Preservation of a meat product by fermentation and drying has been used for hundreds of years. For a long time, the technology of fermentation has been considered an "art"; however, more recently the process has been studied and as a result, products of quality can now be repeatedly produced under controlled conditions.
Dry or semi-dry fermented sausages are prepared by mixing ground meat with various combinations of spices, flavourings, salt, sugar, additives and bacterial cultures. The mixtures, in bulk or after stuffing, are allowed to ferment at different temperatures for varying periods of time. Following fermentation, the product may be smoked and/or dried under controlled conditions of temperature and relative humidity.
Chemical acidification may be used to help lowering the pH. Citric acid or glucono delta lactone are commonly used for this purpose. If product is acidified, controls shall be in place and records kept to ensure that pH of 5.3 or lower is achieved by the conclusion of the process.
The fermentation process involves the growth of lactic acid bacteria in order to acidify the product and produce the desired flavour. The use of commercially prepared starter cultures is recommended to ensure that the correct organisms are added in sufficient numbers. Commercial cultures should be stored according to the culture manufacturer's directions. Although less recommended and much more hazardous, back slopping is an alternative to commercial starter cultures. This technique consists in inoculating a new batch with a portion of a successful batch. Evidently, if pathogens are present in the inoculum, there is a risk of multiplication of those pathogens in the new batch. This is why strict controls shall be in place when using this technique. The inoculum shall be carefully handled and stored to avoid any contamination. The storage temperature for that inoculum shall be maintained at 4°C or less and a pH of 5.3 or less. Samples for microbiological analysis shall be taken to ensure that the process is in line with the specifications. The frequency of that sampling is to be adjusted according to compliance to specifications. Each batch of inoculum which will have a pH > 5.3 shall be analyzed to detect at least S. aureus. Only on satisfactory results, will this inoculum be allowed to be used for back slopping. Providing raw materials are of acceptable microbiological quality, during fermentation the combined effect of curing salts, curing aids and temperature encourages the gradual replacement of the contaminating flora including pathogens (such as salmonella, campylobacter and staphylococci) by lactobacilli, pediococci and micrococci.
Lactobacilli and pediococci are primarily responsible for converting sugars into lactic acid thereby lowering the pH of the meat product. Where nitrate salts are used for curing in slow cured sausages, micrococci present convert nitrate salts to nitrite salts.
Lactobacilli with or without micrococci are components of starter cultures available for use in slow fermentation (25°C) whereas pediococci with or without micrococci are used in starter cultures for rapid fermentation at higher temperatures (25°C to 37°C). Pediococci do not occur in fresh meat products in numbers large enough to be a significant factor in traditional slow fermentation and therefore are only important in meat product fermentation if they are added in starter cultures.
When fermented cured sausages are subjected to an extended drying period, lactobacilli act to significantly reduce the number of undesirable microorganisms including pathogens. The predominant type of fermenting organism combined with the formulation and process schedule will give a product its characteristic flavour.
Contamination by pathogenic organisms at the outset of the process may have a critical effect on finished product. Bacterial competition, pH and aw values are important factors in the control of the development or die-off of pathogenic organisms. Staphylococcus aureus and the production of its enterotoxin are of significant concern. Once the pH of the meat mixtures reaches 5.3 or lower, the environment for Staphylococcus aureus is effectively controlled. During the fermentation and the lowering of the pH to 5.3, it is necessary to limit the time during which the sausage meat is exposed to temperatures higher than 15.6°C. A controlled environment of 15.6°C or less minimizes the opportunity for Staphylococcus aureus reaching levels of public health significance.
In order to obtain a fermented dry sausage (shelf stable product) by the process of fermentation and drying, in addition to a pH of 5.3 or less at the end of the fermentation stage, a finished product's aw of 0.90 or lower is necessary. The physical characteristics of the meat and fat particles (such as particle size, product temperature, etc...) are important in achieving an aw of 0.90 or less. The meat particles must be of such size that would efficiently allow release of moisture and the cut edges must be without fat smearing. Sharp and efficient grinding or chopping equipment and mixers are necessary.
Properly designed and equipped greening and drying rooms are important in achieving both a pH of 5.3 or lower and an aw of 0.90 or less. Coincidental with moisture loss, fermentation must continue through these processing steps.
The two factors primarily responsible for the keeping qualities of fermented meat products are a low pH and low water activity. Although the nominally stated levels are a pH of 5.3 or lower and an aw of 0.90 or less, other combinations of pH and aw may be acceptable provided a lower pH compensates for a higher aw or vice versa. Such processes must be submitted for evaluation and acceptance to the Chief of Foodborne Pathogens. It is important for the firm to maintain processing and quality control records in order to document the safety of the process being used.
To minimize the danger of outgrowth of C. botulinum spores and development of the botulinal toxin in the shelf-stable cured product, nitrite/nitrate shall be added at a minimum level of 100 ppm along with a minimum of 2.5% of salt. The level of nitrate-nitrite should not interfere with the process of fermentation. Processors should consider other hazards such as verotoxic E. coli e.g., E. coli O157:H7 et O111. Some cases of haemorrhagic colitis have been associated with these organisms. Processors must therefore ensure that the fermentation process they are using will address these hazards.
The growth and metabolism of microorganisms demands the presence of water in available form. The most useful measurement of the availability of water in meat products is water activity (aw). The aw may be reduced by adding solutes (salt, sugar) or removing moisture. Aw measurements are therefore a valuable aid in assessing the need for refrigeration in subsequent storage.