Meat Aging and Umami
In order to grasp the foundation of umami in post-mortem meat, it is necessary to delve into two distinct dynamics: nucleic acids and amino acids. These two different umami substances undergo transformative changes during the aging process of the meat. The amount of umami nucleic acids reaches its peak and begins to decline at an earlier stage, while the content of amino acids tends to steadily increase over an extended period of time. This differential behavior contributes to the intricate umami profile that evolves throughout the meat aging process.
The following chart illustrates the fluctuation of inosine-5-monophosphate (IMP) and glutamic acid (Glu), two prominent umami substances, at different points of time in postmortem beef and pork. For beef, the period between the 4th and 12th day showcased a decrease in IMP levels alongside an increase in Glu. Similarly, in the case of pork, the first to sixth day witnessed a decline in IMP and a simultaneous rise in Glu. In this blog, we delve into the underlying mechanisms behind this intriguing phenomenon, shedding light on the factors that contribute to these changes in umami composition during the postmortem aging process of beef and pork.
IMP and Degradation of Nucleotides
Following the death of livestock, adenosine triphosphate (ATP) in muscles undergoes degradation, resulting in the formation of IMP through the intermediate stages of adenosine-5-diphosphate (ADP) and adenosine-5-monophosphate (AMP). Subsequently, IMP continues to degrade, leading to the production of inosine (Ino) and hypoxanthine (Hx).
ATP is an organic compound essential for powering numerous cellular processes in living organisms. It serves as an energy source for crucial functions like muscle contraction, nerve signal transmission, dissolution of cellular components, and chemical synthesis. Significant quantities of free energy are released when a phosphate or pyrophosphate unit is cleaved from ATP, which then turns into ADP or AMP respectively. It leads to IMP, Ino, and Hx with further degradation. It is important to know that umami nucleotides such as IMP are intermediate products in the process and decrease in the long term.
Meat Quality and Nucleotides
Batlle et al. (2001) conducted a study to measure the levels of nucleotides in pork following the death of the pigs. The results showed that in high-quality pork, the concentration of IMP reached its peak 24 hours after the pigs’ death and gradually decreased as it converted into Ino and Hx (Hyp in the chart provided). On the other hand, in low-quality meat, the peak of IMP occurred much earlier, approximately 4 hours postmortem. This difference was attributed to higher levels of upstream nucleotides (ATP and ADP) in the high-quality meat initially, while the initial downstream products (AMP, IMP, Ino, and Hx) were more abundant in the low-quality meat. The degradation of nucleotides had already progressed significantly by 2 hours postmortem in the low-quality meat.
Aging Methods and Nucleotides
The figure presented below from Hwang et al (2019) illustrates the changes in IMP, inosine, and hypoxanthine levels in dry-aged and wet-aged pork cuts over a 21-day aging period. As discussed in previous sections, the content of IMP decreased as the aging period extended. Dry-aged pork cuts exhibited the highest IMP content on the seventh day, while wet-aged cuts reached their peak on the 14th day. In contrast, the content of inosine and hypoxanthine increased progressively as the aging period lengthened, with dry-aged cuts showing a more rapid increase compared to wet-aged cuts. Furthermore, after 21 days of aging, the hypoxanthine content in dry-aged pork cuts was significantly higher than that in wet-aged pork cuts.
IMP May Peak before Aging
Kim et al. (2012) conducted a study to analyze the nucleotide levels in postmortem duck meat. The findings indicated that the upstream nucleotides, including ATP, ADP, and AMP, had been already depleted before the first measurement. At this point, IMP had reached its peak level. It was observed that the process of ATP depletion had already commenced around the time of slaughter. Throughout the aging process, both in duck legs and breasts, IMP exhibited a decline, while Ino and Hx showed an increase. Although the study did not specifically address the conditions of slaughter, it is plausible that livestock may experience stress and expend energy during transportation to the slaughter location or during the actual slaughter process.
Umami Amino Acids and Protein Decomposition
As discussed in Jamón Serrano Aging and Umami, proteins are broken down to peptides and amino acids in meat through a process called proteolysis. Some of those amino acids, such as Glu and aspartic acid (Asp), contribute to the umami taste of meat.
Proteases Decompose Proteins
Calpains and cathepsins are important enzymes involved in the proteolytic degradation of muscle proteins in meat after slaughter. Calpains play a significant role in tenderizing the meat by breaking down certain proteins. On the other hand, cathepsins contribute to the breakdown of myofibrillar proteins, leading to the release of peptides and amino acids. These peptides and amino acids act as water-soluble precursors that contribute to the development of desirable flavors in the meat. In addition to calpains and cathepsins, other proteases like caspases and matrix metalloproteinases have also been identified as participants in the postmortem degradation of muscle proteins in meat.
Aminopeptidases Break down Peptides to Free Amino Acids
Nishimura (1998) conducted a study to investigate the breakdown of proteins into peptides and free amino acids. The research revealed that the highest increase in free amino acids occurred under neutral acidity conditions (pH 7) in pork muscle stored for 5 days. This increase was attributed to the activity of aminopeptidases, which exhibit optimal enzymatic activity in the neutral pH range. The findings supported the notion that the rise in free amino acids during postmortem aging of meat is primarily attributed to the action of aminopeptidases C, H, and P on peptides, which are transformed from meat proteins through the activity of cathepsins and calpains.
Above result suggests that the slaughter process is important for meat aging. When animals experience stress just before slaughter, such as rough handling or fighting, it can lead to the release of muscle glycogen into the bloodstream. After slaughter, while the carcass is still warm, this glycogen is quickly metabolized and converted into lactic acid, causing a fall in pH or a rise in acidity. Such meat is suboptimal as aminopeptidases are less active in a lower pH (higher acidity) condition and less free amino acids are generated.
Dry Aging vs Wet Aging
Hwang et al (2019) examined how aging methods affect the increase in free amino acids. There were no significant differences in the total content of free amino acids between dry-aged and wet-aged pork cuts until the 7th day of aging. However, after this point, there was a significant increase in the total content of free amino acids in the dry-aged pork cuts. The levels of glutamic acid in both wet-aged and dry-aged pork cuts followed a similar increasing pattern as the total free amino acids content. In particular, the shoulder blade and belly cuts of dry-aged pork showed significant increases in glutamic acid compared to wet-aged cuts, starting from the 7th and 14th days of aging, respectively. After 21 days of aging, most of the free amino acids, with the exception of Asp, Cys, and Met, in the dry-aged pork cuts showed a significant increase compared to the wet-aged pork cuts.
The variation between dry-aged and wet-aged meat is not solely attributed to moisture loss during the dry-aging process. Some studies have indicated that dry-aged meat exhibits higher pH levels compared to wet-aged meat. This difference in acidity conditions, which can influence aminopeptidase activity, may also contribute to the observed distinctions between the two aging methods.
Conclusion
Meat aging is a complex subject, and there is no universal method considered optimal for all types of meat. Nonetheless, previous studies have provided valuable insights and key findings that can be applied to our understanding of the aging process in terms of umami.
There May be an Early Umami Peak
While amino acids show a continuous increase over an extended period, umami nucleotides, particularly IMP, reach a peak and then gradually decline as they transform into Ino and Hx. It is worth noting that the concentration of umami may exhibit an early peak coinciding with the highest levels of IMP.
Slaughter Condition Matters
If animals experience distress prior to or during the slaughter process, it can have a negative impact on the suitability of the meat for aging. The stress can deplete ATP, leading to a faster depletion of nucleotides and potentially resulting in an earlier and lower peak of IMP and its synergetic effect. Additionally, increased acidity in the meat due to stress can reduce aminopeptidase activity, resulting in lower levels of free amino acids. Therefore, meat that undergoes optimal aging for maximizing umami tends to come from animals that have been slaughtered in a calm and gentle manner, minimizing stress and energy consumption.
Dry-Aging over Wet-aging
Research has indicated that dry-aged meat tends to have higher concentrations of umami amino acids compared to wet-aged meat. This could be attributed to factors such as moisture loss during the dry-aging process or variations in the acidity conditions. As a result, if maximizing umami flavors is a priority, dry aging is generally considered the preferred method. However, it’s worth noting that the field of meat aging is continually evolving, with ongoing efforts to develop new and innovative aging techniques.
