Freeze-dried food with coating layer and preparation method thereof

Abstract

The present invention provides a freeze-dried food with a coating layer and a preparation method thereof, and belongs to the technical field of food processing. The present invention first coats an aqueous solution on a non-freeze-dried food, and then freeze-dries the food coated with the aqueous solution. In the prepared freeze-dried food, both the outer coating layer and the inner food have a fine porous structure and a silky melting property in the mouth, and have a high thermal stability characteristic of not sticking to the packaging or melting and deformation at a high temperature of 65°C.

Description

Technical Field

[0001] The invention relates to the technical field of food processing, and in particular to a freeze-dried food with a coating layer and a preparation method thereof. Background Art

[0002] Freeze-dried food, especially freeze-dried fruit and vegetable products made from fruits and vegetables, has been welcomed by more and more consumers due to its special tasting characteristics, special taste and convenience. At the same time, in order to enrich the taste and flavor of freeze-dried fruit and vegetable products, products that wrap freeze-dried food with chocolate have also appeared on the market. The preparation method of freeze-dried food wrapped with chocolate under the existing technology is to wrap a layer of chocolate on the outside of the processed freeze-dried food by coating, dipping or spraying, and then cool the outer layer of chocolate fat to crystallize and shape the outer layer of chocolate. However, freeze-dried food wrapped with chocolate produced by the existing technology has the following disadvantages:

[0003] 1. Melting under high temperature environment: Whether it is coated, dipped or sprayed, the chocolate used must have good fluidity, that is, the structure of the chocolate must be a chocolate slurry with a continuous oil phase, and its structure after cooling and forming relies on the fat crystals in the chocolate as the main structure or the adhesive of its non-fat solids. When encountering a temperature environment higher than the melting point of chocolate fat, the fat in the chocolate, such as cocoa butter, milk fat or other fats, will melt, causing the chocolate to soften, and it will stick to the packaging, stick to the hands of consumers, and even drip on the clothes of consumers, which are unpleasant to consumers. For a long time in the past, practitioners and researchers in the chocolate industry have been committed to how to improve the melting point of chocolate fat, and have made many improvements and contributions, including using hydrogenated vegetable oil as a fat to replace cocoa butter to make cocoa butter substitute chocolate. However, the increase in the melting point of chocolate fat or cocoa butter substitute will lead to a decrease in the taste of chocolate, reduce the tasting characteristics and eating pleasure of chocolate, which will lead to the sales of freeze-dried foods wrapped in chocolate under the current technology being restricted, especially in summer, high temperature and tropical areas.

[0004] 2. Health risks: Since the chocolate used in existing freeze-dried foods wrapped in chocolate must maintain the structure of a continuous oil phase in the chocolate slurry to maintain its fluidity, according to experience, the total fat content in the chocolate that can satisfy both the requirement of a continuous oil phase and good fluidity, especially in chocolate used for coating or spraying, must reach ≥35% by mass in the chocolate formula, or even 40% without adding emulsifiers. However, this will result in a high fat content in the chocolate, which may cause consumers to have excessive fat intake. In order to improve solubility, the use of fats treated by hydrogenation may also cause consumers to have excessive intake of trans fatty acids or saturated fatty acids. Summary of the invention

[0005] The object of the present invention is to provide a freeze-dried food with a coating layer and a preparation method thereof. The prepared freeze-dried food with a coating layer has high thermal stability and has no health risks.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides a method for preparing a freeze-dried food with a coating layer, comprising the following steps:

[0008] preparing an aqueous solution consisting of a solvent and a solute;

[0009] The aqueous solution is wrapped around the outside of the food to be freeze-dried, and freeze-dried to obtain the freeze-dried food with a coating layer.

[0010] Preferably, the wrapping method is full wrapping or partial wrapping, or the wrapping method is single-layer wrapping or multi-layer wrapping.

[0011] Preferably, the solvent includes one or more of water, emulsion, water-emulsion mixture, colloid substances, fiber substances, essence substances, emulsifier substances and stabilizer substances, the solvent does not include oil, the water content of the solvent is 70-100% of the total mass of the solvent, and the mass of the solvent in the aqueous phase solution is 10-80% of the total mass of the aqueous phase solution.

[0012] Preferably, the solute comprises one or more of fruits, nuts, grains, vegetables, dairy products, cocoa products, oils, starch, food additives, stabilizers and chocolate products, and the total oil content of the solute is ≤30wt%.

[0013] Preferably, the aqueous phase solution is prepared by one or more of mixing, grinding, emulsification and homogenization, and the particle size of the non-fat solid matter in the aqueous phase solution is ≤50 μm.

[0014] Preferably, the wrapping method includes coating, dipping, spraying or rolling.

[0015] Preferably, the freeze-drying process includes a freezing process and a drying process performed sequentially, and the freezing process includes a single freezing process or multiple freezing processes.

[0016] Preferably, the freezing treatment temperature is -10 to -40°C, and the total freezing time is 0.5 to 10 hours.

[0017] Preferably, the freeze-dried food to be desired includes one or more of fruits, vegetables, grains, nuts, dairy products and meat products.

[0018] The present invention provides a freeze-dried food with a coating layer prepared by the preparation method described in the above technical solution.

[0019] The present invention provides a freeze-dried food with a coating layer prepared by the preparation method described in the above technical solution.

[0020] The present invention provides a method for preparing a freeze-dried food with a coating layer, comprising the following steps: preparing an aqueous solution consisting of a solvent and a solute, coating the aqueous solution on a desired freeze-dried food, and freeze-drying the food to obtain the freeze-dried food with a coating layer. The present invention first coats the unfreeze-dried food with the aqueous solution, and then freeze-dries the food coated with the aqueous solution. In the prepared freeze-dried food, the outer coating layer and the inner food both have a fine porous structure and a silky melting property in the oral cavity, and have a high thermal stability characteristic of not sticking to the packaging or melting and deformation at a high temperature of 65°C.

[0021] Furthermore, the present invention has the following beneficial effects:

[0022] 1. Give the product good anti-melting properties: The total fat content of the outer coating layer is low, such as the total fat content in the chocolate coating layer accounts for ≤30% of the total mass of the chocolate, and the fat in the chocolate wrapped on the outside will not constitute a continuous oil phase. After freeze-drying and molding, like the food inside, non-lipid substances constitute the skeleton of the porous structure, which will not deform, collapse or precipitate fat due to fat melting at high temperatures, making the chocolate have excellent anti-melting properties, so that the freeze-dried food wrapped in chocolate can be transported and sold in all seasons, and is a product that can be transported and sold at room temperature without refrigeration. After the thermostat test, the freeze-dried food with a chocolate coating layer of the present invention, after being placed in a thermostat at 50°C and 65°C for 2 days, the outer layer of chocolate still has a crispy, melt-in-the-mouth taste, and there is no phenomenon of sticking to the packaging bag, sticking to the hands or grease precipitation.

[0023] 2. Give the product unique tasting characteristics: The coating layer of the aqueous solvent, such as the particle size of non-lipid substances in the chocolate coating layer, is ≤50μm, and the interior of the coating layer after freeze-drying presents a porous microporous structure. When the consumer eats it, the freeze-dried chocolate is rehydrated and collapsed by absorbing the consumer's saliva, giving the consumer a sensory feeling of chocolate melting in the mouth without the need for chewing. At the same time, after the chocolate is rehydrated and collapsed, the fat inside the chocolate is melted by the consumer's oral temperature or by contact with the inside of the consumer's mouth, so that the freeze-dried food wrapped in chocolate brings the consumer a silky taste and a full aroma, making the freeze-dried food wrapped in chocolate a silky and fragrant product that only dissolves in the mouth.

[0024] 3. Give the product convenient characteristics: the wrapping layer, such as the chocolate wrapping layer, uses chocolate with low fat content, and the freeze-dried chocolate has a porous microporous structure inside, such as Figure 1 As shown, the mango pieces first wrapped with a chocolate coating layer and then freeze-dried have a chocolate coating layer that also presents a porous microporous structure, and the skeleton constituting the porous structure is almost entirely composed of non-lipid substances, so that the freeze-dried food wrapped with chocolate of the present invention has the characteristics of low specific gravity, will not melt, and is easy to carry. Moreover, even if the structure or appearance is destroyed by external force, the inside of the dispersed particles is still a porous structure, and there will be no sticking to the bag or the hands, and the food can still be eaten directly, which can bring consumers a sense of pleasure when eating, making the freeze-dried food with a chocolate coating layer an easy-to-carry multifunctional convenience food.

[0025] Fourth, give the product functional characteristics: First, the outer layer of the wrapping layer, such as the chocolate wrapping layer, has a low fat content, so that consumers can enjoy chocolate products while reducing their fat intake by nearly 50% compared to conventional chocolate, increasing the pleasure experience brought by chocolate. Second, the present invention uses freeze-drying to maximize the activity and function of functional substances (such as probiotics and vitamins) added to the aqueous solution, making it possible to give chocolate the function of medicine and food, and even use it as a carrier of medicine and food products. Third, the wrapping layer formed by the method of the present invention is ≥1 layer, and the wrapping layer can be an aqueous chocolate solution or other aqueous solutions, such as sweet orange sauce or hazelnut sauce, which can give the freeze-dried food the function of multiple flavors and the function of bringing in multiple nutrients. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a physical picture of freeze-dried mango pieces with a chocolate coating layer prepared in Example 1, which are first coated with a chocolate coating layer and then freeze-dried;

[0027] Figure 2 This is a high temperature experiment diagram of freeze-dried mango chunks with a chocolate coating layer prepared in Example 1, which are first coated with a chocolate coating layer and then freeze-dried;

[0028] Figure 3 This is a high temperature experiment diagram of freeze-dried mango pieces with a chocolate coating layer that were first freeze-dried and then coated with chocolate in Comparative Example 1. DETAILED DESCRIPTION

[0029] The present invention provides a method for preparing a freeze-dried food with a coating layer, comprising the following steps:

[0030] preparing an aqueous solution consisting of a solvent and a solute;

[0031] The aqueous solution is wrapped around the outside of the food to be freeze-dried, and freeze-dried to obtain the freeze-dried food with a coating layer.

[0032] In the present invention, unless otherwise specified, the required materials or reagents are commercially available products well known to those skilled in the art.

[0033] In the present invention, the aqueous phase solution is preferably composed of a solvent and a solute, and the mass of the solvent in the aqueous phase solution is 10-80% of the total mass of the aqueous phase solution, and more preferably 40-60%; the solvent preferably includes one or more of water, emulsion, water-emulsion mixture, colloid substances, fiber substances, flavor substances, emulsifier substances and stabilizer substances; the present invention has no special restrictions on the ratio of different substances in the solvent and the preparation process of the solvent, which can be adjusted according to actual needs.

[0034] In the present invention, the water content of the solvent is preferably 70 to 100% of the total mass of the solvent, and more preferably 80 to 90%.

[0035] In the present invention, the solute in the aqueous solution preferably includes one or more of fruits, nuts, grains, vegetables, dairy products, cocoa products, fats, starch, food additives, stabilizers and chocolate products, and the chocolate product is preferably commercially available chocolate or chocolate powder; the total fat content of the solute is ≤30wt%. The properties of the solute preferably include liquid, sauce or powder.

[0036] In the present invention, the food additive preferably includes at least one of food-grade pigments, flavors and fragrances, emulsifiers and thickeners, the emulsifier preferably includes a single emulsifier, multiple emulsifiers or a composite emulsifier, and the emulsifier preferably includes a natural emulsifier or an artificial synthetic emulsifier. The present invention does not specifically limit the specific type name and dosage of each food additive, and the corresponding substances well known in the art can be adjusted in dosage according to actual needs.

[0037] The present invention has no special limitation on each component of the solute and its content as well as the preparation process of the solute, which can be adjusted according to actual needs.

[0038] In the present invention, the oily substance in the solute preferably includes at least one of vegetable oils, animal oils and synthetic oils; the vegetable oils preferably include at least one of cocoa butter, cocoa butter substitute, refined palm oil, palm kernel oil and refined coconut oil; the animal oils preferably include at least one of butter and anhydrous butter; the synthetic oils are preferably cocoa butter substitute, cocoa butter mixed oil or cocoa butter mixed oil.

[0039] In the present invention, the non-lipid substances in the solute preferably include one or more of plant substances and animal substances; the plant substances preferably include cocoa products, grains, nuts, flowers or fruits and vegetables; the animal substances preferably include dairy products, dairy products, beef, chicken or fish.

[0040] In the present invention, the particle size of the non-fat solid matter in the aqueous solution is preferably ≤50 μm, more preferably 15-35 μm; the total oil content of the aqueous solution is ≤30% of the total mass of the aqueous solution, more preferably 15-25%.

[0041] In the present invention, the water content of the aqueous phase solution is preferably 20 to 80% of the total mass of the aqueous phase solution, and more preferably 30 to 60%.

[0042] In the present invention, the wrapping method is preferably full wrapping or partial wrapping; the present invention has no special limitation on the partial wrapping, which can be adjusted according to the required wrapping requirements.

[0043] In the present invention, the wrapping method preferably includes coating, dipping, spraying or rolling. The present invention has no special limitation on the specific process of wrapping, and it can be carried out according to the corresponding method well known in the art.

[0044] In the present invention, the number of the wrapping layers is preferably ≥1. The present invention has no special limitation on the thickness of each wrapping layer, which can be adjusted according to actual needs.

[0045] In the present invention, when the number of the coating layers is greater than 1, the aqueous phase solutions used in different coating layers are preferably aqueous phase solutions of the same type, flavor and color, or preferably aqueous phase solutions of different types, flavors and colors.

[0046] In the present invention, the flavor of the coating layer preferably includes at least one of chocolate flavor, nut flavor, fruit flavor, floral flavor, plant flavor and meat flavor; the flavor of the coating layer is preferably regulated by the composition of the aqueous phase solution, and the present invention has no special limitation on the regulation process, which can be adjusted according to actual needs.

[0047] In the present invention, the preparation method of the aqueous phase solution preferably includes one or more of mixing, grinding, emulsification and homogenization; when the preparation method of the aqueous phase solution is preferably two or more of the above, the present invention has no special limitation on the specific preparation process, which can be adjusted according to actual needs.

[0048] The aqueous solution is wrapped around the freeze-dried food to be freeze-dried, and after freeze-drying, the present invention preferably applies an outer protective layer to the obtained freeze-dried food with the wrapped layer; the method of applying the outer protective layer preferably includes spraying, flow coating or roller coating; the outer protective layer is used to protect the product from moisture intrusion, reduce the risk of moisture intrusion into the product, and give the product additional outer flavor and color.

[0049] In the present invention, the raw materials used for the outer protective layer preferably include colloid alcohol-based solutions, colloid water-based solutions, lipid solutions or solutions containing flavors and pigments.

[0050] In the present invention, the freeze-dried food to be used preferably includes one or more of fruits, vegetables, grains, nuts, dairy products and meat products. The present invention has no particular limitation on the specific types of the freeze-dried food, and any freeze-dried food known in the art can be used. When the food is of different types, the present invention has no particular limitation on the compounding requirements of different types of food, and can be adjusted according to actual needs.

[0051] In the present invention, the freeze-dried food preferably includes fresh or cooked food; the shape of the freeze-dried food preferably includes flakes, blocks, strips or balls.

[0052] In the present invention, the freeze-drying process preferably includes a freezing process and a drying process performed sequentially.

[0053] In the present invention, the freezing treatment method preferably includes one-time freezing or multiple freezing; the one-time freezing means putting the food with the wrapping layer into the freezing equipment and not taking it out during the freezing process until it is completely frozen and formed, that is, all frozen into a solid ice state, while multiple freezing means taking the food with the wrapping layer out of the freezing equipment once or multiple times during the freezing process, and then putting it into the freezing equipment again, and continuing to freeze until it is completely frozen into a solid ice state. The purpose of multiple freezing in the present invention is to obtain the required ice crystal structure and distribution, and to apply an aqueous solution or other food solution on the surface of the pre-frozen food again to provide a new appearance effect of flavor change and color change.

[0054] In the present invention, the temperature of the freezing treatment is preferably -10 to -40°C, more preferably -30°C, and the total freezing time is preferably 0.5 to 10 hours, more preferably 4 to 6 hours; when the freezing is repeated multiple times, the time of each freezing is independently preferably 0.5 to 4 hours; the temperature of the drying is preferably 40 to 50°C, the vacuum degree is preferably 10 to 15 Pa, and the drying time is preferably 10 to 20 hours.

[0055] In the present invention, the freezing treatment and the drying treatment are preferably completed in the same freeze dryer, or the freezing treatment is completed in one device and the drying treatment is completed in another device.

[0056] The invention freezes the food with the wrapping layer until the food with the wrapping layer is completely frozen and formed into a solid ice state.

[0057] In the present invention, the moisture content of the prepared freeze-dried food with a coating layer is preferably ≤6 wt %, and more preferably 0.5-4 wt %.

[0058] The present invention provides a freeze-dried food with a coating layer prepared by the preparation method described in the above technical solution.

[0059] The technical solutions in the present invention will be described clearly and completely below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0060] Example 1

[0061] The preparation method of freeze-dried mango chunks with a dark chocolate coating comprises the following steps:

[0062] 1) Liquid preparation: The formula of chocolate solution is (by mass percentage): 40% solvent, 60% solute;

[0063] a) The solute of the chocolate solution is dark chocolate powder with a total fat content of 23-24%, and the formula thereof is (by mass percentage): 35% white sugar, 29% cocoa powder, 20% cocoa liquor, 10% cocoa butter, 6% whey powder, and the total cocoa content thereof is ≥58%. The mixture is mixed (1000L horizontal mixer of Chengdu Neste Company) and ground into powdered chocolate crumbs by a two-roller grinder (Bühler Company, Switzerland) and a five-roller grinder (Bühler Company, Switzerland). The particle size of the ground chocolate crumbs is 25 μm. The ground chocolate powders are collected and used as the solute;

[0064] b) preparing a solvent: by weight percentage, 10% fresh milk, 89.5% pure water and 0.5% sodium alginate were mixed evenly to form a solvent; the water content of the solvent was 95.5% of the total weight of the solvent;

[0065] c) preparing solution: the formula of chocolate solution is: 40% solvent and 60% solute; the mixed solvent is added into an emulsifier (Guangzhou Conghua Xinke Light Chemical Equipment Factory, JRH-1100S) according to the proportion, the jacket water temperature of the emulsifier barrel of the emulsifier is set to 60°C, the emulsifier is turned on, the speed is set to 3000rpm, and according to the formula of solvent and solute, fresh milk, purified water and sodium alginate in the solvent are first mixed evenly, mixed for 5 minutes, the speed of the emulsifier is set to 5000rpm, dark chocolate powder is slowly added into the emulsifier according to the composition ratio, after all the materials are added, emulsification is continued for 5 minutes to prepare chocolate solution, in which the particle size of non-fat solid matter in the chocolate solution is ≤30μm, the total oil content is 15.2% of the total mass of the aqueous phase solution, and the water content of the chocolate solution is 35% of the total mass of the aqueous phase solution;

[0066] 2) Wrapping: peel and core the fresh mango, cut it into cubes of 18-20 mm, put it into a freezer at -10--15°C, freeze it for 1 hour, put the prepared chocolate solution into a chocolate tank with insulation, the temperature of the insulation tank is set to 40°C, and dip the frozen mango cubes into the dark chocolate solution, and place them on a plate;

[0067] 3) Freezing: Place the plate with the chocolate-covered mango pieces in a -40°C freezer for 6 hours;

[0068] 4) Drying: The plate was taken out of the freezer and placed in a freeze dryer (Nanjing Yanwo Biotechnology, YTLG50F). The heating plate temperature, i.e., the sublimation temperature, was set to 40° C., the vacuum degree was 10 Pa, and the drying time was 16 h. Freeze-dried mangoes with a chocolate coating were prepared, and the moisture content was 1.2 wt %.

[0069] The freeze-dried mango with a dark chocolate coating in this embodiment has a unique fine and dense porous structure and a melt-in-the-mouth taste characteristic of freeze-dried products, and also has a higher temperature resistance compared to products that are freeze-dried and then coated with a chocolate coating.

[0070] Test: After the product is sealed with aluminum film, it is placed in a constant temperature box at 65℃ for 24 hours. After it is taken out, the product, including the outer layer of chocolate, still has the same tasting characteristics as before the test.

[0071] Comparative Example 1

[0072] After cutting the mango into pieces, freeze it at -40℃ for 6h, and then put it into a freeze dryer, set the sublimation temperature to 40℃, and the drying time to 16h to obtain freeze-dried mango pieces;

[0073] After the chocolate is melted in a 55-60°C insulation tank, the freeze-dried mango pieces are coated with a chocolate coating layer by dipping, and placed in a 5°C refrigerator for 20 minutes. After the chocolate is cooled and set, freeze-dried mango pieces with a chocolate coating layer are obtained.

[0074] Figure 1 This is a physical picture of the mango chunks product prepared in Example 1 that are first wrapped with a chocolate coating layer and then freeze-dried. Figure 1 As shown, the chocolate coating layer presents a porous microporous structure.

[0075] High temperature resistance test

[0076] The mango chunks prepared in Example 1, which were first coated with a chocolate coating and then freeze-dried, were subjected to a high temperature resistance test. After being placed in a 65°C constant temperature box for 48 hours, the results are shown in FIG. Figure 2 , where a is the product status diagram of opening the packaging bag when the constant temperature is completed, and b is the product status diagram of taking out the packaging bag after cooling to room temperature; Figure 2 As shown, no chocolate was found to stick to the packaging bag and no chocolate was found to melt ( Figure 2 In a), the product surface remains intact ( Figure 2 (b)

[0077] The mango chunks prepared in Comparative Example 1, which were freeze-dried and then coated with a chocolate coating, were subjected to a high temperature resistance test. After being placed in a 65°C constant temperature box for 48 hours, the results were shown in Table 1. Figure 3 , where a is the product status diagram of opening the packaging bag when the constant temperature is completed, and b is the product status diagram of taking out the packaging bag after cooling to room temperature; Figure 3 As shown, the outer chocolate coating of the product is completely melted, and the product is adhered to the packaging bag ( Figure 3 a), product appearance is damaged ( Figure 3 (b)

[0078] Example 2

[0079] The preparation method of freeze-dried strawberries with a partial chocolate coating comprises the following steps:

[0080] 1) Liquid preparation: the same chocolate solution was prepared by using the liquid preparation method and formula in Example 1;

[0081] 2) Wrapping: Remove the stems of the whole strawberries and clean them, put them into a freezer at -18 to -22°C, freeze them for 1 hour, put the prepared chocolate solution into a chocolate tank with insulation, the temperature of the insulation tank is set to 40 degrees, and dip the frozen strawberries into the chocolate sauce from the top to 2 / 3 of the tail, and place them on a plate;

[0082] 3) Freeze drying: Place the plate with the partially chocolate-wrapped strawberries into a freeze dryer (Nanjing Yanwo Biotechnology, YTLG50F), set the freezing temperature to -30°C, the freezing time to 4 h, the heating plate temperature to a maximum of 50°C, the vacuum degree to 15 Pa, and the drying time to 20 h to obtain a freeze-dried strawberry product with a partial chocolate-wrapped layer, with a moisture content of 1 wt%.

[0083] Example 3

[0084] The freeze-dried banana chips with double chocolate coating layers are prepared by the following steps:

[0085] 1) Liquid preparation: white chocolate solution 1; dark chocolate solution 2 is the dark chocolate solution in Example 1;

[0086] a) The solute of white chocolate solution 1 is sucrose-free white chocolate powder with a total fat content of 19%, and its formula composition is (by mass percentage): 45% maltitol, 25% whole milk powder, 12% cocoa butter, and 18% whey powder. The white chocolate solution 1 is mixed (100-liter horizontal mixer of Chengdu Neste Company) and ground into powdered chocolate powder by two three-roller fine grinders in series (three-roller fine grinder in Chinese patent, patent number ZL2021.2.0807255.4). The particle size of the ground chocolate powder is 35 μm. The ground chocolate powder is collected and used as the solute;

[0087] The chocolate powder is chocolate powder made by the method for preparing chocolate powder crumbs using patent number ZL201910303370.5, “A chocolate powder crumb and a preparation method thereof”.

[0088] b) preparing a solvent: uniformly mixing 99% pure water and 1% sodium alginate as a solvent;

[0089] c) preparing a solution: according to the formula of the solution: 40% solvent, 60% solute, the mixed solvent is added into an emulsifier (Guangzhou Conghua Xinke Light Chemical Equipment Factory, JRH-1100S) according to the composition ratio, the jacket water temperature of the emulsifier barrel of the emulsifier is set to 60°C, the emulsifier is turned on, the speed is set to 3000rpm, according to the formula of the solvent and the solution, the purified water and sodium alginate in the solvent are first mixed evenly, and the mixture is mixed for 5 minutes, the speed of the emulsifier is set to 6000rpm, the chocolate powder is slowly added into the emulsifier according to the proportion of the solution formula, and after all the materials are added, the emulsification is continued for 5 minutes to prepare a white chocolate solution 1;

[0090] 2) Wrapping: peel the fresh bananas, cut them into banana slices with a diameter of 10 to 12 mm, put them into a freezer at -18 to -22°C, freeze them for 1 hour, put the prepared white chocolate solution 1 into the material trough of the coating machine, set the temperature of the material trough to 40°C, start the circulation pump and mesh belt of the coating machine, set the temperature of the freezing tunnel after the coating machine to -10°C, put the frozen banana slices on the feeding mesh belt of the coating machine, and completely coat the banana slices with the sucrose-free white chocolate solution by coating the banana slices. In the freezing tunnel of the coating machine, freeze them at a temperature of -10°C for 50 minutes to set the coated sucrose-free white chocolate solution layer;

[0091] Place dark chocolate solution 2 in a heat preservation tank and set the temperature to 30°C. Dip banana slices that have been coated with the sugar-free white chocolate solution and frozen on the outside into a layer of dark chocolate solution 2 and place them on a plate.

[0092] 3) Freezing: Place the plate with the banana slices coated with double chocolate in a -40°C freezer for 6 hours;

[0093] 4) Drying: The plate was taken out of the freezer and placed in a freeze dryer (Nanjing Yanwo Biotechnology, YTLG50F). The heating plate temperature, i.e., the sublimation temperature, was set to 40°C, the vacuum degree was 10 Pa, and the drying time was 16 h. Freeze-dried bananas with a double chocolate coating layer were prepared, and the moisture content was 1.5 wt%.

[0094] The freeze-dried banana chips with double different chocolate coating layers of this embodiment not only have the unique fine porous structure, melt-in-the-mouth taste characteristics and good high temperature resistance of freeze-dried products, but also provide consumers with the unique flavor enjoyment of double chocolate and the combination of double chocolate and banana.

[0095] Example 4

[0096] The preparation method of freeze-dried purple sweet potato blocks with a dark chocolate coating layer and a protective layer comprises the following steps:

[0097] 1) Liquid preparation: using the dark chocolate solution of Example 1;

[0098] 2) Wrapping: Steam the purple sweet potato, cool it, cut it into cubes of 15-18 mm, put it into a freezer at -5-0°C, freeze it for 1 hour, put the prepared chocolate solution into a chocolate tank with insulation, the temperature of the insulation tank is set to 40°C, and wrap the refrigerated purple sweet potato cubes with the dark chocolate solution by dipping, and place them on a plate;

[0099] 3) Freezing: Place the plate with the chocolate-covered purple sweet potato cubes in a -40°C freezer for 6 hours;

[0100] 4) Drying: The plate was taken out of the freezer and placed in a freeze dryer (Nanjing Yanwo Biotechnology, YTLG50F). The heating plate temperature, i.e., the sublimation temperature, was set to 40°C, the vacuum degree was 10 Pa, and the drying time was 16 hours to prepare freeze-dried purple potato chunks with a dark chocolate coating layer, with a moisture content of 0.5 wt%.

[0101] 5) Applying a protective layer:

[0102] a) Preparation of protective solution: 70% gum arabic and 30% alcohol. Dissolve the gum arabic in the alcohol and stir evenly. The alcohol used has a volume concentration of 75%.

[0103] b) Applying a protective layer: putting the dried freeze-dried purple sweet potato chunks with a dark chocolate coating layer into a polishing pot, rotating the polishing pot, spraying a protective liquid until the product is completely covered with a layer of protective liquid, blowing dry cold air at 8-12°C into the polishing pot to dry and shape the protective liquid layer, thereby obtaining freeze-dried purple sweet potato chunks with a dark chocolate coating layer and a protective layer.

[0104] After testing, the freeze-dried purple sweet potato with a dark chocolate coating layer and a protective layer of this embodiment was placed naked in an environment with a humidity of 70% and a temperature of 65° C., and after 24 hours, it still maintained a good crispy taste.

[0105] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing a freeze-dried food with a chocolate coating, characterized in that: The following steps are involved: preparing an aqueous solution consisting of a solvent and a solute; Wrapping the aqueous solution on the outside of the freeze-dried food to be processed, and performing freeze-drying treatment to obtain the freeze-dried food with a coating layer; The preparation method of the aqueous phase solution comprises: Liquid preparation: The formula of chocolate solution is: 40% solvent, 60% solute; a) The solute of the chocolate solution is dark chocolate powder with a total fat content of 23-24%, and the formula is: 35% white sugar, 29% cocoa powder, 20% cocoa liquor, 10% cocoa butter, 6% whey powder, and the total cocoa content is ≥58%. The mixture is mixed and ground into powdery chocolate crumbs by a two-roller grinder and a five-roller grinder. The particle size of the ground chocolate crumbs is 25 μm. The ground chocolate powders are collected and used as the solute; b) preparing a solvent: by weight percentage, 10% fresh milk, 89.5% pure water and 0.5% sodium alginate were mixed evenly to form a solvent; the water content of the solvent was 95.5% of the total weight of the solvent; c) preparing solution: the formula of chocolate solution is: 40% solvent and 60% solute; the mixed solvent is added into an emulsifier according to the proportion, the jacket water temperature of the emulsifier barrel of the emulsifier is set to 60° C., the emulsifier is turned on, the speed is set to 3000 rpm, fresh milk, purified water and sodium alginate in the solvent are first mixed evenly according to the formula of solvent and solute, and the mixture is mixed for 5 minutes, the speed of the emulsifier is set to 5000 rpm, dark chocolate powder is slowly added into the emulsifier according to the composition proportion, after all the materials are added, the emulsification is continued for 5 minutes to prepare chocolate solution, in which the particle size of non-fat solid matter in the chocolate solution is ≤30 μm, the total oil content is 15.2% of the total mass of the aqueous phase solution, and the water content of the chocolate solution is 35% of the total mass of the aqueous phase solution.

2. The preparation method according to claim 1, characterized in that: The wrapping method is full wrapping or partial wrapping, or the wrapping method is single-layer wrapping or multi-layer wrapping.

3. The preparation method according to claim 1, characterized in that: The wrapping method includes coating, dipping, spraying or rolling.

4. The preparation method according to claim 1, characterized in that: The freeze-drying process includes a freezing process and a drying process performed sequentially, and the freezing process includes a single freezing process or multiple freezing processes.

5. The preparation method according to claim 4, characterized in that: The freezing treatment temperature is -40 to -10°C, and the total freezing time is 0.5 to 10 hours.

6. The preparation method according to claim 1, characterized in that: The desired freeze-dried food includes one or more of fruits, vegetables, grains, nuts, dairy products and meat products.

7. The preparation method according to claim 6, characterized in that: The desired freeze-dried food includes fresh or cooked food.

8. The preparation method according to claim 1, characterized in that: The freeze-dried food may be in the shape of a sheet, a block, a strip or a ball.

9. The freeze-dried food with a coating layer prepared by the preparation method according to any one of claims 1 to 8.

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