CN114451533B - Preparation method of high-quality raisins - Google Patents

Abstract

The invention discloses a method for making high-quality raisins, which belongs to the technical field of agricultural product processing. The method for making raisins includes the following steps: ① picking fresh "seeless white" grapes without rot, removing the stalks, washing away impurities, and picking and crushed and deformed grape grains during transportation; ②Using vibrating sieve to screen the grapes into three categories: large, medium and small; ③Using the salt solution method to accurately distinguish grapes of different maturity. ④Choose sorted grapes and carry out catalytic infrared heating to dry the surface moisture; ⑤Vacuum pulsation drying for grapes; Thereby, a raisin product with a moisture content and a bacteria content up to the standard and with uniform product quality can be obtained. The method of the invention can produce high-quality raisin products with consistent product quality, moisture content and bacteria content up to the standard, and the product has good quality and no chemical residue, and the method is simple and reliable.

Description

A method for producing high-quality raisins

Technical Field

The invention belongs to the technical field of agricultural product processing and relates to a method for making high-quality raisins.

Background Art

Grapes are highly seasonal and are put on the market at a concentrated time. Fresh grapes have a water content of up to 80% and a high soluble solid content, which makes them prone to microbial growth and rot. In addition to fresh consumption, drying is an important way to process grapes. It not only solves the prominent problems of concentrated grape harvesting, easy rot, short storage period, and high transportation costs, but also produces raisins with a unique taste that fresh grapes do not have. They are deeply loved by consumers and are also an indispensable auxiliary material in the food industry. Seedless white grapes are the most important variety for drying, and about 95% of raisins are processed from "seedless white" grapes.

But the current raisin processing technology generally has the following problems.

1. Natural drying takes a long time. Traditional drying in a drying room can have a greater chance of obtaining a good color (emerald green), but the green product rate is usually less than 35%; and the drying time is generally longer, usually 25 to 40 days, and it is easy to be contaminated by external microorganisms, insects and birds, causing product quality problems.

2. Hot air drying is prone to browning. Color is an important indicator for consumers to choose products. If fresh grapes are not treated with any chemical drying agent, they are directly dried with hot air, and the drying time is about 1 to 7 days. Although it can greatly improve the drying efficiency, the product browning is more serious, and it is difficult to get raisins with good color.

3. The inconsistency in the maturity of fresh grapes leads to inconsistent physical and chemical characteristics of the raw materials. Due to differences in the growth environment of the plants during the growth of agricultural materials, there are usually differences in the size, shape, and maturity of individual grapes. Even the maturity of grapes in the same orchard, the same harvest period, and the same plant can vary significantly, which in turn leads to significant differences in the size and soluble solids content of the grapes. Under the same drying conditions, small grapes are prone to over-drying, affecting the taste; large grapes are usually not dried thoroughly, and are prone to breeding bacteria during storage, reducing quality. In the process of drying grapes, inconsistent physical and chemical characteristics of the raw materials will directly lead to uneven and unstable quality of the dried products. This has become a prominent problem affecting the mechanized drying of grapes.

To ensure the consistency of the quality of raisin products, the most advanced classification method currently uses machine vision technology to classify raisins after they are dried, by judging their color and size one by one. However, it is not possible to accurately distinguish raisins with different soluble solids content based on the differences in the soluble content of the grapes. Moreover, the classification equipment based on machine vision requires a large investment and has high maintenance costs.

Fourth, since it is difficult to kill all microorganisms in the drying process of grapes, raisin products often exceed the microbial standard, which can easily cause food safety problems. At present, the commonly used sterilization methods for dehydrated vegetables are mainly high-temperature steam sterilization, hot water blanching sterilization, chemical reagent sterilization, etc., but high-temperature steam and hot water blanching sterilization can easily lead to loss of raisin nutrients and excessive moisture content. Chemical reagent sterilization may have chemical residues, which is controversial. Therefore, new sterilization technology is urgently needed.

In view of the above problems, a new method for drying grapes was proposed by combining salt solution classification method, vacuum pulsation drying technology and catalytic infrared heating technology.

Combined with the differences in the size of grapes, they can be classified through vibrating screens with different apertures. Grapes of different maturity have different soluble solids content and density. Therefore, grapes of the same size and different maturity can be further classified by preparing salt solutions of different concentrations.

Catalytic infrared heating technology, that is, under the action of a catalyst, promotes the oxidation reaction between natural gas and oxygen to generate carbon dioxide and water, and radiate infrared energy. Catalytic infrared is a flameless reaction, its temperature is lower than the ignition point, there is no open flame, no explosion hazard, safe and reliable. Compared with traditional electric infrared, the energy of catalytic infrared is directly converted from natural gas to infrared heating, which has higher energy utilization efficiency. Catalytic infrared has a strong surface heating effect and can quickly remove moisture from the surface of the material. Compared with traditional high-temperature steam and blanching sterilization, it has the advantages of rapid heating, high heat transfer efficiency, energy saving and environmental protection, and no chemical residue.

Vacuum pulsation drying technology is developed from ordinary vacuum drying technology. It uses continuous periodic pressure switching to continuously break the equilibrium state of moisture evaporation on the surface of the material, thereby accelerating the drying process. In addition, the continuous pulsation of pressure in the drying chamber can also make the microscopic pores of the drying material continuously expand and contract, and even interconnect with each other, which can effectively promote the transfer of internal moisture to the outside and maintain a high drying rate. Compared with ordinary vacuum drying, this technology has many advantages such as high drying efficiency and better product quality.

Therefore, the invention effectively combines salt solution classification, vacuum pulse drying technology, and catalytic infrared heating technology. Under the premise of accurately classifying grapes and ensuring the consistency of the physical and chemical characteristics of grapes, high-quality raisin products with consistent product quality, moisture content, and bacterial content that meet the standards can be quickly produced.

Summary of the invention

The object of the present invention is to provide a method for producing high-quality raisins, which is characterized by comprising the following steps:

(1) Preliminary selection of raw materials: fresh seedless white grapes without rot are selected, the stalks are removed and impurities are washed away, and grapes that are squeezed, damaged or deformed during picking and transportation are discarded;

(2) Grain classification and screening: four layers of vibrating screens are used to screen in sequence, and the grape raw materials are sorted into large, medium and small according to the transverse diameter of the grapes;

(3) Grapes are graded according to the phenomenon of floating or sinking in the salt solution. That is, based on the principle that the density of grape particles of different maturity is different, sodium chloride solutions with concentrations of 120, 140, 160, and 180 g/L are prepared. The grapes after size classification in step (2) are first placed in the salt solution with the lowest concentration. According to their floating and sinking phenomena, the suspended grape particles are fished out and collected. The sinking grapes are fished out and placed in the next salt solution with higher concentration. Similarly, the floating grape particles are fished out, and the sinking samples are fished out and continued to be placed in the salt solution with higher concentration, and this process is repeated until the sample classification is completed. The material is sequentially divided into grapes corresponding to the salt solution concentrations of <120, (120, 140], (140, 160], (160, 180], and >180 g/L, which are defined as grapes of lower, lower, medium, higher, and higher maturity, respectively.

(4) using catalytic infrared heating to remove surface moisture from the grapes of all grades obtained in step (3), selecting grapes classified by size and maturity, and placing them on a double-layer conveyor belt. The conveyor belts are located at different heights, and a catalytic infrared heater is installed on the top. The rolling of the material during the falling process can achieve automatic turning, which is conducive to uniform heating of the material to remove surface moisture; the conveyor belt speed can be adjusted to adjust the sterilization time;

(5) vacuum pulsation drying is performed again on the grapes of each grade from which surface moisture is removed in step (4), and the grapes of each grade from which surface moisture is removed are laid flat in a single layer in a vacuum box, evacuated to a rated vacuum state, and the drying temperature is set to perform vacuum drying; when the vacuum holding time ends, the state is switched to a rated normal pressure state, and normal pressure drying is performed; when the normal pressure holding time ends, the state is switched to a vacuum state again, and the process is repeated in sequence; the process is stopped when the dry basis moisture content of the material is reduced to 0.25±0.05 g/g, and the total drying time is recorded.

(6) After drying, the grapes are sterilized by catalytic infrared and placed on a double-layer conveyor belt. The conveyor belts are located at different heights and a catalytic infrared heater is installed on the top. The rolling of the material during the falling process can achieve automatic turning, which is conducive to uniform heating and sterilization of the material. The conveyor belt speed is adjustable, and the heating time can be adjusted.

(7) Product quality testing: The grapes dried and sterilized in step (6) were sampled using the "nine-point method" and tested for four physical and chemical quality indicators, namely, soluble solid content, color difference value, product moisture content at the end of drying, and total bacterial count residual rate. The average value was taken as the final test result. The logarithmic values of the color difference value and the total bacterial count residual rate, and the uniformity coefficients of the soluble solid content and product moisture content were calculated for comparison of the test results.

(8) Finally, vacuum packaging is performed to obtain a raisin product with a moisture content and a bacterial content that meet the standards and a uniform product quality.

The vibrating screen is made of high-density polyethylene plastic, and the diameters of the circular mesh holes of the vibrating screen are 18, 15, 12, and 9 mm respectively.

The grape raw materials classified into large, medium and small according to the transverse diameter of the grapes are classified into grape particles with diameters of more than 18, (15, 18], (12, 15], (9, 12] and less than 9 mm according to the diameters of the sieve holes of 18, 15, 12 and 9 mm.

The catalytic infrared heating conditions are as follows: the temperature of the catalytic infrared generator is 380-420° C., the distance between the conveyor belt and the upper catalytic infrared generator is 16-20 cm, and the heating time is 60-6000 s.

The vacuum pulsation drying conditions are as follows: the rated vacuum drying chamber pressure is 0-10 kPa, the rated normal pressure drying chamber pressure is 95-101 kPa, the drying temperature is 60-85°C, the vacuum holding time is 1-15 minutes, and the normal pressure holding time is 2-10 minutes;

The catalytic infrared heating conditions are as follows: the temperature of the catalytic infrared generator is 380-420° C., the distance between the conveyor belt and the upper catalytic infrared generator is 16-20 cm, and the heating time is 60-6000 s.

The grape quality detection specifically includes:

1) Soluble solid content: Soluble solid content is determined using NY/T 2637-2014 standard.

The larger the uniformity coefficient of soluble solids content, the better the drying uniformity. The calculation formula is as follows:

表示平均可溶性固形物含量，g·g^-1；j表示不同取样点编号，取值范围1～9；Where, M _1j is the soluble solid content at the jth sampling point, ^o Brix;

represents the average soluble solid content, g·g ^-1 ; j represents the number of different sampling points, ranging from 1 to 9;

2) Color difference value: Take an appropriate amount of raisins and pour them into a white container. Use a colorimeter to test the L, a, and b values of the sample. The L value represents the brightness of the sample, the a value represents the redness and greenness of the sample, and the b value represents the yellowness and blueness of the sample. ΔE represents the color difference value. The larger the value, the greater the change in the color of the dried grapes compared to the fresh grapes. The calculation formula for the color difference value ΔE is as follows:

Wherein: L ⁰ , a ⁰ , b ⁰ represent the color values of fresh grapes, L ^* , a ^* , b ^* represent the color values of dried grape products.

Color difference uniformity coefficient, the larger the value, the better the drying uniformity, the calculation formula is as follows:

表示平均色差值；j表示不同取样点编号，取值范围1～9；Where M _2j is the color difference value of the jth sampling point;

represents the average color difference value; j represents the number of different sampling points, ranging from 1 to 9;

3} Product moisture content: The product moisture content is determined by the drying method in GB 5009.3-2016 national food safety standard. The larger the product moisture content uniformity coefficient value, the better the drying uniformity. The calculation formula is as follows:

表示平均产品含水率；j表示不同取样点编号，取值范围1～9；Where, M _3j is the moisture content of the product at the jth sampling point;

represents the average product moisture content; j represents the number of different sampling points, ranging from 1 to 9;

4) Residual survival rate of total bacteria: The bactericidal effect is expressed by the logarithm of the residual survival rate of total bacteria, and the total colony count determination method is determined in accordance with GB47892-2010 standard;

The larger the uniformity coefficient of the total bacterial count and residual survival rate, the better the drying uniformity. The calculation formula is as follows:

表示平均细菌总数残活率；j表示不同取样点编号，取值范围1～9。Where, M _4j is the residual survival rate of the total number of bacteria at the jth sampling point;

represents the average total bacterial count and the survival rate; j represents the number of different sampling points, ranging from 1 to 9.

The beneficial effect of the present invention is that the present invention provides a high-quality raisin product with consistent product quality, moisture content and bacterial content meeting the standards, and the product quality is good, there is no chemical residue, and the method is simple and reliable. It has the following characteristics:

(1) The effective salt solution classification method, vacuum pulsation drying technology, and catalytic infrared heating technology are used to achieve the drying and sterilization of grapes, which has the advantages of fast drying speed, uniform drying, good product quality, and simple operation.

(2) Realize material classification based on differences in grape physical and chemical characteristics, especially differences in maturity. The salt solution method is used to distinguish the soluble solid content based on differences in grape density, and then accurately distinguish the maturity. Compared with the traditional method of distinguishing the maturity of grapes based on the size and color of the grapes, this method is simple, reliable, and efficient, and it also avoids the destruction of materials during material screening and classification to a large extent.

(3) In the pretreatment of fresh grapes, no chemical reagents are used except for salt solution. The catalytic infrared drying technology is used to quickly dehydrate the surface of the grapes after they are sorted with the help of salt solution, which is conducive to shortening the time spent in the subsequent drying stage and thus reducing energy consumption. Compared with traditional grape drying methods, it has the advantages of being natural, green and pollution-free, and the operation method is simple and easy to understand.

(4) The vacuum pulsation drying technology is used to quickly dehydrate and dry the sorted grapes. Compared with the traditional vacuum drying method, it has the advantage of high efficiency. Compared with the traditional hot air drying, it avoids excessive contact of the material with oxygen for a long time and can effectively curb the oxidation and deterioration of heat-sensitive components. In addition, the quality of the processed grapes is better and the quality consistency is better.

(5) In the later stage of processing, catalytic infrared drying technology is used to sterilize the dried grape products without chemical reagent pollution, which opens up a new way for the sterilization processing of dried grape products and can effectively extend the storage period of the products.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a flow chart showing the specific implementation of raisin production;

Fig. 2 is a schematic diagram of the transverse diameter of grape grains;

Fig. 3 is a schematic diagram of classification by saline solution method;

FIG4 is a schematic diagram of a catalytic infrared heating process;

FIG5 is a schematic diagram of the sample collection method.

1-grapes, 2-lateral diameter, 3-liquid storage tank, 4-low concentration salt solution, 5-high concentration salt solution, 6-catalytic infrared heater, 7-conveyor belt, 8-feeding tray, 9-sampling point.

DETAILED DESCRIPTION

The present invention provides a method for producing high-quality raisins, which will be further described below in conjunction with the accompanying drawings and specific implementation examples.

FIG1 is a flowchart showing a specific implementation of raisin production; the high-quality raisin production shown in FIG1 includes the following steps:

(1) Pick fresh seedless white grapes without rot, remove the stalks and wash away impurities, and discard the grapes that are squeezed, damaged or deformed during picking and transportation;

(2) Particle grading and screening, using 4 layers of vibrating screens for screening in sequence, the vibrating screen is made of high-density polyethylene plastic, and the circular mesh diameters of the vibrating screen are 18, 15, 12, and 9 mm, respectively. As shown in Figure 2, according to the transverse diameter of the grapes (as shown in Figure 2), they can be divided into materials with diameters greater than 18, (15, 18], (12, 15], (9, 12], and less than 9 mm. Since grapes with larger diameters take longer to dry and grapes with smaller diameters have poorer appearance after drying, materials with transverse diameters of (15, 18], (12, 15], and (9, 12] mm are selected as processing raw materials and are defined as large, medium, and small diameter grapes, respectively;

(3) Grapes are graded according to the phenomenon of floating or sinking in the salt solution. That is, based on the principle that grapes of different maturity have different densities, sodium chloride solutions with concentrations of 120, 140, 160, and 180 g/L are prepared, and grapes are graded according to the phenomenon of floating or sinking in the salt solution. As shown in FIG3 , the grapes after size classification are first placed in the salt solution with the lowest concentration. According to their floating and sinking phenomena, the suspended grape particles are fished out and collected. The sinking grapes are fished out and placed in the next salt solution with higher concentration. Similarly, the floating grape particles are fished out, and the sinking samples are fished out and placed in the salt solution with higher concentration, and this process is repeated until the sample classification is completed. The materials are sequentially divided into grapes corresponding to the salt solution concentrations of <120, (120, 140], (140, 160], (160, 180], and >180 g/L, which are defined as grapes of lower, lower, medium, high, and higher maturity, respectively.

(4) Catalytic infrared heating is used to remove surface moisture from the grapes of various grades obtained in step (3). As shown in FIG4 , grapes classified by size and maturity are selected and placed on a double-layer conveyor belt. The conveyor belts are located at different heights and a catalytic infrared heater is installed on the top. The rolling of the material during the falling process can achieve automatic flipping, which is conducive to uniform heating of the material to remove surface moisture; the conveyor belt speed is adjustable, thereby adjusting the sterilization time. The catalytic infrared heating conditions are: the catalytic infrared generator temperature is 380 to 420°C, the distance between the conveyor belt and the upper catalytic infrared generator is 16 to 20 cm, and the heating time is 60 to 6000 seconds.

(5) The grapes are vacuum dried by pulsating the material in a single layer in a vacuum box, pumping to the rated vacuum state, setting the drying temperature, and vacuum drying. When the vacuum holding time is over, switch to the rated normal pressure state and dry at normal pressure. When the normal pressure holding time is over, switch to the vacuum state again, and repeat the process until the dry basis moisture content of the material is reduced to 0.25±0.05g/g, and record the total drying time.

The vacuum pulsation drying conditions are as follows: the rated vacuum drying chamber pressure is 0-10 kPa, the rated normal pressure drying chamber pressure is 95-101 kPa, the drying temperature is 60-85° C., the vacuum holding time is 1-15 min, and the normal pressure holding time is 2-10 min.

(6) After the grapes are dried, they are sterilized by catalytic infrared and placed on a double-layer conveyor belt. The conveyor belts are located at different heights and a catalytic infrared heater is installed on the top. The rolling of the material during the falling process can achieve automatic turning, which is conducive to uniform heating and sterilization of the material. The conveyor belt speed is adjustable, and the heating time can be adjusted. The catalytic infrared heating conditions are: the temperature of the catalytic infrared generator is 380-420°C, the distance between the conveyor belt and the catalytic infrared generator above is 16-20cm, and the heating time is 60-6000s.

(7) Product quality inspection: As shown in Figure 5, the items were laid flat on the tray, and the "nine-point method" was used to take samples. The four physical and chemical quality indicators of soluble solid content, color difference value, material moisture content at the end of drying (product moisture content), and total bacterial count residual rate were tested respectively, and the average value was taken as the final test result. The logarithmic values of the color difference value and the total bacterial count residual rate, and the uniformity coefficients of the soluble solid content and product moisture content were calculated for comparison of the test results.

(8) Finally, vacuum packaging is performed to obtain a raisin product with a moisture content and a bacterial content that meet the standards and a uniform product quality.

In steps (4) and (6) of this embodiment, the temperature of the infrared radiation plate, the irradiation distance and the treatment time can be adjusted as needed. The higher the temperature of the infrared radiation plate and the shorter the irradiation distance, the shorter the treatment time can be.

The specific detection method of grape quality is as follows.

①Soluble solid content: Soluble solid content is determined using NY/T 2637-2014 standard.

The larger the uniformity coefficient of soluble solids content, the better the drying uniformity. The calculation formula is as follows:

表示平均可溶性固形物含量，g·g^-1；j表示不同取样点编号，取值范围1～9；Where, M _1j is the soluble solid content at the jth sampling point, ^o Brix;

represents the average soluble solid content, g·g ^-1 ; j represents the number of different sampling points, ranging from 1 to 9;

② Color difference value: Take an appropriate amount of raisins and pour them into a white container. Use a colorimeter to test the L, a, and b values of the sample. The L value represents the brightness of the sample, the a value represents the red-green degree of the sample, and the b represents the yellow-blue degree of the sample. ΔE represents the color difference value. The larger the value, the greater the change in the color of the dried grapes compared to the fresh grapes. The calculation formula for the color difference value ΔE is as follows:

Wherein: L ⁰ , a ⁰ , b ⁰ represent the color values of fresh grapes, L ^* , a ^* , b ^* represent the color values of dried grape products.

Color difference uniformity coefficient, the larger the value, the better the drying uniformity, the calculation formula is as follows:

表示平均色差值；j表示不同取样点编号，取值范围1～9；Where M _2j is the color difference value of the jth sampling point;

represents the average color difference value; j represents the number of different sampling points, ranging from 1 to 9;

③ Product moisture content: The product moisture content is determined by the drying method in GB 5009.3-2016 national food safety standard.

The larger the uniformity coefficient of product moisture content, the better the drying uniformity. The calculation formula is as follows:

表示平均产品含水率；j表示不同取样点编号，取值范围1～9；Where, M _3j is the moisture content of the product at the jth sampling point;

represents the average product moisture content; j represents the number of different sampling points, ranging from 1 to 9;

④ Total bacterial count residual rate: The bactericidal effect is expressed by the logarithm of the total bacterial count residual rate. The total bacterial count determination method is determined in accordance with GB47892-2010 standard.

The larger the uniformity coefficient of the total bacterial count and residual survival rate, the better the drying uniformity. The calculation formula is as follows:

表示平均细菌总数残活率；j表示不同取样点编号，取值范围1～9；Where, M _4j is the residual survival rate of the total number of bacteria at the jth sampling point;

represents the average total bacterial survival rate; j represents the number of different sampling points, ranging from 1 to 9;

Comparative Example 1

① Pick fresh "seedless white" grapes without rot, remove the stalks and wash away impurities, and discard the grapes that are squeezed, damaged and deformed during picking and transportation;

② Take the washed grapes and place them in a cool and ventilated place to dry until the moisture content of the material on a dry basis drops to 0.25±0.05g/g. Record the total drying time, take samples and measure the color difference value, the logarithm of the total bacterial count and the residual survival rate, the soluble solids content, and the product moisture content, and calculate the color difference value, the logarithm of the total bacterial count and the uniformity coefficient of the soluble solids content and the product moisture content.

Comparative Example 2

① Pick fresh "seedless white" grapes without rot, remove the stalks and wash away impurities, and discard the grapes that are squeezed, damaged and deformed during picking and transportation;

② Take the washed grapes and place them directly in a 75℃ hot air oven for drying until the moisture content of the material on a dry basis drops to 0.25±0.05g/g. Record the total drying time, take samples and measure the color difference value, the logarithm of the total bacterial count and the residual survival rate, the soluble solids content, and the product moisture content, and calculate the color difference value, the logarithm of the total bacterial count and the uniformity coefficient of the soluble solids content and the product moisture content.

Example 1

① Pick fresh "seedless white" grapes without rot, remove the stalks and wash away impurities, and discard the grapes that are squeezed, damaged and deformed during picking and transportation;

② Use 4 layers of vibrating screens to screen in sequence. The vibrating screen is made of high-density polyethylene plastic. The circular mesh diameters of the vibrating screen are 18, 15, 12, and 9 mm, respectively. As shown in Figure 2, the grapes can be divided into materials with diameters greater than 18, (15, 18], (12, 15], (9, 12], and less than 9 mm according to their transverse diameters. Select medium-diameter grapes with a transverse diameter of (12, 15] mm as raw materials;

③ Based on the principle that the density of grape particles at different maturity levels is different, sodium chloride solutions with concentrations of 120, 140, 160, and 180 g/L are prepared, and the grape particles are graded according to the phenomenon of floating or sinking in the salt solution. As shown in Figure 3, the medium-diameter grapes are first placed in the salt solution with the lowest concentration. According to their floating and sinking phenomena, the suspended grape particles are fished out and collected, and the sinking grapes are fished out and placed in the next salt solution with higher concentration. Similarly, the floating grape particles are fished out, and the sinking samples are fished out and continued to be placed in a higher concentration salt solution, and this is done in sequence until the sample classification is completed. The materials are divided into grapes corresponding to salt solution concentrations of <120, (120, 140], (140, 160], (160, 180], and >180 g/L, which are defined as grapes with lower, low, medium, high, and higher maturity levels, respectively. Then, medium-maturity grapes are selected for use.

④ As shown in Figure 4, medium-diameter, medium-maturity grapes are selected and placed on a double-layer conveyor belt. The conveyor belt is located at different heights, and a catalytic infrared heater is installed on the top. The rolling of the material during the falling process can achieve automatic flipping, which is conducive to uniform heating of the material to remove surface moisture; the conveyor belt speed is adjustable to adjust the heating time. The catalytic infrared heating conditions are: the catalytic infrared generator temperature is 400°C, the distance between the conveyor belt and the catalytic infrared generator above is 18cm, and the heating time is 300s.

⑤ Dry the grapes by vacuum pulsation. Lay the material in a single layer in a vacuum box, pump to the rated vacuum state of 5±2kPa, set the drying temperature to 75℃, and perform vacuum drying. Set the vacuum holding time to 10min. When the vacuum holding time is over, switch to the rated normal pressure state of 99±2kPa for normal pressure drying. Set the normal pressure holding time to 4min. When the normal pressure holding time is over, switch to the rated vacuum state again, and repeat in sequence.

The drying process was stopped when the moisture content of the material on a dry basis dropped to 0.25±0.05 g/g, and the total drying time was recorded.

⑥ After drying, the grapes are sterilized by catalytic infrared and placed on a double-layer conveyor belt. The conveyor belts are at different heights and a catalytic infrared heater is installed above. The catalytic infrared heating conditions are: the temperature of the catalytic infrared generator is 400°C, the distance between the conveyor belt and the catalytic infrared generator above is 18cm, and the heating time is 240s.

As shown in Figure 5, the items were laid flat on the tray, and the "nine-point method" was used to take samples. The soluble solid content, color difference value, material moisture content at the end of drying (product moisture content), and total bacterial count residual rate were tested respectively, and the average was taken as the final test result. The logarithmic values of the color difference value and the total bacterial count residual rate, the uniformity coefficients of the soluble solid content and the product moisture content were calculated for comparison of the test results. The specific results are shown in Table 1.

Example 2

① Pick fresh "seedless white" grapes without rot, remove the stalks and wash away impurities, and discard the grapes that are squeezed, damaged and deformed during picking and transportation;

② Use 4 layers of vibrating screens to screen in sequence. The vibrating screen is made of high-density polyethylene plastic. The circular mesh diameters of the vibrating screen are 18, 15, 12, and 9 mm, respectively. As shown in Figure 2, the grapes can be divided into materials with diameters greater than 18, (15, 18], (12, 15], (9, 12], and less than 9 mm according to their transverse diameters. Select small-diameter grapes with a transverse diameter of (9, 12] mm as raw materials;

③ Based on the principle that the density of grape particles at different maturity levels is different, sodium chloride solutions with concentrations of 120, 140, 160, and 180 g/L are prepared, and grape particles are graded according to the phenomenon of floating or sinking in the salt solution. As shown in Figure 3, medium-diameter grapes are first placed in the salt solution with the lowest concentration. According to their floating and sinking phenomena, the suspended grape particles are fished out and collected, and the sinking grapes are fished out and placed in the next salt solution with higher concentration. Similarly, the floating grape particles are fished out, and the sinking samples are fished out and continued to be placed in a higher concentration salt solution, and this is done in sequence until the sample classification is completed. The materials are divided into grapes corresponding to salt solution concentrations of <120, (120, 140], (140, 160], (160, 180], and >180 g/L, which are defined as grapes with lower, low, medium, high, and higher maturity levels, respectively. Then select grapes with higher maturity levels for use.

④ As shown in Figure 4, grapes 1 with small diameter and high maturity are selected and placed on a double-layer conveyor belt 3. The conveyor belt 3 is located at different heights and a catalytic infrared heater 2 is installed on the top. The rolling of the grapes 1 during the falling process can achieve automatic flipping, which is conducive to uniform heating of the grapes to remove surface moisture; the conveyor belt speed is adjustable to adjust the heating time. The catalytic infrared heating conditions are: the catalytic infrared generator temperature is 420℃, the distance between the conveyor belt and the catalytic infrared generator above is 16cm, and the heating time is 180s.

⑤ Perform vacuum pulse drying on grapes. Lay the grapes in a single layer in a vacuum box, pump to the rated vacuum state of 5±2kPa, set the drying temperature to 85℃, and perform vacuum drying. Set the vacuum holding time to 10min. When the vacuum holding time is over, switch to the rated normal pressure state of 99±2kPa for normal pressure drying. Set the normal pressure holding time to 4min. When the normal pressure holding time is over, switch to the rated vacuum state again, and repeat in sequence.

The drying process was stopped when the moisture content of the material on a dry basis dropped to 0.25±0.05 g/g, and the total drying time was recorded.

⑥ After drying, the grapes are sterilized by catalytic infrared and placed on a double-layer conveyor belt. The conveyor belts are located at different heights and a catalytic infrared heater is installed above. The catalytic infrared heating conditions are: the temperature of the catalytic infrared generator is 420°C, the distance between the conveyor belt and the catalytic infrared generator above is 16cm, and the heating time is 180s.

As shown in FIG5 , grapes 1 were spread flat on a tray 4, and the "nine-point method" was used to take samples, and the soluble solid content, color difference value, material moisture content at the end of drying (product moisture content), and total bacterial count residual rate, four physical and chemical quality indicators, were tested respectively, and the average was taken as the final test result. The logarithmic values of the color difference value and the total bacterial count residual rate, the uniformity coefficients of the soluble solid content and the product moisture content were calculated respectively for comparison of the test results, and the specific results are shown in Table 1.

From the results in Table 1, it can be seen that the drying time and the logarithmic value of the total bacterial survival rate of Examples 1 and 2 of the present invention are significantly smaller than those of Comparative Examples 1 and 2, and the quality is better. The color difference value, the logarithmic value of the total bacterial survival rate, the uniformity coefficient of the soluble solid content and the product moisture content index are far better than those of the raisins dried by natural sun drying and hot air drying, and the quality uniformity is better.

Table 1 Comparison between comparative examples and implementation examples

Claims (7)

1. The preparation method of the high-quality raisin is characterized by comprising the following steps of:

(1) Firstly selecting raw materials, picking fresh non-rotten non-white grape, removing fruit stalks, washing away impurities, and removing grape grains which are extruded, damaged and deformed in the picking and transporting processes;

(2) The method comprises the steps of (1) carrying out particle grading screening, sequentially screening by adopting 4 layers of vibrating screens, and storing grape raw materials with large, medium and small transverse diameters;

(3) Grading according to the phenomenon that grape particles float up or sink down in a salt solution, namely, preparing sodium chloride solutions with the concentration of 120 g/L, 140 g/L, 160 g/L and 180g/L according to the principle that the grape particles with different maturity are different, placing the grapes subjected to the size grading in the step (2) in the salt solution with the lowest concentration at first, fishing out suspended grape particles according to the phenomenon that the grapes float up and sink down, collecting the suspended grape particles, fishing out the sinking grape particles, placing the sinking grape particles in the salt solution with the next higher concentration, and similarly, fishing out the floating grape particles, and sequentially placing the sinking sample in the salt solution with the higher concentration until the grading of the sample is completed; the materials are sequentially divided into grapes with salt solution concentration less than 120, (120, 140], (140, 160], (160, 180], > 180g/L, respectively defined as low, medium, high and high maturity;

(4) The surface moisture of each level of grapes obtained in the step (3) is removed by adopting catalytic infrared heating, the grapes with classified sizes and maturity are selected and placed on a double-layer conveyor belt, the positions of the conveyor belt are different, the catalytic infrared heating is arranged above the conveyor belt, and the automatic turning can be realized by rolling in the material falling process, so that the uniform heating of the materials is facilitated, and the surface moisture is removed; the speed of the conveyor belt is adjustable, so that the sterilization time is adjusted;

(5) Carrying out vacuum pulsation drying on all levels of grapes with surface moisture removed in the step (4), respectively laying all levels of grapes with surface moisture removed in a single layer, putting the grapes in a vacuum box, pumping the grapes to a rated vacuum state, setting a drying temperature, and carrying out vacuum drying; when the vacuum keeping time is over, switching to a rated normal pressure state, performing normal pressure drying, and when the normal pressure keeping time is over, switching to the vacuum state again, and sequentially circulating; stopping until the water content of the dry matter is reduced to 0.25+/-0.05 g/g, and recording the total drying time;

(6) The dried grapes are subjected to catalytic infrared sterilization and are placed on a double-layer conveyor belt, the positions of the conveyor belt are different, catalytic infrared heating is arranged above the conveyor belt, and the automatic turning can be realized by rolling in the falling process of materials, so that the uniform heating sterilization of the materials is facilitated; the speed of the conveyor belt is adjustable, and the heating time can be adjusted;

(7) The quality detection of the product, namely sampling the dried and sterilized grapes in the step (6) by a nine-point method, respectively detecting 4 physicochemical quality indexes of the content of soluble solids, the color difference value, the water content of the product after the drying is finished and the total bacterial count residual activity, and taking the average value as a final detection result; calculating the color difference value, the logarithmic value of the total bacterial residual activity rate, the content of soluble solids and the uniformity coefficient of the water content of the product respectively, and comparing the results;

(8) Finally, vacuum packaging is carried out; thus obtaining raisin products with uniform and consistent product quality, wherein the water content and the bacterial content reach the standards.

2. The method for producing high-quality raisins as claimed in claim 1, wherein the vibrating screen is made of high-density polyethylene plastic, and the diameter of the circular mesh of the vibrating screen is 18, 15, 12 and 9mm in order.

3. The method for producing high-quality raisins according to claim 1, wherein the grape raw materials divided into large, medium and small grape transverse diameters are divided into grape particles having diameters of > 18, 15-18, 12-15, 9=12 and < 9mm according to the mesh diameters of 18, 15, 12, 9mm.

4. The method for producing high-quality raisins as claimed in claim 1, wherein said catalytic infrared heating conditions are: the temperature of the catalytic infrared generator is 380-420 ℃, and the distance between the conveyor belt and the upper Fang Cuihua infrared generator is 16-20 cm, and the heating time is 60-6000 s.

5. The method for producing high-quality raisins as claimed in claim 1, wherein said vacuum pulsating drying conditions are: the rated vacuum state drying chamber pressure is 0-10 kPa, the rated normal pressure state drying chamber pressure is 95-101 kPa, the drying temperature is 60-85 ℃, the vacuum holding time is 1-15 min, and the normal pressure holding time is 2-10 min.

6. The method for producing high-quality raisins as claimed in claim 1, wherein said catalytic infrared heating conditions are: the temperature of the catalytic infrared generator is 380-420 ℃, and the distance between the conveyor belt and the upper Fang Cuihua infrared generator is 16-20 cm, and the heating time is 60-6000 s.

7. The method for producing high-quality raisins as claimed in claim 1, wherein said quality detection of raisins comprises:

1) Soluble solids content: the soluble solids content was determined using the NY/T2637-2014 standard,

the greater the value of the uniformity coefficient of the soluble solids content, the better the drying uniformity, the following formula is calculated:

wherein M is _1j For the soluble solids content of the jth sampling point, ^o Brix；

represents the average soluble solids content, g.g ^-1 The method comprises the steps of carrying out a first treatment on the surface of the j represents different sampling point numbers, and the value range is 1-9;

2) Color difference value: pouring a proper amount of raisins into a white vessel, and detecting L, a and b values of a sample by using a color difference meter; the value L represents the brightness of the sample, the value a is the red-green degree of the sample, and the value b represents the yellow Lan Du of the sample; Δe represents the color difference value, the larger this value is, the larger the change in color of the dried grape compared to fresh grape is; the color difference value Δe is calculated as follows:

wherein: l (L) ⁰ 、a ⁰ 、b ⁰ Respectively represent the color value, L of fresh grape ^* 、a ^* 、b ^* Color values representing the grape dry product, respectively;

the larger the color difference value uniformity coefficient value is, the better the drying uniformity is, and the calculation formula is as follows:

wherein M is _2j The color difference value of the j-th sampling point;

representing the average color difference value; j represents different sampling point numbers, and the value range is 1-9;

3} product moisture content: the water content of the product is measured by adopting a drying method in GB 5009.3-2016 national food safety standard, and the larger the water content uniformity coefficient value of the product is, the better the drying uniformity is, and the calculation formula is as follows:

wherein M is _3j The water content of the product at the j-th sampling point;

representing the average product water content; j represents different sampling point numbers, and the value range is 1-9;

4) Total bacterial count survival rate: the sterilization effect is expressed by the logarithmic value of the residual activity rate of the total bacteria, and the total bacterial colony number measuring method is measured by referring to the GB47892-2010 standard;

the higher the value of the residual activity rate uniformity coefficient of the total number of bacteria, the better the drying uniformity, and the calculation formula is as follows:

wherein M is _4j The total bacterial count residual activity rate for the jth sampling point;

represents the average total bacterial count residual activity rate; j represents the number of different sampling points and the value range1 to 9 portions of the periphery. />

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