Effect of salt content on microbiological properties in processed meat products
Salting is one of the most traditional and oldest ways of preserving food. From a preservation point of view, salting is used because most bacteria and fungi cannot survive a highly salty environment. Why is that? To understand the process you first have to understand the concept of water activity.
Water activity (aw)
For microbiologists, water activity is:
the available moisture that best correlates with the growth of microorganisms under conditions of reduced water content.
In other words, it is the ratio of the vapour pressure of the food (or solution) (i) to that of pure water at the same temperature (p0): aw= p/p0.
Water activity is based on a scale of 0 to 1.0 with pure distilled water having aw of 1.00 and fresh meat up to 0.99, but this value is reduced by evaporation during chilling. As a solution (or food) becomes more concentrated, p decreases and aw falls. It is related to the boiling and freezing points, water equilibrium, relative humidity and osmotic pressure.
How does adding salt decrease aw?
Salt is a compound made up of positive sodium (Na+) and negative-charged chloride (Cl-). When salt is added to water, water molecules, which are polar, associate with the salt ions (Na+ and Cl-) and surround them, forming 'spheres of hydration'. This disassociates them from each other and keeps them separated. Consequently, salt dissolves in water, the solution becomes more concentrated, and therefore p declines and aw decreases. Yet, if you keep adding salt there will be a point when there are no longer enough water molecules to surround the ions and some of the Na+ will be directly associated with the Cl- (Strelkauskas et al., 2010).
Effect of salt solutions on micro-organisms
Cells tend to equalize water concentration inside and outside the cell wall by a process of osmosis (water moves through the cell membrane from a low solute concentration to a high solute concentration). Normally, therefore, when micro-organisms are placed in solutions which have high osmotic pressure, such as concentrated salt brine, water inside the microbial cell moves out through the membrane and into the brine, causing a partial dehydration of the cell. This slows metabolic processes and interferes with multiplication of the micro-organisms.
Most micro-organisms cease growth at aw <0.9 and the majority of bacteria that can cause superficial spoilage on meat are very sensitive to reductions in aw. The micro-organisms which are more tolerant of reductions in aw include some Gram-positive bacteria which are used commercially as starter micro-organisms in meat fermentation processes (such as Lactobacilli). In addition, yeasts and moulds can tolerate aw values below 0.95 (Lind et al., 2000; Brown, 1982). Table 2 shows the approximate minimum aw values tolerated by a range of different bacteria.
Table 3.2 Approximate minimum aw values (Adapted from Lind et al, 2000)
|Clostridium botulinum type A||0.94|
|Clostridium botulinum type B||0.94|
|Clostridium botulinum type E||0.97|
Based on their aw, identify from the table the micro-organisms that could either cause superficial spoilage or be present on processed (salted or semi-dried) stored meats.
Can you think how these micro-organisms compensate physiologically for the high osmotic pressure outside the cell?
The next section will explain this.
To grow in a highly saline environment, the cells of these micro-organisms exclude sodium and concentrate potassium within the cell to very high levels. The concentration of potassium and amino acids within cells increases as the aw of the growth medium decreases. Such accumulated potassium and amino acids are called compatible solutes. The enzymes in these micro-organisms are unusually tolerant of sodium and potassium; and intracellular potassium is believed to trigger the induction of the genetic systems involved in osmoregulation. In halophilic bacteria, such as S. aureus, the major compatible solutes besides potassium that accumulate, either by synthesis or transport, are: betaine, proline and trehalose. Similarly, in very salt-tolerant yeasts, such as Debaryomyces hansenii, potassium accumulates in preference to sodium. However, the main solute concentrated during its growth at low aw is glycerol, permitting growth below 0.75 aw(Lind et al., 2000).
Halophilic bacteria have been classified as moderately halotolerant, halotolerant and extremely halotolerant.
- Moderate halophiles are organisms that grow optimally between 3% and 15% salt (such as Halomonadaceae spp.).
- Halotolerant bacteria are able to grow in the absence of salt as well as in the presence of relatively high salt concentration (8% in the case of S. aureus).
- Extreme halophiles grow above 15% salt (for example Actinopolyspora halophila).
Moderate and halotolerant bacteria have been the major spoilage agents in meat-curing brines (Lind et al., 2000; Ventosa et al., 1998) and S. aureus is one of the most typical pathogens found in salted meat products. Contamination of meat and meat products with S. aureus can occur through handlers coughing and sneezing onto the meat, because this bacterium is present in large numbers in the nasal secretions of humans. S. aureus is a Gram-positive facultative anaerobic bacterium which produces five enterotoxins. It is the toxins which are released into the food or the digesta in the gut that are actually responsible for food poisoning and toxic shock syndrome. Their incubation period is 2-6 hours and they cause symptoms such as vomiting and diarrhoea (Feiner, 2006).
What characteristics in a person with food poisoning would lead you to suspect that the poisoning could have been due to S. aureus as distinct from other species of bacteria?