Eben van Tonder
16 April 2024
For the latest update on this article, please visit The Hallstatt Curing Method. I only update my pages and not posts and this will not be the most recent version of this article.
Special Note:
Special thanks to Christa Berger for her advice, translations, and the astronomical volume of information she has provided for this project. Her insights and suggestions, particularly related to meat curing, are of immense value and contribute significantly to our work.
Also, thanks to Richard Bosman for his invaluable insights, encouragement, and ongoing practical testing of the systems developed from this and other related work. He is a true professional and a great friend!
Introduction
Christa Berger contacted me in early 2024 drawing my attention to what is arguably the most important site in Europe (and possibly on earth) for the development of the curing industry in the bronze and iron age. It centres on the Austrian town of Hallstatt.
The Hallstatt area is of interest due to ancient curing vats found there where large quantities of pork were cured presumably to be sold to traders who passed by to buy salt. Hallstatt represents the most detailed source of information on the the most ancient curing practices. As background, there are then two areas of interest to us. Firstly the salt mines that existed there since antiquity and secondly the curing vats which are the main focus of our interest.
Hallstatt Salt Mining
Christa sent me this brief history of salt production in Hallstatt. “An artificially drilled cave bear bone from the Dachstein Giant Ice Cave dates from around 12,000 BC and is considered the oldest evidence of human presence in the Hallstatt area. Due to the brine springs on the mountain, which attracted animals with their salty water then as now, people who hunted these animals were in the valley early on. People settled high above Lake Hallstatt as early as 5000 BC and began extracting salt.
“The first settlement of the area is proven in the Neolithic (around 5000 BC, “Last Wedge Shoe”). It is likely that hunters and fishermen at that time ventured into the primaeval forests of the Salzberg High Valley and discovered salty springs. They began to exploit the salt deposits by boiling the spring brine. The oldest finds proving the presence of humans in the Salzberg Valley and also in the area of today’s town date from the Neolithic. An antler pickaxe found in the Kaiser Josef tunnel in 1838 and since subjected to radiocarbon analysis is about 7,000 years old. Several stone axes have also been found at the Hallstatt salt mine and in the surrounding area of the town. They date from between 5000 and 2000 BC. Their number and the variety of forms show that stone axes were regularly used at the salt mine. The first salt mining in the Hallstatt salt mine can be traced back to around 1500 BC.”
Hallstatt Curing Vats
Block wall constructions were discovered in the Salzberg Valley near Hallstatt. “The initial assumption was that these were building remains, which had long been thought to be log houses. Although Ramsauer wrote at the time of the discovery, “no opening of an entrance to the building or a window in the 4 wooden walls was discovered.” The first structure was discovered in 1877 and excavated in 1878. Later, however, it was recognized that these were sunken basins and believed that they were used for salt production during the Iron Age. This interpretation as collection basins for spring brine (Barth 1998) became obsolete at the latest with the new radiocarbon data (Stadler 1999) because with the simultaneous existing mining of rock salt, the use of low-grade brine sources is quite unlikely. Over time, scientists realized that the basins – like the pig bones also found there – date back to the Bronze Age.” (C. Berger, personal correspondance)
The block wall constructions were identified and studied as part of broader archaeological findings in the region. Researchers like Barth in the years 1976 and 1983, Pauli in 1979, and Weisgerber in 1981 contributed to the understanding of these structures. Their findings combined with the nature of the structures and materials found, led to hypotheses about their use for meat preservation. This was particularly compelling given the anaerobic conditions noted in the dense, clay-rich environment of the constructions, which would be conducive to such processes. The Salzberg Valley’s historical context, being a significant site for salt extraction and processing, further supports the idea that salt-related preservation techniques were likely utilized in these constructions.
Tarh (1998) states that ‘Research has taken over 100 years to properly interpret and date the findings.’ (Barth 1998). As soon as this was thought and written, completely new and surprising aspects emerged. New carbon-14 datings have shifted the beginning of salt mining in Hallstatt back to the 14th century BC (Stadler 1999).
Hypothesized Hallstatt Methods for Meat Salting
Christa sent me a translation last night from Barth, and Lobisser (2002). Their work on the possible function of ancient blockwall constructions found around the salt mines of Hallstatt, dating back as early as 1400 BCE and possibly even to 1500 BCE is volcanic.
Barth and Lobisser wrote that “regarding the possible use of the block wall constructions, many similarities can be observed in the two better-documented block wall constructions in the Salzberg Valley near Hallstatt, leaving no doubt about the similarity of their original use. Therefore, it is permissible to transfer observations made on one building to the other. Up to twelve layers of beams were buried in the ground and sealed on the outside with clay. This clay must have been so-called surface clay, as it was found in 1939 to contain stones. During use, the structure is filled with dense, blue clay containing large amounts of animal bones, shards of thick-walled vessels made of graphite clay, and other finds. Among the bones, pig mandibles and long bones clearly predominate. It is noteworthy that the deposits inside the buildings did not occur in layers as expected, but it is explicitly pointed out that the artefacts were found in pure clay. This indicates that the contents were repeatedly moved and processed over time. The thick cultural layer above cannot be directly linked to the respective structure. The described grey and blue dense clay, which was also encountered in a subsequent excavation at the site in 1998, can be interpreted as dissolved Haselgebirge.” (Barth, Lobisser 2002)
“Given the numerous animal bones, the hypothesis suggests that the Hallstatt block wall constructions were submerged brine pans, in which pork was salted in large quantities with mountain salt. The containers were filled with small pieces of rock salt or rich Haselgebirge, and then the meat was buried in them. After seven to ten days, the salting was completed, and the meat could be exchanged for other meat. This process could be repeated, with the contents of the basins being mixed repeatedly. Over time, the initially granular and salty mass would become softer and richer in clay, and through the water extracted from the meat, it would become richer in protein. Eventually, a turning point would be reached, and the brine would spoil. Such a basin was no longer usable and had to be rebuilt. The pickled meat produced by the above method was certainly not yet a satisfactory final product. Careful drying and ageing were necessary.” (Barth, Lobisser 2002)
“Inside the mining building, a microclimate suitable for prehistoric Hallstatt prevailed. Consistent temperatures of 6-8 degrees Celsius, 60% humidity, strong air circulation, and salty, smoky air from the open light source provide optimal conditions. Practical experiments with pickled meat, hung in suitable places in the current mine and made by laying in small heaps, lost a third of their weight within six months without drying out.” (Barth, Lobisser 2002)
“Of course, this does not prove that high-quality raw ham was produced in Hallstatt in the Bronze Age, but the conditions for it were present. Based on the correctness of the above considerations, the possibility arises to interpret the peculiar separation of a narrow gap discovered on the north side of the building in 1877. Here, the water extracted from the meat as highly concentrated brine with numerous water-soluble components must have been collected. It could easily be scooped out and used for another purpose. The numerous thick shards of graphite clay, found in both block buildings and in the associated cultural layers, can now be meaningfully interpreted: In them, this meat juice could have been concentrated and preserved by heating. The many bones would not simply have been thrown away. They could have been cooked together with little additional effort. If this cooking is continued long enough, a gelling liquid is formed, which can be further dried in the air.” (Barth, Lobisser 2002)
Modern Dry Curing Method (Credit: Mr. Robert Goodrich)
I am interested in the curing mechanisms that may be behind this practice as it would have far-reaching implications for the development of meat curing across Europe. What curing mechanisms may be at work behind the curing of meat starting with the clay pits or vats.
Before we look at possible curing pathways or mechanisms, let’s consider the steps involved in the dry-curing processes which are universally agreed upon that it must be present for the process to be brought to completion and to result in a product that will last a maximum amount of time without refrigeration. For a detailed discussion on the subject, see two chapters from Bacon & the Art of Living, Dry Cured Bacon and The Development of Dry Curing from Salt Only to Salt, Saltpeter and Sugar.
1. Salting: The meat is coated with salt, saltpetre and seasonings where the function of the salt is to draw out moisture and thus inhibit bacterial growth (through reduced water activity).
2. Resting/Drying: After salting, the meat is left to rest in a controlled environment where the salt equalises through the meat and it begins to dry out further, enhancing flavour and texture.
3. Smoking: Smoking adds additional flavour and helps preserve the meat by coating it in smoke’s antimicrobial compounds.
4. Maturing: The meat is aged in a controlled environment to develop flavour and tenderize.
Hallstatt Method (Based on the findings by Fritz Eckart Barth)
Let us now fit the curing vats and their place in this process within the chronology we laid down above.
-> Salting
The first step is salting. The particular salt used was either rock salt mines in the area or salt-rich earth that the meat was packed in.
Barth and Lobisser’s (2002) work takes front and centre stage. The particular system was possibly employed in Hallstatt as early as 1500 BCE. The most intriguing aspect of the possible system employed in Hallstatt is the “sealing in” of the meat in curing vats with salt riach earth.
a. The Possible Function of The First Step in the Hallstatt Method
The proposed Hallstatt system would have been done for a completely different reason from any of the historical instances of burying meat. It is proposed that in the Halstatt method, the burial was done for salting and not drying or protecting it against decay.
a.1 De-Amination or L-Arginine Fermentation
The question comes up as to what the reasons could be for this “burial” of the meat in salt clay. At least two different “curing pathways” emerge if curing was part of the object of the initial salting process and the 3rd possible reason could merely salting and nothing more.
a.1.1. De-Amination
Enzyme-induced deamination of surface proteins in meat, where amino acids are broken down resulting in ammonia production, can occur under certain conditions where specific enzymes are active. Microoganism-induced oxidation of ammonia leads to the formation of nitrites which react chemically in the meat to produce nitric oxide which is responsible for curing the meat. Regarding whether a 7-day period is sufficient for significant ammonia production and subsequent conversion to nitrite (NO2-), enabling a curing process, several factors are essential to consider:
i. Enzyme/ Bacterial Presence and Activity:
– Source of Enzymes: The enzymes that can induce deamination are typically of microbial origin or are endogenous to the meat (i.e., coming from the animal itself). In meat, some enzymes capable of deamination may be present but usually become significant through microbial action.
We know that the environment was anaerobic, so we exclude Micrococcus spp., Nocardia spp. and Flavobacterium spp. since they are strictly aerobic. There are, however certain aerobic microorganisms that can switch to anaerobic metabolism (facultative anaerobes). They are Proteus spp., Pseudomonas spp., Bacillus spp., Escherichia coli, Staphylococcus spp., Arthrobacter spp. and Corynebacterium spp.. These I should also consider, but let’s first look at the anaerobic bacteria.
The following anaerobic microorganisms are involved in deamination.
-> Clostridium spp.: These bacteria are obligate anaerobes, meaning they thrive in environments devoid of oxygen. They are known for their enzyme production, including deaminases, and can be found in various environments such as soil and decaying organic matter (Stackebrandt & Schumann, 2006; Cato et al., 1986). Clostridium species encompass both pathogenic and non-pathogenic microorganisms. Pathogenic species such as Clostridium botulinum, which causes botulism, Clostridium tetani, which causes tetanus, Clostridium perfringens, which causes gas gangrene and food poisoning, and Clostridium difficile, responsible for antibiotic-associated diarrhoea and pseudomembranous colitis, are well-known for their harmful effects on human health. On the other hand, non-pathogenic species such as Clostridium butyricum, which is used in probiotics and found in the intestines and soil, Clostridium acetobutylicum, utilized in industrial fermentation processes to produce acetone and butanol, and Clostridium pasteurianum, which plays a role in nitrogen fixation in soil, highlight the diverse roles of this genus. Given the conditions in the salt clay at Hallstatt, which include high salt concentrations and likely anaerobic conditions, it is plausible that non-pathogenic species like Clostridium butyricum and Clostridium pasteurianum might be present and active in the deamination of surface proteins. These species are well-adapted to anaerobic environments and could contribute to the biochemical processes involved in the preservation and transformation of organic material in such settings.
It is worth taking a much closer look at Clostridium pasteurianum. It is a nitrogen-fixing bacterium that plays a crucial role in converting atmospheric nitrogen (N₂) into ammonia (NH₃). This process is vital for soil fertility and plant growth, as it provides a bioavailable form of nitrogen essential for various biological functions. Nitrogen fixation by Clostridium pasteurianum occurs under anaerobic conditions, using organic matter to produce the energy required for this conversion.
In the specific scenario where pieces of meat are buried in the salt-rich clay of Hallstatt, the decomposition of the meat would indirectly facilitate the nitrogen fixation process. The decaying meat supplies a rich source of organic compounds, such as carbohydrates, proteins, and lipids, which Clostridium pasteurianum can metabolize anaerobically to generate the ATP needed for nitrogen fixation. The burial of meat creates anaerobic conditions because the decomposition process consumes available oxygen, and the high salt concentration limits oxygen diffusion, both of which are essential for the activity of nitrogen-fixing bacteria.
Additionally, decomposing meat releases a variety of nutrients, including amino acids, peptides, and other nitrogenous compounds, enhancing microbial growth and activity. While these nutrients are not directly used in nitrogen fixation, they support the overall metabolic processes of the bacteria involved. The decomposition process involves a complex microbial community where nitrogen-fixing bacteria convert N₂ to NH₃, and other bacteria decompose organic nitrogen compounds in the meat, releasing ammonia and other nitrogenous products, creating a dynamic nitrogen cycle within the buried material.
For buried meat to contribute effectively to nitrogen fixation, the soil or clay must contain nitrogen-fixing bacteria like Clostridium pasteurianum, and anaerobic conditions must be sufficiently maintained. Thus, while buried meat itself does not directly convert atmospheric nitrogen to ammonia, it creates favourable conditions for nitrogen-fixing bacteria to perform this conversion by providing necessary organic matter and maintaining anaerobic conditions.
-> Bacteroides spp.: Commonly found in the intestines of humans and animals, these anaerobic bacteria are capable of deaminating amino acids (Wexler, 2007). Bacteroides species include both pathogenic and non-pathogenic microorganisms. Pathogenic species such as Bacteroides fragilis, known for causing intra-abdominal infections, and Bacteroides thetaiotaomicron, which can contribute to conditions such as appendicitis and abdominal abscesses, are significant due to their potential to cause serious infections. Conversely, non-pathogenic species like Bacteroides vulgatus and Bacteroides ovatus are essential members of the gut microbiota, playing crucial roles in the digestion of complex carbohydrates and maintaining gut health. In the context of the salt clay at Hallstatt, which is characterized by high salinity and anaerobic conditions, non-pathogenic species such as Bacteroides vulgatus and Bacteroides ovatus would likely be present. These species are well-suited to anaerobic environments and could be actively involved in the deamination of surface proteins, contributing to the breakdown and transformation of organic material in such conditions.
-> Fusobacterium spp.: Another group of anaerobic bacteria involved in protein and amino acid degradation, including deamination (Bolstad et al., 1996). Fusobacterium species include both pathogenic and non-pathogenic microorganisms. Pathogenic species such as Fusobacterium necrophorum, which is known for causing Lemierre’s syndrome, a severe infection that typically starts in the throat, and Fusobacterium nucleatum, associated with periodontal diseases, intra-abdominal infections, and adverse pregnancy outcomes, are significant due to their potential to cause serious health issues. On the other hand, non-pathogenic species like Fusobacterium varium and Fusobacterium mortiferum are part of the normal flora of the human gut and oral cavity and are generally not harmful unless they translocate to sterile areas of the body or the host’s immune system is compromised. Given the anaerobic conditions and high salinity of the salt clay at Hallstatt, it is plausible that non-pathogenic species such as Fusobacterium varium and Fusobacterium mortiferum might be present. These species are well-adapted to anaerobic environments and could be involved in the deamination of surface proteins, playing a role in the biochemical processes necessary for the preservation and transformation of organic material in such settings.
ii. Conditions and President
While specific studies on Hallstatt clay are not readily available, research on microbial communities in anaerobic environments similar to clay and soil used for food preservation can provide insights.
-> Microbial Diversity in Anaerobic Environments:
Studies have shown that anaerobic environments, such as deep soil layers and salt mines, host a variety of anaerobic microorganisms, including bacteria like Clostridium spp. and Fusobacterium spp. For example, the study “Microbial diversity and metabolic potential in salt mine environments” highlights the presence of anaerobic bacteria in such conditions (Gunde-Cimerman et al., 2005).
-> Microbial Communities in Archaeological Sites:
Research on ancient preserved food and archaeological sites with anaerobic conditions can reveal the types of microorganisms that thrive in such environments. A study titled “Microbial Communities in Archaeological Environments: The Case of the La Draga Neolithic Site” uses DNA sequencing to identify microbial communities present in anaerobic conditions (Chaves et al., 2019).
The study “Microbial Communities in Archaeological Environments: The Case of the La Draga Neolithic Site” by Chaves et al. (2019) provides insights into the types of microorganisms thriving in anaerobic conditions typical of ancient preservation methods, which can be applied to understand the microbial ecology of the Hallstatt curing methods. The study identified diverse microbial groups, including Clostridia, Bacteroides, and Fusobacterium, known for their roles in protein and fat degradation under low-oxygen conditions. These microbes produce enzymes essential for deaminating proteins and oxidizing L-arginine, leading to the production of ammonia and nitric oxide (NO), crucial for the curing process. In the Hallstatt curing vats, similar anaerobic, salt-rich conditions likely supported these microbial communities, facilitating meat preservation through complex interactions involving microbial deamination, L-arginine oxidation, and fermentation. This multifaceted approach ensured the presence of reactive nitrogen species such as NO, ammonia, nitrite, and nitrate, which collectively contributed to efficient meat curing, enhancing both preservation and flavor. The findings from La Draga offer a valuable framework for hypothesizing the microbial processes that occurred in Hallstatt, highlighting the significant role of these microbial communities in ancient meat curing techniques.
-> Anaerobic Microbial Enzyme Activity in Soil and Clay:
Studies on enzyme activity in anaerobic soil and clay indicate the presence of microorganisms capable of deamination. These studies often focus on the functional roles of microbes in nutrient cycling and organic matter decomposition under anaerobic conditions. For instance, “Anaerobic microbial activity in clay soils” explores the enzyme activity in anaerobic environments (Frossard et al., 2012).
To identify the specific anaerobic microorganisms involved in deamination in the Hallstatt clay, targeted microbiological analyses would be necessary. Based on typical anaerobic environments and processes described, it is plausible that anaerobic bacteria like Clostridium, Bacteroides, and Fusobacterium could be present and involved in deamination.
-> Optimal Conditions:
Enzyme activity is influenced by factors such as pH, temperature, salt concentration, and moisture. Each enzyme has optimal conditions under which it performs efficiently. The conditions in the curing vats in Hallstatt (salt brine from salt clay) need to be conducive to maintaining or enhancing enzyme activity.
iii. Time Frame for Ammonia Production:
The rate of deamination and subsequent ammonia production can vary. In a controlled environment, such as in industrial fermentation or curing processes where conditions are optimized, significant levels of ammonia could potentially be produced within 7 days. However, in a less controlled environment, this rate might differ. The amount of ammonia produced depends on the quantity and activity of the deaminating enzymes, as well as the availability of amino acids as substrates from the meat proteins. The rate of ammonia production will therefore be a key factor in the timeframe required for enough ammonia to be produced.
iv. Conversion of Ammonia to Nitrite:
Let’s assume that the right microorganisms were present and the right conditions existed for the bacteria to flourish, how would the ammonia produced be converted to nitrite or even nitrates and how likely are these to have cured the meat? The conversion of ammonia to nitrite is primarily a microbial process, performed by nitrifying bacteria which are typically aerobic. The presence and activity of these bacteria would be crucial. The problem is, however, that nitrifying bacteria are aerobic, and their activity might be limited in an anaerobic or low-oxygen environment that is possibly created by packing meat in salt clay. Micro-environments within the clay might not sufficiently support the aerobic conditions needed for these bacteria to function effectively.
Ammonia produced by nitrogen-fixing bacteria like Clostridium pasteurianum can diffuse into meat under the high salt content conditions of clay. The diffusion process is influenced by several factors including the concentration gradient of ammonia between the clay and the meat, the salt content in the clay, the physical properties of the meat, and the duration of exposure.
The concentration gradient drives the diffusion of ammonia from the clay, where its concentration is higher, into the meat, where it is lower. High salt content in the clay can affect water activity and osmotic pressure, potentially causing the meat to lose water and increasing the concentration of ammonia within the meat due to reduced water content. Additionally, the high salt content can preserve the meat, slowing down spoilage and microbial activity that might otherwise consume the ammonia.
The physical properties of the meat, such as its structure and porosity, also play a crucial role in determining the depth of ammonia penetration. Denser and less porous meat will slow the diffusion process, whereas more porous meat will allow for deeper penetration. Over time, ammonia will initially penetrate the outer layers of the meat, within the first few millimeters to centimeters, depending on the concentration and exposure time. With extended exposure, ammonia can diffuse several centimetres into the meat.
In high-salt environments like those in Hallstatt, ammonia diffusion might be more limited compared to lower salt environments because the high osmotic pressure can limit microbial activity and slow the diffusion process. However, given sufficient time, ammonia can still penetrate significantly into the meat. Therefore, while ammonia can diffuse into meat under these conditions, the depth of penetration will depend on the concentration gradient, salt content, physical properties of the meat, and duration of exposure, potentially reaching several centimetres over an extended period.
While Nitrosomonas and Nitrosospira are aerobic bacteria that require oxygen to convert ammonia to nitrite, certain anaerobic bacteria can still play a role in nitrogen transformations under low-oxygen conditions. Anammox bacteria, which belong to the Planctomycetes phylum, can perform anaerobic ammonium oxidation by converting ammonia and nitrite directly into nitrogen gas, although they do not convert ammonia directly to nitrite. For ammonia to be converted to nitrite, the right bacteria needs oxygen which may happen during drying or when the brine is stirred as would happen if it was re-used. More about this, later on.
v. Drying/ Maturing
If deamination is the controlling mechanism that produces ammonia that are microbially changed into nitrite which, chemically is reduced to NO in the meat and ultimately cures it, and since the bacteria responsible for this are strictly aerobic, the next step will allow for the conversion of ammonia to nitrites.
If the meat were removed from the salt clay after 7 days of storage and hung inside the salt mine shafts with a relative humidity of 60% and damp conditions, these conditions could be conducive to the activity of microorganisms capable of converting ammonia (NH₃) to nitrite (NO₂⁻) or nitrate (NO₃⁻).
In particular, Nitrosomonas species are known for oxidizing ammonia to nitrite, while Nitrobacter species oxidize nitrite to nitrate. These bacteria thrive in aerobic environments, and the described conditions in the salt mine shafts, if well-ventilated, would provide the necessary oxygen. The relative humidity of 60% and the damp conditions would also supply the moisture needed for their activity.
The initial step of ammonia oxidation to nitrite, carried out by Nitrosomonas species, can begin within a few days, depending on the concentration of ammonia and the environmental conditions. Under ideal circumstances, significant conversion to nitrite can occur within 7 to 14 days. The subsequent oxidation of nitrite to nitrate by Nitrobacter species would follow, and this second step can also take place within a similar timeframe, resulting in a substantial amount of nitrate being produced within 14 to 28 days from the start of nitrification.
The curing process of the meat, which involves drying and ageing, would depend on various factors, including the size and type of meat, as well as the specific conditions within the salt mine shafts. Typically, traditional curing processes can take several weeks to several months. In the described environment, the initial drying phase might take 1 to 2 weeks, during which surface moisture is reduced and microbial activity begins. The subsequent ageing phase could extend from several weeks to several months, allowing for further moisture reduction, flavour development, and textural changes. During this time, nitrifying bacteria could continue to convert ammonia to nitrite and nitrate, contributing to the overall curing process.
Nitrosomonas and Nitrobacter species, the bacteria responsible for nitrification, are pervasive in nature. They are commonly found in a variety of environments, including soil, water, and air, where conditions allow for their growth and activity. These bacteria can colonize new environments given the right conditions of moisture, oxygen, and nutrients. Soil is a major reservoir for both Nitrosomonas and Nitrobacter, especially in areas rich in organic matter. These bacteria are also present in freshwater and marine environments, playing a crucial role in the nitrogen cycle by converting ammonia from decaying organic matter into nitrites and nitrates. Additionally, dust and aerosols can transport these bacteria over long distances, allowing them to settle in new environments and introduce nitrifying bacteria.
Decomposing organic material, such as the meat buried in the salt clay, can harbour a variety of bacteria, including nitrifiers. As the meat decomposes, these bacteria can become active and proliferate. Human and animal activities, such as agriculture and waste disposal, can also introduce nitrifying bacteria to new environments. For example, manure and other organic fertilizers are rich in these bacteria and can contribute to their spread. Given their adaptability, nitrifying bacteria can colonize various environments where the essential conditions for their growth are met, including moisture, oxygen, and the presence of ammonia or nitrite as substrates.
In the specific scenario of the salt mine shafts in Hallstatt, nitrifying bacteria could be introduced through several sources. Soil particles carried into the mines by workers or through natural processes could bring Nitrosomonas and Nitrobacter into the environment. Organic matter, such as decomposing meat or other biological materials, could also introduce these bacteria. Additionally, airborne particles carrying these bacteria could settle in the mines, further contributing to their presence.
Once introduced, if the conditions of relative humidity, moisture, and oxygen availability are favourable, these bacteria can become active and start the process of nitrification. The high salt content in the clay would not inhibit these bacteria as long as sufficient moisture and organic material are present to support their metabolic needs. Thus, Nitrosomonas and Nitrobacter species, being ubiquitous in nature, could readily colonize the salt mine shafts in Hallstatt, contributing to the curing process of the meat by converting ammonia to nitrite and nitrate under the right environmental conditions.
vi. Feasibility of Meat Curing:
Even if ammonia is produced and converted to nitrite, the concentration of nitrite necessary for effective curing (colour development, flavour enhancement, and microbial inhibition) might not sufficient. Enzymatic deamination can lead to ammonia production, and theoretically, this ammonia could be converted to nitrite which could cure the meat. Under the right conditions, 7 days seems to be long enough for this process to take place. This must be validated through experimentation.
Assuming ammonia is produced during the initial 7-day period in which the meat is submerged in a brine and clay mixture and that it penetrates the meat, the subsequent steps involving drying and possible curing in a salt cave involve several specific conditions and processes.
a.1.2. L-Arginine Accessed Microbial Oxidation
-> The Overview
Another option to consider is L-arginine fermentation. In this method of curing, nitrogen is converted through nitric oxide synthase, where neither NO₃⁻ nor NO₂⁻ is used to initiate the reaction sequence to NO.
A combination of amino acids made available through deamination and the extraction of salt-soluble actin and myosin will serve as nutrients for the bacteria, encouraging their metabolism. In the absence of air, the bacteria will favour nitrogen respiration, accessing L-arginine from these proteins for fermentation to produce nitric oxide (NO). Myoglobin, which is also extracted, will be the main recipient of the curing molecule, nitric oxide, produced by the bacteria, thereby undergoing the curing process.
-> What is L-Arganine?
L-arginine is a semi-essential amino acid crucial for various physiological processes, including nitric oxide production, protein synthesis, and immune function. It features a guanidinium group with three nitrogen atoms, making it highly nitrogen-rich. L-arginine serves as a substrate for nitric oxide synthase (NOS) enzymes, which convert it into nitric oxide (NO) and citrulline, playing key roles in vasodilation, neurotransmission, and immune responses. It is also vital in the urea cycle for detoxifying ammonia and stimulating hormone secretion. Found in meat, poultry, fish, dairy, nuts, seeds, soy, and whole grains, L-Arginine is used in medical treatments for cardiovascular conditions, erectile dysfunction, and athletic performance enhancement. In meat curing, it contributes to the formation of nitrosylmyoglobin, stabilizing the colour and flavour of cured meat.
-> The Role of L-Arginine in Nitrogen Storage and Metabolism
A combination of amino acids made available through deamination and the extraction of salt-soluble actin and myosin, which contain L-arginine, will serve as nutrients for the bacteria, encouraging their metabolism. In the absence of air, the bacteria will favour nitrogen respiration, accessing L-arginine from these proteins for fermentation to produce nitric oxide (NO). Myoglobin, which is also extracted, will be the main recipient of the curing molecule, nitric oxide, produced by the bacteria, thereby undergoing the curing process.
-> Why L-Arginine is Used to Store Nitrogen
L-arginine is an amino acid with a high nitrogen content, making it a key player in nitrogen storage and metabolism. It has three nitrogen atoms within its structure, which are crucial for various biological processes. The structure of L-arginine includes a guanidinium group, which is responsible for its high nitrogen content and its role in storing additional nitrogen.
-> Function of L-Arginine in Nitrogen Oxidation through Indigenous Enzymes
L-arginine serves as a substrate for nitric oxide synthase (NOS) enzymes, which oxidize L-arginine to produce nitric oxide (NO) and citrulline. This process is crucial for various physiological functions, including vasodilation, neurotransmission, and immune response. In muscle tissues, the production of NO can also influence muscle contraction and relaxation.
-> Bacterial Production of Similar Enzymes
Bacteria can produce enzymes similar to nitric oxide synthase, enabling them to utilize L-arginine for the production of nitric oxide. This ability is particularly important for bacteria in anaerobic environments, where oxygen is scarce. By using L-arginine, bacteria can carry out nitrogen respiration, which helps them to survive and thrive in such conditions.
-> Function of Nitric Oxide Production in Bacterial Metabolism
In bacterial metabolism, the production of nitric oxide from L-arginine serves several functions. It can act as a signalling molecule, helping to regulate bacterial communication and biofilm formation. Additionally, nitric oxide can serve as a defence mechanism against oxidative stress and immune responses. In the context of meat curing, the bacterial production of nitric oxide plays a critical role in the curing process by reacting with myoglobin to form nitrosylmyoglobin, which stabilizes the colour and flavour of the cured meat.
L-arginine’s role in nitrogen storage and metabolism is facilitated by its high nitrogen content and the presence of the guanidinium group on its side chain. This amino acid is crucial for the production of nitric oxide through the action of nitric oxide synthase enzymes, both in muscle tissues and bacteria. Bacteria leverage this process for nitrogen respiration and survival in anaerobic environments, ultimately contributing to the curing process of meat by producing nitric oxide that reacts with myoglobin. This complex interplay highlights the importance of L-arginine and bacterial enzymes in meat curing and preservation.
-> Hypothetical Mechanism
The system could work in the following manner: the actual protein that needs curing is myoglobin, and L-arginine is present in various proteins within the meat, including sarcoplasmic proteins. As salt penetrates the meat from the salt-rich clay in Hallstatt, these proteins are solubilized. The solubilization of sarcoplasmic proteins, such as actin and myosin, due to the salt releases L-arginine, making it accessible to bacteria. This process allows the bacteria to utilize the solubilized L-arginine without needing to penetrate the meat deeply, which would otherwise slow down the reaction. The bacteria can then use the accessible L-arginine to produce nitric oxide (NO), contributing to the curing process.
Myoglobin may also migrate out of the meat under osmotic pressure. If it is exposed to nitric oxide (NO), it can react with the heme group in myoglobin to form nitrosylmyoglobin, a stable cured form of the protein. This nitrosylmyoglobin could potentially migrate back into the meat, contributing to the curing process and enhancing the flavour profile of the final product. Under osmotic pressure, the initial migration of myoglobin out of the meat and the subsequent reabsorption of nitrosylmyoglobin into the meat can play a significant role in the overall curing process.
The entire process of curing meat through L-arginine fermentation and the subsequent formation and migration of nitrosylmyoglobin can potentially begin within 7 days, but full curing and flavour development typically require a longer period.
The curing process can continue effectively if the meat is removed from the clay and hung in the mine shafts with a relative humidity of 60%. The environment in the mine shafts provides the necessary conditions to support ongoing microbial activity and curing. The salt absorbed into the meat from the initial clay exposure will continue to influence water activity, inhibiting spoilage bacteria while allowing beneficial curing bacteria to thrive. Beneficial microorganisms, such as those from the genera Staphylococcus and Kocuria, will continue to utilize the available nutrients and contribute to the curing process.
As the meat hangs in the mine shafts, the gradual reduction of moisture content will occur, facilitated by the 60% relative humidity and damp conditions. This drying process is crucial for achieving the desired texture and further preserving the meat. During this period, the bacteria will continue to produce nitric oxide (NO) from the accessible L-arginine, which will react with myoglobin to form nitrosylmyoglobin. This reaction will enhance the cured meat colour and flavour. The ongoing enzymatic and microbial activities will deepen the curing effect, resulting in a more complex and desirable flavour profile.
The initial phase of curing, which involves solubilization, microbial activity, and NO production, occurs within the first 7 days while the meat is in the clay. However, the extended curing process after removing the meat and hanging it in the mine shafts is essential for full flavour development and preservation. Over the next several weeks to months, the continued drying and ageing processes will enhance the meat’s texture, flavour, and overall quality. While significant progress can be made within the first 7 days, optimal curing typically requires a longer period, allowing the meat to develop its full range of flavours and characteristics.
Here is a more detailed breakdown:
* Initial Solubilization and Migration:
Day 1-3: As salt from the salt-rich clay penetrates the meat, it begins to solubilize sarcoplasmic proteins, including actin, myosin, and myoglobin. During this period, L-arginine becomes accessible, and some myoglobin may migrate out of the meat under osmotic pressure.
* Microbial Activity and Nitric Oxide Production:
Day 3-7: Bacteria that utilize L-arginine start to ferment it, producing nitric oxide (NO). The NO can react with the heme group in myoglobin to form nitrosylmyoglobin, a stable cured form of the protein. During this time, nitrosylmyoglobin can potentially migrate back into the meat, contributing to the curing process and enhancing the meat’s flavour and colour.
* Drying in the Mine Shafts:
After the initial 7 days, the meat can be removed from the clay and hung in the mine shafts with a relative humidity of 60%. This environment provides the necessary conditions to support ongoing microbial activity and curing. The gradual reduction of moisture content, facilitated by the relative humidity and damp conditions, is crucial for achieving the desired texture and further preserving the meat. The drying process will continue to support the conversion of L-arginine by bacteria, allowing the curing to proceed effectively.
* Factors Affecting the Process:
- Salt Concentration: Higher salt concentrations can accelerate protein solubilization and microbial activity.
- Temperature and Humidity: These environmental factors influence the rate of microbial fermentation and NO production.
While the initial stages of solubilization, microbial activity, and nitrosylmyoglobin formation can occur within the first 7 days, achieving full curing and optimal flavour development usually takes longer. The traditional curing process, which involves extended drying and ageing, typically extends from several weeks to several months. This extended period allows for thorough flavour development and preservation. Therefore, while significant progress can be made within the first 7 days, complete curing likely requires additional time to ensure the meat reaches its optimal flavour and texture.
-> The Bacteria Cultures
Related to the bacterial cultures, Staphylococcus carnosus, Staphylococcus xylosus, and Kocuria spp. appear to be of particular interest. These bacteria are known for their roles in meat curing and fermentation processes. Specific strains that could be present in the salt clay in Hallstatt include:
- Staphylococcus xylosus C2a and Staphylococcus xylosus DSM 20266T: These strains are effective in curing processes and are known for their ability to produce nitric oxide, which is crucial for the formation of nitrosylmyoglobin.
- Strains deposited with the DSMZ, such as:
- Staphylococcus xylosus FASP 2 and FASP 3
- Staphylococcus vitulinus FASP 4
- Staphylococcus equorum FASP 1 and FASP 5
- Kocuria salsicia FASP 6
- Kocuria varians FASP 7
- Staphylococcus carnosus, noted for its upload to the ATCC portal as 5136, is also relevant for its curing properties, although it might not be as prevalent in the Hallstatt environment compared to Staphylococcus xylosus and Kocuria spp.
These strains are likely to be found in the salt-rich clay of Hallstatt, where the anaerobic conditions and availability of nutrients from decomposing organic matter provide an ideal environment for their growth and activity. These bacteria play a significant role in the curing process by facilitating the production of nitric oxide and contributing to the overall flavour and preservation of the meat.
Staphylococcus carnosus is unlikely to be present in the clay-salt environment of Hallstatt due to several factors related to its natural habitat and specific environmental requirements. Staphylococcus carnosus was discovered through research into the microbial ecology of meat fermentation and curing environments. It was not naturally prevalent in the wild but became significant through human cultivation and use in the meat industry. Before its application in meat curing, it would primarily be found in places where meat was processed or stored, highlighting its close association with human activities and meat products. Primarily, Staphylococcus carnosus is associated with meat and meat products, often utilized in modern industrial meat fermentation and curing processes. Its natural habitat is typically linked to domesticated environments where meat processing occurs, rather than natural settings such as soil or clay. Unlike Staphylococcus xylosus and Kocuria spp., which are commonly found in a variety of environments including soil and water, Staphylococcus carnosus is not typically present in unprocessed natural environments. This makes its presence in the clay-salt environment of Hallstatt less likely.
Additionally, Staphylococcus xylosus and Kocuria spp. are known for their adaptability to diverse environments, including high-salt and low-moisture conditions characteristic of the Hallstatt clay-salt environment. These bacteria can thrive in a range of environmental conditions, making them more robust compared to Staphylococcus carnosus, which has more specific habitat requirements. In natural environments like the clay-salt of Hallstatt, bacteria must compete with a wide variety of other microorganisms for resources. Staphylococcus carnosus, being less adaptable to such competitive and varied conditions, would likely be outcompeted by more versatile bacteria such as Staphylococcus xylosus and Kocuria spp.
Furthermore, the historical context of Hallstatt, dating back to the Bronze and Iron Ages, predates the use of Staphylococcus carnosus in meat fermentation and curing processes. The bacteria present in the environment at that time would more likely be those naturally occurring in soil and salt environments, rather than those introduced through human activity in modern meat processing industries. Therefore, it is more plausible that bacteria like Staphylococcus xylosus and Kocuria spp., which are naturally adapted to survive in diverse and challenging conditions, would be more prevalent in the Hallstatt curing vats. These bacteria are well-suited to the anaerobic, high-salt conditions of the Hallstatt curing vats and play a significant role in the meat curing process.
-> Commercial Systems Exploiting L-Arginine Fermentation
The effectiveness of the general approach has been demonstrated in traditional dry curing methods that use only salt (Toldrá, F., 2010, Handbook of Meat Processing). European producers are now developing commercial brine solutions that utilize the same mechanism.
A European producer has developed a system that uses a blend of Staphylococcus xylosus rather than Staphylococcus carnosus. This system operates with minimal nitric oxide synthase and other functionalities like antioxidative effects. It does not add arginine but supplements the culture with special yeast extracts. During the early heating stages, this system stabilizes the natural meat colour without using hidden nitrites or nitrates, and it does not convert natural nitrates from raw materials into nitrites or NO. This prevents the formation of nitrosylheme, nitrosamines, and carcinogenic compounds.
Another European producer has proposed a project outline focusing on the use of different yeasts with the potential to release ascorbic acid from beet leaves, utilizing enzymes such as cellulase and pectinase. They have various lysates with antioxidant potential to retain colour, yeasts with iron-retaining capacity, and other microorganisms that could be useful for zinc protoporphyrin formation, which is also interesting for colour formation.
b. Parallel Hallstatt Method (Based on Fritz Eckart Barth’s findings) with a Speculative Curing Pathway Resumed
We finally return to the proposed mechanism given by Fritz Barth and translated by Christa Berger. We primarily dealt with the burial of the meat or sealing in with clay as the salting step. In our discussions, we have to incorporate the next step given by him, namely drying to arrive at two possible mechanisms that would make at least theoretical sense. His “salting” step includes the re-use of the brine until it becomes unstable.
Both systems proposed namely the deamination/ ammonia formation and the L-Arganine Fermentation pathway would continue if the brine is re-used with the only possible difference that while the meat is being changed, ammonia would have a chance to be oxidised to nitrite and nitrate and myoglobin, enriched with nitric oxide from L-Arganine fermentation would/ could be present and powerfully contribute to the next round of curing.
I do not doubt that the old brine would be far more effective than the new brine where the curing vat is used for the first time. This re-use of the old brine is exactly the method used in the Wiltshire or Live Brine system which I have researched over many years and is summarised in Bacon & the Art of Living, Wiltshire Cured or Tank Cured Bacon.
Let’s review the system proposed by Barth once more.
1. Salting in an Anaerobic Environment
- Meat is buried in brine pans filled with small pieces of rock salt or rich Haselgebirge.
- The containers are sealed with clay to create an anaerobic environment, facilitating a unique curing process involving both salting and potential fermentation due to microbial action.
- After 7-10 days, the initial salting phase is complete, though the meat is not yet fully preserved.
2. Extracting and Utilizing Meat Juices:
- The extracted brine, enriched with proteins like myoglobin and potentially beneficial bacteria like staphylococci and lactic acid bacteria, remains in the clay.
3. Use of Extracted Brine:
- The highly concentrated brine, rich in water-soluble components, is collected and could be used for further salting or as a flavour enhancer.
4. Further Processing:
- The meat would then need to be dried and matured in the mine’s microclimate, benefiting from consistent temperatures and high humidity, mimicking modern dry curing chambers.
c. Impact of Excess Sodium Hydroxide Production on Brine Stability in Hallstatt Curing Vats
Barth’s observations about the curing vats in Hallstatt reveal that these vats were often rebuilt when the brine became unstable, leading to a decline in their effectiveness. One significant factor contributing to this instability could be the excess production of sodium hydroxide (NaOH) by lactic acid bacteria (LAB) in the brine. This excess NaOH can create a series of problems that destabilize the curing environment, necessitating the reconstruction of the vats.
Lactic acid bacteria, while beneficial for producing organic acids that aid in meat preservation, can also produce NaOH under certain conditions. These conditions could indeed exist in the clay vats in Hallstatt after repeated use of the brine. Repeated cycles of curing and the accumulation of organic material can alter the microbial balance and chemistry of the brine. Over time, the depletion of readily fermentable carbohydrates and the buildup of nitrogenous waste products can lead to shifts in LAB metabolism, potentially increasing the production of NaOH.
When NaOH production is excessive, it raises the pH of the brine, making it more alkaline. This elevated pH can significantly impact the oxidation of fats within the meat. Alkaline conditions accelerate the breakdown of fatty acids into free radicals and peroxides, which then degrade into rancid compounds such as aldehydes and ketones. These rancid compounds not only deteriorate the flavor and aroma of the meat but also reduce its overall quality and shelf life.
In the context of Hallstatt’s curing vats, the occurrence of fat oxidation and resultant rancidity would render the brine ineffective for its intended purpose of preserving and flavouring meat. The sensory properties of the meat would suffer, making it unpalatable and decreasing its market value. Furthermore, the oxidative degradation of fats would destroy essential fatty acids and vitamins, diminishing the nutritional value of the meat.
Barth’s account suggests that the rebuilding of vats was necessary when the brine reached a point of instability. This instability could be directly linked to the excess NaOH production, which disrupts the delicate balance required for effective curing. An unstable brine would also affect the microbial ecosystem within the vat. LAB thrive in mildly acidic environments, and a shift toward alkalinity can inhibit their growth and activity. This inhibition reduces the production of beneficial organic acids that are crucial for the curing process, further destabilizing the brine.
Moreover, an alkaline environment can promote the growth of spoilage bacteria and pathogens that are normally suppressed in acidic conditions. This shift not only poses food safety risks but also accelerates the degradation of the curing environment, making the vats unsuitable for continued use.
In conclusion, Barth’s observations about the periodic rebuilding of the Hallstatt curing vats highlight the critical impact of brine instability. Excess production of sodium hydroxide by LAB can lead to elevated pH levels, causing fat oxidation and rancidity, which compromise meat quality. The conditions that promote excessive NaOH production can indeed develop in the clay vats after repeated use. Maintaining a balanced pH in the brine is essential to prevent instability, ensure the effectiveness of the curing process, and avoid the need for frequent vat reconstruction.
d. Re-Use of Old Brine
I already addressed this matter to some extent in reference to Wiltshire or the Live Brine System where brine is re-used indefinitely. The question of whether the Hallstatt curing called for the brine to be re-used. Christa Berger addresses this matter in personal correspondence. She writes, “One question is whether the collected brine with the meat juice was renewed after each procedure. And here I think, yes. The reason is that the clay releases salt into the meat, and the salt concentration decreases with each new batch. In the large vessels, a salt solution could have been prepared and then transferred to the basin. The Hallstatt people likely had knowledge of the correct concentration. I also think this because a whisk was found in the basin, which was probably accidentally thrown in after mixing. These whisks are not robust enough to stir the clay itself; I know this from my own experience, as my grandfather made such whisks from old Christmas trees. Incidentally, similar whisks were also found in Northern Italy, 3,500 years old, in the stilt house museum at Lake Carera near Trento, made of fir wood (as it is still today and was in Hallstatt, too). If this was the case, the brine-meat juice mixture could indeed be further processed into soup – first jelly – then dried. Otherwise, the liquid would have “broken,” as we say, meaning the acid-base ratio would no longer be correct; in other words, it would be spoiled. Actually, only the clay needs to be re-enriched with salt.”
As this re-salting of the brine takes place, the brine will be enriched with another key component, namely oxygen. Earlier, we noted that Nitrosomonas and Nitrobacter convert ammonia to nitrite under aerobic conditions, and stirring the brine will facilitate oxygen to be mixed into the old brine, which could promote this conversion.
Christa places another startling possibility on the table when she writes, “The piece of leather found in the basin also intrigued me. It is conceivable that it was used to introduce bacteria, perhaps to rub onto the meat. The leather piece could have been used to introduce beneficial bacteria, to inoculate the meat and so to aid in the curing process.”
I uncovered a sophisticated meat preservation technique from West Africa using “beneficial bacteria” originating from the skin of the animal. An agriculture chemist once told me that a Log 6 size colony of healthy bacteria is sufficient to keep any pathogens at bay. Students in Veterinary sciences in Lagos confirmed to me that when they investigated for pathogens at abattoirs that use this slaughtering technique, no pathogens could be found on freshly slaughtered carcasses despite the complete absence of any form of refrigeration. The meat is transported to markets across the city in ambient vehicles and it was only after a few days at the markets that they started to encounter pathogens in small numbers.
Nitrite in Aqueous Solution and Its Role in Meat Curing
Some revision is in order. Nitrite (NO₂⁻) in aqueous solution can form nitrous acid (HNO₂). When nitrite salts dissolve in water, they react with the water to form nitrous acid through the equilibrium reaction:
NO₂⁻ + H₂O ⇌ HNO₂ + OH⁻
In this reaction, the nitrite ion (NO₂⁻) interacts with water to produce nitrous acid (HNO₂) and hydroxide ions (OH⁻). Nitrous acid is a weak acid that partially dissociates in water, producing nitrite ions and hydrogen ions (H⁺):
HNO₂ ⇌ H⁺ + NO₂⁻
The formation of nitrous acid is significant in various chemical and biological processes, including food preservation and curing. Nitrous acid can further decompose to produce nitric oxide (NO) and nitrogen dioxide (NO₂):
2 HNO₂ → NO + NO₂ + H₂O
Curing Reactions Involving Nitric Oxide
Nitric oxide (NO) plays a critical role in the curing of meat. When NO reacts with myoglobin, a muscle protein, it forms nitrosylmyoglobin, which is responsible for the characteristic pink color of cured meat. The reaction can be represented as follows:
Myoglobin + NO → Nitrosylmyoglobin
This reaction stabilizes the colour and enhances the flavour of the cured meat. However, nitric oxide is a reactive species and can be further oxidized to nitrite (NO₂⁻) or nitrate (NO₃⁻) in the presence of oxygen or through microbial action:
2 NO + O₂ → 2 NO₂
NO₂ + O₂ + H₂O → 2 HNO₃
Interplay of Reactive Nitrogen Species in Curing
The curing process in Hallstatt likely involved a complex interplay of various reactive nitrogen species, including nitric oxide (NO), ammonia (NH₃), nitrite (NO₂⁻), and nitrate (NO₃⁻). These species are part of what is collectively known as reactive nitrogen species. The oxidation and reduction reactions involving these species are critical to the curing process.
The presence of both deamination and bacterial oxidation of L-arginine suggests that multiple pathways could be at work simultaneously. Deamination of proteins can produce ammonia, which in turn can be oxidized to nitrite and nitrate by nitrifying bacteria. The bacterial oxidation of L-arginine to nitric oxide adds another layer to the curing process.
Implications for Meat Curing in Hallstatt
This multifaceted approach to meat curing has profound implications for understanding the history and techniques used in ancient meat preservation. The combination of chemical and microbial actions likely created an environment where all four nitrogen species were present in varying concentrations. The equilibrium between these reactive nitrogen species ensured that the curing process was efficient and effective.
Nitrite (NO₂⁻) in aqueous solution existing as nitrous acid (HNO₂) plays a crucial role in these reactions. Nitrous acid can decompose to form nitric oxide, which then reacts with myoglobin to produce nitrosylmyoglobin, stabilizing the meat’s colour. The continual cycling of these nitrogen species, through both chemical and microbial pathways, ensures the meat remains preserved and develops the desired flavour and colour.
Conclusion
The Hallstatt curing vats represent a remarkable intersection of ancient technology and sophisticated meat preservation techniques. The site offers invaluable insights into early European curing methods, emphasizing the importance of maintaining brine stability to ensure successful meat curing. As we continue to explore and understand these ancient practices, we uncover the foundations of an industry that has shaped culinary traditions for millennia. The Hallstatt curing vats not only provide a glimpse into the past but also inform modern practices, highlighting the enduring legacy of this extraordinary site.
References
Barth, F. E., & Lobisser, W. (2002). Research on the function of ancient block wall constructions around the salt mines of Hallstatt.
Bolstad, A.I., Jensen, H.B., & Bakken, V. (1996). Taxonomy, biology, and periodontal aspects of Fusobacterium nucleatum. Clinical Microbiology Reviews, 9(1), 55-71.
Cato, E.P., George, W.L., & Finegold, S.M. (1986). Genus Clostridium. In Bergey’s Manual of Systematic Bacteriology (Vol. 2).
Chaves, S. et al. (2019). Microbial communities in archaeological environments: The case of the La Draga Neolithic site. PLOS ONE, 14(7), e0219397.
Correspondence with a European producer regarding the “Natural Rose” system.
Frossard, A., Gerull, L., Mutz, M., & Gessner, M.O. (2012). Anaerobic microbial activity in clay soils. Soil Biology and Biochemistry, 50, 58-68.
Gunde-Cimerman, N., Oren, A., & Plemenitaš, A. (2005). Microbial diversity and metabolic potential in salt mine environments. FEMS Microbiology Ecology, 53(1), 55-66.
“Mechanisms of protein solubilization in curing processes,” Food Science Journal.
“Microbial cultures in meat fermentation,” European Microbiology Review.
“Microbial safety in starter cultures,” Food Safety Journal.
“Nitric oxide synthase and its role in L-arginine fermentation,” European Producer Research Documentation.
Project outline from another European producer on yeasts and lysates for colour retention.
Stackebrandt, E., & Schumann, P. (2006). Introduction to the taxonomy of Clostridia. Anaerobe, 12(5-6), 253-260.
Toldrá, F. (2010). Handbook of Meat Processing.
Wexler, H.M. (2007). Bacteroides: The good, the bad, and the nitty-gritty. Clinical Microbiology Reviews, 20(4), 593-621.
Berger, Christa. Private Communication.
Featured Image: https://nhm-wien.ac.at/hallstatt/en/trading_hub/meat_processing_industry/meat_curing_vats
to 
to NO 
. Nitrous acid is formed. This hydration of nitrous acid is an important time-consuming reaction (Krause, B. L.; 2009: 9).
