Our experiments show that the effect of various measures is influenced by many factors. Mild heat treatment is a particularly robust method which ensures a high reduction of E. coli while safeguarding good taste. This method is also rapid and technologically simple to implement in a production line.
Finding efficient measures that also keep the good taste and high quality of the sausages is a major issue. The implementation of any new measure also entails changes in logistics and additional costs.
New technology and simpler traditional measures both work, and will help ensure safer cured sausages for Norwegian consumers when they are implemented in the industry.
From recipe to consumer
There is a great variety of cured sausages on the Norwegian market. What most of the industrially produced cured sausages have in common is the use of starter cultures to achieve a rapid and controlled pH reduction and also to obtain the desired taste and aroma. The pH reduction occurs during the first 2-3 days of the production process and also ensures that undesirable bacteria do not proliferate. (Some sausages are also smoked once or twice during this period.) Salt and often also nitrite is added in order to optimise the fermentation process. Water activity is reduced during the maturing and drying period of about three weeks, which takes place under controlled temperature and humidity, and then the sausages are ready for transportation to the stores or to short-term intermediate storage (for up to two weeks). Measures to ensure safe sausages can be implemented at various stages in this process from the determination of the recipe to the sausages are eaten by the consumer. Such measures may include changing the recipe, changes in the production process and how or whether the sausages are treated after they are finished but before they are eaten.
Changes in recipe and production process
Changing the recipe or the production process has limited effect on reducing E. coli. Combining the two, however, can optimally reduce E. coli by 1-2 log (90-99%) more during the production process. Conditions that favour the reduction of E. coli include a low pH in the end product, a low water activity in the finished sausages, high concentrations of salt and nitrite and fermentation at high temperatures. Increasing the temperature from 20 to 30°C during fermentation leads to increased reduction of E. coli. Using optimal fermentation temperatures for the starter culture also ensures a more robust fermentation under tougher conditions (higher levels of salt and nitrite). An interesting observation was that 100 ppm nitrite did not cause increased E. coli death in the sausages compared to sausages without added nitrite (organic cured sausages), but that levels of 300-500 ppm nitrite did have such an effect.
In addition to having a limited effect on E. coli, there are other factors that speak against introducing some of these measures in industrial production: an increased salt content is not a health benefit. High levels of salt and nitrite can also inhibit the starter culture, and thus the desired decrease in fermentation and pH. Taste wise there was surprisingly little difference between sausages with various levels of salt, nitrite and glucose.
Heat treatment of cured sausages is common in many countries, but not much used in Norway. Heat treatment can be carried out at relatively low temperatures that will only have a limited effect on the taste of the product while significantly reducing E. coli levels. Industrial production of sausages which includes heat treatment at 43°C for 24 hours gives about a 5 log (99,999%) reduction in E. coli. Higher temperatures need shorter heat treatment in order to have the same effect, but will have increasingly negative effects on the taste of the product.
High pressure treatment
High pressure treatment is a relatively new technology which can be used to ensure increased food safety. The treatment is carried out at 600 MPa (6000 atm) for several minutes. Extra E. coli killing of 2-4 log can be expected. Our experiments show that high pressures can have a good effect on reducing E. coli in cured sausages with a low water activity. The process requires investment in special equipment and logistic rearrangements in the industrial production of cured sausages, but operating costs are low. Sensory scientific studies on untreated sausages and sausages who had received pressure treatment revealed no differences in taste and texture.
In today's production process, cured sausages are considered ready to eat after about three weeks. Short time storage of cured sausages at the manufacturers' does occur, but is not widespread. Storage of cured sausages can reduce E. coli levels, but the storage conditions must be regulated. The right combinations of storage time and storage temperatures are important factors. An increased reduction is attained at high temperatures and longer storage time. Very little reduction is attained at cold storage (4°C) while reductions of around 1-2 log and 2-3 log are attained at storage at 20°C for 1 and 2 months respectively. The level of reduction also depends on the recipe and other conditions in the sausages. Storage is increasingly effective as a measure to reduce E. coli in sausages depending on how tough these conditions are (low pH, low water activity, high nitrite, high salt).
Freezing is a method that is not much used in food safety, but which can be used on cured sausages without reducing sensory properties. Freezing in itself does not kill bacteria to a great extent, but freezing and subsequent thawing does. Freezing (at -20°C for 24 hours) and subsequent thawing gives about an extra 1 log reduction in E. coli. Four repeated cycles of this freezing and thawing process increased the effects a little bit. Combining freezing with subsequent storage for 1 month can give a 2-4 log reduction depending on storage temperatures.