During processing the microorganisms in the sausage mince are subjected to a large number of parameters that will influence their performance.
The effect of the more important factors – fermentation temperature, sugar concentration and sugar type, salt concentration, microbial contamination and pH of the raw materials, physical form of the starter culture – will be further described in the following paragraphs, specifically with relation to the Chr. Hansen cultures.
Temperature is the factor mostly influencing the fermentation process. An increase in temperature increases the rate of pH-lowering if below or close to the optimum growth temperature for the specific lactic acid bacteria.
In general, it has been found that a 5°C increase in temperature approximately doubles the rate of acid formation. The increase in acid formation is due to an accelerated flux through the enzymatic pathway converting sugar to lactic acid.
Figures 5 and 6 show typical profiles for the pH-decrease brought about by four different Chr. Hansen cultures, containing either fast or traditional P. pentosaceus and L. sakei strains.
It is clearly seen that increasing temperature from 18 to 38°C (64 to 100°F) increases acid formation by all cultures, but that the effect of increasing temperature is less in the fast cultures than in the traditional cultures.
Also, it is evident that P. pentosaceus is faster than L. sakei at the highest fermentation temperature, but has a similar or a lower speed at the lower temperatures. This corresponds to P. pentosaceus having an optimum growth temperature around 35°C (95°F), whereas the optimum temperature for L. sakei is approximately 30°C (86°F).
In general, higher fermentation speed will result in a lower pH even if the added amount of sugar is the same. Details on the temperature sensitivity of other Chr. Hansen cultures are available on request.
The growth of Staphylococcus, Debaryomyces and Penicillium strains added besides the lactic acid bacteria will in general proliferate from increased temperatures. The most important effect of temperature is probably related to the influence that temperature has on the acidifying culture and the resulting pH-profile.
Staphylococcus strains are very sensitive to low pH, and at pH below 5.0 the nitrate reductase activity starts becoming hampered. In order to ensure a fast on-set of Penicillium growth on the sausage surface both temperature and humidity must be high; above 20°C (68°F) and 90% relative humidity, respectively.
The type and amount of sugar directly affect the pH decrease and the time to achieve the lowest pH. Simple sugars, such as glucose, are readily utilized by all lactic acid bacteria, whereas other more complex sugars, such as lactose or maltose, are less easily fermented.
Table 6 compiles the amount of lactic acid produced from different sugars by Lactobacillus pentosus at optimum temperature conditions.
The data distinctly show why it is recommendable to add the most simple sugars to the sausage mince if a fast pH-drop is needed and how it is possible to control the rate and the extend of the pH-drop in a more subtle way.
By using a mixture of fast and slowly fermen- table sugars, a rapid but small pH decrease can be achieved at the beginning of the fermentation period and a slower rate to the final pH can be achieved in the end of the fermentation period.
This is recommendable in sausages added nitrate in order to inhibit unwanted bacteria in the beginning of the cycle, without suppressing Staphylococcus, nitrate reduction and color formation by a too fast, extensive pH-drop.
It has been found that increasing glucose concentration above 0.15% does not change the rate of lactic acid formation, in general (i.e. the slope of the pH-curve), but only the extend of the pH-decrease.
However, the final pH will also depend on the actual lactic acid bacteria and its sensitivity to low pH and the decline in aw obtained during the fermentation period.
Figures 7 and 8 show the pH-profiles made by four different Chr. Hansen cultures containing either fast or traditional P. pentosaceus and L. sakei strains.
Salt concentration and water activity
The acidifying bacteria of the sausage mince are only active at aw above a certain level. Decreasing the aw of the mince by adding salt or high amounts of fat will prolong the lag phase of the culture, resulting in an extension of the total fermentation time.
Decreasing the aw of the sausage during drying will inevitable reduce the activity of the cultures over time. To achieve maximum acidification it is therefore important to regulate the amount of salt in the mince and the drying rate correspondingly, keeping in mind the salt-in-water limit of the specific acidifying bacteria.
Staphylococcus and Debaryomyces strains added to sausage mince are much less sensitive to high salt concentrations than lactic acid bacteria. The a of most dried sausages do not reach Aw levels that totally inactive those species, though their growth and metabolic activity will be partly hampered.
The optimum salt concentration for growth of many Staphylococcus species is close to the salt-in-water content of the fresh mince.
Raw material quality
The indigenous bacterial flora of meat consists of microorganisms unintentionally added during butchering and preparation of the meat. The composition of this bacterial flora is influenced by the treatment and storage of the meat before it is used for sausage production.
Traditionally, meat was pre-salted in order to favor a bacterial flora consisting mainly of lactic acid bacteria. The method often resulted in fermentation faults as the composition of the lactic acid bacterial flora was always random. Typical fermentation faults included gas formation, bitterness and sourness, the latter especially as a consequence of the formation of acetic acid.
The use of pre-salting will in most cases result in the dominance of the starter culture organism and the desired pH-development. However, sometimes the indigenous lactic acid bacteria are so dominating that they control the fermentation process and this may cause problems.
This may also be the case if vacuum-packed meat is used. Therefore, it is of utmost importance that the bacterial load of the raw materials, even if just lactic acid bacteria, is as low as possible, preferably lower than 105 CFU/g.
Meat that has been left in production rooms before freezing or kept in cold storage for a long time at aerobic conditions contains high numbers of spoilage bacteria. Those organisms are characterized by their ability to break down fat and protein and produce putrid off-flavors.
At the same time pH of the meat is slightly raised. When a starter culture is used with such meat, the lag phase of the lactic fermentation may increase. The small pH-rise caused by the spoilage bacteria increases the buffer capacity of the meat and in order to obtain the same pH-drop more acid must be produced by the starter culture.
It may even be necessary to add extra sugar to produce the desired amount of acid. All in all, the total processing time is prolonged.
Physical form of the starter culture
Starter cultures can be applied as frozen liquid cultures, frozen pellets or as freeze-dried cultures. In general, the physical form has no great influence on the fermentation pattern, except that the lag phase of the frozen cultures is somewhat shorter.
Only for the very fast fermentation procedures, such as for the US style technology, this delay is of significant importance. The difference due to the delayed on-set of fermentation is approximately 5 hours.
In some cases, it is possible to shorten the lag phase of a freeze-dried culture by making a suspension of the culture in tap water (chlorine free) before application to the mince. But the method is not altogether reliable and is not recommendable.
Also, one should be aware that this step will not make up for the difference between a frozen and a freeze-dried culture. If a shorter lag phase is needed, it is recommended to increase inoculation level, change culture or change processing procedure.
Mold spores for surface inoculation should always be re-activated in water before use, but in order to avoid contamination with other microorganisms and loss of activity the re-activation time should be kept as short as possible.