The survival and growth of Bacillus cereus and Listeria monocytogenes, during the manufacture of Ricotta Salata cheese : a thesis presented in partial fulfilment of the requirements for the degree of Masters in Food Technology at Massey University, Palmerston North, New Zealand

Abstract

This study was conducted with the following objectives: 1) to investigate the survival and growth of Bacillus cereus during the manufacture of Ricotta Salata cheese; and 2) to investigate the survival and growth of Listeria monocytogenes during the manufacture of Ricotta Salata cheese. The Ricotta Salata cheese was made by heating the whole milk to 95oC, and adding citric acid to coagulate the cheese curd. The cheese curd was inoculated with 7 log10 CFU/g B. cereus broth and 8 log10 CFU/g L. monocytogenes broth. After moulding for 12h, Ricotta Salata cheese was stored at 4oC for 1 week. During manufacture, the physico-chemical properties [pH, water activity (aw), and Sodium chloride (NaCl) concentration] and bacterial counts were recorded. The pH change fluctuated between 6.00 to 6.10 on the surface and 6.00 to 5.95 in the centre; the lowest aw was approximately 0.96 on the surface and 0.97 in the centre; and the highest NaCl concentration was 3.3% on the surface and 3% in the centre. The survival and growth of the two B. cereus strains (D1 and ATCC 13061) during the manufacture of Ricotta Salata cheese were similar. The B. cereus grew from approximately 5 log10 CFU/g to a maximum of 7.7 log10 CFU/g of cheese curd during moulding (20h at room temperature). The survival and growth of the two L. monocytogenes strains (W1 and ATCC 35152) during the manufacture of Ricotta Salata cheese were similar. The difference between the bacteria count on the surface and in the centre was very small. L. monocytogenes increased from 5 to 6 log10 CFU/g to a maximum of 8.6 log10 CFU/g during manufacture and maintained a level of around 8 log10 CFU/g in the final product. The Ricotta Salata supported the survival and growth of B. cereus and L. monocytogenes during manufacture. It is important to improve the management of process hygiene for reducing the environmental contamination. Ideally, some lethal treatments should be applied after the packaging of the cheese, to limit the contamination of Ricotta Salata with these two bacteria.