Growth of E. coli in reduced salt cheddar cheese

Abstract

Modern day consumers have become more health conscious and there has been a movement towards reducing sodium intake in their diets. This is due to the risk of the development of hypertension and cardiovascular diseases, as well as other diet related non-communicable diseases associated with excessive sodium intake. Cheddar cheese is one of the most popular cheeses consumed globally and has a relatively high sodium content (2% w/w). A possible way of reducing the sodium content is by making use of replacement salts such as KCl and MgCl2. Partial substitution of NaCl with KCl and MgCl2 has been shown to be possible without compromising on key quality parameters, however very little work has been conducted on the effects of partial salt replacement on the growth of pathogenic bacteria such as E. coli. The first phase of the study focused on replicating the model employed by Grummer & Schoenfuss (2011), to determine equivalent water activities amongst the cheese samples made with different partial salt replacers. The model was adjusted accordingly, and any deviations were noted and taken into account for the second phase of the study. The second phase of the study involved the manufacture of reduced salt cheeses and their inoculation with three different serotypes of non-O157:H7 shiga toxin-producing E. coli. The effect that the alternative sources of salt, as well as reduced NaCl levels had on the growth of E. coli were studied. Physicochemical analyses for the water activity, moisture content and salt-in-moisture (S/M) content of all cheeses were carried out. All three E. coli serotypes were able to grow at water activities greater than 0.95, irrespective of the type of salt treatment used. Even though the Full NaCl control cheeses (2% NaCl) were salted to bring about water activities of less than 0.95, E. coli was still able to grow and increased for 14 days. No differences were found between E. coli growth in the different salt treated cheeses. A correlation was found between the S/M ratio and E. coli growth, with a higher S/M ratio resulting in less E. coli growth. Although water activity is a critical parameter with respect to the inhibition of E. coli growth in cheddar cheese, the S/M ratio was found to be just as crucial a consideration. A combination of hurdle technology is therefore required for ensuring the safety of cheddar cheese products. Salt content, in addition to low pH and low storage temperature work synergistically to exclude the growth of pathogenic bacteria, however much care must be taken when reducing the salt content of cheddar cheese. Reduction of the salt content may interfere with the balance of inhibition of other currently non-problematic bacteria, which may result in the necessity for the replacement of its antimicrobial action. It is therefore apparent that further research on the effects of salt reduction as well as its partial replacement on the growth of E. coli and other pathogens is required, before the implementation of potential salt reduction regulations in cheese products is considered.