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Feasibility assessment of vacuum cooling followed by immersion vacuum cooling on water-cooked pork
Meat Science 90 (2012) 199–203
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Meat Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m e a t s c i
Feasibility assessment of vacuum cooling followed by immersion vacuum cooling on water-cooked pork Xiaoguang Dong, Hui Chen, Yi Liu, Ruitong Dai, Xingmin Li ⁎ College of Food Science and Nutritional Engineering, China Agricultural University, No. 17 Qinghua East Road, Beijing 10083, China
a r t i c l e
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Article history: Received 3 March 2011 Received in revised form 4 May 2011 Accepted 1 July 2011 Keywords: Immersion vacuum cooling Vacuum cooling Pork Combined cooling
a b s t r a c t Vacuum cooling followed by immersion vacuum cooling was designed to cool water-cooked pork (1.5± 0.05 kg) compared with air blast cooling (4± 0.5 °C, 2 m/s), vacuum cooling (10 mbar) and immersion vacuum cooling. This combined cooling method was: vacuum cooling to an intermediate temperature of 25 °C and then immersion vacuum cooling with water of 10 °C to the final temperature of 10 °C. It was found that the cooling loss of this combined cooling method was significantly lower (P b 0.05) than those of air blast cooling and vacuum cooling. This combined cooling was faster (P b 0.05) than air blast cooling and immersion vacuum cooling in terms of cooling rate. Moreover, the pork cooled by combined cooling method had significant differences (P b 0.05) in water content, color and shear force. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Cooked meats are a significant segment of the meat industry to be used in ready-meals or as an ingredient in meat-based food products. Although there are many commercial cooling methods being extensively used, such as air blast cooling, water immersion cooling and slow chilling, techniques for obtaining a fast cooling method to satisfy safety requirements have been of great importance to the meat industry. Thus, some new cooling methods like vacuum cooling and immersion vacuum cooling have been studied. Vacuum cooling (Huber & Laurindo, 2006; McDonald & Sun, 2000; Wang & Sun, 2001, 2002) has been studied. The principle of vacuum cooling process was based on the evaporation of water contained in cooked products under vacuum condition. This process enables cooked food products such as meat products to be chilled in an extremely short period of time (Sun & Zheng, 2006; Zheng & Sun, 2004). However, the water evaporating from cooked food represents the undesired mass loss. Moreover, since most products are sold by weight, it means less profit for manufacturers. For large meat products, high moisture loss during vacuum cooling would lead to a decrease in tenderness and juiciness (Desmond, Kenny, Ward, & Sun, 2000; McDonald, Sun, & Kenny, 2000). Various methods have been reported to reduce or compensate the moisture loss, such as adjusting injection level (Desmond, Kenny, & Ward, 2002; McDonald, Sun, & Kenny, 2001; McDonald & Sun, 2001b), adjusting the evacuation rate (Huber &
⁎ Corresponding author. Tel.: + 86 010 62737547. E-mail address: [email protected] (X. Li). 0309-1740/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.meatsci.2011.07.002
Laurindo, 2005; McDonald & Sun, 2001a) and cooling the meat in soup (Houska, Sun, Landfeld, & Zhang, 2003). Among these methods, cooling beef in soup obtained the best result with no weight loss. Instead, it was found that the mass of meat cooled in soup increased by 7.7% due to water penetration at the end of the vacuum cooling. And immersion vacuum cooling was studied and the results showed that immersion vacuum cooling can substantially reduce the weight loss and improve the quality of the cooled products (Cheng & Sun, 2006a, 2006b; Schmidt, Aragão, & Laurindo, 2010). Houska, Landfeld, and Sun (2005) found pork and beef ripened in the brine and cooled by immersion vacuum cooling would have a better quality. To prevent the solution from boiling over during immersion vacuum cooling, the rate of pressure decrease was adjusted following the curve of saturated steam and according to the temperature of the cooking solution. Therefore, a bleed valve was needed to regulate the pressure during immersion vacuum cooling (Cheng & Sun, 2006a). During immersion vacuum cooling, the thermal conduction played a more important role than water evaporation for most of the time. Because the thermal conductivity of meat was low, the cooling rate of immersion vacuum cooling was smaller compared to that of vacuum cooling. As Drummond, Sun, Vila, and Scannell (2009) showed in his experiment, the cooling time of immersion vacuum cooling was longer than that of vacuum cooling. And as the way of heat release, the size of the meat cooled by immersion vacuum cooling had a great influence on the cooling time. Drummond and Sun (2008) illustrated that the size of the meat significantly affected the cooling rate and meat product larger than the sample (4 kg) in their study would take more time to cool down which may no longer comply with safety guidelines. Moreover, as the remaining hot solution after cooking was placed together with cooked meat into the vacuum chamber, the cooling load was increased (Cheng & Sun, 2006a).
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The study conducted by Cheng, Sun, and Scannell (2005) showed that water cooking compared with dry air cooking and wet air cooking would process pork ham with high yield and compatible nutritional and textural qualities. Drummond and Sun (2006) also illustrated beef samples cooked in water were significantly tender and had higher water holding capacity values than dry heat oven cooked samples. So water cooking could be an advantageous method combined with cooling to render safety and high quality meat products. Based on the above, it was proposed that it would be possible to design a combined cooling method of vacuum cooling followed by immersion vacuum cooling for the water-cooked pork. This combined cooling method involved the extremely rapid temperature drop in the beginning of vacuum cooling to an intermediate temperature and then immersion vacuum cooling to compensate the cooling loss of vacuum cooling until the final temperature. This study was to assess the feasibility of the vacuum cooling followed by immersion vacuum cooling (VC–IVC), compared to air blast cooling (AB), vacuum cooling (VC) and immersion vacuum cooling (IVC).
cooking was cooled with ice water bath to 10 °C. After vacuum cooling to 25 °C, the cooking solution (10 °C) was injected into the container and sufficiently covered the sample. Then the sample was cooled with immersion vacuum cooling till core temperature reached 10 °C. The vacuum cooler (model WBN-5, Beinuo Machinery Co., Ltd., Wenzhou, China) used was specially designed for research. It consists of a cylindrical metal chamber with door (free volume of chamber was about 0.12 m3). The equipment is completed with a condenser and a vacuum pump (model 2ZX(S), China) with a pumping rate of 14.4 m 3/h. The vacuum cooler was run to cool the vacuum chamber before cooking was completed. During all the cooling process, pork temperature and weight of the pork samples after cooling were recorded. The core temperature was recorded using 3 K-type thermocouples (model DM6801B, Tondaj Instruments and Meters Co., China) inserted into an appropriate position around the geometric center of the samples to record the temperature of the samples during cooking and cooling process. 2.4. Cooking and cooling process parameters
2. Materials and methods 2.1. Sample preparation The ellipsoid-shaped pork samples were cut from hindquarters (M. semimembranosus group) which were purchased from a local butcher (Fifth meat factory, Beijing, China). The pork samples were free of skin, visible fat and connective tissue. The pH of the meat was measured at three different points using a pH probe (pH211, HANNA instruments Inc., USA). Only muscles with pH between 5.70 and 6.10 and with weights of 1.5 ± 0.05 kg were used. Finally the pork samples were vacuum packed and stored at 4 ± 1 °C for aging before cooking (duration time was about 2 days).
The parameters determined for each process were cooking loss, cooling loss, cooling rate and the mean temperature reduction per unit of percentage weight loss (ηT). The cooking loss was considered to be the percentage weight loss between the uncooked and the cooked sample. The cooling loss was the percentage weight loss between the cooked and the cooked–cooled sample. The yield (%) was calculated as the weight after cooling divided by the weight of raw sample. The ηT was calculated as the quotient between the core temperature reduction during the cooling stage (ΔT) and the cooling loss. The cooling rate was the core temperature reduction during the cooling stage (ΔT) divided by the cooling time. 2.5. Water content
2.2. Sample cooking Before cooking, the vacuum bags were removed from the samples. All pork samples were cooked in 2% salt solution bath (Model DK-8B, Jing Hong Laboratory Instrument, China) to core temperature of 72 °C. Additionally, the weights of the pork samples before and after cooking were recorded. 2.3. Sample cooling For air blast cooling (AB), immediately after the cooked sample was weighed, it was placed in a laboratory scale air-blast cooler (model PRx-350, Ningbo Haishu Safe Experimental Instrument Factory, China) with 0.35 m 3 chamber capacity at an air temperature of 4 ± 0.5 °C and air velocity 2 m/s. Then the sample was cooled with air blast cooling till core temperature reached 10 °C. For vacuum cooling (VC), the cooked pork sample was placed on a perforated stainless steel table in the vacuum chamber. After the equipment door was closed, the pressure of the chamber was allowed to drop to the working pressure of 10 mbar. For the immersion vacuum cooling (IVC), samples were transferred into a container, together with the hot salt solution from cooking enough to cover the sample. Samples cooled with the solution from 72 °C to 10 °C of core temperature under the controlled pressure in the vacuum chamber. To avoid spilling of the water from the container during cooling, the chamber pressure was controlled precisely, according to saturated steam pressure (Cheng & Sun, 2006a), to reach the final working pressure of 10 mbar. For vacuum cooling followed by immersion vacuum cooling (VC– IVC), immediately after the cooked sample was weighed, it was transferred into the container of vacuum chamber. The pork sample was vacuum cooled until the thermocouples read the target intermediate temperature of 25 °C, in the meantime, the hot salt solution from
The water content of the cooled pork was measured in triplicate by oven drying at 104 °C, 24 h (AOAC, 1980). 2.6. Warner Bratzler shear (WBS) Strips (10 mm × 10 mm × 30 mm) were cut parallel to the muscle fiber orientation from the cooled pork samples. Strips were sheared at room temperature using the Warner-Bratzler shear device (Model WARNER-BRATZLER, G-R Manufacturing Co., USA) at the shear head speed of 4 mm/s. 2.7. Texture profile analysis (TPA) Samples were cut parallel to the longitudinal orientation of the muscle fibers into 20 mm × 20 mm × 20 mm size and examined by textural instrument (Model TMS-Pro, Food Technology Corporation, USA). Two cycles of 6 mm target distance were applied to the samples using a 60 mm circular flat disk attached to the textural instrument at cross-head speed 30 mm/min. The outputs from TPA included hardness, gumminess, springiness, chewiness, and adhesiveness. 2.8. Color The color of cooled pork was measured by the CIE L*a*b* (L*— lightness, a*—red/green and b*—yellow/blue) system using a reflectance colorimeter (WSC-S, Shanghai Precision & Scientific Instrument Co., Ltd., China). 2.9. Statistical analysis All experiments were performed three times in triplicates. All data was analyzed by one-way analysis of variance (ANOVA) using SPSS
X. Dong et al. / Meat Science 90 (2012) 199–203
Temperature (°C)
Table 2 Effect of different cooling methods on color and water content of the water-cooked pork.
AB VC VC-IVC IVC
70 60 50 40 30 20
50
100
150
200
250
300
Cooling methods
Color L*
a*
b*
Water content (%)
AB VC IVC VC–IVC
69.29 ± 0.29c 67.25 ± 0.39d 70.10 ± 0.38b 72.45 ± 0.36a
4.45 ± 0.10b 5.14 ± 0.15a 4.30 ± 0.18b 3.04 ± 0.11c
16.34 ± 0.12b 16.67 ± 0.23a 16.14 ± 0.06b 15.60 ± 0.20c
67.03 ± 0.24c 64.05 ± 0.17d 71.44 ± 0.36a 69.67 ± 0.59b
Each value is the mean ± standard deviations of three replicate analyses. Values in the same column with different letters are significantly different (P b 0.05), n = 9. AB, air blast cooling; VC, vacuum cooling; IVC, immersion vacuum cooling; VC–IVC, vacuum cooling followed by immersion vacuum cooling.
10 0
201
350
Time (min) The cooking and cooling process parameters are summarized in Table 1, which indicated that there was no significant difference (P N 0.05) about the cooking loss and the core temperature reduction during the cooling stage (ΔT) among different cooling methods. However, the cooling loss, cooling rate and ηT were all significantly affected by the cooling methods (P b 0.05), which can be attributed to the different heat transfer mechanisms of different cooling methods. During AB, the heat is transferred from the core of the sample to the surface by conduction and then from the surface to the environment by convection (Sun & Wang, 2000). However, the VC, which enabled meat to be cooled faster (Desmond et al., 2000; McDonald et al., 2000), is mainly based on the evaporation of water. Many studies (McDonald et al., 2000; McDonald & Sun, 2001a) showed that the evaporation of water represented the undesired mass loss. During IVC, water evaporation from the meat surface and pores was limited because of the solution surrounding meat. Thus, thermal conduction was the main way to release heat which explained that the cooling rates of AB and IVC are smaller (P b 0.05) in comparison to VC because of low thermal conductivity of meat. It was also observed that immersion vacuum cooling obviously reduced the weight loss while increased the cooling time (Cheng & Sun, 2006b; Drummond et al., 2009; Schmidt et al., 2010). The VC–IVC initially cooled the meat in vacuum cooling at a high cooling rate to the intermediate temperature but leaded high cooling loss. Then immersion vacuum cooling compensates the cooling loss with lower cooling rate. Therefore, the cooling loss of VC–IVC (6.69%) was significantly lower (P b 0.05) than those of the AB (7.28%) and VC (11.82%). The cooling rate of VC–IVC was significantly higher (P b 0.05) than those of AB and IVC, as shown in Table 1. The ηT value was used to evaluate the process efficiency. The process VC–IVC presented a significantly higher (P b 0.05) ηT value (ηT = 9.26 °C/1%) than those of AB (ηT = 8.46 °C/1%) and VC (ηT = 5.24 °C/1%), but still kind of lower (P b 0.05) than that of IVC (ηT = 9.49 °C/1%) which had the lowest cooling loss. And the comparison of ηT between VC and IVC coincided with the results showed by Schmidt et al. (2010) in which the value of IVC (ηT = 22.2 °C/1%) was significantly higher than that of VC (ηT = 8.0 °C/1%) for the chicken breast with the same previous cooking. The rate of VC obviously decreased at the point of 25 °C, so the intermediate temperature was set at 25 °C. But it was still not approved whether 25 °C was the ideal operating point that balances the
Fig. 1. Temperature profile of the water-cooked pork with four different cooling methods.AB, air blast cooling; VC, vacuum cooling; IVC, immersion vacuum cooling; VC–IVC, vacuum cooling followed by immersion vacuum cooling.
15.0 (Statistical Package for the Social Sciences). Duncan's multiplerange tests were used to compare the significant differences among the mean values, and differences at P b 0.05 were considered to be statistically significant. Data are reported as the mean ± standard deviations of three experiments performed in triplicate.
3. Results and discussion 3.1. Cooking and cooling Time-temperature profiles of four different cooling processes (AB, VC, IVC and VC–IVC) are presented in Fig. 1. The results obtained in the replicates of each process were similar, so for simplicity and better data visualization, the results of only one experimental run are shown in Fig. 1. From Fig. 1, the shapes of the curves of AB were distinctively different from those of VC, IVC and VC–IVC, which showed that the cooling rate of AB was significantly lower (P b 0.05) than those of VC, IVC and VC–IVC. One can observe that during the initial step of the VC– IVC, the pork was cooled as fast as VC. However, after reaching the intermediate temperature of 25 °C, which was immersion vacuum cooling section, the cooling rate of pork was slower than that of VC. The cooling time of VC–IVC was 85 min from 72 °C to 10 °C, significantly shorter than those of AB (346 min) and IVC (143 min) as indicated in Fig. 1, which was in agreement with the cooling rate (Table 1). For the purpose of food safety, the temperature zone ranging from 72 °C to 10 °C is very dangerous, for the surviving organism after cooking can readily multiply. So the meat should be cooled fast to let the temperature go through the danger zone quickly. Just as Cheng and Sun (2006a) showed the governments of many European countries have made strict guidelines for the cooling process to ensure the safety of cooked meat products. The results indicated that the VC–IVC was a better method to satisfy the safety requirements than AB and IVC because of the higher cooling rate (P b 0.05).
Table 1 Process parameters for the different cooling methods of the water-cooked pork. Cooling methods AB VC IVC VC–IVC
Cooking loss (%) a
23.10 ± 0.61 23.23 ± 0.25a 22.97 ± 0.45a 23.07 ± 0.25a
Cooling loss (%) b
7.28 ± 0.04 11.82 ± 0.03a 6.54 ± 0.06d 6.69 ± 0.03c
ΔT (cooling stage) (°C)
Yield (%) b
69.62 ± 0.64 64.95 ± 0.22c 70.49 ± 0.48a 70.24 ± 0.28ab
a
61.60 ± 0.36 62.2 ± 0.26a 62.07 ± 0.32a 61.9 ± 0.21a
ηT (°C/1%)
Cooling rate (°C/min) c
8.46 ± 0.10 5.24 ± 0.01d 9.49 ± 0.06a 9.26 ± 0.01b
0.18 ± 0.001d 1.27 ± 0.005a 0.44 ± 0.002c 0.73 ± 0.002b
Each value is the mean ± standard deviations of three replicate analyses. Values in the same column with different letters are significantly different (P b 0.05), n = 9. AB, air blast cooling; VC, vacuum cooling; IVC, immersion vacuum cooling; VC–IVC, vacuum cooling followed by immersion vacuum cooling.
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Table 3 Effect of different cooling methods on instrumental properties of the water-cooked pork. Cooling methods
WBS (kg)
Ha (N)
Gu (N)
Sp
Ch (N)
Ad (N·sec)
AB VC IVC VC–IVC
0.79 ± 0.09c 1.28 ± 0.08a 0.88 ± 0.12c 1.11 ± 0.04b
19.63 ± 3.72a 24.38 ± 0.12a 22.05 ± 4.27a 22.96 ± 2.27a
12.95 ± 2.51a 15.98 ± 0.21a 15.15 ± 2.98a 14.36 ± 0.59a
1.83 ± 0.06a 1.81 ± 0.01a 1.84 ± 0.02a 1.87 ± 0.01a
23.81 ± 5.11a 28.91 ± 0.45a 27.96 ± 5.44a 26.78 ± 0.92a
2.69 ± 2.61a 1.06 ± 1.11a 2.02 ± 1.90a 1.80 ± 0.41a
Each value is the mean ± standard deviations of three replicate analyses. Values in the same column with different letters are significantly different (P b 0.05), n = 9. AB, air blast cooling; VC, vacuum cooling; IVC, immersion vacuum cooling; VC–IVC, vacuum cooling followed by immersion vacuum cooling. WBS, Warner-Bratzler shear force; Ha, hardness; Gu, gumminess; Sp, springiness; Ch, chewiness; Ad, adhesiveness.
competing criteria of reducing cooling loss and cooling time. Further research over a wider range of temperatures is required to identify such a point.
hardness and cohesiveness had no significant difference (P N 0.05) between the water-cooked beef joints cooled by AB, VC and IVC. 4. Conclusions
3.2. Color and water content analysis The measurement of color given in Table 2 indicated that the cooling method affected the color of the water-cooked pork. Vacuum cooled pork was found to be slightly darker (P b 0.05), with a lower L* value than those of AB, IVC and VC–IVC (P b 0.05). The value of a* of pork cooled by VC was significantly higher (P b 0.05) than that of AB, IVC and VC–IVC, which is a further proof to show that high a* value for pork with VC was due to the concentration of pigments caused by high cooling loss (Desmond et al., 2002; McDonald et al., 2001; Zheng & Sun, 2004). The relationship of color between samples cooled by AB, VC and IVC has the coincidence with values of water content (Table 2). However, VC–IVC had higher L* and lower a* (P b 0.05) than AB, VC and IVC, which did not coincide with the water content. Moisture loss might cause the pork darker because it would increase adhesion of muscles fibers, concentrate the pork pigments and increase light penetration. However, migration of water would lead solutes with color pigments to the surface and has a hypochromic effect. What's more, the removal of water would increase separation between muscle fibers and thus has a hypochromic effect (McDonald et al., 2000). In VC–IVC, the water evaporated in the vacuum cooling section, and in immersion vacuum cooling section the water in the soup might penetrate to the meat pores (Houska et al., 2003). And the complicated movement of water might change the microstructure of the meat. Thus to accurately explain the color of the pork cooled by VC– IVC, further research on the water movement and microstructure is required. 3.3. TPA and WBS analysis Warner-Bratzler shear (WBS) force values are a measurement of the physical force necessary to cut through a standardized piece of the pork sample. From the results presented in Table 3, shear force values were highest (P b 0.05) for vacuum cooled samples with a mean value of 1.28 kg, indicating that vacuum cooled samples were less tender. Cheng and Sun (2006a) showed that vacuum cooled samples had higher WBS values which were supposed to be associated with the lower water content. Water loss and compression of muscle fibers were believed to be responsible for the increased shear force during vacuum cooling (Desmond et al., 2000; McDonald et al., 2000). Therefore, it was required to decrease water loss because high water content would be helpful for improving the tenderness of meat products. The method of VC–IVC decreased water loss and got more tender products with lower WBS value than those from VC, but still higher than those of AB and IVC. There were no significant differences (P N 0.05) between samples cooled by the four cooling methods in terms of TPA, although the values of VC–IVC had a tendency to be lower compared with those of VC. Palka and Daun (1999) found that the changes of microstructure and moisture had a close relationship with parameters of TPA. Drummond et al. (2009) also showed that
The combined cooling method of vacuum cooling followed by immersion vacuum cooling (VC–IVC) has been studied. The feasibility of this combined cooling method was evaluated in terms of the process parameters, water content and physical properties compared with air blast cooling (AB), vacuum cooling (VC) and immersion vacuum cooling (IVC). The results showed that VC–IVC could compensate the cooling loss of VC within less cooling time than IVC (P b 0.05). And ηT value of VC–IVC was bigger than those of AB and VC (P b 0.05). The pork with VC–IVC had higher L* and lower a* (P b 0.05) than those of AB, VC and IVC. The pork with VC–IVC obtained improvements in the physical attributes, such as lower shear force value (P b 0.05). The results indicated that VC–IVC could be an effective method in getting lower cooling loss (P b 0.05) than those of AB and VC and higher cooling rate (P b 0.05) compared with those of AB and IVC. The lower water content and higher shear force of VC had bad effect on the products, VC–IVC could compensate these inferiorities of products caused by VC. Acknowledgment We gratefully acknowledge the financial support received in the form of research grant (Project No. 200903012) from Ministry of Agriculture of China. References AOAC (1980). Official methods of analysis. Washington: Association of Official Analytical Chemists. Cheng, Q., & Sun, D. -W. (2006). Feasibility assessment of vacuum cooling of cooked pork ham with water compared to that without water and with air blast cooling. International Journal of Food Science and Technology, 41(8), 938–945. Cheng, Q., & Sun, D. -W. (2006). Improving the quality of pork ham by pulsed vacuum cooling in water. Journal of Food Process Engineering, 29(2), 119–133. Cheng, Q., Sun, D. -W., & Scannell, A. G. M. (2005). Feasibility of water cooking for pork ham processing as compared with traditional dry and wet air cooking methods. Journal of Food Engineering, 67(4), 427–433. Desmond, E. M., Kenny, T. A., & Ward, P. (2002). The effect of injection level and cooling method on the quality of cooked ham joints. Meat Science, 60, 271–277. Desmond, E. M., Kenny, T. A., Ward, P., & Sun, D. -W. (2000). Effect of rapid and conventional cooling methods on the quality of cooked ham joints. Meat Science, 56(3), 271–277. Drummond, L. S., & Sun, D. -W. (2006). Feasibility of water immersion cooking of beef joints: Effect on product quality and yield. Journal of Food Engineering, 77(2), 289–294. Drummond, L., & Sun, D. -W. (2008). Immersion vacuum cooling of cooked beef — Safety and process considerations regarding beef joint size. Meat Science, 80(3), 738–743. Drummond, L., Sun, D. -W., Vila, C. T., & Scannell, A. G. M. (2009). Application of immersion vacuum cooling to water-cooked beef joints — Quality and safety assessment. LWT—Food Science and Technology, 42, 332–337. Houska, M., Landfeld, A., & Sun, D. -W. (2005). Eating quality enhancement of cooked pork and beef by ripening in brine and vacuum cooling. Journal of Food Engineering, 68(3), 357–362. Houska, M., Sun, D. -W., Landfeld, A., & Zhang, Z. (2003). Experimental study of vacuum cooling of cooked beef in soup. Journal of Food Engineering, 59, 105–110. Huber, E., & Laurindo, J. B. (2005). Weight loss of precooked chicken breast cooled by vacuum application. Journal of Food Process Engineering, 28(3), 299–312.
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Meat Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m e a t s c i
Feasibility assessment of vacuum cooling followed by immersion vacuum cooling on water-cooked pork Xiaoguang Dong, Hui Chen, Yi Liu, Ruitong Dai, Xingmin Li ⁎ College of Food Science and Nutritional Engineering, China Agricultural University, No. 17 Qinghua East Road, Beijing 10083, China
a r t i c l e
i n f o
Article history: Received 3 March 2011 Received in revised form 4 May 2011 Accepted 1 July 2011 Keywords: Immersion vacuum cooling Vacuum cooling Pork Combined cooling
a b s t r a c t Vacuum cooling followed by immersion vacuum cooling was designed to cool water-cooked pork (1.5± 0.05 kg) compared with air blast cooling (4± 0.5 °C, 2 m/s), vacuum cooling (10 mbar) and immersion vacuum cooling. This combined cooling method was: vacuum cooling to an intermediate temperature of 25 °C and then immersion vacuum cooling with water of 10 °C to the final temperature of 10 °C. It was found that the cooling loss of this combined cooling method was significantly lower (P b 0.05) than those of air blast cooling and vacuum cooling. This combined cooling was faster (P b 0.05) than air blast cooling and immersion vacuum cooling in terms of cooling rate. Moreover, the pork cooled by combined cooling method had significant differences (P b 0.05) in water content, color and shear force. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Cooked meats are a significant segment of the meat industry to be used in ready-meals or as an ingredient in meat-based food products. Although there are many commercial cooling methods being extensively used, such as air blast cooling, water immersion cooling and slow chilling, techniques for obtaining a fast cooling method to satisfy safety requirements have been of great importance to the meat industry. Thus, some new cooling methods like vacuum cooling and immersion vacuum cooling have been studied. Vacuum cooling (Huber & Laurindo, 2006; McDonald & Sun, 2000; Wang & Sun, 2001, 2002) has been studied. The principle of vacuum cooling process was based on the evaporation of water contained in cooked products under vacuum condition. This process enables cooked food products such as meat products to be chilled in an extremely short period of time (Sun & Zheng, 2006; Zheng & Sun, 2004). However, the water evaporating from cooked food represents the undesired mass loss. Moreover, since most products are sold by weight, it means less profit for manufacturers. For large meat products, high moisture loss during vacuum cooling would lead to a decrease in tenderness and juiciness (Desmond, Kenny, Ward, & Sun, 2000; McDonald, Sun, & Kenny, 2000). Various methods have been reported to reduce or compensate the moisture loss, such as adjusting injection level (Desmond, Kenny, & Ward, 2002; McDonald, Sun, & Kenny, 2001; McDonald & Sun, 2001b), adjusting the evacuation rate (Huber &
⁎ Corresponding author. Tel.: + 86 010 62737547. E-mail address: [email protected] (X. Li). 0309-1740/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.meatsci.2011.07.002
Laurindo, 2005; McDonald & Sun, 2001a) and cooling the meat in soup (Houska, Sun, Landfeld, & Zhang, 2003). Among these methods, cooling beef in soup obtained the best result with no weight loss. Instead, it was found that the mass of meat cooled in soup increased by 7.7% due to water penetration at the end of the vacuum cooling. And immersion vacuum cooling was studied and the results showed that immersion vacuum cooling can substantially reduce the weight loss and improve the quality of the cooled products (Cheng & Sun, 2006a, 2006b; Schmidt, Aragão, & Laurindo, 2010). Houska, Landfeld, and Sun (2005) found pork and beef ripened in the brine and cooled by immersion vacuum cooling would have a better quality. To prevent the solution from boiling over during immersion vacuum cooling, the rate of pressure decrease was adjusted following the curve of saturated steam and according to the temperature of the cooking solution. Therefore, a bleed valve was needed to regulate the pressure during immersion vacuum cooling (Cheng & Sun, 2006a). During immersion vacuum cooling, the thermal conduction played a more important role than water evaporation for most of the time. Because the thermal conductivity of meat was low, the cooling rate of immersion vacuum cooling was smaller compared to that of vacuum cooling. As Drummond, Sun, Vila, and Scannell (2009) showed in his experiment, the cooling time of immersion vacuum cooling was longer than that of vacuum cooling. And as the way of heat release, the size of the meat cooled by immersion vacuum cooling had a great influence on the cooling time. Drummond and Sun (2008) illustrated that the size of the meat significantly affected the cooling rate and meat product larger than the sample (4 kg) in their study would take more time to cool down which may no longer comply with safety guidelines. Moreover, as the remaining hot solution after cooking was placed together with cooked meat into the vacuum chamber, the cooling load was increased (Cheng & Sun, 2006a).
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The study conducted by Cheng, Sun, and Scannell (2005) showed that water cooking compared with dry air cooking and wet air cooking would process pork ham with high yield and compatible nutritional and textural qualities. Drummond and Sun (2006) also illustrated beef samples cooked in water were significantly tender and had higher water holding capacity values than dry heat oven cooked samples. So water cooking could be an advantageous method combined with cooling to render safety and high quality meat products. Based on the above, it was proposed that it would be possible to design a combined cooling method of vacuum cooling followed by immersion vacuum cooling for the water-cooked pork. This combined cooling method involved the extremely rapid temperature drop in the beginning of vacuum cooling to an intermediate temperature and then immersion vacuum cooling to compensate the cooling loss of vacuum cooling until the final temperature. This study was to assess the feasibility of the vacuum cooling followed by immersion vacuum cooling (VC–IVC), compared to air blast cooling (AB), vacuum cooling (VC) and immersion vacuum cooling (IVC).
cooking was cooled with ice water bath to 10 °C. After vacuum cooling to 25 °C, the cooking solution (10 °C) was injected into the container and sufficiently covered the sample. Then the sample was cooled with immersion vacuum cooling till core temperature reached 10 °C. The vacuum cooler (model WBN-5, Beinuo Machinery Co., Ltd., Wenzhou, China) used was specially designed for research. It consists of a cylindrical metal chamber with door (free volume of chamber was about 0.12 m3). The equipment is completed with a condenser and a vacuum pump (model 2ZX(S), China) with a pumping rate of 14.4 m 3/h. The vacuum cooler was run to cool the vacuum chamber before cooking was completed. During all the cooling process, pork temperature and weight of the pork samples after cooling were recorded. The core temperature was recorded using 3 K-type thermocouples (model DM6801B, Tondaj Instruments and Meters Co., China) inserted into an appropriate position around the geometric center of the samples to record the temperature of the samples during cooking and cooling process. 2.4. Cooking and cooling process parameters
2. Materials and methods 2.1. Sample preparation The ellipsoid-shaped pork samples were cut from hindquarters (M. semimembranosus group) which were purchased from a local butcher (Fifth meat factory, Beijing, China). The pork samples were free of skin, visible fat and connective tissue. The pH of the meat was measured at three different points using a pH probe (pH211, HANNA instruments Inc., USA). Only muscles with pH between 5.70 and 6.10 and with weights of 1.5 ± 0.05 kg were used. Finally the pork samples were vacuum packed and stored at 4 ± 1 °C for aging before cooking (duration time was about 2 days).
The parameters determined for each process were cooking loss, cooling loss, cooling rate and the mean temperature reduction per unit of percentage weight loss (ηT). The cooking loss was considered to be the percentage weight loss between the uncooked and the cooked sample. The cooling loss was the percentage weight loss between the cooked and the cooked–cooled sample. The yield (%) was calculated as the weight after cooling divided by the weight of raw sample. The ηT was calculated as the quotient between the core temperature reduction during the cooling stage (ΔT) and the cooling loss. The cooling rate was the core temperature reduction during the cooling stage (ΔT) divided by the cooling time. 2.5. Water content
2.2. Sample cooking Before cooking, the vacuum bags were removed from the samples. All pork samples were cooked in 2% salt solution bath (Model DK-8B, Jing Hong Laboratory Instrument, China) to core temperature of 72 °C. Additionally, the weights of the pork samples before and after cooking were recorded. 2.3. Sample cooling For air blast cooling (AB), immediately after the cooked sample was weighed, it was placed in a laboratory scale air-blast cooler (model PRx-350, Ningbo Haishu Safe Experimental Instrument Factory, China) with 0.35 m 3 chamber capacity at an air temperature of 4 ± 0.5 °C and air velocity 2 m/s. Then the sample was cooled with air blast cooling till core temperature reached 10 °C. For vacuum cooling (VC), the cooked pork sample was placed on a perforated stainless steel table in the vacuum chamber. After the equipment door was closed, the pressure of the chamber was allowed to drop to the working pressure of 10 mbar. For the immersion vacuum cooling (IVC), samples were transferred into a container, together with the hot salt solution from cooking enough to cover the sample. Samples cooled with the solution from 72 °C to 10 °C of core temperature under the controlled pressure in the vacuum chamber. To avoid spilling of the water from the container during cooling, the chamber pressure was controlled precisely, according to saturated steam pressure (Cheng & Sun, 2006a), to reach the final working pressure of 10 mbar. For vacuum cooling followed by immersion vacuum cooling (VC– IVC), immediately after the cooked sample was weighed, it was transferred into the container of vacuum chamber. The pork sample was vacuum cooled until the thermocouples read the target intermediate temperature of 25 °C, in the meantime, the hot salt solution from
The water content of the cooled pork was measured in triplicate by oven drying at 104 °C, 24 h (AOAC, 1980). 2.6. Warner Bratzler shear (WBS) Strips (10 mm × 10 mm × 30 mm) were cut parallel to the muscle fiber orientation from the cooled pork samples. Strips were sheared at room temperature using the Warner-Bratzler shear device (Model WARNER-BRATZLER, G-R Manufacturing Co., USA) at the shear head speed of 4 mm/s. 2.7. Texture profile analysis (TPA) Samples were cut parallel to the longitudinal orientation of the muscle fibers into 20 mm × 20 mm × 20 mm size and examined by textural instrument (Model TMS-Pro, Food Technology Corporation, USA). Two cycles of 6 mm target distance were applied to the samples using a 60 mm circular flat disk attached to the textural instrument at cross-head speed 30 mm/min. The outputs from TPA included hardness, gumminess, springiness, chewiness, and adhesiveness. 2.8. Color The color of cooled pork was measured by the CIE L*a*b* (L*— lightness, a*—red/green and b*—yellow/blue) system using a reflectance colorimeter (WSC-S, Shanghai Precision & Scientific Instrument Co., Ltd., China). 2.9. Statistical analysis All experiments were performed three times in triplicates. All data was analyzed by one-way analysis of variance (ANOVA) using SPSS
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Temperature (°C)
Table 2 Effect of different cooling methods on color and water content of the water-cooked pork.
AB VC VC-IVC IVC
70 60 50 40 30 20
50
100
150
200
250
300
Cooling methods
Color L*
a*
b*
Water content (%)
AB VC IVC VC–IVC
69.29 ± 0.29c 67.25 ± 0.39d 70.10 ± 0.38b 72.45 ± 0.36a
4.45 ± 0.10b 5.14 ± 0.15a 4.30 ± 0.18b 3.04 ± 0.11c
16.34 ± 0.12b 16.67 ± 0.23a 16.14 ± 0.06b 15.60 ± 0.20c
67.03 ± 0.24c 64.05 ± 0.17d 71.44 ± 0.36a 69.67 ± 0.59b
Each value is the mean ± standard deviations of three replicate analyses. Values in the same column with different letters are significantly different (P b 0.05), n = 9. AB, air blast cooling; VC, vacuum cooling; IVC, immersion vacuum cooling; VC–IVC, vacuum cooling followed by immersion vacuum cooling.
10 0
201
350
Time (min) The cooking and cooling process parameters are summarized in Table 1, which indicated that there was no significant difference (P N 0.05) about the cooking loss and the core temperature reduction during the cooling stage (ΔT) among different cooling methods. However, the cooling loss, cooling rate and ηT were all significantly affected by the cooling methods (P b 0.05), which can be attributed to the different heat transfer mechanisms of different cooling methods. During AB, the heat is transferred from the core of the sample to the surface by conduction and then from the surface to the environment by convection (Sun & Wang, 2000). However, the VC, which enabled meat to be cooled faster (Desmond et al., 2000; McDonald et al., 2000), is mainly based on the evaporation of water. Many studies (McDonald et al., 2000; McDonald & Sun, 2001a) showed that the evaporation of water represented the undesired mass loss. During IVC, water evaporation from the meat surface and pores was limited because of the solution surrounding meat. Thus, thermal conduction was the main way to release heat which explained that the cooling rates of AB and IVC are smaller (P b 0.05) in comparison to VC because of low thermal conductivity of meat. It was also observed that immersion vacuum cooling obviously reduced the weight loss while increased the cooling time (Cheng & Sun, 2006b; Drummond et al., 2009; Schmidt et al., 2010). The VC–IVC initially cooled the meat in vacuum cooling at a high cooling rate to the intermediate temperature but leaded high cooling loss. Then immersion vacuum cooling compensates the cooling loss with lower cooling rate. Therefore, the cooling loss of VC–IVC (6.69%) was significantly lower (P b 0.05) than those of the AB (7.28%) and VC (11.82%). The cooling rate of VC–IVC was significantly higher (P b 0.05) than those of AB and IVC, as shown in Table 1. The ηT value was used to evaluate the process efficiency. The process VC–IVC presented a significantly higher (P b 0.05) ηT value (ηT = 9.26 °C/1%) than those of AB (ηT = 8.46 °C/1%) and VC (ηT = 5.24 °C/1%), but still kind of lower (P b 0.05) than that of IVC (ηT = 9.49 °C/1%) which had the lowest cooling loss. And the comparison of ηT between VC and IVC coincided with the results showed by Schmidt et al. (2010) in which the value of IVC (ηT = 22.2 °C/1%) was significantly higher than that of VC (ηT = 8.0 °C/1%) for the chicken breast with the same previous cooking. The rate of VC obviously decreased at the point of 25 °C, so the intermediate temperature was set at 25 °C. But it was still not approved whether 25 °C was the ideal operating point that balances the
Fig. 1. Temperature profile of the water-cooked pork with four different cooling methods.AB, air blast cooling; VC, vacuum cooling; IVC, immersion vacuum cooling; VC–IVC, vacuum cooling followed by immersion vacuum cooling.
15.0 (Statistical Package for the Social Sciences). Duncan's multiplerange tests were used to compare the significant differences among the mean values, and differences at P b 0.05 were considered to be statistically significant. Data are reported as the mean ± standard deviations of three experiments performed in triplicate.
3. Results and discussion 3.1. Cooking and cooling Time-temperature profiles of four different cooling processes (AB, VC, IVC and VC–IVC) are presented in Fig. 1. The results obtained in the replicates of each process were similar, so for simplicity and better data visualization, the results of only one experimental run are shown in Fig. 1. From Fig. 1, the shapes of the curves of AB were distinctively different from those of VC, IVC and VC–IVC, which showed that the cooling rate of AB was significantly lower (P b 0.05) than those of VC, IVC and VC–IVC. One can observe that during the initial step of the VC– IVC, the pork was cooled as fast as VC. However, after reaching the intermediate temperature of 25 °C, which was immersion vacuum cooling section, the cooling rate of pork was slower than that of VC. The cooling time of VC–IVC was 85 min from 72 °C to 10 °C, significantly shorter than those of AB (346 min) and IVC (143 min) as indicated in Fig. 1, which was in agreement with the cooling rate (Table 1). For the purpose of food safety, the temperature zone ranging from 72 °C to 10 °C is very dangerous, for the surviving organism after cooking can readily multiply. So the meat should be cooled fast to let the temperature go through the danger zone quickly. Just as Cheng and Sun (2006a) showed the governments of many European countries have made strict guidelines for the cooling process to ensure the safety of cooked meat products. The results indicated that the VC–IVC was a better method to satisfy the safety requirements than AB and IVC because of the higher cooling rate (P b 0.05).
Table 1 Process parameters for the different cooling methods of the water-cooked pork. Cooling methods AB VC IVC VC–IVC
Cooking loss (%) a
23.10 ± 0.61 23.23 ± 0.25a 22.97 ± 0.45a 23.07 ± 0.25a
Cooling loss (%) b
7.28 ± 0.04 11.82 ± 0.03a 6.54 ± 0.06d 6.69 ± 0.03c
ΔT (cooling stage) (°C)
Yield (%) b
69.62 ± 0.64 64.95 ± 0.22c 70.49 ± 0.48a 70.24 ± 0.28ab
a
61.60 ± 0.36 62.2 ± 0.26a 62.07 ± 0.32a 61.9 ± 0.21a
ηT (°C/1%)
Cooling rate (°C/min) c
8.46 ± 0.10 5.24 ± 0.01d 9.49 ± 0.06a 9.26 ± 0.01b
0.18 ± 0.001d 1.27 ± 0.005a 0.44 ± 0.002c 0.73 ± 0.002b
Each value is the mean ± standard deviations of three replicate analyses. Values in the same column with different letters are significantly different (P b 0.05), n = 9. AB, air blast cooling; VC, vacuum cooling; IVC, immersion vacuum cooling; VC–IVC, vacuum cooling followed by immersion vacuum cooling.
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Table 3 Effect of different cooling methods on instrumental properties of the water-cooked pork. Cooling methods
WBS (kg)
Ha (N)
Gu (N)
Sp
Ch (N)
Ad (N·sec)
AB VC IVC VC–IVC
0.79 ± 0.09c 1.28 ± 0.08a 0.88 ± 0.12c 1.11 ± 0.04b
19.63 ± 3.72a 24.38 ± 0.12a 22.05 ± 4.27a 22.96 ± 2.27a
12.95 ± 2.51a 15.98 ± 0.21a 15.15 ± 2.98a 14.36 ± 0.59a
1.83 ± 0.06a 1.81 ± 0.01a 1.84 ± 0.02a 1.87 ± 0.01a
23.81 ± 5.11a 28.91 ± 0.45a 27.96 ± 5.44a 26.78 ± 0.92a
2.69 ± 2.61a 1.06 ± 1.11a 2.02 ± 1.90a 1.80 ± 0.41a
Each value is the mean ± standard deviations of three replicate analyses. Values in the same column with different letters are significantly different (P b 0.05), n = 9. AB, air blast cooling; VC, vacuum cooling; IVC, immersion vacuum cooling; VC–IVC, vacuum cooling followed by immersion vacuum cooling. WBS, Warner-Bratzler shear force; Ha, hardness; Gu, gumminess; Sp, springiness; Ch, chewiness; Ad, adhesiveness.
competing criteria of reducing cooling loss and cooling time. Further research over a wider range of temperatures is required to identify such a point.
hardness and cohesiveness had no significant difference (P N 0.05) between the water-cooked beef joints cooled by AB, VC and IVC. 4. Conclusions
3.2. Color and water content analysis The measurement of color given in Table 2 indicated that the cooling method affected the color of the water-cooked pork. Vacuum cooled pork was found to be slightly darker (P b 0.05), with a lower L* value than those of AB, IVC and VC–IVC (P b 0.05). The value of a* of pork cooled by VC was significantly higher (P b 0.05) than that of AB, IVC and VC–IVC, which is a further proof to show that high a* value for pork with VC was due to the concentration of pigments caused by high cooling loss (Desmond et al., 2002; McDonald et al., 2001; Zheng & Sun, 2004). The relationship of color between samples cooled by AB, VC and IVC has the coincidence with values of water content (Table 2). However, VC–IVC had higher L* and lower a* (P b 0.05) than AB, VC and IVC, which did not coincide with the water content. Moisture loss might cause the pork darker because it would increase adhesion of muscles fibers, concentrate the pork pigments and increase light penetration. However, migration of water would lead solutes with color pigments to the surface and has a hypochromic effect. What's more, the removal of water would increase separation between muscle fibers and thus has a hypochromic effect (McDonald et al., 2000). In VC–IVC, the water evaporated in the vacuum cooling section, and in immersion vacuum cooling section the water in the soup might penetrate to the meat pores (Houska et al., 2003). And the complicated movement of water might change the microstructure of the meat. Thus to accurately explain the color of the pork cooled by VC– IVC, further research on the water movement and microstructure is required. 3.3. TPA and WBS analysis Warner-Bratzler shear (WBS) force values are a measurement of the physical force necessary to cut through a standardized piece of the pork sample. From the results presented in Table 3, shear force values were highest (P b 0.05) for vacuum cooled samples with a mean value of 1.28 kg, indicating that vacuum cooled samples were less tender. Cheng and Sun (2006a) showed that vacuum cooled samples had higher WBS values which were supposed to be associated with the lower water content. Water loss and compression of muscle fibers were believed to be responsible for the increased shear force during vacuum cooling (Desmond et al., 2000; McDonald et al., 2000). Therefore, it was required to decrease water loss because high water content would be helpful for improving the tenderness of meat products. The method of VC–IVC decreased water loss and got more tender products with lower WBS value than those from VC, but still higher than those of AB and IVC. There were no significant differences (P N 0.05) between samples cooled by the four cooling methods in terms of TPA, although the values of VC–IVC had a tendency to be lower compared with those of VC. Palka and Daun (1999) found that the changes of microstructure and moisture had a close relationship with parameters of TPA. Drummond et al. (2009) also showed that
The combined cooling method of vacuum cooling followed by immersion vacuum cooling (VC–IVC) has been studied. The feasibility of this combined cooling method was evaluated in terms of the process parameters, water content and physical properties compared with air blast cooling (AB), vacuum cooling (VC) and immersion vacuum cooling (IVC). The results showed that VC–IVC could compensate the cooling loss of VC within less cooling time than IVC (P b 0.05). And ηT value of VC–IVC was bigger than those of AB and VC (P b 0.05). The pork with VC–IVC had higher L* and lower a* (P b 0.05) than those of AB, VC and IVC. The pork with VC–IVC obtained improvements in the physical attributes, such as lower shear force value (P b 0.05). The results indicated that VC–IVC could be an effective method in getting lower cooling loss (P b 0.05) than those of AB and VC and higher cooling rate (P b 0.05) compared with those of AB and IVC. The lower water content and higher shear force of VC had bad effect on the products, VC–IVC could compensate these inferiorities of products caused by VC. Acknowledgment We gratefully acknowledge the financial support received in the form of research grant (Project No. 200903012) from Ministry of Agriculture of China. References AOAC (1980). Official methods of analysis. Washington: Association of Official Analytical Chemists. Cheng, Q., & Sun, D. -W. (2006). Feasibility assessment of vacuum cooling of cooked pork ham with water compared to that without water and with air blast cooling. International Journal of Food Science and Technology, 41(8), 938–945. Cheng, Q., & Sun, D. -W. (2006). Improving the quality of pork ham by pulsed vacuum cooling in water. Journal of Food Process Engineering, 29(2), 119–133. Cheng, Q., Sun, D. -W., & Scannell, A. G. M. (2005). Feasibility of water cooking for pork ham processing as compared with traditional dry and wet air cooking methods. Journal of Food Engineering, 67(4), 427–433. Desmond, E. M., Kenny, T. A., & Ward, P. (2002). The effect of injection level and cooling method on the quality of cooked ham joints. Meat Science, 60, 271–277. Desmond, E. M., Kenny, T. A., Ward, P., & Sun, D. -W. (2000). Effect of rapid and conventional cooling methods on the quality of cooked ham joints. Meat Science, 56(3), 271–277. Drummond, L. S., & Sun, D. -W. (2006). Feasibility of water immersion cooking of beef joints: Effect on product quality and yield. Journal of Food Engineering, 77(2), 289–294. Drummond, L., & Sun, D. -W. (2008). Immersion vacuum cooling of cooked beef — Safety and process considerations regarding beef joint size. Meat Science, 80(3), 738–743. Drummond, L., Sun, D. -W., Vila, C. T., & Scannell, A. G. M. (2009). Application of immersion vacuum cooling to water-cooked beef joints — Quality and safety assessment. LWT—Food Science and Technology, 42, 332–337. Houska, M., Landfeld, A., & Sun, D. -W. (2005). Eating quality enhancement of cooked pork and beef by ripening in brine and vacuum cooling. Journal of Food Engineering, 68(3), 357–362. Houska, M., Sun, D. -W., Landfeld, A., & Zhang, Z. (2003). Experimental study of vacuum cooling of cooked beef in soup. Journal of Food Engineering, 59, 105–110. Huber, E., & Laurindo, J. B. (2005). Weight loss of precooked chicken breast cooled by vacuum application. Journal of Food Process Engineering, 28(3), 299–312.
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