The general objective of achieving a reduced pH by fermentation and limited drying necessary for semidry sausages can be approached in two ways. Semidry sausages in North America are most often fermented rapidly (12 hr or less) at high temperature (32-46 °C) and to a relatively low pH (well below 5.0). In Europe and many parts of the world, fermentation tends to be slow (24 hr or more) at lower temperatures and to somewhat higher final pH. These differences in fermentation speed and final pH result in significantly different product flavor (5).
While the flavor, and sometimes texture, of semidry sausages can vary considerably, and can be highly acceptable in all cases, all of these products must meet a rather narrow range of conditions to assure that shelf life and safety are achieved. The reduced pH of fermented sausage is, by itself, a significant means of extending the shelf life of products and, when combined with low storage temperatures of 2°C or less, is a very effective means of preservation. However, as noted earlier, semidry sausages in the United States may also be considered shelf-stable without refrigeration if the pH is 5.0 or less and the M:P ratio is 3.1:1 or less. In addition, shelf-stable products have been defined by others in Europe as having, pH of 5.2 or less with water activity of 0.95 or less (6). Products that meet either of these requirements are very unlikely to spoil from bacterial growth, and shelf life is usually limited only by chemical or physical spoilage.
Safety from the hazards of pathogenic microorganisms is a separate issue from shelf life and one that is of critical concern to both meat processors and consumers. For semidry sausages, the relatively high temperatures often used for rapid fermentation can also accentuate the growth of pathogenic bacteria, particularly if acid production is slower than expected. Several pathogenic microorganisms are recognized as probable contaminants of raw meat under normal conditions. However, many of these are very unlikely to grow in the presence of salt, nitrite, and a dominant lactic acid culture. Salmonella spp., for example, are inhibited by salt and by growth of the starter culture; Clostridium botulinum and Clostridium perfringens are very effectively controlled by the presence of nitrite. These pathogens have very rarely been associated with food-borne illnesses originating from semidry sausages. However, pathogens that are salt- and nitrite- tolerant have the potential to survive and grow in semidry sausages. Two pathogens that have been long recognized as significant risks in fermented sausage are Trichinae spiralis and Staphylococcus aureus.
T. spiralis, if present, will survive fermentation but is easily killed by heat. Practically speaking, this pathogen is a risk only in unheated products, and because of recent requirements to inactivate other ''new'' pathogens such as Escherichia coli O157:H7, heat treatments for semidry sausages have become common and are typically more than adequate to inactivate T. spiralis. In addition, modern swine production practices have resulted in near-elimination of T. spiralis from pork. Consequently, concern for T. spiralis in semidry sausages is no longer a major issue, but it should be noted that regulations still apply for trichinae control in pork products. These regulations specify that trichinae-free pork (''certified pork'' in the United States) must be used for products that do not receive sufficient heating to kill the organism. Heating products to an internal temperature of 62.2°C is sufficient to inactivate this organism.
Staphylococcus aureus has been one of the ''trademark'' pathogens associated with fermented sausage, and care is required to prevent food-borne illness outbreaks from this organism. Staphylococcus is a relatively common contaminant in raw meat and one that is very salt- and nitrite-tolerant (1). This organism can produce a heat-stable enterotoxin (7) that survives heat processes even when the organism does not. It is not unusual to have a scenario in which a product has caused illnesses from staphylococcal toxin but no organisms can be recovered because they were killed by heating while the toxin remained viable. Fortunately, Staphylococcus aureus is not very tolerant of acid conditions, and a critical control point for control of this organism in fermented sausage is achieving pH 5.3 before the organism can produce toxin. Because staphylococcal toxin can be produced in fermented sausage when temperature exceeds 15.6°C, use of "degree-hour'' limits are used to prevent toxin production. These limits are defined as the time in hours multiplied by the temperature in degrees Fahrenheit in excess of 60°F (15.6°C) (1). For example, a product that reached pH 5.3 in 20 hr at 100 °F would have experienced (l00°F-60°F) x 20 hr = 800 degree-hours. Table 2 shows the maximum degree-hours allowed to reach pH 5.3 to assure control of Staphylococcus aureus toxin production. For semidry sausages fermented at 90-100 °F (32.2-37.8 °C) or higher, the degree-hours required is 1000; for fermentation temperatures of 100-110°F (37.8-43.0°C), the requirement is 900. Any product that reaches pH 5.3 in 18 hr or less would be considered safe from staphylococcal enterotoxin production during fermentation. The degree-hour concept assumes a constant temperature process during fermentation.
Since the mid-1990s, when E. coli O157:H7 was found in fermented sausage and determined to be the cause of food-borne illness outbreaks, additional requirements have
Maximum degree/hours3 to achieve pH 5.3
Maximum hours to meet degree/hour limit
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