Processing method of nutritional and healthy kiwi fruit slices

Abstract

The invention discloses a processing method of nutritional and healthy kiwi fruit slices, the kiwi fruit slices are treated by blanching, color-protecting hardening and freezing, and raw materials are preserved by adopting frozen storage, so that the loss of nutritional ingredients is less, the storage life of the raw materials is long, the quality of the raw materials is stable along with the prolonging of the storage life, and the stable quality of fruit slice products produced throughout the year is ensured; the raw materials are processed by vacuum sugar infusion and ultrasonic sugar infusion in sequence, so that the sugar infusion rate of the fruit slices is increased, the damage to the nutrition of the fruit slices is less, and the sugar is uniformly distributed in the fruit slices; freezing, unfreezing and ultrasonic technologies are applied to fruit slice treatment, the material drying dynamic process is changed, and drying energy consumption is reduced. The kiwi fruit slices processed by the method are yellow green, full in shape, chewy and moderate in sweetness and sourness, the original nutrition and unique flavor of kiwi fruits are kept, the palatability is high, stachyose serves as a penetrating agent, and the requirements of specific people such as diabetics, obesity patients and hyperlipidemia patients for low-sugar healthy food are met.

Description

Technical Field

[0001] The invention belongs to the technical field of food processing, and in particular relates to a method for processing nutritious and healthy kiwi fruit slices, which improves the quality of the kiwi fruit slices. Background Art

[0002] my country ranks first in the world in terms of the cultivated area and yield of kiwifruit. Kiwifruit is a plant of the genus Actinidia in the family Actinidia. It is also known as vine pear, sheep peach, hairy wood fruit, kiwi fruit, etc. The fruit contains a variety of nutrients such as polyphenols, polysaccharides, proteins, amino acids, etc., and the vitamin C content far exceeds that of other fruits. It is called the "king of fruits" and "world precious fruit" and is deeply favored by consumers. Because most of its fruits mature in the hot season, the peel is thin and has a high water content, and the fresh fruit is very easy to soften and rot after harvest. Although cold storage and controlled atmosphere storage can effectively solve the storage resistance problem of fresh kiwifruit, in the face of large kiwifruit production, concentrated harvest period and the characteristics of the fruit itself, the storage cost and requirements of fresh fruit are high, and it is difficult to fully meet the needs of the development of the kiwifruit industry. In addition, according to statistics, the loss rate of fresh fruit during transportation and storage can reach 20% to 30%. Therefore, deep processing of kiwifruit can significantly extend the shelf life, reduce waste, alleviate the pressure of unsalable caused by the concentrated listing of fresh fruit, and increase added value, which is of great significance to the healthy development of the kiwifruit industry. At present, the main products of kiwifruit deep processing include fruit slices, juice, fruit wine and jam, etc. Among them, kiwifruit slices have become the first choice of food processing companies due to their relatively simple processing technology, high raw material utilization rate, low one-time investment, and ability to retain the nutritional components and unique flavor of the fruit to a certain extent. They have become processed products with large demand in the international and domestic markets, and have extremely broad development prospects.

[0003] After more than 20 years of development, the processing technology of fruit and vegetable chips has formed two major process technology systems, namely vacuum frying technology and non-frying technology. Non-frying technology includes freeze-drying technology, vacuum drying, microwave drying, microwave-pressure difference puffing, etc. At present, most of the fruit and vegetable chips in my country are produced by low-temperature vacuum frying technology. Due to its low processing temperature and short processing time, the flavor and most of the nutrients of the original fruits and vegetables are retained. However, the oil content of vacuum fried fruit and vegetable chips is still above 10%, which not only increases the cost and is not easy to preserve, but also is not good for human health if consumed for a long time. Therefore, the research and development of non-fried fruit and vegetable chips is a future development trend.

[0004] Kiwi fruit slices are currently a processed product with a large demand in the international and domestic markets. However, in the production of kiwi fruit slices, companies soak the slices in liquid for a long time and store them. The raw materials are taken out for production according to production needs. The raw materials are prone to softening and nutritional loss during storage. In addition, the cooking and sugar infiltration time is long and the temperature is high during production, resulting in serious nutritional loss and a high sugar content in the product. With the construction of the "big health" pattern of the nutrition and health industry, nutritious and healthy foods have become the cornerstone of people's livelihood to meet the needs of people for a better life. Green and nutritious fruit slices with low sweetness, good sensory quality and higher nutritional value have become the development trend of kiwi fruit slices.

[0005] CN102742711A discloses a method for preparing kiwi preserved fruit by vacuum freeze drying, wherein the kiwi fruit is peeled, sliced, and then soaked in a color-protecting liquid, and then placed in a hardening liquid for hardening; after hardening, ultrasonic pretreatment is performed in a sugar solution; thereafter, vacuum sugar infiltration, prefreezing, vacuum freeze drying, and packaging are performed to obtain the kiwi preserved fruit. This method uses a method of first ultrasonicating and then vacuum-permeating sucrose, which has the following problems: (1) Ultrasonic cavitation generates bubbles, which are difficult to completely eliminate in the subsequent vacuum, affecting the entry of the sugar solution. In addition, premature cell rupture will cause sugar to be lost with the juice, causing reverse osmosis of the sugar solution and reducing sugar penetration efficiency; (2) Ultrasound generates active oxygen in an oxygen-rich environment, which accelerates V C (3) Ultrasound directly destroys cell walls, and vacuum suction intensifies tissue shrinkage, which deteriorates the texture of the product and makes it easy to become soft and rotten.

[0006] CN105124455A discloses a method for preparing FD ripe kiwifruit in-situ slices, wherein ripe fruits are selected, peeled, and cut into slices with a thickness of 0.5 to 1 cm, and the kiwifruit slices are quickly frozen using a freeze dryer, and the quickly frozen kiwifruit slices are kept for 30 minutes to 2 hours, and then sublimated and dried. The method slices the ripe kiwi fruit. Although the ripe kiwi fruit has a good taste, there are the following problems in slicing the ripe kiwi fruit: (1) The ripe kiwi fruit flesh is soft and juicy, and is easily deformed or broken due to squeezing during slicing, resulting in incomplete slices. In addition, cells rupture during the cutting process, and juice oozes out, causing the surface of the fruit slices to become wet and sticky, resulting in a poor taste; (2) Kiwi fruit contains polyphenol oxidase, and after slicing, it is exposed to the air, and the surface of the flesh darkens in a short time (about 10 to 15 minutes). At the same time, the oxidation reaction accelerates the degradation of nutrients such as vitamin C; (3) After slicing, the flesh contacts the knife, cutting board or bacteria in the environment, and is easy to breed microorganisms (especially at room temperature), shortening the shelf life. In addition, the oozing juice provides nutrition for the reproduction of microorganisms, accelerating corruption.

[0007] CN107047738 A discloses a method for preparing active Actinidia arguta fruit slices, which comprises slicing Actinidia arguta, quick-freezing with liquid nitrogen, storing at -15 to -30°C, and then performing vacuum freeze drying. In this method, liquid nitrogen is used to quick-freeze the fruit slices, which has the following problems: (1) the ultra-low temperature of liquid nitrogen causes a large temperature difference between the inside and outside of the fruit slices, and the rapid crystallization of water generates mechanical stress, leading to cell wall rupture; (2) the edges of the fruit slices are more directly exposed to liquid nitrogen, resulting in over-freezing, while there is still space for ice crystal growth in the center, resulting in structural stratification. Summary of the invention

[0008] The invention aims to overcome the shortcomings of current kiwi fruit slice processing and provide a method for processing nutritious and healthy kiwi fruit slices.

[0009] The processing method of the nutritious and healthy kiwi fruit slices provided by the present invention comprises the following steps:

[0010] Step 1: Peel and slice the kiwi fruit

[0011] The harvested kiwifruit is peeled and stalked, and cut into 6-8 mm kiwifruit slices.

[0012] Step 2: Blanching

[0013] Place the cut kiwi fruit slices in 90-95℃ hot water for 0.5-2 minutes. After blanching, quickly cool them with cold water and absorb the surface moisture.

[0014] Step 3: Color hardening

[0015] The blanched kiwi fruit slices are immersed in a mixed aqueous solution containing citric acid, ascorbic acid and calcium chloride for color protection and hardening treatment.

[0016] Step 4: Freeze

[0017] Freeze the color-protected and hardened kiwifruit slices at -60 to -20°C for 2 to 3 hours to make the center temperature of the kiwifruit slices less than -18°C.

[0018] Step 5: Freeze

[0019] The frozen kiwi fruit slices are stored at -20 to -16°C for fresh-keeping.

[0020] Step 6: Thawing

[0021] According to the processing volume requirements, take out the corresponding amount of frozen kiwi fruit slices for thawing.

[0022] Step 7: Vacuum first and then ultrasonic sugar infiltration treatment

[0023] The thawed kiwifruit slices are placed in a stachyose aqueous solution with a mass fraction of 30% to 50%, and are subjected to sugar infiltration treatment under vacuum conditions. The kiwifruit slices are then allowed to return to normal pressure, and then cycled through vacuum treatment-restoration to normal pressure, wherein a single vacuum treatment time is 10 to 20 minutes, a single normal pressure time is 5 to 10 minutes, and the total vacuum treatment time is 1 to 2 hours. After the cycle, the kiwifruit slices are placed in a stachyose aqueous solution with a mass fraction of 30% to 50% for ultrasonic sugar infiltration treatment, the ultrasonic power is 200 to 400 W, and the ultrasonic treatment time is 2 to 3 hours.

[0024] Step 8: Rinse

[0025] Rinse the residual solution on the surface of the kiwi fruit slices after sugar infiltration with running water and drain the water.

[0026] Step 9: Drying

[0027] The drained kiwi fruit slices are vacuum freeze-dried, vacuum microwave-dried or gas jet impact-dried to reduce the moisture content to 10% to 20%.

[0028] Step 10: Packaging

[0029] The dried kiwi fruit slices are sealed and packaged to obtain nutritious and healthy kiwi fruit slices, so as to extend the shelf life, maintain the quality, prevent pollution and enhance the commodity value.

[0030] In the above step 3, preferably, the mass fraction of citric acid in the mixed aqueous solution is 0.1% to 0.2%, the mass fraction of ascorbic acid is 0.1% to 0.3%, the mass fraction of calcium chloride is 0.05% to 0.15%, and the soaking time is 20 to 40 minutes.

[0031] In the above step 6, the thawing method is room temperature thawing, low temperature thawing or vacuum steam thawing.

[0032] In the above step 7, it is preferred that the material-liquid ratio in the vacuum sugar infiltration treatment stage is 1 g:6-10 mL, the temperature is 30-50° C., and the vacuum degree is 0.06-0.10 MPa.

[0033] In the above step 7, the material-liquid ratio in the ultrasonic infiltration sugar treatment stage is preferably 1 g: 6-10 mL and the temperature is 30-40°C.

[0034] In the above step 10, the packaging is vacuum packaging, modified atmosphere packaging or deoxidizer packaging.

[0035] The beneficial effects of the present invention are as follows:

[0036] 1. The present invention utilizes blanching, color protection and hardening, and freezing to treat kiwi fruit slices, and adopts frozen storage to preserve the raw materials. Under the synergistic effect, the loss of nutrients is small, the shelf life of the raw materials is long, and the quality of the raw materials is stable as the shelf life is extended, thereby ensuring the stable quality of kiwi fruit slices produced all year round.

[0037] 2. In the kiwi fruit slice processing method of the present invention, the raw materials are processed at low temperature by vacuum first and then ultrasonic sugar infiltration treatment and drying, thereby maintaining the original nutrition of kiwi fruit, and the Vc content is greater than 50 mg / 100 g. Therefore, the kiwi fruit slices produced by the present invention are green and safe.

[0038] 3. The present invention uses stachyose as an osmotic agent. Stachyose, as a prebiotic, is known as a "natural super bifidobacterial factor". After entering the human body, it can be directly decomposed and utilized by intestinal microorganisms to play a variety of physiological functions. In particular, after entering the human body, stachyose will not be decomposed by digestive enzymes, and its metabolic process does not rely on insulin. Therefore, the kiwi fruit slices produced by the present invention are nutritious and healthy, and are an ideal choice for specific groups of people such as diabetic patients, obese patients and hyperlipidemia patients.

[0039] 4. The present invention applies the technology of vacuum first and then ultrasonic sugar infiltration to the sugar infiltration process of kiwi fruit slices. Vacuum first removes intercellular gas to reduce the osmotic resistance of sugar solution, and the ultrasonic stage can promote sugar diffusion more efficiently; vacuum allows sugar solution to initially fill the tissue, and ultrasound only needs fine-tuning to avoid local over-osmosis or reverse osmosis, so that sugar distribution is more uniform; vacuum reduces oxygen, inhibits oxidation reaction and chlorophyllase activity, and subsequent ultrasonic low-temperature treatment can effectively retain nutrients such as Vc and chlorophyll in the fruit slices; vacuum gentle dehydration, ultrasound-assisted infiltration, avoid cell network collapse, sugar solution supports intercellular gaps, reduces collapse, and has high fullness; vacuum forms uniform micropores, ultrasound refines channels, improves rehydration rate, and product rehydration is better. Therefore, the combination of vacuum sugar infiltration and then ultrasonic sugar infiltration technology and their synergistic effect improves the sugar infiltration rate of kiwi fruit slices, shortens the sugar infiltration time, and makes the sugar evenly distributed in kiwi fruit slices. In addition, the vacuum sugar infiltration and ultrasonic sugar infiltration processes have little effect on the fruit slice tissue, which helps to maintain the nutrition and quality of the fruit slices and the fruit slices are full.

[0040] 5. The present invention applies freezing, thawing and ultrasonic technology to the treatment of kiwi fruit slices. The freezing causes the water in the kiwi fruit slices to form fine ice crystals, and more pores are formed during thawing. The cavitation effect of ultrasound destroys the microstructure. Under the synergistic effect of these treatments, the water holding capacity of the material is reduced, the water binding state is changed, the mass transfer path of the water is optimized, and finally the material drying kinetic process is changed, the drying efficiency is improved, the drying time is further shortened, and the drying cost and energy consumption are reduced.

[0041] 6. The present invention optimizes the synergistic effect of 10 factors, including vacuum temperature, mass fraction of sugar solution in vacuum stage, material-liquid ratio in vacuum stage, vacuum degree, single normal pressure time, total processing time in vacuum stage, ultrasonic temperature, ultrasonic power, mass fraction of sugar solution in ultrasonic stage, and material-liquid ratio in ultrasonic stage, through factorial experiments and response surface experiments, thereby reducing nutritional loss, improving sugar infiltration efficiency, saving energy consumption, and the sugar infiltration time is 3 to 5 hours.

[0042] 7. The kiwi fruit slices processed by the present invention are yellow-green in color, full in shape, chewy, moderately sweet and sour, retain the original nutrition of kiwi fruit, have the unique flavor of kiwi fruit, and are highly palatable. The Vc content is greater than 50 mg / 100 g, the mass fraction of stachyose is greater than 34%, the rehydration rate is greater than 182%, and the sensory score is greater than 80 points, which meets the needs of modern consumers for low-sugar healthy food. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is the Pareto chart of the comprehensive score of fruit slice quality in the factorial experiment. DETAILED DESCRIPTION

[0044] Qinmei kiwifruit was used in the following experiments and examples. High-quality kiwifruits with uniform size, uniform hardness and no mechanical damage and pests were selected. The samples were placed in plastic baskets and stored at 0°C.

[0045] The following experiments and examples are used to determine the following indicators:

[0046] (1) The total sugar content was determined by anthrone colorimetry; the titratable acid content was determined by NaOH titration, see GB / T 12456-2021; the Vc content was determined by 2,6-dichloroindophenol titration, see GB 5009.86-2016.

[0047] (2) Sensory evaluation of kiwi fruit slices: A professional evaluation panel was formed by randomly selecting 12 people (6 males and 6 females) who had received sensory evaluation training. The panel evaluated the taste, color and appearance of the products using their senses. The sensory evaluation criteria are shown in Table 1.

[0048] (3) Determination of the texture of kiwi fruit slices: TPA test was performed using a texture analyzer with a P 0.5 probe. The TPA test was performed on kiwi fruit slice samples at a speed of 3 mm / s before, 2 mm / s during, and 2 mm / s after the test. The compression degree was 50% of the sample deformation.

[0049] (4) Using NS810 colorimeter to measure the L * 、a * 、b * , the color difference value ΔE is calculated according to the following formula (1).

[0050]

[0051] Where, L 0 、a 0 , b 0 Indicates the fresh sample measurement value, L * 、a * , b * It indicates the measured value after the sample is freeze-dried.

[0052] (5) Soak the freeze-dried kiwifruit slices in a 30°C distilled water bath for 30 min. After the slices have fully absorbed water, remove them and drain the surface water. Calculate the rehydration rate according to the following formula (2).

[0053]

[0054] Where, Rr represents the rehydration rate, %; m 0 Indicates the mass of the sample before rehydration, g; m f Indicates the mass of the sample after rehydration, g.

[0055] (6) Determination of chlorophyll content: Spectrophotometer colorimetry was used. Take 1.0 g of sample, add 15 mL of 80% acetone aqueous solution, let stand for 30 minutes, filter to obtain the supernatant, dilute to 50 mL, and measure the absorbance at 663 nm and 645 nm respectively. Calculate the content of chlorophyll a and chlorophyll b according to formula (3) and (4). Repeat the measurement 3 times for each group and take the average value.

[0056]

[0057] In the formula, m represents the mass of the fruit slice, g; OD 663 Express the absorbance value at 663nm; OD 645 Indicates the absorbance value at 645 nm.

[0058] (7) Determination of fullness: Refer to the method of Shang Fanzhen et al. (Optimization of sugar-free probiotic kiwifruit preserves processing and nutritional flavor analysis [J]. Food Industry Science and Technology, 2021, 42(19): 226-237.) for determination, and calculate the fullness according to formula (5).

[0059]

[0060] In the formula, A represents fullness, %; V 1 Indicates the volume of kiwi fruit slices before drying, mL; V 2 Indicates the volume of kiwi fruit slices after drying, mL.

[0061] (8) Determination of yield rate: The yield rate was determined according to the method of Wang Haiou et al. (Effects of immersion treatment in glucose-citric acid solution on the quality of freeze-dried apple slices [J]. Journal of Jiangsu Agricultural Sciences, 2020, 36(02): 477-486.) and calculated according to formula (6).

[0062]

[0063] Where, W represents the output rate, %; m 1 Indicates the mass before freeze-drying, g; m 2 Indicates the mass after freeze-drying, g.

[0064] Table 1 Sensory evaluation standards for kiwi fruit slices

[0065]

[0066] In order to determine the processing method of the present invention, the inventors conducted a large number of laboratory research experiments, and the specific experimental results are as follows:

[0067] 1. Determine the sugar penetration method of kiwi fruit slices

[0068] (1) Pretreatment of kiwi fruit slices

[0069] Take kiwifruit of the same hardness, wash it, remove the skin and the stems at both ends, cut it into 8 mm kiwifruit slices, blanch the sliced ​​kiwifruit slices in 95°C hot water for 1 min, quickly cool them with cold water after blanching, and then soak the kiwifruit slices in a mixed aqueous solution containing 0.2wt% citric acid, 0.15wt% ascorbic acid and 0.1wt% calcium chloride for 30 min, freeze them at -40°C for 2h, and then put them into a -20°C refrigerator for frozen storage.

[0070] (2) Method for incorporating sugar into kiwi fruit slices

[0071] Take out the frozen kiwi fruit slices, thaw them at room temperature for 45 minutes, add them into a 50% stachyose aqueous solution at a solid-liquid ratio of 1g:5mL, and perform sugar infiltration treatment according to the following 9 methods:

[0072] Normal pressure sugar osmosis: Place in a 40°C incubator for 2 hours;

[0073] Hot-boiled infiltration sugar: Place in a 90℃ constant temperature water bath for 2 hours;

[0074] Microwave infiltration of sugar: placed in a microwave drying oven at 40°C for 2 h, microwave power at 210 W;

[0075] Ultrasonic sugar infiltration: Place in an ultrasonic generator at 40°C for 2 h, with an ultrasonic power of 200 W;

[0076] Vacuum infiltration of sugar: placed in a vacuum drying oven at 40°C, cycled by vacuum treatment-restoration to normal pressure, vacuum degree 0.09MPa, single vacuum time 15min, single normal pressure time 15min, cycle number 4 times;

[0077] Microwave first, then ultrasound sugar infiltration: first microwave-assisted sugar infiltration for 1 hour, then ultrasound-assisted sugar infiltration for 1 hour, where the microwave and ultrasound setting conditions remain the same as above;

[0078] Ultrasound followed by vacuum infiltration: First, ultrasound-assisted infiltration was performed for 1 hour, followed by vacuum-assisted infiltration for 1 hour, where the vacuum and ultrasound setting conditions remained the same as above;

[0079] Vacuum first and then ultrasound infiltration: vacuum-assisted sugar infiltration was performed for 1 hour first, followed by ultrasound-assisted sugar infiltration for 1 hour, wherein the vacuum and ultrasound setting conditions remained the same as above;

[0080] Ultrasonic microwave synergistic sugar infiltration: Place in an ultrasonic microwave synergistic workstation for 2 hours, ultrasonic power 200W, microwave temperature 40℃.

[0081] (3) Drying of kiwi fruit slices

[0082] Vacuum freeze drying technology was used to dry the kiwi fruit slices after sugar infiltration, and the cold trap temperature was set to -45°C and the vacuum degree was 10 Pa. When the sample temperature displayed on the freeze dryer operation interface reached a constant, the drying process was considered to be completed.

[0083] According to the above method, the sugar content, titratable acid content, Vc content, hardness, chewiness, ΔE, chlorophyll content, rehydration rate, fullness, yield rate and sensory score of the dried kiwifruit slices were comprehensively scored. Among them, the higher the sugar content means the better mass transfer effect during the sugar infiltration process, the higher the Vc content reflects the better quality, the enhancement of hardness and chewiness indicates that the kiwifruit slices have better taste and toughness, the higher the chlorophyll content means the product has brighter color and better quality, the higher the fullness, yield, rehydration rate and sensory score indicates the better quality, and the membership of the above indicators is calculated by formula (7); for the titratable acid content and ΔE, the lower the value, the higher the quality of the kiwifruit slices, and the membership is determined by formula (8). Finally, the indicators are weighted by formula (9) to obtain the comprehensive evaluation score S of the kiwifruit slices.

[0084]

[0085] S=aY 1 +bY 2 +cY 3 +dY 4 +eY 5 +fY 6+gY 7 +hY 8 +iY 9 +jY 10 +kY 11 (9)

[0086] In the formula, Xmax and Xmin represent the maximum and minimum values ​​of each indicator, and Xi represents the actual value of each indicator. 1 , Y 2 , Y 3 , Y 4 , Y 5 , Y 6 , Y 7 , Y 8 , Y 9 , Y 10 and Y 11 are the membership of sugar content, titratable acid content, hardness, chewiness, ΔE, chlorophyll content, rehydration rate, fullness, Vc content, yield rate and sensory score respectively; a, b, c, d, e, f, g, h, i, j and k represent the weight coefficients of each index, and a+b+c+d+e+f+g+h+i+j+k=1.

[0087] The measurement results are shown in Table 2.

[0088] Table 2 Effects of different sugar infiltration methods on kiwifruit slice quality and comprehensive score

[0089]

[0090]

[0091] As shown in Table 2, the sugar infiltration methods had significant differences in the sugar content, titratable acid content, Vc content, texture, ΔE, chlorophyll content, rehydration rate, plumpness, yield rate and sensory score of kiwi fruit slices (p<0.05). Normal pressure sugar infiltration performed well in Vc retention and color, but its sugar content and yield rate were significantly lower than those of other methods. Hot boiling sugar infiltration performed best in yield rate and plumpness, but hot boiling sugar infiltration caused great damage to Vc and texture, thus significantly reducing the nutritional value and overall quality of the product. Microwave sugar infiltration and ultrasonic sugar infiltration were effective in increasing sugar content and improving color, but microwave and ultrasonic treatments also led to a decrease in Vc content, hardness and chewiness of the product, affecting the taste of the product. Vacuum sugar infiltration and vacuum followed by ultrasonic sugar infiltration performed well in Vc retention and sensory scores, but the vacuum sugar infiltration process would result in relatively low yield rate and rehydration rate. Sugar infiltration with microwave followed by ultrasound significantly improves the sugar penetration efficiency and has a high yield. However, the combined treatment softens the texture of the fruit slices, resulting in a decrease in hardness, chewiness and rehydration performance. Ultrasonic-microwave synergistic sugar infiltration significantly improves the rehydration performance of kiwifruit slices, but also leads to a decrease in Vc content and fullness. Sugar infiltration with ultrasound followed by vacuum generates active oxygen in an oxygen-rich environment, which accelerates the degradation of Vc and pigments. In addition, long-term ultrasound heat generation can also aggravate the decomposition of chlorophyll and heat-sensitive components. Ultrasound directly destroys cell walls, and vacuum suction aggravates tissue contraction, making the product easy to become soft and rotten. Among the 9 sugar infiltration methods, the kiwifruit slices in the group of sugar infiltration with vacuum followed by ultrasound have better overall quality and the highest overall quality score because of the low oxygen and structural stability established in the vacuum stage and the precise assistance in the ultrasound stage. Therefore, the most suitable sugar infiltration method is determined to be sugar infiltration with vacuum followed by ultrasound.

[0092] 2. Optimization of process parameters of vacuum followed by ultrasonic sugar infiltration

[0093] Stachyose was infiltrated into kiwifruit slices by vacuum first and then ultrasound. A factorial experiment was used to take the comprehensive quality score of kiwifruit slices after sugar infiltration and drying as the response value to explore the effects of 10 experimental factors including vacuum degree, vacuum temperature, single normal pressure time, sugar solution mass fraction in vacuum stage, material-liquid ratio in vacuum stage, total time of vacuum treatment stage, ultrasonic power, ultrasonic temperature, sugar solution mass fraction in ultrasonic stage and material-liquid ratio in ultrasonic stage on the comprehensive quality score of kiwifruit slices. Through screening, three key factors with significant influence on the quality of kiwifruit slices were identified. The experimental design is shown in Table 3, and the experimental results are shown in Table 4.

[0094] Table 3 Factorial experimental design

[0095]

[0096]

[0097] Table 4 Factorial experiment results

[0098]

[0099] By analyzing the results in Table 4, we can obtain Figure 1 The Pareto chart of the comprehensive quality score of kiwi fruit slices is shown. Figure 1 The data can determine that the key factors affecting the comprehensive quality score of kiwifruit slices are ranked as follows: vacuum stage material-liquid ratio> vacuum stage sugar solution mass fraction> vacuum temperature> ultrasonic stage material-liquid ratio> total time of vacuum stage> vacuum degree> single normal pressure time> ultrasonic temperature> ultrasonic stage sugar solution mass fraction> ultrasonic power. Among them, the effects of vacuum stage material-liquid ratio, vacuum stage sugar solution mass fraction and vacuum temperature on the comprehensive quality score of kiwifruit slices reached a significant level (P < 0.05). In view of this, these three factors were selected as key factors for the next step of response surface optimization design and analysis. For the remaining factors, the optimal values ​​were selected based on the results of their effect analysis, specifically: vacuum degree of 0.10MPa, single normal pressure time of 7.5min, total vacuum stage processing time of 1h, ultrasonic power of 350W, ultrasonic temperature of 30℃, ultrasonic stage sugar solution mass fraction of 30%, and ultrasonic stage material-liquid ratio of 1g:6mL.

[0100] According to the results of the above factorial experiment, under the conditions of vacuum degree of 0.10MPa, single normal pressure time of 7.5min, single vacuum time of 15min, total vacuum stage treatment time of 1h, ultrasonic power of 350W, ultrasonic temperature of 30℃, sugar solution mass fraction of 30% in ultrasonic stage, material-liquid ratio of 1g:6mL in ultrasonic stage, and ultrasonic stage treatment time of 3h, three significant influencing factors, namely vacuum temperature, sugar solution mass fraction in vacuum stage, and material-liquid ratio in vacuum stage, were selected as independent variables, and the comprehensive score of kiwifruit slice quality was used as the response value. A response surface experiment based on three factors and three levels was conducted to analyze and optimize the sugar infiltration process conditions of kiwifruit slices. The specific response surface experimental design and results are shown in Table 5.

[0101] Table 5 Response surface experimental design and results

[0102]

[0103]

[0104] The optimization results show that the best process parameters of vacuum first and then ultrasonic sugar infiltration are: vacuum temperature 40℃, sugar solution mass fraction 37% in vacuum stage, material-liquid ratio 1g:8mL in vacuum stage, vacuum degree 0.10MPa, single normal pressure time 7.5min, single vacuum time 15min, total vacuum stage treatment time 1h, ultrasonic power 350W, ultrasonic temperature 30℃, sugar solution mass fraction 30% in ultrasonic stage, material-liquid ratio 1g:6mL in ultrasonic stage, and ultrasonic stage treatment time 3h. Under this process, the comprehensive quality score of kiwi fruit slices is 0.636.

[0105] The technical solution of the present invention is further described below in conjunction with embodiments, but the implementation method thereof is not intended to limit the present invention.

[0106] Example 1

[0107] Step 1: Peel and slice the kiwi fruit

[0108] Kiwi fruits of the same hardness were selected, the kiwi fruits were manually peeled and stems were removed, and then the kiwi fruits were cut into 8 mm kiwi fruit slices.

[0109] Step 2: Blanching

[0110] Place the cut kiwi fruit slices in 95℃ hot water for 1 minute, quickly cool them with cold water after blanching, and absorb the surface moisture.

[0111] Step 3: Color hardening

[0112] The blanched kiwi fruit slices were immersed in a mixed aqueous solution containing 0.2 wt % citric acid, 0.15 wt % ascorbic acid and 0.1 wt % calcium chloride for 30 minutes to perform color protection and hardening treatment.

[0113] Step 4: Freeze

[0114] Freeze the color-hardened kiwifruit slices at -40°C for 2 hours to make the center temperature of the kiwifruit slices less than -18°C;

[0115] Step 5: Freeze

[0116] The frozen kiwi fruit slices are placed in a refrigerator at -20°C for storage.

[0117] Step 6: Thawing

[0118] Thaw the frozen kiwi fruit slices at room temperature for 45 minutes and absorb the surface moisture.

[0119] Step 7: Vacuum first and then ultrasonic sugar infiltration treatment

[0120] The thawed kiwifruit slices were placed in a 37% stachyose aqueous solution heated to 40°C at a material-liquid ratio of 1g:8mL, and sugar infiltration treatment was performed under a vacuum degree of 0.10MPa. Then, it was allowed to return to normal pressure, and then the vacuum treatment-restoration to normal pressure cycle was performed, wherein the single vacuum treatment time was 15min, the single normal pressure time was 7.5min, and the total vacuum treatment time was 1h. After the cycle, the kiwifruit slices were placed in a 30% stachyose aqueous solution heated to 30°C at a material-liquid ratio of 1g:6mL for ultrasonic sugar infiltration treatment, the ultrasonic power was 350W, and the ultrasonic treatment time was 3h.

[0121] Step 8: Rinse

[0122] Rinse the residual solution on the surface of the kiwi fruit slices after sugar infiltration with running water and drain the water.

[0123] Step 9: Drying

[0124] The drained kiwi fruit slices are vacuum freeze-dried to reduce the moisture content to 10% to 20%.

[0125] Step 10: Packaging

[0126] The dried kiwi fruit slices are vacuum-sealed and packaged to obtain nutritious and healthy kiwi fruit slices.

[0127] Example 2

[0128] In step 7 of this embodiment, the thawed kiwifruit slices are placed in a 30% stachyose aqueous solution heated to 40°C at a material-liquid ratio of 1g:10mL, and the sugar is infiltrated under a vacuum degree of 0.10MPa, and then restored to normal pressure, and then cycled by vacuum treatment-restoration of normal pressure, wherein the single vacuum treatment time is 15min, the single normal pressure time is 7.5min, and the total vacuum treatment time is 1h; after the cycle, the kiwifruit slices are placed in a 30% stachyose aqueous solution heated to 30°C at a material-liquid ratio of 1g:6mL for ultrasonic sugar infiltration treatment, the ultrasonic power is 350W, and the ultrasonic treatment time is 3h. The other steps are the same as in Example 1, and nutritious and healthy kiwifruit slices are obtained.

[0129] Example 3

[0130] In step 7 of this embodiment, the thawed kiwifruit slices are placed in a 30% stachyose aqueous solution heated to 40°C at a material-liquid ratio of 1g:6mL, and the sugar is infiltrated under a vacuum degree of 0.10MPa, and then restored to normal pressure, and then cycled by vacuum treatment-restoration of normal pressure, wherein the single vacuum treatment time is 15min, the single normal pressure time is 7.5min, and the total vacuum treatment time is 1h; after the cycle, the kiwifruit slices are placed in a 30% stachyose aqueous solution heated to 30°C at a material-liquid ratio of 1g:6mL for ultrasonic sugar infiltration treatment, the ultrasonic power is 350W, and the ultrasonic treatment time is 3h. The other steps are the same as in Example 1, and nutritious and healthy kiwifruit slices are obtained.

[0131] The results of the texture characteristics, color, and physical and chemical indicators of the kiwi fruit slices processed in Examples 1 to 3 above were carefully compared with the products processed by traditional hot boiling and sugar infiltration (control). The specific data are shown in Table 6.

[0132] Table 6 Test results of kiwi fruit slices obtained by different processing methods

[0133]

[0134] The results of the quality index determination of kiwi fruit slices in Table 6 show that the kiwi fruit slices prepared by the method of Example 1 are yellow-green in color, with a translucent and shiny surface, a full shape, no crystal precipitation, a balanced sweet and sour taste, a moderately hard and soft texture, elasticity and chewiness, and a sensory score of 92, the highest comprehensive score. The comprehensive scores of the kiwi fruit slices processed in Examples 1 to 3 of the present invention are better than those of the products processed by traditional hot boiling and sugar infiltration. It is particularly noteworthy that the Vc content, sugar content, and sensory evaluation are significantly higher, and the hardness and rehydration ability also have better performance. Therefore, the processing method of the present invention can not only effectively retain the nutritional components of fruits and vegetables, ensure the good quality of fruit slices, but also significantly shorten the time of sugar infiltration.

[0135] In addition, the kiwi fruit slices produced by the present invention present a natural yellow-green hue, the flesh is full, the edges are slightly wrinkled but do not affect the appearance, the surface is smooth and crystal-free, and the touch is dry and non-sticky. In terms of taste, the product has a soft and chewy texture, moderate sourness and sweetness, perfectly retains the original flavor characteristics of kiwi fruit, and meets the needs of modern consumers for low-sugar healthy food.

Claims

1. A method for processing nutritious and healthy kiwi fruit slices, characterized in that: The processing method comprises the following steps: Step 1: Peel and slice the kiwi fruit The harvested kiwifruit is peeled and stalked, and cut into 6-8 mm kiwifruit slices; Step 2: Blanching Place the sliced ​​kiwifruit in hot water at 90-95°C for 0.5-2 minutes, cool it down quickly with cold water, and dry the surface moisture; Step 3: Color hardening The blanched kiwi fruit slices are immersed in a mixed aqueous solution containing citric acid, ascorbic acid and calcium chloride for color protection and hardening treatment; Step 4: Freeze Freeze the color-hardened kiwifruit slices at -60 to -20°C for 2 to 3 hours, so that the center temperature of the kiwifruit slices is less than -18°C; Step 5: Freeze The frozen kiwi fruit slices are stored at -20 to -16°C for freshness preservation; Step 6: Thawing According to the processing volume requirements, take out the corresponding amount of frozen kiwi fruit slices for thawing; Step 7: Vacuum first and then ultrasonic sugar infiltration treatment The thawed kiwi fruit slices are placed in a stachyose aqueous solution with a mass fraction of 30% to 50%, and the kiwi fruit slices are subjected to sugar infiltration treatment under vacuum conditions, and then the kiwi fruit slices are allowed to return to normal pressure, and then the vacuum treatment-normal pressure return cycle is performed, wherein the single vacuum treatment time is 10 to 20 minutes, the single normal pressure time is 5 to 10 minutes, and the total vacuum treatment time is 1 to 2 hours; after the cycle, the kiwi fruit slices are placed in a stachyose aqueous solution with a mass fraction of 30% to 50% for ultrasonic sugar infiltration treatment, the ultrasonic power is 200 to 400W, and the ultrasonic treatment time is 2 to 3 hours; Step 8: Rinse Rinse the residual solution on the surface of the kiwi fruit slices after the infiltration of sugar with running water and drain the water; Step 9: Drying The drained kiwi fruit slices are vacuum freeze-dried, vacuum microwave-dried or gas jet impact-dried to reduce the moisture content to 10% to 20%; Step 10: Packaging The dried kiwi fruit slices are sealed and packaged to obtain nutritious and healthy kiwi fruit slices.

2. The method for processing the nutritious and healthy kiwi fruit slices according to claim 1, characterized in that: In step 3, the mass fraction of citric acid in the mixed aqueous solution is 0.1% to 0.2%, the mass fraction of ascorbic acid is 0.1% to 0.3%, the mass fraction of calcium chloride is 0.05% to 0.15%, and the soaking time is 20 to 40 minutes.

3. The method for processing the nutritious and healthy kiwi fruit slices according to claim 1, characterized in that: In step 6, the thawing method is room temperature thawing, low temperature thawing or vacuum steam thawing.

4. The method for processing the nutritious and healthy kiwi fruit slices according to claim 1, characterized in that: In step 7, the material-liquid ratio in the vacuum sugar infiltration treatment stage is 1 g: 6-10 mL, the temperature is 30-50° C., and the vacuum degree is 0.06-0.10 MPa.

5. The method for processing the nutritious and healthy kiwi fruit slices according to claim 1, characterized in that: In step 7, the material-liquid ratio in the ultrasonic infiltration sugar treatment stage is 1 g: 6-10 mL, and the temperature is 30-40° C.

6. The method for processing the nutritious and healthy kiwi fruit slices according to claim 1, characterized in that: In step 10, the packaging is vacuum packaging, modified atmosphere packaging, or deoxidizer packaging.

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