CN104839318A - Liquid nitrogen quick freezing and freezing storage combined litchi preservation method - Google Patents

Abstract

The invention discloses a liquid nitrogen quick freezing and freezing storage combined litchi preservation method. The liquid nitrogen quick freezing and freezing storage combined litchi preservation method comprises the following steps of arranging fresh litchis in liquid nitrogen to be quickly frozen and then performing freezing storage under the freezing condition. The fresh litchis can be whole litchis or can be peeled litchis or peeled and denucleated litchis. After liquid nitrogen treatment, the litchis are taken out in the freezing (a freezing box or a refrigerator) preservation process, the hardness can be recovered to be similar to the hardness of the fresh litchis after 2-5 minutes, and the texture is similar to the texture of ice-cream. After the litchis are taken out, the litchis can be directly eaten or unfrozen for eating, and the nutrition, taste and flavor of the fresh litchis are kept. By means of the liquid nitrogen quick freezing and freezing storage combined litchi preservation method, the litchis preserved in July can be kept to the April of the second year, the nutrition, taste and flavor of the fresh litchis are still kept, the litchi preservation effect is good, cost is low, and the method has wide industrial prospect.

Description

A method of liquid nitrogen quick-freezing combined with frozen storage and preservation of fresh litchi

technical field

The invention belongs to the field of fresh-keeping of fruits and vegetables, and in particular relates to a method for preserving fresh lychees by using liquid nitrogen quick-freezing technology combined with frozen storage.

Background technique

Litchi is a good edible product in southern summer. The sugar content in lychee pulp is as high as 20%, and the vitamin C content is as high as 70mg/100g. In addition, it also contains protein, fat, phosphorus, calcium, iron and other ingredients, with excellent flavor. However, litchi is difficult to preserve. There is a saying that "the color changes in one day, the fragrance changes in two days, the taste changes in three days, and the color and fragrance disappear in four days". Therefore, the best consumption of litchi should be within 24 hours.

At present, the storage and preservation methods of litchi include normal temperature, refrigeration and air conditioning. Normal temperature storage generally does not exceed 8 days, cold storage can generally be stored for 20 to 30 days, and controlled atmosphere storage can last up to 56 days (Research status and analysis of environmental parameters for litchi preservation. Guangdong Agricultural Science, 2012 (18): 105-107; Research progress on postharvest physiology and preservation technology of litchi. Agricultural Products Processing, 2012(11): 71-75). Frozen lychees can achieve the purpose of long-term preservation of lychees, but due to the formation of large ice crystals during the freezing process, the fruit cells are damaged, and the juice will flow out and change color after thawing, and the taste and appearance will deteriorate.

Fresh lychees start to go on the market in May every year, and June to July is the peak period for lychees to be harvested. Since the current preservation technology does not keep the lychees fresh for a long time, it is difficult to eat fresh lychees after October, so canned lychees can only be used instead. Freezing can achieve long-term preservation of fresh lychees, but directly frozen lychees are very hard and difficult to eat directly; after thawing, lychees will flow out juice and change color, completely losing the taste of fresh lychees.

The short listing period of lychees not only shortens the time for people to taste fresh lychees, but also often brings immeasurable losses to fruit farmers, especially in harvest years of lychees. Therefore, it is of great social and economic significance to adopt scientific methods and techniques to preserve litchi.

Contents of the invention

In order to overcome the shortcomings and deficiencies of the prior art, the object of the present invention is to provide a method for liquid nitrogen quick-freezing combined with frozen storage to preserve fresh litchi.

The invention utilizes the principle of quick-freezing technology, so that the moisture in the litchi is rapidly frozen into tiny ice crystals in liquid nitrogen, so that the texture of the litchi is not changed due to freezing. After lychees are quick-frozen, they can be preserved for a long time under frozen storage conditions without reducing the content of lychee nutrients (sugar, vitamins, minerals and other nutrients), and the taste is similar to ice cream.

The object of the present invention is achieved through the following technical solutions:

A method for liquid nitrogen quick-freezing combined with frozen storage and preservation of fresh lychees comprises the following steps: placing the fresh lychees in liquid nitrogen for quick-freezing, and then performing frozen preservation under freezing conditions.

Described fresh litchi comprises the fruit of all litchi varieties;

Described fresh lychee can be whole lychee, also can be the lychee of peeling or peeling and pitting;

Described dehulling or dehulling is to adopt manual or mechanical method to realize;

The consumption of described liquid nitrogen is 2～10 times of fresh litchi weight;

The time for quick freezing is preferably 2 to 20 minutes;

Described freezing condition can adopt freezer or refrigerator to realize;

The quick freezing in liquid nitrogen can adopt batch or continuous process.

The storage temperature is preferably -4°C to -30°C;

The storage time is preferably 1-240 days; during the storage period, it can be taken out at any time and eaten directly.

The scientific basis of the present invention is as follows: polysaccharides such as pectin in lychees form a gel structure, and they can prevent moisture from moving to form tiny ice crystals in the lychee liquid nitrogen quick-freezing process; ) taken out during storage, the hardness can be restored to that of fresh lychees in 2-5 minutes, and the texture is similar to ice cream. After being taken out, it can be eaten directly or after thawing, and the lychee keeps the nutrition, taste and flavor of the fresh fruit.

Compared with the prior art, the present invention has the following advantages and effects:

(1) The research of the present invention shows that the lychee preserved in July by this technology will still retain the nutrition, flavor and mouthfeel of the fresh fruit in April of next year.

(2) The lychee preserved by the method of the present invention can be eaten directly or after thawing.

(3) The liquid nitrogen preparation technology has been industrialized, and the cost price is not much 4 yuan/kg. Therefore, adopting the fresh-keeping lychee of the present invention has good effect, low cost and broad industrialization prospect.

Description of drawings

Fig. 1 is the lychee of Huaizhi, Nuomici and Heiye varieties after quick-freezing or freezing for 8 months; wherein, Fig. 1 (a) is Nuomici peeled and quick-frozen in liquid nitrogen for 2 minutes; Fig. 1 (b) is Peel and cut the glutinous rice into two halves and freeze them in liquid nitrogen for 2 minutes; Figure 1(c) shows the peeled glutinous rice and freeze them in liquid nitrogen for 5 minutes; Figure 1(d) shows the peeled Nuomi and freeze them at -18°C for 8 months; Figure 1(e) It is quick-frozen in liquid nitrogen for 5 minutes with peeled branches; Figure 1(f) is quick-frozen with liquid nitrogen for 5 minutes in whole Huaizhi; Figure 1(g) is quick-frozen in liquid nitrogen for 5 minutes with peeled black leaves.

Fig. 2 is the standard curve diagram of determining vitamin C content by HPLC method.

Figure 3 is an HPLC chromatogram of a vitamin C standard.

Fig. 4 is the HPLC chromatogram of vitamin C in the glutinous rice cake lychee of embodiment 1 liquid nitrogen quick-freezing treatment 5min and then frozen preservation 6 months.

Fig. 5 is the HPLC chromatogram of vitamin C in Nuomi Ci litchi frozen for 6 months in Example 1.

Detailed ways

The present invention will be further described in detail below in conjunction with the embodiments and the accompanying drawings, but the embodiments of the present invention are not limited thereto.

Example 1

Take 250g of lychee fruits of the Nuomi variety, set whole, peeled, peeled + pitted, put them in a foam box with a cover, add 750g of liquid nitrogen, quick-freeze and keep warm for 2 to 5 minutes, take them out and place them at -18°C After 6-8 months in the refrigerator, sensory evaluation is carried out, and the content of main nutrients is determined. Litchi that was not quick-frozen in liquid nitrogen was used as a control. The main results are as follows:

(1) See Table 1 for the nutrient content of Nuomi Ci lychee fresh pulp.

Table 1 Nutrient composition list of litchi*

heat 70 kcal Vitamin E 0mg protein 0.9g cholesterol 0mg Fat 0.2g Potassium 151mg carbohydrate 16.1g sodium 1.7mg Dietary fiber 0.5g calcium 2mg Vitamin A 2μg magnesium 12mg carotene 0.4μg iron 0.4mg Retinol 81.9μg manganese 0.09mg Thiamine 0.1mg zinc 0.17mg riboflavin 0.04mg copper 0.16mg niacin 1.1mg phosphorus 24mg Vitamin C 41mg selenium 0.14mg

*Nutrient content of 100g lychee fresh pulp.

(2) The lychees of the Nuomi variety were quick-frozen or frozen and stored for 8 months. The specific results are shown in Figure 1; Figure 1(a) is the result of quick freezing in liquid nitrogen for 2 minutes after peeling the glutinous rice, Figure 1(b) is the result of quick freezing in liquid nitrogen for 2 minutes after peeling and cutting the glutinous rice, Figure 1(c) is the result of quick freezing of glutinous rice The results of peeled glutinous rice cakes were frozen in liquid nitrogen for 5 minutes. Figure 1(d) is the result of peeled glutinous rice cakes frozen at -18°C for 8 months.

Liquid nitrogen quick-frozen for 2 minutes, stored for 240 days after the glutinous rice magnetic sample, the pulp is basically white wax color, some parts are light yellow, and there is no obvious ice crystal on the surface; liquid nitrogen quick-frozen for 5 minutes, the sample after 240 days of storage, the pulp is white wax color, the surface is not obvious Ice crystals; after the sample was peeled and frozen at -18°C for 240 days, the end of the fruit stalk and the lower end of the aril showed obvious yellowish browning, and the flesh was mostly yellowish waxy, with a large number of ice crystals adhering to the surface; after the whole litchi was quick-frozen in liquid nitrogen for 5 minutes, The appearance keeps the color of fresh fruit without obvious ice crystals.

(3) The weighted scoring method was used to evaluate the flavor of the samples, mainly investigating the five indicators of pulp color, meat quality, sweetness, juice percentage and aroma. See Table 2 for the evaluation items and weights of litchi sensory evaluation; see Table 3 for the scoring rules of litchi sensory evaluation items; see Table 4 for the weighted scoring statistics of frozen Nuomi Ci litchi; for the weighted scoring statistics of liquid nitrogen quick-frozen Nuomi Ci litchi, See Table 5. The evaluation results of 9 appraisers show that the lychee treated with liquid nitrogen quick freezing is obviously better than the lychee treated with freezing. (The lychees of the Nuomi variety were quick-frozen and frozen for 8 months)

Table 2 Evaluation items and weights of litchi sensory evaluation

evaluation item flesh color succulent sweetness juice rate aroma Weights 20 40 15 15 10

Table 3 Grading rules for each item of litchi sensory evaluation

Grading Items Rating content Score flesh color cream 90～100 white wax 80～90 yellow wax 70～80 succulent crisp 90～100 soft 80～90 rough 70～80 other 60～70 sweetness thick sweet 90～100 sweet 80～90 Moderately sweet and sour 70～80 acid 60～70 juice many 90～100 medium 80～90 few 70～80 aroma honey fragrance 90～100 slightly fragrant 80～90 special fragrance 70～80 Fragrance free 60～70

Table 4 Weighted score statistics of frozen Nuomici lychee

evaluator 1 2 3 4 5 6 7 8 9 pulp fruit color 85 65 77 85 75 80 65 82 75 succulent 77 80 80 78 80 77 80 80 80 sweetness 73 75 75 75 70 85 65 80 80 juice rate 72 80 80 81 80 82 70 85 80 aroma 74 95 85 85 60 75 80 75 80 weighted score 76.95 77.75 79.15 80.1 75.5 79.35 73.25 80.65 79

Table 5 Weighted scoring statistics of liquid nitrogen quick-frozen Nuomi Ci litchi

evaluator 1 2 3 4 5 6 7 8 9 pulp fruit color 87 88 89 80 75 90 85 80 75 succulent 85 88 88 85 80 95 82 87 83 sweetness 80 78 92 75 85 75 80 85 85 juice rate 83 89 88 85 84 80 83 80 89 aroma 84 70 89 85 88 86 87 85 86 weighted score 84.25 84.85 88.9 82.5 81.15 87.85 82.95 84.05 82.9

(4) After the lychees were squeezed, the content of vitamin C in the liquid nitrogen quick-frozen samples was 102.47 μg/mL, while that in the frozen samples was 89.44 μg/mL; the results are shown in Table 6. High performance liquid chromatography (HPLC) measures the impact of two kinds of freezing processes on vitamin C content in litchi, and the results are shown in Fig. 4 and Fig. 5; Wherein, HPLC method measures the standard curve of vitamin C content, see Fig. 2; The HPLC chromatogram of product is shown in Figure 3. (The results of glutinous rice magnetic lychees were frozen for 5 minutes and then frozen for 6 months)

Table 6 Effects of two freezing processes on the content of vitamin C in Nuomi Ci litchi

sample Peak area Vitamin C content (μg/mL) Liquid nitrogen quick freezing treatment 914996 102.47 Freezing 799771 89.44

Note: The vitamin C content in the juice was measured after the lychee sample was squeezed directly; the peak area was measured at 267nm.

Example 2

Take 250g of lychee fruits of Huaizhi variety, set whole, peeled, peeled + pitted, put them in a foam box with a cover, add 750g of liquid nitrogen, quick-freeze and keep warm for 5min, take them out and place them in a -18°C refrigerator , Carry out sensory evaluation after 6 to 8 months, and determine the content of main nutrients. Litchi that was not quick-frozen in liquid nitrogen was used as a control. The main results are as follows:

(1) Litchi of Huaizhi variety were quick-frozen and stored for 8 months after being frozen. Fig. 1(e) is the result of quick-freezing in liquid nitrogen for 5 minutes of peeled twigs, and Fig. 1(f) is the result of quick-freezing of whole twigs in liquid nitrogen for 5 minutes.

Liquid nitrogen quick-frozen for 5 minutes, the sample after 240 days of preservation, the pulp was white wax color, and there was no obvious ice crystal on the surface.

(2) The flavor of the sample was evaluated by the weighted scoring method, mainly investigating the five indicators of pulp color, meat quality, sweetness, juice rate and aroma; the weighted scoring statistics of frozen lychees are shown in Table 7; liquid nitrogen quick-frozen lychees See Table 8 for the weighted score statistics of litchi. The evaluation results of 9 appraisers show that the litchi treated with liquid nitrogen quick-frozen is obviously better than the litchi treated with freezing. (Litchi of Huaizhi variety has been frozen or frozen for 8 months)

Table 7 Weighted scoring statistics of frozen Huaizhi litchi

evaluator 1 2 3 4 5 6 7 8 9 pulp fruit color 78 85 75 80 75 75 70 85 75 succulent 80 78 88 80 70 85 80 85 85 sweetness 75 88 80 75 60 80 75 80 80 juice rate 72 80 80 75 80 85 80 80 80 aroma 70 80 75 70 60 80 70 65 80 weighted score 76.65 81.4 81.7 77.5 70 81.75 76.25 81.5 81

Table 8 Weighted scoring statistics of liquid nitrogen quick-frozen Huaizhi litchi

evaluator 1 2 3 4 5 6 7 8 9 pulp fruit color 85 95 86 85 75 90 75 85 75 succulent 83 95 85 80 70 95 90 80 85 sweetness 75 87 82 80 80 90 75 85 90 juice rate 75 71 88 85 70 80 80 80 85 aroma 78 85 90 90 75 75 75 75 75 weighted score 80.5 89.2 85.7 82.75 73 89 81.75 81.25 82.75

(3) After lychee juice was squeezed, the content of vitamin C in the lychee samples that were frozen in liquid nitrogen for 5 minutes and then refrigerated for 6 months was 97.51 μg/mL, while that in the frozen samples was 90.26 μg/mL.

Example 3

Take 250g of lychee fruits of the black leaf variety, set whole, peeled, peeled + pitted, put them in a foam box with a cover, add 750g of liquid nitrogen, quick-freeze and keep warm for 5min, take them out and place them in a -18°C refrigerator , Carry out sensory evaluation after 6 to 8 months, and determine the content of main nutrients. Litchi that was not quick-frozen in liquid nitrogen was used as a control. The main results are as follows:

(1) Litchi of the black-leaf variety was quick-frozen and stored for 8 months after being frozen. Fig. 1(g) is the result of quick freezing in liquid nitrogen for 5 minutes of peeled black leaves.

Liquid nitrogen quick-frozen for 5 minutes, the sample after 240 days of preservation, the pulp was white wax color, and there was no obvious ice crystal on the surface.

(2) The flavor of the sample was evaluated by the weighted scoring method, mainly investigating the five indicators of pulp color, meat quality, sweetness, juice percentage and aroma; the weighted scoring statistics of frozen black leaf lychee are shown in Table 9; liquid nitrogen quick-frozen black leaf See Table 10 for the weighted score statistics of Litchi Ye. The evaluation results of 9 appraisers show that Litchi Heiye treated with liquid nitrogen quick-frozen is significantly better than litchi treated with freezing. (Litchi of the black leaf variety has been frozen or stored for 8 months)

Table 9 Weighted scoring statistics of frozen black leaf litchi

evaluator 1 2 3 4 5 6 7 8 9 pulp fruit color 80 70 88 75 75 75 80 80 75 succulent 80 73 83 80 70 87 70 80 80 sweetness 85 83 87 70 60 85 75 75 80 juice rate 85 79 85 80 70 75 85 70 70 aroma 85 68 88 70 60 70 72 70 75 weighted score 82 74.3 85.4 76.5 68.5 80.8 75.2 76.75 77

Table 10 Weighted scoring statistics of liquid nitrogen quick-frozen Heiye litchi

evaluator 1 2 3 4 5 6 7 8 9 pulp fruit color 85 75 92 80 75 70 67 90 78 succulent 90 93 85 85 80 85 75 90 80 sweetness 80 90 88 75 70 75 75 70 83 juice rate 80 83 85 85 70 85 80 75 85 aroma 80 75 89 75 60 90 75 70 85 weighted score 85 85.65 87.25 81.5 74 81 74.15 82.75 81.3

(3) After lychee juice was squeezed, the content of vitamin C in the black leaf samples frozen in liquid nitrogen for 5 minutes and refrigerated for 6 months was 98.02 μg/mL, while that in the frozen black leaf samples was 90.11 μg/mL.

The above-mentioned embodiment is a preferred embodiment of the present invention, but the embodiment of the present invention is not limited by the above-mentioned embodiment, and any other changes, modifications, substitutions, combinations, Simplifications should be equivalent replacement methods, and all are included in the protection scope of the present invention.

Claims (10)

1. liquid nitrogen flash freezer is in conjunction with a method for refrigerated storage Preservation of Litchi chinensis Fruits, after it is characterized in that comprising the steps: that fresh lichee is placed in liquid nitrogen quick-frozen, more under freezing conditions carries out freezing.

2. liquid nitrogen flash freezer according to claim 1 is in conjunction with the method for refrigerated storage Preservation of Litchi chinensis Fruits, it is characterized in that: the consumption of described liquid nitrogen is 2 ~ 10 times of fresh lichee weight.

3. liquid nitrogen flash freezer according to claim 1 is in conjunction with the method for refrigerated storage Preservation of Litchi chinensis Fruits, it is characterized in that: the time of described quick-frozen is 2 ~ 20min.

4. liquid nitrogen flash freezer according to claim 1 is in conjunction with the method for refrigerated storage Preservation of Litchi chinensis Fruits, it is characterized in that: the temperature of described freezing is-4 DEG C ~-30 DEG C.

5. liquid nitrogen flash freezer according to claim 1 is in conjunction with the method for refrigerated storage Preservation of Litchi chinensis Fruits, it is characterized in that: the time of described freezing is 1 ~ 240 day.

6. liquid nitrogen flash freezer according to claim 1 is in conjunction with the method for refrigerated storage Preservation of Litchi chinensis Fruits, it is characterized in that: described fresh lichee comprises the fruit of all Litchi Varieties.

7. the liquid nitrogen flash freezer according to claim 1 or 6, in conjunction with the method for refrigerated storage Preservation of Litchi chinensis Fruits, is characterized in that: described fresh lichee is whole lichee, peeling lichee or peeling stoning lichee.

8. liquid nitrogen flash freezer according to claim 7 is in conjunction with the method for refrigerated storage Preservation of Litchi chinensis Fruits, it is characterized in that: described peeling lichee or peeling stoning lichee adopt artificial or mechanical means realization.

9. liquid nitrogen flash freezer according to claim 1 is in conjunction with the method for refrigerated storage Preservation of Litchi chinensis Fruits, it is characterized in that: described freezing conditions adopts to freeze storehouse or refrigerator realization.

10. liquid nitrogen flash freezer according to claim 1 is in conjunction with the method for refrigerated storage Preservation of Litchi chinensis Fruits, it is characterized in that: in described liquid nitrogen, quick-frozen adopts technique in batches or continuously.