Method for improving texture and color of freeze-dried recombined multi-layer fruit crisp blocks and fruit crisp blocks

Abstract

The invention discloses a method for improving texture, color and luster of freeze-dried recombined multilayer fruit crisps and fruit crisps, and the method comprises the following steps: step 1, preparing lower-layer fruit pulp, adding beet pectin and laccase into the lower-layer fruit pulp, stirring, pouring into a mold to form a gel state, and freezing to obtain a frozen bottom-layer material; 2, spraying a calcium chloride solution with the temperature of 10-15 DEG C to the surface of the frozen bottom layer material to form a middle layer, standing and freezing to obtain a bottom material containing the middle layer; and step 3, preparing upper-layer fruit pulp, adding low-ester pectin into the upper-layer fruit pulp, standing, adding calcium lactate and gluconolactone into the upper-layer fruit pulp, stirring, and pouring the upper-layer fruit pulp onto the base material containing the middle layer to obtain the freeze-dried recombined multilayer fruit crisp. The laccase is utilized to catalyze the beet pectin to construct a network based on covalent bonds to fix the lower-layer fruit pulp, and the lower-layer fruit pulp is always in a gel state when the upper-layer fruit pulp is poured by combining with the difference of eutectic points, so that the texture quality of the freeze-dried recombined multilayer fruit crisp blocks is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of food preparation, and more particularly to a method for improving the texture and color of freeze-dried reconstituted multi-layered fruit crisps and the fruit crisps. Background Art

[0002] Vacuum freeze drying refers to the process of obtaining dried food by using the principle of sublimation to remove the water in the pre-frozen material directly from ice to water vapor without melting the ice. It can be assisted or not assisted by other technologies for further dehydration. Since vacuum freeze drying technology is carried out in a low-temperature, low-oxygen environment, most biological reactions are stagnant, and there is no liquid water in the treatment process. The water is directly sublimated in a solid state, so that the original structure and shape of the material are protected to the greatest extent, and finally a high-quality dried product with both appearance and internal quality is obtained. This drying method is particularly suitable for the processing of fruits rich in heat-sensitive nutrients and functional substances such as vitamins and polyphenols.

[0003] From 2021 to 2024, the average annual compound growth rate of freeze-dried food in my country exceeded 15%. In 2023, the total scale of the freeze-dried food industry exceeded 26 billion yuan. As people pay more attention to nutritional health and balanced diet, the combination of multiple fruit raw materials has become a new development trend in recent years based on the freeze-drying of single fruit raw materials. By combining texture reconstruction and freeze-drying technology, high-end composite fruit freeze-dried snacks containing multiple layers of different fruits can be processed and manufactured.

[0004] At present, freeze-dried reconstituted multi-layered fruits are generally made by layered pouring, that is, first pouring a kind of fruit pulp into the mold to freeze it into a solid state, and then pouring the second layer of fruit pulp, and repeating this process. Due to the different material compositions of different fruit pulp raw materials, this method of pouring the second layer directly on the frozen material cannot form a tight cross-linking between the layers, and the interlayer bonding force is extremely weak. The product is prone to interlayer separation during subsequent transportation and consumption, which reduces the texture quality and integrity of the product. Summary of the invention

[0005] An object of the present invention is to provide a method for improving the texture and color of freeze-dried reconstituted multi-layer fruit crisps and fruit crisps, using laccase to catalyze beet pectin to construct a covalent bond-based network to fix the lower layer of fruit pulp, and combining the difference in eutectic points to ensure that the lower layer of fruit pulp is always in a gel state when pouring the upper layer of fruit pulp, thereby improving the texture quality of the freeze-dried reconstituted multi-layer fruit crisps.

[0006] In order to achieve these objects and other advantages according to the present invention, a method for improving the texture and color of freeze-dried reconstituted multi-layered fruit crisps is provided, comprising the following steps:

[0007] Step 1, preparing a lower layer of pulp, adding beet pectin, laccase, and low-ester pectin to the lower layer of pulp, stirring, pouring into a mold, standing to form a gel, freezing, and obtaining a frozen bottom layer material, wherein the mass fraction of beet pectin in the lower layer of pulp is 0.8-1.2%, the concentration of laccase is 200-600 U / g, and the mass fraction of low-ester pectin is 0.5-1%;

[0008] Step 2: Spraying a calcium chloride solution at a temperature of 10 to 15° C. onto the surface of the frozen bottom material to form an intermediate layer, allowing it to stand and freeze to obtain a bottom material containing an intermediate layer, wherein the concentration of the calcium chloride solution is 15 to 25 mmol / / mL;

[0009] Step 3, preparing an upper layer of fruit pulp, adding low-ester pectin to the upper layer of fruit pulp, letting it stand, adding calcium lactate and gluconolactone to the upper layer of fruit pulp and stirring, and pouring the upper layer of fruit pulp on a base material containing an intermediate layer, letting it stand to form a gel, and freezing, freeze-drying, and demolding in sequence to obtain freeze-dried reconstituted multi-layer fruit crisp blocks, wherein the concentration of calcium lactate in the upper layer of fruit pulp is 15-25 mmol / mL, the concentration of gluconolactone is 40-50 mmol / mL, and the mass fraction of low-ester pectin is 0.8-1.2%.

[0010] Preferably, in step 1, the temperature of pouring into the mold and standing to form a gel is 37° C. and the time is 2 to 3 hours.

[0011] Preferably, in step 1, the freezing temperature is -18°C and the freezing time is 4 to 6 hours.

[0012] Preferably, in step 2, the calcium chloride solution is sprayed onto the frozen bottom material at -2 to 2°C, the standing time is 4 to 8 minutes, the freezing temperature is -18°C, and the time is 1 to 2 hours.

[0013] Preferably, in step three, low-ester pectin is added to the upper layer of fruit pulp and then allowed to stand at -2 to 2° C. for 4 to 6 hours.

[0014] Preferably, in step three, the upper layer of pulp is poured onto the base material containing the middle layer at -2.5 to -1.5°C and allowed to stand at -2.5 to -1.5°C for 2 to 3 hours.

[0015] Preferably, in step three, the freezing temperature is -18°C and the freezing time is 4 to 6 hours.

[0016] Preferably, in step three, the product is placed in a freeze dryer for freeze drying, and the temperature of the heating plate of the freeze dryer is set as follows: first stage, 65-85°C, 6-8h; second stage, 50-70°C, 8-12h; third stage, 40-50°C, drying until the moisture content is less than 4%.

[0017] Preferably, in step three, ethanol is added when adding low-ester pectin to the upper layer of pulp, and the mass fraction of ethanol in the upper layer of pulp is 6-8%.

[0018] The present invention also provides a fruit crisp block prepared by the method of improving the texture and color of freeze-dried reconstituted multi-layer fruit crisp blocks.

[0019] The present invention has at least the following beneficial effects:

[0020] First, the present invention adopts a step-by-step method to prepare a texture reconstructed multi-layer fruit crisp block, uses laccase to catalyze beet pectin to construct a network based on covalent bonds to fix the lower layer of fruit pulp, and combines the difference in eutectic points to ensure that the lower layer of fruit pulp is always in a gel state when pouring the upper layer of fruit pulp, thereby solving the problem of poor color and reduced appearance quality caused by interlayer interpenetration of freeze-dried reconstructed multi-layer fruit crisp blocks. The specific principle is as follows: First, the present invention selects beet pectin as the gel network matrix of the lower layer of fruit pulp. The beet pectin has many ferulic acid ester cross-linking sites, high covalent bond cross-linking strength, and good thermal stability of the formed gel, which fluctuates and is stable within the temperature range of room temperature and below room temperature, which plays a great role in improving the strength and cohesion of the lower layer of fruit pulp. Secondly, after adding ethanol to the upper layer of fruit pulp, the freezing point of the upper layer of fruit pulp can be reduced to -2 to -3°C, thereby controlling the fruit pulp temperature when pouring the upper layer of fruit pulp at a lower temperature close to its freezing point, thereby achieving the completion of pouring when the lower layer of fruit pulp is still in a frozen state. As for the added ethanol, the ethanol is very volatile and will not remain in the fruit crisps after freeze-drying, and will not cause ethanol residue in the fruit crisps.

[0021] Second, the present invention introduces an intermediate spray layer, and uses calcium ion-mediated pectin cross-linking to form a skeleton network to cross-link different layers, thereby solving the problem that the freeze-dried reconstituted multi-layer fruit crisps are not firmly adhered and easy to fall off. The specific principles are as follows: First, in the lower layer of fruit pulp, not only a covalent network based on ferulic acid ester cross-linking is constructed by beet pectin, but also low-ester pectin is added. This is because beet pectin has more side chains and relatively fewer carboxyl groups. By adding low-ester pectin, the concentration of free carboxyl groups on the surface of the lower layer of fruit pulp gel is increased, so that it can more fully participate in interlayer cross-linking; secondly, by spraying calcium chloride solution, a thin layer with a high calcium ion concentration is constructed as an intermediate layer. Calcium ions can simultaneously react with the carboxyl groups of different pectin molecules to form calcium bridges, thereby bonding different fruit pulp layers together. Finally, the upper layer of fruit pulp contains low-ester pectin, which will gradually release calcium ions in calcium lactate to form a gel as gluconolactone is hydrolyzed after pouring. However, the fruit pulp near the middle layer will instantly come into contact with the high concentration of calcium ions in the base material containing the middle layer, and then form cross-links with it. As time goes on, the upper layer gradually forms a gel and turns into a solid state. Finally, the upper and lower layers of fruit pulp are combined with each other through the surface pectin molecules, forming a strong adhesion between the layers at the macro level, effectively enhancing the adhesion between the layers of freeze-dried reconstituted multi-layer fruit crisps, and effectively solving the problem of weak adhesion and easy falling off between the layers of freeze-dried reconstituted multi-layer fruit.

[0022] Third, in the present invention, the lower layer of pulp is cross-linked under a certain temperature (37° C.) based on laccase catalysis of beet pectin ferulic acid ester to form a covalent bond gel, and the gel strength is high and the gel is stable; when the upper layer of pulp forms a gel, in order to keep the lower layer of pulp in a gel state, the upper layer of pulp needs to form a gel at a low temperature, so calcium ions are introduced into the upper layer of pulp so that the upper layer of pulp is cross-linked under low temperature conditions to form an ionic bond gel, and at the same time, calcium lactate can slowly release calcium ions under the acidification of gluconolactone, thereby avoiding the upper layer of pulp from forming a gel before pouring, and allowing the upper layer of pulp to form a gel after pouring.

[0023] Other advantages, objectives and features of the present invention will be embodied in part through the following description, and in part will be understood by those skilled in the art through study and practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a diagram showing the placement state of the fruit crisp pieces when detecting the interlayer shear force in the present invention;

[0025] Figure 2 The diagram is a schematic diagram of the sampling process and test points for detecting the color difference of the bonding surface in the present invention. DETAILED DESCRIPTION

[0026] The present invention is further described in detail below in conjunction with embodiments so that those skilled in the art can implement the invention with reference to the description.

[0027] <Example 1>

[0028] A method for improving the texture and color of freeze-dried reconstituted multi-layer fruit crisps, comprising the following steps:

[0029] Step 1, lower layer injection molding: prepare the lower layer pulp (the preparation method of the lower layer pulp is to clean the blueberries, cut them into fruit blocks, and add sodium isoascorbate to the fruit blocks, wherein the sodium isoascorbate in the fruit blocks accounts for 0.1% of the total mass of the fruit blocks, and use a pulper to make pulp to obtain the lower layer pulp), add beet pectin, laccase, and citrus low-ester pectin to the lower layer pulp, stir, pour into a mold, let stand at 37° C. for 2 hours to form a gel, and freeze at -18° C. for 4 hours to obtain a frozen bottom layer material, wherein beet pectin accounts for 0.8% of the total mass of the lower layer pulp, the concentration of laccase in the lower layer pulp is 200 U / g, and the citrus low-ester pectin accounts for 0.5% of the total mass of the lower layer pulp;

[0030] Step 2: Spray the middle layer: Spray a 15°C calcium chloride solution onto the frozen bottom layer at 2°C. The spray volume of the calcium chloride solution is 0.1 ml / cm 2 , forming an intermediate layer, standing at 2°C for 4 minutes, and then placed at -18°C for 1 hour to obtain a base material containing an intermediate layer, wherein the concentration of the calcium chloride solution is 15mmol / / mL;

[0031] Step 3, lower layer injection molding: prepare an upper layer of fruit pulp (the preparation method of the upper layer of fruit pulp is to wash the apples, cut them into fruit pieces, and add sodium isoascorbate to the fruit pieces, wherein the sodium isoascorbate in the fruit pieces accounts for 0.1% of the total mass of the fruit pieces, and use a pulper to make pulp to obtain the upper layer of fruit pulp), add citrus low-ester pectin and ethanol to the upper layer of fruit pulp, mix them and let them stand at 2° C. for 4 hours, and add calcium lactate and gluconolactone (GDL) to the upper layer of fruit pulp and stir;

[0032] At -1.5°C, pour the upper layer of fruit pulp onto a base material containing an intermediate layer (the base material containing the intermediate layer is in a frozen state) and cover the surface of the base material containing the intermediate layer, then place it at -1.5°C for 1 hour to form a gel, and freeze, freeze-dry, and demold it in sequence to obtain freeze-dried reconstituted multi-layer fruit crisp blocks, wherein the concentration of calcium lactate in the upper layer of fruit pulp is 15 mmol / mL, the concentration of gluconolactone is 40 mmol / mL, citrus low-ester pectin accounts for 0.8% of the total mass of the upper layer of fruit pulp, and ethanol accounts for 6% of the total mass of the upper layer of fruit pulp.

[0033] In step 3, the freezing temperature is -18°C and the time is 4 hours.

[0034] In step three, the mixture is placed in a freeze dryer for freeze drying, and the temperature of the heating plate of the freeze dryer is set as follows: first stage, 85°C, 6h; second stage, 70°C, 8h; third stage, 50°C, drying until the moisture content is less than 4%.

[0035] <Example 2>

[0036] A method for improving the texture and color of freeze-dried reconstituted multi-layer fruit crisps, comprising the following steps:

[0037] Step 1, lower layer injection molding: prepare the lower layer pulp (the preparation method of the lower layer pulp is to clean the blueberries, cut them into fruit blocks, and add sodium isoascorbate to the fruit blocks, wherein the sodium isoascorbate in the fruit blocks accounts for 0.1% of the total mass of the fruit blocks, and use a pulper to make pulp to obtain the lower layer pulp), add beet pectin, laccase, and apple low-ester pectin to the lower layer pulp, stir, pour into a mold, let stand at 37° C. for 2.5 hours to form a gel, and freeze at -18° C. for 5 hours to obtain a frozen bottom layer material, wherein beet pectin accounts for 1% of the total mass of the lower layer pulp, the concentration of laccase in the lower layer pulp is 400 U / g, and the apple low-ester pectin accounts for 0.7% of the total mass of the lower layer pulp;

[0038] Step 2: Spray the middle layer: Spray a 12°C calcium chloride solution onto the surface of the frozen bottom layer at 0°C. The spray volume of the calcium chloride solution is 0.15 ml / cm 2 , forming an intermediate layer, standing at 0°C for 6 minutes, and then placed at -18°C and frozen for 1.5 hours to obtain a base material containing an intermediate layer, wherein the concentration of the calcium chloride solution is 20mmol / / mL;

[0039] Step 3, lower layer injection molding: prepare an upper layer of fruit pulp (the preparation method of the upper layer of fruit pulp is to wash the apples, cut them into fruit pieces, and add sodium isoascorbate to the fruit pieces, wherein the sodium isoascorbate in the fruit pieces accounts for 0.1% of the total mass of the fruit pieces, and use a pulping machine to make pulp to obtain the upper layer of fruit pulp), add apple low-ester pectin and ethanol to the upper layer of fruit pulp, mix them and let them stand at 0°C for 5 hours, then add calcium lactate and gluconolactone (GDL) to the upper layer of fruit pulp and stir;

[0040] At -2°C, pour the upper layer of fruit pulp onto a base material containing an intermediate layer (the base material containing the intermediate layer is in a frozen state) and cover the surface of the base material containing the intermediate layer, then place it at -2°C and let it stand for 2.5 hours to form a gel, and freeze, freeze-dry and demold it in sequence to obtain freeze-dried reconstituted multi-layer fruit crisp blocks, wherein the concentration of calcium lactate in the upper layer of fruit pulp is 20 mmol / mL, the concentration of gluconolactone in the upper layer of fruit pulp is 45 mmol / mL, the upper apple low-ester pectin accounts for 1% of the total mass of the upper layer of fruit pulp, and ethanol accounts for 7% of the total mass of the upper layer of fruit pulp.

[0041] In step 3, the freezing temperature is -18°C and the time is 5 hours.

[0042] In step three, the mixture is placed in a freeze dryer for freeze drying, and the temperature of the heating plate of the freeze dryer is set as follows: first stage, 75°C, 7h; second stage, 60°C, 1h; third stage, 45°C, drying until the moisture content is less than 4%.

[0043] <Example 3>

[0044] A method for improving the texture and color of freeze-dried reconstituted multi-layer fruit crisps, comprising the following steps:

[0045] Step 1, lower layer injection molding: prepare the lower layer pulp (the preparation method of the lower layer pulp is to clean the blueberries, cut them into fruit blocks, and add sodium isoascorbate to the fruit blocks, wherein the sodium isoascorbate in the fruit blocks accounts for 0.1% of the total mass of the fruit blocks, and use a pulper to make pulp to obtain the lower layer pulp), add beet pectin, laccase, and apple low-ester pectin to the lower layer pulp, stir, pour into a mold, let stand for 3 hours at 37° C. to form a gel, and freeze at -18° C. for 6 hours to obtain a frozen bottom layer material, wherein beet pectin accounts for 1.2% of the total mass of the lower layer pulp, the concentration of laccase in the lower layer pulp is 600 U / g, and the apple low-ester pectin accounts for 1% of the total mass of the lower layer pulp;

[0046] Step 2: Spray the middle layer: Spray a 10°C calcium chloride solution onto the surface of the frozen bottom layer at -2°C. The spray volume of the calcium chloride solution is 0.2 ml / cm 2 , forming an intermediate layer, standing at -2°C for 8 minutes, and then freezing at -18°C for 2 hours to obtain a base material containing an intermediate layer, wherein the concentration of the calcium chloride solution is 25mmol / / mL;

[0047] Step 3, lower layer injection molding: prepare an upper layer of fruit pulp (the preparation method of the upper layer of fruit pulp is to wash the apples, cut them into fruit pieces, and add sodium isoascorbate to the fruit pieces, wherein the sodium isoascorbate in the fruit pieces accounts for 0.1% of the total mass of the fruit pieces, and use a pulping machine to make pulp to obtain the upper layer of fruit pulp), add apple low-ester pectin and ethanol to the upper layer of fruit pulp, mix them, and let them stand at -2°C for 6 hours, and add calcium lactate and gluconolactone (GDL) to the upper layer of fruit pulp and stir;

[0048] At -2.5°C, pour the upper layer of fruit pulp onto a base material containing an intermediate layer (the base material containing the intermediate layer is in a frozen state) and cover the surface of the base material containing the intermediate layer, then place it at -2.5°C for 3 hours to form a gel, and freeze, freeze-dry, and demold it in sequence to obtain freeze-dried reconstituted multi-layer fruit crisp blocks, wherein the concentration of calcium lactate in the upper layer of fruit pulp is 25 mmol / mL, the concentration of gluconolactone is 50 mmol / mL, apple low-ester pectin accounts for 1.2% of the total mass of the upper layer of fruit pulp, and ethanol accounts for 8% of the total mass of the upper layer of fruit pulp.

[0049] In step 3, the freezing temperature is -18°C and the time is 6 hours.

[0050] In step three, the mixture is placed in a freeze dryer for freeze drying, and the temperature of the heating plate of the freeze dryer is set as follows: first stage, 65°C, 8h; second stage, 50°C, 12h; third stage, 40°C, drying until the moisture content is less than 4%.

[0051] <Example 4>

[0052] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0053] Select strawberry as the raw material of the lower layer of pulp, and select banana as the raw material of the upper layer of pulp.

[0054] <Example 5>

[0055] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0056] Dragon fruit is selected as the raw material of the lower layer of pulp, and grape is selected as the raw material of the upper layer of pulp.

[0057] <Example 6>

[0058] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0059] Select kiwi fruit as the raw material for the lower layer of fruit pulp, and select yellow peach as the raw material for the upper layer of fruit pulp.

[0060] <Example 7>

[0061] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0062] Papaya and Hami melon were selected as the raw materials for the lower layer of pulp (the papaya and Hami melon were pulped separately and mixed in a ratio of 1:1), and pineapple and lychee were selected as the raw materials for the upper layer of pulp (the pineapple and lychee were pulped separately and mixed in a ratio of 1:1).

[0063] <Example 8>

[0064] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0065] Papaya and Hami melon were selected as the raw materials for the lower layer of pulp (the papaya and Hami melon were pulped separately and mixed in a ratio of 1:1), and pineapple and lychee were selected as the raw materials for the upper layer of pulp (the pineapple and lychee were pulped separately and mixed in a ratio of 1:1).

[0066] <Example 9>

[0067] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0068] Durian, orange and passion fruit were selected as the raw materials for the lower layer of fruit pulp (durian, orange and passion fruit were pulped separately and mixed according to the ratio of 1:1:0.2), and mango, yellow peach and coconut meat were selected as the raw materials for the upper layer of fruit pulp (mango, yellow peach and coconut meat were pulped separately and mixed according to the ratio of 1:1:0.1).

[0069] <Example 10>

[0070] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0071] Watermelon, orange and red raspberry were selected as the raw materials for the lower layer of fruit pulp (watermelon, orange and red raspberry were pulped separately and mixed according to the ratio of 1:1:0.2), and pear, carambola and grape were selected as the raw materials for the upper layer of fruit pulp (pear, carambola and grape were pulped separately and mixed according to the ratio of 1:1:0.3).

[0072] <Comparative Example 1>

[0073] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0074] In step 1, no beet pectin was added to the lower layer of pulp, that is, laccase was added to the lower layer of pulp and stirred, and then poured into a mold. The other conditions were the same as those in Example 2.

[0075] <Comparative Example 2>

[0076] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0077] In step 1, no laccase was added to the lower layer of pulp, that is, beet pectin was added to the lower layer of pulp and stirred, and then poured into a mold. The remaining operations were the same as those in Example 2.

[0078] <Comparative Example 3>

[0079] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0080] In step 1, if the lower layer of pulp does not form a gel at 37° C. for 2.5 h, beet pectin and laccase are added to the lower layer of pulp, stirred, poured into a mold, and frozen at -18° C. for 5 h. The remaining operations are the same as in Example 2.

[0081] <Comparative Example 4>

[0082] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0083] In step 2, the calcium chloride solution is not sprayed, that is, the upper layer of fruit pulp prepared in step 3 is poured onto the surface of the frozen bottom layer material, and the remaining operations are the same as those in Example 2.

[0084] <Comparative Example 5>

[0085] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0086] A calcium chloride solution with a spray temperature of 4°C was formed into an intermediate layer, which was directly placed at -18°C and frozen for 1.5 hours to obtain a base material containing an intermediate layer. The remaining operations were the same as in Example 2.

[0087] <Comparative Example 6>

[0088] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0089] Step 1: After the gel is formed, beet pectin and laccase are added to the lower layer of pulp without freezing, and the mixture is poured into a mold and allowed to stand at 37° C. for 2.5 hours to form a gel to obtain a frozen bottom layer material. The remaining operations are the same as those in Example 2.

[0090] <Comparative Example 7>

[0091] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0092] Step 2 is to spray a calcium chloride solution at a temperature of 25°C onto the surface of the frozen bottom material at 25°C to form an intermediate layer, let it stand at 25°C for 30 minutes, and place it at -18°C for 1.5 hours to obtain a bottom material containing a middle layer. The remaining operations are the same as Example 2.

[0093] <Comparative Example 8>

[0094] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0095] In step 2, no freezing is performed. At -2°C, the upper layer of fruit pulp is poured onto the bottom material containing the middle layer after standing. That is, at 0°C, a calcium chloride solution at a temperature of 12°C is sprayed onto the surface of the frozen bottom material to form a middle layer. The bottom material is allowed to stand for 6 minutes to obtain a bottom material containing the middle layer. At -2°C, the upper layer of fruit pulp is poured onto the bottom material containing the middle layer (at this time, the bottom material containing the middle layer is not in a frozen state) and covers the surface of the bottom material containing the middle layer.

[0096] <Comparative Example 9>

[0097] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0098] In step three, apple low-ester pectin (without ethanol) is added to the upper layer of pulp, stirred, and allowed to stand at 0°C for 5 hours. Calcium lactate and gluconolactone (GDL) are then added to the upper layer of pulp, stirred. At -0.5°C, the upper layer of pulp is poured onto the surface of the base material containing the middle layer, allowed to stand at -0.5°C for 2.5 hours, and then frozen, freeze-dried, and demolded in sequence.

[0099] <Comparative Example 10>

[0100] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0101] In step 3, ethanol (without adding apple low-ester pectin) is added to the upper layer of pulp, stirred, and allowed to stand at 0° C. for 5 hours, and then calcium lactate and gluconolactone (GDL) are added to the upper layer of pulp, stirred.

[0102] <Comparative Example 11>

[0103] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0104] In step three, apple low-ester pectin and ethanol are added to the upper layer of pulp with stirring (without calcium lactate and gluconolactone), and the mixture is equilibrated at 0°C for 5 hours. Then, the upper layer of pulp is poured onto the base material containing the middle layer (the base material containing the middle layer is in a frozen state) at -2°C to cover the surface of the base material containing the middle layer.

[0105] <Comparative Example 12>

[0106] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0107] In step three, pour the upper layer of fruit pulp onto the base material containing the middle layer (the base material containing the middle layer is in a frozen state) at -2°C and cover the surface of the base material containing the middle layer (not standing still), and freeze, freeze-dry and demold in sequence to obtain freeze-dried reconstituted multi-layer fruit crisp blocks.

[0108] <Comparative Example 13>

[0109] The method of Example 2 is used to prepare the fruit crisp pieces, except that:

[0110] In step three, pour the upper layer of fruit pulp onto the base material containing the middle layer (the base material containing the middle layer is in a frozen state) at -2°C and cover the surface of the base material containing the middle layer, then place it at 25°C for equilibration for 3 hours and freeze, freeze-dry and demold in sequence to obtain freeze-dried reconstituted multi-layer fruit crisp blocks.

[0111] <Comparative Example 14>

[0112] The direct step-by-step pouring process commonly used in the industry is as follows:

[0113] (1) preparing the lower layer pulp: adding 0.1% sodium isoascorbate to the blueberry fruit pieces and beating into a homogenous pulp using a pulping machine;

[0114] (2) Lower layer injection molding: Pour the lower layer of fruit pulp into the mold and freeze it at -18℃ for 5h;

[0115] (3) preparing the upper layer pulp: adding 0.1% sodium isoascorbate to the apple fruit pieces, blending them into a homogenous pulp using a pulping machine, and placing them at 4°C;

[0116] (4) Upper layer injection molding: the lower layer of fruit pulp frozen in step (2) is placed at 4°C, and the upper layer of fruit pulp in step (3) is poured into the mold and covered on the frozen lower layer of fruit pulp, and then placed at -18°C for freezing for 5 hours. After the freezing is completed, freeze drying is performed, and the temperature of the freeze dryer heating plate is set as follows: first stage, 75°C, 7 hours; second stage, 60°C, 10 hours, third stage, 45°C; finally drying to a moisture content of less than 4%; after the freeze drying is completed, the freeze-dried material is taken out from the mold, and the freeze-dried texture reconstructed multi-layer fruit crisp blocks are obtained after packaging.

[0117] The freezing treatments of Examples 1-10 and Comparative Examples 1-14 were all performed in a cold storage.

[0118] <Experimental Characterization>

[0119] The interlayer shear force and the color difference of the bonding surface of the fruit crisp blocks were detected. The test results are shown in Table 1. For each test method, a total of 24 groups of experimental methods of Examples 1-10 and Comparative Examples 1-14 were used to prepare fruit crisp blocks. Each group of experiments was repeated 12 times to obtain 12 fruit crisp blocks (6 blocks were used to detect the interlayer shear force, and the remaining 6 blocks were used to detect the color difference of the bonding surface). The test method is:

[0120] Interlayer shear force: The interlayer adhesion of freeze-dried reconstituted multi-layer fruit crisps is expressed by interlayer shear force, which is measured by a texture analyzer. Fix one side of the interface of each fruit crisp on the sample clamp of the texture analyzer sample table in a vertical direction (the sample is placed as shown in the figure). Figure 1 Specific test conditions are set as follows: the test type is a single test, the test type is downward pressure, the target mode is displacement, the speed before and after the test is 2.00 mm / s, the test speed is 1.00 mm / s, the trigger force is 0.05 N, and the detection distance is 50.00 mm. Each piece of fruit crisp is measured 3 times, and the average interlayer shear force of the 6 fruit crisps in each group of experiments is calculated to obtain the data in Table 1. The maximum force (the maximum force required to cause obvious shear damage between layers during the test) represents the interlayer shear force of the freeze-dried crisp (unit, N).

[0121] Adhesive surface color difference value: Adhesive surface color difference value is used to characterize the change of color of the interface between material layers. It is measured by a colorimeter. The specific measurement method is as follows: Use a CM-5 colorimeter to measure the color of the fruit crisps, select an 8mm target mask, and use white and black calibration devices to perform 0% and 100% calibration. For each piece of fruit crisp, use a blade to cut it evenly along the interface of the fruit crisp to separate the fruit pulp layer A, the fruit pulp layer B, and the middle adhesive layer sandwiched between the fruit pulp layer A and the fruit pulp layer B, and use a hard-bristle brush to scrape off the middle layer on the adhesive surface of the fruit pulp layer A and the fruit pulp layer B. The fruit pulp layer A and the fruit pulp layer B of each fruit crisp are measured according to Figure 2 The color difference analysis was performed by taking 5 test points on the adhesive surface and the non-adhesive surface, and measuring the L*, a* and b* values ​​of each point. The color difference value of the adhesive surface of the material is calculated as shown in (1). Each piece of fruit crisp is taken according to Figure 2 The average value of the color difference values ​​of each test point in the test shown represents the color difference value (ΔE) of the bonding surface of the material, wherein the color difference values ​​are calculated one by one for the 5 test points on the bonding surface and the 5 test points on the non-bonding surface.

[0122]

[0123] Wherein, ΔE is the color difference value of the bonding surface of the material; a1, b1, and L1 are the chromaticity values ​​on the non-bonding surface respectively; a2, b2, and L2 are the chromaticity values ​​on the bonding surface respectively. The test results are expressed by the average value of the color difference values ​​of the bonding surface of the material of 6 pieces of fruit crisps in the same group of experiments (5 points are taken on the bonding surface and non-bonding surface of each layer of fruit pulp).

[0124] Table 1 shows the interlaminar shear force and color difference of fruit crisps

[0125]

[0126]

[0127] The freeze-dried reconstituted multi-layered fruit crisps prepared in Example 1, on the one hand, introduce an intermediate spray layer, and use calcium ion-mediated pectin cross-linking to form a skeleton network to cross-link different layers. Due to the influence of the cross-linking effect of polysaccharides, the product has strong interlayer adhesion, which effectively solves the problem that the freeze-dried reconstituted multi-layered fruit crisps are not firmly adhered and easy to fall off. The different fruit layers of the product are not easy to separate during transportation and consumption, the product has good integrity, and the product texture quality is excellent; on the other hand, laccase is used to catalyze beet pectin to construct a network based on covalent bonds to fix the lower layer, and combined with the difference in eutectic points, it is ensured that the lower layer of fruit pulp is always in a gel state when pouring the upper layer of fruit pulp, thereby solving the problem of poor color and reduced appearance quality caused by mutual penetration between freeze-dried multi-layered reconstituted fruit layers. The fruit crisps obtained in Example 1 have clear color boundaries between layers, no color blur or gradient color, and the product has excellent appearance quality.

[0128] Compared with Example 1, Examples 2 and 3 further increase the content of beet pectin and laccase, making the constructed polysaccharide network denser than that of Example 1, thereby obtaining relatively stronger interlaminar shear force and lower material color difference value, indicating that the texture and appearance quality of the product are further improved. From the perspective of product quality, the texture and color improvement of the fruit crisps in Example 3 are the best, but from an economic point of view, Example 2 has the best industrial application value, so Examples 4-10 and Comparative Examples 1-13 are all prepared based on Example 2.

[0129] ,Comparative analysis of Example 2 and Example 4-10, Example 4-10 adjusted the fruit types of the upper layer of fruit pulp and the lower layer of fruit pulp. The texture and color test results of the fruit crisp blocks were comparable to those of Example 2, which shows that the preparation method of the freeze-dried reconstituted multi-layer fruit crisp blocks of the present invention can be applied to different types of fruits and can also be used for composite fruit pulp.

[0130] Comparative analysis of Example 2 and Comparative Examples 1-3 shows that the data of Example 2 is better than that of Comparative Examples 1-3. Comparative Example 1 does not add beet pectin when preparing the lower layer of pulp, and Comparative Example 2 does not add laccase when preparing the lower layer of pulp. Comparative Example 3 does not allow the lower layer of pulp to form a gel in the mold. On the one hand, beet pectin and laccase are not added to the lower layer of pulp, resulting in the lower layer of pulp being unable to cross-link to form a gel, which enhances the fluidity of the lower layer of pulp and is not conducive to stabilizing the pigment in the pulp, causing the bonding surface color difference value of Comparative Example 1 to be much higher than that of Example 2. At the same time, freezing is performed directly without forming a gel, which results in the inability of the polysaccharide in Comparative Example 3 to be cross-linked. On the other hand, since the lower layer of pulp in Comparative Examples 1-3 does not form an effective cross-linked network, the cross-linking strength between the middle layer and the lower layer is reduced to a certain extent, which reduces the interlayer shear force and makes it easy to separate the fruit crisps.

[0131] A comparative analysis of Example 2 and Comparative Example 4 shows that the data of Example 2 is better than that of Comparative Example 4. In the preparation process of Comparative Example 4, no intermediate layer of calcium chloride solution is sprayed, which makes it impossible for the upper layer of fruit pulp and the lower layer of fruit pulp to form calcium bridge cross-linking through calcium ions, and the bonding strength between the upper layer of fruit pulp and the lower layer of fruit pulp is relatively small, the interlayer shear force is reduced, and the fruit crisps are easily separated from each other.

[0132] Comparative analysis of Example 2 and Comparative Example 5 shows that the data of Example 2 is better than that of Comparative Example 5. Comparative Example 5 was not allowed to stand. Since the spraying liquid temperature was too low and the calcium ions in the spraying liquid did not stand when the calcium chloride solution was sprayed, there was too little penetration exchange between the calcium ions in the spraying liquid and the surface of the lower layer of fruit pulp, resulting in insufficient cross-linking strength on the upper surface of the lower layer of fruit pulp, thereby reducing the interlayer cross-linking strength of the fruit crisps.

[0133] A comparative analysis of Example 2 and Comparative Example 6 shows that the data of Example 2 is better than that of Comparative Example 6. Comparative Example 6 is not frozen, and the lower layer of pulp is not frozen after forming a gel, but is directly sprayed with calcium chloride solution, which leads to serious penetration between the middle layer and the lower layer of pulp. Calcium ions penetrate into the lower layer of pulp, reducing the calcium ion concentration in the middle layer, which is insufficient to form sufficient cross-linking with the pectin in the upper layer of pulp, thereby affecting the cross-linking strength between the upper layer of pulp and the lower layer of pulp, and reducing the interlayer shear force.

[0134] A comparative analysis of Example 2 and Comparative Example 7 shows that the data of Example 2 is better than that of Comparative Example 7. In Comparative Example 7, the spray liquid and ambient temperature are too high and the standing time is too long, which leads to serious penetration of the middle layer and the lower layer of pulp. Calcium ions penetrate into the lower layer of pulp, reducing the calcium ion concentration in the middle layer, which is insufficient to form sufficient cross-linking with the pectin in the upper layer of pulp, thereby affecting the cross-linking strength between the upper layer of pulp and the lower layer of pulp and reducing the interlayer shear force.

[0135] A comparative analysis of Example 2 and Comparative Example 8 shows that the data of Example 2 is better than that of Comparative Example 8. Comparative Example 8 is not frozen again after spraying the calcium chloride solution, and the upper layer of fruit pulp is directly poured, which increases the surface temperature of the lower layer of fruit pulp, resulting in enhanced material fluidity, which causes the pigment to diffuse when the upper layer of fruit pulp is poured, which is not conducive to the stability of the pigment between layers. At the same time, the calcium ion concentration of the middle layer will partially penetrate into the lower layer of fruit pulp, thereby reducing the calcium ion concentration in the middle layer, reducing the cross-linking strength of the calcium ions between the layers, causing the interlayer adhesion to decrease, and affecting the interlayer stability of the fruit crisps.

[0136] A comparative analysis of Example 2 and Comparative Example 9 shows that the data of Example 2 is better than that of Comparative Example 9. In Comparative Example 9, no ethanol is added to the upper layer of fruit pulp, and the upper layer pouring and balancing are completed under relatively high temperature conditions. In Comparative Example 9, no ethanol is added to the upper layer of fruit pulp, resulting in a relatively high freezing point, which leads to unstable ice crystal structure on the surface of the upper layer of fruit pulp, improved fluidity of the system, easy diffusion of pigments, and increased color difference of the bonding surface. At the same time, due to the increase in pouring temperature, the penetration of calcium ions from the upper layer into the middle layer and the lower layer of fruit pulp is also enhanced, and the interlayer crosslinking of the upper layer and the lower layer of fruit pulp is strengthened compared with Example 2, and the interlayer shear force is increased.

[0137] A comparative analysis of Example 2 and Comparative Example 10 shows that the data of Example 2 is better than that of Comparative Example 10. In Comparative Example 10, apple low-ester pectin is not added to the upper layer of fruit pulp. On the one hand, because apple low-ester pectin is not added to the upper layer of fruit pulp, the upper layer of fruit pulp cannot be cross-linked to form a gel, which enhances the fluidity of the upper layer of fruit pulp and is not conducive to stabilizing the pigment in the fruit pulp, resulting in the color difference value of the bonding surface of Comparative Example 10 being much higher than that of Example 2. On the other hand, since the upper layer of fruit pulp does not form an effective cross-linking network, the cross-linking strength between the upper layer and the middle layer is reduced to a certain extent, which reduces the interlayer shear force and makes it easy to separate the fruit crisps from each other.

[0138] A comparative analysis of Example 2 and Comparative Example 11 shows that the data of Example 2 is better than that of Comparative Example 11. In Comparative Example 11, calcium lactate and gluconolactone are not added to the upper layer of fruit pulp. Since the upper layer of fruit pulp lacks calcium lactate and gluconolactone, the upper layer of fruit pulp lacks calcium ions required for cross-linked polysaccharide macromolecules, and thus the upper layer of fruit pulp cannot form a gel, which increases the fluidity of the upper layer of fruit pulp and is not conducive to stabilizing the pigment in the upper layer of fruit pulp. On the other hand, since the upper layer of fruit pulp does not form an effective cross-linked network, the cross-linking strength between the middle layer and the upper layer of fruit pulp is reduced, which reduces the interlayer shear force and makes it easy for the fruit crisps to separate from each other.

[0139] A comparative analysis of Example 2 and Comparative Example 12 shows that the data of Example 2 is better than that of Comparative Example 12. Comparative Example 12 is not balanced after pouring the upper layer of fruit pulp, and is directly frozen, freeze-dried, and other operations. On the one hand, it is not balanced after pouring the upper layer of fruit pulp, resulting in a lack of cross-linking of pectin polysaccharides in the upper layer of fruit pulp, which enhances the fluidity of the lower layer of fruit pulp and is not conducive to stabilizing the pigment in the fruit pulp; on the other hand, since the upper layer of fruit pulp does not form an effective cross-linking network, the cross-linking strength between the upper layer of fruit pulp and the lower layer of fruit pulp is reduced to a certain extent, which reduces the interlayer shear force and makes it easy to separate the fruit crisps from each other.

[0140] A comparative analysis of Example 2 and Comparative Example 13 shows that the data of Example 2 is better than that of Comparative Example 13. In Comparative Example 13, the equilibrium temperature of the upper layer of fruit pulp is high and the time is long, which causes the melting of the lower layer of fruit pulp. The structure of the lower layer of fruit pulp is maintained only by covalent bonds, and the interlayer migration of pigment substances cannot be completely controlled or avoided, resulting in a significant increase in the color difference value; at the same time, the migration of calcium ions in the upper layer of fruit pulp is enhanced, and the upper and lower layers of fruit pulp are tightly cross-linked through the pectin network, thereby increasing the interlayer shear force.

[0141] A comparative analysis of Example 2 and Comparative Example 14 shows that the data of Example 2 is better than that of Comparative Example 14. Comparative Example 14 adopts a common step-by-step casting process, lacks an effective interlayer cross-linking network, and has weak interlayer adhesion. The fruit crisps are very likely to separate from each other during transportation and consumption, which seriously affects the texture and appearance quality of the fruit crisps. In Comparative Example 14, the pigments between the layers migrate with each other, and the color difference value is high, which is manifested as blurred color boundaries between layers and serious color mixing, resulting in poor color appearance quality of the product, affecting the appearance quality of the fruit crisps.

[0142] Although the embodiments of the present invention have been disclosed as above, they are not limited to the applications listed in the specification and the implementation modes. They can be fully applied to various fields suitable for the present invention. For those familiar with the art, additional modifications can be easily implemented. Therefore, without departing from the general concept defined by the claims and the scope of equivalents, the present invention is not limited to the specific details and the embodiments shown and described herein.

Claims

1. A method for improving the texture and color of freeze-dried reconstituted multi-layered fruit crisps, characterized in that: The following steps are involved: Step 1, preparing a lower layer of pulp, adding beet pectin, laccase, and low-ester pectin to the lower layer of pulp, stirring, pouring into a mold, standing to form a gel, freezing, and obtaining a frozen bottom layer material, wherein the mass fraction of beet pectin in the lower layer of pulp is 0.8-1.2%, the concentration of laccase is 200-600 U / g, and the mass fraction of low-ester pectin is 0.5-1%; Step 2: Spraying a calcium chloride solution at a temperature of 10 to 15° C. onto the surface of the frozen bottom material to form an intermediate layer, allowing it to stand and freeze to obtain a bottom material containing an intermediate layer, wherein the concentration of the calcium chloride solution is 15 to 25 mmol / / mL; Step 3, preparing an upper layer of fruit pulp, adding low-ester pectin to the upper layer of fruit pulp, letting it stand, adding calcium lactate and gluconolactone to the upper layer of fruit pulp and stirring, and pouring the upper layer of fruit pulp on a base material containing an intermediate layer, letting it stand to form a gel, and freezing, freeze-drying, and demolding in sequence to obtain freeze-dried reconstituted multi-layer fruit crisp blocks, wherein the concentration of calcium lactate in the upper layer of fruit pulp is 15-25 mmol / mL, the concentration of gluconolactone is 40-50 mmol / mL, and the mass fraction of low-ester pectin is 0.8-1.2%.

2. The method for improving the texture and color of freeze-dried reconstituted multi-layered fruit crisps according to claim 1, characterized in that: In step 1, the temperature of pouring into a mold and letting it stand to form a gel is 37° C. and the time is 2 to 3 hours.

3. The method for improving the texture and color of freeze-dried reconstituted multi-layered fruit crisps according to claim 1, characterized in that: In step 1, the freezing temperature is -18°C and the time is 4 to 6 hours.

4. The method for improving the texture and color of freeze-dried reconstituted multi-layered fruit crisps according to claim 1, characterized in that: In step 2, calcium chloride solution is sprayed onto the frozen bottom material at -2 to 2°C, the standing time is 4 to 8 minutes, the freezing temperature is -18°C, and the time is 1 to 2 hours.

5. The method for improving the texture and color of freeze-dried reconstituted multi-layered fruit crisps according to claim 1, characterized in that: In step 3, low-ester pectin is added to the upper layer of fruit pulp and then allowed to stand at -2 to 2°C for 4 to 6 hours.

6. The method for improving the texture and color of freeze-dried reconstituted multi-layered fruit crisps according to claim 1, characterized in that: In step three, pour the upper layer of pulp onto the base material containing the middle layer at -2.5 to -1.5°C and let it stand at -2.5 to -1.5°C for 2 to 3 hours.

7. The method for improving the texture and color of freeze-dried reconstituted multi-layered fruit crisps according to claim 1, characterized in that: In step 3, the freezing temperature is -18°C and the time is 4 to 6 hours.

8. The method for improving the texture and color of freeze-dried reconstituted multi-layered fruit crisps according to claim 1, characterized in that: In step three, the product is placed in a freeze dryer for freeze drying, and the temperature of the heating plate of the freeze dryer is set as follows: first stage, 65-85°C, 6-8h; second stage, 50-70°C, 8-12h; third stage, 40-50°C, drying until the moisture content is less than 4%.

9. The method for improving the texture and color of freeze-dried reconstituted multi-layered fruit crisps according to claim 1, characterized in that: In step three, ethanol is added when adding low-ester pectin to the upper layer of pulp, and the mass fraction of ethanol in the upper layer of pulp is 6-8%.

10. A fruit crisp block prepared by the method for improving the texture and color of freeze-dried reconstituted multi-layered fruit crisp blocks according to any one of claims 1 to 9.

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