Understanding the impacts of cooking and freezing processes on meat quality and physiochemical properties of beef steaks

Abstract

This project aimed to understand 1) the physiochemical changes that occur throughout the cooking process by determining myoglobin denaturation percentages between three degrees of doneness and three muscles and 2) the impact of freezing on eating quality, perception of quality, and physiochemical properties of beef Longissimus lumborum steaks aged for 21, 28, or 35 d. The objective of the first study was to determine the change in myoglobin denaturation through three degrees of doneness (medium rare, medium, and well-done) within the Longissimus lumborum, Gluteus medius, and Biceps femoris muscles. Beef strip loins (n=12), and top butts (n=12) were collected at a Midwest beef processing plant and brought to Kansas State University for processing. The strip loins (LL) were denuded and the top butts were denuded and separated into the Biceps femoris (BF) and Gluteus medius (GM). The muscles were sliced into 2.5-cm steaks and assigned to one of the following treatments: raw, medium rare (MR), medium (MED), or well-done (WD). All three muscles were aged for 28 d at 4°C and then frozen and held at -20°C. The steaks assigned for lab assays were cooked to the appropriate degree of doneness (DOD) using a Thermopen to monitor peak temperatures to reach 62.8°C, 71°C or 76.7°C for each DOD, respectively. L*,a* , b* color readings were taken using a Hunter Lab Miniscan, and samples were powdered immediately for moisture, fat, pH, myoglobin denaturation, metmyoglobin-reducing activity, and lipid oxidation. Myoglobin denaturation was determined using a modified protocol provided in the AMSA color guidelines. The change in absorbance was used to calculate the myoglobin denaturation,% of each sample. The metmyoglobin-reducing activity was performed following a modified protocol provided in the AMSA color guidelines to determine the change from metmyoglobin to oxymyoglobin using absorbance at 525 nm. Lastly, lipid oxidation was determined by finding the concentration of malondialdehyde per kg of meat tissue. Data were analyzed using SAS Proc GLIMMIX with a split plot design and an α of 0.05 was considered significant. Myoglobin was denatured 29.08%, 48.34% or 70.17% at each DOD, respectively. The metmyoglobin-reducing activity was the highest (P < 0.05) for the raw treatment and decreased (P < 0.05) with each DOD. As expected, the a* values decreased (P < 0.05) with each different DOD, however, the pH was not impacted (P > 0.05). Similarly, the cooking loss percentages increased (P < 0.05) with each DOD and the Warner-Brazler shear force values were higher (P < 0.05) for well-done steaks in comparison to the other treatments. Lastly, there was an interaction (P < 0.05) for lipid oxidation as the DOD impacted the level of oxidation of the three muscles. As expected, the myoglobin denaturation percentage increased with increasing DOD and behaved similarly to changes in the a* and metmyoglobin-reducing activity values. This research gives more insight to the impacts of cooking and the changes that proteins, especially myoglobin, undergoes between different DOD. The objective of the second study was to determine the impact of freezing on eating quality, perception of quality, and physiochemical properties of beef Longissimus lumborum steaks aged for 21, 28, or 35 d. Beef carcasses (N = 72; n = 18 per collection; 6 per aging period) were selected from a Midwestern beef processing plant on two different kill dates 1 week apart. The strip loins were cut into 2.5-cm steaks, given a random four-digit code, and assigned to either 21 , 28, or 35 d aged with one of the following designations: trained sensory panels, consumer sensory panels, shear force, or lab assays. All 18 loins from the first kill date represented the frozen samples while the later kill date represented the fresh samples. All steaks were aged to their appropriate aging period at 2-4°C in the absence of light. After aging, the frozen samples were blast frozen at -20°C for 24 h before being placed in a 2-4°C refrigerator to thaw. At exactly 21, 28, and 35 d aged, the steaks were fed in trained and consumer sensory panels to eight panelists at a time for a total of three trained and consumer panel sessions per day. Trained sensory panelists were asked to rate initial juiciness, sustained juiciness, myofibrillar tenderness, connective tissue amount, overall tenderness, beef flavor intensity, and off-flavor intensity. Consumer panelists were asked to rate the liking of juiciness, tenderness, flavor, and overall liking while determining the acceptability of each factor. The consumers were fed two of each treatment with no additional information and two of each treatment with the statement “previously frozen”, or “fresh, never frozen”. One pairing correctly described the sample and one pairing did not. This data was used to determine the perception of quality for a fresh or frozen beef steak. Internal color, Warner-Brazler shear force, slice shear force, purge loss, and cook loss were measured on the same day as the sensory panels. Also, on the same day, an additional steak was powdered to determine proximate analysis as well as the determination of metmyoglobin-reducing activity, and lipid oxidation. Overall, there were no interactions between the aging period and freezing treatment. There were only minute differences between the trained and consumer sensory data. Trained panelists determined the fresh samples were juicier (P < 0.05), but the frozen samples were more tender (P < 0.05) overall. Similarly, the consumer panelists determined the frozen samples were more tender (P < 0.05) but did not distinguish a difference (P > 0.05) in overall liking. Additionally, 89.3% of consumers found the frozen samples to be acceptable for tenderness, compared to only 83.9% of consumers for the fresh samples (P < 0.05). However, there were no differences (P > 0.05) found within the percentage of fresh and frozen samples characterized as “unsatisfactory”, “everyday quality”, “better than everyday quality”, or “premium quality”, with the majority of samples falling within the “better than everyday quality” category. These results were supported by the physiochemical properties evaluated. The frozen samples had lower (P < 0.05) shear force values, higher (P < 0.05) cook loss, purge loss, L* values, and a greater (P < 0.05) metmyoglobin-reducing activity. However, no differences (P > 0.05) were found within a* values and lipid oxidation amounts. Similarly, the consumer ratings were not altered when given additional information about the preservation method. Therefore, the consumer’s perception of eating quality was not impacted when the statements “previously frozen” or “fresh, never frozen” were included. Based on this study, the actual eating quality and perception of quality are not impacted by freezing beef steaks.