Effect of lauric acid and monolaurin on the multiplication of Listeria monocytogenes and Listeria innocua at 10°C in bi-phasic systems.

The effect of monolaurin (0.35 mM) and lauric acid (5.0 mM) alone and in combination has been tested on growth of three cheesebome strains of Listeria monocytogenes and two strains of L. innocna. The Listeria spp. were grown in Tryptose Soya Yeast Glucose Broth (TSGYB) in shake culture at 10 °C and an initial pH of 7.0. The additives were dissolved in butter oil 10 % (w/v). Lauric acid (5.0 mM) increased the doubling time of the five Listeria spp. by 3 - 8 h at 10 °C. Monolaurin by itself was found to slightly increase or decrease the doubling time depending on the microbial strain. Monolaurin had an augmentative effect when combined with lauric acid in the presence of butter oil, where the doubling time increased between 10 to 15 h depending on the strain.Inhibition of Listeria spp. was seen in the model bi-phasic broth system. A model food system was developed to test the antimicrobial properties of lauric acid and monolaurin, where the fat soluble additives were dissolved in cream and milk with 3.6% (w/v) fat. The milk was reconstituted from skim milk and cream 40 % (w/v) fat containing lauric acid or monolaurin. This milk was used to make a soft-ripened cheese of the Brie Camembert type. Two strains of L. innocna were added to the reconstituted milk. During production of the soft-ripened cheese, a draining table was designed to comply with COSHH regulations so that the whey containing L. innocua could be removed and disposed of by heating for 30 min at 121 °C.In cheeses without lauric acid or monolaurin the population of L. innocua increased from 10[3] g[-1] to 10[7] cfu g[-1] on the surface of the cheese. The counts in the centre of the cheese were less at 10[5] cfu g[-1] after ripening for 28 d at 10 °C. Addition of 0.9 mM monolaurin reduced the count to 10[5] cfu g[-1] after 28 d ripening at 10 °C on the surface of the cheese. The effect of increasing the initial draining time at ambient temperature from 24 h to 48 h reduced the population to 0 after 28 d ripening at 10 °C. Unlike experiments in broth culture, addition of lauric acid changed the aroma of the Camembert-type cheese to give a blue cheese aroma. This was due to the conversion of lauric acid to a methyl ketone (2-undecanone) by the starter fungus Penicillium camembertii. Due to lack of stability of lauric acid in this system, lauric acid was omitted from the reconstituted milk in further experiments. During production of cheese, lactose was converted to mainly lactic acid by metabolism of the lactic acid starter. The presence of lactic acid combined with the added monolaurin resulted in a significant reduction in the population of L. innocua particularly when the draining time was increased from 24 h to 48 h.The unusual approach in this study was to dissolve the biocide in the non-aqueous phase, butter oil in the experiments in broth culture and in cream in the model cheese experiments. An untrained taste panel detected monolaurin (0.9 mM) in soft-ripened cheese. Some respondents liked the 'mature' taste whilst others described it as 'farmyard like'. In food systems the use of antimicrobials which result in an increase in the lag phase or a reduction in the overall population of pathogens, has a significant role in promoting the microbiological safety of a product which is eaten without further heat treatment.