Method for improving flavor of cedrela sinensis based on freeze-thaw modification

Abstract

The invention relates to a method for improving the flavor of toona sinensis based on freeze-thaw modification, which comprises the following steps: (1) selecting fresh toona sinensis sprouts, removing unedible parts, removing impurities, and cleaning; (2) performing ultrasonic-permeation treatment on the cedrela sinensis; (3) freezing the cedrela sinensis obtained in the step (2); (4) unfreezing the cedrela sinensis obtained in the step (3); and (5) repeating the step (3) and the step (4) on the cedrela sinensis obtained in the step (4), wherein the cycle number is 1-5 times. Compared with untreated cedrela sinensis, the method for treating the cedrela sinensis by combining ultrasonic-permeation and freeze-thaw modification has the advantages that the content of flavor amino acid is increased by 42.5%; the activity of a key enzyme for forming the characteristic aroma of the cedrela sinensis is improved by 0.5-2.4 times; the content of sulfur-containing compounds is increased by 47.3%, and the content of total volatile compounds is increased by 51.1%. The method has the advantages of obvious flavor enhancing effect, simple operation, greenness and health, and provides a new method for high-value cedrela sinensis processing.

Description

Technical Field

[0001] The present invention belongs to the technical field of toona sinensis processing, and particularly relates to a method for improving the flavor of toona sinensis based on freeze-thaw modification. Background Art

[0002] Toona sinensis ( Toona sinensis (A.Juss.) Roem), also known as toona sinensis head and toona sinensis bud, is a perennial deciduous tree of the genus toona in the meliaceae family. It originated in China and has a cultivation history of more than 2,300 years. It is distributed and planted in subtropical to temperate zones in China. The existing cultivation area is more than 2 million mu, with an annual output of more than 120,000 tons of toona sinensis buds and about 600,000 cubic meters of toona sinensis wood annually. 3 The annual output value exceeds 10 billion yuan, and the industrial scale shows an increasing trend year by year. It has developed into a modern characteristic agriculture of the "rich people project" in mountainous and hilly areas of China.

[0003] Toona sinensis bud is a tree-borne vegetable on the market in early spring and is also one of the representatives of healthy forest vegetables popular in the world, ranking first among famous, special and excellent vegetables. Toona sinensis bud is rich in nutrients and functional components such as protein, various vitamins, trace elements, flavonoids, polysaccharides, saponins, alkaloids, essential oils, etc., and has high edible and medicinal values. Therefore, there is a saying among the people that "eating toona sinensis buds regularly can prevent various diseases". In addition, toona sinensis can secrete insect-repellent substances during the growth process and is free of pesticide pollution. The tender buds of toona sinensis are favored because of their strong and unique flavor and are the most popular woody vegetables in China. In recent years, toona sinensis buds have been exported to many different countries.

[0004] Toona sinensis buds cannot be produced and supplied throughout the year like other vegetables. Generally, a large number of tender buds of toona sinensis are on the market around the Tomb-Sweeping Festival every year. About 2 weeks after budding, if not harvested in time, they will become fibrous and lignified and are no longer suitable for consumption. The water content of fresh toona sinensis buds reaches 85% - 90%, and they are extremely easy to spoil, with the flavor lost, seriously affecting their commercial value. In addition, the natural flavor contained in toona sinensis has unstable characteristics such as easy loss, heat sensitivity, and easy destruction. Improper treatment is very likely to cause the loss of its unique flavor. The characteristic flavor of toona sinensis is its most important sensory quality index, which determines its edible value, commercial value and industrial prospect. Therefore, taking effective technical measures to maximize the retention of its original flavor and nutritional components, as well as extending the storage period of toona sinensis, alleviating market demand, and enhancing the commercial value of fresh toona sinensis buds is of great significance.

[0005] The volatile aroma components of toona sinensis are complex and diverse. At present, more than 100 volatile compounds have been identified, mainly sulfides, terpenes, aldehydes, alcohols, ketones, etc. It is reported that 2-mercapto-3,4-dimethyl-2,3-dihydrothiophene, ( E , E )-di-1-propylene disulfide and ( E , Z)-Bis-1-propenyl disulfide and other sulfur-containing compounds are the key aroma substances of fresh toon sprouts. Li Jiaxiao et al. (J. Agric. Food Chem. 2013, 61, 7470-7476) speculated that the characteristic sulfur-containing flavor compounds of toon sprouts are also an enzymatic reaction, and the flavor precursor substance ( γ γ-Glutamyl-S-propenylglycine) forms allyl mercaptan under the action of γ γ-glutamyltranspeptidase, thus producing the characteristic flavor of toon sprouts. Previous studies in our group found that the sulfur-containing flavor precursor substrates and γ γ-glutamyltranspeptidase of toon sprouts were separately isolated and purified, and by constructing an enzymatic reaction system, the characteristic flavor of toon sprouts was successfully prepared, and the enzymatic reaction mechanism of the formation of sulfur-containing characteristic aroma of toon sprouts was verified in vitro.

[0006] At present, the research on the flavor substances of toon sprouts mainly focuses on the extraction and separation of its characteristic aroma substances, the differences in key aroma components such as different varieties, drying, and blanching processes, etc. Traditional processing methods of toon sprouts often face problems such as loss of flavor components and unstable taste, and the flavor of toon sprouts itself has seasonal fluctuations, making it difficult to maintain a stable flavor quality during non-seasonal periods. Existing methods for enhancing the flavor of toon sprouts include pickling, drying, seasoning, etc., which have problems such as insignificant flavor improvement, changing the original characteristic flavor, and cumbersome processing procedures. Therefore, based on the enzymatic reaction mechanism of the formation of toon sprout characteristic flavor, making its flavor precursor substrate and γ γ-glutamyltranspeptidase fully react to stimulate flavor formation, and developing a simple, effective and able to significantly enhance the flavor of toon sprouts treatment technology has important significance for improving the market value of toon sprouts and the acceptance of consumers.

[0007] Ultrasound-permeation treatment technology is a multi-physical field combined technology that combines ultrasound and permeation (or diffusion). Its basic principle is to change the physical properties of materials through the cavitation effect, mechanical effect and thermal effect of ultrasound, so as to improve the permeability or diffusion rate. It is mostly used in the food drying process, which can reduce energy consumption and improve drying efficiency. Freeze-thaw treatment has been proven to be able to change the structure of materials and has the significant advantage of modification effect. For example, Chinese patent CN111205374B, a physical modified highly lipophilic starch and its preparation method and application, uses combined ultrasound-freeze-thaw treatment to prepare physical modified starch, which improves the lipophilicity of the modified starch and can be used as a green adsorbent, as well as developing new low-GI foods, but the influence on flavor has not been studied. Applying freeze-thaw modification technology to enhance the flavor of toon sprouts has not been reported, and its operating process conditions cannot be directly borrowed from other application fields. Based on this, this application was developed. Summary of the Invention

[0008] The object of the present invention is to overcome the defects of the prior art and provide a method for enhancing the flavor of toona sinensis based on freeze-thaw modification. This method uses a multi-physical field combined treatment technology of ultrasonic-permeation and freeze-thaw cycles to physically modify the water state, its distribution and cell structure in toona sinensis, and promote the release and transformation of flavor substances, thereby enhancing the flavor of toona sinensis.

[0009] To achieve the above object, the present invention adopts the following technical solutions: A method for enhancing the flavor of toona sinensis based on freeze-thaw modification, which comprises the following steps: (1) Pretreat the toona sinensis buds. (2) Pour NaCl solution into an ultrasonic water bath as the permeating liquid, and then add the toona sinensis buds pretreated in step (1) for ultrasonic-permeation treatment. (3) Freeze the toona sinensis buds obtained in step (2). (4) Thaw the toona sinensis buds obtained in step (3) at room temperature. (5) Repeat the treatments of step (3) and step (4) for the toona sinensis buds obtained in step (4). (6) Collect the toona sinensis obtained in step (5), and the toona sinensis with modified and enhanced flavor is obtained.

[0010] Specifically, in step (1), the pretreatment of the toona sinensis buds can be: select fresh toona sinensis buds, remove the inedible parts, remove impurities, wash, and drain the water.

[0011] Specifically, in step (2), the mass concentration of the NaCl solution can be 1% - 8%. Further, during the ultrasonic-permeation treatment, the ultrasonic frequency is 40 - 80 KHz, the power is 280 - 720 W, and the treatment time is 5 - 20 min. The material-liquid ratio of the toona sinensis buds to the NaCl solution can be 1g:5 - 20mL.

[0012] Specifically, in step (3), during the freezing treatment, the freezing temperature is preferably -80 °C to -18 °C, and the freezing time is preferably 0.5 - 3 h.

[0013] Further, in step (4), during the thawing treatment, the thawing temperature is preferably 20 °C - 25 °C, and the thawing time is preferably 0.5 - 1.5 h.

[0014] Further, in step (5), when repeating the treatments of step (3) and step (4), the preferred number of repetitions is 1 - 5 times.

[0015] The present invention provides toona sinensis with modified and enhanced flavor prepared by the above method. For the toona sinensis obtained by this method, the activity of the key flavor enzyme is increased by 0.5 - 2.4 times, and the content of the characteristic sulfur-containing compounds is increased by 47.3%.

[0016] Compared with the prior art, the technical solution of the present invention has the following beneficial effects and remarkable progress: 1) The present invention applies the combined treatment technology of multi-physical fields of ultrasonic-infiltration and freeze-thaw modification to the improvement of the flavor of toon. Ultrasonic waves can promote the rupture and infiltration of the cell wall of toon, thereby improving the release efficiency of flavor substances. The addition of NaCl can promote the release of flavor components that interact with various proteins, and enhance the overall flavor perception through synergistic effects with other substances. During the freezing process, water crystallizes inside the cells, causing changes in the cell structure. The thawing process further ruptures the cell wall, enabling the characteristic flavor precursors and peptidases to come into full contact. At the same time, freeze-thaw can change the activity of enzymes related to aroma metabolism. In short, in the toon samples treated by freeze-thaw, the rupture of the cell wall, the release of liquid, and the acceleration of enzymatic reactions will promote the generation of volatile organic compounds and flavor substances. It also helps to release taste amino acids. In addition, components such as glutamic acid and glycine play important roles in the formation of toon flavor precursors ( γ -glutamyl-propenylthioglycine).

[0017] 2) The present invention combines the three core technologies of ultrasonic wave, infiltration, and freeze-thaw modification to synergistically improve the release of flavor components of toon, maximize the presentation of aroma flavor substances, and significantly enhance the flavor enhancement effect.

[0018] 3) The operation process of the present invention is simple, the production cost is low, and no chemical additives are introduced, making it safer and more reliable, and in line with the concept of green and healthy consumption.

[0019] 4) Compared with the untreated toon, the total amino acid content increased by 38.4% and the content of umami amino acids increased by 42.5% by using the method of ultrasonic-infiltration and freeze-thaw modification combined provided by the present invention; the activity of the key enzyme for the formation of sulfur-containing characteristic aroma in toon increased by 0.5 - 2.4 times; the total content of volatile compounds increased by 34.8% - 51.1%, and the sulfur-containing substances increased by 21.7% - 47.3%. Therefore, the method of ultrasonic-infiltration and freeze-thaw modification combined provided by the present invention can significantly improve the flavor quality of toon, with simple operation, being green and healthy, and providing a new method for the high-value processing of toon. Description of the Drawings

[0020] Figure 1 is the inversion diagram of the transverse relaxation time of toon moisture; Figure 2 is the influence of freezing temperature and freeze-thaw cycles on γ the activity of -glutamyl transpeptidase; Figure 3 is the sensory score of toon samples treated with different freeze-thaw modifications; Figure 4For the types and contents of volatile aroma components of toon sprouts under different freeze-thaw treatments; Figure 5 For the fingerprint maps of volatile organic compounds of toon sprouts under different freeze-thaw treatments. Specific implementation manners

[0021] To make the objectives, technical solutions and advantages of the present invention easier to be understood, the following further details the technical solutions of the present invention in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0022] Room temperature generally refers to 25±5°C.

[0023] The detection methods involved in the present invention are listed as follows. For operations or steps not described in detail, conventional techniques in the art can be adopted.

[0024] Moisture distribution: Use a low-field nuclear magnetic resonance analyzer (LF-NMR) to measure the moisture distribution of toon sprout samples. Take about 3.0 g of toon sprout samples and place them in a nuclear magnetic tube, then put them into a low-field nuclear magnetic device. Use a CPMG (Carr-Pur-cell-Meiboom-Gill) pulse train to collect the transverse relaxation time of the samples. The magnetic field strength is 0.5T, and the ambient temperature is 32 °C. Test parameters: proton resonance frequency is 20 MHz; sampling frequency is 100 kHz; 90-degree pulse width P1 = 9.00 μs; 180-degree pulse width P2 = 17.04 μs; repeated sampling waiting time TW = 1500 ms; echo time TE = 0.3 ms; number of echoes NECH = 3000; number of accumulations NS = 8. Each sample is measured 3 times repeatedly, and the average value is taken for inversion and graphing.

[0025] Determination of amino acid content: For the extraction and determination of amino acids, refer to the national standard GB / T 5009.124-2016) and use an L-8900 full-automatic amino acid analyzer (ninhydrin post-column derivatization ion exchange chromatograph) to perform. Accurately weigh about 0.500 g of toon sprout samples, place them in a sealed glass tube, add 10 ml of HCl (6 mol / L) and 4 drops of phenol (0.2 mL), and place the sealed glass tube in an electric heating blast thermostatic oven at 110±1°C for hydrolysis for 24 h. After hydrolysis, pass through a 0.22 μm water-based filter membrane for determination. Detection conditions of the amino acid analyzer: the chromatographic column is a cation exchange column; the detector is ultraviolet; detection wavelengths: 570, 440 nm; buffer solution: sodium citrate buffer solution, flow rate 0.45 mL / min, gradient elution; derivatization reagent: ninhydrin solution, flow rate 0.25 mL / min; injection volume: 20 μL; analysis time: 60 min; post-column reaction temperature: 135 °C.

[0026] γ Determination of γ-glutamyl transpeptidase activity: The p-nitroaniline colorimetric method was adopted. Take 2 g of toon samples in a 50 mL centrifuge tube, add 10 mL of 0.05 mol / L Tris-HCl buffer solution, perform ultrasonic treatment in an ice bath for 30 min, centrifuge at 4 °C and 10000 r / min for 10 min, take the supernatant, filter it to obtain a crude enzyme extract. Take 2.5 mL of the working solution (1.3 mL of 0.1 mol / L Tirs-HCl, 0.6 mL of 0.1 mol / L glycylglycine, 0.6 mL of 5 mmol / L L-γ-glutamine-p-nitroaniline) in a stoppered test tube, add 0.5 mL of the crude enzyme extract, mix evenly, incubate in a water bath at 37 °C for 30 min, quickly add 2 mL of 2 mol / L acetic acid to terminate the transpeptidation reaction, cool to room temperature, measure its absorbance at 410 nm, and calculate the concentration according to the p-nitroaniline standard curve. γ The definition of γ-glutamyl transpeptidase activity is: 1 unit of enzyme activity is defined as the production of 1 μmol / L p-nitroaniline per gram of toon per minute.

[0027] In the formula, C is the concentration of p-nitroaniline obtained by referring to the standard curve (μmol / L); V is the total volume of the enzyme solution, mL; V0 is the reaction volume, mL; V1 is the volume of the enzyme solution taken for measurement, mL; t is the reaction time, min; m is the sample mass, g; N is the dilution factor of the enzyme solution.

[0028] Sensory evaluation: According to the requirements of aroma evaluation, personnel who have received systematic training and have sensory evaluation experience were selected as the evaluation group. Nine healthy non-smokers were recruited, including 4 males and 5 females, consisting of college students / employees aged 23 - 40 years old. Weigh 5 g of toon samples and place them in white disposable centrifuge tubes labeled with random three-digit codes, equilibrate at room temperature for 15 min, and then let the group members evaluate each sample. The samples were supplied individually, and there was a 3-minute break between two evaluations. A 10-point system was adopted, and the strength level of the aroma attribute ranged from 0 (none) to 10 (extremely strong). The average score of the evaluation members was taken as the final strength of the aroma attribute of the sample.

[0029] Gas chromatography-ion mobility spectrometry (GC-IMS) analysis: Accurately weigh 0.500 g of toon samples into a 20 mL headspace vial, heat and incubate at 40 °C for 10 min, with an incubation rotation speed of 250 r / min; the temperature of the headspace injection needle is 80 °C; the injection volume is 200 μL, in splitless mode; carrier gas: high-purity nitrogen (purity ≥99.999%).

[0030] GC conditions: The chromatographic column is an MXT-5 column (15 m × 0.53 mm, 0.53 μm); the column temperature is 60 °C; the carrier gas is high-purity nitrogen (purity ≥ 99.999%); the carrier gas flow rate program: initially 2.0 mL / min, held for 2 min, linearly increased to 5.0 mL / min from 2 - 10 min, linearly increased to 50.0 mL / min from 10 - 20 min, and linearly increased to 100.0 mL / min from 20 - 30 min.

[0031] IMS conditions: The ion source is deuterium (6.5 KeV); positive ion mode; the drift tube length is 9.8 cm; the linear voltage inside the tube is 500 V / cm; the drift tube temperature is 45 °C; the drift gas flow rate is 150 mL / min (nitrogen purity ≥ 99.999%).

[0032] GC-IMS result processing: Use n-ketones C4 - C9 as external standards to calculate the retention index (RI) of volatile compounds. Qualify volatile components through two dimensions of migration time and retention time using the NIST database and IMS database built into the VOCal software supporting GC-IMS. Analyze the intensity of volatile compounds based on the peak volume of the selected signal peaks.

[0033] Example 1 A method for enhancing the flavor of toon sprouts based on freeze-thaw modification, which comprises the following steps: 1) Select fresh toon sprouts, remove inedible parts, impurities, wash, and drain the water. 2) Pour a 5% NaCl solution by mass concentration into an ultrasonic water bath as the osmotic solution, put the processed toon sprouts into the ultrasonic water bath according to the material-liquid ratio of 1 g:15 mL with the osmotic solution, and perform ultrasonic-osmotic treatment on the toon sprouts at an ultrasonic power of 360 W and a frequency of 40 KHz for 15 min. 3) Then lay the toon sprouts in a single layer on a tray and place them in a -20 °C refrigerator for freezing treatment for 1 h; after freezing, place the toon sprouts at 25 °C for thawing, and end thawing when the internal temperature is stable (about 0.5 h). This process is recorded as 1 freeze-thaw cycle treatment -20①, and repeat this cycle 2 times and 3 times, which are respectively recorded as 2 freeze-thaw cycles -20② and 3 freeze-thaw cycles -20③. Take samples for each freeze-thaw cycle to measure the water distribution, reducing sugar content, amino acid content, γ-glutamyl transpeptidase activity, and volatile flavor components of the toon sprouts after modification treatment.

[0034] Example 2 A method for enhancing the flavor of toon sprouts based on freeze-thaw modification, which comprises the following steps: 1) Select fresh tender Chinese toon shoots, remove the inedible parts, impurities, wash, and drain the water. 2) Pour a 5% NaCl solution by mass concentration into the ultrasonic water bath as the osmotic solution. Put the treated Chinese toon shoots into the ultrasonic water bath according to the material-liquid ratio of 1 g:15 mL with the osmotic solution, and perform ultrasonic-osmotic treatment on the Chinese toon with an ultrasonic power of 360 W and a frequency of 40 KHz for 15 min. 3) Then lay the Chinese toon shoots in a single layer on a tray and place them in a -40°C refrigerator for freezing treatment for 1 h. After freezing, thaw the Chinese toon at a temperature of 25°C until the internal temperature stabilizes (about 0.5 h). This process is recorded as 1 freeze-thaw cycle treatment -40①. Repeat this cycle 2 times and 3 times, which are respectively recorded as 2 freeze-thaw cycles -40② and 3 freeze-thaw cycles -40③. Take samples for each freeze-thaw cycle to measure the water distribution, reducing sugar content, amino acid content, γ γ-glutamyl transpeptidase activity, and volatile flavor components of the modified Chinese toon.

[0035] Example 3 A method for enhancing the flavor of Chinese toon based on freeze-thaw modification, which includes the following steps: 1) Select fresh tender Chinese toon shoots, remove the inedible parts, impurities, wash, and drain the water. 2) Pour a 5% NaCl solution by mass concentration into the ultrasonic water bath as the osmotic solution. Put the treated Chinese toon shoots into the ultrasonic water bath according to the material-liquid ratio of 1 g:15 mL with the osmotic solution, and perform ultrasonic-osmotic treatment on the Chinese toon with an ultrasonic power of 360 W and a frequency of 40 KHz for 15 min. 3) Then lay the Chinese toon shoots in a single layer on a tray and place them in an -80°C refrigerator for freezing treatment for 1 h. After freezing, thaw the Chinese toon at a temperature of 25°C until the internal temperature stabilizes (about 0.5 h). This process is recorded as 1 freeze-thaw cycle treatment -80①. Repeat this cycle 2 times and 3 times, which are respectively recorded as 2 freeze-thaw cycles -80② and 3 freeze-thaw cycles -80③. Take samples for each freeze-thaw cycle to measure the water distribution, reducing sugar content, amino acid content, γ γ-glutamyl transpeptidase activity, and volatile flavor components of the modified Chinese toon.

[0036] Comparative Example 1 Fresh Chinese toon shoots, without any treatment, are denoted as RAW.

[0037] Effect evaluation: The T2 relaxation time distribution spectra of the samples of Comparative Example 1, Example 1, Example 2 and Example 3 are as follows Figure 1 shown. Each peak represents one type of moisture, and the peak area characterizes the relative amount of water in this phase state. The larger the peak area, the more water content in this phase state. According to the different moisture relaxation times, the T2 relaxation curve is divided into three parts: T21 (0.01 - 2.768 ms), T22 (3.917 - 505.263 ms) and T3 (622.257 - 2171.118 ms), which represent the degree of interaction between moisture and other substances in the tissue, namely strongly bound water, weakly bound water and free water. As can be seen from Figure 1 it, compared with the T2 relaxation time curve of the untreated toon samples, the T2 relaxation time curves of the toon samples treated at different temperatures and freeze-thaw cycles all shift to the left, that is, the T2 relaxation time becomes shorter, and the lower the freezing temperature and the more the number of freeze-thaw cycles, the greater the distance that the T2 relaxation time curve of the sample shifts to the left, and the shorter the T2 relaxation time. This is mainly because freeze-thaw treatment causes the loss of toon juice and the reduction of water content, resulting in a shorter T2 relaxation time. With the increase of the number of freeze-thaw cycles, the peak area M21 decreases, while the peak area values of M22 and M23 increase, indicating that after freeze-thaw treatment, bound water transfers to weakly bound water and free water, increasing their relative content.

[0038] Figure 2 For the γ γ-glutamyl transpeptidase activity in toon samples, γ γ-glutamyl transpeptidase plays an important role in the metabolic pathway of glutathione and shows extensive substrate specificity for the intermediates in the synthesis pathway of sulfur-containing flavor compounds, so it plays an important role in the formation of key flavor substances in toon. The γ γ-glutamyl transpeptidase activity of the toon samples treated by freeze-thaw modification is significantly higher than that of the untreated group, and shows a consistent trend at different freezing temperatures, first rising and then falling with the increase of freeze-thaw cycles, and the activity is the highest after 2 freeze-thaw cycles. This enzyme is mainly present on the outer surface of the cell membrane and is connected to the cell membrane through subunits. The first freeze-thaw usually causes mechanical damage to toon cells, promoting γ the release of γ-glutamyl transpeptidase, so it is significantly higher than that of the untreated group. However, with the increase of the number of freeze-thaw cycles, the damage of cell tissue will be further aggravated, destroying the spatial structure of enzyme molecules, resulting in a certain decrease in enzyme activity. The enzyme activity of the group treated at -20°C with 2 freeze-thaw cycles is the highest at 6.96 U / g, which is 2.4 times higher than that of the untreated group, and the enzyme activity of other treatment groups is increased by more than 0.5 times.

[0039] Table 1 shows the amino acid contents of the untreated samples and the samples treated by freeze-thaw modification. It can be seen from the table that the total amino acid content of the toon samples treated by freeze-thaw modification increases significantly ( P(<0.05). The toon samples contain 8 essential amino acids for the human body, and the content of essential amino acids is rich. According to the taste characteristics of amino acids, they can be divided into 4 categories. Among them, the amino acids with umami taste are Asp and Glu, the sweet ones are Thr, Ser, Gly, Ala, and Pro, the bitter ones are Val, Met, Ile, Leu, Phe, His, and Arg, and the tasteless ones are Tyr and Lys. The total amount of umami amino acids in each treatment group is the highest, ranging from 1.79 to 2.55 g / 100g, which is also the main reason for the delicious taste of toon. After freeze-thaw modification treatment, the content of umami amino acids in the toon samples increased by 42.5%, and the total amount of amino acids increased by 38.4%. It shows that the freeze-thaw modification treatment has a significant effect on enhancing the taste of toon.

[0040] Table 1 Effects of freeze-thaw modification treatment on the amino acid content of samples The sensory evaluation of the aroma attributes of toon samples by 9 group members is as Figure 3 shown. It can be seen from the figure that the scores of the freeze-thaw modified toon samples all reached above 8.2, and the score of the samples treated with 2 freeze-thaw cycles at -40°C was as high as 9.33. The modified samples were all significantly higher than the untreated group, indicating a significant effect on enhancing the aroma.

[0041] Furthermore, the aroma attributes of the samples were analyzed by instrumental analysis. GC-IMS was used to analyze the volatile compounds in the toon samples treated with 2 freeze-thaw cycles at different freezing temperatures. Ion migration time and retention index were used to qualitatively identify the volatile organic compounds. A total of 80 chromatographic signal peaks were detected, and 54 volatile compounds were qualitatively identified, including 5 alcohols, 15 aldehydes, 2 acids, 11 terpenes, 3 ketones, 5 esters, 10 heterocyclics, and 3 others. Figure 4 Table 2 shows the types and contents of volatile aroma components of toon samples with different treatments. Figure 4 It can be seen from Table 2 that the total content of volatile compounds increased significantly after freeze-thaw modification treatment, increasing by 34.8% - 51.1% compared with the untreated group. The peak volume ratio of aldehyde substances was the highest in each sample, ranging from 29.6% to 37.6%, followed by terpenes with a ratio of 17.2% - 22.1%. The detected sulfur-containing substances accounted for 9.6% - 12.2%, which was increased by 21.7% - 47.3% compared with the sulfur-containing substances in the untreated toon.

[0042] To more intuitively and visually explore the differences in volatile compounds in toon samples with different freeze-thaw treatments, the GalleryPlot plug-in was used to generate a fingerprint map ( Figure 5), the shade of the color represents its concentration level. Brighter spots indicate higher concentrations of volatile compounds. Monomers and dimers of the compounds are represented by different columns with the same compound name. Undetermined substances are numbered with Area and Arabic numerals. Region A is composed of volatile compounds common to all tested samples and with relatively high peak volumes, mainly aldehydes, alcohols, and terpene compounds, including (E)-hexanal, (E)-2-hexenal-M, linalool, 2-phenylethanol, α -pinene-M / D / T, myrcene-M, α -phellandrene. Region B is composed of volatile compounds with relatively high peak volumes in the fresh untreated group, mainly esters and alcohols, including heptanol, amyl formate, ethyl acetate, etc. Region C is composed of volatile compounds with relatively high peak volumes that appear in the groups treated with 2 freeze-thaw cycles at -20 °C and -40 °C, mainly 4-hydroxy-4-methyl-2-pentanone, 5-methyl-2-formylthiophene-D, ( E , E )-2,4-heptadienal, 1-hydroxy-2-propanone, tetrahydrofuran, etc. Region D is composed of volatile compounds with relatively high peak volumes that appear in the group treated with 2 freeze-thaw cycles at -80 °C, mainly triethylamine, 2-methyl-2-pentenal-M, 3-methylbutyraldehyde-D, acetic acid, 2-ethylpyridine, 5-methylfurfural, 3-methylbutyraldehyde-D, 2-methylbutyraldehyde-D, butyraldehyde, 2-butanone, etc. It can be intuitively seen from the fingerprint that after freeze-thaw modification treatment, many flavor compounds are newly added (Regions C and D) and the levels of various flavor compounds are increased, and the flavor improvement effect is significant.

[0043] The above results show that by using the technical method of the present invention, the total amino acid content is increased by 38.4%, and the content of umami amino acids is increased by 42.5%; the activity of the key enzyme for the formation of sulfur-containing characteristic aroma in toona sinensis is increased by 0.5 - 2.4 times; the total content of volatile compounds is increased by 34.8% - 51.1%, and the sulfur-containing substances are increased by 21.7% - 47.3% compared with the original. Therefore, the method of the present invention that combines ultrasonic-infiltration and freeze-thaw modification can significantly improve the flavor quality of toona sinensis, and the operation is simple, green and healthy, providing a new method for the high-value processing of toona sinensis.

Claims

1. A method for improving the flavor of Chinese toon based on freeze-thaw modification, characterized in that: The steps include: (1) Pre-treating the toon buds; (2) Pour NaCl solution into an ultrasonic water bath, and then add the toon buds pretreated in step (1) for ultrasonic-osmotic treatment; (3) freezing the toon sprouts obtained in step (2); (4) thawing the toon buds obtained in step (3) at room temperature; (5) Repeating steps (3) and (4) on the toon buds obtained in step (4); (6) Collect the Chinese toon obtained in step (5), that is, obtain the modified Chinese toon with enhanced flavor.

2. The method for improving the flavor of Toona sinensis based on freeze-thaw modification according to claim 1, characterized in that: In step (1), the pretreatment of the Chinese toon sprouts is as follows: selecting fresh Chinese toon sprouts, removing inedible parts, removing impurities, washing, and draining water.

3. The method for improving the flavor of Chinese toon based on freeze-thaw modification according to claim 1, characterized in that: In step (2), the mass concentration of the NaCl solution is 1% to 8%.

4. The method for improving the flavor of Toona sinensis based on freeze-thaw modification according to claim 1, characterized in that: In step (2), during the ultrasonic-permeation treatment, the ultrasonic frequency is 40 to 80 KHz, the power is 280 to 720 W, and the treatment time is 5 to 20 min.

5. The method for improving the flavor of Toona sinensis based on freeze-thaw modification according to claim 1, characterized in that: In step (2), the solid-liquid ratio of the toon sprouts to the NaCl solution is 1 g: 5-20 mL.

6. The method for improving the flavor of Toona sinensis based on freeze-thaw modification according to claim 1, characterized in that: In step (3), during the freezing treatment, the freezing temperature is -80 to -18°C and the freezing time is 0.5 to 3 hours.

7. The method for improving the flavor of Toona sinensis based on freeze-thaw modification according to claim 1, characterized in that: In step (4), during the thawing process, the thawing temperature is 20°C to 25°C, and the thawing time is 0.5 to 1.5 h.

8. The method for improving the flavor of Toona sinensis based on freeze-thaw modification as claimed in claim 1, characterized in that: In step (5), the number of repetitions is 1 to 5 times.

9. The modified Chinese toon with enhanced flavor prepared by the method according to any one of claims 1 to 8.

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