Fresh-keeping method for keeping unique flavor and nutritional ingredients of postharvest allium vegetables

Abstract

The invention discloses a fresh-keeping method for keeping the unique flavor and nutritional ingredients of picked allium vegetables. The fresh-keeping method comprises the following steps: storing the allium vegetables in mixed gas of nitrogen and oxygen at low temperature; the volume fraction of oxygen in the mixed gas of nitrogen and oxygen is 60-80%; the low-temperature storage is performed at the temperature of 3-8 DEG C; the storage process also comprises a step of controlling the humidity to be 70-80%; the low-temperature storage process refers to low-temperature storage in a dark environment. According to the invention, by screening the optimal gas and the concentration ratio thereof, packaging and then storing at a proper temperature, not only is the decay inhibited and the shelf life significantly prolonged to 28 days, but also the unique spicy flavor is effectively maintained, and the mechanism of flavor maintenance is further clarified by metabonomics research, so that the taste of the preserved fruit is improved. And the research blank of the postharvest flavor quality maintaining mechanism of the allium vegetables is filled.

Description

Technical Field

[0001] The invention belongs to the technical field of vegetable preservation, and in particular relates to a preservation method for maintaining the unique flavor and nutritional components of allium vegetables after harvest. Background Art

[0002] Allium vegetables are mostly biennial or perennial herbs, known for their unique onion and garlic flavors, and have been widely eaten and cultivated around the world since ancient times. Common Allium vegetables on the table include leeks, chives, garlic sprouts, garlic sprouts and shallots, which are not only unique in flavor, but also rich in a variety of nutrients and bioactive ingredients such as sulfides, polyphenols, vitamins, minerals and dietary fiber. Modern medical research has confirmed that Allium plant extracts are dietary phytochemicals for the treatment of cardiovascular and heart diseases, inflammation, diabetes and cancer. Due to their high nutritional and medicinal properties, Allium vegetables have become increasingly popular as a promising functional food. In recent years, people have increasingly recognized the potential of Allium vegetables in improving human health, which has driven the growth of people's demand for such functional crops. The planting area of ​​Allium vegetables in China, other Asian countries and even many European countries has steadily expanded year by year, occupying an indispensable and important position in the global vegetable production and supply chain.

[0003] Allium vegetables have a special spicy aroma because they are rich in S-alk(en)yl cysteine ​​sulfoxides (CSOs). CSOs are the precursors of the unique flavor of Allium plants, including S-2-propylene-L-cysteine ​​sulfoxide (allicin, ACSO), S-(E)-1-propylene-L-cysteine ​​sulfoxide (isoallicin, PeCSO), S-methyl-L-cysteine ​​sulfoxide (methiin, MCSO) and S-propyl-L-cysteine ​​sulfoxide (propiin, PCSO). CSOs exist in the cytoplasm and are tasteless, non-volatile and chemically stable substances. Alliinase is the key enzyme that is ultimately responsible for the formation of the unique flavor compounds of Allium vegetables. When the tissues of Allium plants are crushed or cut, the alliinase in the cell fluid quickly hydrolyzes the CSOs stored in the cytoplasm to produce sulfinic acid, pyruvic acid and ammonia. Subsequently, the spontaneous degradation of sulfinic acid produces a series of volatile organosulfur compounds with a distinctive odor, which make an important contribution to the final flavor, giving Allium vegetables their unique flavor. The consumption of Allium vegetables is mainly related to their unique flavor and ability to enhance the flavor of other foods. They can be eaten fresh or heat-processed, cooked alone or used as a condiment to enhance the flavor of other ingredients. The unique flavor of Allium vegetables has given them a place in cooking, especially in Asian cuisine, where they are a key element in the diversity and depth of flavor of ingredients.

[0004] However, leafy vegetables such as Allium vegetables are extremely perishable after harvest. Their tender leaves and high water content make their respiration and transpiration extremely strong, which accelerates the aging process. Therefore, during the post-harvest storage and transportation process, they will quickly lose their freshness, and the flavor quality will drop sharply. They often wilt, turn yellow, and rot in a very short time, accompanied by the disappearance of the characteristic flavor and the generation of odor. This high perishability of Allium vegetables not only seriously restricts their storage capacity, but more importantly, it leads to the rapid loss of characteristic flavor and nutrients, especially under high temperature climate conditions in summer, which seriously affects their edible quality and potential commodity value. In addition, since the edible parts of common Allium vegetables such as leeks, leeks, garlic sprouts, garlic sprouts, and shallots are tender leaves, they cannot withstand vacuum packaging to "fix the flavor"; therefore, the "flavor preservation" of Allium vegetables is extremely difficult, which seriously affects the quality upgrade of its industry.

[0005] Therefore, the key to improving the storage performance of allium vegetables is not only to prevent them from rotting, but more importantly to ensure that their unique flavor substances and nutrients are effectively maintained and avoid loss. Taking leek as an example, there are many methods for maintaining the nutritional quality of leek in the prior art. For example, the Chinese patent with publication number CN111466437A and the name "A method for maintaining the nutrition and flavor of leek" discloses a method for preserving leek by treating with a sodium magnesium chlorophyll solution and then combining with LED light irradiation. The storage time can reach 7-8 days under 20°C storage conditions. Although the above method plays a certain role in preserving leek, these methods mostly use treatment methods such as soaking, which brings difficulties such as inconvenience in operation to actual production and limits its scope of application. In addition, there are technologies such as low-temperature storage, cytokinin compound spraying and molecular hydrogen treatment, but low-temperature storage is not ideal for the preservation effect of leek, and the cytokinin compound spraying technology is not environmentally friendly enough, and the molecular hydrogen treatment cost is relatively high. Conventional controlled atmosphere preservation technology generally inhibits vegetable spoilage by regulating the ratio of oxygen, carbon dioxide or nitrogen. However, for Allium vegetables such as leeks, it can only delay the decay time, while the effect of preserving flavor substances and nutrients is still unsatisfactory. Summary of the invention

[0006] In order to solve the above technical problems, the present invention proposes a preservation method for maintaining the unique flavor and nutritional components of allium vegetables after harvest. By screening the optimal gas and its concentration ratio, packaging and storing at a suitable temperature, not only the decay is inhibited and the shelf life is significantly extended, but also its unique spicy flavor is effectively maintained. Further metabolomics analysis is used to clarify the mechanism of maintaining its unique flavor, filling the research gap in the mechanism of maintaining the flavor quality of allium vegetables after harvest.

[0007] In order to achieve the above object, the present invention adopts the following technical solution:

[0008] The invention discloses a preservation method for maintaining the unique flavor and nutritional components of allium vegetables after harvest. The allium vegetables are stored at low temperature in a nitrogen and oxygen mixed gas; the volume fraction of oxygen in the nitrogen and oxygen mixed gas is 60% to 80%.

[0009] Technical principle of the present invention:

[0010] The leaf cells of leafy vegetables, Allium vegetables, enter the senescence stage quickly after harvesting due to their extreme tenderness, and are extremely vulnerable to adverse external environmental factors, thus suffering damage. This fragility not only accelerates the loss of its unique flavor, but also leads to a large loss of nutrients. Oxygen can maintain the organizational structure and function of cells, but different species have different sensitivities to oxygen. For example, Chinese patent CN116172051A studied that the suitable oxygen concentration for the preservation of pakchoi is 5% (volume concentration), the suitable oxygen concentration for the preservation of spinach is 6% (volume concentration), and the suitable oxygen concentration for the preservation of lettuce is 10% (volume concentration). For another example, Chinese patent CN115152831A discloses that celery, tomatoes and carrots have a better preservation effect when the oxygen concentration is 30%-42% (volume concentration). Although these existing methods for improving the preservation effect also include some other technical means as auxiliary, ensuring that the oxygen concentration is within an appropriate range is a critical factor. The invention firstly establishes a new atmosphere-controlled fresh-keeping storage system for allium vegetables after harvest based on the formation pathway of the unique spicy flavor of allium vegetables and the function of oxygen in maintaining cells, by matching a reasonable nitrogen-oxygen ratio and storage temperature; by real-time observation of changes in the appearance color of leeks, the presence or absence of tissue damage, the detection of spiciness, and nutritional components during the storage period, and by using high performance liquid chromatography and metabolomics to analyze its mechanism, so that the shelf life of allium vegetables after harvest is greatly extended. Under optimal conditions, the shelf life of leeks is significantly extended to 28 days, and the spiciness is effectively maintained. The spiciness of leeks stored for 28 days can still reach 76.10% of that of fresh leeks; metabolomics analysis shows that an appropriate atmosphere-controlled mode can effectively inhibit the activity of alliinase and stabilize the content of CSOs, a precursor substance of the spicy flavor of leeks; therefore, the technical bottleneck problem that the flavor quality of allium vegetables after harvest is difficult to maintain is solved, and on this basis, the optimal atmosphere-controlled storage parameters are fully clarified, so that the mechanism of maintaining the spicy taste of leeks is achieved. In summary, the present invention creatively discovered that high-oxygen controlled refrigeration treatment plays an important regulatory role in maintaining the flavor quality of Allium vegetables, which can significantly extend their shelf life and maintain their unique flavor. The gas-controlled flavor preservation technology is studied to determine the optimal parameters, and the material components and contents that affect the flavor quality of Allium vegetables are clearly defined. Based on this, the regulatory mechanism of high-oxygen controlled refrigeration on the maintenance of Allium vegetable flavor substances is clarified through metabolomics analysis, in order to provide a theoretical basis and technical support for the research on the preservation of perishable flavor vegetables. The present invention solves the technical bottleneck problem that the flavor quality of Allium vegetables is difficult to maintain after harvest.

[0011] As a further preferred embodiment of the present invention, the volume fraction of oxygen in the mixed gas of nitrogen and oxygen is 60%.

[0012] As a preferred embodiment of the present invention, the low temperature storage is storage at 3-8°C.

[0013] As a further preferred embodiment of the present invention, the low-temperature storage is storage at 4°C.

[0014] As a preferred embodiment of the present invention, the storage process also includes a step of controlling the humidity to 70-80%.

[0015] As a further preferred embodiment of the present invention, the storage process further includes a step of controlling the humidity to 75%.

[0016] As a preferred embodiment of the present invention, the Allium vegetables are stored in a nitrogen and oxygen mixed gas by sealing and packaging with a high barrier film; the thickness of the high barrier film is 105 μm, and the water vapor permeability is 1.9530 g / m 2 ·24h, carbon dioxide permeation is 0.000019460cm 3 / m 2 .d.Pa, oxygen permeability is 0.000028620cm 3 / m 2 .d.Pa.

[0017] As a preferred embodiment of the present invention, the low-temperature storage process is performed in a dark environment.

[0018] As preferred embodiments of the present invention, the Allium vegetables are leek, leek greens, garlic sprouts, garlic seedlings and shallots.

[0019] As a further preferred embodiment of the present invention, the Allium vegetable is leek.

[0020] The present invention has the following technical effects:

[0021] The present invention has found through research that high-oxygen controlled atmosphere treatment plays an important regulatory role in maintaining the flavor quality of Allium vegetables, which can significantly prolong their shelf life and maintain their unique flavor. The controlled atmosphere flavor preservation technology is studied to determine the optimal parameters, and the material components and contents that affect the flavor quality maintenance of Allium vegetables are clarified. Based on this, metabolomics analysis is used to clarify the regulatory mechanism of high-oxygen controlled atmosphere refrigeration on the maintenance of Allium vegetable flavor substances, in order to provide a theoretical basis and technical support for the research on the preservation of perishable flavor vegetables.

[0022] The present invention aims at the key problem that the flavor quality of allium vegetables deteriorates quickly after harvest, and carries out systematic research. By comparing the effects of various temperature conditions, gas components and their concentration ratios on the flavor quality of leeks after harvest, a set of controlled atmosphere preservation technology system for allium vegetables represented by leeks is established. The focus of the system is the screening of gas types and the determination of key gas concentrations. Under the controlled atmosphere storage system of the present invention, the flavor and quality of allium vegetables are significantly maintained, and the year-round supply of "good flavor and excellent quality" specialty vegetables represented by leeks is achieved. The shelf life of representative allium vegetables, leeks, can be significantly extended to 28 days, and the pungency of leeks stored for 28 days is maintained at 76.10% of fresh leeks. Further analysis by high performance liquid chromatography (HPLC) and non-targeted LC-TOF-MS metabolomics (Non-targeted Liquid Chromatography-Time of Flight Mass Spectrometry Metabolomics) showed that the appropriate concentration of high oxygen maintained the unique spicy flavor of leek by stabilizing the level of CSOs, the unique flavor precursor compound of leek, and reducing the activity of alliinase. At the same time, under the optimal technology and parameters, the content of umami substances in leek, aspartic acid, glutamate, inosinic acid (IMP), guanylic acid (GMP) and uridylic acid (UMP), cytidylic acid (CMP), and adenosine monophosphate (AMP) was maintained, thus maintaining the umami taste of leek. These findings fill the gap in the study of the post-harvest preservation mechanism of the characteristic flavor of Allium vegetables, and at the same time provide technical support and theoretical basis for the preservation of flavor quality of other perishable vegetables. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0024] Figure 1 The radar graphs of volatile flavors of different treatment groups measured by electronic noses of leeks treated in different embodiments of the present invention are shown;

[0025] Figure 2 The main component analysis diagram of volatile flavor of different treatment groups measured by electronic nose of leeks treated in different embodiments of the present invention;

[0026] Figure 3 The sensory scoring radar chart of leeks in the treatment groups of different embodiments of the present invention is shown;

[0027] Figure 4 This is a graph showing the shelf life results of leeks in different treatment groups according to the present invention;

[0028] Figure 5 These are three CSO bar graphs of leeks treated with different embodiments of the present invention;

[0029] Figure 6 The alliinase activity diagram of leek treated with different embodiments of the present invention;

[0030] Figure 7 The pungency of leeks in different treatment groups according to the present invention;

[0031] Figure 8 The glutamate content of leeks in the treatment groups of different embodiments of the present invention;

[0032] Fig. 9 The aspartic acid content of leeks in the treatment groups of different embodiments of the present invention;

[0033] Fig.10 This is a graph showing the relative content of umami nucleotides in leeks treated with different embodiments of the present invention;

[0034] Fig.11 The morphological diagram of leeks stored for 28 days in different embodiment treatment groups;

[0035] Fig.12 The multivariate statistical analysis results of leek metabolites in different treatment groups of embodiments, where (a) is principal component analysis (PCA); (b) is partial least squares discriminant analysis (PLS-DA); (c) is hierarchical cluster analysis (HCA); and (d) is a visualization result diagram. DETAILED DESCRIPTION

[0036] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0037] It should be understood that the terms described in the present invention are only for describing special embodiments and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0038] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0039] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to those skilled in the art. The present invention description and examples are exemplary only.

[0040] The embodiment of the invention discloses a preservation method for maintaining the unique flavor and nutritional components of allium vegetables after harvest, wherein the allium vegetables are stored at low temperature in a nitrogen and oxygen mixed gas; the volume fraction of nitrogen in the nitrogen and oxygen mixed gas is 60% to 80%.

[0041] In some more preferred embodiments of the present invention, the volume fraction of oxygen in the mixed gas of nitrogen and oxygen is 60%.

[0042] In some preferred embodiments of the present invention, the low-temperature storage is storage at 3-8°C.

[0043] In some more preferred embodiments of the present invention, the low-temperature storage is storage at 4°C.

[0044] In some preferred embodiments of the present invention, the storage process further includes a step of controlling the humidity to 70-80%.

[0045] In some more preferred embodiments of the present invention, the storage process further includes a step of controlling the humidity to 75%.

[0046] In some preferred embodiments of the present invention, the Allium vegetables are stored in a nitrogen and oxygen mixed gas by sealing and packaging with a high barrier film; the thickness of the high barrier film is 105 μm, and the water vapor permeability is 1.9530 g / m 2 ·24h, carbon dioxide permeation is 0.000019460cm 3 / m 2 .d.Pa, oxygen permeability is 0.000028620cm 3 / m 2 .d.Pa.

[0047] In some preferred embodiments of the present invention, the low-temperature storage process is low-temperature storage in a dark environment.

[0048] In an optional embodiment of the present invention, the Allium vegetables are leek, leek greens, garlic sprouts, garlic sprouts and shallots.

[0049] In the following embodiments of the present invention, leek is taken as an example to study the preservation effect.

[0050] The preservation method in a preferred embodiment of the present invention is as follows: 250 grams of freshly harvested leeks are weighed and put into a plastic box (195 mm × 135 mm × 50 mm, Shanghai Liyue Machinery Co., Ltd.), and then the plastic box containing the leeks is placed on a modified atmosphere packaging machine (DZM-450, Shanghai Liyue Machinery Co., Ltd.) with pre-set parameters (volume fraction, 60% oxygen + 40% nitrogen) to perform vacuuming, inflation and film packaging in succession. The cover film used is a high barrier film (thickness of 105 μm, water vapor permeability of 1.9530 g / m 2 ·24h, carbon dioxide permeation is 0.000019460cm 3 / m 2 .d.Pa, oxygen permeability is 0.000028620cm 3 / m 2 .d.Pa); store the leeks packaged in modified atmosphere in a dark environment at 4±1°C and relative humidity of 75±5%.

[0051] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0052] The raw material leek used in the embodiment of the present invention is Hangyan 998 variety, which is harvested in the early morning at Beijing Taihua Lucun Planting Professional Cooperative (Fangshan District, Beijing), and immediately placed in a sealed foam box with an ice bottle, and transported to Beijing Fruit and Vegetable Storage and Processing Key Laboratory within 1 hour. Leeks with uniform size, uniform color, and no wilting and pests were selected for the experiment.

[0053] In the following examples of the present invention, "%" represents volume percentage.

[0054] Example 1

[0055] Weigh 250 g of freshly harvested leeks and put them into a plastic box (195 mm × 135 mm × 50 mm, Shanghai Liyue Machinery Co., Ltd.). Then put the plastic box containing the leeks on a modified atmosphere packaging machine (DZM-450, Shanghai Liyue Machinery Co., Ltd.) with pre-set parameters (60% oxygen + 40% nitrogen, recorded as Y60) for vacuuming, inflation and film packaging. The film used was a high barrier film (thickness of 105 μm, water vapor permeability of 1.9530 g / m2 ·24h, carbon dioxide permeation is 0.000019460cm 3 / m 2 .d.Pa, oxygen permeability is 0.000028620cm 3 / m 2 .d.Pa); store the leeks packaged in controlled atmosphere in the dark at 4±1°C and relative humidity of 75±5%.

[0056] Example 2

[0057] The only difference from Example 1 is that the gas system composition is 40% oxygen + 60% nitrogen, denoted as Y40.

[0058] Example 3

[0059] The only difference from Example 1 is that the gas system composition is atmospheric air, denoted as CG.

[0060] Example 4

[0061] The only difference from Example 1 is that the gas system composition is 80% oxygen + 20% nitrogen, denoted as Y80.

[0062] Example 5

[0063] The only difference from Example 1 is that the gas system composition is 100% nitrogen, denoted as Y0.

[0064] Example 6

[0065] The only difference from Example 1 is that the gas system composition is 100% oxygen, denoted as Y100.

[0066] The leek of embodiment is adopted the following method to carry out fresh-keeping effect evaluation:

[0067] 1. Electronic nose analysis of volatility characteristics

[0068] The method described by Zhang Ning (Zhang Ning. Study on the characteristic flavor of fried scallion oil [D]. China Agricultural University, 2017. DOI: 10.27628 / d.cnki.gzndu.2017.000096.) was modified with some modifications. The electronic nose used was PEN3 model, purchased from Airsense Analytics GmbH (Schwerin, Germany).

[0069] 1.1 Sample pretreatment

[0070] Weigh 12 g of leek, cut into 0.5 cm pieces, and immediately put into a sealed container and equilibrate at 25°C for 60 minutes.

[0071] 1.2 Detection parameters

[0072] The sampling time interval is 1s, the sensor self-cleaning time is 120s, the sensor zeroing time is 5s, the analysis sampling time is 150s, the carrier gas flow rate is 300mL / min, and each sample is repeated 3 times. The PEN3 electronic nose consists of 10 sensor arrays, and the specific information is shown in Table 1.

[0073] Table 1 Electronic nose system sensor types and performance description

[0074]

[0075] The analysis results are shown in Figures 1-2 It can be seen that the retention effect of various volatile substances in Y60 among the six treatment groups is closest to that of fresh leek.

[0076] 2. Shelf life and sensory evaluation

[0077] Refer to Sun et al. (Sun B, Di H, Zhang J, et al. Effect of light on sensory quality, health-promoting phytochemicals and antioxidant capacity in post-harvest baby mustard [J]. Food Chemistry, 2020. DOI: 10.1016 / j.foodchem.2020.128057.) and Wang et al. (Wang MS, Fan M,ZhengAR,etal.Characterization of a fermented dairy,sour cream:Lipolysis and the releaseprofile offlavor compounds[J].Food Chemistry, 2023, 423. DOI: 10.1016 / j.foodchem.2023.136299.) with slight adjustments. We organized a sensory evaluation panel consisting of 10 well-trained panelists (half male and half female) to conduct a comprehensive evaluation of the leek samples using a 9-point characteristic scale (1 point is very poor and 9 points is the best), including appearance, aroma, taste, texture and overall acceptability. During the evaluation, the leek samples were placed on coded plastic trays and tested under standard laboratory lighting conditions. The shelf life is determined by the texture changes of the leek and the presence of tissue damage. The shelf life is before the sample tissue becomes soft or there is slight tissue damage.

[0078] The analysis results are shown in Figure 4 ,Depend on Figure 4It can be seen that Y60 has the best preservation effect and the longest shelf life, up to 28 days. The shelf life of the low-oxygen treatment groups such as Y0 and CG is 6 days and 8 days respectively. This may be due to the low oxygen concentration, and the high concentration of carbon dioxide produced by respiration leads to anaerobic fermentation and cell tissue damage. It can be seen that high-concentration oxygen treatment can significantly improve the preservation effect of leeks.

[0079] The morphology of the samples stored at different oxygen concentrations for 28 days is as follows Fig.11 As shown in the figure, it can be seen that the leeks in the Y60 and Y80 groups are brighter and more uniform in color than those in other groups, with almost no wilting and yellowing. Among them, the Y60 treatment group has the best morphology and the highest sensory score, which is closest to fresh leeks. The leeks in the Y0, CG, Y40 and Y100 treatment groups all rotted to varying degrees, which shows that the preservation effect of too high and too low oxygen concentration treatments is not good.

[0080] Sensory evaluation results Figure 3 It shows that the sensory attributes of leek, such as appearance, aroma and taste, change dynamically with the change of oxygen concentration. When the oxygen concentration is in the range of 0% to 60%, the oxygen concentration is positively correlated with the overall acceptability of leek, and the sensory score increases accordingly. The 60% oxygen treatment group showed a pleasant aroma, refreshing taste and the highest overall acceptability, which is closest to fresh leek. However, when the oxygen concentration reaches 80% and 100%, a negative correlation occurs, showing different degrees of tissue damage, lacking the unique aroma and texture of fresh leek, and therefore the overall score is low. It can be seen that the key technical parameter for maintaining the flavor quality of leek after harvest lies in the determination of the optimal oxygen concentration. According to the above flavor quality detection and evaluation results, leek treated with 60% oxygen is more popular with consumers. When high-respiration rate fruits and vegetables are packaged with low oxygen conditioning, it is difficult to match the air permeability of the existing packaging film with the respiration rate of fruits and vegetables, which can easily lead to too low oxygen concentration or too high carbon dioxide concentration in the package, causing anaerobic fermentation or physiological damage to fruits and vegetables.

[0081] 3. Analysis of spicy flavor related indicators

[0082] 3.1 HPLC analysis of pungent taste precursors

[0083] The content determination of MCSO, ACSO and PeCSO was carried out according to the method of Yabuki et al. (Yabuki Y, Mukaida Y, Saito Y, et al. Characterization of volatile sulphur-containing compounds generated incrushed leaves of Chinese chive (Allium tuberosum Rottler) [J]. Food Chemistry, 2010, 120 (2): 343-348. DOI: 10.1016 / j.foodchem.2009.11.028.) with some modifications. 50 grams of frozen fresh samples were inactivated at high temperature for 2 minutes and then cooled to room temperature. The mass loss during the inactivation process was supplemented with high-purity water, and 50 grams of high-purity water was added to the cooled sample, and the mixture was mashed into a paste. Then, 2 grams of the paste sample was placed in a 50 ml centrifuge tube and 40 ml of mobile phase was added. After shaking for 1 minute, the paste sample was placed in an ultrasonic device and extracted at a power of 300 watts for 15 minutes, followed by centrifugation at 10,000 rpm for 15 minutes. The supernatant was collected and filtered through a 0.22 μm filter membrane for HPLC analysis. The combined device was an Agilent 1200 HPLC system (Agilent Technologies, Palo Alto, CA, USA), equipped with a binary pump, degasser, autosampler, and diode array detector (DAD). The chromatographic column used for analysis was an Eclipse XDB-C18 column (250×4.6 mm, 5 μm) (Agilent, CA, USA) at 30°C. The injection volume was 10.0 μL, which was diluted with the mobile phase before HPLC injection. Mobile phase A: methanol, mobile phase B: 0.1% phosphoric acid solution. The mobile phase was 0.1% phosphoric acid solution and methanol (90:10, v / v), and isocratic elution was performed at a flow rate of 1.0 mL / min. The detection wavelength was 208 nm and the elution time was 10 min.

[0084] 3.2 Determination of alliinase activity

[0085] The activity of alliinase was measured using the cysteine ​​sulfoxide lyase (CSL) activity assay kit (Solaibo Technology Co., Ltd., Beijing, China). Samples, standards, and HRP-labeled detection antibodies were added to the pre-coated microwells of the CSL antibody, incubated and washed thoroughly. The substrate TMB (3,3',5,5'-tetramethylbenzidine) was added for color development. TMB was converted to blue under the catalysis of peroxidase and finally converted to yellow under the action of acid. The depth of color was positively correlated with the content of CSL in the sample. The absorbance (OD value) was measured with the enzyme marker at a wavelength of 450 nm, and the enzyme activity of the sample was calculated.

[0086] Test results such as Figures 5-6 As shown by Figure 5 It can be seen that the contents of MCSO, ACSO, and PeCSO in the Y60 and Y80 groups are close to those in fresh leeks, among which the Y60 group is the best. Figure 6 It can be seen that the alliinase activity in the Y60 and Y80 groups was low, especially in the Y60 group. The inhibition of alliinase activity can avoid the loss of flavor substances.

[0087] 3.3 Determination of pungency

[0088] The flavor of garlic vegetables is produced by the hydrolysis of its precursor CSOs by alliinase when the tissue is damaged. The amount of pyruvate generated during the hydrolysis reaction is equal to the amount of CSOs. Therefore, the pungency as an indicator of the flavor of allium plants is indirectly determined by the pyruvate method, and the total amount of pyruvate produced by enzymatic means can be used as a surrogate estimate of different sulfur volatiles. The pungency of leek was tested with slight modifications to the onion pungency test method. The supernatant of leek was obtained by homogenization and centrifugation. The 20-fold diluted supernatant was divided into two halves, one half was immediately mixed with an equal volume of 2.5% trichloroacetic acid to inactivate alliinase; the other half was incubated at room temperature for 3 minutes, and both were reacted with an equal volume of 0.0125% 2,4-dinitrophenylhydrazine at 37°C for 5 minutes. Then 5 volumes of 0.6M sodium hydroxide were added, and the absorbance was measured at a wavelength of 520nm using a spectrophotometer. The calibration curve was obtained using sodium pyruvate as the standard.

[0089] The analysis results are as follows Figure 7 As shown, the enzymatic pyruvate content of the Y60 and Y80 groups was closer to that of fresh leek, among which the Y60 group was the best.

[0090] 3.4 Determination of free amino acids

[0091] The free amino acid content was determined using a Hitachi L-8900 fully automatic amino acid analyzer (Hitachi Ltd, Tokyo, Japan). The free amino acid concentration was expressed as grams of free amino acids per 100 grams of fresh weight (g / 100g).

[0092] Depend on Figures 7-10 It can be seen that the relative contents of enzymatic pyruvate, glutamate, and aspartate in groups Y60 and Y80 were closer to those in fresh leek.

[0093] Depend on Fig.10 It can be seen that the contents of umami-related nucleotides such as IMP, GMP, AMP, UMP and CMP in the Y60 and Y80 groups are closer to those in fresh leeks.

[0094] 3.5 Non-targeted metabolomics analysis

[0095] Total metabolites were extracted from leek samples. 50 mg of sample was mixed with acetonitrile / methanol / water (2:2:1, volume ratio) and then vortexed for 30 seconds. Ultrasonic extraction was performed in an ice-water bath for 10 minutes. Subsequently, the extract was centrifuged at 12,000 rpm for 20 minutes at 4°C. The supernatant was filtered through a 0.22 μm filter membrane into a sample bottle and then used for liquid chromatography-mass spectrometry (LC-MS) analysis. Five replicates were performed for each sample. In order to test the reliability and stability of the metabolomics method, quality control (QC) samples were prepared by mixing equal amounts of each sample extract, and QC sample analysis was performed once every six samples.

[0096] The LC / MS system used for metabolomics analysis consisted of a Waters Acquity I-Class PLUS ultra-high performance liquid phase coupled to a Waters Xevo G2-XS QT high-resolution mass spectrometer. The chromatographic column was purchased from Waters Acquity UPLC HSS T3 (2.1×100 mm, 1.8 μm). The mobile phase consisted of 0.1% formic acid in water (A, v / v) and 0.1% formic acid in acetonitrile (B, v / v), and the gradient elution program was as follows: 0 min, 98% A+2% B; 0.25 min, 98% A+2% B; 10 min, 2% A+98% B; 13 min, 2% A+98% B; 13.5 min, 98% A+2% B; 15 min, 98% A+2% B. The flow rate of the mobile phase was 0.4 mL / min, and the injection volume was 1 μL. The Waters Xevo G2-XS QTOF high-resolution mass spectrometer can collect primary and secondary mass spectrometric data in MSe mode under the control of the acquisition software (MassLynx V4.2, Waters). In each data acquisition cycle, dual-channel data acquisition can be performed simultaneously for low collision energy and high collision energy. The low collision energy is 2 V, the high collision energy range is 10-40 V, and the mass spectrometry scanning frequency is 0.2 seconds. The parameters of the ESI ion source are as follows: capillary voltage: 2000 V (positive ion mode) or -1500 V (negative ion mode); cone voltage: 30 V; ion source temperature: 150 ° C; desolvation gas temperature 500 ° C; backwash gas flow rate: 50 L / h; desolvation gas flow rate: 800 L / h.

[0097] After filtering the metabolite ion features detected in positive and negative ion modes for all samples, a total of 2076 metabolites were identified in samples IM (fresh leek sample), Y0, CG, Y40, Y60, Y80 and Y100. These metabolites were classified into 12 categories ( Fig.12(c) shows a visual representation of these classifications), of which the top 11 categories include 629 lipids and lipid-like molecules (30.30%), 344 organic acids and derivatives (16.57%), 259 phenylpropanoids and polyketides (12.48%), 257 organic heterocyclic compounds (12.38%), 212 organic oxygen-containing compounds (10.21%), 154 benzene ring compounds (7.42%), 116 nucleosides, nucleotides and their analogs (5.59%), 39 alkaloids and their derivatives (1.88%), 28 organic nitrogen compounds (1.35%), 16 lignans, neosemaphorins and related compounds (0.77%), 4 organic sulfur compounds (0.19%) and 18 other major metabolites that are not clearly classified (0.87%). The specific statistical results are shown in Table 2.

[0098] Table 2 Statistics of the number of differential metabolites

[0099]

[0100]

[0101] Note: Non-targeted metabolomics testing is relatively comprehensive because it covers a wider range of differential metabolites and a larger number than targeted metabolomics.

[0102] 3.6 Statistical analysis

[0103] The processed data were imported into MetaboAnalyst 5.0 (https: / / genap.metaboanalyst.ca / ) for multivariate statistical analysis, including data normalization, principal component analysis (PCA), and partial least squares discriminant analysis (PLS-DA) to identify differential changes in metabolites. Heat maps, volcano plots, and Venn diagrams were constructed using Origin 9.0 software (OriginLab Corporation, USA). One-way analysis of variance (ANOVA) was performed using IBM SPSS Statistics 21.0 software (SPSS Inc., Chicago, IL, USA), and Tukey's HSD test was used to determine the level of significant differences. All experiments were performed at least three times, and the results are expressed as mean ± standard deviation (SD).

[0104] The analysis results are shown in Fig.12 . Principal component analysis (PCA) Fig.12(a) shows that the first two principal components explained 55.6% of the total variance. In the PCA plot, the packaged leek samples with the same oxygen concentration showed obvious clustering and were clearly distinguished from samples with other oxygen concentrations. This indicates that the metabolite profile of packaged leek changed significantly with the change of oxygen concentration. Partial least squares discriminant analysis (PLS-DA) ( Fig.12 (b)) obtained the same result as PCA. In order to better understand the changes in leek metabolites under different oxygen concentration treatments, we performed hierarchical cluster analysis (HCA). Heat map ( Fig.12 (d)) was used to visualize the results of HCA. The HCA results showed that the fresh leek (IM) samples formed an independent cluster, while the clustering of the Y60 samples was relatively close to it. In addition, the clustering between the Y40, Y80, and Y100 samples was relatively tight, and the clustering between the Y0 and control group (CG) samples was also tight. This clustering pattern was intuitively reflected in the heat map. These findings further indicate that the metabolite profile of leek fluctuates with increasing oxygen concentration, but when the oxygen concentration reaches 60%, its metabolite profile becomes more similar to that of fresh leek. This trend is consistent with the results of PCA and PLS-DA.

[0105] In the statistical analysis of differential metabolites, we set the following parameters: fold change FC ≥ 2 or ≤ 0.5, P-value = 0.01, and variable importance in projection VIP = 1. As shown in Table 2, compared with the fresh leek (IM) group, the Y60 treatment group had the least number of differential metabolites, a total of 682, including 405 up-regulated differential metabolites and 277 down-regulated differential metabolites. This result further verified that the 60% oxygen treatment had the best preservation effect, making the metabolite profile of leek closest to that of fresh leek.

[0106] The above are only preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A method for preserving the unique flavor and nutritional components of Allium vegetables after harvest, characterized in that: The allium vegetables are stored at low temperature in a nitrogen and oxygen mixed gas; the volume fraction of oxygen in the nitrogen and oxygen mixed gas is 60% to 80%.

2. The preservation method according to claim 1, characterized in that: The volume fraction of oxygen in the mixed gas of nitrogen and oxygen is 60%.

3. The preservation method according to claim 1, characterized in that: The low temperature storage is storage at 3-8°C.

4. The preservation method according to claim 3, characterized in that: The low temperature storage is storage at 4°C.

5. The preservation method according to claim 1, characterized in that: The low-temperature storage process also includes a step of controlling the humidity to 70-80%.

6. The preservation method according to claim 1, characterized in that: The storage of the Allium vegetables in a nitrogen and oxygen mixed gas is achieved by sealing and packaging with a high barrier film; the thickness of the high barrier film is 105 μm, and the water vapor permeability is 1.9530 g / m 2 ·24h, carbon dioxide permeation is 0.000019460cm 3 / m 2 .d.Pa, oxygen permeability is 0.000028620cm 3 / m 2 .d.Pa.

7. The preservation method according to claim 1, characterized in that: The low-temperature storage process is performed in a dark environment.

8. The preservation method according to claim 1, characterized in that: The allium vegetables are leek, leek greens, garlic greens, garlic sprouts and shallots.

9. The preservation method according to claim 8, characterized in that: The Allium vegetable is leek.

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