In this article we will discuss about the fermentation process of sausages.
Fermented sausages are sometimes claimed to have originated in the Mediterranean region, although traditional products in China and Southeast Asia suggest that they probably developed independently in several locations.
Like cheese making, meat fermentation is a method for improving the keeping qualities of an otherwise highly perishable commodity. Key features in this are the combination of lactic fermentation with salting and drying which, in many cases, produces a product which is shelf-stable at ambient temperatures.
A further similarity to cheese is the bewildering variety of different types, 330 produced in Germany alone. In the United States fermented sausages are divided into two categories: dry, which have a moisture content of 35% or less, and semi-dry typically containing about 50% moisture.
Spreadable fermented sausages, produced in Germany, such as Teewurst, and Mettwurst are not dried during production and in this respect are similar to the Thai product nam.
The ingredients of a European-style fermented sausage may comprise:
lean meat, 55-70%
curing salts, 3%
fermentable carbohydrate, 0.4-2.0%
spices and flavouring, 0.5%
starter, acidulant, ascorbic acid, etc. 0.5%
Pork is most commonly used in southern Europe but elsewhere beef, mutton and turkey meat are also used. The meat should always be of high quality since the products are usually consumed without cooking and so are essentially a raw-meat product.
Unlike fermented milk products, it is not possible to heat treat the meat before processing as this would destroy the sausage’s textural characteristics, but some are given a final pasteurization to ensure safety.
The curing salts added are a similar mixture of sodium chloride and sodium nitrate and/or nitrite to that used in the production of cured meats such as ham and bacon. Here too, they contribute to the taste, colour, safety, stability and texture of the product.
Spices are added primarily for reasons of flavour but are known to have potentially important roles in retarding microbial spoilage and promoting lactic fermentation. The antimicrobial effect of spice components, could help inhibit the normal spoilage microflora of the meat. Spices have also been shown to stimulate the growth of lactic acid bacteria.
This is a result of their manganese content, spice extracts or spices low in manganese do not have this stimulatory effect. Most aerobes have micro molar quantities of the enzyme superoxide dismutase (SOD) to scavenge the toxic superoxide anion radical produced by a one-electron reduction of molecular oxygen.
Aero tolerant lactic acid bacteria do not have SOD but have developed an alternative protective mechanism based on the accumulation of milli-molar concentrations of manganous (Mn2+) ion.
The Mn2+ is regenerated by a subsequent reduction step. Increasing the manganese content of the medium can stimulate LAB growth.
Other ingredients which may be included are glucono-δ-lactone which improves acidulation by slowly hydrolyzing to produce gluconic acid, ascorbic acid to improve colour production and stability, and glucose to supplement the available fermentable sugar in the starting mix.
Ingredients are blended together in a bowl chopper at low temperature. When the ingredients have been blended together they are packed into casings of the appropriate diameter. Traditionally collagen from the gastrointestinal tract of animals has been used but nowadays fibrous cellulose and regenerated collagen produced from animal hides are more common.
The packing material must have certain properties: it must adhere to the meat mix and must shrink with it during processing and must be permeable to moisture and smoke.
Fermented sausages are still often made by natural fermentations in which the selectivity of the starting mix determines which components of the heterogeneous microflora dominate. Starter cultures are however being increasingly used for the greater assurance of a satisfactory fermentation they provide.
The principal components of commercial starters are lactic acid bacteria and nitrate-reducing bacteria. Some will also include yeasts and moulds such as Debaryomyces hansenii and Candida famata, and moulds, usually Penicillium spp. LAB included in early starters were mainly Lactobacillus plantarum, Pediococcus acidilactici and Ped. pentosaceus, not necessarily those most important in the natural fermentation.
Surveys of naturally fermented products demonstrated that the dominant LAB were psychrotrophic, facultatively heterofermentative lactobacilli (see page 257) that were slightly less acid tolerant than usual (minimum pH 3.9 compared with 3.7-3.8). Most of these are now assigned to the species Lactobacillus sake and Lactobacillus curvatus and strains of these have been incorporated into commercial starters.
Members of the Micrococcaceae such as Micrococcus variants and Staphylococcus carnosus are important with respect to the reduction of nitrate to nitrite although this activity has also been demonstrated in some lactobacilli. Their presence would not be required in nitrite-cured products.
Sausage fermentations are conducted at temperatures ranging from 15 °C up to in excess of 40 °C, depending on the starter used, and last for 20-60 h. The relative humidity is also controlled to ensure that a slow drying of the product commences.
Acid production and the decrease of the pH to below 5.2 promote die coagulation of meat proteins and this aids moisture expulsion and development of the desired texture and flavour. It also contributes to the microbiological stability and safety of the product.
North American and northern European sausages are often smoked. This confers a characteristic flavour but phenolic components of the wood smoke also have important anti-oxidative and antimicrobial properties which improve shelf-life. Fungal growth may occur on the surface of un-smoked sausages providing a particular character to these products as a result of fungal lipolytic, proteolytic and anti-oxidative activity.
In the final drying stage which can last up to 6 weeks, the moisture content is reduced further by storage at low temperatures, 7-15 °C, and at low relative humidity (65-85%). The combination of antimicrobial hurdles or barriers introduced during sausage fermentation is normally sufficient to ensure product safety.
Staphylococcus aureus with its ability to tolerate reduced aw and pH and grow anaerobically would seem well suited to growth in these products. Occasional outbreaks of Staph, aureus food poisoning have been reported from the United States where higher fermentation temperatures are used.
However studies suggest that Staph, aureus does not compete well with the LAB present, particularly if the latter have a large numerical superiority as a result of starter addition. The risk is also reduced since enterotoxin production appears to be more susceptible than growth to inhibition by adverse conditions.
Numbers of Salmonella and other Enterobacteriaceae have been shown to fall throughout fermentation and drying. At present though, our knowledge of such processes is insufficient to allow us to predict this lethal effect reliably. It is therefore, most important that only good quality raw materials are used so that undue reliance is not placed upon these factors.
In the United States semi-dry sausages are heated to 74 °C which controls the risk from vegetative bacteria (though not from pre-formed Staph, aureus toxin). Following an outbreak of salmonellosis in the UK associated with a salami stick product imported from Germany, the production process was changed to incorporate a final pasteurization step.
Nam, the Thai fermented sausage, differs in several respects from European fermented sausages. It is a low-fat product which is subjected to a short fermentation and is not dried. It is also wrapped in water-impermeable plastic material or, traditionally, banana leaves.
As the fermentation proceeds and the pH drops the moisture is expelled but is trapped within the packaging giving the consumer an indication of the age of the product. It is not always stored chilled and its largely anecdotal association with food poisoning has prompted test marketing of irradiated nam in some areas of Thailand.