By Eben van Tonder and Christa Berger, 10 January 2025
For the Complete work on the Hallstatt Curing reaction, see The Hallstatt Curing Method. All subsequent updates and relevant articles are listed there.
Introduction
This article builds on earlier work published in April 2024 on the Hallstatt curing method, exploring ancient industrial-scale meat preservation practices. The findings presented here to progress the insights shared in a follow-up article two days ago, in which we investigated the role of lysine-rich pork skin in ammonia production during curing. These investigations stem from trials conducted in Cape Town, where putrefaction was found to occur earlier than deamination in unoptimized conditions. The current focus expands on the critical role of lysine and introduces two other amino acids—glutamine and glutamate—as potential contributors to deamination and ammonia production. By examining their roles and comparing them to lysine, we aim to determine which amino acid holds the most importance for ammonia production in Hallstatt-style curing systems.
Amino Acids in Boiled Pork Skin: Nitrogen Chemistry and Ammonia Production
Boiled pork skin, rich in collagen, releases amino acids into the water during cooking. This nutrient-rich solution forms the basis of the curing liquid. The three amino acids of interest—lysine, glutamine, and glutamate—are examined for their roles in nitrogen chemistry and ammonia production.
Lysine is a basic amino acid with a high nitrogen content. It undergoes rapid microbial deamination, yielding large amounts of ammonia. This ammonia raises the pH, creating an alkaline environment that prevents spoilage. Lysine’s high efficiency in ammonia production makes it the most critical amino acid for rapid deamination.
Glutamine, a derivative of glutamic acid, acts as a nitrogen transporter in biological systems. It hydrolyzes into glutamate, releasing ammonia, though at a slower rate than lysine. Glutamate, the most abundant amino acid in pork skin, can also produce ammonia through microbial deamination. However, its conversion is less efficient than lysine’s, making its impact more gradual.
The water derived from boiled pork skin contains these amino acids, providing a nitrogen-rich medium for microbial activity in curing vats.
Microbial Communities in Curing Vats
For deamination and ammonia production to occur, specific bacterial populations must be present. Key players include:
1. Ammonia-Producing Bacteria: Species such as Bacillus and Pseudomonas are known for their ability to deaminate amino acids rapidly. These bacteria thrive in the nutrient-rich pork skin solution.
2. Nitrifying Bacteria: Nitrosomonas and Nitrobacter convert ammonia into nitrites (NO₂⁻) and nitrates (NO₃⁻), crucial for meat curing. Their activity requires oxygen, emphasizing the importance of aeration or stirring.
3. Salt-Resistant Bacteria: Natural salts from sources such as the Hallstatt mines, Celtic sea salt, or Baja salt (mined from Californian sea salt flats) may harbor halophilic bacteria capable of contributing to the curing process. These salts, often used in traditional curing methods, may carry bacterial species that act as natural starter cultures.
In earlier work on Bay Salt in Seventeenth-Century Meat Preservation, the importance of salt as a carrier of microbial starter cultures was highlighted. Natural salts, particularly those exposed to seawater and evaporation processes, often contain robust halophilic microbial communities. These communities can include bacteria capable of deaminating amino acids and producing nitrites. Baja salt and Celtic sea salt, in particular, are known for their association with these microbial reservoirs, offering a plausible explanation for their traditional use in curing. The use of natural salt in the Hallstatt vats could have inoculated the curing environment with these beneficial bacteria, speeding up ammonia production and enhancing preservation.
From Ammonia to Nitrite: The Chemistry of Curing
The curing process begins with the microbial deamination of lysine, glutamine, and glutamate, producing ammonia. Ammonia raises the pH, preventing spoilage bacteria from thriving. Nitrifying bacteria then oxidize ammonia to nitrite, a process requiring oxygen. Stirring the curing vats or leaving them exposed to air would introduce oxygen, supporting this critical step. Nitrite reacts with meat myoglobin, forming nitrosomyoglobin, which imparts the characteristic cured meat color and flavor. Over time, nitrite oxidizes to nitrate, stabilizing the curing environment.
Proposed Timeline for Hallstatt-Style Curing
1. Day 1–2: Boiled pork skin water is prepared and combined with meat in curing vats. Microbial deamination begins, producing ammonia and creating an alkaline environment.
2. Day 3–5: Ammonia production peaks, and nitrifying bacteria begin converting ammonia to nitrite, supported by oxygen introduced through stirring.
3. Day 6–10: Nitrite levels stabilize. Meat proteins interact with nitrite, initiating the curing process.
4. Day 11–20: Nitrite oxidizes to nitrate. The meat is either hung to dry or stirred periodically to maintain oxygenation.
Why Lysine is the Most Important Amino Acid
While glutamine and glutamate contribute to ammonia production, lysine is the most important amino acid in this system for several reasons:
1. Efficiency in Ammonia Production: Lysine yields more ammonia per molecule than glutamine or glutamate, making it the dominant contributor to alkalinity.
2. Microbial Preference: Bacteria deaminate lysine more readily due to its simple metabolic pathway.
3. Critical Role in pH Stabilization: Rapid ammonia production from lysine deamination ensures spoilage is prevented during the early stages of curing.
Conclusion
This investigation deepens our understanding of the Hallstatt curing process, emphasizing lysine’s dominant role in ammonia production. While glutamine and glutamate offer supporting contributions, lysine’s rapid deamination and high ammonia yield make it indispensable. Natural salts and microbial communities further enhance the process, highlighting the sophistication of ancient curing techniques.
These findings build on previous work and suggest a nuanced interplay between amino acids, microbial activity, and environmental factors in Late Bronze Age meat preservation. Future experimental archaeology could validate these hypotheses and offer additional insights into ancient food science.
References
1. Van Tonder, E. (2024, April). The Hallstatt Curing Method. EarthwormExpress. Link
2. Van Tonder, E. (2024, December). The Hallstatt Meat Curing Method: An Update on Late Bronze Age Industrial Processes. EarthwormExpress. Link
3. Van Tonder, E. (2024). Bay Salt in Seventeenth-Century Meat Preservation: How Ethnomicrobiology and Experimental Archaeology Help Us Understand Historical Tastes. EarthwormExpress. Link
4. Experimental Trials, Cape Town (2024). Internal Research on Deamination with Richard Bosman.