By Eben van Tonder, 22 March ’25

Introduction

Ancient meat preservation techniques offer remarkable insights that resonate with modern biochemical understanding. Among the most intriguing of these is the Hallstatt salt-curing method, practised during the Bronze Age in the salt mines of Hallstatt, situated in present-day Austria. Here, meat was preserved in clay-lined salt vats long before the development of deliberate nitrite curing. Archaeological discoveries at Hallstatt, including wooden brine vats, animal bones, large ceramic and bronze boiling vessels, suggest that prehistoric meat curers may have intuitively exploited complex biochemical processes. These likely included autolysis (the self-digestion of cells), deamination (the removal of amino groups from amino acids), and the subsequent production of ammonia.

Contemporary science demonstrates that ammonia can be microbially oxidised into nitrite, a key agent in meat curing. This study explores the Hallstatt curing method through an interdisciplinary approach, combining archaeological findings with biochemical and microbiological principles. It investigates whether the Hallstatt curers, through the boiling of pig skins and other animal tissues, may have produced protein-rich broths that contributed directly to ammonia formation in curing vats.

The paper examines historical yeast extract and meat extract production to understand early methods of controlled autolysis and draws parallels with possible techniques used at Hallstatt. It evaluates the optimal conditions for autolysis, including temperature, salinity, and pH, and considers whether such conditions could have been achieved within Hallstatt’s curing systems. Quantitative estimates are provided to assess how much protein would be required to generate meaningful levels of ammonia, and how the addition of concentrated protein broths might have enhanced this process.

Finally, the study reviews the microbial pathways capable of converting ammonia into nitrite in salt-rich environments and considers whether such organisms might have naturally colonised Hallstatt’s curing vats. The potential role of pre-cooking, or light boiling, of meat to accelerate protein breakdown is assessed, alongside implications for modern nitrite-free curing technologies. In so doing, the Hallstatt curing method provides a compelling example of ancient innovation with relevance for today’s clean-label meat processing practices.

A. Yeast Extraction and Historical Autolysis Methods

A clue to the ancient knowledge of the “extraction” power of low cooking first came to me when I studied yeast in The History, Science, and Cultural Significance of Yeast: From Ancient Leaven to Modern Meat Extracts, followed by From Sacred Ferment to Scientific Extract: The Evolution of Yeast’s Value from Antiquity to Biotechnology.

Yeast extract is a food ingredient made by breaking yeast cells and extracting their contents (proteins, amino acids, vitamins) while discarding the cell walls. Yeast extraction yields the soluble components of the yeast (peptides, amino acids, nucleotides) rather than whole intact yeast cells. After autolysis breaks down the yeast cells and releases the soluble components (peptides, amino acids, nucleotides), the mixture contains both solubilised materials and insoluble cell wall fragments. The separation happens in one of two ways:

1. Centrifugation is most commonly used in modern production. It rapidly separates the heavier, insoluble cell walls from the lighter, soluble extract.
2. Filtration can also be used, especially in earlier or smaller-scale systems, where the liquid portion is strained or filtered off to leave behind the solids.

The goal is to isolate the clear liquid yeast extract, which contains the desirable soluble compounds while discarding the remaining insoluble debris. This concentrated essence provides a savoury, umami flavour and nutritional value. It became popular in the late 19th and early 20th centuries as a way to utilise brewery waste yeast. The process relies on autolysis, where the yeast’s own enzymes digest its proteins into smaller compounds.

Ancient Evidence of Yeast Extraction

Yeast extracts drew my attention to the sophisticated manipulation of protein-rich sources, and in my search for insights that might further illuminate the Hallstatt curing system of the Late Bronze Age, I was eager to explore this intriguing science more deeply.

There is limited but fascinating evidence suggesting ancient cultures may have practised rudimentary forms of yeast extraction. In ancient Egypt (as early as 3000 BCE), yeast was separated from beer fermentations and possibly dried and used as a nutrient-rich food additive. Bread and beer production were often interlinked, and it is theorised that surplus yeast biomass was repurposed. In ancient China, records from the Han dynasty (206 BCE – 220 CE) mention fermentation starters (qu) that contain yeast and moulds. These starters produced fermentation products often dried and used in food applications.

The discovery of the beneficial effects of low-temperature processing in yeast extraction likely occurred empirically. Ancient practices often included gentle warming for prolonged periods to prevent spoilage or enhance flavour, such as bone broth production. The realisation that prolonged mild heating (around 50°C) could promote autolysis without denaturing enzymes came much later with Liebig’s and Marmite’s experimentation in the 19th century.

Benefits of Yeast Extracts in L-Arginine or Ammonia Curing Systems

Yeast extracts can play a supportive role in ammonia- and arginine-based curing systems, offering multiple functional benefits.

-> Rich Source of Free Amino Acids (Including L-Arginine)

Yeast extracts are often cited as being naturally high in L-arginine, a precursor for nitric oxide (NO) production, either enzymatically via nitric oxide synthase in some bacteria or through microbial metabolism in fermentation systems. However, the actual quantity of L-arginine introduced into a curing brine via yeast extract is limited and typically not sufficient to have a primary role in driving NO formation.

-> How Much L-Arginine Is Actually Added?

Commercial yeast extracts typically contain 1.5% to 4% free L-arginine by weight. If a formulation includes 3% yeast extract in a curing brine (a relatively high inclusion level for flavour), the calculation is as follows:

3% yeast extract × 3% L-arginine = 0.09% L-arginine

This equals 0.9 g of L-arginine per litre of brine, or 900 ppm. In contrast, dedicated arginine supplementation in modern curing systems can range from 2,000 to 5,000 ppm to ensure effective NO production.

-> Is This Amount Significant?

While 900 ppm L-arginine in the brine is measurable, it is generally not sufficient to drive robust nitric oxide formation on its own. Thus, in both historical and modern curing systems, yeast extract would not serve as a significant source of L-arginine for nitric oxide production. In fact, both L-arginine-based curing systems and nitrite-curing systems often function better in tandem, with yeast extracts offering microbial support rather than substantial contributions to nitric oxide production.

-> Buffering and pH Control

Yeast extracts can stabilise the curing environment by buffering pH, which favours the survival and activity of beneficial bacteria.

-> Nutrient Source for Microbes

Yeast extracts are rich in B vitamins, peptides, and growth factors. These nutrients support microbial populations that play key roles in both ammonia-to-nitrite conversion and nitric oxide production.

-> Flavour Enhancement

In addition to their functional benefits, yeast extracts add umami depth and enhance the overall sensory profile of long-term cured meats, especially in nitrite-free systems.

I had hoped these factors might point toward an alternative curing pathway to the ammonia-based method I currently favour. However, it seems unlikely that yeast or L-Arginine played a significant role in the curing processes used in the Hallstatt vats other than being supportive to bacteria.

Development of Meat Extracts

The discussion on yeast has not been an unfruitful one at all. The next subject on the menu was to extend the inquiry to meat extracts. Meat extracts parallel yeast extracts in concentrating the soluble, nutrient-rich components of biological material. The origins of meat extracts trace back to reducing broths and bone stocks in ancient societies like Rome. Justus von Liebig formalised meat extract production in the 19th century, advocating for simmering meat at low temperatures (70°C) to extract nutrients without coagulating proteins excessively.

Both yeast and meat extracts rely on:

• Mild heat over time to liberate water-soluble components.
• Salt to promote osmotic extraction and preservation.
• Removal of insoluble material to concentrate nutrients and flavours.

There is compelling evidence that the ancient practice of low and slow cooking, especially in bone broth and soup preparation, contributed directly to the conceptual underpinnings of modern extract production. Ancient civilisations such as the Chinese, Egyptians, and Romans were known to use simmering techniques for many hours, often at temperatures not exceeding 60–70°C, to create nourishing broths and medicinal concoctions. These slow-cooked broths concentrate flavour and nutrients without the harsh taste changes that high-heat cooking can introduce.

The low temperatures prevented the denaturing of delicate amino acids and peptides, preserving their nutritional value and taste. This empirical knowledge was undoubtedly passed through generations. While there is no direct documentation that Liebig, Marmite, or Fred Walker (the Australian) explicitly cited ancient methods as their inspiration, it is reasonable to surmise that their work built upon traditional practices of slow extraction. Liebig was certainly aware of Roman cooking techniques and commented on their empirical understanding of nutrition in his lectures.

This is of huge interest to me as it points to an understanding that by altering boiling techniques and levels of ingredients like salt, the protein-rich material can be manipulated and this was well understood in antiquity.

Ancient Yeast and Meat Extracts: Insights for Hallstatt Curing and Modern Applications

Ancient Knowledge of Low-Temperature Extraction

Ancient food practices suggest an empirical understanding of low-temperature extraction. Slow simmering, common in Rome and China, aligns with modern autolysis techniques. In many cases, broths were cooked at 50–60°C for periods ranging from several hours to multiple days. These methods extracted gelatin, amino acids, and minerals without introducing bitter flavours that often accompany high-temperature processing.

Liebig’s 19th-century work systematised gentle temperatures (~50–70°C) for protein breakdown, and his meat extract process was a formalisation of what had been empirical knowledge for millennia. Fred Walker’s development of Vegemite in Australia drew inspiration from the success of Marmite, and while their advertising framed it as modern nutrition, they were tapping into this ancient tradition of concentrated nutrition.

Evidence of Boiling Technology used on Skins

I first investigated the possibility that skins were boiled and the water used in the curing process in Amino Acids in Late Bronze Age Curing: Investigating The Role of Lysine, Glutamine, and Glutamate in Ammonia Production. I now have the opportunity to look more closely at this line of thinking. How plausible is it really?

1. Cooking Vessels and Cauldrons

Evidence from bronze and ceramic cauldrons found in burials and settlements demonstrates that boiling was a central food preparation method. Although located in southwestern Germany, the Hochdorf burial mound (ca. 550 BCE) presents analogous practices to Hallstatt elites. A massive bronze cauldron, capable of holding over 500 litres, suggests the preparation of large quantities of boiled meat and skin stews (Biel, 1996, pp. 51–54).

> “The massive cauldron from Hochdorf was designed for communal feasting, likely containing stews of pork and beef, in which skin and cartilage contributed to the meal’s consistency and nutrition” (Biel, 1996, p. 52).

At Hallstatt itself, bronze cauldrons and ceramic vessels show burn marks and residues consistent with boiling fatty tissues, including skin (Harding, 2000, p. 195). Lipid residue analysis by Kern et al. (2009) identifies animal fats, indicative of long-term boiling processes.

> “Fatty acid profiles from ceramic cooking pots excavated at Hallstatt imply the slow rendering of animal fats, likely derived from boiling tissues rich in collagen, such as skin and connective tissue” (Kern et al., 2009, p. 243).

2. Nutritional and Medicinal Implications of Boiled Skins

Gelatin Extraction and Bone Broths

Boiling skins and bones releases gelatin, a source of collagen, which has nutritional and medicinal properties. While Hallstatt texts are absent, ethnographic parallels from early Celtic and Germanic sources (Cunliffe, 1997) indicate bone and skin broths were used to promote healing and restore strength.

> “The therapeutic use of broths, made from long-simmered bones and hides, is a well-documented phenomenon in early Celtic medical practices, likely reflecting continuity with earlier Hallstatt traditions” (Cunliffe, 1997, p. 241).

Furthermore, the high protein and mineral content of boiled skin broths would have been valuable for salt miners, whose intense physical labour demanded nutrient-dense diets (Stöllner & Köstler, 2013).

3. Parallels in Hallstatt Mining Communities

Excavations in the Hallstatt salt mines recovered wooden bowls and containers, interpreted as lunch boxes for miners. Given the seasonal conditions and energy requirements, it is plausible that these workers consumed gelatinous stews incorporating boiled skins (Harding, 2000, p. 198).

> “Miners’ food must have been high in calories and proteins; the evidence of preserved animal tissues suggests the consumption of skin-rich preparations, either as soups or stews thickened by gelatin” (Harding, 2000, p. 199).

4. Industrial and Technological Use of Boiled Skins

– Animal Glues and Adhesives

Boiled skins were the principal source of collagen-based adhesives, essential in the hafting of tools and weapons, as well as the construction of composite bows. While no composite bows have been directly recovered from Hallstatt, glue residues have been found on hafted tools (Kern et al., 2009).

> “Adhesives derived from collagen, obtained by boiling hides and sinews, played an indispensable role in weapon manufacturing and craft production in the Hallstatt culture” (Kern et al., 2009, p. 247).

The production process—boiling hides to extract gelatinous glue—was an established technology by this period, as evidenced by analogous finds from the Scythian cultures of the same era (Harding, 2000, p. 202).

– Leatherworking and Hide Softening

Leather artefacts recovered from Hallstatt salt mines, including shoes, belts, and bags, show advanced tanning and softening techniques. Boiling hides was part of the dehairing and softening process, although cold processing may have been more common for delicate items (Stöllner & Köstler, 2013, p. 81).

> “Leather preparation included soaking, heating, and possibly brief boiling to facilitate dehairing and pliability, as suggested by the condition and texture of recovered leather goods” (Stöllner & Köstler, 2013, p. 81).

5. Ritualistic and Communal Significance of Boiled Skins

– Feasting and Sacrificial Consumption

The consumption of boiled meats, including skin-on portions, was central to elite feasting in Hallstatt society. The presence of animal bones with cut marks, along with cauldrons in grave goods, indicates the ritual significance of these meals (Harding, 2000, p. 203).

> “Feasting involving the boiling of entire carcasses, including the skins, reflected both social hierarchy and religious observance in Hallstatt society” (Harding, 2000, p. 204).

– Symbolic Use of Animal Parts

The boar had particular symbolic importance in Celtic cultures, extending back to Hallstatt. The boar’s hide, sometimes boiled and softened, was used in ritual garb or feasting (Green, 1992).

> “The boar, as a sacred animal, was often consumed in its entirety, including the skin, during ritual feasts. The hide may have been boiled to render it edible or to prepare it for symbolic reuse” (Green, 1992, p. 151).

B. Autolysis as Main Mechanism to Ammonia?

The possibility that skins were boiled in the cooking vessels and that the products from that process played an important role in the Hallstatt curing method suddenly looks extremely plausible.

Already in the first work I did on the subject, The Hallstatt Curing Method, I speculated that deamination and autolysis would be the key mechanisms initiating the curing process. The working hypothesis is that the Hallstatt curing system involves the generation and utilisation of ammonia as an intermediate step in preserving meat. For ammonia to accumulate in the curing environment, proteins must first be broken down into free amino acids. This process can occur via autolysis, where endogenous enzymes within the meat initiate the breakdown of muscle proteins, and through microbial proteolysis, particularly under anaerobic conditions. Once the amino acids are available, deamination reactions can liberate ammonia, which may accumulate in the sealed environment of a storage pit. Understanding this sequence is critical, as it provides a foundation for hypothesising how ancient curers may have facilitated preservation through ammonia production, eventually leading to the potential formation of nitrites via microbial action.

The fact that the technology of yeast extraction relies on the exact same mechanism made me look a lot closer at it.

Autolysis and Amino Acid Hydrolysis: The Mechanism

Autolysis is the primary biochemical process by which amino acids are liberated from native proteins, particularly in low-oxygen, anaerobic environments. Within meat tissues, endogenous enzymes such as cathepsins and calpains break down muscle proteins into peptides and free amino acids (Toldrá, 2010). Once these amino acids are freed, especially those containing amine groups like glutamine and asparagine, they undergo deamination. This process results in the release of ammonia (NH₃).

The Role of Boiled Skins and Bones in Accelerating Amino Acid Liberation

The discovery of large flat-based ceramic cooking vessels and bronze cauldrons near the Hallstatt brine curing installations, documented by Kern in 1930 and further described in detail by Stöllner and his team in 2003, raised important questions about their intended use. These vessels, located in close proximity to the curing vats, displayed heavy soot deposits and substantial capacity, suggesting they were used for prolonged boiling or rendering of animal products. One of the central challenges in the ammonia curing hypothesis was understanding how enough ammonia could be produced in a short time from the breakdown of surface proteins. This would have been necessary to penetrate the meat and allow microbial conversion of ammonia to nitrite, enabling effective curing. This question became central to the hypothesis under investigation.

The widespread use of the boing of skins now become interesting. When skins and bones are boiled, the liberation of amino acids and peptides occurs through a different, more immediate pathway. During prolonged simmering or boiling, typically between 80°C and 100°C, the collagen present in skin and connective tissues undergoes thermal hydrolysis. This breaks down the collagen into gelatine, smaller peptides, and free amino acids (Toldrá, 2010; Heinz & Hautzinger, 2007).

This pre-digestion process takes place outside the anaerobic pit environment. The result is a broth highly enriched with soluble nitrogenous compounds. By coating the meat or lining the storage pit with this collagen-rich, amino acid-loaded broth, ancient curers effectively introduced substrates that had already been partially hydrolysed. This significantly accelerated the subsequent breakdown to ammonia by microbial action.

Ammonia Production Under Anaerobic Conditions: The Reaction

Under anaerobic conditions, particularly in sealed storage pits, microbial proteolysis continues the breakdown process initiated by autolysis. Anaerobic bacteria such as Clostridium species dominate these environments. These bacteria carry out proteolytic fermentation, where they degrade peptides and free amino acids via deamination. The typical reaction involves removing the amino group (-NH₂) from amino acids, forming an organic acid and releasing ammonia (NH₃) in the process. For example:

• Glutamine → Glutamate + NH₃
• Glutamate → α-Ketoglutarate + NH₃

This pathway is known as anaerobic deamination. Because oxygen is absent, the ammonia cannot be oxidised directly to nitrite or nitrate without the later intervention of nitrifying bacteria in an aerobic phase. However, the ammonia accumulates in the anaerobic phase, often giving rise to the strong ammonia odours historically associated with fermented or pit-cured meats.

Impact on Ammonia Production: Quantitative and Environmental Considerations

The practice of storing meat in pits dug into the earth is well documented across numerous ancient cultures. I previously explored these traditions, including the pit fermentation practices of the Inughuit people and the bog storage methods employed by the Celts, in Sacred Sustenance: The Celebration of Meat Preservation in Northern Cultures and Its Integration into Christian Traditions and Survival, Spirituality, and Celebration: Inughuit Meat Preservation Practices and the Quviasukvik Winter Feast.

Ancient food storage technologies often relied on the use of sealing agents derived from boiling animal skins and bones. Akkermans and Schwartz (2003) describe early Neolithic sites in Syria where similar gelatinous adhesives were applied to storage vessels, not only for structural purposes but also for limiting oxygen ingress. Pollard and Heron (2008) demonstrate through residue analysis that fat and collagen derivatives were commonly used to line storage vessels and pits. In Europe, Brothwell and Brothwell (1998) document the use of bone broths and rendered fats in sealing grain pits and food stores among pre-Roman and Iron Age cultures. These sealing methods served dual purposes: physical protection and the establishment of anaerobic environments.

In the context of meat preservation, these sealing practices had important biochemical implications. By coating the walls of pits and the meat itself with gelatinous substances, ancient societies effectively reduced oxygen exposure, creating anaerobic conditions ideal for proteolytic bacteria. Once sealed, endogenous enzymes within the meat tissues (via autolysis) and microbial proteases initiated the hydrolysis of proteins into peptides and free amino acids (Toldrá, 2010). Amino acids like glutamine and asparagine, which possess amine groups, were especially susceptible to microbial deamination under these anaerobic conditions, releasing ammonia (NH₃).

In essence, the integration of boiled skin and bone broths into pit sealing practices did more than preserve the stored meat; it contributed significantly to the biochemical environment that favoured ammonia generation. The resulting ammonia smell, often cited in historical and ethnographic descriptions, serves as both sensory evidence and a marker of the deamination processes that occurred in these ancient storage systems.

The direct application of amino acid-rich broth to the meat and the pit interior would have led to faster and more intense ammonia production compared to relying only on the slow autolytic activity of the meat under cold, anaerobic conditions.

Cold earth temperatures, typical of subterranean storage environments, would have greatly slowed down enzymatic autolysis and microbial proteolysis. If the process depended only on the meat’s endogenous enzymes, ammonia production would proceed slowly and the quantity of ammonia generated would have been limited over short storage periods.

By introducing pre-hydrolysed proteins and amino acids through the gelatinous sealing material, much of the rate-limiting steps were bypassed. Anaerobic bacteria, such as Clostridium species, would have rapidly deaminated the available amino acids, generating higher concentrations of ammonia in a shorter time frame.

This two-stage process combined thermal hydrolysis carried out prior to burial with anaerobic fermentation inside the pits. It represents a simple yet effective biotechnological approach that leveraged both heat-based extraction and anaerobic conditions to enhance ammonia generation.

So, the addition of collagen-rich broths directly into pits or over meat likely accelerated ammonia production. By pre-liberating amino acids during the boiling process, the concentration of free amino substrates in the pit environment would have increased, hastening the microbial breakdown and ammonia production. This hypothesis is supported by ethnographic parallels from Inuit kiviaq and Scandinavian fermented fish practices, where fermentation pits emit intense ammonia odours (Berlant, 2017; Høy-Petersen, 1931).

Outram (2004) and Richards & Mellars (1998) note elevated nitrogen levels in ancient pit sites, consistent with ammonia accumulation. Moreover, historical accounts from Wells (1999) and Cunliffe (1997) mention the distinctive, often offensive odours associated with preserved meat stores among Iron Age and Celtic societies, likely a reference to ammonia-laden vapours.

The practice of storing meat underground was already well established by the time the Hallstatt curing vats were in use. Long-term boiling of skins and bones, along with the preparation of sealing materials derived from these broths, was a familiar and widely applied technique. It was common for meat to be sealed with such collagen-rich substances to protect it during storage. The generation of ammonia within these storage pits is well documented, both through historical accounts and archaeological investigations.

Summary of Ammonia Concentrations and Process Efficiency

Pre-processing the skins and bones into a nutrient-dense broth would have significantly increased the nitrogen load within the storage pit environment. This resulted in faster ammonia accumulation, higher ammonia concentrations measured in parts per million, and a more predictable preservation process. These outcomes could have been achieved even under colder ambient conditions.

Without the application of the broth, ammonia production would have depended solely on the autolysis of the buried meat. This process was much slower and would have resulted in lower ammonia yields, particularly in colder climates or during the winter months.r months.

C. Parallels to Hallstatt Curing

The detailed study of yeast extraction and ancient broth preparation shows that ancient societies understood far more about the potential of slow and low-temperature boiling than we often credit them for. These cultures recognised that prolonged simmering extracted more than just flavours from protein-rich materials; it released substances invisible to the eye, with wide-ranging applications. Techniques to manipulate and concentrate these broths were developed and refined, resulting in products that served culinary, medicinal, industrial, and ritualistic purposes. The knowledge of how to extract and repurpose these broths was widespread.

As we explored in Section B, the practice of storing meat underground in pits was well-established across many ancient societies. In Europe, this practice was common among Celtic peoples, and in the far north, similar methods were used by the Inuit. Sealing these storage pits with products derived from the long boiling of animal skins and bones not only provided a protective physical barrier but also introduced a complex biochemical substrate. This sealing material, rich in amino acids and peptides, would have accelerated ammonia production within these anaerobic storage environments.

By applying gelatinous sealants created through long-term boiling, the ancients effectively introduced pre-hydrolysed proteins into the storage system. This, combined with the anaerobic conditions of the sealed pits, provided an ideal environment for microbial deamination processes. The resulting ammonia production was significantly more rapid and abundant than would have occurred through the slow autolysis of meat alone. Ethnographic and historical records confirm that the stored meat often exhibited intense ammonia odours, indicative of high levels of ammonia accumulation. These odours were noted by the Inughuit of Greenland and the Saami peoples of Scandinavia, as well as among the Celts, who likely used similar practices in their bog pits and storage systems (Berlant, 2017; Cunliffe, 1997; Høy-Petersen, 1931).

Now that we understand these foundational technologies and biochemical processes, we can return to the Hallstatt curing vats and reconsider the possible curing process used there. Archaeological finds of large boiling vessels in Hallstatt suggest that boiled water, likely enriched with soluble compounds from pig skins, was incorporated into the brine curing system. Boiling pig skins at low temperatures yields gelatin, amino acids, and peptides. If this boiled skin water was cooled and added to the brine vats, it would have served multiple purposes:

• It increased the organic nitrogen load, providing abundant substrates for microbial deamination.
• It supported the growth of nitrifying microbes in the oxygenated zones of the vats, particularly when stirring was employed.
• The addition of this water would have lowered the brine’s salinity, optimising conditions for autolysis and microbial ammonia-nitrite cycling.

This combination of biochemical and environmental manipulation, applying concentrated protein broths, using clay-lined vats, stirring for oxygenation, and potentially following the pit phase with an aerobic drying or curing step, forms a coherent and plausible hypothesis for an ammonia-initiated curing process in Hallstatt. This method foreshadows modern efforts to develop nitrite-free curing systems and highlights the innovative practices of Bronze and Iron Age societies.

D. Complete Reconstruction of the Hallstatt Curing Method (Hypothetical Workflow)

Earlier this year, in Amino Acids in Late Bronze Age Curing: Investigating the Role of Lysine, Glutamine, and Glutamate in Ammonia Production, I speculated that the Hallstatt curing system may have followed a timeline such as this:

Proposed Timeline for Hallstatt-Style Curing

1. Day 1–2: Boiled pork skin water is prepared and combined with freshly butchered meat in curing vats. Microbial deamination begins, producing ammonia and creating an increasingly alkaline environment.
2. Day 3–5: Ammonia production peaks. Nitrifying bacteria, aided by oxygen introduced through stirring or surface exposure, begin converting ammonia to nitrite.
3. Day 6–10: Nitrite levels stabilize as the curing reaction between nitrite and meat proteins progresses, preserving colour and preventing spoilage.
4. Day 11–20: Nitrite is gradually oxidized to nitrate. During this phase, the meat is either removed for drying or stirred periodically in the vats to maintain micro-aerobic conditions.
5. Day 21 onward (optional): If the meat is hung to dry, residual nitrifying activity on the surface further enhances preservation. The drying process completes the transformation into a shelf-stable, cured product.

Based on this timeline, the following is a complete reconstruction of the Hallstatt curing method.

Updated Step-by-Step Workflow for the Hallstatt Curing Method

1. Arrival of the Meat
The animals processed at Hallstatt were domesticated pigs, as confirmed by osteological remains. These pigs were likely slaughtered close to the saltworks, either within the Hallstatt Valley or nearby settlements. They were transported to the curing areas shortly after slaughter to ensure minimal spoilage.

2. Initial Preparation
After slaughter, the pigs were butchered, trimmed, and cleaned. While there is no direct evidence that the meat itself was par-boiled or pre-cooked, the discovery of large ceramic and bronze cauldrons (Kern, 1930; Stöllner & Köstler, 2013) suggests that skins and bones were boiled. This process yielded nutrient-dense broths rich in gelatin, peptides, and amino acids. The broth was likely poured into the curing vats or applied to the meat, creating a sealing layer and providing a substrate that promoted ammonia production during curing.

3. Immersion Curing (Day 1–10)
The trimmed meat was submerged in clay-lined vats filled with brine, possibly enriched with the amino acid-rich broth from boiled skins and bones. Microbial proteolysis and autolysis within the meat released free amino acids into the brine, accelerating ammonia production. Anaerobic conditions prevailed in the lower vat layers, while limited oxygen at the surface—introduced by occasional stirring—created micro-aerobic zones that supported nitrifying bacteria.

4. Ammonia Production and Alkaline Shift (Day 2–5)
During the first few days, microbial deamination produced significant amounts of ammonia, elevating the pH and creating an alkaline environment in the brine. This alkaline condition was essential for the initial stage of preservation and microbial control.

5. Onset of Nitrification (Day 4–10)
As ammonia levels peaked, nitrifying bacteria such as Nitrosomonas spp. began converting ammonia into nitrite. Periodic stirring introduced sufficient oxygen to the upper layers of the brine to sustain nitrification. Nitrite levels stabilized by Day 10, initiating the formation of stable cured pigments in the meat.

6. Estimating Substrate Requirements (Throughout Process)
Boiling 10 kg of pig skins and bones produced a broth capable of contributing approximately 320 g of ammonia nitrogen when added to a 1000-litre brine system. This could yield an estimated 480 g of sodium nitrite (NaNO₂) through microbial conversion. Such concentrations provided around 480 ppm nitrite in the brine. During curing, the meat could absorb between 48–144 ppm nitrite, consistent with modern curing standards (Pegg & Shahidi, 2000).

7. Resting and Microbial Activation Phase (Day 10–12)
After about 10 days, the meat likely underwent a resting period within the vats, lasting another 12 to 48 hours. This allowed endogenous enzymes to further break down proteins (autolysis), releasing additional amino acids. Anaerobic bacteria in the brine continued to deaminate these amino acids, while gentle stirring aerated the surface and supported further nitrification.

8. Ripening and Aeration (Day 12–20)
Following immersion curing, the meat was removed from the vats and hung in the cool, ventilated shafts of the Hallstatt salt mines. Exposure to air facilitated continued nitrification on the meat surfaces, converting residual ammonia to nitrite and ensuring further preservation. This phase was critical for colour development and flavour enhancement.

9. Drying and Final Preservation (Day 20 onward)
After ripening, the meat underwent a slow drying process in the mine shafts. The stable, cool conditions in the mines controlled moisture loss and slowed spoilage. Residual nitrites in the meat provided ongoing antimicrobial protection, while the high salt content prevented microbial growth. The result was a shelf-stable, cured product suitable for long-term storage and trade.

Conclusion

This hypothetical reconstruction of the Hallstatt curing method, grounded in archaeological, biochemical, and ethnographic evidence, proposes a plausible workflow for ancient meat preservation. The use of boiled pig skins and bones to produce nutrient-rich broths aligns with known ancient culinary and technological practices. By integrating this broth into the curing vats, Hallstatt curers may have enhanced protein breakdown and accelerated ammonia production.

Subsequent nitrification, aided by aeration during ripening and drying phases, could have converted ammonia to nitrite, completing a natural curing process long before the deliberate addition of nitrites was understood.

Parallels with ancient yeast and meat extract production illustrate the sophisticated empirical knowledge these early societies possessed regarding temperature control, extraction techniques, and fermentation processes.

Modern clean-label curing methods continue to draw inspiration from these ancient practices, using protein hydrolysates and controlled microbial environments to replicate the preservation, flavour, and safety of traditional cured meats.

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