the subunit resulting from it, significantly influences the drying time of the sub-unit. Since different sub-units of cooked food require different drying times, the heights of the individual sub-units are determined according to their required drying times.

[0049] The following layer heights were experimentally determined for the sub-units of Tikka Masala cooked food, corresponding to the following weights of these sub-units on the tray: meat: 35 mm / 5 kg sauce: 20 mm / 5, 5 kg rice: 35 mm / 5, 5 kg. Considering that, as already described in the section of the present invention application relating to the subject matter of the invention, it is preferable from the point of view of the efficiency of the entire vacuum drying method according to the invention to dry different sub-units of cooked food in the vacuum drying chamber separately, the following example of a method according to the invention in which three different subunits of cooked food are to be vacuum dried, i.e. meat, rice and sauce, is described for one of the sub-units, i.e. meat, and it should be understood that the same process also applies to the remaining sub-units of the cooked food, i.e. rice and sauce.

[0050] The trays filled with one and the same sub-unit of cooked food, i.e. meat, are inserted into the respective loading carriage, whereupon the carriage is taken to the cooling device in which the meat is frozen / cooled to a temperature of -5°C to 0°C.

[0051] Next, the trays with cooled / frozen meat are transferred to the vacuum drying chamber on the appropriate heating shelves. The vacuum drying chamber is closed, and the drying cycle is started, which of the drying cycle is controlled by the control system for controlling the vacuum drying, wherein the control system operates according to a control program selected according to the kind of sub-unit of the cooked food, in this case meat.

[0052] The drying cycle involves two steps, namely:

[0053] - the pressure decreasing step, in which the pressure in the drying chamber is decreased to 2 to 5 mbar, wherein this step takes about 1 hour.

[0054] - a drying step in which the temperature of the heating shelves is gradually increased depending on the amount of water vapour removed, wherein the temperature of the heating shelves is increased in increments of 5°C up to a maximum value of 35°C (can be higher). During the drying step, it is necessary to ensure that the chamber pressure is maintained at a maximum of 5 mbar. This is achieved by the control system regulating the temperature of the heating shelves according to the amount of water vapour removed, so that when the control system detects, for example, an increase in pressure in the drying chamber, it will set the temperature of the heating shelves to a lower value, thereby reducing the amount of water vapour removed.

[0055] The resulting drying time for the meat was 56 h, for the other sub-units of the cooked food, i.e., sauce and rice, 48 h and 46 h, respectively. It should be noted that the drying times achieved are determined by the technical characteristics of the vacuum drying chamber used in this example embodiment of the method and the available power input of the building supplied to this chambre. In contrast, in industrial vacuum drying chambers, drying times that are one-half to one-third of the drying times of this example are to be expected due to the much more powerful exhauster pumps and cooling devices that can be used to remove much more vapour.

[0056] Drying for all three sub-units, i.e. meat, sauce and rice, was stopped when none of the heating shelves showed a heating power above 2% of the rated heating power, at which heating power none of the sub-units of the cooked food were removing heat from the heating shelves, indicating that none of the sub-units contained water that could sublimate. This limit of heating power of the heating shelves at which drying was stopped had previously been experimentally determined and verified by subsequent measurements of water activity for most cooked food processed in the method according to the invention.

[0057] After drying is complete, air is reintroduced into the drying chamber to create an atmospheric pressure environment inside the chamber, whereupon the chamber is opened.

[0058] The trays with the dried cooked food sub-units were tipped into a stainless steel container and then packed into airtight boxes in which they were stored for as short a period as possible (maximum 14 days) until all the cooked food sub-units, i.e., meat, rice, and sauce, were dried.

[0059] Once all the sub-assemblies of the cooked food have been dried, the production of the dried cooked food is completed by bringing the sub-units together and finally packaging the combined sub-units in their final packaging.

[0060] The production of the dried cooked food and its packing in the final package was completed as follows:

[0061] Firstly, bags for the type of food were prepared, in this example embodiment for the food Chicken Tikka Masala with Basmati Rice, preferably doypacks, i.e., bags made of multi-layered materials with an extreme oxygen and moisture barrier. Each bag was opened and in three steps all three sub-units of the cooked food, i.e. dried meat, sauce and rice, were individually poured into the bag in the exact quantities required, with a tolerance of + / - 1 g, after which the filled bag was placed in a vacuum packing machine in which all the air was sucked out of the bag and at the same time hermetically sealed.

[0062] A second example embodiment of a method according to the invention will be described herein.

[0063] In this example, the cooked dish comprises one selected sub-unit, namely, a cooked Dhal lentil soup with chopped tomatoes and spices.

[0064] The cooked soup was spilled onto stainless steel trays intended for vacuum drying in such a volume that the soup had a predetermined optimum layer height, i.e., weight per tray. For the particular Dhal lentil soup, the height and weight per tray were 20 mm and 5,5 kg, respectively.

[0065] The filled soup trays placed in the loading carriage were then transferred to the cooling device, where the cooked soup was cooled to a temperature between -5°C and - 0°C.

[0066] The trays with the cooled / frozen cooked soup were placed in a vacuum drying chamber on heating shelves. The chamber was closed and a drying cycle specifically programmed for that cooked soup was started.

[0067] As in the first example embodiment of the method according to the invention, the drying cycle was carried out in two steps, namely a pressure decreasing step, in which the pressure in the drying chamber was decreased to 2-3 mbar within about 1 hour, and a drying step, in which the temperature of the heating shelves was increased depending on the amount of vapour removed, wherein this temperature was increased in increments of 5°C up to a maximum temperature of 35°C (which may be higher), while controlling the temperature of the heating shelves according to the amount of vapour removed maintained the pressure inside the drying chamber at a maximum of 5 mbar. For this cooked soup, the total drying time was 49 hours.

[0068] As in the first example, the drying of the cooked soup was stopped when none of the heating shelves showed a heating power of 2%.

[0069] After drying was completed, air was introduced into the drying chamber to create an internal environment with atmospheric pressure and then the drying chamber was opened. The dried soup was tipped from the trays into a stainless steel container, after which it was stored in airtight boxes for as short a period as possible.

[0070] Finally, the dried soup was packed into the final containers as described in the previous example.

[0071] Industrial applicability

[0072] The vacuum drying method according to the invention can be used in all applications where lightweight dried cooked food with the quality of the original cooked food, rapid rehydration and preparation without cooking are produced. This method is particularly useful in applications where significant savings in production costs are required when using existing technological devices of the manufacturers of lyophilized cooked food.

Claims

P A T E N T C L A I M S1 . A method of vacuum drying of cooked food, characterized in that the cooked food is cooled in a cooling device to a temperature from +5°C to negative temperature, at which temperature all the components of the cooked food have already been completely frozen and have transitioned to the solid phase, whereupon the cooked food is subjected to a maximally 5 mbar pressure in the vacuum drying chamber and subsequently heated to a temperature from 0°C to 60°C, wherein the heating is carried out to such a temperature within that temperature range at which the pressure in the vacuum drying chamber is not undesirably increased and the temperature within that temperature range is gradually increased depending on the remaining water content in the cooked food until the cooked food is dried.

2. The method of vacuum drying of cooked food according to claim 1 , characterized in that the cooked food is divided into selected sub-units of cooked food, whereupon these selected sub-units are cooled in the cooling device and dried in the vacuum drying chamber separately.