Abstract Pasteurization, 116 and 121 ℃ sterilization were used in the development of Sichuan Industrial Huiguorou. The sensory, color, texture, lipid oxidation, preservation and flavor substance types and contents of the products were determined, and the effects of different sterilization methods on product characteristics were compared. The results showed that pasteurization was the best way to maintain the edible quality of products, but the storage period of products was very short, so the products could only be quickly distributed and consumed. Sterilization at 116 ℃ and sterilization at 121 ℃ could ensure the shelf life of Huiguorou under non-refrigeration conditions, and effectively solve the problem of long-term storage and transportation of Sichuan meat dishes. However, higher temperature had more or less adverse effect on the flavor of the product, and the product texture was too soft. It suggested that the better way was to sterilize at 116 ℃, which had less adverse effect on the flavor of the product, and could greatly prolong the non-refrigerated storability of the product, and increase the content of aldehydes and esters, so as to improve the flavor.
　　Key words Sichuan cuisine; Industrialization; Huiguorou; Sterilization process; Product characteristics; Shelf life
　　Received: April 29, 2021  Accepted: June 28, 2021
　　Supported by Open Project of Sichuan Key Laboratory of Culinary Science of Sichuan Tourism University (PRKX201905); Open Fund Project of Sichuan Key Laboratory of Meat Processing (20-R-11); Transformation of Scientific and Technological Achievements and Patent Implementation Project of Liangshan Prefecture (20CGZH0003).
　　Lili JI (1982-), female, P. R. China, associate professor, devoted to research about food science and engineering.
　　*Corresponding author. Wei WANG (1958-), male, P. R. China, professor, devoted to research about food science and engineering.
　　 Huiguorou is a traditional Sichuan dish, which is considered to be one of the most representative characteristics of Sichuan cuisine［1］. With the advancement of urbanization and the acceleration of the pace of modern life, the demand for flavor, nutrition and convenient food is increasing. The industrialization of Sichuan cuisine is in line with this market development trend, and its key technologies are quantitative instead of fuzzy, standard instead of personality, machinery instead of handicraft, automatic control instead of manual production, and continuous instead of intermittent processing［2］. 　　In the processing technology of industrialized Huiguorou, the sterilization of dishes is very important to the quality and shelf life of products. Sterilization methods include low temperature sterilization and high temperature sterilization. Low temperature sterilization can maintain the nutrients, taste and original flavor of dishes to a large extent, but the shelf life of products is not long, and most of them need cold chain transportation［3］. High temperature sterilization can greatly extend the shelf life of products, which is convenient for normal temperature storage and long-distance transportation, but it is easy to denature muscle protein, resulting in the decline of texture quality and flavor decay［4-5］. In the process of heat treatment, proper oxidation of fat and protein can also promote the formation of flavor compounds, such as aldehydes and nitrogenous compounds.
　　In this study, pasteurization, 116 ℃ sterilization and 121 ℃ sterilization methods were used to determine the sensory, color, texture, fat oxidation and preservation of the product in the processing of industrial Huiguorou. The effects of different sterilization methods on the quality characteristics of Huiguorou can provide reference for the quality of industrialized Huiguorou.
　　Materials and Methods
　　Huiguorou processing
　　The pork was cut into strips, 6 cm in width, and pre-cooked in boiling water for 24 min. The ratio of material to liquid was 1∶3. Then the pre-cooked strips were sliced, 3 mm thickness, and fried at 159 ℃ for 32 s. Three sterilization methods commonly used in the industrial production of dishes, pasteurization, 116 ℃ sterilization, and 121 ℃ high temperature sterilization, were selected to sterilize the cooked meat. The specific sterilization parameters are shown in Table 1.
　　Sensory measurement
　　According to GB 22210-2008［6］, the appearance, flavor, texture and taste of Huiguorou were evaluated by scoring method. The evaluation was conducted by 10 professional evaluation personnel, and the scoring standard is shown in Table 2.
　　Texture determination
　　According to the method of Yu Jing［7］, some modifications were made. The hardness and chewiness of the lean part of Huiguorou were determined by P/5S spherical probe. The descending speed of the probe is 1.00 mm/s before the test, 2.00 mm/s test the sample, 5.00 mm/s after the test, with 70% deformation and normal temperature.
　　Shear force measurement 　　The lean part of Huiguorou was made into 3.0 cm×2.0 cm×0.3 cm. According to the method of Tian［8］, the shear force was measured by texture analyzer.
　　Chromaticity value determination
　　CR-400 colorimeter was used to determine the chromaticity value of pork［9-10］. The probe of color difference meter was pressed vertically on the lean part of Huiguorou for determination. After the value was stable, the reading was repeated 3 times for each sample.
　　Moisture content
　　According to GB 5009.3-2016［11］ , a 1 g of the sample was accurately weighed, placed in a dryer with constant weight, and baked to constant weight in an oven at 105 ℃.
　　Peroxide value
　　Referring to GB 5009.227-2016［12］, a 5 g of sample was put into a titration cup of potentiometric titrator, added with 50 ml of isooctane glacial acetic acid mixture, and mixed well. Then, 0.5 ml of saturated potassium iodide solution was accurately added, and a magnetic stirrer was started. The mixture reacted for (60±1) s at an appropriate stirring speed. Next, 30-100 ml of water was immediately added into the titration cup, and inserted with an electrode and titration head. The titration parameters were set to run the titration program using the dynamic titration mode for titration, and the titration curve and point change were implemented. The dosage of sodium thiosulfate standard solution is controlled at 0.05-0.20 ml/drop. After reaching the end point of titration, record the volume V of standard solution consumed in titration. Control the volume of blank test at 0.005 ml/drop, record the volume of standard solution V0 after reaching the key point, and express the peroxide value with the mass fraction of peroxide equivalent to iodine, unit: g/100 g.
　　TBARS value
　　Referring to the method of Cao et al.［13］, a 4 g of minced meat sample was added with 20 ml of 7.5% trichloroacetic acid (containing 0.1% EDTA-Na2), homogenized in an ice bath for 15 s twice, leveled and centrifuged at 12 000×g for 5 min. Then, 2 ml of supernatant was add with 2 ml of 0.02 mol/L 2-thiobarbituric acid solution, then water bath at 90℃ for 40 min . The system was taken out and cooled with flowing water, and centrifuged at 1 000×g for 5 min. Next, 2 ml of chloroform was added to the supernatant, followed by shaking well and standing for stratification. After static stratification, the supernatant was taken and the absorbance was measured at 532 nm and TBARS value was calculated by the content of malondialdehyde in meat sample, unit: mg/kg. 　　Microbiological testing
　　According to GB 4789.2-2016［14］, the total bacterial count of the three groups of samples was determined. After treatment, the three groups of samples were diluted to the appropriate concentration, and the bacterial solution was evenly spread on the PCA plate, cultured at (37±1) ℃ for 48 h and counted.
　　Product shelf life
　　The sterilized meat samples were cultured in a constant temperature climate box with (37±1) ℃ and 60% humidity for 21 d［15］. The total bacterial count was detected every 3 d. When the total bacterial count exceeded the standard or the bags swelled, the experiment was stopped immediately.
　　Statistical analysis
　　Office 2016, spss25.0 and graphpad prism 8 were used to process, analyze and plot the test data. Values of P<0.05 were considered statistically significant.
　　Results and Discussion
　　Sensory
　　It can be seen from table 3 that the sterilization method had a significant effect on the sensory indexes of the meat (P<0.05).Through data analysis, compared with the control group, the appearance, meat quality and overall sensory score of the first group was 87.11±0.09, which was better than the second group and the third group. The odor and taste of the second group were 17.39±0.06 and 27.39±0.06, which were better than other two groups. Compared with groups 2 and 3, the appearance and texture of the first group were better, which indicated that pasteurization had less influence on appearance, smell, texture and taste, and was more conducive to maintaining the edible quality of the second group, which was consistent with the research results of Zhao et al.［16］. The formation of flavor and taste in the first group was not as good as that in the second group, but better than that in the third group. It might be due to the lower pasteurization temperature and the lower Maillard reaction intensity. When the sterilization temperature continued to rise to 121 ℃, the score of flavor and taste decreased again. It can be seen that the increase of sterilization temperature is conducive to the formation of fragrance and taste, but it has a decreasing effect when the temperature exceeds 116 ℃.
　　Chromaticity value
　　Fig. 1 shows the effect of sterilization methods on the color value of Huiguorou. The results show that sterilization caused significant changes in the color of the meat (P<0.05). Compared with the control group, the brightness of the first, second and third groups decreased by 8.49%, 9.06% and 9.08%, respectively. The brightness value of the first group was higher than that of the second and third groups, but the redness and yellowness values were slightly lower, indicating that the color of the second group became darker after sterilization. It had a great relationship with the sterilization temperature, high temperature caused a strong Maillard reaction, produced a lot of taste and aroma, and also deepened the color of the cooked meat. Too high temperature even led to dark color. According to the results of color detection, sterilization has a certain impact on the color, several sterilization methods will make the color darker, but overall, the brightness drop is less than 10%, only with the naked eye, no obvious difference. 　　Texture
　　Fig. 2 shows that there were significant differences in the shear force, hardness and chewiness of the Huiguorou under different sterilization methods (P<0.05). Compared with the control group, the shear force, hardness and chewiness of the sterilized pork decreased significantly. The shear force, hardness and chewiness of the sterilized pork in groups 1, 2 and 3 decreased by 35.35%, 35.73% and 38.61%, 22.32%, 32.19% and 36.44%, and 19.31%, 32.57% and 47.63%, respectively. The results showed that sterilization could result in the rupture of muscle fiber and the destruction of meat structure. Through the analysis of the data, it was found that the texture characteristics of the first group of products decreased the least in the sterilization group. It could be seen that among the several sterilization methods, pasteurization could better maintain the texture characteristics of meat followed by sterilization at 116 ℃ for 20 min, and sterilization at 121 ℃ for 20 min had the greatest impact on the texture.
　　Physical and chemical indicators
　　By analyzing table 4, it was found that different sterilization methods had significant effects on its moisture content, chloride content, TBARS and POV (P<0.05). Generally, high temperature can decrease the water holding capacity of meat, dehydrate meat products to a certain extent, and decrease the water content of products［17］. Therefore, compared with the control group, the moisture content of the experimental group decreased significantly,However, the higher the sterilization temperature, the less water content decreased. The chloride content of the second and third groups was significantly higher than that of the control group and the first group. It is because too high temperature sterilization leads to muscle fiber rupture, resulting in the increase of muscle tissue gap. The higher the temperature is, the larger the tissue gap is, the Huiguorou absorbs a large amount of soup in the seasoning during sterilization, resulting in the phenomenon that the higher the sterilization temperature is, the higher the water content and chloride content are. The POV of group 3 was higher, and TBARS was the lowest, which indicated that high temperature at 121 ℃ accelerated the oxidation rate of oil in Huiguorou, increased the content of peroxide, the reaction of secondary product malondialdehyde with amino group, and reduced the reaction substrate of TBA reagent, and TBARS began to decline［18］. 　　Agricultural Biotechnology2021
　　Shelf life
　　Table 5 shows the effect of different sterilization methods on the total number of bacterial colonies. According to GB2726-2016 Health Standard for Cooked Meat Products［19］, the total number of colonies in cooked meat products ≥ 104 CFU/g (4 lg CFU/g) was taken as the limit standard for the deterioration and putrefaction of Huiguorou. It can be seen from Table 5 that the total number of colonies in the control group exceeded the limit standard in GB 2726-2016 Hygienic Standard for Cooked Meat Products. After 18 d of storage at 37 ℃, the total number of colonies of the first group of Huiguorou exceeded the limit. After 21 d of storage, the total number of colonies in the second group was higher than the standard limit, while the number of colonies in the third group was lower than the standard limit 96 lg CEU/g, but not more than 4 lg CFU/g, within the national limit. The shelf life of pasteurization was 15 d, that of sterilization at 161 ℃ was 18 d, and that of sterilization at 121 ℃ was 21 d. These data show that sterilization can significantly slow down the growth rate of microorganisms, and the antibacterial effect of different sterilization methods is also significantly different. Several sterilization methods can inhibit the growth of microorganisms and prolong the shelf life of Huiguorou. The higher the sterilization temperature is, the longer the shelf life is. However, the selection of the best sterilization method should also take into account the product texture, physical and chemical indicators, sensory evaluation and so on.
　　Volatile flavor compounds
　　As shown in Fig. 3, after sterilization, the contents of some volatile flavor compounds were increased, especially malononitrile, limonene, linalool and aryl acetate, which also produced a large number of substances that were not found in the unsterilized group. As shown in Table 6, a total of 58 substances were detected in the three groups of sterilized pork. Aldehydes, hydrocarbons, alcohols and esters were the main flavor compounds of sterilized pork. In the first group, 36 flavor compounds were detected, including 10 aldehydes, 8 alkanes, 5 alcohols, 6 alkenes, 5 esters and 1 nitrogen, with a total content of 23 099.61 ng/g. In the second group, 44 flavor compounds were detected, including 9 aldehydes, 11 alkanes, 7 alcohols, 6 alkenes, 5 esters, 3 ketones and 3 nitrogenous compounds, with a total content of 35 083.43 ng/g. In the third group, 29 flavor compounds were detected, including 9 aldehydes, 3 alkanes, 6 alcohols, 5 alkenes, 3 esters, 2 ketones and 1 nitrogen, with a total content of 16 497.84 ng/g. Compared with the first group, the volatile flavor components of the second group increased by 8, while the third group decreased by 7. In contrast, 37 flavor compounds were detected in the control group, which indicated that sterilization at 116 ℃ for 25 min could enrich the volatile flavor components of the pork, and sterilization at 121 ℃ for 20 min could destroy the volatile flavor components of the pork, which is consistent with the research results of Zhang［20］. 　　 Compared with the control group, the change of aldehydes composition was small, but the content of aldehydes increased. Compared with group 1, the content of aldehydes in group 3 decreased by 26.81%. It indicated that sterilization at 121 ℃ for 20 min could reduce the type and content of aldehydes in the Huiguorou, which might be caused by the decrease of the content of hexanal, heptanal and nonanal under the action of high temperature and high pressure, and the decrease of hexanal content was the most obvious, which is consistent with the research results of Wang［21］. After sterilization, the content of alcohol increased significantly. Compared with the first group, the content of alcohols in the second group increased by 79.04%, while that in the third group decreased by 20.96%, indicating that sterilization at 116 ℃ for 25 min could promote the formation of alcohols. Compared with the control group, the types of esters in the first and second groups increased by 269.36% and 497.58%, respectively, while the types of esters in the third group decreased by 1, but the content of esters increased by 81.66%, indicating that sterilization at 121 ℃ for 30 min caused great damage to the composition of esters. The content of hydrocarbons increased significantly after sterilization. Although the threshold value of hydrocarbon compounds is high, the hydrocarbon compounds in the total content is relatively high, which has a certain effect on the flavor of Huiguorou.
　　After different ways of sterilization, the absolute content of flavor substances in instant Huiguorou was greatly improved, and the absolute total content of flavor substances in the three experimental groups was much higher than that in the control group. In different sterilization methods, the types and absolute contents of flavor substances in the two groups were significantly higher than those in other two groups, and the absolute contents of each type of flavor substances were significantly higher than those in group 1 and 2. It can be seen that sterilization at 116 ℃ for 25 min is the most favorable way for the formation of flavor compounds in instant Huiguorou.
　　Conclusion
　　Three kinds of sterilization processes were used to study the effects of different sterilization methods on the product characteristics of the industrial Huiguorou. The results showed that pasteurization was the best way to maintain the quality of food products, but the product storage period was very short, only 15 d, only for the fast distribution and circulation consumption. The shelf life of the two high-temperature sterilization methods of 116 ℃ and 121 ℃ reached 18 and 21 d, respectively, which could effectively solve the problem of long-term storage, transportation and circulation of Sichuan meat dishes. However, higher temperature had more or less adverse effects on the flavor of the product, especially the sterilization at 121 ℃ caused the product to have overheated taste similar to lunch meat, and the texture of the product was too soft. In the three processes, the best way was sterilization at 116 ℃. Compared with sterilization at 121 ℃, sterilization at 116 ℃ had less adverse effect on the flavor of the product, and could greatly extend the non refrigeration storability of the product. Compared with pasteurization, it could also significantly increase the content of aldehydes and esters, which could help to the improvement of its flavor. 　　References
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