Abstract
Part 1
To promote the application of sous-vide cooking in the production process of sweet duck, this study was based on the traditional cooking methods of sweet duck, combined with modern low-temperature cooking, and optimized its cooking method through single-factor experiments and orthogonal experiments. The optimal process parameters for low-temperature cooking of sweet duck were finally determined. The slow cooking temperature was 75°C, the slow cooking time 50 minutes, the frying temperature 150°C, and the frying time 1.0 minutes. This study not only provides a theoretical reference for cooking sweet duck at a low temperature, but also provides a reference for the application of low-temperature cooking in cooking other food.
Part 2
Sweet duck is a traditional delicacy in Leshan City, Sichuan Province. It is marinated and then fried. The sweet duck has a bright red color, tender meat, crispy and slightly sweet skin, and is very popular among consumers. The traditional sweet duck is heated and braised. This method can make it crispy, soft and palatable, but it will greatly damage the aroma and tenderness of the duck. Choosing a suitable cooking method to further improve the duck has become an urgent problem to be solved in manufacturing sweet duck. Sous-vide cooking is a relatively popular cooking method in recent years.
Part 3
It can preserve the taste of the ingredients to the greatest extent, without destroying the color and nutrients of the ingredients, minimize the weight loss of the food in the heating process, and improve the taste of the meat. This technology can effectively solve the problems existing in the production of traditional sweet duck. Applying the sous-vide cooking to the production of sweet duck to prepare a new type of sweet duck can not only enrich the variety of sweet duck, but also satisfy consumers’ demand for new tastes.
The sweet duck was the research object in this article. The traditional process characteristics were analyzed. The differences between the sweet duck produced by the traditional method and the sous-vide cooking were analyzed by combining the sous-vide cooking technology and adopting single-factor experiment and orthogonal experiment
1. Materials and methods
1.1 Ingredients
Raw duck and salad oil purchased from Wal-Mart, brine purchased from Leshan Fuluwei Luwei Processing Factory, crispy syrup prepared by us, magnesium oxide and hydrochloric acid made in China
1.2 Instruments and equipment
CT050P117W Low-temperature Slow Cooking Equipment, Vacuum Sealers, TG-16W-I Centrifuges, PWN125DZH Electronic scales, S8156X constant temperature water bath box, ZP100L electric fryers. Other equipment is provided by the Food Science Laboratory of Lingnan Normal University.
1.3 Steps of making sweet duck
1.3.1 Preparation of traditional sweet duck
Remove the internal organs of the raw duck; wash the whole duck, and put it into a centrifuge for centrifugal dehydration. Put the duck in the brine to cook for 50 minutes and then stop heating. Soak it for 1 hour, and drain it. Spray crispy syrup on the duck and dry it. Deep-fry the duck.
1.3.2 Preparation of Sous-vide sweet duck
Remove the internal organs of the raw duck. Wash the whole duck, and put it into a centrifuge for centrifugal dehydration. Put the duck and brine into a vacuum bag and seal them. Cook the duck at a low temperature. Spray the crispy syrup and let the duck dry. Deep-fry the duck.
1.4 Experimental Design
This study determined the basic process conditions based on the basic process conditions of the pre-experiment. The slow cooking temperature is 75℃, slow cooking time 55 minutes, frying temperature 145℃ and frying time 2 minutes. Single-factor experiment was carried out with sensory evaluation as an index. The variable values of each factor are as follows: the slow cooking temperatures are 65℃, 70℃, 75℃, 80℃ and 85℃, slow cooking time 45 minutes, 50 minutes, 55 minutes, 60 minutes and 65 minutes, frying temperatures135℃, 140℃, 145℃, 150℃ and 155℃, frying time 1.0 min, 1.5 minutes, 2.0 minutes, 2.5 minutes and 3.0 minutes. The specific level factors are shown in Table 1.
Table 1 Factor level tables
| Levels | A Slow cook temperatures/℃ | B Slow cook time/minutes | C Fry temperatures/℃ | D Fry time/minutes |
| 1 | 70 | 50 | 145 | 1.0 |
| 2 | 75 | 55 | 150 | 1.5 |
| 3 | 80 | 60 | 155 | 2.0 |
1.5 Sensory evaluations
| Sensory descriptions | Sensory evaluation criteria | Scores | |||
| Color (25%) | Aroma (25%) | Taste (25%) | Texture states (25%) | ||
| Excellent | Red and bright | Excellent natural salty and no peculiar smell | Good aftertaste | The meat is good with crispy skin. | 21 to 25 |
| Good | Light red but acceptable | Excellent natural salty and no peculiar smell | Good aftertaste | The meat is good with crispy skin. | 15 to 20 |
| Average | Light red with little other colors | Average natural salty and little peculiar smell | Average aftertaste | The meat is average with poor crispy skin. | 10 to 14 |
| Bad | Little red and uneven color | No natural salty and serious peculiar smell | Poor taste | The meat is rough without crispy skin. | 0 to 9 |
1.6 Determination of physical and chemical indicators
The weight of sweet duck was weighed before and after cooking, and the difference between before and after weight was divided by the weight before cooking to obtain the cooking loss rate; the determination of volatile base nitrogen (TVB-N) was based on the method of Tao Hu and others.1.7 Data processing
Data statistics and graphing were carried out through Excel 2010.
2. Results and analysis
2.1 Single factor experimental results and analysis
2.1.1 The effect of slow cooking temperature on sensory quality of sweet duck
Figure 1 The effect of slow-cooking temperature on sensory scores of sweet duck
2.1.2 The effect of slow cooking time on sensory quality of sweet duck
Figure 2 The effect of slow cooking time on sensory scores of sweet duck
2.1.3 The effect of frying temperature on sensory of sweet duck
After the sweet duck is slow-cooked, uniformly put syrup on it. The main purpose is to use the caramelization reaction of sugar. In the frying process, different oil temperatures will have a certain impact on the degree of caramelization reaction. In addition, the degree of caramelization reaction also affects the sweetness and crispness of the skin of sweet duck. Through the analysis in Figure 3, it was found that in the heating process, the sensory score has been on an upward trend, reaching the highest value at a temperature of 150°C, indicating that the sweet duck fried at this frying temperature has the best quality.
The evaluation at a temperature of 135℃ scored the lowest, mainly because it could not effectively promote the caramelization reaction, and it was difficult to form a bright red color and crispness for the skin of sweet duck. After a comprehensive analysis, the frying temperatures of 145 ℃, 150 ℃ and 155 ℃ were selected as the third level of orthogonal experiment.
Figure 3 The effect of the frying temperature on the sensory score of sweet duck
2.1.4 The effect of frying time on sensory quality of sweet duck
The frying time not only affects the brightness of the skin of the sweet duck, but also affects the texture of the sweet duck. If the time is too short, the caramelization reaction will be insufficient, resulting in a less expected color. Crispy skin is not easily formed for the duck. If the frying time is too long, the color is too dark, and the caramelization reaction is too much. There will be a bitter taste, and the surface will be burnt. Through the analysis in Figure 4, it is found that the sensory quality for frying for 1.5 minutes is the best. With the increase in frying time, the sensory score shows a downward trend. After a comprehensive analysis, 1.0 minutes, 1.5 minutes and 2.0 minutes are selected as the three levels of orthogonal experiment of this factor.
Figure 4 The effect of frying time on sensory scores of sweet duck
2.2 Orthogonal experimental results
Part 1
In the process of making sweet duck by sous-vide cooking, slow cooking temperature, slow-cooking time, frying temperatures and frying time are the key factors that affect the quality of the sous-vide sweet duck. Through the analysis of Table 3 and Table 4, it can be seen that the frying time has the greatest impact on the quality of sweet duck, mainly because frying is the final process, and the frying time directly affects the color and texture of sweet duck.
The slow-cooking temperature of the sous-vide cooking is the most direct factor affecting the quality of the duck, followed by the frying temperature and the slow-cooking time. From the orthogonal experiment results in Table 3 and the variance analysis in Table 4, it can be seen that the best technological combination of sous-vide cooking sweet duck is A2B1C2D1, and the sensory score is 97.23, that is, the slow-cooking temperature was 75°C, the slow-cooking time 50 minutes, the frying temperature 150°C, and the frying time 1 min.
Part 2
Table 3 Orthogonal experiment results
| Items | A Slow cooking temperature | B Slow cooking time | C Frying temperature | D Frying time | Sensory scores |
| 1 | 1 | 1 | 1 | 1 | 93.23 |
| 2 | 1 | 2 | 2 | 2 | 87.23 |
| 3 | 1 | 3 | 3 | 3 | 86.32 |
| 4 | 2 | 1 | 2 | 3 | 93.25 |
| 5 | 2 | 2 | 3 | 1 | 96.23 |
| 6 | 2 | 3 | 1 | 2 | 89.32 |
| 7 | 3 | 1 | 3 | 2 | 87.21 |
| 8 | 3 | 2 | 1 | 3 | 89.36 |
| 9 | 3 | 3 | 2 | 1 | 94.25 |
| K1 | 88.9267 | 91.2300 | 90.6367 | 94.5700 | |
| K2 | 92.93333 | 90.9400 | 91.5767 | 87.9200 | |
| K3 | 90.2733 | 89.9633 | 89.9200 | 89.6433 | |
| R | 4.0067 | 1.2667 | 1.6567 | 6.6500 |
| Variance sources | Deviation sum of squares | Degrees of freedom | Variance | F values | Significant levels |
| Slow cooking temperature | 24.9245 | 2 | 12.4622 | 7.334 | ** |
| Frying temperature | 4.1745 | 2 | 2.0872 | * | |
| Frying time | 71.4453 | 2 | 35.7227 | ** | |
| Error e | 2.6224 | 2 | 1.3112 | ||
| Correction error e | 6.7969 | 4 | 1.6992 | ||
| Sum | 103.167 |
2.3 The comparative analysis of the quality of sweet duck by low-temperature vacuum cooking and traditional methods
2.3.1 Comparative analysis of sensory evaluation
The sensory evaluation scores of sweet duck prepared by the two cooking methods were similar in taste, indicating that the two cooking methods had little influence on the taste of sweet duck. The aroma index of sweet duck prepared by sous-vide cooking is significantly lower than that of sweet duck prepared by traditional cooking, mainly because low-temperature cooking is not conducive to the formation of aroma. To sum up, the two cooking methods have their advantages. Sous-vide cooking can effectively improve the sensory quality of the finished product. It is slightly worse in aroma, but has little effect on the overall sensory quality.
Figure 5 Sensory evaluation results
2.3.2 The comparative analysis of cooking loss rates
The cooking loss rate mainly reflects the loss of moisture in meat in the cooking process. Through the analysis in Table 5, the cooking loss rate of sweet duck prepared by sous-vide cooking is significantly lower than that of sweet duck prepared by traditional cooking methods, which is mainly because the high-temperature heating of traditional cooking methods makes the duck contain a lot of moisture in the heating process and causes the duck to lose its moisture. Compared with traditional cooking methods, sous-vide cooking enhances the water keeping of the duck.
Table 5 Cooking loss rates
| Cooking methods | Cooking loss rates/% |
| Traditional cooking methods | 18.65±0.65 |
| Sous-vide cooking | 15.21±0.32 |
2.3.3 Determination of Volatile Base Nitrogen (TVB-N)
| Cooking methods | TVB-N values (mg/100 g) |
| Traditional cooking methods | 7.31±0.21 |
| Sous-vide cooking | 6.28±0.19 |
