Fermentation is a processing technology that utilizes microbial action under natural or artificially controlled conditions to endow meat with unique flavor, color and texture, so as to produce meat products with an extended shelf life.
Two Generations of Starter Bacteria
The first-generation starters are derived from plants, represented by Lactobacillus plantarum and Pediococcus pentosaceus. The second-generation starters are isolated from meat products, which are more suitable for fermented sausage production. Their predominant microorganisms include Lactobacillus sakei and Lactobacillus curvatus. With strong competitive advantages, these two strains can inhibit wild lactic acid bacteria in the natural environment and dominate the entire fermentation and drying process.
The second-generation starters also have the following characteristics: they can produce enzymes that contribute to color formation and aromatic substances. The flavor and sensory quality of fermented sausages result from the combined effects of lactic acid bacteria, micrococci and yeasts inside the sausage. At present, the β-galactosidase gene, catalase gene and bacteriocin gene of lactobacilli have been cloned, which can improve the properties of bacterial strains. The application of bacteriocin-producing lactic acid bacteria in fermented sausages can enhance the competitiveness of starters and inhibit the growth of pathogenic bacteria. Lactobacillus plantarum, Lactobacillus sakei and Lactobacillus curvatus are all capable of producing bacteriocins.
Functions of Microorganisms in Fermented Meat Products
To reduce pH value, inhibit spoilage, improve tissue texture and flavor; promote color development; prevent oxidative discoloration; reduce nitrosamine formation; and suppress the growth and toxin production of pathogenic microorganisms.
Microorganisms in fermented sausages mainly include lactic acid bacteria, micrococci, molds and yeasts, each playing a unique role in flavor formation and food safety of fermented sausages.
Fermentation Methods
① Natural Fermentation
In traditional processing, fermentation completely relies on indigenous lactic acid bacteria in raw meat. Lactic acid bacteria are ubiquitous in raw meat but with an extremely low initial count, unless the raw meat has been stored in vacuum packaging for a period of time.
The initial conditions of sausage batter are generally unfavorable to the growth of dominant Gram-negative bacteria in meat, but conducive to the proliferation of Gram-positive bacteria, coagulase-positive and coagulase-negative staphylococci, as well as lactic acid bacteria. Evidence shows that lactic acid fermentation involves a sequential microbial succession from Enterobacteriaceae to Enterococci, and finally to Lactobacilli and Pediococci. With smooth fermentation, lactic acid bacteria multiply rapidly, reaching a colony count of 106∼108 cfu/g within 2 to 5 days. The consequent pH decline causes the death of Pseudomonas and other acid-sensitive Gram-negative bacilli in 2 to 3 days, while acid-resistant bacteria such as Salmonella may survive longer.
After reaching the peak quantity, the population of lactic acid bacteria gradually declines. However, mold-ripened sausages often show a second growth peak after about 15 days, consistent with the pH rise caused by lactate metabolism. Delayed initiation of lactic acid fermentation and slow pH reduction will facilitate the growth and enterotoxin production of Staphylococcus aureus, and the proliferation of miscellaneous bacteria will deteriorate sausage flavor. Fermented sausages usually contain nitrates instead of nitrites, allowing the growth of a wide range of microorganisms, which is beneficial to improving the flavor quality of dry fermented sausages.
To improve the stability and reliability of natural fermentation, the back-slopping method was widely adopted in early production, which refers to inoculating fresh sausage batter with partially fermented materials from the previous production batch. This method effectively enhanced fermentation stability but had obvious drawbacks. Firstly, the lactic acid bacteria in back-slopped materials were physiologically aged and failed to initiate rapid fermentation. Secondly, the uncontrollability of this method might introduce undesirable strains, such as peroxide-producing bacteria, which would severely compromise sausage quality once they became dominant.
Among lactic acid bacteria isolated from naturally fermented sausages, Lactobacillus accounts for the majority, followed by Pediococcus, which even dominates in the fermentation of certain sausage varieties. Key Pediococcus species include Pediococcus acidilactici, Pediococcus damnosus and Pediococcus pentosaceus. Except for low-quality sausages with abundant Leuconostoc, the quantities of Lactococcus and Leuconostoc are generally low.
② Starter Culture Fermentation
Due to the unreliability and uncontrollability of natural fermentation, modern processing increasingly adopts pure microbial cultures, namely commercial starter cultures, to precisely control the fermentation process and ensure product safety and stable quality. Fermentation initiated by lactic acid bacteria starters is basically consistent with successful natural fermentation, except that starter cultures enable lactic acid bacteria to become dominant strains more rapidly.
Commercial meat starter cultures are available in frozen or freeze-dried forms, including single-strain and mixed-strain preparations of lactobacilli, pediococci and molds. Active starters are generally added during the batching stage. Although most manufacturers add starters after dry ingredients, uniform distribution requires mixing starters thoroughly with raw meat prior to adding other ingredients.
Crucially, viable microbial cultures must not come into direct contact with high-salinity ingredients such as salt and nitrites, otherwise the viability and activity of strains will be reduced. Most starters are sold in concentrated form and can be evenly distributed after dilution with water. Freeze-dried starters require hydration to achieve optimal activity.
Fermentation Process Conditions
Temperature, humidity and air circulation in the fermentation chamber collectively affect the flavor, color and final pH of fermented sausages.
Industrial starters are mostly freeze-dried and need rehydration before use. Rehydrated starters should be placed at room temperature for 18 to 24 hours to restore microbial activity before being incorporated into sausage batter. The conventional inoculation dosage is 106∼107 cfu/g of meat batter, and a higher dosage up to 108 cfu/g is required for high-temperature short-time fermentation.
Fermentation temperatures are classified into three categories: high temperature (＞40℃), traditional European temperature (20～24℃) and low temperature (10～15℃), selected according to product types. In general, a slightly higher temperature accelerates pH reduction; the acid production rate doubles with every 5℃ temperature rise. Nevertheless, high temperature increases the risk of pathogenic bacterial growth (especially Staphylococcus aureus) if fermentation is delayed. Temperature also regulates the ratio of lactic acid to acetic acid produced, with higher temperatures favoring lactic acid synthesis.
In practical production, fermentation parameters vary greatly for different sausage types. Dry sausages are commonly fermented at 15～27℃ for 24 to 72 hours; spreadable sausages at 22～30℃ for 48 hours; semi-dry sliced sausages at 30～37℃ for 14 to 72 hours. Processing conditions differ drastically across regions: for instance, Hungarian salami is fermented below 10℃, while low-pH semi-dry smoked sausages in the United States are fermented at up to 40℃.
Ambient relative humidity is critical for initiating drying and preventing excessive growth of surface yeasts and molds, and thus requires strict control. Proper humidity management also avoids the formation of a hard surface crust during drying. A hardened crust hinders internal moisture removal and prolongs drying time; meanwhile, excessive surface moisture in crusty sausages leads to mold growth during storage. For high-temperature short-time fermentation, the relative humidity is usually set at approximately 98%. For low-temperature fermentation, the relative humidity in the chamber should be 5% to 10% lower than the equilibrium moisture-related humidity inside sausages (around 90%).
In modern production, sausage fermentation is carried out in sealed chambers with strictly controlled temperature and humidity. Mild smoking can be applied to certain sausage varieties at this stage without interfering with fermentation progress. In the past, due to the lack of precise environmental control, specific measures were adopted in some countries to prevent spoilage during fermentation. Though redundant for modern production, these traditional methods are still applied to specialty products to obtain unique sensory characteristics. For example, certain German sausages are fermented at 25℃ under high humidity, with excessive surface microorganisms removed by regular washing.
Dry sausages ferment faster in still air than in rapidly circulating air. The acidification degree of fermented sausages varies by product type. Semi-dry sausages have the highest acidity, especially American semi-dry sausages with a post-fermentation pH below 5.0. The pH of German dry sausages ranges from 5.0 to 5.3, while dry sausages from France, Italy and other regions show mild acidification. Vacuum-filled and large-diameter sausages exhibit the strongest acidification due to hypoxic conditions. However, ammonia accumulation in large-diameter sausages counteracts the pH decline caused by lactic acid production.
Two Generations of Starter Bacteria
The first-generation starters are derived from plants, represented by Lactobacillus plantarum and Pediococcus pentosaceus. The second-generation starters are isolated from meat products, which are more suitable for fermented sausage production. Their predominant microorganisms include Lactobacillus sakei and Lactobacillus curvatus. With strong competitive advantages, these two strains can inhibit wild lactic acid bacteria in the natural environment and dominate the entire fermentation and drying process.
The second-generation starters also have the following characteristics: they can produce enzymes that contribute to color formation and aromatic substances. The flavor and sensory quality of fermented sausages result from the combined effects of lactic acid bacteria, micrococci and yeasts inside the sausage. At present, the β-galactosidase gene, catalase gene and bacteriocin gene of lactobacilli have been cloned, which can improve the properties of bacterial strains. The application of bacteriocin-producing lactic acid bacteria in fermented sausages can enhance the competitiveness of starters and inhibit the growth of pathogenic bacteria. Lactobacillus plantarum, Lactobacillus sakei and Lactobacillus curvatus are all capable of producing bacteriocins.
Functions of Microorganisms in Fermented Meat Products
To reduce pH value, inhibit spoilage, improve tissue texture and flavor; promote color development; prevent oxidative discoloration; reduce nitrosamine formation; and suppress the growth and toxin production of pathogenic microorganisms.
Microorganisms in fermented sausages mainly include lactic acid bacteria, micrococci, molds and yeasts, each playing a unique role in flavor formation and food safety of fermented sausages.
Fermentation Methods
① Natural Fermentation
In traditional processing, fermentation completely relies on indigenous lactic acid bacteria in raw meat. Lactic acid bacteria are ubiquitous in raw meat but with an extremely low initial count, unless the raw meat has been stored in vacuum packaging for a period of time.
The initial conditions of sausage batter are generally unfavorable to the growth of dominant Gram-negative bacteria in meat, but conducive to the proliferation of Gram-positive bacteria, coagulase-positive and coagulase-negative staphylococci, as well as lactic acid bacteria. Evidence shows that lactic acid fermentation involves a sequential microbial succession from Enterobacteriaceae to Enterococci, and finally to Lactobacilli and Pediococci. With smooth fermentation, lactic acid bacteria multiply rapidly, reaching a colony count of 106∼108 cfu/g within 2 to 5 days. The consequent pH decline causes the death of Pseudomonas and other acid-sensitive Gram-negative bacilli in 2 to 3 days, while acid-resistant bacteria such as Salmonella may survive longer.
After reaching the peak quantity, the population of lactic acid bacteria gradually declines. However, mold-ripened sausages often show a second growth peak after about 15 days, consistent with the pH rise caused by lactate metabolism. Delayed initiation of lactic acid fermentation and slow pH reduction will facilitate the growth and enterotoxin production of Staphylococcus aureus, and the proliferation of miscellaneous bacteria will deteriorate sausage flavor. Fermented sausages usually contain nitrates instead of nitrites, allowing the growth of a wide range of microorganisms, which is beneficial to improving the flavor quality of dry fermented sausages.
To improve the stability and reliability of natural fermentation, the back-slopping method was widely adopted in early production, which refers to inoculating fresh sausage batter with partially fermented materials from the previous production batch. This method effectively enhanced fermentation stability but had obvious drawbacks. Firstly, the lactic acid bacteria in back-slopped materials were physiologically aged and failed to initiate rapid fermentation. Secondly, the uncontrollability of this method might introduce undesirable strains, such as peroxide-producing bacteria, which would severely compromise sausage quality once they became dominant.
Among lactic acid bacteria isolated from naturally fermented sausages, Lactobacillus accounts for the majority, followed by Pediococcus, which even dominates in the fermentation of certain sausage varieties. Key Pediococcus species include Pediococcus acidilactici, Pediococcus damnosus and Pediococcus pentosaceus. Except for low-quality sausages with abundant Leuconostoc, the quantities of Lactococcus and Leuconostoc are generally low.
② Starter Culture Fermentation
Due to the unreliability and uncontrollability of natural fermentation, modern processing increasingly adopts pure microbial cultures, namely commercial starter cultures, to precisely control the fermentation process and ensure product safety and stable quality. Fermentation initiated by lactic acid bacteria starters is basically consistent with successful natural fermentation, except that starter cultures enable lactic acid bacteria to become dominant strains more rapidly.
Commercial meat starter cultures are available in frozen or freeze-dried forms, including single-strain and mixed-strain preparations of lactobacilli, pediococci and molds. Active starters are generally added during the batching stage. Although most manufacturers add starters after dry ingredients, uniform distribution requires mixing starters thoroughly with raw meat prior to adding other ingredients.
Crucially, viable microbial cultures must not come into direct contact with high-salinity ingredients such as salt and nitrites, otherwise the viability and activity of strains will be reduced. Most starters are sold in concentrated form and can be evenly distributed after dilution with water. Freeze-dried starters require hydration to achieve optimal activity.
Fermentation Process Conditions
Temperature, humidity and air circulation in the fermentation chamber collectively affect the flavor, color and final pH of fermented sausages.
Industrial starters are mostly freeze-dried and need rehydration before use. Rehydrated starters should be placed at room temperature for 18 to 24 hours to restore microbial activity before being incorporated into sausage batter. The conventional inoculation dosage is 106∼107 cfu/g of meat batter, and a higher dosage up to 108 cfu/g is required for high-temperature short-time fermentation.
Fermentation temperatures are classified into three categories: high temperature (＞40℃), traditional European temperature (20～24℃) and low temperature (10～15℃), selected according to product types. In general, a slightly higher temperature accelerates pH reduction; the acid production rate doubles with every 5℃ temperature rise. Nevertheless, high temperature increases the risk of pathogenic bacterial growth (especially Staphylococcus aureus) if fermentation is delayed. Temperature also regulates the ratio of lactic acid to acetic acid produced, with higher temperatures favoring lactic acid synthesis.
In practical production, fermentation parameters vary greatly for different sausage types. Dry sausages are commonly fermented at 15～27℃ for 24 to 72 hours; spreadable sausages at 22～30℃ for 48 hours; semi-dry sliced sausages at 30～37℃ for 14 to 72 hours. Processing conditions differ drastically across regions: for instance, Hungarian salami is fermented below 10℃, while low-pH semi-dry smoked sausages in the United States are fermented at up to 40℃.
Ambient relative humidity is critical for initiating drying and preventing excessive growth of surface yeasts and molds, and thus requires strict control. Proper humidity management also avoids the formation of a hard surface crust during drying. A hardened crust hinders internal moisture removal and prolongs drying time; meanwhile, excessive surface moisture in crusty sausages leads to mold growth during storage. For high-temperature short-time fermentation, the relative humidity is usually set at approximately 98%. For low-temperature fermentation, the relative humidity in the chamber should be 5% to 10% lower than the equilibrium moisture-related humidity inside sausages (around 90%).
In modern production, sausage fermentation is carried out in sealed chambers with strictly controlled temperature and humidity. Mild smoking can be applied to certain sausage varieties at this stage without interfering with fermentation progress. In the past, due to the lack of precise environmental control, specific measures were adopted in some countries to prevent spoilage during fermentation. Though redundant for modern production, these traditional methods are still applied to specialty products to obtain unique sensory characteristics. For example, certain German sausages are fermented at 25℃ under high humidity, with excessive surface microorganisms removed by regular washing.
Dry sausages ferment faster in still air than in rapidly circulating air. The acidification degree of fermented sausages varies by product type. Semi-dry sausages have the highest acidity, especially American semi-dry sausages with a post-fermentation pH below 5.0. The pH of German dry sausages ranges from 5.0 to 5.3, while dry sausages from France, Italy and other regions show mild acidification. Vacuum-filled and large-diameter sausages exhibit the strongest acidification due to hypoxic conditions. However, ammonia accumulation in large-diameter sausages counteracts the pH decline caused by lactic acid production.
