AU2018200085B2

AU2018200085B2 - Method for prolonging shelf life of prepared eight-treasure duck by utilizing combined flexible sterilization

Info

Publication number: AU2018200085B2
Application number: AU2018200085A
Authority: AU
Inventors: Yuchuan Wang; Chaohui YANG; Fanli Yang; Min Zhang; Weiming Zhang
Original assignee: Jiangnan University
Current assignee: Jiangnan University
Priority date: 2017-01-16
Filing date: 2018-01-04
Publication date: 2019-10-17
Anticipated expiration: 2038-01-04
Also published as: AU2018200085A1; CN106819064A

Abstract

A method for prolonging the shelf life of a prepared eight-treasure duck by utilizing combined flexible sterilization relates to the field of meat product processing technologies. According to the present invention, during a process of making a staffing heart of an 5 eight-treasure duck, a nano preservative containing a composite extract of clove, cinnamon, and onion is added to the stuffing heart. After being cooked, the eight-treasure duck is vacuum packed by using a retort pouch, and then the packed eight-treasure duck is placed in a radio frequency working chamber to be sterilized by heating. Subsequently, the packed eight-treasure duck is sterilized by short-time high temperature and high pressure 10 sterilization, and then is quickly cooled. According to the method, a problem that the product quality is damaged by using a traditional high temperature and high pressure sterilization method and complexity of quality preservation that is caused because the eight-treasure duck has both the duck body and stuffs are resolved. After flexible sterilization, quality preservation rates of the color, aroma, taste, and shape of the duck 15 body and the stuffing heart are respectively 90% and 88%. Moreover, under the premise that no artificial preservative is added, commercial sterility is achieved, thereby obviously prolonging the high-quality shelf life of the product. The high-quality shelf life at a room temperature (25°C to 35C) in summer may be 60 days, and the high-quality shelf life at a low temperature (4°C) may be 180 days.

Description

METHOD FOR PROLONGING SHELF LIFE OF PREPARED EIGHT-TREASURE DUCK BY UTILIZING COMBINED FLEXIBLE STERILIZATION

Technical Field

The present invention relates to a method for prolonging the shelf life of a prepared eight-treasure duck by utilizing combined flexible sterilization, mainly used for vacuum packing meat products, specifically relates to a meat product sterilization process, pertaining to the field of meat product processing technologies.

Related Art

Eight-treasure duck is a traditional, delicious, and famous Huaiyang dish. A key point of the dish is that bones of the duck are removed from the duck by using a whole duck bone removal technology without breaking the duck skin. Stuffs (sticky rice, myotonin, diced bamboo shoots, diced mushrooms, diced ham, lotus seeds, red dates, diced chestnuts, diced cooked chicken, and cooked pine nuts) are stuffed inside the duck belly, and the duck is made to a gourd shape. The duck is fresh and tender, the stuffs are sticky and scattered, and the taste is salty, fresh, and mellow. Quality deterioration of a vacuum packed eight-treasure duck during storage is mainly caused by physical factors, chemical factors, microorganism factors, and action of endogenous enzymes in the duck. Because the endogenous enzymes in the duck can be deactivated during cooking and sterilization of the product, the physical and chemical factors have relatively small impact on the quality deterioration of the duck. Microorganisms and physicochemical quality deterioration caused by the microorganisms become main factors affecting the shelf life of the product. In addition, because the eight-treasure duck has two parts, that is, the duck body and the stuffs, quality preservation of the eight-treasure duck is relatively complex, and there is a requirement on a sterilization manner. Currently, for most eight-treasure duck products on the market, high temperature and high pressure sterilization at 121°C is applied, a preservative is added, or cryopreservation is applied to prolong the shelf life. Because costs of chemical preservatives are relatively low, preservatives that are usually used in the food industry in the world are mostly chemical synthetic preservatives. However, some chemical synthetic preservatives have problems, for example, the preservatives have cancerogenesis, carcinogenicity, and teratogenicity, and the preservatives may easily cause food poisoning. Therefore, a natural preservative substance is used in combination with flexible sterilization, so that preservation and bacteriostasis are ensured while reducing a sterilization strength, thereby prolonging the shelf life of the eight-treasure duck at a room temperature and at a low temperature while ensuring the product quality.

Science reports on the use of spices in food due to antibacterial properties thereof was first appeared in 1880, reporting that mustard, clove, cinnamon, onion, and essential oils thereof have bacteriostatic effects. Martindale studied the antibacterial property of the spice oils by using a phenol coefficient, and found that cinnamon oil and clove oil have strong bacteriostasis capacities. A clove bud contains 15% to 20% essential oil, and 85% to 92% of the essential oil is the main bacteriostatic component eugenol. The eugenol has an inhibition effect on colibacillus, salmonella, dysentery bacillus, proteus, and the like. Similarly, cinnamon contains 1% to 3.5% essential oil, and cinnamic aldehyde occupies up to 80% to 95% of the essential oil, and the cinnamic aldehyde plays a major role in bacteriostasis. The cinnamic aldehyde has a good inhibition effect on mould, has strong bacteriostatic action, and can effectively kill germs and colibacillus. An onion contains 0.05% hot essential oil having a pungent odor. The hot essential oil contains a large amount of sulfur compounds, and components thereof have an obvious bacteriostatic effect. Kang Meiling (2012) reported that onion juice has a good bacteriostatic effect on Gram-negative bacteria (bacillus subtilis) and Gram-positive bacteria (colibacillus).

A radio frequency refers to a high-frequency alternating current electromagnetic wave whose frequency ranges from 3 MHz to 300 MHz. Radio frequency heating is a new quick heating technology by means of which a material absorbs electromagnetic wave energy and converts the energy into heat energy without any heat conduction process, and both the inside and surface of the material are heated and heated up almost at the same time. The mechanism of radio frequency sterilization is that, under action of a radio frequency having a particular strength, germs in a material can absorb radio frequency energy and be heated up at the same time due to molecular polarization, so that protein of the material is denatured and then deactivated. On the other hand, a non-thermal effect of the radio frequency energy plays a special role in the sterilization compared with common physical sterilization, and is also one of reasons of germ death (Shi Huojie et al. in 2014). Compared with traditional heating manners, the heating rate of the radio frequency heating is high, and heating is relatively uniform. Compared with microwave heating, for the radio frequency heating, heating is more uniform, the penetration depth is larger, temperature control is more stable, and product quality is better. In addition, the radio frequency is a non-ionizing radiation technology. High energy generated by X-rays or gamma rays can change molecular structures. However, energy generated by the radio frequency does not ionize water molecules. Therefore, the radio frequency is considered as a safe heat treatment method. Because the radio frequency technology has a selective heating property, and a treatment process has advantages such as high efficiency, a high speed, low energy consumption, and a low material temperature, application of the radio frequency technology in the field of agricultural product and food sterilization becomes a research focus in China and foreign countries (Liu Yanhong et al. in 2010). It was reported that the radio frequency sterilization technology has relatively good sterilization effects in canned lunch meats (Zhao Hui in 2015), canned pineapples (Hou Quan in 2015), kiwi juice (Lv Xiaoying et al. in 2015), white breads (Liu Yanhong et al. in 2009), and the like.

High temperature and high pressure sterilization mainly refers to sterilization at 120°C for 30 min, and is a necessary process link of most meat product at present. The high temperature and high pressure sterilization can kill all microorganisms including brood cells, and is one of methods having the best sterilization effect. Although the sterilization method can ensure safety of a product, amino acids that are contained in meat and that are required by the human body are damaged, and the protein nutrition value is also reduced. In addition, for the product, the meat is flaccid, the juice runs off, and the aroma is seriously damaged. Quality of the product is significantly affected.

Flexible sterilization is a sterilization method in which a sterilization strength is reduced by combining different sterilization technologies or adding a preservative under a premise of ensuring safety of a product, thereby keeping the original color, aroma, and taste of the product as much as possible.

Zhang Min et al. (Chinese Patent, Patent Application No.: 201310578719.9) invented a method for preparing a preservative containing a composite extract of clove and cinnamon and an application of the preservative in sauce product preservation. For the application of the preservative in sauce product preservation, preservation and bacteriostasis are implemented by means of addition of 0.2% of the weight of the preservative in combination with 85°C sterilization for 20 min to 30 min, so that quality assurance shelf lives of sauce products can reach one year, thereby achieving a good bacteriostasis and preservation effect. A difference between the present invention and this invention is that, in the present invention, three substances, that is, clove, cinnamon, and onion, are selected, so that a bacteriostatic spectrum is wider. In addition, the particle size of a prepared nano preservative containing a composite extract of clove, cinnamon, and onion is smaller, thereby facilitating uniform distribution of the preservative in food ingredients.

Zhang Min et al. (Chinese Patent, Patent Application No.: 201510746427.0) invented a method for preparing a nano preservative containing a composite extract of clove, cinnamon, and ginger and an application of the preservative in chicken stock preservation. A combination of addition of 0.5 g/kg of the nano preservative and a condition of 85°C sterilization for 15 min to 20 min may be applied to preservation and bacteriostasis of baste products, so that storage shelf lives of the products can reach one year. A difference between the present invention and this invention is that, in the present invention, three substances, that is, clove, cinnamon, and onion, are selected, and ultrasonic extraction is performed twice. Therefore, the extraction efficiency is higher, effective concentration of a bacteriostatic substance is higher, and a preservation effect is better.

Ye Jian (Chinese Patent, Patent Application No.: 201410427261.1) invented a method for preparing a flavored spiced duck. Artificial preservatives such as sodium nitride and potassium sorbate are used in the duck. A difference between the present invention and this invention is that, in the present invention, a natural nano preservative containing a composite extract of clove, cinnamon, and onion is used. Therefore, a preservation effect is achieved and a product flavor is added. In addition, the preservative is harmless to the human body.

Wu Guanghe et al. (1997) invented a preparation technology of an eight-treasure duck packed with a soft package. A sterilization method is that the traditional high temperature and high pressure sterilization is used and a sterilization formula is (10-40-15) min/120°C. The product meets a commercial sterility requirement on canned foods. A difference between the present invention and the invention is that, in the present invention, the radio frequency sterilization is used in combination with short-time high temperature and high pressure sterilization, so that a sterilization strength is reduced, thereby avoiding damage to the product quality caused by the traditional high temperature and high pressure sterilization. In addition, an overlong sterilization time causes reduction of the processing efficiency, and goes against quick and efficient industrial production. In the present invention, an action time of the method that combines the radio frequency sterilization and short-time high temperature and high pressure sterilization is short, thereby improving the production efficiency.

Zhang Min et al. (Chinese Patent, Patent Application No.: 200910213553.4) invented a mycelium repressing and quality assurance method for reducing a thermal sterilization strength for a prepared fowl or fish product. A bacteriostatic spectrum is expanded by mixing nisin and potassium sorbate. Freezing sterilization is combined with flexible thermal sterilization, thereby achieving a good sterilization effect by means of a large temperature difference change within a short time. Therefore, damage to nutritional components of fowl meat caused by pure high temperature sterilization is reduced, and quality of the fowl meat is kept. Finally, under a premise that the shelf life of the product is ensured, a cooked fowl product with mycelium repressing and quality assurance is obtained. A difference between the present invention and the invention is that, in the present invention, the used nano preservative containing the composite extract of clove, cinnamon, and onion is safer, and the product flavor is not obviously changed while reducing the strength of the traditional high temperature and high pressure sterilization.

Zhang Min et al. (Chinese Patent, Patent Application No.: 201610009458.2) invented a method for performing flexible sterilization on a ready-to-eat surimi product by means of a radio frequency-hot air combination. An edible surimi product is placed on a polyetherimide resistance block inside a radio frequency working chamber after the product is vacuum packed, and radio frequency-hot air combined sterilization is directly performed. The obtained surimi product can keep relatively good flavor, texture, color, and nutritional quality, and the shelf life at a normal temperature may be more than 9 months. A difference between the present invention and this invention is that, in the present invention, the radio frequency is used to reduce a high strength of the traditional high temperature and high pressure sterilization, so that an objective of the sterilization effect of the product is achieved at a lower strength.

Guo Zhuozhao et al. (China Patent, Patent Application No.: 200910009361.1) invented a method for processing a brine fowl product. In this invention, a sterilization technology combining microwave and high temperature (118°C to 121°C, and 10 min to 15 min) is used, so that a time for high temperature sterilization of a product is reduced by 5 min to 10 min, and the product quality is improved by approximately 20%. Therefore, commercial sterility of the product is achieved, and the product can be stored for a long period. A difference between the present invention and the invention is that, in the present invention, the radio frequency is used, and the penetration depth of the radio frequency is much greater than that of a microwave, so that a problem of nonuniform sterilization caused due to the relatively small penetration depth of the microwave is avoid, and the strength of the high temperature sterilization is also reduced, thereby ensuring the product quality.

Li Zhijian (Chinese Patent, Patent Application No.: 201410447763.0) disclosed a method for preserving a salted meat product. In this invention, a manner of sterilizing a salted meat product, is microwave sterilization and pulse strong light sterilization. However, the penetration depth of the microwave is relatively small. A nonuniform sterilization problem may be caused due to an insufficient penetration depth of the microwave. In the present invention, the penetration depth of the radio frequency is much

2018200085 04 Jan 2018 greater than that of the microwave, and can effectively resolve a nonuniform sterilization phenomenon.

Li Xin and Xiong Guangquan et al. (Chinese Patent, Patent Application No.: 201210174940.3) disclosed a preparation process and a sterilization method of a ready-to-eat anchovy. In this invention, irradiation sterilization is used, an irradiation dosage is 3 Kgy, and an irradiation temperature is a normal temperature. Compared with the traditional high temperature and high pressure sterilization, the hardness and chewiness of a product are increased. Therefore, this patent provides a new idea for performing flexible sterilization on a meat product. However, the meat product obtained after the irradiation sterilization generates an irradiation odor, affecting the acceptability of the product. In the present invention, because the radio frequency is characterized by an extremely large penetration depth and uniform action, quick sterilization is implemented without generating another unfavorable odor, and the original quality of the product can also be kept.

Xia Xiaodong and Lv Xiaoying et al. (China Patent, Patent Application No.: 201410314527.1) disclosed a method for sterilizing kiwi juice. The method includes steps of placing kiwi juice between pole plates of a radio frequency apparatus and performing radio frequency sterilization, thereby effectively preserving nutritional components in the kiwi juice and improving the nutritional quality of the kiwi juice. However, the sterilization is excessive long by using only the radio frequency heating, and neither energy saving nor high efficiency can be achieved. In the present invention, by combining the radio frequency sterilization and the high temperature and high pressure sterilization, a sterilization time can be shortened, energy consumption can be reduced. Therefore, the method is and economy and environmental protection are implemented.

According to the present invention, a new preservative containing a composite of clove, cinnamon, and onion is added to the stuffing heart of the eight-treasure duck that is difficult to be sterilized and preserved, and the flexible sterilization is performed on the eight-treasure duck product by combining the radio frequency sterilization and the high temperature and high pressure sterilization, so that not only an effect of killing microorganisms is achieved, but also the nutrition and texture of the product can be protected from being damaged by the high temperature, thereby implementing the flexible sterilization with high efficiency and low damage.

SUMMARY

Embodiments of the present invention provide a method for prolonging the shelf life of a prepared eight-treasure duck by utilizing combined flexible sterilization.

A method for prolonging the shelf life of a prepared eight-treasure duck by utilizing combined flexible sterilization, wherein a nano preservative is added to a stuffing heart of an eight-treasure duck, and after the prepared eight-treasure duck is vacuum packed, the prepared eight-treasure duck is placed inside a radio frequency working chamber to be sterilized by heating in combination with short-time high temperature and high pressure sterilization; and specific steps are as follows:

(1) addition of the nano preservative: adding the nano preservative in a process of preparing the stuffing heart of the eight-treasure duck, wherein an addition amount is 0.2% of the weight of the stuffing heart of the eight-treasure duck wherein the nano preservative is a nano preservative containing a composite extract of clove, cinnamon, and onion having a nanoscale particulate size;

(2) cooking the eight-treasure duck; and (3) combined flexible sterilization: performing radio frequency heating sterilization after the prepared eight-treasure duck is vacuum packed, wherein a working frequency of a radio frequency apparatus is 27.12 MHz, a power is 1500 W, a distance between pole plates is 30 cm to 35 cm, and a time for the sterilization is 10 min to 15 min; and then performing the short-time high temperature and high pressure sterilization, wherein a sterilization temperature is 115°C to 121°C, and a time for the sterilization is 8 min to 10 min.

According to one embodiment, the average particle size of the nano preservative is 8.52 nm, and the polydispersity index PDI is 0.26.

According to one embodiment, the method for preparing the nano preservative may be as follows:

(a) preparation of extracts: preparation of extracts of clove, cinnamon, and onion: separately grinding clove bud, cinnamon bark, and dehydrated onion by using a high-speed plant grinder, and obtaining powder after screening with a 80-mesh screen; separately taking 50 g clove bud powder, 50 g cinnamon bark powder, and 50 g onion powder, and performing extraction twice, wherein for the extraction of each time, a ratio g/mL of solid to liquid is 1:2, that is, a total ratio of solid to liquid is 1:4; and subsequently, adding an alcohol solution whose mass concentration is 95%, performing extraction for 15 min with assistance of an ultrasonic wave of 200 W and a low frequency of 27 kHZ, and obtaining the extracts after the solution is concentrated to 10 mL;

(b) preparation of the composite extract of clove, cinnamon, and onion: uniformly mixing the clove extract, the cinnamon extract, and the onion extract that are obtained in the step (a) in a volume ratio of 1:1.8:1.8, to obtain the composite extract of clove, cinnamon, and onion; and (c) preparation of the nano preservative, wherein the temperature is set to 50°C, and mixing and preparation are performed by using a magnetic stirrer based on mass percentages: dissolving 2% of the composite extract of clove, cinnamon, and onion that is obtained in the step (b) in 3% ethyl acetate, adding 10% alcohol and 30% tween-80, stirring uniformly until the mixture gradually changes from thick to thin, and finally dripping 55% distilled water, until the emulsion becomes clear and transparent, that is, the nano preservative containing the composite extract of clove, cinnamon, and onion is obtained by means of preparation.

According to one embodiment, the method for preparing the prepared eight-treasure duck may be as follows:

(a) pretreatment: removing bones of a duck and turning skin of the duck inside out, taking off two pieces of duck breast meat, blanching the duck breast meat with hot water, and washing the blanched duck breast meat for use;

(b) preparation of eight-treasure stuffs: taking sticky rice and soaking the sticky rice for 2 h to 3 h; removing plumules from lotus seeds and soaking the sticky lotus seeds for 2 h; steaming and dicing the duck breast meat that is obtained in the step (a); adding sesame oil to a frying pan and heating the oil, adding chopped green onion and chopped ginger, then adding the sticky rice, the lotus seeds, diced bamboo shoots, diced mushrooms, diced ham, red dates, diced chestnuts, myotonin, cooked pine nuts, diced cooked chicken, and the duck breast meat; adding spices, wherein powdered pepper 3g, chicken powder 50g, and light soya sauce 300g; stir frying the spices with the nano preservative for 5 min, wherein the nano preservative is 0.2% of the weight of the stuffing heart; and placing the fried materials in a tray for cooling and use;

(c) frying: turning back the whole duck skin that is obtained in the step (a), placing 250 g/1 kg of the eight-treasure stuffs and correctly reversing the whole duck, tying the duck in a gourd shape by using a straw rope, applying the light soya sauce to duck skin, and then placing the duck in an oil pot to fry the duck until the duck become golden; and (d) smouldering: placing the eight-treasure duck that is obtained in the step (c) in a pot having ginger and green onion at the bottom, adding water, 250 g light soya sauce, 100 g dark soya sauce, 500 g yellow wine, and 200 g crystal sugar; boiling the water with big fire; turning down the fire and smouldering the duck for 3 h; and taking out the duck, that is, obtaining the prepared eight-treasure duck.

According to one embodiment, the prepared eight-treasure duck is vacuum packed, the size of the vacuum package is within 20 cm* 13 cm* 10 cm, and the vacuum degree is 0.1 MPa.

Beneficial effects of the present invention are: according to the present invention, the nano preservative containing the composite extract of clove, cinnamon, and onion is added to the stuffing heart of the eight-treasure duck that is relatively difficult to be sterilized, so that microorganism growth can be effectively inhibited, and a flavor of the product is added.

According to one embodiment, the radio frequency sterilization is combined with the high temperature and high pressure sterilization, so that a sterilization strength is reduced, and better keeping product quality is better kept.

According to the radio frequency sterilization method of the present invention, the sterilization can penetrate an interior of a material. The penetration depth of the radio frequency sterilization is much greater than that of microwave sterilization, so that a satisfied sterilization effect can be achieved at a relatively low material temperature.

According to one embodiment, the radio frequency sterilization is first performed on the eight-treasure duck, to achieve objectives of quick temperature rise and initial sterilization. After the radio frequency sterilization is performed, a whole temperature of the product is raised, and then the short-time high temperature and high pressure sterilization is performed. This is helpful for quickly reaching a targeted sterilization temperature, and shortening a time required by the sterilization.

DETAILED DESCRIPTION

Embodiment 1 Method for prolonging the shelf life of a prepared eight-treasure duck at room temperature by utilizing combined flexible sterilization

During making an eight-treasure duck, 0.2% (mass percent) of a new nano preservative containing composite clove, cinnamon, and onion was added when a stuffing heart was fried. After the eight-treasure duck was cooked, the prepared eight-treasure duck (1 kg duck with 250 g stuffs) was vacuum packed by using a retort pouch, then the duck was placed in a radio frequency working chamber to be sterilized. A standard working frequency of a radio frequency apparatus was 27.12 MHz, a power was 1500 W, and a distance between pole plates was 35 cm. Sterilization lasted 10 min. Subsequently, the prepared eight-treasure duck was placed in a retort to be sterilized. A sterilization temperature was 115°C, the sterilization lasted 10 min, and a sterilization formula was (6-10-1) min/115°C. After the sterilization, spraying was immediately performed to cool the prepared eight-treasure duck to a normal temperature. A total amount of bacterial colonies in the prepared eight-treasure duck was less than 10 cfu/g, which was reduced by 6.2 orders of magnitude, and no disease germ was detected. The hardness, color, and flavor of the eight-treasure duck did not obviously change, preservation rates of the color, aroma, taste, and shape of the duck body and the stuffing heart were respectively 91.3% and 88.2%. A high-quality shelflife of an eight-treasure duck at room temperature (25°C to 35°C) in summer could reach 60 days.

Embodiment 2 Method for prolonging the shelf life of a prepared eight-treasure duck at low temperature by utilizing combined flexible sterilization

During making an eight-treasure duck, 0.2% (mass percent) of a new nano preservative containing composite clove, cinnamon, and onion was added when a stuffing heart was fried. After the eight-treasure duck was cooked, the prepared eight-treasure duck (1 kg duck with 250 g stuffs) was vacuum packed by using a retort pouch, then the duck was placed in a radio frequency working chamber to be sterilized. A standard working frequency of a radio frequency apparatus was 27.12 MHz, a power was 1500 W, and a distance between pole plates was 35 cm. Sterilization lasted 10 min. Subsequently, the prepared eight-treasure duck was placed in a retort to be sterilized. A sterilization temperature was 115°C, the sterilization lasted 9 min, and a sterilization formula was (6-10-1) min/115°C. After the sterilization, spraying was immediately performed to cool the prepared eight-treasure duck to a normal temperature. A total amount of bacterial colonies in the prepared eight-treasure duck was less than 10 cfu/g, which was reduced by 6.1 orders of magnitude, and no disease germ was detected. The hardness, color, and flavor of the eight-treasure duck did not obviously change, preservation rates of the color, aroma, taste, and shape of the duck body and the stuffing heart were respectively 91.8% and 88.7%. A high-quality shelf life of an eight-treasure duck at low temperature (4°C) could reach 180 days.

Throughout the specification and the claims that follow, unless the context requires otherwise, the words “comprise” and “include” and variations such as “comprising” and “including” will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that such prior art forms part of the common general knowledge.

2018200085 20 Sep 2019

It will be appreciated by those skilled in the art that the invention is not restricted in its use to the particular application described. Neither is the present invention restricted in its preferred embodiment with regard to the particular elements and/or features described or depicted herein. It will be appreciated that the invention is not limited to the embodiment or 5 embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention.

Claims

What is claimed is:

(1) addition of the nano preservative: adding the nano preservative in a process of preparing the stuffing heart of the eight-treasure duck, wherein an addition amount is 0.2% of the weight of the stuffing heart of the eight-treasure duck, wherein the nano preservative is a nano preservative containing a composite extract of clove, cinnamon, and onion having a nanoscale particulate size;

1. A method for prolonging the shelf life of a prepared eight-treasure duck by utilizing combined flexible sterilization, wherein a nano preservative is added to a stuffing heart of an eight-treasure duck, and after the prepared eight-treasure duck is vacuum packed, the prepared eight-treasure duck is placed inside a radio frequency working chamber to be sterilized by heating in combination with short-time high temperature and high pressure sterilization; and specific steps are as follows:
2. The method for prolonging the shelf life of a prepared eight-treasure duck by utilizing combined flexible sterilization according to claim 1, wherein the average particle size of the nano preservative is 8.52 nm, and a polydispersity index PDI is 0.26.

(2) cooking the eight-treasure duck; and (3) combined flexible sterilization: performing radio frequency heating sterilization after the prepared eight-treasure duck is vacuum packed, wherein a working frequency of a radio frequency apparatus is 27.12 MHz, a power is 1500 W, a distance between pole plates is 30 cm to 35 cm, and a time for the sterilization is 10 min to 15 min; and then performing the short-time high temperature and high pressure sterilization, wherein a sterilization temperature is 115°C to 121°C, and a time for the sterilization is 8 min to 10 min.
3. The method for prolonging the shelf life of a prepared eight-treasure duck by utilizing combined flexible sterilization according to claim 1, wherein a method for preparing the nano preservative is as follows:

(a) preparation of extracts: preparation of extracts of clove, cinnamon, and onion: separately grinding clove bud, cinnamon bark, and dehydrated onion by using a high-speed plant grinder, and obtaining powder after screening with a 80-mesh screen; separately taking 50 g clove bud powder, 50 g cinnamon bark powder, and 50 g onion powder, and performing extraction twice, wherein for the extraction of each time, a ratio g/mL of solid to liquid is 1:2, that is, a total ratio of solid to liquid is 1:4; and subsequently, adding an alcohol solution whose mass concentration is 95%, performing extraction for 15 min with assistance of an ultrasonic wave of 200 W and a low frequency of 27 kHz, and obtaining the extracts after the solution is concentrated to 10 mL;

(b) preparation of the composite extract of clove, cinnamon, and onion: uniformly mixing the clove extract, the cinnamon extract, and the onion extract that are obtained in the step (a) in a volume ratio of 1:1.6-2:1.6-2, to obtain the composite extract of clove, cinnamon, and onion; and (c) preparation of the nano preservative, wherein the temperature is set to 45°C to 55°C, and mixing and preparation are performed by using a magnetic stirrer based on mass percentages: dissolving 2% of the composite extract of clove, cinnamon, and onion that is obtained in the step (b) in 3% ethyl acetate, adding 10% alcohol and 30% tween-80, stirring uniformly until the mixture gradually changes from thick to thin, and finally dripping distilled water to form 100% emulsion, until the emulsion becomes clear and transparent, that is, the nano preservative containing the composite extract of clove, cinnamon, and onion is obtained by means of preparation.
4. The method for prolonging the shelf life of a prepared eight-treasure duck by utilizing combined flexible sterilization according to claim 1, wherein a method for preparing the prepared eight-treasure duck is as follows:

(a) pretreatment: removing bones of a duck and turning skin of the duck inside out, taking off two pieces of duck breast meat, blanching the duck breast meat with hot water, and washing the blanched duck breast meat for use;

(b) preparation of eight-treasure stuffs: taking sticky rice and soaking the sticky rice for 2 h to 3 h; removing plumules from lotus seeds and soaking the sticky lotus seeds for 2 h; steaming and dicing the duck breast meat that is obtained in the step (a); adding sesame oil to a frying pan and heating the oil, adding chopped green onion and chopped ginger, then adding the sticky rice, the lotus seeds, diced bamboo shoots, diced mushrooms, diced ham, red dates, diced chestnuts, myotonin, cooked pine nuts, diced cooked chicken, and the duck breast meat; adding spices, wherein a mass ratio of powdered pepper, chicken powder, and light soya sauce is 3:50:300; stir frying the spices with the nano preservative for 5 min, wherein the nano preservative is 0.2% of the weight of the stuffing heart; and placing the fried materials in a tray for cooling and use;

and wherein cooking the eight-treasure duck comprises:

(c) frying: turning back the whole duck skin that is obtained in the step (a), placing 250 g/1 kg of the eight-treasure stuffs and correctly reversing the whole duck, tying the duck in a gourd shape by using a straw rope, applying the light soya sauce to duck skin, and then placing the duck in an oil pot to fry the duck until the duck become golden; and (d) smouldering: placing the eight-treasure duck that is obtained in the step (c) in a pot having ginger and green onion at the bottom, adding water, 250 g light soya sauce, 100 g dark soya sauce, 500 g yellow wine, and 200 g crystal sugar; boiling the water with big fire; turning down the fire and smouldering the duck for 3 h; and taking out the duck, that is, obtaining the prepared eight-treasure duck.
5. The method for prolonging the shelf life of a prepared eight-treasure duck by utilizing combined flexible sterilization according to claim 5, wherein the prepared eight-treasure duck is vacuum packed, the size of the vacuum package is within 20 cm* 13 cmxlO cm, and the vacuum degree is 0.1 MPa.