Production method of frozen pears

Abstract

The invention discloses a production method of frozen pears, which is characterized by comprising the following steps: (1) selecting pear fruits without mechanical damage and pest and disease damage, cleaning and wiping water for later use; (2) freezing the pear fruits obtained in the step (1) in an environment of-38 + / -1 DEG C for 20-24 hours; and (3) putting the frozen pear fruits in an environment of 4 DEG C, and unfreezing for 8-10 hours to obtain the frozen pears. The method is simple and high in efficiency, and the prepared frozen pears are high in nutritional value.

Description

Technical Field

[0001] The invention belongs to the technical field of food processing, and particularly relates to a production method of frozen pears. Background Art

[0002] Pears are a delicious fruit that grows widely in temperate regions and come in a variety of sizes, shapes, textures and flavors. China is the world's largest producer of pears, producing more than 15 million tons per year. Pears contain vitamins A, B1, B2, B3, C and minerals such as sodium, potassium, phosphorus, calcium, magnesium and iron. Pears are low in acidity and nutrients and are mainly used in the production of baby food such as purees and various fruit juices. Due to their high dietary fiber content, pears are also used to treat cystitis, kidney stones, constipation and intestinal inflammation.

[0003] Frozen pears are a traditional specialty food originating from Northeast China. They are pears frozen under low temperature conditions into black fruits. The history of making frozen pears can be traced back to a long time ago. It is a way for residents in northern China to store and eat fruits during the cold winter weather. Traditionally, the production process of frozen pears is relatively simple. Specifically, after the pears are picked, they are washed and placed directly outdoors at below zero degrees for freezing. The pears will freeze quickly, and the internal water will condense into ice, and then thaw to become frozen pears. With the development of food cold chain technology, frozen pears are no longer limited to winter supply in Northeast China, but can be supplied throughout the country throughout the year. Modern technology has also made the production of frozen pears more standardized and efficient, allowing more people to enjoy this traditional delicacy. The current methods for making frozen pears are too complicated, or the process is not optimized enough, making it difficult to maintain the nutritional value of frozen pears.

[0004] China's published patent CN108541914A investigated the effects of post-ripening hardness, thawing times, thawing time, and thawing interval on the quality indexes of frozen pears of Nanguo Pear through single factor and orthogonal experiments, and determined that the preparation method of frozen pears of Nanguo Pear is to ripen the pears to a hardness of 1.5kg / 0.5cm 2 <Y2≤2.5kg / 0.5cm 2 The pears are frozen at -20℃ for 10 days, then the temperature is adjusted to 0℃ and thawed for 36 hours. The freezing-thawing process is repeated 3 times in total. After the third thawing, the temperature is adjusted to -20℃ and frozen to make Nanguoli frozen pears. China's published patent CN109793042A discloses a method for preventing frozen pears from browning. Specifically, Qiuzi pears are blanched, cooled, placed in an anti-browning liquid, frozen at -20℃ for 48 to 72 hours, and then thawed at 4℃ for 48 to 72 hours.

[0005] The frozen pear production process disclosed in the first patent document is too complicated and time-consuming, and the purpose of the complex process is unclear. The second patent document is dedicated to solving the problem of frozen pear browning, but does not analyze the various parameters of the frozen pear freezing and thawing process, that is, the selection of parameters has no specific basis and optimization; in addition, the unique black appearance of frozen pears is its own characteristic, and people will evaluate whether the frozen pears are successfully produced based on the darkness of their color. The frozen pears produced by the production process to prevent browning are likely to cause misunderstandings among consumers. Summary of the invention

[0006] The invention aims to provide a method for producing frozen pears, which is simple and efficient, and the prepared frozen pears have high nutritional value.

[0007] The purpose of the present invention is achieved through the following technical solutions.

[0008] A method for producing frozen pears, characterized in that it comprises the following steps:

[0009] (1) selecting pear fruits without mechanical damage and pests and diseases, washing them, drying them and setting them aside;

[0010] (2) placing the pear fruit obtained in step (1) in a -38±1°C environment and freezing for 20 to 24 hours;

[0011] (3) The frozen pear fruits are placed in a 4°C environment for thawing for 8 to 10 hours to obtain frozen pears.

[0012] As a specific embodiment of the present invention, the pear fruit includes snow pear and crown pear.

[0013] As a preferred embodiment of the present invention, the freezing rate of pear fruit in an environment of -38±1°C is 1 cm / h, and the time to pass through the ice crystal zone is ≤90 min.

[0014] As a preferred embodiment of the present invention, in step (3), the frozen pear fruits are thawed in a high oxygen atmosphere consisting of 20% CO2 and 80% O2.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] The present invention is based on the juice loss rate of frozen pears, the brightness value L *, hardness, soluble solids, vitamin C, total polyphenols, titratable acid, stone cells, juice yield and respiration intensity and other indicators, and optimized the process parameter factors of frozen pears such as freezing temperature, freezing time, thawing temperature and thawing time; on the one hand, the present invention simplifies the production process of frozen pears, and frozen pears are obtained through a single freezing + thawing process, thereby shortening the production time and improving the production efficiency; on the other hand, the nutritional components of frozen pears are retained to the maximum extent during the production process, and the appearance and taste quality of the frozen pears are improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is the appearance of the frozen pears prepared in Example 1, wherein A is a crown pear and B is a snow pear.

[0018] Figure 2 This is the appearance of the frozen pears prepared in Example 2, wherein A is a crown pear and B is a snow pear.

[0019] Figure 3 This is the appearance of the frozen pears prepared in Comparative Example 1, where A is a crown pear and B is a snow pear.

[0020] Figure 4 This is the appearance of the frozen pears prepared in Comparative Example 2, where A is a crown pear and B is a snow pear. DETAILED DESCRIPTION

[0021] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] The following are the juice loss rate and brightness value L of frozen pears * , hardness, soluble solids, vitamin C, total polyphenols, titratable acid, stone cells, juice yield and respiratory intensity indicators.

[0023] 1. Determination method of juice loss rate

[0024] Before the pears were frozen at -38°C, their weight W1 was weighed. After freezing, the test samples were taken out and the surface juice was removed after thawing and weighed W2 (W1 and W2 were measured in parallel 3 times, and the arithmetic mean of the 3 data was taken as the result). The juice loss rate indicates the percentage of the juice lost after the pears were thawed to the original weight, reflecting the water retention capacity and tissue structure stability of the fruit; a lower juice loss rate indicates that the frozen pears have a tighter tissue structure, better water retention and better quality.

[0025] The juice loss rate is calculated according to the formula shown in (1):

[0026]

[0027] 2. Brightness value L * Determination method

[0028] The colorimeter is based on CIEL * a * b * In this mode, the brightness value L of the pear peel after thawing was tested using a colorimeter (calibrated before use). * , randomly select 6 fruits of uniform size, randomly select two symmetrical points for each fruit, and calculate the average brightness of the peel. For frozen pears, the dark brown skin is its most basic feature, and the L* value reflects the color and visual quality of the frozen pear skin.

[0029] 3. Hardness determination method

[0030] Refer to NY / T 2009-2011 for the determination of hardness. Select a fruit hardness tester to carry out the hardness measurement test. Peel off a thin layer of peel (2cm) of frozen pear. 2 ) After measuring, hold the hardness tester perpendicular to the surface of the fruit being tested, and press the indenter evenly into the peeled fruit surface; the pointer will rotate, and it will stop when the indenter hits the scale line. At this time, read the value. Repeat each hardness measurement test 5 times, record the test data, and calculate its average value. Hardness refers to the resistance of the fruit flesh when subjected to pressure, reflecting the texture and chewing feel of the fruit flesh. For frozen pears, when the hardness is small, the taste is relatively good.

[0031] 4. Determination of soluble solids

[0032] On the day of sampling, cut the frozen pear along the equator, evenly sample 3 pulp tissues from the edible part, drip the juice on the refractometer, adjust to zero with distilled water, drip the filtrate on the refractometer, read and record the data, and repeat 3 times. Soluble solids mainly refer to the total amount of water-soluble substances such as sugars, acids, and minerals in the fruit. The higher the content, the higher the nutritional value of the fruit.

[0033] 5. Determination method of vitamin C

[0034] The determination was carried out according to the 2,6-dichloroindophenol titration method in GB5009.86-2016. Vitamin C is an important antioxidant with high nutritional value, and its content reflects the nutritional value of frozen pears.

[0035] 6. Determination method of total polyphenols

[0036] Using spectrophotometry, refer to Dewanto et al. (Journal of Agricultural & Food Chemistry), a certain mass of frozen pears was weighed, quick-frozen with liquid helium, ground into powder, added 60% ethanol, the volume mass ratio of ethanol to sample was 30:1, extracted in a 70°C water bath in the dark for 2 hours, then centrifuged at 12000r / min for 20 minutes, and the supernatant was taken as the crude extract of pear polyphenols. 0.125mL of phenolic substance extract was taken, 0.5mL of distilled water and 0.125mL of Folin-Schoka reagent were added, vortexed and mixed, and then allowed to react at room temperature for 6 minutes, then 1.25mL of 7g / 100mL NaCO3 solution and 1mL of distilled water were added, and allowed to react at room temperature in the dark for 90 minutes, and the absorbance was measured at a wavelength of 760nm, and 0.125L of methanol was used instead of the extract as a blank control. According to the gallic acid standard curve (y=0.0088x+0.0054(R 2 =0.999)) to calculate the phenol content in the frozen pear extract. The total phenol content is expressed as gallic acid equivalent per kilogram of fresh sample, in mg GAE / kg. Total polyphenols refers to the total content of polyphenolic compounds in fruits. These compounds have biological activities such as antioxidant and anti-inflammatory. The higher the better, indicating that the frozen pear has stronger antioxidant capacity and is more beneficial to health.

[0037] 7. Determination of titratable acid

[0038] Determine with reference to GB 12456-2021. Weigh 10.0g frozen pears and grind them in a mortar. Transfer to a 100mL volumetric flask and dilute to the mark with double distilled water. Shake well and filter after standing for 30 minutes. Take 20.0mL of the filtrate and transfer to a conical flask for titration.

[0039] 8. Determination method of stone cells

[0040] Stone cells are a hard cell structure in fruits (especially pears) that affect the taste and texture of the fruit. A high content of stone cells will make the fruit taste worse and have a rough texture. The determination of stone cells in frozen pears was carried out with reference to NY / T 1388-2007.

[0041] 9. Juice yield determination method

[0042] Randomly select 3 frozen pears and use a juicer to extract juice. The juice yield is calculated according to juice yield (%) = juice mass / pulp mass × 100, and repeat 3 times.

[0043] 10. Method for determining respiratory intensity

[0044] Respiration intensity refers to the respiratory metabolic rate of fruit during storage, which affects its storage life and preservation performance. It is measured using an infrared carbon dioxide meter. After the pears are thawed, they are tested at room temperature. A certain amount of frozen pears are placed in a sealed container and the carbon dioxide content released within 1 hour is tested. The respiration intensity is expressed as the amount of carbon dioxide released per kilogram of fruit per hour, in ng / (kg·h).

[0045] Example 1

[0046] A method for producing frozen pears comprises the following steps:

[0047] (1) Select fresh snow pears and crown pears without mechanical damage and pests and diseases, wash them with clean water, wipe off the surface moisture with kitchen towels and set aside.

[0048] (2) The pear fruit obtained in step (1) is placed in a -38°C environment with a freezing rate of 1 cm / h and a time to pass through the ice crystal zone of no more than 90 min, and frozen for 20 hours.

[0049] (3) The frozen pear fruits are placed in an environment of 4° C. for thawing for 8 hours to obtain frozen pears; wherein the thawing environment atmosphere is composed of 20% CO2 and 80% O2 (volume percentage).

[0050] (4) Determine the juice loss rate and brightness value L of frozen pears using the above method * , hardness, soluble solids, vitamin C, total polyphenols, titratable acid, stone cells, juice yield and respiration intensity. The results are shown in Table 1.

[0051] Example 2

[0052] The difference between this embodiment and embodiment 1 is that the freezing is performed for 24 hours and the thawing is performed for 10 hours; the other steps are the same as those in embodiment 1.

[0053] The above method was used to determine the juice loss rate and brightness value L of frozen pears. * , hardness, soluble solids, vitamin C, total polyphenols, titratable acid, stone cells, juice yield and respiration intensity. The results are shown in Table 1.

[0054] As can be seen from Examples 1 and 2, the method of the present invention improves the efficiency of producing frozen pears and reduces the cost to the maximum extent. After the pears are quickly frozen at -38°C, they are slowly thawed at low temperature in a high-oxygen atmosphere environment, so that the oxidation reaction can fully occur, and the production is completed after the surface of the frozen pears turns black. Compared with the existing method, not only the production time is greatly shortened, but the obtained frozen pears are also very nutritious. The high-oxygen environment can inhibit the respiration of the plant itself. The frozen pears produced by the present invention contain L * The value and hardness value are reduced by more than 50%, and the appearance is Figure 1 and 2As shown, the requirements of frozen pears are clearly met. Among them, the soluble solid content and vitamin C content of frozen pears are retained by more than 90%, and the total polyphenol content and titratable acid content are retained by more than 80%, which means that the frozen pears retain rich nutritional value; the stone cell content is less than 5g / 100g, the juice loss rate is less than 2%, the juice yield is more than 80%, and the respiration intensity during storage is less than 200ng / (kg·h).

[0055] Comparative Example 1

[0056] The difference between this comparative example and Example 1 is that the pear fruits are stored frozen at -18°C; the other steps are the same as Example 1.

[0057] The above method was used to determine the juice loss rate and brightness value L of frozen pears. * , hardness, soluble solids, vitamin C, total polyphenols, titratable acid, stone cells, juice yield and respiration intensity. The results are shown in Table 1.

[0058] The results show that the frozen pears prepared by the method of Comparative Example 1 have a lower juice yield and a lower degree of blackening. Figure 3 As shown, the unique color effect of the ideal frozen pear cannot be achieved, and more nutrients such as soluble solids, vitamin C, polyphenols, etc. are lost. Furthermore, because the pears are frozen at -18°C, the stone cell content is not fully destroyed, and more is retained, and the juice loss rate is also higher than that of the embodiment.

[0059] Comparative Example 2

[0060] The difference between this comparative example and Example 1 is that the thawing atmosphere is composed of 80% CO2 and 20% O2; the other steps are the same as Example 1.

[0061] The above method was used to determine the juice loss rate and brightness value L of frozen pears. * , hardness, soluble solids, vitamin C, total polyphenols, titratable acid, stone cells, juice yield and respiration intensity. The results are shown in Table 1.

[0062] The results show that the frozen pears prepared by the method of Comparative Example 2 have a low degree of darkening of the pear skin. Figure 4 As shown, there was a large loss of nutrients such as soluble solids, vitamin C, and polyphenols, indicating that higher CO2 concentrations may lead to further metabolic disorders in the fruit, and the pears may have undergone respiration, resulting in excessive nutrient loss.

[0063] Comparative Example 3

[0064] The difference between this comparative example and Example 1 is that the frozen pear fruits are thawed at room temperature for 6 hours; the other steps are the same as Example 1.

[0065] The above method was used to determine the juice loss rate and brightness value L of frozen pears. * , hardness, soluble solids, vitamin C, total polyphenols, titratable acid, stone cells, juice yield and respiration intensity. The results are shown in Table 1.

[0066] The results show that the frozen pears prepared by the method of Comparative Example 3 have serious nutritional losses, and because they are thawed quickly at room temperature, the peel is not fully oxidized, resulting in a lower degree of dark brown color of the peel.

[0067] Comparative Example 4

[0068] The difference between this comparative example and Example 1 is that the thawing atmosphere is air; the other steps are the same as Example 1.

[0069] The above method was used to determine the juice loss rate and brightness value L of frozen pears. * , hardness, soluble solids, vitamin C, total polyphenols, titratable acid, stone cells, juice yield and respiration intensity. The results are shown in Table 1.

[0070] The results showed that the pears prepared by the method of Comparative Example 4, when thawed in an air environment, had a lower degree of oxidation of the peel, resulting in a lower degree of dark brown color of the peel, and more nutrient loss in the frozen pears.

[0071] Comparative Example 5

[0072] The difference between this comparative example and Example 1 is that the frozen pear fruits are thawed in a 4° C. cold water bath for 8 h; the other steps are the same as those in Example 1.

[0073] The above method was used to determine the juice loss rate and brightness value L of frozen pears. * , hardness, soluble solids, vitamin C, total polyphenols, titratable acid, stone cells, juice yield and respiration intensity. The results are shown in Table 1.

[0074] The results show that the pears prepared by the method of Comparative Example 5 have a relatively high brightness value because they are soaked in water and isolate most of the oxygen. Correspondingly, their hardness also has a high value; the loss of nutritional content such as juice loss rate, soluble solids, and vitamin C is also relatively serious.

[0075] Comparative Example 6

[0076] The difference between this comparative example and Example 1 is that the pear fruits are placed in a -38±1°C environment with a freezing rate of 0.3 cm / h and a time of passing through the ice crystal zone of >90 min; the other steps are the same as in Example 1.

[0077] The above method was used to determine the juice loss rate and brightness value L of frozen pears. *, hardness, soluble solids, vitamin C, total polyphenols, titratable acid, stone cells, juice yield and respiration intensity. The results are shown in Table 1.

[0078] The results showed that the pears prepared by the method of Comparative Example 6 had a slow freezing speed, resulting in serious nutritional loss.

[0079] Comparative Example 7

[0080] The difference between this comparative example and Example 1 is that the pear fruits are placed in an environment of 4° C. for thawing for 48 hours; the other steps are the same as those in Example 1.

[0081] The above method was used to determine the juice loss rate and brightness value L of frozen pears. * , hardness, soluble solids, vitamin C, total polyphenols, titratable acid, stone cells, juice yield and respiration intensity. The results are shown in Table 1.

[0082] The results showed that the thawing time of the comparative example 7 method was longer, which resulted in the loss of some nutrients in the frozen pears during the process. The data showed that some indicators such as vitamin C, soluble solids, and titratable acid were significantly less than those in the embodiment.

[0083] Table 1 Data of frozen pears of Examples 1 to 2 and Comparative Examples 1 to 7

[0084]

[0085]

[0086] Note: The same index without the same letter indicates significant difference, P < 0.05.

Claims

1. A method for producing frozen pears, characterized in that: The following steps are involved: (1) selecting pear fruits without mechanical damage and pests and diseases, washing them, drying them and setting them aside; (2) placing the pear fruit obtained in step (1) in a -38±1°C environment and freezing for 20 to 24 hours; (3) The frozen pears are placed in a 4°C environment for thawing for 8 to 10 hours to obtain frozen pears.

2. The method for producing frozen pears according to claim 1, characterized in that: The freezing rate of pear fruit in an environment of -38±1℃ is 1cm / h, and the time to pass through the ice crystal zone is ≤90min.

3. The method for producing frozen pears according to claim 2, characterized in that: In step (3), the frozen pear fruits are thawed in a high oxygen atmosphere consisting of 20% CO2 and 80% O2.

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